Recently, due to increasing energy and environment problems, the development of energy-saving and emission-reducing technology has become urgent. The most efficient method which can solve the current dilemma is the development and implementation of cleaner energy. Solar energy has attracted significant attention for its green, widespread, and inexhaustible characteristics. In recent years, photocatalytic technology has become an increasingly popular research topic due to its capability of converting solar into chemical energy, degrading organic pollutants, and producing organic fuel [1-8]. In the field of photocatalysis, photoelectrochemical (PEC) water-splitting is considered to be a promising solution which could provide enough clean energy and satisfy the increasing energy demands of modern society [11, 12]. The biggest challenge to date for PEC water-splitting is the preparation of highly active photoanodes and photocathodes. Though a large number of n-type semiconductor materials, such as BiVO₄, ZnO, and Fe₂O₃, have been developed for preparing photoanodes [13-15], only a few candidates including p-Si, Cu₂O, Cu₂ZnSnS₄, and CuIn_xGa_1–xSe [16-19] have been reported as p-type semiconductors for photocathodes. However, these materials have some unavoidable problems which limit their applications and include high cost, difficult preparation, and instability. The exploration of photocathode materials is crucial for further developments in the field of PEC water-splitting.

CuBi₂O₄ is a p-type semiconductor which is naturally stable and exhibits a band gap of approximately 1.70 eV with excellent visible light responses, allowing it to utilize a significant part of light energy [20-23]. The conduction band minimum (CBM) of CuBi₂O₄ is estimated to be at a more negative potential than the thermodynamic potential for H₂ production, and its valence band maximum (VBM) at a more positive potential than that of p-type Si or Cu₂O. This appropriate band gap structure makes it a promising photocathode material. Few studies have been reported for PEC hydrogen production using CuBi₂O₄ photocathodes. However most of them focused on synthetic methods and theoretical calculations [21-23], and not on promoting interfacial carrier separation.

In this study, the p-type CuBi₂O₄ photocathodes were prepared using a simple electrochemical synthesis method. The CuBi₂O₄/TiO₂ p-n heterojunction was first prepared by spin-coating a TiO₂ layer on the surface of the CuBi₂O₄ photocathodes. The experiment results showed that the activity of the prepared CuBi₂O₄ photocathode was substantially improved by the formation of the CuBi₂O₄/TiO₂ heterojunction. It is believed that the successful construction of Pt/CuBi₂O₄/TiO₂ will act as a guide for future modification studies of CuBi₂O₄ photocathodes.

All chemicals were of analytical grade and were used without further purification.

A typical three-electrode system composed of an F-doped Tin Oxide (FTO) working electrode, Ag/AgCl (saturated KCl) reference electrode, and Pt counter electrode was used. Preparation of the Cu/Bi metal film was achieved via electrodeposition in a dimethyl sulfoxide (DMSO) electrolyte solution containing 20 mmol/L Bi(NO₃)₃, 10 mmol/L Cu(NO₃)₂, and 100 mmol/L KClO₄. Deposition was performed for 10 cycles at 0.05 C/cm² (E = –1.5 V vs Ag/AgCl) for each cycle. A total charge of 0.50 C/cm² was applied. Subsequently, the Cu/Bi films were heated at 500 ℃ for 3 h in air (ramping rate = 3 ℃/min) to form CuBi₂O₄ [27]. After annealing, the organic residue from DMSO was removed from the surface.

Preparation of Pt/CuBi₂O₄ photocathodes. Pt was deposited onto the CuBi₂O4 films through photoelectrochemical reduction in 20 vol% methanol aqueous solution containing 0.3 mmol/L H₂PtCl₆. A 300 W Xe arc lamp was used as the light source and was equipped with an AM 1.5G filter. CuBi₂O₄ photoelectrodes were used as working electrodes. The deposition amount was approximately 0.02 C/cm² at E = 0 V vs Ag/AgCl.

