The excessive combustion of fossil fuels results in the production of CO₂, contributing to the greenhouse effect and exacerbating energy scarcity. To address this issue, it is essential to reduce carbon dioxide emissions, advance the utilization of renewable energy sources, and decrease reliance on conventional fuels^[1−3]. Photocatalytic CO₂ reduction is a green technology that converts carbon dioxide into valuable chemicals and fuels^[4]. Through the photocatalytic process, CO₂ can be reduced to a variety of products, including monocarbon compounds (CO^[5], CH₄^[6], CH₃OH^[7], HCOOH^[8]) and polycarbon compounds (C₂H₄^[9], C₂H₆^[10], CH₃CHO^[11]). The core challenge in photocatalysis technology is the development of efficient photocatalytic systems. Most photocatalysts exhibit high surface charge recombination and low energies for CO₂ adsorption and activation, leading to inefficient CO₂ conversion and low selectivity for reduction products.

Over the past decade, bismuth-containing compounds have garnered significant attention as promising candidates for photocatalytic applications. A diverse array of bismuth-based materials has been extensively investigated, including, but not limited to, bismuth oxyhalides (BiOX, where X represents Cl, Br, or I)^[12−14], bismuth molybdate (Bi₂MoO₆)^[15−16], bismuth vanadate (BiVO₄)^[17], bismuth manganese oxide (Bi₁₂MnO₂₀)^[18], bismuth silicate (Bi₂SiO₅)^[19], bismuth tungstate (Bi₂WO₆)^[20] and bismuth oxycarbonate (Bi₂O₂CO₃)^[21]. These materials have demonstrated remarkable potential in various photocatalytic reactions, thereby paving the way for innovative solutions in environmental remediation and energy conversion. Among these photocatalysts, Bi₂MoO₆ (BMO) stands out because of its non-toxic properties, simple preparation process, and economical raw materials. Its layered structure and appropriate band gap further enhance its promise as a photocatalyst^[22−23]. As a member of the layered Aurivillius oxide family, Bi₂MoO₆ is made up of alternating [MoO₄]²⁻ and [Bi₂O₂]²⁺ layers. Additionally, given its adjustable band gap ranging from 2.5 to 2.8 eV, BMO is capable of more efficiently absorbing visible light and also offers sufficient reaction sites to enhance photocatalytic activity^[7]. Key factors affecting photocatalytic efficiency include the semiconductor’s band gap, which influences solar light absorption, the speed of photogenerated carrier transfer affecting redox reactions, and the conduction band’s position for thermodynamic feasibility. However, single BMO development is limited by weak visible light absorption, slow carrier migration, rapid recombination of electron-hole pairs and low quantum yield^[24].

There remains significant potential for the modification and exploration of BMO to improve its photocatalytic performance. Various strategies can be employed to enhance the photocatalytic activity of BMO. The formation of heterojunctions is particularly effective in facilitating light absorption and charge transfer^[25−26]. Introducing oxygen vacancies can alter the band gap of BMO, thereby creating additional active sites for photocatalysis^[27−29]. Elemental doping is another useful approach for introducing impurity energy levels and enhancing the intrinsic activity of BMO^[30−31]. Due to its unique variable band gap, BMO can be easily combined with other semiconductor materials to form heterojunctions. The integration of BMO-based semiconductor heterojunctions is advantageous for extending the range of visible light absorption. To overcome existing limitations, researchers often utilize heterojunction construction and noble metal modification to enhance photocatalytic performance. For instance, BiOBr (BOB), with its layered structure, facilitates electronic transfer and improves photocatalytic activity^[32−34]. The integration of BMO/BOB heterojunctions significantly enhances catalytic efficacy, as demonstrated in previous studies^[35−37]. Additionally, precious metal-modified semiconductor catalysts have garnered significant attention in research and application due to their excellent electrical conductivity and unique optical properties. For example, the incorporation of precious metals such as platinum (Pt)^[38], palladium (Pd)^[39−40], silver (Ag)^[41−42], and others has been shown to enhance electron transfer and introduce Schottky barriers, thereby inhibiting the recombination of photogenerated electron-hole pairs and ultimately improving the photocatalytic efficiency. Among these, Ag has attracted significant interest because of its unique characteristics and cost-effectiveness. The localized surface plasmon resonance (LSPR) effect observed in Ag NPs can substantially enhance the visible light absorption and utilization of catalysts, facilitating the generation of active species, promoting carrier separation efficiency, and accelerating carrier regeneration, thereby boosting photocatalytic performance^[43−50]. Moreover, traditional powder-like photocatalysts suffer from drawbacks such as difficult solid-liquid separation, aggregation, and potential secondary pollution. Addressing these challenges, this study utilizes bismuth plates as substrates for the in situ preparation of bismuth-based films via electrochemical methods. Therefore, this approach, combined with the construction of Bi-based heterojunctions and the decoration of catalysts with Ag NPs, represent effective strategies for enhancing the performance of photocatalytic CO₂ reduction.

