Article Cite This: Chem. Mater. 2017, 29, 9472-9479
pubs.acs.org/cm
Investigation of Pristine and (Mo, W)-Doped Cu11V6O26 for Use as Photoanodes for Solar Water Splitting Margaret A. Lumley and Kyoung-Shin Choi* Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: The development of new and inexpensive semiconductor electrodes that possess suitable band gap energies and band positions for solar water splitting is of great interest in the field of solar fuel production. In this study, n-type Cu11V6O26 that has a band gap energy of 1.9 eV was produced as a pure, high-quality photoanode, and its properties and stability for photoelectrochemical water splitting were systematically investigated in pH 9.2 and 13 solutions. As Cu11V6O26 photoanodes appeared to suffer from poor charge transport properties, Mo and W doping into the V site was also examined, which considerably improved the photocurrent generation of Cu11V6O26. The band gap energy, band edge positions, flatband potential, photocurrent generation, and photostability of pristine and doped Cu11V6O26 electrodes are discussed in comparison to elucidate the effect of Mo and W doping and to evaluate the promise and limitations of Cu11V6O26 as a photoanode for use in a water splitting photoelectrochemical cell.
1. INTRODUCTION Developing Cu-based semiconductor electrodes for use in solar fuel production (i.e., water splitting photoelectrochemical cells (PECs)) is highly advantageous because of the low cost, abundance, and environmentally benign nature of Cu. However, most Cu-containing semiconductor electrodes (i.e., Cu2O, CuO) are p-type and are commonly known to suffer from cathodic photocorrosion.1−6 This is because when they serve as photocathodes, the reduction of Cu(I) or Cu(II) ions can readily occur on the electrode surface by surface reaching minority carriers (electrons) that are generated under illumination. Although the presence of Cu in photocathodes almost always results in photocorrosion, we postulated that the situation may be different for n-type semiconductor electrodes that contain Cu(II). The surface reaching minority carriers in an n-type semiconductor acting as a photoanode during photoelectrochemical operation are holes.7 Since the oxidation of Cu(II) is not known to be as facile as the reduction of Cu(I) or Cu(II), the presence of Cu(II) in a photoanode may not necessarily cause photocorrosion, unlike Cu(I) or Cu(II) in a photocathode. This motivated us to develop Cu(II)-containing n-type semiconductor electrodes. Recently, a few n-type Cu(II) vanadates have been investigated as photoanodes for solar water splitting, including γ-Cu3V2O8, α-CuV2O6, α-Cu2V2O7, and β-Cu2V2O7.8−14 These compounds commonly possess a band gap energy of ∼2 eV, which is favorable for utilizing a significant portion of visible light.15−17 Cu11V6O26 has also been identified from a highthroughput study of the Cu1−xVxOz photoanodes and has been reported as another potentially promising Cu(II) vanadate phase for use in solar water splitting.18 However, Cu11V6O26 has not yet been prepared as a high-quaility photoelectrode for © 2017 American Chemical Society
systematic and thorough investigation of its photoelectrochemical properties and stability. Preparing Cu11V6O26 as a pure phase may be challenging because of the existence of many Cu−V−O phases that contain Cu and V in very similar ratios (e.g., Cu3V2O8, CuV2O6, and Cu2V2O7) to that of Cu11V6O26. In this study, we report a synthesis method that allows for the production of Cu11V6O26 as a pure, high surface area film, which enabled an accurate and thorough evaluation of its photoelectrochemical properties. Furthermore, we prepared Mo- and W-doped Cu11V6O26 electrodes, which showed drastically improved photoelectrochemical performances compared to pristine Cu11V6O26. The investigation of these electrodes enabled us to evaluate the advantages and limitations of using Cu11V6O26 as a photoanode for a water splitting PEC.
