ZnO Photoanodes for High

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Three-Dimensional Bicontinuous BiVO4/ZnO Photoanodes for High Solar Water-Splitting Performance at Low Bias Potential Kiwon Kim and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, Sinsu-dong 1, Seoul 04107, Republic of Korea

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ABSTRACT: A photoanode capable of high-efficiency water oxidation at low bias potential is essential for its practical application for photocathode-coupled tandem systems. To address this issue, a photoanode with low turn-on voltage for water oxidation and high charge separation efficiency at low bias potential is essential. In this study, we demonstrate the photoanode of the BiVO4/ZnO three-dimensional (3D) bicontinuous (BC) structure. ZnO has a relatively cathodic flat-band potential, which leads to low turn-on potential; the BiVO4/ZnO 3D BC photoanode shows an onset potential of 0.09 V versus the reversible hydrogen electrode (VRHE). Moreover, we achieve remarkably high charge separation efficiency at low bias potential (78% at 0.6 VRHE); this is attributed to the application of thin-film BiVO4 shells by high light-scattering properties of the 3D BC structure. As a result, the BiVO4/ZnO 3D BC photoanode generates a high water oxidation photocurrent of up to 3.4 ± 0.2 mA cm−2 (with CoPi catalyst coating). This photocurrent value is reproducible, and the photocurrent-to-O2 conversion efficiency is over 90%. To the best of our knowledge, this is the highest value among the values of the photocurrent at 0.6 VRHE in previous BiVO4-based heterojunction photoanodes. KEYWORDS: heterojunction photoanodes, photoelectrochemical water splitting, bicontinuous structures, low bias potentials, zinc oxide

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potential, is provided by the photocathode;13−15 this tandem system enables stand-alone operation. In this system, the operating voltage is determined by the point at which the photocurrent−voltage curves of the photoanode and photocathode cross. In common practice, the operation voltage should be less than the onset potential of the photocathode; various conventional photocathodes, including Si, InP, Cu2O, CdTe, and CIGS exhibited an onset potential in the range of 0.6−0.7 V versus the reversible hydrogen electrode (VRHE).14,16−18 Thus, photoanodes with high water oxidation performance at low bias potential (i.e., potentials below 0.6− 0.7 VRHE) are required. However, most of the PEC photoanode studies have focused on water oxidation at the maximum thermodynamic water oxidation potential, 1.23 VRHE.8,19,20 Indeed, in many of these studies, the photocurrent density for water oxidation at 0.6 VRHE was very low (see Table S1).

INTRODUCTION Photoelectrochemical (PEC) water splitting is a promising method to convert solar energy into chemical energy (i.e., hydrogen).1,2 The key component in this system is a photoanode that induces water oxidation through the absorption of solar light.1,3 Water oxidation is kinetically slow and involves the transfer of multiple electrons and protons, thus requiring an efficient and stable photoanode system.4,5 Various metal oxides such as TiO2, WO3, BiVO4, and Fe2O3 have been applied,3,6,7 but these materials are limited in that the light absorption band is narrow because of the large band gap, and/or the charge transfer is poor.3 Recent studies have employed heterojunctions of two or more metal oxides to overcome these limitations. The BiVO4/WO3 is a typical heterojunction photoanode; various nanostructures of the BiVO4-based heterojunction, including nanorods, nanowire arrays, and nanohelixes have been studied.8−12 The practical application of PEC water splitting should take into account that a two-electrode system with coupled photoanodes and photocathodes, that is, an additional bias © XXXX American Chemical Society