In this research, we utilized the photodeposition method to deposit the precious metal silver onto the prepared BMO/BOB film, leading to the creation of an Ag/BMO/BOB composite film that exhibited enhanced efficiency in the photocatalytic reduction of CO₂ into CO. In order to study the effect of different Ag deposition amounts on the photocatalytic CO₂ reduction performance, we deposited silver with different mass fractions onto the BMO/BOB composite film and subsequently evaluated its performance in the photocatalytic reduction of CO₂ to CO. The film was immobilized on a Bi plate, effectively addressing issues commonly associated with powder catalysts such as poor dispersion and separation difficulties in solid-liquid systems. This immobilization technique not only overcomes these challenges but also promotes environmentally friendly photocatalytic CO₂ reduction. We conducted comprehensive characterization and CO₂ reduction performance tests on both single and composite films. Based on the characterization data, we propose potential mechanisms for film formation and CO₂ photoreduction. The study further elucidates a plausible mechanism for the photocatalytic reduction of CO₂, occurring over the Z-scheme heterojunction of the Ag/BMO/BOB film. Our findings hold significant implications for catalyst fixation and the development of highly efficient Bi-based halide Z-scheme heterostructures to boost CO₂ photoreduction.

The crystal structure of the synthesized films is analyzed by XRD, as shown in Figure 2. The positions of the principal diffraction peaks of the synthesized BiOBr aligned with those in the standard reference card PDF#78-348 for BiOBr. Similarly, for the Bi₂MoO₆ thin film, the figure clearly shows that each peak matched the standard reference card PDF#21-102 for Bi₂MoO₆. A series of diffraction peaks at 10.896°, 28.252°, 32.612°, 33.098°, 47.15°, 55.559° and 58.432° can be assigned to the (020), (131), (002), (060), (062), (133) and (262) planes of Bi₂MoO₆. For the XRD pattern of the BMO/BOB composite thin film, the peaks at 10.900°, 21.928°, 32.220° and 46.865° are indexed to the (001), (002), (110) and (113) planes of BiOBr. Correspondingly, the peaks at 10.927°, 27.334°, 56.251° and 55.585° could all be attributed to the (020), (140), (191) and (133) crystal planes of Bi₂MoO₆. In addition, XRD was carried out on the composite films prepared using various mass fractions of deposited Ag NPs. The resulting diffraction patterns, illustrated in Figure 2, demonstrate that the photodeposition of Ag at various mass loadings does not induce significant structural changes in the prepared composite film. This suggests that the crystal structure of the BMO/BOB film was not altered after the depositon of Ag NPs. No characteristic diffraction peaks of Ag NPs were detected in the Ag/BMO/BOB films, which is typically caused by the low Ag loading content^[41] and its high dispersion^[52].

