Boosting Charge Separation and Transfer by Plasmon-Enhanced

Apr 13, 2018 - School of Physical Science and Technology, ShanghaiTech University , Shanghai 201210 , China. § University of Chinese Academy of ...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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Boosting Charge Separation and Transfer by Plasmon-Enhanced MoS2/BiVO4 p−n Heterojunction Composite for Efficient Photoelectrochemical Water Splitting Qingguang Pan,†,‡,§ Chi Zhang,† Yunjie Xiong,†,‡,§ Qixi Mi,‡ Dongdong Li,† Liangliang Zou,† Qinghong Huang,† Zhiqing Zou,† and Hui Yang*,†,‡

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Shanghai Advanced Research Institute, Chinese Academy of Sciences, No. 99, Haike Road, Zhangjiang Hi-Tech Park, PuDong, Shanghai 201210, China ‡ School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China § University of Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: The poor separation and significant recombination of electron−hole pairs and slow transfer mobility of charge carriers limit the performance of BiVO4 for photoelectrochemical (PEC) water splitting. To ameliorate the above problems, a novel integrated Ag-embedded MoS2/BiVO4 p−n heterojunction ternary composite electrode is fabricated and applied. Surface plasmon resonance (SPR) of Ag nanoparticles (NPs) by the near-field electromagnetic enhancement or abundant hot electrons injection and p−n heterojunction of MoS2/BiVO4 by the built-in electrical potential synergistically boost the electron−hole pair separation, transfer properties and suppress the recombination of the electron−hole pairs. Consequently, the BiVO4−Ag−MoS2 electrode among four of the BiVO4-based electrodes achieves the largest photocurrent density of 2.72 mA cm−2 at 0.6 V vs RHE, which is 2.44 times higher than that of pure BiVO4 electrode (0.79 mA cm−2), and possesses the largest IPCE of 51% at 420 nm. This work proposes a worthy design strategy for a plasmon enhanced p−n heterojunction for efficient PEC water splitting. KEYWORDS: BiVO4, p−n heterojunction, SPR, Charge separation, Carrier transfer, Water splitting

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INTRODUCTION Exploring new methods of utilizing solar energy is essential to solve problems associated with the energy crisis, sustainability, and environmental emissions of our society. One of the best pathways of harvesting solar energy is photoelectrochemical (PEC) water splitting via semiconductor electrodes, which has the grand prospect of exploiting photon energy to drive reactions directly from water to storable hydrogen energy.1 Recently, a promising candidate, monoclinic scheelite bismuth vanadate (m-BiVO4) with a direct, low band gap energy (Eg) of ∼2.4 eV, has been a research hotspot because of its capable photoactivity in the visible light region for PEC water splitting.2 BiVO4 presents the following virtues: nontoxicity, inexpensiveness, and chemical stability.2 However, several issues need to be addressed, including poor separation of electron− hole pairs,2 slow charge-carrier transfer mobility, short carrier © 2018 American Chemical Society

diffusion lengths, and significant recombination of electron− hole pairs,3 which have limited the performance of BiVO4 for PEC water splitting. Owing to suffering from these problems, BiVO4 is not able to reach its theoretical maximum photocurrent density of 7.5 mA cm−2.4 Recently, various strategies have been investigated to improve the performance of BiVO4, for example, shortening the charge-carrier transfer length by morphology control,5 increasing the active surface ratio by crystal facet studies,6,7 broadening the spectral absorption by metal ion doping,3,8,9 enhancing oxygen evolution kinetics by oxygen evolution catalyst loading,3,5,10 improving the charge separation and transfer by heterojunction engineering,11−14 and Received: January 12, 2018 Revised: March 14, 2018 Published: April 13, 2018 6378

