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Rapid Screening of Photoanode Materials using SPECM Technique and Formation of Z-scheme Solar Water Splitting System by Cou-pling p- and n-type Heterojunction Photoelectrodes Pravin S. Shinde, Xiaoniu Peng, Jue Wang, Yanxiao Ma, Louis Edward McNamara, Nathan I Hammer, Arunava Gupta, and Shanlin Pan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00381 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Energy Materials

Rapid Screening of Photoanode Materials using SPECM Technique and Formation of Z-scheme Solar Water Splitting System by Coupling p- and n-type Heterojunction Photoelectrodes Pravin S. Shinde,†, ‡ Xiaoniu Peng,§ Jue Wang,†, ‡ Yanxiao Ma,†, ‡ Louis E. McNamara,# Nathan I. Hammer,# Arunava Gupta, †, ‡,⟘ and Shanlin Pan *, †,‡ †

Department of Chemistry and Biochemistry, ‡ Center for Materials for Information Technology, ⟘ Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, Alabama 35487, United States § Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Physics and Electronic Science, Hubei University, Wuhan 430062, P. R. China # Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38655, United States

KEYWORDS: SPECM, Z-scheme Solar Water Splitting, Photoelectrodes, Onset potential, ABSTRACT: A scanning photoelectrochemical microscopy (SPECM) technique is applied to rapidly screening the photoelectrochemical (PEC) activities of cobalt (Co)-incorporated bismuth vanadate (BiVO4) photocatalyst arrays with varying Co concentrations on conducting FTO and Ti substrates. The SPECM screening study is successfully utilized to determine an optimal Co concentration of 6% to improve the photocatalytic performance of BiVO4. Subsequently, pristine and Co-doped BiVO4 thin film photoanodes are fabricated by spin-coating/drop-casting methods with optimal precursor concentrations of Co, Bi and V to validate the results of SPECM. X-ray diffraction structural characterization shows 6% Co-BiVO4 contains a photocatalytically active scheelitemonoclinic phase of BiVO4. Scanning electron microscopy images and EDAX show that 6% Co is partly incorporated into the BiVO4 lattice and the remaining accumulates on the surface in the form of cobalt oxide, which is further evidenced by X-ray photoelectron spectroscopy (XPS) and Raman studies. Co-doped BiVO4 thin film photoanode prepared by spin-coating method exhibits similar remarkable PEC response with ~150% increase in photocurrent density at 1.0 V vs. RHE with respect to the pristine BiVO4 photoanode. Additionally, such photoanode exhibited a cathodic shift of ~200 mV in the onset of water oxidation photocurrent. The mott-schottky analysis confirms a relative improvement in charge carrier density for Co-doped BiVO4 photoanode. Thus, the enhanced water splitting performance by Co is attributed to largely due to 1) enhancement in water oxidation kinetics via formation of cobalt oxide (Co3O4) on the surface of BiVO4, and partially due to 2) enrichment in electronic conductivity via Co doping BiVO4. An unbiased Z-scheme solar water splitting system is demonstrated at the outlet of this work by combining optimized Co-doped BiVO4/WO3 photoanode and CuO/CuBi2O4 photocathode in a two-electrode configuration. INTRODUCTION Direct solar energy driven photoelectrochemical (PEC) water splitting by employing photoanodes, photocathodes, or both in a tandem cell configuration holds great promise toward direct and sustainable generation of hydrogen fuel.1-2 Ever since the discovery of electrochemical photolysis on TiO2,3 the photocatalytic and PEC water splitting processes using semiconducting materials have been extensively studied for the development of renewable hydrogen energy system.4-7 However, the performance of photocatalysts for practical applications is greatly limited by narrow band gap requirement of semiconductors to promote the efficient utilization of solar light and respective locations of valence band and/or the conduction band edges in the redox potential range of the O2 and/or H2. In addition, the recombination of the photoinduced pairs seriously reduce the utilization efficiency of solar energy.8-9 With the aim of obtaining highly efficient devices for water splitting, catalysts are required that decrease the overpotential necessary for the hydrogen evolution reaction (HER) and the oxygen

