Transparent Co3FeOx Film Passivated BiVO4 Photoanode for Efficient Photoelectrochemical Water Splitting①

FANG Ming QIN Qi CAI Qian LIU Wei②

a (Fujian Normal University, Fuzhou 350007, China)

b (CAS Key Laboratory of Design and Assembly of Functional Nanostructures,Fujian Provincial Key Laboratory of Nanomaterial, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT Photoelectrochemistry that use semiconductors to absorb sunlight for water splitting provides an effective method for the development of renewable hydrogen energy in the future. In this paper, a transparent and highly-efficient cobalt-iron oxide (Co3FeOx) nano-film was fabricated through hydrothermal method by directional adjustment of atomic ratio to promote the kinetics of BiVO4 (BVO) photoanode water oxidation. As a result, the Co3FeOx-modified BVO photoanode (Co3FeOx/BVO) exhibits an impressive photocurrent density of 4.0 mA·cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), approximately 2.17-fold higher than that of bare BVO, as well as a cathodically shifted onset potential of 320 mV. Transparent catalyst nanolayer structure is clarified by ultraviolet-visible spectroscopy. In addition, the Co3FeOx/BVO photoanode has better stability, and there is no obvious activity degradation after 10 hours of reaction. This strategy provides a broad prospect for the use of water oxidation co-catalyst to achieve effective water splitting.

Keywords: photoelectrochemical water splitting, bismuth vanadate, co-catalyst, cobalt-iron oxide;

1 INTRODUCTION

Photoelctrochemical (PEC) for water splitting has great potential in the production of sustainable hydrogen fuel and climate change issues[1-3]. The water splitting process consists of two different half reactions, namely the oxygen evolution reaction (OER) of the positive electrode and the hydrogen evolution reaction (HER) of the negative electrode[4-7]. Therefore, the development of an efficient photoanode is an essential part of PEC water splitting.Effective photoanode requires excellent light response,effective charge separation and long-term stability in an aqueous medium and does not react with the aqueous solution[8-10]. Among various available metal oxide semiconductors, bismuth vanadate is among the most promising materials fit for photoanode semiconductors because of proper conduction band edge position and narrow band gap[11,12]. However, due to serious surface reorganization and slow water oxidation kinetics, practical photocurrent densities of the pure BVO are much smaller than the theoretical value (7.5 mA·cm-2at AM 1.5 G)[13,14]. Therefore,how to significantly improve the kinetics of BVO photoanode OER water oxidation still faces huge challenges.Lately, many efforts such as element doping[15-17], cocatalyst deposition[18-20], heterojunction construction[21-23]and nanostructured control[24-26]have been made to improve the kinetics of BVO photoanode. Among various methods, the introduction of OER catalyst can accelerate water oxidation reaction and consumption of photogenerated holes, thereby inhibiting the recombination of photogenerated carriers[27,28].Recently, because of their particular electronic structure and orbital, nickel- and cobalt-based materials have obtained wide applications in promising OER non-noble metal catalysts, particularly layered double hydroxides (LDH) and metal oxides with Fe and Co species[29-33]. However, the disadvantages of nickel- and cobalt-based LDH are poor electronic conductivity, which hinders the catalytic activity of the catalyst. As we all know, doping Fe in NiOOH may increase the catalytic activity of OER[34-36]because this doping possibly affects NiOOH’s local electronic structure[37], and the addition of Fe heteroatoms may gain active site points and reinforce the charge transfer kinetics between Ni and Fe sites[38]. In addition, OER catalyst can also improve the stability of the photoanode. The uniform load on the surface of the photoanode by blocking the direct contact with the electrolyte, which can effectively protect the photoanode from photocorrosion[39].

