Silicon photocathode functionalized with osmium complex catalyst for selective catalytic conversion of CO2 to methane

Synthesis and characterization of [Os] complexesMultiple nitrogen heterocyclic structure ligands of 5,5’-bis(trifluoromethyl)−2H,2’H−3,3’-bipyrazole (bpzH2) and 5,5′-bis(trifluoromethyl)−2H,2′H−3,3′-bi(1,2,4-triazole) (btzH2) were prepared according to literatures54,55. The designed triazole ligand has additional nitrogen heteroatoms that can be used as binding sites for CO2 substrate and proton to promote the proton-coupled electron transfer (PCET) process. Subsequently, these bipyrazole and triazole ligands reacted with Os3(CO)12 and 1,10-phenanthroline (phen) to form osmium complexes of przpOs and trzpOs (Fig. 1a). The successful preparation of przpOs and trzpOs was confirmed by hydrogen nuclear magnetic resonance (1H-NMR) spectroscopy (see Supplementary Fig. S1, S2), mass spectroscopy (MS) (see Supplementary Fig. S3) and Fourier transform infrared (FTIR) spectroscopy (see Supplementary Fig. S4). The 1H NMR spectroscopic analysis confirmed the presence of 32 protons in complexes przpOs and 30 protons in trzpOs, each associated with their respective ligands. Specifically, the 1H NMR spectrum of przpOs displayed a distinctive singlet at 6.76 ppm, attributed to the two protons on the 3,3′-bipyrazole ligand, while the methyl protons corresponding to the PhPMe2 ligands were observed at 0.76 (s, 12H). High-resolution mass spectrometry (HR-MS) studies revealed a molecular mass of 916.1683 for przpOs, compared to that of 918.1588 observed for trzpOs. The molecular ion peak signals of both complexes were prominent in the HR-MS analysis (see Supplementary Fig. S3), with minimal ion residue peaks suggesting their stability and resistance to decomposition into ion fragments. The stretching vibration signals of aromatic C-H and methyl C-H were confirmed in FTIR spectra, and the stretching vibration characteristics of the aromatic ring skeleton were observed (see Supplementary Fig. S4). The solid-state of przpOs was further confirmed by the single-crystal X-ray structural characterization (Fig. 1b, Supplementary S1). For the side view of przpOs, two P ligands are in the axial direction with Os-P lengths of 2.3388 and 2.3429 Å, respectively, and P-Os-P dihedral angle is 174.59°. Moreover, the phenyl group in the P ligand is parallel to the phen ligand due to the π-π stacking. From the top view, the Os atom locates at the center of the plane constructed by phen and bipyrazole ligands, and the distances between Os and N atoms are 2.0852, 2.0791, 2.0760, and 2.0649 Å, respectively (Fig. 1b). It is particularly noteworthy that the uncoordinated N4 atom in the structure of przpOs exposes the ideal binding sites for CO2 and proton substrates.Fig. 1: Synthesis and characterizations of [Os] complexes.a Synthesis of przpOs and trzpOs. b Single-crystal X-ray structure of przpOs from the side and top views (thermal ellipsoids: 30%). c UV-visible (UV-vis) absorption and photoluminescence (PL) spectra of 10-5 M przpOs and trzpOs in dichloromethane. d PL lifetimes of 10-5 M przpOs and trzpOs in N2 and CO2 atmosphere.The photophysical properties of przpOs and trzpOs were characterized by the ultraviolet-visible (UV-vis) absorption and steady-state photoluminescence (PL) spectroscopy in dichloromethane (CH2Cl2) solution. As shown in Fig. 1c, the absorption bands <350 nm mainly originated from the ligand π–π* transitions, while the absorptions ranging from 400 to 550 nm are ascribed to the metal-to-ligand charge transfer (MLCT) transitions. The board absorption bands across 550–700 nm region are attributed to their d–d transitions52. Consequently, the optical band gaps (Eg,op) of przpOs and trzpOs calculated from the edge of MLCT absorption peaks are 2.30 and 2.45 eV, respectively (see Supplementary Table S1). Interestingly, trzpOs has a larger Eg,op value than that of przpOs, indicating that its excited state possesses higher energy for transferring photogenerated charge to the CO2 substrate, potentially leading to more efficient CO2 reduction activity. This finding aligns with the results obtained from the electrochemical studies. The PL spectra of przpOs and trzpOs show board emissions centered at 839 and 783 nm, with quantum yields (QY) of 0.2% and 2.5%, respectively (Fig. 1c, Table S1). By contrast, trzpOs shows a blue-shift emission, which has a similar trend with its absorption character. Moreover, time-resolved