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Hybrid CuFe–CoFe Prussian Blue Catalysts on BiVO4 for Enhanced Charge Separation and Injection for Photoelectrochemical Water Oxidation

  • Emre Usman
    Emre Usman
    Materials Science and Engineering, Koç University, 34450Istanbul, Turkey
    Koç University Tüpraş Energy Center (KUTEM), 34450Istanbul, Turkey
    More by Emre Usman
  • Mahsa Barzgar Vishlaghi
    Mahsa Barzgar Vishlaghi
    Materials Science and Engineering, Koç University, 34450Istanbul, Turkey
    Koç University Tüpraş Energy Center (KUTEM), 34450Istanbul, Turkey
  • Sina Sadigh Akbari
    Sina Sadigh Akbari
    UNAM-National Nanotechnology Research Center, Institute of Materials Science and Nanotechnology, Bilkent University, 06800Ankara, Turkey
    Department of Chemistry, Bilkent University, 06800Ankara, Turkey
  • Ferdi Karadaş*
    Ferdi Karadaş
    UNAM-National Nanotechnology Research Center, Institute of Materials Science and Nanotechnology, Bilkent University, 06800Ankara, Turkey
    Department of Chemistry, Bilkent University, 06800Ankara, Turkey
    *Email: [email protected]
  • , and 
  • Sarp Kaya*
    Sarp Kaya
    Materials Science and Engineering, Koç University, 34450Istanbul, Turkey
    Koç University Tüpraş Energy Center (KUTEM), 34450Istanbul, Turkey
    Department of Chemistry, Koç University, 34450Istanbul, Turkey
    *Email: [email protected]
    More by Sarp Kaya
Cite this: ACS Appl. Energy Mater. 2022, 5, 12, 15434–15441
Publication Date (Web):December 1, 2022
https://doi.org/10.1021/acsaem.2c03038

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

The utilization of cocatalysts on the photoelectrode surface is a feasible strategy to achieve a high photocurrent density in the photoelectrochemical water oxidation process. The catalysts can enhance the activity by improving the reaction kinetics, retarding charge carrier recombination, or accumulating charge carriers. In this work, we have utilized a CuFe–CoFe Prussian blue (PB) catalyst layer on the BiVO4 photoanode surface to enhance its water oxidation activity. The hybrid catalyst, in which the semiprecious cobalt ions are partially substituted with earth-abundant copper ions, exhibits 56% higher photocurrent density than the CoFe PB-modified BiVO4. We show that photogenerated hole accumulation is present in the CuFe PB layer, which results in higher charge extraction from the BiVO4 surface. The CoFe PB layer on top of the CuFe one facilitates the charge transfer due to its catalytic activity toward the oxygen evolution reaction (OER).