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

Research Article

ACS Sustainable Chemistry & Engineering

processes using a modified method that was initially demonstrated by Kim and Choi in 2014.5 Ag NPs were synthesized by a revised method described by Ruditskiy and Xia in 2016.34 The detailed fabrication process of BiVO4 and Ag NPs are presented in the Supporting Information (SI). Ag NPs-decorated BiVO4 electrodes were prepared using an electrophoretic deposition process. Anodic deposition was potentiostatically performed at 2 V vs fluorine-doped tin oxide (FTO) counter electrode at room temperature for 4−8 min with a two-electrode system to deposit Ag NPs. Then, the BiVO4−Ag electrodes were rinsed with distilled water and dried under ambient environment. Construction of the BiVO4−MoS2 and BiVO4−Ag−MoS2 Heterojunction Electrodes. Similarly, the BiVO4−MoS2 and BiVO4−Ag−MoS2 heterojunction electrodes were prepared using an electrophoretic deposition process. Anodic deposition was potentiostatically conducted at 1 V vs FTO electrode, and the BiVO4-based electrodes were used as a counter electrode at room temperature for 1−5 min with a two-electrode system to deposit the layered MoS2 nanosheets purchased from Nanjing Jcnano Co., Ltd. Then, the BiVO4−MoS2 and BiVO4−Ag−MoS2 electrodes were rinsed with distilled water and dried under ambient environment. Finally, the electrodes were annealed in 10% H2/N2 at 350 °C for 2 h with a ramping rate of 2 °C per min to ensure that the MoS2 nanosheets tightly contacted with the BiVO4-based electrodes. Characterization. The purity and crystallinity of BiVO4 was determined by X-ray diffraction (XRD, Bruker AXS D8 Advance) with Cu Kα radiation (λ= 1.5418 Å). The morphologies of the samples were examined by field-emission scanning electron microscopy (SEM, Hitachi Situation-4800) at an accelerating voltage of 5 kV. Elemental compositions were determined by energy dispersive X-ray spectroscopy (EDX, EDX detector on the Hitachi Situation-4800). Transmission electron microscopy (TEM, JEOL 2100F) images were obtained at an accelerating voltage of 200 kV. The layered MoS2 nanosheets on the Si platform were confirmed by atomic force microscopy (AFM, NT-MDT). The valence states of the elements were determined by X-ray photoelectron spectroscopy (XPS, PHI 5400) using Mg Kα radiation (1253.6 eV). All XPS spectra were calibrated to C 1s = 284.8 eV. UV−vis spectra were recorded using a Cary 5000 UV−vis−NIR spectrophotometer in diffuse reflectance mode. Photoelectrochemical Measurements. All PEC water splitting reaction measurements were carried out with an electrochemical workstation (CHI 760E, CH Instruments Inc.) using a standard threeelectrode system with a flat quartz window cell. The as-prepared BiVO4-based electrodes were used as the working electrode, an Ag/ AgCl (4 M KCl) electrode was used as the reference electrode, and platinum foil was used as the counter electrode in a 0.1 M potassium phosphate buffer with and without 1 M Na2SO3 solution (pH 7). The illumination measurements were conducted under simulated AM 1.5G solar illumination with a 500 W xenon lamp, and the light intensity was calibrated to 100 mW cm−2 by the standard reference of a Newport 91150 silicon solar cell. The linear scanning voltammetry (LSV) curves were obtained with a scan rate of 10 mVs−1. The Mott− Schottky experiments were conducted using a setting frequency of 1000 Hz in the dark. Electrochemical impedance spectroscopy (EIS) measurements were conducted under AM 1.5G (100 mW cm−2) illumination from 100 kHz to 1 Hz. The entire working electrodes were illuminated through the back of the FTO glass. Incident phototocurrent conversion efficiency (IPCE) was measured on a commercial monochromator amplifier assembly (Zolix Solar Cell Scan 100) in a three-electrode cell at 0.6 V vs the reversible hydrogen electrode (RHE) from 380 to 600 nm from a Xe arc lamp referenced to a calibrated standard Si solar cell at room temperature. The gas products were measured using an online gas chromatograph (GC9860, Shanghai Fanwei Equipment Co., Ltd.) and before the equipment for testing, air was thoroughly purged by argon (Ar) for 1 h.

restraining the electron−hole pair recombination by noble metal deposition.15,16 To ameliorate the poor separation of electron−hole pairs, improve the transfer properties, and restrict the recombination of electron−hole pairs, BiVO4based heterojunctions have attracted scientific attention. For example, various heterojunctions, such as Al-doped ZnO/ BiVO4,12 CuWO4/BiVO4,13 and WO3/BiVO4,14 have been constructed. These heterojunctions are composed with two ntype semiconductors with matching band gaps bonded to build a highly efficient heterostructure, in which the most suitable structure is a staggered band gap type (type-II).17 Based on the above considerations, it is more effective for PEC water splitting to apply a p−n-type semiconductor heterojunction with a built-in electrical potential whose direction from an ntype semiconductor to a p-type semiconductor than the n-ntype semiconductor heterojunction, besides benefiting from type-II heterojunctions.18 For the p−n-type semiconductor heterojunction, electrons and holes move in the opposite directions by the built-in electrical potential from the diffusion of electrons and holes in the space-charge region to achieve separation of electron−hole pairs and attain robust transfer properties. For example, BiOI/ BiVO4,18 BiOCl/BiVO4,19 Co3O4/BiVO4,20 NiO/BiVO4,10 and Cu2O/BiVO421 have been identified BiVO4-based p−n-type heterojunction as the efficient architectures to enhance PEC performance; however, these p-type materials do not deliver a broad-spectrum absorption due to large band gap energy. Considering that two-dimensional few-layered molybdenum disulfide (MoS2) nanosheets are a feasible material for heterojunctions, they not only exist suitable band gap energy with a direct Eg of ∼1.9 eV to absorb visible light and match with BiVO4 but also exhibit an ultrahigh specific surface area and numerous exposed active edge sites, in contrast with bulk MoS2 with an indirect Eg of 1.3 eV.22−24 Recently, the MoS2based composite electrodes for the PEC water splitting have been reported,25,26 and MoS2/BiVO4 has been utilized for photocatalytic oxidation and reduction with theoretical support proving its feasibility.23,27,28 However, MoS2/BiVO4 has been rarely studied for PEC water splitting. Furthermore, Ag nanoparticles (NPs) with surface plasmon resonance (SPR) could serve as antennas to control the location of generated charge carriers and localize the optical energy distribution by near-field electromagnetic enhancement or abundant hot electrons injection for PEC water splitting.15,29−33 In this work, we design a novel integrated Agembedded MoS2/BiVO4 p−n heterojunction ternary composite electrode. This electrode possesses a nanoporous n-type BiVO4 using a modified electrodeposition and annealing route to prepare a high specific surface area and shorten diffusion lengths for photoexcited charge carriers. BiVO4 and MoS2 construct a p−n heterojunction to boost its poor electron− hole separation and transport properties using the built-in electrical potential. Simultaneously, the enhanced PEC performance of BiVO4 for the BiVO4−Ag−MoS2 electrode would be verified by the synergistic effect of the p−n heterojunction and SPR of Ag NPs. This work might prompt further exploration of diverse plasmon enhanced p−n heterojunctions devoted to PEC water splitting and photocatalysis.

