Strategic Modification of BiVO4 for Improving Photoelectrochemical

Article pubs.acs.org/JPCC

Strategic Modification of BiVO4 for Improving Photoelectrochemical Water Oxidation Performance Hye Won Jeong,† Tae Hwa Jeon,‡ Jum Suk Jang,§ Wonyong Choi,‡ and Hyunwoong Park*,† †

School of Energy Engineering, Kyungpook National University, Daegu 702-701, Korea School of Environmental Science and Engineering, §Beamline Research Division, Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Korea

‡

S Supporting Information *

ABSTRACT: To improve the photoelectrochemical (PEC) performance of BiVO4, three different modifications (doping, heterojunction, and catalyst deposition) using earth-abundant elements are performed and their effects are compared in a 0.1 M phosphate electrolyte at pH 7 under AM1.5 light (100 mW/ cm2). When a hexavalent element (Cr6+, W6+, or Mo6+) is doped at various levels, the Mo6+-doping effect is most significant at 10 atomic % with about two times higher photocurrent generation at the oxygen evolution potential (1.23 VRHE). Such enhancement is attributed to a decrease in charge transfer resistance (Rct) by donor doping, resulting in an approximate 2-fold increase in charge separation efficiency (ηsep) to about 25%. W6+ is less effective than Mo6+, whereas Cr6+ has a detrimental effect. To further improve the charge separation efficiency of Mo6+-doped BiVO4 (Mo-BiVO4), a approximate 600 nm thick WO3 layer is deposited under a similarly thick Mo-BiVO4 layer. This binary heterojunction (WO3/Mo-BiVO4) exhibits ηsep of about 50% along with more than 3 times higher photocurrent generation. On the other hand, an oxygen evolving cobalt-phosphate (Co-Pi) catalyst electrodeposited to Mo-BiVO4 (Mo-BiVO4/Co-Pi) enhances charge injection efficiency (ηinj) from ∼50 to ∼70% at 1.23 VRHE. These two binaries are coupled into a ternary heterojunction (WO3/Mo-BiVO4/Co-Pi) in order to improve the charge transfer efficiencies (ηsep and ηinj). The PEC performance of this ternary is significantly high with photocurrent density of about 2.4 mA/cm2 at 1.23 VRHE (corresponding to the solar-to-hydrogen efficiency of ca. 3%) due to ηsep and ηinj of ∼60 and 90%, respectively.

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INTRODUCTION Photoelectrochemical (PEC) water oxidation is essential for sustainable solar hydrogen and artificial photosynthesis.1−3 Water oxidation with simultaneous oxygen evolution is a fourproton-coupled electron transfer process with an approximately 3 orders of magnitude lower rate than the hydrogen evolution reaction in water electrolysis. A range of semiconductor photoanodes for PEC water oxidation are available. Among these, BiVO4 is unique and promising4−7 owing to its bandgap of ∼2.4 eV (monoclinic) and its ability to produce a maximum photocurrent of ∼7.5 mA/cm2 at 1.23 VRHE (volts vs a reversible hydrogen electrode) under AM1.5G light (1 sun: 100 mW/cm2). Therefore, the maximal solar-to-hydrogen (STH) conversion efficiency is approximately 9.2%, which is close to the entrance level for commercialization. Nevertheless, the typical STH of BiVO4 is disappointingly low ( 1.5 VRHE for bare and Cr-BiVO4. This suggests that the bare and Cr-BiVO4 electrodes require more sufficient band-bending to achieve comparable charge separation. The high ηsep of W- and MoBiVO4 is due most likely to the increased charge mobility resulting from decrease in charge transfer resistance (Rct). Electrochemical impedance analyses were carried out on the bare and doped BiVO4 electrodes to determine how the dopants affect the charge transfer resistance. In this study, the

The low PEC performance of n-type oxide semiconductor electrodes is mainly attributed to the low mobility of holes,1,2,5 resulting in rapid charge recombination. To improve the PEC water oxidation photocurrent, three donor elements (W6+, Mo6+, and Cr6+) were doped into the V5+ site of BiVO4 at dopant concentrations of up to 15 atomic %. XPS of the bare and doped BiVO4 samples was performed to determine if the elements were doped successfully (Figure 2). The doped

Figure 2. XPS spectra of (a) Bi4f, (b) O1s, and (c) W4f, Mo3d, and Cr2p for bare and doped BiVO4 films. W6+, Mo6+, and Cr6+ were doped at 8, 10, and 2 atomic %, respectively.