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Introduction

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Limited resources of fossil fuels as the main energy supply in the world and their unsustainability have raised a worldwide concern. Photoelectrochemical water splitting, which converts solar energy into chemical fuels, has been regarded as a promising method for sustainable energy production. (1−3) In semiconductor-based solar water splitting devices, photogenerated electrons in the cathode run the reduction reaction ending up in hydrogen production, while photogenerated holes in the anode drive the water oxidation reaction. (3) Such semiconductors have band edge positions straddling the water oxidation and water reduction electrochemical potentials. Since the water oxidation process is the rate-limiting process, limiting the overall hydrogen production rate due to the sluggish kinetics, a great deal of effort has been devoted to developing photoanode materials with faster water oxidation kinetics. (4) As a promising photoanode material for solar water splitting, BiVO4 has attracted attention due to its proper band gap for sunlight absorption (2.4 eV). Moreover, it exhibits appropriate proper valence band edge position (5) compared to the electrochemical potential of the water oxidation reaction, providing enough driving force for the photogenerated holes to run the oxygen evolution reaction. (6,7) However, it suffers from slow charge transfer kinetics at the surface. A wide variety of catalysts including CoOx, NiO, NiOOH, FeOOH, CoBi, and CoPi (8−12) have been used to overcome the poor catalytic activity of BiVO4 by increasing the charge transfer kinetics at the solid–liquid interface. The relatively poor catalytic activity of BiVO4 is also due to the charge recombination at the surface defects. It is shown that CoPi enhances the water oxidation kinetics at the BiVO4 surface by blocking the surface defects that serve as charge recombination sites rather than promoting the hole transfer to the electrolyte. (12) Interestingly, some oxide materials that are not OER catalysts have been observed also to boost BiVO4 performance. An ultrathin TiO2 layer is shown to enhance the photocurrent of BiVO4 due to suppression of electron trapping at surface states and enhancement of the electron photoconductivity. (13) Another type of catalyst, which has shown a strong water oxidation activity is metal-based Prussian blue (PB) analogues that are successfully implemented for BiVO4 photoanodes. (14−16)
Cobalt-based PB analogues, CoM PBs (M = Co, Fe), have recently emerged as promising materials for the water oxidation reaction due to their high stabilities and activities as water oxidation catalysts. (17−20) The metal ions are connected through a cyanide group (CN) to construct a robust coordination polymer that is stable and active in a wide pH range, pH 1–13. (16) In a typical CoM PB structure, M sites, which are coordinated by six cyanide groups, are catalytically inactive. In contrast, cobalt sites are surrounded by a combination of nitrogen atoms of cyanide groups and water molecules, making them suitable catalytic centers for the water oxidation process. Coupling PB with an appropriate photosensitizer and/or a semiconductor to use them in the light-driven water oxidation process is the general approach due to the poor photocurrent efficiency of PBs. (18,21−24) In such a design, the valence band of the semiconductor should be located at lower energy than the highest occupied molecular orbital (HOMO) level of the PB for an efficient charge transfer. Goberna-Ferron and co-workers studied a catalyst screening of PB with various metals (Mn–Fe, Fe–Fe, Co–Fe, Ni–Fe, Cu–Fe, Mn–Co, Fe–Co, and Co–Co) for the photocatalytic water oxidation process in the presence of a ruthenium photosensitizer. Their study showed that PBs containing Mn2+ or Co2+ have catalytic activities for the OER (18) with slower kinetics for MnFe PB than CoFe PBs. They reported that the presence of Co in a PB-type structure serves as an efficient catalytic site for the OER, which is compatible with classical metal oxides. A CoFe PB layer on a semiconductor is also observed to improve the hole transfer at the surface via different mechanisms. Hegner and co-workers compared the photoelectrochemical activity of CoFe PB with other Co-based catalysts to reveal that a CoFe PB layer raises the photocurrent significantly due to a proper valence band alignment with BiVO4 (25) and an adequate interface between the catalyst and BiVO4. Moss and co-workers showed that an irreversible charge transfer from CoFe PB to BiVO4 occurs, resulting in a hole accumulation in the CoFe PB layer and retardation of charge recombination. (26) Similarly, Li et al. reported that CoFe PB acts as a hole storage layer on BiVO4 photoanode and accelerates charge transfer through the interface. (27) Despite the growing number of efforts on improving the activity of BiVO4 with Co-based PBs, the effect of a second metal ion on the activity of Co-based PB layers has not been investigated yet. Furthermore, partial substitution of semiprecious cobalt ions with relatively nontoxic, abundant, and low-cost 3d metal ions could pave the way for the development of low-cost PB-based photoanodes.
The insertion of a CuFe layer into a Co-based PB layer could be a novel approach to unify the strengths of Cu and Co, which exhibit remarkable charge transfer properties and high catalytic activities, respectively. Recently, Yamada et al. showed that the photocatalytic activity of CoFe PB can be enhanced by increasing the intrinsic defect sites with the formation of a CoFe/CuFe PB core–shell structure. (28) In this work, we prepared a series of PB-coated BiVO4 photoelectrodes with different Cu:Co atomic ratios to elucidate the effect of Cu and Co sites on the performance of the photoanode. We showed that CuFe–CoFe PB layers on BiVO4 enhance the photocurrent and IPCE more than CoFe PB despite the better catalytic activity of Co for the OER compared to Cu. A CuFe layer between BiVO4 and CoFe has been shown to ease the charge transfer from BiVO4 to CoFe and improve the charge separation, while the CoFe layer acts as an OER catalyst at the BiVO4 surface. This study shows the importance of catalysts and replacing rare and toxic catalysts with earth-abundant and safe ones, which can be a motivation for optimum catalyst design.

