Fabrication of BiVO4 photoanode cocatalyzed with NiCo-layered double hydroxide for enhanced photoactivity of water oxidation

Applied Catalysis B: Environmental 263 (2020) 118280

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Fabrication of BiVO4 photoanode cocatalyzed with NiCo-layered double hydroxide for enhanced photoactivity of water oxidation Houde Shea,c, Pengfei Yuea, Xiaoyu Maa, Jingwei Huanga, Lei Wanga, Qizhao Wanga,b,c,

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a College of Chemistry and Chemical Engineering, Research Center of Gansu Military and Civilian Integration Advanced Structural Materials, Gansu International Scientific and Technological Cooperation Base of Water-Retention Chemical Functional Materials, Northwest Normal University, Lanzhou 730070, China b School of Environmental Science and Engineering, Chang'an University, Xi’an, 710064, China c Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China

A R T I C LE I N FO

A B S T R A C T

Keywords: BiVO4 NiCo-LDH Photoelectrochemical water splitting Electric deposition

Modifying a semiconductor photoanode with an oxygen evolution cocatalyst is an effective method for improving the photoelectrochemical (PEC) water decomposition efficiency. Here, PEC efficiency of BiVO4 electrode is enhanced by constructing a NiCo-LDH/BiVO4 heterojunction. Under the synergistic effect of NiCo-LDH and BiVO4, the photocurrent, incident photon-to-current conversion efficiency and hydrogen production of NiCoLDH/BiVO4 electrode have been significantly improved, which are mainly attributed to the enhancement of internal light absorption, photogenerated carriers transfer and separation.

1. Introduction Solar-driven photoelectrochemical (PEC) water splittingto produce hydrogen has aroused widespread concern [1–5] since it is viewed as a promising alternative to increasingly depleted fossil fuels [6–9]. It can provide clean, economical hydrogen fuel with high solar chemical energy conversion [10–14]. In the past decade, intensive efforts have been devoted in searching for eligible candidates with improved solar to hydrogen (STH) conversion efficiency for industrial application of PEC process. For this purpose, various photoelectrode materials such as TiO2 [15–19], WO3 [20–22], Fe2O3 [23–25], and BiVO4 [26–29], have been well studied.Among them, BiVO4 exhibits great potential due to itsappropriate band position, abundant resources, and high stability against photocorrsion. With external bias to counterbalance the overpotential of the half reaction of water splitting, it is thermodynamically favorable for water oxidizing half reactionto occur with a theoretical maximum STH efficiency ∼9.1% under AM 1.5 G irradiation [7,30]. Although BiVO4 is an ideal photoanode light absorber, there is still an urgent need to improve its poor charge separation efficiency and slack oxygen evolution kinetics due to its short carrier diffusion length and slow oxidation kinetics.Fabrication of BiVO4 photoanode consisted of mesoporous nanoparticles can help to improve bulk charge separation efficiency. In searching of appealing BiVO4-based photoanode, various strategies, such as morphological control [26,29], metal and non-metal doping [31,32] have been brought out to enhance surface

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reaction kinetics and charge separation efficiency [33,34]. At the same time, some very effective but expensive cocatalysts like IrO2 and RuO2were adopted to improve the PEC performance of BiVO4 [1]. Driven by the incentive to lower the cost, researchers began to put more emphasis on developing non-noble-metal OER cocatalysts (e.g. Co-Pi [35,36], CoOx [37,38]), which can improve PEC performance in aspect of water oxidation kinetics. Transition metal hydroxides (oxy)such as Ni (OH)2 [39], CoOOH [40] and NiCo layered double hydroxides (NiCoLDH) have also been widely used to accelerateoxygen evolution kinetics [41,42]. Layered double hydroxides(LDHs) are classified as anion clay and can be used as OER cocatalysts when transitional metals (e.g. Fe, Co, Ni, Zn and Mn) are doped in, which can provide active sites due to optimal adsorption energy achieved at the doping sites [43]. LDHs have also been reported to have a narrow band gap to enhance light harvesting [44], and a metal feature to promote charge separation at the BiVO4/ LDH phase interface. Inspired by above mentioned merits, here we report the preparation of NiCo-LDH/BiVO4 heterostructure via a simple electrodeposition method. The NiCo-LDH/BiVO4 photoanode exhibited a significantly enhanced photocurrent compared to pure BiVO4. The photocurrent of composite electrode reached 3.4 mA/cm2 at 1.23 V vs. RHE with onset potential shifted 230 mV cathodically, which can be accounted to the enhanced charge separation efficiency and the hole injection efficiency.

