CuO layers were fabricated on FTO substrates via electrodeposition, following a method similar to previously reported protocols^[24]. A solution of 0.43 g of copper(II) chloride dihydrate (CuCl₂·2H₂O) and 0.15 g of potassium chloride (KCl) was dissolved in 50 mL of distilled water, followed by 20 min of magnetic stirring to form the electrodeposition electrolyte. The CuO films were prepared using a three-electrode system, where FTO conductive glass served as the working electrode, a platinum sheet as the counter electrode, and an Ag/AgCl (saturated KCl) electrode as the reference. Electrodeposition was performed at a constant potential of −0.5 V (vs. Ag/AgCl) for durations of 150, 200, 250 and 300 s. After deposition, the FTO electrode was rinsed with deionized water and air-dried to obtain a red copper film. The samples were then annealed in a muffle furnace at 450 °C for 2 h, transforming the copper film into a CuO film.

CuO/CuBi₂O₄ photocathodes were fabricated on FTO/CuO substrates via electrodeposition. To prepare the copper precursor solution, 0.02 g of copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O) was dissolved in 20 mL of distilled water and magnetically stirred for 20 min. Subsequently, 0.08 g of bismuth(III) nitrate pentahydrate (Bi(NO₃)₃·5H₂O) was dissolved in 20 mL of distilled water and stirred for an additional 20 min. To promote the dissolution of the bismuth salt, 10 mL of a 10% nitric acid (HNO₃) solution was added to the mixture, yielding the bismuth precursor solution. The combined solution was then sonicated for 30 min to ensure uniform dispersion, resulting in the electrodeposition precursor solution. The electrodeposition of the Bi-Cu film followed the same procedure as for the Cu film, using a three-electrode system with FTO/CuO as the working electrode, a platinum sheet as the counter electrode, and an Ag/AgCl (saturated KCl) electrode as the reference. Electrodeposition was conducted at a constant potential of −0.4 V (vs. Ag/AgCl) for 10 min, resulting in a co-deposited Bi-Cu film. After electrodeposition, the sample was removed, dried at room temperature, and then calcined in a muffle furnace at 550 °C for 2 h to obtain the CuO/CuBi₂O₄ photocathode. The composite photoanodes prepared with different CuO electrodeposition durations (100, 150, 200 and 250 s) were denoted as CuO/CuBi₂O₄-100, CuO/CuBi₂O₄-150, CuO/CuBi₂O₄-200 and CuO/CuBi₂O₄-250, respectively. For comparison, the CuBi₂O₄ sample was prepared using the same method, except without the CuO interlayer step.

The experimental reagents, material characterization, PEC testing, and designed formulas involved in this study can be found in the supplementary materials.

The phase structure of the samples was characterized by X-ray diffraction (XRD). As shown in Figure 1, for CuBi₂O₄, diffraction peaks appeared at 20.92°, 28.09°, 33.36° and 46.80° can be indexed to the (200), (211), (310) and (411) planes, respectively, in good agreement with the standard JCPDS No. 79-1810^[25]. For CuO, the peaks located at 32.52°, 35.57°, 37.53° and 38.77° correspond to the (022), (311), (−222) and (440) planes, respectively, matching well with the standard JCPDS No. 80-1268^[26]. Furthermore, the XRD patterns of CuO/CuBi₂O₄-150, CuO/CuBi₂O₄-200, CuO/CuBi₂O₄-250 and CuO/CuBi₂O₄-300 clearly showed an increase in the intensity of CuO-related peaks with increasing CuO content, which is consistent with the experimental expectations. These results confirmed the successful introduction of CuO layer.

The surface and cross-sectional morphologies of the films were characterized using scanning electron microscopy (SEM). As shown in Figure 2(a), the CuO film electrodeposited onto the FTO substrate exhibited an irregular, block-like structure. In contrast, the CuBi₂O₄ film (Figure 2(b)) presented a uniformly distributed, worm-like morphology. Upon deposition of CuBi₂O₄ onto the FTO/CuO substrate, a dense and homogeneous CuBi₂O₄ layer is formed that uniformly covers the CuO surface, confirming the successful construction of the CuO/CuBi₂O₄ photocathode (Figure 2(c)). Cross-sectional SEM images further revealed that the CuO layer is completely covered by CuBi₂O₄, resulting in a compact and well-integrated composite structure (Figure 2(d)−(f)).

Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were employed to examine the interfacial structure between CuO and CuBi₂O₄. The TEM images revealed a distinct and well-defined junction between the two phases (Figure 2(g)). Clear lattice fringes with spacings of 0.246 and 0.289 nm are observed, corresponding to the (–111) plane of CuO and the (002) plane of CuBi₂O₄, respectively, consistent with the standard PDF cards for CuO (#80-1268) and CuBi₂O₄ (#79-1810) (Figure 2(h)). The presence of locally disordered lattice fringes at the interface further confirmed the good contact between CuO and CuBi₂O₄. Moreover, energy-dispersive X-ray spectroscopy (EDS) analysis verifies the coexistence of Cu, O and Bi within the CuO/CuBi₂O₄ film, indicating the successful synthesis of the heterostructured photocathode (Figure 2(i)).

