Photoelectrochemical Properties and Photostabilities of High Surface

Jun 9, 2016 - Electrochemical synthesis methods were developed to produce CuBi2O4, a promising p-type oxide for use in solar water splitting, as high ...
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Photoelectrochemical Properties and Photostabilities of High Surface Area CuBi2O4 and Ag-Doped CuBi2O4 Photocathodes Donghyeon Kang,† James C. Hill,‡ Yiseul Park,† and Kyoung-Shin Choi*,† †

Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

‡

S Supporting Information *

ABSTRACT: Electrochemical synthesis methods were developed to produce CuBi2O4, a promising p-type oxide for use in solar water splitting, as high surface area electrodes with uniform coverage. These methods involved electrodepositing nanoporous Cu/Bi films with a Cu:Bi ratio of 1:2 from dimethyl sulfoxide or ethylene glycol solutions, and thermally oxidizing them to CuBi2O4 at 450 °C in air. Ag-doped CuBi2O4 electrodes were also prepared by adding a trace amount of Ag+ in the plating medium and codepositing Ag with the Cu/Bi films. In the Ag-doped CuBi2O4, Ag+ ions substitutionally replaced Bi3+ ions and increased the hole concentration in CuBi2O4. As a result, photocurrent enhancements for both O2 reduction and water reduction were achieved. Furthermore, while undoped CuBi2O4 electrodes suffered from anodic photocorrosion during O2 reduction due to poor hole transport, Ag-doped CuBiO4 effectively suppressed anodic photocorrosion. The flat-band potentials of CuBi2O4 and Ag-doped CuBi2O4 electrodes prepared in this study were found to be more positive than 1.3 V vs RHE in a 0.1 M NaOH solution (pH 12.8), which make these photocathodes highly attractive for use in solar hydrogen production. The optimized CuBi2O4/Ag-doped CuBi2O4 photocathode showed a photocurrent onset for water reduction at 1.1 V vs RHE, achieving a photovoltage higher than 1 V for water reduction. The thermodynamic feasibility of photoexcited electrons in the conduction band of CuBi2O4 to reduce water was also confirmed by detection of H2 during photocurrent generation. This study provides new understanding for constructing improved CuBi2O4 photocathodes by systematically investigating photocorrosion as well as photoelectrochemical properties of high-quality CuBi2O4 and Ag-doped CuBi2O4 photoelectrodes for photoreduction of both O2 and water.

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INTRODUCTION CuBi2O4 is a p-type semiconductor that possesses several attractive features for use as a photocathode for solar water splitting. First, it is reported to have a band gap of 1.5−1.8 eV.1−5 As a result, it can utilize a significant portion of the visible spectrum. Second, its conduction band minimum (CBM) is estimated to be at a more negative potential than the thermodynamic potential for water reduction, enabling solar H2 production.4−6 Third, its valence band maximum (VBM) is located at a much more positive potential than those of p-type Si and p-type Cu2O, which are currently considered the most promising photocathodes.2−6 As a result, it can have a more positive flat-band potential (>1.0 V vs RHE)3,6 and, therefore, has the possibility of achieving a photovoltage (i.e., the difference between the thermodynamic reduction potential of water and the photocurrent onset potential) greater than 1 V for H2 evolution. For comparison, the photovoltage achieved by recently reported p-type Si photocathodes for H2 production ranges from 0.2 to 0.6 V.7−11 Having a more positive photocurrent onset for water reduction is an advantageous feature for a photocathode in maximizing operating photocurrent density when it is combined with an n-type photoelectrode to assemble a p/n-photoelectrochemical cell for overall water splitting.12 © 2016 American Chemical Society

CuBi2O4 has previously been prepared as a powder-type photocatalyst by metal organic decomposition,1,13 solid-state synthesis,14−16 sol−gel method,17 and hydrothermal synthesis.18−20 However, reports on preparing high-quality CuBi2O4 films that possess uniform coverage, high surface area, and good electrical continuity are rare. Photoelectrochemical properties of polycrystalline films can be significantly different depending on morphological details and interfacial structures even for materials of the same composition. Therefore, establishing synthesis methods to produce highquality CuBi2O4 films to investigate and optimize their photoelectrochemical properties would be highly beneficial. Another important issue for investigating CuBi2O4 is understanding and alleviating its corrosion phenomena, because Cubased photoelectrodes typically suffer from photocorrosion. Understanding the type and cause of photocorrosion and assessing the feasibility to suppress photocorrosion would be critical for the further development of CuBi2O4 photocathodes. Herein, we report electrochemical synthesis methods to prepare high surface area CuBi2O4 photocathodes composed of Received: March 31, 2016 Revised: May 29, 2016 Published: June 9, 2016 4331