Deposition of TiO₂. The sol-gel precursor was prepared by adding 0.3 mL of isopropyl titanate to 20 mL ethanol under stirring. Subsequently, 250 μL of the precursor was dropped on the CuBi₂O₄ photoelectrodes for spin-coating (1000 r/min, 30 s) several times and the photoelectrodes were annealed at different temperatures for 1 h.

Preparation of TiO₂ powder. The sol-gel precursor was dried in glass beakers and annealed at different temperatures. The preparation of the Pt/TiO₂/CuBi₂O₄ photocathodes was performed using the same method as that of the Pt/CuBi₂O₄ photocathodes.

Morphology observation was performed using an S-4800 field emission scanning electron microscope (FE-SEM, Hitachi, Japan). The purity and crystallinity of the CuBi₂O₄ and TiO₂/CuBi₂O₄ electrodes were examined by X-ray diffraction (XRD) using a Bruker AXS D8 ADVANCE diffractometer (Cu K_α X-ray radiation, λ = 0.154056 nm). The light source was a 300 W Xe arc lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) equipped with an AM 1.5 filter (100 mW/cm²). UV-vis diffuse reflectance spectra were recorded using a Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere, and BaSO₄ was used as a reference. The X-ray photoelectron spectroscopy were got through Thermo Scientific ESCALAB 250.

Photoelectrochemical measurements were performed using an electrochemical analyzer (CHI660C, CH Instruments) with a standard three-electrode system in 0.1 mol/L NaOH electrolyte solution. The reference electrode was Ag/AgCl (saturated KCl), the counter electrode was a Pt plate, and the prepared photocathodes were used as working electrodes.

XRD was used to investigate the phase structure of the samples. Fig. 1 shows the XRD of the CuBi₂O₄ sample prepared without a spin coated TiO₂ layer. The observed diffraction peaks at 20.87°, 28.01°, 29.68°, 30.74°, and 33.26° arose from (200), (211), (220), (002), and (310), respectively which correspond to JCPDS No. 72-493. The obvious peak at approximately 26.4° is characteristic of the FTO substrate. The XRD patterns of the TiO₂/CuBi₂O₄ photocathodes treated at different temperatures are shown in Fig. S1 and exhibited no obvious TiO₂ peaks.

To clarify the status of the TiO₂ on CuBi₂O₄, we prepared TiO₂ using the same sol-gel precursor under different temperature heat treatment conditions, as shown in Fig. S2. Only when the temperature was raised over 400 ℃ did the reaction products show good crystallinity, indicating that the TiO₂ obtained at 200 ℃ on CuBi₂O₄ was amorphous.

The surface morphologies and elemental distributions of the CuBi₂O₄ and TiO₂/CuBi₂O₄ photocathodes were characterized by SEM, as shown in Fig. 2. From the images of the CuBi₂O₄ photocathodes shown in Fig. 2(a)–(c), we can see that the CuBi₂O₄ film was composed of uniform nanoparticles with a diameter of 100–150 nm. Fig. 2(d)–(f) show the SEM images of the TiO₂/CuBi₂O₄ photocathodes after modification with a TiO₂ layer and annealing at 200 ℃. The boundary of the nanoparticles was blurred and the dispersibility deteriorated. The elemental distribution in the TiO₂/CuBi₂O₄ photocathodes is shown in Fig. 2(g)–(i). The mapping results indicate that the TiO₂ layer was uniformly deposited on the surface of the CuBi₂O₄ film. The morphologies of photocathodes treated at other temperatures are shown in Fig. S3, and no obvious difference was observed. The thickness of the TiO₂/CuBi₂O₄-200 ℃ photocathode was determined as shown in Fig S4. From the results, we can see that the thickness of the TiO₂/CuBi₂O₄ layer was approximately 350 nm.

The optical properties of the prepared samples were characterized by UV-vis diffuse reflectance spectroscopy, as shown in Fig. 3. From their light absorption properties, it can be concluded that the CuBi₂O₄ samples showed excellent light response in the visible region. The light absorption of the CuBi₂O₄ photocathode was not influenced after modifying the TiO₂ layer. The outstanding light response of CuBi₂O₄ highlights its potential as an outstanding photocathode material.

