Photocorrosion Inhibition of Semiconductor-Based Photocatalysts

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Photocorrosion Inhibition of Semiconductor-Based Photocatalysts: Basic Principle, Current Development and Future Perspective Bo Weng, Ming-Yu Qi, Chuang Han, Zi-Rong Tang, and Yi-Jun Xu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Photocorrosion Inhibition of Semiconductor-Based Photocatalysts: Basic Principle, Current Development and Future Perspective Bo Wenga,b, Ming-Yu Qia,b, Chuang Hana,b, Zi-Rong Tangb, and Yi-Jun Xua,b,* a State

Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350116, P. R. China

b College

of Chemistry, New Campus, Fuzhou University, Fuzhou, 350116, P. R. China E-mail: [email protected]

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Abstract Heterogeneous photocatalysis is a promising strategy for addressing the worldwide environmental pollution and energy shortage issues. However, unlike TiO2 with good photostability, the intrinsic drawback of photoinduced decomposition, i.e., photocorrosion, of semiconductors significantly challenges the durable photocatalysis. In this review, the photocorrosion mechanisms of typical semiconductors and different characterization methods proposed for monitoring the photocorrosion process of semiconductor-based composite photocatalysts are elaborated. Dedicated emphasis is put on the strategies for improving the anti-photocorrosion property of semiconductor-based photocatalysts including modifying crystal structure or morphology of semiconductors, doping heteroatom, hybridizing with various semiconductors and/or cocatalysts, and regulating photocatalytic reaction conditions. Finally, we cast a personal prospect on future development of rational design of corrosion-controlled semiconductor-based photocatalysts toward versatile photoredox applications.

Keywords: Photocatalysis; semiconductor; stability; photocorrosion mechanism; anti-photocorrosion.

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1. Introduction With the growing energy demand and worsening environmental pollution, searching renewable and clean energy resources is an urgent task to develop pollution-free technologies for environmental remediation and alternative energy supplies for the sake of sustainable development of human society.1-8 Semiconductor-based heterogeneous photocatalysis, a green and sustainable technology for direct conversion of abundant solar energy into chemical energy, provides a perspective strategy to address the energy and environmental issues in the future.2, 3, 9-15 Since seminal work regarding the photoelectrochemical water splitting using TiO2 electrode,16 various photocatalysts have been developed in diverse photocatalytic fields including photocatalytic elimination of pollutants,1 selective organic transformations to fine chemicals,17 photocatalytic reforming,18-21 water splitting2, 13, 22 and CO2 reduction.23-26 Particularly, the commercialization of TiO2-based photocatalyst is now proceeding not only for mineralizing pollutants in water but also destructing organics in gas phase.27-30 For a practical system, chemical stability is another critical issue for realizing large-scale implementation of solar energy conversion. Unlike TiO2 with good photostability, many semiconductors, such as metal sulfide materials, silver-containing compounds, copper-based materials and zinc oxide et al., exhibit relatively low chemical stability due to the light induced photocorrosion. For instance, metal sulfides-based photocatalysts are susceptible to corrosion under light illumination and the surface sulfide ions (S2-) would be oxidized by photogenerated holes to form sulfate (SO42-) and/or sulphur (S0) depending on whether or not the molecular oxygen is present, thus resulting in the deactivation of photocatalysts.31-35 In addition, the Ag+ in silver-containing compounds can be reduced to metallic Ag0 by photogenerated electrons, which leads to the decomposition of compounds and decrease of photoactivity. Considering that the long-term mechanical and chemical stability of photocatalyst is a prerequisite for technological applications, it is therefore highly required to surmount the photocorrosion of semiconductor-based composites in order to achieve stable photocatalysts for long-time photocatalytic redox reactions. Thus far, numerous efforts is devoted to restraining the photoinduced instability of semiconductorbased composites, with a focus on parameters, such as modifying the crystal structure, size and morphology of semiconductors,36, 37 doping with anion and/or cation,38 combining with other semiconductors,39, 40 hybridizing with various cocatalysts41, 42 and tuning conditions of different reaction systems.43-45 However, to the best of our knowledge, the relative review available to thoroughly summarize the mechanisms and strategies for inhibiting the photocorrosion of semiconductors in photocatalysis is scarce although this topic has been gaining continuous attention.46 Consequently, it is a strong incentive for us to provide such a state-of-the-art review to present the related principle and recent progress in enhancing the photostability of semiconductor-based photocatalysts. This review aims to provide a panorama of the photocorrosion over semiconductor-based composite photocatalysts and systematically summarize recent progress with regard to improving their anti-photocorrosion properties by means of different strategies. It begins by introducing the various photocorrosion mechanisms of different semiconductors. Then, we describe the general characterizations that have been proposed to verify the photostability of semiconductor-based -3-


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photocatalysts for providing some references to justify the photodecomposition of semiconductors after photocatalytic redox reactions. Subsequently, the special focus of this review is paid to classify the various strategies for inhibiting the photocorrosion of semiconductor-based composite photocatalyts with representative examples. In the end, we would cast a personal prospect on future development of corrosion-controlled semiconductor-based composite photocatalysts toward targeting artificial photoredox applications. 2. The photocorrosion mechanisms of semiconductors It has been well documented that the semiconductors may suffer from photocorrosion under light irradiation. Both the photogenerated electrons and holes can participate in decomposing the semiconductors, which thus leads to various photocorrosion mechanisms over different semiconductor-based photocatalysts. In addition, it is worth noting that photocorrosion mechanisms for semiconductor-based photocatalysts, such as metal sulfides, would be altered depending on whether or not molecular oxygen is contained in the reaction systems. Therefore, in-depth understanding on photocorrosion mechanisms of different semiconductor-based composites is of significant importance for designing photocatalysts with high photostability. In the following, we will give a concise description of different photocorrosion mechanisms in terms of typical semiconductor photocatalysts. 2.1. Photogenerated holes induced instability It is well-known that for metal sulfides (e.g., CdS), upon the suitable light illumination, electrons and holes are generated from the conduction band (CB) and valence band (VB) respectively, during which photoexcited electrons could smoothly transfer to the surface, while the transfer of holes is problematic.47, 48 The photogenerated holes could enrich on the outer surface of metal sulfides photocatalysts before consumed by the electron donors in the reaction systems.34 Under such circumstances, the photocorrosion originating from the irreversible hole-driven oxidation reactions in metal sulfides has frequently been observed, which leads to the oxidization of surface sulfide ions (S2-) to sulphur (S0) and/or sulfate (SO42-),31-35 thereby resulting in low photostability of metal sulfides and greatly restricting its practical applications. Therefore, the accumulation of excess photoinduced holes on metal sulfides surface is the main reason causing photo-dissolution, and the photocorrosion behavior of metal sulfides can be expressed as the following processes in the presence of oxygen (taking CdS as an example):35, 49-51 hν  CdS   eCB (CdS)+h +VB (CdS)

CdS  4h   2H 2 O  O 2  Cd 2  SO 24  4H 

(1) (2)

O 2  4e   2H   2OH 

(3)

CdS+2O 2  Cd 2+ +SO 24

(4)

An overall reaction is described as Equation (4). While the photocorrosion of metal sulfides (e.g., CdS) changes in the absence of oxygen, which is caused by two holes, the formation of possible -4-


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photooxidation product is sulfur by the photocorrosion reaction below:51-53

CdS  2h   Cd 2  S

(5)

O 2  4e   2H   2OH 

(6)

Zinc oxide (ZnO) is another typical, widely studied semiconductor photocatalyst. Upon band-gap photoexcitation, the photogenerated holes could cause the collapse of ZnO structure, and such photocorrosion behaviour has been known as a serious drawback for ZnO-based photocatalysts. Generally, the photo-driven dissolution of semiconductor ZnO can be described as the following with two slow steps where two holes are trapped on the surface (Equation 6 and 7) followed by the fast formation of an oxygen molecule (Equation 8) and the fast expulsion of Zn2+ from the surface (Equation 9).54-56 2  Osurface  h   Osurface

(6)

 Osurface  3O 2  3h   2(O-O 2 )

(7)

(O-O 2 )  2h   O 2

8)

2 Zn 2   2 Zn 2  (aq)

(9)

O 2  4e   2H   2OH 

(10)

The reaction for photo-driven decomposition of ZnO is: 2ZnO+4h   2Zn 2  O 2

(11)

Cu-oxide semiconductor (such as Cu2O) photocatalysts have received increasing attention for different types of photocatalytic applications.57-59 Nevertheless, the presence of photogenerated holes could lead to the change of Cu2O at different oxidation state under light irradiation and tend to corrode Cu2O during the photocatalytic redox reactions. Specifically, the semiconductor Cu2O has been demonstrated to be oxidized by photogenerated holes,60-62 which leads to the formation of CuO and thus the variation of physicochemical property, as shown below:63 (12) Cu2O + 2OH ― +2h + → 2CuO + H2O O 2  4e   2H   2OH 

(13)

Additionally, the photoinduced holes have been demonstrated to be detrimental for the metal oxynitride semiconductors, which have been extensively explored as potential candidates for visible light induced water splitting.64-66 The self-oxidative decomposition of oxynitride materials, in which nitrogen anions are oxidized to N2 by photogenerated holes (2N3− + 6 h+ → N2),67-69 not only compete with the water oxidation process but also lead to the serious photocorrosion under light illumination, which is unfavorable for the construction of efficient and stable metal oxynitride-based composite photocatalysts. Based on the above discussion, in order to enhance the stability and activity of these photocatalysts, it is imperative to restrict the reaction between the photogenerated holes and semiconductor as well as to facilitate the separation of photoinduced electron-hole pairs for promoting various photocatalytic redox reactions. -5-


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2.2. Photogenerated electrons induced instability Silver-containing compounds, such as silver orthophosphate, silver halides have received much attention due to their facile syntheses and high photocatalytic performance in redox reactions.70-74 However, the main obstacle for the applications of silver-containing photocatalysts is the serious defect of unwanted and uncontrolled photocorrosion.71 Specifically, most silver-based compounds are light sensitive and prone to decompose into metallic Ag when exposed to light.70 Using Ag3PO4 as a selected example, the decomposition of Ag3PO4 induced by electrons under visible light irradiation can be described as following.

Ag 3 PO 4  3e   3Ag 0  PO34 2H 2 O+4h   O 2 +4H 

(11) (11)

The overall reaction for decomposition of Ag3PO4 is:

4Ag 3 PO 4  12e  +6H 2 O+12h   12Ag 0  3H 3 PO 4  3O 2

(11)

The reduction of Ag+ in the lattice of silver-based photocatalysts by the photogenerated electrons will definitely dissolve the composite structure, which leads to relatively low chemical stability and obvious deactivation after repetitive cycles.75, 76 Additionally, the semiconductor CuO has also been confirmed to be unstable under light illumination and could be reduced into Cu2O and/or metallic Cu by photogenerated electrons, which thus affects the photocatalytic performance of CuO-based composites.58, 77-79 3. Characterizations to verify the stability of semiconductor-based photocatalysts The stability of a photocatalyst is of much importance for its practical applications in sustainable reuse. In most cases, the stability and reusability of semiconductor-based catalysts are investigated by performing the recycle experiments, among which the photocatalysts are collected and reused for several cycles toward target photocatalytic reactions.80-82 Notably, the cycle activity as an indication of photostability is not the conclusive proof for semiconductor corrosion since other events such as active site blocking, loss of sample during recycle, intermediates adsorption or surface changes could also lead to the activity loss of photocatalysts during recycle tests. Therefore, the stability of photocatalysts should be further confirmed by characterizing the samples after recycle experiments. Generally, the morphology information, crystal structure and surface property of the reused samples are analyzed by electron microscopy (such as SEM and TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively, for proving whether or not the structures of photocatalysts are well-maintained under light irradiation and confirming the photostability of semiconductor-based composites. Beside the characterizations of collected samples after photocatalytic reactions, to monitor the reaction solution by inductively coupled plasma emission spectrometry (ICP) and/or linear sweep voltammetry for detecting the dissolution of photocatalysts is also an efficient way for demonstrating the stability of the samples. In the following section, we will elaborate how these technical characterizations judge the photostability of semiconductor materials with typical examples. -6-


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(i) The morphology and microstructure of semiconductor-based catalysts may be destroyed or changed due to the photocorrosion during the light irradiation process. Therefore, electron microscopy (such as SEM and TEM) is an efficient tool for studying the stability of semiconductor-based composites by analyzing the crystalline structures and morphology changes of the samples before and after photocatalytic tests. For example, Guo and co-workers have reported that the Au@MoS2-ZnO sample exhibits stable photoactivity for H2 generation (Figure 1A) under UV-Vis light illumination using a 300 W Xe arc lamp with 0.3 M Na2S and 0.3 M Na2SO3 as holes scavengers.80 The morphologies of Au@MoS2-ZnO sample before and after photocatalysis is nearly intact with no corrosion observed, as confirmed by SEM in Figure 1B and C, which suggests the effective photocorrosion inhibition of the composites. In addition, Cui et al. have demonstrated that the CdS encapsulated in carbon nanotubes (CdS-in-CNTs) photocatalyst shows excellent photocatalytic stability, as displayed in Figure 1D.81 The TEM images of CdS-in-CNTs reveal that CdS particles in CNTs show no great loss after photocatalytic recycle tests (Figure 1E and F), demonstrating that the CNT shell is able to protect the semiconductor CdS from being photo-corroded.

Figure 1. Photocatalytic hydrogen production rate of Au@MoS2-ZnO under UV-Vis light illumination using 300 W Xe arc lamp with 0.3 M Na2S and 0.3 M Na2SO3 as holes scavengers (A); SEM images of Au@MoS2-ZnO before (B) and after (C) photocatalytic reactions. Reprinted with permission from ref.80. Copyright 2016 John Wiley & Sons, Inc.80 Photocatalytic recycle degradation of MB as a function of the irradiation period over CdS-in-CNTs under visible light irradiation (λ ≥ 420 nm, 300 W Xe arc lamp) (D); TEM images of CdS-in-CNTs before (E) and after (F) four reaction cycles. Reprinted with permission from ref.81. Copyright 2014 Royal Society of Chemistry.81 (ii) As for the photocorrosion process of metal sulfide-based catalysts, the surface lattice S2− in metal sulfide could be oxidized to S0 and/or SO42- by photogenerated holes, which will lead to the change of surface structures of metal sulfide in the composite catalysts. Consequently, X-ray photoelectron spectroscopy (XPS) measurement has been employed to investigate the surface valence state of the element S in the sample of metal sulfide-based catalysts before and after recycle tests for -7-


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evaluating their photostability.33, 83 For example, Yu and co-workers33 have reported that the Ti(IV)−Ni(II)/CdS sample exhibits higher photostability toward H2 generation than the bare CdS (Figure 2A) under visible light irradiation (λ ≥ 420 nm) using a 350 W Xe lamp in a Na2SO3-Na2S mixed solution. The surface microstructures of various CdS samples after photocatalytic reactions have been studied by XPS, as shown in Figure 2B. The XPS spectrum of S 2p shows that a small amount of S0 originated from the oxidation of surface lattice S2− by photogenerated holes has been detected, suggesting the photocorrosion of semiconductor CdS. The X-ray diffraction (XRD) of silvercontaining compounds will be greatly changed once the metal Ag particles are formed due to the photocorrosion process, which is a distinct characterization to monitor the photoinduced instability of silver-containing composites. Teng et al. have observed the stable activity over branched tetrapods Ag3PO4 (BTA) sample for the degradation of RhB under natural indoor weak light irradiation, as shown in Figure 2C, and the XRD pattern of the sample maintains unchanged after three recycles reactions. However, under visible light irradiation (λ ≥ 420 nm) provided by an artificial Xe lamp, the degradation ratios of RhB over BTA sample decrease gradually, which indicates poor stability of BTA under visible light and a significant amount of metallic Ag has been detected for the Ag3PO4 sample after catalytic reactions (Figure 2D).84

Figure 2. Cycle runs of the photocatalytic hydrogen generation activity under visible light irradiation (λ ≥ 420 nm) using a 350 W Xe lamp in a Na2SO3-Na2S mixed solution for various samples: (a) CdS, (b) Ti(IV)/CdS, (c) Ni(II)/CdS, and (d) Ti(IV)–Ni(II)/CdS (A); high-resolution XPS spectra of S 2p for the various samples after photocatalytic reactions: (a) CdS(after), (b) Ti(IV)/CdS(after), (c) Ni(II)/CdS(after), and (d) Ti(IV)−Ni(II)/CdS(after) (B). Reprinted with permission from ref.33. Copyright 2016 American Chemical Society.33 Cycle curves of branched tetrapods Ag3PO4 sample for degradation of RhB under natural indoor weak light irradiation (CC); XRD patterns of branched -8-


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tetrapods Ag3PO4 sample after reaction (D). Reprinted with permission from ref.84. Copyright 2015 American Chemical Society.84 (iii) According to the photocorrosion mechanisms of ZnO-based photocatalysts, the photogenerated holes induced collapse of ZnO directly lead to the leakage of Zn2+ in the reaction solution.55 Therefore, the measurement of the leakage content of Zn2+ by inductively coupled plasma emission spectroscopy (ICP) during the reaction process is one of effective approaches to monitor the photodecomposition of ZnO-based photocatalysts. For instance, the Chen et al. have employed ICP for measuring the concentration of Zn2+ in the solution to study the photocorrosion effect of ZnO/RGO catalysts.82 After exposure under UV light for 80 h, the concentration of Zn2+ for ZnO/RGO composites is lower than that of pure ZnO, indicating that the photoinduced instability of ZnO is effectively hindered in the photocatalyst of ZnO/RGO. Similarly, the holed induced photocorrosion over metal sulfides has also been investigated by ICP. Huo and co-workers have evaluated the photoinduced dissolution of CdS-doped TiO2 under visible light irradiation via the amount of Cd2+ leakage.85 According to the ICP analysis,85 the supercritical treated CdS fast leaches off while only a few CdS in CdS-TiO2 with supercritical treatment leaks out even after being used repetitively for 6 times, exhibiting a remarkable inhibition of the photocorrosion of CdS (Table 1), which is attributed to the strong interaction between the components of CdS and TiO2. Furthermore, linear sweep voltammetry has been recently developed by Xu et al. to measure the concentration of Cd2+ with a bismuth film electrode,86 which provides another method to detect the concentration of Cd2+ in the reaction solutions, thereby judging the corrosion process of semiconductor CdS under light illumination. Table 1. The ICP analysis of residual Cd2+ in the different used photocatalysts a Sample Recycles Residual Cd2+ (%) CdS-TiO2 6 90 CdS 3 9.4 a Reaction conditions: 0.080 g of photocatalyst, 50ml 1.0×10-4 M p-chlorophenol (4-CP) aqueous solution, a 500 W Xe lamp (λ > 420 nm) at 18 cm away from the reaction solution, reaction temperature = 303 K, stirring rate = 800 rpm, reaction period = 6 h. Reprinted with permission from ref.85. Copyright 2011 Elsevier.85 4. Strategies for improving the stability of semiconductor-based photocatalysts To implement semiconductor-based composite photocatalysts into practical applications, one of the most important issues that need to be addressed is photocorrosion. As mentioned in Section 2, the corrosion of semiconductors could be induced by both photogenerated electrons and holes,34 55, 71 which thus leads to the loss of photoactivity. Under such circumstances, efficiently exporting photogenerated charge carriers from the surface of semiconductors seems to be a crucial method for improving their stability and inhibiting the photocorrosion effect. Hitherto, research efforts have been devoted to designing semiconductor-based composite materials with stable photoactivity (Figure 3), such as modifying the crystal structure, size and morphology of semiconductors,36, 37 doping with heteroatoms,38 elaborating hterojunction,39, 40 hybridizing with cocatalysts41, 42 and tuning reaction conditions.43-45, which will be detailedly summarized in the following section with typical examples.

