Improving Stability and Photoelectrochemical Performance of BiVO4

Jan 19, 2016 - ... photoanodes for use in photoelectrochemical cells for solar water splitting. Dong Ki Lee , Dongho Lee , Margaret A. Lumley , Kyoung...
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Letter pubs.acs.org/JPCL

Improving Stability and Photoelectrochemical Performance of BiVO4 Photoanodes in Basic Media by Adding a ZnFe2O4 Layer Tae Woo Kim†,‡ and Kyoung-Shin Choi*,† †

Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States Energy Materials Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Korea

‡

S Supporting Information *

ABSTRACT: BiVO4 photoanodes have mainly been investigated under neutral conditions because BiVO4 gradually dissolves under extreme pH conditions. In this study, the possibility of utilizing ZnFe2O4 as a protection layer to stabilize BiVO4 in a 0.1 M KOH solution was investigated. A 10−15 nm thick ZnFe2O4 layer was conformally placed on a nanoporous BiVO4 electrode by photodepositing a FeOOH layer, followed by drop casting a zinc nitrate solution and annealing. The resulting BiVO4/ZnFe2O4 electrode generated a photocurrent density of >2 mA/cm2 at 1.23 V versus RHE with a significantly improved stability compared with the pristine BiVO4 electrode. The incident and absorbed photonto-current conversion efficiencies along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron−hole separation. These results suggest that further investigation of protection and catalyst layers can enable more stable and efficient operation of BiVO4based photoanodes in basic media.

when an additional thin layer of Ni was deposited on the surface to serve as an oxygen evolution catalyst (OEC).13 This result was exciting in that it demonstrated the possibility of operating BiVO4 photoanodes in basic media, although the performances of BiVO4 in a pH 13 solution were not as good as those demonstrated under neutral conditions. In this study we examined the possibility of utilizing a thin ZnFe2O4 layer as a protection layer to stabilize BiVO4 in a basic solution (pH 13). ZnFe2O4 is an n-type semiconductor that is chemically stable in basic media. In addition, ZnFe2O4 has a smaller bandgap (∼2.0 eV) than BiVO4 with the valence band maximum (VBM) and conduction band minimum (CBM) positions more negative than those of BiVO4.14,15 Therefore, adding a ZnFe2O4 layer on the surface of a BiVO4 photoanode may increase photon absorption and electron−hole separation in addition to enhancing the chemical stability of BiVO4 in basic media; however, compared with CoOx and TiO2 that are compositionally simpler binary compounds, producing a uniform pinhole-free conformal coating layer of ZnFe2O4, which is a ternary compound, may be more difficult. In addition, the BiVO4 electrodes protected by CoOx or TiO2 layers were composed of compactly packed polycrystals that did not have high surface areas.12,13 As the film morphology becomes more complex to increase surface areas and electron− hole separation, depositing a pinhole-free protection layer is expected to become more challenging. Here we demonstrate successful formation of a thin conformal coating layer of

N-type bismuth vanadate (BiVO4) has recently been identified as a promising photoanode for use in a water-splitting photoelectrochemical cell (PEC) because in addition to its relatively narrow bandgap (2.4 to 2.5 eV), its conduction band minimum is located near 0 V versus RHE.1,2 This enables BiVO4 to exhibit a photocurrent onset potential for water oxidation more than 1 V more positive than the thermodynamic potential for water oxidation (1.23 V vs RHE) and generate considerable photocurrent in the low bias region (V < 1.23 V vs RHE) when coupled to proper oxygen evolution catalysts.2−5 As a result, BiVO4 photoanodes have recently been combined with various solar cells and photocathodes to achieve unassisted solar water splitting;5−9 however, BiVO4 has mainly been investigated for use under neutral conditions (pH ∼7) because it is chemically unstable and gradually dissolves in strong basic and acidic solutions. When the operating conditions of BiVO4 can be extended to basic or acidic media, BiVO4 can be coupled to more diverse catalysts or photocathodes, which perform optimally only under basic or acidic conditions. This will provide more freedom in developing BiVO4-based PECs. Additionally, using basic or acidic media may offer an advantage of achieving higher solution conductivities without using additional supporting electrolytes or buffers for PEC operation.10,11 Recently, nanometer-thick (∼1 nm) CoOx or amorphous TiO2 layers prepared by atomic layer deposition (ALD) have been examined as protection layers for polycrystalline BiVO4 photoanodes operated in a pH 13 solution.12,13 The BiVO4 photoanode protected with an amorphous TiO2 layer was particularly interesting in that it generated a photocurrent density of ∼1.4 mA/cm2 in a stable manner up to 120 min © 2016 American Chemical Society