Corresponding author at: School of Environmental Science and Engineering, Chang’an University, Xi’an, 710064, China. E-mail addresses: [email protected], [email protected] (Q. Wang).

https://doi.org/10.1016/j.apcatb.2019.118280 Received 9 June 2019; Received in revised form 14 September 2019; Accepted 10 October 2019 Available online 18 October 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. SEM images of BiOI (A–B), BiVO4 (C–D) and NiCo-LDH/BiVO4 (E–F) electrode. NiCo-LDH/BiVO4 (G–K)electrode elemental mapping images of Bi, V, O, Ni and Co respectively.

source was a xenon lamp simulating sunlight AM 1.5 G (100 mW/cm2), and a 0.5 M Na2SO4 solution (pH∼7.3) was used as the electrolyte. Photocurrent was measured by linear sweep voltammetry from −0.6 to +1.2 V(vs. Ag/AgCl) with a scan rate of 10 mV s−1.For all cases, light is irradiated from the back side of the fluorine doped tin oxide (FTO) glass. Electrochemical impedance spectroscopy (EIS) was operated by applying an AC voltage amplitude of 50 mV over a frequency range of 105-0.1 Hz at an open circuit potential. The Applied bias photon-current efficiency (ABPE) calculation formula is as follows:

2. Experiment section 2.1. Materials VO(acac)2, Bi(NO3)3·5H2O, Ni(NO3)3·6H2O, Co(NO3)3·6H2O and KI are purchased from Sinopharm Chemical Reagent Co., Ltd., China. All reagents are analytical grade and used without further purification. Pbenzoquinone (99.0%) is purchased from Tianjin Institute of Fine Chemicals. 2.2. Preparation of NiCo-LDH/BiVO4 photoanode

ABPE = [J × (1.23-Vapp)]/Plight where J is the photocurrent density (mA/cm2), Vapp is the applied bias (vs RHE), and Plight is the incident light intensity (100 mW/cm2). The incident-to-electrical conversion efficiency (IPCE) was measured at 1.23 V vs. RHE by a xenon lamp (PLS-SXE300C) equipped with a monochromator (71SWS, Beijing NBT Technology Co., Ltd.). The IPCE calculation formula is as follows:

BiVO4 electrode was prepared according to previous reported method [45] and can be found at supporting information. The as-prepared BiVO4 electrode was placed in an mixed solution of 0.1 M Ni (NO3)2, 0.2 M Co(NO3)2 and 0.075 M NaNO3. Cathodic deposition was performed potentiostatically at -0.7 V vs. Ag/AgCl for 290 s at RT. Then, the electrode wassoaked with DI water for a few minutes and dried.The solution was purged with N2 gas for 0.5 h before the experiment. -1.0 V vs. Ag/AgCl is also used for deposition of NiCo-LDH. The composite photoanodes are denoted as NiCo-LDH/BiVO4 (−0.7 V) and NiCo-LDH/BiVO4 (−1.0 V). Without other states, NiCo-LDH/BiVO4 represents the NiCo-LDH coated at −0.7 V.

IPCE = (1240 × J)/(λ × P) where J is the current density (mA/cm2) measured at each specific wavelength, λ is the wavelength (nm) of the incident light, and P is the power density of the incident light (mW/cm2). The absorbed photon-to-current efficiency (APCE) was obtained by dividing the IPCE by the light harvesting efficiency (LHE) using equation.