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EXPERIMENTAL SECTION

Preparation of the BiVO4−Ag Electrodes. The BiVO4 electrodes were fabricated by electrochemical deposition and annealing 6379

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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ACS Sustainable Chemistry & Engineering

Figure 1. Electron microscopic characterization of the BiVO4, BiVO4−MoS2, BiVO4−Ag, and BiVO4−Ag−MoS2 electrodes. SEM top-view images of (a) BiVO4, (b) BiVO4−MoS2, (c) BiVO4−Ag, (e) BiVO4−Ag−MoS2 and cross-sectional views image of (d) BiVO4; (f) SEM image of BiVO4−Ag−MoS2 and the corresponding SEM-EDX elemental mapping images for S, Mo, Ag, Bi, V, O; (g) TEM (inset is HRTEM) image of Ag NPs, (h) TEM image of MoS2 and (i) AFM image of MoS2.

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RESULTS AND DISCUSSION The columnar particles of BiVO4 with a thickness of 1.2 μm (Figure 1e) were prepared on FTO substrates from SEM topview images of the BiVO4 electrode, as well as the electrodes decorated with MoS2, Ag, and Ag/MoS2 (Figure 1a−c,e). Ag NPs, as indicated by the red dashed arrow (Figure 1c,e), are dispersed on the BiVO4 electrodes with a mean size of 50 nm (Figure 1g), which presents a lattice spacing of 0.14 nm corresponding to the (220) planes of Ag (JCPDS #01-1164) (Figure 1g, inset image). MoS2 layered structure of ∼1 nm confirmed by TEM (Figure 1h) and AFM (Figure 1i), as indicated by the blue dashed rectangle (Figure 1b,e), is decorated on the BiVO4 and BiVO4−Ag electrodes to construct MoS2/BiVO4 heterojunction. However, the part of the layered MoS2 nanosheets aggregated when the composite electrodes were annealed in 10% H2/N2 at 350 °C for 2 h, but there are still the few-layered MoS2 sheets even after they aggregate (Figure 1f). The elemental mapping images for S, Mo, Ag, Bi, V, O of the BiVO4−Ag−MoS2 electrode were verified by EDX from SEM image (Figure 1f), and the signals of the different elements are clearly observed.

The purity and highly crystalline nature of the nanoporous monoclinic scheelite structure were confirmed by sharp peaks of XRD pattern (Figure 2a), along with SnO2 peaks originating from FTO substrate. Figure 2b−d display XPS spectra of Ag 3d, Mo 3d, and S 2p for the BiVO4−Ag−MoS2 electrode, respectively. The characteristic Ag 3d3/2 and 3d5/2 peaks are located at 273.8 and 267.8 eV, which corresponds to metallic Ag with a surface concentration of 3%. Additionally, the Mo 3d3/2, Mo 3d5/2, S 2p1/2, and S 2p3/2 peaks are located at 232.1, 229.0, 162.8, and 161.8 eV, respectively, which are lower than those values reported for pure MoS2 (232.5, 229.3, 163.3, and 162.3 eV, respectively).35 The binding energies of the BiVO4−Ag−MoS2 electrode decrease comparing to those of pure MoS2 due to the formation of the MoS2/BiVO4 p−n heterojunction.28 In the corresponding survey scan (Figure S1), the Bi 4f, V 2p, and O 1s peaks for the BiVO4−Ag−MoS2 electrode agree with previous reports.2 The significance of the BiVO4 electrodes decorated with MoS2, Ag, and Ag/MoS2 was confirmed by the systemic PEC performance of the BiVO4-based electrodes in 0.1 M phosphate buffer (pH 7) within and without 1 M Na2SO3 aqueous 6380

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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Figure 2. (a) XRD pattern of BiVO4. Vertical lines indicate the JCPDS diffraction peaks of FTO (blue) and monoclinic scheelite BiVO4 (red, JCPDS #14-0688). XPS spectra of (b) the Ag 3d peaks, (c) Mo 3d peaks, and (d) S 2p peaks of the BiVO4−Ag−MoS2 electrodes.

