Accepting Excited High-Energy-Level Electrons and Catalyzing H2

May 25, 2016 - ... Fei Chen , Yingying Sun , Shuwang Duo , Rongbin Zhang , Quanhong Hu , Wenkui Li , Yongfang Kang. Molecular Catalysis 2017 438, 55-6...
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Accepting Excited High-Energy-Level Electrons and Catalyzing H2 Evolution of Dual-Functional Ag-TiO2 Modifier for Promoting VisibleLight Photocatalytic Activities of Nanosized Oxides Ji Bian, Yang Qu,* Raziq Fazal, Xinlei Li, Ning Sun, and Liqiang Jing* Key Laboratory of Functional Inorganic Materials Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials Science, International Joint Research Center for Catalytic Technology, Harbin 150080, P. R. China S Supporting Information *

ABSTRACT: To improve the photocatalytic activities of narrow band gap oxide semiconductors for H2 evolution under solar irradiation, it is highly desired to develop effective acceptors for visible light-excited high-energy-level electrons. Herein, we have successfully fabricated Ag-modified TiO2/BiVO4 nanocomposites by putting nanosized BiVO4 into the Ag modified TiO2 sol. Both steady-state and transient-statesurface photovoltage spectra demonstrate that photogenerated charge separation and lifetime of nanosized BiVO4 is promoted when coupling with TiO2 and modifying an appropriate amount of Ag, while the lifetime of photogenerated electrons got prolonged. Interestingly, the resulting Ag-TiO2/BiVO4 nanocomposites exhibit excellent visible light activities for H2 evolution, although the visible light activities of TiO2/BiVO4 one, Ag/BiVO4 and bare BiVO4 are neglectable, indicating that AgTiO2 could be utilized as effective acceptors for hydrogen production. It is suggested based on the experimental data that the effective acceptors be attributed to the used TiO2 for accepting the high-energy-level electrons generated from BiVO4 and to the modified Ag for being reduced then to catalyze H2-evolution reactions. The developed strategy is versatile for other narrow band gap semiconductors, like WO3 and Fe2O3.

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INTRODUCTION Photocatalytic hydrogen evolution is regarded as a potential technique that can solve the current increasingly serious energy crisis through effective solar utilization.1 Despite a great deal of effort made, the quantum efficiencies of state-of-art works are still low, far from the practical requirement for industrialization.2,3 Photocatalytic reactions for H2 evolution are mainly involved with complex thermodynamics and kinetic processes. In general, efficient photocatalysts are needed to possess strong solar-light absorption, high charge separation and superior catalytic capacity. It is well-known that cheap, stable, and environment friendly narrow band gap oxide semiconductors, such as BiVO4, Fe2O3, and WO3, can greatly absorb solar light, and the electrons will be excited from valence band (VB) to conduction band (CB) bottom and even over the level under solar irradiation.4−6 The excited electrons at much higher energy level than the CB bottom usually relax to the CB bottom quite quickly (within picosecond time scale).7,8 Since their CB bottom levels are usually located below 0 eV versus NHE,9 so that the excited electrons are not suitable for energetic H2 evolution, it is understandable that such narrowband gap oxides always exhibit low photocatalytic activities. However, it is worth noting that those visible-light-excited highenergy-level (HEL) electrons would naturally possess strong capacity to induce reduction reactions with water molecules thermodynamically.10,11For this, the key would be how to prolong the lifetimes of HEL electrons by altering the fast © XXXX American Chemical Society