evolution reaction (OER). Particularly, photoelectrochemical systems based on semiconductor materials have shown high efficiencies for the conversion of solar energy to fuels or electricity. While the HER is a comparatively easily catalyzed process featuring a low overpotential, the OER requires a significant overpotential even with the best known electrocatalysts due to the four-electron/four-proton transfer process.10 Metal oxides have been extensively investigated as potential electrocatalyst candidates for PEC water oxidation11 because they are capable of promoting OER reaction via photogenerated holes.10 Narrow band gap, visible-light–active semiconductors have been extensively studied to improve the water oxidation performance.12-14 Narrow band gap provides efficient utilization of solar light, and the type II band alignment promotes spatial charge separation and decreases charge carrier recombination.10, 15-19 Due to this reason, n-type semiconductor metal oxides such as bismuth vanadate (BiVO4), tungsten oxide (WO3), etc. can serve as effective photoanodes for splitting water under solar light to generate oxygen and hydro-

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gen fuel. BiVO4 (Eg~2.4 eV) is known as a promising O2 evolution photocatalyst.20-23 Nevertheless, its PEC conversion efficiency is inadequately low for practical applications because of poor electron-hole separation yield. To improve its water oxidation efficiency, several approaches such as heterojunction,15-16, 24 elemental doping,25-27 and coupling with oxygen evolving catalysts28-30 have been used. Mo or W doping has improved the electronic properties by enriching the carrier density of BiVO4.31-33 In another effort, simultaneous doping of Mo and W metals in BiVO4 improved the water splitting performance dramatically.31 It should be noted that cobalt or cobalt phosphate has been commonly employed as a cocatalyst when loaded on top of a photocatalyst such as BiVO4 to improve its water oxidation.34-39 Several other studies have been reported on Co-doping effect on photocatalytic performance of BiVO4.40-44 However, none of these shows photocatalytic characteristics of the modified photoanodes for water oxidation. All these studies are focused on synthesis of Codoped BiVO4 in powdered form and their utilization for photocatalytic degradation of organic additives in water. BiVO4 containing 5% Co exhibited 85% photocatalytic activity versus 65% by pristine BiVO4 in oxidative removal of methylene blue (MB) in 5 h, which has been attributed to the formation of Co3O4 oxide catalyst on the surface of BiVO4 and increase in visible light absorption.40 BiVO4 doped with 6% Co2+ (0.06:1 mole ratio) significantly improved the photocatalytic MB degradation performance under visible light irradiation.44 Incorporation of a small fraction of Co2+ in BiVO4 structure has resulted in narrowing of band gap that enhanced the photocatalytic photo-degradation activity of MB and dichlorophenol by suppressing the recombination of photogenerated electrons and holes.42 Co-doped BiVO4 samples prepared by wet impregnation method exhibited significant enhancement in the photocatalytic decolorization of MB44 and phenol.43 The enhancement has been attributed to the suppression of electron-hole recombination due to formation of Co3O4-BiVO4 p-n heterojunction with few nanometer thick Co3O4. WO3 with band gap of ~ 2.8 eV is a promising candidate that can be combined with BiVO4 to enhance the electronic conductivity and thereby improve its PEC performance. Thus, rational design and transformation of electronic structures of existing semiconductor electrode materials would help address sluggish kinetics and stability issues of known electrode materials for energy conversion devices with improved efficiency. The main challenge in studying many of these energy systems lies in the complexity of the electrode-electrolyte interface. The efficiency of such systems is determined by the interplay between the nature (material and structure) of the electrode, mass transport of reactants and products to and from the electrode, and the surface reactivity, all of which are strongly localized on the nanoscale. In this regard, scanning photoelectrochemical microscopy (SPECM) technique has proven to be a powerful tool for the analysis of a wide variety of surfaces and has been used for rapid screening of different photocatalysts,10, 45-47 evaluating libraries of photocatalysts.48 SPECM is a scanning probe technique that can be used to rapidly screen large number of catalytic samples and study the local electrochemical and PEC properties of a photocatalyst and is in principle capable of providing in-depth characterization of photoactive systems.49 Bard group has been instrumental in exploring the versatility of SPECM technique for synthesizing and characterizing new materials.31 SPECM has been successfully used to demonstrate 10 times improvement in the photooxidation cur-

