WO3 Direct Z-Scheme Photocatalyst for

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Materials and Interfaces

Construction of Ag2S/WO3 Direct Z-Scheme Photocatalyst for Enhanced Charge Separation Efficiency and H2 Generation Activity Shuai Zhang, Jinming Wang, Shengtao Chen, Renjie Li, and Tianyou Peng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02335 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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Industrial & Engineering Chemistry Research

Construction of Ag2S/WO3 Direct Z-Scheme Photocatalyst for Enhanced Charge Separation Efficiency and H2 Generation Activity Shuai Zhang,†,‡ Jinming Wang,†,‡ Shengtao Chen,† Renjie Li,† and Tianyou Peng*,†,‡

†College

of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China

‡Research

Institute of Wuhan University in Shenzhen, Wuhan University, Shenzhen 518057, P.

R. China

*E-mail address: [email protected] (T.Y.P.). ORCID: 0000-0002-2527-7634

ABSTRACT: Efficient separation and utilization of the photoinduced charge is crucial to phtocatalytic H2 generation reaction, especially for semiconductors with narrow bandgap energies that suffer severe charge recombination. Herein, a novel Ag2S/WO3 direct Z-scheme photocatalyst was fabricated by depositing Ag2S nanoparticles (NPs) on hexagonal WO3 (h-WO3) nanorods (NRs) via a precipitation process, which can effectively prevent the agglomeration of Ag2S NPs, consequently shortening the charge

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migration pathway and reducing the recombination probability. Also, the Z-scheme mechanism further promotes the charge separation without crippling the redox ability of electron-hole pairs. In virtue of the synergistic effect between morphology and Zscheme mechanism, the optimal Ag2S/WO3 photocatalyst exhibits a superior charge separation efficiency and 4 times higher photocatalytic H2 generation activity than that of Ag2S NPs alone. The present study provides a promising strategy to alleviate the charge recombination and improve the photoactivity of narrow bandgap semiconductors by designing hybrid photocatalysts. KEYWORDS: Ag2S, WO3, Z-scheme photocatalyst, Charge separation efficiency, Hydrogen generation

■ INTRODUCTION

As a sustainable and carbon-free energy source, hydrogen (H2) has become an alternative solution to the increasing worldwide energy crisis and environmental pollution. Since the pioneering work of Fujishima and Honda was reported in 1972,1 the


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photocatalytic H2 generation over semiconductor that can capture the solar energy and convert water to valuable hydrogen energy, has been considered as a promising method of clean energy production from renewable sources.2-6 Among the various semiconducting photocatalysts developed, silver sulfide (Ag2S) is an eligible candidate for the photocatalytic H2 generation due to its lower toxicity, strong absorption in the visible-to-near-infrared (Vis/NIR) region, and suitable conduction band (CB) position for the photocatalytic H2 generation reaction.7-9 Particularly, the energy band structure of Ag2S can be tuned conveniently due to the quantum confinement effect when its particle size decreases to 4.4 nm (two times of its exciton Bohr radius).10-12 Therefore, Ag2S would be an attractive photocatalyst for water splitting. Nevertheless, as a transition metal chalcogenide with narrow bandgap energy, Ag2S usually has severe photoinduced charge recombination and intrinsic photocorrosion, which cast a pall over its application in the field of photocatalytic H2 generation.9 In order to promote the photoinduced charge separation, numerous strategies have been proposed to modify the components and morphologies of photocatalysts.2-6,13,14


3


Among which, morphology tailoring has been proved an effective method for promoting the charge separation since the photoexcited electrons and/or holes with suitable morphology can fast migrate to the reaction sites, and thus causing a suppressed charge recombination.13,14 Besides, constructing Z-scheme photocatalyst is also a common approach to achieve an improved charge separation efficiency since it has the advantage of simultaneously possessing the high photoinduced charge separation efficiency and strong redox ability.15-21 Compared to the conventional Z-scheme photocatalysts that need a shuttle redox mediator, direct Z-scheme systems are more efficient to promote the photoinduced charge separation since it can not only shorten the electron transfer path but also avoid the undesirable backward reactions caused by the necessary redox couples.22-27 Xie’s group23 reported a CdS/WO3 direct Z-scheme system in which the photoinduced electrons and holes can be maintained at the CB of CdS and the valence band (VB) of WO3, respectively. This spatial charge separation together with the un-weakened reducibility of photoinduced electrons led to ca. 5 times higher H2 generation activity than the single CdS. Inspired by this result, we


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hypothesized that constructing a direct Z-scheme system would be a promising strategy for separating and utilizing the photoinduced electrons of Ag2S with narrow bandgap energy even though there is few investigation on the Ag2S-based direct Z-scheme system for H2 generation. A judicious selection of the second complementary semiconductor to Ag2S for constructing a direct Z-scheme system might be tungsten oxide (WO3) since it has been widely coupled with other semiconductors due to its considerable merits such as nontoxicity, facile synthesis, resistance to photo-corrosion, and high oxidation ability of its photoinduced holes.17,23 Particularly, WO3 with nanorod-like morphology is usually a benefit to the photoinduced charge separation and thus the enhanced photocatalytic performance due to the lager specific surface area and shorter charge diffusion distance.28-31 By decorating Ag2S nanoparticles (NPs) on hydrothermally synthesized WO3 nanorods (NRs), herein, a series of novel Ag2S/WO3 composites (denoted as AW-

x composites, in which x value is the weight percentage of Ag2S NPs) are successfully constructed for the first time. It was found that the Ag2S NPs with sizes in the range of


5


5-50 nm are distributed on the well-defined WO3 NRs with diameter of 100-200 nm and length of 1-5 μm. The resultant AW-x composites exhibited a better H2 generation performance than the single Ag2S NPs, implying that the photoinduced electrons of Ag2S NPs can be utilized efficiently. According to the energy band structure and microstructure analyses as well as the radical trapping experiments, a direct Z-scheme charge transfer mechanism was demonstrated, and the positive influences of morphology and direct Z-scheme system on the charge separation and photocatalytic performance were discussed in detail.

■ EXPERIMENTAL SECTION

Material preparation. WO3 nanorods (NRs) were synthesized through a hydrothermal process. Typically, Na2WO4·2H2O (8.4 mmol) and Na2SO4 (16.8 mmol) were dissolved into 67.5 mL of water under stirring, then the pH value of the solution was adjusted to 2.00 using HCl solution (3.0 M). After stirring for 30 min, the above mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL) which was kept at


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180C for 24 h, and then cooled down to room temperature naturally. The creamy product was separated by centrifugation and washed several times with water and ethanol, followed by drying at 70C in the air. Ag2S nanoparticles (NPs) were deposited on WO3 NRs via a facile solution precipitation reaction. In a typical process, 0.2 g of WO3 NRs were dispersed in an AgNO3 solution (30.0 mL) with certain concentration to obtain the homogeneous suspension. Subsequently, a certain amount of Na2S solution (40.0 mM) was added dropwise into the above suspension under stirring. After stirring for 30 min, the resultant precipitates was centrifuged and washed repeatedly with water and ethanol, and finally dried at 70C in the air to yield the Ag2S NPs/WO3 NRs composite. The obtained composites with different weight percentages of Ag2S NPs are denoted as AW-x composites, and the x value is the weight percentage of the Ag2S NPs, which is varied from 1.0 to 30 wt% by changing the amount of its sources during the precipitation process. For comparison, the single Ag2S NPs were also prepared by the same method in the absence of WO3 NRs. Meanwhile, the single Ag2S NPs and WO3 NRs were