PL spectra of przpOs and trzpOs in N2 saturated solution display average lifetimes (τN2) of 1445.4 and 1604.0 ns, respectively, which significantly decreased to 866.4 and 815.8 ns, respectively, after purging the solution with CO2 (Fig. 1d, Table S2). This result indicates that the excited state of [Os] complexes could deliver photogenerated electrons to CO2 substrate for its reduction process. Thus, the electron transfer constant (ket) from [Os] complex to CO2 is estimated to be 4.62 × 105 of przpOs and 6.02 × 105 s−1 of trzpOs, respectively (see Supplementary Table S2). Therefore, trzpOs have a faster electron transfer rate than that of przpOs, which is conducive to efficient photocatalytic CO2 reduction.Electrochemical activity of CO2 reductionCyclic voltammetry (CV) of przpOs and trzpOs in acetonitrile solvent exhibit two reversible redox peaks (E1 and E2) at 0.64 and −1.44 V versus normal hydrogen electrode (VNHE) and 0.82 and −1.44 V vs. NHE, respectively (Fig. 2a, Table S3). Then, the electrochemical redox bandgap (Eg,redox) was determined to be 2.08 and 2.26 eV, respectively, which has a similar trend of their optical bandgaps. Moreover, the E2 values are lower than the thermodynamic equilibrium required potentials for hydrogen evolution reaction (HER) and CO2-to-CH4 conversion18, enabling that [Os] complexes could be used as photocatalysts for H2 and CH4 generation via proton reduction and CO2R. Their electrochemical behaviors were conducted in pure N2 and CO2 saturated solvents to investigate their HER and CO2R activities. As shown in Fig. 2b, przpOs exhibits a reduction peak current (i0) of 2.7 μA at E2 potential in N2 atmosphere. After purging CO2, an irreversible reduction peak appeared at 1.3 VNHE, which is assigned to CO2R catalytic current (ic), and the corresponding current increased to 7.9 μA. In contrast, the trzpOs has a i0 of 2.3 μA at E2 with purging N2, and ic of 7.3 μA at 1.3 VNHE with purging CO2. In order to verify whether the catalytic process is related to protons, 1 mM trifluoroacetic acid (TFA) was added as a proton source. The catalytic current with protons (ic-H) of przpOs and trzpOs was significantly enhanced to 18.1 and 52.9 μA, respectively. The catalytic current enhancement of trzpOs with proton source is higher than that of przpOs, indicating that the N atoms on the triazole ligand provide binding site for protons to promote the PCET process, resulting in more efficient catalytic reduction. The results also imply that CO2R is a mass transfer-controlled reaction, meaning that the reaction process could be improved by enhancing mass transferring.Fig. 2: Electrochemical behaviors and electrochemical spectra of [Os] complexes.a CV curves of 1.0 mM of przpOs and trzpOs in electrolyte of 0.1 M n-Bu4NPF6 acetonitrile solution, in N2 atmosphere, at scan rate of 0.1 V s−1 and without iR correction. The arrows indicate the scanning direction. CV curves of przpOs (b) and trzpOs (c) in N2 and CO2 saturated electrolyte with the addition of 1.0 mM TFA. d Current-time curves of przpOs and trzpOs in N2 and CO2 at −1.4 VNHE, under chopped AM 1.5 G illumination. e Gas products of przpOs and trzpOs in CO2 at −1.4 VNHE determined by GC measurements. In situ UV-vis spectro-electrochemistry of 1.0 mM przpOs (f) and trzpOs (g) in N2 and CO2 saturated electrolyte at different applied potentials. The inset figure shows the proposed catalytic cycles.Gas chromatography measurements confirmed that the main electrocatalytic CO2R product of [Os] complexes at −1.4 VNHE was CH4, with trace of CO and H2 (Fig. 2d–f). The CO2R activity and selectivity of trzpOs are higher than przpOs, which is consistent with their electrochemical properties. It’s worth noting that both trzpOs and przpOs present repeatable photo-response currents for electrocatalysis CO2R at −1.4 VNHE, indicating that [Os] complex can be used as the light harvesting units as well as the CO2-to-CH4 conversion catalyst. Consequently, the first-order catalytic constant (kcat) of [Os] complex, usually referred to the turnover frequency (TOF) of catalyst, can be assessed by Eq. (1):$$\frac{{i}_{c}}{{i}_{0}}=\frac{{{{\rm{n}}}}}{0.4463}{\left(\frac{{RT}{k}_{{cat}}}{{Fv}}\right)}^{1/2}$$
(1)