BiVO4 films display different binding energies of Bi4f, V2p, and O1s (Figure 2a,b; for the V2p spectra, see Figures S5) with 0.2−0.3 eV shifts to higher binding energies compared to bare BiVO4. These shifts are attributed to the stronger interaction of doped hexavalent elements with Bi, V, and O atoms. Cr, W, and Mo are located at binding energies of approximately 578 eV (Cr2p), 34.5 eV (W4f), and 231.5 eV (Mo3d; Figure 2c), which is indicative of all hexavalent ions (Cr6+, W6+, and Mo6+) being doped. On the other hand, the atomic percentage of the elements (Cr, 0.47%; W, 4.93%; Mo, 3.34%) are lower than those of the precursor solutions (2, 8, and 10%, respectively). The reduced dopant atomic percentages are attributed to their inhomogeneous distribution throughout the BiVO4 structure. D

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Figure 3. FE-SEM images (top views) of (a) bare BiVO4, (b) Mo-BiVO4 (Mo: 10 atomic %), and (c, d) WO3. Insets show the side views of the corresponding films. All scale bars are identically 200 nm, except 1 μm for (c).

Figure 4. Comparison of the bare and doped BiVO4 electrodes in terms of the charge separation efficiency (ηsep, solid lines) and charge injection efficiency (ηinj, broken lines) under AM1.5 light (100 mW/ cm2). W6+, Mo6+, and Cr6+ were doped at 8, 10, and 2 atomic %, respectively.

Figure 5. Impedance analysis (Nyquist plots) of bare and modified BiVO4 electrodes in a 0.1 M phosphate solution (pH 7). W6+, Mo6+, and Cr6+ were doped at 8, 10, and 2 atomic %, respectively. Inset shows the flat band potentials of bare and doped BiVO4 in a 0.1 M phosphate solution (pH 7).

electrochemical cell was considered to represent the classical Randles circuit composed of the solution resistance (Rs), interfacial charge transfer resistance, and capacitance associated with the Helmholtz layer at the electrode/solution interface (CH; Figure S8). As shown in the Nyquist plots in Figure 5, the electrodes show semicircles with different radii. The fitting of the semicircles indicate that bare BiVO4 has an Rct of ∼4.9 kΩ, which is decreased to ∼3.9 and ∼3.1 kΩ by W and Mo doping, respectively. In contrast, Cr doping increases the resistance by approximately 40% (∼6.8 kΩ). The reverse tendency of the change in Rct from that of the PEC performance strongly suggests that photogenerated charge transfer is a critical factor in determining the photoelectrochemical performance of BiVO4. The increase in Rct by Cr doping may result from heavier electrons. The Mott−Schottky analysis further shows

that W and Mo doping shift the flat band potential of bare BiVO4 cathodically by ∼0.15 V (from 0.4 to 0.25 VRHE: see Figure 5 inset). The cathodic shift of the potential suggests that W- and Mo-BiVO4 electrodes have larger band-bending compared to bare BiVO4, consequently enhancing charge separation. Figure 4 also shows ηinj changes as a function of the bias potential. Bare BiVO4 exhibits similar ηinj values of 30−40% over the entire potential range, whereas ηinj of W-BiVO4 becomes greater than that of bare BiVO4 from ∼1.3 VRHE. This suggests that the positive PEC effect of W6+ doping is due to the enhanced charge separation efficiency, not from the charge injection efficiency at 1.23 VRHE. On the other hand, the Mo6+ doping effect on ηinj appears from ∼1 VRHE, and the injection efficiency reaches a plateau of ca. 50% at 1.2 VRHE. E