1 M Na2SO3. A comparison of the results (Figure 3a) reveals that the BiVO4−Ag−MoS2 electrode possesses the largest photocurrent densities of 4.16 and 5.94 mA cm−2 at 0.6 and 1.23 V vs RHE, respectively (Figure 3b). Applied bias phototo-current efficiency (ABPE) of the BiVO4-based electrodes was calculated using LSV curves (Figure 3a) with practical Faradaic efficiency for H2 evolution (see Figure 5b),5,37 and the results are plotted in Figure 3c. The BiVO4−Ag−MoS2 electrode shows a much larger PEC water splitting efficiency of 1.50% at a much smaller bias voltage of 0.57 V vs RHE, and this electrode achieves the highest PEC water splitting efficiency of 2.67% at 0.53 V vs RHE in the presence of the hole scavenger (Figure S4) than others’ BiVO4based electrodes, which is an extremely beneficial characteristic for constructing a PEC diode or a tandem cell.5 Surface-charge-transfer efficiency of water oxidation [ηtrans (H2O)] was calculated from LSV curves (Figure 3a, b)5,36,38 and the curves were plotted in Figure 3d. ηtrans (H2O) of 67% for the BiVO4−Ag−MoS2 electrode at 1.23 V vs RHE display the best performance, followed by 59% for the BiVO4−Ag electrode, 54% for the BiVO4−MoS2 electrode, and 43% for the pure BiVO4 electrode. In addition, charge separation efficiency (ηsep) was calculated from IPCE curves with the standard solar spectrum of the electrodes and plotted (Figure S5), which represents the yield of electron−hole pair separation. The BiVO4−Ag−MoS2 electrode achieves the greatest ηsep of 75% at 1.23 V vs RHE, followed by 70% for the BiVO4−Ag and BiVO4−MoS2 electrodes, 62% for the pure BiVO4 electrode. These indicate MoS2/BiVO4 heterojunction can improve surface-charge-transfer efficiency of PEC water splitting for electron−hole pair separation and charge carrier transport at the interfaces of BiVO4 and MoS2 by the built-in potential of MoS2/BiVO4 heterojunction. Moreover, electrodes decorated with Ag NPs demonstrate a more superior catalysis property owing to SPR effect of Ag NPs.

solution as a hole scavenger, in the dark or under AM 1.5G illumination (Figure 3). Figure 3a compares LSV curves of the BiVO4, BiVO4−MoS2, BiVO4−Ag, and BiVO4−Ag−MoS2 electrodes in 0.1 M phosphate buffer (pH 7). The pure BiVO4 electrode reveals photocurrent densities of 0.79 and 2.14 mA cm−2 at 0.6 and 1.23 V vs RHE, respectively, that are much larger than those in previous reports5,36 owing to the thermal pretreatment to promote the crystal growth and create the probably increased active areas. Both the BiVO4−Ag and BiVO4−MoS2 electrodes display larger photocurrent densities than that of the pure BiVO4 electrode under the same conditions due to the decoration with Ag NPs or MoS2. The photocurrent densities of the BiVO4−Ag−MoS2 electrode are the highest among all the BiVO4-based electrodes, reaching 2.72 and 4.02 mA cm−2 at 0.6 and 1.23 V vs RHE, respectively, which are 3.44 and 1.88 times compared with those of the pure BiVO4 electrode. The optimal contents of the Ag NPs and MoS2 were chosen on the basis of the results (Figure S2), and the optimal deposition time of Ag NPs and MoS2 were 6 and 3 min, respectively. All the electrodes used for testing were prepared under the optimal deposition time. Examining the photocurrent density for sulfite oxidation can provide information on the PEC performance of the BiVO4-based electrodes irrespective of its poor water oxidation kinetics because sulfite oxidation is kinetically and thermodynamically easier than water oxidation. In sulfite oxidation, the rate of charge transfer to the electrolyte interface is very quick, and thus, surface recombination of charges can be considerable negligible. Thus, the PEC performance were further measured containing 1 M Na2SO3 as a hole scavenger (Figure 3b). Surface-charge-transfer efficiency of sulfite oxidation [ηtrans (Na2SO3)] are close to 100%,2,5 as confirmed in Figure S3, and the photocurrent densities of the BiVO4-based electrodes are stable under potentiostatic conditions containing 1 M Na2SO3 as a hole scavenger; however, the photocurrent densities of the BiVO4-based electrodes gradually decay without 6381