where R is universal gas constant (8.314 J mol−1 K−1), F is Faradaic constant (96485 C mol−1), T is the temperature (298.13 K), v is the scan rate (0.1 V s−1), and n is the number of electrons involved in the catalytic reaction (8 for CO2-to-CH4 conversion). Therefore, the kcat of przpOs is 0.104 s−1 under neutral conditions, and kcat-H increases to 0.545 s−1 under proton source conditions with TFA. In contrast, trzpOs exhibits higher kcat of 0.122 s−1 without proton and kcat-H of 6.41 s−1 with proton source. Electrochemical analysis has revealed distinct differences in the kcat-H values of przpOs and trzpOs catalysts when an additional proton source is introduced. We hypothesized that an extra nitrogen atom on the 3,3’-bi(1,2,4-triazole) ligand in trzpOs serves as a proton-relay that facilitates the proton transfer for the sequential CO2 reduction process. The N atom can provide binding sites for protons, which was verified in subsequent comparative experiments. Under N2 atmosphere, as TFA was incrementally added to the acetonitrile solution of przpOs, its E2 peak became irreversible with a notable increase in the cathodic current, pointing towards a characteristic of the hydrogen evolution process (HER) (see Supplementary Fig. S5). Conversely, the HER current of trzpOs demonstrated a considerably greater enhancement compared to that of przpOs, thus confirming the superior proton-binding capability and showcasing trzpOs’ heightened proton-reduction activity.Potential-dependent UV−vis spectra of przpOs and trzpOs were carried out in the range of −1.0 to −2.0 V in N2 and CO2 saturated environment to detect the intermediates during the catalytic cycle (Fig. 2f, g). As the voltage decreases from −1.0 to −2.0 V, [Os] complexes in N2 atmosphere exhibit enhanced absorption signals at 372 and 294 nm, 360 and 290 nm, and bleaching signals at 483 and 453 nm for the przpOs and trzpOs, respectively, which is attributed to the formation of reduced state [Os] complexes. Furthermore, these signals decrease sharply after purging CO2 with other peaks appearing and gradually increasing, indicating the interaction between the excited state [Os] and CO2 to form CO2 adducts. Accordingly, we propose the catalytic cycle shown inset of Fig. 2f, g, the CO2 molecule firstly interacts with the N atoms on the open site of bipyrazole and triazole ligands, and further forms CO2 adduct via hydrogenation reaction. The CO2R products (e.g., CO, CH4) then desorb from active sites and diffuse to bulk solution. Compared with przpOs, trzpOs has a synergistic catalytic effect, which can promote the proton transfer from the solvent to the active sites via N atoms on triazole ligand, resulting in enhanced CO2R activity.Photoelectrochemical CO2 reduction on Si-based electrodeSubsequently, [Os] complexes were deposited on a p-n heterojunction Si/TiO2 photocathode to construct solar-driven CO2 conversion system. Si-based photocathode has advantages of element abundance, sunlight absorption, and high saturated photocurrent, as well as that its CB ideally straddles the required potential for CO2-to-CH4 conversion30,56. Herein, a black Si photocathode with a nano-porous surface was prepared by a PEC HF etching method developed in our previous works57,58, which provides a large specific surface area for the deposition of abundant catalysts. Moreover, n-type TiO2 layer was coated on the black Si surface by magnetron sputtering to form p-n heterojunction and protective layer, which can further improve the charge separation and stability of Si-based electrode33,38. The Si/TiO2/[Os] photocathodes were prepared by dropping the acetonitrile solution of the [Os] complexes onto the surface of Si/TiO2 electrode and drying at room temperature. The uniform distribution of Os and F elements on the surface of Si/TiO2/[Os] photocathode was confirmed by the scanning electron microscopy (SEM) and the corresponding energy dispersive X-ray spectroscopy (EDX) (see Supplementary Fig. S6). In the Si/TiO2/przpOs and Si/TiO2/trzpOs electrode, the composition of Os element is similar, 0.95 % and 1.01 %, respectively.Under the illumination of simulated sunlight (AM1.5 G, 100 mW/cm2), Si/TiO2/trzpOs photocathode exhibits enhanced photocurrents for CO2R with a gradually increasing of trzpOs catalyst (see Supplementary Fig. S7). The optimal Si/TiO2/trzpOs with 1.6 nmol/cm2 of trzpOs catalyst exhibits a high jph of −14.11 mA/cm2 VRHE without external bias (0.0 VRHE) and −25.8 mA/cm2 at potential of −0.3 VRHE, which are 140 and 92 times that of Si/TiO2 with jph of −0.02 mA/cm2 at 0.0 VRHE and −0.28 mA/cm2 at −0.30 VRHE, respectively (Fig. 3a, Table S4). Moreover, the onset potential (Eon) for photo-response current is positive-shifted from 