Inspired by the above analysis, in the research, we propose one easy and efficient hydrothermal immersion synthesis method, which can construct the ultrathin and transparent CoFe-based oxide nanolayers on the BVO photoanode as a water oxidation catalyst to enhance the water decomposition ability of PEC. The best OER activity is achieved by adjusting the ratio of Co/Fe atoms. As a result,under the AM 1.5G sunlight, the photocurrent density of BVO photoanode modified with the Co/Fe atomic ratio of 3:1 catalyst reached 4.0 mA-2at 1.23 V versus RHE, which was 2.17 times that of the original BVO. Moreover, the photoanode also shows satisfactory stability under high current density and could achieve stable water splitting within 10 hours, without a significant drop in photocurrent density, mainly originating from the protection of ultrathin Co3FeOxfilm. Detailed experimental research and PEC test show that proper Fe element doping can fine-tune the valence and electronic state of Co element, thereby exposing more active sites. In this work, the efficient and stable Co3FeOx/BVO photoanode provides new insights into the metal oxide co-catalyst.

2 EXPERIMENTAL

2. 1 Synthesis of BVO films

The BVO film used our previous method. First, 0.13 mol/L tartaric acid aqueous solution was perepared, then 5 mL acetic acid was added and stirred for 10 minutes, and nitric acid (76wt%, Sigma-Aldrich) was added to adjust PH to 2. Next, 8.9 × 10-2mol/L bismuth nitrate pentahydrate(98%, Sigma-Aldrich) was added until dissolution. After that,a three-electrode electrodeposition method was used, with the bias voltage kept at 2.6V and the electroplating performed for 40 minutes. Finally, 0.132 mmol VO(acac)2(98%, Sigma-Aldrich) and the bismuth precursor thin-film electrode were annealed in the air at 500 °C for 2 hours.Excessive bismuth oxide was soaked and removed in 1 mol/L NaOH solution.

2. 2 Synthesis of CoxFeyO co-catalyst deposition on BVO films

The CoxFeyO co-catalysts were deposited using a hydrothermal method. In a typical procedure, a predetermined amount of 2.0 mmol cobalt nitrate hexahydrate and ferric nitrate nonahydrate are added to 50 mL of deionized water. Then, the ratio of Co2+and Fe3+was adjusted to 4:0,3:1, 2:2, 1:3 and 0:4 for comparison. Next, 2.24 g of urotropine was added to the mixed solution. Finally, it was kept in a Teflon-lined stainless-steel autoclave at 90 °C for 2 h, cooled to room temperature and rinsed with deionized water for drying, and stored under vacuum for later testing.

3 RESULTS AND DISCUSSION

The fabrication process of Co3FeOx/BVO photoanode is illustrated in Fig. 1a. In detail, the BVO nanoarray synthetic onto a fluorine-doped tin oxide (FTO) by electrodeposition and chemical vapor deposition (CVD) method in our recent study[40-42]. Afterwards, Co3FeOxfilm deposits on BVO photoanode by the hydrothermal method. Briefly,Co(NO3)2·6H2O and Fe(NO3)3·9H2O needed to be dissolved in 40 mL of DI water with a 3:1 stoichiometric ratio of Co2+and Fe3+. Then 2.24 g hexamethylenetetramine and 1.5 mmol of triethanolamine (TEA) were added to the mixture under stirring. Finally, the above solution was kept at 90 °C for 2 h in a Teflon-lined stainless-steel autoclave. The structure diagram is shown in Fig. 1b. Upon illumination,photogenerated holes can be transferred from the conduction band of BVO to the ultrathin Co3FeOxfilm because ultrathin structure could facilitate the holes migration and effectively restrain the charge recombination. Moreover, Co element can serve as conductive and reactive stable host that can reinforce Fe active site activity. The holes can quickly migrate to the catalyst layer and participate in the water oxidation reaction owing to its rapid water oxidation kinetics and lower onset potential. Simultaneously, the electrons migrate to the Pt electrode where H+ions are reduced to from H2. Owing to the more efficient charge separation efficiency and more photogenerated holes accumulation, the higher OER performance is achieved by Co3FeOxcoating,which is confirmed by higher photocurrent density (Fig. 4a).The presence of Co3FeOxcan quickly transfer photogenerated holes from BVO to its outer layer, where metal active sites can utilize these holes. The Co species considered to be the active site can accept the holes generated by BVO and be oxidized to Co3+. The resulting high-valent Co species can effectively oxidize water to generate O2.

