The conversion of solar energy into hydrogen by photoelectrochemical (PEC) water splitting is a promising way to produce hydrogen, which can simultaneously solve the current energy shortage and environmental problems caused by fossil fuel combustion [1-3]. In the process of PEC water splitting, water oxidation (producing oxygen) and water reduction (producing hydrogen) reaction are involved. The water oxidation reaction involves a kinetic unfavorable four-electron transfer process, which determines the entire water splitting reaction rate [4, 5]. Researches are mainly focused on photoanodes where water oxidation reaction takes place. Metal oxides such as TiO₂ [6, 7], Fe₂O₃ [8, 9], WO₃ [10], ZnO [11] and BiVO₄ [12-14]are often used as photoanode materials for PEC water oxidation. Among them, BiVO₄ is a very excellent photoanode material with beneficial energy band structure and a band gap of 2.4 eV, enabling it to absorb part of visible light [15-17]. However, the slow migration rate of photogenerated electrons and holes in BiVO₄ and sluggish kinetics for water oxidation render BiVO₄ far below its theoretical photocurrent density [18, 19]. Strategies such as controlling morphology [20], constructing heterostructures [10] have been used to improve the performance of BiVO₄. Besides, loading cocatalyst is an effective way to improve the surface reaction kinetics of BiVO₄ for PEC water splitting [21, 22].

Cobalt-based cocatalysts, such as cobalt oxides [23-25], cobalt hydroxides [26, 27], cobalt phosphide [28, 29], Co-Pi [30, 31], Co-based double hydroxide (LDH) [32], have been intensively studied as water oxidation catalysts due to their outstanding catalytic performance. Specially, ternary spinel MCo₂O₄ (M = Ni, Mn, Zn, etc.) have improved bearing conductivity and possessing more active sites than Co₃O₄, resulting in a better water oxidation capability [33-35]. Among them, ZnCo₂O₄ has a definite structure in which Zn²⁺ only replaces Co²⁺ in the tetrahedral sites in Co₃O₄, rarely affecting the Co³⁺ active site in the octahedral sites in Co₃O₄ [36]. Therefore, as compared to Co₃O₄, the use of ZnCo₂O₄ could reduce the use of rare and relative expensive Co element without the compromise of catalytic activity.

In addition to loading cocatalyst, enhancing built-in electric field of a photoanode by constructing a heterojunction structure is also usually adopted. WO₃@BiVO₄ heterojunction photoanode [37], Fe₂O₃/BiVO₄ heterojunction photoanode [38], g-C₃N₄@BiVO₄ heterojunction photoanode [39] etc. are recently reported to be favorable for the separation of photo-generated carriers due to the formation of built-in electric field between these different semiconductors in heterojunction. However, the water oxidation reaction on surface of these photoanodes is still sluggish. To address this problem, cocatalysts are still needed. For example, NiFe-LDH was used to load on Fe₂O₃/BiVO₄ heterojunction photoanode [40]. WO₃/BiVO₄ was modified by FeOOH to construct ternary WO₃/BiVO₄/FeOOH hierarchical photoanode [41]. Considering the complicated fabrication process of above mentioned photoanodes, loading a semiconductor cocatalyst that could enhance the built-in electric field as well as act as cocatalyst is a new guiding ideology to construct a high performance photoanode for water oxidation.

In this work, ZnCo₂O₄ nanoparticles, acting as a p-type semiconductor and a cocatalyst simultaneously, were loaded on BiVO₄ to form ZnCo₂O₄/BiVO₄ composite photoanode by electrophoretic deposition. The formation of p-n junctions between ZnCo₂O₄ and BiVO₄ is systematically confirmed, which introduces built-in electric field that is beneficial to the separation of photogenerated carriers. This composite photoanode shows high photocurrent for PEC water oxidation due to the improvement in charge separation efficiency and surface reaction efficiency.

As can be seen from Fig. 1a, BiVO₄ presents a coral-like structure composed of porous nanoparticles. The nanoparticles are interconnected on the surface of the FTO to form pores and channels. These abundant channels facilitate the contact between the semiconductor and electrolyte. From Fig. 1b, it can be clearly seen that ZnCo₂O₄ nanoparticles are uniformly loaded onto the surface of BiVO₄ film by the electrophoretic deposition method. TEM image (Fig. 1c) shows that BiVO₄ has a pore structure and smooth surface. However, a thin layer of nanoparticles appears on the surface of BiVO₄ after loading ZnCo₂O₄ as shown in Fig. 1d. Figs. 1e and f show the HR-TEM images of ZnCo₂O₄/BiVO₄ photoanode. The spacing of 0.20 nm and 0.48 nm correspond to the (400) plane of ZnCo₂O₄ (PDF card No. 23-1390) and the (110) plane of BiVO₄ (PDF No. 14-0688), respectively, demonstrating the successful combination of ZnCo₂O₄ nanoparticles and BiVO₄ film to form ZnCo₂O₄/BiVO₄ photoanode.