relaxation process. Unfortunately, it has seldom been reported until now. Constructing a heterojuntional nanocomposite might be a protocol to the relaxation process of HEL electrons. It is assumed that the improved photoactivities are from the transfer of visible-light-excited HEL electrons to acceptors, and such acceptors (like TiO2 and ZnO) with a high-level CB bottom would play important roles in maintaining the energy and prolonging the lifetime of transferred HEL electrons. To evolve H2, noble metal, like Pt, is often used as an effective acceptor and cocatalyst.1,12 Thus, it is expected that to introduce a highenergy-level platform by coupling a wide-band gap oxide and then a cocatalyst by modifying a kind of noble metal are feasible to greatly improve the photocatalytic activities of narrow-band gap oxides for H2 evolution under visible-light irradiation. Similarly, it is possibly feasible to only modify a noble metal since it behaves as effective acceptors, especially to trap the HEL electrons, and as cocatalysts.13 To date, those expectations have not been clarified. Among various investigated visible-response oxides, BiVO4 has been regarded as a potential photocatalyst for water splitting.14,15 However, most works focus on O2 evolution, while photocatalytic H2 production over BiVO4 has been Received: April 11, 2016 Revised: May 25, 2016

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DOI: 10.1021/acs.jpcc.6b03664 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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vigorous stirring for 30 min. Eventually, the mixture was kept at 80 °C in an oven and then calcined at 450 °C in air for 2 h. 7Ag-TiO2/WO3 nanocomposite was obtained, which 7 is the mole ratio percentage of Ag to TiO2. The molar percentage of TiO2 to WO3 is 5%. Characterization. XRD was employed to characterize the crystal structure and phases of products with a Bruker D8 (German), using Cu K α radiation (α = 0.15418 nm). The model of UV−vis DRS equipment is Shimadzu UV2550. Transmission electron microscopy (TEM) observation was implement on a JEOL JEM-2010 instrument operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) was tested by Kratos-AXIS ULTRA DLD. Al (Mono) was used as the X-ray source. The SS-SPS and TS-SPV measurement for samples were implemented on home-built equipment, which has been described in detail elsewhere.23 Photocatalytic Hydrogen Production. The photocatalytic hydrogen evolution was carried out in an online hydrogen production system (Perfectlight, Beijing, LabSolar II). Eighty milliliters of deionized water and 20 mL of methanol were mixed in the cubic glass cell and keep stirring. Then, 0.1 g of photocatalyst powder was putted into the above solution. Ahead of the reaction, the mixture was deaerated by evacuation to remove the gas dissolved in water. The experiments were performed by illuminating the mixture solution, using a xenon lamp (300 W) with a 420 CUT filter (420−780 nm) as the visible light source. The amount of evolved H2 in the photocatalysis were performed in an inline gas chromatograph (7900, TCD, molecular sieve 5 Å, N2 carrier, Techcomp). Electrochemical Reduction Measurement. Electrochemical reduction measurement were carried out in a traditional three-electrode system. The working electrode was a 0.3 cm diameter glassy carbon (GC) electrode, saturated calomel electrode (SCE) as the reference electrode, and a Pt sheet was used as the counter electrode. Five milligrams of different samples mixed with 20 μL of 5 wt % Nafion ionomer was dissolved in 0.18 mL of ethanol aqueous solution. The catalyst ink was ultrasound for 30 min, and a suitable mass of the ink was uniformly dropped onto the clean GC electrode surface and dried in air. A BAS 100B electrochemical workstation was employed to test electrochemical activity and stability of a series of catalysts. At the beginning, electrode potentials were cycled between two potential limits until perfectly overlapping; afterward the I−V curves were obtained. The electrolytes for test were 1 M NaClO4. All the experiments were performed at room temperature (about 25 °C).