XPS was used to verify the successful deposition of Ag on the 1.5% Ag/BMO/BOB composite film, and the results are compared with the unmodified BMO/BOB film (Figure 3). The presence of Mo, O, Br, Bi and Ag in the 1.5% Ag/BMO/BOB composite is verified by the survey spectrum (Figure 3(a)), which validates the successful photodeposition of Ag. High-resolution spectra further delineate characteristic peaks for Mo 3d (Figure 3(b)), O 1s (Figure 3(c)), Br 3d (Figure 3(d)), Bi 4f (Figure 3(e)) and Ag 3d (Figure 3(f)). The Mo 3d XPS spectrum (Figure 3(b)) showed a doublet with peaks at 231.44 (Mo 3d_5/2) and 234.57 eV (Mo 3d_3/2), characteristic of Mo^6+[7]. The O 1s spectrum ((Figure 3(c)) of BMO exhibited peaks at 529.17 and 530.61 eV, attributed to lattice oxygen and surface hydroxyl groups, respectively^[51]. In Figure 3(d), the Br 3d peaks located at 67.53 and 68.52 eV were associated with Br^– in Br 3d_5/2 and Br 3d_3/2^[53]. The Bi 4f spectrum (Figure 3(e)) showed the characteristic doublet of Bi³⁺ at 158.96 (Bi 4f_7/2) and 164.35 eV (Bi 4f_5/2) ^[27]. Similarly, the Ag 3d region (Figure 3(f)) exhibited peaks at 368.18 (Ag 3d_5/2) and 374.18 eV (Ag 3d_3/2), with a characteristic 6 eV splitting, corresponding to the presence of metallic silver (Ag⁰)^[54].

Compared with the BMO/BOB composite film, the XPS peaks of all the elements in the Ag-deposited composite film catalyst show the slight shifts toward to the lower binding energies. This negative shift primarily stems from the increased electron density on the surface of the main heterojunction catalyst, revealing that the incorporation of Ag alters the chemical environment of the BMO/BOB heterojunction composite film. Here, Ag acts as an electron donor, transferring electrons to the host catalyst, which further confirms the successful incorporation of Ag^[29]. This comprehensive XPS analysis provides compelling evidence for the formation of a well-integrated composite film with synergistic interactions among its components.

To verify the effective deposition of Ag and to detail the morphological traits of the composite films, SEM and TEM analyses are carried out. SEM micrographs (Figure 4(a), (b)) reveal a distinctive fusiform morphology for the composite film with 1.5% Ag/BMO/BOB. The flake-like structures was clearly visible at higher magnification (Figure 4(b)) correspond to the morphological composition of the BMO/BOB composite films.

In contrast, a pure BiOBr film prepared under identical conditions exhibited a sheet-like morphology (Figure 4(c)). Cross-sectional SEM images (Figure 4(f)) revealed that the composite film with 1.5% Ag/BMO/BOB possesses a thickness of approximately 41 µm. TEM analysis (Figure 4(d)) identifies lattice fringes with d-spacings of 0.230 nm (Ag (111)) ^[55] and 0.328 nm (Bi₂MoO₆ (131)). In addition, the (102) plane of BiOBr is linked to lattice fringes with d-spacings of 0.2842 and 0.2858 nm. Figure 4(e) directly shows the distribution of Ag nanoparticles on the composite film. EDX mapping (Figure 4(g)) confirms the homogeneous distribution of Bi, Mo, O, Br and Ag throughout the composite film, thereby verifying the successful synthesis for the composite film with 1.5% Ag/BMO/BOB.