DOI: 10.1021/acs.chemmater.6b01294 Chem. Mater. 2016, 28, 4331−4340

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Chemistry of Materials

generating the highest photocurrent was prepared when the plating solution contained 0.06 mM silver nitrate (Figure S1a and Table S1). CuBi2O4 films were also prepared by depositing Cu/Bi films from an ethylene glycol (EG) solution containing 100 mM Bi(NO3)3·5H2O and 30 mM Cu(NO3)2·2.5H2O. The deposition was carried out by passing 0.40 C/cm2 at a constant potential of E = −1.8 V vs Ag/AgCl without resting time during deposition. The aforementioned heating condition was used to convert Cu/Bi films to CuBi2O4 films. Since deposition in DMSO solutions allows for effective Ag doping while deposition in EG solutions allows for the deposition of more uniform Cu/Bi films, Cu/Bi films were also prepared by a two-step procedure, which combines the two procedures, to construct optimum CuBi2O4 electrodes (denoted as CuBi2O4/Ag-doped CuBi2O4 electrodes). To prepare CuBi2O4/Ag-doped CuBi2O4 electrodes, electrodeposition of a Cu/Bi film was first carried out in the EG plating solution using the same conditions described above with a total charge of 0.24 C/cm2 passed. A consecutive deposition of a Cu/Bi/Ag layer was then carried out using the DMSO plating solution containing 0.06 mM silver nitrate. A potential pulse of E = −1.3 V vs Ag/AgCl was applied while passing 0.04 C/cm2, followed by a resting time of 2 s. This cycle was repeated 4 times to pass a total charge of 0.16 C/cm2. The as-deposited films were heated at 450 °C for 3 h in air (ramping rate = 3.5 °C/min) to form CuBi2O4/Ag-doped CuBi2O4 electrodes. Photodeposition of Pt. Pt nanoparticles were photodeposited on the CuBi2O4/Ag-CuBi2O4 films from a solution of water and methanol (4:1 volume ratio) containing 4 mM H2PtCl6 at pH 12.8. A 300 W Xe arc lamp was used as the light source, and the light passed through AM 1.5G and neutral density filters. The light intensity that reached the surface of the CuBi2O4/Ag-CuBi2O4 films was ca. 400 mW/cm2. A typical three-electrode cell composed of a CuBi2O4/Ag-CuBi2O4 working electrode, a Pt counter electrode, and a Ag/AgCl (4 M KCl) reference electrode was used. Under illumination, a current pulse of 100 μA/cm2 was applied for 3 s, followed by 2 s of resting time. This cycle was repeated for 160−180 cycles to pass a total charge of ca. 0.05 C/cm2. The amount of Pt deposited on the CuBi2O4/AgCuBi2O4 surface was estimated to be ca. 25 μg/cm2, assuming 100% Faradaic efficiency. Characterization. The purity and crystallinity of the as-deposited Cu/Bi, CuBi2O4, and Ag-doped CuBi2O4 electrodes were examined by powder X-ray diffraction (XRD) (Bruker D8 Advanced PXRD, Nifiltered Cu Kα radiation, λ = 1.5418 Å) at room temperature. The surface morphologies of the electrodes were examined with scanning electron microscopy (SEM) using a LEO 1530 at an accelerating voltage of 5 kV. The atomic ratios in all films were obtained by the same SEM equipped with an energy-dispersive X-ray spectrometer (EDS) (Noran System Seven, Thermo Fisher) at an accelerating voltage of 20 kV. UV−vis-NIR absorption spectra were recorded using a Cary 5000 UV−vis-NIR spectrophotometer (Agilent), in which the sample electrode was placed in the center of an integrating sphere to measure all light reflected and transmitted. X-ray photoelectron spectroscopy (XPS) spectra were measured using a K-Alpha X-ray photoelectron spectrometer (Thermo Scientific) equipped with Al Kα excitation. The binding energies were calibrated with respect to the residual carbon 1s peak at 284.6 eV. Photoelectrochemical and Electrochemical Characterization. Photocurrent measurements were carried out using an SP-200 potentiostat/EIS (BioLogic Science Instrument) and simulated solar illumination obtained by passing light from a 300 W Xe arc lamp through neutral density filters, an AM 1.5G filter, and an IR filter (water filter) into an optical fiber. Illumination through the FTO (back-side illumination) was used. The light’s power density was calibrated to be 100 mW/cm2 at the FTO surface (before the light passed through FTO) by using both a thermopile detector (International Light) and an NREL certified reference cell (Photo Emission Tech. Inc.). All oxide electrodes were masked with epoxy resin to make the exposed geometrical area (ca. 0.04−0.05 cm2) smaller than the illuminated area (0.06 cm2). An undivided threeelectrode cell composed of a working electrode (CuBi2O4), a Pt counter electrode, and a Ag/AgCl (4 M KCl) reference was used.

CuBi2O4 nanocrystals that uniformly cover the substrate. Previously, two electrodeposition-based methods to prepare CuBi2O4 films were reported.2,3 One involved cathodic deposition of Cu/Bi films in aqueous media, followed by thermal annealing.2 This method, however, could not produce CuBi2O4 films with a good coverage. The other method involved anodic deposition of CuO and Bi2O3, followed by thermal treatment.3 This method resulted in a dense and compact CuBi2O4 film, but its photocurrent generation was limited probably because the electron−hole recombination loss in the dense film was considerable. The method reported in this study allows for the synthesis of CuBi2O4 films that achieve both a high surface area and good surface coverage. We also report the synthesis of Ag-doped CuBi2O4 films where Ag+ ions substitutionally replace Bi3+ ions, which increases the hole concentration. Since our methods produced CuBi2O4 and Ag-doped CuBi2O4 electrodes with identical morphologies, the effect of Ag doping on photoelectrochemical properties and photostability of CuBi2O4 could be accurately investigated. Previously, Berglund et al. reported photocurrent enhancement of CuBi2 O 4 when Ag was incorporated during a combinatorial screening study where samples were prepared by drop-casting/heating methods.21 However, due to the difference in the synthesis methods, the Ag incorporation achieved in the previous study (i.e., incorporation of Ag metal) appears to be different from Ag+ doping (i.e., substitutional doping in Bi3+ sites) achieved in this study. Therefore, the origins of the photocurrent enhancement reported in the previous and current studies are different. The systematic examinations of photoelectrochemical and photocorrosion properties of high-quality CuBi2O4 and Ag-doped CuBi2O4 electrodes for O2 reduction and water reduction contained in this study will provide new opportunities to examine the possibilities and limitations of using CuBi2O4 photocathodes for stable solar water splitting.

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EXPERIMENTAL SECTION

Materials. Copper(II) nitrate (Cu(NO3)2·2.5H2O, 98.0%), bismuth(III) nitrate (Bi(NO3)3·5H2O, ≥98.0%), and dimethyl sulfoxide ((CH3)2SO, ≥99.0%) were purchased from Sigma-Aldrich. Potassium perchlorate (KClO4, 99.0−100.5%), silver nitrate (AgNO3, 99.9+%), silver(I) oxide (Ag2O, 99.99%), and silver(II) oxide (AgO, 98%) were purchased from Alfa Aesar. Ethylene glycol (HOCH2CH2OH, ≥99.8%) was purchased from Fisher Scientific. All chemicals were used as purchased without further purification. Preparation of CuBi2O4 Electrodes. The electrodeposition was carried out in an undivided cell using a VMP2 multichannel potentiostat (Princeton Applied Research). A typical three-electrode system composed of an FTO working electrode, a Ag/AgCl (4 M KCl) reference electrode, and a Pt counter electrode was used. The Pt counter electrode was prepared by depositing 20 nm of titanium, followed by 100 nm of platinum, on clean glass slides by e-beam evaporation. For the preparation of Cu/Bi bimetallic films with the Cu:Bi ratio of 1:2 from dimethyl sulfoxide (DMSO), a DMSO solution containing 20 mM Bi(NO3)3·5H2O, 10 mM Cu(NO3)2· 2.5H2O, and 100 mM KClO4 was used as the plating solution. The deposition was carried out by passing 0.04 C/cm2 at E = −1.5 V vs Ag/AgCl, followed by a resting time of 2 s. This cycle was repeated 10 times to pass a total charge of 0.40 C/cm2. The Cu/Bi films were heated at 450 °C for 3 h in air (ramping rate = 3.5 °C/min) to form CuBi2O4 films. After annealing, no carbon-related organic residue from DMSO was found in the film. To prepare Ag-doped CuBi2O4 films, silver nitrate (0.04−0.10 mM) was added to the aforementioned DMSO plating solution and the same deposition conditions were used to produce Cu/Bi/Ag films, followed by the same heat treatment. The Ag-doped CuBi2O4 sample 4332