2. EXPERIMENTAL SECTION Materials. All chemicals used in this study were reagent-grade purity. Copper(II) sulfate pentahydrate (≥98%), p-benzoquinone (≥98%), sodium hydroxide (≥97%), potassium hydroxide (≥85%), vanadyl acetylacetonate (VO(acac)2) (98%), ammonium tetrathiomolybdate ((NH4)2MoS4) (99.97%), ammonium tetrathiotungstate ((NH4)2WS4) (≥99.9%), boric acid (≥99.5%), and sodium sulfite (≥98%) were purchased from Sigma-Aldrich and used as received. Sodium sulfate (ACS grade) was purchased from DOT Scientific and used as received. Dimethyl sulfoxide (DMSO) (99.9%) was purchased from VWR Analytical and used as received. All solutions were prepared using deionized water further purified using a Barnstead Epure (model D4631) purification system (resistivity > 18MΩ). Received: August 24, 2017 Revised: October 21, 2017 Published: November 3, 2017 9472
DOI: 10.1021/acs.chemmater.7b03587 Chem. Mater. 2017, 29, 9472−9479
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Chemistry of Materials Synthesis of Cu11V6O26 Electrodes. Cu11V6O26 electrodes were prepared using high surface area fibrous CuO electrodes as precursor films following a recently published literature procedure for the electrochemical deposition of Cu−hydroxy double salt (Cu−HDS) films with subsequent annealing in air.4 All depositions were performed in a conventional undivided three-electrode cell using fluorine-doped tin oxide (FTO) coated glass (Hartford Glass, Inc.) as the working electrode and Pt as the counter electrode. The Pt electrodes were prepared by sputter coating a 100 nm thick Pt layer over a 20 nm thick Ti layer onto clean glass slides (LGA Thin Films). The reference electrode used was a single-junction Ag/AgCl electrode in 4 M KCl. Cu−HDS films were deposited at 0.1 V vs Ag/AgCl (4 M KCl) from an unstirred aqueous solution of 20 mM CuSO4·5H2O, 100 mM Na2SO4, and 50 mM p-benzoquinone at a temperature of 80 °C. The plating solution was adjusted to a pH of 5 with NaOH before performing the deposition. The FTO working electrodes were masked to expose a circular area of 0.5 cm2, and the depositions were carried out to pass 0.5 C/cm2, with an average deposition time of 5 min. The detailed deposition mechanisms can be found elsewhere.4 To convert the as-deposited Cu−HDS electrodes to CuO, the films were first placed in a 1 M NaOH solution for 10−15 s at room temperature to form Cu(OH)2 followed by annealing in air at 500 °C for 3 h (ramp rate = 2.25 °C/min). To convert the fibrous CuO electrodes to Cu11V6O26 electrodes, 40 μL of a DMSO solution containing 150 mM VO(acac)2 was placed onto the CuO electrodes (area = 0.5 cm2) followed by heating in a furnace at 600 °C for 3 h (ramp rate = 1 °C/min). This slow ramp rate was necessary to ensure the complete conversion of CuO to Cu11V6O26. After annealing, the films were washed in pH 13 NaOH for 30 min with gentle stirring to remove excess V2O5 present in the Cu−V−O electrodes. This was followed by washing in pH 14 NaOH for 5 min with gentle stirring to remove α- and β-Cu2V2O7 impurity phases that were identified from X-ray diffraction. After these two washing steps, light brown Cu11V6O26 films remained. The resulting pure Cu11V6O26 electrodes were rinsed with DI water and dried at room temperature. Preparation of Mo-Doped and W-Doped Cu11V6O26. In order to investigate the effect of doping on the photoelectrochemical properties of Cu11V6O26, various amounts of ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 4 ) and ammonium tetrathiotungstate ((NH4)2WS4) were added to the VO(acac)2 solution as Mo and W precursors, respectively. Doping levels of 1%, 3%, and 5% were investigated for both Mo and W. The concentrations of the precursor solution used to achieve each doping level and the atomic percent of the dopant atom in the resulting Cu11V6O26 electrodes identified using energy-dispersive X-ray spectroscopy (EDS) can be found in Table S1. For Mo doping, it was not possible to incorporate Mo into the lattice beyond 3 atomic %. In both cases, 3 atomic % was found to be the optimum doping level to achieve the best photoelectrochemical performance. Structural and Optical Characterization. X-ray diffraction (XRD) patterns were collected using a Bruker D8 diffractometer with Ni-filtered Cu Kα radiation (λ = 1.5418 Å). The film morphology was investigated using a LEO 1530 Scanning Electron Microscope (SEM) operated at an accelerating voltage of 2 kV. Energy-dispersive X-ray spectroscopy (EDS) was performed using the same SEM equipped with an EDS (Noran System Seven, Thermo Fisher) at an accelerating voltage of 20 kV. UV−vis absorption data were obtained with a Cary-5000 UV−vis−NIR spectrophotometer (Agilent) equipped with an integrating sphere attachment. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific K-Alpha XPS to confirm the absence of sulfur incorporation into the films from the DMSO solvent and from the Mo and W precursors, (NH4)2MoS4 and (NH4)2WS4. Electrochemical and Photoelectrochemical Measurements. Mott−Schottky analysis was performed in an undivided threeelectrode cell composed of a working electrode (Cu11V6O26, 3% Mo:Cu11V6O26, or 3% W:Cu11V6O26), a Pt counter electrode, and a Ag/AgCl (4 M KCl) reference electrode. Mott−Schottky measurements were performed in a solution containing 0.1 M boric acid