The photocatalytic performance of composite films with different Ag contents (0.5%, 1.0%, 1.5% and 2.0%) is evaluated for CO₂ reduction under 300 W xenon lamp irradiation. Figure 5(a) presents a comparison of the CO yield from these composites with that of unmodified BMO, BOB and BMO/BOB films. The CO production rates for the single BMO, BOB and BMO/BOB composite films are determined to be 10.45, 8.64 and 11.71 μmol/(g·h), respectively. The BMO/BOB composite outperforms single-component films, while Ag modification further enhances catalytic activity. The yields for Ag/BMO/BOB composite films with varying amounts of Ag mass fractions (0.5%, 1.0%, 1.5% and 2.0%) are recorded as 11.99, 12.31, 13.65 and 12.44 μmol/(g·h), respectively. By optimizing the amount of deposited Ag, the Ag/BMO/BOB composite film exhibits a significantly higher activity for CO₂ reduction to CO than the pure BMO/BOB composite film. Detailed analysis shows that the composite film’s photocatalytic performance initially rises with Ag deposition, hits a maximum, and then falls. A maximum CO yield of 13.65 μmol/(g·h) is attained by the film with 1.5% Ag loading. This enhancement can be attributed to the optimal Ag loading, which functions as an electron-trapping cocatalyst, thereby increasing the surface electron concentration^[12]. Conversely, excessive Ag loading compromises photocatalytic efficiency, as the surplus Ag not only forms recombination centers for charge carriers^[56−57] but also agglomerates into larger particles, reducing the effective contact area and active sites^[58]. Figure 5(b) illustrates the transient photocurrent responses, revealing that the 1.5% Ag/BMO/BOB film demonstrates the highest current density and superior light responsiveness. This observation indirectly demonstrates that the composite film containing 1.5% Ag/BMO/BOB possesses the most efficient photogenerated electron-hole pair separation capability, resulting in a greater number of electrons participating in reduction processes^[59].

As shown in Figure 5(c), the electrochemical impedance spectroscopy (EIS) reveals that the arc radius is inversely proportional to the material’s conductivity. The Nyquist circle diameter of the composite film with 1.5% Ag/BMO/BOB is significantly smaller than that of the BMO/BOB composite film, indicating lower interface charge transfer resistance in the former. This reduction in resistance facilitates the migration and separation of photogenerated carriers. The smaller arc radius observed for the Ag/BMO/BOB composite film indicates its enhanced ability to effectively separate photogenerated electron-hole pairs^[54,60].

Both the photocurrent and impedance analyses confirm that the Ag/BMO/BOB composite film further demonstrates superior carrier separation efficiency, thereby augmenting the photocatalytic efficiency of the composite material. This enhancement in carrier separation is a crucial factor contributing to the improved CO₂ to CO photocatalytic reduction performance observed in the composite film following noble metal Ag deposition.

The films’ optical absorption characteristics were examined using UV-vis diffuse reflectance spectroscopy (Figure 6(a)). It can be observed that the intrinsic absorption edges of Ag/BMO/BOB, BMO/BOB and BOB films are largely consistent, with Ag/BMO/BOB exhibiting a minor red shift. Furthermore, the Ag/BMO/BOB composite film demonstrates enhanced light absorption intensity in the visible region (400–800 nm). This phenomenon can be attributed to the localized surface plasmon resonance (LSPR) effect of Ag NPs. The characteristic LSPR absorption peak of Ag is located around 400–430 nm, which overlaps with the intrinsic absorption edge of the semiconductor itself. As a result, no distinct characteristic peak was observed in Figure 6^[61−62]. Additionally, the existence of irradiation-induced hot electrons on Ag NPs promotes the transfer of photogenerated carriers, thus enhancing CO₂ photocatalysis^[63].