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This is only about 12% enhancement compared to the ηinj value of bare BiVO4 (ca. 40%), suggesting that Mo6+ doping primarily increases ηsep. The hexavalent W and Mo dopants appear to trap photogenerated holes (e.g., Mo6+ + h+ → Mo5+), effectively inhibiting bulk recombination. Similar to ηsep, Cr6+ doping is quite detrimental to ηinj. Improving the PEC Performance of Mo-BiVO 4. Codoping. Codoping with two different elements is often performed to further improve the photocatalytic and photoelectrocatalytic performance of single element-doped semiconductors.37,38 Recently, Berglund et al. reported that the coincorporation of 6% Mo6+ and 2% W6+ into BiVO4 enhances the photoelectrochemical performance significantly.10 In this study, various atomic percentages of W6+ (2−10%) were doped into Mo-BiVO4 electrodes at 10% of Mo6+. However, only 2% W6+ doping enhances the PEC performance marginally, and higher doping levels of W6+ reduce the performance significantly (Figure S9). The different result from literature may result from the different codoping process and the reduced performance may be due to the degeneration of BiVO4. The doping effect of 2% W6+ on photocurrent generation is increased slightly with increasing bias potential. Heterojunction. A heterojunction WO3/BiVO4 structure was tested as well (Scheme 1b). In the construction of an effective heterojunction, WO3 should be located beneath BiVO4 (i.e., FTO/WO3/BiVO4; see Figure S3) because the bandgap of WO3 is ∼0.3 eV wider. Under the condition of back illumination (to the FTO side), WO3 absorbs photons of λ < ∼440 nm and BiVO4 absorbs the photons of ∼440 nm < λ < ∼500 nm if all the high energy photons are absorbed by the WO3 layer. In addition, the valence and conduction bands of WO3 are positive of those of BiVO4, respectively. This relative energy level configuration will allow the photogenerated holes to transport to and be collected at BiVO 4 with the photogenerated electrons moving to WO3. As a result, electron−hole pairs are separated more effectively with enhanced photocurrents. Before coupling BiVO4 and WO3, their linear sweep voltammograms are compared under irradiation (Figure 6a). The photocurrent generation of BiVO4 follows exponential growth-like behavior, whereas the photocurrent increases linearly at WO3. This suggests that WO3 may be better for generating photocurrent in the region of E < 1.8 VRHE and exhibit a similar photocurrent value at 1.23 VRHE compared to BiVO4 (Figure S10) When WO3 and BiVO4 are coupled, the shape of the linear sweep voltammogram was more similar to that of bare WO3 than BiVO4. Because the most photons of λ < ∼440 nm are absorbed primarily by WO3, the contribution of BiVO4 to photocurrent generation might be limited in that wavelength range. A comparison of the WO3 + BiVO4 (photocurrent sum) and WO3/BiVO4 samples reveals the superiority of the heterojunction in that the photocurrent of the latter is approximately 50% higher than that of the former (Figure S10). Although BiVO4 may exhibit limited absorption of high energy photons due to underlying WO3, the potential gradient created across WO3 and BiVO4 should enhance charge transfer of both semiconductors. In this heterojunction, WO3 serves as an electron acceptor, whereas BiVO4 acts as a hole acceptor. To enhance the PEC performance further, Mo-BiVO4 and WBiVO4 are used for the heterojunction because the hole mobility of bare BiVO4 is inherently low. As shown in Figure 6a (also see Figure S10), WO3/Mo-BiVO4 and WO3/W-BiVO4 exhibit ∼60 and 25% higher photocurrents than WO3/BiVO4,

Figure 6. Light-irradiated linear sweep voltammograms of bare and doped BiVO4 with (a) WO3 underlayer and (b) Co-Pi catalyst and WO3 underlayer in 0.1 M phosphate electrolyte (pH 7) under AM1.5 light (100 mW/cm2). W6+ and Mo6+ were doped at 8 and 10 atomic %, respectively. In (a), “+” refers to a simple sum.

respectively. These photocurrent values are also greater than those of WO3 + Mo-BiVO4 and WO3 + W-BiVO4, respectively, indicating synergistic effects. Analyses of WO3/Mo-BiVO4 for charge transfer also show that ηsep of Mo-BiVO4 (∼25%) is doubled by coupling with WO3 (Figure 4 vs Figure 7),

