BiVO4 Hybrid Nanostructure

Subscriber access provided by UNIVERSITY OF SASKATCHEWAN LIBRARY

Article 2

3

4

In-situ noble fabrication of BiS/BiVO hybrid nanostructure through photoelectrochemical transformation process for solar hydrogen production Mahadeo A. Mahadik, Hee-Suk Chung, Su Yong Lee, Min Cho, and Jum Suk Jang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03140 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 45 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

ACS Sustainable Chemistry & Engineering

In-situ noble fabrication of Bi2S3/BiVO4 hybrid nanostructure through photoelectrochemical

transformation

process

for

solar

hydrogen

production Mahadeo A. Mahadika, Hee-Suk Chungb, Su-Yong Leec, Min Choa,* and Jum Suk Janga,* a

Division of Biotechnology, Safety, Environment and Life Science Institute, College of

Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea. b

Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do, 54907, Republic of Korea c

Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea.

*Corresponding authors.

Tel.: +82 63 850 0846; fax: +82 63 850 0834.

E-mail addresses: [email protected] (J.S. Jang), [email protected] (Min Cho)

Abstract Here, we establish a methodology and phenomena for systematically assessing the photoelectrochemical transformation of the Bi2S3/BiVO4 hybrid heterostructure during in-situ solar hydrogen generation. The photoelectrochemical transformation involves a facile anion exchange process by reacting BiVO4 photoelectrodes with Na2S/Na2SO3 electrolyte. X-ray photoelectron spectroscopy and transmission electron microscopy analyses confirmed the successful transformation of BiVO4 into the Bi2S3/BiVO4 nanostructure matrix. The photocurrent density of the ABV3 photoelectrode is optimized to be 3.3 mA.cm−2 and

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

hydrogen generation activity (~417 µmol cm-2 h-1) under simulated sunlight at 0.67 V vs RHE. The impact of other factors such as crystal structure, enhancing light harvesting and increasing electron-hole separation on the photoelectrochemical performance and stability of Bi2S3/BiVO4 hybrid photoanodes are highlighted. A model based on the surface transformation is also proposed to explain the growth and change transport phenomenon in Bi2S3/BiVO4 hybrid nanostructured photoanodes. These findings are expected to shed light on further understanding and design of novel strategy of engineering of electronic band structure through photoelectrochemical transformation, which plays a key role in the enhanced performance of hybrid nanostructures in the fields of energy conversion.

Keywords: BiVO4 ternary oxides; Photo-electro-transformation; heterojunction; hydrogen generation.


Page 2 of 45

Page 3 of 45 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


Introduction Hydrogen is one of the clean and renewable energy fuel, which has been perceived as a source of energy for the future and is receiving increased attention to overcome the possible energy crisis and environmental concerns.1-3 Apart from the water spitting, interest in the production of hydrogen from hydrogen sulphide (H2S) continues unabated, due to the production of large quantities of toxic H2S as a byproduct in the coal and petroleum industries.4 Additionally, to deal with H2S there has been numerous attempts devoted to photocatalytic hydrogen production from H2S5-7, but the conversion efficiency is still low. Also, compared with the photooxidation kinetics in water, the sulfide and sulfite ions decompose at lower energy and can be realized as a sacrificial agent during the photocatalytic hydrogen production.8 Metal oxides have been focused as photoanode materials in PEC hydrogen generation, due to their better stability under oxidizing conditions.9 However, most of the binary metal oxides such as TiO210 and WO311 have not achieved high efficiency because of their poor light harvesting due to large band gaps and poor charge transport properties resulting from low mobility and/or short carrier lifetimes.12 On the other hand, low band gap materials such as Fe2O3 (Eg 2.2 eV) have a conduction band edge farther from the vacuum level than desired for a PEC photoanode and led to extremely poor charge transport properties.11,13 To overcome these limitations of binary metal oxides, recent research in the context of solar-to-chemical energy conversion, the ternary oxide BiVO4 has been attracting attention as they provide the moderate band gap of ~2.4 eV. This band gap permits promising visible light absorption, conduction, and valence band edges (ECB and EVB) (~4.77 and 7.27 eV) that are relatively less than the vacuum level of water redox potentials and moderate charge transport properties.14-16 This signifies that BiVO4 could be resistant to photo-corrosion under



water splitting conditions.17 Despite the band positions and band gaps, it has been demonstrated that the photoelectrochemical properties of BiVO4 could be adjusted by changing its structure.18 However, due to the relatively fast recombination of photogenerated electron–hole pairs, the photoactivity of BiVO4 alone is very poor.19 To address these issues, several strategies such as hydrogen reduction,20 doping with foreign elements21 have been developed to improve the photoelectrochemical activity of BiVO4. Thus, in order to improve the photoelectrochemical performance of the BiVO4 photoanode, there is need to shorten the distance of photogenerated holes which are collected at the electrode/solution interface. In addition to the metal oxides, sulfides possess relatively high conduction band positions suitable for H2O splitting and better sunlight absorption than oxides and hence emerged as a new promising class of active materials that exhibit higher photocatalytic activity.22 This provides lower chances of the electrons-holes recombination before the holes participating in electrochemical reactions.23 Among the metal sulfides, due to its large absorption coefficient (104-105 cm−1) and reasonable incident photon to current conversion efficiency,24 Bi2S3 with a narrow direct bandgap (1.3-1.7 eV) has attracted great interest as a semiconductor material for applications in photovoltaic cells. The ECB and EVB values of Bi2S3 correspond to 0.08 and 1.48 eV vs. NHE, respectively.25 However, the photo-corrosion of metal sulfide is a challenging problem that hinders the use of metal sulfide for photoelectrochemical (PEC) water splitting. Therefore, to solve the problem of photo-corrosion of metal sulfide and light absorption in transition metal oxides, the fabrication of metal sulfide and proper metal oxides heterostructure is of great interest. The band positions of both components indicate that the conduction band of Bi2S3 is less anodic than BiVO4, while its valence band is more cathodic than BiVO4 and has acts as a sensitizer for BiVO4 in PEC and photovoltaic cells.26 Although there are few approaches to fabricate the Bi2S3/BiVO4 heterostructure, however, these were usually prepared by a simple coating structures. For example, Liu et al reported the BiVO4–


Page 4 of 45

Page 5 of 45 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


Bi2S3 heterojunction by two step methods (drop casting and hydrothermal method). However, in this case, even though they reported the enhanced photocurrent density in 0.35 M Na2SO3 and 0.25 M Na2S electrolyte, but the photocurrent stability is very poor. 27 Also, Wang et al. prepared the slurry of BiVO4 nanosheet/Bi2S3 nanorod heterostructures using two step hydrothermal methods and then the prepared slurry deposited on FTO by doctor-blade method. The PEC measurements was very low under 300 W Xe lamp in 0.1 M Na2SO4, however they reported very lower photocurrent density (1.2 µA.cm-2 at 0.1V vs. Ag/AgCl).28 Moreover, the formation of the contact quality at the interface of Bi2S3/BiVO4 heterojunction and understanding of the factors governing stability during photoelectrochemical hydrogen generation remains a question to be studied. The design of new heterostructured photocatalysts using a simple, efficient technique is an existing challenge in materials science and chemical engineering fields.29 The in-situ fabrication of Bi2S3/BiVO4 heterostructure photoanodes from photoelectrochemical transformation of BiVO4 and rationally tune their interface quality during PEC solar hydrogen generation have not been reported yet. The present work reports, for the first time, a methodology and phenomena for systematically assessing the in situ photoelectrochemical transformation of BiVO4 to Bi2S3/BiVO4 hybrid heterostructure and it’s photoelectrochemical stabilities PEC solar hydrogen generation. Firstly, due to suitable morphology of BiVO4 films, the photogenerated holes can travel a shorter distance to be collected at the electrode/solution interface for participating in electrochemical reactions. Secondly, during photoelectrochemical solar hydrogen generation, the BiVO4 photoelectrodes transform into Bi2S3/BiVO4 hybrid heterostructure through the controlled vanadium leaching into the electrolyte and sulfide indiffusion. The amount of the Bi2S3 phase on the surface of the hybrid nanostructure help to produce more photogenerated charge carriers in the hybrid structure and amount of the Bi2S3 can be easily adjusted during the photoelectrochemical hydrogen generation process. XPS



and FIB-TEM analyses indicate that during PEC process vanadium from BiVO4 loss into solution and in-diffusion of sulfur from the electrolyte causes structural destabilization and formation of Bi2S3/BiVO4 hybrid nanostructure. Our results reveal that the transforming surface of drop coated BiVO4 into Bi2S3/BiVO4 hybrid nanostructure leads to engineering the interfaces of promising new semiconductors for solar hydrogen generation applications. A comparison of the photoelectrochemical activity of photoelectrochemical transformed Bi2S3/BiVO4 heterostructures for different Bi to V (Bi/V) molar ratios was made. The ABV3 hybrid nanostructure photocatalysts exhibit significantly improved photoelectrochemical response (3.3 mA.cm−2 at 0.67 V vs RHE) during solar hydrogen generation among all other structures built by this process in the Na2S/Na2SO3 aqueous solution. Also, the mechanisms of the photoelectrochemical transformation of BiVO4 to Bi2S3/BiVO4 hybrid heterostructure and the associated charge transfer during the solar hydrogen generation have been proposed.