DOI: 10.1021/acs.chemmater.6b01294 Chem. Mater. 2016, 28, 4331−4340

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Chemistry of Materials All photocurrent data were obtained either while sweeping the potential to the negative direction with a scan rate of 10 mV/s (for J− V plots) or while applying a constant bias (for J−t plots). All measurements were performed in a 0.1 M sodium hydroxide solution (pH 12.8) with continuous O2 (for O2 reduction) or N2 (for water reduction) bubbling and vigorous stirring. While all measurements were carried out using a Ag/AgCl (4 M KCl) reference electrode, all results in this work are presented against the reversible hydrogen electrode (RHE) for ease of comparison with other reports that used electrolytes with different pH conditions. The conversion between potentials vs Ag/AgCl and vs RHE is performed using the equation below.

results in a decrease in net cathodic photocurrent generated by the CuBi2O4 photocathode. We postulated that the poor coverage of the Cu/Bi films is mostly associated with the deposition of Bi in an aqueous solution on an oxide-based working electrode such as fluorinedoped tin oxide (FTO).23 The Bi(III)-containing salts are soluble only in strongly acidic solutions. Therefore, the deposition of Bi in aqueous media requires the use of acidic plating solutions.2,4,5,24 However, because Bi metal also dissolves in acid, the acidic plating solution necessary for Bi deposition can dissolve Bi deposits when the deposition ends and Bi metal deposits are no longer under cathodic protection.24 The dissolution of Bi can occur immediately, especially at the Bi/substrate interface, resulting in poor adhesion and coverage of Bi deposits.23,24 This problem may not be as pronounced when Bi is deposited on a metal substrate where the substrate−Bi interaction is relatively strong.23 However, when Bi is deposited on an oxide substrate such as FTO to utilize the transparent nature of the substrate for optical and photocurrent measurements, the weak oxide−Bi interaction is more prone to dissolution of Bi at the Bi/ substrate interface in acid. In order to overcome this issue, we investigated electrodeposition of Cu/Bi films using nonaqueous DMSO solutions. The optimum plating solution to deposit Cu/Bi films with the Cu:Bi ratio of 1:2 was composed of 10 mM Cu(NO3)2·2.5H2O, 20 mM Bi(NO3)3·5H2O, and 100 mM KClO4. The deposition was carried out by passing 0.04 C/cm2 at E = −1.5 V vs Ag/ AgCl, followed by a resting time of 2 s. This cycle was repeated 10 times to pass a total charge of 0.40 C/cm2. Introducing a resting time between the short deposition periods allowed the electrode interface to be replenished with Bi and Cu ions, before each new deposition period. This process resulted in the formation of films with a higher coverage by impeding severe dendritic growth caused by mass transport limited growth. The SEM image of the as-deposited Cu/Bi film showed that the film was composed of porous nanoparticulate networks (Figure 1a), and the coverage was significantly improved

E (vs RHE) = E (vs Ag/AgCl) + EAg/AgCl (reference) + 0.0591 V × pH (EAg/AgCl (reference) = 0.1976 V vs NHE at 25 °C) Incident photon-to-current efficiency (IPCE) at each wavelength was measured using AM 1.5G illumination from a 300 W Xe arc lamp through neutral density filters. Monochromatic light was generated by using an Oriel Cornerstone 130 monochromator with a 10 nm bandpass, and the output was measured with a photodiode detector. IPCE was measured at 0.6 V vs RHE in 0.1 M NaOH (pH 12.8) with continuous O2 (for O2 reduction) or N2 (for water reduction) bubbling and vigorous stirring using the same three-electrode setup described above for the photocurrent measurement. Absorbed photonto-current efficiency (APCE) was obtained by dividing the IPCE by the light harvesting efficiency (LHE) at each wavelength using the equations below.

APCE (%) = IPCE (%)/LHE

LHE = 1 − 10−A(λ) (A(λ): absorbance at wavelength λ) Capacitances were measured to obtain Mott−Schottky plots using an SP-200 potentiostat/EIS (BioLogic Science Instrument). The same three-electrode cell used for the photocurrent measurement was used in a 0.1 M sodium hydroxide solution (pH 12.8). All electrodes were masked with epoxy resin to expose the same geometrical area (0.04 cm2). A sinusoidal modulation of 10 mV was applied at frequencies of 0.5 and 1 kHz. Hydrogen evolution was measured in a custom-built airtight twocompartment cell divided by a glass frit. A CuBi2O4 working electrode and a Ag/AgCl (4 M KCl) reference electrode are held in one side, while another side contains a Pt counter electrode. Both sides were filled with 0.1 M NaOH saturated with O2. (The reason why O2 was dissolved for water reduction reaction to detect hydrogen is explained later in the main text.) The electrolyte in the working compartment was 19.5 mL, and the headspace was 22 mL. Photocurrent was generated at 0.6 V vs RHE. The light power density was calibrated to be ca. 400 mW/cm2 after light from a 300 W Xe arc lamp passed through an AM 1.5G filter and a water filter. The evolved gas was sampled by a gastight syringe and analyzed using an SRI 8610C GC equipped with a column packed with molecular sieve 13X, a HID (Helium Ionization Detector), and He as a carrier gas.

Figure 1. SEM images of (a) as-deposited Cu/Bi film, (b) CuBi2O4 film, and (c) Ag-doped CuBi2O4 film prepared from DMSO solutions.

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RESULTS AND DISCUSSION CuBi2O4 and Ag-Doped CuBi2O4 Photocathodes Prepared by Electrodeposition in DMSO Solutions. The main problem reported in the previous study on preparing CuBi2O4 electrodes by thermal oxidation of Cu/Bi films electrodeposited in aqueous plating solutions was the poor coverage of the as-deposited Cu/Bi films,2 which limited photon absorption of the resulting CuBi2O4 electrodes. Furthermore, the uncovered FTO surface can create pathways for photoexcited holes to be used at the FTO/electrolyte interface for back reactions (i.e., oxidation of reduced species) before they are transferred to the counter electrode.22 This

compared with films deposited from aqueous media.2 When the as-deposited films were annealed at 450 °C for 3 h, high surface area nanoporous CuBi2O4 electrodes were obtained (Figure 1b). Ag-doped CuBi2O4 films were also prepared by adding silver nitrate as the Ag source to the plating solution to deposit Cu/Bi films containing a trace amount of Ag while using the same deposition and annealing conditions. We postulated that, because the size of Ag+ is similar to the size of Bi3+ (1.15 Å for Ag+ and 1.03 Å for Bi3+ when the coordination number is 6), Ag+ ions can substitutionally replace Bi3+ ions in the CuBi2O4 structure.5,25,26 When Ag+ ions replace Bi3+ ions, the 4333