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rent of BiVO4 by Mo/W-doping.31 However, there are no reports on optimization of Co-containing BiVO4 using SPECM technique and/or detailed understanding of the structural and morphological effect on PEC characteristics upon Co incorporation. There is no study of Co-containing BiVO4 with enhanced PEC response upon the formation of a heterojunction with WO3 for complete water splitting in the absence of applied bias. In this work, we employed a high-throughput screening and optimizing technique based on combinatorial chemistry and SPECM technique to prepare photocatalytically active Codoped BiVO4 in combination with WO3 to form a heterojunction photoanode. No prior studies pertaining to PEC water splitting using thin films of Co-doped BiVO4 in combination with WO3 photoelectrodes have been reported. Through this study, we have demonstrated that if optimally incorporated, cobalt performs a dual role of improving the electronic conductivity as well as catalytically performing the water oxidation reaction at lower applied potential. To support these results, we have carried out detailed Raman and XPS characterizations on the doped BiVO4 samples used for SPECM study and validated the SPECM results with their bulk counterparts by fabricating and characterizing thin film photoelectrodes. Finally, a Z-scheme solar water splitting system is independently established by connecting Co-BiVO4/WO3 photoanode to a CuO/CuBi2O4 photocathode in a series configuration to split the water at zero external voltage bias. MATERIALS AND METHODS Bismuth (III) nitrate (Bi(NO3)3·5H2O, 98%) and cobalt (II) nitrate (Co(NO3)2·6H2O, 98%) from Alfa Aesar, ammonium metavanadate (NH4VO3, 99.5%) from Acros Organics, potassium phosphate (K2HPO4, 99.4%) from Fisher Scientific, ethylene glycol (HOCH2CH2OH, 99%) from BDH, sodium hydroxide (beads, VWR), sodium sulfate anhydrous (Na2SO4, 99%, Fisher Scientific), polyvinyl alcohol (PVA), acetone, absolute ethanol, and ethanol, etc. were used as-received without further purification. All the solutions were freshly prepared using high-purity deionized water (Resistivity 515 nm) should not contribute to the light absorption in pristine BiVO4 (Eg ~2.4 eV). Hence, the absorbance data between 550 and 800 nm is considered as a background due to the contribution from the thicker nature of the films. A red-shift in the absorption (by ~100 nm) is seen for 7% Co-doping with enhanced absorptivity in the entire spectral region. Additionally, Co-doped BiVO4 shows additional absorbance extending from 550 to 800 nm. A red-shift in UV-vis spectrum with enhanced visible light absorption beyond 500 nm has also been observed for the Codoped BiVO4 prepared by the amorphous heteronuclear complexing method.42 Such extended absorbance has previously been reported to be due to the contribution of Co3O4 formed on BiVO4.43, 59 or Therefore, it is likely that cobalt in our sample has turned partially into cobalt oxide contributing such absorbance. However, IPCE shows that photogenerated charge carriers beyond 550 nm are not utilized for photocurrent conversion. This could be due to loss photo-generated charge carriers from low-energy photons due to thicker nature of the heterojunction films. The utilization of photons to current conversion is higher for 7% Co-doped BiVO4/WO3 photoanode in the wavelength region of 400-550 nm, reaching an IPCE value of 72% at 400 nm as against 55% for the pristine sample. Similar IPCE response is observed for pristine/CoPimodified BiVO4.36, 60 Figure 7 shows the Mott-Schottky (M-S) plot of drop-casted pristine and Co-doped BiVO4 samples obtained at an AC frequency of 0.5 kHz in 0.5 M Na2SO4 solution. The flat band potential of the semiconductor film in a liquid junction can be estimated from the Mott-Schottky equation,61 1 2 (1) where Csc is the space charge capacitance in F cm−2; e is the electronic charge in C (1.602 × 10−19 C); ε is the dielectric constant of the semiconductor; ε0 is the permittivity of free space (8.854 × 10−12 Fm−1); ND is the carrier density in cm−3; E is the applied potential in V; Efb is the flat band potential in V; kb is the Boltzmann constant (1.381 × 10−23 J K−1); and T represents the temperature in K. The temperature related term (kbT/e) in eq. (1) is negligibly small (0.0257 eV) at room tem-


