A Critical Review on Energy Conversion and Environmental

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A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface and Interface Jinming Luo, Shuqu Zhang, Meng Sun, Lixia Yang, Shenglian Luo, and John C. Crittenden ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03649 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface and Interface Jinming Luoa, Shuqu Zhangb,*, Meng Sunc, Lixia Yangb, Shenglian Luob and John C. Crittendena,* a

Brook Byers Institute for Sustainable Systems and School of Civil and Environmental Engineering, Georgia Institute of Technology, 828 West Peachtree Street, Atlanta, GA 30332, United States b Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, Jiangxi Province, People’s Republic of China c Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States

* Corresponding author: Email: [email protected]; [email protected]

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ABSTRACT: Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability for aiming at environmental problems and producing renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites and (iii) control of the electron-hole (e--h+) recombination, while these three processed could be influenced by remodeling crystal lattice, surface and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO2 reduction, nitrogen fixation and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion, and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application. KEYWORDS: semiconductor photocatalyst, solar energy, energy conversion, environmental remediation, photocatalytic activity, large-scale preparation, sustainable, practical application Sustainable development requires the growth of practical and clean energy to avoid subsequent environmental problems.1 Solar energy is a clean energy and is generally accepted as an abundant, endless and renewable source that meets the current and future energy needs of humans.2,3 The semiconductor photocatalyst is an ideal candidate in solar energy applications, as it can efficiently utilize the solar energy with simultaneous energy conversion and environmental remediation. The energy conversion aspect mainly includes hydrogen evolution, CO2 reduction, nitrogen fixation and oriented synthesis to fulfill the sustainable strategy. Additionally, its environmental remediation applications are more likely to focus on air purification (i.e., NOx and volatile organic compounds (VOCs) conversion) and wastewater decontamination (i.e., organic pollutants degradation), which has been reported by recent studies.7,12 Hence, the development of efficient artificial photocatalysts is highly desirable, and it is deemed to be “Holy Grail of Chemistry”.4,5 However, ever since the Fujishima-Honda effect of the TiO2 photoelectrode was first reported in 1972,6 none of photocatalysts developed have completely satisfied all the practical requirements. The ideal photocatalysts need to fulfill several key factors, such as (i) photogenerated charges with long lifetimes, (ii) an appropriate band gap, (iii) full range of sunlight utilization and (iv) low cost, high efficiency and stability.7-9 The previous strategies have mainly focused on morphologies and structural features regulation that cannot fully satisfy the current demand of designing efficient and stable photocatalysts. By constructing abundant active sites and tuning the electron structure, 2


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the photocatalytic performance can be enhanced and stablized.10 Meanwhile, it is accepted that the photocatalytic activity is governed by thermodynamics and kinetics of reaction.11 Hence, a comprehensive analysis of light harvesting, active sites and carrier generation/separation should be further explored, combined with thermodynamics and kinetics to design an efficient and stable photocatalyst that can realize the practical application requirements. Overall, we have detailed the state-of-the-art methods and strategies that have enhanced the photocatalytic performance and the photocatalytic application achievements on two aspects: energy conversion and environmental remediation (Figure 1). Finally, suggestions are provided on how to promote the practical application of the solar-driven photocatalyst.

Strategies to Improve Photocatalytic Performance Photons can excite the photocatalyst when incident solar energy is great than or equal to the photocatalyst band gap (Eg). As a result, electrons can be excited from the valence band (VB) to the conduction band (CB) and holes are left on the VB. Generally, the photocatalytic reactions involve the following four major steps: (i) light harvesting and absorption, (ii) photogenerated carriers (e--h+ pairs) generation, (iii) separation and migration of carriers and (iv) electrons or holes utilization for reduction or oxidation. In recent decades, researchers have done tremendous efforts to devote themselves to exploiting photocatalysts with a broader absorption spectrum of solar energy (step i), increasing the quantity and quality of active sites (step ii) as well as separating the charge efficiently in the bulk and interface of the photocatalyst system (step iii) by a series of designs, constructions and modifications.12,13 Hence, Enhancing light absorption, development of active sites and the building of a tunnel for efficient charge separation need to be simultaneously fulfilled to maximally utilize the solar energy.

Enhance and Broaden Light Absorption For efficient utilization of solar energy, it is still one of most challenge to explore semiconductor photocatalysts which can capture the wide spectrum, from ultraviolet (UV) to visible light, and even the near‐infrared (NIR) region to enhance and broaden light absorption.14-17 The Lambert Beer law and the absorption coefficient determine the ability to absorb light for photocatalysts. The light to be trapped will be lost because light can reflect directly from a smooth and flat surface. However, a rough surface can improve light scattering. Surface modifying or alteration and structure design of photocatalysts can optimize light utilization since it can enhance the distribution degree of incident light via light scattering.18 This compels researchers to devote themselves to nanotechnology and even single atoms. Surface Reconstruction Surface Hybridization. Owing to the surface modification can largely optimize the light utilization, the combination of one semiconductor with other semiconductors or cocatalysts was always the first choice by most of the researchers. As results, the light absorption ability has been strengthened accompany with better longevity for the composite photocatalysts Based on TiO2 was the first photocatalyst that has been 3


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reported back to 1972, and it has been well studied and applied in some areas, here we take TiO2 for example. In the case of TiO2, plenty methods were implemented to enhance the photocatalytic activity of TiO2 by broadening its light absorption ability. Till today, researchers usually combine metal oxides with a series of photocatalysts, such as C3N4,19-21 CdS,22,23 Bi2MoO6,24 BiVO4,25 ZnIn2S4,26,27 graphene28,29 and MoS230,31. Surface reconstruction can be achieved by hybridizing the metal oxides, which can broaden and enhance photon-absorption abilities. Specifically, we thought that a new bandgap was formed by the introduction of other semiconductors, which was responsive for longer wavelength in order to improve its light scattering ability. Meanwhile, surface plasmons during the hybridization process is another common technique that has been widely used to enhance photocatalytic absorption. The resonance in the absorption takes place at the plasmon frequency when the dielectric function verges to zero. Intense electric fields at the surface of metal nanoparticles will generate when light irradiates metal nanoparticles at the plasmon frequency.32 Different materials have different resonance frequencies, which can be tuned by modifying the sizes and morphologies of the materials.33-36 The plasmon resonance can be moved into the visible range from UV range by tuning nanoparticles’ size in nanosize or in a single atom.37 Analogously, plasmon resonance can be shifted from visible to the infrared wavelength by further minimizing the size. Meanwhile, assembling the metallic nanoparticles (i.e., Au, Ag, Pt, Pd or alloys) on the semiconductor’s surface can largely enhance and broaden the light absorption using the surface plasmon resonance (SPR) effect of the metallic nanoparticles.38-42 It was accepted traditionally that SPR was only induced by metallic nanoparticles. While, Yang et al. reported that MoS2/TiO2 heterostructures can act as nonmetal plasmonic photocatalysts by regional growth of MoS2 inside TiO2 nanotube for efficient photocatalytic hydrogen evolution.17 This research greatly enriched the field of SPR. Therefore, it is a wise way to producing SPR effect by controlling confined growth of semiconductors. Surface Hydrogenation. To be noted, the poor light absorption ability is owing to the unacceptable wide band gap. Adjustable bandgap to improve photo-adsorption could be achieved by bandgap engineering. Recent years, it was reported that the band gap could be narrowed by hydrogenation on the surface of semiconductor to enhance the light absorption capability.43-48 For example, Mao et al. increased the light absorption of with black hydrogenated TiO2 nanocrystals.43 Huang et al. focused on the black titania with an amorphous shell/crystalline core structure (TiO2-xHx@TiO2) assisted by hydrogen plasma, which revealed intense light absorption abilities in visible and infrared range.44 Furthermore, the absorption edge and intensity of hydrogenated ZnO,45 γ-TaON,46 Cu2WS4,47 In2O348, etc. were broadened and enhanced immensely as well after hydrogenation. Hydrogenation could give rise to disorder of surface atom to improve light scattering, enhancing and broadening the light absorption. Structure Design To achieve an efficient light absorption ability, constructing a photocatalyst with an on-limits nanostructure to increasing the illumination area can take full advantage of reflection and refraction. A hierarchical assembly of the nanomaterials with different dimensions is a crucial way to fabricate an on-limits nanostructure that can exhibit 4


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superior performance.49-50 Researchers have built the structure vertically, hierarchically, omnidirectionally and architecturally in reasonable space configurations of components and meticulous designs.11,51-56 For example, Xu et al. hierarchically decorated zerodimensional (0D) CdS nanoparticles on the hybrids of one-dimensional (1D) ZnO nanorods (NRs) grown vertically on two-dimensional (2D) graphene to enhance its overall photocatalytic performance.57 Hierarchical nanostructures include ternary component of 0D, 1D and 2D. Additionally, graphene oxide acts as a flexible substrate for the formation of ZnO NR arrays (Figure 2a) and meanwhile, it is reduced in the process of ZnO growth. Furthermore, CdS nanoparticles were clearly doped along on 1D ZnO NRs (Figure 2b and 2c), which was a typical hierarchical structure that existed vertically. This nanostructure showed an excellent optical light absorption property, which is mainly due to this structure having excellent performance regarding reflection, refraction and scattering of photons.