Experimental Synthesis of BiVO4 nanostructured thin films BiVO4 films were prepared using a modified metal–organic decomposition method.30 Various concentrations (0.025 M, 0.05 M, and 0.1 M) of Bi(NO3)3.5H2O (99.8%, Kanto Chemicals) were dissolved in acetic acid (99.7%, Kanto Chemicals) with the assistance of ultrasonication for 10 min, and marked as a solution (A). The corresponding amount of vanadyl acetylacetonate (C10H14O5V) (98.0%, Sigma–Aldrich) was then prepared in the acac=acetylacetonate and marked as the solution (B). Then to form 1:1 molar ratio of Bi/V, the specific amount of solution B was added dropwise to solution A under vigorous stirring. For the fabrication of the BiVO4 film onto FTO glass, the 85 µL of as prepared resulting mixture of Bi/V solution ( solution A + solution B) was drop coated on 1 × 2.5 cm2 FTO and dried for 30 min under ambient atmosphere. This drop coating process was repeated twice to


Page 6 of 45

Page 7 of 45 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


acquire the desired film thickness. This as-prepared film was then heated at 100 °C in an electric oven for 10 min and then quenched at 470 °C for 30 min to form a BiVO4. These deposited films were further labeled as BV1, BV2, and BV3, corresponding to the concentrations (0.025 M, 0.05 M, and 0.1 M) of Bi(NO3)3.5H2O respectively.

Photoelectrochemical transformation of BiVO4 to Bi2S3/BiVO4 hybrid nanostructure The

Bi2S3/BiVO4

hybrid

nanostructure

photoanodes

were

synthesized

by

photoelectrochemical transformation through ion exchange reactions in the aqueous electrolyte containing two sulfur species (Na2S/Na2SO3). The photoelectrochemical transformation process was performed under illumination (100 mW.cm-2) at 0.67 V vs. RHE, in a standard photoelectrochemical cell comprising a three-arm glass compartment with a circular quartz window for light illumination. The BiVO4 on FTO glass was used as the working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode. The photoanode was illuminated with the simulated sunlight of one sun illumination using a solar simulator (Abet Technologies). The J-t curves were measured at 0.67 V vs. RHE using a potentiostat

(COMPACTSTAT.e,

Ivium,

Netherlands).

The

Photoelectrochemical

transformation of BiVO4 was conducted for 180 min in the chronoamperometry mode. Finally, yellow colored BiVO4 heterojunctions were converted into black colored Bi2S3/BiVO4 heterojunctions under the photoelectrochemical transformation. A facile synthesis strategy of the photoelectrochemical transformation of BiVO4 to Bi2S3/BiVO4 hybrid nanostructure is shown in Scheme S1. The BV1, BV2, and BV3 samples after PEC measurements are termed ABV1, ABV2, and ABV3, respectively.

Characterization The X-ray diffraction measurements of deposited BiVO4 based films were performed at



Page 8 of 45

the 5A beamline of the Pohang Light Source II (PLS-II) in Korea. A two-theta scan with Xrays of 11.57 keV (λ=0.1072 nm), while we fixed the theta at 3° throughout the scan to enhance diffraction signals from the near-surface region. X-ray photoelectron spectroscopy (XPS) was used for determining the chemical state and elemental quantification. XPS analysis was performed using a PHI Quantera II XPS spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The morphologies of the deposited films were observed by a field emission scanning electron microscope (FESEM; SUPRA 40VP, Carl Zeiss, Germany) equipped with an X-ray energy dispersive spectrometer (EDX). The UV−Vis−DRS spectra were measured using a dual-beam spectrophotometer (Shimadzu, UV-2600 series) in the wavelength range of 300−800 nm. Room temperature photoluminescence (PL) spectra of the samples were also recorded with the F-4500 FL spectrophotometer using a Xe arc lamp as the excitation source. The formation of Bi2S3/BiVO4 hybrid nanostructures was analyzed using transmission electron microscopy (TEM; JEOL ARM-200F) combined with focus ion beam (FIB; FEI Quanta 2D) lift-out cross-sectional TEM sampling. Also, annular dark field-scanning TEM (ADF-STEM) were performed with a spherical aberration corrector (CEOS GmbH) at 200 kV.

The STEM-

energy dispersive X-ray spectroscopy (EDS) analysis was carried out with EDAX detector (SDD type 80T). Photoelectrochemical (PEC) measurements Photoelectrochemical measurements were carried out using a conventional threeelectrode configuration with the BiVO4 based samples, platinum (Pt) wire, and Ag/AgCl electrodes were used as the working electrode, counter electrode, and reference electrode, respectively. The working electrode was illuminated from front side by a solar light source with 100 mW.cm−2 intensity. The scan rate for the current–voltage curve was 20 mV/s and all potentials were recorded with correction by the Nernst relation ERHE = EAg/AgCl + 0.0591 pH+


Page 9 of 45 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


E0Ag/AgCl, where EAg/AgCl is the experimentally measured potential against Ag/AgCl reference and E0Ag/AgCl = 0.1976 at 25 °C. The electrochemical impedance spectra (EIS) were measured using an IVIUMSTAT electrochemical workstation in a potentiostatic mode with an AC voltage amplitude of 10 mV and an AC frequency range of 0.1−300 kHz under AM 1.5G illumination. The experimental EIS data were fitted to the suitable equivalent circuit model using the Z View (Scribner Associates Inc.) software. An air-tight three-electrode PEC cell with an aqueous solution containing 0.1 M Na2S/0.02 M Na2SO3 as an electrolyte (pH ≈ 12.8) and Ag/AgCl reference electrode and a Pt wire counter electrode was used for PEC solar hydrogen generation under a simulated one sun illumination (100 mW.cm−2). Before the light irradiation, N2 gas was purged through the PEC cell containing 65 ml electrolyte to act as a carrier gas and to complete the removal of the dissolved oxygen in the electrolyte. All experiments were performed at an applied bias of 0.67 V vs. RHE. The evolved hydrogen was quantitatively analyzed using gas chromatography with a thermal conductivity detector (TCD), using a 5-Å molecular sieve column and N2 as the carrier gas.

Results and discussion Figure 1 depicts the change in surface morphologies of the BiVO4 thin films under different Bi/V concentrations (BV1, BV2 and BV3). Figure 1a shows at a low concentration of 0.025 M bismuth nitrate Bi(NO3)2.5H2O and 0.0037 vanadyl acetylacetonate (C10H14O5V), the surface of the BV1 thin film consists of the nano-islands of BiVO4. As the concentration of Bi/V were doubled (BV2, 0.05 M/0.0075 M), the diameter and thickness of the uniformly distributed nano-islands are also observed to be increased (diameter 250 nm and thickness 150-163 nm). In addition, Figure 1b shows that, to large-sized islands, the small-sized BiVO4 nanograins are also observed on the FTO surface, which assists in the light-trapping with the



nano-islands. Further, when the concentration of Bi/V reached the value of 0.1 M/0.015 M (BV3), the surface morphology images of BiVO4 differ from those of the lower Bi/V concentrations (Figure 1c). The BiVO4 layer presented a continuous structure, possibly due to the formation of the surface swells.31 The FESEM cross-sectional views of BV1, BV2, and BV3 are shown in Figures S1 (b)–(c), respectively. It is found that with an increase in the initial precursor concentration (Bi/V ratio) can be attributed to the increase in thickness of the drop-coated photoelectrodes from 58 nm to 167 nm. Thus, these results may suggest that various morphologies of BiVO4 could be controlled by adjusting the synthesis methods and experimental conditions.32, 33 The phase and composition of the BiVO4 thin films synthesized with different Bi/V concentrations were first examined by X-ray diffraction (XRD) analysis and are shown in Figure 2A. The peaks appeared at 8.4°, 12.86°, 13.57°, and 23.21° for BV1, which can be indexed to the diffractions of the (101), (112), (004), and (116) planes of tetragonal BiVO4, respectively (denoted as “T,” JCPDS No. 04-010-5710). Additionally, the presence of the doublet peaks in BV2 at 18.5° indicates the formation of monoclinic BiVO4.34,35 The presence of the (011), (110), (013), and (004) planes with lattice constants of a =5.196 Å, b = 5.0935 Å, and c = 11.7044 Å (denoted as “M,” JCPDS card no.04-010-5713) confirms the formation of monoclinic BiVO4 (Space Group: I2/a (15)) in BV2 drop-coated samples. The same diffraction peaks were also observed for BV3, indicating that monoclinic structures of BiVO4 were well preserved in these samples. The (011), (110), and (004) facets are equally as important as the (121) facet for the photocatalytic activity for splitting water as reported in the previous literature.36, 37 Figure 2B (a) shows low magnification bright field (BF) TEM image of BV2 sample before the PEC measurements fabricated by cross-sectional FIB lift-out technique, which displays about 130 nm thick BiVO4 layer on the FTO surface. Figure 2B (b) reveals an enlarged ADF-STEM image from the interface between BiVO4 and FTO in Figure 2B (a). Figure 2B (c) depicts the high-resolution ADF-STEM (HR-ADF-