The band gap energy is determined from the Tauc relation, (αhν)^1/n =A (hν−E_g), where the symbols α, h, ν, E_g and A denote the absorption coefficient, Planck constant, photon frequency, energy gap of photons, and a constant. The value of n is 1/2 for direct and 2 for indirect transitions^[64]. Both BOB and BMO are the typical indirect band gap semiconductors. Based on the Tauc plots in Figure 6(b), (c), the direct band gaps of the BMO and BOB films are determined to be 2.58 and 2.71 eV, respectively, obtained by extrapolating the linear portion of the (αhν)^1/2 vs. hv plots. The Mott-Schottky plots, presented in Figure 6(d), (e), reveal a positive slope relationship between the potential values and 1/C² for both BMO and BOB films, indicative of their n-type semiconductor characteristics^[40,65]. The BMO film demonstrates a flat band potential (E_FB) of −0.31 eV regarding the Ag/AgCl standard electrode. When referenced to the normal hydrogen electrode (NHE), the corresponding potential is −0.09 eV (equivalent to + 0.22 V vs. NHE)^[66]. As depicted in the VB-XPS spectrum of the BMO film (Figure 6(f)), the energy gap between the valence band and Fermi level is 1.76 eV. Given the fact that the flat band potential (E_FB) corresponds to the Fermi energy level (E_f), the E_VBM of the BMO film has been definitively ascertained to be 1.67 eV^[67]. Utilizing the formula E_g=E_VB−E_CB^[18], the conduction band value for BMO is calculated to be −0.91 eV. For the BOB film, we employ the equations E_VBM=X−E_C+0.5E_g and E_g=E_VB−E_CB to determine its valence and conduction band energies. The calculations yielded E_VB=3.025 and E_CB=0.315 eV, where X represents the absolute electronegativity of the element (X=6.17), and E_C denotes the electrode potential of a standard hydrogen electrode for free electrons (E_C=4.5)^[68].

Drawing from the aforementioned data analysis, we propose a Z-scheme heterojunction photocatalytic reduction mechanism for CO₂, as depicted in Figure 7. Electrons in the valence bands of BMO and BOB were excited under conditions of simulated solar irradiation and subsequently migrated to their respective conduction bands, leaving corresponding holes in the valence bands. The noble metal Ag, through its localized surface plasmon resonance (LSPR) effect, broadens the material’s light absorption spectrum. Additionally, the high work function of Ag acts as an electron trap, successfully promoting the electron transfer from the semiconductor to the noble metal^[69]. The Ag NPs serves as a charge-transfer bridge, accepting electrons from the BOB valence band and facilitating their recombination with holes in the BMO valence band. This critical step preserves the valuable electrons and holes with higher redox potential, which in turn enhances photocatalytic performance.

The computational analysis reveals that the electron valence band (E_VB) of the BOB film exhibits a more positive potential compared to the oxidation potential for H₂O/O₂ (E^ɵ = +0.82 V vs. NHE)^[40]. Concurrently, the electron conduction band (E_CB) of the BMO film displays a potential that is more negative than the reduction potential of CO₂/CO (E^ɵ = −0.53 V vs. NHE)^[70]. These energetic configurations are sufficiently favorable to facilitate the occurrence of redox reactions, thereby enabling the reduction of CO₂ to CO. Therefore, in the BMO conduction band, electrons facilitate the reduction of CO₂ (CO₂ + 2H⁺ + 2e⁻→ CO + H₂O), and in the BOB valence band, holes are involved in the oxidation of H₂O (2H₂O + 4h⁺ → 4H⁺ + O₂)^[23,71]. Based on the results, the deposited Ag plays a dual function. Firstly, it forms a Z-scheme heterojunction for efficient charge separation. Secondly, the LSPR effect induced by Ag NPs broadens the light absorption range. These combined effects lead to a significant improvement in photocatalytic performance. In the Ag/BMO/BOB composite films, the Schottky barrier function of the Ag NPs inhibits reverse charge transfer, thereby enhancing the stability of the photocatalytic system^[72]. This study provides critical insights into the rational design of multicomponent semiconductor-based photocatalytic systems for CO₂ reduction, facilitating future advancements in sustainable energy technologies.

In summary, an efficient Ag/BMO/BOB composite film for photocatalytic CO₂ reduction was successfully synthesized using an electrochemical method with a Bi plate substrate. The composite films, modified with 1.5% Ag, exhibited significantly enhanced photocatalytic performance, yielding an optimal CO production rate of 13.65 μmol/(g·h). Photocurrent and impedance measurements corroborate the superior charge separation in the Ag-modified composites. The proposed Z-scheme mechanism elucidates the synergistic interactions among the components, thereby providing a scientific basis for the rational design of advanced photocatalysts and the development of immobilized systems for CO₂ reduction.