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Figure 3. Strategies for inhibiting the photocorrosion of semiconductor-based photocatalyst. 4.1. Modification on the crystallinity and morphology of photocatalysts It has been well recognized that the photoactivity and stability of semiconductor-based composites could be optimized by engineering the physical properties of semiconductor materials, such as crystallinity, size and morphology.36, 87-89 In general, the semiconductor with high crystallinity shows better photoactivity and stability than that with poor one since the increase of crystallinity could gain efficient charge migration and diminish the recombination centers for photoexcited electron-hole pairs.36, 90 Additionally, since the physical/chemical properties of semiconductors are intimately correlated to their microscopic structural/morphology factors (e.g., size, shape, crystal facet), the “structure-dictates-function” has been regarded as a basic principle in chemistry.87, 91, 92 Therefore, sustained effort and interest from researchers has been devoted to designing materials with desirable architectural structural morphology for improving their photoactivity and stability toward diverse photocatalytic applications.93-99 A literature survey indicates that the modification on the crystallinity and morphology of semiconductor-based photocatalysts can minimize the photocorrosion of these materials which will be elucidated in the following text. 4.1.1 Modification on the crystallinity of photocatalysts. Improving the crystallinity of semiconductors is beneficial for ameliorating their photocatalytic activity and stability.87, 90 For instance, Bao et al. have reported a novel method for preparing CdS photocatalysts with different crystalline phases,100 among which the freshly prepared CdO has been subjected to thermal treatment in the presence of H2S gas (denoted as CdS-S-m, where m represents the thermal treatment temperature). The high intense of XRD peaks over CdS-S-400 sample indicates its good crystallinity and only hexagonal phase CdS is detected. In contrast, all the diffraction peaks of CdS-C prepared by the precipitation method without thermal treatment are well indexed as cubic phase CdS with poor - 10 -


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crystallinity. The results of photocatalytic hydrogen production rate after five runs suggest that the hexagonal Pt/CdS-S-400 composite exhibits decreased photocorrosion trend as compared with cubic Pt/CdS-C sample. The enhanced anti-photocorrosion property is attributed to the high crystallinity of hexagonal CdS-S-400 with less surface oxygen atoms to efficiently separate the photogenerated electrons-holes pairs and decrease their recombination opportunity, thus hampering the photocorrosion process of CdS. Another work reported by Chong et al. further confirms the effect of crystallinity on the stability of semiconductor.101 Specifically, the Ag3PO4 photocatalysts synthesized by coprecipitation method are calcined under different temperature (200-500 ℃), which leads to the improvement of crystallinity and enhances the photocatalytic performance toward methylene blue (MB) degradation. After five runs of MB degradation under visible light irradiation (λ ≥ 420 nm) using a 300 W Xe lamp, only slight decrease of photoactivity has been observed over the optimized Ag3PO4400 sample, while the photocatalytic efficiency of uncalcined Ag3PO4 is deteriorated due to the presence of photocorrosion, which clearly suggests the effect of crystallinity on the photostability of Ag3PO4. Additionally, since various semiconductor crystals expose different polar surfaces, the photostability of ZnO exhibits pronounced orientation dependence, as demonstrated by Kislov and coworkers.102 The photostability test toward methyl orange (MO) under UV irradiation shows that the photocorrosion of ZnO(000-1) surface is more pronounced than that of ZnO(0001) which only exhibits localized dissolution at defect sites. With these findings in mind, the stable semiconductor-based composites could be designed for photocatalysis by selecting the photostable surface orientations. Xu et al.103 have proved that the photostability of silicon wafers is determined as a function of crystal orientation owing to the anisotropic etching behavior of single-crystal silicon. In detail, the porous Si wafers with crystal orientation of (100) show not only higher photocatalytic performance but also more stable activity than that with crystal orientation of (111) for MO degradation under a 30 W fluorescent lamp. Similarly, the (100) Si nanowires (NWs) show more preferable photostability than the Si NWs with the crystal orientation of (111) as reported by Song et al.,104 which is attributed to the anisotropy of optical and physical properties of Si NWs. 4.1.2 Modification on the size and morphology of photocatalysts. Considering the correlation between the size (morphology) factors and photocatalytic performance, research efforts have been paid to design semiconductor-based photocatalysts with controlled size (morphology) for achieving efficient and stable composites toward various photocatalytic redox reactions. For example, Kitture and co-workers105 have synthesized polydispersed ZnO nanoparticles (ZnO1000 and ZnO600) with two different size distributions (30 and 120 nm) by controlling the annealing treatments at 600 and 1000 ℃, respectively. The size effect on photostability of ZnO nanoparticles has been evaluated by performing MO decolorization under ambient sunlight. When the photocatalyst of ZnO1000 is used for four times, the degradation efficiency decreases from 99.2% to 99.12%, indicating the high stability of ZnO1000. However, only 20% dye has been eliminated after four reuses over ZnO600, which is attributed to the photocorrosion effect of ZnO600 due to the comparatively large surface area and high surface energy. Despite the size effect of ZnO has been regarded as the main reason for the enhanced photostability, the morphologies of ZnO1000 and ZnO600 are different and their effect on the photocorrosion of ZnO has been neglected. Subsequently, Zhang et al. have prepared reduced graphene oxide/ZnO composites by integrating different sized ZnO particles with reduced graphene - 11 -


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oxide (RGO) for degradation of MB and reduction of Cr(VI) under UV light illumination (λ = 365 ± 15 nm).106 The composites of 5% RGO decorated ZnO with size of 20-100 nm (referred as 5%RGO/ZnO-S1 in Figure 4A) exhibit high stability toward photocatalytic reduction of Cr(VI) and conversion of MB, as displayed in Figure 4C and D, whereas the case is not for 5%RGO/ZnO-S2 sample with ZnO particle size of 50-500 nm (Figure 4B). During the recycle experiments for four times, the activity deactivation of 5%RGO/ZnO-S2 indicates the serious photocorrosion of ZnO-S2. The efficient anti-photocorrosion observed for 5%RGO/ZnO-S1 sample is attributed to the strengthened interfacial contact and formed chemical bonding between ZnO-S1 and RGO, thus resulting in the enhanced photostability of 5%RGO/ZnO-S1 as compared with that of 5%RGO/ZnOS2.

Figure 4. Typical SEM images of 5%RGO/ZnO-S1 composite (A) and 5%RGO/ZnO-S2 composite (B); recycle photoactivity testing of 5%RGO/ZnO-S1 composite, 5%RGO/ZnO-S2 composite, bare ZnO-S1 and bare ZnO-S2 toward degradation of MB (C) and reduction of Cr(VI) to Cr(III) (D) under UV light irradiation (λ = 365 ± 15 nm) using a 300 W Xe arc lamp as light source. Reprinted with permission from ref.106. Copyright 2013 Elsevier.106 In addition to the size effect, the optimization on structure and morphology of semiconductor has also been demonstrated to hamper its photocorrosion under light illumination.60, 89, 105, 107, 108 For instance, Wan and co-workers have synthesized Ag3PO4 porous nanotubes (PNTs) via a surface anion exchange reaction, which show enhanced photocatalytic stability as compared with irregular Ag3PO4 (IRs) toward RhB degradation under visible light illumination (λ > 400 nm).109 Specifically, after five repetitive cycles, the Ag3PO4 IRs exhibit apparent downtrend activity, while the photoactivity of Ag3PO4 PNTs can be well maintained, which confirms the effect of morphology on the photostability of Ag3PO4 photocatalysts. The photostability enhancement is ascribed to the fact that the porous nanotubes structure of Ag3PO4 is favorable for separating photogenerated charge carriers and improving the adsorption ability towards dye molecules. The Ag3PO4 PNTs sample obtained after - 12 -


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cycle experiments have been characterized by XRD, which suggests the inappreciable change in the phase structure of Ag3PO4, further confirming the structure stability of Ag3PO4 PNTs during the photocatalytic reactions. In another work, the visible-light-energized plasmonic photocatalysts of Ag/AgCl with controlled morphology, i.e., cube and sphere, have been hybridized with graphene oxide (GO).110 The durability of cube-like and sphere Ag/AgCl/GO composites has been investigated by performing MO degradation reaction repeatedly four times under visible light illumination (λ > 400 nm). The results show that the cube-like Ag/AgCl/GO is more stable than Ag/AgCl/GO sphere, which verifies the morphology dependent photostability. A series of characterizations including SEM, Raman, and XPS confirm the trivial change of cube-like Ag/AgCl/GO photocatalysts after the recycle experiments, further manifesting the excellent stability of the sample. The photostability of cuprous oxide (Cu2O), ZnO and silicon has also been demonstrated to be morphology dependent. As confirmed by Kwon and co-workers,60 under irradiation of a 300 W Xe lamp, the Cu2O crystals with different morphology including cubes, octahedra and rhombic dodecahedra present photocorrosion at the surface, but the degree of corrosion differs by the shape. A slight corrosion over Cu2O octahedra is observed while the Cu2O rhombic dodecahedra exhibits the most serious photocorrosion among these samples. The work reported by Zheng111 et al. has shown that the Cu2O microcrystals transform into nanosheets as the irradiation time increases during the MO bleaching along with photoactivity decrease. Although the photocatalytic activity of nanosheets is only half of that from microcrystals in the first run, the Cu2O nanosheets remain stable at subsequent runs of MO bleaching under visible light irradiation (λ > 400 nm) using a 300 W Xe arc lamp. Yi and coworkers have evaluated the photostability of three kinds of zinc oxides, i.e., micro-sized ZnO particle (c-ZnO), nano-sized ZnO particle (n-ZnO) and tetra-needle like ZnO (t-ZnO) toward phenol degradation under UV light illumination105 and the Zn2+ concentration in the solution is regarded as criterion to monitor the photodecomposition of ZnO photocatalyst. The results show that the photocorrosion of n-ZnO and c-ZnO is more serious than that of t-ZnO, and the activity of recycled tZnO remains unchanged after sixth times of photodegradation experiments, elucidating the relationship between the morphology and stability of ZnO. In another work, ZnO photonic crystals (ZnO-PCs) have been prepared for MO mineralization under UV-Vis light irradiation,108 which exhibit higher photostability than the commercial ZnO NPs and disordered porous ZnO, indicating the effect of morphology on photostability of ZnO. Notably, the photocorrosion resistant mechanisms of ZnO in these works have not been well investigated yet. Hierarchically porous silicon with nanopores in macropores structure (NP-MPSi) has been demonstrated to exhibit stable photocatalytic performance toward phenol conversion using a Xe lamp equipped with a 420 nm optical filter.112 The NP-MPSi can be used for five times and removal efficiency of phenol remains higher than 90%, indicating the good photocatalytic stability in aqueous solution. The photostability of NP-MPSi is originated from the quantum confinement effect due to the presence of nanometer pores, among which the some dangling bonds of Si would be saturated by hydrogen atoms instead of be terminated fully by oxygen, thus reducing the passivation of these nanosized Si materials. Similar results have also been reported by Wang et al.,113 who have prepared nanoporous three-dimensional silicon nanowire arrays (SiNWAs) for methyl red (MR) degradation under a 150 W halogen lamp with the optical range of 400-800 nm and the nanoporous SiNWAs show excellent photocatalytic stability after the HF-treatment. Additionally, silicon monoliths with - 13 -


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mesoporous exhibit excellent photocatalytic ability and stability for degrading MO under visible light irradiation (λ > 400 nm) using an 8 W fluorescent lamp,114 which is ascribed to its unique mesoporous network, abundant surface hydrides and efficient light harvesting. From the above discussion, we can find that, although the influence of size and morphology on photostability of semiconductor-based composites has been well confirmed, the underlying mechanism for explaining such size- and morphology-dependent stability is still elusive and needs further studies. Moreover, the strict comparison of morphologies effect on the photostability over semiconductor-based composites should be conducted in a reasonable framework. Specifically, the preparation methods for diverse architectures of photocatalysts are generally variable, which may lead to the different crystallinity and surface property of semiconductors, whereas their effect on the photostability has not been excluded. 4.2. Heteroatom-doped photocatalysts Heteroatom doping, including cation and anion doping, has been widely investigated to adjust the energy structure of semiconductor for achieving high efficiency and stable photocatalysts.115-120 Generally, the doping of foreign elements (anion or cation) could regulate the band gap of semiconductor by introducing impurity levels in the forbidden band or forming solid solution,9, 121-123 and then extend its light absorption and improve the separation efficiency of photogenerated carriers, thus improving the photocatalytic activity and stability of semiconductor.116, 117, 124-131 4.2.1. Cation-doped photocatalysts. The replacement of cations in the crystal lattice of semiconductor could create impurity energy levels within the bandgap. For instance, Lee et al. have prepared Ni-doped ZnS microspheres,132 which exhibit enhanced durability for photocatalytic H2 evolution under visible light irradiation (λ > 400 nm) with a 350 W Xe light source using sacrificial reagents of Na2S. The H2 production rate over the catalyst maintains the same after 48 h reaction and XRD patterns of Ni-doped ZnS samples before and after the photocatalytic recycles have no notable differences, which suggests the hampered photocorrosion of Ni-doped ZnS sample. Additionally, Huang and co-workers have demonstrated that the photocatalytic stability of CdS can be improved through metal Ag doping,133 which is explained by the following two reasons. First, the substitution of Cd2+ by Ag+ will produce a doping level near the VB of CdS, namely, the acceptor energy level, which could accept the photogenerated holes. The oxidative ability of holes will be lowered due to the up-migration and thus hinder the S2- oxidation process, thereby leading to the resistance of photocorrosion over Ag-CdS sample. Second, the chemical bond between Ag+, substituted for Cd2+ in the hexagonal crystal lattice, and S2- (Ksp (Ag2S) = 6.3 × 10−50) is more stronger than that between Cd2+ and S2- (Ksp (CdS) = 8.0 × 10−27), which is also beneficial for the photostability improvement of Ag-CdS sample. Different metal ions (such as Bi3+, Mo2+, Ni2+, Ba2+ and various lanthanide cations) have also been doped into the crystal lattice of silver-containing photocatalysts for improving their photocatalytic efficiency and durability.117, 123, 128, 134-137 For example, Zhang and co-workers have reported the Bi doped Ag3PO4 photocatalyst (Bi-Ag3PO4). The density of states (DOS) and band structure calculation suggest that the Bi dopants inset doping bands in the primary band structure and reduce the band gap - 14 -


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ACS Catalysis

of Ag3PO4.135 The reusability of Bi-Ag3PO4 composite has been evaluated by preforming recycle experiments for MO degradation under visible light irradiation (λ > 420 nm). The results show no significant deactivation after ten repetitive cycles, suggesting the durable photoactivity of Bi-Ag3PO4 sample. The photostability enhancement of Bi-Ag3PO4 composite is ascribed to the formation of stable Ag2O component during the catalytic reaction. However, the effect of Bi doping on the corrosionresistance has not been taken into consideration in this work. Similarly, the Bi-doped Ag/AgCl composites have also been synthesized by the same group,117 which exhibits stable photoactivity toward MO degradation under visible light irradiation (λ > 420 nm), further confirming that the cation doping is an efficient strategy for delaying the photocorrosion of silver-containing photocatalysts. In another work, Song and co-workers have successfully doped Ag/AgCl with V5+ ion (VAg/AgCl) for decomposition of MO under simulated solar light illumination.138 The V-Ag/AgCl photocatalyst exhibits higher photocatalytic activity and stability than bare Ag/AgCl. Especially, the results of MO degradation cycle experiments suggest that the activity of optimized V-Ag/AgCl sample maintain at 97% for five cycles, confirming the effective photocorrosion inhibition of V-Ag/AgCl. Such enhancement of photostability is attributed to the introduction of V5+ ion which could trap the photogenerated electrons to suppress the charge recombination and retard the reduction reaction between electrons and Ag+ in the lattice of AgCl. Nanoporous Ba-doped Ag3PO4 hollow sheets (BaAg3PO4) have been fabricated by a cation exchange process for degradation organic dyes under visible light irradiation (λ ≥ 400 nm).123 The Ba2+ cation doping not only increases the amount of surface hydroxyl but also creates the oxygen defects to inhibit the recombination of photogenerated electronhole pairs, which both contribute to the improvement of photocatalytic stability. By reusing the BaAg3PO4 composites for five cycles, the durability experiments suggest that the photoactivity toward RhB degradation is well maintained, which indicates the anti-photocorrosion property of Ba-Ag3PO4 sample. The improved photostability is attributed to the unique hollow structure and the porous morphology as well as the Ba doping. Khataee and co-workers have produced the dysprosium (Dy)-doped ZnO nanoparticles (NPs) by a simple sonochemical method for photocatalytic decolorization of Acid Red 17 under visible light irradiation using a 100 W visible lamp.139 The XRD results suggest that the Dy3+ ions have been doped into the lattice of ZnO structure and have an influence on the lattice deformation and strain since the XRD peaks of Dy-doped ZnO shift to lower angle. The doping of Dy also leads to a strong photoabsorption in the visible light range. Therefore, the decolorization efficiency over Dy-doped ZnO composites is higher than that of undoped ZnO because the doped Dy could serve as an electron trapper for reducing the recombination of photogenerated electrons-holes. Notably, the results of reusability test indicate that 3%Dy-doped ZnO NPs exhibit stable photoactivity. However, the mechanism on the enhanced photostability remains unclear and has not been well investigated in this work. To understand the effect of cation doping in hampering photocorrosion of ZnO, Yu et al. have introduced different concentrations Mo ion into ZnO NPs for degradation of acid orange II under UV light irradiation (λ = 365 nm) using a 110W UV lamp.140 After being used four times, the photocatalytic performance of Mo(2%)ZnO remains unchanged while the photoactivity of bare ZnO gradually decrease, which suggests the enhanced photostability properties of Mo(2%)ZnO sample. The improvement of anti-photocorrosion over Mo(2%)ZnO photocatalyst is attributed to the following two - 15 -


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reasons. First, the surface atomic structure of ZnO has been changed by doping of Mo, which could decrease its surface energy, thus stabilizing ZnO by reducing the possibility of hole attacking.141 Second, the undoped Mo species (i.e., MoO3) grafted on the surface of ZnO form the MoO3/ZnO heterojunction to transfer the photogenerated charge carriers and inhibit the recombination of electronhole pairs, thereby resulting in the improved photostability of Mo(2%)ZnO sample as compared to blank ZnO under UV light illumination. Notably, considerable research works have been reported regarding the substitution of metal ions in ZnO lattices, such as Cd,142 Cu,126, 143 La,144, 145 Al,146 Cr,147 Zr,148 Ga149 for reducing band gap energy and enhancing the charge separation efficiency, and thus leading to the photocatalytic performance improvement. Nevertheless, the effect of metal cation doping on the photostability of ZnO is generally neglected and more attention should be paid to elucidate the underlying anti-photocorrosion mechanism of cation-doped ZnO composites. 4.2.2. Anion-doped photocatalysts. Substitution of anion ions (e.g., P,38 N,119, 125, 150 S,124 C120, 151, F118) in inorganic materials is expected to bring about significant changes in the electronic structure properties of materials.38, 152, 153 Recently, Kouser and co-workers154 have demonstrated the effects of P and Cl substitution on the properties of CdS in hexagonal and cubic structures by carrying out density functional theoretical (DFT) calculations. The results suggest that P and Cl atoms preferentially occupy the sites bonding to Cd in the lowest energy configuration, which thus reduces the band gap of CdS and enhances its dielectric properties via creating an isolated sub-band at the top of valence band. Experimentally, a gradient phosphorus-doped CdS homojunction nanostructure has been constructed, which generates an oriented built-in electric-field to facilitate the charge extraction from inside to surface of photocatalyst.38 As shown in Figure 5A, the sample of CdS-P1 exhibits efficient visible light (λ > 420 nm) activity toward photocatalytic H2 generation in the presence of 1.0 M Na2S and 1.0 M Na2SO3 as sacrificial reagents and the activity has no significant decrease after six runs (Figure 5B), revealing the good stability and photocorrosion resistance of CdS-P1 sample. The distribution of P dopants in the near surface of CdS-P1 NRs detected by XPS spectrum with the help of Ar+ beam etching in Figure 5C displays that the signal of P 2p is the mirror image of S 2p, indicating the gradient distribution of nonmetal elements P and the substitution of P in place of element S. The as-proposed schematic energy level in CdS-P1 composites, as displayed in Figure 5D, suggests that the gradient doping of P on the surface of CdS-P1 could extract the photoexcited charge carriers to the surface defect sites by the built-in oriented electric field, which thus efficiently promotes the separation of electron-hole pairs and improves the photocatalytic stability. Consequently, the photocorrosion of CdS can be delayed since the photogenerated holes are extracted adequately and S sites are “hidden” inside. Additionally, Zhou and co-workers have recently fabricated N-doped ZnS by nitriding ZnS nanopowder in NH3 atmosphere for H2 generation under simulated sunlight irradiation (300 W Xe lamp) in an aqueous solution containing methanol (10 vol%).155 The N-doped ZnS sample exhibits improved photocatalytic stability compared to pristine ZnS and no peaks attributed to S0 have been detected in the XPS spectrum of S 2p over the N-doped ZnS after photocatalytic reaction, suggesting the corrosion resistance of N-doped ZnS. Moreover, by monitoring the ion concentration of Zn2+ upon photocatalysis, the same conclusion can be reached. Namely, the doping of N is an efficient method to enhance the photostability of ZnS against photocorrosion under light illumination.

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Figure 5. Time courses of H2 evolution and apparent quantum efficiency (AQY) on CdS-P1 under monochromatic 420 nm light irradiation (A); cycle runs of CdS-P1 for photocatalytic H2 evolution under visible light illumination (λ > 420 nm) using 280 W Xe lamp in the aqueous solution of 1.0 M Na2S and 1.0 M Na2SO3 (B); elements depth distribution recorded by Ar+ beam etching (C); schematic energy level and exciton transfer pathways in CdS-P1 sample with a gradient distribution of P element from inner to surface of the catalyst (D). Reprinted with permission from ref.38. Copyright 2017 American Chemical Society.38 Because the N element has similar size with O and its low electronegativity as compared to O, element N seems to be a suitable dopant for ZnO. For instance, Sudrajat et al. have synthesized Ndoped ZnO (N-ZnO) through a combustion reaction for degradation of amaranth (AM) and MB under visible and UV light illumination.125 It is found that the N-ZnO exhibits extended light absorption to visible region, which is attributed to the formation of a mid-band state by N 2p orbital. The recycle experiments have been performed to investigate the long-term stability of photocatalyst. The results suggest that, during six successive runs, there is no significant decrease on photocatalytic performance for MB and AM degradation under both UV and visible light irradiation, confirming impeded photocorrosion of N-ZnO. The relatively high crystallinity of N-ZnO has been regarded as the main reason for stable photoactivity. However, the photostability of N-ZnO is significantly affected by the preparation methods. Muthulingam et al. argue that the N-doped ZnO synthesized by a simple reflux method under 333K exhibits relatively poor stability for malachite green (MG) degradation under daylight irradiation.119 The recycle experiment results show that the decolorization efficiency of Ndoped ZnO is decreased from 83% to 60% with three cycles. Notably, the photostability of N-doped ZnO can be alleviated by carbon quantum dots (CQDs) modification, and the CQD/N-ZnO composites can be reused as stable daylight photocatalysts toward MG degradation.