Page 10 of 45

Page 11 of 45 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


STEM) image of the BiVO4. The lattice fringes with the spacing of 0.254 and 0.26 nm are in good agreement with the (020) and (200) planes of monoclinic BiVO4, respectively. Corresponding fast Fourier transform (FFT) image is shown in insets of the Figure 2B (c). Furthermore, the EDS elemental mapping images (Figure 2B (d-g)) show uniform signals for Bi, V, elements indicating the homogeneous distribution of m-BiVO4 on the FTO substrate. The optical absorption property of an oxide semiconductor is recognized as the key factor in determining its photocatalytic activity.38 Absorbance spectra of the BiVO4 photoelectrodes prepared using different concentration and ratios of bismuth to vanadium in the starting material (BV1, BV2 and BV3) were measured, the results of which are shown in Figure 3a. In the characteristic spectrum of BiVO4, a strong absorption at a wavelength shorter than around 500 nm was observed and is attributed to the intrinsic band gap absorption.39 In addition, each spectrum exhibits two shoulders at about 352 and 431 nm, respectively, which are ascribed to the charge-transfer transition involving the V−O component and Bi and V centers.31

Closer inspection of the absorption spectra (Figure 3a), shows the onset of the

absorption edge for BV1 at 476 nm, whereas the onset of the absorption edge for BV2 and BV3 was about 480 nm, which shows there is a red shift in BV2 and BV3. This is due to the phase change from tetragonal to monoclinic phase BiVO4 for BV1 to BV2 and BV3.32 It was also reported that the electronic structure of BiVO4 changes with the crystal phase transition, which further leads to the different mobility of photogenerated holes.40,

41

A linear

relationship between (αhν)2 and hν indicates that BiVO4 has a direct energy band gap. The band gap (Eg) for different BiVO4 photoanode films can be estimated from the absorption edges (αhν=A (hν-Eg)n), where n = 2 for direct semiconductors, as shown in the inset of Figure 3a.42 The estimated values of the bandgap energies for BiVO4 ranged from 2.59 to 2.6 eV. These results demonstrate that the bandgap values of these as-prepared BiVO4 photoanodes slightly larger than the reported values of BiVO4 photoanodes.43,44 Higher



bandgap energies for the BiVO4 samples may be ascribed to the “quantization-like” effect.34 Figure 3b shows the photocurrent density vs. potential (J–V) curves for the BiVO4 photoanode measured in the dark and under AM 1.5 irradiation. It can be clearly seen that a saturated current density is not obtained within the applied potentials range of the J–V curve, suggesting a substantial amount of surface recombination.45 The comparisons of the linear sweeps of BiVO4 confirm that the photocurrent densities increase from BV1 to BV2. This is attributed to the fact that, with increasing the precursor concentration from BV1 to BV2, the phase of BiVO4 changes from tetragonal to the monoclinic phase which exhibit higher photocatalytic activity than the tetragonal.46 However, compared to the BV2 sample, the photocurrent of BV3 decreased because the increased thickness of BiVO4 leads to an increase in the bulk recombination. Thus, the BV2 photoanode showed a photocurrent value of 2.1 mA.cm-2 at 0.67 V vs. RHE, which is highest photocurrent density value amongst (BV1, BV2 and BV3) and is considered to be optimized. This indicates that an appropriate phase structure, electronic band structure and morphology are essential key factors for harvesting the sunlight with high efficiency.47

Further to verify the optimization of the concentration

of the Bi and V precursors, the transient photocurrent, and electrochemical impedance spectroscopy measurements were conducted under AM 1.5 illuminations (100 mW.cm-2) for all deposited BiVO4 photoelectrodes. As shown in Figure S2a, compared to the BV1 and BV3 electrodes, the BV2 electrodes showed the highest photocurrent density (2.1 mA.cm-2 at 0.67 V vs. RHE). In addition to light absorption, the highest photocurrent mainly originated from the effective separation of photogenerated charges within the photoelectrode.48 Hence, further to elucidate enhanced electron-transfer and recombination processes of the photogenerated charges in BiVO4, electrochemical impedance spectroscopy (EIS) was carried out at the applied potential of 0.67 vs RHE in 0.1 M Na2S/ 0.025 M Na2SO3 electrolyte. The EIS parameters were obtained by fitting the Nyquist plots using equivalent circuit (EC) as


Page 12 of 45

Page 13 of 45 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


shown in the inset of Figure S2b. The arcs in the Nyquist plot in lower frequency range are related to charge transfer at the interface of the electrode/electrolyte. The lower semicircle arc of the BV2 confirms the lower charge transfer resistance in BV2 than that in BV1 and BV3. The EIS fitted parameters were shown in Table S1, which clearly shows lower charge transfer resistance value for BV2 (R1= 1360 Ω ) than the BV1 (R1= 1552 Ω) and BV3 (R1= 2039 Ω), which facilitates better charge carrier separation in BV2 and contributes to significantly enhance the overall photocurrent under solar-light irradiation. The photoelectrochemical stability of the BiVO4 photoanode was also evaluated for a long time (180 min) as it is one of the essential factors during photoelectrochemical hydrogen generation. Figure 3c shows the J-t behaviors of the BV1, BV2 and BV3 photoanodes under a potentiostatic bias of 0.67 V vs. RHE over 180 min and these BV1, BV2, and BV3 samples after PEC measurements are denoted as ABV1, ABV2, and ABV3, respectively. It is worth to note that the photocurrent is shown to decrease with time. This is due to during the PEC measurements, vanadium leaches from the BiVO4 into the electrolyte and at the same time the photogenerated holes in VB of BiVO4 interacts with the S2− ions and produces Bi2S3 at the surface of BiVO4. Initially, both the components Bi2S3 and BiVO4 may help to produce the photogenerated charges. However, with illumination time the amount of Bi2S3 is thicker than the BiVO4, which led to maximum contribution from the Bi2S3 than BiVO4, thus causing a decrease in the photocurrent with illumination however total; photocurrent is higher than the initial value (BiVO4 alone). In addition to this, the dissolved oxygen in the electrolyte solution scavenge the photogenerated electrons immediately following photo-illumination, contributing to the decrease in the photocurrent.49 Under dark conditions, the current densities measured with respect to time for the Bi2S3/BiVO4 heterostructure electrodes shows negligible values (less than 60 µA.cm-2). This indicates that in the Bi2S3/BiVO4 heterostructure electrodes separation of electrons and holes almost does not occur in dark.



However, under visible light irradiation, the current densities of ABV photoanodes generate instantly which are due to photoelectrochemical conversion, effective light absorption, separation efficiency of the photoinduced electron-hole pairs and a lower recombination rate in such hybrid structures and not due to the photocorrosion of BiVO4. Figure S3 shows an actual photograph of the photoelectrochemical cell used for the photoelectrochemical transformations of BiVO4 to Bi2S3/BiVO4 photoanodes under solar light illumination. The morphologies and microstructures of the photoelectrodes after PEC measurements were revealed by FESEM and are shown in Figure 3(d-f). The FESEM images further reveal that after long-term PEC measurements, the surface morphology is slightly modified. A close investigation of the samples showed that, after the PEC measurement, the surfaces of the nano-islands of BiVO4 have effectively covered with a Bi2S3 layer. It is anticipated that during the PEC measurements, an anion exchange reaction was initiated between BiVO4 and the sulfate ions of the Na2S/Na2SO3 electrolyte ions, which generated a thin Bi2S3 layer on the surface of BiVO4. In addition, the space between the BiVO4 nano-islands was gradually filled with Bi2S3 layers during the photoelectrochemical reaction. This will further closely attach to form a unique wrapped structure on BiVO4. Such growth of the Bi2S3 layer on the BiVO4 surface led to the formation of a hybrid nanostructure framework as well as it forms a bridge between the adjacent nano-island building blocks. Such structure is favorable for the effective charge transport, and hence the photoelectrochemical performance of the Bi2S3/BiVO4 hybrid nanostructure photoanode. UV-visible spectra of ABV1, ABV2, and ABV3 samples were collected to understand the influence of Bi2S3/BiVO4 hybrid nanostructure on the light harvesting capability. Figure S4a shows the absorption spectra of ABV1, ABV2, and ABV3 samples prepared after photoelectrochemical transformation were red shifted compared with pure BiVO4 (BV1, BV2, and BV3) films which was attributed to formation of Bi2S3 on the surface of BiVO4 and acting as a sensitizer to extend the optical