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mechanically mixed by grinding in the agate mortar with the same mass ratio of Ag2S NPs and WO3 NRs of AW-10, and the resultant mixture was denoted as MAW-10. Material Characterization. Crystal phase analyses of the products were based on the X-ray powder diffraction (XRD) patterns obtained from a Miniflex 600 X-ray diffractometer with Cu Kα irradiation (λ = 0.154 nm) working at 40 kV, 15 mA, a scan rate of 5 min-1 and a step of 0.02. The elemental compositions of the products were detected using a JXA-8530F Plus filed emission electron probe microanalyzer (FEEPMA) without standard sample. The morphology and microstructure observations were performed with a Zeiss-Sigma filed emission scanning electron microscope (FESEM) and a LaB6 JEOL JEM-2100 (HR) high-resolution transmission electron microscope (HRTEM) working at 200 kV. The element mappings were obtained using JEM-2100F transmission electron microscope equipped with an energy dispersive X-ray spectrometric microanalysis (EDS) unit. UV-vis-NIR diffuse reflectance absorption spectra (DRS) were recorded on a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere with BaSO4 as the reference sample. X-ray photoelectron


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spectroscopy (XPS) was measured on a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al Kα) operated at 300 W and calibrated with C1s. Steady-state photoluminescence (PL) spectra were carried out on a Hitachi Model F-4500 fluorescence spectrophotometer. Liquid N2 adsorption-desorption experiments were performed on an ASAP 2460 nitrogen adsorption instrument operated at 77 K after the sample was degassed at 120 C. Electrochemical measurements were performed by using a CHI Model 618C electrochemical analyzer and a standard three-electrode system, in which a platinum plate and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. The sample films, acting as working electrodes, were fabricated on the commercial fluorine-doped tin oxide (FTO) glass which was sequentially cleaned by water, ethanol, acetone and isopropanol in an ultrasonic bath and then dried. Typically, 20.0 mg of sample was dispersed in 0.2 mL of Nafion ethanol solution (0.05 wt%) to obtain the homogeneous suspension, then the suspension was spread onto the FTO


9


substrate, followed by drying in the air naturally. Finally, the three electrodes were immersed into a Na2SO4 solution (0.5 M) which was continuously purged by N2 for 30 min to remove O2, then the transient photocurrent response and electrochemical impedance spectroscopy (EIS) measurements were performed. For the analyses of flatband potential (Efb), the Mott-Schottky plots of products were measured in dark by using the three-electrode system and an acidic Na2SO4 solution (0.5 M), which pH value was adjusted to ~1.75 (the same as that of 10 vol% lactic acid (LA) solution in the photoreaction system) using H2SO4 solution. Photocatalytic activity measurement. The photocatalytic H2 generation reaction was carried out in a sealed reactor under a 300 W Xe-lamp irradiation.33-35 In a typical run, 15 mg of photocatalyst was dispersed in 10 mL of lactic acid (LA) solution (10 vol%) that acted as sacrificial reagent. Prior to the irradiation, the above suspension was thoroughly degassed to remove air completely, and then the reactor was irradiated from the top. The H2 generation amount was analyzed using a gas chromatograph (GC, SP6890, thermal conductivity detector, 5 Å molecular sieve columns, and Ar as carrier


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gas). The long-term photocatalytic experiment was conducted as follow: the photocatalyst was centrifuged and washed several times with water and ethanol after each 5 run (5 h). Subsequently, the rinsed photocatalyst was dried at 70C in the air and kept in the dark until the next 5 runs of photoactivity measurements by adding new LA solution. Radical

trapping

determination.

Hydroxyl

radical

(•OH)

formed

during

the

photocatalystic process was determined using a fluorescence technique with terephthalic acid (TA) as probe molecule.36 Since •OH radicals can react with TA to produce a highly fluorescent product (2-hydroxyterephthalic acid), the fluorescence intensity of 2-hydroxyterephthalic acid corresponds to the amount of formed •OH radicals. Typically, TA was dissolved into a dilute NaOH solution (2.0 mM) to obtain a TA solution (0.5 mM), then a homogeneous suspension containing 20 mg of photocatalyst and 100 mL of TA solution was subjected to light irradiation (300 W Xelamp). Subsequently, aliquot (5 mL) was withdrawn from the irradiated suspension by syringe at 20 min intervals. After centrifuged to separate the sample, each aliquot was


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analyzed by the Hitachi Model F-4500 fluorescence spectrophotometer with the excitation wavelength of 315 nm.

■ RESULTS AND DISCUSSION

Crystal phase and composition analyses. The X-ray powder diffraction (XRD) patterns of the obtained Ag2S/WO3 composites (AW-x) are depicted in Figure 1. As seen from Figure 1a, the diffraction peaks of the single WO3 NRs are well conformed to hexagonal WO3 (h-WO3) (JCPDS No. 33-1387).30 No diffraction peak ascribable to other WO3 or hydrous WO3 can be detected from the XRD pattern. It indicates that the as-prepared WO3 NRs have high phase purity, which is well consistent with our previous works.30,31 Also, the single Ag2S NPs display a XRD pattern of monoclinic Ag2S (JCPDS No. 140072) (Figure 1a).37 With regard to the AW-x composites when the Ag2S-loading contents is larger than 10 wt%, obvious diffraction peaks ascribable to monoclinic Ag2S appear in those XRD patterns (Figure 1b), indicating the formation of Ag2S NPs during the deposition process. However, no obvious characteristic diffraction peak of Ag2S NPs


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appears in the XRD patterns of the AW-x composites with Ag2S-loading contents lower than 10 wt% (Figure 1b), possibly owing to their relatively low content, small size and well dispersion on WO3 NRs.10,38 In general, only diffraction peaks ascribable to h-WO3 and/or Ag2S can be visualized from the XRD patterns of those AW-x composites, suggesting that the deposition process does not influence the crystal phase of WO3 NRs, and the sharp diffraction peaks of h-WO3 in those AW-x composites demonstrate that WO3 NRs also maintains its high crystallinity, which will be propitious to the photoreactions.31

Figure 1. (a) Typical XRD patterns of the AW-x composites, the single WO3 NRs and Ag2S NPs. (b) Comparison of XRD patterns at 2 = 25-45o of those AW-x composites.


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The element compositions of the obtained AW-x composites, the single WO3 NRs and Ag2S NPs were determined using filed emission electron probe microanalysis (FEEPMA) technique, and the relative mass percentages and atom ratios of W, O, Ag and S in the products are listed in Table 1. As seen, the W:O and Ag:S atom ratio in those AW-x composites are about 1:3 and 2:1, respectively. It demonstrates that the AW-x composites may contain WO3 and Ag2S, which is consistent with the above XRD analyses. Besides, the mass ratios of Ag2S in AW-x composites (Table 1) are similar to the reckoned ratio during the sample preparation process, which further proves that the deposition process of Ag2S NPs dose not lead to any other side-products.