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Additionally, Hsu and co-workers have grown S-doped ZnO nanorods on stainless steel mesh as immobilized hierarchical photocatalyst for H2 production in the aqueous solution containing 0.1 M Na2S and 0.04 M Na2SO3 under UV light irradiation (UV lamps).124 The XRD peaks of S-doped ZnO nanorods shift toward low diffraction angle position, which is attributed to the substituted oxygen by sulfur and lattice expansion since the radius of sulfur atoms is larger than that of oxygen. The immobilized S-doped ZnO photocatalyst can be easily recycled after being rinsed with water. After five repeated H2 production experiments, limited activity loss over S-doped ZnO nanorods coated stainless-steel wire mesh photocatalyst is observed, which indicates the high photostability of S-doped ZnO. The photostability of ZnO can also be enhanced by carbon doping, as demonstrated by Zhou et al.,120 who have prepared carbon-doped ZnO by calcining the organic zinc compound synthesized by the reaction between ZnCl2 and glycol. The durability of carbon-doped ZnO composites has been confirmed by recycle experiments for removal of gaseous formaldehyde under irradiation of a 15 W indoor fluorescent lamp, in which the concentration of produced CO2 remains unchanged throughout all four cycles, suggesting the high corrosion resistant property of C-doped ZnO photocatalyst. Besides, it has been reported that by the introduction of F, the unwanted photoinduced dissolution of Ag/AgBr will be hampered, and the F-Ag/AgBr photocatalyst is reused for five cycles without obvious deactivation toward MO degradation under visible light irradiation.118 In addition to the anion doped semiconductor, oxoanion groups have also been reported to modulate the optical properties and electronic structure of semiconductors.156, 157 Experimentally, Cao et al. have successfully designed sulfate-doped Ag3PO4 photocatalysts via a simple precipitation method for photocatalytic degradation of organic dyes (RhB and MB) under visible light irradiation (λ ≥ 400 nm).158 The SO42--doped Ag3PO4 catalyst exhibits enhanced photocatalytic performance than bare Ag3PO4 photocatalyst, which is attributed to the doping of SO42- to inhibit the recombination of photogenerated electron-hole pairs. Moreover, the results of recycle experiments for RhB degradation indicate that the SO42--doped Ag3PO4 photocatalyst can be used repeatedly to degrade RhB for six cycles without obvious photoactivity decrease, confirming the reinforced photostability of SO42-doped Ag3PO4 composites. It is the doping of SO42- which not only facilitates the transfer of charge carriers but also shifts the bottom of conduction band (CB) towards lower energy regions to suppress the reduction potential of photogenerated electrons, thereby resulting in the photostability enhancement. 4.2.3. Formation of solid solution. Formation of solid solutions with controllable band structure and tunable electrical conductivity could significantly improve their photocatalytic stability and activity as compared with the single component.159-167 For example, Xie et al. have fabricated the ZnxCd1-xS@ZnO nanorod arrays via a facile two-step process.159 As revealed in Figure 6A, the photocatalytic activities of ZnxCd1-xS@ZnO nanorods exhibit good stability after five cycles under Xe lamp light irradiation. This is because ZnxCd1-xS possesses a suitable band gap, which is smaller than that of ZnS but larger than that of CdS, and the photogenerated electrons could transfer to ZnO nanorods easily (Figure 6B), thus facilitating the separation of electron-hole pairs and enhancing photocatalytic stability. In addition, the photostability of ZnxCd1-xS solid solution can be further improved by doping with Cu ions, as reported by Zhang and co-workers.163 A solid solution of (Zn0.95Cu0.05)1-xCdxS has been examined for H2 production under visible light illumination (λ ≥ 420 nm) using 300 W Xe lamp in the aqueous solution containing 0.35 M Na2SO3 and 0.25 M Na2S, which - 18 -


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ACS Catalysis

is durable after three consecutive runs of accumulative 12 h, showing good photocatalytic stability. In addition to ZnxCd1-xS, other solid solutions, such as Mn1-xCdxS,116, 122, 131, 168, 169 are also found to possess long-term and stable photocatalytic activity. For instance, Liu et al. have fabricated Mn1-xCdxS solid solutions for H2 production under irradiation of a 300 W Xe lamp equipped with a cut-off filter (λ > 420 nm) using 0.1 M Na2S and 0.5 M Na2SO3 as sacrificial reagents and no significant deactivation of activity has been observed after 4 recycles (Figure 6C).116 Additionally, the similar XRD patterns of the fresh and used Mn0.24Cd0.76S solid solutions further demonstrate its anti-photocorrosion capability during the photocatalytic H2 evolution. In a recent example, Lai and co-workers have obtained a series of Mn1-xCdxS solid solutions hollow spheres by a simple hydrothermal route.169 The recycle test displays that the Mn0.33Cd0.67S does not show obvious decrease of RhB degradation activity after six recycles under visible light irradiation (λ ≥ 400 nm) with a 350 W Xe lamp, revealing the excellent photostability of Mn0.33Cd0.67S sample. This is ascribed to the tunability of its band gap by regulating the contents of Cd and Mn, which is beneficial to the separation and transportation of photogenerated electron-hole pairs.

Figure 6. Photocatalytic activity and stability of ZnxCd1-xS@ZnO nanorod films for MO degradation under Xe lamp light irradiation: ZnO (line a); CdS@ZnO (line b); ZnS@ZnO (line c); Zn0.34Cd0.66S@ZnO (line d); Zn0.71Cd0.29S@ZnO (line e); Zn0.16Cd0.84S@ZnO (line f) (A); schematic diagram showing the movement of photogenerated electrons and holes in ZnxCd1-xS@ZnO films (B). Reprinted with permission from ref.159. Copyright 2012 American Chemical Society.159 Photocatalytic H2 evolution during the stability study over Mn0.24Cd0.76S solid solution under irradiation of a 300 W Xe lamp equipped with a cut-off filter (λ > 420 nm) using 0.1 M Na2S and 0.5 M Na2SO3 as sacrificial reagents (C). Reprinted with permission from ref.116. Copyright 2014 Royal Society of Chemistry.116 4.3. Hybridization with cocatalysts - 19 -


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It is generally accepted that cocatalysts are capable of serving as reaction sites to catalyze various reactions, promoting the charge separation and suppressing the photocorrosion of semiconductorbased photocatalysts, which in turn achieves high photocatalytic performance.22, 170-176 Often, the loading of suitable cocatalysts on semiconductor can effectively enhance the photostability of composite photocatalysts by timely consuming or transporting photogenerated charge carriers.170, 172, 177-184 In this section, we focus on the development of multiple cocatalysts to ameliorate the robustness of semiconductor-based photocatalysts for various artificial photoredox reactions. 4.3.1. Metal and transition metal ions cocatalysts. The introduction of noble metals for designing semiconductor-metal photocatalyst systems not only promote the photocatalytic efficiency,185-189 but also greatly hamper the photocorrosion of semiconductors.173, 190 For example, Kakuta and co-workers have found that the photoinduced dissolution of AgBr semiconductor is inhibited under successive UV irradiation after the formation of Ag species at the early stage of reaction since the electron-hole separation could occur smoothly in the presence of Ag0 species.191 Wang et al. have demonstrated that, after one cycle of photocatalytic test, the metallic Ag in situ deposit on Ag2O particle to form a stable Ag2O-Ag structure and only a slight decreased photoactivity has been observed for the degradation of MO throughout all five cycles, which is ascribed to the formation of metallic Ag to act as a photoelectron mediator and promote the multielectron reactions between electrons and oxygen to produce H2O2, thereby reducing the photocorrosion process of Ag2O.185 Similarly, metal Ag doped Ag-based composite photocatalysts, such as Ag/Ag3PO4,75, 192 Ag/AgBr,193, 194 Ag@AgCl195 and Ag/AgI196, 197 have been widely investigated to enhance the photocatalytic stability under light irradiation. The introduction of Ag NPs has been demonstrated to inhibit the corrosion of ZnO photocatalyst. As reported by Hsu and co-workers, the Ag NPs have been decorated on the surface of ZnO nanorods anchored on stainless-steel wire meshes for photocatalytic decolorization of organic dyes under visible light irradiation.173 During the three repeated photocatalytic decolorization experiments, no photoactivity loss has been observed over Ag-doped ZnO nanorods, while the photocatalytic activity of ZnO nanorods decreases about 8% after recycle experiments. The enhanced photostability of Ag-doped ZnO nanorods is ascribed to the facilitated charge separation efficiency by transferring photogenerated electrons from ZnO nanorods to Ag NPs through interfacial interaction between Ag and ZnO interface, thereby reducing the photocorrosion of ZnO. Besides the metal Ag, hybridization of other noble metal cocatalysts (e.g., Au, Pd, Pt) with semiconductor components is able to accelerate the separation of photogenerated electron-hole pairs due to the low Fermi energy levels of noble metal, and thus improve their photocatalytic activity and stability.183, 198, 199 For instance, Wang and co-workers have fabricated Au/Ag3PO4 heterostructure for photocatalytic degradation of RhB under visible light illumination (λ > 420 nm), which exhibits stable photoactivity with only 5% decrease in the fifth round cyclic experiments.199 The presence of Au NPs plays an important role in restraining the reduction reaction of Ag+ by trapping the photoinduced electrons, thus resulting in enhanced photostability of Ag3PO4. Yuan and co-workers have observed that the structural stability of Cu2O can be enhanced by introducing metal Au NPs,200 whereas the underlying mechanism for the stability improvement is still unclear. Liu et al. have deposited Au NPs onto the surface of CdS nanowire (NW) via an electrostatic self-assembly method for photocatalytic - 20 -


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selective reduction of nitroaromatic compounds in the aqueous phase under visible light irradiation (λ > 420 nm).201 The results of cycle experiments over CdS NW-Au NPs composites suggest that no obvious activity decay has been observed in five successive recycles, which indicates that the composites is stable during the photocatalytic reduction reaction. The presence of excess amount of ammonium formate to consume the photogenerated holes timely could inhibit the photocorrosion of CdS NWs. Notably, similar conclusions have been drawn over the Pt/CdS nanorods composites system for selective alcohol dehydrogenation and hydrogenolysis under visible light irradiation (λ > 420 nm).202 The Pt NPs acting as an electron reservoir is able to preserve the integrity of BiNbO4 photocatalyst under UV light irradiation, as reported by Dunkle et al..203 In another work, different noble metals (Au, Pd, Pt) have been deposited onto semiconductor Ag3PO4 using NaBH4 as a reduction agent.183 During the repeated six experiments, the photocatalytic activity toward MO degradation over Pt/Ag3PO4 is more stable than that of bare Ag3PO4. No obvious deactivation has been observed for Pt/Ag3PO4 under continuous light irradiation, which suggests the key roles of noble metal in ameliorating the photocatalytic efficiency and stability of Ag3PO4 photocatalyst. When the noble metals are contacted with Ag3PO4, the Schottky barrier will form due to the difference in work functions, by which recombination rate of photogenerated electrons is effectively suppressed and the transportation of photogenerated electrons from Ag3PO4 to noble metals is accelerated. Therefore, the photoinduced instability of M-Ag3PO4 (M= Au, Pd, Pt) is hampered. Moreover, the dissolution process of Ag3PO4 in aqueous solution is effectively inhibited by the coating of insoluble noble metal layer, thereby enhancing the structural stability of M-Ag3PO4 composites during the photocatalytic process. Generally, the metal nanoparticles (NPs) are deposited on the out surface of semiconductor, in which the metal NPs are directly exposed to reactants and the surrounding medium. Thus, during the photocatalytic reaction process, the dissolution of noble metal NPs may take place, which could lead to the loss of photoactivity and decrease the stability of photocatalysts. Yu et al. have reported hollow Pt-ZnO microspheres with hierarchical structure for degradation of acid orange II, among which the noble metal Pt with size of 4 nm is embedded in ZnO crystal.178 The results of five times recycle experiments suggest that Pt-embedded ZnO nanostructures exhibit a much higher stability than the bare ZnO and Pt deposited ZnO photocatalyst, demonstrating the improved anti-photocorrosion property of ZnO. In addition, the nano-sized ZnO-metal (Au, Ag, Pt, Pd) hollow nanoparticles (HNPs) with thin shell have also been fabricated.176 After photocatalytic reaction, the TEM results suggest that pure ZnO HNPs disintegrate into ultrafine nanoparticles due to the serious photocorrosion, while no obvious change in the morphology of Pt/ZnO HNPs has been observed, which indicates the enhanced photostability of Pt/ZnO HNPs sample. The authors claim that the presence of noble metal acting as a reservoir to accept the photogenerated electrons hinders the photoreduction of Zn2+ to Zn, which thus depress the corrosion process of ZnO. Nevertheless, the holes gathered on the surface of ZnO may oxidize the oxygen atom in the lattice of ZnO, thereby leading to the dissolution of ZnO during photocatalytic reactions, which has not been taken into consideration in this work. Actually, for the noble metal modified hole-sensitive-semiconductors systems (such as ZnO, CdS, Cu2O et al.), the effect of holes accumulation on the stability of these semiconductors is generally neglected. Even though the accumulated holes could be consumed timely by the sacrificial agents and/or reactants and the hole-induced dissolution process of these composites is inhibited, the accumulation of holes in - 21 -


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hole-sensitive-semiconductor should be unfavorable for their photostability once the oxidation reactions between photogenerated holes in the valence band (VB) of hole-sensitive-semiconductors and the sacrificial agents and/or reactants are delayed or blocked. In this regard, the research attention should be paid to the composite photocatalyst systems consisting of hole-sensitive-semiconductor and electron cocatalysts. Providing sufficient sacrificial agents, enhancing the adsorption ability toward reactants or introducing hole-cocatalysts may be feasible for further reducing the possibility of photogenerated hole induced corrosion of these hole-sensitive-semiconductors. It can be evidently seen that the decoration of noble metal could efficiently ameliorate the photoinduced instability of semiconductor-based photocatalyst. However, noble metals are high cost and scarce, which greatly obstructs their large scale utilization. Therefore, the development of lowcost cocatalysts to enhance the photocatalytic stability of semiconductor-based materials is of great importance and has attracted considerable research attentions. In recent years, the graft of earthabundant transition metal ions (e.g., Co(II), Cu(II) and Fe(III)) on the surface of photocatalyst has been demonstrated to ameliorate its photocatalytic performance by photoinduced interfacial charge transfer (IFCT) process.204-212 The surface grafted ions act as cocatalysts to accept photogenerated electrons from semiconductor and thus reduce the recombination possibility of electron-holes pairs, leading to the photoactivity and stability improvement.211, 212 For example, Yu and co-workers have fabricated Fe(III) grafted AgBr photocatalysts for decolorization of MO under visible light irradiation (λ ≥ 400 nm).211 The introduction of Fe(III) cluster not only improves the photocatalytic efficiency of AgBr but also greatly hampers the photocorrosion of AgBr. More specifically, during repeated photocatalytic reactions, the catalytic efficiency of Fe(III)/AgBr composites firstly decrease and then become constant after three cycles of photocatalytic tests, which indicates that the Fe(III)/AgBr sample can serve as a stable photocatalyst. The photostability enhancement over Fe(III)/AgBr composites has been systematically investigated by a series of characterizations including XRD, DRS and XPS, and is ascribed to the Fe(III) cocatalysts to trap the photogenerated electrons from the CB of AgBr , which thus enhances the photostability of Fe(III)/AgBr composites. Yu’s group has loaded Cu(II) cocatalyst onto various Ag-based compounds (including Ag3PO4, AgCl, AgBr, AgI, Ag2O and Ag2CO3) for photocatalytic decomposition of phenol and MO under light irradiation.212 Taking Cu(II)/AgCl as a typical example, its photostability has been investigated by repeating the photoactivity for five times. The results show that Cu(II)/AgCl photocatalyst exhibits higher recycle stability than blank AgCl toward degradation of phenol, indicating that the Cu(II) ion grafted on the AgCl surface greatly retard the photocorrosion of AgCl. In the case of Cu(II)/AgCl composites, the light induced photoelectrons in the CB of AgCl could transfer to Cu(II) cocatalyst to produce unstable Cu(I) ions, which subsequently react with oxygen under an ambient condition, thus ameliorating the photoactivity and stability of Ag-based photosensitive materials. In another work, the Fe(III) cocatalysts have been grafted on the surface of ZnO nanostructures to form Fe(III)/ZnO photocatalysts for degradation of RhB under simulated sunlight illumination.213 The as-formed Fe(III)/ZnO composites exhibit higher photoactivity than blank ZnO sample and the reusability of Fe(III)/ZnO catalysts is investigated by recycle experiments. Specifically, only 9% decrease of photocatalytic efficiency has been observed over Fe(III)/ZnO sample after three recycles tests, indicating the good stability of the catalyst. The samples of Fe(Ш)/ZnO before and after catalytic reaction have been characterized by XPS, which demonstrates that the peak positions of Fe(III) are - 22 -


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ACS Catalysis

unchanged and the amount of Fe in the Fe(Ш)/ZnO photocatalyst is similar, suggesting the high photostability of Fe(Ш)/ZnO composites. The mechanism of stability enhancement is associated with the direct IFCT, in which the Fe(III) cocatalysts not only serve as electron sinks to separate the photoinduced electron-hole pairs but also elevate the amount of holes to produce hydroxyl radical with high oxidizability for RhB degradation. 4.3.2. Transition metal oxides and hydroxide cocatalysts. Transition metal oxides, such as RuO2214-216, IrO2217 and Co3O4,218, 219 are well-known as excellent cocatalysts for protecting semiconductors from photo-dissolution by efficiently removing the photogenerated charge carriers. Early work by Grätzel and co-workers have indicated that colloidal CdS loaded with ultrathin layers of RuO2 as a oxidation cocatalyst inhibit its photocorrosion.214 The cocatalyst of RuO2 has also been reported by Maeda et al. to enhance the photostability of GaN:ZnO solid solution toward overall water splitting under UV and/or visible light irradiation.220 Moreover, the bimetallic cocatalyst of Rh2−yCryO3 has been demonstrated to retard the photocorrosion of (oxy)nitrides by effectively facilitating the hydrogen evolution reaction while inhibiting the oxygen reduction reaction.221-223 Recently, Yuan et al. have fabricated Co3O4-CdS nanorods via a two-step solvothermal/hydrothermal method for visible light (λ ≥ 420 nm) H2 production irradiated with a Xe lamp (300 W) in an aqueous solution containing 0.5 M Na2S and 0.5 M Na2SO3.218 No significant decrease in the catalytic performance is observed after three-cycles, and the morphology of CdS-3% Co3O4 photocatalyst after 3 cycles is still similar to that before reaction, confirming the critical role of amorphous Co3O4 nanoparticles in enhancing the stability of CdS nanorods under visible light irradiation. The main cause of increased stability is ascribed to the accelerated holes transfer from the VB of CdS to that of Co3O4. The cobalt oxide species contained both Co(II) and Co(III) (referred as CoOx) have been decorated onto the surface of singlecrystalline meso- and macroporous LaTiO2N (LTON) for oxygen generation under visible light irradiation (λ > 420 nm) by employing silver nitrate as electron sacrificial agent.224 The CoOx/LTON sample acts as a stable photocatalyst for water oxidation without any noticeable change in its structure, and only a low level of N2 evolution (∼2-8 μmol) has been detected in the initial stage of the reaction (first 1-2 h) but production of N2 is completely suppressed as the reaction progresses, indicating the stability of this composite material. In addition to metal oxides, transition metal hydroxides, e.g., Ni(OH)2,225, 226 and Co(OH)2227 have also proved to be effective in ameliorating the photocatalytic activity and stability of semiconductors. For instance, Zhou and co-workers have decorated Co(OH)2 onto CdS nanowires (NWs) to form coreshell structure.227 The sample of Co(OH)2/CdS NWs with 6.5% molar ratio of Co(OH)2 exhibits stable H2 generation rate of 7.52 mmol h-1 g-1 within 12 hours in an aqueous solution containing 30 vol% tri(2-hydroxyethyl)amine under visible light irradiation (λ ≥ 420 nm), which is ascribed to the loading of Co(OH)2 to facilitate the separation of photogenerated electron-hole pairs, confirming the avoided light-induced corrosion of core-shell Co(OH)2/CdS NWs composites. 4.3.3. Transition metal sulfide, phosphide, carbide, nitride and boride cocatalysts. Hitherto, transition metal sulfides have been regarded as highly efficient cocatalysts to integrate with semiconductor-based photocatalysts for high stability and durability toward solar energy conversion.32, 228-235 For example, Zhong et al. have combined ZnS NPs with WS nanosheets (ZWS) for 2 photocatalytic H2 generation under Xe lamp (300 W) irradiation with 0.1 M Na2S and 0.1 M Na2SO3 - 23 -