Experimental Methods

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Materials

Vanadyl acetyl acetonate (VO(acac)2, 98%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, ≥99.99%), p-benzoquinone (C6H4O2, ≥98%), ethanol (C2H5OH, ≥99.8%), DMSO (C2H6OS, ≥99.5%), sodium iodide (NaI, ≥99.5%), potassium hexacyanoferrate (K3[Fe(CN)6], ≥99.5%), and nitric acid solution (HNO3, 70% w/w) were purchased from Sigma-Aldrich. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, ≥99.9%) and copper nitrate trihydrate (Cu(NO3)2·3H2O, ≥99.9%) were obtained from Alfa Aesar. Sodium hydroxide (NaOH, 99%), dipotassium phosphate (K2HPO4, 98–101%), monopotassium phosphate (KH2PO4, 99.5–100.5%), and sodium sulfite (Na2SO3, ≥98.0%) were purchased from ISOLAB. FTO-coated glasses (fluorinated tin oxide, 6–9 Ω·sq–1) were purchased from Ossila. Ultrapure water (18.2 MΩ·cm) from ELGA Purelab Flex was used to prepare the solutions. All of the chemicals were used without any other purification procedure.

BiVO4 Photoanode Fabrication

The BiVO4 photoanodes were prepared using the electrodeposition method, which was previously reported elsewhere. (9) First, the FTOs on glass substrates were cleaned by sonication in ethanol, acetone, and ultrapure water for 10 min each. To prepare the plating solution for electrochemical deposition, first, 1.49 g of NaI (0.4 M) was dissolved in 25 mL of ultrapure water. The pH of the solution was adjusted to 1.8 with diluted HNO3 solution. Then, 485 mg of bismuth nitrate pentahydrate (0.04 M) was added to the solution and stirred for 30 min, until complete dissolution. Another solution was prepared by dissolving 249 mg of p-benzoquinone (0.23 M) in 10 mL of ethanol. Then, these solutions were mixed and stirred for additional 30 min. The electrodeposition was done using a VSP-300 potentiostat and a three-electrode configuration. A platinum wire was used as the counter electrode, an Ag/AgCl (sat’d KCl) electrode was utilized as the reference electrode, and the FTO substrate was used as the working electrode. The electrodeposition was done with −0.1 V bias vs the reference electrode for 2 min to obtain a BiOI film. After electrodeposition, the electrodes were cleaned with ultrapure water and dried in air at room temperature. Then, 530 mg of vanadyl acetyl acetonate (0.4 M) was dissolved in 5 mL of DMSO, and 2 μL of this solution was dropped on BiOI films with a 1 cm2 geometric area. After 5 min of waiting for VO(acac)2 solution to penetrate into the porous BiOI film, the excess solution was removed by touching the edge of the substrate with a filter paper. The samples were annealed in a muffle furnace in air at 450 °C for 2 h with a ramping rate of 2 °C·min–1. Then, the photoanodes were soaked in 1 M sodium hydroxide solution for 30 min to remove the excess V2O5 impurity at the surface. As a final step, the photoanodes were further annealed at 200 °C with flowing O2 gas in a tubular furnace for 2 h. (29)

Prussian Blue Coating

The deposition of PB on the BiVO4 electrodes was performed according to a two-stage procedure. First, the electrodes were dipped in a K3[Fe(CN)6] solution (0.02 M in H2O) for 10 min with gentle stirring to bind Fe(CN)63– on the surface of the BiVO4 layer. After thoroughly rinsing the electrodes with deionized water, they were dipped in the M2+(NO3)2 solution (0.04 M in H2O; M2+: Co2+ or Cu2+) for 10 min under stirring to form the PB structure. The sequence was repeated six times for CoFe PB- and CuFe PB-modified electrodes. CoCuFe/BiVO4 electrodes were coated first with five layers of CuFe PB and then one layer of CoFe PB.