Page 14 of 45

Page 15 of 45 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


response.50 Figure S4b shows the band gaps of BiVO4 samples after the PEC measurements vary from 2.0 to 1.7 eV, which correspond to the amount of transformation to Bi2S3. Thus, it is shown that the Bi2S3/BiVO4 nanostructure can be efficiently excited by visible-light irradiation, which might lead to enhanced photoelectrochemical performance. Figure S5 shows the X-ray diffraction (XRD) patterns of Bi2S3/BiVO4 hybrid nanostructures. The BV1 exhibits no peaks correspond to the tetragonal phase of BiVO4, this is due to during the photoelectrochemical transformation process the vanadium leaches from the BiVO4 into the electrolyte and Bi2S3 forms at the surface of BiVO4. On the other hand, thicker BV2 and BV3 after PEC measurements shows characteristic peaks of monoclinic BiVO4 (Space Group: I2/a (15)) and Bi2S3 (Space group: 63Pmcn) phases, indicating the structural variation between the samples before and after PEC measurements. Thus, the presence of Bi2S3 diffraction peaks in XRD pattern of Bi2S3/BiVO4 heterostructure indicates that the Bi2S3 phase exists in the heterostructure by a transformation from BiVO4. To further investigate the chemical state of the surface species of the BiVO4 samples before and after the photoelectrochemical performance, the samples were analyzed using X-ray photoelectron spectroscopy (XPS). Figure 4a shows the survey scans of BiVO4 before and after PEC measurements (i.e. BV2 and ABV2). It is clearly seen that the BV2 are mainly composed of Bi, V and O, However, the presence of Bi and S elements with a lower amount of V and O in ABV2, indicates that Bi2S3 formed on the surface of BiVO4. The high-resolution XPS spectrum (Figure 4b) of Bi 4f of BV2 consist of two intense and symmetrical peaks at the binding energy (EBE) = 159.7 and 164.5 eV, corresponding to Bi4f7/2 and Bi4f5/2, respectively, in accordance with the Bi ions in their trivalent oxidation state (Bi3+ peaks).51 However, high-resolution scans of Bi4f of ABV2 reveals in addition to Bi4f7/2 and Bi4f5/2 peaks, two weak peaks centered at of 161.1 eV and 162.3 eV are appointed to S2p3/2 and S2p1/2, which are consistent with the values reported in the literature.52 The results of FESEM and XPS unanimously prove that the Bi2S3



have been successfully covered on the surface of BiVO4 and forms the Bi2S3/BiVO4 heterostructure. However, in ABV2, the Bi4f7/2 and Bi4f5/2 peaks shift toward the high banding energy, which could be attributed to the electronic shielding effect of probably formed the chemical bonds of O–Bi–S in Bi2S3/BiVO4 heterostructure.27 As shown in Figure 4c, the XPS patterns of V 2p show splitting peaks at 516.5 and 524.2 eV of V2p3/2 and V2p1/2 are attributed to the surface V5+ species, respectively. The lower peak intensities of V2p3/2 and V2p1/2 and O1s in ABV2 indicate the transformation of the surface of BiVO4 into the Bi2S3/BiVO4 heterostructure. In addition, Figure 4d shows that the 2S peak at around 226.1 eV was attributed to S2s, which confirms the formation of Bi2S3.53 However, in ABV2 the asymmetric peak centered at 530 eV was observed with lower peak intensity than the BV2 sample, indicating the presence of Bi2S3 layer on BiVO4 after PEC measurements. The O 1s peak of ABV2 can be deconvoluted into two components as shown in the supporting information, Figure S6. The intense O 1s peak located at 530.1 eV was assigned to the lattice oxygen(Olatt) in BiVO4, while another relatively lower peak at 532.4 eV was usually assigned to surface adventitious species such as hydrocarbon (C–H), carbonate species (C–O, C═O and adsorbed water (OHads) bonds due to contamination from environment during characterization.54, 55 This also provides evidence of the existence of BiVO4; thus, the XPS results show the probability of the presence of a Bi2S3/BiVO4 hybrid nanostructure during the photoelectrochemical transformation of BiVO4 under solar light illumination. The PL analysis is used to explain the fade of electron-hole pairs, which could greatly affect the photoelectrochemical performance of the photoelectrodes.56 The photoluminescence emission originates from the radiative recombination of photogenerated holes and electrons in photoelectrodes. Figure S7 shows the broad emission peaks of the of optimized BV2, and samples prepared after PEC measurements (ABV1, ABV2 and ABV3, respectively) centered at 535 nm related to the presence of surface defects.57


It was found that the relative

Page 16 of 45

Page 17 of 45 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


intensities of the PL spectra of the ABV1 and ABV2 samples were lower than that of BV2 and reached the minimum at ABV2, which indicates that the photogenerated electrons in the Bi2S3 can easily transfer from Bi2S3 to BiVO4, the Bi2S3/BiVO4 structure can diminish the recombination of photoinduced electron–hole pairs.58, 59 Also, the PL intensity at 640 nm of the photoelectrochemically transformed Bi2S3/BiVO4 samples (ABV1, ABV2, and ABV3) results from the recombination of the holes formed in the S1s band and the electrons in the Bi4f band.60 Interestingly, the intensities of the PL spectra of ABV3 do not decrease, suggesting that ABV2 can effectively restrain the recombination of electrons and holes. The increased PL emission intensity of ABV3 is caused by the increase in the bulk recombination in thicker films, and the defects on the surface of the films. These results reflect the slight discrepancy in the photocatalytic performance of the samples. Therefore, these observations indicate that the crystallinity, surface morphology, and band gap energy should be considered as having a combined effect on the photocatalytic performance.61 To further investigate the photoelectrochemical transformation of BiVO4 into Bi2S3/BiVO4 hybrid nanostructure after PEC measurements, the cross-sectional transmission electron microscopy was employed with FIB lift-out manner. A representative BF TEM and ADF-STEM images of the cross-sectional views of the ABV2 sample at the two different places to confirm the Bi2S3/BiVO4 heterojunctions material indicates the distinct small grains of BiVO4 are seen in the interface between the FTO and Bi2S3 layers (Figure 5(a, b)). However, the HR-ADF-STEM images representing the two different set of lattice fringes corresponding to the BiVO4 and Bi2S3 phases as shown in Figure 5 (c and d). The lattice spacing observed to be 0.254 and 0.26 nm corresponds to (020) and (200) planes of monocline phase BiVO4 (JCPDS 040105713) and the lattice spacing of 0.311 nm is assigned to the (023) plane of Bi2S3 (JCPDS No. 010840273). Additionally, two corresponding fast Fourier transform patterns (inset in Figure 5 (c and d) confirm the co-existence of BiVO4, Bi2S3 phases with the heterojunction interface



Page 18 of 45

between the two BiVO4 and Bi2S3 phases leading to the formation of heterojunction, which helps to transport of charge carriers in ABV2. Furthermore, low magnification TEM image of Bi2S3/BiVO4 hybrid nanostructure in Figure S8 (a) clearly indicates that the Bi2S3 layer is distributed uniformly on the surface of the BiVO4. A clear interface between Bi2S3 and BiVO4 can be observed. Also, the HR-TEM image clearly shows that the BiVO4 and Bi2S3 domains form a highly interconnected interface that would be beneficial for the charge separation and electronic mobility.62 The interlocked lattice fringes of both Bi2S3 (2.8 Å corresponding to the 212 plane) and BiVO4 (3.12 Å corresponding to the 103 plane) indicating that Bi2S3 was tightly attached to the surface of BiVO4 and are shown in one of the representative

HR-TEM

images

(Figure

S8b)

obtained

for

this

heterojunction

photoelectrode.63 Figure 5 (e-j) shows ADF-STEM image, and corresponding compositional analysis of the resulting ABV2 heterostructure was carried out using EDS mapping. As can be seen from Figure 5 (f, g, and h) the elemental distribution of Bi, S and O elements in Bi2S3/BiVO4 heterostructure confirms the phase transformation of the BiVO4 to Bi2S3/BiVO4 after photoelectrochemical transformation. The element mapping image vividly shows (Figure 5j) that the Sn signal is mainly confined from the FTO substrates. Thus, it was seen that at two different places HR STEM images representing the two different set of lattice fringes corresponding to the BiVO4 and Bi2S3 phases, indicating the homogeneous distribution of Bi2S3 on the surface of BiVO4 in Bi2S3/BiVO4 heterojunction photoelectrode. Thus, interpretation of BF TEM and ADF-STEM images are in good agreement with the XPS analyses. To get more insight into hybrid heterostructure of ABV2, the ADF-STEM and point EDS spectra of ABV2 at two different points (the top surface and the bottom of ABV2 samples) are shown in Figure S9 (a-d). The ADF-STEM and point EDS results confirm the BiVO4 sample is converted into the Bi2S3/BiVO4 nanostructure after photoelectrochemical transformation. The corresponding results of the EDS mapping of Bi, V, and S elements well


Page 19 of 45 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


agree with the ADF-STEM and point EDS and are shown in Figure S9 (e-g). Besides, crosssectional TEM analyses of ABV2 films, the TEM and ADF-STEM image with EDS results of the ABV2 powder samples obtained by scratching the ABV2 films from the FTO are also shown in Figure S10 (a-g). The typical TEM elemental mapping images of the ABV2 (Figure S10 (d-f)) clearly shows the surface layers of well-formed BiVO4 nanograins were homogeneously converted into the thin Bi2S3 layer. Moreover, the EDS spectrum is shown in order to determine the composition of BV2 after the PEC measurements (Figure S10b). Besides the three elements (Bi, V, and O) of as-prepared BV2, an additional element, S, was observed in ABV2, this suggests the formation of Bi2S3 on the surface of BiVO4. Thus, comparing the elemental mapping images of BV2 and ABV2 (i.e. Figure 2B (d-g) and Figure 5 (d-g)), a relatively larger decrease is shown in the vanadium content at the ABV2 compared to BV2. Also, the ratios between Bi2S3 and BiVO4 in each sample (ABV1, ABV2 and ABV3) were measured using FESEM-EDAX and compared with the BV1, BV2 and BV3 samples. The change in ratios with the processing of photoelectrochemical hydrogen generation experiment has been shown in the Figure S11. Table inset of Figure S11 shows that, during the photoelectrochemical transformation process, leaching of vanadium anion (VO34-) from the BiVO4 into the electrolyte takes place which gives less content of Vanadium after PEC measurements and the same time S2− react with the BiVO4 and led to the formation of Bi2S3 layer on the surface of BiVO4. These results clearly show that the photoelectrochemical transformation process successfully converts the surface of BiVO4 into a Bi2S3 layer, which yields a Bi2S3/BiVO4 hybrid nanostructure using a facile, economical, and green process. To demonstrate

the

photoelectrochemical

activity

during

the

photoelectrochemical

transformation of BiVO4 to Bi2S3/BiVO4 hybrid nanostructure, the hydrogen generation was measured under solar-light irradiation. During the long-term photocurrent stability test, H2 bubbles were visibly observed on the counter electrode (see Video S1 in the Supporting