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as the sacrificial agent,236 during which the WS2 nanosheets act as electronic transporters, hydrogen evolution activity sites and scaffold for the immobilization of ZnS nanoparticles. Pure ZnS photocatalysts slightly loss their photocatalytic activity with prolonged reaction time, while the ZWS composites show excellent reusability and no catalytic activity is decreased after 50 h light irradiation. Hao’s group has recently proposed a novel photocorrosion-recrystallization strategy to improve the stability of CdS/WS2 composite system.230 Figure 7A and B show that the H2 generation activity of 40 wt% CdS/WS2 composites maintains more than 200 h with nearly no activity loss under visible light irradiation (λ > 420 nm) with a 300 W Xe lamp in the aqueous solution containing lactic acid as sacrificial agents, which validates the superb stability of CdS/WS2 photocatalysts. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) in Figure 7C suggests that the concentration of Cd2+ and S2− increases during the photocatalytic reaction process over bare CdS, while the concentration of those ions for the sample of CdS/WS2 is lower than that of CdS (Figure 7D) and remains unchanged after 90 h irradiation. It is believed that the dissolved S2− ions are easy to interact with the WS2 nanosheets with S-vacancies, which recrystallize with Cd2+ ions to form small CdS nanoparticles (Figure 7E), thereby leading to the equilibrium state between the dissolution of CdS and recrystallization process of CdS on the S-vacancies of WS2 nanosheets and thus the excellent photostability of CdS/WS2 composites. Similar photocorrosion-recrystallization strategy is also efficient for enhancing the anti-photocorrosion property of CdS/MoS2 composites.32

Figure 7. H2 evolution during the photocatalytic stability study (A, B). Conditions: 0.1 g CdS/WS2 nanocomposites, 100 mL aqueous solution with 20 mL lactic acid, and 300 W Xe lamp with an optical cut off filter (λ > 420 nm). The concentration of S2− and Cd2+ decomposed from CdS nanoparticles during visible light irradiation (C); the concentration of S2−, Cd2+, and W4+ decomposed from CdS/WS2 - 24 -


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ACS Catalysis

composites during visible light irradiation (D). (ICP-AES Condition: Sampling at different times and centrifuged to remove the photocatalyst powder). Schematic of CdS photocorrosion and recystallization on the surface of WS2 nanosheets (E). Reprinted with permission from ref.230. Copyright 2016 Elsevier.230 Guan and co-workers have synthesized the MoS2-RGO/ZnO composite by a facile two-step solvothermal process for H2 production using Na2S and Na2SO3 as sacrificial reagents under light irradiation (300 W Xe lamp).234 The MoS2-RGO/ZnO composite exhibits better photocatalytic performance than the sample of ZnO loaded with the same amount of MoS2 or RGO alone. More importantly, the cycle experiments over MoS2-RGO/ZnO composite suggest that H2 evolution rate maintains ca. 93% after three cycle reactions (12 h), which indicates the high stability of MoS2RGO/ZnO photocatalyst. Notably, for blank ZnO after 12 h cycle tests, a very small amount of ZnS is detected by XPS analysis, which originates from the reaction between the released Zn2+ due to the corrosion of ZnO and the S2- ions in solution, indicating the poor stability of blank ZnO. Constructing core-shell structure has been demonstrated to inhibit the release of ions in the core component over composite photocatalyst, which thus enhances the photostability and avoids light-induced corrosion process. Bearing this in mind, a novel MoS2 nanosheets-coated ZnO heterostructure photocatalyst has been prepared for H2 evolution under irradiation of 300 W Xe lamp in the aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as the sacrificial reagents.237 The ZnO surface is coated by layered MoS2 with intimate interfaces, which confirms as-formed core-shell structure. The photocatalytic stability of MoS2-ZnO has been investigated by recycle experiments for three consecutive runs, among which the H2 production rate is well maintained after 12 h of photocatalytic reaction without noticeable degradation. The XRD pattern of MoS2-ZnO composites after recycle reaction is similar to that of MoS2-ZnO sample before reaction, suggesting the MoS2-loaded ZnO core-shell composite photocatalyst with high stability for H2 evolution. Additionally, the earth-abundant transition metal phosphate, e.g., Co-Pi, has also been reported to separate photogenerated holes for retarding electronhole recombination and enhancing the photocatalytic performance and stability of ZnO.167 Apart from transition metal sulfides, transition metal phosphides,238-240 carbides,241-244 nitride245 and borides246 have also attracted increasing attention as efficient cocatalysts to ameliorate the stability of semiconductor-based composite for various photocatalytic reactions. Recently, CoP has been reported as a highly robust and inexpensive cocatalyst to improve the photostability of CdS by Cao et al..238 After 50 h of light irradiation, a slight decrease of H2 generation activity over CoP/CdS composites has been observed, which may be attributed to the consumption of L(+)-lactic acid and photocorrosion of CdS. However, the activity can be restored rapidly when a small amount of CdS and L(+)-lactic acid are supplemented into the reaction system and the CoP/CdS sample exhibits excellent stability even after 100 h of light irradiation. In addition, Cheng and co-workers have combined various proportions of FeP and CdS particles by strongly grinding them together in agate mortar.239 The FeP/CdS composite photocatalysts exhibit high photostability toward H2 generation under visible light irradiation using LEDs (λ > 420 nm) in aqueous solution containing lactic acid (10% v/v), as demonstrated by the cycle tests. The enhanced photocorrosion-resistance of FeP /CdS composites is ascribed to the band bending between CdS and FeP, which facilitates the separation of electron-hole pairs and significantly depresses the recombination rate of charge carriers. This work further enriches the promising application scope of transition metal complexes with inexpensive and earth-abundant - 25 -


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elements for improving the photostability of semiconductor-based composite photocatalysts toward various catalytic reactions. Recently, Ran and co-workers have decorated Ti3C2 nanoparticles onto the surface of various metal sulfides, including CdS, ZnS and ZnCdS, for visible light H2 production under irradiation of 300 W Xe arc lamp (λ ≥ 420 nm) in the aqueous solution containing of lactic acid (25 vol%).243 Both the photocatalytic activity and stability of CdS-Ti3C2 sample are meliorated due to the introduction of Ti3C2 that serves as an efficient cocatalyst. The seven successive cycle tests toward H2 generation show that the CdS-Ti3C2 composites exhibit outstanding stable capability without deterioration of photocatalytic activity. Furthermore, no apparent alterations have been observed by comparison of the crystalline phase and morphology of the original and used photocatalysts, confirming the antiphotocorrosion property of CdS-Ti3C2 composites. Nevertheless, it still is unclear how the Ti3C2 nanoparticles stabilize these metal sulfides. In the following work reported by Xie et al., the “Cd2+ confinement effect” of Ti3C2Tx has been proposed, among which Ti3C2Tx shows strong adsorption affinity with Cd2+, which leads to the local confinement of photocorrosion-released Cd2+ around CdS instead of rapid diffusion into the solution, thereby inhibiting the serious photocorrosion of CdS.244 4.3.4. Molecular complex cocatalysts. To further develop highly efficient cocatalysts for ameliorating photocatalytic efficiency and stability of semiconductor-based composites, one approach is to downsize the cocatalysts to create more active sites, large surface areas and low barriers for the interfacial charge transfer from semiconductor to cocatalysts, thus remarkably improving their catalytic performance.247 Consequently, molecular complex cocatalysts have been integrated with semiconductors for constructing highly stable composite photocatalysts toward target photocatalytic reactions.248-254 For instance, Zhao and co-workers have anchored well-dispersed transition-metal species on CdS support by controlling the decarboxylation of the ethylenediaminetetraacetate (EDTA) ligands in mixtures of CdS precursors and metal-EDTA complexes (M-EDTA, M = Co or Ni) for H2 production in the Na2S/Na2SO3 solution (0.35 M) using a 300 W Xe arc lamp with a UV‐cut filter (λ > 420 nm) as the light source.252 As shown in Figure 8A-C, the surface of CdS particles is uniformly wrapped by a thin amorphous layer of Co and Ni species to give CdS-CoE and CdS-NiE, respectively. Notably, the turnover frequency (TOF) up to 600 h−1 (calculated as the number of moles of H2 divided by the number of moles of Co(II)) is achieved over the hybrid of CdS-CoE-300, where 300 represents the decarboxylation temperature, and the performance is rather stable without apparent decay (Figure 8D). For various CdS-NiE samples (initial mole proportion of Ni-EDTA to CdS: 0.7%), the highest H2 production rate is obtained over the CdS-NiE-350 composite with a TOF as high as 864 h−1. The photoactivity maintains unchanged during the six recycle tests, as exhibited in Figure 8E, which suggests the super-high photostability of as-prepared CdS-NiE-350 hybrid. Such high stability of metal molecular species modified CdS is attributed to the passivation of the surface sulfur and the efficient charge transfer from CdS to the reactive Co(II) or Ni(II) sites, both of which lead to enhanced stability under visible light irradiation.

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ACS Catalysis

Figure 8. The structure of CdS modified with M-EDTA (M = Co or Ni) (A); high resolution transmission electron microscopy (HRTEM) images of as prepared CdS-CoE (B) and CdS-NiE (C); cycle test photocatalytic H2 evolution over CdS-NiE-350 with 0.7% mole proportion of Co-EDTA to CdS (D) and CdS-NiE-350 with 0.7% mole proportion of Co-EDTA to CdS (E) in the Na2S/Na2SO3 solution (0.35 M) using a 300 W Xe arc lamp with a UV‐cut filter (λ > 420 nm) as the light source. Reprinted with permission from ref.252. Copyright 2017 John Wiley & Sons, Inc.252 Additionally, the [Fe2S2] hydrogenase mimic [(μ-SPh-4-NH2)2Fe2(CO)6],254 molecular cobaltsalen complex (Co complex)251 and Ni-based complex255 have also been reported to serve as efficient cocatalysts for enhancing the photocatalytic efficiency and stability of metal sulfides. For instance, using [Fe2S2] hydrogenase mimic as a molecular cocatalyst, semiconductor ZnS as a light harvester, and ascorbic acid as sacrificial electron donors, a robust photocatalytic system has been developed by Wen et al. for H2 evolution under illumination of a Xe lamp (300 W).254 A relatively constant rate for H2 formation sustains for more than 38 h with a TON (based on [Fe2S2] complex) of about 2607, suggesting a superior photocatalytic stability. Notably, even though the exceptional stability of this complex catalyst in such reaction conditions has been achieved, the detailed mechanism for the inhibited photocorrosion of semiconductors has not been elucidated. 4.3.5. Dual cocatalysts. Recently, loading dual cocatalysts, including electron cocatalysts and hole cocatalysts, has been proved to be a superior way to suppress photocorrosion of semiconductorbased photocatalysts.182, 256-262 When dual cocatalysts are loaded onto the surface of semiconductor supports, the photogenerated electrons migrate to reduction cocatalysts and the holes transfer to oxidation cocatalysts, which leads to the efficient spatial separation of photoexcited charge carriers, thereby enhancing the photocatalytic activity and stability. For example, Li’s group has fabricated the - 27 -


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Pt-PdS/CdS composites with PdS and Pt as the oxidation and reduction cocatalysts, respectively, for H2 evolution irradiated under visible light (λ > 420 nm) by a 300-W Xe lamp using 0.5 M Na2S and 0.5 M Na2SO3 as the sacrificial reagents.256, 257 The H2 evolution activity of Pt-PdS/CdS displays a very stable trend for over 25 h and even longer than 100 h for a scale up test, which indicates that the simultaneous loading of Pt and PdS is able to significantly prohibit photocorrosion of CdS. The TEM images reveal that the bulk structure and surface morphology of PdS and Pt loaded CdS remain nearly unchanged after reaction for 25 h. Additionally, Gaikwad and co-workers have reported the Pt-RuS2CdZnS system along with Pt and RuS2 as dual cocatalysts.259 By repeating the photocatalytic experiment for 60 h, the reusability of photocatalysts has been investigated. The unchanged photoactivity during the repeated use of catalyst suggests the photocorrosion inhibition of Pt-RuS2CdZnS samples. The anti-photocorrosion of Pt-RuS2-CdZnS is attributed to RuS2 loading to extract photogenerated holes from the VB of CdZnS, which thus reduces the oxidation of CdZnS and minimizes the photocorrosion of CdZnS catalysts. In another study, amorphous TiOXH/SiOXH deposited RhCrOy/LaMg1/3Ta2/3O2N photocatalysts have been reported by Pan and co-workers for overall water splitting under 300 W Xe lamp (λ ≥ 300 nm),263 among which RhCrOy acts as electron-cocatalysts for H+ reduction, while the TiOXH/SiOXH shell serves as hole-capture centers. As a result, the stoichiometric H2 and O2 evolution proceeding at a constant rate suggests the stable photocatalytic performance over TiOXH/SiOXHRhCrOy/LaMg1/3Ta2/3O2N, which is attributed to the fact that the amorphous oxyhydroxide layer could accept the holes to prevent the excessive accumulation of holes in nitrogen species on the photocatalyst surface, thereby suppressing the N2 evolution and retarding the photocorrosion of LaMg1/3Ta2/3O2N photocatalyst. Similarly, the Ti-oxyhydroxide-deposited RhCrOy/CaTaO2N photocatalyst for overall water splitting under UV and visible light irradiation has been designed.264 The stability of CaTaO2N is significantly enhanced by introduction of dual cocatalysts of Ti-oxyhydroxide and RhCrOy. Additionally, the dual cocatalysts modified photocatalyst systems of Cr2O3/RuOx/ZrO2/TaON,264 TiO2/SiO2/RhCrOx/LaMg1/3Ta2/3O2N,265 IrO2/Cr2O3/RuOx/ZrO2/TaON266 with stable photocatalytic performance under light illumination have also been reported. Considering the high cost and scarcity of noble metals cocatalysts (such as Ag, Pt, RuS2, RhCrOy etc.), it is highly desirable to develop economical and efficient noble-metal-free cocatalysts materials. Very recently, Yu and co-workers33 have reported the fabrication of dual amorphous Ti(IV)-Ni(II)modified CdS (Ti(IV)-Ni(II)/CdS) by a facile impregnation method for photocatalytic H2 evolution under visible light irradiation (λ ≥ 420 nm) in a Na2SO3-Na2S mixed solution. The recycle experiments have been performed to assess the photostability of pristine CdS and Ti(IV)-Ni(II)/CdS nanocomposites under identical conditions. As compared to pure CdS with serious photocorrosion, no noticeable photoactivity changes for Ti(IV)-Ni(II)/CdS composites are observed even after five recycles, suggesting the obviously inhibited photocorrosion of CdS by simultaneous modification of CdS with amorphous Ti(IV) and Ni(II). More specifically, the Ti(IV) hole-cocatalyst could reduce the photocorrosion effect of CdS by rapidly transferring and capturing the photogenerated holes from the CdS surface, while the Ni(II) electron-cocatalyst could capture the photogenerated electrons and then function as the reduction active sites to promote the interfacial H2 evolution reaction. The development of low-cost, nontoxic, and earth-abundant dual cocatalysts is beneficial for the preparation of stable semiconductor-based photocatalysts and their large-scale applications toward solar energy conversion. - 28 -


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ACS Catalysis

Co-Pi as a hole cocatalyst and Cu(II) as an electron cocatalyst have been successfully decorated onto the semiconductor Ag3PO4 surface by an in situ photo-deposition method and an impregnation method, respectively.267 According to the five-cycle test results in Figure 9A, the as-formed CoPiCu(II)/Ag3PO4 photocatalyst shows stable photocatalytic activity toward MO degradation under visible light illumination (λ > 400 nm) by a 350-W Xe lamp with a UV cutoff filter, suggesting the suppressed photocorrosion. Figure 9B elucidates the photocatalytic mechanism over CoPiCu(II)/Ag3PO4, in which the Cu(II) cocatalyst acts as oxygen-reduction active sites to consume the photoelectrons and hampers the reduction of surface lattice Ag+, while the Co-Pi cocatalyst serves as hole-capture centers to promote the oxidation reaction between holes and organic pollutants. It is the synergistic action of Cu(II) and Co-Pi cocatalysts that facilitates the charge separation and therefore endows the CoPi-Cu(II)/Ag3PO4 composites with high photocatalytic efficiency and stability. Similarly, Fe(III) and amorphous Ti(IV) have been employed as electron and hole cocatalysts, respectively, to ameliorate the catalytic performance and stability of AgBr materials for photodegradation of phenol.181 The presence of amorphous Ti(IV) efficiently transfer holes to oxidize organic contaminants, and Fe(III) cocatalyst is able to rapidly capture photogenerated electrons from AgBr surface to hinder the reduction of Ag+ ions to metallic Ag, which thereby suppress the deactivation of AgBr and enhance its photostability. In addition to both the electron cocatalysts and hole cocatalysts, dual electron cocatalysts modified semiconductor photocatalysts have also been demonstrated to exhibit high photostability during light irradiation. The ternary Ag/AgVO3/reduced graphene oxide (Ag/AgVO3/RGO) composites have been constructed by Chen and co-workers for photocatalytic degradation of Basic Fuchsin (BF) and Bisphenol A (BPA) under visible light irradiation (λ > 420 nm).262 The Ag/AgVO3/RGO sample exhibits excellent photocatalytic activity and stability since both the Ag particles and RGO can serve as electron mediators to promote the charge transfer and decrease the recombination of electron-hole pairs via a multiple-electron transfer process. Chen et al. have encapsulated Ag2O octahedral with ultrathin carbon quantum dots (CQDs) layer which serves as support for Ag NPs to construct CQDs/Ag/Ag2O ternary photocatalyst.182 Such designed architecture provides a vectorial charge transfer pathway for expediting charge separation. The cycle catalytic performance of different samples in Figure 9C confirms the advantage of CQDs/Ag/Ag2O ternary structure in improving the anti-photocorrosion properties since no downtrend activity of CQDs/Ag/Ag2O photocatalyst has been observed throughout all five cycles under the irradiation of visible light (λ ≥ 420 nm) using 250W Xe lamp. The increase in photostability is ascribed to two key factors: (1) the dissolution of Ag2O is prevented by encapsulating insoluble CQDs layer; (2) the photoelectrons in the CB of Ag2O are effectively extracted by CQDs and Ag, which thus retards the photocorrosion and improves photostability of catalyst during the catalytic process.

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Figure 9. Cycle runs of CoPi-Cu(II)/Ag3PO4 for the MO degradation under visible light illumination (λ > 400 nm) by a 350-W Xe lamp (A); schematic diagram illuminating the charge behavior at the interface for CoPi/Ag3PO4 and CoPi-Cu(II)/Ag3PO4 photocatalysts (B). Reprinted with permission from ref.267. Copyright 2017 Royal Society of Chemistry.267 The repeated photocatalytic experiments for pure Ag2O, CQDs/Ag2O and CQDs/Ag/Ag2O under the irradiation of visible light (λ ≥ 420 nm) using 250W Xe lame (C). Reprinted with permission from ref.182. Copyright 2016 Elsevier.182 4.4. Hybridization with carbon materials 4.4.1. Graphene and/or its derivatives. Graphene (GR) and/or its derivatives with unique twodimensional (2D) structure have been regarded as an ideal electron collector and transporter due to its high chemical stability and superior electrical conductivity.3, 12, 268 Therefore, coupling semiconductors with graphene has been demonstrated to be a feasible way to reduce the recombination of charge carriers and an effective strategy to ameliorate their photocatalytic performance and antiphotocorrosion for various photocatalytic applications.3, 55, 96, 228, 269-278 For example, Wang and co-workers have draped CdS with few layers graphene,83 which considerably delays the photoinduced dissolution process of CdS film photocatalyst. As displayed in Figure 10A, the digital images of CdS and different-layer-graphene-draped CdS electrodes (i.e., CdS/1LG, CdS/2LG and CdS/3LG) illuminated by visible light suggest that graphene draping effectively improves the photocorrosion resistance of semiconductor CdS. X-ray photoelectron spectroscopy (XPS) measurements in Figure 10B show that the anion of S2- in CdS can be readily oxidized into SO42− by photoinduced holes in the presence of H2O and O2. In contrast, as for the sample of CdS/3LG, no SO42− ions are detected (Figure 10D), while the peaks at binding energies of 163.6 and 164.7 eV reveal the presence of S0 element, resulting from the direct oxidation of CdS by photogenerated holes, which suggests sequestering the direct interaction of CdS with H2O and O2 molecules due to the introduction of three layers ultrathin graphene. Density functional theory (DFT) calculations in Figure 10C indicate that the barrier energies of 37.4 eV and 111.2 eV are required for - 30 -


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an O2 molecule to perpendicularly go through hollow and bridge sites on a monolayer defect-free graphene. This suggests that highly impermeable graphene physically blocks the passage of O2 due to the steric hindrance effect, thereby endowing the CdS/3LG composites with relatively high photostability upon light exposure.