In order to clearly grasp the crystal structure of the sample, we performed XRD testing on the photoanode material. As shown in Fig. 2a, FTO substrate has the diffraction peaks located at 26.6°, 37.8°, 51.8°, 61.7° and 65.7° corresponding to the (110), (200), (211), (310) and (301) lattice diffractions of SnO₂ (JCPDS No. 46–1088), respectively. BiVO₄ shows diffraction peaks located at 18.7°, 28.8°, 30.5°, 34.5°, 35.2°, 40.0°, 42.5°, 46.0°, 47.3°, 50.3°, 53.3°, 58.3° and 59.3°, indicating the formation of monoclinic phase BiVO₄ (JCPDS: 14-0688). The synthesized ZnCo₂O₄ powders have diffraction peaks mainly located at 19.0°, 31.2°, 36.8°, 44.7°, 59.3° and 65.1°. These diffraction peak positions correspond to the (111), (220), (311), (400), (511) and (440) crystal planes of the ZnCo₂O₄ (JCPDS: 23-1390), proving that the ZnCo₂O₄ nanoparticles were successfully synthesized. The XRD pattern of ZnCo₂O₄/BiVO₄ composite electrode does not show obvious characteristic diffraction peaks of ZnCo₂O₄ due to small loading amount of ZnCo₂O₄ nanoparticles on BiVO₄. X-ray photoelectron spectroscopy (XPS) test was used to determine the elements status of ZnCo₂O₄/BiVO₄ electrode. It can be seen from Fig. 2b that two peaks of Co 2p in ZnCo₂O₄ are located 795.0 and 780.0 eV, respectively belonging to the Co 2p_1/2 and Co 2p_3/2 peaks of Co³⁺ in the octahedral site, and proving that the cobalt in the compound is +3 valence [36]. The binding energies of the Zn 2p_3/2 and Zn 2p_1/2 peaks in the compounds are located at 1021.1 eV and 1044.2 eV, indicating that Zn in the ZnCo₂O₄ is +2 valence in the compound (Fig. 2c) [42]. The O 1s spectrum in the ZnCo₂O₄/BiVO₄ photoanode can be fitted into two diffraction peaks (Fig. 2d). The peak at 531.4 eV in the spectrum is derived from adsorbed OH⁻ on the electrode, while the peak at 529.7 eV is assigned to oxide lattice [43].

The light absorption range of ZnCo₂O₄/BiVO₄ photoanode is broadened slightly by supporting ZnCo₂O₄ cocatalyst (Fig. S1a in Supporting information). The light absorption intensity of the composite is also higher than that of BiVO₄, indicating that ZnCo₂O₄ improves the light absorption capacity of BiVO₄. The band gap of BiVO₄ is about 2.46 eV, whilst ZnCo₂O₄/BiVO₄ electrode shows an apparent band gap of 2.43 eV, demonstrating ZnCo₂O₄ can improve the light utilization efficiency of BiVO₄ photoanode (Fig. S1b in Supporting information). In order to investigate whether the catalytic active area of BiVO₄ film would be changed after loading ZnCo₂O₄, cyclic voltammetry (CV) tests of BiVO₄ and ZnCo₂O₄/BiVO₄ electrodes were carried out at different scan rates as shown in Figs. S1c and e (Supporting information). The catalytic active area of BiVO₄ and ZnCo₂O₄/BiVO₄ electrodes can be estimated from double layer capacitances that are linearly related to the slopes of difference values between anode current (J) and cathode current (J) in CVs against the scan rate as shown in Figs. S1d and f (Supporting information). Taking the current difference as y-axis and scanning rate as x-axis, the linear slope is twice over the double layer capacitances [44]. It can be seen from Figs. S1d and f that the slope of ZnCo₂O₄/BiVO₄ is larger than that of BiVO₄, demonstrating the catalytic active area of BiVO₄ photoanode increases after loading ZnCo₂O₄.