Received: July 6, 2018 Accepted: September 14, 2018

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DOI: 10.1021/acsami.8b11241 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Characterization. The morphologies were examined by fieldemission scanning electron microscopy (FE-SEM, JEOL) and transmission electron microscopy (TEM, JEM-3010, JEOL). The chemical compositions were determined using energy-dispersive X-ray spectroscopy. X-ray photoelectron spectroscopy (XPS) was performed using a Leybold spectrometer. (ESCALAB 250 XPS system, Theta Probe XPS system). The crystal structure was analysed by Xray diffraction (XRD, Rigaku). The UV−visible transmittance and diffuse reflectance were measured by UV−vis spectrophotometry (UV-2550, Shimadzu) with an integrating sphere. PEC Characterization. The PEC photocurrent−voltage performance was examined using a potentiostat (VersaSTAT, Ametek) in a three-electrode system, with the photoanode as the working electrode, a Pt wire as the counter electrode, and an Ag/AgCl as the reference electrode. The scan rate was 10 mV/s. The illuminated area defined by the mask was approximately 1 cm2. All PEC currents were averaged over 3−5 samples. A solution of 0.2 M Na2SO4 (pH 6.5) was used as the electrolyte. Linear sweep voltammetry was conducted under simulated solar illumination using a 150 W Xe lamp (PEC-L01, Peccell Technologies) and AM 1.5G filters. All measurements were carried out using an Ag/AgCl (saturated KCl) reference electrode, and all results in this work are presented against RHE. The conversion between the potential versus Ag/AgCl and the potential versus RHE was performed using the equation E (vs RHE) = E (vs Ag/AgCl) + EAg/AgCl (reference) + 0.059 V × pH, where EAg/AgCl (reference) = 0.197 V versus NHE at 25 °C. The incident photon-to-current conversion efficiency (IPCE) was measured using a 300 W Xe light source (Oriel) with a monochromator (Cornerstone 130, 1/8 m, Newport) at 0.6 V versus RHE. The incident light intensity was measured using a photodiode detector (silicon calibrated detector, Newport). O2 evolution was measured using a fluorescence O2 sensor (NEOFOX-KIT-PROBE). Mott−Schottky measurements were conducted using impedance spectroscopy in a dark environment in 0.2 M Na2SO4 solution at a frequency of 1 kHz. Electrochemical impedance spectroscopy (EIS) was performed using a potentiostat (VersaSTAT, Ametek) in a three-electrode system in the potentiostatic mode with an ac voltage amplitude of 5 mV over a frequency range of 0.1−100 kHz under AM 1.5G illumination. The charge transfer resistance was obtained using Z-view software (Scribner Associates).

Achieving a high water-splitting performance at lower bias potential is more challenging, as a low bias potential is relatively not enough to separate charge carriers and drive photocurrent generation.21 Thus, water oxidation at low bias potential is strongly affected by intrinsic properties of the photoanodes, such as band energy levels, charge mobility, and nanostructure. The BiVO4/WO3 photoanode has a high onset potential because the position of flat band potential (Efb) in WO3 is relatively positive; thus, it is difficult to obtain high water oxidation at 0.6 VRHE (see Table S1). Several studies have used heterojunctions with SnO2, CoO2, and ZnO instead of WO3.22−26 Because these materials have a relatively negative Efb relative to WO3, the heterojunction of these materials with BiVO4 showed a lower onset potential than the BiVO4/WO3 photoanode (see Table S2). However, the highest photocurrent at 0.6 VRHE was about 2.6 mA cm−2 obtained by Sbdoped SnO2/BiVO4 nanorods;27 this value is only about 35% of the value at 1.23 VRHE. Thus, the development of highperformance heterojunction photoanodes at low onset potential still remains a great challenge. In this study, we introduce a three-dimensional (3D) bicontinuous (BC) BiVO4/ZnO heterojunction photoanode with high water-splitting performance at a low bias potential of 0.6 VRHE. ZnO has a relatively negative Efb of 0.2−0.3 VRHE (pH 6.5−7.4),22,28,29 compared to WO3 (0.4−0.7 VRHE, pH 7);30−32 the potential loss upon electron transfer toward ZnO is relatively small, and thus, a cathodic shift of more than 0.1 V compared to WO3 is allowed. In particular, 3D BC BiVO4/ ZnO showed a remarkably high charge separation efficiency (ηsep) of 78% at 0.6 VRHE. As a result, the 3D BC BiVO4/ZnO photoanode generates a water oxidation photocurrent density of 3.4 ± 0.2 mA cm−2 at 0.6 VRHE. To the best of our knowledge, this is the highest level of water oxidation photocurrent at 0.6 VRHE among the previous BiVO4/WO3 heterojunction photoanodes.