PEC Measurements

PEC tests were carried out using a VSP-300 Biologic potentiostat using a three-electrode configuration. A Pt wire was used as the counter electrode, and Ag/AgCl (sat’d KCl) was used as the reference electrode, similar to the electrodeposition setup. As a solar simulator, an Oriel LCS-100 with a 100 W Xe lamp and an AM1.5G filter was used. The intensity of light on the photoanodes was calibrated to 100 mW·cm–2 with a thermal power sensor. The potential values were converted to vs reversible hydrogen electrode (RHE) using the Nernst equation, which is given below
VRHE=VAg/AgCl+0.059×pH+VAg/AgCl°
(1)
where VRHE is the potential with respect to the RHE, VAg/AgCl is the potential with respect to the reference electrode, pH is the pH of the electrolyte solution, and VAg/AgCl° is the standard potential of the reference electrode, which is 0.197 VNHE. Potassium phosphate buffer (KPi) (0.1 M) at pH = 7, with or without 0.5 M Na2SO3, was used as the electrolyte solution. The linear sweep voltammetry (LSV) measurements were performed using 50 mV·s–1 potential scan rate. For the transient photocurrent (TPC) measurements, a white light-emitting diode (LED) light was used with a pulse duration of 50.0 ms. The distance between the white LED source and the sample was kept at 10 cm, where the intensity of light was measured to be 15.0 mW·cm–2. The photocurrent responses were measured every 400.0 μs. The electrochemical impedance spectroscopy (EIS) measurements were done potentiostatically at different potentials with 10 mV signal amplitude and frequencies between 105 and 10–1 Hz. Incident photon-to-current efficiency (IPCE) measurements were done using a Newport monochromator with a 300 W Xenon lamp. IPCE values were calculated with the equation below
IPCE=[j(mA·cm2)×1240(V·nm)][Pλ(mW·cm2)×λ(nm)]
(2)
where j is the photocurrent density, λ is the wavelength of the incident light, and Pλ is the power of the incident light at wavelength λ.

Structural Characterizations

X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo K-alpha laboratory XPS system with a monochromatized Al Kα X-ray source. Raman spectra were obtained using a Renishaw INVIA spectrometer with a 532 nm laser as an excitation source.

Results and Discussion

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The bare BiVO4 photoanodes were modified with three different PB catalysts. The photoanodes, which were dip-coated with six layers of CoFe PB, are denoted by CoFe/BiVO4. CoCuFe/BiVO4 photoanodes were modified with five layers of CuFe and then one layer of CoFe. The CuFe/BiVO4 photoanodes were dip-coated with six layers of CuFe PB (Figure S1).
The BiVO4 photoanodes modified at the surface with PBs were characterized by XPS, as shown in Figure 1. The Co 2p3/2 peak located at 781 eV for CoFe/BiVO4 and CoCuFe/BiVO4 (Figure 1a) suggests that Co2+ sites are present on the surface of both electrodes. (30) The binding energy difference between Co 2p3/2 and 2p1/2 can be informative regarding the oxidation state of Co ions. A difference of 15 and 16 eV is indicative of 3+ and 2+ states, respectively. (31,32) The binding energy difference of 15.6 eV in the PB-modified BiVO4 indicates the presence of a mixture of 2+ and 3+ oxidation states, which is typically observed for CoFe PBs. (18,33,34) Cu 2p3/2 has two distinct features; a peak at 935 eV with a broad satellite peak at around 943 eV (Figure 1b) is characteristic of a Cu2+ state, while the peak at 933 eV may be related to metallic Cu or Cu+. (35) To distinguish these species from each other, we look at the Cu LMM Auger spectra (Figure 1c), revealing a broad peak at 916 eV that can be attributed to the Cu+ state. (36) Fe 2p spectra in Figure 1d show peaks for Fe 2p3/2 at 708.3 eV, suggesting that the majority of Fe3+ sites is reduced to Fe2+ during the fabrication of electrodes either by oxidizing Co sites, by transferring holes to BiVO4, or a combination of both. (37) Bi 4f, V 2p, N 1s, and O 1s spectra are shown in Figure S2. Bi 4f and V 2p XPS spectra suggest Bi3+ and V5+ valence states, (38) respectively, which is consistent with the previous reports. (39,40) N 1s spectra show a peak at 397.7 eV attributed to N in cyanide groups. (41) In the O 1s spectra, there are two distinct peaks located at 529.8 and 531.0 eV that can be attributed to lattice oxygen in the bulk of the BiVO4 and the surface hydroxyl groups, respectively. (42,43) The Raman spectra given in Figure S3 show a peak at 2110 cm–1 for all three photoanodes, which is associated with the vibration of the cyanide group coordinated to Fe2+, consistent with the XPS results. (44,45)