(H3BO3) adjusted to pH 9.2 with KOH and in a 0.1 M (pH 13) KOH solution. An SP-200 potentiostat/EIS (BioLogic Science Instrument) was used to apply a sinusoidal modulation of 10 mV at 0.5, 1, and 2 kHz. All potentials used for electrochemical and photoelectrochemical measurements in this study are reported versus the reversible hydrogen electrode (RHE) and were converted using the equation
E(vs RHE) = E(vs Ag/AgCl) + E(Ag/AgCl)(reference) + 0.0591 V × pH(at 25 °C) EAg/AgCl (reference, 4 M KCl) = 0.1976 V vs NHE at 25 °C
EAg/AgCl (reference, 3 M NaCl) = 0.209 V vs NHE at 25 °C Photocurrent measurements were performed using the same threeelectrode cell setup described above for Mott−Schottky analysis. Simulated solar illumination was obtained by passing light from a 300 W Xe arc lamp through neutral density filters, an AM 1.5G filter, and an IR (water) filter into an optical fiber (0.06 cm2). The power intensity of the light was calibrated to be 1 Sun (100 mW/cm2) at the FTO surface using an NREL-certified Si reference cell (Photo Emission Tech. Inc.). All electrode samples were masked to ensure that the exposed area was smaller than the illuminated area, and films were illuminated through the FTO (back-side illumination). All photocurrent measurements were performed in pH 9.2 borate buffer and pH 13 KOH, and in some cases 0.1 M Na2SO3 was added to the electrolyte as a kinetically fast hole acceptor. J−V measurements were obtained using manually chopped light and a sweep rate of 10 mV/s, and J−t measurements were obtained by applying a constant potential. As noted above, all photocurrent measurements are reported versus RHE. Oxygen Detection Measurement. To confirm that the photocurrent generated by pristine and doped Cu11V6O26 photoelectrodes was truly associated with water oxidation, evolved oxygen was measured and quantified during photocurrent generation. The O2 measurement was performed in a custom-built airtight, undivided cell using a three-electrode configuration. A 3% W-doped Cu11V6O26 electrode was used as the working electrode with a high surface area Pt mesh as the counter electrode. The reference electrode used in this case was a single-junction Ag/AgCl electrode in 3 M NaCl, which is a commercially available electrode that is thin enough to fit into the opening of the airtight cell. The electrolyte used was pH 9.2 borate buffer, and the cell was purged with N2 for 1 h before the O2 detection measurement began. The amount of O2 evolved was quantified using a fluorescence-based oxygen sensor (Ocean Optics, Neofox, FOSPOR-R 1/16 in.) that was placed in the headspace of the airtight cell. The electrode area was masked to ensure that the exposed area was smaller than the illuminated area and the film was illuminated through the FTO (back-side illumination). In order to generate a sufficient amount of O2, which enables a more reliable measurement, a light intensity of 3 Suns was used and a constant potential of 1.6 V vs RHE was applied to the WE. The measurement was maintained for 2 h, and the Faradaic efficiency was calculated by dividing the actual amount of O2 detected by the expected amount of O2 produced based on the charge passed from the photocurrent measurement during illumination.19
3. RESULTS AND DISCUSSION Cu11V6O26 electrodes used in this study were prepared using high surface area fibrous CuO electrodes as precursor films.4 The conversion of CuO to Cu11V6O26 was achieved by annealing the CuO electrodes covered with a DMSO solution containing VO(acac)2 as the vanadium source. The film obtained after annealing contained Cu11V6O26 and both αand β-Cu2V2O7 phases. Since excess vanadium source was used in the annealing solution, unreacted vanadium was also present in the sample as V2O5. The V2O5 could be easily removed by immersing the film in pH 13 NaOH for 30 min. Cu2V2O7, which is very soluble in strong base, could also be completely 9473
DOI: 10.1021/acs.chemmater.7b03587 Chem. Mater. 2017, 29, 9472−9479
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Chemistry of Materials removed by subsequently immersing the film in pH 14 NaOH for 5 min. Cu11V6O26 is chemically more stable in basic media than Cu2V2O7 because Cu11V6O26 has a higher Cu:V ratio.14,18 Since the formation of Cu−V−O phases in our synthesis method is achieved by vanadium ions diffusing into CuO particles by a solid state diffusion reaction, the more vanadiumrich Cu2V2O7 is formed only on the surface of Cu11V6O26 particles, which allows for the facile removal of Cu2V2O7. The dissolution of Cu11V6O26 under this condition appeared to be negligible. After these dissolution processes a light brown film composed of pure Cu11V6O26 was obtained. The SEM image of the fibrous CuO electrode used as a precursor film is shown in Figure 1a. The fibrous morphology
Figure 1. SEM images of (a) CuO and (b) Cu11V6O26 films.