2.3. Photoelectrochemical measurements PEC properties were measured by an electrochemical workstation (CHI 650E) in a standard three-electrode system. The illumination

APCE = IPCE/LHE 2

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LHE = 1-10−A(λ) (A(λ) is the absorbance at wavelength λ) The conversion between potentials vs. Ag/AgCl and vs. RHE is calculated using the formula below：

respectively (Fig. 3A) [46]; while the V2p3/2 and V2p1/2 orbital peakslocate at 517.1 and 522.8 eV, respectively (Fig. 3B). For the O1s core level spectra, three peaks can be clearly identified (Fig. 3C).In detail,the peak at 529.9eV is due to bonding of oxygen atoms to metals; the peak at 532.7 eV is associated with hydroxy species of surface-adsorbed water molecules; the peak at 531.4 eV can be ascribed to O 1s that is in accordance with the lattice oxygen [47,48]. The characteristic peaks of Ni and Co were also detected. Besides two satellite peaks (represented as Sat), peaks at 855.9 eV and 873.4 eV observed in Fig. 3E can beassigned to Ni 2p3/2 and Ni 2p1/2 signals of Ni2+, respectively [49,50]. The 15 eV spacing between 781.3 eV (Co 2p3/2) and 796.3 eV(Co 2p1/2) indicates the coexistence of Co2+ and Co3+ in NiCo-LDH/BiVO4 [50] (Fig. 3F). NiCo-LDH nanosheet can be obtained at bias voltages of -1.0 V (Fig. S2). However, the photocurrent of NiCo-LDH/BiVO4 (−1.0 V) is smaller than NiCo-LDH/BiVO4 (-0.7 V) as shown in Fig. S3. This can be explained from the aspect of transfer resistance of hole. It is obvious that large LDH nanosheet is disadvantageous for hole transfer, while hole is easy to transfer from BiVO4 to LDH nanoparticle surface and subsequently for water oxidation. The electrodeposition time was optimized as shown in Fig. S4. Three samples with NiCo-LDH deposition time of 280 s, 290 s and 300 s were prepared. The photocurrent density of composite electrodesobtained by electrodeposition of 280 s, 290 s and 300 s were larger than pure BiVO4 electrodes. The photocurrent density of the composite electrode obtained by electrodeposition of 290 s has the largest photocurrent density. The subsequent experiments were carried out on NiCo-LDH/BiVO4 obtained by electrodeposition of 290 s if not described. The effect of NiCo-LDH loading on the PEC performance of BiVO4 was first studied by linear sweep voltammetry (LSV) in a 0.5 M Na2SO4 solution under AM 1.5 G. As shown in Fig. 4A, the photocurrent of the NiCo-LDH/BiVO4 electrode reaches 3.4 mA/ cm2, which is 3 times that of the pure BiVO4 electrode at 1.23 V (vs. RHE). While, compared with the BiVO4 electrode, the onset potential of NiCo-LDH/BiVO4 significantly reduces from 0.47 V to 0.24 V with a cathodic shift of 230 mV, indicating that NiCo-LDH is a good water oxidation promoter, which is also corroborated by the PEC results in the case of no light. As shown in Fig. 4B, the initial potentials of the BiVO4 and NiCo-LDH/BiVO4 electrode are 2.14 and 2.03 V (vs. RHE), respectively. Fig. 4C shows the LSV curves of NiCo-LDH/BiVO4 and BiVO4 electrodes in the presence of xenon light and all photoanodes have excellent optical switching characteristics and fast response. It is clear that NiCo-LDH/BiVO4 shows the highest photo response over the entire voltage range, which is consistent with the results in Fig. 4A. Electrochemical impedance spectroscopy (EIS) tests were performed to investigate the charge transport kinetics of these photoanodes. As shown in Fig. 4D, the EIS results show that only one semicircle is observed for each sample. Since a smaller semicircle radius indicates a better charge transfer capability (i.e. faster surface reaction kinetics), the much smaller semicircular radius of the NiCo-LDH/BiVO4 electrode indicates greatly enhanced transportation and inhibited recombination of photogenerated charge-carrier achieved on NiCo-LDH/BiVO4 photoanode as compared to BiVO4 electrode. To evaluate the efficiency in which electrical energy was subtracted, ABPE was used as shown in Fig. 5A. NiCo-LDH/BiVO4 achieves a maximum efficiency of 0.66% at 0.8 V, while BiVO4 can onlyget 0.12% at 1.0 V. The IPCE value of the electrodes was measured at a bias of 1.23 V vs. RHE as shown in Fig. 5B. All photoanodes have photo response in the wavelength range of less than 520 nm, which is consistent with their UV–vis spectra (Fig. 8A). The IPCE of the NiCo-LDH/BiVO4 electrode is higher than that of the BiVO4 electrode in the whole test range, reaching 59% at 380 nm and 53–56% at 360–450 nm. In addition, the photocurrent density of NiCo-LDH/BiVO4 was estimated to be 2.95 mA/cm2 by integrating IPCE on the AM 1.5 G solar spectrum. As shown in Fig. S5, this value is close to the actual measurements in the LSV, which means that all test procedures are reliable. The photonelectron conversion efficiency is improved due to the enhancement of