X-ray photoelectron spectroscopy (XPS) was employed to analyze the composition of the films. As shown in Figure 3, the XPS spectra of the pristine CuBi₂O₄ and the CuO/CuBi₂O₄ film were compared. In the Bi 4f spectra (Figure 3(a)), the Bi 4f_7/2 and Bi 4f_5/2 peaks of the pristine CuBi₂O₄ were located at approximately 159 and 164 eV, respectively, corresponding to the typical oxidation state of Bi³⁺. In contrast, the peaks in the composite sample underwent a negative shift of about 0.10−0.20 eV, indicating an increase in the electron cloud density around Bi due to the presence of CuO and suggesting a redistribution of charge at the interface^[27]. In the Cu 2p spectra (Figure 3(b)), the Cu 2p_3/2 peak of the pristine CuBi₂O₄ film was centered at 933−934 eV, accompanied by a pronounced satellite peak (~942 eV), which is characteristic of Cu²⁺ species. For the composite sample, the main peak shifted positively by approximately 0.20−0.30 eV, while the intensity of the satellite peak is significantly reduced. This indicated that the local electronic environment of Cu was modified upon the introduction of CuO, which was consistent with the role of CuO as a hole transport layer that facilitated interfacial charge transfer^[28]. In the O 1s spectra (Figure 3(c)), the main peak for the pristine CuBi₂O₄ appeared at 529.5 eV (lattice oxygen). In the composite sample, a notable increase in peak intensity was observed at approximately 531 eV, which was attributed to surface-adsorbed hydrogen species. This suggested that the CuO interlayer promoted the surface passivation of oxygen, thereby enhancing the stability and interfacial properties^[29]. Overall, the XPS results confirmed the formation of a stable interface interaction between CuO and CuBi₂O₄, which facilitated charge redistribution and separation. Furthermore, these results provided further validation of the role of CuO as an effective hole transport layer in the FTO/CuBi₂O₄ system.

The PEC performance of CuBi₂O₄ and CuO/CuBi₂O₄ composite films was evaluated using a three-electrode electrochemical analysis system. As shown in Figure 4(a), the photocathodes of CuBi₂O₄, CuO/CuBi₂O₄-150, CuO/CuBi₂O₄-200, CuO/CuBi₂O₄-250 and CuO/CuBi₂O₄-300 achieved photocurrent densities of approximately −0.49, −1.12, −1.71, −1.22 and −1.01 mA/cm², respectively, at 0 V vs. RHE. Notably, CuO/CuBi₂O₄-200 exhibited the highest photocurrent density among all samples, which is ~3.5 fold higher than that of pristine CuBi₂O₄. Further analysis using transient photocurrent (I−t) curves (Figure 4(b)) revealed that all photocathodes exhibited typical photoresponse behavior, and CuO/CuBi₂O₄-200 maintained stable output over multiple on/off cycles. These results indicated a significant reduction in interfacial charge recombination, with more efficient charge separation and transfer processes.

To evaluate the energy conversion capabilities of the photoelectrodes, IPCE and ABPE were measured. As shown in Figure 4(c), pristine CuBi₂O₄ exhibited an IPCE of only ~3.5% at 365 nm, whereas CuO/CuBi₂O₄-200 delivered a markedly enhanced value of ~13%, corresponding to a ~3.7 fold improvement. In comparison, the IPCE value of CuO/CuBi₂O₄-150, CuO/CuBi₂O₄-250 and CuO/CuBi₂O₄-300 is 4.2%, 7.1% and 4.5%, respectively.

The ABPE curves (Figure 4(d)) reveal that the maximum value of CuBi₂O₄ is 0.075%, while it increases to 0.17% for CuO/CuBi₂O₄-200. These results confirm again that CuO modification layer significantly enhanced the overall energy conversion performance of CuBi₂O₄ photocathodes.

In addition, photoelectrochemical impedance spectroscopy (PEIS) was employed to probe the charge-transport kinetics within the photocathodes and across the interface between semiconductor and electrolyte (Figure 4(e)). The equivalent circuit fitting (Table S1) revealed that the optimized CuO interlayer significantly reduced the interfacial charge transfer resistance, thereby facilitating the efficient transport of photogenerated holes to the FTO substrate and effectively inhibiting bulk charge recombination in CuBi₂O₄. Further analysis of the carrier separation (ƞ_sep) and injection (ƞ_inj) efficiencies (Figure 4(f)) demonstrated that the introduction of CuO as a hole transport layer substantially improved bulk carrier separation and reduced surface recombination, thereby enhancing the overall PEC performance.