Table 1. Element compositions of the AW-x composites detected using FE-EPMA technique Products

h-WO3 NRs

AW-1

AW-5

AW-10

AW-20

AW-30

Ag2S NPs

W / wt%

79.15

78.45

75.37

71.39

63.29

55.39

–

O / wt%

20.85

20.59

19.62

18.62

16.66

14.53

–

Ag / wt%

–

0.83

4.37

8.70

17.46

26.22

87.14

S / wt%

–

0.12

0.64

1.28

2.59

3.86

12.86


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W:O atom ratio

1:3.03

1:3.02

1:2.99

1:3.00

1:3.02

1:3.01

–

Ag:S atom ratio

–

2.01:1

2.03:1

2.02:1

2.00:1

2.02:1

2.01:1

Ag2S / wt%

0

0.96

5.01

9.98

20.05

30.08

100

The survey X-ray photoelectron spectra (XPS, Figure S1) indicate the existence of W, O, Ag and S elements in the AW-10 composite in addition to the binding energy peak ascribable to C1s, which might be due to the surface adsorbed contaminations in the air. The valence states of W, O, Ag and S in those samples can be determined from the corresponding high-resolution XPS spectra (Figure 2). As seen, the Ag3d spectra (Figure 2a) of both Ag2S NPs and AW-10 composite deliver two characteristic binding energy peaks at 368.2 and 374.3 eV, ascribable to Ag3d5/2 and Ag3d3/2 orbits of Ag+ in Ag2S,39 respectively. It indicates that the dominant oxidation state of Ag in AW-10 composite and single Ag2S NPs is Ag+ species. The O1s spectra (Figure 2b) of AW-10 and WO3 NRs can be deconvoluted into two peaks (530.7 and 531.8 eV), ascribable to the lattice oxygen in WO3 and the oxygen in surface adsorbed –OH groups, respectively.40


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The single WO3 NRs deliver a W4f spectrum (Figure 2c) with two symmetrical peaks at binding energies of 35.8 and 38.0 eV, which are corresponding to W4f7/2 and W4f5/2 of W6+ species in WO3,40 respectively. Although the AW-10 composite shows a similar W4f7/2 and W4f5/2 binding energy peaks, a shift (~0.2 eV) towards lower binding energy values can be observed. Differently, the AW-10 composite show a S2p spectrum with the binding peaks shifting towards higher binding energy (~0.1 eV) as compared to the single Ag2S NPs (Figure 2d), which binding energy peaks with a spin-orbit splitting of 1.2 eV can be ascribable to the S2p3/2 (161.7 eV) and S2p1/2 (162.9 eV) of S2- in Ag2S,39,41 respectively. The above shifts towards lower (W4f) and higher (S2p) binding energy sides for the AW-10 composite imply that the electrons can transfer from Ag2S to WO3 phases,25 which may further result in the formation of a built-in electric field from Ag2S to WO3 phase, and then accelerate the transfer of photoexcited electrons in the CB of WO3 into the VB of Ag2S for promoting an efficient Z-scheme charge transfer.42,43


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Figure 2. Typical high resolution XPS of the AW-10 composite, single WO3 NRs and Ag2S NPs. (a) Ag 3d, (b) O 1S, (c) W 4f, (d) S 2p, as well as the corresponding XPS of the recovered AW-10 composite after the 15 h photocatalytic H2 generation reaction.


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Figure 3. Typical FESEM images of the single WO3 NRs (a), AW-1 (b), AW-5 (c), AW10 (d), AW-20 (e), as well as the single Ag2S NPs (f).

Morphology and Microstructure Analyses. Figure 3 shows the FESEM images of the as-prepared AW-x composites, the single WO3 NRs and Ag2S NPs. As can be seen in Figure 3a, the WO3 derived from the hydrothermal reaction solution exhibits welldefined nanorod-like morphology with diameter of 100-200 nm and length of 1-5 μm. After the deposition process of Ag2S NPs, those WO3 NRs also maintain the 1D nanostructures (Figure 3b-e), which is beneficial to the photogenerated charge separation due to the shorter charge diffusion distance.29-31 Further, some small


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particles can be observed on the smooth surfaces of NRs, which are related to the Ag2S nanoparticles on the basis of above analyses. When the x value is 1, 5 and 10 for the AW-x composites (Figure 3b-d), the number of Ag2S NPs decorated on WO3 NRs increases with enhancing the Ag2S-loading content. However, further enhancing the Ag2S-loading level to 20 wt% (Figure 3e), the Ag2S NPs on WO3 NRs do not increase and some obvious aggregates can be observed. It is reasonably inferred that a high Ag2S-loading level (x ≥ 20) leads to the relatively oversized Ag2S NPs that can easily abscise from the surface of WO3 NRs due to the weakened contacts between Ag2S NPs and WO3 NRs, and then aggregate into the dissociative Ag2S particles. The FESEM image of AW-30 composite (Figure S2) confirms the above conjecture. As seen, more aggregates surround the WO3 NRs than that of AW-20 composite, while the Ag2S NPs on WO3 NRs do not increase compared with AW-10 composite. As for the single Ag2S NPs, which can be observed in Figure 3f, the irregular Ag2S particles also adhere with each other forming large aggregates. Unfortunately, those Ag2S aggregates may cause the negative effects on the light harvesting of WO3 NRs and the charge


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separation of Ag2S, which would further lead to a decreased photocatalytic performance of whole system.9,38 In contrast, the Ag2S NPs decorating on the WO3 NRs have much smaller sizes than the single Ag2S NPs as well as the dissociative Ag2S aggregates in AW-20 and AW-30 composites. Those dispersed Ag2S NPs could effectively promote the charge separation due to the fast transfer of photoinduced charge from bulk to surface, and may achieve an improved photoactivity.15 The liquid N2 adsorption-desorption experiments (Figure S3) indicate that the single Ag2S NPs have a type I-like isotherm without hysteresis loop, while the WO3 NRs and AW-10 composite exhibit similar type IV-like isotherms with obvious hysteresis loops. It can be ascribed to the capillary condensation of N2 molecules among the accumulated pores of those nanorods in WO3 NRs and AW-10 composite. In addition, the BrunauerEmmett-Teller (BET) specific surface area (SBET) of WO3 NRs (35.2 m2 g-1) and AW-10 composite (33.1 m2 g-1) as shown in Figure S3 are much larger than that of the single Ag2S NPs (3.0 m2 g-1), which can be attributed to the severe aggregation of Ag2S NPs (Figure 3f). The larger surface area of AW-10 composite than Ag2S NPs could provide


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sufficient photocatalytic reaction sites and ensure an improved photoactivity accordingly.9 To gain further insight into the microstructures, TEM images of AW-10 composite, single WO3 NRs and Ag2S NPs are shown in Figure 4 and S4. As can be seen from Figure S4a, the individual nanorod with diameter of 200 nm and length of 5 μm in the single WO3 NRs has relatively smooth surfaces. From the HRTEM image (Figure S4b), the lattice spacing of 0.390 nm between the regular lattice fringes corresponds to the

d-spacing of (001) facets of hexagonal WO3, which can be further confirmed by the fast Fourier transform (FFT) patterns (inset in the Figure S4b).31 Meanwhile, it can be observed from the TEM images of single Ag2S NPs (Figure S4c and d) that the Ag2S aggregates is larger than 1 μm and consists of Ag2S particles (with the sizes larger than 50 nm) adhering to each other, in accordance with the above FESEM image (Figure 3f). As for the AW-10 composite, the nanorod-like morphology of WO3 NRs is maintained (Figure 4a), and the lattice fringes with a d-spacing of 0.390 nm (Figure 4b), combined with the corresponding FFT patterns (inset in the right lower corner of Figure 4b),


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demonstrates that the microstructure of h-WO3 does not deteriorate during the deposition process.31 Moreover, the AW-10 composite exhibits many dispersive particles decorated on WO3 nanorod with the sizes in the range of 5-50 nm (Figure 4a and b), which are much smaller than the aggregates in the single Ag2S NPs. Additionally, those tiny nanoparticles on WO3 nanorod have interlayer spacings of 0.241 and 0.239 nm, corresponding to the (121) and (-103) planes of monoclinic Ag2S (Figure 4b),44,45 respectively. Besides, the FFT patterns (inset in the right upper of Figure 4b) indicate the decorated nanoparticles are Ag2S NPs which are intimately contacted with WO3 NRs. The high-angle annular dark field (HAADF) image of AW-10 composite and the corresponding energy dispersive X-ray (EDX) element mappings are shown in the Figure 4c-h. As seen, the distribution of W element is well-corresponding to the elemental O distribution, which perfectly matches WO3 nanorod-like morphology (Figure 4c). On the other hand, the element Ag mapping is also in accordance with the elemental S mapping, corresponding to the small Ag2S NPs loaded on WO3 NRs, implying all Ag elements exist in the Ag2S form and the intimate contact between Ag2S


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NPs and WO3 NR. On the basis of above observation, it can be concluded that Ag2S/WO3 nanostructure in which tiny Ag2S NPs are decorated on WO3 NRs has been successful fabricated in this work.