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RESULTS AND DISCUSSION To prepare the BiVO4/ZnO 3D BC films, we first fabricated a ZnO 3D porous structure using a polymer colloidal crystal template. Subsequently, a continuous shell layer of BiVO4 was formed on the ZnO surface by coating a precursor solution, resulting in the 3D BC structure of ZnO and BiVO4, as described in Figure 1a. The ZnO porous film has a pore structure with a face-centred cubic symmetry and macropores of approximately 360 nm in size, as shown in Figure 1b. The conformal BiVO4 layer in the BiVO4/ZnO BC film has a thickness of 40−50 nm, as observed in Figure 1c. We controlled the thickness of the BiVO4 layer by controlling the concentration of the BiVO4 precursor coating solution (Figure S1a). The thickness of the BC film was controlled to be 2 μm, as this thickness showed the highest photocurrent value (Figure S1b). The BiVO4/ZnO 3D BC film exhibited a uniform thickness without cracks, as shown in Figure 1d. Elemental analysis of the cross-section of the film confirmed that BiVO4 was uniformly formed inside the film, as observed in Figure 1e. The crystal structure of the BiVO4/ZnO 3D BC film was confirmed by lattice analysis using high-resolution TEM and XRD. The high-resolution TEM image reveals lattice spacings of 0.24 and 0.46 nm, which correspond to the (101) plane of wurtzite ZnO and the (011) plane of monoclinic BiVO4, respectively, as observed in Figure 2a.33 Figure 2b shows an atomic lattice illustration at the interface of the (101) plane of

EXPERIMENTAL PROCEDURES

Fabrication of BiVO4/ZnO 3D BC Photoanodes. The ZnO 3D BC structures were fabricated using colloidal crystal templates, where the colloidal crystals were obtained by self-assembly of monodisperse polystyrene (PS) spheres of approximately 550 nm in size. ZnO deposition was performed using atomic layer deposition. Diethylzinc and deionized water were used as precursors and were sequentially fed into the reactor using N2 gas. A chamber pressure of 2.0 Torr, an exposure time of 1 s, and a purge time of 2 s were used throughout the deposition for both ZnO and water. Subsequently, the ZnOdeposited PS colloidal crystals were heated at 500 °C for 2 h in air to remove the PS templates, leaving behind the ZnO 3D BC structures. The BiVO4 shell was prepared by spin-coating a precursor solution of bismuth nitrate pentahydrate [Bi(NO3)3·5H2O] (Sigma, 99.9%) and vanadyl acetylacetonate [VO(acac)2] (Fisher, 99%) in 2-methoxyethanol (Sigma, 99.9%). The BiVO4/ZnO bilayer film was prepared the same as the 3D bicontinuous film, except that no colloidal crystal template was used. The BiVO4/ZnO bilayer was fabricated by first coating ZnO on FTO by atomic layer deposition and spin coating the BiVO4 precursor solution to form the BiVO4 layer. After each layer was coated, heat treatment was performed in air at 450 °C. The BiVO4/WO3 bilayer was fabricated by spin coating a 0.25 M H4WO3 solution and then coating the BiVO4 layer with the same precursor solution. After each layer was coated, heat treatment was performed in air at 450 °C. A cobalt−phosphate (CoPi) complex was deposited by electrodeposition using a three-electrode cell with an Ag/AgCl reference electrode and a Pt counter electrode. The electrodeposition was performed in 0.5 mM cobalt nitrate in 0.1 M phosphate buffer (pH 7) for 300 s. B