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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Figure 3. PEC performance of the BiVO4 (black), BiVO4−MoS2 (green), BiVO4−Ag (red), and BiVO4−Ag−MoS2 (blue) electrodes. LSV curves in the dark (dashed lines) or under AM 1.5G (100 mW cm−2) illumination (solid lines) in 0.1 M phosphate buffer (pH 7) (a) without and (b) with 1 M Na2SO3 as a hole scavenger. (c) ABPE curves calculated from (a) LSV curves. (d) Surface-charge-transfer efficiency (ηtrans) calculated from (a,b) LSV curves. (e) UV−vis absorption spectra (inset is the corresponding Tauc plot of the BiVO4 electrode) and (f) IPCE spectra collected at an incident wavelength range from 380 to 600 nm (inset shows the wavelength range from 500 to 600 nm) at 0.6 V vs RHE in 0.1 M phosphate buffer (pH 7).

photoactive range of the BiVO4-based electrodes cover 380− 550 nm, slight shorter than light absorption spectra but longer than that of the pure BiVO4 electrode. Evidently, IPCE of the BiVO4−Ag−MoS2 electrode reaches 51% at 420 nm, which is superior to those of the BiVO4−Ag (32%), BiVO4−MoS2 (24%), and pure BiVO4 (18%) electrodes. Similarly, IPCE of the BiVO4−Ag−MoS2 electrode is ∼75% at 420 nm, superior to those of the BiVO4−Ag, BiVO4−MoS2 and pure BiVO4 electrodes in the presence of the hole scavenger (Figure S7). In addition, APCE is used to represent the efficiency of photocurrent collected per incident photon absorbed.37 The BiVO4−Ag−MoS2 electrode exhibits the highest APCE of 57% and 86% at 420 nm among all the BiVO4-based electrodes without and with 1 M Na2SO3 as a hole scavenger, respectively (Figure S8). As stated above, the absorption spectra of the BiVO4 electrodes could be broadened, and the photon-tocharge conversion efficiency could be improved by introducing Ag and MoS2 owing to the synergistic beneficial effect of SPR excitation of Ag NPs and p−n heterojunction of MoS2/BiVO4.

The light absorption ability and the photon-to-charge conversion efficiency of the BiVO4-based electrodes are considered to evaluate the effect of incorporating Ag NPs and MoS2. The absorption edges of the BiVO4 electrodes are at ∼505 nm, and the Eg of the BiVO4 electrode is approximately 2.46 eV from the corresponding inset Tauc plots in agreement with a previous report (Figure 3e).5 At wavelengths higher than 505 nm, the BiVO4−Ag, BiVO4−MoS2, and BiVO4−Ag−MoS2 electrodes demonstrate slightly larger absorption intensity than the pure BiVO4 electrode due to the absorption from MoS2 and SPR absorption of Ag NPs because the absorption of Ag NPs covered from 300 to 650 nm and the absorption peak was located at 420 nm (Figure S6a). Meanwhile, IPCE is measured to understand the photon-to-charge conversion efficiency, involving photon absorption, charge-carrier excitation, electron−hole pair separation, charge transport from the solid to the solid−liquid interface, and interfacial charge transfer across the solid−liquid interface, to provide understanding of the PEC performance of the electrodes.37 IPCE was measured at 0.6 V vs RHE in 0.1 M phosphate buffer (pH 7) (Figure 3f). The 6382

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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Figure 4. (a) EIS Nyquist plots of the BiVO4 (black), BiVO4−MoS2 (green), BiVO4−Ag (red), and BiVO4−Ag−MoS2 (blue) electrodes in 0.1 M phosphate buffer (pH 7). (b) Mott−Schottky plots of the BiVO4 (black), BiVO4−MoS2 (green), BiVO4−Ag (red), and BiVO4−Ag−MoS2 (blue) electrodes in 0.1 M phosphate buffer with 1 M Na2SO3 (pH 7).

Figure 5. Operational stability of the BiVO4−Ag−MoS2 electrode. (a) The J−t curve collected at 1.23 V vs RHE under AM 1.5G illumination in 0.1 M phosphate buffer (pH 7). (b) Detection of H2 and O2 evolution at 1.23 V vs RHE (dotted) and the calculated H2 and O2 evolution from curve a (dashed lines).