DOI: 10.1021/acs.chemmater.6b01294 Chem. Mater. 2016, 28, 4331−4340

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Chemistry of Materials oxidation states of Cu2+ or Bi3+ need to increase to compensate for the charge difference between Bi3+ and Ag+. This can result in an increase in hole concentration of CuBi2O4 and, therefore, leads to enhanced hole transport properties. It is unlikely that Ag+ ions with a d10 configuration are stabilized in the square planar sites of Cu2+ ions with a d9 configuration exerting the Jahn−Teller effect. Also, it is not likely that Ag2+ and Ag3+ ions form under the synthesis conditions used in our study (i.e., oxidation at 450 °C at ambient pressure). Ag-doped CuBi2O4 films with varying Bi:Ag atomic ratios were prepared by systematically changing the composition of the plating solution. (Table S1). Using quantitative analysis of X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS) measurements, the atomic content of Ag in the various Ag-doped CuBi2O4 films (i.e., [mol of Ag/mol of (Bi + Ag)] × 100%) was determined to be 0.5, 1.0, and 1.6 at. %. Films with a Ag content higher than 1.6% appeared to have Ag not completely incorporated into the CuBi2O4 lattice and were not included in this study. Among the three Ag-doped samples, the one containing 1.0 at. % of Ag in the Bi site (equivalent to CuBi1.98Ag0.02O4) demonstrated the most significant enhancement in photocurrent generation (Figure S1a). Thus, CuBi1.98Ag0.02O4 will be primarily discussed in comparison with the undoped sample to examine the effect of Ag doping in this study. Figure 1c shows that the morphology of CuBi1.98Ag0.02O4 is identical to that of CuBi2O4, ensuring that any performance difference shown between CuBi2O4 and CuBi1.98Ag0.02O4 should be unarguably due to Ag doping. The side-view SEM and elemental mapping EDS results of Agdoped CuBi2O4 showing uniform distributions of Cu, Bi, and Ag throughout the depth of the film can be found in Figure S2. X-ray diffraction (XRD) patterns of as-deposited Cu/Bi, CuBi2O4, and CuBi1.98Ag0.02O4 films are shown in Figure 2. The

CuBi2O4, but this is expected if Ag is present as Ag+ in the Bi3+ site because the sizes of Ag+ and Bi3+ ions are comparable. The oxidation state of Ag in the Ag-doped sample was examined by XPS. The XPS peak positions of metallic Ag and cationic Ag(I) and Ag(III) are known to appear within a 1.2 eV range in binding energy,27,28 which may make it difficult to unambiguously assign the oxidation state of Ag using literature reported values. Therefore, we obtained the Ag XPS of the Agdoped CuBi2O4 along with those of Ag metal, Ag2O, and AgO using the same equipment operating under identical measurement conditions to minimize any shifts of the peaks caused by a difference in experimental conditions (Figure S3 and Table S2). Figure 3a shows that the Ag 3d5/2 and Ag 3d3/2 peaks of the Ag-

Figure 3. XPS spectra of (a) Ag 3d, (b) Cu 2p, and (c) Bi 4f obtained from CuBi2O4 (black) and Ag-doped CuBi2O4 (red). Shake-up satellite peaks are denoted by asterisks in (b).

doped sample appear at 368.4 and 374.4 eV, respectively, which shows the best agreement with the peaks obtained from Ag2O (Figure S3), suggesting that the oxidation state of Ag in the Agdoped CuBi2O4 is +1. The peak positions of Ag metal and Ag2O differ by only 0.14 eV (Figure S3 and Table S2), but a more distinctive feature between these peaks is the difference in full width at half-maximum (fwhm).28 The fwhm of the Ag peak is ca. 0.83, whereas that of Ag2O is ca. 1.3 eV. The fwhm of Ag in Ag-doped CuBi2O4 was measured to be 1.4 eV, confirming that the oxidation state of Ag in the Ag-doped sample is +1. Changes in the oxidation states of Cu2+ and Bi3+ resulting from Ag doping were also examined by comparing the XPS of Cu and Bi peaks in the pristine and Ag-doped samples. As shown in Figure 3b, the Cu peaks of the pristine and Ag-doped CuBi2O4 appear at the exactly same locations, 934.1 eV (Cu 2p3/2) and 954.0 eV (Cu 2p1/2), indicating that Ag doping does not affect the oxidation state of Cu 2+ in the CuBi2 O4 structure.21,29 However, the Bi 4f peaks in the Ag-doped sample were shifted slightly to higher binding energy by ca. 0.1 eV, suggesting that substitutional doping of Ag+ into the Bi3+ sites resulted in an increase in oxidation state of Bi to compensate the charge imbalance (Figure 3c).30,31

Figure 2. XRD patterns of (a) as-deposited Cu/Bi, (b) CuBi2O4, and (c) Ag-doped CuBi2O4 prepared from DMSO solutions. The peaks originated from the FTO substrate are denoted by asterisks.

as-deposited film shows only crystalline Bi peaks while the EDS confirms the presence of both Cu and Bi in a 1:2 ratio. This means that Cu metal is present as an amorphous phase in the as-deposited film, suggesting that the codeposition of Bi prevents the formation of crystalline Cu domains. The CuBi2O4 film and CuBi1.98Ag0.02O4 film obtained after annealing generate diffraction peaks, all of which can be indexed as peaks generated by CuBi2O4 (JCPDF 48-1886). This suggests that the CuBi1.98Ag0.2O4 film does not contain any Ag-related crystalline impurities (Ag, Ag2O, AgO). The peak positions of CuBi1.98Ag0.02O4 did not shift from those of 4334

DOI: 10.1021/acs.chemmater.6b01294 Chem. Mater. 2016, 28, 4331−4340

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Chemistry of Materials

present in the CuBi2O4 lattice, increasing the hole concentration. The UV−vis-NIR absorption spectra of CuBi2O4 film and CuBi1.98Ag0.02O4 film coincide with each other, suggesting that Ag doping does not affect photon absorption of CuBi2O4 by altering the band-gap energy or creating interband states (Figure 4c). The spectra show that the earliest absorption onset appears at around 1.53 eV, but the Tauc plots show that the most pronounced direct band gap is ∼1.83 eV (inset of Figure 4c). The current density−potential (J−V) characteristics of CuBi2O4 and Ag-doped CuBi2O4 photoelectrodes were first examined for O2 reduction under AM 1.5G, 100 mW/cm2 illumination in an O2-saturated 0.1 M sodium hydroxide solution (pH 12.8) with constant bubbling of O2. Photoreduction of O2 is thermodynamically and kinetically easier than photoreduction of water on oxide photocathodes. Therefore, measuring a J−V plot for the photoreduction of O2 can enable us to examine the photoactivity of CuBi2O4 based only on its ability for photon absorption and electron− hole separation while decoupling the issue associated with the poor catalytic nature of the CuBi2O4 surface for hydrogen evolution. Comparing photocurrents of CuBi2O4 and Ag-doped CuBi2O4 for O2 reduction also allows us to clearly identify the effect of Ag doping on electron−hole separation, independently from possible effects on catalytic activity for hydrogen evolution. Figure 5a shows that the photocurrent onset potentials of CuBi2O4 and Ag-doped CuBi2O4 photoelectrodes are 1.25 and 1.26 V, respectively, which are very close to the flat-band

The effect of the Ag doping on the majority carrier density of p-type CuBi2O4 was examined by Mott−Schottky analysis (Figure 4). The Mott−Schottky plots of CuBi2O4 obtained at

Figure 4. Mott−Schottky plots of (a) CuBi2O4 and (b) CuBi1.98Ag0.02O4 in 0.1 M NaOH (pH 12.8) solution (open circles for 500 Hz and filled circles for 1 kHz). (c) UV−vis-NIR absorption spectra of CuBi2O4 (black) and CuBi1.98Ag0.02O4 (red). The inset shows the corresponding Tauc plots.