Figure 10. Digital images of as-prepared CdS, CdS/1LG, CdS/2LG, and CdS/3LG taken at different visible light irradiation times (A); high resolution X-ray photoelectron spectroscopy (XPS) spectra of S2p for as-prepared CdS electrode after a 10 min visible light irradiation provided by a 500W tungstenhalogen lamp (B) and CdS/3LG electrode after a 60 min visible light irradiation (D); potential barrier of an O2 molecule passing through graphene with perfect carbon lattices (C). Reprinted with permission from ref.83. Copyright 2017 American Chemical Society.83 Similarly, Zhang et al. have constructed reduced graphene oxide (RGO)-ZnCdS composite by a facile co-precipitation hydrothermal reduction strategy for photocatalytic H2 formation using Na2S and Na2SO3 as sacrificial agents under simulated solar irradiation.279 By performing the recycle experiments, the photostability of RGO-ZnCdS and blank ZnCdS has been studied, which suggests that RGO-ZnCdS sample exhibits no activity loss after four recycles while the H2 generation of bare ZnCdS is gradually decreased due to the photocorrosion effect. The photostability enhancement of RGO-ZnCdS is ascribed to the facilitated electrons transfer between ZnCdS and RGO. The antiphotocorrosion property of Cu2O concave cubes with oxygen vacancies has also been improved by the introduction of RGO, as reported by Zhang and co-workers.280 The Cu2O-RGO composite exhibits stable H2 generation rate during three recycle experiments under irradiation of a 300 W Hg lamp (λ > 420 nm) in deionized water containing methanol (20%) as sacrificial reagent, which is ascribed to the presence of RGO and oxygen vacancies in Cu2O to effectively transfer photogenerated elections and retard the recombination of electron-hole pairs.

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An analogous phenomenon has also been confirmed by Chen et al.,269 who have fabricated the ZnO nanorods (NRs)-GR composites via a facile hydrothermal reaction of GO and ZnO NRs toward MB degradation under UV light irradiation. The photocatalytic activity of GR-ZnO NRs composite is enhanced as compared with blank ZnO NRs and the photocorrosion of ZnO NRs is also inhibited due to the hybridization of GR as verified by well-maintained photoactivity during three successive recycle tests. The improved photoactivity and anti-photocorrosion of GR-ZnO NRs is ascribed to the integrative synergistic effect of enhanced adsorption capacity and prolonged lifetime of photogenerated electron-hole pairs. Notably, the above GR-ZnO NRs composite is prepared via the hard integration of GR with ZnO NRs, which exhibits poor interfacial contact between, thus leading to the limited activity enhancement of GR-ZnO NRs photocatalyst. Subsequently, Weng and coworkers have design the core@shell reduced graphene oxide (RGO) sheets wrapped cationic surfactant (i.e., APTES) modified ZnO nanospheres (NSs) with intimate interfacial contact for RhB degradation under UV light irradiation (λ = 350 ± 15 nm).96 The photostability of as-prepared RGO-ZnS NSs composite has been evaluated by three successive recycle tests. The results show that the photoactivity of blank ZnO is decreased gradually due to the photocorrosion of ZnO NSs induced, while the RGOZnO NSs composite exhibit high photostability due to the fact that RGO sheets could passivate the ZnO surface and work as the protective shield to avoid the photocorrosion of ZnO. Moreover, the presence of RGO nanosheets enhances the adsorption of RhB molecules through the π–π conjugation, which could directly react with photoinduced holes to compete with the photocorrosion processes, thereby resulting in the anti-photocorrosion improvement of ZnO NSs. The chemically modified graphene with abundant hydroxyl and carboxyl groups, i.e., graphene oxide (GO), has been widely applied for fabrication of GO-based hybrid composites toward photocatalytic redox reactions since GO could separate charge carriers and inhibit the recombination of electron-hole pairs in the photocatalysts. A facile ion-exchange method has been developed to synthesize GO enwrapped Ag3PO4 (GO-Ag3PO4) composites for degradation of pollutant dye AO7 and phenol under visible light irradiation (420 nm < λ ≤ 630 nm) using a 300 W Xe arc lamp.277 The stability of Ag3PO4 and GO-Ag3PO4 composite has been evaluated and the results show that a significant loss of photocatalytic activity over bare Ag3PO4 has been observed after the first cycle, while GO-Ag3PO4 composite maintains high photocatalytic efficiency without significant activity decrease, indicating the stability of GO-Ag3PO4 composite. The photostability enhancement of GOAg3PO4 samples is sustained by the enwrapping of GO, in which the photogenerated electrons in Ag3PO4 could be swept away by efficient electron transfer from Ag3PO4 to GO sheets. Similar results have also been reported by Zhu et al., who have hybridized Ag/AgX (X=Cl, Br) with GO to serve as a stable photocatalyst for MO photodegradation under visible light irradiation.278 Notably, the abundance of oxygenated functional groups endows GO with the accessible wetchemistry processability, while the electronic property of GO will be significantly changed.3 Therefore, the reduction of GO to remove surface oxygenated functional groups for ameliorating its electrical conductivity has been widely adopted in order to promote the photocatalytic performance of graphenebased composites.281-283 For instance, nano-sized Ag3PO4 particles (NA) decorated graphene sheets (NAG) have been fabricated toward photodegradation of MB under visible light irradiation (λ > 420 nm).281 The recycle experiments for MB degradation over NA and NAG sample have been carried out to study the photocatalytic stability of these samples, as illustrated in Figure 11A. After five recycles, - 32 -


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ACS Catalysis

the activity loss over the NAG sample is neglectable, suggesting the anti-photocorrosion property of NAG photocatalyst. However, the photocatalytic activity of NA decreases to 45% and the color of NA sample becomes black after the fifth cycle because the formation of metallic Ag nanoparticles. Notably, in the XRD pattern of NAG sample after five cycles, no peak belonged to metallic Ag is detected (Figure 11B), while a relatively weak peak attributed to Ag for the NA sample after reaction indicates the presence of Ag NPs. The above results confirm that the electron-conductive platform of graphene can efficiently impede the reduction of Ag+ by expediting the transfer of photoinduced electrons from CB of Ag3PO4 to graphene (Figure 11C), thus preventing the corrosion and improving the photocatalytic stability of Ag3PO4.

Figure 11. Cycle degradation curves for degradation of MB over NA and NAG samples under visible light irradiation (λ > 420 nm) using a 350 W Xe arc lamp (A); XRD patterns of NA and NAG samples after five cycles (B); proposed photocatalytic mechanism for Ag3PO4-graphene composite under visible light illumination (λ > 420 nm) (C). Reprinted with permission from ref.281. Copyright 2015 Elsevier281 In addition to graphene, some graphene derivatives have also been incorporated with metal sulfides to improve their photostability.41, 284 For example, N-graphene/CdS heterostructures have been fabricated by Jia and co-workers and applied for water splitting under visible light illumination (λ ≥ 420 nm) using a 300 W Xe lamp in an aqueous solution containing 0.1 M Na2S/0.1 M Na2SO3.41 There is no deactivation for H2 evolution over N-graphene/CdS photocatalyst ever after 30 h reaction, and the structure of N-graphene/CdS shows no noticeable difference before and after recycle photocatalytic tests. The results reveal that the N-graphene is able to act as a protective layer to protect semiconductor CdS from photocorrosion under light irradiation. Besides the binary composites, the ternary semiconductor-graphene photocatalysts have also been constructed. Sarkar et al. have engineered Ag and ZnO nanoparticles with RGO via a one-step hydrothermal technique for methyl orange (MO) degradation under UV light irradiation (λ = 365 - 33 -


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nm).285 The result of stability tests reveal that the Ag-ZnO-RGO composite is stable after 3 cycles without significant activity loss. In another work, Kumar and co-workers have synthesized ternary heterojunction composites by anchoring ZnO nanoparticles on MoS2-RGO nanosheets for photocatalytic H2 evolution irradiated under natural sunlight in an aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3 as sacrificial reagents.286 The composite of ZnO-MoS2-RGO with addition amount of 4 wt% MoS2-RGO (denoted as ZMG4) exhibits the highest photocatalytic activity and the activity of H2 evolution is well maintained during cycle testing, which suggests the enhanced anti-photocorrosion properties ZMG4 photocatalyst. The photostability and activity enhancement of ZMG4 is attributed to two factors: (1) The synergistic effect of MoS2-RGO cocatalyst not only separates the photogenerated electron-hole pairs due to the favorable band potentials, but also provides abundant active sites to promote generation of H2. (2) The dissolution of ZnO in alkaline sulfide solutions in situ generates ZnS during the photocatalytic reaction. The as-formed ZnS on the surface of ZnO further facilitates the transportation of photogenerated charge carriers, thus improving the photocatalytic stability of ZnO. 4.4.2. Graphitic carbon nitride. Recently, carbon nitride (g-C3N4) with strong covalent bonds between carbon and nitride atoms has been widely investigated for photocatalysis and photoelectrocatalysis applications due to its abundance, high thermal and chemical stability.287-289 The hybridization of semiconductor materials with g-C3N4 has proved to be an effective strategy for manufacturing composite photocatalysts with high corrosion resistance under light irradiation.86, 168, 290-302 For example, Liu and co-workers have synthesized g-C N /ZnO composite photocatalysts for 3 4 photooxidation of RhB and reduction of Cr(VI) under visible light irradiation (λ > 420 nm).295 To evaluate the stability of the photocatalyst, the repeated experiments of g-C3N4/ZnO sample have been performed, which indicates that the g-C3N4/ZnO composite can serve as a stable photocatalyst. In addition, no obvious difference in the XRD patterns of the fresh and used photocatalyst after fifth cycle experiment has been observed, confirming the high stability of the g-C3N4/ZnO composite. This is reasonable since the semiconductor ZnO cannot be photoexcited under visible light illumination and no holes on ZnO participate in the photocorrosion process. Therefore, to confirm the role of g-C3N4 in promoting the photostability of ZnO, the recycle experiments of ZnO/g-C3N4 composite should be performed under simulated sunlight irradiation, during which both the ZnO and g-C3N4 can be photoexcited to generate electron-hole pairs, as reported by He et al..299 The results of six-run cycle test of photocatalytic CO2 reduction over ZnO/g-C3N4 composite indicate that CO2 reduction rate changes little after the first three cycle runs under the irradiation of 500 W Xe lamp in the presence of water. The used ZnO/g-C3N4 composite has been evaluated by the XRD and XPS, in which no obvious change in the structure of sample has been detected, suggesting the photostability of ZnO/g-C3N4 composite. The anti-photocorrosion property of ZnO/g-C3N4 composite is ascribed to the enhanced separation of photogenerated electron-hole pairs between ZnO and g-C3N4, among which the photogenerated holes in the VB of ZnO will transfer to that of g-C3N4, thereby reducing the reaction possibility between holes and the lattice oxygen in ZnO and inhibiting the dissolution of ZnO. As well known, Ag3PO4 is slightly soluble in aqueous solution due to its high Ksp (1.6 × 10-16),303 so coating the surface of Ag3PO4 with a suitable material to construct core@shell nanostructures could be effective to ameliorate the structural instability of Ag3PO4-based photocatalysts under light irradiation. The g-C3N4 sheets have been reported for the surface coating of semiconductors for enhancing their photostability and activity.304 Therefore, the core@shell Ag3PO4@g-C3N4 composite - 34 -


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ACS Catalysis

photocatalyst has been designed for degradation of MB under visible light irradiation.291 The recycle runs of MB degradation are performed to assess the photostability of Ag3PO4 and Ag3PO4@g-C3N4. The Ag3PO4@g-C3N4 composite shows higher stability than bare Ag3PO4, which is ascribed to the fact that the enwrapping of g-C3N4 sheets is able to protect Ag3PO4 from dissolution in the aqueous solution. In addition, the core@shell Cu2O@g-C3N4 octahedra with superior activity and stability for H2 production under visible light irradiation (λ ≥ 420 nm, 300 W Xe lamp) in an aqueous solution containing 10 vol.% triethanolamine (TEOA) and 0.5 wt.% H2PtCl·6H2O has been reported.305 The intensified photostability is attributed to the synergistic effect at the interface between Cu2O octahedra and g-C3N4 for effectively separating photogenerated charge carriers. However, the photogenerated holes in the VB of g-C3N4 transfer to that of Cu2O, which is unfavorable for the long-term photostability of Cu2O@g-C3N4 octahedra composite. Subsequently, Bao et al. have introduced the Cu NPs between the Cu2O and g-C3N4 to construct Z-scheme heterojunction,306 among which the photoexcited holes in the VB of Cu2O recombine with the electrons in the CB of g-C3N4, thus improving the photostability of Cu2O/Cu/g-C3N4 composites for degradation of MO and phenol under visible light irradiation (λ ≥ 420 nm). In another work, CdS/g-C3N4 core/shell nanowires (CdS/g-C3N4) have been synthesized by Zhang and co-workers via a combined solvothermal and chemisorption method.304 The photostability for H2 generation over CdS/g-C3N4 is evaluated by cyclic photocatalytic reaction tests under visible light illumination (λ ≥ 420 nm) using a 350 W Xe arc lamp in a mixed aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3. The rate of H2 evolution maintains unchanged after four recycles, suggesting the excellent photostability of CdS/g-C3N4 composites during the photocatalytic reaction. The concentrations of Cd2+ ions in the solution of pristine CdS and 2 wt% g-C3N4-coated CdS nanowires (CN2) after irradiation have been monitored (Table 2) and are calculated to be 329.23 mg L-1 and 29.04 mg L-1, respectively, which indicates the serious photocorrosion of CdS and the improved photostablity of CN2 sample. TEM image shows that the morphology of CN2 is essentially unchanged, whereas bare CdS is photocorroded obviously leaving a porous aggregation, which elusidates that the photocorrosion of CdS can be effectively inhibited by the g-C3N4 surface coating strategy. The photostability enhancement of CN2 is ascribed to the well matched band structure between CdS and g-C3N4, among which excited energetic electrons transfer from LUMO level of g-C3N4 to the CB of CdS, while the photogenerated holes on CdS rapidly migrate to the coated g-C3N4 and thus alleviate the photocorrosion of CdS. Table 2 Concentrations of Cd2+ (mg L-1) in the solutions of pristine CdS and CN2 in anaerobic water before (0 h) and after irradiation304 Sample

Irradiation time (h) 0

1

4

8

CdS

0.40

18.16

232.76

329.23

CN2

0.88

2.26

7.33

29.04

4.4.3. Other carbon materials. Besides graphene and g-C3N4, the utilization of other carbon materials (such as amorphous carbon, C60, carbon nanofiber, graphite-like carbon, carbon nanotube, carbon dots and graphdiyne) have been studied to effectively ameliorate the photostability of - 35 -


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semiconductor-based photocatalysts.73, 307-327 For example, Hu et al. have synthesized a series of CdSC petalous particles for degradation of MO and RhB under irradiation of visible light (λ ≥ 420 nm).307 The stability tests for CdS-C-0.5 photocatalyst studied by collecting and reusing the photocatalyst for four cycles suggest that no significant loss in the photocatalytic activity is observed since the introduction of amorphous carbon layer can protect CdS from photocorrosion by facilitating the separation of photogenerated charges and inactivating the surface of CdS. C60 molecules with monomolecular layer state have been dispersed on the surface of ZnO to produce ZnO-C60 composites for degradation of MB under UV-light illumination (λ = 254 nm).316 The attachment of C60 on the surface of ZnO could reduce the vacant sites in ZnO, which substantially inhibits the activation of surface oxygen atom in ZnO, thus resulting in photocorrosion inhibition of ZnO-C60 composites. Under UV light irradiation for 50 h, the photocatalytic performance of C60-hybridized ZnO sample has not been affected whereas bare ZnO is nearly deactivated, clearly confirming the enhanced photostability of ZnO-C60 composite catalyst. Hierarchical core-shell carbon nanofiber@ZnIn2S4 composites (CNF@ZnIn2S4) have been fabricated via a two-step method for H2 generation under irradiation of a 300 W Xe lamp with an optical filter (λ > 420 nm) in a mixed aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3.310 The H2 evolution rate of blank ZnIn2S4 sample obviously decreases after 10 h of reaction owing to severe photocorrosion. As for CNF@ZnIn2S4, it can be well maintained even after four times of usage, confirming that the presence of CNF is favorable for prohibiting the photocorrosion of ZnIn2S4. In another representative example, the graphite-like carbon has also been utilized to integrate with ZnO for enhancing its photostability.321 The recycle experiments toward MB degradation under UV light irradiation (λ = 254 nm) manifest that the photocorrosion of ZnO is hampered after hybridizing with carbon layers, which is ascribed to the covering of graphite-like carbon layers as a firm barrier to inhibit the photo-dissolution process of ZnO. Moreover, the enhanced adsorption ability toward MB molecules over ZnO-carbon composites to consume photoinduced holes is also beneficial for enhancing the photostability of ZnO nanoparticles. One-dimensional (1D) CNT has been confirmed previously to ameliorate the photocatalytic activity and stability of semiconductors.317, 319, 328-333 For instance, the CNT/Ag3PO4 composites have been synthesized and its photocatalytic stability has been evaluated by repeated degradation of RhB under visible light irradiation.73 Notably, the photoactivity of CNT/Ag3PO4 composites maintains at about 90% after 5 cycles, while the pure Ag3PO4 is unstable to degrade RhB dye. The XRD pattern of used CNT/Ag3PO4 sample indicates the unchanged crystalline structure, implying the effective photocorrosion inhibition due to the presence of CNT. Such observed photostability enhancement is attributed to the good electron conductivity of CNT, by which the photoinduced electrons in Ag3PO4 are captured and transferred to CNT efficiently, thereby reducing the reaction between electrons and Ag+ in the lattice Ag3PO4 and inhibiting the decomposition of Ag3PO4. Dai and co-workers have synthesized multi-walled carbon nanotube (MWCNT)/ZnO composites for MB degradation under UV light irradiation at 365 nm.334 The results of recycle experiments show that MWCNT/ZnO exhibits high corrosion-inhibition property and no noticeable activity change is observed after ten recycles, whereas the serious photocorrosion is observed for blank ZnO, indicating that the incorporation of MWCNT with ZnO effectively inhibits its photocorrosion during the photocatalytic reactions. The improved photoactivity and stability of MWCNT/ZnO is related to the strong interaction between ZnO - 36 -


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ACS Catalysis

and MWCNT to separate the photoinduced electron-hole pairs and prolong the lifetime of charge carriers. Carbon quantum dots (CQDs) have been reported to accept and transport photoelectrons conveniently, which could hamper the recombination of charge carriers and thus improve the photocatalytic efficiency over CQDs-based composites.335, 336 Zhang et al. have synthesized CQDs/Ag3PO4 and CQDs/Ag/Ag3PO4 composites for photodecomposition of MO under the irradiation of visible light (λ > 420 nm) by a 150 W Xe lamp.324 The ten cycles experiments suggest that CQDs/Ag3PO4 and CQDs/Ag/Ag3PO4 composites show higher photocatalytic activity and stability than bare Ag3PO4 particles. The XRD patterns of CQDs/Ag3PO4 and CQDs/Ag/Ag3PO4 samples after recycle experiments further confirm the stability of their crystalline structures, while the metallic Ag is detected in the XRD pattern of Ag3PO4 particles, implying the decomposition of bare Ag3PO4. The photostability improvement over CQDs/Ag3PO4 and CQDs/Ag/Ag3PO4 photocatalysts is because: (1) the dissolution of Ag3PO4 and Ag/Ag3PO4 in aqueous solution could be effectively decreased due to the presence of insoluble CQDs layer; (2) the CQDs separate the photoelectrons in the CB of Ag3PO4 and Ag/Ag3PO4, thereby protecting Ag3PO4 and Ag/Ag3PO4 from photocorrosion through electron reduction process, and thus enhancing their photostability. In addition to the improved visible light stability, the modification of carbon dots has also been reported to greatly inhibit the photocorrosion of Ag/AgBr composites for tetracycline degradation under illumination of nearinfrared light.325 Recently, a composite photocatalyst of ordered mesoporous carbon (OMC)-CuO has been reported by Tuerdi and co-workers for photocatalytic degradation of RhB under visible light irradiation (λ > 420 nm).337 The results of five times repetitive experiments show that only 12% activity loss is observed for OMC-CuO, while the pure CuO only remain 22% of photocatalytic activity, which suggests the improved stability and anti-photocorrosion of OMC-CuO as compared with that of CuO. The XRD patterns of pure CuO change obviously but the crystalline phase structure of OMC-CuO is well maintained after photocatalytic recycles. The evidence from the TEM analysis further confirms the high stability of OMC-CuO since no remarkable changes on the morphology have been detected after photocatalytic recycles. The photostability improvement over OMC-CuO is attributed to the OMC channels to accelerate the transfer of electrons, which thus contributes to decreasing the recombination of electron-hole pairs and inhibits the photocorrosion of CuO. In the work of Hossain et al.,338 the porous carbon has been employed as a platform for electron transfer to ameliorate the stability CuO nanoribbons for decomposing MB under visible light irradiation (λ > 420 nm). These studies demonstrate the multifunctional roles of carbon materials in hindering the photocorrosion of semiconductor-based photocatalysts. Graphdiyne, as a new member of carbon materials with highly π-conjugated structure, has also been employed as a hole-transfer layer for perovskite solar cells and/or photocathodes due to its high hole mobility.339-341 However, the application of graphdiyne as a hole-transfer material for photocatalytic reactions has been rarely achieved. Lv and co-workers have recently synthesized CdS/graphdiyne composite for photocatalytic H2 generation driven by LED light (λ = 450 nm) in the aqueous solution (0.3 M triethanolamine).309 The recycle photocatalytic experiments of CdS/graphdiyne composite exhibit no obvious deactivation after four consecutive cycles, which - 37 -


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indicates the high stability and anti-photocorrosion property of CdS/graphdiyne composites. The injection of photoinduced holes from CdS to graphdiyne to suppress the recombination of photogenerated electron-hole pairs has been regarded as the key reason explaining the high photostability of CdS/graphdiyne composites. 4.5. Construction of heterojunctions Since the photoinduced charge carriers suffer from serious recombination in a single semiconductor,72 which leads to low photocatalytic activity and stability, an effective strategy to improve charge separation efficiency is the fabrication of junction structure. Many important findings have been reported on the construction of composites comprising multicomponent or multiphase heterojunctions during the past few years for broadening the light absorption range and elongating the lifetime of charge carriers to design highly active and stable semiconductor-based composite photocatalyst systems.196, 292, 342-362 In the following sections, we will discuss the construction of heterojunctions with particular focus on type-I heterojunction, type-II heterojunction, Z-scheme combination and p-n junction, as shown in Figure 12, for promoting the photocatalytic stability enhancement of semiconductor-based composites.