Fig. 1. (a) Synthetic process of Co3FeOx/BVO photoanode. (b) Structure schematic of Co3FeOx/BVO

The BVO photoanode crystal structure was characterized by SEM and TEM technologies, as shown in Fig. 2. The SEM image (Fig. 2a and Fig. S1, ESI) shows that BVO crystalline is tightly connected to each other to form a nano-flower-like structure and evenly distributed on the FTO substrate. Furthermore, the transmission electron microscopy(TEM) and high-resolution transmission electron microscopy (HRTEM) images (Fig. 2d and Fig. 2e) were also performed. The high-crystalline structure with a lattice spacing of 0.47 nm could be clearly observed, which matches well with the (110) lattice plane of monoclinic scheelite BVO. The coating of Co3FeOxcatalyst layer was synthesized by the hydrothermal method (Supporting Information, Experimental Section). From SEM picture (Fig.2b and Fig. S2, ESI) we can see that Co3FeOxfilm uniformly covers the surface of BVO crystal. Additionally, the photoelectric performance of Co3FeOx/BVO and BVO photoanode was characterized by ultraviolet-visible spectroscopy (UV-Vis) (Fig. 2c). Clearly, the position of the edge of the absorption band is basically the same after the deposition of OER co-catalyst compared with pure BVO. It can be concluded that the loading of Co3FeOxcan not expand or weaken the range of its response to light. Moreover, the high resolution TEM images (Fig. 2d) obviously demonstrate that the surface of BVO nanocrystal is evenly covered with a layer of Co3FeOxcatalyst with a thickness of about 13 nm.The catalyst layer is closely attached to the surface of BVO,which is conducive to the transfer of surface carriers and improves the water oxidation kinetics of PEC and the stability of photoanode. X-ray diffraction (XRD) was taken for the characterization of the crystal structure and phase in all samples obtained (Fig. 2f). The SEM mapping and energy dispersive X-ray spectroscopy (EDS) pattern of Co3FeOx/BVO photoanode are displayed in Fig. S3 and Fig.S4. Due to the trace amount of catalyst layer, no information about Co and Fe elements was detected, which will be further verified by XPS analysis. The peak of SnO2on the FTO glass substrate is marked with a heart-shaped symbol.In addition, in these samples, the typical diffraction peak of the monoclinic BVO crystal (JCPDS No. 14-0688) can be well detected. In contrast, no evident peak of Co3FeOxin Co3FeOx/BVO has been observed, which can be attributed to the amorphous and ultra-thin structure of the catalyst layer.

Fig. 2. (a) SEM image of the BVO film. Inset: digital image of BVO photoanode. (b) SEM image of the Co3FeOx/BVO film. Inset:digital image of Co3FeOx/BVO photoanode. (c) UV-Vis spectra of BVO and Co3FeOx/BVO photoanode. (d, e) TEM and high-resolution TEM image of the Co3FeOx/BVO film. (f) XRD patterns of the BVO and Co3FeOx/BVO films

In order to explore the chemical state and composition of the modified Co3FeOx/BVO and BVO photoanode surface,we used X-ray photoelectron spectroscopy (XPS) for characterization. Typical O 1s, V 2pand Bi 4fpeaks are found from the XPS spectrum of the two species (Fig. S5,ESI). The characteristic peaks of Bi 4f7/2(159 eV), Bi 4f5/2(164.2 eV), V 2p3/2(516.4 eV) and V 2p1/2(524.2 eV) are attributed to Bi3+and V5+in BVO (Fig. 3a and 3b). In particular, the XPS spectrum of Bi and V (Fig. 3a-b) in Co3FeOx/BVO is significantly shifted to a higher binding energy than BVO, indicating that the electronic environment around BVO changes with the loading of the catalyst. For the O 1sXPS spectrum of Co3FeOx/BVO films (Fig. S6,ESI), it is fitted into three peaks. In detail, the peak at 529.9 eV can be assigned to O2−species in the lattice (OL), that at 531.4 eV results from oxygen vacancies (OV), and that at 532.4 eV should be attributed to adsorbed hydroxyl group on the surface (OC)[43]. The Co 2pspectrum comprises two major peaks at 781.1 and 796.9 eV, respectively, and the spin orbital split is about 16 eV, corresponding to Co 2p3/2and Co 2p1/2(Fig. 3c). In addition, two satellite peaks are located at 785.8 and 806.0 eV (denoted as “sat” in Fig. 3c), which are the characteristics of Co2+species. The Fe spectrum (Fig. 3d)contains two primary peaks at 710.6 and 724.5 eV, attributed to Fe 2p3/2and Fe 2p1/2of Fe3+, respectively. Furthermore,two satellite peaks located at higher binding energies 716.9 and 732.5 eV (denoted as “sat” in Fig. 3d), which can be ascribed to the charge-transfer or shakeup processes associated with Fe3+[44]. These results further indicate the successful synthesis of Co3FeOx/BVO photoanode.