500 Hz and 1 kHz show different slopes, but they show identical flat-band potentials. The Mott−Schottky plots of Agdoped CuBi2O4 electrodes also show two different slopes at 500 Hz and 1 kHz, but each value corresponds to ca. 2/3 of the slope of CuBi2O4 electrodes measured at the corresponding frequency. Although these results cannot be used for reliable quantitative analysis of carrier density concentrations, they can be used to qualitatively conclude that the carrier density of the Ag-doped CuBi2O4 electrode is higher than that of the undoped CuBi2O4 electrode because these two electrodes have identical film morphologies (e.g., surface area and particle size). The flatband potential of the Ag-doped CuBi2O4 electrode shows a slight shift to the positive direction by ∼15 mV, which agrees with an increase in hole concentration (Figure 4b). The flatband potentials of CuBi2O4 and Ag-doped CuBi2O4 electrodes, which are more positive than 1.3 V vs RHE, make these photocathodes highly attractive for use in solar hydrogen production. Mott−Schottky plots of CuBi2O4 electrodes with varying amounts of Ag can be found in Figure S1b where the slope gradually decreases as the Ag content increases. This supports the postulation that Ag+ ions in these samples are

Figure 5. (a) J−V plots (scan rate = 10 mV/s) and (b) J−t plots at 0.6 V vs RHE of CuBi2O4 (black) and Ag-CuBi2O4 (red) measured in 0.1 M NaOH solution (pH 12.8) saturated with O2 under AM 1.5G (100 mW/cm2) illumination. Dotted lines in (a) represent dark currents for O2 reduction. The inset in (b) shows photographs of CuBi2O4 and AgCuBi2O4 electrodes after the J−t measurement. 4335

DOI: 10.1021/acs.chemmater.6b01294 Chem. Mater. 2016, 28, 4331−4340

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Chemistry of Materials

CuBi2O4 Photocathodes Prepared by Electrodeposition in EG Solutions. The CuBi2O4 films deposited from DMSO solutions showed significantly improved coverage compared to those obtained from aqueous media.2 However, low magnification SEM images of as-deposited Cu/Bi or CuBi2O4 films still showed regions where the coverage was not completely uniform and the surface of the FTO substrate was exposed (Figure S5a,b). This may be due to the dendritic growth nature of the Cu/Bi film in DMSO solutions, which is caused by mass transport limited growth.38−40 We attempted to increase the coverage of the Cu/Bi film by changing various synthesis conditions in DMSO media (e.g., changing Cu2+ and Bi3+ concentrations, deposition temperature, deposition time, deposition potential). However, the coverage shown in Figure S5a,b could not be further improved. Therefore, we investigated the use of other organic solvents that may change the nucleation and growth patterns of Cu and Bi to more uniform nanostructure formation. We identified that using ethylene glycol (EG) as a plating medium produced Cu/Bi films composed of nanocrystals that evenly cover the substrate (Figure S5c). It appears that the nucleation process of Cu and Bi metals is easier in EG than in DMSO, and therefore, nucleation of nanocrystals is favored over dendritic growth in EG. The CuBi2O4 films obtained by annealing Cu/Bi films deposited from the EG solution (denoted by CuBi2O4-EG) were also composed of nanocrystals uniformly covering the substrate (Figure S5d). Because of the even coverage of the film, the CuBi2O4-EG film showed much higher absorption compared to the CuBi2O4 films obtained from the DMSO solution (denoted as CuBi2O4-DMSO) even though these two films were prepared by passing the same amount of charge to deposit Cu/Bi films (Figure S5e). XRD patterns of Cu/Bi and CuBi2O4 films prepared from the EG solution showed no impurity phases as in the case of films prepared from the DMSO solution (Figure S6a,b). The photoelectrochemical properties of the CuBi2O4-EG electrode for O2 reduction in comparison with the CuBi2O4DMSO and Ag-doped CuBi2O4-DMSO electrodes are shown in Figure S7a. Since the CuBi2O4-EG electrode absorbs more photons while eliminating the exposed FTO surface, which may enable back reactions, the CuBi2O4-EG electrode generates higher photocurrent than the CuBi2O4-DMSO and Ag-doped CuBi2O4-DMSO electrodes. For example, it achieves 1.25 mA/ cm2 at 0.6 V vs RHE. However, in J−t plots, the CuBi2O4-EG electrode shows a gradual decrease in photocurrent similar to the CuBi2O4DMSO electrode (Figure S7b) because the hole transport of the CuBi2O4-EG electrode is poor. Preparation of Ag-doped CuBi2O4 was also attempted using EG media; however, the resulting Ag-containing films did not show an increase in carrier density or in photocurrent (Figure S8). We believe that this is because EG can chemically reduce Ag+ ions to Ag colloidal particles. In this case, during the deposition of Cu/Bi films in EG, the already formed Ag colloidal particles are embedded in the Cu/Bi deposits, which is quite different from codepositing Cu/Bi/Ag where Ag0 is uniformly mixed with Cu and Bi at the atomic level. For Ag-containing films prepared from EG solutions, although the EDS analysis shows the presence of Ag in the film, the Mott−Schottky analysis did not show an increase in carrier density, suggesting that Ag is not present as Ag+ in the CuBi2O4 lattice (Figure S8). Photocurrent also decreased after Ag was incorporated, suggesting that Ag is