The charge-transfer kinetics in the heterostructured composites and the interfacial properties between the electrodes and electrolyte were investigated by EIS responses of BiVO4-based electrodes at 0.6 V vs RHE under illumination (AM 1.5 G) (Figure 4a). EIS Nyquist plots fitted according to the equivalent Randles circuit model, as shown in the right inset of the figure, where RS, CPE, and RCT represent the solution resistance, constant phase element, and charge-transfer resistance at the interface of semiconductor and electrolyte, respectively.39 The fitted RS values are 32, 28, 26, and 25 Ω cm2 for the pure BiVO4, BiVO4−MoS2, BiVO4−Ag, and BiVO4− Ag−MoS2 electrodes, respectively, which demonstrates that the interfaces among BiVO4, Ag and MoS2 decreased the solution resistance (Table S1). The fitted RCT values extracted from semicircle radii are 448, 295, 243, and 195 Ω cm2 for the pure BiVO4, BiVO4−MoS2, BiVO4−Ag, and BiVO4−Ag−MoS2 electrodes, respectively (Table S1). The RCT values of the BiVO4−MoS2 and BiVO4−Ag electrodes are smaller than that of the pure BiVO4 electrode. The RCT of the BiVO4−Ag−MoS2 electrode exhibits the lowest value among all the BiVO4-based electrodes because the p−n heterojunction of MoS2/BiVO4 and SPR excitation of Ag NPs promote charge transfer at the interface of the semiconductor and electrolyte. Similar results are demonstrated in Figure S10a with measurements in the presence of the hole scavenger. The intrinsic electronic structures of the electrodes were analyzed by Mott−Schottky

plots. As shown in Figure 4b, the Mott−Schottky plots of the four different electrodes in 0.1 M phosphate buffer (pH 7) with 1 M Na2SO3 were measured. If surface charge recombination is ignored, the BiVO4−Ag−MoS2 and BiVO4 electrodes show the smallest and largest photocurrent onset potential among four different BiVO4-based electrodes, respectively, indicating that the electrodes with Ag NPs and MoS2 decoration are easier to react. Meanwhile, the flat-band potential (EFB) approaches the photocurrent onset potential for sulfite oxidation due to fast oxidation kinetics,5 and thus, we can deduce that the BiVO4− Ag−MoS2 electrode has the lowest EFB, and the BiVO4 electrode has the highest EFB, where the flat-band potential was obtained from the x-axis intercept. In addition, the charge carrier density (ND) in these electrodes can be calculated from the slopes of the Mott−Schottky plots.2 The ND values of the BiVO4, BiVO4−MoS2, BiVO4−Ag, and BiVO4−Ag−MoS2 electrodes gradually increase from 1.85 × 1018 cm−3 to 2.74 × 1018 cm−3 (Table S2). Meanwhile, the Fermi level of BiVO4 for the BiVO4−Ag-MoS2, BiVO4−Ag, and BiVO4−MoS2 electrodes will increase as the charge carrier density increases, which causes increased effective band bending in the spacecharge region than pure BiVO4, owing to a bigger difference between the redox potential of the electrolyte and the Fermi level of BiVO4.2 The water splitting results are shown in Figure S10b. 6383

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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Figure 6. Spatial distributions of the electric field by FDTD simulations for (a) the BiVO4−Ag and (b) BiVO4−Ag−MoS2 electrodes with excitation wavelengths of 420 nm. Incident light is along the z-axis direction. (c) Linear distributions of the electric field at x = 40 nm along the z axis of the BiVO4−Ag and BiVO4−Ag−MoS2 electrodes (red dashed lines in Figure 7a,b). (d) Linear distributions of the electric field at Z = 50 and 100 nm along the x axis of the interfaces of the BiVO4−Ag and BiVO4−Ag−MoS2 electrodes (yellow dashed lines in Figure 7a,b). The color bar shows the electric field intensity.

The BiVO4−Ag−MoS2 composite after the PEC test is examined by SEM (Figure S13) to study the impact of the prolonged photoelectrolysis on morphology. The main structure of the BiVO4−Ag−MoS2 composite after the tests does not significantly change, but it is rougher than that before the tests (Figure 1e), and the sizes of BiVO4 and Ag NPs only increase a little, which might decrease the activity site density and be the reason for the decay of activity as shown in the durability test. The physicochemical properties are characterized after the PEC. As shown in Figure S14, all the diffraction peaks of BiVO4 still exist after the PEC test, indicating that PEC tests do not change the crystal phase of BiVO4, while no obvious peaks for MoS2 and Ag NPs could be detected in the composite before and after the test, probably due to the high dispersion and small percentage of MoS2 and Ag NPs. Figure S15 shows XPS spectra of Ag 3d, Mo 3d, and S 2p. One could not observe an obvious shift in the spin orbital peaks although their contents become smaller than those before test (Figure 2b−d), thus demonstrating the stable physicochemical states for MoS2 and Ag NPs after PEC tests. The related possible enhancement mechanisms of the p−n heterojunction and SPR of Ag NPs are further elaborated.