DOI: 10.1021/acsami.8b11241 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

bilayer film were comparable; the flat-band potentials of BiVO4/ZnO 3D BC and BiVO4/ZnO bilayer samples were almost identical as observed in Figure S4. In Figure 2c, the XRD peaks of the bilayer are exactly the same as those of the 3D BC. We measured Efb to confirm the alignment of the conduction band position of BiVO4 and ZnO. The Efb of ZnO and BiVO4 was approximately −0.25 and −0.45 V versus Ag/AgCl at pH 7.0, respectively, as shown in Figure S2. Because the conduction band position of n-type semiconductors is close to the flat-band potential, the alignment of the conduction band between BiVO4 and ZnO is favorable for electrons to drain from BiVO4 to ZnO. The water oxidation of the photoanode is affected by the various efficiencies of light harvesting (LHE), electron−hole separation (ηsep), and the efficiency of charge transfer at the semiconductor/electrolyte interface (ηtrans).27 Because ηtrans can achieve unity by coating oxygen evolution catalysts (OECs),5,37 LHE and ηsep determine the performance of the photoanode. First, we evaluate the LHE of the BiVO4/ZnO 3D BC photoanode. The LHE was obtained by using the formula, LHE = 1 − 10−A(λ), where A(λ) is the absorbance at the wavelength λ (see Figure 3a). The reflection and transmission spectra are shown in Figure S3a,b, respectively. For comparison, we obtained the LHE of a BiVO4/ZnO bilayer film. The LHE of the 3D BC and bilayer film sharply decreases near the wavelength of about 450 nm, whereas the 3D BC exhibits an extended absorption tail of up to 550 nm, as observed in Figure 3a. Specifically, the LHE of 3D BC in the wavelength range of 450−500 nm is about 50% higher than that of the bilayer film; this represents a strong light-scattering property in the 3D BC structure. The 3D BC photoanode (2 μm thick) shows the LHE of about 90% over the wavelength range of 300−450 nm. Previously, one of the top-performing BiVO4/WO3 nanohelix (2 μm film thickness) photoanodes obtained the LHE of 80−90% in this wavelength range;8 thus, the 3D BC structure reveals highly efficient LHE properties. The ηsep was obtained by using the photocurrent for sulfite oxidation (Jsulfite) in the 0.1 M Na2SO3-containing electrolyte.38 The charge transfer rate to oxidize sulfite ions is very fast,39,40 and thus 100% of ηtrans can be assumed.41,42 Then, the Jsulfite can be expressed as Jsulfite = Jmax × LHE × ηsep. The ηsep value is obtained by dividing Jsulfite by (Jmax × LHE). Jmax × ηabs was calculated from the integration of the product of the LHE and the spectral irradiance (see the detailed calculation in Figure S5). Figure 3b,c show the Jsulfite and ηsep values of BiVO4/ZnO 3D BC and the bilayer film for various potentials, respectively. At 0.6 VRHE, the Jsulfite value of the 3D BC was approximately

Figure 1. (a) Schematic for the fabrication of the BiVO4/ZnO 3D BC structure. SEM images of (b) ZnO 3D porous scaffold and (c) BiVO4/ZnO 3D BC and (d) Low-magnification image of BiVO4/ ZnO 3D BC. The inset shows a digital camera image of the BiVO4/ ZnO BC film. (e) Elemental analysis of the cross-section of the BiVO4/ZnO 3D BC film. The scale bars in the images are 1 μm.

ZnO and the (011) plane of BiVO4, showing an intact interface with well-matched lattice distances. As shown in Figure 2c, the XRD pattern of the BiVO4/ZnO BC film exhibits peaks at 33°, 35°, and 37°, corresponding to the (100), (002), and (101) planes of wurtzite ZnO, respectively.34 The peaks at 20°, 30.5°, 32°, and 42.5° correspond to the (011), (121), (040), and (211) planes of monoclinic BiVO4, respectively.35,36 The average crystallite size is estimated using the Scherrer equation; the average crystallite size was approximately 10 nm. For comparison, the BiVO4/ZnO bilayer film was prepared; the masses of BiVO4 and ZnO in the 3D BC structure and the

Figure 2. TEM image of (a) BiVO4/ZnO 3D BC structure. The inset shows a low-magnification image of BiVO4/ZnO 3D BC. The scale bars in the images are 10 nm. (b) Schematic illustration of the ZnO (101) plane along the BiVO4 (011) plane. (c) XRD patterns of the BiVO4/ZnO 3D BC structure, BiVO4 (JCPDS no. 14-0688), and ZnO (JCPDS no. 36-1451). C

DOI: 10.1021/acsami.8b11241 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) LHEs of the BiVO4/ZnO bilayer film and BiVO4/ZnO 3D BC. (b) Sulfite oxidation photocurrent density (Jsulfite) vs voltage curves of the BiVO4/ZnO 3D BC and the bilayer film. (c) Separation efficiency and (d) EIS Nyquist plot, and (e) water oxidation photocurrent (JH2O) vs voltage curves of the BiVO4/ZnO 3D BC and the bilayer film.