Developing Active Sites It is accepted that the catalysis occurs on the active sites generally, while the active sites exist in the edges, unsaturated steps, terraces, kinks and/or corner atoms. Meanwhile, the quantity of carriers is directly determined by the quantity and quality of the active sites.58 The exploration of methods to develop active sites is unceasing. From conventional viewpoints, a larger surface area was usually regarded as having rich active sites. In fact, we believe that the design of an efficient photocatalyst system needs to develop the active sites more adequately and completely by basal engineering and quantum size engineering from lengthways and breadthwise, respectively (Figure 3). Basal Engineering On the one hand, the 2D catalyst with atomic thickness has attracted extensive attention and has become the optimum foundation of next generation materials due to its novel photochemical properties, etc.59 Lengthways, the bulk catalyst can be peeled into few-layer or monolayer nanosheets to increase the ratio of exposed surface atoms to develop active sites. The general principle of exfoliating layered catalysts is by weakening interlayer forces, either through an enlargement of the interlayer distance by intercalation or slippage of layer along the in-plane direction by assistance of an external force.60 Currently, many strategies have been well set up to exfoliate various layered photocatalysts, such as (ⅰ) mechanical exfoliation,61,62 (ⅱ) intercalated expansion and exfoliation63,64 and (ⅲ) etched exfoliation.65 Regarding (ⅰ) mechanical exfoliation, Wang et al. reported that graphitic carbon nitride (g‐C3N4) nanosheets with 2 nm thickness were obtained via simple sonicated exfoliation from bulk g‐C3N4.66 Ball milling is another mechanical mean to exfoliate bulk photocatalysts, and it can produce layered catalysts in large scale, which is one of the most notable advantages.67,68 Regarding (ⅱ) intercalated expansion and exfoliation, the interlayer spacing and Van der Waals force of layered catalyst is enlarged and broken by means of an intercalator, such as tetrabutylammonium and propylammonium hydroxide,63 n-butyl lithium (nBuLi)64 and so forth. Regarding (ⅲ) etched exfoliation, MXenes, with chemical formula Mn+1Xn(n=1, 2, 3), are a term of 2D transition-metal carbides and nitrides, 5


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where M represents a transition metal (Ti, Ta, Nb, V, Mo) and the X can be C and/or N.65 Thin MXenes are typically obtained by chemical etching from bulk MAX phases, 69 which cannot be assisted by the general exfoliating mean due to strong covalent bonds among of the ions and layers., It was reported that isolated 2D Ti3C2 could be obtain through hydrofluoric acid (HF) to etch aluminum layers in Ti3AlC2 bulk materials and intercalation of organic molecules by Naguibet et al. in 2011.65 However, the HF-etched process is not an environmentally friendly process, and decreases the material performance, especially the capacitance owing to the inert F terminals.70,71 Recently, Zhang et al. reported on a high-purity fluorine-free Ti3C2Tx (T = OH, O) that can be synthesized under an alkali condition, which is considered as an innovative method to synthesize functional MXenes.72 The other hand, pores mainly include two types, i.e., stack pores and in-phase pores. The stack pores are crucial to preserve a hierarchical and firm construction, which can largely enhance the cycling stability of photocatalyst.73 The in-phase pores give rise to the formation of low coordinated atoms, edges, terraces, kinks and corners where the active sites locate. Previous studies have proved that the construction of inphase pores on ultrathin 2D nanosheets showed enhanced photocatalytic performance due to more active sites induced by more in-phase pores, such as ultrathin In2O3 porous sheets,74 uniform 2D Co3O4 porous sheets,75,76 BN porous sheets77, etc. Furthermore, it was an effective strategy to solve the exposure of active sites by constructing ultrathin nanosheets with abundant in-phase nanopores. Xie et al. reported on an etchingintralayered Ostwald ripening process to achieve porous β-Ni(OH)2 ultrathin nanomesh to manufacture more active sites in the basal phase (Figure 4).78 After etching under NaOH hydrothermal conditions, the Al atoms and some Ni atoms surrounded with Al atoms were gotten rid of from the basal plane, resulting in porous β-Ni(OH)2 nanosheets with different pore sizes (Figure 4b). At last, β-Ni(OH)2 ultrathin nanomeshes with uniform in-phase nanopores were fabricated (Figure 4c). The Gibbs energy (△G) of vacancies formation with different nanopore sizes was calculated by density function theory (DFT) to demonstrate the Ostwald ripening process (Figure 4d). The larger the vacancy was, the higher energy was required in order to form a vacancy-free nanomesh equally. Thermodynamically, larger pores were more unstable than smaller ones. As a result, under coexistence of larger and smaller pores, the larger pores were tended to shrink, and the smaller pores would continue to compress. Finally, nanomeshes with uniform in-phase nanopore distributions could be produced (Figure 4e-h). Quantum Size Engineering Active sites are generally situated on the low coordinated edges.58 Hence, reduce the size to expose and increase the edges can increase the quantity of active sites. The bulk catalysts can turn into nanoparticles, quantum dots (QDs), sub-nanoclusters, trimer, dimer, single metal atoms by quantum size engineering (Figure 3). Nanoparticles, QDs and sub-nanoclusters contain multiple active centers, but their catalytic activity cannot be ideally achieved. Herein, it is necessary to search well-defined single active sites for photocatalysts to improve its performance and understand the catalytic mechanisms.79 Accordingly, increasing the atom-utilization efficiency is the most effective and reasonable way to use metal resources and facilitate atomic economy. The ultimate goal 6


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of fine dispersion is single-atom catalysts by downsizing the metal nanostructures, which can be well distributed on metal active centers. However, surface tension of particles markedly increases with size decreases, which can largely promote its aggregation.59 Inspired by nanoparticles and QDs anchored on different 2D photocatalyst, it was necessary to search for the adaptive template for catching single atoms controllably and efficiently. 80-83 Zhang et al. firstly reported that the practical platinum (Pt) single atom catalyst supported on iron oxide (FeOx), named Pt1/FeOx.84 Xie et al. reported that the independent single Pt atoms as an efficient cocatalyst were highly dispersed on g-C3N4 (Pt-CN) successfully (Figure 5a and b).85 The extended X-ray absorption fine structure (EXAFS) spectroscopy confirmed that isolated Pt atoms was anchored on the g-C3N4 network (Figure 5c and d). When single-atom Pt was presented as the cocatalyst, PtCN showed an excellent photocatalytic H2 generation, which showed 50 times higher pristine g-C3N4. Furthermore, single Cu atoms acted as cocatalyst could deposited on polymeric carbon nitride (p-CN), which was first reported recently by Xie et al.86 Meanwhile, in 2016, for the first time, Du et al. unraveled the path and mechanism of CO2 reduction using single palladium (Pd) and Pt atoms anchored on g-C3N4 photocatalysts by DFT.87 The individual metal single atoms (i.e., Pd and Pt) functioned as the active sites. In addition, depositing single atom catalysts on g-C3N4 can markedly enhance the light absorption in visible range. Xiong et al. also reported heterogeneous single-atom catalyst for visible-light-driven high-turnover CO2 reduction.88 Ye et al. successfully decorated isolated single Pt atoms onto the surface of CdS nanowires (Pt/CdS), and its structure has been confirmed by EXAFS spectroscopy and HAADFSTEM images (Figure 6b-f).89 The DFT results verified that the Pt/CdS structure could enhance the excited carrier density (Figure 6f-i). The single-atom cocatalyst with high stability can maximize atom utilization and can obviously boost the separation efficiency of e--h+. Hence, the overall photocatalytic hydrogen evolution performance has been greatly enhanced, which is 63.77 times than bare nanowires, respectively. Li et al. recently reported that bulk copper could transform to a Cu single atom via directly emitting atoms. This simple method can be used at industrial levels.90 Hence, it is reasonable to believe that the trimers and dimers of semiconductor photocatalysts could become the next generation of photocatalysts. By basal engineering and quantum size engineering from lengthways and breadthwise, there are more edges, unsaturated steps, terraces, kinks and/or corner atoms forming, increasing the external atom-utilization efficiency is the effective and reasonable way to use metal resources and facilitate atomic economy. Finally, the quantity and quality of the active sites can be improved tremendously.

Separating Charges Efficiently Semiconductor photocatalysts absorb photons to induce photogenerated carriers, which separate or recombine in the process to reach at surface reaction sites. The efficiency of charge separation determines the conversion efficiency. In the photocatalytic process, e--h+ pairs generate within several femtoseconds (fs); the time span from bulk to reactive sites need hundreds of picoseconds (ps). However, the tine of catalytic 7


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reactions between carriers and the adsorbed reactants take place changing from nanoseconds (ns) to microseconds (ms).91,92 In contrast, photogenerated carriers recombine ranging from ps to ns.93 Particularly, the e--h+ recombination requires several ps, which is much rapid than charge migration and carrier utilization to participate reaction.94 Therefore, separation photogenerated carriers that are more effective are highly desirable.95-98 Hereinafter, the charge separation will be discussed into the following six different categories: (i) loading cocatalyst, (ii) doping engineering, (iii) vacancy engineering, (iv) crystal engineering, (v) interface engineering and (vi) the Zscheme photocatalyst system. Loading Cocatalyst The cocatalyst can afford trapping sites for the photogenerated carriers and promote carriers separation to improve the performance of the semiconductor photocatalyst, thus enhancing the quantum efficiency.10 Most importantly, the cocatalysts deposited on surface of semiconductor photocatalyst accelerate the reactions through lowering the activation energy.2,3 Many reported studies have confirmed that the photocatalytic activities can be significantly enhanced by compositing suitable cocatalysts on the semiconductor photocatalysts. The cocatalysts mainly include the oxidation cocatalysts (i.e., RuO2,99 RhOx,100 IrO2,101 MnOx,102 CoOx,103 etc.) and reduction cocatalysts (i.e., Ru,104 Rh,105 Pd,106 Pt,107,108 Au,109 graphene,110,101 etc.). Two dimensional cocatalysts are interesting for researchers to explore and modify.111-115 Furthermore, the transition metal dichalcogenides (TMDs), such as MoS2 and MoSe2, are representing the inexpensive, earth-abundant and functional cocatalysts have been extensively investigated in recent years. Typically, TMDs were loaded with graphene with better conductivity,116 and it has shown great performance. Moreover, owing to the interesting structures of TMDs (the existence of semiconducting phase (2H) and metallic phase (1T)), the efforts are still endeavored regarding on the exfoliation of expose active edges,117 oxygen-incorporation,118 construction of S vacancy,119 and phase engineering120,121 etc. What is more, it is interesting to research if the deposition sites of cocatalysts can be precise and controllable. Doping Engineering Heterogeneous doping in crystal lattice is one of the most valid methods to regulate the electronic structure, separating the charges efficiently.122,123 However, the effect that dopants act in the photocatalyst remains controversial. Dopants act as the recombination centers for the photoexcited carriers, which can impede photocatalytic reation.124,125 Moreover, the dopant’s type, content and distribution also significantly influence the host semiconductor’s properties. Consequently, it is of great concern to figure out the latent mechanism of dopants and how it can influence the photocatalytic performance. Metal Doping. The generation of the impurity levels can be achieved by incorporating metal ions into the crystal lattice, which can largely benefit the carrier separation.126-128 It has been reported that the bulk/surface dopants could bring in regulation sites for the separation of e--h+ pairs and enhance the photocatalytic activities.126 Atomic layers confined with dopant is salutary for determining the role of 8