Page 20 of 45

Information). Figure 6 shows the evolved hydrogen (H2) gas measured by gas chromatography (GC) during the long-term chronoamperometry for various Bi2S3/BiVO4 hybrid nanostructure photoelectrodes. It is demonstrated that during PEC solar hydrogen generation, the photocurrents increased initially at about 110 min for all the samples, and then started to decrease. This increase in photocurrent can be due to the fact that under the light illumination and photoelectrochemical transformation condition, the surface of BiVO4 starts to convert into Bi2S3 by the out-diffusion of Vanadium ions in Na2S/Na2SO3 and efficient indiffusion of Sulfur in BiVO4.51 These photoelectrochemically transformed Bi2S3 layer on the surface of BiVO4 can helps to produce photogeneration of electrons and holes in addition to the BiVO4, however, this enhancement depends upon the thickness of Bi2S3 in Bi2S3/BiVO4 heterojunction and the charge separation process.12 Thus, once the optimum Bi2S3 layer formed on the BiVO4, which forms the uneven block to the vanadium leaching process and hence the photocurrent is started to suppress at the later stage. However, the BV3 sample exhibits the highest photocurrent density and solar hydrogen generation activity (~417 µmol h-1cm-2) compared to BV1 and BV2. To investigate the origin of the photocurrent, Faradaic efficiencies of hydrogen evolution need to be measured. The theoretical number of moles of hydrogen evolved can be calculated from Faraday's 2nd law of electrolysis and used to measure the faradaic efficiency.64 t

Idt 1 ∫0 Mole of hydrogen evolved (theoretical) = n F

(1)

where, I is the measured photocurrent, t is time, n is the number of electrons transferred during hydrogen evolution reaction (2e-) and F is the Faraday constant. The faradaic efficiencies corresponding to measured and calculated hydrogen production for BV1, BV2 and BV3 are 68, 75 and 80%, respectively at 30 min and further increases to 85-88% (Figure


Page 21 of 45 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


S12). The results suggest that the Bi2S3/BiVO4 heterojunctionformed after 180 min significantly influenced the electron injection than the BiVO4 (lower time 30) and substantially enhancing the charge separation and transfer in BV3 than the BV1 and BV2 photoanodes. Thus, most of the photogenerated electrons were consumed for hydrogen production in the current system. The superior photoelectrochemical performance of BV3 could be explained as follows. At the optimum thickness of Bi2S3 in BV3, the chance for recombination of photogenerated charges reduced. Also due to appropriate the conduction band and valence band (VB) positions of Bi2S3 and BiVO4, the Bi2S3 can serve as a hole acceptor which are generated in the BiVO4 semiconductor and transfer of electrons form CB of Bi2S3 to BiVO4.23 However, the excessive Bi2S3 in the BV3 heterostructure is due to the unfavorable morphology and led to poor charge transport in thicker Bi2S3 produced on the surface of BiVO4 during the hydrogen generation.65 The obtained PEC results in present work using the Bi2S3/BiVO4 hybrid nanostructure were compared with other BiVO4 based heterojunctions and composites prepared by a variety of methods (Table S2). The presence of Bi2S3 on the surface of BiVO4 was also observed by the color change of BiVO4 from yellow to black and further confirmed by X-ray diffraction (XRD) patterns as well as by the XPS, cross-sectional TEM analyses with FIB, and the optical spectra analyses. Finally, on the basis of these results, the charge transfer mechanism in the BiVO4 and Bi2S3/BiVO4 hybrid nanostructure is shown in Figure 7. It can be seen that the intergrain boundary between the adjacent building blocks of BiVO4 provides the resistive path for photogenerated charge

carriers

(Figure

7a).

Figure

7c

shows that during the

photoelectrochemical measurements under illumination and applied potential conditions, the photogenerated electrons in conduction band (CB) of BiVO4 were collected at the FTO substrate, and then moved to the counter electrode via the external circuit, where they reacted with H+ in the electrolyte and generated hydrogen on the surface of counter electrode. At the



Page 22 of 45

same time, the photogenerated holes in the valence band (VB) of BiVO4 interact with the SO32− and S2− radicals in the Na2S/Na2SO3 sacrificial reagent and oxidize them to form S22− and S2O32−. Further, the S22− ions react with the SO32− and regenerate the S2− ions in the electrolyte (path 1, Figure 7c). Also, S2− ions interact with the BiVO4 and produces Bi2S3 at the surface of BiVO4 (see Figure 7b). Thus, after the formation of Bi2S3/BiVO4 hybrid nanostructure by photoelectrochemical transformation process, the possible cascade charge transfer process takes place between the Bi2S3 and BiVO4, under visible light illumination. The photogenerated electrons in the conduction band of Bi2S3 transfer to the conduction band of BiVO4 and at the same time the photogenerated electrons are also produced in the conduction band of BiVO4 itself.66 At the same time, the photogenerated holes in the valence band (VB) of BiVO4 will efficiently transport to VB of Bi2S3 due to the effects of built-in electric fields. These holes in VB of Bi2S3 then effectively transport to the electrolyte as shown in path 1 in Figure 7d. Also, at the photogenerated holes produced in the conduction band of BiVO4 itself can react with the SO32− and produces the SO42− (path 2, figure 7d). Finally, on the basis of our results and the available literature,25, 66-68 the charge transfer mechanism in the BiVO4 and Bi2S3/BiVO4 hybrid nanostructure is shown in Figure 7 (c, d). Thus, these holes (path 1 and path 2, figure 7d) further interact with the SO32− and 2S2− radicals in the Na2S/Na2SO3 sacrificial reagent and produce S2O32− and SO42−, which promote redox reactions in the electrolyte. The possible reactions occurring during the photoelectrochemical transformation are summarized below (equation 2-9):67 Reaction at the counter electrode: 2(H+) + (e-) → H2

(2)

Reaction at the working electrode: BiVO4 + hν → BiVO4 (h+) + BiVO4 (e-)


(3)

Page 23 of 45 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


BiVO4 (h+) +S2- → Bi2S3 + VO43-

(4)

Bi2S3 + hν → Bi2S3 (e-) + Bi2S3 (h+)

(5)

Bi2S3 (e-) + BiVO4 → BiVO4 (e-)

(6)

2S2− + Bi2S3/BiVO4 (2h+) → S22−

(7)

S22− + SO32− → S2O32−+ S2−

(8)

SO32− + 2OH− + 2h+ → SO42− + H2O

(9)

Also, the sulfite (SO32-) from the Na2S/Na2SO3 electrolyte acts as an efficient hole scavenger and can prevent the anodic photocorrosion of Bi2S3. However, the formation of Bi2S3/BiVO4 heterostructure from BiVO4 is generally very slow because of the high activation energies for the diffusion of atoms and ions from the BiVO4.69 Furthermore, the photoelectrochemical transformation mechanism of BiVO4 to Bi2S3/BiVO4 hybrid nanostructure under illumination of solar light is illustrated in Figure S13. During photoelectrochemical transformation reactions, the leaching of vanadium at the surface of BiVO4 into the electrolyte (Figure S13 b1) and in-diffusion of S2− anions in BiVO4 (Figure S13 b2) led to the surface transformation of BiVO4 into Bi2S3/BiVO4 heterostructures.51 Thus, the presence of the Bi2S3 coating on the surface of BiVO4 offers a pathway to photogenerated charge carriers in Bi2S3/BiVO4 hybrid nanostructure by enabling the connectivity between neighboring grains through the formation of a bridge from Bi2S3 and BiVO4, as shown in Figure 7 (b and d). This can significantly improve the charge transfer and thus the photoelectrochemical performance of the prepared photoanodes.