Figure 12. Schematic illustration of the type-I heterostructure (A); type-II heterostructure (B); Zscheme (C) and p-n heterostructure (D) band alignments, and the transfer process of photoinduced electron-hole pairs of semiconductor-based heterojunction photocatalysts. SC-1: semiconductor 1; SC2: semiconductor 2. 4.5.1. Straddling alignment (type-I). In type-I heterojunction, both CB and VB edges of one semiconductor localized within the energy gap of another, forming straddling band alignment. For example, a type-I core-shell structure photocatalyst synthesized by coating mesoporous ZnS shell on - 38 -


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CdS core through a one pot surfactant-free hydrothermal route has been reported.39 The CdS-ZnS coreshell structure exhibits excellent photocatalytic stability toward H2 generation from an aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 sacrificial agents under the irradiation of 300 W Xe lamp (λ > 400 nm). As shown in Figure 13A, after sixth reaction cycle (60 h), 87% of the initial H2 evolution rate over CdS-ZnS composites is retained without renewing sacrificial agents. According to the band edges of CdS core and ZnS shell, a type-I heterojunction is formed in CdS-ZnS core-shell composites, which indicates that both the photogenerated electrons and holes in CdS core are hardly transferred to ZnS shell under visible light illumination. Therefore, the different intrinsic defects including zinc vacancies (VZn) and interstitial sulfur (IS) in ZnS shell has been considered as localized acceptor states to accept photogenerated holes from the CdS core to ZnS shell, as illustrated in Figure 13B, which suppress the recombination of charge carriers and enhance the photostability of CdS-ZnS. Similarly, Jiang and co-workers363 have coated ZnS nanoparticles as protective shell on the surface of CdS nanorods (NRs) to construct core-shell photocatalysts for H2 generation irradiated by a 300 W Xe arc lamp with a wavelength range of 420-800 nm in the mixed solution containing Na2S (0.35 M) and Na2SO3 (0.25 M). The optimal CdS/ZnS NRs exhibit good stability, which is attributed to the facilitated holes transfer from the VB of CdS to the VZn and IS in ZnS shells, thereby stimulating efficient H2 production and inhibiting the photocorrosion of CdS NRs. However, the effect of holes accumulation on the photocorrosion of ZnS shell has not been taken into consideration in these works. By tuning the molar ratios of Cd/Zn, the ZnCdS/CdS heterostructures have been prepared by an in situ synthesis method with organic amines as templates.364 The presence of localized acceptor states (such as zinc vacancies and sulfur vacancies), which could accept photogenerated holes from semiconductor CdS, is favorable for enhancing the corrosion resistance of the photocatalyst. Consequently, the ZnCdS/CdS heterostructures exhibit high photostability over 100 h, as displayed in Figure 13C. Additionally, Shi and co-workers have prepared sea-urchin shaped Bi2S3/CdS hierarchical heterostructures via a convenient one-pot growth rate-controlled route.345 Since the CB potential of Bi2S3 is more positive than that of CdS, the photoexcited electrons migrate from Bi2S3 to CdS driven by the contact electric field. Meanwhile, as the VB potential of CdS is deeper than that of Bi2S3, the photoexcited holes transfer from CdS to Bi2S3, forming typical type-I heterojunction. The cycle experiments toward RhB degradation under visible light illumination (λ > 420 nm) over Bi2S3/CdS composite in Figure 13D suggest that no evident loss of activity after four cycles has been observed, indicating the good stability of composites. The enhanced photostability is ascribed to the intimate interaction between Bi2S3 and CdS, which is able to transfer photogenerated holes from CdS to Bi2S3, thereby inhibiting the photocorrosion of CdS. In the work of Bandara et al., CuO incorporated TiO2 composite photocatalyst has been prepared for hydrogen production under UV light illumination using 125 W medium pressure mercury lamp.365 The TiO2/CuO composite exhibits stable photocatalytic activity during five recycle tests and only slight decrease of catalytic activity has been observed. The stable photoactivity is attributed to the rapid inter-particle electron transfer from the CB of CuO to that of TiO2 and efficient water reduction reaction on the surface of CuO, which thereby inhibits the dissolution of CuO photocatalyst. However, in this work, the electron transfer process over the TiO2/CuO type-I heterojunction is unclear and the presence of Cu2O after photocatalytic reaction should be excluded to further confirm the anti-photocorrosion of the TiO2/CuO photocatalyst.

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Figure 13. Hydrogen evolution as a function of time from an aqueous solution containing Na2SNa2SO3 agents by CdS-ZnS evacuated per 10 h without renewing the sacrificial agents under visible light irradiation (λ > 400 nm) by a 300 W Xe arc lamp (A); band structure alignments of CdS-ZnS core-shell structure (B). Reprinted with permission from ref.39. Copyright 2014 Royal Society of Chemistry.39 Photocatalytic stability of ZCS0.1 in mixed solution containing Na2S (0.35 M) and Na2SO3 (0.25 M) under visible light irradiation (λ > 400 nm) by a 300 W Xe arc lamp (C). Reprinted with permission from ref.364. Copyright 2016 American Chemical Society.364 Repeated photocatalytic test of RhB over the recycled Bi2S3/CdS composite under visible light irradiation using 150 W Xe arc lamp as a light source with a 420 nm cutoff filter (D). Reprinted with permission from ref.345. Copyright 2014 Royal Society of Chemistry.345 4.5.2. Staggered alignment (type-II). In type-II heterojunction band alignment, two different semiconductors 1 and 2 with well-matched band positions are tightly bonded to construct the efficient heterostructure, among which the CB and VB positions of semiconductor 1 is both higher than that of semiconductor 2, and the steps in the CB and VB go in the same direction.342, 366 Then, the photogenerated electrons in the CB of semiconductor 1 are transferred to the CB of semiconductor 2, and the holes are migrated in opposite directions simultaneously, which thus spatially separates the photoexcited charge carriers and reduces the probability of electron-hole recombination, subsequently resulting in enhanced photocatalytic activity and stability.354, 367-372 Yu and co-workers have decorated paramagnetic ZnFe2O4 nanocrystal onto the surface CdS nanorods (NRs) for constructing type-II heterojunction.373 The photostability of CdS/ZnFe2O4 toward H2 evolution has been investigated by running four cycles under visible light (λ > 400 nm, 400 W high pressure mercury lamp) using Na2S and Na2SO3 as sacrificial agents. The constant H2 formation rate suggests the excellent photochemical stability of CdS/ZnFe2O4 composite while the H2 evolution amount over bare CdS NRs is significantly reduced due to the serious photocorrosion. The - 40 -


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morphologies of blank CdS and CdS/ZnFe2O4 composites after the cycle test show that the shape of CdS/ZnFe2O4 maintains well-defined, whereas the bare CdS turns blurred due to the photocorrosion process. The stability enhancement over CdS/ZnFe2O4 is ascribed to the construction of type-II heterojunction structure, during which photogenerated holes of CdS tend to transfer to the photochemically stable ZnFe2O4 domain, thereby protecting CdS NRs from being decomposed. Additionally, Wang and co-workers have prepared In2S3-CdIn2S4 for photocatalytic CO2 reduction under visible light irradiation (λ > 400 nm) using a 300W Xe lamp with triethanolamine as sacrificial agents.374 The recycle experiments suggest that no evident deactivation after 6 cycles tests has been observed over In2S3-CdIn2S4, confirming the corrosion resistance of the sample. Furthermore, the XRD characterizations of used In2S3-CdIn2S4 after photocatalysis further prove the high stability of In2S3CdIn2S4 photocatalysts. In another study, the composites of Ag2Mo2O7@AgBr-Ag have been prepared through an in situ ion exchange reaction between Ag2Mo2O7 rods and Br-, followed by visible light irradiation (λ ≥ 400 nm).375 The Ag2Mo2O7@AgBr-Ag composites exhibit stable photoactivity toward degradation of methylene blue (MB) during the cycle experiments. The synergistic effects originated from the well-aligned type-II heterojunction band structures between semiconductors Ag2Mo2O7 and AgBr as well as the SPR of Ag NPs are considered as the main reasons for achieving the good photostability over Ag2Mo2O7@AgBr-Ag composite catalysts. Since the architectures of semiconductor-based composites affect their photocatalytic activity and stability, type-II heterojunction photocatalysts with different morphologies have been widely reported. For instance, ZnO/ZnS microflowers have been fabricated by a scalable microwave-assisted method for decomposition of eosin B under simulated sunlight irradiation.368 According to the Anderson model, the ZnO/ZnS heterostructures belong to “staggered” type-II heterojunction band configuration. When ZnO/ZnS is exposed under simulated sunlight light, the photogenerated holes tend to transfer from VB of ZnO to the corresponding band of ZnS with a higher potential, while the electrons transfer from CB of ZnS to that of ZnO. Therefore, the separation of holes from ZnO reduce its photocorrosion and facilitate the stability of ZnO/ZnS heterostructure photocatalyst. Similar phenomenon has also been observed in ZnO/ZnS nanocable and nanotube arrays.354 The complex ZnO/ZnS arrays exhibit high reusability toward MB degradation under UV light irradiation and ca. 94.4% photoactivity is maintained even after ten recycles. In addition, a lollipop‐shaped, uniform Cu@Cu2O/ZnO composite with ultrasmall Cu@Cu2O NPs partially coated with ZnO nanorods has been well designed.376 The Cu@Cu2O/ZnO composites exhibit stable H2 generation activity for repeated cycles under UV-Vis irradiation in the presence of Na2S and Na2SO3 as sacrificial agents, which is attributed to the fact that the unique design of Cu@Cu2O/ZnO heterojunction promotes the charge transfer from plasmonic Cu to the CB of Cu2O and then that of ZnO nanorods, and the holes generated in the VB of ZnO are transferred to the VB of Cu2O. BaTaO2N nanoparticles decorated 1D Ta3N5 nanorods (denoted as Ta3N5/BTON) have been prepared for overall water splitting under visible light irradiation (λ > 420 nm).377 In the type-II heterostructure of Ta3N5/BTON, the photoexcited holes in the VB of BTON transfer to that of Ta3N5 and the photogenerated electrons in the CB of BTON are transferred oppositely, thereby leading to the spatial charge separation. As a result, the Ta3N5/BTON composite exhibits stable photoactivity. Zhang and co-workers have fabricated SnO2 nanoparticles (NPs) decorated dodecahedral-like Ag3PO4 for photocatalytic degradation of MO under visible light irradiation (λ ≥ 420 nm).378 The - 41 -


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introduction of SnO2 NPs not only increases the photocatalytic activity of Ag3PO4 but also greatly inhibits its photocorrosion, as demonstrated by the recycle experiments in Figure 14A. This stability enhancement is attributed to the virtue of type-II heterojunction, among which the SnO2 could accept photogenerated electrons from Ag3PO4 to facilitate separation of electron-hole pairs (Figure 14B) and avoid the reduction of Ag3PO4 by photoelectrons (Ag+ + e- → Ag). In another study, the NaNbO3/CdS type-II core/shell heterostructures have been employed as visible light photocatalysts toward MB degradation.379 The results show that the efficient migration of photoinduced electron-hole pairs resulting from the type-II band gap structure and core/shell configuration contribute to the effective corrosion resistance of NaNbO3/CdS composites. According to the above discussion, the synthesis of type-II heterostructure could enhance the photostability of semiconductor-based materials. Nevertheless, for most research works, the recyclable separation of composite powder from the aqueous phase is difficult and leads to catalysts loss during the recycle experiments, which also impedes their practical applications. Sui et al. have recently employed the electrospun TiO2 nanofibers as supports for fabricating Ag-AgBr/TiO2 heterostructures via solvothermal technique and photoreduction method.380 The advantage of this photocatalyst is that the support matrix of TiO2 nanofibrous web enables the photocatalyst to be separated from solution easily, which is beneficial for recovering the photocatalysts for long-term repeated use. The stability of Ag-AgBr/TiO2 heterostructures has been investigated by performing recycle experiments toward MB degradation. As illustrated in Figure 14C, upon irradiation with visible light (λ ≥ 420 nm) for five cycles, the change on the photoactivity of Ag-AgBr/TiO2 heterostructures is neglectable. The crystalline structure and morphology of Ag-AgBr/TiO2 photocatalyst after five times of cycle experiments are well maintained, as confirmed by the XRD and SEM analysis in Figure 14D.

Figure 14. The repeated photodegradation of MO over Ag3PO4/SnO2 under visible light irradiation (λ ≥ 420 nm) (A); schematic model for the roles of SnO2 for the enhanced photocatalytic activity and stability of Ag3PO4/SnO2 (B). Reprinted with permission from ref.378. Copyright 2012 Royal Society - 42 -


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of Chemistry.378 Cycle runs for photocatalytic degradation of MB over Ag-AgBr/TiO2 nanofibers under visible light irradiation (λ ≥ 420 nm) using a 150 W high-pressure xenon lamp (C); XRD patterns and SEM images of Ag-AgBr/TiO2 nanofibers before and after five photocatalytic runs (D). Reprinted with permission from ref.380. Copyright 2015 Elsevier.380 4.5.3. Z-scheme combination. The Z-scheme photocatalysis system mimicking natural photosynthesis could efficiently widen the optical absorption range, facilitate the charge separation and enhance the redox ability the photocatalysts.357, 381 The Z-scheme system is that the photoexcited electrons in one semiconductor with a lower conduction band is able to recombine with the holes in the other semiconductor with a higher valence band (VB) in two contacted semiconductors.382-386 Numerous semiconductor-based Z-scheme systems have been constructed for achieving long-term and robust photocatalysts.13, 260, 351, 381, 385, 387-391 Generally, a charge carrier mediator (such as Au, Ag, Cu, Pt, Cd) is used to facilitate charge carrier transfer between two different semiconductors in the Zscheme system photocatalysts.392-394 For example, Pan and co-workers have reported the Z-scheme system consisting of RhCrOx-loaded LaMg1/3Ta2/3O2N (RhCrOx/LaMg1/3Ta2/3O2N) as hydrogen evolution photocatalysts and Mo-doped BiVO4 (BiVO4:Mo) as oxygen evolution photocatalysts with the protect layer of amorphous TiO2 for overall water splitting under UV and visible light irradiation (λ ≥ 300 nm).395 Stable water splitting is achieved over the RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet without N2 release originated from the corrosion of LaMg1/3Ta2/3O2N. This is attributed to the well-designed Z-scheme heterostructure, during which the majority of the holes in LaMg1/3Ta2/3O2N recombine with electrons in BiVO4:Mo via the underlying Au layer, thus enabling the composites with stable photocatalytic activity. Similarly, the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/rutile Z-scheme system has also been constructed for overall water splitting in TiO2 precursor solution under UV and visible light (λ ≥ 300 nm),396 which simultaneously produces H2 and O2 at a ratio close to 2:1 and almost completely prevents the N2 evolution, suggesting the high photostability of (RhCrOx/LaMg1/3Ta2/3O2N)/Au/rutile composite photocatalysts. One-dimensional Ag2CO3/Ag/AgBr ternary nanorods have been developed for photocatalytic degradation of RhB under visible light irradiation (λ > 420 nm).360 The Ag2CO3/Ag/AgBr exhibits high stability and reusability for RhB degradation. Additionally, the SEM and XRD analysis of Ag2CO3/Ag/AgBr composites after recycle photocatalysis experiments confirm the intact morphology and stable structure. The Z-scheme mechanism has been proposed to explain the photostability and activity enhancement, in which the lifespan of holes in the VB of Ag2CO3 and electrons in the CB of AgBr is prolonged due to the recombination of electron from the CB of Ag2CO3 and holes from the VB of AgBr. However, the presence of a large amount of electrons in the CB of AgBr is detrimental for its stability since the reduction possibility of Ag+ in the lattice of AgBr may increase under continuous light illumination. In the work of He et al., the Z-scheme Cu2O/Cu/AgBr/Ag photocatalyst has been prepared for MO degradation under visible light irradiation (λ > 420 nm),397 among which the photogenerated electrons in the CB of AgBr combine with holes in the VB of Cu2O using Cu NPs as the electron mediators, which thereby leads to the high photocatalytic stability of Cu2O/Cu/AgBr/Ag composite. Recently, Wang and co-workers have developed ZnO-CdS@Cd heterostructure consisting of a metal Cd core and a ZnO-CdS shell,398 where the CdS shell is directly grown on a Cd core and ZnO - 43 -


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nanoparticles are embedded in CdS shell, for photocatalytic H2 generation in aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 under irradiation of a 300 W Xe lamp. During a long-term photocatalytic reaction (20 h), no noticeable degradation of activity after four recycles has been detected. Moreover, the XPS analysis reveals that the binding energies of Zn 2p, Cd 3d, S 2p, and O 1s in ZnO-CdS@Cd heterostructure after reaction maintain unchanged except for the S 2p due to the Na2SO3 absorption. Also, there is no clear change over the morphology of ZnO-CdS@Cd before and after 20 h photocatalytic reactions. The improved anti-photocorrosion property over ZnO-CdS@Cd heterostructure is proposed as follows: (i) in the ZnO-CdS@Cd heterostructure, the metal Cd core serves as an effective channel to facilitate the Z-scheme charge carrier transport, where photoexcited electrons from the CB of ZnO recombine with the holes from VB of CdS, thus prolonging the lifetime of charge carriers; (ii) the thin ZnO-CdS shell (15 nm) is able to shorten the diffusion length of charge carrier transport in ZnO-CdS@Cd to hinder the bulk combination of charge carriers in ZnO or CdS. Subsequently, GO with excellent electron conductivity has been introduced to construct Ag2MoO4/Ag/AgBr/GO heterostructure photocatalyst for visible light degradation of tetracycline hydrochloride (λ > 420 nm).359 The photodegradation experiments are repeated for eight times to evaluate the photostability of Ag2MoO4/Ag/AgBr and Ag2MoO4/Ag/AgBr/GO composites. The results show that the Ag2MoO4/Ag/AgBr/GO composite remains higher photostability than Ag2MoO4/Ag/AgBr composites and only 4.5% activity decrease has been observed. Moreover, no diffraction peaks of Ag0 are detected in the XRD spectrum of Ag2MoO4/Ag/AgBr/GO after recycle experiments, which confirms its structural stability under light irradiation. According to the Z-scheme mechanism, the photoinduced electrons will gather in CB of AgBr while the holes leave to the VB of Ag2MoO4. The introduction of GO with specific π-conjugated structures quickly transfer the gathered photoelectrons in AgBr to suppress the decomposition process of AgBr, which thus leads to obviously improved photostability. In another work, the Cu2O/graphene/α-Fe2O3 nanotube arrays (Cu2O/G/FNA) composite has been constructed for photocatalytic degradation of MB under visible light irradiation (λ ≥ 400 nm).399 The Cu2O/G/FNA composite exhibits stable photoactivity due to formation of Z-scheme charge transport, which efficiently quench the strong oxidative holes on the VB of Cu2O by using graphene as an electron mediator, thus resulting in prevented photocorrosion of Cu2O. Additionally, the Z-scheme composites of BiVO4-carbon-Cu2O,400 BiVO4(010)-Au-Cu2O401 and 402 CuO/Ag3AsO4/GO have also been well designed with high stability for solar energy conversion. Notably, direct Z-scheme photocatalysts, where a direct contact between two semiconductors is ensured without any charge carrier transfer mediators, have been widely investigated by numerous research groups.348, 403 For instance, semiconductor Ag3PO4 has been extensively studied for construction of direct Z-scheme systems.357, 381, 383 More specifically, Luo and co-workers have prepared hierarchical In2S3/Ag3PO4 hybrid photocatalysts through an in situ chemical precipitation method for photodegradation of MO under visible light irradiation (λ > 420 nm).404 During the photocatalytic reaction, a small fraction of Ag3PO4 is reduced to metallic Ag due to the photocorrosion, which serve as electron mediators to capture photogenerated electrons from the CB of Ag3PO4 and then recombine with photogenerated holes of In2S3 through a Z-scheme charge transfer mechanism. As expected, the In2S3/Ag3PO4 composites retain at over 82% photoactivity after four successive cycle reactions, indicating the stability of In2S3/Ag3PO4 photocatalyst. In this work, the photoinduced electron in the CB of Ag3PO4 recombine with the holes in the VB of In2S3, which is a win-win situation - 44 -