Herein, the coating of the TiO₂ layer was used to promote the transmission of electrons generated from CuBi₂O₄. The effect of TiO₂ layer thickness is a key factor for photocathode activity. The TiO₂/CuBi₂O₄ samples with different thicknesses were fabricated using different spin-coating times, as shown in Fig. 4(a). From the current-potential curves, it is clear that the photoelectric activity initially increases and then decreases with increasing spin-coating times. The Pt/TiO₂/CuBi₂O₄ photocathodes showed the best performance at four cycles of spin-coating. Fig. 4(b) shows the photocurrent-time curves of the Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄ photocathodes prepared with four cycles of spin-coating. Fig. 4(a) shows that the photocurrent onset potentials of the Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄ photoelectrodes are approximately 1.15 and 1.20 V, respectively, and that the TiO₂ layer did not significantly affect the onset potential. The photocurrent densities of the Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄ electrodes at 0.60 V vs RHE were 0.22 and 0.35 mA/cm², respectively. The comparison of the Photocurrent-time curves of Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄ at 0.60 V vs RHE showed a more significant effect of the TiO₂ layer (Fig. 4(b)). For the Cu-based photocathodes, photoinduced corrosion is commonly encountered and difficult to solve. In these experiments, the pristine Pt/CuBi₂O₄ photocathode showed an initial photocurrent density of 0.22 mA/cm², which decreased to 0.20 mA/cm² over 100 s of reaction, corresponding to a 9% decrease. In contrast, Pt/TiO₂/CuBi₂O₄ exhibited a pristine photocurrent density of 0.39 mA/cm², which is 1.77 times that of the Pt/CuBi₂O₄ photocathode. In addition, the crystallinity of TiO₂ was examined, and the results are shown in Fig. 4(c). The activity of the Pt/TiO₂/CuBi₂O₄ photocathodes decreased with increasing annealing temperature, and the amorphous TiO₂ modified photocathodes prepared at 200 ℃ showed the best activity. Compared to that of the crystalline TiO₂, amorphous TiO₂ has a disordered structure resulting in a larger surface and more active sites [30, 31]. This leads to better contacting both for the CuBi₂O₄ and Pt and more efficient carrier separation. The Faradic efficiency and stability of the photocathode in photoelectrochemical H₂ generation was also measured, as shown in Fig S5. The total Faradic efficiency over 60 min was approximately 96%, and the normalized current indicated the high stability of the photocathode.

The incident photon to current efficiency (IPCE) of the photoelectrodes was characterized at 0.60 V vs. RHE in 0.1 mol/L NaOH electrolyte (Fig. 4(d)). The IPCE of the CuBi₂O₄ photocathode at different wavelengths shows similar tendencies and increased significant after decorating TiO₂ on the photocathode. The prepared Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄-200 ℃ photocathodes showed their highest IPCE values of 1.4% and 2.9%, respectively, at 400 nm. These photoelectrochemical tests indicated that modifying amorphous TiO₂ improved the activity of the CuBi₂O₄ photocathode.

To further understand the role of TiO₂ in the improved PEC performance, electrochemical impedance spectroscopy (EIS) analysis and Mott-Schottky experiments were used to investigate the interfacial properties of the Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄ photocathodes. The diameter of the semicircle in the EIS measurements reflects the resistance of electron transfer, which can directly indicate the photogenerated electron transfer efficiency at the interface between the electrodes and electrolyte solution [24, 25]. Fig. 5(a) shows the EIS curves of the Pt/CuBi₂O4 and Pt/TiO₂/CuBi₂O₄ photocathodes measured in 0.1 mol/L NaOH under light illumination. The Pt/TiO₂/CuBi₂O₄-200 ℃ photocathode exhibited a smaller EIS curve diameter compared to that of the Pt/CuBi₂O₄ photocathode. This indicates that the resistance between the Pt/TiO₂/CuBi₂O₄-200 ℃ photocathode and electrolyte interface is smaller than that of the Pt/CuBi₂O₄ photocathode and electrolyte interface, allowing photogenerated electrons to transfer more efficiently upon modification with TiO₂. Mott-Schottky analysis was used to confirm the flat band potential (Efb) and understand the changing interface, using a 1 kHz frequency and 10 mV amplitude. For p-CuBi₂O₄, a negative slope was obtained, as shown in Fig. 5(b), and the intersection point with the x-axis represents the Efb which was approximately 1.47 V. A negative shift of 0.11 V was observed after deposition of amorphous TiO₂. From previous reports regarding the band gap structure of CuBi₂O₄ [20-23], we can conclude that n-TiO₂ and p-CuBi₂O₄ construct a single kind of p-n heterojunction at the interface, which promotes carrier separation.