of CuO allowed for facile incorporation of vanadium to form ternary Cu−V−O phases. The SEM image of the Cu11V6O26 electrode after the removal of V2O5 and Cu2V2O7 phases is shown in Figure 1b. This image reveals that the Cu11V6O26 electrode is composed of micrometer size globular networks, which creates a high surface area. XRD of the Cu11V6O26 film is shown in Figure 2a. Due to the low symmetry of Cu11V6O26 (triclinic, space group P1̅), its XRD pattern is rather complicated but the absence of other copper vanadate phases (Cu3V2O8, CuV2O6, and Cu2V2O7) was carefully confirmed. EDS of the Cu11V6O26 film verifies a Cu:V ratio of 1.8:1 that is expected for the pure Cu11V6O26 phase. Our synthesis method also provided a simple way to prepare Mo- and W-doped Cu11V6O26 electrodes. Doping that substitutionally replaces V5+ ions with Mo6+ or W6+ ions in ternary vanadate oxides, such as BiVO4 and Cu3V2O8, has been shown to effectively increase the majority carrier densities of the host oxide materials.8,20−22 In our procedure, Mo or W ions with varying concentrations (Table S1) could be added to the vanadium solution that was used to convert CuO to Cu11V6O26. Mo and W ions that did not incorporate into the Cu11V6O26 lattice may form MoO3 and WO3 on the surface; however, these oxides could be easily removed when excess V2O5 was removed since both MoO3 and WO3 are also very soluble in base.23 For Mo doping, we prepared Cu11V5.94Mo0.06O26 and Cu11V5.82Mo0.18O26 electrodes where the atomic % of Mo in the V site [mol of Mo/mol of (V + Mo)] are 1% and 3%, respectively. It was not possible to incorporate Mo into the V site of the Cu11V6O26 lattice beyond 3 atomic %. For W doping, we prepared Cu 11 V 5.94 W 0.06 O 26, Cu 11 V 5.82 W 0.18 O 26 , and Cu11V5.70W0.30O26 electrodes where the atomic % of W in the V site [mol of W/mol of (V + W)] are 1%, 3%, and 5%, respectively. When the photoelectrochemical properties of the films with varying amounts of Mo or W dopant atoms were preliminarily tested (Figure S1), the 3% doped samples for
Figure 2. (a) XRD pattern of Cu11V6O26 on FTO. Vertical red lines correspond to the calculated diffraction pattern for triclinic Cu11V6O26 (PDF No. 36-0431). FTO substrate peaks are marked with an asterisk. XRD patterns of (b) Mo:Cu11V6O26 (blue) and (c) W:Cu11V6O26 (green) in comparison with an undoped sample (black) showing slight shifts of the peaks to lower two theta values.
both Mo and W doping showed the best performance. Therefore, hereafter, we will mainly discuss the 3% Mo- and 3% W-doped samples, and they will be referred to as Mo:Cu11V6O26 and W:Cu11V6O26, unless noted otherwise. We note that the morphologies of the doped samples are comparable to that of the pristine sample up to 3 atomic % incorporation of Mo and W, although the particle sizes of the doped samples are slightly larger than those of the pristine sample (Figures S2 and S3). Since the pristine and the doped samples were prepared using fibrous CuO electrodes containing the same amount of CuO, the amount of Cu present in these samples must be the same. Therefore, the larger grain sizes of the doped samples do not mean that they contain more Cu11V6O26. It simply means that the Mo and W ions present in the V solution affected the growth of Cu11V6O26, resulting in the formation of films with larger grain sizes. The fact that Mo and W ions are truly incorporated into the Cu11V6O26 lattice could be confirmed by carefully analyzing the positions of the Bragg diffraction peaks in the XRD patterns. The doped samples showed slight shifts of the peaks to lower two theta values, indicating a subtle but evident increase in cell 9474
DOI: 10.1021/acs.chemmater.7b03587 Chem. Mater. 2017, 29, 9472−9479
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Chemistry of Materials parameters due to the replacement of V5+ ions with larger Mo6+ or W6+ ions24 (Figure 2b and 2c). UV−vis absorption spectra of pristine and doped Cu11V6O26 electrodes are shown in Figure 3. All of the electrodes show a
Figure 3. UV−vis absorption spectra for undoped Cu11V6O26 (black), Mo:Cu11V6O26 (blue), and W:Cu11V6O26 (red), indicating a band gap of 1.9 eV (noise at 1.5 eV is an artifact due to the change of the detector and grating of the spectrophotometer).