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ACS Catalysis

for reducing the possible photocorrosion of both semiconductor Ag3PO4 and In2S3. In another work, Zhou et al. have designed hierarchical CdS/BiVO4 hybrid composed of CdS nanoparticles decorated on BiVO4 nanowires with Z-scheme heterostructures for photocatalytic H2 evolution in 1 M Na2SO3 solution and irradiated under 300 W Xe lamp with 420 nm cutting off filter.387 The photocatalytic activity for H2 generation over CdS/BiVO4 only shows 17% fading during 5 h irradiation, while bare CdS exhibits almost 53% activity fading because of heavy photocorrosion. The improved stability of CdS/BiVO4 is attributed to Z-scheme charge separation mechanism, in which the photoexcited holes in CdS could be timely removed by reacting with the electrons generated from BiVO4. In another example, Han and co-workers have synthesized CdS/BiVO4 heterostructures for photocatalytic reduction of nitro compounds under visible light irradiation (λ > 420 nm).405 The four times recycle experiments demonstrate that the formation of CdS/BiVO4 Z-scheme heterostructure is able to prohibit the photocorrosion of CdS to a certain extent. Additionally, Ma et al. have prepared the CdS@ZnO core-shell heterostructure with accurately controlled growth of ZnO shell on CdS core using an atomic layer deposition technology for photocatalytic H2 evolution under irradiation of a 225 W Xe arc lamp with 0.35 M of sodium sulfate and 0.25 M of sodium sulfide as sacrificial agents.406 The long-term recycle test over Pt deposited CdS@ZnO sample suggests that the amount of produced H2 maintains for five cycles without decay, verifying the high stability and durability of CdS@ZnO core-shell heterostructure. By performing the hydroxyl radicals trapped photoluminescence experiment, the charge carrier transfer in CdS@ZnO heterostructure has been validated to comply with direct Z-scheme mechanism without an electron mediator, in which photoinduced electrons and holes tend to keep in the CB of CdS and VB of ZnO, respectively, while the electrons transfer from the CB of ZnO to the VB of CdS. However, with the prolonged irradiation time, the reserved holes in the VB of ZnO may cause self-oxidization, leading to photocorrosion. Therefore, PdS as an excellent hole-captured cocatalyst has been in situ decorated onto the surface of ZnO for collecting the holes in the VB of ZnO.407 As a result, the developed CdS@ZnO/PdS photocatalyst exhibit highly stable anti-photocorrosion behavior for H2 generation. In another work, the Z-scheme system of CdS nanoparticles coated 1D rod-like ZnO core-shell structure has been fabricated for water splitting into H2 under the irradiation of simulated solar light from 300 W Xe lamp in the aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3.408 The results show that in repeated runs for photocatalytic reaction of 30 h, no noticeable degradation of H2 generation rate has been detected, indicating a good anti-photocorrosion capability of ZnO@CdS core-shell nanorods. In another work, the Co3O4 nanoparticles modified silicon nanowires arrays (SiNWs@Co3O4) composites have been prepared by Lu et al. for photocatalytic H2 generation under UV-Vis light (320 nm < λ < 780 nm) using a 300 W Xe arc lamp.409 Recycle photoactivity experiments suggest that no obvious inactivation for SiNWs@Co3O4 composites has been observed while the activity of bare SiNWs deteriorates by ca. 30% during recycle photoactivity tests. A direct Z-scheme band alignment in SiNWs@Co3O4 film composites to efficiently separate the photoinduced electron-hole pairs has been considered as the main reason to the enhanced photostability of SiNWs@Co3O4 photocatalysts. 4.5.4. Homojunctions. Apart from the abovementioned heterojunctions made by two different semiconductors, the junction constructed by two different crystalline phases of a single semiconductor is defined as homojunction or phase junction, which has aroused substantial interest since it can deliver - 45 -


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much higher activity than that of single phase catalyst.342, 410 The well-matched interfaces and favorable band alignments are beneficial for building an internal field in homojunctions, which thus efficiently separates the photogenerated electron-hole pairs and hampers the recombination of charge carriers, thereby leading to the high photocatalytic activity and stability. Bao et al. have demonstrated that the hexagonal-cubic CdS phase junctions exhibit worse photocatalytic performance than the bare hexagonal or cubic phase CdS nanocrystals alone, which is ascribed to grain boundaries in the phase junctions.88 Notably, this issue could be circumvented by interlinking cubic CdS and hexagonal CdS with metal platinum, by which both the photocatalytic activity and corrosion resistance of CdS are enhanced. Nevertheless, the fabrication of high-quality CdS-based phase junction with long-term stability remains challenging.411, 412 In this regard, Li and co-workers have integrated the hexagonal-cubic core-shell architecture with nanorod morphology to construct CdS nanorod phase junctions (NRPJs) via a facile hydrothermal method for H2 generation under irradiation of 300 W Xe arc lamp with a wavelength range of 420-800 nm with Na2S (0.35 M) and Na2SO3 (0.25 M) as sacrificial agents.412 The HRTEM image in Figure 15A shows the nanorod morphology of CdS NRPJs. The phase junction of CdS NRPJs is confirmed by the high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image, as depicted in Figure 15B, among which the staking mode of CdS lattice changes from the cubic phase (ABCABC) to hexagonal phase (ABABAB). Notably, the recycle photocatalytic H2 generation over core-shell CdS NRPJs indicates an excellent stability over 400 h (Figure 15E), suggesting an extremely long lifetime of CdS phase junction photocatalyst. The XRD patterns and HRTEM image in Figure 15C and D further confirm the stable phase junction of CdS NRPJs after 400 h photocatalytic reactions. Such superior photocatalytic stability of CdS NRPJs is ascribed to the unique CdS hexagonal-cubic core/ultrathin shell phase junction, among which the ultrathin cubic shell not only enhances the separation of photogenerated charge carriers but also passivates the surface state of hexagonal core. Although the phase junction has been demonstrated to enhance the photostability of semiconductor CdS, the importance of phase junction has not been taken much concern really. Ai et al. argue that the photocatalytic stability of CdS phase junction is closely related to the bonding region width between cubic and hexagonal phase of CdS.411 Therefore, a novel CdS photocatalyst with bonding-regionwidth-controlled phase junction have been designed. The photocatalytic stability test over the optimal CdS nanoparticles indicates the rate of H2 generation is well maintained after 100 h. The phase junction with suitable bonding region width is able to separate the photogenerated charge carriers efficiently. Moreover, the presence of suitable bonding region width could be the shield for phase junction, thereby leading to the long-term photostability of CdS nanoparticles.411

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Figure 15. The HRTEM image (A) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image (B) of an individual concentric hexagonal-cubic CdS nanorod phase junctions (NRPJs); X-ray diffraction patterns of the CdS NRPJs before and after the irradiation for 200 and 400 h (C); HRTEM images of the CdS NRPJs after irradiated for 400 h (D); the photocatalytic stability of CdS NRPJs in vacuum under irradiation of 300 W Xe arc lamp with a wavelength range of 420-800 nm with Na2S (0.35 M) and Na2SO3 (0.25 M) as sacrificial agents (E). Reprinted with permission from ref.412. Copyright 2016 John Wiley & Sons, Inc.412 Additionally, the homojunctions of ZnCdS with zinc blende/wurtzite twin-induced crystal structure have also been demonstrated to prohibit the corrosion of metal sulfides photocatalysts due to the efficient transportation of photoexcited electron-hole pairs.213, 413, 414 For instance, Ng et al. have decorated sub-2 nm Pt onto twinned Zn0.5Cd0.5S nanocrystals for photocatalytic H2 evolution in both acidic and alkaline sacrificial reagents using a 500 W Xe arc lamp (λ > 400 nm) as light source.413 After five repeated photocatalytic reactions, no significant deterioration of photocatalytic activity over Pt-Zn0.5Cd0.5S homojunction composites has been observed, suggesting the high photostability and recyclability of the sample. The catalysts of Pt-Zn0.5Cd0.5S after photocatalytic reaction have been characterized by XRD and TEM analysis, which clarifies the unchanged crystallinity and morphology, further demonstrating that the construction of homojunctions is favorable for ameliorating the antiphotocorrosion property of semiconductor-based composites. A p-n homojunction in the same material has also be used for enhancing its photocatalytic activity and stability. Wang and co-workers have fabricated ZnO homojunctions by decorating n-type ZnO NPs on p-type ZnO through an in situ secondary-growth method.415 The existence of ZnO p-n homojunction is confirmed by both the “V-shaped” Mott-Schottky plots and anodic shift of onset potentials. The photocatalytic activity and stability of ZnO p-n homojunction have been evaluated by - 47 -


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degradation of MO and phenol irradiated by UV light. The results show that the ZnO p-n homojunction is more efficient in mineralizing pollutants than pure type ZnO. In addition, the recycle photocatalytic reactions confirm the stable performance of ZnO p-n homojunction since no obvious deactivation after five cycle reactions has been observed. 4.5.5. p-n Heterojunctions. With contact of p-type and n-type semiconductors, the different pand n-type electronic structures are capable of accumulating electrons in coupling interface of p-type side, and holes in n-type ingredient, which thus leads to the bands bending and Fermi level equilibration.342, 343, 416, 417 In such case, concentration gradients of electrons and holes are formed across the hetero-interface and restrain the recombination of photogenerated charge carriers by the built-in electric field.342, 343, 417, 418 Therefore, the fabrication of p-n junction represents another alternative strategy to enhance the stability of semiconductor-based photocatalysts.349, 352, 419-421 For example, Abdullah et al. have loaded p-type Ag2S on n-type (AgIn)xZn2(1−x)S2 solid solution (denoted as (AgIn)xZn2(1−x)S2/Ag2S) for Cr(VI) detoxification under visible light irrradiation.422 Due to the different electronic structures of p-type Ag2S and n-type (AgIn)xZn2(1−x)S2, the holes commonly accumulate on Ag2S and electrons on (AgIn)xZn2(1−x)S2 to form p-n junctions with inner electric field. The Cr(VI) reduction efficiency over (AgIn)xZn2(1−x)S2/Ag2S is stable during four consecutive runs, which indicates the good stability of (AgIn)xZn2(1−x)S2/Ag2S composite, confirming that the formation of p-n junction to separate photoinduced charge carriers plays a key role in improving antiphotocorrosion property of (AgIn)xZn2(1−x)S2. In another example, Cu3P/CdS NRs p-n junction has been synthesized by a simple solvothermal method.240 The yield of H2 amount over Cu3P/CdS NRs increases steadily without an apparent decrease in 12 hours of cycle tests, demonstrating the durability of Cu3P/CdS NRs. This improved photostability is attributed to the fact that the inner electric field formed by p-n junction promote the migration of photogenerated holes from the VB of CdS to that of Cu3P, which greatly restricts the recombination of electron-hole pairs and reduces the possibility of CdS corrosion induced by hole oxidation. In addition, Lin and coworkers have reported p-n CdS quantum dots (QDs) decorated silicon nanowire arrays (SiNWs@CdS) composites for photocatalytic hydrogen evolution under visible light illumination (λ ≥ 420 nm) using a 300 W Xe arc lamp.423 The H2 evolution rates over p-n SiNWs@CdS composites remain relatively stable after four cycles as compared with bare SiNWs and CdS QDs. The enhanced photostability of SiNWs@CdS is ascribed to the established p-n junction to promote the migration of photogenerated electron-hole pairs.423 Tang and coworker have decorated Co3O4 NPs on the surface of Ag3PO4 using an impregnation method for MB degradation under visible light irradiation (λ ≥ 400 nm) using a 300 W Xe lamp.424 The Co3O4/Ag3PO4 composites with p-n heterojunction exhibit high reusability after five cycle runs, whereas the photoactivity of bare Ag3PO4 is significantly reduced during the repeated photocatalytic reactions. Two possible reasons have been proposed to explain the improved photostability of Co3O4/Ag3PO4 composites. First, metallic Ag in the Co3O4/Ag3PO4 composites, which is originated from the partial decomposition of Ag3PO4 during photocatalytic reactions, could serve as excellent acceptors to trap photoexcited electrons, thus suppressing the further formation of more Ag0 species during the photocatalytic process. In addition, when p-type Co3O4 is contact with Ag3PO4, the Fermi level of Co3O4 will raise up and simultaneously the Fermi level of Ag3PO4 is lowered until an equilibrium state to establish an inner electric field of p-n heterojunction, which blocks the transfer of - 48 -


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ACS Catalysis

photogenerated electrons from the CB of Co3O4 to that of Ag3PO4, thus decreasing the possibility of Ag3PO4 corrosion. Second, the Co3O4 with low solubility intimately contacts with the outer surface of Ag3PO4 crystals, which hinders the dissolution of Ag3PO4 and enhances its structural stability in solution. Recently, a novel p-n AgBr/Bi2WO6 heterojunction by decorating AgBr quantum dots (QDs) on the surface of Bi2WO6 has been synthesized for photodegradation of MB dye and phenol under irradiation of 500 W Xe arc lamp equipped with a cutoff filter to cut off the light below 420 nm.425 The contact of n-type AgBr and p-type Bi2WO6 leads to the formation of an internal electric field due to the thermodynamic equilibrium of p-n heterojunction. Therefore, the photoexcited electrons in the CB of AgBr will inject into that of Bi2WO6, thereby enhancing the seperation efficiency of charge carriers. More improtantly, the photostability of AgBr in the AgBr/Bi2WO6 composites is ameriolated by decreasing the reduction reaction between the electrons and lattice Ag+ in AgBr. The results of recycle experiments suggest that no apparent downtrend activity over AgBr/Bi2WO6 sample is deteced in five successive MB degradation runs. In another work reported by Zhang et al.,426 p-type Ag2O decorated n-type TaON microspheres have been constructed for degradation of RhB, MO and parachlorophenol (4-CP) under visible light irradiation (λ > 400 nm, 300 W Xe lamp). The results of four times recycle tests toward RhB degradation suggest that around 92% degradation efficiency is well maintained, indicating the durable stability of p-n Ag2O/TaON heterojunction. Because of the asformed p-n junction with inner electric field over Ag2O/TaON composites, the photogenerated electrons in the CB of Ag2O flow into that of TaON and photogenerated holes in the VB of TaON drift into that of Ag2O simultaneously. This consequently leads to both the enhanced photocorrosion resistance and the photoactivity of Ag2O/TaON composite photocatlaysts. Zhang and co-workers have synthesized one-dimensional p-type NiO/n-type ZnO heterojunctions nanofibers for degradation of RhB under UV light illumination by a 50 W high pressure mercury lamp with the main emission wavelength 313 nm.420 The contact between p-type NiO and n-type ZnO leads to the formation of p-n heterojunction, in which the photogenerated electrons transfer from ZnO to NiO while the holes transfer conversely to build an inner electric field. The as-formed p-n junction between NiO and ZnO drive the fast separation of photogenerated electron-hole pairs, which is beneficial for the photostability enhancement. The NiO/ZnO composite maintains durable photocatalytic activity after three repetitive cycles, indicating the corrosion-resistance of NiO/ZnO with p-n heterojunction. Zhou et al. have constructed the ZnO nanodisks (NDs) grafted CuxO nanowires (NWs) with the modification of Au NPs for photocatalytic H2 evolution in a mixed Na2S/Na2SO3 aqueous solution under a Xe lamp irradiation.349 Figure 16A(a, b) shows the TEM and HRTEM images of CuxO/ZnO@Au heterostructure, where the CuxO NWs are combined with ZnO NDs on which Au NPs are decorated. The three-dimensional (3D) branched structure has been further confirmed by the elemental mappings, as displayed in Figure 16A(c, d). The photostability of optimized CuxO/ZnO@Au composites has been investigated by the recycle experiments, as illustrated in Figure 16B. No obvious activity decay during four successive cycles over CuxO/ZnO@Au suggests the relatively stable photochemical stability. The presence of abundant p-n junctions between CuxO and ZnO drive the fast separation of photogenerated electron-hole pairs, and the holes in the VB of ZnO could be fast transferred and consumed (Figure 16C), thus suppressing the photocorrosion of CuxO/ZnO@Au composites. Additionally, a 3D hetero hierarchical device consisting of n-type ZnO - 49 -


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nanorods and p-type CuO nanosheets has been synthesized by Yu et al.,427 which exhibits stable recyclability toward reduction of Cr(VI) under visible light irradiation (λ > 420 nm). The enhanced photostability of 3D CuO-ZnO composite is ascribed to the formation of p-n heterojunction to retard the recombination of photogenerated electron-hole pairs. Additionally, the p-n heterojunctions of CuOIn2O3,428 CuO-BiVO4,429 CuO/BiFeO3430 and Cu2O-TiO2431, 432 have been well documented to ameliorate the photostability of Cu-based semiconductors for various photocatalytic redox reactions.

Figure 16. (a) TEM image, (b) HRTEM images, (c) Cu, Zn, O and Au elemental mappings, and (d) proposed model of an individual CuxO/ZnO@Au heterostructure (A); recycle test of photocatalytic hydrogen production over CuxO/ZnO@Au under a Xe lamp irradiation (λ > 420 nm) in a mixed Na2S/Na2SO3 aqueous solution (B); schematic illustration of photoexcited carrier dynamics in CuxO/ZnO@Au heterostructure (C). Reprinted with permission from ref.349. Copyright 2015 American Chemical Society.349 Apart from the binary composites, ternary semiconductor-based photocatalysts with p-n junction have demonstrated to exhibit high photoactivity and anti-photocorrosion performance toward target applications.433 Specifically, Wu et al. have prepared the n-CdS/n-TiO2/p-BDD hybrid (n-TiO2 cube tubes are vertically grown on p-type boron-doped diamond (BDD) film) with triple heterojunction.433 This triple heterojunction hybrid reveals 78% reduction of photocorrosion rate in comparison with the coupled CdS/TiO2 hybrid. The remarkably inhibited photocorrosion is ascribed to the fact that TiO2 acts as an acceptor for photogenerated electrons due to the lower CB position of TiO2 than the corresponding band positions of CdS and BDD, while BDD acts as a sink for photogenerated holes due to the higher VB position of BDD than those of CdS and TiO2. Thus, photogenerated electrons on the CB of CdS flow to that of TiO2, whereas photogenerated holes on the VB of CdS and TiO2 inject - 50 -


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to the VB of BDD. Namely, the more cathodic edge of the VB of BDD than the corresponding bands of both CdS and TiO2 bring about a more powerful force to drive photogenerated holes on CdS and TiO2 to transfer to BDD instead of accumulating on CdS. Consequently, with the assistance of p-n heterojunctions between BDD and CdS or TiO2, the corrosion of CdS induced by photogenerated holes is greatly mitigated. We can learn from the above discussion that the construction of various heterojunctions, such as type-I heterojunction, type-II heterojunction, Z-scheme combination, homojunction and p-n junction, is able to hamper the photoinduced corrosion of semiconductor-based photocatalysts. Despite the respective advantages of heterojunction fabrication, the further improvement and optimization of some heterojunction systems are still needed for achieving long-term photostability. For instance, under visible light irradiation (λ ≥ 400 nm), photogenerated holes transfer from the VB of CdS to that of ZnS in the Type-I CdS-ZnS junction,39 which is unfavorable for the stabilization of semiconductor ZnS although the amount of holes is reduced in semiconductor CdS. Similarly, photoinduced electrons in the CB of AgBr will flow and accumulate in Ag2Mo2O7 in the Type-II Ag2Mo2O7@AgBr-Ag systems,375 which may accelerate the reduction of Ag+ in Ag2Mo2O7 lattice and lead to the corrosion. Also, the design of CdS phase junctions412 and ZnCdS homojunction413 encounters the disadvantage of photogenerated holes concentration. Therefore, theoretically, the above photocatalyst systems are still facing the risk of photocorrosion under continuous light irradiation. The addition of another component to further effectively trap the concentrated charge carriers could be an option to reduce the possibility of semiconductors photocorrosion. 4.6. Hybridization with conducting polymers Conducting polymers have attracted increasing interest in artificial photoredox reactions for their advantages of high conductivity and stability, unique electronic and optical properties.434, 435 Various polymers, such as polyaniline (PANI),436-445 poly(3-hexylthiophene),446 nafion membranes447, 448 and poly(diallyl dimethylammonium) chloride (PDDA),449 polypyrrole,450 polyacrylonitrile,451, 452 polyvinyl alcohol,453 poly(N-isopropylacrylamide),454 sulfonated polystyrene,455 have been reported to enhance the photocatalytic performance and stability of semiconductor-based photocatalysts. Zhang and co-workers have dispersed monomolecular-layer PANI on the surface of ZnO for degradation of MB under UV light irradiation (λ = 254 nm).443 The results of recycle experiments suggest that the degradation efficiency is decreased from 92% to 16% after three recycles over bare ZnO, indicating poor stability due to serious photocorrosion effect under light illumination, while the photocatalytic activity of PANI-ZnO composites has not been conspicuously affected during recycle reactions, suggesting the improved anti-photocorrosion of PANI-ZnO sample. The enhancement of photostability is attributed to rapidly transfer of photogenerated holes by PANI monolayer. This has further been confirmed by Pei et al.,444 who have reported that the PANI layer modified defective ZnO composite exhibits stable activity toward MO degradation after 40 h irradiation under UV light. Additionally, Ag2CO3 nanorods integrated with Ag NPs have been coated with a sheath of PANI and the as-formed Ag2CO3/Ag/PANI composite shows good photostability during recycle degradation of MO under visible light illumination (λ > 420 nm).441 In another work, PANI modified TaON composite has been fabricated for degradation of RhB under visible light irradiation (λ > 420 nm) using a 300 W - 51 -