Figure 1

Figure 1. XPS measurements of CoFe-, CuFe-, and CoCuFe-modified BiVO4 photoanodes. (a) Co 2p, (b) Cu 2p, (c) Cu LMM, and (d) Fe 2p XPS spectra.

To assess the water oxidation activities of these photoelectrodes, we have carried out linear sweep voltammetry (LSV), incident photon-to-current efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) measurements in the absence of a sacrificial agent. Figure 2a shows that CoFe/BiVO4 exhibits a photocurrent density of 1.07 mA·cm–2 at 1.23 VRHE. CoCuFe/BiVO4 on the other hand shows an enhanced photocurrent density of 1.67 mA·cm–2, demonstrating 56% higher activity than CoFe/BiVO4. CuFe/BiVO4 shows the lowest photocurrent among these photoanodes with a photocurrent density of 0.77 mA·cm–2 at 1.23 VRHE. The IPCE results, which are given in Figure 2b, also confirm the superior activity of CoCuFe-modified photoanode as it results in a 17% efficiency, whereas CoFe has a 12% and CuFe has a 3% efficiency at 400 nm. EIS measurements were also carried out, and the Nyquist plots were fitted using the Randles model with a constant phase element. The equivalent circuit is given as an inset in Figure 2c, which consists of Rs (the Ohmic resistance in the electrochemical cell because of the electrolyte, wires, and electrical contacts), Rct (the charge transfer resistance across the interface), and CPE (the capacitance at the photoanode/electrolyte interface). (46) The Nyquist plots show that the charge transfer resistance for the CoFe/BiVO4 is 790 Ω, whereas CoCuFe/BiVO4 demonstrates a lower resistance of 520 Ω. On the other hand, CuFe modification increases the resistance to 1030 Ω. These results suggest that the presence of CoFe on the surface is crucial for the enhancement of BiVO4 water oxidation activity. However, this activity can be further enhanced by adding an interfacial layer of CuFe between BiVO4 and the CoFe PB. The lowest activity is observed for CuFe/BiVO4, confirming that the surface Co ions are critical for the high water oxidation performance.

Figure 2

Figure 2. For CoFe, CuFe, and CoCuFe PB-modified BiVO4 photoanodes, (a) LSV with chopped illumination, (b) IPCE at 1.23 VRHE, and (c) EIS measurements at 0.8 VRHE under front illumination in 0.1 M KPi buffer solution (pH = 7).

To decouple the catalytic activity and the charge separation efficiency of the PB-modified photoanodes, we measured LSVs in the presence of 0.5 M Na2SO3 as a hole scavenger, which is given in Figure S4. The charge injection and charge separation efficiency values are given in Figure 3a,b, respectively. The details of the calculation are given in the SI. CoFe/BiVO4 shows the highest charge injection efficiency among all photoanodes. Such behavior is expected since the enhancement of the catalytic activity of Co-based catalysts, (8,47−49) and more specifically, CoFe PB (14,25,26) on BiVO4 has been well studied. CoCuFe/BiVO4 on the other hand shows lower charge injection efficiency than CoFe/BiVO4 at potentials between 0.4 and 0.9 VRHE and very similar injection efficiency in the 1.0–1.2 VRHE range. CuFe/BiVO4 injection efficiency is observed only in the 0–0.2 potential range, suggesting very low catalytic turnover. Interestingly, these photoanodes demonstrate the highest charge separation efficiency, as shown in Figure 3b. The charge separation efficiency shows the following order: CuFe/BiVO4 > CoCuFe/BiVO4 > CoFe/BiVO4. These findings suggest that the presence of CuFe PB at the BiVO4 surface can enhance its charge separation properties without improving the catalytic activity. The role of CoFe PB, on the other hand, is attributed to its catalytic activity toward OER, resulting in an enhanced charge injection efficiency.