Figure 7. Comparison of modified BiVO4 electrodes in terms of the charge separation efficiency (solid lines) and charge injection efficiency (broken lines) under AM1.5 light (100 mW/cm2). W6+ and Mo6+ were doped at 8 and 10 atomic %, respectively.

suggesting that bulk recombination is significantly inhibited due to cascaded charge transfer. The inhibition of the bulk recombination may be attributed to reduced charge transfer resistance. As shown in Figure 5, the charge transfer resistance of WO3/Mo-BiVO4 is approximately 1.8 kΩ, corresponding to ∼58% of that of Mo-BiVO4. WO3 also enhances moderately the charge injection efficiency of Mo-BiVO4 (∼48%) to ∼70% at 1.23 VRHE. The synergistic effect of this heterojunction can be attributed to a so-called necking effect. If BiVO4 particles penetrate into F

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Co-Pi and WO3/Mo-BiVO4) and is maintained at more than 80% in the region between 1.0 and 1.8 VRHE. This indicates that the ternary configuration of WO3/Mo-BiVO4/Co-Pi is effective in enhancing ηsep and ηinj. Decrease in ηinj of the ternary at high bias potentials (E > ∼1.8 VRHE) may be attributed to the relatively enhanced photocurrent for H2O2 oxidation (or ηsep) compared to that for H2O oxidation (eqs 4 and 5). Co-Pi can effectively catalyze the single electron oxidation of H2O2 without a kinetic limit even at high bias potentials, whereas the four sequential electron oxidation of water is kinetically limited by the reduction rate of high valence cobalt (Co3+ and Co4+) to low valence cobalt (Co2+). This kinetic bottleneck effect of Co-Pi is commonly observed in photoelectrochemical reactions.24,26,41,42

the pores created by WO3 particles (if WO3 layer is sufficiently porous), then photogenerated charges can migrate from one particle (WO3 and BiVO4) to neighboring particles through the filled BiVO4 particles. This necking effect is often observed in photocatalysis and photoelectrocataysis.8,39,40 To examine this possibility, as-prepared WO3 film was analyzed by FE-SEM and compared to BiVO4 (Figure 3). WO3 film is composed of closely packed particles of ∼200 nm (Figure 3c,d). They are interconnected (i.e., necked) among themselves and no pores are observed. BiVO4 particles (Figure 3a,b) are also fully packed, leaving no obvious void. Due to their large particle size of ∼100 nm, BiVO4 particles are unlikely filled into WO3 layer. In addition, Figure S3 (the side view of WO3/Mo-BiVO4) obviously shows that WO3 and BiVO4 layers are distinctly separated and BiVO4 particles do not intrude into the WO3 layer. In this regard, the necking effect may be minor. Coupling with Oxygen Evolving Catalyst. Finally, cobaltphosphate (Co-Pi) is electrodesposited to bare and doped BiVO4. As shown in Figure 6b, BiVO4/Co-Pi generates a dark current from ∼1.75 VRHE, which is approximately 0.15 V negative of the dark onset potential at bare BiVO4, due to the catalytic effect of Co-Pi. Mo6+ doping shifts cathodically the dark onset potential further to ∼1.6 VRHE, which is attributed to the more metallic properties of BiVO4 (i.e., increase in electrical conductivity). Upon irradiation, BiVO4/Co-Pi generates a photocurrent from ∼0.5 VRHE but still requires a large overpotential to obtain high photocurrent. The onset potential is relatively unaffected by Mo6+ doping but the photocurrent is increased dramatically. The stabilized photocurrent region at 1.0−1.6 VRHE also suggests that almost full charge separation is accomplished in the potential region. An increase in photocurrent from ∼1.6 VRHE results from the catalytic effect of CoPi. The effect of Co-Pi is due primarily to an increase in the charge injection efficiency of Mo-BiVO4. As shown in Figure 7, ηinj of Mo-BiVO4/Co-Pi is approximately 80% at 1.23 VRHE, suggesting that around 80% of the interfacial holes (i.e., photogenerated and separated) are used for water oxidation. The primary effect of Co-Pi therefore is the inhibition of surface recombination.30 The volcano-shaped ηinj is due to the maximized Co-Pi effect at the low potential region and the kinetic bottleneck effect of Co-Pi at the high potential region. It is usually found that the Co-Pi effect is reduced with increasing bias potential over 1.23 VRHE, because the charge transfer efficiency is already sufficient without Co-Pi due to the large band-bending and positive shift of the valence band.5,14,30 Despite the superior catalytic effect, ηsep of Mo-BiVO4/Co-Pi is very similar to that of Mo-BiVO4 (30%) (Figure 4 vs 7), indicating the insignificant effect of Co-Pi on the inhibition of bulk recombination. The impedance analysis shows that the effect of Co-Pi on the charge transfer resistance is insignificant (Figure 5). This confirms that bulk recombination is less affected by Co-Pi. To enhance ηsep of Mo-BiVO4/Co-Pi along with high ηinj, WO3 was coupled with Mo-BiVO4 and Co-Pi was then deposited on the WO3/Mo-BiVO4 binary. This ternary structure is found to be very promising in generating photocurrents (Figure 6b). Although the onset potential is similar to that of Mo-BiVO4/Co-Pi, the photocurrent of the ternary increases exponentially, reaching approximately 2.4 mA/cm2 at 1.23 VRHE (corresponding to the STH efficiency of ∼3%). As expected, ηsep of the ternary is increased significantly to ∼60% at 1.23 VRHE (Figure 7). In particular, ηinj of the ternary is even higher than those of the binaries (Mo-BiVO4/