ERHE = 0.197 + 0.059 * pH + EθAg/AgCl The hydrogen generation was evaluated in a single cell photo-reactor. Xenon lamp was employed as light source while 0.5 M Na2SO4 was used as electrolyte. The photo-reactor cell was purged with argon gas for 30 min to remove the air in the reaction system before the light was turned on. Produced hydrogen was collected with the trace injection instrument and measured by gas chromatography (GC-9560). 2.4. Materials characterization The X-ray diffraction patterns of all the electrodes were recorded on a Rigaku X-ray diffractometer D/Max-2200/PC equipped with Cu-Kα radiation (40 kV, 20 mA). The optical properties of all samples were estimated by a dual beam UV–vis spectrophotometer (PuXin TU1901).The structure and morphology of all membranes were observed using a JSM6701E field emission scanning electron microscope. TEM test was performed on a TECNAI TF20 instrument. XPS analysis was recorded on a PHI5702 photoelectron spectrometer. 3. Results and discussion The morphologies of pristine BiVO4 and NiCo-LDH/BiVO4 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1(A,B) shows that the original BiOI film is composed of uniformly deposited nanoflakes with the thickness of 30–40 nm. The nanoflakes are so loosely arranged that it leaves enough space to the subsequent expansive step of intercalation of vanadium ions. After the phase-transformation to BiVO4, a porous film stacked by 200–300 nm wormlike structure with smooth surface was developed from the original plate-like structure as shown in Fig. 1(C,D). After loading NiCo-LDH as shown in Fig. 1(E,F), no significant changes could be observed but the surface of wormlike structure turned rather rough compared with pure BiVO4, which implies that the loaded NiCoLDH nanoparticles is small. From cross-sectional SEM images, there is no obvious changes can be found after loading NiCo-LDH nanoparticles (Fig. S1). This rough porous wormlike structure is expected to increase the contact area betweenelectrodeand electrolyte. Elemental mappings of NiCo-LDH/BiVO4 shows that the Bi, O, V, Ni, and Co elements was uniformly distributed in the composites, suggesting the successful preparation of composite electrode, as shown in Fig. 1(G–K). As shown in Fig. 2(A,B), the lattice fringe spacings of 0.30, 0.25 and 0.26 nm correspond to the (−121), (002) planes of BiVO4 and the (101) plane of NiCo-LDH, respectively. Through the energy-dispersive X-ray spectroscopy (EDX) analysis, it can be seen that Ni, Co, O elements are detected and no impurity elements are detected on NiCo-LDH/BiVO4 after loading NiCo-LDH (Fig. 2C). The X-ray diffraction (XRD) patterns of the bare BiVO4 and NiCoLDH/BiVO4 electrodes are shown in Fig. 2D. It can be seen from the spectra that the diffraction peaks derived from BiVO4 can be attributed to the monoclinic scheelite crystal series (JCPDS No. 14-0688), and the peak at 26.8° corresponds to (110) plane of conductive substrate SnO2 (JCPDS No. 41-1445). After deposition of NiCo-LDH on the BiVO4 electrode, no discernable changes can be observed in the pattern of the NiCo-LDH/BiVO4 compared with that of bare BiVO4, indicating that the content of NiCo-LDH is below the lower limit of XRD detection. X-ray photoelectron spectroscopy (XPS) measurement was carried out to probe the surface composition and chemical states of NiCo-LDH/ BiVO4 (Fig. 3). High-resolution spectra show the characteristic peaks of Bi4f, V2p, O1s and C1s detected on NiCo-LDH/BiVO4. Taking the C 1s peak at 284.8 eV as the reference standard,the binding energies of 159.4 eV and 164.7 eV can be ascribed to the Bi4f2/7 and Bi4f2/5, 3

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Fig. 2. HRTEM images of BiVO4 (A) and NiCo-LDH/BiVO4 (B) films; Spectra of EDX of NiCo-LDH/BiVO4(C). XRD patterns of BiVO4 and NiCo-LDH/BiVO4 composite samples (D).