As shown in Figure 5a, the operational stability of the BiVO4−Ag−MoS2 electrode was tested by chronoamperometry (J−t curve) at 1.23 V vs RHE under AM 1.5G illumination in 0.1 M phosphate buffer (pH 7). The H2 and O2 evolution are measured by integration of the GC (Figure 5b), and the total amounts of H2 evolution and O2 evolution are 578 and 297 μmol after a 10 h test. The molar ratio of produced H2/O2 is 1.95:1 with about 90% and 88% Faradaic efficiencies for O2 and H2 evolution, respectively. Such a ratio of H2/O2 is very close to the theoretical one of 2:1 and expounds that nearly no sidereaction occurred during water splitting. Besides, the operational stability of the BiVO4, BiVO4−Ag, and BiVO4−MoS2 electrodes are shown in Figure S12. The amounts of H2 and O2 evolution increase after Ag NPs or MoS2 decorated on BiVO4. Thus, Ag NPs and MoS2 combined with BiVO4 could efficiently promote OER, consistent with PEC performance as shown in Figure 3. After BiVO4 was decorated with MoS2 and Ag NPs, the photocurrent stability of the BiVO4-based electrodes is enhanced and shows the superlative stability with a decay of approximately 14% after 10 h (Figure 5), which demonstrates that MoS2/BiVO4 p−n heterojunction and SPR effect of Ag NPs boost a collaborative enhanced PEC stability. 6384

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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ACS Sustainable Chemistry & Engineering

Figure 7. Schematic diagrams of the proposed band gap diagram of BiVO4, Ag, and MoS2 (a) before and (c) after contact with each other and (b) formation of the MoS2/BiVO4 p−n heterojunction. Evac, vacuum energy; Ef, Fermi energy; CB, conduction band; VB, valence band; ϕm, metal work function; ϕSB, semiconductor work function; χs, electron affinity of the semiconductor.

staggered band gap, and it exists as the built-in potential between the CBM of BiVO4 and MoS2 (Figure 7b).23 As presented by the schematic diagrams of the proposed band gap diagrams in Figure 7c. The photogenerated electrons could be excited from the VB into the CB, and electrons transfer from the CB of MoS2 to the CB of BiVO4 and accumulate on the CB of BiVO4 for the hydrogen evolution reaction on the Pt counter electrode. At the same time, the holes for the oxygen evolution reaction could transfer from the VB of BiVO4 to the VB of MoS2, which could readily transfer charge carriers and transfer the photogenerated electrons from MoS2 to the BiVO4 surface, resulting in effective separation (Figure 7b,c). Briefly, the separation, recombination, and interfacial transfer of photoinduced charge carriers could be rewardingly influenced by the built-in potential and matched band gap energy. Further, Ag NPs are embedded in MoS2/ BiVO4 heterojunction. The Schottky barriers could be formed when the metal and semiconductor combine, and then, the band edge energies of the semiconductors could be constantly shifted due to the electric field at the interface, leading to charge transfer (i.e., band bending) in the space-charge region. The Fermi energy (Ef) of the different nanostructures would attain the same level,41 which would prevent back electron transfer to Ag NPs and favor electron accumulation in the CB of MoS2 or BiVO4. Meanwhile, under light illumination, Ag NPs absorb resonant photons, and then, energetic hot electrons would be generated and induce an enhanced local electric field from SPR excitation.31 The local electric field would facilitate the separation of electron−hole pairs and carrier transport from the Ag NPs to BiVO4 or MoS2. The hot electrons would pass over the Schottky barrier at the BiVO4/Ag and MoS2/Ag interfaces and inject into the CB of BiVO4 and MoS2 to promote charge-carrier accumulation and contribute to an escalated charge carrier concentration in the two kinds of semiconductors; thus, SPR of Ag NPs would further enhance the built-in potential of p−n heterojunction. Finally, the prominent separation and delayed recombination of electron− hole pairs and rapid charge carrier mobility would be achieved owing to the MoS2/BiVO4 heterojunction with a staggered

Finite difference time domain (FDTD) simulations were implemented to simulate the electric field spatial distributions across the interfaces of the different substances and understand the energy-transfer processes as a function of the incident light wavelength to explore the potential mechanism of SPR excitation of Ag NPs enhancing the PEC water-splitting performance. Considering the complexity of the structure and simulation, Figures 6 and S16 show the representative FDTD simulations of the most probable construction for the BiVO4− Ag and BiVO4−Ag−MoS2 electrodes at an excitation wavelength of 420 nm, which corresponds to the absorption peak of Ag NPs (Figure S6a). The electric field intensity at Ag and BiVO4 interface is higher than that at Ag and MoS2 interface because light radiates from the back of BiVO4 (Figures 6a−c and S16), and the electric field possesses a spatially inhomogeneous distribution with the highest intensity near the interface of the nanostructures, reducing with the distance from the interface. This result would deduce the function of plasmonic Ag NPs as antennas to control the location of generated charge carriers and localize the optical energy distribution by near-field electromagnetic near the interface of the nanostructures (Figure 6a,b,d). Meanwhile, hot-electron injection might play a dominant role at the interface of the nanostructures under the enhanced electric field. Furthermore, the enhancement mechanism of the p−n heterojunction is elucidated. The Eg of 2.42 eV for m-BiVO4 estimated by the hybrid HSE06 functional agrees well with the experimental result (2.46 eV).23,40 Few-layered MoS2 is a direct Eg (1.92 eV) semiconductor, consistent with experiments (1.90 eV).23,24 Because of the introduced strains and the van der Waals (vdW) interactions of MoS2 (001) (1.64 eV) and mBiVO4 (010) (2.24 eV) surface, the Eg of the MoS2 (001)/mBiVO4 (010) heterojunction narrows to 2.05 eV, thus achieving a typical band gap value for visible-light-driven photoactivity.23,24,40 The partial density of states (PDOS) of the structures shows the valence band maximum (VBM) mainly originates from the MoS2 states, while the conduction band minimum (CBM) is dominated by BiVO4 states.23 The MoS2/ BiVO4 heterojunction forms a type-II band alignment with a 6385