To further assess the hydrogen-production capability of the photocathodes, the H₂ and O₂ evolution rates of CuBi₂O₄ and CuO/CuBi₂O₄-200 photocathodes under AM 1.5 G (100 mW/cm²) illumination were quantitatively analyzed. As shown in Figure 5(a) and (c), both photocathodes exhibited a stable increase in H₂ and O₂ production over time. Within 1 h, the CuBi₂O₄ photocathode produced 0.93 μmol/cm² of H₂ and 0.45 μmol/cm² of O₂, whereas the CuO/CuBi₂O₄-200 photocathode achieved significantly higher yields of 2.05 and 0.96 μmol/cm², respectively (Figure 5(b) and (d)). Additionally, the CuO/CuBi₂O₄-150, CuO/CuBi₂O₄-250 and CuO/CuBi₂O₄-300 photocathodes also exhibited enhanced water-splitting activity relative to bare CuBi₂O₄ (Figure S1). These results were in agreement with the photocurrent and efficiency measurements, further indicating that the introduction of an optimized CuO interlayer between the FTO substrate and CuBi₂O₄ significantly enhanced interfacial charge separation and transfer, thereby effectively promoting the utilization of photogenerated carriers and improving PEC water-splitting performance.

Given its high H₂ evolution capability, the CuO/CuBi₂O₄-200 photocathode was further integrated in series with a BiVO₄ photoanode to construct a bias-free tandem water-splitting device (Figure 6(a)). The preparation method and structural characterization of the BiVO₄ photoanode were provided in the Supporting Information (Figure S2 and S3). The BiVO₄ photoanode supplied photogenerated carriers, while FeOOH used as a co-catalyst improved charge separation and transfer efficiency. In addition, FeOOH promoted the oxygen evolution reaction, enhancing the catalytic efficiency of the photoanode. When combined with the CuO/CuBi₂O₄-200 photocathode, the tandem system delivered a maximum photocurrent density of 0.56 mA/cm² under solar illumination (Figure 6(b)), demonstrating its capability to drive overall water splitting under completely bias-free conditions.

Figure 7 presents the optical absorption characteristics and corresponding bandgap calculations for CuBi₂O₄ and CuO/CuBi₂O₄-200 photocathodes. As shown in the UV-vis diffuse reflectance spectra (UV-vis DRS) (Figure 7(a)), all electrodes exhibited prominent absorption features in the visible light range (300−900 nm). Notably, CuO/CuBi₂O₄-200 shows significantly higher absorption intensity compared to CuBi₂O₄, indicating that the CuO layer effectively enhances light-harvesting capability. Further analysis of the Tauc plot (Figure 7(b)) was performed to determine the optical bandgap. CuBi₂O₄ exhibited a bandgap of approximately 1.79 eV, while that of CuO/CuBi₂O₄-200 showed a slightly reduced value of 1.76 eV. This reduction of the bandgap facilitated light absorption in the visible region, thereby enhancing the excitation efficiency of photogenerated carriers.

To further elucidate the role of the CuO interlayer in the charge transfer process, the band structure of the composite photocathodes was analyzed using UV-vis and Mott-Schottky (M-S) testing. The UV-vis results indicated that the bandgap of CuBi₂O₄ was approximately 1.79 eV, while that of CuO is 1.81 eV (Figure S4). Additionally, the flat-band potential from the Mott-Schottky curves was used to estimate the Fermi levels (E_f) of CuBi₂O₄ and CuO (Figure 8(a) and (b)). Based on these results, the band structure diagram for the CuO/CuBi₂O₄ photocathode was constructed (Figure 8(c)). Based on the band structure, a mechanism for photogenerated charge carrier separation and transmission was proposed. Under illumination, CuBi₂O₄ generated photogenerated electrons and holes. However, its intrinsic band structure led to significant bulk and interfacial recombination, limiting the PEC efficiency. The introduction of the CuO interlayer provided a favorable energy-level alignment that facilitated the rapid extraction of photogenerated holes from CuBi₂O₄ into CuO, effectively inhibiting the recombination at the CuBi₂O₄ interface. These holes were subsequently transported through the FTO substrate to the counter electrode, where they participated in oxidation reactions. Simultaneously, the reduced recombination within the CuBi₂O₄ bulk allowed a greater number of electrons to migrate to the electrode surface and drive the hydrogen-evolution reaction. Thus, in the CuO/CuBi₂O₄ photocathode, CuO served as an efficient interfacial “bridge,” lowering the charge-transfer barrier and inhibiting electron-hole recombination. This synergistic interfacial modulation significantly improved carrier separation efficiency and enhanced the overall PEC performance of the photocathode.