Figure 4. TEM and HRTEM images of the AW-10 composite (a, b). HAADF image of the AW-10 composite (c), as well as the corresponding elemental mappings of W (d), O (e), S (f), Ag (g) and their overlay mappings (h).


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Optical absorption and energy band structure analyses. The optical absorption properties of the AW-x composites were monitored using the UV-vis-NIR diffuse reflectance spectra (DRS). As can be seen from Figure 5a, the single WO3 NRs show an absorption in UV and a small part of visible region with an absorption edge at 416 nm, corresponding to a bandgap energy (Eg) of 2.98 eV which is consistent with the previous report.31 However, the absorption edge of the single Ag2S NPs can be up to 1365 nm (corresponding to an Eg of 0.91 eV, Figure 5b), indicating that the single Ag2S NPs can significantly absorb more photons in vis/NIR region compared with the single WO3 NRs as a result of its lower Eg.4 Therefore, most AW-x composites also exhibit a strong absorption in vis/NIR region due to the coexistence of Ag2S. In particular, two absorption edges, corresponding to h-WO3 NRs and Ag2S NPs, appear in the DRS spectra of those AW-x composites except AW-1, possibly resulting from its low Ag2S-loading content. Owing to the increasing Ag2S-loading content (from 1.0 to 30 wt%), both of the absorption edges display redshift and absorption intensities in vis/NIR region are also enhanced,23,36 which may be beneficial to the photocatalytic


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performance of AW-x composites since more electron-hole couples will be generated under illumination.9

Figure 5. Typical diffuse reflectance absorption spectra (DRS) of the AW-x composites, the single WO3 NRs and Ag2S NPs in the UV-vis (a) and NIR (b) regions.

The flat-band potentials (Efb) of the as-prepared h-WO3 NRs and Ag2S NPs were measured by the Mott-Schottky relation:33,46

C-2

=

(2

/

eεε0Nd)[Va

–

Efb

(1)


25

–

kT

/

e]


where C is space charge layer capacitance, e is electron charge, ε is dielectric, ε0 is permittivity of vacuum, Nd is electron donor density, Va is applied potential, k is the Boltzmann constant (1.38 × 10-23 J K-1), and T is the temperature. From the Mott-Schottky plots with various frequencies (Figure S5), the Efb values of hWO3 NRs and Ag2S NPs can be determined to be 0.21 V and –0.44 V vs. Ag/AgCl (saturated KCl aqueous solution, Eθ = 0.198 V vs. NHE).33 That is, the Efb values of hWO3 NRs and Ag2S NPs are 0.41 V and –0.24 V vs. NHE, respectively. Meanwhile, the positive slope of all Mott-Schottky plots implies that both h-WO3 NRs and Ag2S NPs are n-type semiconductor whose conduction band (CB) position is usually more negative by

ca. –0.1 V than the Efb.33,47 Therefore, the CB levels (ECB) of the h-WO3 NRs and Ag2S NPs can be estimated to be 0.31 V and –0.34 V vs. NHE, respectively. According to the

Eg determined from DRS spectra (Figure 5), the VB levels (EVB) of the h-WO3 NRs and Ag2S NPs can be calculated to be ca. 3.29 and 0.57 V vs. NHE from formula Eg = EVB –

ECB.33


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Photocatalytic H2 production activity analyses. The photocatalytic H2 production activities of the AW-x composites, the single WO3 NRs and Ag2S NPs are evaluated in a photocatalyst suspension containing lactic acid (LA) as the hole scavenger under an artificial sunlight irradiation. As can be seen in Figure 6a, the single WO3 NRs exhibit no photocatalytic H2 generation activity, which is due to its lower CB potential (ECB = 0.31 V

vs. NHE as mentioned above) than the potential of H+/H2 redox couple.23 However, the single Ag2S NPs only shows a low H2 generation activity of 7.97 μmol h-1, indicating that the photogenerated electrons from the CB of Ag2S NPs have enough reducibility for the H2 generation reaction.8 Interestingly, all of those AW-x composites show enhanced photocatalytic H2 generation activity in contrast to the single Ag2S NPs. Specifically, the H2 generation activity displays an increasing trend with enhancing the Ag2S-loading content from 1.0 wt% to 10 wt%, and then goes downhill with further enhancing the Ag2S-loading content to 20 wt% and 30 wt% (Figure 6a). Namely, the AW-10 composite containing 10 wt% Ag2S exhibits the best H2 generation activity (32.9 μmol h-1), which is 4 times higher than that of the single Ag2S NPs.


27


The typical time course for photocatalytic H2 generation (Figure 6b) demonstrates that the photocatalytic activity of the AW-10 composite relatively remained stable in the three consecutive runs (15 h). The average H2 generation activity of the first run (32.1 μmol h1)

1),

is slightly higher than that of the second run (29.9 μmol h-1) and third run (27.9 μmol hwhich are 93.1% and 86.9% activity maintained as compared with the first run,

respectively. The XRD patterns (Figure S6) of the AW-10 before and after the three cycling photoreaction indicate no apparent change of characteristic diffraction peaks ascribed to h-WO3 NRs and Ag2S NPs.

Figure 6. (a) Photocatalytic H2 generation rates of the AW-x composites, the single WO3 NRs and Ag2S NPs as well as the mechanical mixed sample (MAW-10). (b) Time


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course of the H2 generation of the AW-10 composite. Conditions: Photocatalysts (0.015 g) in 10 mL lactic acid aqueous solution (10 vol%).