well-defined absorption onset at ∼1.9 eV, which can be assigned as the band gap energy of Cu11V6O26. The UV−vis spectrum also shows high absorbance below the band gap energy, suggesting the presence of interband states. This phenomenon has been observed in the absorbance spectra of other Cu(II) vanadate phases as well.8,9,11 We also note that the W-doped sample showed a slightly higher absorbance than the pristine and Mo-doped samples. As we mentioned earlier, all of these samples contain comparable amounts of Cu11V6O26. Also, it is unlikely that a doping level of 3 atomic % would significantly affect the extinction coefficient of Cu11V6O26. Therefore, we believe that the observed difference in absorbance is mainly due to the morphology change described earlier (i.e., particle size), which affects the scattering of light on the surface of the film. Mott−Schottky plots of the Cu11V6O26, Mo:Cu11V6O26, and W:Cu11V6O26 films were obtained in 0.1 M borate buffer (pH 9.2) at three different frequencies (0.5, 1, and 2 kHz) to compare the flatband potentials and the majority carrier densities of these films (Figure 4a−c). All films showed positive slopes, confirming their n-type nature.19,25,26 The x intercepts and slopes of these plots showed a dependence on the frequency for all samples. However, the pristine and doped samples showed clear differences in their x intercepts and the magnitude of their slopes, which could be used for qualitative evaluation of doping effects. The x intercept of the pristine sample was found to lie between 0.64 and 0.73 V vs RHE. However, the x intercepts of the Mo- and W-doped samples were clearly shifted to the negative direction (0.53−0.60 V vs RHE for Mo:Cu11V6O26 and 0.51−0.60 V vs RHE for W:Cu11V6O26). Furthermore, the slopes of the doped samples were consistently smaller than those of the pristine sample. The change in flatband potentials agrees well with the change in the slopes of the doped samples, unambiguously confirming that Mo and W doping increased the carrier density of Cu11V6O26. We note that the x intercepts of the pristine and doped samples obtained from Mott−Schottky plots are very close to the photocurrent onset potentials of these samples obtained by using a hole acceptor with fast oxidation kinetics, which will be discussed below. This suggests that the flatband potentials even
Figure 4. Mott−Schottky plots for (a) Cu11V6O26, (b) Mo:Cu11V6O26, and (c) W:Cu11V6O26 in 0.1 M borate buffer (pH 9.2) at three different frequencies (squares for 0.5 kHz, circles for 1 kHz, and triangles for 2 kHz).
for the doped samples are more than 500 mV more positive than the water reduction potential. This also means that Cu11V6O26 will require a significant external bias to achieve total water splitting. When a three-electrode cell is used for evaluation, the positive flatband potential of Cu11V6O26 will significantly limit the photovoltage that can be achieved for water oxidation (the difference between the thermodynamic water oxidation potential and the photocurrent onset potential) since the photocurrent onset potential of Cu11V6O26 cannot be more negative than its flatband potential. The small band gap of Cu11V6O26 appeared to be a clear advantage when comparing it with BiVO4, which is currently considered to be one of the most promising photoanodes but has a relatively wide band gap (2.4−2.5 eV).20,27,28 However, our result showing that even the flatband potentials of doped Cu11V6O26 are a few hundred millivolts more positive than that of BiVO4 suggests that the smaller band gap of Cu11V6O26 is achieved not by raising the valence band maximum (VBM) but by lowering the conduction band minimum (CBM) compared to BiVO4.20,27 In this case, the decrease in band gap would not result in an increase in solar-to-hydrogen (STH) efficiency, as photoelectrochemical H2 production becomes thermodynamically more difficult as the CBM is lowered. We note that all 9475
DOI: 10.1021/acs.chemmater.7b03587 Chem. Mater. 2017, 29, 9472−9479
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were shifted to 0.94 V vs RHE for the Mo-doped sample and 0.87 V vs RHE for the W-doped sample. This trend agrees well with the trend of the flatband potential shifts of the doped samples that were determined from Mott−Schottky analysis (Figure 4). However, there still exists a considerable disparity between the photocurrent onset potentials for water oxidation and the flatband potentials of the doped samples. The doped samples also showed transient photocurrent comparable to that of the undoped sample, suggesting a considerable loss of surface reaching holes due to surface recombination.29,30 This suggests that the doped electrodes have equally poor catalytic abilities for water oxidation. Thus, the increase in photocurrent observed for the doped samples is mainly due to the improvement of the electron transport properties and electron−hole separation increasing the number of holes reaching the surface. To investigate the photoelectrochemical stability of these electrodes during water oxidation, J−t measurements were performed at 1.4 V vs RHE in pH 9.2 borate buffer. Although this potential is past the potential for the thermodynamic oxidation potential of water, it was chosen to generate appreciable photocurrent for all samples to make the stability test more meaningful. At this potential, the Cu11V6O26, Mo:Cu11V6O26, and W:Cu11V6O26 electrodes generated stable photocurrents of 10, 60, and 95 μA/cm2 for 2 h (Figure 5b). This indicates that Cu11V6O26 is stable against anodic photocorrosion during photoelectrochemical water oxidation, even without the aid of oxygen evolution catalysts or a protection layer. To confirm that the photocurrent generated in this condition was truly due to water oxidation to O2, a fluorescence-based probe was used to quantify the amount of O2 produced during the J−t measurement of the W:Cu11V6O26 electrode (Figure 6).