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dopant on photocatalysis at the atomic-level. Co-doped In2S3 atomic layers with good dispersity were controllably prepared by Xie et al., which can implement the strategy of the lamellar hybrid intermediate.127 Furthermore, the DFT calculations demonstrated that the introduction of Co atoms can increase the charges density at the CB minimum (CBM), which might be observed from the splitting of the Co 3d states (Figure 7a-d). Since so, the photoinduced electrons could be easily aroused via the d→d internal converting of Co atoms, which would allow the possibility of achieving higher photoconversion efficiency. Meanwhile, the light absorption ability of the Co-doped In2S3-based photoelectrode was definitely improved with observed values from 600 to 2000 nm (Figure 7e). Notably, the incident photon-to-electron conversion efficiency (IPCE) of the Co-doped In2S3-based photoelectrode can reached as high as 46% at 450 nm, which was 2 and 9 times higher than that of the bare In2S3 atomic layer and bulk In2S3, respectively. What is more, the Co-doped In2S3 atomic layer exhibits an obvious increase in IPCE in wavelength ranges between 750-900 and 1500-1750 nm, demonstrating their remarkably improved light conversion efficiency in visible range. Nonmetal Doping. In addition to metal doping, efficient charge separation can also achieve by nonmetal doping.128 Oxygen-doped ZnIn2S4 (O-doped ZIS) nanosheets with five S-Zn-S-In-S-In-S layers were prepared by Xie et al. A highly distorted structure was formed by replacing sulfur atoms with oxygen atoms.129 Due to the adjustment of pristine atomic structures by replacing the S atom with the O atom, the electronic structure was evidently changed. Based on the DFT calculation, O doping can enhance the DOS at the VB maximum (VBM) compare to bare ZnIn2S4, indicating the generation of an enhanced charge density around the VBM (Figure 8a-d). Apart from the effect of electronic structure, O doping can also impact the atomic assignment, and the structure distortion of atoms could be distinguished from the TEM images and further confirmed by X-ray absorption fine structure spectroscopy (XAFS) (Figure 8e and f). Furthermore, the average lifetime of photogenerated electrons in O-doped ZnIn2S4 was increased by 1.53 times compared to bare ZnIn2S4 nanosheets, which was verified by ultrafast transient absorption spectroscopy (UTAS) (Figure 8g and h). This phenomenon can further confirm that O doping can regulate the bandgap by atomic arrangement to separate charges to increase the lifetime of electrons. Vacancy Engineering Under the ideal condition, vacancies confined in lattices have tremendous potential for operating the electron properties without destroying the pristine lattices and exposing a high proportion of surface atoms. The electron properties are generated by modulating the mutual effect between intracell and the movement of outer valence electron. While, the band-gap structure, concentration and density of carrier and so on are related to the electron properties.64 Therefore, the development of vacancy-confined semiconductor photocatalyst could be the future direction to regulate the bandgap and the migration of photogenerated carriers, optimizing the overall photocatalytic performance of photocatalysts.130-132 Anion Vacancy. As one of the anion vacancies, sulfur vacancies (Vs) have been extensively researched since they can tune the electronic structure to guide the carrier flows.133-137 The role of the anion vacancy was more than that. New photocatalyst 9


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systems could be constructed by induction of the anion vacancy. For example, MoS2 QDs was induced by Vs to grow on ZnIn2S4 monolayer (Vs-M-ZnIn2S4) selectively to construct an atomic-level heterostructure was applied in hydrogen evolution by our group.138 The Vs-M-ZnIn2S4 nanosheets were produced by exfoliating bulk ZnIn2S4 through n-butyllithium intercalation (Figure 9a). The monolayer property of Vs-MZnIn2S4 was measured by AFM with a thickness of approximately 1.0 nm (Figure 9b). The HRTEM image of Vs-M-ZnIn2S4 confirmed that atoms were absent and pores were formed (Figure 9c). The HRTEM image indicated that MoS2 QDs were pointed on the Vs area (S vacancy-rich area) of Vs-M-ZnIn2S4, which was proven by the existence of the {100} facet of MoS2 (Figure 9d). Moreover, Vs-M-ZnIn2S4 showed a strong S electron spin resonance (ESR) response compared to bulk ZnIn2S4, which suggested that Vs-M-ZnIn2S4 owned the rich S vacancies (Figure 9e). By analyzing In 3d and Zn 2p of Vs-M-ZnIn2S4 and ZnIn2S4, it further suggested that the S vacancies were around the Zn atoms not the In atoms (Figure 9f and g). The regional charge density calculations demonstrated that the charge density of the Zn atom without S atoms was more positive than the other Zn atoms (Figure 9h). The Vs on the Zn facet served as electron traps and regulated the electron cloud.139-141 Under illumination, the Vs on the Zn facet served as electron traps, which can enrich the e- on the Zn facet to hold back vertical migration of e- to recombine with h+. At last, separation of photogenerated charge carriers was efficient because of e- transferring by means of the Zn−S intimate interface. The other side, the h+ was left on Vs-M-ZnIn2S4 (Figure 9i). Hence, Vs can not only efficiently achieve spatial photogenerated charge separation but also act on constructing smart photocatalytic hydrogen reaction systems. Cation Vacancy. Beside of anion vacancy, the cation vacancy is the other effective way to build attractive electronic-nanostructures due to its diverse electron configuration and orbit.142 For the first time, the freestanding orthorhombic-BiVO4 (oBiVO4) atomic layers with vanadium vacancies (Vv) were successfully synthesized via Xie et al.143 A V-defective orthorhombic-BiVO4 atomic layers and a perfect o-BiVO4 atomic layers along the {001} orientation are shown in Figure 10b and d. The introduction of Vv generated a defect level and induced higher hole concentration near the Fermi level revealed by DFT calculation; the result is that the e- can be easily transited into the CB under solar irradiation. Moreover, the higher DOS at the VB edge was due to the engineering-modified Vv (Figure 10a and c). Positron annihilation spectrometry (PAS) was performed to investigate the species and amounts of defects in o-BiVO4 (Figure 10e). The higher lifetime of positron suggested that a higher concentration of Vv existed. It is precisely because of the existence of Vv that the carriers’ lifetime was increased from 74.5 to 143.6 ns, as demonstrated by time-resolved fluorescence emission decay spectra (Figure 10f). The theoretical and experimental data demonstrated that Vv could separate carriers efficiently. Crystal Engineering Facet Engineering. The active sites of reduction and oxidation reaction are probably situated close to each other on the photocatalyst surface. Under the circumstances, recombination of photogenerated carriers maybe occur before they are migrated to active sites of reduction and oxidation reaction. It has been reported that 10


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different crystal facets of semiconductor photocatalysts with low symmetry can separate photogenerated carriers.144 Monoclinic BiVO4 showed ascendant light absorbance, benign photocatalytic performance and nontoxic properties. Thus, it was chosen as a typical photocatalyst to figure out the facet functionality regarding on the reduction and oxidation catalytic sites by tailoring the deposition methods (Figure 11).145,146 As shown in Figure 11a, BiVO4 is a polyhedron. Under irradiation, the photogenerated electrons are abundant on the {010} facets to participate in the reduction reaction. Reduction cocatalysts (such as Au, Pt, Ag, etc.) were easily and individually anchored on the {010} facets by photodepositions. The holes are abundant on the {110} facets to participate in the oxidation reaction. Oxidation cocatalysts (such as Co3O4, MnOx, PbO2, etc.) tended to be deposited on the {110} facets selectively. The photo-deposition of metals was proceeded with water as a hole scavenger. The photo-deposition of MnOx or PbO2 on BiVO4 were performed by using IO3- as the electron acceptors (Figure 11b-d). All these results clearly revealed that the photogenerated e- and h+ tended to migrate on the {010} and {110} facets of BiVO4, respectively. It could lead to reduction and oxidation reactions occurring on the {010} and {110} facets respectively. Facet engineering can steer photogenerated carriers orderly, anisotropically and spatially by separating catalytic sites of reduction and oxidation reaction. Meanwhile, facet engineering can also help to control nucleation and selectively grow the activated sites. In fact, the facets of semiconductor that can accumulate charges would change photocatalytic reaction because it can change the surface state of the semiconductor. Recently, Chen et al. have reported photoexcited charge transport and accumulation in anatase TiO2.147 They have indicated that anatase TiO2 can be highly delved if the directions of charge separation can be clearly understood. Specifically, the reduction and oxidation reactions were preferably to be happened on the crystal facets that charge accumulate preferred They gained a comprehensive cognition of the charge transport and preferred facets of charge accumulations by analyzing electronic band structure calculations of various facets in the k space of anatase TiO2 (Figure 12A). The calculation showed that the order of the preferred facet for oxidation reaction follows: {110} > {445} > {001} > {100} > other facets (including {101}) and {000} > {001} > {110} > {112} > {101} > other facets for reduction reaction. Meanwhile, the results of surface electronic structure gained from surface energy were different from to the results based on calculation. The oxidation facet preferred the order follows: {110} > {445} > {001} > {100} > other facets (including {101} facet), and the reduction facet tended to the order of {001} > {110} > {112} > {011} > {101} > other facets. It showed the comparisons of charges aggregation trend for the bulk/surface calculations with experimental observations (Figure 12B). Figure 12B(a) illustrated that the accumulated trend of holes and electrons was {110} > {445} > {001} and {001} > {110} > {112}, respectively, which was based on the assumption of all the related facets were exposed on the surface of the anatase TiO2 in the bulk calculation. The {001} and {101} was favorable for holes and electrons, respectively, according to the experimental observation (Figure 12B(b)), which was against with the results of surface calculation. 11


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On the basis of the bulk calculation, Figure 12B(c) displayed the ending facets on the surface for charge aggregation, the electrons and holes were preferred on {001} and {110} facet, respectively. However, if the surface was ended with {001} and {101} facets, both electron and holes could be aggregated on (001) (Figure 12B(d)). While in experimental observation, if the surface was ended with {001} and {101} facets, holes favored on the {001} facet and electrons on {101} facet, consistent with the results of surface calculation (Figure 12B(e)). Therefore, photoexcited charges might tend to transport following the order of the bulk electronic structures by considering its thermodynamic energy. Nevertheless, the direction of charge transport and the exposure of ultimate facets are responsible for the charge accumulation and would result in the surface energy alteration. Insightful understanding on the charge transport and accumulation properties of semiconductor can benefit the probing into the photo-corrosion phenomenon. This could benefit the synthetic procedure on preparing the highly active and stable photocatalysts to enhance the photocatalytic performance, which was highly desirable. Phase Engineering. A crystal facet can steer the e- and h+ flow in a macrostructure. Moreover, on the microstructure, the management of crystal phase is crucial to flow of photogenerated carriers.148 For example, a twin crystal refers to a mirror-symmetric orientation relationship (or a specific orientation relationship) that is formed along a common plane between two crystals (or two parts of a crystal); these two crystals are known as the "twins", and this common plane is known as the twin plane. Some reports have stated that the twinsstructure could regionalize the distribution of photogenerated charges.149 Typically, nanotwins Cd1-xZnxS with strip-like lattice structures (Figure 13a) have a tunnel for free charge transportation, and it can also prevent the recombination of carriers.148 The photogenerated carriers could be efficiently separated due to the formation of bending band edge and the ‘‘back to back’’ barrier potential (Figure 13b). Further researches revealed that malposed crystal lattice was formed between the zinc-blende (ZB) and wurtzite (WZ) segments, generating the enormous homojunctions in a specific orientation (Figure 13c).149 A similar type-II malposed crystal lattice was acquired in Cd0.5Zn0.5S through linear estimation (Figure 13d). The upshift of both CB and VB of the WZ segment can result in the efficient separation of both photoexcited e- and h+ compared to the ZB segments. The phase engineering with the “twins-structure” could regionalize and vectorize the charge transfer to enhance the photocatalytic performance.150,151 Interface Engineering The photocatalytic performance does not solely depend on the properties of the photocatalysts, the electron transfer on the interface is another key factor that needs to be considered. Therefore, it is momentous to form an intimate interface by spatial integration of the photocatalyst systems.152 Generally, a large contact surface on the interface can furnish abundant channels for transfer and separation of photo-induced carriers.153 To realize optimal contact, research on how to improve the construction of low contact resistance at the appropriate contact region is needed.154 The interfacial energy level is generally relevant to the electron migration among components, which 12