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perature. The flat band potential is obtained from the xintercept of the tangent line of the M-S plot on the potential axis at Csc=0. The Efb values of pristine and Co-doped BiVO4 are 0.63 and 0.51 V vs. RHE, respectively, which are close to their respective onset potentials. The donor density estimated from the slope of the M-S plot is increased from 0.59 × 1021 to 1.02 × 1021 cm−3 after cobalt incorporation in BiVO4. This suggests that the additional conductivity arises from cobalt doping of the BiVO4. To further elucidate on the chemical state of cobalt into the BiVO4 lattice, XPS analysis was performed on freshly synthesized pristine BiVO4, Co-doped BiVO4 and cobalt oxide thin film samples fabricated on FTO substrate under identical an

(531.8 eV) on the surface.64 The relatively higher area of the Co2+ peak as compared to Co3+ in Co-doped BiVO4 suggests the formation of oxygen vacancies. The slight shifting of O1s lattice peak towards higher BE by ~ 0.2 eV is again consistent with Co doping. Similar effects have been observed for W and Mo-doped BiVO4.65 Figure 8D shows the deconvolutions of Co2p peak in the Co-doped BiVO4 sample. The presence of shake-up satellite peaks at 786.8 and 803.1 eV and a spin orbit splitting distance of 16 eV between the Co2p3/2 (796.4 eV) and Co2p1/2 (780.4 eV) lines confirms the existence of Co2+ state.66 Co2p additionally shows a peak at 782.9 eV, which corresponds to an oxidation state of +3 suggesting the formation of Co3O4. Excess incorporation of cobalt content in BiVO4 is observed to change the oxidation state of cobalt from +2 to +3 forming different oxides (CoO, Co3O4, or

Figure 7. Mott-Schottky plot of drop-casted pristine and Co-doped BiVO4 electrodes from 0.5 M Na2SO4 electrolyte (pH ~7) under dark condition at 0.5 kHz AC frequency.

nealing conditions. Figure S6 (Supporting Information) shows the survey spectra of BiVO4, Co-doped BiVO4 and CoxOy samples prepared on FTO substrate. In the survey spectra, characteristic peaks are identified as Bi4f, V2p, O1s, Co2p, and C1s together with Auger lines of O KLL and Co LMM, confirming that other metals and/or impurities are not present. The appearance of C is atmospheric in origin. Figure 8 shows the high-resolution XPS spectra of Bi4f, V2p, O1s, Co2p for pristine BiVO4, 6% Co-doped BiVO4 and cobalt oxide (CoxOy) samples. The high-resolution XPS data were calibrated using standard adventitious C1s peak at 284.8 eV. The highresolution XPS spectrum of Bi (Figure 8A) shows the binding energies of Bi 4f centered at 158.4 and 163.7 eV corresponding to the Bi 4f7/2 and Bi4f5/2, respectively, which are consistent with the reported values for pristine BiVO4.62 However, these doublet peaks shift towards higher binding energy by 0.3 eV for Co-incorporated BiVO4. Similarly, the V2p binding energy peaks V2p3/2 and V2p1/2 also shift towards higher binding energy by similar margin respectively from 516.0 and 523.7 eV, respectively (Figure 8B). The deconvolution of V2p3/2 peak with binding energies of 520.6 and 522.7 eV are normally assigned to the V5+ and V4+ states, respectively. The shifting of Bi4f and V2p peaks towards higher BE by ~ 0.3 eV suggests doping in BiVO4. The asymmetric nature of O1s peak (Figure 8C) indicates the presence of different oxygen species in the near surface region.63 The deconvoluted O1s spectrum reveals a peak for lattice oxygen at 529.6 eV in BiVO4 or CoxOy, the chemisorbed OH− (530.6 ± 0.2 eV) and adsorbed water molecules

Figure 8. High-resolution XPS spectra of (A) Bi4f, (B) V2p, (C) O1s, (D) Co2p for BiVO4, Co-BiVO4, and (E) O1s and (F) Co2p for cobalt oxide on FTO substrate.

Co2O3).40 The radius of Co2+ (0.072 nm) is only slightly higher than that of V5+ (0.059 nm), it is easier to replace V5+ than Bi3+ (0.103 nm) in the internal lattice to form Bi-Co-O bonds. Thus, it is possible that some of the V5+ are reduced to V4+ state to maintain charge neutrality. The presence of mixed V4+ and V5+ valence state results in high electronic conductivity and hence supports the catalytic surface reaction.67 Thus, a part of cobalt is doped into BiVO4 and excess cobalt coexists as Co3O4 in the BiVO4 or on the surface of BiVO4, thereby serving as a biphasic water splitting catalyst for BiVO4 photoanode. To distinguish between the state of cobalt with and without BiVO4, cobalt oxide was prepared on bare FTO substrate by a spin-coating method. Figure 8E shows O1s peaks in cobalt oxide, which can be deconvoluted to four peaks respectively for lattice oxygen, chemisorbed oxygen, adsorbed water,



and organics. Figure 8F shows the deconvolutions of Co2p peak in CoxOy sample. No distinct shake-up satellite peaks are observed, unlike for CoO. A Co2p3/2 (795.0 eV) -Co2p1/2 (780.0 eV) splitting difference of 15.0 eV suggests the presence of mixed Co(II) and Co(III) oxidation states. The deconvoluted intense peaks around 779.9 eV and 781.8 eV are assigned to the Co3+ and Co2+ configuration of Co3O4, respectively. The observed Co3+/Co2+ peak area ratio of 1:2 further confirms the formation Co3O4, which is basically a wellknown water oxidation catalyst. Alternatively, the chemical state of cobalt in Co-doped BiVO4/WO3 heterojunction samples is also confirmed and contains mixed states of cobalt. Figure 9A shows the XRD patterns of BiVO4, 6% Co-doped BiVO4 and cobalt oxide films on FTO substrate. All the peaks in the XRD patterns are indexed. The crystalline BiVO4 phase with monoclinic scheelite structure is confirmed. No clear evidence from XRD regarding BiVO4 lattice deformation due to cobalt doping is noted, unlike Raman and XPS studies. No dif

Figure 9. (A) XRD patterns and (B) FESEM images of BiVO4, 6% Co-doped BiVO4 and cobalt oxide films drop-casted on FTO substrate. SEM magnification: 30,000X.

fraction peaks of Co species are apparent in the XRD pattern of the Co-doped BiVO4 sample. However, the width of XRD peaks of BiVO4 is noticeably larger, which is an indication of decreased particle size. Figure 9B-D shows the respective surface morphological SEM images of drop-casted BiVO4, 6% Co-doped BiVO4 and cobalt oxide thin films prepared on FTO at 500°C. BiVO4 sample shows a typical porous worm-like morphology. Upon cobalt incorporation, a decrease in porosity is observed with small sized grains, which is in line with the XRD study. Additionally, the surface of BiVO4 is covered with dark patches, which are confirmed to be of cobalt oxide. This indicates that the excess cobalt remains on the surface of BiVO4 in the form of Co3O4, which is also in agreement with the XPS study. Figure S7 (Supporting Information) shows the EDS spectra of BiVO4, 6% Co-doped BiVO4 and cobalt oxide films. Figure 10 shows the cross-sectional film thickness (~120 nm) of drop-casted Co-doped BiVO4 obtained from FIB analysis. The same sample is further used to check the heterogeneity of cobalt doping into the BiVO4 lattice. Figure 10 also shows the

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Figure 10. FIB cross-section image of drop-casted Co-doped BiVO4 film with an average film thickness of ~120 nm and elemental mapping showing constituent elements.

elemental mapping of the cross-sectional SEM image of the drop-casted Co-doped BiVO4 sample with heterogeneity in its morphology and Co distribution inside the BiVO4. In a similar fashion, the pristine and cobalt-doped WO3/BiVO4 heterojunction composite films are analyzed structurally and morphologically. Figure 11A shows the XRD patterns of pristine and 7% Co-doped BiVO4/WO3 composite film. The peaks in the XRD patterns of BiVO4/WO3 are consistent with standard diffraction