Also, the survey XPS spectra (Figure S1) and the high-resolution XPS spectra of O1s, W4f and S2p (Figure 2b-d) exhibit scarcely any difference between the AW-10 before and after the cycling photoreaction, implying that the chemical state of element O, W and S in the AW-10 composite have barely changed during the photocatalytic process. However, the binding energy peaks in the Ag3d spectrum of AW-10 after the cycling photoreaction shift lightly when compared with the spectrum before irradiation, and can be deconvoluted into two pair of apparent asymmetrical peaks (Figure 2a). The peaks with binding energies at 368.2 and 374.3 eV correspond to Ag3d5/2 and Ag3d3/2 of Ag+ as discussed above, while the peaks centered at 367.9 and 374.0 eV can be assigned to Ag0 which is derived from the photoreduction of Ag2S.37,48,49 Although this metal Ag species are resulted from the photolysis of Ag2S NPs, it possibly has positive influence on the charge separation efficiency of present photocatalytic system by acting as an electron trap to attract the photogenerated electrons of Ag2S, and thus suppressing their


29


recombination with the holes in Ag2S NPs.37,50 From the FESEM images (Figure S7) of the AW-10 composite before and after the photoreaction, it can be observed that the WO3 NRs still maintain the nanorod-like morphology after the 15 h photoreaction and their sizes barely changed. Nevertheless, the sizes of Ag2S NPs (Figure S7b) are slightly larger than that of their counterparts before the photoreaction (Figure S7a). It implies that a handful of Ag2S NPs aggregate with each other into bigger sizes, which may cause the negative effects on the light harvesting of WO3 NRs and the charge separation, and then the slightly decreased photocatalytic H2 generation activity. Anyway, the above results suggest that only slight structural deterioration of Ag2S NPs occurred during the photocatalytic reaction process, and it can be concluded that the present AW-x composite has an enhanced photoactivity and a relatively good stability as compared with the Ag2S NPs. Discussion on the photocatalytic mechanism of AW-x composites. To understand the reasons that those AW-x composites deliver an enhanced H2 generation activity compared to the single Ag2S NPs, the photoexcited charge transfer process between


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WO3 NRs and Ag2S NPs should be explored. As mentioned above, the ECB/EVB levels of h-WO3 NRs and Ag2S NPs are 0.31/3.29 and –0.34/0.57 V (vs. NHE), respectively. Therefore, the energy band structures of the AW-10 composite can be illustrated as shown in Figure 7a. At such an energy diagram, there are usually two possible charge transfer mechanisms: (1) type II heterojunction, and (2) direct Z-scheme.5 If the AW-10 composite forms a type II heterojunction system (left in Figure 7a), the photogenerated holes will transfer from the VB of h-WO3 NRs to the VB of Ag2S NPs, while the photogenerated electrons transfer from the CB of Ag2S NPs to the CB of h-WO3 NRs with a more positive potential.5 However, those accumulated electrons in the CB of hWO3 NRs cannot reduce H+ to produce H2, since its CB potential (0.31 V) is much lower than the potential of H+/H2 redox couple.23 It means that AW-x composites would show much lower photocatalytic H2 generation activity than the single Ag2S NPs, which conflicts with the fact that AW-x composites deliver better photocatalytic performance as shown in Figure 6a. Obviously, the photogenerated charge transfer mechanism in AW-x composites would be direct Z-scheme process as shown in right part of Figure 7a.17-22


31


To further validate the direct Z-scheme process in the present system, hydroxyl radical (•OH) formed under illumination was determined using a fluorescence technique with terephthalic acid (TA) as probe molecule.36 The changing trend of fluorescence intensity at 425 nm along with prolonging the illumination time are depicted in Figure 7b based on the corresponding fluorescence spectra shown in Figure S8. As seen, the AW-10 composite in dark only shows very weak fluorescence and barely increase along with prolonging the time, but it exhibits an extremely strong fluorescence at 425 nm under illumination, demonstrating that AW-10 composite cannot yield •OH in dark, and the formation of •OH is related to the photocatalytic process. Moreover, the fluorescence intensity of AW-10 composite under illumination is much stronger than that of single WO3 NRs, which suggests that the AW-10 composite would be beneficial for the formation of •OH radical.51 Nevertheless, the single Ag2S NPs show very weak fluorescence without obvious increase, suggesting no •OH radical is yielded from the suspension, which can be due to its more negative position of VB (0.57 V V) than H2O/•OH (2.40 V).36,52


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Figure 7. (a) Schematic illustration of the possible type II heterojunction (left) and direct Z-scheme (right) charge transfer mechanisms in AW-x composites; (b) Fluorescence intensities at 425 nm of the suspensions illuminated with an excited wavelength of 285 nm as a function of illumination time. Conditions: the suspensions contained basic solution of terephthalic acid and different products.

On the contrary, the photogenerated holes in the VB of h-WO3 NRs have sufficient oxidizing ability to form •OH radicals under irradiation as shown in the right part of Figure 7a, and thus the corresponding photoreaction system can exhibit modest fluorescence intensity. As for the AW-10 composite, the significantly enhanced fluorescence intensity along with prolonging the illumination time demonstrates that the photoinduced holes in the VB of h-WO3 NRs are not transferred to the VB of Ag2S NPs


33


following the type II heterojunction mechanism since the holes in VB of Ag2S NPs would not produce •OH radicals owing to its too negative EVB of Ag2S NPs compared to the potential of H2O/•OH (2.40 V) as shown in Figure 7a. Moreover, those photoexcited holes remained in the WO3 NRs of AW-10 composite are more efficient in reacting with TA than that in the single WO3 NRs, being well consistent with the retarded charge recombination rate in the direct Z-scheme system.22 From the aforementioned results and discussion, it can be concluded that the charge transfer mechanism between the Ag2S NPs and WO3 NRs in AW-x composites follows a direct Z-scheme process as shown in the right part of Figure 7a. After the photoexcitation process, the photogenerated electrons in the CB of h-WO3 NRs can migrate to the VB of Ag2S NPs via the solid-solid heterojunction interface, and quench the local photoinduced holes, leaving the electrons in the CB of Ag2S NPs and holes in VB of the h-WO3 NRs,22 respectively. This transfer and spatial separation processes of the photogenerated charge not only can maintain the reducibility of photoindued electrons in the CB of Ag2S NPs, but also ensure the high charge separation efficiency


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of the Ag2S NPs, and thus cause the AW-10 composite achieving 4 times higher H2 generation activity than the single Ag2S NPs as shown in Figure 6a.15-19 Charge behaviour analyses of AW-x composites. The above conjecture on the improved charge separation efficiency in the AW-10 composite can be proved by the electrochemical measurements since the photoactivity primarily relies on the transfer/separation efficiency of photogenerated charge.5,53 As can be seen from the transient photocurrent curves under repeatedly switching on and off the incident light (Figure 8a), the weakest photocurrent response of the Ag2S NPs reveals its severe charge recombination,37 while the AW-10 composite exhibits higher photocurrent density than that of the single WO3 NRs and Ag2S NPs. Compared to the single Ag2S NPs, this increased photocurrent response suggests that the AW-10 composite can achieve a superior charge transfer/separation efficiency, which then leads to the enhanced photocatalytic performance.54 The electrochemical impedance spectra (EIS) shown in Figure S9 also indicate that the AW-10 composite has the smallest radius of Nyquist plot, implying it has the lowest electronic resistance among the samples tested.


35


Therefore, it can be concluded that the AW-10 composite has most efficient charge transfer/separation in contrast to the single WO3 NRs and Ag2S NPs.53 On the basis of the SEM and TEM observations, the improved charge transfer/separation efficiency of the AW-10 composite may partly originate from the decreased size and well dispersion of Ag2S NPs on WO3 NRs. Compared to the single Ag2S NPs, those Ag2S NPs in AW-10 composite have a shorten distance for the photoinduced charge migrating to the surface, which can facilitate the space separation of the charge carriers.13,15 In succession, the fast separated charge carriers are collected by the conductive substrate or participate in the photoreaction, and thus resulting in the higher photocurrent response (Figure 8a) or photocatalytic activity (Figure 6a).13-15


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Figure 8. (a) Photocurrent-time curves of the AW-10 composite, WO3 NRs and Ag2S NPs illuminated with a 300 W Xe-lamp. (b) Steady-state PL spectra of the AW-10 composite, WO3 NRs and Ag2S NPs with an excited wavelength of 285 nm.