The LSV curves of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes were tested under AM 1.5G simulated sunlight (100 mW/cm²) from back side of the sample. As shown in Fig. 3a, the photocurrent density of BiVO₄ is 1.4 mA/cm² at 1.23 V vs. RHE, while the photocurrent density of 3.0 mA/cm² is obtained for the ZnCo₂O₄/BiVO₄ photoanode. Besides, the onset potential of BiVO₄ photoanode shifts ca. 0.1 V after loading ZnCo₂O₄. Co₃O₄/BiVO₄ photoanode was also fabricated for comparison using the same electrophoretic deposition method as shown in Fig. 3a, giving a photocurrent of 2.2 mA/cm² at 1.23 V vs. RHE at same reaction condition. Obviously, ZnCo₂O₄ shows more higher catalytic activity than that of Co₃O₄. The photocurrent density of ZnCo₂O₄/BiVO₄ is 2.1 times that of BiVO₄. This indicates that the performance of BiVO₄ for PEC water splitting can be enhanced by ZnCo₂O₄. The comparison of ZnCo₂O₄/BiVO₄ photoanode with other BiVO₄ and Co based photoanodes is shown in Table S 1 (Supporting information). In order to investigate the effect of deposition time on the catalytic performance of ZnCo₂O₄/BiVO₄, the electrophoretic deposition time of ZnCo₂O₄ was set be to 15, 20, 25, 30, 35 and 40 s for comparison. These composite photoanodes are named as ZnCo₂O₄/BiVO₄-15, ZnCo₂O₄/BiVO₄-20, ZnCo₂O₄/BiVO₄-25, ZnCo₂O₄/BiVO₄-30, ZnCo₂O₄/BiVO₄-35 and ZnCo₂O₄/BiVO₄-40 accordingly. As demonstrated in Fig. S 2 (Supporting information), the photocurrent density of ZnCo₂O₄/BiVO₄-35 has the highest photocurrent at 1.23 V vs. RHE, indicating that ZnCo₂O₄/BiVO₄-35 has the best PEC water splitting performance. When the deposition time is less than 35 s, small photocurrents were obtained probably due the lower content of ZnCo₂O₄. However, when the amount of the supported ZnCo₂O₄ is excessive, the transport of holes to the electrode surface is hindered, leading to low photocurrents. ZnCo₂O₄/BiVO₄-35 was used in all the experiments without further description. Fig. 3b is LSV curves of the electrodes in the absence of light. The initial potential of BiVO₄ is about 2.4 V, while the initial potential of the ZnCo₂O₄/BiVO₄ composite photoanode is about 2.1 V. The initial potential of ZnCo₂O₄/BiVO₄ negatively shifts 300 mV compared to that of BiVO₄. This indicates that ZnCo₂O₄ can reduce the overpotential of PEC water splitting. The chopped I-t curves of the photoanodes were tested at 1.23 V vs. RHE. It can be clearly seen from the I-t curves in Fig. 3c that the photocurrent density of the composite ZnCo₂O₄/BiVO₄ photoanode is stable and higher than that of BiVO₄ at constant voltage. Fig. 3d shows the LSV curves of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes tested in 0.5 mol/L Na₂SO₄ solution with 1 mol/L Na₂SO₃. As a hole trapping agent, Na₂SO₃ is easily oxidized by holes. When Na₂SO₃ is present, the photogenerated holes reaching the surface of BiVO₄ and ZnCo₂O₄/BiVO₄ are all involved in the oxidation of Na₂SO_3. Therefore, the photocurrent density for Na₂SO₃ oxidation represents the amount of photogenerated holes reaching on the surface of BiVO₄ and ZnCo₂O₄/BiVO₄. In the presence of Na₂SO₃, the photocurrent density of ZnCo₂O₄/BiVO₄ is higher than that of BiVO₄, indicating that the amount of photogenerated holes reaching the surface of ZnCo₂O₄/BiVO₄ is higher than that of BiVO₄. This means that the ZnCo₂O₄/BiVO₄ heterojunction structure promotes the separation of photogenerated charges. Fig. S 3 (Supporting information) shows the chopped LSV curves of BiVO₄ and ZnCo₂O₄/BiVO₄ with chopped light illumination. When the simulated sunlight was used to illuminate the photoanode, the photocurrent density of the BiVO₄ and ZnCo₂O₄/BiVO₄ electrodes immediately increased. When the light is blocked, the photocurrent density of the both photoanodes immediately reduced to almost 0 mA/cm², suggesting that the photoanodes is very sensitive to light. The photocurrent densities of the electrodes are disparate at different voltages. At the same potential, the photocurrent density of the ZnCo₂O₄/BiVO₄ photoanode is distinctly higher than that of the BiVO₄ electrode, implying ZnCo₂O₄ nanoparticles can significantly improve the PEC water oxidation performance of BiVO₄ photoanode.