Fig. 3. (a-b) High-resolution XPS spectra for BVO and Co3FeOx/BVO: (a) Bi 4f,(b) V 2p, (c-d) High-resolution XPS spectra of the Co3FeOx/BVO film: (c) Co 2p, and (d) Fe 2p

To investigate the PEC water splitting performance,photocurrent density-potential (J-V) curves of BVO and Co3FeOx/BVO photoanodes needed to be documented by 1 mol/L K3BO3electrolyte (pH = 9.5) in AM 1.5 G simulated sunlight (100 mW·cm-2). As shown in Fig. 4a, the linear sweep voltammetry (LSV) curve shows that the original BVO film only exhibits a photocurrent density of 1.84 mA·cm-2at 1.23 V versus RHE, and also an onset potential of ～0.4 V versus RHE. After loading the Co3FeOxlayer as co-catalyst, it is gratifying that the photocurrent density was increased by 1.17 times at 1.23V versus RHE, reaching 4.0 mA cm-2(Fig. 4a). Besides, this photocurrent density is better than most of the previously reported BVO electrodes modified by co-catalyst (Table S1, ESI). In addition,compared with pure BVO, it shows a significant cathodic displacement of the starting potential. At the same time, the PEC water oxidation performance of CoPi/BVO was also measured under the same conditions for comparison. Clearly,BVO and Co3FeOx/BVO films show a similar onset potential,while Co3FeOx/BVO photoanode exhibits the higher current density at a voltage of 1.23 V versus RHE.