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can lead to electronic redistribution and modulation.155,156 Accordingly, due to the different type of contact sites of two semiconductors, two representative interfaces are existed, which is homojunction and heterojunction interface, respectively. For homojunction, it could be constructed by phase engineering in one semiconductor, as above twins-lattice,149 to regionalize the distribution of photogenerated charges. Ye et al. reported that cyano terminal C≡N groups endowed the defect-modified g-C3N4 with both n- and p-type conductivities, which could generate p-n homojunctions.157 Enhanced photocatalytic activity was demonstrated that this homojunction structure was highly efficient in charge transfer and separation. Moreover, Chen et al. indicated that anatase/rutile TiO2 composites have displayed enhanced photocatalytic performance, attributing to a synergistic effect between these two phases.158 However, this effect and the interfacial structure has not been confirmed. It was agog to revealed that the influence of interfacial structures and properties on reactivity and performance. They found distinct differences in the thermal properties of anatase and rutile by using Raman spectroscopy, it has confirmed that anatase has become more thermal dynamic than rutile under laser irradiation. They also analyzed the atomic structure and electronic properties to elucidate the process of directional heat diffusion at the interface level by using the first-principles calculations. This asymmetric heat diffusion across interface could be the reason to facilitated phase transition of anatase to rutile after nucleation, which was attributed to the distinctly different elastic properties of two interfacial phases. This research could be useful for the development of two-phase even multiphase semiconductor with enhanced performance at the homojunction interface level. For heterojunction, there are two interfaces (including basal interface and lateral interface) according to contact site of two semiconductors. The heterointerface constructed by epitaxial growth, where charge-carriers can be subsequently vectorially transferred, can benefit the existence of long-lived e--h+ pairs.159,160 And details were given in the following sections. Basal Interface. There is a great quantity of active sites on the interarea of 2D layered photocatalysts. For a semiconductor with a different contact type, the “face to face” 2D-2D stack exhibits greater stability, a larger contact area and more flux to allow higher charge transfers on the interface.161,162 Overall, the 2D-2D structured basal interface can largely improve the photocatalyst’s performance. Zhang et al. prepared MoS2 monolayers constructed with single-layered Bi12O17Cl2 accompanied by interarea oxygen vacancies (Vo) assisted by lithiumintercalation.64 The internal electric field, which was generated by the polarization of the nonuniform distribution of charges among the different layers, could enable efficient e--h+ separation.163 In this system, electrons were driven by the internal electric field and transferred to the MoS2 monolayer via internal Bi-S bonds. Moreover, 2D-2D stacking of the basal interface between the cocatalyst and photocatalyst was reported by numerous studies, such as MoS2/TiO2,164 MoS2/CdS or WS2/CdS,165 MoS2/C3N4,166 and MoSe2/ZnIn2S4.167 A van der Waals (vdW) heterostructure, which is one of basal interfaces, has been put forward to alter the properties of 2D materials.168,169 Strong band linking maintain the basal stability of the 2D structure and the vdW interaction 13


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can maintain the stack structure. More importantly, the vdW heterostructure brings promotions about photoelectric properties by building monolayers together to construct the basal interface.170 In the future, it is quite desirable to strengthen the interfacial acting force to trigger the charger separation by building an intimate basal interface. Lateral Interface. Lateral heterostructures with lateral oriented attachment lead to unparalleled architectures and tunable properties.171 Experimental and theoretical studies revealed that active lateral side or edge have the potency to open nucleation with heterogeneous components and multiple functions.172-174 The steerable selective nucleation of site in catalyst on lateral directions of the 2D photocatalyst is sorely needed but formidable. Wang et al. investigated the reaction kinetics by ingenious construction of the site selectivity between 2D photocatalyst and other nanoparticles at the atomic level.175 Specifically, CdS nanoparticles (NPs) can controllably and basalselectively grow onto 2D Bi2Se3 nanosheets through solution phase reactions. S-related precursors are prone to inert sodium diethyldithiocarbamate and react with thiourea, which can benefit that lateral phase of Se-rich Bi2Se3 nanosheets was modified with CdS NPs (Figure 14a and 14b). The HRTEM images showed lattice orientation of CdS and Bi2Se3 from basal and lateral nanoheterostructures (NHS), and the crystal structures were both hexagonal (Figure 14c). This signified the faultless match of these two components (Figure 14d). Based on the DFT calculations, lateral phase of Se-rich Bi2Se3 nanosheets was the most mair and functional facet (Figure 14e). The DFT calculation indicated that electron coupling was present in vicinity of the CBM between these two components in the NHS, which was strong proof of electron migration from CdS to Bi2Se3 (Figure 14f). The injected electrons from CdS NPs can promptly shift away from interface due to special surface states of Bi2Se3 nanosheets that fell into the bulk energy gap.176 Finally, the existence of an intimate and efficient lateralheterojunction yielded a moderate improvement in the photocatalytic performance. Z-scheme System The Z-scheme was driven by a two parts photoexcitation in photocatalytic system , which was firstly introduced in 1979.177 Strong reductive electrons and oxidative holes can simultaneously exist, which is the most notable phenomenon in Z-scheme photosystems (Figure 15).178 This is mainly because photosystems I (PS I) and II (PS II) are connected by using a mediator. The holes from PS I and the electrons from PS II can be guided directionally. In recent decades, the Z-scheme system has been extensively studied and applied in the photocatalytic field.179-184 Two main questions still remain unclear and need to be resolved for indirect Z-scheme systems. (i) First, the plasma resonance effect has been overlooked. Those elements exhibiting the plasma resonance effect have been used as mediators (i.e., Au, Ag), and it exists widely in the Z-scheme system. The electrons generated from the mediator will inject into the PS Ⅰ and PS Ⅱ because of the SPR. (ii) Second, the recombination site of the holes from PS Ⅰ and the electrons from PS Ⅱ is still unclear. Some studies declare that the electrons transfer to recombine with the holes on the mediator or on the VB of PS Ⅰ through the mediator.180,185,186 If the recombination site occurs on the mediator, the deposited site of mediator must be located at a region that can affect both the values of highest occupied molecular orbital (HOMO) of PS Ⅰ and the lowest unoccupied molecular 14


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orbital (LUMO) of PS Ⅱ. Systematic studies are needed to prove this concept. The intrinsic property and externality of PS Ⅰ, PS Ⅱ and the mediator can affect the migration rate and the recombination site. For direct Z-scheme systems, it is reported that the electrons would recombine with the holes on VB of PS Ⅰ or on the interface.27,187-189 PS Ⅰ needs to be more stable to stand if the recombination sites are located on the VB of PS Ⅰ. Other crucial factors need to be taken into consideration as well (such as the thickness, interfacial acting force and compactness of the interface) if the recombination sites are located on interface. Strategies Associating The individual strategy can effectively, directionally, vectorially, anisotropically and spatially separate photogenerated carriers. Furthermore, combining with the individual strategy can achieve great photocatalytic synergy. Pan et al. enabled high-activity photocatalytic H2 generation under visible-light illumination through vacancies associated with doping engineering in Zn0.5Cd0.5S (ZCS) nanocrystals (Figure 16a).126 The complex of Cu+ and Vs confined into mesoporous ZCS would be locations of active sites, which could improve the photocatalytic activities for H2 evolution. Once a Vs occurred, it affected on the distribution of electrons (CBM states) (Figure 16b). The other side, Cu+-dopant influenced the allocation of holes which located on the {110} and {111} facets (Figure 16c). Hence, as we can see, the oriented separation of photoexcited carriers can largely benefit the photocatalytic activities.

Energy Conversion Photocatalytic Hydrogen Evolution H2 has been deemed to an ideal fuel due to its high energy capacity and environmentally friendly nature. Meanwhile, solar energy is currently identified as an abundant, endless and renewable source that could meet human energy requirements.11 Photocatalytic hydrogen generation has been a broad prospect to convert solar energy into H2. In recent years, researchers have been devoted to constructing an ideal photocatalyst system to improve H2 production. Janus bilayer junctions via steering charge flow at atomic level was newly reported by Zhang et al., which has a superior hydrogen evolution ability with a hydrogen production rate of 33 mmol h-1g-1 and quantum yield was approximate to be 36% at 420 nm.64 Asymmetric Vo-rich Bi12O17Cl2 (BOC) monolayer nanosheets could make full use of the internal electric field to channel holes to the [Cl2] and electrons to the [Bi12O17-x] end-faces, respectively.190 It was reported that Vo had the intriguing capability to entice the metals depositing on Vo sites selectively.191 MoS2 monolayers were selectively assembled on [Bi12O17‑x] facet to prompt electrons migration which was driven by the internal electric field precisely (Figure 17a-h). Moreover, Vo can adjust the coordination of exposed Bi atoms around the Vo to bond with the S atoms on the MoS2 surface. The coordination of Bi-S bonds can act as an efficient highway on interface between MoS2 and Bi12O17Cl2 with Vs (Figure 17i and j). Overall, the ingenious basal design enabled atomic-level control for 15


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efficient charge migration. Furthermore, other investigated photocatalytic systems for H2 production are listed in Table 1. High-profile that modifying and reassembly photocatalysts to obtain efficient photocatalyst systems are used for improving hydrogen production.