The efficiency of PEC water splitting is determined by light capture efficiency, charge separation efficiency in the bulk (η_bulk) and surface reaction efficiency (η_surface) of photoelectrode. To analyze these efficiencies, the maximum theoretical photocurrents J_abs under AM 1.5G simulated sunlight was calculated from UV diffuse reflection data of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes (Fig. S4 in Supporting information). η_bulk was calculated through dividing the photocurrent for Na₂SO₃ oxidation (J_Na, Data in Fig. 3d) by J_abs [44]. η_surface represents the ratio of surface holes participated in the reaction, which is obtained through dividing photocurrent for water oxidation (data in Fig. 3a) by J_Na. Fig. 4a shows that η_bulk of ZnCo₂O₄/BiVO₄ reaches 70% at 1.23 V vs. RHE, while the η_bulk of pure BiVO₄ is 50%, indicating the loading of ZnCo₂O₄ can enhance charge separation efficiency of BiVO₄. Fig. 4b represents the η_surface of the ZnCo₂O₄/BiVO₄ photoanode reaches 50% at 1.23 V vs. RHE, while the η_surface of the BiVO₄ photoanode is merely 25%. As a water oxidation cocatalyst, ZnCo₂O₄ improves the kinetics of water splitting reaction. Based on LSV curve shown in Fig. 3a, applied bias photon-to-current efficiency (ABPE) values can be obtained [45]. In Fig. 4c, the maximum efficiency of the BiVO₄ film is 0.1% at 1.1 V vs. RHE, while the ZnCo₂O₄/BiVO₄ photoanode attains a maximum ABPE value of 0.6% at 0.9 V vs. RHE. The maximum efficiency of ZnCo₂O₄/BiVO₄ composite photoanode is 6 times that of BiVO₄. This indicates that ZnCo₂O₄ improves the photoelectrochemical performance of BiVO₄. The IPCE values of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes were tested at 1.23 V vs. RHE. As can be seen from Fig. 4d, the IPCE value of ZnCo₂O₄/BiVO₄ electrode is higher than that of BiVO₄ in the wavelength range of 320~510 nm. The IPCE value of ZnCo₂O₄/BiVO₄ electrode can reach 40% at 380 nm. Absorbed photon-to-current conversion efficiency (APCE) of BiVO₄ and ZnCo₂O₄/BiVO₄ electrodes were calculated through dividing IPCE values by light absorption values and the resulting APCE values are given in Fig. S 5 (Supporting information). Almost half absorbed photons can be converted to current by ZnCo₂O₄/BiVO₄ electrode, about 20% higher than that of BiVO₄, indicating that the light utilization rate of the electrode is improved after loading ZnCo₂O₄. To confirm the reliability of our tests, the integrations of IPCE of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes with the AM 1.5G solar current were carried out. As shown in Figs. 4e and f, the photocurrents of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes obtained by integrating AM 1.5G solar currents are 1.8 and 3.2 mA/cm², which are approximate equivalent to the photocurrents in LSV measurement, indicating the tests are reliable.

The relative magnitude of the charge transfer rate of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes was investigated by electrochemical impedance spectroscopy (EIS). The Nyquist plots of EIS spectra of BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes are a semi-circular arc whatever in dark conditions or under AM 1.5G simulated sunlight (Figs. S6a and b in Supporting information). The impedance of the circuit can be explained by the simplified Randles circuit model, in which R_Ω represents the solution resistance. R_ct represents charge-transfer resistance at the interface of semiconductor and electrolyte and C_ct represents the capacitance of the bulk BiVO₄ [46]. The smaller the diameter of the arc, the stronger the charge transfer capability is. ZnCo₂O₄/BiVO₄ exhibits a smaller arc diameter than BiVO₄, indicating that the charge transfer rate in the photoanode increases after the ZnCo₂O₄ cocatalyst is supported. It is worth noting that the arc diameter is smaller under light irradiation, indicating charge transfer become easy with light irradiation. The PEC water splitting reaction of ZnCo₂O₄/BiVO₄ photoanode was carried out in a closed reactor. Prior to the water splitting reaction, the reactor was purged with N₂ for 1 hour to remove air from the reactor. The PEC water splitting reaction was carried out for 3 h at 1.23 V vs. RHE. 1 mL of gas was withdrawn from the reactor every half hour, and the gas was quickly pumped into the gas chromatograph to detect the amount of hydrogen and oxygen produced. After 3 h of reaction, 153.7 µmol of H₂ and 79.8 µmol of O₂ were produced with the mole ratio of H₂ to O₂ of 1.9:1 (Fig. S6c in Supporting information). The average Faradaic efficiency of PEC cell for H₂ and O₂ production (Fig. S6d in Supporting information) is about 90% and 86%, respectively, which indicates that the photoanode ZnCo₂O₄/BiVO₄ has good water splitting efficiency.