For understanding the intrinsic mechanism of improved PEC performance, the incident photon-current conversion efficiency (IPCE) curve for BVO and Co3FeOx/BVO electrodes should be assessed at 1.23 V versus RHE (Fig. 4b).Both BVO and Co3FeOx/BVO films exhibit photo-current response from 350 to 520 nm, conforming to the UV-vis data(Fig. 2c) at the edge of light absorption. Compared with BVO, Co3FeOx/BVO shows great efficiency in the entire wavelength range. The maximum IPCE of Co3FeOx/BVO is 41.7%, while the maximum IPCE of original BVO is 21%.Fig. 4c demonstrates the bias photon-to-current efficiency(ABPE) in the sample. The ABPE value of BVO film is 0.28% at 0.7 V, which is gratifyingly that after the Co3FeOxcatalyst deposited the ABPE value is increased to 1.2% at 0.77 V. The excellent photoelectric properties of the Co3FeOx/BVO electrode are caused by the decrease of surface charge recombination, which can be further verified through measuring the chopping transient photocurrent density (Fig. 4d). When the lamp is cycled on or off, an upward current spike will appear, which is caused by the accumulation of holes between the electrode and the electrolyte interface. In this case, the surface state and reaction kinetics of the electrode photoresponse affect the smoothness of the current spike. As shown in Fig. 4b, the pure BVO electrode has a strong spike, indicating the charge recombination caused by the slow water oxidation kinetics on the original BVO surface. In contrast, Co3FeOx/BVO has a much smoother peak, suggesting the inhibition of surface modification on the recombination of photogenerated carriers. Moreover, electrochemical impedance spectroscopy(EIS) was measured at 0.7 V versus RHE to further evaluate the reaction kinetic. The EIS Nyquist curves of two photoanodes comprise a semicircle fitted to the series resistance (Rs), charge transfer resistance between electrode surface and electrolyte (Rct) and constant interface phase element (CPE) (Fig. 4e). A smaller diameter reflects the smaller charge-transfer resistance, which is used to compare the electron transfer kinetics. Therefore, the Co3FeOx-coating can effectively enhance charge transport capability, thus further improving the performance of water splitting. The photoelectric performance of the electrode is characterized by evaluating the surface charge separation efficiency (ηsep) associated with the hole reaching the surface.Sodium sulfite (Na2SO3) is used as a hole scavenger. Due to the fast kinetics of SO32-, the η trans of SO32-oxidation can be assumed to be 100%[45](see the supporting information section for details). According to the results of Fig. 4f,compared with pure BVO (35%), the ηsepof Co3FeOx/BVO is as high as 78%, which proves that a good interface is formed with high efficiency photoelectric conversion efficiency between Co3FeOx/BVO and BVO. In order to confirm the hydrogen evolution reaction at the Pt electrode as well as oxygen evolution reaction at the Co3FeOx/BVO photoanode, the amount of hydrogen and oxygen released was analyzed by gas chromatography (GC) (Fig. 4g). With prolonging light time, the contents of H2and O2showed a linear increase. The amounts of H2and O2released after 30 minutes were 43.3 and 20.1 μmol, respectively with the stoichiometric ratio to be about 2:1, which was basically consistent with the theoretical results. Mott-Schottky (MS)was measured in a 1 mol/L potassium borate solution under dark conditions (Fig. 4h). The BVO and Co3FeOx/BVO demonstrate the positive slope in MS curves, showing n-type characteristics. Compared with the BVO film, the MS plot of Co3FeOx/BVO shows a gentler slope, indicating an increase in carrier concentration after Co3FeOxis deposited, which is just in line with the above experimental results. The high stability of photoanode also plays an important role in future practical application. Therefore, we studied the stability of the prepared photoanode. Fig. 4i shows the water oxidation stability of the original BVO and Co3FeOx/BVO membrane PEC in a 1 mol/L potassium borate buffer solution. It can be clearly seen that after 10 hours of stability test, the photocurrent density of BVO dropped to 75.9%. We believe that the photocurrent density degradation of BVO in water oxidation should be attributed to its poor kinetics, leading to the serious recombination of photogenerated electron and hole pairs. In addition, after assembling the catalyst, the stability of BVO photoelectrode is significantly improved.Compared with BVO, the photocurrent density of Co3FeOx/BVO maintains 95.5%, indicating the prominent role of Co3FeOxcocatalysts in reinforcing the OER activity and stability of BVO photoanodes.

Fig. 4. PEC performance for BVO and Co3FeOx/BVO photoanodes. (a) Photocurrent density versus applied potential curves,(b) IPCE curves obtained at 1.23 V versus RHE, (c) ABPE curves, (d) Transient photocurrent density as a function of time under illumination at an applied potential of 1.23 V versus RHE, (e) EIS spectra measured under 1.23 V versus RHE and AM 1.5G illumination (Inset: the equivalent circuit model) (f) Charge transfer efficiency versus applied potential curves. (g) Fluorescence probe measurement of O2 and H2 generation from Co3FeOx/BVO photoanode. (h) Mott-Schottky plots of BVO, and Co3FeOx/BVO photoanodes, (i) photocurrent density versus time curves at 1.23 V versus RHE under AM 1.5 G illumination of the photoanodes. All measurements were performed in a 1 mol/L potassium borate electrolyte (pH 9.5)

4 CONCLUSION

In summary, we have prepared a transparent cobalt-iron oxide layer on the surface of BVO by the hydrothermal method to construct a composite structure photoanode. The Co3FeOx/BVO photoanode achieves a remarkable photocurrent density of 4.0 mA·cm-2at 1.23 V versus RHE by optimizing the proportion of cobalt iron ions, and a low onset potential (～320 mV) for water splitting also achieved.In addition, the Co3FeOx/BVO thin film photoanode also showed a more stable photoactivity, and it still maintained a photocurrent density of 95.5% after a 10h stability test. Our research shows that the excellent PEC activity is attributable to the protection of Co3FeOxand the inhibition of charge recombination to improve the surface reaction efficiency, as well as the timely consumption of photo-generated holes reaching the surface to improve the overall stability. Our work may provide a promising strategy for improving the PEC performance of BVO photoanode in solar water splitting.