Figure 3

Figure 3. (a) Charge injection and (b) separation efficiency values for PB-modified BiVO4 photoanodes.

As shown in our earlier works on BiVO4 photoanodes, (13,50) TPC measurements performed with 50.0 ms white LED short pulses can be used to acquire information about surface charge carrier dynamics in the millisecond time regime. We have carried out these measurements in the potential regime of 0.4–1.0 VRHE with 0.1 V increments. Figure 4a–c shows the TPC results for CoFe/BiVO4, CuFe/BiVO4, and CoCuFe/BiVO4, respectively. When we turn the light on at t = 0.0 ms, we observe an anodic increase in photocurrent in the first few milliseconds, resulting from charge generation and hole migration toward the surface. For each photoanode, the anodic increase in photocurrent increases with applied potential due to increasing band bending with the anodic polarization. Then, the photocurrent decays to a steady-state value due to the hole accumulation at the interface and catalytic limitations toward OER. The steady-state photocurrent density of the photoanodes follows the trend CoCuFe > CoFe > CuFe, which is consistent with the LSV, EIS, and IPCE results. For CoFe/BiVO4 photoanode, the photocurrent decays when the light is turned on for applied potentials of 0.4–0.8 VRHE. However, we do not observe a significant decay in photocurrent at potentials between 0.8 and 1.0 VRHE, which suggests low hole accumulation and decent catalytic activity at these potentials. For CuFe/BiVO4, we observe a decay in photocurrent at all potentials with a lower photocurrent density, suggesting low catalytic activity. For CoCuFe/BiVO4, we observe a decay at potentials between 0.4 and 0.8 VRHE and no decay at 0.8–1.0 VRHE similar to CoFe/BiVO4. When the light is turned off at t = 50.0 ms, holes accumulated on the surface recombine with the electrons, and the photocurrent decays to zero. It is also noteworthy that we observe a small anodic peak when we turn the light off for the CuFe/BiVO4 photoanode. We previously attributed this peak to the photogenerated electron detrapping from the BiVO4 surface due to the V5+/V4+ surface state, where we showed that such phenomena could limit the water oxidation activity. (13) However, the extent of the detrapping phenomena is not significant; therefore, we leave it out of this discussion. The rate of the photocurrent decay after the light is turned off can give information about the existing charge carrier trapping or localization on the surface. (51) The localized holes in midgap states may recombine slower than the holes in the valence band. We have fitted the decay kinetics of the TPC measurements using two exponential decay functions and calculated the time constants, which are given in Figure S5. The average time constants for the CuFe/BiVO4 photoanodes are higher at all potentials, suggesting that the decay is slower. On the other hand, CoFe and CoCuFe time constants are lower and comparable to each other. The slower decay can be an indication of hole trapping in the CuFe-modified photoanodes. The Cu 2p XPS and LMM Auger spectra given in Figure 1b,c show that Cu+ species are present in the photoanode surfaces, as well as Cu2+, suggesting that hole localization is viable in these sites. For CoFe- and CoCuFe-modified photoanodes, the hole trapping is not as significant as CuFe/BiVO4. Therefore, we attribute the role of the CuFe PB layer as a hole extraction layer from the BiVO4 surface, which enhances the charge separation properties of BiVO4. The role of the CoFe PB layer is mostly catalytic. In the CoFe photoanodes, the photogenerated holes from the BiVO4 surface are transferred to the CoFe layer to be utilized for the catalytic water oxidation process on the Co sites. For the CuFe-modified electrode, the photogenerated holes may transfer to the CuFe layer; however, due to sluggish OER kinetics of the Cu ions, the holes cannot be transfered to the electrolyte effectively, resulting in poor photocurrent. For the CoCuFe/BiVO4 photoanode, we propose that two layers of PB can have a synergistic effect, in which the photogenerated holes near the surface region of BiVO4 can get trapped in the CuFe layer, to be transferred efficiently to the CoFe PB layer for the OER process.