Figure 4. (a) Schematic image of the water-splitting reaction by the CoPi/BiVO4/ZnO 3D BC structure. Band diagram of CoPi/BiVO4/ZnO 3D BC upon light irradiation. (b) Water oxidation photocurrent vs voltage curve by CoPi/BiVO4/ZnO 3D BC measured under 100 mW/cm2 AM 1.5G illumination (scan rate, 10 mV/s) and (c) IPCE spectra measured in the same solution at 0.6 VRHE. (d) Oxygen and hydrogen evolution and the photocurrent-to-O2 conversion efficiency of CoPi/BiVO4/ZnO 3D BC photoanodes at 0.6 VRHE.

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DOI: 10.1021/acsami.8b11241 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces 4.1 mA cm−2 and ηsep was 78%; the 3D BC photoanode showed about 7 times higher ηsep than the bilayer film photoanode. The low ηsep of the bilayer photoanode is due to the thickness of the BiVO4 layer being 400 nm, which is more than the charge diffusion length of BiVO4, which is about 70− 100 nm.43,44 On the other hand, the 3D BC structure has a BiVO4 film thickness of 40−50 nm, which is much smaller than the charge diffusion length, and thus exhibits a very high ηsep. Notably, the ηsep of the BiVO4/ZnO 3D BC photoanode at 0.6 VRHE is remarkably higher than those of the previously reported BiVO4/WO3 photoanodes. For example, the BiVO4/ WO3 core/shell nanowire photoanode achieved approximately 38% at 0.6 VRHE,19 and the BiVO4/WO3 nanohelix displayed approximately 58% at 0.6 VRHE.8 In Figure 3d, we obtained the interfacial charge transfer resistance (Rct) of BiVO4/ZnO 3D BC and the bilayer film by EIS at 0.6 VRHE. We obtained the Rct value using the equivalent Randle circuit (R-RC); the Rct values of the 3D BC and the bilayer were 877 and 2614 Ω, respectively. These results confirm the improved charge transfer in the 3D BC structure at 0.6 VRHE. The water-splitting performance of the BiVO4/ZnO 3D BC photoanode was evaluated. The photocurrent density versus voltage (J−V) curves was obtained under 1 sun AM 1.5G illumination, which is shown in Figure 3e. Similarly, the J−V curve of the BiVO4/ZnO bilayer photoanode was measured for comparison. Figure S6 also displays the J−V for 3D ZnO and compares it with these samples. In the dark conditions, all samples show negligible photocurrent generation up to 1.23 VRHE (see Figure S7). The water-splitting photocurrent of the BiVO4/ZnO 3D BC photoanode at 0.6 VRHE is 0.92 mA cm−2. This value was about 7 times higher than the photocurrent density of the bilayer film; this photocurrent ratio is similar to the ratio of the IPCE of each photoanode integrated (Figure S8). The high photocurrent value of 3D BC at 0.6 VRHE is because of the superior LHE and ηsep. We coated a CoPi OEC on the surface of BiVO4 to evaluate the maximum water-splitting photocurrent of BiVO4/ZnO 3D BC photoanodes.45 The charge transport in the OEC-coated BiVO4/ZnO 3D BC photoanode is depicted in Figure 4a.5,46 The presence of CoPi improves oxygen evolution kinetics, achieving unity of ηtrans. CoPi was coated by anodic electrodeposition.22 XPS analysis confirmed the presence of cobalt and phosphorus (see Figure S9). The CoPi/BiVO4/ ZnO 3D BC electrode achieve a water oxidation photocurrent of approximately 3.4 ± 0.2 mA cm−2 at 0.6 VRHE, as observed in Figure 4b. Figure S10 compares the J−V of CoPi/BiVO4/ ZnO 3D BC obtained from front-side illumination and backside illumination. The photocurrent output is reproducible as observed in Figure S11. The stability of photocurrent is also exhibited in Figure S12. To the best of our knowledge, this water oxidation photocurrent at 0.6 VRHE is greater than those of previous BiVO4/WO3 photoanodes (see Tables S1 and S2). Besides the excellent LHE and ηsep of BiVO4/ZnO 3D BC, the high photocurrent value at 0.6 VRHE is attributed to a lower turn-on potential of about 0.09 VRHE. Note that this onset potential is lower than the onset potential range of various BiVO4/WO3 photoanodes47,48 (see Table S1). Moreover, this value is even comparable to the value of 0.11 VRHE from the recently reported top-performing BiVO4 photoanode.49 The onset potential is related to the position of Efb of the core material of the heterojunction;50 the Efb of ZnO measured from Mott−Schottky plots in Figure S2 is more negative than