Conclusion



In summary, noble Bi2S3/BiVO4 hybrid nanostructure was successfully synthesized via photoelectrochemical transformation approach. Uniform nano-islands of BiVO4 particles are firstly synthesized and then photoelectrochemically transformed to Bi2S3/BiVO4 hybrid nanostructure in aqueous Na2S/Na2SO3 electrolyte. A detailed TEM and XPS results confirm the leaching of Vanadium and growth of Bi2S3 on the surface of BiVO4 during PEC process. The amount of BiVO4 transformed to Bi2S3/BiVO4 hybrid nanostructures can be easily varied by controlling the concentration of bismuth and vanadium precursors in drop coating of BiVO4 in addition to photoelectrochemical transformation time. The transformation of BiVO4 to the Bi2S3/BiVO4 hybrid nanostructure enhances the visible light absorption, fast charge transportation, thus delivering a photocurrent of 3.3 mA.cm-2 at 0.67 V (vs. RHE) hydrogen generation for ABV3 which is significantly higher than that of the ABV1 and ABV2. Thus, during the photoelectrochemical transformation process, the obtained hydrogen evolution rate was ~417 µmol cm-2 h-1) for the ABV3 photoanode and can drive the hydrogen evolution reaction with a Faradaic efficiency of 87%. These findings could be extended to design of various complex nanostructures with tailored optical and electronic properties for photoelectrocatalysis, electrochemical energy storage, and solar cell applications.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Schematics of photoelectrochemical transformation of BiVO4 to Bi2S3/BiVO4 hybrid nanostructure, FESEM, Photocurrent density−time (J−t) curves and (b) Nyquist plots of BiVO4, and Absorbance, XRD and PL spectra of Bi2S3/BiVO4 hybrid nanostructure, Highresolution XPS spectra of O1s of spectra of BV2 sample, HR-TEM images of the interface


Page 24 of 45

Page 25 of 45 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


between Bi2S3/BiVO4 photoanode, ADF-STEM image and EDS element of ABV2 photoanode, Faradaic efficiency vs. irradiation time. EIS fitting parameters of BV2 samples before PEC measurements. Recent reports on the PEC performances of BiVO4 heterojunction photocatalysts

AUTHOR INFORMATION

Corresponding Authors *Fax: +82 63 850 0834. Tel.: +82 63 850 0846. E-mail: [email protected] (J.S. Jang), [email protected] (Min Cho).

ORCID J.S. Jang: 0000-0001-6874-8216

Notes The authors declare no competing financial interest.

Acknowledgements This research was supported by the Korean National Research Foundation (NanoMaterial Fundamental Technology Development, 2016M3A7B4909370) and the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (2017H1D3A1A02014020). Also, this work was supported by the Korea Ministry of the Environment as Eco-Innovation project (2016000140001).



References [1] Xiang, Y.; Lu, S.; Jiang, S. P. Layer-by-layer self-assembly in the development of electrochemical energy conversion and storage devices from fuel cells to supercapacitors, Chem. Soc. Rev. 2012, 41, 7291–7321, DOI:10.1039/C2CS35048C [2] Singh, W. M.; Pegram, D.; Duan, H. F.; Kalita, D.; Simone, P.; Emmert, G. L.; Zhao, X. Hydrogen Production Coupled to Hydrocarbon Oxygenation from Photocatalytic Water Splitting, Angew. Chem., Int. Ed. 2012, 51, 1653-1656, DOI.org/10.1002/anie.201106494 [3] Qiu, Y. C.; Yan, K. Y.; Deng, H.; Yang, S. H. Secondary Branching and Nitrogen Doping of ZnO Nanotetrapods: Building a Highly Active Network for Photoelectrochemical Water Splitting, Nano Lett. 2012, 12, 407-413, DOI: 10.1021/nl2037326 [4] Xie, S.; Zhai, T.; Li, W.; Yu, M.; Liang, C.; Gan, J.; Lu, X.; Tong, Y. Hydrogen production from solar driven glucose oxidation over Ni(OH)2 functionalized electroreducedTiO2 nanowire arrays, Green Chem. 2013, 15, 2434-2440, DOI:10.1039/C3GC40973B [5] Ma, G.; Yan, H.; Shi, J.; Zong, X.; Lei, Z.; Li, C. Direct splitting of H2S into H2 and S on CdS-based photocatalyst under visible light irradiation, J. Catal. 2008, 260, 134-140, DOI.org/10.1016/j.jcat.2008.09.017 [6] Gao, L.; Cui, Y.; Wang, J.; Cavalli, A.; Standing, A.; Vu, T. T. T.; Verheijen, M. A.; Haverkort, J. E. M.; Bakkers, E. P. A. M.; Notten, P. H. L. Photoelectrochemical Hydrogen Production on InP Nanowire Arrays with Molybdenum Sulfide Electrocatalysts, Nano Lett.

2014, 14, 3715-3719, DOI: 10.1021/nl404540f [7] Gomathisankar, P.; Hachisuka, K.; Katsumata, H.; Suzuki, T.; Funasaka, K.; Kaneco, S. Photocatalytic Hydrogen Production from Aqueous Na2S + Na2SO3 Solution with B-Doped ZnO, ACS Sustainable Chem. Eng. 2013, 1, 982-988, DOI: 10.1021/sc400061w


Page 26 of 45

Page 27 of 45 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


[8] Kim, J. H.; Lee, B. J.; Wang, P.; Son, M. H.; Lee, J. S. Facile surfactant driven fabrication of transparent WO3 photoanodes for improved photoelectrochemical properties, Appl Catal A Gen. 2016, 521, 233-239, DOI.org/10.1016/j.apcata.2016.01.003 [9] Woodhouse, M.; Parkinson, B. A. Combinatorial approaches for the identification and optimization of oxide semiconductors for efficient solar photoelectrolysis, Chem. Soc. Rev.

2009, 38, 197-210, DOI: 10.1039/b719545c [10] Cho, I. S.; Lee, C. H.; Feng, Y.; Logar, M.; Rao, P. M.; Cai, L.; Kim, D. R.; Sinclair, R.; Zheng, X. Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical

performance,

Nat.

Commun.

2013,

4,

17231-17238,

DOI:

10.1038/ncomms2729 [11] Dotan, H.; Sivula, K.; Graetzel, M.; Rothschild, A.; Warren, S. C. Probing the photoelectrochemical properties of hematite (α-Fe2O3) electrodes using hydrogen peroxide as a hole scavenger, Energy Environ. Sci. 2011, 4, 958-964, DOI:10.1039/C0EE00570C [12] Zhou, L.; Zhao, C.; Giri, B.; Allen, P.; Xu, X.; Joshi, H.; Fan, Y.; Titova, L. V.; Rao, P. M. High light absorption and charge separation efficiency at low applied voltage from Sbdoped SnO2/BiVO4 core/shell nanorod-array photoanodes, Nano Lett. 2016, 16, 3463-3474, DOI: 10.1021/acs.nanolett.5b05200 [13] Kay, A.; Cesar, I.; Graetzel, M. New Benchmark for Water Photooxidation by Nanostructured α-Fe2O3 Films, J. Am. Chem. Soc. 2006, 128, 15714-15721, DOI: 10.1021/ja064380l [14] Cooper, J. K.; Gul, S.; Toma, F. M.; Chen, L.; Glans, P. A.; Guo, J.; Ager, J. W.; Yano, J.; Sharp, I. D. Electronic Structure of Monoclinic BiVO4, Chem. Mater. 2014, 26, 5365-5373, DOI: 10.1021/cm5025074 [15] Rettie, A. J. E.; Lee, H. C.; Marshall, L. G.; Lin, J. F.; Capan, C.; Lindemuth, J.; McCloy, J. S.; Zhou, J.; Bard, A. J.; Mullins, C. B. Combined charge carrier transport and



Page 28 of 45

photoelectrochemical characterization of BiVO4 single crystals: intrinsic behavior of a complex metal oxide, J. Am. Chem. Soc. 2013, 135, 11389-11396, DOI: 10.1021/ja405550k [16] Park, Y.; McDonald, K. J.; Choi, K. S. Progress in bismuth vanadate photoanodes for use

in

solar

water

oxidation,

Chem.

Soc.

Rev.

2013,

42,

2321-2337,

DOI:

10.1039/C2CS35260E [17] Chen, S.; Wang, L. W. Thermodynamic oxidation and reduction potentials of photocatalytic semiconductors in aqueous solution, Chem. Mater. 2012, 24, 3659-3666, DOI: 10.1021/cm302533s [18] Su, J.; Guo, L.; Yoriya, S.; Grimes, C. A. Aqueous Growth of Pyramidal-Shaped BiVO4 Nanowire arrays and structural characterization: application to photoelectrochemical water splitting, Cryst. Growth Des. 2010, 10, 856-861, DOI: 10.1021/cg9012125 [19] Zhang, M.; Shao, C. ; Li, X.; Zhang, P.; Sun, Y.; Su, C.; Zhang, X.; Ren, J.; Liu, Y. Carbon-modified BiVO4 microtubes embedded with Ag nanoparticles have high photocatalytic activity under visible light, Nanoscale 2012, 4, 7501-7508, DOI: 10.1039/c2nr32213g [20] Wang, G.; Ling, Y.; Lu, X.; Qian, F.; Tong, Y.; Zhang, J. Z.; Lordi, V.; Rocha Leao, C.; Li, Y. Computational and Photoelectrochemical Study of Hydrogenated Bismuth Vanadate, J. Phys. Chem. C 2013, 117, 10957-10964, DOI: 10.1021/jp401972h [21] Parmar, K. P.; Kang, H. J.; Bist, A.; Dua, P.; Jang, J. S.; Lee, J. S. Photocatalytic and photoelectrochemical water oxidation over metal-doped monoclinic BiVO4 photoanodes, ChemSusChem 2012, 5, 1926-1934, DOI: 10.1002/cssc.201200254 [22] Wang, X.; Xie, J.; Li, C. M. Architecting smart “umbrella” Bi2S3/rGO-modified TiO2 nanorod array structures at the nanoscale for efficient photoelectrocatalysis under visible light, J. Mater. Chem. A 2015, 3, 1235-1242, DOI:10.1039/C4TA05846A