Photocatalytic CO2 Reduction Solar-driven CO2 reduction can produce carbon fuels with a simultaneous reduction in the greenhouse effect.206 Under solar light illumination, CO2 molecules could be reduced into various reduction products by the photoexcited electrons (i.e., CO, CH4, CH3OH, HCHO, and HCOOH) at a certain potential, as shown in Table 2. Developing an efficient photocatalyst is needed to enhance the performance of CO2 photoreduction for practical implementation. Quantity efforts have been done to improve the conversion efficiency of CO2 photoreduction using efficient strategies to construct photocatalysts in recent years (Figure 18). (ⅰ) Efficient and metal-free cocatalysts. Ran et al. reviewed CO2 reduction cocatalysts in recent years and advocated probing into cheap and efficient cocatalysts to enhance activity of photocatalysts on CO2 reduction.207 Pt,208 Ag,209 Pd,210,211 Ru,212 Rh,213 and Au214 have been generally acted as highly suitable and active cocatalysts for CO2 photoreduction. However, the rarity and expensive price of the abovementioned cocatalysts of noble metal have significantly hindered their large scale adhibition for solar conversion. Various cocatalysts are including of cheap and earthplentiful elements, such as Co(II) species,215,216 graphene,217 MoS2,218 and MXene.219 (ⅱ) Vacancy engineering. Zhang et al. employed defect-engineering of a 2D nanolayer to enhance the photocatalytic performance, such as defective BiVO4,143 BiOBr,220 ZnAl-layered double hydroxides (ZnAl-LDH)221 and BiOCl222. (ⅲ) Dopant to adjust bandgaps and charge-carrier separation kinetics. For instance, partially oxidized SnS2 atomic layers,223 phosphorus-doped g-C3N4,224 and Ce doped ZnFe2O4.225 (ⅳ) Core/hell hierarchical heterostructures. Lou et al. rationally designed ZnIn2S4-In2O3,226 In2S3-CdIn2S4,227 N-doped carbon@NiCo2O4,228 and hierarchical heterostructures. (ⅴ) Z-scheme systems. Zhu et al. and some other researchers adopted Z-scheme photocatalysts for CO2 photoreduction, such as ZnIn2S4/TiO2,27 Ag3PO4/g‑C3N4,183 α‑Fe2O3/g‑C3N4,229 α‑Fe2O3/Cu2O,230 BiOI/g‑C3N4,231 and metal Sulfide-RGOCoOx/BiVO4.232 (ⅵ) Construction of interfacial heterojunction, such as MXene/Bi2WO6,219 g‑C3N4/NiAl-LDH,233 and P25@CoAl LDH.234 CO2 photoreduction indicates a potential pathway to convert into hydrocarbon fuels by harvesting solar energy friendly. Currently, the efficiency of CO2 photoreduction is still lower than the demands for practical application. Moreover, it is still a challenge to obtain the pure and single product of CO2 photoreduction, which needs further exploration in the future.

Photocatalytic Nitrogen Fixation NH3 is one of the most highly produced inorganic chemicals in the world, due to its large demand in pharmaceutical, fertilizer and many other industrial prodution.235,256 The Haber-Bosch process is applied into nitrogen fixation in industrial production; 16


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however, this process needs to expend a lot of energy because of the rigorous reaction conditions required (i.e., 673-873 K and 15-25 MPa).237,238 Moreover, over 1% of CO2 is generated and emitted to the atmosphere from this industrial process, which promotes the effect of global warming.239,240 Therefore, it is highly desirable to seek an environmentally friendly and sustainable technology to fix the nitrogen with low energy consumption and no CO2 emissions. Semiconductor photocatalysis has attracted widespread interest because of its potential to capture solar energy and drive thermodynamically unfavorable chemical reactions. However, two points need to be considered when designing photocatalysts for N2 fixation, which are as follows: (i) active adsorption sites for N2 and (ii) N≡N activation by photoexcited electrons. Zhang and coworkers found that engineering Vo into BiOBr nanosheets could adsorb the N2 molecule effectively and prolong N≡N to active N2 to simultaneously promote the nitrogen fixation reaction.199 It has been proven that vacancy can efficiently guide and trap the electrons. The electrons around the Vo can efficiently migrate to the adsorbed N2, which can be activated. Ye et al. and Zhang et al. constructed Bi5O7Br nanotubes and CuCr-LDH nanosheets with rich Vo, which also showed satisfactory performance for nitrogen fixation.241,242 Dong et al. reported that the N vacancy (VN) in C3N4 could also relax the bond length of N≡N from 1.117 Å to 1.214 Å.243 Photocatalytic N2 fixation provides an environmentally friendly and cost-efficient method to produce NH3.244 The mechanisms of N2 reduction and the fundamental parameters for efficient catalytic systems need to be further explored. It is desirable to optimize the performance of photocatalytic N2 fixation by developing active sites for N2 adsorption and N≡N activation under ambient conditions.

Photocatalytic Synthesis Converting low value-added products (i.e., methanol) into high value-added products (i.e., ethylene glycol) using “green technology” is highly preferable, and it can meet the requirements of sustainable development. However, the chemical transformation cannot occur via conventional thermocatalytic conversion and needs rigorous conditions to operate. Thus, the solar-light-driven photocatalysis reaction is a very attractive approach to meet this requirement. Recently, photocatalysts displayed great potential in terms of selective chemical transformation and directional synthesis.245,246 Methanol is chosen to be the sacrificial in process of photocatalytic H2 generation; however, the oxidation reaction of methanol and the products oxidized by h+ have been always overlooked. For the first time, Wang et al. emphasized this typical question.245 They studied the visible-light-driven dehydrogenative coupled with the transformation of methanol into ethylene glycol using a MoS2 foam/CdS nanorod photocatalyst. The HRTEM and HAADF-STEM images indicated that MoS2 nanosheets anchored on the CdS nanorod could reveal more active sites. Meanwhile, MoS2 foam can form a closer contact with CdS compared to MoS2 sheets (Figure 19a-c). The EXAFS data indicated that the MoS2 foam on CdS owned less Mo-Mo compared to the bare MoS2, which suggested that more edge sites exist (Figure 19d). The time-resolved PL spectroscopy (TRPL) confirmed that the charge separation and photogenerated excitations have been 17


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enhanced in the presence of MoS2 foam (Figure 19e). Systematic researches revealed a prior excitation of the C-H bond rather than the O-H bond of methanol by photoexcited holes from CdS. This was mainly due to the formation of a hydroxymethyl radical (•CH2OH), which could easily desorb from photocatalyst surfaces for futher reactions (Figure 19f). With separating EG, the MoS2 foam/CdS photocatalyst could be easily recovered without apparent inactivation (Figure 19g). Additionally, in the conventional reactor, many byproducts were detected, such as HCHO HCOOH and oxalic acid. However, relatively pure products can be achieved by adding EG in this system. (Figure 19h). Moreover, Hu et al. reported the evolution of C-H amination and arylation driven by photocatalytic process.246 It showed high efficiency of conversion and selectivity by cerium salts photocatalysts. Photocatalytic synthesis provided a unique strategy to activate C-H to obtain specific product with excellent conversion and selectivity by photocatalysis.

Environmental Remediation Photocatalytic Air Purification Typical air pollutants both outdoor and indoor are generating increasing environmental concern.247,248 NOx, including nitric oxide and nitrogen dioxide, are blamed to atmospheric pollutions, such as ozone depletion, acid rain and photochemical smog, even in an infinitesimal concentration (i.e., g/L). For NOx generated from industrial process, conventional strategies (i.e., adsorption, selective catalytic reduction and thermo-catalysis) have been widely applied and show great performance at relatively higher concentrations (i.e., mg/L). Unfortunately, these techniques are not uneconomical for NOx removal at a low concentration level (i.e., g/L).249-252 However, semiconductor photocatalysis can utilize real sunlight or manmade light source to eliminate air pollutants even at low concentrations under mild environment. Dong et al. reported on a g-C3N4/Al2O3 composite photocatalyst that could effectively remove 77.1% of NO (initial concentration is 600 g/L).253 The reaction mechanism was proposed as follows: g‐C3N4 + hʋ → e− + h+ e− + O2 → •O2− − •O2 + 2H+ + e− → H2O2 H2O2 + e− → •OH + OH− NO + •O2− → NO3− 2•OH + NO → NO2 + H2O NO2 + •OH → NO3− + H+ To further improve the perfomance of NO removal, in 2015, Dong et al. newly prepared an advanced Bi/g-C3N4 photocatalyst.254 The Bi/g-C3N4 composite photocatalyst presented excellent high performance with excellent stability for NO removal due to SPR endowed with Bi metal. Avoiding NO2 emission is the key challenge during the oxidation process from NO to nitrate. It has been proven that •O2− could directly oxidized NO to NO3−; however, •OH is not capable of doing this.253,254 Zhang et al. reported that a well-designed BiOCl 18


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with Vo (Vo-BiOCl) on the prototypical {001} surface can efficiently generate •O2−. This is mainly because the electrons enrichment around Vo with can obviously enhance the surface activity of BiOCl.255 As a result, it facilitated the generation of •O2−. As shown in Figure 20a, Surface bridged •O2− could convert NO into monodentate nitrate directly in route II, which was more thermodynamic than pathway I. The oxidation of NO through route I or II was autocephalous (Figure 20b). The deduction based on the DFT calculation explained the mechanism of •O2− for the thorough oxidation of NO without NO2 release. After O2 was captured on the Vo of BiOCl {001}, the electrons enrichment around the Vo were depleted on two Bi atoms (Figure 20c). Along with the nitrate formation, it was found that the •O2− interacted rigorously with the NO manifested (Figure 20d). The concentration of NO was evidently decreased within 5 min, and gradually reached 70% removal efficiency after 15 min (Figure 20e). Sure enough, just 4 μg/L NO2 was detected, which is much lower than the specified standard of indoor NO2 (0.24 mg/L). Overall, NO was oxidized to NO3- directly with near-perfect selectivity. A schematic diagram of photocatalytic NO elimination over Vo-BiOCl is shown in Figure 20f. The existence of volatile organic compounds (VOCs) in the air is harmful to the health of humans. Photocatalysis presents potential among various technologies for air purification. It can mineralize VOCs into CO2 and H2O completely under ambient conditions.256,257 Choi et al. synthesized TiO2 nanotubes with {001} facet exposed (001-TNT) by electrochemical anodization.258 The performance of 001-TNT on the VOCs removal was examined into both reactor and air cleaner (Figure 21a). The 001TNT presented a better photocatalytic performance than the TNT for toluene degradation under same condition (Figure 21b). The 001-TNT kept its better activity after five continuous operation without any deactivations (Figure 21c). Moreover, the 001-TNT can also degrade acetaldehyde and formaldehyde, which was affirmed by the generation of CO2. However, this phenomenon was not observed using TNT (Figure 21d and e). The 001-TNT filters equipped into air cleaner reached of 72% efficiency for VOCs removal on average within a 30 min operation time in an 8 m3 test chamber.