Photoluminescence (PL) spectra is also a useful technique for assessing the charge recombination.55,56 Generally, the enhanced PL intensity implies a higher charge recombination probability, while the efficient photoexcited charge separation can be deduced from the weak PL emission peaks.54 As can be seen from Figure 8b, the PL spectra of WO3 NRs and AW-10 composite exhibit similar emission peaks in the range of 375-525 nm, while no obvious PL peak can be observed from Ag2S NPs, and thus it can be conjectured that the PL phenomenon of the single WO3 NRs is mainly responsible for the PL profile of the AW-10 composite.25 By comparing the PL peak


37


intensity of WO3 NRs and AW-10 composite, a fact hidden behind the results of the above photoactivity and electrochemical measurements emerges. That is, the charge recombination in WO3 NRs, which is inert to the photocatalytic H2 generation reaction, is also retarded by coupling with Ag2S NPs. Therefore, it is reasonable to anticipate that there is a charge interaction existing between WO3 NRs and Ag2S NPs in AW-10 composite, which then increases the charge separation efficiency between the WO3 NRs and Ag2S NPs simultaneously. Namely, besides of the size and dispersion of Ag2S NPs in AW-10 composite, the charge transfer between WO3 NRs and Ag2S NPs may also be a crucial reason for the slow charge recombination kinetics and the enhanced photocatalytic performance of the AW-10 composite. Furthermore, the relatively good stability of AW-10 composite and the changing trend in photoactivity of AW-x composites along with the increase of x value can also be explained. On the one hand, the photogenerated holes in the VB of Ag2S NPs of AW-10 composite will recombine with the electrons migrating from the CB of WO3 NRs during the photoexcitation process, and this consumption of photogenerated holes in the VB of


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Ag2S NPs can avoid the irreversible photoanodic decomposition of Ag2S,24 then improving the stability of the photocatalyst. On the other hand, the WO3 NRs and Ag2S NPs should produce the same number of photogenerated electrons and holes to recombine with each other in a direct Z-scheme charge transfer mechanism as mentioned above, while the consumption of photoinduced holes and electrons in the oxidation and reduction reaction should be at a similar rate.22 Otherwise, the surplus quantity of charge will be probably combined in the bulk, thus deteriorating the charge separation efficiency and the photoactivity sequentially.22 When the Ag2S-loading content in AW-x composites is relatively low (1.0-10 wt%), only less active sites for the photocatalytic H2 generation and the photogenerated holes for recombination are supplied. Therefore, increasing the Ag2S-loading content is helpful for the enhancement of photocatalytic performance (Figure 6a). However, further enhancing the Ag2S-loading content (20-30 wt%) will form dissociative Ag2S aggregates which not only decrease the amount of photogenerated charges in WO3 NRs due to their strong absorption in the vis/NIR region, but also result in severe charge recombination in the Ag2S,


39


consequently leading to a weakened activity (Figure 6a). Hence, the highest H2 generation activity is obtained from AW-10 composite with an optimal weight percentage (10 wt%) of Ag2S NPs. In addition, the AW-10 composite delivers a much better photocatalytic H2 generation activity than that of the mechanical mixture (MAW-10) containing the same amount of Ag2S NPs and WO3 NRs as shown in Figure 6a. This issue may be due to the intimate contact interface between Ag2S NPs and WO3 NRs in AW-10 composite (Figure 3), which facilitates the charge migration from the h-WO3 NRs to Ag2S NPs, thus ensuring the operation of Z-scheme mechanism.56 Also, the tiny size of the dispersed Ag2S NPs and the nanorod-like morphology of WO3 NRs can shorten the transfer distance of photogenerated charge carriers,15,29 which then enhance the charge separation efficiency of the present direct Z-scheme photocatalysts. Therefore, it can be concluded that the morphology and Z-scheme mechanism collectively contribute to the enhancement of charge separation efficiency and photocatalytic activity of the AW-x composites, and thus the present work paves a new way for the high photoinduced


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charge separation efficiency and the efficient photocatalytic H2 generation over Ag2Sbased photocatalysts by means of constructing direct Z-scheme system and morphology tailoring.

■ CONCLUSION

In summary, a novel Ag2S/WO3 direct Z-scheme photocatalyst was fabricated for the first time by decorating Ag2S nanoparticles (NPs) on the hydrothermally prepared hWO3 nanorods (NRs) via a facile solution-based precipitation process. The resultant Ag2S/WO3 composites (AW-x) display enhanced photocatalytic H2 generation activity and charge separation efficiency compared with the single Ag2S NPs, demonstrating that the photoinduced electrons in the Ag2S NPs can be effectively utilized. Those improvements of AW-x composites in the charge separation and photocatalytic performance can be attributed to the synergistic effect between morphology and Zscheme mechanism. Among them, the tiny size and well dispersion of Ag2S NPs, combined with the nanorod-like morphology of WO3 NRs, can shorten the photoexcited


41


charge migration path to the surface, thus decreasing the probability of charge recombination. Meanwhile, the charge separation efficiency of AW-x composites can be further improved by the direct Z-scheme charge transfer mechanism without weakening the redox ability of the photoinduced electron-hole couples. Moreover, the Z-scheme mechanism also avoids the accumulation of photogenerated holes in Ag2S NPs, thus suppressing the irreversible photoanodic decomposition of Ag2S, which is responsible for the relatively good stability of Ag2S NPs/WO3 NRs. The results presented in this work may provide a promising strategy for the effective separation of the electron-hole pairs and the efficient H2 generation on Ag2S-based photocatalysts.

■ ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.iecr.xxxxxxx.


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Survey

XPS

spectra/EIS

spectra/liquid

N2

adsorption-desorption

isotherms/fluorescence intensity changing trends of AW-10, single WO3 NRs and Ag2S NPs, TEM/HRTEM images and Mott-Schottky plots of single WO3 NRs and Ag2S NPs, Survey XPS spectra and XRD patterns of AW-10 before and after photoreaction, SEM image of AW-30 (PDF)

■ AUTHOR INFORMATION

Corresponding Author *E-mail address: [email protected].

ORCID Tianyou Peng: 0000-0002-2527-7634 Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest.


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■ ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21573166, 21871215, and 21271146), the Science & Technology Program of Science, Technology

and

Innovation

Commission

of

Shenzhen

Municipality

(JCYJ20180302153921190), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (BK20151247), China.

■ REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238, 37-38. (2) Chen, X. B.; Li, C.; Grätzal, M.; Kostecki, R.; Mao, S. S. Nanomaterials for Renewable Energy Production and Storage. Chem. Soc. Rev. 2012, 41, 7909-7937. (3) Zou, X.; Zhang, Y. Noble Metal-free Hydrogen Evolution Catalysts for Water Splitting. Chem.

Soc. Rev. 2015, 44, 5148-5180.