Received: December 14, 2015 Accepted: January 15, 2016 Published: January 19, 2016 447

DOI: 10.1021/acs.jpclett.5b02774 J. Phys. Chem. Lett. 2016, 7, 447−451

Letter

The Journal of Physical Chemistry Letters

deposition using the condition described in the SI). Assuming 100% Faradaic efficiency, the amount of FeOOH deposited onto BiVO4 was estimated to be ca. 60−64 μg/cm(geometric)2. The results discussed later were obtained from BiVO4/ ZnFe2O4 electrodes prepared from this optimum condition. X-ray diffraction and selected area electron diffraction (SAED) showed that the as-deposited FeOOH layer on the BiVO4 surface is amorphous (Figures S1 and S2). The conversion of FeOOH to ZnFe2O4 was achieved by covering the surface of the BiVO4/FeOOH film by drop casting a 0.5 M zinc nitrate solution and heating the film at 550 °C in air. The amount of Zn2+ present in the zinc nitrate solution covering the FeOOH layer was more than necessary to convert FeOOH to ZnFe2O4. Therefore, the resulting film contained excess ZnO on the ZnFe2O4-coated BiVO4 electrode. The ZnO could be easily removed by soaking the BiVO4/ZnFe2O4/ZnO films in a 1 M NaOH solution for 30−60 min at 30−35 °C, leaving a pure BiVO4/ZnFe2O4 electrode. The advantage of using excess Zn2+ ions and removing residual ZnO after the synthesis instead of using a stoichiometric amount of Zn2+ is the formation of a more uniform ZnFe2O4 coating by ensuring the sufficient supply of Zn2+ throughout the film during the solidstate reaction, where diffusion of Zn2+ ions may be limited. The scanning electron microscopy (SEM) images of the BiVO4 surface with and without a ZnFe2O4 layer show that the ZnFe2O4 coating looks uniform and thin enough not to noticeably change the morphology or reduce the porosity of the BiVO4 electrode (Figure 1a,b). The high-magnification SEM images clearly show that the surface of the BiVO4 became uniformly rougher after the deposition of ZnFe2O4 (Figure 1c,d). The high-resolution transmission electron microscopy (HRTEM) images indicate that ZnFe2O4 is forming a conformal coating layer on a BiVO4 particle, and the thickness of the ZnFe2O4 layer is ∼10−15 nm (Figure 1e). While the ZnFe2O4 coating layer was too thin to generate diffraction peaks of ZnFe2O4 detectable by a conventional powder X-ray diffractometer (Figure S1), the phase and polycrystalline nature of the ZnFe2O4 layer could be confirmed by SAED (Figure 1f). The formation and composition of the ZnFe2O4 in the BiVO4 film were additionally confirmed by X-ray photoelectron spectroscopy (XPS) (Figure S3),18−20 which show Zn 2p and Fe 2p peaks corresponding to those expected for Zn2+ and Fe3+ ions present in ZnFe2O4, with the ratio of Zn/Fe being 0.51 ± 0.02. The optical properties of BiVO4 and BiVO4/ZnFe2O4 samples were examined by UV−vis absorption spectroscopy (Figure 2). While the pristine BiVO4 electrode shows a bandgap absorption at ∼2.5 eV, the absorption of the BiVO4/ ZnFe2O4 electrode initiates at ∼2.0 eV. This means that the thin coating layer of ZnFe2O4 can contribute to the absorption of more visible photons; however, whether the enhanced photon absorption truly results in enhanced photocurrent cannot be confirmed by the absorption data alone and requires measurements of wavelength-dependent photocurrent generation, which is discussed later in the photocurrent section. The effect of the ZnFe2O4 layer on the prevention of chemical dissolution of BiVO4 in basic media in the dark was first tested by immersing BiVO4 and BiVO4/ZnFe2O4 electrode in a 0.1 M KOH solution (pH 13) for 72 h. The SEM images taken after 72 h of immersion show that the pristine BiVO4 electrode was dissolved considerably exposing the surface of bare FTO substrate (Figure 3a). On the contrary, the ZnFe2O4coated BiVO4 electrode did not show any detectable sign of