Wastewater Decontamination Process and Achievements of Water Decontamination With the economic development and accelerated industrialization, water contamination has raised public concerns due to its negative impact on human health. Currently, photocatalysis technology is applied to alleviate water contamination. In terms of mechanism, e- is motivated and migrated from VB to CB and form e--h+ pairs (Figure 22) under illumination. The generated h+ can directly oxidize/mineralize organic contaminants to CO2 and H2O or create •OH to indirectly oxidize/mineralize organic contaminants. If photocatalytic degradation is performed in an aerobic environment, e- can be trapped by O2 to generate •O2− and other reactive species, which can also oxidize organic pollutants. In application, water contaminants, such as dye,259 pesticides,260 herbicides,261 antibiotics,262 detergents,263 pathogens,264, 265 viruses,266 coliforms267 and spores268 can be effectively removed by using the photocatalytic process. Hence, photocatalysis based on semiconductor has been determined to be a 19


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promising art to eliminate water contamination.184, 269-279 Furthermore, many byproducts are formed during the mineralization process using photocatalysis approaches, which is particularly important aspect that need to be considered owing to the toxicity of byproducts generated from the recalcitrant organics. Hence, it is worthwhile to seriously consider the oxidation potential of contaminants, adsorption manner matched by electron clouds and the attacked sites of free radicals. (I) The oxidation potential of the contaminant. The degree of oxidation byproducts is risen gradually. It is critical to definite if the free radicals can also oxidize these byproducts. (II) Adsorption manner matched by electron clouds. Adsorption manner of contaminants on photocatalysts can also determine by electron clouds. The optimal adsorption mode can perfectly matched electron clouds, which makes the entire process much more efficient (III) The attacked sites of free radicals. For the target contaminant, it is better to know the mineralized pathway in order to comprehensively evaluate poisonousness of byproducts. Loop opening or not are related to attacked sites of free radicals, which influence the degree of mineralization. Fukui Function is a function that describes the electron density in a frontier orbital in computational chemistry has determined to be the essential tool to further understand organic contaminants degradation behaviors, owing to it can represents the electrophilic (f -), nucleophilic (f +) and radical attack (f 0) that has been widely used to evaluate the reactive sites of organic contaminants.280 Recently, Liang et al. and Liu et al. have researched attacked C atom of different organic contaminants (such as rhodamine B (RhB), bisphenol A (BPA), diclofenac (DCF)) by using the natural population analysis (NPA) charge distribution and Fukui index in order to fully determine the degradation pathways.281 For example, according to calculated f 0 of the atoms in BPA (Figure 23a), the C1 and C1’, C4 and C4’, C6 and C6’ of BPA with positive and high f 0 values were the reactive sites for radical attacks. Whereas, the C4, C4’, C6 and C6’ were determined to be the most reactive sites because of C1 and C1’ occupied by O and no further attacked. (Figure 23b). These researches can largely benefit the highly active photocatalytic systems design and screening nucleophilic and electrophilic free radicals to realize hypersalinity of contaminants. Several Key Issues in the Applicability for Water Decontamination (I) Operation costs. Operation costs depend on the actual water quality, technological processes (i.e., reactor, performance period and desired effect) and photocatalyst (performance, large-scale preparation and lifetime). (II) The turbidity, substrate concentration, pH, temperature and metal ions in the real environment shall be taken into account. Especially for those insoluble particulate matters (i.e., mud, silt, fine organic matter, inorganic matter, plankton, other suspended matter and colloidal matter) that can interfere with the penetration of solar energy. As reported by previous studies, the degradation efficiency would be significantly reduced when organic contaminants with high concentration were presented in the aqueous solution.282 The pH of the solution not only changes the surface properties of the photocatalysts but also influences the bandgap of semiconductor. Therefore, it is necessary to design photocatalysts with great photocatalytic performance in a wide pH range.283, 284 Meanwhile, the photocatalytic degradation can proceed at normal temperatures and 20


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pressure, which is beneficial for water treatment to save energy. (III) Technological process. The photocatalytic process acts either as a forepart of technology or an individual system.282 As forepart, the hydraulic retention time and the quantity of the sludge could be obviously reduced from the biological treatment.282 As an individual system, the requirement of the retention time might be drawn-out because of microbiology inactivation or mineralization.285,286 Moreover, the photocatalytic reactor system needs to be fabricated scientifically and systematically to utilize solar energy efficiently to reduce the electricity costs. (IV) Photocatalyst. The scalable technology of the photocatalyst should be explored, as well as its activity and lifetime. Meanwhile, the stability of the photocatalyst is another key factor that should be taken into consideration; it is highly related to the continual performance and can also cause the photocatalyst to have a leakage problem. (V) Recovery method. Immobilization strategy of photocatalyst can provide a separating strategy of solids and liquids with low-cost and high efficiency.

Capturing Energy from Wastewater Decontamination Wastewater treatment techniques are currently only focused on water purification or decontamination. However, to some extent, wastewater can be defined as a renewable resource. This is mainly because we can simultaneously obtain clean water and energy (i.e., hydrogen, methane, methanol, ethanol, etc.) using photocatalytic approaches.287,288 Panagiotis Lianos has firstly reviewed that the electricity and hydrogen can be produced by photocatalytic degradation of organic wastes in photoelectrochemical cell in 2011.289 The viewpoint was aroused that this whole process was very attractive on both ending points with electricity and hydrogen production and organic wastes degradation, and it should be a re-emerging research field. However, some aspects of this research area need to be further undertook. Firstly, the choice of electrocatalysts for the cathode electrode was rather limited and need tremendous efforts to improve its overall performance and energy efficiency. Secondly, the final products which generated from the organic wastes (i.e., higher polyols and sugars) may arouse great interests in the next step research. Thirdly, more attentions were needed to be paid regarding on the byproducts generation during the photodegradation process. Choi et al. introduced an ideal mineralization process (Equation 1) that can capture energy in the process of wastewater decontamination.290 In fact, the organic molecules in wastewater can be completely mineralized to CO2 and H2O, and the generated CO2 can be further reduced to methane, methanol and ethanol by accepting electrons.291 The electrons in this process are mainly from the following two pathways: (i) oxidation of organic pollutants and (ii) photoinduced generation. Furthermore, the electrons could be injected into the CB of the semiconductor photocatalyst to reduce the absorbed H+ to H2. Herein, the cooccurrence of wastewater and solar-driven semiconductor photocatalysts can not only mineralize organic pollutants to purify wastewater but can also generate energy substances (i.e., H2, CH4, CH3OH and C2H5OH) from wastewater (Equation 2). This process was further described in Figure 24. CxHyOz + (2x-z) H2O→ xCO2 + (2x+0.5y-z) H2 (1) 21


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𝑆𝑜𝑙𝑎𝑟 𝑒𝑛𝑒𝑟𝑔𝑦

CxHyOz + H2O𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡H2 + CH4+ CH3OH + C2H5OH+

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(2)

Solar energy driven reactions (Equation 2) coupled with photocatalysis can realize the purpose of wastewater decontamination synergistically with energy conversion, which can largely reduce the overall cost of wastewater treatment. The hydrogen production efficiency of photocatalytic water splitting is mainly limited to the fourelectron kinetics process of the O2 evolution. The four-electron process of producing the O2 evolution converted into the two electron processes of H2O2/•OH can further improve the kinetic rate.292 While the H2O2/•OH generated by the two electronic mechanisms is the active species for pollutants degradation. Specifically, organic pollutants, as electron donors, can promote the hydrogen production of water splitting, and the active species (i.e., •OH) with a strong oxidizing ability can oxidize/mineralize the organic pollutants to CO2.293 In 2006, Patsoura et al. reported Pt/TiO2 suspensions that can degraded azo-dyes with simultaneous hydrogen production.294 Since then, Pt/CdS/TiO2,295 F- and P-TiO2/Pt290,296,297, and Pt/C3N4 nanotubes293 have been prepared to fulfill this goal. Although photocatalysts deposition with Pt is efficient for hydrogen evolution reactions, Pt is easily poisoned when it is surrounded by organic substance.298-300 By further considering the overall efficiency, both the proton reduction reaction and pollutant destruction reaction should be taken into consideration. TiO2/CuO photocatalyst composites were structured using the electrospinning method by Liu et al. and were used for efficient photocatalytic dye degradation with simultaneous H2 generation. The mechanism is detailed in Figure 25a.301 Furthermore, as shown in Figure 25b, the H2 generation over TiO2/CuO photocatalyst composite was significantly better than with pristine TiO2. Moreover, as shown in Figure 25c, the H2 evolution rate was measured to be 62.7 mol h-1 using TiO2/CuO composite nanofibers, which was about five times higher than pristine TiO2. The removal of acid orange II (AO7) was significantly improved over TiO2/CuO photocatalyst (Figure 25d). Complete AO7 removal was observed within 20 min. AO7 degradation was due to the destroy of the azo-bond into smaller molecules by the h+ and •OH. Figure 25e showed a stable photocatalytic activity of H2 generation within 3 cycles, which indicated that the TiO2/CuO photocatalyst composite were excellent stability and reutilization. Zou et al. prepared the one-pot conversion of methylene blue (MB) into useful fuels over GQDs/V-TiO2 using photogenerated CO2 as the carbon source (Figure 26).302 The e- was excited from the VB to the CB of vanadium-doped TiO2 (V-TiO2) and subsequently migrated to graphene QDs (GQDs). The holes in the VB reacted with the MB into CO2 and H2O. Meanwhile, the photoinduced e- on GQDs was responsible for the CO2 reduction, which could lead to the generation of CH3OH, C2H5OH and CH4. Ion chromatography detected the existence of CO32-, which was from the mineralization of MB (Figure 25a and b). CH3OH, C2H5OH and CH4 were produced due to CO2 photoreduction in the system (Figure 25c). The proposed process of the one-pot conversion of MB to useful fuels using GQDs/V-TiO2 is shown in Figure 25d. It was a novel project to harvest and convert solar energy to generate reusable liquid fuels with simultaneous organic wastewater purification. Zhang et al. enriched the area of photocatalytic organic degradation and 22


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simultaneous H2 production over MoS2 QD-decorated hybrid of ZnIn2S4 assembled on RGO.303 The LUMO and HOMO of different organic pollutants were investigated by the DFT calculation (Figure 27a). The band matching mechanism was first proposed to take the organic compound structure into consideration to determine the degradation rate and amount of H2 production. In terms of reaction kinetics. TRPL was also applied to further indicate the lifetime and migrating behavior of photogenerated carriers (Figure 27b). The emission lifetime for 5MoS2QDs@ZnIn2S4@RGO1 decreases to 12.05 ns, which is 6.27 times shorter than ZnIn2S4 (75.57 ns), thus indicating a faster interfacial charge transfer (Figure 27c). Schematic illustration of pollutant conversion and energy flow was shown in Figure 27d.