seldom reported. To improve the surface catalytic kinetic of semicondutor oxides, Pt is a widely used noble metal cocatalyst for H2 evolution. Compared with Pt, Ag is much cheaper. Based on the above analyses, along with the expected crucial roles of TiO2 and Ag, it is indeed meaningful to develop the Ag-TiO2 multifunctional nanocomposite as a modifier to improve the photocatalytic activities of BiVO4 for H2 production, especially under visible-light irradiation, simultaneously to clarify the activity-enhanced mechanisms, and also to expand this to other photocatalyst systems. Herein, under visible light illumination, the nanosized BiVO4 for H2 evolution in the methanol solution has been significantly improved after coupling with a certain mass (5% in mole) of nanocrystalline anatase TiO2 premodified with a proper amount of Ag (Ag-TiO2), and it is confirmed that the coupled TiO2 could accept the HEL electrons from BiVO4 to prolong their lifetimes so that the transferred HEL electrons could effectively induce reduction reactions with H2O to evolve H2 with photoreduced Ag as the cocatalyst. Interestingly, this is also applicable to other visible-light-response oxides, like αFe2O3 and WO3, and it is also feasible if Ag is substituted with other precious metals, like Pt, Au, and Pd, with similar effects. This work provides a practicability strategy to design and prepare highly reactive oxide nanophotocatalysts for water decomposition under solar irradiation.

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EXPERIMENTAL SECTION Materials. Synthesis of BiVO4. In a typical synthesis, 4.85 g of Bi(NO3)3·5H2O was dissolved in 100 mL of HNO3 aqueous solution with a concentration of 2 mol/L under vigorous magnetic stirring. Then, 1 g of PEG and 1.17 g of NH4VO3 were added to the above mixture, and after that, NH3·H2O was used to adjust the pH to 7. Then, the mixture solution was ultrasound for 30 min, followed by continuous stirring for 1 h. Finally, the solution was centrifuged and washed by deionized water and absolute ethyl alcohol by turns, and kept at 60 °C in an oven. Synthesis of Ag-TiO2/BiVO4. In the typical synthesis, AgTiO2 sol was prepared first. Twenty milliliters of absolute ethanol, 5 mL of water, 1 mL of HNO3 (16 mol/L), and AgNO3 (1.5, 4.6, 7.7, 1.08, and 1.54 × 10−5 mol) were mixed to form a clear solution at room temperature (20 ± 3 °C) under magnetic stirring. Then, 0.05 mL of Ti(OBu)4 and 0.45 mL of absolute ethyl alcohol were dropwise added into the mixture. Finally, 1 g of BiVO4 powder was put into the above mixture solution under vigorous stirring for about 0.5 h. Finally, the mixture was dried at 80 °C for 6 h and calcined at 450 °C in air for 2 h. Different Ag-modified TiO2/BiVO4 nanocomposites were obtained, this series of samples named as X Ag-T/BV in which X (1, 3, 5, 7 and 10) is the mole ratio percentage of Ag to TiO2. The molar percentage of TiO2 to BiVO4 is 5%. Synthesis of Ag-TiO2/Fe2O3. In the typical synthesis, the above Ag-TiO2 sol was prepared first. Fe2O3 powder was prepared according to the reported method.11Then, 1 g of Fe2O3 powder was put into the Ag-TiO2 sol under stirring for about 0.5 h. Finally, the mixture was kept at 80 °C in an oven and then calcined at 450 °C in air for 2 h. 7Ag-T/Fe2O3 nanocomposite was obtained, in which 7 is the mole ratio percentage of Ag to TiO2. The molar percentage of TiO2 to Fe2O3 is 5%. Synthesis of Ag-TiO2/WO3. In the typical synthesis, the above Ag-TiO2 sol was prepared first. WO3 powder was prepared according to the reported method.16 Then, 1 g of WO3 powder was put into the above mixture solution under

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RESULTS AND DISCUSSION TiO2/BiVO4 (T/BV) and Ag-modified TiO2/BiVO4 (Ag-T/ BV) nanocomposites have been prepared by the wet chemical method. One can see that all the peaks from the X-ray diffraction could be attributed to the monoclinic BiVO4 (JCPDC no. 14-688) in Figure S1A.17 After coupling with TiO2 or Ag-TiO2, the crystal phase and crystallinity of monoclinic BiVO4 are not changed, and no related peaks of TiO2 or Ag are found, which is possibly due to the tiny amount beyond the detection sensitivity. However, the band edge from UV-vis DRS of Ag-T/BV slightly shifts to the red compared with that of T/BV at around 570 nm. (Figure S1B), which is associated with Ag2O.18 This is supported by the Ag 3d X-ray photoelectron spectroscopy (XPS) centering at 367.4 eV in Figure S2.19 Obviously, the characterizations including XRD, DRS, and XPS results are in accordance with each other. It B