DOI: 10.1021/acssuschemeng.8b00170 ACS Sustainable Chem. Eng. 2018, 6, 6378−6387

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ACS Sustainable Chemistry & Engineering band gap and built-in potential whose direction from n-type BiVO4 to p-type MoS2, along with SPR enhancement of Ag NPs by the near-field electromagnetic enhancement or abundant hot electrons injection.

Ag−MoS2 electrodes before and after PEC tests; Figure S14. XRD pattern of the BiVO4−Ag−MoS2 electrode before and after PEC tests; Figure S15. XPS spectra of survey, the Ag 3d peaks, Mo 3d peaks and S 2p peaks of the BiVO4−Ag−MoS2 electrodes after PEC tests; Figure S16. Spatial distributions of the electric field by FDTD simulations, Table S1. The fitted results of EIS data, and Table S2. Carrier densities (ND) of the different electrodes (PDF)

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CONCLUSIONS In conclusion, the BiVO4-based electrodes were designed and confirmed to possess enhanced PEC water splitting activity. The electrodes were prepared using a convenient electrodeposition and electrophoretic deposition process. The PEC performance of the BiVO4−Ag−MoS2 electrode is noticeably enhanced comparing to the BiVO4 electrode. More facile water oxidation is achieved, displaying the lowest onset potential and largest photocurrent density of 2.72 mA cm−2 at 0.6 V vs RHE among all the BiVO4-based electrodes, which is 2.44 times larger than that of the pure BiVO4 electrode (0.79 mA cm−2). In addition, we deduce the effective separation of electron− hole pairs (ηsep of 75% at 1.23 V vs RHE), the rapid transfer mobility of charge carriers (ηtrans of 67% at 1.23 V vs RHE), and the prominent photon-to-current conversion efficiency (IPCE of 51% and APCE of 57% at 420 nm) that is due to the synergistic effect between SPR of Ag NPs and p−n heterojunction of MoS2/BiVO4. It is noteworthy that the built-in potential from BiVO4 to MoS2 and matched band gap energy of MoS2/BiVO4 p−n heterojunction, as well as the nearfield electromagnetic enhancement and hot-electrons injection of Ag NPs with SPR excitation would play a crucial role for enhanced catalytic performance. Furthermore, the plasmon enhanced BiVO4-based p−n heterojunction electrodes are promising research subjects for PEC water splitting.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hui Yang: 0000-0001-5013-0469 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research programs (2017YFA0206500), the National Natural Science Foundation of China (21533005, 21503262)

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00170. Further details of the benchmark electrodes fabrication simulation and calculation; Figure S1. XPS spectra of Survey scan, Bi 4f peaks, V 2p peaks and O 1s peaks of the BiVO4−Ag−MoS2 electrode; Figure S2. LSV curves of the BiVO4−Ag electrodes with different electrophoresis time (4, 6, 8 min) and the BiVO4−Ag−MoS2 electrodes with different electrophoresis time (1, 3, 5 min) without and with 1 M Na2SO3; Figure S3. J−t curves of the different electrodes at 1.23 V vs RHE without and with 1 M Na2SO3; Figure S4. ABPE curves of the different electrodes with 1 M Na2SO3; Figure S5. ηsep of the different electrodes; Figure S6. UV−vis absorption spectrum of Ag NPs and light harvesting efficiencies (ηAbs) of the different electrodes; Figure S7. The corresponding Tauc plots of the different electrodes; Figure S8. IPCE spectra with 1 M Na2SO3 of different electrodes, Figure S9. APCE spectra without and with 1 M Na2SO3 of the different electrodes; Figure S10. Calculated photocurrent densities curves corresponding to different wavelength and the standard solar spectrum of different electrodes; Figure S11. EIS Nyquist plots of the different electrodes with 1 M Na2SO3 and Mott− Schottky plots of the different electrodes; Figure S12. Operational stability of the BiVO4, BiVO4−Ag and BiVO4−MoS2 electrodes. J−t curves and H2 and O2 evolution; Figure S13. SEM images of the BiVO4− 6386

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