Figure 4

Figure 4. TPC measurements done using 50.0 ms white light LED short pulses in 0.1 M KPi buffer solution for (a) CoFe/BiVO4, (b) CuFe/BiVO4, and (c) CoCuFe/BiVO4.

We have carried out EIS measurements in the potential region of 0.4–1.0 VRHE to gain insights into the interfacial charge transfer resistance and the capacitance of the PB-modified photoanodes. Figure 5a,b shows the charge transfer resistance and the capacitance values at the interface as a function of the applied potential, respectively. The Nyquist plots of the EIS measurements and the fittings are given in Figure S6. The charge transfer resistance for the CoCuFe/BiVO4 photoanode is the lowest, whereas CuFe/BiVO4 shows significantly higher resistance. Such a behavior is expected as the Co ions can transfer holes more rapidly due to their catalytic activities, resulting in lower resistance for CoFe/BiVO4 than CuFe/BiVO4. CoCuFe/BiVO4 has even lower Rct, indicating superior OER activity. Capacitance measurements given in Figure 5b show higher Cint for CuFe/BiVO4 photoanodes at all potentials, whereas CoFe/BiVO4 photoanodes have the lowest capacitance. The capacitive behavior for a photoelectrode can result from the surface states, electrical double layer, and the space–charge capacitance. (52)CDL measurements given in Figure S7 do not vary significantly between the photoanodes. The Mott–Schottky analysis in Figure S8 also does not yield a significantly different slope between the photoanodes, suggesting that the Nd values and thus the potential drop across the space–charge region are similar. The Cint under illumination can be associated with the accumulation of the positively charged species, namely, photogenerated holes, at the interface. (53) We propose that the Cint is larger for the CuFe-modified photoanode because of the photogenerated hole accumulation at the CuFe layer, resulting from available Cu sites for hole localization, consistent with the previous TPC and charge separation efficiency results. CoFe/BiVO4 shows the lowest Cint, as the holes are effectively utilized for water oxidation and no significant accumulation occurs. CoCuFe/BiVO4 on the other hand has both features, as the holes are extracted and accumulated in the CuFe PB layer and then transferred to the CoFe PB layer and utilized effectively for OER.

Figure 5

Figure 5. Potential-dependent EIS measurements and fitting parameters of (a) Rct, and (b) Cint performed under front illumination in 0.1 M KPi buffer solution.

LSV measurements were carried out for CuFe/BiVO4 and CoFe/BiVO4 in the absence of illumination, which are given in Figure S9. The oxidation onset potentials can be used to determine the HOMO levels for the PB catalysts. (18) For CuFe and CoFe PB, HOMO levels can be estimated as 2.1 and 1.9 VRHE, respectively. Thus, the relative band positions of BiVO4, CuFe PB, and CoFe PB can be demonstrated as given in Figure 6. The relative positions of the valence band edge of BiVO4 and the HOMO levels of CuFe and CoFe PB show that the hole transfer toward the surface in such a configuration is feasible.

Figure 6

Figure 6. Schematic representation of the CoCuFe/BiVO4 band diagram and the charge transfer pathway.

Conclusions

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We have modified the BiVO4 surface with a novel CoCuFe hybrid PB catalyst layer to enhance the charge injection and separation properties of BiVO4. The CoCuFe PB modification yields a photocurrent density of 1.67 mA·cm–2, showing 56% higher activity than that of the CoFe-coated photoanode. We corroborated the performance enhancement with the decrease of charge transfer resistance and increase of the IPCE. The CuFe PB-modified photoanode shows a slow photocurrent decay in the TPC measurements, higher capacitance under illumination, and enhanced charge separation. We attribute the enhancement with the CoCuFe PB modification to the synergistic effect of CuFe and CoFe PB catalyst layers. The CuFe PB layer can extract photogenerated holes from the BiVO4 surface, and the CoFe layer facilitates the reaction kinetics of the OER, resulting in an enhanced photocurrent. Adding a CuFe layer between the BiVO4 surface and the catalytic CoFe layer not only enhances the performance but also decreases the cost of the photoanode. In conclusion, we show that adding an interfacial layer could be a feasible strategy to enhance the activity of cocatalyst-modified BiVO4 photoanodes further and decrease their cost.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.2c03038.