Mott-Schottky tests were performed to study the semiconductor type of ZnCo₂O₄, BiVO₄ and ZnCo₂O₄/BiVO₄ photoanodes. As demonstrated in Fig. 5a, ZnCo₂O₄ shows a negative slope, indicating that ZnCo₂O₄ is p-type semiconductor. While BiVO₄ photoanode shows positive slope, meaning the n-type property of BiVO₄ (Fig. 5b). The intersection of Mott-Schottky curves and x-axis are close to 0.38 V vs. RHE with/without loading ZnCo₂O₄, demonstrating the flat band potentials (V_fb) of BiVO₄ photoanodes are 0.38V.

The V_fb value of BiVO₄ is 0.38V. Considering that the V_fb of n-type semiconductor is slightly positive (~0.1−0.3 V) than the CB potential, the conduction band (CB) potential of BiVO₄ is about 0.18 V [47]. The band gap of BiVO₄ is 2.46 eV (Fig. S1b). According to the relationship between CB and valence band (VB), the VB of BiVO₄ can be calculated to be 2.64 V. The p-type semiconductor ZnCo₂O₄ has a V_fb value of 1.98 V (Fig. 5a). Because VB position of a p-type semiconductor is approximately 0.2 V higher than its V_fb value, the VB position of ZnCo₂O₄ is 2.18 V. ZnCo₂O₄ has a band gap width of 2.10 eV, so the CB position of ZnCo₂O₄ is calculated to be 0.08 V. The staggered energy levels of BiVO₄ and ZnCo₂O₄ allows them to form p-n junction.

The formation of p-n junctions between BiVO₄ and ZnCo₂O₄ was proved by input voltage-output current test as shown in Fig. 5c. BiVO₄ photoanode shows a linear relationship between input voltage and output current due to the ohmic contact between BiVO₄ and FTO substrate [48]. On the contrary, ZnCo₂O₄/BiVO₄ anode represents a nonlinear relation between output current and input voltage, indicating the interface contact type between ZnCo₂O₄ and BiVO₄ is p-n junction [49]. Built-in electric field in the p-n junction favors the transfer of holes from BiVO₄ to ZnCo₂O₄ for water oxidation.

The PEC water splitting mechanism of ZnCo₂O₄/BiVO₄ is proposed as shown in Fig. 5d. BiVO₄ and ZnCo₂O₄ are excited to generate electrons and holes by illuminating from the back of the photoanode. The electrons in conduction band of ZnCo₂O₄ migrate to the conduction band of BiVO₄, and then transfer to the counter electrode under external circuit for H₂ evolution reaction. The holes in valence band of BiVO₄ transfer to the valence band position of ZnCo₂O₄ with the assistance of built-in electric field for water oxidation reaction (O₂ evolution reaction). After a long-time reaction, the photocurrent of ZnCo₂O₄/BiVO₄ anode presents only a small decrease (Fig. S7 in Supporting information).

In summary, we have constructed a p-n heterojunction ZnCo₂O₄/BiVO₄ photoanode by a simple electrophoretic deposition method. The formation of p-n junction between BiVO₄ and ZnCo₂O₄ is proved by input voltage−output current test. Compared with BiVO₄, this composite photoanode ZnCo₂O₄/BiVO₄ has better photoelectrochemical water splitting performance. The photocurrent density of ZnCo₂O₄/BiVO₄ reaches 3.0 mA/cm² at 1.23 V vs. RHE, about 2.1 times greater than that of BiVO₄. The formation of a p-n heterojunction between the BiVO₄ and ZnCo₂O₄ improves the separation efficiency of carriers while the cocatalyst ZnCo₂O₄ accelerates the surface reaction kinetics, leading to enhanced charge separation efficiency and surface reaction efficiency. After 3 h reaction, the produced H₂ and O₂ achieve 153.7 and 79.8 µmol, respectively. All results demonstrate that ZnCo₂O₄ can boost the photoelectrochemical water oxidation performance of BiVO₄.

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.

This work was financially supported by the National Natural Science Foundation of China (Nos. 21808189 and 21663027), Natural Science Basic Research Fund of Shaanxi Province (No. 2020JZ20), and Fundamental Research Funds for the Central Universities of Chang'an University (No. 300102299304).

Supplementary material associated with this article can befound, in the online version, at doi:10.1016/j.cclet.2021.08.082