potentials determined by Mott−Schottky plots. The photocurrent densities of undoped CuBi2O4 and Ag-doped CuBi2O4 electrodes at 0.6 V vs RHE are 0.8 and 1.0 mA/cm2, respectively. Since Ag doping did not increase photon absorption according to UV−vis-NIR spectra, the enhanced photocurrent should be due solely to Ag doping increasing electron−hole separation by improving hole transport. The J− V plots of CuBi2O4 with different amounts of Ag doping can be found in Figure S1a where a doping content greater than or less than 1 at. % resulted in a less significant photocurrent enhancement. The comparison of J−t plots of CuBi2O4 and Ag-doped CuBi2O4 at 0.6 V vs RHE revealed a more exciting effect of Ag doping (Figure 5b). The pristine CuBi2O4 photocathode showed an initial photocurrent density of 0.75 mA/cm2, but it gradually decreased to 0.55 mA/cm2 over the course of 2 h, corresponding to a 27% reduction. However, Ag-doped CuBi2O4 generated a photocurrent density of ca. 1 mA/cm2 at the same potential for 2 h in a stable manner. As mentioned in the Introduction, photocorrosion is a common problem for Cu-based photocathodes. After the 2 h of photocurrent measurement, the illuminated area of the pristine CuBi2O4 electrode looked thinner, suggesting the dissolution loss of the film, while the illuminated area of the Ag-doped CuBi2O4 showed no change and was not distinguishable from the unilluminated area (Figure 5b, inset). The dissolution loss indicated that CuBi2O4 suffered not from cathodic photocorrosion but from anodic photocorrosion during photoreduction of O2. Cathodic photocorrosion of CuBi2O4 is expected if the interfacial electron transfer kinetics at the CuBi2O4/electrolyte interface is slow and the surface accumulated electrons are used for reduction of Cu2+ or Bi3+ ions in the CuBi2O4 lattice to form Cu2O, Cu, or Bi. Since Cu2O, Cu, or Bi are not soluble in a pH 13 solution, cathodic corrosion would result in the change in color of the illuminated area but not in the dissolution of CuBi2O4. Indeed, the XPS results of CuBi2O4 before and after 2 h of photocurrent measurement for O2 reduction showed that the oxidation states of Cu2+ and Bi3+ did not change at all, confirming that cathodic photocorrosion did not occur (Figure S4). This is because, unlike water reduction, the O2 reduction with fast kinetics does not accumulate electrons at the CuBi2O4 surface that can be used for cathodic photocorrosion. On the other hand, anodic photocorrosion can involve the use of photogenerated holes for oxidation of lattice oxygen (O2−),37 which leads to dissolution loss of metal ions and, therefore, thinning of the oxide film in the illuminated area. Although anodic photocorrosion is more commonly observed for n-type semiconductors by surface accumulated holes, it can occur in nanoparticulate-type p-type semiconductors that are thermodynamically unstable against anodic photocorrosion and suffer from poor hole transport. Anodic photocorrosion of a photocathode may not be easily noticeable when the illuminated area is simultaneously darkened by cathodic photocorrosion. However, the photocurrent measurement for O2 reduction, which kinetically suppresses cathodic photocorrosion, provided us with an opportunity to observe the effect of anodic photocorrosion on CuBi2O4. When Ag doping was performed to enhance the hole concentration and, therefore, hole transport in Ag-doped CuBi2O4, anodic photocorrosion was effectively suppressed. This confirms that the anodic photocorrosion of CuBi2O4 is truly due to the poor hole transport and it can be overcome by improving hole transport. 4336

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(Figure 7a). The photocurrent onset was also shifted to the positive direction. This is caused by the presence of the Ag-

present as Ag or Ag2O particles that may hinder charge transport or serve as recombination centers. CuBi2O4/Ag-Doped CuBi2O4 Photocathodes Prepared by Consecutive Electrodeposition in EG and DMSO Solutions. In order to combine the advantages of using a DMSO plating solution that allows for effective Ag doping and an EG plating solution that allows for the preparation of more uniform Cu/Bi films, we prepared Cu/Bi films by a two-step procedure (Figure 6a). The first step was the deposition of a

Figure 7. (a) J−V plot (scan rate = 10 mV/s) and (b) J−t plot at 0.6 V vs RHE of CuBi2O4/Ag-CuBi2O4 photocathode (red) measured in 0.1 M NaOH solution (pH 12.8) saturated with O2 under AM 1.5G (100 mW/cm2) illumination. Dotted lines represent dark currents for O2 reduction. For comparison, the performances of CuBi2O4-EG (black) are also shown. (c) IPCE (black) and APCE (red) for CuBi2O4/AgCuBi2O4 electrodes measured for O2 reduction at 0.6 V vs RHE.

doped CuBi2O4 layer, which improves hole transport and increases electron−hole separation. Furthermore, the photostability of Ag-doped CuBi2O4-DMSO was reproduced in the CuBi2O4/Ag-CuBi2O4 electrode (Figure 7b). The incident photon-to-current conversion efficiency (IPCE) and absorbed photon-to-current conversion efficiency (APCE) of CuBi2O4/ Ag-CuBi2O4 electrodes for O2 reduction obtained at 0.6 V vs RHE are shown in Figure 7c. The maximum IPCE and APCE values are obtained at 340 nm, which are 15.6% and 18.6%, respectively. The J−V behavior of the CuBi2O4/Ag-CuBi2O4 electrode for water reduction was also examined using the same electrolyte purged with N2. The corresponding J−V plots of CuBi2O4DMSO, Ag-doped CuBi2O4-DMSO, and CuBi2O4-EG are shown for comparison (Figure 8a). As expected, the CuBi2O4/Ag-CuBi2O4 electrode showed the highest photocurrent for water reduction. Previously reported CuBi2O4 electrodes prepared by electrodeposition methods could not generate photocurrent densities for water reduction greater than 0.08 mA/cm2 at 0.6 V vs RHE.2,3 Compared with these results, the performance of the CuBi2O4/Ag-CuBi2O4 electrode (i.e., 0.46 mA/cm2 at 0.6 V vs RHE) is considerably high, suggesting that preparation of uniform, high surface area CuBi2O4 electrodes and Ag doping effectively increase photon absorption and electron−hole separation. The photocurrent onset demonstrated by the CuBi2O4/AgCuBi2O4 electrode was 1.1 V vs RHE, achieving a photovoltage (i.e., the difference between the thermodynamic reduction potential of water and the photocurrent onset potential) of 1.1 V for H2 evolution. For comparison, other promising

Figure 6. SEM images of (a) as-deposited Cu/Bi(EG)/Cu/Bi/ Ag(DMSO) film and (b) the resulting CuBi 2 O4 /Ag-CuBi 2 O 4 electrodes. (c) Comparison of UV−vis-NIR absorption spectra of CuBi2O4-EG (red dashed) and CuBi2O4/Ag-CuBi2O4 (red solid).