the Efb values of WO3 reported in the literature,47,48 which lead to the low onset potential at the heterojunction with ZnO. The IPCE of CoPi/BiVO4/ZnO 3D BC is measured at 0.6 VRHE as shown in Figure 4c. The value obtained by integrating the IPCE with respect to the wavelength corresponds to the photocurrent density at 0.6 VRHE. Meanwhile, at the wavelength of 470 nm corresponding to the indirect bandgap of BiVO4,51 the IPCE value was up to 20%. At the indirect bandgap wavelength, low IPCE is obtained by a high recombination rate. Therefore, the high IPCE value in the indirect transition wavelength reveals an efficient charge separation in the 3D BC photoanodes. To confirm the measured photocurrent which was generated by water splitting, the amount of H2/O2 gases was measured, as observed in Figure 4d. The efficiency of the photocurrentto-O2 conversion efficiency was calculated to be 90%. The molar ratio of the evolved H2/O2 was 1.9:1, which is similar to the 2:1 stoichiometric ratio of H2/O2. Thus, most of the photogenerated charge was consumed for water splitting.

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CONCLUSIONS Our study presents a heterojunction photoanode that simultaneously satisfies low turn-on potential and excellent charge separation at low bias voltages. Our BiVO4/ZnO 3D BC showed a low onset potential of 0.09 VRHE by ZnO with a relative cathodic Efb. The use of a thin BiVO4 layer by high LHE in the 3D BC structure achieved a remarkably high ηsep of 78% at 0.6 VRHE. As a result, the BiVO4/ZnO 3D BC photoanode obtained a water splitting photocurrent of 3.4 ± 0.2 mA cm−2 at 0.6 VRHE under 1 sun illumination; the photocurrent-to-O2 conversion efficiency was 90%. Considering that many previous BiVO4-based photoanodes showed even negligible water splitting photocurrent output at 0.6 VRHE, our results promise practical water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11241.

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J−V curves of 3D BC of the BiVO4 layer by various concentration of BiVO4 precursor coating, J−V curves of 3D BC of various thicknesses, Mott−Schottky plots of ZnO and BiVO4 films, transmittance and reflectance spectra, Tauc plot SEM/elemental mapping of BiVO4/ ZnO bilayer films, details of separation efficiency calculation, J−V curve of 3D ZnO, J−V curve of CoPi3D BC, 3D BC and bilayer photoanodes in the dark, IPCE spectra of CoPi-3D BC, 3D BC and bilayer photoanodes, XPS of CoPi catalysts, J−V curves of CoPi-3D BC under FTO and BiVO4 illumination, photocurrent values for independent runs of CoPi-3D BC photoanodes, photocurrent output over time of CoPi-3D BC photoanodes, and a table comparing the results of previous BiVO4 heterojunction photoanodes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jun Hyuk Moon: 0000-0002-4776-3115 E

DOI: 10.1021/acsami.8b11241 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (2016M3D3A1A01913254, 20110030253). The Korea Basic Science Institute is also acknowledged for the SEM and EDS measurements.

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DOI: 10.1021/acsami.8b11241 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX