CN116732567A - CuBi2O4/CuO photocathode surface modification method - Google Patents

Abstract

The invention discloses a CuBi ₂ O ₄ CuO photocathode surface modification method by electrodepositing ZnO as CuBi ₂ O ₄ Protection layer of CuO photocathode. The invention improves the PEC performance of the photocathode and improves the stability of the photocathode.

Description

CuBi2O4/CuO photocathode surface modification method

Technical field

The invention belongs to the technical field of photoelectrochemistry, and in particular relates to a CuBi ₂ O ₄ /CuO photocathode surface modification method.

Background technique

Photoelectrochemical (PEC) water splitting systems provide high light-to-hydrogen conversion efficiency at a reasonable cost and are a technology with development potential. In the PEC system, the photocathode is used to produce hydrogen, and it is very important to develop efficient and stable photocathode materials. Due to its narrow band gap (1.5~1.8eV), CuBi2O4's conduction band position is close to the thermodynamic hydrogen evolution potential (0V _RHE ), and its relatively positive photocurrent starting potential (>1V _RHE ), it has high solar energy conversion efficiency. Its theory The photocurrent density can reach up to 20mA/cm ² , which has great application potential. However, due to its low hole mobility, limited electron diffusion length, severe photogenerated carrier recombination, and susceptibility to photocorrosion, the photocurrent density of CuBi ₂ O ₄ is far lower than the theoretical photocurrent density. In the field of photoelectrocatalysis The application is greatly restricted.

Contents of the invention

The technical problem to be solved by the present invention is to provide a CuBi2O4/CuO photocathode surface modification method to improve the PEC performance of the photocathode and improve the stability of the photocathode in view of the above-mentioned deficiencies in the prior art.

The invention discloses a method for surface modification of CuBi ₂ O ₄ /CuO photocathode, which uses electrodeposition of ZnO as a protective layer for the CuBi ₂ O ₄ /CuO photocathode.

The above technical solution uses appropriate deposition methods to prepare a high-quality protective layer on the surface of the CuBi ₂ O ₄ /CuO photocathode, which can effectively reduce the photocorrosion of the CuBi ₂ O ₄ /CuO photocathode. ZnO is electrodeposited as a protective layer for the CuBi ₂ O ₄ /CuO photocathode. Under AM1.5G illumination and 0.2VRHE potential, the CuBi ₂ O ₄ /CuO/ZnO photocathode exhibits a photocurrent density of 1.18mA/cm2, which is approximately twice the current density of the CuBi ₂ O ₄ /CuO photocathode. After coating the ZnO layer, the current retention rate of CuBi ₂ O ₄ /CuO increased from 58.3% to 84% during the 12h stability test. Experimental results show that the amorphous ZnO covering layer not only serves as a protective layer, but also effectively improves the charge transfer of the photocathode and increases the electrochemically active area of the photocathode.

Compared with the existing technology, the present invention has the following advantages: by depositing a high-quality protective layer on the surface of the CuBi ₂ O ₄ /CuO photocathode, the photocorrosion of the CuBi ₂ O ₄ /CuO photocathode can be reduced and the stability of the photocathode can be improved. .

The technical solution of the present invention will be further described in detail below through the accompanying drawings and examples.

Description of drawings

In Figure 1, (a) is the XRD pattern of the CuBi ₂ O ₄ /CuO/ZnO photocathode, (bc) is the SEM image of the CuBi ₂ O ₄ /CuO photocathode, and (df) is the CuBi ₂ O ₄ /CuO/ZnO photocathode. SEM image of the cathode, (gi) is the EDS pattern of the CuBi ₂ O ₄ /CuO/ZnO photocathode.

Figure 2 shows the UV-visible absorption spectra of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode. The inset is the Tauc of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode. Figure, and photos of CuBi ₂ O ₄ /CuO/ZnO photocathode samples.

Figure 3 is a TEM image of CuBi ₂ O ₄ /CuO/ZnO (ad), in which the inset in (d) is the crystal structure of the CuBi ₂ O ₄ (730) crystal plane.

In Figure 4 (a) is the Mott-Schottky diagram of CuBi ₂ O ₄ and CuO electrodes, and (b) is a schematic diagram of the energy band structure of CuBi ₂ O ₄ /CuO photocathode.

Figure 5 shows the XPS patterns of CuBi2O4/CuO and CuBi2O4/CuO/ZnO photocathode: (a) Cu2p; (b) Bi4f; (c) O1s; (d) Zn2p.

Figure 6 (a) shows the chopping LSV curve of CuBi ₂ O ₄ /CuO/ZnO electrode deposited at different deposition potentials for 100 seconds; (b) shows the relationship between ZnO deposition potential and photocurrent density at -0.4V _{Ag/AgCl. The influence of _} Effect of density.

In Figure 7 (a) is the chopping LSV curve of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode; (b) HC-STH efficiency of the electrode calculated based on chopping LSV; (c) CuBi Stability test curves of ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode, conducted in 0.4VRHE, 0.5MNa ₂ SO ₄ and 1% (v/v) H ₂ O ₂ electrolyte; Figure ( The inset in c) shows the stability test curve of the CuBi ₂ O ₄ /CuO photocathode in 0.4V _RHE , 0.5M Na ₂ SO ₄ electrolyte.

Figure 8 shows the SEM images after the stability test (a) CuBi ₂ O ₄ /CuO; (b) CuBi ₂ O ₄ /CuO/ZnO.

Figure 9 shows the electrochemical active area (ECSA) measurement chart, (a) CuBi ₂ O ₄ /CuO; (b) cyclic voltammetry curve of CuBi ₂ O ₄ /CuO/ZnO, the scan rate is 20 ~ 100mV/s; ( c) Calculated electric double layer capacitance (EDLC) to reflect ECSA.

In Figure 10 (a) η _inj curve; (b) η _sep curve; (c) Mott-Schottky curve of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO electrodes; (d) CuBi ₂ O ₄ / Nyquist plots of tested (scatter) and fitted (line) CuO and CuBi ₂ O ₄ /CuO/ZnO electrodes; inset is the equivalent circuit used for the fitting.

Figure 11 is the Bode plot of the test (scattered points, squares correspond to resistance, circles correspond to phase angle) and fitting (line) of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO electrodes.

In Figure 12, (a) is a schematic diagram of the electrode process of CuBi ₂ O ₄ /CuO photocathode; (b) is a schematic diagram of the electrode process of CuBi ₂ O ₄ /CuO/ZnO photocathode; (c) is CuBi ₂ O ₄ /CuO/ Schematic diagram of the energy band diagram of ZnO.

Detailed ways

Example 1

A method for surface modification of CuBi ₂ O ₄ /CuO photocathode, which uses electrodeposition of ZnO as a protective layer for CuBi ₂ O ₄ /CuO photocathode.

The CuBi ₂ O ₄ /CuO photocathode can be prepared by the following method: a nanoporous CuBi ₂ O ₄ /CuO photocathode is prepared on an FTO glass substrate through a two-step electrodeposition method. Electrodeposition uses a standard three-electrode system, with FTO as the working electrode, Ag/AgCl as the reference electrode, and platinum wire as the counter electrode. In the first step, BiOI nanosheets are electrodeposited on FTO. Prepare a solution containing 0.23M p-benzoquinone in absolute ethanol and stir until it becomes a clear solution (Solution 1). At the same time, prepare a 0.4M KI solution, add HNO ₃ to the KI solution, and adjust the pH to 1.7. Then, Bi(NO ₃ ) ₃ ·5H ₂ O was dissolved in the pH-adjusted KI solution to prepare a 0.04MBi(NO ₃ ) ₃ solution (solution 2). Mix solution 1 with solution 2 and stir well. The obtained mixed solution was used to electrodeposit BiOI on FTO at a constant potential of -0.1 V _Ag/AgCl at room temperature, with a deposition time of 600 s. After electrodeposition, the resulting BiOI film was carefully washed with deionized water and dried at room temperature. In the second step, a copper source is deposited on the BiOI film. The copper source was electrodeposited on the BiOI photocathode from an aqueous solution of 0.2MCu(CH ₃ COO) ₂ ·H ₂ O. Deposition was performed at room temperature for 300 s at a constant potential of -0.3 V _Ag/AgCl . After electrodeposition of the copper source, the photocathode was washed with deionized water and dried at room temperature. Finally, the electrode was annealed at 450°C for 3h (heating rate 2°C/min) to obtain a CuBi ₂ O ₄ /CuO photocathode (as shown in Figure 1).

ZnO is deposited on the surface of CuBi ₂ O ₄ /CuO photocathode by electrodeposition to prepare CuBi ₂ O ₄ /CuO/ZnO photocathode. Electrodeposition adopts a three-electrode system, with CuBi ₂ O ₄ /CuO photocathode as the working electrode, Ag/AgCl electrode as the reference electrode, and platinum wire electrode as the counter electrode. Prepare a 0.05MZnCl ₂ solution, and use the 0.05MZnCl ₂ solution to electrodeposit ZnO on the CuBi ₂ O ₄ /CuO electrode. Deposition was performed at room temperature at a constant potential of -0.2 V _Ag/AgCl , with a deposition time of 150 s. After electrodeposition, the obtained CuBi ₂ O ₄ /CuO/ZnO photocathode was washed with deionized water and placed in an oven to dry at 60°C.

A layer of amorphous ZnO is coated on the surface of CuBi ₂ O ₄ /CuO photocathode by electrodeposition method. The photocurrent density of CuBi ₂ O ₄ /CuO/ZnO photocathode is 2.1 times that of CuBi ₂ O ₄ /CuO photocathode. At the same time, after 12 hours of stability testing, the photocurrent retention rate of the CuBi ₂ O ₄ /CuO/ZnO photocathode was 84%. The amorphous ZnO layer can provide more active sites, improve charge transfer, and protect the CuBi ₂ O ₄ /CuO photocathode. This work proves that electrodeposited ZnO coating can alleviate photocorrosion of photocathode to a certain extent while improving PEC (photoelectrochemical water splitting system) performance.

Material characterization analysis: by X-ray diffraction (XRD, BRUKER AXS D8 ADVANCE) and Cu Kα radiation Identify the phase composition, crystal structure, and growth orientation of the electrode. Scanning electron microscopy (SEM, ThermoScientific Apreo 2C) was used to obtain surface morphology images of the electrodes. Energy dispersive spectroscopy (EDS) images were recorded using OXFORD ULTIM Max65. The UV-visible absorption spectrum of the electrode was obtained using a Shimadzu UV-3600 plus spectrophotometer in the range of 300 to 1200 nm. A transmission electron microscope (Talos F200S G2) was used to obtain TEM images of the electrodes and analyze the internal structure of the electrodes. X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALABXi+) was used to determine the chemical state of elements on the electrode surface.

In order to determine the composition of the electrode, the CuBi ₂ O ₄ /CuO/ZnO photocathode was characterized by XRD. As shown in Figure 1(a), the diffraction peaks located at 20.8°, 27.9°, 30.7°, 33.3°, 37.4°, 46.7° and 52.9° match the tetragonal phase CuBi ₂ O ₄ (JCPDSNO.71-1774), Corresponding to the (200), (211), (002), (130), (202), (141) and (123) crystal planes respectively. At the same time, the diffraction peak of monoclinic phase CuO can be observed from the XRD pattern of CuBi ₂ O ₄ /CuO/ZnO. The diffraction peaks located at 35.5°, 38.6° and 48.8° correspond to the (-111), (111) and (-202) crystal planes of CuO respectively. Due to the low loading of ZnO, no obvious ZnO diffraction peak can be observed. The morphology of the prepared CuBi ₂ O ₄ /CuO photocathode characterized by SEM is shown in Figure 1(b) and (c). Nanosheets and octahedral structures can be clearly observed. Considering the fabrication process, it can be deduced that the nanosheets are CuBi ₂ O ₄ and the latter is CuO. In order to improve the stability of the CuBi ₂ O ₄ /CuO heterojunction photoelectrode, an electrodeposited ZnO covering layer was coated on the surface of the above photocathode. The SEM image of CuBi ₂ O ₄ /CuO/ZnO photocathode is shown in Figure 1(df). Compared with the uncoated CuBi ₂ O ₄ /CuO electrode, CuBi ₂ O ₄ /CuO/ZnO is covered by an ultrathin layer of nanosheet-like ZnO (Fig. 1(d) and (e)), which shows some wrinkles (See arrows in Figure 1(d) and (e)). The element distribution of the cross-section CuBi ₂ O ₄ /CuO/ZnO film was measured using EDS spectrum. The EDS pattern in Figure 1(gi) clearly confirms that CuBi ₂ O ₄ and CuO are nanosheet and octahedral structures respectively, which supports the previous inference. The EDS results also show that the ZnO layer covers the entire film of CuBi ₂ O ₄ /CuO. As shown in Figure 1(f), the average thickness of the CuBi ₂ O ₄ /CuO/ZnO film is approximately 1.2 μm. The light absorption characteristics of the photoelectrode are one of the important indicators to characterize the performance of the photoelectrode. The light absorption capacity of the photoelectrode directly affects the generation of photogenerated carriers. The UV-visible absorption spectra of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode are shown in Figure 2. It can be observed that in the range of 300 to 1100 nm, the light absorption of the electrode decreases slightly after depositing ZnO, which is due to the wide optical band gap of ZnO. The optical band gaps of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO, deduced from the Tauc diagram (inset of Figure 2), are 1.80 eV and 1.81 eV, respectively.

TEM is not only used to determine the structure of the ZnO covering layer but also to identify the CuBi ₂ O ₄ /CuO heterojunction. The TEM image of CuBi ₂ O ₄ /CuO/ZnO photocathode is shown in Figure 3. As shown in Figure 3(a), the surface of the photoelectrode is covered with a thin layer of ZnO. The high-resolution TEM (HRTEM) image of the marked area in Figure 3(a) is shown in Figure 3(b). A lattice spacing of 0.31 nm can be clearly observed, which can match the interplanar spacing _of the (220) crystal plane of _CuBi2O4 . In Figure 3(c), the lattice distance of 0.25 nm can be indexed as the interplanar spacing of the (-111) crystal plane of CuO. At the same time, an amorphous ZnO layer can be observed. CuBi ₂ O ₄ and CuO form a heterojunction in close contact, which promotes charge transfer at the CuBi ₂ O ₄ /CuO interface. The larger stripe spacing of 0.36nm may be attributed to the lattice distortion _{of CuBi2O4} caused by oxygen vacancies. Furthermore, a stripe spacing of 1.12 nm is observed in Figure 3(c). The HRTEM image of the marked area in Figure 3(c) is shown in Figure 3(d). The image is the high refractive index (730) crystal plane of CuBi ₂ O ₄ . The structure of the (730) crystal plane is shown in the inset in Figure 3(d), which closely matches the real spatial atomic arrangement.

In order to elucidate the energy band structure of CuBi ₂ O ₄ /CuO heterojunction, CuBi ₂ O ₄ and CuO electrodes were prepared. According to the UV-visible absorption spectrum in Figure 3-5(a), the Tauc diagrams of CuBi ₂ O ₄ and CuO are obtained (as shown in Figure 3-5(b) and (c)). From the Tauc diagrams of CuBi ₂ O ₄ and CuO, the optical band gaps of CuBi ₂ O ₄ and CuO are 1.81eV and 1.43eV respectively. According to the Mott-Schottky diagram of CuBi ₂ O ₄ and CuO electrodes (Fig. 4(a)), by the formula

Among them, C is the space charge capacitance of the semiconductor (F/cm2), e is the charge of the electron, ε ₀ is the vacuum dielectric constant, ε is the dielectric constant of the semiconductor, N _A is the acceptor concentration, and E is the applied bias. Pressure, E _fb is the flat band potential, k _B is Boltzmann's constant, T is the thermodynamic temperature (K), calculate the carrier concentration ( _NA ) of the electrode. For _CuBi2O4 and CuO, ε is ₈₀ and 10.26 respectively. The linear extrapolation slope of the Mott-Schottky plot can be used to estimate N _A . The N _A of CuBi ₂ O ₄ and CuO are 2.80×10 ¹⁹ cm ^-3 and 5.82×10 ¹⁹ cm ^-3 respectively. Then, the valence band edge E _V can be expressed by the formula It is calculated that E _Ag/AgCl is 0.197V. E _F is the Fermi energy level, which is equal to E _fb in vacuum, and N _V is the effective density of states in the valence band. Among them, the N _V of CuBi ₂ O ₄ is 5×10 ¹⁹ cm ^-3 and the N _V of CuO is 5.57 ×10 ²⁰ cm ^-3 . The calculated _EV - _EF values _{of CuBi2O4} and CuO are then 0.02eV and 0.06eV respectively. When CuBi ₂ O ₄ and CuO contact to form a heterojunction, the Fermi levels of CuBi ₂ O ₄ and CuO will be at the same position, which is equivalent to the E _fb of the CuBi ₂ O ₄ /CuO photocathode (Figure 10(c) shown). Therefore, type II CuBi ₂ O ₄ /CuO heterojunction is established. The energy band structure of the CuBi ₂ O ₄ /CuO electrode is shown in Figure 4(b).

XPS testing was used to analyze the detailed composition and valence state of elements in CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO electrodes. Compared with CuBi ₂ O ₄ /CuO, the XPS spectrum of CuBi ₂ O ₄ /CuO/ZnO shows an obvious Zn2p peak. This further proves that ZnO is successfully deposited on the CuBi ₂ O ₄ /CuO heterojunction photocathode surface. For the Cu2p energy spectrum (Fig. 5(a)), the binding energy peaks appearing at 933.5eV and 953.5eV are attributed to Cu2p _3/2 and Cu2p _1/2 respectively. The energy difference between the double peaks is 20 eV, which proves the existence of Cu ²⁺ ions. In addition, the XPS peak of Cu2p _3/2 can be fitted by two separate peaks of Cu ²⁺ and Cu ⁺ . The ratio of Cu ⁺ /(Cu ⁺ +Cu ^{2 +} ) is shown in Table 1. After ZnO is electrodeposited, part of Cu ²⁺ is converted into Cu ⁺ . The content of Cu ⁺ is related to the concentration of oxygen vacancies. A higher Cu ⁺ content indicates the presence of more oxygen vacancies. In addition, the satellite peaks of Cu2p _3/2 and Cu2p _1/2 are located at 942eV and 962eV respectively, which are attributed to the 3d ⁹ shell of Cu2 ⁺ . In Figure 5(b), two peaks appear at 158.8eV and 164.1eV, corresponding to the binding energies of Bi4f _7/2 and Bi4f _5/2 of Bi ³⁺ ions, respectively. As shown in Figure 5(c), the O1s energy spectrum is subdivided into three peaks of 529.5eV, 531.1eV and 532.0eV. The main O1s peak at 529.5 eV is attributed to lattice oxygen (O _L ). The peak located at 531.1 eV is related to the presence of oxygen vacancies ( _OV ). The peak at 532.0 eV is related to surface-adsorbed oxygen and hydroxyl groups ( _OA ). After ZnO deposition, the ratio of O _V /( _OL + O _V + O _A ) increased from 20.1% to 37.5%, indicating that O _V was generated during ZnO electrodeposition. At the electrodeposition potential _of -0.2V _Ag/AgCl , there is a reducing environment on the _CuBi2O4 /CuO electrode, which leads to the generation of Cu ⁺ and O _V. As shown in Figure 5(d), the Zn2p energy spectrum of CuBi ₂ O ₄ /CuO/ZnO has two peaks at 1021.7eV and 1044.9eV, which are related to the binding energies of Zn 2p _3/2 and Zn2p _1/2 respectively. , proving the existence of Zn ²⁺ ions

Table 5-1 Summary of Cu2p and O 1s XPS energy spectrum data analysis of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO

Electrochemical performance analysis: All PEC tests were performed on an electrochemical workstation (CHI 660E) using a standard three-electrode system. In this system, the prepared photocathode was used as the working electrode, platinum wire was used as the counter electrode, and Ag/AgCl electrode ( 3MKCl) as the reference electrode. During the test, a 300W xenon lamp (CEL-HXF300) coupled with a filter was used as the light source, and the light intensity on the photocathode surface was approximately 100mW/cm ² . Light is incident from the back side of the electrode. The electrolyte is 0.5M Na ₂ SO ₄ . Linear sweep voltammetry (LSV) curves were recorded at a scan rate of 20 mV/s. Photoelectrochemical impedance spectroscopy (PEIS) was measured under light conditions, in the frequency range of 0.1 to 10 ⁵ Hz, under a disturbance of 10mV, and at a potential of 0.4 VRHE. Moreover, Z-View software was used to fit the photoelectrochemical impedance spectrum. Mott-Schottky (MS) plots were measured in the dark at a fixed frequency of 2 kHz. For charge injection efficiency and separation efficiency measurements, the electrolyte used was a mixed electrolyte of 0.5M Na ₂ SO ₄ and 1% (v/v) H ₂ O ₂ .

The PEC performance of the prepared CuBi ₂ O ₄ -based photocathode was tested in 0.5M Na ₂ SO ₄ electrolyte under AM1.5G illumination. First, the electrodeposition potential and electrodeposition time of ZnO synthesis were optimized, and the corresponding LSV curve is shown in Figure 6. The optimal electrodeposition potential is -0.2V _Ag/AgCl . The optimal electrodeposition time is 150 seconds. Secondly, the PEC performance of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode was compared. Figure 7(a) shows the chopping LSV curves of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode in 0.5MNa ₂ SO ₄ at a scan rate of 20mV/s. Under 0.2V _RHE , the photocurrent density of CuBi ₂ O ₄ /CuO photocathode is 0.56mA/cm ² . After coating the amorphous ZnO layer, the photocurrent density reaches 1.18mA/cm ² under 0.2V _RHE , which is almost 1.1 times the photocurrent density of the original CuBi ₂ O ₄ /CuO. As shown in the inset of Figure 7(a), the onset potentials of both CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode are greater than 1.0V _RHE . In addition, both photocathode showed clear transient current spikes, indicating the recombination of electron-hole pairs. The cathode spike (lit state) indicates the occurrence of photogenerated electron-hole pair recombination, while the anode spike (off state) indicates that photogenerated electrons are stored in trapping sites on the surface of these electrodes and then recombine with holes in the dark. Therefore, the ZnO coating can inhibit the recombination of electron-hole pairs on the CuBi ₂ O ₄ /CuO photocathode. In Figure 7(b), the HC-STH efficiency, by using the formula

where J _ph is the measured photocurrent density (mA/cm ² ), E _bias is the applied potential (V _RHE ), E _H+/H2 is the standard potential of the hydrogen evolution reaction (0 V _RHE ), and P _light is the power density of light. (100mW/cm ² ), calculated based on the chopping LSV curve. The maximum HC-STH efficiency of the CuBi ₂ O ₄ /CuO photocathode is 0.11% at 0.2V _RHE . Under 0.3V _RHE , the maximum HC-STH efficiency of the CuBi ₂ O ₄ /CuO/ZnO photocathode is 0.26%, which is 1.4 times higher than that of the CuBi ₂ O ₄ /CuO photocathode. Finally, the PEC stability of the above two photocathode was studied using 1% (v/v) H ₂ O ₂ as electron scavenger. As shown in Figure 7(c), after 12 hours of stability testing, for CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO, the photocurrent retention rates of the photocathode were 58.3% and 84%, respectively, and the current of the electrode The density and stability are at a better level among the reported CuBi ₂ O _4- based photocathode (see Table 2). The reason for the fluctuations in the curve is that hydrogen bubbles were formed and released on the photocathode surface during the stability test. The trend of photocurrent decay is similar to _that reported for _CuBi2O4 / _TiO2 photocathode. The inset _of Figure 7(c) shows that when tested in an electrolyte without sacrificial reagents, the current density of the _CuBi2O4 /CuO photocathode decreases rapidly within seconds after the start of illumination. After 500 seconds of testing, the photocurrent retention rate is only 40%. It should be noted that _H2O2 used as an electron scavenger can cause a current multiplication effect due to additional hole injection without absorbing additional _photons . The initial decrease and then increase in the photocurrent density of CuBi ₂ O ₄ /CuO may be due to photocorrosion of CuBi ₂ O ₄ /CuO. Under the same conditions, CuBi ₂ O ₄ /CuO/ZnO showed a higher photocurrent retention rate after 12 hours of testing. This confirms that the amorphous ZnO coating alleviates the photocorrosion of CuBi ₂ O ₄ /CuO as a protective layer to a certain extent. The photocorrosion of CuBi ₂ O ₄ /CuO/ZnO should be attributed to the imperfect coating of the ZnO layer. The morphology characterization of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO after the stability test (Figure 8) shows that without the protection of the ZnO covering layer, it can be observed on the surface of CuBi ₂ O ₄ /CuO Some tiny nanostructures; the ZnO protective layer can effectively protect the nanostructures of the CuBi ₂ O ₄ /CuO photocathode.

A ZnO capping layer can be further continuously deposited to completely cover and protect the internal CuBi ₂ O ₄ /CuO from electrolyte contact.

Table 5-2 Summary of reported PEC performance of CuBi ₂ O ₄ (CBO)-based photocathode.

Mechanistic analysis: Electrochemical active area (ECSA) of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode, measured by cyclic voltammetry (CV) in the scan rate range of 20 to 100mV/s And calculated, as shown in Figure 9. The electric double layer capacitance values of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO are 0.029mF/cm ² and 0.044mF/cm ² respectively. This shows that the ZnO layer can not only protect the CuBi ₂ O ₄ /CuO heterojunction, but also provide more active sites.

Next, an electron scavenger (H ₂ O ₂ ) was added to the electrolyte for charge injection and separation efficiency measurements. The charge injection efficiency (η _inj ) and separation efficiency (η _sep ) are calculated by the formula and/> Calculation, where J _H2O is the photocurrent density measured in the electrolyte without electron scavenger (H ₂ O ₂ ) at the electrode, and J _H2O2 is the photocurrent density measured at the electrode in the electrolyte with electron scavenger (H ₂ O ₂ ) present. The obtained photocurrent density, J _abs , is the theoretical photocurrent density calculated based on the UV-visible absorption spectrum of the electrode. J _abs is the theoretical photocurrent density calculated based on the UV-visible absorption spectrum in Figure 2, which is 26.25mA/cm ² . As shown in Figure 10(a), under 0.2V _RHE , the eta _inj value of the CuBi ₂ O ₄ /CuO photocathode is 15.3%. After depositing the ZnO coating, the eta _inj value of the CuBi ₂ O ₄ /CuO/ZnO photocathode can reach 21.5%. The eta _sep plots of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode are shown in Figure 10(b), and no obvious suppression from the ZnO layer was observed. Mott-Schottky curves were measured at a fixed frequency of 2 kHz in the dark to reveal changes in acceptor density and flat band potential (V _fb ) of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO electrodes. A negative slope can be observed in Figure 10(c), which is a typical characteristic of p-type semiconductors. After ZnO layer coating, V _fb changed from 1.16V _RHE to 1.06V _RHE . At the same time, after coating the ZnO layer on the CuBi ₂ O ₄ /CuO photocathode surface, a decrease in the slope of the Mott-Schottky curve was observed, indicating a decrease in the electrode carrier concentration. The change in the Mott-Schottky curve is attributed to the increase in oxygen vacancies. The acceptor energy level in the CuBi ₂ O ₄ /CuO photocathode originates from Cu ²⁺ vacancies. The formation of oxygen vacancies will compensate for the acceptor energy level in the CuBi ₂ O ₄ /CuO photocathode and push the Fermi level away from the valence With maximum value.

In order to study the charge transfer kinetics, the CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode were characterized by electrochemical impedance spectroscopy under 0.4V _RHE and AM1.5G illumination conditions. As shown in Figure 10(d), the measured Nyquist plots of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO photocathode were fitted using the equivalent circuit (inset in Figure 10(d)). It consists of series resistance (R _s ), bulk resistance (R _bulk ), bulk constant phase element (CPE _bulk ), surface charge transfer resistance (R _ct ) and constant phase element from the surface state (CPE _ct ). Table 3 summarizes the fitting parameters. After depositing the ZnO coating, the resistance of CuBi ₂ O ₄ /CuO/ZnO dropped from 30.1Ω to 29.4Ω. This indicates that the ZnO capping layer cannot limit charge transport. The R _bulk of CuBi ₂ O ₄ /CuO/ZnO dropped from 1153.0Ω of the original CuBi ₂ O ₄ /CuO electrode to 467.7Ω. At the same time, the R _{ct value} of CuBi ₂ O ₄ /CuO/ZnO (~15541Ω) is also significantly lower than the R _ct value of CuBi ₂ O ₄ /CuO (~17931Ω). This indicates that ZnO coating can significantly improve charge transfer at the electrode/electrolyte interface, which may be attributed to the higher electron mobility of ZnO. In addition, the Bode plots of CuBi ₂ O ₄ /CuO and CuBi ₂ O ₄ /CuO/ZnO (Fig. 11) show that the ZnO layer can shorten the characteristic time of charge transfer at the electrode/electrolyte interface (higher characteristic frequency).

Table 5-3 Parameters of each component in the equivalent circuit obtained through fitting

The possible mechanism of the ZnO coating on the CuBi ₂ O ₄ /CuO surface is shown in Figure 12. In Figure 12(a), the surface of CuBi ₂ O ₄ /CuO is directly exposed to the Na ₂ SO ₄ electrolyte. The self-reduction potential of CuO is lower than E(H ⁺ /H ₂ ). Therefore, CuO is thermodynamically unstable. As shown in Figure 12(a), the photocorrosion phenomenon may occur severely. When a ZnO covering layer is electrodeposited on the surface of CuBi ₂ O ₄ /CuO (Fig. 12(b)), the above photocorrosion process can be effectively prevented. In addition, the ZnO covering layer can increase the electrochemically active area while improving charge transfer at the electrode/electrolyte interface. From the energy band structure of CuBi ₂ O ₄ /CuO/ZnO photocathode (Fig. 12(c)), the n-type ZnO layer can produce small energy band bending like the photoanode. More importantly, the ZnO layer can suppress surface charge recombination due to its high valence band edge. In addition, amorphous ZnO may introduce some defect states that can allow electron injection through valence state-mediated transport.

The use of chemical methods to deposit thin films is not as dense as the physical method, but it is low cost and has good scalability. The present invention proves the feasibility of electrodepositing a ZnO covering layer as a protective layer for CuBi ₂ O ₄ /CuO photocathode. At the same time, under AM 1.5G illumination, the CuBi ₂ O ₄ /CuO/ZnO photocathode shows a photocurrent density of 1.18mA/cm ² at a 0.2V _RHE potential in 0.5M Na ₂ SO ₄ , which is approximately that of CuBi ₂ Twice the current density of O ₄ /CuO photocathode. After coating the ZnO layer, the current retention rate of CuBi ₂ O ₄ /CuO increased from 58.3% to 84% during the 12h stability test. Experimental results show that the amorphous ZnO covering layer can not only serve as a protective layer, but also serve as a "cocatalyst", effectively improving the charge transfer of the electrode and increasing the electrochemically active area of the electrode.

The above are only preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modifications, changes and equivalent structural changes made to the above embodiments based on the technical essence of the present invention still belong to the technology of the present invention. within the protection scope of the scheme.

Claims (1)

1.CuBi ₂ O ₄ A method for modifying the surface of CuO photocathode, which is characterized in that ZnO is electrodeposited as CuBi ₂ O ₄ Protection layer of CuO photocathode.