Figure 11. Structural and surface morphological features of pristine and 7% Co-doped BiVO4-WO3 heterojunction thin film photoanodes. SEM magnification: 3,000X (Inset: 30,000X).

patterns of the triclinic WO3 (PDF#: 00-020-1323) and monoclinic BiVO4 (PDF#00-014-0688). Upon Co-doping, the intensity of peaks is decreased with slight increase in width of the diffraction peaks as compared to the pristine sample, indicating that diffusion of Co2+ ions into the BiVO4/WO3 decreases the particle size. Figure 11B shows the surface morphology of the pristine and 7% Co-doped BiVO4/WO3 composite films. The surface morphology of pristine sample shows a mesoporous architecture with the aggregated network of particles, similar to the BiVO4 film in this work and earlier report.68 For 7% Co-doped sample, a finer mesoporous architecture is formed with more smooth surface morphology and decreased grain size, which is consistent with XRD study. Figure 12 shows a schematic energy band diagram illustrating the mechanism charge transfer in Co-doped BiVO4 photoanode. The cobalt exists as a dopant in the BiVO4 lattice as well as on the surface in the form of Co3O4, respectively as confirmed by various analyses. The former contributes to en-


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hancing the electronic conductivity of BiVO4 while the latter acts as a

Figure 12. Schematic of energy band diagram illustrating the mechanism charge transfer in Co-doped BiVO4 photoanode depicting the bi-phasic role of cobalt oxide for water splitting.

water oxidation co-catalyst and speeds up the sluggish oxidation reaction kinetics of BiVO4 photoanode. As seen from the schematic figure, the valance band position of BiVO4 is lower than that of Co3O4. Similarly, the valence band position of BiVO4 is lower than them. Since Co3O4 exists on the BiVO4 surface in patches, it allows sufficient contact of water molecules with BiVO4 for water oxidation reactions to occur. Upon illumination, the photogenerated electrons and holes are transferred to their respective band positions. The holes are easily traversed to the BiVO4 surface near the electrolyte and are collected by Co3O4 providing a fast path for the charge transfer between the BiVO4 light absorber and the electrolyte, where they perform water oxidation to produce oxygen. The electrons in the conduction band of Co3O4 readily jump to the conduction band of BiVO4 and eventually to the back contact. The electron and hole diffusion lengths of BiVO4 are 10 and 100 nm, respectively.69 The catalytic properties of cobalt oxide are reported mainly due to its Lewis acid nature.70 Moreover, several other studies suggest that the Co3O4 catalysts utilized in oxidation reactions have a spinel structure, which is mixed valance compound containing both Co (II) and Co(III) oxidation states.59, 70-72 The stability domain of Co3O4 spinel lies well below the thermodynamic oxidation potential for water under all pH conditions.73 The high catalytic activity of the spinel in oxidation processes has been related to the higher oxidation state of the cation (i.e. weaker Co(III)-O bond).71 The studies suggest that Co3+ within a CoO(OH) structure is the active phase that catalyzes the OER in alkaline solutions. A recent study demonstrates the formation of CoO(OH) as the catalytically active phase with a subnanometer thickness on the surface of the Co3O4−MWCNT OER catalyst that exists only under operational conditions and is rapidly converted back into Co3O4 when no voltage is applied.74 A recent transient absorption spectroscopy study confirmed that a visiblelight-induced transfer of holes from a light absorber such as free-base porphyrin to Co3O4 cocatalyst occurs at the ultrafast rate.75 Such rapid hole transfer can improve the separation efficiency and catalyze the water oxidation efficiently. Therefore, in the presence of Co3O4 cocatalyst, the photo-generated holes are transported much faster to the surface than the photogenerated electrons to the back contact. Additionally, the electronic conductivity due to Co-doping further contributes to current. However, the electron transport becomes a limiting factor for photocurrent density in the absence of Co3O4 cocatalyst, which defines the photocurrent in pristine BiVO4. Thus, doping of cobalt in BiVO4 plays a biphasic dual role in enhancing solar water splitting performance of BiVO4.