The internal elemental composition and the chemical state of the Pt/CuBi₂O₄ and Pt/TiO₂/CuBi₂O₄ photocathodes were further analyzed by X-ray photoelectron spectroscopy (XPS). Fig. 6(a)–(d) show the XPS spectra of Cu 2p, Bi 4f, O 1s, and Ti 2p, respectively. In the XPS spectra of the CuBi₂O₄ photocathode shown in Fig. 6(a), the peaks at 934.3 and 954.1 eV correspond to the binding energies of Cu 2p_3/2 and Cu 2p_1/2, respectively, and the peaks at 942.3 and 962.4 eV correspond to satellite peaks of Cu 2p. The Cu 2p peak positions and the presence of satellite peaks confirm that the copper in CuBi₂O₄ exists as Cu²⁺. The Bi 4f peaks located at 158.8 and 164.1 eV were assigned to the Bi 4f_7/2 and Bi 4f_5/2, respectively, of Bi³⁺. The peak at 529.8 eV originates from the oxygen in the crystal lattice and the higher binding energy of 531.6 eV indicates the presence of oxygen defects or absorbed chemical oxygen. Two weak peaks were observed at approximately 458.0 and 464.9 eV, originating from Ti 2p in TiO₂ [26-28]. The Cu 2p, Bi 4f, and O 2p peaks showed negligible changes after TiO₂ layer decorating, which indicates that the TiO₂ deposition did not destroy the CuBi₂O₄ layer and increased the activity of the photocathode. In addition, FTIR spectra were used to investigate the surface of the TiO₂/CuBi₂O₄ electrode, as shown in Fig. S6. The peak at approximately 1070 cm^–1 was attributed to oxygen deficiency, and with increasing annealing temperature, the peak became weaker, indicating the loss of oxygen deficiency at higher temperatures [29].

Based on the above experimental results, we proposed a possible mechanism to explain the higher efficiency of the Pt/TiO₂/CuBi₂O₄ photocathodes for PEC hydrogen production, as shown in Fig. 7. After modifying with the n-type semiconductor TiO₂, a kind of type Ⅱ p-n CuBi₂O₄/TiO₂ heterojunction formed. The heterojunction aided in the separation of electrons and holes generated by the CuBi₂O₄ film. As a result, the Pt/TiO₂/CuBi₂O₄ photocathode showed much better performance than that of the Pt/CuBi₂O₄ photocathode.

In summary, a simple electrodeposition method was used to prepare highly crystalline CuBi₂O₄ films on FTO glass for PEC hydrogen production. The CuBi₂O₄ photocathodes were modified with an amorphous n-TiO₂ layer by spin-coating, and with Pt by PEC reduction. Characterization of the photoelectrodes indicated that the TiO₂ layer played a key role in the separation of the photogenerated electrons and holes because of the p-n heterojunction formed. A suitable thickness of TiO₂ greatly enhanced the activity of CuBi₂O₄ photocathodes. This study provides a reference for the preparation of high-performance CuBi₂O₄ photocathodes.

This work was supported by the National Natural Science Foundation of China (51602179, 21333006, 21573135, and 11374190), and the National Basic Research Program of China (973 Program, 2013CB632401). Peng Wang acknowledges support from the Recruitment Program of Global Experts, China, and Baibaio Huang acknowledges support from the Taishan Scholars Program of Shandong Province.