other Cu(II) vanadates reported to date (Cu3V2O8, CuV2O6, and Cu2V 2O7) commonly show very positive flatband potentials or photocurrent onset potentials.8−11 Also, there seems to be a limitation for doping to shift the flatband potential to the negative direction. This suggests that all Cu(II) vanadates may have CBM positions that are a few hundred millivolts more positive than the water reduction potential, which could be an inevitable result of combining Cu and V orbitals in the oxide matrix. Although other limitations of Cu(II) vanadates may be improved by further studies, their positive CBM positions and flatband potentials will likely become the fundamental limitations of using them for solar water splitting. The photoelectrochemical performances of pristine and doped Cu11V6O26 electrodes for water oxidation were investigated in pH 9.2 borate buffer by first measuring J−V plots (Figure 5a). The photocurrent generated by the pristine
Figure 5. (a) J−V plots and (b) J−t plots at 1.4 V vs RHE for water oxidation: Cu 11 V 6 O 26 (black), Mo:Cu 11 V 6 O 26 (blue), and W:Cu11V6O26 (red). Photocurrent onset potentials are indicated with arrows in a. All measurements were performed in pH 9.2 borate buffer using AM 1.5G (100 mW/cm2) illumination.
Figure 6. Comparison of experimental (open circles) and theoretical (solid line) O2 produced by W:Cu11V6O26 during the J−t measurement in pH 9.2 borate buffer under 3 Sun illumination at 1.6 V vs RHE.
sample was not impressive, as it was less than 10 μA/cm at 1.23 V vs RHE. The fact that the photocurrent onset potential of 0.99 V vs RHE is ∼300 mV away from the flatband potential estimated from Mott−Schottky analysis combined with the considerable transient photocurrent observed under chopped illumination indicates that the surface of Cu11V6O26 is poorly catalytic for water oxidation. The doped samples generated significantly enhanced photocurrents compared to the pristine sample. For example, steadystate photocurrent densities of 35 and 75 μA/cm2 at 1.23 V vs RHE were achieved by the Mo:Cu11V6O26 and W:Cu11V6O26 electrodes, respectively. Also, the photocurrent onset potentials 2
For this experiment, in order to ensure the generation of a sufficient amount of O2 to enable a reliable measurement, a light intensity of 300 mW/cm2 was used while applying 1.6 V vs RHE to the W:Cu11V6O26 electrode. The result showed a Faradaic efficiency of 95% for O2 production after 2 h. The photoelectrochemical properties of pristine and doped Cu11V6O26 electrodes were also investigated in pH 9.2 borate buffer containing 0.1 M Na2SO3 as a hole acceptor. The use of 9476
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truly regarded as the photocurrent onset potentials. In this case, accurately determining the photocurrent onset potentials requires sweeping the potential from the OCP to the negative direction until the photocurrent completely disappears (Figure 7a, inset). However, during this scan, nonzero dark current appears, which is most likely due to the electrochemical reduction of Cu11V6O26 on the electrode surface, since there are no solution species present that can be electrochemically reduced at 0.7−0.8 V vs RHE. When such a scan is used to determine the photocurrent onset potential, the surface composition of the photoelectrode is concurrently altered, and so, the photoelectrode after this measurement should not be used for other measurements. Also, the flatband potentials obtained from this method should be verified with those obtained by other methods. We compared and confirmed that the photocurrent onset potentials shown in the inset of Figure 7a (0.65 V vs RHE for Cu11V6O26, 0.62 V vs RHE for Mo:Cu11V6O26, and 0.58 V vs RHE for W:Cu11V6O26) are comparable to