CONCLUSION AND PERSPECTIVES Benefiting from the advancements with various characterization tools and the growing demands for both environment and energy sustainability, the evolution of photocatalysts has dramatically increased in the past decades. Herein, we summarized the state of art methodologies and strategies comprehensively and systematically to improve photocatalytic performance in the following aspects: (i) how to enhance and broaden light absorption, (ii) how to develop active sites, and (ⅲ) how to separate charges efficiently. The photocatalytic application achievements in energy conversion, environmental remediation and capturing energy in wastewater decontamination by photocatalysis were also discussed. Although great achievement has been obtained in intriguing field, studies still must be further developed, and at least the following aspects should be considered: (1) The relationship among the photocatalytic performance, the reaction mechanism and structure-property need to be deepened, especially at the molecular level. Moreover, In Situ characterization techniques are urgently needed to reveal the photocatalysis reaction process. (2) Compatible architecture is required to achieve high-efficiency and great stability. The active sites must be located, defined and maximized. The focus on the active sites needs to be deepened into the basal plane microscopically and not just on the macro level. The photoelectric conversion efficiency is needed further improvement due to unavoidable e--h+ recombination. It is necessary to regulate the charge flow from microscopically especially at the atomic level and, achieving the charge separation directionally, vectored, anisotropically and spatially. The photocatalytic studies on energy conversion and environmental remediation were studied, while the long-term stability and durability need to be further tested. It is possible to improve the durability in the practical experiment by coating and wrapping the surface of the photocatalyst with a porous inert layer. (3) Scale production. The large-scale and high-yield production of photocatalysts is another important prerequisite for practical application. Unceasing development of science and technology infuse fresh energy to photocatalysis, like by means of 3D printing and femtosecond laser to prepare photocatalysts. (4) Practical application is needed. Cost is another key factor when considering practical application after satisfying the high efficiency, stability and large-scale 23


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preparation requirements. Overall, this comprehensive and systematic review can benefit the future studies on the application of the solar-driven photocatalyst in energy conversion and environmental remediation. ORCID Jinming Luo: 0000-0001-8698-7624 Shuqu Zhang: 0000-0003-3392-8705 Meng Sun: 0000-0002-8188-9264 John C. Crittenden: 0000-0002-9048-7208 ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51868051, 51662031, 51720105001). The views and ideas expressed herein are solely those of the authors and do not represent the ideas of the funding agencies in any form. We appreciate from Dr. Shuqu Zhang from Nanchang Hangkong University specially for his provided ideas, thoughts and the supports. Additionally, we thank the ChemWorx for the English editing. VOCABULARY photocatalysis, the acceleration of a photoreaction in the presence of a catalyst; electron hole, often simply called a hole, is the lack of an electron at a position where one could exist in an atom or atomic lattice; energy conversion, the process of changing energy from one form to another; environmental remediation, a way to deal with the removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water; crystal structure, a description of the ordered arrangement of atoms, ions or molecules in a crystalline material; REFERENCES (1) Weaver, P.; Jansen, L.; Grootveld, G.; Spiegel, E. and Vergragt, P. Sustainable technology development. Routledge 2017. (2) M.H. Abdel-Haleem, A.; A. Seleem, H.; K. Galal, W., Assessment of Kamut® Wheat Quality. World J. Sci., Technol. Sustain. Dev. 2012, 9, 194-203. (3) Pacesila, M.; Burcea, S. G.; Colesca, S. E. Analysis of Renewable Energies in European Union. Renew. Sust. Energ. Rev. 2016, 56, 156-170. (4) Shabangoli, Y.; Rahmanifar, M. S.; El-Kady, M. F.; Noori, A.; Mousavi, M. F.; Kaner, R. B. An Integrated Electrochemical Device Based on Earth-Abundant Metals for Both Energy Storage and Conversion. Energy Storage Mater. 2018, 11, 282-293. (5) Brinkert, K. Energy Conversion in Natural and Artificial Photosynthesis. Springer: 2018; Vol. 117. (6) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (7) Wang, Q.; Hisatomi, T.; Jia, Q.; Tokudome, H.; Zhong, M.; Wang, C.; Pan, Z.; Takata, T.; Nakabayashi, M.; Shibata, N.; Li, Y.; Sharp, I. D.; Kudo, A.; Yamada, T.; 24


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A Strategy for One-Pot Conversion of Organic Pollutants into Useful Hydrocarbons through Coupling Photodegradation of MB with Photoreduction of CO2. ACS Catal. 2016, 6, 6861-6867. (303) Zhang, S.; Wang, L.; Liu, C.; Luo, J.; Crittenden, J.; Liu, X.; Cai, T.; Yuan, J.; Pei, Y.; Liu, Y. Photocatalytic Wastewater Purification with Simultaneous Hydrogen Production Using MoS2 QD-Decorated Hierarchical Assembly of ZnIn2S4 on Reduced Graphene Oxide Photocatalyst. Water Res. 2017, 121, 11-19.

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Figure 1. The state-of-the-art methods and strategies for improved photocatalytic energy conversion and environmental remediation.

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Figure 2. (a) Schematic illustration of hierarchical nanostructures of binary CdS nanostars@ZnO NRs and ternary CdS@ZnO NRs@RGO. (b) and (c) SEM images of ternary CdS@ZnO NRs@RGO-5% (RGO: 5wt%). Reprinted with permission from ref. 57. Copyright 2014 WILEY-VCH.

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Figure 3. Strategies to develop active sites by basal engineering and quantum size effect.

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Figure 4. (a) Schematic diagram of Ni–Al LDH atomic structure. (b) After etching by NaOH hydrothermal conditions. (c) β-Ni(OH)2 nanomeshes with uniform inphase nanopore after Ostwald ripening process. (d) Values of the Gibbs free energy variation of the calculated by DFT calculation about △G of vacancies formation with different nanopore sizes. (e-h) HRTEM (high-resolution TEM) images about the morphology evolution of Ni–Al LDH, Scale bars: 5 nm ((g) is the β-Ni(OH)2 ultrathin nanomesh). Reprinted with permission from ref. 78. Copyright 2017 WILEY-VCH.

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Figure 5. (a) HAADF‐STEM image of Pt‐C3N4. The inset shows the size statistics of the Pt single atom. (b) Schematic diagram of Pt‐C3N4. (c) Fourier transforms of the Pt L3‐edge EXAFS oscillations over Pt‐C3N4, K2PtCl6, and Pt foil. (d) FT‐EXAFS curves of experimental data and fit of the Pt‐C3N4. Reprinted with permission from ref. 85. Copyright 2016 WILEY-VCH.

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Figure 6. (a) The schematic diagram of Pt/CdS atomic structure. (b) Fourier transform magnitudes of the experimental Pt L3-edge EXAFS spectra over four samples. (c) TEM images and (e) HAADF-STEM image of Pt/CdS. (d) Cd, S and Pt mapping images of Pt/CdS. (f) Calculated density of states (DOS) and (g-i) the charge density distribution of the conduction band edge over Pt/CdS, Pt-NP-CdS and CdS, respectively. Reprinted with permission from ref. 89. Copyright 2018 ELSEVIER.

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Figure 7. The calculated DOS of (a) Co‐doped In2S3 atomic layer and (c) perfect In2S3. (b) The distribution of charge density of. (b) Co‐doped In2S3 and (d) perfect In2S3. (e) UV-Vis diffuse reflectance spectra and (f) IPCE over Co‐doped In2S3 atomic layer, In2S3 atomic layer and bulk In2S3. Reprinted with permission from ref. 127. Copyright 2015 Wiley-VCH.

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Figure 8. Calculated DOS of (a) O-doped ZIS nanosheet and (b) the bare ZnIn2S4. The corresponding charge density distributions on the VBM of (c) the O-doped ZnIn2S4 nanosheet and (d) the bare ZnIn2S4. (e) K-edge EXAFS oscillation function and (f) the corresponding Fourier transforms around Zn of the O-doped ZnIn2S4 nanosheets, bare ZnIn2S4 and ZnO. Ultrafast transient absorption spectroscopy (UTAS) of (g) bare ZnIn2S4 nanosheets and (h) O-doped ZnIn2S4 nanosheets. Reprinted with permission from ref. 129. Copyright 2016 Wiley-VCH.

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Figure 9. (a) Prepared scheme diagram of MoS2QDs@Vs-M-ZnIn2S4. (b) AFM image of Vs-M-ZnIn2S4 (the inset shows the height profiles of line 1 and 2). (c) False-color image of the HRTEM image. (d) HRTEM image of MoS2QDs@Vs-MZnIn2S4. (e) ESR spectra of prepared samples. High-resolution XPS spectra of (f) In 3d and (f) Zn 2p in MoS2QDs@Vs-M-ZnIn2S4, Vs-M-ZnIn2S4, and bulk ZnIn2S4. (h) Charge density distributions of the side view structural model of VsM-ZnIn2S4 (the insert shows the corresponding distribution of the charge density at the edge of the conduction). (i) Schematic illustration of the hydrogen generation over MoS2QDs@Vs-M-ZnIn2S4. Reprinted with permission from ref. 138. Copyright 2018 American Chemical Society.

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Figure 10. Calculated DOS of (a) the V-defective orthorhombic-BiVO4 atomic layers and (c) the perfect orthorhombic-BiVO4 atomic layers along the {001} orientation. Atomic structure of (b) the V-defective orthorhombic-BiVO4 atomic layer and (d) the perfect orthorhombic-BiVO4 atomic layer. Defect characterization for the orthorhombic-BiVO4 atomic layers with rich and poor Vv. (e) Positron lifetime spectrum. (f) Time-resolved fluorescence emission decay spectra. Reprinted with permission from ref. 143. Copyright 2017 American Chemical Society.

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ACS Nano

Figure 11. (a) The schematic diagram of selective anchoring of the reduction and oxidation cocatalysts on the {010} and {110} facets of BiVO4. (b) Photo-deposition of noble metals on the {010} facet, (c) photo-deposition of metal oxides on the {110} facet and (d) coinstantaneous photo-deposition of metal and metal oxides on the different facets. Reprinted with permission from ref. 145. Copyright 2014 Royal Society of Chemistry and ref. 146. Copyright 2013 Springer Nature.

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Figure 12. (A) Calculated electronic structure of anatase TiO2 (on CB, point 1-6 represent G {000}, Z {001}, {110}, {112}, R {011} and M {101}, respectively. On VB, point 1-6 represent {110}, M {110}, G {000}, {445}, Z {001} and {100}, respectively). (B) Schematic diagram of the photoexcited charges accumulation on various facets of the (a, c, d) bulk and (e) surface calculations, and (b, e) experimental observations of anatase TiO2. Reprinted with permission from ref. 147. Copyright 2018 American Chemical Society.