ZnFe2O4 on a high surface area nanoporous BiVO4 photoanode using a solution-based photodeposition method, which results in enhanced photocurrent generation as well as enhanced stability of BiVO4 photoanodes operating in a pH 13 solution. The nanoporous BiVO4 electrodes used in this study were prepared using a recently published method where BiOI electrodes were electrodeposited first and were converted to BiVO4 by a mild chemical and heat treatment.4,16 The resulting electrodes were composed of BiVO4 nanoparticles with a diameter of ∼80 nm, creating a 3D nanoporous structure, which achieves a surface area of 31.8 ± 2.3 m2/g (Figure 1a).4

Figure 1. SEM images of (a) BiVO4 and (b) BiVO4/ZnFe2O4; high magnification SEM images of (c) BiVO4 and (d) BiVO4/ZnFe2O4; (e) high-resolution TEM image and (f) SAED pattern of the ZnFe2O4 layer deposited on BiVO4.

A thin layer of ZnFe2O4 was placed on the surface of nanoporous BiVO4 by photodepositing a FeOOH layer, followed by a chemical and thermal treatment. (See the SI for experimental details.) During photodeposition, photogenerated holes in BiVO4 oxidized Fe2+ ions in a 0.1 M FeSO4 solution (pH 3.2 to 3.8) to Fe3+ ions (eq 1), which precipitated as FeOOH on the BiVO4 surface due to their limited solubility in the given pH condition (eqs 2 to 3).3,17 Fe2 +(aq) + h+ → Fe3 +(aq)

(1)

Fe3 +(aq) + 2H 2O → FeOOH(s) + 3H+

(2)

log[Fe3 +] = 4.84 − 3pH

(3)

A ZnFe2O4 layer with an optimum thickness in terms of maximizing both chemical protection and photocurrent generation was achieved when the total charge passed during photodeposition of FeOOH was 65−70 mC/cm2 (∼40 min 448

DOI: 10.1021/acs.jpclett.5b02774 J. Phys. Chem. Lett. 2016, 7, 447−451

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The Journal of Physical Chemistry Letters

Figure 2. UV−vis spectra of BiVO4 (black) and BiVO4/ZnFe2O4 (red) electrodes. The inset shows Tauc plots (top) and photographs (bottom) of pristine BiVO4 and BiVO4/ZnFe2O4 electrodes.

Figure 3. SEM images of (a) BiVO4 and (b) BiVO4/ZnFe2O4 electrodes after 72 h of immersion in 0.1 M KOH (pH 13).

Figure 4. (a) J−V plots obtained in 0.1 M KOH (pH 13) under AM 1.5G, 100 mW/cm2 illumination: pristine BiVO4 (black), BiVO4/ Co 2+ (green), BiVO 4 /ZnFe 2 O 4 (red), and BiVO 4 /ZnFe 2 O 4 / Co2+(blue). The dark current coincides with the x axis. (b) IPCE and (c) APCE spectra of BiVO4/Co2+(green) and BiVO4/ZnFe2O4/ Co2+(blue) measured at 1.23 V versus RHE in the same solution. (d) J−t plots measured at 1.23 V vs RHE in the same solution. The color codes used here are the same as those used in panel a.