Fig. 3. XPS spectra of NiCo-LDH/BiVO4 sample. 4

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Fig. 4. LSV curves of electrodes in illumination (A) and non-illuminated (B); transient photocurrent density under chopped light (on or off cycle: 5 s) (C) and EIS diagram (D) of electrodes under illumination.

generated by BiVO4 to the electrode/electrolyte interface. As a result, surface recombination is greatly reduced by forming NiCo-LDH/BiVO4 heterojunction, facilitating the successful injection of holes into the electrolyte to participate in the water oxidation reaction, as evidenced by an increase in the yield of hole injection. The charge separation efficiency of BiVO4 and NiCo-LDH/BiVO4 electrodes was obtained by ηsurface = JH2O/JNa2SO3. As shown in Fig. 6B, the charge separation efficiency of the NiCo-LDH/BiVO4 electrode at the 1.23 V vs. RHE is 81%, which is almost 1.8 times that of the BiVO4 electrode. This result further reconfirms the enhanced separation of electrons and holes after loading the cocatalyst. The relative electrochemical surface area (ECSA) of the BiVO4 and NiCo-LDH/BiVO4 samples was determined by the capacitive region of the cyclic voltammogram. As shown in Fig. 7(A,B), cyclic voltammograms were carried out at scan rates of 10, 20, 30, 40, 50, 60, 70, 80 and 90 mV/s in 0.5 M Na2SO4 (pH～7.3). The electrochemically active surface area was then determined by measuring the capacitive current associated with double layer charging from the scan rate dependence of the CV. The double layer capacitance (Cdl) was estimated from the relationship between ΔJ = (Ja-Jc) of Ag/AgCl at 0.8 V and the scan rate. As shown in Fig. 7(C,D), the linear slope is equivalent to twice the Cdl and can be used to represent the electrochemically active surface area. Moreover, the linear slope of the NiCo-LDH/BiVO4 electrode is 58 times that of the BiVO4 electrode, which further proves that loading NiCoLDH increases the specific surface area and enriches the active site. In order to measure optical properties, the optical absorption curves of BiVO4 and NiCo-LDH/BiVO4 electrodes were measured by UV–vis diffuse reflectance spectroscopy, and the band gap values were calculated according to Kubelka-Munk Function. As shown in Fig. 8(A,B), pure BiVO4 shows strong absorption in the wavelength range of

the light harvesting efficiency of the composite electrode. As shown in Fig. 5C, in the wavelength range of 350–600 nm, the light harvesting efficiency (LHE) of the composite electrode is higher than that of the BiVO4 electrode. The calculated APCE of the NiCo-LDH/BiVO4 electrode is 5 times higher than that of the BiVO4 electrode over the entire wavelength range (Fig. 5D). The APCE results show that NiCo-LDH/ BiVO4 anode can make use of absorbed light more efficiently. In order to evaluate the photoelectrocatalytic ability of the BiVO4 electrode, Na2SO3 was used as a hole scavenger. As shown in Fig. S6, the water oxidation initiation potential of the BiVO4 electrode in 0.5 M Na2SO4 solution containing 1 M Na2SO3 is only 0.12 V (vs. RHE), and the current density at 1.23 V (vs. RHE) reaches 4.5 mA/cm2. But the photo-oxidation current density is much smaller (1.1 mA/cm2) in the absence of Na2SO3, which implies that the water oxidation process occurring on the BiVO4 electrode is mainly limited by the surface reaction kinetics. By contrast, the NiCo-LDH/BiVO4 electrode exhibited a much higher current density (3.4 mA/cm2 at 1.23 V vs. RHE) and a smaller onset potential than the BiVO4 electrode in the absence of Na2SO3 scavenger. The above results indicate that NiCo-LDH can reduce the recombination of photogenerated charges on the surface of BiVO4 and promote the oxidation of water. As demonstrated in Fig. 6A, the hole injection efficiency of the BiVO4 photoelectrode is kept below 24%, indicating that more than half of the generated holes are wasted due to bulk recombination. The hole injection efficiency of the NiCo-LDH/BiVO4 photoanode could reach 71% at 1.23 V, which is almost 2.9 times that of the BiVO4 electrode. On the one hand, electrons can easily recombine with holes on the surface of the BiVO4. On the other hand, the valence band of NiCo-LDH is much more negative than BiVO4 and just below the water oxidation potential. Therefore, it can be used as a stepping stone to transfer holes 5

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Fig. 5. ABPE (A) and IPCE (B) of electrodes. Light harvesting efficiency of BiVO4 and NiCo-LDH/BiVO4 photoanodes (C), APCE of BiVO4 and NiCo-LDH/BiVO4 photoanodes measured by monochromatic light (D).