uniform Cu/Bi film from the EG solution by passing 0.24 C/ cm2, which uniformly covers the FTO surface. The second step was the consecutive deposition of a Cu/Bi/Ag film from the DMSO solution by passing 0.16 C/cm2. The total charge used for this two-step process was 0.40 C/cm2, which was the same as that used to deposit metal films from the DMSO solution or the EG solution alone. This ensures that the resulting CuBi2O4 electrode contains an amount of CuBi2O4 comparable to CuBi2O4-DMSO and CuBi2O4-EG electrodes, allowing for a straightforward comparison of their performances. Thermal treatment of this film at 450 °C for 3 h resulted in a CuBi2O4 film coated with a Ag-doped CuBi2O4 layer (referred to as CuBi2O4/Ag-CuBi2O4), which achieves a high surface area and complete coverage of the FTO substrate as well as Ag doping (Figure 6b). The UV−vis-NIR absorption spectrum of the CuBi2O4/Ag-CuBi2O4 electrode was comparable to that of CuBi2O4-EG, confirming that the coverage and the quantity of CuBi2O4 contained in these two electrodes are comparable (Figure 6c). The XRD of the CuBi2O4/Ag-CuBi2O4 electrode is shown in Figure S6c. The J−V plot of the CuBi2O4/Ag-CuBi2O4 electrode for O2 reduction shows enhanced photocurrent compared with the CuBi2O4-EG electrode, achieving 1.5 mA/cm2 at 0.6 V vs RHE 4337

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kinetically suppressed and stable photocurrent generation for water reduction may be possible. Finally, the thermodynamic feasibility of photoexcited electrons in CuBi2O4 for reducing water to H2 was investigated. This was to ensure that the CBM of CuBi2O4 is located above the water reduction potential. Since the photocurrent for water reduction was not stable and decayed quickly, it was not possible to generate a reliably detectable amount of H2 from water reduction using our set up. However, we found that, during photocurrent generation for O2 reduction, approximately 1% of photocurrent generated was associated with H2 production. Since photocurrent for O2 reduction can be generated in a stable manner for a long period, by increasing the duration of photocurrent generation and by increasing the light intensity to 400 mW/cm2, we were able to detect H2 and confirm the thermodynamic feasibility of photoexcited electrons in the conduction band of CuBi2O4 to reduce water to H2 (Figure S10). We have preliminarily attempted to place Pt as a hydrogen evolution catalyst (HEC) on the CuBi 2 O 4 /Ag-CuBi 2 O 4 electrode using photodeposition. The J−V plot of the resulting CuBi2O4/Ag-CuBi2O4/Pt electrode showed an enhancement in photocurrent, particularly in the potential region near the photocurrent onset, which significantly increased the fill factor (Figure S11a). The J−t plot measured at 0.6 V vs RHE also showed an enhancement in photocurrent due to the presence of Pt (Figure S11b); however, the CuBi2O4/Ag-CuBi2O4/Pt electrode still suffered from photocorrosion and the photocurrent decayed gradually. This suggests that either the distribution of Pt or the interface between Pt and CuBi2O4 is not optimal and a considerable portion of surface reaching photoexcited electrons are still not used for water reduction but are instead accumulated at the electrode surface, resulting in photocorrosion. SEM shows that Pt was deposited as nanoparticles on the CuBi2O4 surface (Figure S11c). We believe that, in order to achieve a greater improvement in photocurrent and photostability for water reduction, the CuBi2O4 surface needs to be conformally coated with an HEC layer. Conformal deposition of oxide-based HECs that may interface better with the CuBi2O4 surface will be investigated in the future to further improve the photoelectrochemical water reduction performance of CuBi2O4.

Figure 8. (a) J−V plots (scan rate = 10 mV/s) and (b) J−t plots at 0.6 V vs RHE for CuBi2O4-DMSO (gray), Ag-doped CuBi2O4-DMSO (black), CuBi2O4-EG (blue), and CuBi2O4/Ag-CuBi2O4 (red) photocathodes for water reduction. (c) IPCE (black) and APCE (red) for CuBi2O4/Ag-CuBi2O4 photocathode measured for water reduction at 0.6 V vs RHE.

photocathodes such as Cu2O, Si, Cu2ZnSnS4, and CuGaS2 typically show a photocurrent onset of ca. 0.6 V vs RHE for water reduction.11,32−36 However, the photocurrent for water reduction by the CuBi2O4/Ag-CuBi2O4 electrode was still significantly lower than the photocurrent observed for O2 reduction by the same electrode due to the poor catalytic nature of the CuBi2O4 surface for water reduction. For example, the photocurrent density achieved by CuBi2O4/Ag-CuBi2O4 at 0.6 V vs RHE for water reduction was 0.46 mA/cm 2, only 30% of the photocurrent density achieved at the same potential for O2 reduction. Since the majority of the surface reaching electrons are not used for photocurrent generation, they should be lost to surface recombination or to cathodic photocorrosion. The J−t plot obtained at 0.6 V vs RHE showed a rapid photocurrent decay in 10 min, and the area that had been illuminated showed a noticeably darker color after the measurement. The XPS analysis of the Ag-doped CuBi2O4-DMSO layer after 30 min of photocurrent measurement shows shifts of both Cu and Bi peaks to lower binding energies (Figure S9). This is a clear sign of cathodic photocorrosion, which was not observed during photoreduction of O2. This result shows that Ag doping cannot alleviate cathodic photocorrosion, which is expected because cathodic photocorrosion is caused not by poor hole transport but by poor interfacial electron injection kinetics. While stable water reduction was not achieved with CuBi2O4 photocathodes, the fact that the Ag-doped CuBi2O4 electrode did not suffer from cathodic or anodic photocorrosion during photoreduction of O2 provides valuable information. If CuBi2O4 is coupled with a hydrogen evolution catalyst that can increase the water reduction rate to the level of the oxygen reduction rate and the hole transport in CuBi2O4 is improved by doping, both cathodic and anodic corrosion processes can be

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SUMMARY In summary, we have established synthesis conditions to prepare uniform, high surface area CuBi2O4 photocathodes composed of CuBi2O4 nanocrystals by optimizing deposition conditions for Cu/Bi films in DMSO and EG solutions. We demonstrated that the codeposition of Cu/Bi films with a trace amount of Ag in DMSO solutions offered an effective means to prepare Ag-doped CuBi2O4 electrodes. We also demonstrated that the deposition of Cu/Bi films in EG solutions resulted in a more uniform and complete coverage. On the basis of these results, we deposited a Cu/Bi film in EG solution first and consecutively deposited a Cu/Bi/Ag layer in DMSO solution to prepare an optimum CuBi2O4 photocathode composed of a CuBi2O4 layer uniformly covering the FTO substrate, coated by a Ag-doped CuBi2O4 layer. In the Ag-doped CuBi2O4 layer, Ag+ ions substitutionally replaced Bi3+ ions and increased the hole concentration. Improved hole transport caused by Ag doping resulted in not only an increase in photocurrent but also suppression of anodic photocorrosion of CuBi2O4 during photoreduction of O2. As a result, a stable photocurrent of 1.3 4338