DOI: 10.1021/acs.jpcc.6b03664 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C should be noted that surface plasmon resonance peaks of Ag are not found in UV-vis DRS, due to the too small amount and size of Ag nanoparticles. This gives the evidence that the visible light response of the Ag-T/BV composite is only original from BiVO4 itself, while Ag is just to catalyze. High charge separation is crucial for efficient photocatalysis.20 The surface photovoltage technique, including SSSPS and TS-SPV response, are believed to be effective tools to study the behavior of photogenerated charge.8,21,22 To well reveal the photogenerated charge separation, the SS-SPS responses in N2 of Ag-T/BV nanocomposites are recorded in Figure 1A. One can notice that, differently from bare BiVO4, an

Figure 2. (A) Amounts of evolved H2 under visible- and UV−visible light irradiation for 6 h on different Ag-T/BV nanocomposites in the methanol-containing water; the inset means the amount of evolved H2 on 7Ag-T/BV under light-on and light-off conditions. (B) The fitted amounts of evolved H2 on 7Ag-T/BV under different-wavelength-light irradiation with increasing irradiation time; the inset implies the amount of evolved H2 on 7Ag-T/BV under visible-light irradiation.

Interestingly, the fabricated Ag-T/BV nanocomposites could exhibit high cocatalyst-free activities for photocatalytic H2 evolution, even under visible-light irradiation, which is very obvious for the 7Ag-T/BV one. Thus, it is concluded that the improved activity of Ag-T/BV nanocomposite for photocatalytic H2 evolution mainly depends on the enhanced separation of photogenerated charges. Moreover, four points should be noticed for 7Ag-T/BV as follows. One is that the amount of evolved H2 is increased only when the light is on by four recycles (inset in Figure 2A), indicating that it is a photocatalytic reaction. Two is that the H2 quantities rely on linear growth with increasing illumination time (inset in Figure 2B), indicating that the fabricated nanocomposite, 7Ag-T/BV, is stable. Three is that the evolved H2 is detectable after irradiating for a certain time, which is related to the preferential reduction of Ag+ to form Ag. Four is that it has photoactivity with short 520 nm light and no photoactivity with a 560 nm one, all shorter than the threshold wavelength of ∼570 nm. Also, the higher the used light energy, the larger the amount of produced H2. This implies that the photoactivity for H2 evolution depends on the irradiation energy. From the XRD patterns (Figure S4) it can be seen that the crystal phase and crystallinity remain unchanged before and after photocatalytic reactions. For the chemical states of Bi, V, O, and Ti are nearly unchanged by means of related XPS spectra (Figure S5). These indicate that it is stable in photocatalysis for TiO2 and BiVO4. However, it is confirmed that the valence state of Ag is changed from Ag+ to Ag0 according to the binding energy change.26 This is responsible for detectable H2 evolution after irradiating for a certain time. According to the TEM (Figure S6) and HRTEM image (Figure 3A) of 7Ag-T/BV after the photocatalytic reaction, it is confirmed that the nanoparticle surface of BiVO4 is partly capped by the resulting low-crystallinity TiO2 with ∼1 nm thickness (consistent with XRD result), and the photodeposited Ag with ∼5 nm size in diameter is dispersed on the surfaces of TiO2. This indicates that a novel threecomponent closely contacted nanocomposite is formed so as to favor charge transfer and separation among them. Hence, it is naturally expected that the coupled Ag-TiO2 plays a bifunctional role in the H2 production of BiVO4 in the presence of methanol as hole scavengers. Or rather, the visible-light-excited HEL electrons of BiVO4 would first transfer to TiO2 so as to keep high-level energy and then to the Ag, where the catalytic reaction for H2 evolution could take place. Along with no photocatalytic activities for H2 evolution on BiVO4, TiO2/