44

Page 45 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520-7535. (5) Bai, S.; Jiang, J.; Zhang, Q.; Xiong, Y. Steering Charge Kinetics in Photocatalysis: Intersection of Materials Syntheses, Characterization Techniques and Theoretical Simulations. Chem. Soc. Rev. 2015, 44, 2893-2939. (6) Zhang, X.; Peng, T.; Song, S. Recent Advances in Dye-Sensitized Semiconductor Systems for Photocatalytic Hydrogen Production. J. Mater. Chem. A 2016, 4, 2365-2402. (7) Sadovnikov, S. I.; Gusev, A. I. Recent Progress in Nanostructured Silver Sulfide: from Synthesis and Nonstoichiometry to Properties. J. Mater. Chem. A 2017, 5, 17676-17704. (8) Chen, C.; Li, Z.; Lin, H.; Wang, G.; Liao, J.; Li, M.; Lv, S.; Li, W. Enhanced Visible Light Photocatalytic Performance of ZnO Nanowires Integrated with CdS and Ag2S. Dalton Trans. 2016,

45, 3750-3758. (9) Ong, W. L.; Lim, Y. F.; Ong, J. L. T.; Ho, G. W. Room Temperature Sequential Ionic Deposition (SID) of Ag2S Nanoparticles on TiO2 Hierarchical Spheres for Enhanced Catalytic Efficiency. J. Mater. Chem. A 2015, 3, 6509-6516.


45


(10) Zhang, Y.; Liu, Y.; Li, C.; Chen, X.; Wang, Q. Controlled Synthesis of Ag2S Quantum Dots and Experimental Determination of the Exciton Bohr Radius. J. Phys. Chem. C 2014, 118, 49184923. (11) Lin, S.; Feng, Y.; Wen, X.; Zhang, P.; Woo, S.; Shrestha, S.; Conibeer, G.; Huang, S. Theoretical and Experimental Investigation of the Electronic Structure and Quantum Confinement of Wet-Chemistry Synthesized Ag2S Nanocrystals. J. Phys. Chem. C 2015, 119, 867-872. (12) Zhang, X.; Liu, J.; Johansson, E. M. J. Efficient Charge-Carrier Extraction from Ag2S Quantum Dots Prepared by the SILAR Method for Utilization of Multiple Exciton Generation.

Nanoscale 2015, 7, 1454-1462. (13) Yu, J.; Yu, Y.; Zhou, P.; Xiao, W.; Cheng, B. Morphology-Dependent Photocatalytic H2Production Activity of CdS. Appl. Catal., B 2014, 156-157, 184-191. (14) Zhang, P.; Gao, L.; Song, X.; Sun, J. Micro- and Nanostructures of Photoelectrodes for SolarDriven Water Splitting. Adv. Mater. 2015, 27, 562-568. (15) Wang, K.; Zhang, G.; Li, J.; Li, Y.; Wu, X. 0D/2D Z-Scheme Heterojunctions of Bismuth Tantalate Quantum Dots/Ultrathin g-C3N4 Nanosheets for Highly Efficient Visible Light Photocatalytic Degradation of Antibiotics. ACS Appl. Mater. Interfaces 2017, 9, 43704-43715.


46

Page 46 of 55

Page 47 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Bard, A. J. Photoelectrochemistry and Heterogeneous Photocatalysis at Semiconductors. J.

Photochem. 1979, 10, 59-75. (17) Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D. J.; Higashi, M.; Kong, D.; Abe, R.; Tang, J. Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems. Chem. Rev. 2018, 118, 5201-5241. (18) Ye, L.; Liu, J.; Gong, C.; Tian, L.; Peng, T.; Zan, L. Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge. ACS Catal. 2012, 2, 1677−1683. (19) Yu, W.; Xu, D.; Peng, T. Enhanced Photocatalytic Activity of g-C3N4 for Selective CO2 Reduction to CH3OH via Facile Coupling of ZnO: A Direct Z-Scheme Mechanism. J. Mater. Chem. A 2015, 3, 19936-19947. (20) Cong, Y.Q.; Ge, Y.H.; Zhang, T.T.; Wang, Q.; Shao, M.L.; Zhang, Y. Fabrication of Z-Scheme Fe2O3-MoS2-Cu2O Ternary Nanofilm with Significantly Enhanced Photoelectrocatalytic Performance. Ind.

Eng. Chem. Res. 2018, 57, 881-890. (21) Jin, J.; Yu, J.G.; Guo, D.P.; Cui, C.; Ho, W.K. A Hierarchical Z-Scheme CdS-WO3 Photocatalyst with Enhanced CO2 Reduction Activity. Small 2015, 39, 5262-5271.


47


(22) Zhou, P.; Yu, J.; Jaroniec, M. All-Solid-State Z-Scheme Photocatalytic Systems. Adv. Mater. 2014, 26, 4920-4935. (23) Zhang, L. J.; Li, S.; Liu, B. K.; Wang, D. J.; Xie, T. F. Highly Efficient CdS/WO3 Photocatalysts: Z-Scheme Photocatalytic Mechanism for Their Enhanced Photocatalytic H2 Evolution under Visible Light. ACS Catal. 2014, 4, 3724-3729. (24) Guo, H. L.; Du, H.; Jiang, Y. F.; Jiang, N.; Shen, C. C.; Zhou, X.; Liu, Y. N.; Xu, A. W. Artificial Photosynthetic Z-Scheme Photocatalyst for Hydrogen Evolution with High Quantum Efficiency. J. Phys. Chem. C 2017, 121, 107-114. (25) Yu, W.; Chen, J.; Shang, T.; Chen, L.; Gu, L.; Peng, T. Direct Z-Scheme g-C3N4/WO3 Photocatalyst with Atomically Defined Junction for H2 Production. Appl. Catal., B 2017, 219, 693704. (26) Li, C.; Song, C.D.; Wamg, X.; Zhang, J.; Chen, X.B. Enhanced Performance of Direct Z-Scheme CuS-WO3 System towards Photocatalytic Decomposition of Organic Pollutants under Visible Light. Appl.

Surf. Sci. 2017, 425, 788-795. (27) Hu, T.P.; Li, P.F..; Zhang, J.F.; Liang, C.H.; Dai, K. Highly Efficient Direct Z-Scheme WO3/CdSDiethylenetriamine Photocatalyst and Its Enhanced Photocatalytic H2 Evolution under Visible Light Irradiation. Appl. Surf. Sci. 2018, 442, 20-29.


48

Page 48 of 55

Page 49 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Zheng, F.; Song, S.; Lu, F.; Li, R.; Bu, N.; Liu, J.; Li, Y.; Hu, P.; Zhen, Q. Hydrothermal Preparation, Growth Mechanism and Supercapacitive Properties of WO3 Nanorod Arrays Grown Directly on a Cu Substrate. CrystEngComm 2016, 18, 3891-3904. (29) Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X. Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano Lett. 2014, 14, 1099-1105. (30) Peng, T.; Ke, D.; Xiao, J.; Wang, L.; Hu, J.; Zan, L. Hexagonal Phase WO3 Nanorods: Hydrothermal Preparation, Formation Mechanism and Its Photocatalytic O2 Production under VisibleLight Irradiation. J. Solid State Chem. 2012, 194, 250-256. (31) Zhang, S.; Wu, S.; Wang, J.; Jin, J.; Peng, T. Controllable Syntheses of Hierarchical WO3 Films Consisting of Orientation-Ordered Nanorod Bundles and Their Photocatalytic Properties. Crys.

Growth Des. 2018, 18, 794-801. (32) Aslam, I.; Cao, C.; Tanveer, M.; Khan, W. S.; Tahir, M.; Abid, M.; Idrees, F.; Butt, F. K.; Ali, Z.; Mahmood, N. The Synergistic Effect between WO3 and g-C3N4 towards Efficient Visible-LightDriven Photocatalytic Performance. New J. Chem. 2014, 38, 5462-5469.


49


(33) Fu, W.; Wang, J.; Zhou, S.; Li, R.; Peng, T. Controllable Fabrication of Regular HexagonShaped SnS2 Nanoplates and Their Enhanced Visible-Light-Driven H2 Prodution Activity. ACS Appl.