350–500 nm with the absorption edge at 500 nm. The composite exhibits higher absorption intensity compared to that of pure BiVO4 after the deposition of NiCo-LDH. The band gap is calculated by extending the maximum slope of the Tauc curve (αhv vs. hν) to the axis. Thus, the intersection of the tangent line with the abscissa is the forbidden band value of the composite sample. It can be seen that there is no significant change of the forbidden band value after NiCo-LDH deposition. The evolution ratesof produced H2 and O2 in the photoelectrolysis on NiCo-LDH/BiVO4 electrode are shown in Fig. 9A. After three hours of reaction, the produced H2 to O2 have a molar ratio of approximately 2:1, which agrees with the passed charge during the decomposition of water (Fig. 9B). The calculated Faradaic efficiency for H2 and O2

production is about 95%, indicating that the photogenerated charges are almost entirely used to decompose water. According to the above characterization and performance analysis, the possible reaction mechanism of PEC water decomposition process of NiCo-LDH/BiVO4 electrode is proposed as follows (Fig. 10). First, the energy level matching between NiCo-LDHand BiVO4 facilitates the transmission of photogenerated electrons to the FTO substrate and the transportation of holes to the surface of NiCo-LDH. Therefore, the electrons are subsequently oxidized at the cathode through the external circuit to generate hydrogen gas, while the holes migrate to the valence band of NiCo-LDH and generate oxygen at the same time. Second, the enhanced light absorbance of the composite electrode increases the

Fig. 6. Charge injection efficiency (A) and charge separation efficiency of electrodes (B). 6

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Fig. 7. CV of BiVO4 (A) and NiCo-LDH/BiVO4 (B) photoanodes performed at different scan rates in the non-Faradaic potential range; Charging current density differences of BiVO4 (C) and NiCo-LDH/BiVO4 (D) photoanodes plotted against scan rates.

performance of NiCo-LDH/BiVO4 photoanodes can be attributed to the catalytic/light absorbing/synergy triple function.

utilization of light, as evidenced by UV–vis absorption and IPCE measurements. This configuration of two materials with different bandgaps benefits better utilization of solar light. In addition, the M2+ (Ni2+,Co2+/Co3+ based on the XPS analysis) can be oxidized to higher valence state M3+δ (Co3+δ and Ni3+δ) via the capture of holes [32,34,51,52], which subsequently act as active sites to oxidize H2O to generate O2, while the electrons are transferred to Pt counter electrode via external circuit and participated in water reduction reaction to generate H2. Third, NiCo-LDH as cocatalyst not only can reduce the onset potential but also can further facilitate the charge transfer process, accelerating the migration of carriers and improving the charge injection efficiency. Therefore, the significant increase in PEC

4. Conclusions In summary, we successfully loaded NiCo-LDH on BiVO4 by method of electrodeposition for the first time. The composite electrode achieved photocurrent of 3.4 mA/cm2 at 1.23 V vs. RHE, much larger than that of the BiVO4 electrode (1.1 mA/cm2). It has been proved that the heterojunction structure formed between the NiCo-LDH and BiVO4 electrode contributes to the enhancement of light absorption, the transfer of photogenerated carriers and the separation between electrons and

Fig. 8. UV–vis diffused reflectance spectra of BiVO4 and NiCo-LDH/BiVO4 (A) and the energy gap of BiVO4 and NiCo-LDH/BiVO4 (B). 7

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Fig. 10. Schematic illustration of NiCo-LDH/BiVO4 photoanode for efficient PEC water splitting.

holes. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21663027, 21808189, 21962018), the Science and Technology Support Project of Gansu Province (1504GKCA027), the Program for the Young Innovative Talents of Longyuan, the Program for Innovative Research Team (NWNULKQN-15-2) and the Fundamental Research Funds for the Central Universities of Chang’an University (300102299304). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118280. References [1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473.

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