  • Schematic representations of the photoanodes, XPS and Raman spectra, performance comparison with the literature, LSV in scavenger solution, TPC average decay constants, Nyquist plots, CDL measurements, Mott–Schottky analysis, stability measurements, and CV with the absence of illumination (PDF)

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Author Information

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  • Corresponding Authors
    • Ferdi Karadaş - UNAM-National Nanotechnology Research Center, Institute of Materials Science and Nanotechnology, Bilkent University, 06800Ankara, TurkeyDepartment of Chemistry, Bilkent University, 06800Ankara, Turkey Email: [email protected]
    • Sarp Kaya - Materials Science and Engineering, Koç University, 34450Istanbul, TurkeyKoç University Tüpraş Energy Center (KUTEM), 34450Istanbul, TurkeyDepartment of Chemistry, Koç University, 34450Istanbul, TurkeyOrcidhttps://orcid.org/0000-0002-2591-5843 Email: [email protected]
  • Authors
    • Emre Usman - Materials Science and Engineering, Koç University, 34450Istanbul, TurkeyKoç University Tüpraş Energy Center (KUTEM), 34450Istanbul, TurkeyPresent Address: Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
    • Mahsa Barzgar Vishlaghi - Materials Science and Engineering, Koç University, 34450Istanbul, TurkeyKoç University Tüpraş Energy Center (KUTEM), 34450Istanbul, TurkeyPresent Address: Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany
    • Sina Sadigh Akbari - UNAM-National Nanotechnology Research Center, Institute of Materials Science and Nanotechnology, Bilkent University, 06800Ankara, TurkeyDepartment of Chemistry, Bilkent University, 06800Ankara, Turkey
  • Author Contributions

    E.U., M.B.V., and S.S.A. have contributed to this study equally.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors would like to thank The Scientific and Technical Research Council of Turkey (TUBITAK) for support (Grant Number: 221Z070). The authors thank KUYTAM for the characterization measurements. The authors thank Elif Öykü Alagöz for her support.

References

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This article references 53 other publications.

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Cited By

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This article is cited by 1 publications.

  1. Naeimeh Sadat Peighambardoust, Sina Sadigh Akbari, Rana Lomlu, Umut Aydemir, Ferdi Karadas. Tunable Photocatalytic Activity of CoFe Prussian Blue Analogue Modified SrTiO3 Core–Shell Structures for Solar-Driven Water Oxidation. ACS Materials Au 2024, 4 (2) , 214-223. https://doi.org/10.1021/acsmaterialsau.3c00090
  • Abstract

    Figure 1

    Figure 1. XPS measurements of CoFe-, CuFe-, and CoCuFe-modified BiVO4 photoanodes. (a) Co 2p, (b) Cu 2p, (c) Cu LMM, and (d) Fe 2p XPS spectra.

    Figure 2

    Figure 2. For CoFe, CuFe, and CoCuFe PB-modified BiVO4 photoanodes, (a) LSV with chopped illumination, (b) IPCE at 1.23 VRHE, and (c) EIS measurements at 0.8 VRHE under front illumination in 0.1 M KPi buffer solution (pH = 7).

    Figure 3

    Figure 3. (a) Charge injection and (b) separation efficiency values for PB-modified BiVO4 photoanodes.

    Figure 4

    Figure 4. TPC measurements done using 50.0 ms white light LED short pulses in 0.1 M KPi buffer solution for (a) CoFe/BiVO4, (b) CuFe/BiVO4, and (c) CoCuFe/BiVO4.

    Figure 5

    Figure 5. Potential-dependent EIS measurements and fitting parameters of (a) Rct, and (b) Cint performed under front illumination in 0.1 M KPi buffer solution.

    Figure 6

    Figure 6. Schematic representation of the CoCuFe/BiVO4 band diagram and the charge transfer pathway.

  • References

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    This article references 53 other publications.

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    • Schematic representations of the photoanodes, XPS and Raman spectra, performance comparison with the literature, LSV in scavenger solution, TPC average decay constants, Nyquist plots, CDL measurements, Mott–Schottky analysis, stability measurements, and CV with the absence of illumination (PDF)


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