Page 29 of 45 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


[23] He, H.; Berglund, S. P.; Rettie, A. J. E.; Chemelewski, W. D.; Xiao, P.; Zhang, Y.; Mullins, C. B. Synthesis of BiVO4 nanoflake array films for photoelectrochemical water oxidation, J. Mater. Chem. A 2014, 2, 9371-9379, DOI:10.1039/C4TA00895B [24] Tahir, A. A.; Ehsan, M. A.; Mazhar, M.; Wijayantha, K. G. U.; Zeller, M.; Hunter, A. D. Photoelectrochemical and Photoresponsive Properties of Bi2S3 Nanotube and Nanoparticle Thin Films, Chem. Mater. 2010, 22, 5084-5092, DOI: 10.1021/cm101642b [25] He, H.; Berglund, S. P.; Xiao, P.; Chemelewski, W. D.; Zhang, Y.; Mullins, C. B. Nanostructured Bi2S3/WO3 heterojunction films exhibiting enhanced photoelectrochemical performance, J. Mater. Chem. A 2013, 1, 12826-12834, DOI:10.1039/C3TA13239K [26] Desai, J. D.; Lokhande, C. D. Nonaqueous chemical bath deposition of Bi2S3 thin films, Mater. Chem. Phys. 1993, 34, 313-316, DOI.org/10.1016/0254-0584(93)90054-P [27] Wang, W.; Wang , X.; Zhou, C.; Du, B.; Cai, J.; Feng, G.; Zhang, R. Bi2S3-NanowireSensitized BiVO4 Sheets for Enhanced Visible-Light Photoelectrochemical Activities, J. Phys. Chem. C 2017, 121, 19104-19111, DOI: 10.1021/acs.jpcc.7b06838 [28] Liu, C.; Li, J.; Li, Y.; Li, W.; Yang, Y.; Chen, Q. Epitaxial growth of Bi2S3 nanowires on BiVO4 nanostructures for enhancing photoelectrochemical performance, RSC Adv. 2015, 5, 71692-71698, DOI:10.1039/C5RA13171E [29] Chen, L.; He, J.; Yuan, Q.; Liu, Y.; Au, C. T.; Yin, S. F. Environmentally benign synthesis of branched Bi2O3–Bi2S3 photocatalysts by an etching and re-growth method, J. Mater. Chem. A 2015, 3, 1096-1102, DOI:10.1039/C4TA05346J [30] Kim, J. H.; Magesh, G.; Kang, H. J.; Banu, M.; Kim, J. H.; Lee, J.; Lee, J. S. Carbonatecoordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode tailored for CO2 reduction to fuels, Nano Energy 2015, 15, 153-163, DOI.org/10.1016/j.nanoen.2015.04.022



Page 30 of 45

[31] Liu, C.; Yang, Y.; Li, W.; Li, J.; Li, Y.; Chen, Q. In situ synthesis of Bi2S3 sensitized WO3 nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performance, Sci. Rep. 2016, 6, 2345101-2345112, DOI: 10.1038/srep23451 [32] Yu, J. Q.; Kudo, A. Effects of Structural Variation on the Photocatalytic Performance of Hydrothermally

Synthesized

BiVO4,

Adv.

Funct.

Mater.

2006,

16,

2163-2169,

DOI.org/10.1002/adfm.200500799 [33] Yin, W. Z.; Wang, W. Z.; Zhou, L.; Sun, S. M.; Zhang, L. CTAB-assisted synthesis of monoclinic BiVO4 photocatalyst and its highly efficient degradation of organic dye under visible-light

irradiation,

J.

Hazard.

Mater.

2010,

173,

194-199,

DOI.org/10.1016/j.jhazmat.2009.08.068 [34] Toma, F. M.; Cooper, J. K.; Kunzelmann, V.; McDowell, M. T.; Yu, J.; Larson, D. M.; Borys, N. J.; Abelyan, C.; Beeman, J. W.; Yu, K. M.; Yang, J.; Chen, L.; Shaner, M. R.; Spurgeon, J.; Houle, F. A.; Persson, K. A.; Sharp, I. D. Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes, Nature Comm. 2016, 7, 1201201-12011211, DOI: 10.1038/ncomms12012 [35] Kho, Y. K.; Teoh, W. Y.; Iwase, A.; Madler, L.; Kudo, A.; Amal, R.

Flame

Preparation of Visible-Light-Responsive BiVO4 Oxygen Evolution Photocatalysts with Subsequent Activation via Aqueous Route, ACS Appl. Mater. Interfaces 2011, 3, 1997-2004, DOI: 10.1021/am200247y [36] Kim, H. R.; Kim, G.; In, S.; Park, Y. Optimization of porous BiVO4 photoanode from electrodeposited Bi electrode: Structural factors affecting photoelectrochemical performance, Electrochim. Acta 2016, 189, 252-258, DOI.org/10.1016/j.electacta.2015.12.106 [37] Thalluri, S.M.; Hernández, S.; Bensaid, S.; Saracco, G.; Russo, N. Green-synthesized W- and Mo-doped BiVO4 oriented along the {040} facet with enhanced activity for the sundriven water oxidation, Appl. Catal. B. 2016, 180, 630-636, DOI: 10.1021/ie403999g


Page 31 of 45 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] Oshikiri, M.; Boero, M.; Ye, J.; Zou, Z.; Kido, G. Electronic structures of promising photocatalysts InMO4 (M=V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region, J. Chem. Phys. 2002, 177, 7313-7318, DOI.org/10.1063/1.1507101 [39] Zhao, W.; Liu, Y.; Wei, Z.; Yang, S.; He, H.; Sun, C. Fabrication of a novel p–n heterojunction photocatalyst n-BiVO4@p-MoS2 with core–shell structure and its excellent visible-light photocatalytic reduction and oxidation activities, Appl. Catal. B. 2016, 185, 242252, DOI.org/10.1016/j.apcatb.2015.12.023 [40] Meng, X.; Zhang, L.; Dai, H. X.; Zhao, Z. X.; Zhang, R. Z.; Liu, Y. X. Surfactantassisted hydrothermal fabrication and visible-light-driven photocatalytic degradation of methylene blue over multiple morphological BiVO4 single-crystallites, Mater. Chem. Phys.

2011, 125, 59-65, DOI.org/10.1016/j.matchemphys.2010.08.071 [41] Jiang, H. Y.; Meng, X.; Dai, H. X.; Deng, J. G.; Liu, Y. X.; Zhang, L. High-performance porous spherical or octapod-like single-crystalline BiVO4 photocatalysts for the removal of phenol and methylene blue under visible-light illumination, J. Hazard. Mater. 2012, 217–218, 92-99, DOI.org/10.1016/j.jhazmat.2012.02.073 [42] Walsh, A.; Yan, Y.; Huda, M. N.; Al-Jassim, M. M.; Wei, S. H. Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals, Chem. Mater. 2009, 21, 547-551, DOI: 10.1021/cm802894z [43] Luo, H.; Mueller, A. H.; McCleskey, M.; Burrell, A. K.; Bauer, E.; Jia, Q. X. Structural and Photoelectrochemical Properties of BiVO4 Thin Films, J. Phys. Chem. C 2008, 112, 6099-6102, DOI: 10.1021/jp7113187 [44] Kudo, A.; Omori, K.; Kato, H. A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties, J. Am. Chem. Soc. 1999, 121, 11459-11467, DOI: 10.1021/ja992541y



Page 32 of 45

[45] Trześniewski, B. J.; Smith, W. A. Photocharged BiVO4 photoanodes for improved solar water splitting, J. Mater. Chem. A 2016, 4, 2919-2926, DOI:10.1039/C5TA04716A [46] Fan, H.; Jiang, T.; Li, H.; Wang, D.; Wang, L.; Zhai, J.; He, D.; Wang, P.; Xie, T. Effect of BiVO4 Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity, J. Phys. Chem. 2011, 116, 2425-2430, DOI: 10.1021/jp206798d [47] Chae, S. Y.; Jung, H.; Jeon, H. S.; Min, B. K.; Hwang, Y. J.; Joo, O. S. Morphology control of one-dimensional heterojunctions for highly efficient photoanodes used for solar water splitting, J. Mater. Chem. A 2014, 2, 11408-11416, DOI:10.1039/C4TA00702F [48] Peerakiatkhajohn, P.; Butburee, T.; Yun, J. H.; Chen, H.; Richards, R. M.; Wang, L. A hybrid

photoelectrode

with

plasmonic

Au@TiO2

nanoparticles

for

enhanced

photoelectrochemical water splitting, J. Mater. Chem. A 2015, 3, 20127-20133, DOI:10.1039/C5TA04137F [49] Lu, L.; Shen, Y.; Chen, X.; Qian, L.; Lu, K. Ultrahigh Strength and High Electrical Conductivity in Copper, Sci. 2004, 304, 422-426, DOI: 10.1126/science.1092905 [50] N. Liang, J. Zai, M. Xu, Q. Zhu, X. Wei and X. Qian, Novel Bi2S3/Bi2O2CO3 heterojunction photocatalysts with enhanced visible light responsive activity and wastewater treatment, J. Mater. Chem. A 2014, 2, 4208-4216, DOI:10.1039/C3TA13931J [51] Gao, X. ; Huang, G.; Gao, H.; Pan, C.; Wang, H.; Yan, J.; Liu, Y.; Qiu, H.; Ma, N. ; Gao, J. Facile fabrication of Bi2S3/SnS2 heterojunction photocatalysts with efficient photocatalytic activity

under

visible

light,

J.

Alloy.

Compd.

2016,

674,

98-108,

DOI.org/10.1016/j.jallcom.2016.03.031 [52] Fang, Z.; Liu, Y.; Fan, Y.; Ni, Y.; Wei, X.; Tang, K.; Shen J.; Chen, Y. Epitaxial Growth of CdS Nanoparticle on Bi2S3 Nanowire and Photocatalytic Application of the Heterostructure, J. Phys. Chem. C 2011, 115, 13968-13976, DOI: 10.1021/jp112259p


Page 33 of 45 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


[53] Ren, X.; Ma, Q.; Fan, H.; Pang, L.; Zhang, Y.; Yao, Y. ; Ren, X.; Liu, S. A Se-doped MoS2 nanosheet for improved hydrogen evolution reaction, Chem. Commun. 2015, 51, 15997-16000, DOI:10.1039/C5CC06847A [54] Booshehri, A. Y.; Goh, S. C. K.; Hong, J.; Jiang, R.; Xu, R. Effect of depositing silver nanoparticles on BiVO4 in enhancing visible light photocatalytic inactivation of bacteria in water, J. Mater. Chem. A 2014, 2, 6209-6217, DOI: 10.1039/C3TA15392D [55] Zhang, L.; Tan, P. Y.; Lim, C. K.; Guo, X.; Tse, M. S.; Tan, O. K.; Chang, V. W. C. N– TiO2-coated polyester filters for visible light-Photocatalytic removal of gaseous toluene under static and dynamic flow conditions, J. Environ. Chem. Eng. 2016, 4, 357-364, DOI.org/10.1016/j.jece.2015.11.027 [56] Han, P. L.; Mihi, A.; Ferre-borrull, J.; Pallarés, J.; Marsal, L. F. Interplay Between Morphology, Optical Properties, and Electronic Structure of Solution-Processed Bi2S3 Colloidal

Nanocrystals,

J.

Phys.

Chem.

C

2015,

119,

10693–10699,

DOI:

10.1021/acs.jpcc.5b01305 [57] Panmand, R. P.; Kumar, G.; Mahajan, S. M.; Kulkarni, M. V.; Kale, B. B.; Gosavi, S. W. Novel and Stable Mn2+@Bi2S3 Quantum Dots–Glass System with Giant Magneto Optical Faraday Rotations, J. Mater. Chem. C 2013, 1, 1203–1210, DOI:10.1039/C2TC00262K [58] Yue, W. J.; Han, S. K.; Peng, R. X.; Shen, W.; Geng, H. W.; Wu, F.; Tao, S. W.; Wang, M. T. CuInS2 quantum dots synthesized by a solvothermal route and their application as effective electron acceptors for hybrid solar cells, J. Mater. Chem. 2010, 20, 7570-7578, DOI: 10.1039/C0JM00611D [59] Chen, Y; Tian, G.; Guo, Q, Li, R.; Han, T.; Fu, H. One-step synthesis of a hierarchical Bi2S3 nanoflower\In2S3 nanosheet composite with efficient visible-light photocatalytic activity, CrystEngComm 2015, 17, 8720-8727, DOI:10.1039/C5CE01747E



Page 34 of 45

[60] Ijaz, S.; Ehsan, M. F.; Ashiq, M. N.; He, T. Synthesis of a Bi2S3/CeO2 nanocatalyst and its visible light-driven conversion of CO2 into CH3OH and CH4, Catal. Sci. Technol. 2015, 5, 5208-5215, DOI:10.1039/C5CY00955C [61] Guo, M.; He, Q.; Wang, A.; Wang, W.; Fu, Z. A novel, simple and greenway to fabricate BiVO4 with excellent photocatalytic activity and its methylene blue decomposition mechanism, Crystals 2016, 6, 8101-8111, DOI: 10.3390/cryst6070081 [62] Zou, L.; H. Wang, X. Wang, High Efficient Photodegradation and Photocatalytic Hydrogen Production of CdS/BiVO4 Heterostructure through Z‑Scheme Process, ACS Sustainable Chem. Eng. 2017, 5, 303−309, DOI: 10.1021/acssuschemeng.6b01628 [63] Samanta, S.; Khilari, S.; Pradhan, D.; Srivastava, R. An Efficient, Visible Light Driven, Selective Oxidation of Aromatic Alcohols and Amines with O2 Using BiVO4/g-C3N4 Nanocomposite: A Systematic and Comprehensive Study toward the Development of a Photocatalytic Process, ACS Sustainable Chem. Eng. 2017, 5 2562–2577, DOI: 10.1021/acssuschemeng.6b02902 [64] Mosa, I. M.; Biswas, S.; El-Sawy, A. M.; Botu, V.; Guild, C.; Song, W.; Ramprasad, R.; Rusling, J. F.; Sui, S. L. Tunable mesoporous manganese oxide for high performance oxygen reduction

and

evolution

reactions,

J.

Mater.

Chem.

A

2016,

4,

620-631,

DOI:10.1039/C5TA07878D [65] Kim, J.; Kang, M. High photocatalytic hydrogen production over the band gap-tuned urchin-like Bi2S3-loaded TiO2 composites system, Int. J. Hydrogen Energy 2012, 37, 82498256, DOI.org/10.1016/j.ijhydene.2012.02.057 [66] Gao, X.; Wang, Z.; Fu, F.; Li X.; Li, W. 2D double-layer-tube-shaped structure Bi2S3/ZnS heterojunction with enhanced photocatalytic activities, Phys. B 2015, 474, 81-89, DOI.org/10.1016/j.physb.2015.06.002


Page 35 of 45 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


[67] Mahadik, M. A.; Shinde, P. S.; Cho, M.; Jang, J. S. Fabrication of a ternary CdS/ZnIn2S4/TiO2 heterojunction for enhancing photoelectrochemical performance: effect of cascading

electron–hole

transfer,

J.

Mater.

Chem.

A

2015,

3,

23597-23606,

DOI:10.1039/C5TA07454A [68] Reber, J. F.; Rusek, M. Photochemical hydrogen production with platinized suspensions of cadmium sulfide and cadmium zinc sulfide modified by silver sulfide, J. Phys. Chem. 1986, 90, 824-834, DOI: 10.1021/j100277a024 [69] Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Cation Exchange Reactions in Ionic Nanocrystals, Sci. 2004, 306, 1009-1012, DOI: 10.1126/science.1103755



Page 36 of 45

Figure captions Figure 1. FESEM images of BiVO4 photoanodes prepared by varying Bi/V precursor concentrations, (a) BV1, (b) BV2, (c) BV3, and (d) magnified view of BV2 respectively.

Figure 2. (A) XRD patterns of BiVO4 photoanodes prepared by varying Bi/V precursor concentrations, (B) low and magnified FIB-TEM (a, b), HRTEM and inset shows FFT pattern, ZA=zone axis (c), and (d, g) the elemental mappings of

Bi, V, O and Sn

elements in BV2.

Figure 3. (a) UV-vis absorption spectra of BiVO4 films prepared using different concentration ratios of vanadium to Bismuth. The inset shows the band gap extrapolation. (b) J-V plots of BiVO4 samples prepared at different Bi/V ratios. (c) Comparative study of transient photocurrent (applied potential: ndia time RHE, Electrolyte: 0.1 M Na2S/ 0.025 M Na2SO3 electrolyte, Light: One sun illumination), (d-f) FESEM top images of BiVO4 after PEC measurements (ABV1, ABV2, and ABV3, respectively).

Figure 4. (a) XPS survey scan spectra and high resolution XPS spectra of (b) Bi 4f, (c) V 2p, of ABV2 of spectra of BV2 sample before and after PEC measurements, and (d) S2s spectrum of ABV2.

Figure 5. (a) BF TEM and ADF-STEM image of the BV2; (b) magnified TEM image of the BV2; (c and d) high-resolution TEM image and insets in (c–d) are fast Fourier transform (FFT) images, (e-j) EDX-TEM elemental mapping for Bi, V, S, O and Sn elements in a thin slice of Bi2S3/BiVO4 deposited on FTO.


Page 37 of 45 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


Figure 6. Photocurrent and hydrogen generation as a function of time for BV1, BV2, and BV3 photoelectrodes, respectively under irradiation of one sun illumination. (applied potential: 0.67 vs RHE, Electrolyte: 0.1 M Na2S/ 0.025 M Na2SO3 electrolyte, Nitrogen purging time: 30 min)

Figure 7. (a-d) Possible photoelectrochemical transformation and charge transport mechanism of Bi2S3/BiVO4 heterostructure photoelectrodes.



Figures

Figure 1


Page 38 of 45

Page 39 of 45 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


Figure 2



Figure 3


Page 40 of 45

Page 41 of 45 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


Figure 4



Figure 5


Page 42 of 45

Page 43 of 45 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


Figure 6



Figure 7


Page 44 of 45

Page 45 of 45 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


Graphic for Manuscript

In-situ photoelectrochemical transformation of BiVO4 to Bi2S3/BiVO4 nanostructures photoanode for improved charge transfer and light harvesting.


BiVO4 Hybrid Nanostructure

Recommend Documents