the x-intercept values obtained from the Mott− Schottky plots. The dark current observed near the flatband potential of these electrodes is most likely associated with the reduction of Cu(II) to Cu(I), as the reduction of V(V), W(VI), and Mo(VI) cannot occur in this potential region.31 Judging from the fact that the doped samples show more pronounced dark current than the pristine sample, the incorporation of dopant atoms into the Cu−V−O lattice may make it easier to reduce Cu(II) on the electrode surface. The photocurrent onset potential of W:Cu11V6O26 demonstrated in this study (0.58 V vs RHE) is the most negative of any Cu(II) vanadate phase reported to date. However, it is still only ∼0.65 V more negative than the thermodynamic water oxidation potential (1.23 V vs RHE). Since the photocurrent onset potential for water oxidation cannot be more negative than the photocurrent onset potential for sulfite oxidation (or the flatband potential),19 the maximum photovoltage gain by Cu11V6O26 systems for solar water oxidation cannot exceed 0.65 V, even after optimum doping levels are achieved. This will be the major fundamental limitation of using Cu11V6O26 photoelectrodes for solar water splitting, as discussed with the Mott− Schottky results. J−t plots for sulfite oxidation of these electrodes at 1.0 V vs RHE are also shown in Figure 7b, where all of them showed stable photocurrent generation over the 2 h measurement period (55 μA/cm 2 for Cu 11 V 6 O 26 , 85 μA/cm 2 for Mo:Cu11V6O26, and 145 μA/cm2 for W:Cu11V6O26). The stability of these electrodes demonstrated during photoelectrochemical oxidation of sulfite and water, along with previously reported photostability of CuV2O6, Cu2V2O7, and Cu3V2O8, confirms that the presence of Cu(II) in photoanodes does not necessarily result in photocorrosion, unlike the presence of Cu(I) and Cu(II) in photocathodes.9,11 Other Cu−V−O electrodes reported to date have been investigated only in pH 9.2 borate buffer because they were found to be chemically unstable in all other pH conditions.8 However, Cu11V6O26 appears to be stable in more alkaline conditions,14,18 and so, we also investigated its photoelectrochemical properties in 0.1 M KOH (pH 13) solution. Mott−Schottky plots of the Cu11V6O26, Mo:Cu11V6O26, and W:Cu11V6O26 films obtained in pH 13 KOH solution are shown in Figure S4. The flatband potentials obtained in this solution were comparable to those obtained in 0.1 M borate buffer (pH 9.2).
sulfite, which has fast oxidation kinetics, makes it possible to examine the photoelectrochemical properties of photoanodes independently of their slow water oxidation kinetics.19,27 The photocurrent obtained from this experiment can be regarded as the maximum photocurrent that can be generated by the Cu11V6O26 electrodes when they are coupled with an appropriate oxygen evolution catalyst. Indeed, the photocurrent generated for sulfite oxidation was considerably higher than the photocurrent generated for water oxidation for all electrodes, and the transient photocurrent became negligible with the addition of sulfite. The photocurrent densities at 1.0 V vs RHE were 55, 85, and 145 μA/cm2 for Cu11V6O26, Mo:Cu11V6O26, and W:Cu11V6O26, respectively (Figure 7a). However, the observed photocurrents are still
Figure 7. (a) J−V plots and (b) J−t plots at 1.0 V vs RHE for sulfite oxidation: Cu 11 V 6 O 26 (black), Mo:Cu 11 V 6 O 26 (blue), and W:Cu11V6O26 (red). (Inset in a) Photocurrent obtained by sweeping the potential from the OCP under illumination to the negative direction to accurately determine the photocurrent onset potentials (indicated with arrows). All measurements were performed in pH 9.2 borate buffer containing 0.1 M Na2SO3 using AM 1.5G (100 mW/ cm2) illumination.