Figure 13. (A) Corresponding J-V curves of n-type Co-doped WO3/BiVO4 and p-type CuO/CuBi2O4 heterojunctions toward establishing a Z-scheme water splitting system. (B) J-V curves of two-electrode configuration water splitting system involving 0 and 7% Co-doped BiVO4/WO3 and CuO/CuBi2O4 electrodes.

Further, a Z-scheme water splitting system is established by combining n-and p-type heterojunction composites such as Co-doped BiVO4/WO3 and CuO/CuBi2O4 to obtain a costeffective and efficient PEC water splitting system to produce hydrogen fuel. Figure 13A shows the photocurrent responses of individual components. It is demonstrated that by establishing a Z-scheme system, it is possible to generate oxygen and hydrogen fuel without applying any external bias. To realize such a Z-scheme system, the PEC activity of p-n junction formed using composite films such as BiVO4/WO3 photoanode and CuO/CuBi2O4 photocathode connected in series in a two-electrode PEC cell setup was measured in 0.1 M potassium phosphate buffer electrolyte (pH 7). Figure 13B shows the J-V curves of water splitting system measured in buffered neutral electrolyte using a two-electrode PEC cell setup that involves 0% or 7% Co-doped BiVO4/WO3 and CuO/CuBi2O4 electrodes connected in series. The presence of 7% Co not only doubles the photocurrent at 0 V but also shifts the onset of water splitting reaction towards the more cathodic region. Thus, using such system, it is possible to obtained greatly improved PEC solar water splitting should optimal optical densities and thicknesses of all layers among the heterojunction structure are obtained for efficient light absorption, charge separation, charge transport, and collection at the electrode surface via PEC reactions of water splitting. CONCLUSIONS In conclusion, the SPECM technique was successfully used to screen molar compositions constituent elements to prepare efficient Co-doped BiVO4 photoanodes using Ti and FTO substrates for efficient solar water splitting. The heterojunction composite WO3 and Co-doped BiVO4 further improved the photoactivity for water oxidation. The molar doping concentration of 6-7% was found to be optimum for enhancing the PEC performance of BiVO4 thin film photoanode. A photocatalytically active scheelite-monoclinic phase of BiVO4 is confirmed by X-ray diffraction. Scanning electron microscopy images and EDAX suggest that cobalt partly incorporates into the BiVO4 lattice and the remaining accumulates on the surface in the form of cobalt oxide, which is further evidenced by X-ray photoelectron spectroscopy and Raman studies. PEC measurements of Co-doped BiVO4 revealed ~150% higher photocurrent response at 1.0 V vs. RHE with a cathodic shift in the onset of water oxidation and the flat band potential. Mott-Schottky analysis revealed that Co-doping contributes to the electronic conductivity of BiVO4. Thus, the enhanced water splitting performance is attributed to the dual role of cobalt: enhancement in water oxidation catalytic activity via forming



a heterojunction by remaining as a cobalt oxide on the surface of BiVO4 and contributing to electronic conductivity via partially doping in BiVO4. Further, a Z-scheme water splitting system was established by combining n-and p-type heterojunction electrode structures of Co-doped WO3/BiVO4 and CuO/CuBi2O4, respectively, to obtain a PEC water splitting system with zero external voltage bias to produce hydrogen fuel.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: Photostability curves, Optical image of 10×10 array; SEM micrographs of spots; J-V curves; Survey XPS spectra; EDS spectra (PDF)

AUTHOR INFORMATION Corresponding Author * (Shanlin Pan) E-mail: [email protected]. Tel.: +1-205-348-6381. Fax: +1-205-348-6381.

ORCID: 0000-0003-2226-9687 Notes The authors have no competing financial interests to declare.

ACKNOWLEDGMENT We acknowledge the support of National Science Foundation (NSF) under award numbers OIA-1539035 and CHE-1508192. S.P. and A. G. are also thankful for support from the University of Alabama through the RGC-level 2 award. X.P. acknowledges the National Nature Science Foundation of China (No. 11704107) for supporting his contribution to work.

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