Figure 1. Responses in N2 of SS-SPS (A) and TR-SPV (B) of different Ag-T/BV nanocomposites, in which BV means BiVO4 and T means TiO2 with 5% mole ratio to BiVO4, and the number is the mole ratio percentage of Ag to TiO2. The wavelength of used laser is 532 nm for TS-TPV measurement.

obvious SS-SPS response occurs in T/BV, indicating that the photoinduced electrons could transfer to TiO2 from BiVO4 under visible-light irradiation.23 Interestingly, Ag-T/BV displays much higher SS-SPS response compared with T/BV, especially for 7Ag-T/BV, meaning the high charge separation. This is attributed to the role of modified Ag for capturing electrons to promote charge separation. Similar to T/BV, the SS-SPS response of 7Ag-T/BV is gradually enhanced with increasing the content of O2 (Figure S3), indicating that the presence of O2 is favorable for charge separation by capturing electrons.8,22 The kinetic manners of photogenerated electrons and holes are studied by TS-SPV in N2. Generally speaking, the TS-SPV response is involved with two time ranges: fast (10−4 s) from the charge diffusion process.23,24 For nanosized semiconductor, the built-in electric field could be neglectable since the fast time response is very weak.25 As shown in Figure 1B, the slow-time TS-SPV response becomes gradually strong as the amount of modified Ag is increased, along with the prolonged carrier lifetime, and the 7Ag-T/BV nanocomposite displays the highest response among Ag-T/BV ones. If the modified Ag continues to increase, the TS-SPV signal will begin to decrease. Obviously, the TS-SPV results are in accordance with the SS-SPS ones. Basically, the photogenerated charge separation becomes better, and the SS-SPS (TS-SPV) signal is enhanced as well. Thus, it is confirmed that the modified Ag would be favorable to promote charge separation of the T/BV nanocomposite. It is naturally predicted that the prepared Ag-T/BV will exhibit high photocatalytic activities compared with the T/BV one. A series of photocatalytic H2 evolution activities tests have been carried out in the methanol solution, as shown in Figure 2. There is no H2 evolution for nanosized BiVO4 (BV) alone and T/BV nanocomposite,23 and the Ag-deposited BV (Ag−BV) also exhibits no photocatalytic activity for H2 evolution. C

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Figure 3. HRTEM image of selected area of 7Ag-T/BV after photocatalytic reaction for 6 h (A). Electrochemical I−V curves in the dark on different BV-based nanoparticles (B). HER performance is measured in a 0.5 M NaClO4 solution which bubbled N2 during the experiments. Hg/Hg2Cl2 (saturated KCl) electrode as the reference electrode. Ag−BV is the BV deposited Ag by a photoreduction process.

Figure 4. Mechanism of the photogenerated HEL electrons on 7AgT/BV nanocomposite about the transfer and separation under visible light illumination and their induced reactions for H2 evolution with Ag as cocatalyst.