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CONCLUSIONS Strategic modification is employed to improve the photoelectrochemical water-oxidation performance of BiVO4 by decreasing bulk and surface recombination. BiVO4 is doped with hexavalent elements (W6+, Mo6+, and Cr6+), among which Mo6+ is found to be most effective in enhancing the photocurrent generation at 1.23 VRHE. The doping effect is attributed primarily to the more than 2-fold improved charge separation efficiency (from ∼12 to ∼25%), resulting from a decrease in charge transfer resistance, along with a marginal increase in charge injection efficiency (from ∼40 to ∼50%). The binary heterojunction of Mo-BiVO4 with either WO3 or Co-Pi is constructed to further increase the charge transfer efficiency. The primary effect of the former is an increase in the charge separation efficiency (∼50%) due to cascaded charge transfer, whereas the latter results from enhanced charge injection efficiency (∼80%) due to Co-Pi mediated catalysis. Therefore, a ternary heterojunction of Mo-BiVO4 with both WO3 and Co-Pi is very effective in charge separation (∼60%) and charge injection (∼90%) at 1.23 VRHE by inhibiting the bulk and surface recombination, respectively. This ternary structure produces approximately 2.4 mA/cm2 at 1.23 VRHE in a phosphate electrolyte (pH 7), corresponding to a STH of ∼3%, without the use of noble metals or strong acids/bases. Although the basic concept of the modification is not so novel, this study obviously shows that a systematic experimental approach promises a high efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of Mo-BiVO4 (Figures S1 and S2), SEM image of WO3/Mo-BiVO4 (Figure S3), linear sweep voltammograms of bare and modified BiVO4 (Figure S4), XPS spectra of V2p for doped BiVO4 (Figure S5), photocurrent time profiles of bare and doped BiVO4 (Figure S6), comparison of AM1.5 light spectrum and UV−vis absorption spectra of bare and modified BiVO4 (Figure S7), Randle circuit (Figure S8), linear sweep voltammograms of W and Mo codoped BiVO4 (Figure S9), and photocurrent time profiles of single and heterojunction electrodes (Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-53-950-8973. E-mail: [email protected]. Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Programs (Nos. 2012R1A2A2A01004517, 2010-0002674, and 2011-0021148) and the Korea Center for Artificial Photosynthesis (KCAP; No. 2012M1A2A2671779) through the National Research Foundation (NRF) funded by MEST, Korea.

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