dissolution (Figure 3b). The fact that the 10 nm thick ZnFe2O4 coating prevented the dissolution of BiVO4 confirms the conformal and pinhole free nature of the ZnFe2O4 coating prepared by photodeposition. The effect of the ZnFe2O4 layer on the photoelectrochemical properties and photostabilities of BiVO4 was tested by measuring J−V plots in 0.1 M KOH (pH 13) under simulated AM 1.5G irradiation (100 mW/cm2), using a three-electrode configuration. The pristine BiVO4 electrode showed a poor fill factor in the low bias region (V < 0.9 V vs RHE) and achieved a photocurrent density of 1.04 mA/cm2 at 1.23 V versus RHE (Figure 4a). It is well known that BiVO4 is not catalytic for O2 evolution, and adding OECs can significantly enhance its photocurrent for water oxidation. To make the photoanode surface more catalytic for water oxidation, we employed the simplest method to place an OEC on the BiVO4 surface, which is dipping the BiVO4 electrode in a solution containing Co2+ ions.21−23 It was reported that Co2+ ions adsorbed on the Fe2O3 surface can serve as OECs for photocurrent generation in basic media.21−23 Most likely upon illumination in a basic solution, the adsorbed Co2+ ions get oxidized to Co3+ by photogenerated holes in the photoanode, forming a Co3+containing oxide or hydroxide, which is catalytic for oxygen evolution. After the simple Co2+ modification, photocurrent of BiVO4 was indeed considerably increased and a photocurrent density of 1.53 mA/cm2 was achieved at 1.23 V versus RHE. The J−V plots of BiVO4/ZnFe2O4 and Co2+-modified BiVO4/ZnFe2O4 are also shown in Figure 4a. The results show that the addition of the ZnFe2O4 layer increased the photocurrent significantly. For example, photocurrent densities at 1.23 V for BiVO4/

ZnFe2O4 and BiVO4/ZnFe2O4/Co2+ electrodes are 2.76 and 2.84 mA/cm2, respectively. The presence of Co2+ on the ZnFe2O4 surface did not considerably improve the photocurrent density in the high bias region (V > 1.2 V vs RHE), but it improved the fill factor in the low bias region. To examine if the enhanced photocurrent demonstrated by BiVO4/ZnFe2O4 is due to the enhanced photon absorption by ZnFe2O4, incident photon-to-current conversion efficiencies (IPCEs) of the BiVO4/Co2+ and BiVO 4/ZnFe2O 4/Co2+ electrodes were measured at 1.23 V versus RHE in 0.1 M KOH (pH 13) (Figure 4b). The onset of IPCE for the BiVO4/ ZnFe2O4/Co2+ electrode is 570 nm, while the onset of IPCE for BiVO4/Co2+electrode is 510 nm. These results confirm that the ZnFe2O4 layer serves as an additional photon absorber, allowing the BiVO4/ZnFe2O4/Co2+ electrode to utilize more visible light for photocurrent generation. Because the VBM and CBM of ZnFe2O4 are more negative than those of BiVO4,14 the photogenerated holes in the VB of BiVO4 can be transferred to the VB of ZnFe2O4, while the photoexcited electrons in the CB of ZnFe2O4 can be transferred to the CB of BiVO4, allowing for the utilization of photogenerated electron−hole pairs in both layers for photocurrent generation. According to the IPCE result, the photons absorbed by the BiVO4/ZnFe2O4 electrode with λ > 570 nm do not result in detectable photocurrent generation. 449

DOI: 10.1021/acs.jpclett.5b02774 J. Phys. Chem. Lett. 2016, 7, 447−451

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The Journal of Physical Chemistry Letters

ZnFe2O4/Co2+ electrode, which generated an initial photocurrent density higher than 2.5 mA/cm2, was effectively slowed down by the presence of a thin ZnFe2O4/Co2+ layer is encouraging. This result suggests that the solution-based photodeposition technique can provide a high-quaility, pinhole-free, thin passivation layer and that the photocurrent decay may be further suppressed when more efficient protection and OEC layers are developed. In summary, we demonstrated conformal coating of a 10−15 nm thick ZnFe2O4 layer on a nanoporous BiVO4 electrode using a photodeposition method combined with a mild chemical and thermal treatment. Although the nanoporous BiVO4 electrode had a highly complex morphology, a uniform ZnFe2O4 layer could be deposited, which served as an excellent protection layer against chemical dissolution in a pH 13 solution. The IPCE and APCE results along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron−hole separation of the BiVO4 layer. When the surface of the ZnFe2O4 layer was modified with Co2+ ions, the resulting BiVO4/ZnFe2O4/Co2+ electrode generated a photocurrent density of >2 mA/cm2 at 1.23 V versus RHE with a significantly improved stability compared with the pristine BiVO4 electrode. Although the BiVO4/ZnFe2O4/Co2+ electrode did not completely suppress photocorrosion, the preliminary results demonstrated in this study suggest that further investigation of proper protection and OEC layers as well as optimization of photodeposition-based synthesis conditions may enable more stable and efficient operation of BiVO4-based photoanodes in basic solutions.