quite low considering the band gap of ∼1.9 eV. This suggests that these electrodes suffer from considerable electron−hole recombination in the bulk. The interband states that are responsible for the high absorption below the band gap transition (Figure 3) may be involved with this substantial bulk recombination, but further studies would be necessary to elucidate the factors that govern bulk recombination in Cu11V6O26 electrodes. The photocurrent onset potential obtained with a kinetically fast hole acceptor that suppresses surface recombination can be used to assess the flatband potentials of photoelectrodes.19,26 We note that in the J−V plots shown in Figure 7a, which were obtained by sweeping the potential from the open circuit potential (OCP) of each electrode under illumination to the positive direction, all electrodes possess nonzero photocurrent (i.e., the difference in current when the light is on and off) at their OCPs. This means that the OCPs in these plots cannot be 9477
DOI: 10.1021/acs.chemmater.7b03587 Chem. Mater. 2017, 29, 9472−9479
Article
Chemistry of Materials J−V plots measured in pH 13 KOH solution without and with 0.1 M sulfite are shown in Figure 8a and 8b. The
oxidation identified during the investigation in pH 9.2 solution still remains the same in basic media. Since all Cu(II) vanadates reported to date show very positive flatband potentials or photocurrent onset potentials, further investigation of this family of compounds for solar water splitting would require an effective way to shift the CBM and the flatband potential to the negative direction.
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CONCLUSION In summary, we developed synthesis conditions to produce high-quality Cu11V6O26, Mo:Cu11V6O26, and W:Cu11V6O26 electrodes. In order to evaluate the promise and limitations of Cu11V6O26 as a photoanode for use in a water splitting PEC, its photoelectrochemical properties were systematically investigated in pH 9.2 and 13 solutions. Mo and W doping were shown to increase the majority carrier density and improved electron−hole separation in Cu11V6O26, which directly resulted in enhancement of the photocurrent. However, photocurrents generated by the doped samples were still significantly lower than the photocurrent expected from the band gap energy of Cu11V6O26 (1.9 eV). This suggests that considerable bulk recombination via interband states may occur in these electrodes. The photostability of the electrodes shown in pH 9.2 solution confirmed that the presence of Cu(II) does not automatically cause anodic photoinstability in Cu(II)-containing photoanodes. Furthermore, these electrodes demonstrated photostability in a strongly basic condition (pH 13), which was exceptional compared to other vanadates reported to date. However, the major limitation of using these electrodes for solar water oxidation was identified to be their CBM and flatband potential positions. Our results showed that the flatband potentials, even for the doped samples, were ∼0.62 Vand ∼0.58 V vs RHE for Mo:Cu11V6O26 and W:Cu11V6O26, respectively. Judging from similar flatband potentials reported for other Cu(II) vanadates, the mixing of Cu d and V d orbitals in the oxide matrix appears to result in a CBM that is a few hundred millivolts more positive than the water reduction potential. This limits the flatband potential positions that can be achieved by Cu(II) vanadates and, therefore, the photovoltage gained by these electrodes for solar water oxidation. This will in turn limit the photocurrent that is produced for overall water splitting when these materials are coupled with a photocathode for unassisted solar water splitting. This work, which has identified the advantages and limitations of using Cu11V6O26 for solar water oxidation, will be useful for selecting future studies to improve photoelectrode materials.
Figure 8. J−V plots for (a) water oxidation and (b) sulfite oxidation, and J−t plots for (c) water oxidation at 1.4 V vs RHE and (d) sulfite oxidation at 1.0 V vs RHE: Cu11V6O26 (black), Mo:Cu11V6O26 (blue), and W:Cu11V6O26 (red). All measurements were performed in pH 13 KOH without and with 0.1 M Na2SO3 under AM 1.5G (100 mW/ cm2) illumination.
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photocurrent generated by all electrodes under these conditions is quite similar to that obtained in pH 9.2 solution, suggesting that the Cu11V6O26 electrodes can be used in strongly basic media without a decrease in performance. The J−t plots obtained in the same conditions are also show in Figure 8c and 8d. The Mo:Cu11V6O26 and W:Cu11V6O26 electrodes both showed a slight decrease in photocurrent during water oxidation at 1.4 V vs RHE over 2 h. However, all electrodes appear to be photostable for a period of 2 h for sulfite oxidation at 1.0 V vs RHE (45 μA/cm2 for Cu11V6O26, 65 μA/cm2 for Mo:Cu11V6O26, and 95 μA/cm2 for W:Cu11V6O26). Considering that V2O5 and most vanadium-containing ternary oxides are not stable in pH 13 solution,8,18,31 the photostabilities of these electrodes in pH 13 solution achieved under illumination and anodic bias are quite exceptional. Nonetheless, the major limitation of using Cu11V6O26 electrodes for solar water
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03587. Quantitative analysis of Mo and W doping, J−V plots comparing the photoelectrochemical performance of electrodes with different doping levels, SEM images of doped samples, and Mott−Schottky analysis in pH 13 KOH solution (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 9478
DOI: 10.1021/acs.chemmater.7b03587 Chem. Mater. 2017, 29, 9472−9479
Article
Chemistry of Materials ORCID
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Kyoung-Shin Choi: 0000-0003-1945-8794 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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