Nano Mater. 2018, 1, 2923-2933. (34) Wang, J.; Luo, J.; Liu, D.; Chen, S.; Peng, T. One-Pot Solvothermal Synthesis of MoS2Modified Mn0.2Cd0.8S/MnS Heterojunction Photocatalysts for Highly Efficient Visible-Light-Driven H2 Production. Appl. Catal., B 2019, 241, 130-140. (35) Song, S.; Liang, Z.; Fu, W.; Peng, T. Preparation of Single-Crystalline AgIn5S8 Octahedrons with Exposed {111} Facets and Its Visible-Light-Responsive Photocatalytic H2 Production Activity.

ACS Appl. Mater. Interfaces 2017, 9, 17013-17023. (36) Jo, W. K.; Natarajan, T. S. Facile Synthesis of Novel Redox-Mediator-free Direct Z-Scheme CaIn2S4 Marigold-Flower-like/TiO2 Photocatalysts with Superior Photocatalytic Efficiency. ACS Appl.

Mater. Interfaces 2015, 7, 17138-17154. (37) Jiang, D.; Chen, L.; Xie, J.; Chen, M. Ag2S/g-C3N4 Composite Photocatalysts for Efficient Ptfree Hydrogen Production. The co-Catalyst Function of Ag/Ag2S Formed by Simultaneous Photodeposition. Dalton Trans. 2014, 43, 4878-4885.


50

Page 50 of 55

Page 51 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Li, K.; Peng, T.; Ying, Z.; Song, S.; Zhang, J. Ag-Loading on Brookite TiO2 quasi Nanocubes with Exposed {210} and {001} Facets: Activity and Selectivity of CO2 Photoreduction to CO/CH4.

Appl. Catal., B 2016, 180, 130-138. (39) Liu, J.; Jing, L.; Gao, G.; Xu, Y.; Xie, M.; Huang, L.; Ji, H.; Xie, J.; Li, H. Ag2S Quantum Dots

in situ Coupled to Hexagonal SnS2 with Enhanced Photocatalytic Activity for MO and Cr(VI) Removal. RSC Adv. 2017, 7, 46823-46831. (40) Barreca, D.; Carta, G.; Gasparotto, A.; Rossetto, G.; Tondello, E.; Zanella, P. A Study of Nanophase Tungsten Oxides Thin Films by XPS. Surf. Sci. Spectra 2001, 8, 258-267. (41) Fan, W.; Jewell, S.; She, Y.; Leung, M. K. H. In situ Deposition of Ag-Ag2S Hybrid Nanoparticles onto TiO2 Nanotube Arrays towards Fabrication of Photoelectrodes with High Visible Light Photoelectrochemical Properties. Phys. Chem. Chem. Phys. 2014, 16, 676-680. (42) Liu, J.; Cheng, B.; Yu, J. A New Understanding of the Photocatalytic Mechanism of the Direct Z-Scheme g-C3N4/TiO2 Heterostructure. Phys. Chem. Chem. Phys. 2016, 18, 31175-31183. (43) Xu, F.; Zhang, L.; Cheng, B.; Yu, J. Direct Z-Scheme TiO2/NiS Core-Shell Hybrid Nanofibers with Enhanced Photocatalytic H2-Production Activity. ACS Sustainable Chem. Eng. 2018, 6, 1229112298.


51


(44) Liu, B.; Wang, D.; Zhang, Y.; Fan, H.; Lin, Y.; Jiang, T.; Xie, T. Photoelectrical Properties of Ag2S Quantum Dot-Modified TiO2 Nanorod Arrays and Their Application for Photovoltaic Devices.

Dalton Trans. 2013, 42, 2232-2237. (45) Xiong, J.; Han, C.; Li, W.; Sun, Q.; Chen, J.; Chou, S.; Li, Z.; Dou, S. Ambient Synthesis of A Multifunctional 1D/2D Hierarchical Ag-Ag2S Nanowire/Nanosheet Heterostructure with Diverse Applications. CrystEngComm 2016, 18, 930-937. (46) Cardon, F.; Gomes, W. P. On the Determination of the Flat-Band Potential of a Semiconductor in Contact with a Metal or an Electrolyte from the Mott-Schottky Plot. J. Phys. D:

Appl. Phys. 1978, 11, L63. (47) Ding, S.; Xiong X.; Liu, X.; Shi, Y.; Jiang, Q.; Hu, J. Synthesis and Characterization of SingleCrystalline Bi2O2SiO3 Nanosheets with Exposed {001} Facets. Catal. Sci. Technol. 2017, 7, 37913801. (48) Zhang, H.; Li, K.; Ma, Z. Simple Fabrication of Morphology-Tunable Ag2S and Ag Architectures Assisted by Denatured Bovine Serum Albumin. Colloids Surf. A 2012, 402, 94-101. (49) Basu, M.; Nazir, R.; Mahala, C.; Fageria, P.; Chaudhary, S.; Gangopadhyay, S.; Pande, S. Ag2S/Ag Heterostructure: A Promising Electrocatalyst for the Hydrogen Evolution Reaction.

Langmuir 2017, 33, 3178-3186.


52

Page 52 of 55

Page 53 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) Sadovnikov, S. I.; Kozlova, E. A.; Gerasimov, E. Y.; Rempel, A. A.; Gusev, A. I. Enhanced Photocatalytic Hydrogen Evolution from Aqueous Solution on Ag2S/Ag Heteronanostructure. Int. J.

Hydrogen Energy 2017, 42, 25258-25266. (51) Ahmed, A. Y.; Kandiel, T. A.; Oekermann, T.; Bahnemann, D. Photocatalytic Activities of Different Well-Defined Single Crystal TiO2 Surfaces: Anatase versus Rutile. J. Phys. Chem. Lett. 2011, 2, 2461-2465. (52) Jia, X.; Tahir, M.; Pan, L.; Huang, Z. F.; Zhang, X.; Wang, L.; Zou, J. J. Direct Z-Scheme Composite of CdS and Oxygen-Defected CdWO4: An Efficient Visible-Light-Driven Photocatalyst for Hydrogen Evolution. Appl. Catal., B 2016, 198, 154-161. (53) Yu, W.; Zhang, S.; Chen, J.; Xia, P.; Richter, M. H.; Chen, L.; Xu, W.; Jin, J.; Chen, S.; Peng, T. Biomimetic Z-Scheme Photocatalyst with a Tandem Solid-State Electron Flow Catalyzing H2 Evolution. J. Mater. Chem. A 2018, 6, 15668-15674. (54) Song, S.; Wang, J.; Peng, T.; Fu, W.; Zan, L. MoS2-MoO3-x Hybrid Cocatalyst for Effectively Enhanced H2 Production Photoactivity of AgIn5S8 Nano-Octahedrons. Appl. Catal., B 2018, 228, 3946.


53


(55) Ding, J.; Chai, Y.; Liu, Q.; Liu, X.; Ren, J.; Dai, W. L. Selective Deposition of Silver Nanoparticles onto WO3 Nanorods with Different Facets: The Correlation of Facet-Induced Electron Transport Preference and Photocatalytic Activity. J. Phys. Chem. C 2016, 120, 4345-4353. (56) Katsumata, H.; Sakai, T.; Suzuki, T.; Kaneco, S. Highly Efficient Photocatalytic Activity of gC3N4/Ag3PO4 Hybrid Photocatalysts through Z-Scheme Photocatalytic Mechanism under Visible Light. Ind. Eng. Chem. Res. 2014, 53, 8018-8025.


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