We also note that even for the region where the UV−vis spectrum of the BiVO4/ZnFe2O4 electrode does not show additional photon absorption due to the presence of ZnFe2O4 (460 ≤ λ ≤ 480 nm), IPCE and APCE of the BiVO4/ZnFe2O4/ Co2+ electrode are higher than those of the BiVO4/Co2 electrode (Figures 4b,c). This suggests that even without additional photon absorption the presence of the ZnFe2O4 layer can be advantageous in increasing electron−hole separation in the BiVO4 layer by allowing rapid hole extraction from the VB of BiVO4 to the VB of ZnFe2O4. When the IPCEs of BiVO4/Co2+ and BiVO4/ZnFe2O4/Co2+ shown in Figure 4b were integrated over the wavelength considering photon flux of standard AM 1.5G spectrum (ASTM G173−03) at each wavelength, photocurrent density values of ∼1.09 and ∼2.90 mA/cm 2 were estimated, respectively, which were very close to those of ca. 1.04 and ca. 2.83 mA/cm2 at 1.23 V versus RHE from the J−V plots shown in Figure 4a for BiVO4/Co2+ and BiVO4/ZnFe2O4/ Co2+, respectively. The photocurrent densities of BiVO4, BiVO4/Co2+, BiVO4/ ZnFe2O4, and BiVO4/ZnFe2O4/Co2+ electrodes measured at 1.23 V versus RHE as a function of time are plotted in Figure 4d. The BiVO4 electrode showed rapid photocurrent decay within 600 s. The photocurrent loss observed by BiVO4 is partly due to chemical instability and partly due to photoelectrochemical instability. The photoelectrochemical instability is related to the slow interfacial hole transfer kinetics of BiVO4 for water oxidation, resulting in accumulation of holes at the interface, which results in anodic photocorrosion of BiVO4. When Co2+ was added, the photocurrent decay was retarded because the hole-transfer kinetics for water oxidation was improved, alleviating photocorrosion of BiVO4. The BiVO4/ ZnFe2O4 electrode, which can suppress chemical dissolution of BiVO4, showed significantly improved photocurrent stability over time, but a gradual photocurrent loss due to photocorrosion of BiVO4 was still observed. When Co2+ was added, the resulting BiVO4/ZnFe2O4/Co2+ electrode generated much more stable photocurrent with the photocurrent density measured after 3000 s being 2.21 mA/cm2; however, an SEM image taken after the J−t measurement (Figure S4) shows a partial loss of BiVO4, suggesting that the addition of Co2+ as an OEC could not completely prevent the accumulation of holes in the BiVO4 layer, which resulted in photoanodic dissolution of BiVO4. Identifying and optimally interfacing a better OEC layer on the protection layer will be necessary to avoid hole accumulation in BiVO4. The effect of the thickness of the ZnFe2O4 layer was also investigated. A high photocurrent could be obtained when a thinner ZnFe2O4 layer (i.e., 20 min deposition passing 15−19 mC/cm2) was deposited, but the stability of the resulting BiVO4/ZnFe2O4 electrode was limited, suggesting the presence of pinholes in the thinner ZnFe2O4 layer. Making the ZnFe2O4 layer thicker (i.e., 60 min deposition passing 87−95 mC/cm2) than the optimal thickness resulted in a decrease in photocurrent, suggesting that the poor charge transport in the ZnFe2O4 layer becomes a main limiting factor for photocurrent generation when a thick ZnFe2O4 layer is present (Figure S5). The photocurrent generated by the BiVO4/ZnFe2O4/Co2+ electrode here is the highest achieved by BiVO4-based photoanodes in a pH 13 solution. In general, preventing photocorrosion and generating photocurrent in a stable manner is more difficult when generating a higher photocurrent density. In this context, the fact that the photocorrosion of BiVO4/

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02774. Experimental methods, XRD patterns, TEM and SAED of FeOOH, XPS data, J−V and J−t plots of BiVO4/ ZnFe2O4 electrodes with varying ZnFe2O4 thickness, and an SEM image after J−t measurement. (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 and by the Research and Development Program of Korea Institute of Energy Research (B6-2452).

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