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Chemistry of Materials mA/cm2 could be generated at 0.6 V vs RHE for 2 h for O2 reduction. Photoreduction of water by CuBi2O4 photocathodes was also investigated. Ag doping also improved the photocurrent of CuBi2O4 for water reduction. However, during water reduction, which has significantly slower interfacial electron transfer kinetics than O2 reduction, electrons accumulated at the surface resulted in cathodic corrosion. Nonetheless, the observation that stable photocurrent could be generated by Agdoped CuBi2O4 for O2 reduction showed that it is possible to kinetically suppress both cathodic and anodic photocorrosion of CuBi2O4. If the rate of electron injection for water reduction is improved to the level of that for O2 reduction by adding a catalyst and the hole transport does not limit photocurrent generation, stable photocurrent generation by CuBi2O4 for water reduction may be achieved. Preparing high-quality pristine and Ag-doped CuBi2O4 photoelectrodes and comparing their photoelectrochemical performances and photostabilities for O2 and water reduction were critical for evaluating and gaining a better understanding of CuBi2O4 photocathodes.

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(5) Sharma, G.; Zhao, Z.; Sarker, P.; Nail, B. A.; Wang, J.; Huda, M. N.; Osterloh, F. E. Electronic Structure, Photovoltage, and Photocatalytic Hydrogen Evolution with p-CuBi2O4 Nanocrystals. J. Mater. Chem. A 2016, 4, 2936−2942. (6) Park, H. S.; Lee, C.-Y.; Reisner, E. Photoelectrochemical Reduction of Aqueous Protons with a CuO|CuBi2O4 Heterojunction under Visible Light Irradiation. Phys. Chem. Chem. Phys. 2014, 16, 22462−22465. (7) Hou, Y.; Abrams, B. L.; Vesborg, P. C. K.; Björketun, M. E.; Herbst, K.; Bech, L.; Setti, A. M.; Damsgaard, C. D.; Pedersen, T.; Hansen, O.; Rossmeisl, J.; Dahl, S.; Nørskov, J. K.; Chorkendorff, I. Bioinspired Molecular Co-Catalysts Bonded to a Silicon Photocathode for Solar Hydrogen Evolution. Nat. Mater. 2011, 10, 434−438. (8) Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F. Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Adv. Energy Mater. 2014, 4, 1400739. (9) Bao, X.-Q.; Liu, L. Improved Photo-Stability of Silicon Nanobelt Arrays by Atomic Layer Deposition for Efficient Photocatalytic Hydrogen Evolution. J. Power Sources 2014, 268, 677−682. (10) Zhao, Y.; Anderson, N. C.; Zhu, K.; Aguiar, J. A.; Seabold, J. A.; Lagemaat, J. v. d.; Branz, H. M.; Neale, N. R.; Oh, J. Oxidatively Stable Nanoporous Silicon Photocathodes with Enhanced Onset Voltage for Photoelectrochemical Proton Reduction. Nano Lett. 2015, 15, 2517− 2525. (11) Kast, M. G.; Enman, L. J.; Gurnon, N. J.; Nadarajah, A.; Boettcher, S. W. Solution-Deposited F:SnO2/TiO2 as a Base-Stable Protective Layer and Antireflective Coating for Microtextured BuriedJunction H2-evolving Si Photocathodes. ACS Appl. Mater. Interfaces 2014, 6, 22830−22837. (12) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (13) Arai, T.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. HighThroughput Screening Using Porous Photoelectrode for the Development of Visible-Light-Responsive Semiconductors. J. Comb. Chem. 2007, 9, 574−581. (14) Wei, L.; Shifu, C.; Sujuan, Z.; Wei, Z.; Huaye, Z.; Xiaoling, Y. Preparation and Characterization of p-n Heterojunction Photocatalyst p-CuBi2O4/n-TiO2 with High Photocatalytic Activity under Visible and UV Light Irradiation. J. Nanopart. Res. 2010, 12, 1355−1366. (15) Elaziouti, A.; Laouedj, N.; Bekka, A. Synergetic Effects of SrDoped CuBi2O4 Catalyst with Enhanced Photoactivity under UVALight Irradiation. Environ. Sci. Pollut. Res. 2015, 1−15. (16) Abdelkader, E.; Nadjia, L.; Ahmed, B. Synthesis, Characterization and UV-A Light Photocatalytic Activity of 20 wt %SrO− CuBi2O4 Composite. Appl. Surf. Sci. 2012, 258, 5010−5024. (17) Zhang, J.; Jiang, Y.; Gao, W.; Hao, H. Synthesis and Visible Photocatalytic Activity of New Photocatalyst MBi2O4(M = Cu, Zn). J. Mater. Sci.: Mater. Electron. 2015, 26, 1866−1873. (18) Deng, Y.; Chen, Y.; Chen, B.; Ma, J. Preparation, Characterization and Photocatalytic Activity of CuBi2O4/NaTaO3 Coupled Photocatalysts. J. Alloys Compd. 2013, 559, 116−122. (19) Oh, W.-D.; Lua, S.-K.; Dong, Z.; Lim, T.-T. Rational Design of Hierarchically-Structured CuBi2O4 Composites by Deliberate Manipulation of the Nucleation and Growth Kinetics of CuBi2O4 for Environmental Applications. Nanoscale 2016, 8, 2046−2054. (20) Yuvaraj, S.; Karthikeyan, K.; Kalpana, D.; Lee, Y. S.; Selvan, R. K. Surfactant-Free Hydrothermal Synthesis of Hierarchically Structured Spherical CuBi2O4 as Negative Electrodes for Li-Ion Hybrid Capacitors. J. Colloid Interface Sci. 2016, 469, 47−56. (21) Berglund, S. P.; Lee, H. C.; Núñez, P. D.; Bard, A. J.; Mullins, C. B. Screening of Transition and Post-Transition Metals to Incorporate into Copper Oxide and Copper Bismuth Oxide for Photoelectrochemical Hydrogen Evolution. Phys. Chem. Chem. Phys. 2013, 15, 4554−4565. (22) Eisenberg, D.; Ahn, H. S.; Bard, A. J. Enhanced Photoelectrochemical Water Oxidation on Bismuth Vanadate by Electro-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01294. Quantitative analysis, J−V plots, and Mott−Schottky plots of CuBi2O4 samples containing various amounts of Ag prepared from DMSO and EG solutions; XPS of Ag, Ag2O, and AgO; XPS of CuBi2O4 and Ag-doped CuBi2O4 before and after photoreduction of O2 and water; XRD of Cu/Bi and CuBi2O4 films prepared from EG; photoelectrochemical properties and SEM images of CuBi2O4-EG and CuBi2O4/Ag-CuBi2O4/Pt; and H2 detection (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DESC0008707.

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REFERENCES

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