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Xe lamp.456 The photogenerated holes from the VB of TaON will be accepted by PANI, which thus inhibits the photocorrosion of PANI/TaON composite. In another work, it has been shown that the photocatalytic H2 rate of bare Ta3N5 is obviously decreased during the recycle experiments due to the self-photocorrosion,457 whereas the activity of thickness tunable PANI shell coated Ta3N5 composite maintains almost unchanged after six recycle tests. The PANI shell prevents the contact between the holes and nitrogen anions in Ta3N5, thereby effectively inhibiting the photocorrosion of Ta3N5. Considering the flexibility of PANI polymer, coating various semiconductors with PANI to design core-shell structure for alleviating the observed photoinduced dissolution of semiconductors have been well investigated. For instance, Wang et al. have investigated PANI thin layer coated CdS spheres (PANI@CdS) with core-shell structure toward photocatalytic H2 production under visible light (λ ≥ 420 nm) with a 300 W Xe lamp using 0.1 M Na2S and 0.1 M Na2SO3 as sacrificial reagents.34 The high-resolution XPS spectrum of C1s suggests the formation of a new C-S bond between PANI and CdS. Additionally, the coating of PANI leads to the binding energy decrease of N1s peak, which elucidates the formation of N-Cd bond. The PANI@CdS sample exhibits stable H2 evolution, which is ascribed to two key factors: (1) the formation of C-S and/or N-Cd bonds between PANI and CdS avoids the self-oxidization of S2- by photogenerated holes; (2) PANI acts as the shell and separates the photoexcited charge carriers via migrating photogenerated holes from VB of CdS surface to HOMO of PANI, thereby leading to photocorrosion inhibition of CdS. Similarly, Liu et al. have reported the core@shell Ag3PO4@PANI composite for degradation of phenol under visible light irradiation (λ > 420 nm).458 The Ag3PO4@PANI exhibits enhanced photostability. The amounts of Ag+ from the dissolution of Ag3PO4 and Ag3PO4@PANI in solution determined by the ICP analysis are about 109.30 mg/g and 28.58 mg/g for Ag3PO4 and Ag3PO4@ PANI, respectively, after 100 h light irradiation, confirming the coating of PANI shell onto Ag3PO4 can significantly hamper the dissolution of Ag3PO4. In addition to PANI, Yang and co-workers have reported the fabrication of poly(diallyl dimethylammonium) chloride (PDDA) coated CdS pre-incorporated hexagonal mesoporous silica (HMS) spheres (CdS/HMS-PDDA), which is demonstrated to be a stable and regenerable photocatalyst toward visible light degradation of Eosin B.449 Specifically, the CdS/HMS-PDDA composite can degrade Eosin B completely for over 22 cycle runs. Despite the photoactivity gradually decreases after 23 runs, the CdS/HMS-PDDA composites are easily to regenerate by H2S treatment. The cadmium leakage in supernate collected after photocatalytic reaction is measured by atomic absorption spectroscopy. The results show that, with the coating of PDDA, the cadmium leakage in CdS/HMS-PDDA composite is effectively resisted even after 150 runs. The improved photostability of CdS/HMS-PDDA is ascribed to the electrostatic repulsion and/or mechanical obstruction between cationic PDDA layer and cationic cadmium species. Additionally, the polymer of 7,7,8,8Tetracyanoquinodimethane (TCNQ) has also been reported to enhance the photocatalytic stability of Ag3PO4 toward phenol degradation under visible light illumination.459 The TCNQ nanosheets wrapped Ag3PO4 particles (Ag3PO4@TCNQ) exhibit high stability and the degradation rate still reaches 90% after five cycles, while only 43% activity retains for bare Ag3PO4 particles. The enhanced corrosionresistance over Ag3PO4@TCNQ is originated from the excellent electron transfer property of TCNQ to efficiently separate photogenerated charge carriers and reduce the reduction of Ag+, therefore greatly improving the stability of Ag3PO4@TCNQ photocatalyst. - 52 -


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4.7. Control of reaction conditions In general, the photocatalytic activity and stability of semiconductors are known to be strongly dependent on reaction conditions. To enhance the stability of semiconductor-based composite photocatalysts, it is of crucial importance to control the photocatalytic reaction conditions. In the following section, the effects of pH values,44, 460, 461 sacrificial agents462 and the oxygen extraction45, 463 on the photostability of semiconductor-based composites will be discussed with choosing recent typical examples. 4.7.1 pH values. The pH value of the reaction system plays an auxiliary but important role in determining the catalytic efficiency and stability of semiconductor-based photocatalysts. As for CdS, it is known that the acidic conditions encourage the dissociation of Cd2+ ions by taking H2S away from CdS even without light irradiation.461, 464-466 Davis et al. have investigated the correlation between pH values and CdS photo-oxidative dissolution process by monitoring the concentration of Cd2+ ions in solution,44 which suggests that the release of Cd2+ ions is a strong function of pH values. At low pH values, the maximum dissolution of CdS occurs, which leads to the high concentration of Cd2+ ions in solution, while the high pH values of reaction solution inhibit photocatalyst corrosion and prevent the Cd2+ ion leaching. In another study, during the photocatalytic mineralization of phenazopyridine over CdS-sensitized rutile TiO2 (TiO2/CdS) nanoparticles composite under solar-simulator radiation, Zyoud and co-workers have studied the tendency of Cd2+ ions leaching at different pH values.466 With the increase of irradiation time, it has been found that a higher pH value corresponds to lower Cd2+ ions concentration, demonstrating that the TiO2/CdS sample shows relatively high stability in basic system as compared to acidic and neutral media. Additionally, Maeda et al. has also observed that the photostability and activity of (Ga1−xZnx)(N1−xOx) solid solution are significantly affected by reaction pH.467 The Rh-Cr mixed-oxide (Rh2-yCryO3) nanoparticles have been decorated onto the surface of (Ga1−xZnx)(N1−xOx) sample for overall water splitting at various pH values under visible light irradiation (λ > 400 nm) using a 450 W high-pressure mercury lamp. It has been found that the Rh2-yCryO3/(Ga1−xZnx)(N1−xOx) photocatalyst exhibits relatively stable gas evolution behavior for 72 h at pH 4.5, whereas the activity decreases markedly as the reaction proceeds in the aqueous solution of pH 3.0 and 6.2, indicating the strong pH dependent photocorrosion behaviour of (Ga1−xZnx)(N1−xOx) composite photocatalysts. Such pH dependence of photocatalytic activity and long-term stability has also been well documented for ZnObased composites photocatalysts.468, 469 In addition to photoinduced corrosion, the semiconductor ZnO suffers from dissolution in acidic or alkaline medium.321 Specifically, ZnO nanoparticles exhibit a tendency to dissolve in an acidic environment: (14) ZnO +2H + →Zn2 + + H2O And in an alkaline environment, ZnO undergo dissolution according to the following equation: ZnO + H2O + 2OH ― →Zn(OH)24 ―

(15)

Therefore, particular attention should be paid to the control of pH for photocatalytic applications of semiconductor-based composites toward achieving both high activity and high photostability. - 53 -


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4.7.2 Sacrificial agents. Adding sacrificial agents to consume photogenerated holes in the aqueous phase reaction system is another important approach to prevent semiconductor-based photocatalyst from being corroded.43, 311, 470-472 These electron donors react more easily with holes than water due to its less positive oxidation potential, which results in accelerated hole consumption on the surface of photocatalysts.473 As described by Equation (1), the electrons and holes are generated in the CB and VB of semiconductor under visible light irradiation, respectively. The effective consumption of holes in the VB by adding electron donors or hole scavengers could result in enhanced photocatalytic stability and activity of semiconductors. For example, when using Na2SO3 and Na2S as electron donors, SO2－ 3 and S2- will be oxidized into SO2－ 4 and S2－ 2 directly by the photogenerated holes in the VB of semiconductor according to the flowing reactions:88, 121, 470, 471, 473, 474

SO 32   H 2 O  2h VB  SO 24   2H  2S2   2h VB  S22 

(16) (17)

2S22   SO 32   S2 O 32   S2 

(18)

SO 32   S2   2h VB  S2 O 32 

(19)

Notably, the efficiency of hole elimination during the photocatalytic reaction is tunable by choosing different hole scavengers with variable redox potentials. Berr and co-workers have demonstrated that increasing redox potential of hole scavengers could decrease photogenerated electron-hole recombination probability, which leads to the enhanced hydrogen evolution activity and stability over the Pt-decorated CdS nanorods photocatalysts.43 As shown in Figure 17A, the photocatalytic H2 generation activity as a function of hole scavengers suggests that the quantum efficiency is the highest in the presence of SO2－ 3, while the H2 formation amount is below the detection limit when MeOH is employed as a hole scavenger under UV-Vis illumination (360-440 nm). Through comparing the redox potentials of different hole scavengers (Figure 17B), it is found that the H2 evolution activity is correlated with the oxidation potentials of different hole scavengers: the larger of the oxidation potential, the higher of photocatalytic efficiency. The comparison between the morphology of Pt-decorated CdS nanorods before and after illumination using SO2 － 3as hole scavengers in Figure 17C-D indicates that no obvious morphological change occurs, which is attributed to the high redox potential of SO2－ 3 leading to fast hole scavenging. However, the welldefined CdS nanorods are aggregated when EDTA4- with low oxidation potential is employed as hole scavengers (Figure 17E-F), suggesting the destabilization of CdS nanorods through photooxidation. This work suggests that the redox potentials of hole scavengers play a critical role in determining the photocatalytic stability of Pt-decorated CdS nanorods.

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ACS Catalysis

Figure 17. Comparison of quantum efficiencies for Pt-decorated CdS nanorods using SO2－ 3, TEA, EDTA4－ , and MeOH as hole scavengers under UV-Vis light illumination (360-440 nm) (A). The inset shows the hydrogen evolution as a function of time for MPA-stabilized nanorods and SO2－ 3, TEA, and EDTA4- as hole scavengers, respectively; schematic plot of the energy levels of the electrons of SO2－ 3, TEA, EDTA4－ , and MeOH vs. vacuum and on normal hydrogen electrode (NHE) scale (B); typical TEM images of Pt-decorated CdS nanorods before and after four hours of hydrogen generation when SO2－ 3(C-D) and EDTA4－ (E-F) are used as hole scavengers and MPA as stabilizing ligands. Reprinted with permission from ref.43. Copyright 2012 AIP Publishing.43 4.7.3 Oxygen extraction. Generally, the photogenerated charge carriers have been regarded as the main cause for the photocorrosion of semiconductor-based catalysts. However, as for the photocatalytic oxygen evolution reaction, the effect of nascent oxygen on the stability of semiconductors is often ignored.45, 463 Tian and co-workers have reported the assembly of Ce-doped WO3 and CdS powders with the artificial blood component perfluorodecalin (PFDL) for photocatalytic H2 generation without sacrificial reagents under visible light (λ ≥ 420 nm) with a 300 W Xe lamp,45 among which the PFDL with high oxygen combining capacity acts as oxygen carrier to remove the asformed O2 from reaction system, thereby leading to the excellent photostability. As shown in Figure 18A, the Ce-doped WO3 and CdS powders with Pt nanoparticles distributed between WO3 and CdS (CPWC) exhibit excellent photostability with the help of PFDL and no significant decrease of activity has been observed for more than 24 h. The XRD patterns of CPWC photocatalyst before and after reaction in Figure 18B display the similar diffraction peaks, which implies the stable structure of CPWC sample. Additionally, the result of BaCl2 titration suggests that no precipitate is detected in the photocatalyst dispersion after 24 h reaction, further confirming the inhibited photoinduced dissolution of CdS in reaction systems. Recently, an ultrathin NiO layer has been assembled onto Zn0.8Cd0.2S (NiO/Zn0.8Cd0.2S) by the same group through an in situ photodeposition method for overall water splitting in the presence of oxygen transfer reagent (PFDL) under visible light illumination (λ ≥ 420 nm).475 With the aid of PFDL, the activity and stability over NiO/Zn0.8Cd0.2S catalyst have been increased since the recombination of the evolved H2 and O2 are markedly inhibited and the oxidation - 55 -


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of NiO/Zn0.8Cd0.2S by O2 or H2O2 is effectively suppressed. As is well known, fish gill is the respiratory organ of fish, which can extract the dissolved oxygen from water, as pictured in Figure 18C. Inspired by this, the artificial gill has been applied in photocatalytic overall water splitting system.463 It has been demonstrated that the utilization of artificial gill not only separates the nascent oxygen from the reaction system to prevent the hydrogen and oxygen back recombination to water but also inhibits the oxygen induced oxidation of CdS (Figure 18E), thereby leading to both high photocatalytic activity and stability toward overall water splitting.463 The cyclic experiments in Figure 18D suggest that the Pt-TiO2/CdS photocatalysts with the addition of artificial gill show excellent activity and stability under visible light irradiation (λ ≥ 420 nm). In the absence of artificial gill, the photocatalytic activity and stability of Pt-TiO2/CdS is low. Inductively coupled plasma-optical emission spectrometer (ICP) analysis shows that the Cd2+ concentration in PtTiO2/CdS dispersion after several cycles is much lower than that in bare CdS dispersion, implying the excellent photocatalytic stability of Pt-TiO2/CdS composites, as displayed in Figure 18F. Similarly, it has also been demonstrated that the photostability of water splitting over the Pt/CdS@Al2O3 composite is obviously enhanced using artificial gill to remove nascent formed O2 from water under visible light irradiation (λ > 420 nm).476

Figure 18. Cycle runs for photocatalytic hydrogen evolution in the presence of 50 mg CdS/0.5 wt% Pt NPs/WO3-CeOx photocatalyst in a 100 mL pure water at room temperature under visible light (λ ≥ 420 nm) with a 300 W Xe lamp. After every 4 h, the produced H2 is evacuated (A); the XRD patterns of CdS/0.5 wt% Pt NPs/WO3-CeOx photocatalyst before and after reaction (B). Reprinted with permission from ref.45. Copyright 2017 Elsevier.45 The structure of fish gill (C) and the schematic diagram of photocatalytic reaction system (E); cycle runs of the photocatalytic activity for various samples: (a) CdS, (b) TiO2/CdS, (c) Pt-TiO2/CdS without artificial gill photocatalytic system and (d) Pt-TiO2/CdS irradiated by a 300 W Xe lamp with a 420 nm cut-off filter (D); the concentration of Cd2+ for each run in the reaction solution of CdS and Pt-TiO2/CdS with artificial gill (F). Reprinted with permission from ref.463. Copyright 2017 Elsevier.463

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4.8. Hybridization photocatalysts with inert support/component When the particle size of photocatalysts decreases to nanoscale, the separation of these catalysts during the recycle experiments is difficult,477 which often leads to the decrease of durability. The immobilization of small-sized catalysts on appropriate inert supports has been reported to efficiently ameliorate their stability since the aggregation of nanoparticles into micrometer-sized particles could be inhibited together with the enhanced adsorption toward reactants. Ma and co-workers have intercalated fine Ag3PO4 nanoparticles into bentonite interlayers (Ag3PO4-Ben) for photocatalytic decomposition of Orange II solution under Xe arc lamp irradiation.478 The recyclability of Ag3PO4 and Ag3PO4-Ben has been compared in 9 recycle runs, among which the bare Ag3PO4 shows high degradation rate of Orange II in the first run, but loses its activity at the last recycle runs, indicating the poor photostability of Ag3PO4 due to silver reduction reaction. In contrast, the Ag3PO4-Ben composite not only exhibit higher activity than blank Ag3PO4, but also show no significantly activity loss after nine recycle runs. The photostability improvement results from the special structure of bentonite with strong net-negative ionic charge compensated by adsorbing Ag+ cations between lamellar layers. The adsorbed Ag+ is oxidized to Ag2O and then reacts with H3PO4 to yield Ag3PO4 salt deposited in the bentonite interlayers. On the other hand, the compensated H+ cations easily capture photoexcited electrons to hamper the formation of metallic Ag, which consequently enhances their photostability during photocatalytic reactions. The small-pore silicon-substituted silicon aluminum phosphate (SAPO-34) molecular sieve has also been employed as a matrix support for Ag-contained photocatalysts due to its large specific surface areas, shape selectivity within small pore entrance and hydrothermal stability. As reported by Wu et al., both the activity and life span of Ag3PO4 modified SAPO-34 photocatalyst have been significantly increased toward RhB degradation under visible light irradiation (λ > 400 nm).477 The enhanced photostability of Ag3PO4/SAPO-34 compared to pure Ag3PO4 is primarily attributed to the chemical adsorption between Ag+ cations in Ag3PO4 and phosphorus-oxy in SAPO-34 frame structure. In addition, the SAPO-34 with micropores provides uniform sites for the growth of Ag3PO4 nanoparticles to prevent the aggregation, thereby enhancing the stability of Ag3PO4/SAPO-34 photocatalyst. Considering the importance of large specific surface area on promoting the photostability of Ag-contained semiconductors, clays and clay minerals, such as layered double hydroxides,479-481, attapulgite,482 sepiolite,483 and montmorillonite484, have also been extensively used as supports for designing efficient and stable photocatalyst toward versatile photocatalytic applications. Additionally, other inert supports, such as mordenite zeolite,485 SiO2,486 chitosan,487, 488 have been demonstrated to enhance the corrosion-resistant of Ag-contained photocatalysts. 5. Summary and future outlook In this review, we attempt to provide a panorama of basic principle and mechanisms with regard to improving the photostability and durability of semiconductor-based photocatalysts. Particularly, special focus has been paid on various strategies implemented to tackle problems associated with the photodecomposition of semiconductors, which has covered the modification on crystal structure or morphology of semiconductors, heteroatom-doped photocatalysts, hybridization with various semiconductors and/or cocatalysts, and regulation of photocatalytic reaction conditions. Based on the - 57 -


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current progress in this area, some perspectives on future development in the aspects of improving the photostability of semiconductor-based photocatalysts have been proposed. Firstly, controlling the crystal structure, size, morphology and shape of semiconductors can tailor their structural and/or electronic properties, which is favorable for manufacturing semiconductorsbased composite photocatalysts with anti-photocorrosion property. Thus, further efforts should be paid to the optimization of individual semiconductors. For example, the CdS nanowires generally exhibit higher photocorrosion resistance than CdS nanoparticles under visible light irradiation. Secondly, the insights into interface interactions and charge carrier dynamics between individual components need to be further deepened. Engineering the interface by the combination of tailored individual components to inhibit the recombination of photogenerated electron-hole pairs and facilitate charge separation could effectively hamper the photoinduced corrosion of semiconductors-based heterostructures. Thirdly, the effect of reaction conditions on photocatalytic stability of semiconductor-based composites is often neglected. By adjusting the pH value, controlling reaction atmosphere and selecting different sacrificial agents, the light-induced dissolution of photocatalysts should be further alleviated to some extent. Last but not least, to fundamentally reveal the underlying corrosion mechanisms of semiconductors-based composites, various advanced detection techniques should be developed. At present, the in-depth understanding on photocorrosion mechanisms is still insufficient. For example, the decomposition process of CdS photocatalyst under nonaqueous conditions cannot be explained by current photocorrosion mechanisms, i.e., hole-induced oxidation process. Therefore, an acidic reaction intermediates driven corrosion mechanism has been verified.489 Recently, new advanced technologies, such as high spatially resolved Kelvin probe force microscopy (KPFM) and surface photovoltage (SPV) spectra, have been employed for visualizing the distribution of photoexcited holes of photocatalysts, which could help to explain the holes induced photocorrosion process of semiconductors-based photocatalysts. Acronyms 1D 2D 3D AQY AM BDD BF BPA CB CNF CNT CQD DFT EDTA

One-dimensional Two-dimensional Three dimensional Apparent quantum efficiency Amaranth Boron-doped diamond Basic fuchsin Bisphenol A Conduction band Carbon nanofiber Carbon nanotube Carbon quantum dots Density functional theoretical Ethylenediaminetetraacetate - 58 -


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DOS Density of states GO Graphene oxide GR Graphene HAADF-STEM High-angle annular dark field-scanning transmission electron microscopy HMS Hexagonal mesoporous silica HNPs Hollow nanoparticles HOMO Highest occupied molecular orbital HRTEM High resolution transmission electron microscopy ICP Inductively coupled plasma emission spectrometry ICP-AES Inductively coupled plasma-atomic emission spectrometry IFCT Interfacial charge transfer KPFM Kelvin probe force microscopy LUMO Lowest unoccupied molecular orbital MB Methylene blue MG Malachite green MO Methyl orange MR Methyl red MWCNT Multi-walled carbon nanotube NDs Nanodisks NPs Nanoparticles NP-MP Nanopores in macropores NRs Nanorods NRPJ Nanorod phase junction NSs Nanospheres NWs Nanowires NWAs Nanowire arrays OMC Ordered mesoporous carbon PANI Polyaniline PC Photonic crystal PDDA Poly(diallyl dimethylammonium) chloride PFDL Perfluorodecalin PNT Porous nanotube QDs Quantum dots RhB Rhodamine B RGO Reduced graphene oxide SAPO-34 Silicon-substituted silicon aluminum phosphate SEM Scanning electron microscopy SPV Surface photovoltage TCNQ 7,7,8,8-Tetracyanoquinodimethane TEM Transmission electron microscopy TEOA Triethanolamine TON Turnover of number TOF Turnover frequency UV Ultraviolet - 59 -


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Valence band X-ray photoelectron spectroscopy X-ray diffraction

Acknowledgements. The support from the National Natural Science Foundation of China (21872029, U1463204, and 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (NO. 2014A05), the 1st Program of Fujian Province for Top Creative Young Talents, and the Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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Photocorrosion Inhibition of Semiconductor-Based Photocatalysts

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