BiVO4 and Ag/BiVO4, it is deduced that the simultaneous existence of TiO2 and Ag is indispensable to evolve H2 on BiVO4. To further prove the opinion, electrochemical hydrogen evolution reactions (HER) on different BiVO4 electrodes were measured (Figure 3B). The observed cathode current in the range of −1.3−1.6 V versus Hg/Hg2Cl2 is attributed to the H2 evolution.27 By contrast, it is found that Ag and TiO2 both are favorable for H2 evolution. This is well responsible for the observed largest current of fabricated Ag-T/BV nanocompsite, indicating that Ag-T/BV possesses the highest activity for H2 evolution. Obviously, the electrochemical HER results are in good agreement with the H2-evolution photoactivities. Moreover, photogenerated charge separation of different BiVO4 has been investigated by means of the SS-SPS and TSSPV responses (Figure S7). Apparently, the response intensities of SS-SPS and TS-SPV of BiVO4 could be enhanced respectively after Ag deposition and TiO2 coupling, implying that the separation of photogenerated electrons and holes was strengthened. In particular, the highest charge separation for Ag/BiVO4 is noticed, which is attributed to the role of Ag as an effective acceptor. However, no visible-light activities for H2 evolution are observed for Ag/BiVO4 and TiO2/BiVO4. Interestingly, the high charge separation of Ag-T/BV, even Ag0-T/BV, could be seen compared to BiVO4, with efficient photocatalytic reactions for H2 evolution. According to the above analyses, it can be sure that the lifetime and separation of photogengrated electrons and holes of nanosized BiVO4 excited by visible light have prolonged and enhanced through a suitable mass of Ag modified on TiO2, and both of Ag and TiO2 are indispensable for H2 evolution under visible light irradiation. Hence, a mechanism schematic is suggested in the presence of methanol in Figure S8 and Figure 4. After Ag is deposited on the surfaces of BiVO4, the photogenerated charge separation is enhanced by electrons transferring to Ag. However, it is possible for excited highenergy-level electrons to first relax quickly to the CB bottom of BiVO4 and then transfer to Ag. In this case (Figure S8A), the excited electrons transferred to Ag lack enough energy to induce the reduction reactions with H2O for H2 production, to say that it is unfavored thermodynamically although an effective catalyst Ag exists. After TiO2 is coupled to BiVO4, the photogenerated charge separation is enhanced by electrons transferring to TiO2, in which the energies of visible-lightexcited HEL electrons could be maintained (Figure S8B).

Thermodynamically, the transferred HEL electrons could induce the reduction reactions with H2O. However, it is difficult for H2 evolution because of the great overpotential in the absence of a catalyst Ag, that is, it is unfavored kinetically.1,13 Naturally, it is accepted that the resulting Ag-T/BV nanocomposite exhibits high charge separation, and high visible-light activity for H2 evolution. When visible light is passed to BiVO4, HEL electrons could be produced, and then transfer to TiO2 with enough energy. Subsequently, they could migrate to Ag (Figure 4). As a result, H2 evolution becomes feasible. Obviously, TiO2 plays an important role by introducing a high-energy-level platform to keep HEL electrons enough energies, while Ag could continuously accept the transferred HEL electrons and catalyze the reduction reactions with H2O for H2 evolution. Thus, Ag-modified TiO2 behaves a comprehensively crucial action for H2 production on BiVO4. According to the reported energy band positions of TiO2 and BiVO4,23,28 it has been evaluated that the beginning wavelength threshold is 534 nm for producing HEL electrons, while the band edge of BiVO4 is positioned at 570 nm. This agrees well with the fact that Ag-T/BV has photoactivity with short-520 nm light and no photoactivity with 560 nm one. TiO2 in the fabricated ternary system (Ag-T/BV) is regarded as the platform for accepting the HEL electrons and maintain their energy in a relatively high level, due to its proper conduction band. To make much sure, Ag-modified TiO2 is also applied to nanosized WO328 (Figure S9) and α-Fe2O3 (Figure S10) under similar conditions. It is confirmed that the modification of Ag-TiO2 does not change the crystal structure and optical property of WO3 and Fe2O3, from the XRD and DRS. However, the increased SS-SPS response means the enhanced charge separation after Ag-TiO2 modification. As expected, both the Ag-TiO2/WO3 and Ag-TiO2/Fe2O3 show the obvious photocatalytic activities for H2 production under visible-light irradiation, although it is not active for WO3 or Fe2O3 alone. Thus, it is demonstrated that the Ag-TiO2 modification is generally applicable to narrow bandgap oxide semiconductors with two functions including HEL electrons acceptor and cocatalyst. Similar to Ag, other precious metals, like Pt, Au and Pd, should play important cocatalytic roles in the H2-evolution process. For this, Ag in Ag-T/VB nanocomposite is replaced by Pt, Au and Pd respectively as the same mole ratio relative to TiO2. As assumed, it is noticed from Figure S11 that all of them D