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Figure 13. (a) The HRTEM image of the nanotwin structure in a crystal; the insets show the FFT pattern. (b) Schematic illustration of nanotwin boundary in a zinc blende (ZB) nanocrystal to shape a ZB–WZ-ZB homojunction. (c) Mechanism for separation of photogenerated carriers over Cd0.5Zn0.5S with malposed crystal lattice. (d) Schematic diagram of the parallel homojunctions in Cd0.5Zn0.5S; black arrows and pink squares represent the fragments of the ZB and WZ. Reprinted with permission from ref. 148. Copyright 2011 Royal Society of Chemistry and ref. 149. Copyright 2013 Springer Nature.

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Figure 14. (a) TEM and (b) HAADF STEM image of the lateral Bi2Se3-CdS NHS. (c) HRTEM image, scale bar 2 nm and (d) schematic diagram of interface between the Bi2Se3-CdS, along the {001} direction. (e) and (f) Scheme of the charge transfer in lateral Bi2Se3-CdS NHS, HOMO-LUMO and the band structure of Bi2Se3 and CdS based on DFT calculations. Reprinted with permission from ref. 175. Copyright 2015 American Chemical Society.

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Figure 15. Schematic diagram of the electron migration in the Z-scheme.

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Figure 16. (a) Schematic diagram of pathway of e- and h+ over mesoporous ZCS with complex of Cu+ and Vs. The charge density of the electronic states of VBM and CBM on {110}: (b) ZCS with Vs, (c) Cu+-doped ZCS. Reprinted with permission from ref. 126. Copyright 2016 ELSEVIER.

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Figure 17. (a) and (b) AFM images. (c) Side view of the HAADF-STEM images, and (d-h) the respective elemental maps. (i) Schematic diagram of the photocatalytic behavior over BOC-MS. (j) Charge-flow processes between Janus bilayer. Reprinted with permission from ref. 64. under a Creative Commons CCBY License. Copyright 2016 Springer Nature.

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Figure 18. Rated strategy for photocatalytic CO2 conversion.

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Figure 19. (a) TEM image and (b) HAADF-STEM image of MoS2 foam/CdS. (c) Schematic diagram of photocatalytic synthesis and H2 evolution over MoS2 foam/CdS. (d) The EXAFS spectra of photocatalyst samples. (e) TRPL spectrum of different photocatalyst samples. (f) Energy barrier profiles of Reaction via different free radical. (g) Reaction Mode with EG separation. (h) Performance of EG generation over MoS2 foam/CdS photocatalyst. Reprinted with permission from ref. 245. under a Creative Commons CC-BY License. Copyright 2018 Springer Nature.

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Figure 20. (a) Energy barrier evolution of NO by •O2− over the BiOCl {001}. (b) Nitrate conversion from peroxynitrite. The charge density and DOS of the BiOCl {001} surface with (c) O2 and (d) nitrate. (e) Photocatalytic NO elimination over BiOCl. (f) Schematic illustration of photocatalytic NO removal on the Vo-BiOCl. Reprinted with permission from ref. 255. Copyright 2018 American Chemical Society.

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ACS Nano

Figure 21. (a) Schematic diagram of TiO2 nanotube filter with {001} facet exposed to remove VOCs. (b) Toluene degradation over TNT and 001-TNT under UV illumination. (c) Stability of toluene degradation over TNT and 001-TNT. (d) Acetaldehyde and (e) formaldehyde degradation over TNT and 001-TNT under visible light illumination. Reprinted with permission from ref. 258. Copyright 2018 American Chemical Society.

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Figure 22. Process and achievements and several key issues in the applicability of water decontamination.

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ACS Nano

Figure 23. (a) NPA charge distribution and Fukui index, (b) chemical structure of BPA. Reprinted with permission from ref. 280. Copyright 2018 ELSEVIER.

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Figure 24. Organic wastewater recycling process driven by solar energy based on a semiconductor photocatalyst (Enclosed and anaerobic condition).

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Figure 25. (a) Schematic illustration of the migration of electrons and holes over TiO2/CuO heterojunctions. (b) Photocatalytic H2 generation with the increasing irradiation time of 3 h over photocatalyst samples and (c) Comparison of H2 rate over photocatalyst samples. (d) Removal of AO7 under UV-visible illumination. (e) Recycle for photocatalytic H2 generation over TiO2/CuO photocatalyst composite. Reprinted with permission from ref. 301. Copyright 2013 ELSEVIER.

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Figure 26. (a) Ions types in the process of MB degradation over 5%GQDs/V-TiO2. (b) Accumulation of CO32− in the MB solution with increasing irradiation time. (c) Comparison of CH3OH, C2H5OH, and CH4 generation rate. (d) Proposed mechanism of conversion of MB into useful fuels over the 5%GQDs/V-TiO2. Reprinted with permission from ref. 302. Copyright 2016 American Chemical Society.

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Figure 27. (a) LUMO and HOMO of pollutants: RhB (Rhodamine B), EY (Eosin Y), MB (Methylene blue) and PNP (p-nitrophenol). (b) Bandgap diagram of photocatalysts: ZnIn2S4, MoS2 QDs and RGO, pollutants: RhB, EY and MB, (c) TRPL spectra of photocatalyst samples from, and (d) schematic diagram of pollutant conversion and energy flow. Reprinted with permission from ref. 303. Copyright 2017 ELSEVIER.

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Table 1. The List of Photocatalytic H2 Production Systems to Date. Photocatalyst

MoS3@CdSe/CdS CuO/TiO2

Reaction condition (Light source, Sacrificial agent,

Performance (mmol h-1g-1)

Concentration of photocatalyst)

and AQE

Hg/Xe 50-500 W DC arc lamp (λ>450 nm), 0.1 M triethanolamine 400 W high pressure Hg lamp, 10 vol% methanol, 1 g/L

Graphene-Cu@TiO2

500 W Xe lamp (λ>400 nm), Water: methanol (v/v = 2 : 1) 20 mg/30 mL

100

192

71.6

193

63.750

194 195

Pd/C3N4

300 W Xe lamp (λ>400 nm), 0.1 M formic acid, 20 mg/10 mL

53.4

MoS2/Bi12O17Cl2

300 W Xe arc lamp, 0.3 M ascorbic acid, 10 g/80 mL

33

750 W Xenon lamp (λ>420 nm, 100 mW

Co-Ni/CdS-NR

cm-2)

Ref.

36% at 420 nm

64

32.6

196

29.2

197

0.1 M Na2S and 0.1 M Na2SO3, 5 mg/45 mL

28.616

198

NiOx

300 W Xe arc lamp (λ>420 nm), 20% vol of ethanol

26.940

Ni

100 mg/500 mL

11.038

AM 1.5 sunlight, 2.7 M formic acid, 5 mg/10 mL

17.7

FA (containing NaHCO2) (1.04 mM=150 μg mL-1) 400 W high-pressure Hg lamp (λ>250 nm), 10% vol of

[email protected]

methanol 500 mg/1500 mL

ZnO-MoS2-RGO CdStitanate

AuPd/TiO2

Fe-g-CN

Cu(OH)2@TiO2

[email protected]−ZnxCd1−xS Cu/TiO2 Pt@O-g-C3N4

white-light LED (λ=400-750 nm), TEA (10% v/v) and EY (4.0×10−4 M), 0.025 g/L 400 W high pressure Hg lamp as UV-light source 10 vol% methanol, 150 mg/500 mL

16.2

13.533

300 W Xe lamp, 10 vol% methanol, 40 mg/40 mL

12.779

50 mg/100 mL

MoS2QDs@Vs-

300 W Xe lamp (λ>420 nm), 10 vol% Lactic acid

MZnIn2S4

50 mg/80 mL

MoSe2/ZnIn2S4 MoS2/Cu-ZnIn2S4

28.1% at 420 nm 0.8%

14.940

0.1 M Na2S and 0.1 M Na2SO3, 20 mg/50 mL

300 W Xe lamp (λ>400 nm), 10 vol% triethanolamine

199

8.875

200

201

202 26.4% at 420 nm

203 204

13.7% at 420 nm

205

6.884

138

300 W Xe lamp (λ>400 nm), 10 vol% lactic acid, 5 mg/10 mL

6.454

167

λ = 420 nm, 0.1 M ascorbic acid, 50 mg/250 mL

5.463

153

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Table 2. Standard Redox Potentials for the Mentioned Reactions in This Article. E0(V) vs. Reaction

NHE (pH = 0)

2H+

+

2e-

→ H2(g)

2H2O(aq) +

4h+

0

→O2(g) +

4H+

1.230

2CO2(g)+2H+ + 2e- → HOOCCOOH(aq)

-0.481

CO2(g) + 2H+ + 2e- → HCOOH(aq)

-0.199

CO2(g) + 2H+ +2e- → CO(g) + H2O

-0.117

CO2(g) + 4H+ + 4e- → HCHO(aq) + 2H2O

-0.07

CO2(g) + 8H+ + 8e- → CH4(g) + 2H2O

0.169

CO2(g) + 6H+ + 6e- → CH3OH(aq) + H2O

0.03

2CO2(g) +

8H+

+

12e-

2CO2(g) + 9H2O +

→ C2H4(g) +

12e-

12OH-

0.07

→ C2H5OH(aq) +

0.08

3CO2(g) + 13H2O + 18e- → C3H7OH(aq) +

0.09

12OH-

18OH-

N2 + e- → N2-

-4.16

N2 + H+ + e- → N2H

-3.2

N2 + 2H+ + 2e- → N2H2

-1.32

N2 + 4H+ + 4e- → N2H4

-0.36

N2 +

5H+

N2 +

6H+

+

4e-

+

→ N2H5

-0.23

+

6e-

→ 2NH3

-0.137

NO (g) + 2H2O + 3h+ → NO3- + 4H+

0.957

h+

0.80

NO2 (g) + H2O +

-

→ NO3 +

2H+

NH3 + H2O → NH3·H2O ⇋ NH4+ + OH−

O2 + e− → •O2−

-0.284

O2 + e− + H+ → HO2•

-0.046

HO2• + e− + H+ →H2O2

1.44

OH-

2.403

+

h+

→ •OH

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Graphic for Manuscript

The most advanced methods and strategies in terms of broadening the light absorption wavelengths, design of active reaction sites and control of the electron-hole (e--h+) recombination were reviewed, while these three processed could be influenced by remodeling crystal lattice, surface and interface. Additionally, the reconstructed photocatalysts were applicate in the aspects of energy conversion (i.e., hydrogen evolution, CO2 reduction, nitrogen fixation and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment).


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A Critical Review on Energy Conversion and Environmental

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