DOI: 10.1021/acs.jpcc.6b03664 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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exhibit obvious visible-light activities. This indicates that the noble metal in the fabricated novel nanocomposite could be regarded as the important HEL-electron acceptors and cocatalysts for H2 evolution. Surprisingly, their photoactivities of the nanocomposites with different noble metals are rather close to each other, implying that the used noble metals in the nanocomposites exhibit a similar catalytic capacity for H2 evolution, and the transfer step of HEL electrons to TiO2 is very crucial. Comparing with noble metals, Ag takes the advantage due to its low cost, therefore it is meaningful to develop an Ag-TiO2 multifunctional nanocomposite modifier to improve photocatalytic activities of visible-light-response oxides for H2 production. Noteworthily, the fabricated Ag-TiO2/ BiVO4 nanocomposite could utilize visible light from 400 to about 530 nm so that it is meaningful to utilize solar energy efficiently. In addition, We have confidence that the H2 production performance, even for overall water splitting, could be further improved through optimizing the size, morphology, ratio, and connection of three necessary components in the specific composite.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the NSFC (U1401245, 21501052), the National Key Basic Research Program of China (2014CB660814), the Chang Jiang Scholar Candidates Programme for Heilongjiang Universities(2012CJHB003), the Project of Chinese Ministry of Education (213011A), the Specialized Research Fund for the Doctoral Program of Higher Education (20122301110002), the Science Foundation for Excellent Youth of Harbin City of China (2014RFYXJ002) and the Program for Innovative Research Team in Chinese Universities (IRT1237),

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CONCLUSIONS In summary, Ag-TiO2/BiVO4 nanocomposites have successfully been fabricated, and it is clearly demonstrated via a series of steady-state and transient-state- surface photovoltage responses that the separation and lifetime of the charge carriers of nanosized BiVO4 have enhanced after being complexed with a suitable mass of Ag-modified TiO2. Interestingly, the resulting Ag-TiO2/BiVO4 nanocomposites exhibit rather high visiblelight photocatalytic activity for H2 evolution, compared with TiO2/BiVO4 and Ag/BiVO4, while bare BiVO4 nearly has no activities. It is suggested that the utilization of HEL electrons of nanosized-oxides excited by visible light has been improved with Ag-modified TiO2, leading to an efficient visible-light photocatalytic H2 evolution. In this case, Ag-modified TiO2 plays a bifunctional role to accept the excited high-energy-level electrons from oxides by TiO2 and then catalyze the H2 evolution by photoreduced Ag. This uncommon mechanism is believed to afford important influence to the visible-lightdriven photocatalytic hydrogen production, by providing a feasible way to synthesize efficient visible-response oxide-based nanophotocatalysts for solar utilization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03664. XRD, UV−vis spectra, XPS, and SS-SPS of different AgT/BV nanocomposites; XRD, XPS, and SS-SPS of 7AgT/BV nanocomposite before and after photocatalytic reactions; mechanism of transfer and separation of photogenerated HEL electrons on Ag−BV and T/BV; XRD patterns (A), DRS spectra (B), and SS-SPS responses of Ag-T/WO3 and Ag-T/Fe2O3 nanocomposites (PDF)

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*E-mail: [email protected]. TEL:+86 0451-86604760 (Y.Q.). *E-mail: [email protected]. TEL:+86 0451-86604760 (L.J.). E

DOI: 10.1021/acs.jpcc.6b03664 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b03664 J. Phys. Chem. C XXXX, XXX, XXX−XXX