0.24 VRHE of Si/TiO2 to 0.52 VRHE of Si/TiO2/trzpOs. In contrast, the Si/TiO2/przpOs photocathode shows a jph −4.11 mA/cm2 in N2 and −8.43 mA/cm2 in CO2 without external bias (Fig. 3a), which is lower than values performed by Si/TiO2/trzpOs electrode at the same conditions. As a systematic comparison, the Si/TiO2 without [Os] complex displays low photocurrents of −0.34 and −0.39 mA/cm2 at 0.0 VRHE in N2 and CO2 atmosphere, respectively (Fig. 3a). These results indicate that [Os] complexes are efficient catalysts for CO2R, manifesting with the increased photocurrent and mitigated voltage loss38.Fig. 3: PEC performance of [Os] complexes on Si photocathode.a LSV curves performed by Si/TiO2, Si/TiO2/przpOs and Si/TiO2/trzpOs electrode in N2 and CO2 atmosphere, in 0.5 M Na2SO4 (pH 6.8 ± 0.3) solution, at scan rate of 30 mV/s, with 1.6 nmol/cm2 of [Os] complexes and without iR correction. J-t curves (b) and Faradaic efficiency (c) of Si/TiO2, Si/TiO2/przpOs and Si/TiO2/trzpOs at 0 VRHE. The Faraday efficiency was tested three times repeatedly. d GC-MS analysis of Si/TiO2/trzpOs in 0.5 M Na2SO4 (pH 6.8 ± 0.3) solution with 13CO2 and CO2, respectively.Current-time (j-t) measurements were carried out to evaluate the CO2R stability of Si/TiO2/[Os] photocathodes under continuous illumination of AM 1.5 G. As shown in Fig. 3b, the photocurrent densities of Si/TiO2/[Os] photocathodes stabilize within 6000 s. Meanwhile, gas products were monitored by gas chromatography (GC) analysis, presenting H2, CO, and CH4. Accordingly, the Si/TiO2 photocathode without [Os] complex exhibits a high Faradaic Efficiency of H2 (FEH2) over 95% and a low Faradaic Efficiency of CO product (FECO) (Fig. 3c), which is consistent with the reported results of a mainly CO product from Si/TiO2-based electrodes (see Supplementary Table S5). After the deposition of trzpOs catalyst, a main product of CH4 is detected with a high FECH4 of 91.8 ± 3.1%, while FECO and FEH2 are only 6.5 ± 3.5% and 1.7 ± 1.8%, respectively (Fig. 3c). Besides, the Si/TiO2/przpOs also show a high FECH4 of 89.8 ± 3.0%, and low FECO and FEH2 of 8.6 ± 3.1% and 1.5 ± 1.3%, respectively (Fig. 3c). These results indicate that [Os] complexes are efficient CO2-to-CH4 conversion catalysts. In 13CO2 isotope experiments,13CH4 and 13CO products with m/z of 17 and 29 are detected for the Si/TiO2/trzpOs at 0.0 VRHE in 13CO2 atmosphere, in 0.5 M Na2SO4 solution, and under illumination (Fig. 3d). In contrast, CH4 and CO products with m/z of 16 and 28 are detected in a CO2 atmosphere at the same conditions. Gas chromatograph-mass spectrometry (GC-MS) results determine that CH4 and CO products are generated from the reduction of CO2 molecules. To the best of our knowledge, this is the first report of PEC systems using [Os] complex as CO2R catalyst, which exhibits ultra-high activity and CH4 product selectivity, exceeding all previously reported Si-based photocathode for CO2R (see Supplementary Table S5).Stability of photocathode and [Os] catalystAfter the PEC tests, the cross-sectional structure of Si/TiO2/przpOs and Si/TiO2/trzpOs electrodes were characterized using SEM images and the related elemental mappings. As illustrated in Fig. S8, the post-PEC CO2 reduction displayed a distinct TiO2 protection layer and an [Os] catalyst layer, signifying the structural stability of the photoelectrodes. The XRD pattern exhibited characteristic diffraction peaks of TiO2 and Si were observed, while no distinctive diffraction signals was observed for the [Os] catalysts due to their low loading. Notably, after the PEC test, there were no significant changes in the diffraction peaks of both the electrodes (see Supplementary Fig. S9a). Furthermore, the [Os] catalysts deposited on the Si/TiO2 electrodes were eluted by acetonitrile, and UV-vis spectroscopic studies indicated that the solution contained przpOs and trzpOs (see Supplementary Fig. S9b, c). These experimental findings confirmed that the [Os] complex remained stable on the Si- electrode.X-ray photoelectron spectroscopy (XPS) was used to compare the element composition and electronic states of Si/TiO2/[Os] electrode before and after PEC CO2R measurements. In Fig. 4a, XPS survey spectra verified the existence of elemental Os, F, P, N, C, O, Si in Si/TiO2/przpOs and Si/TiO2/trzpOs electrodes. In high-resolution XPS spectra, przpOs shows two peaks at 50.4 and 53.2 eV, corresponding to Os 4f7/2 and 4f5/2 spin-orbit levels, respectively (Fig. 4b). While, trzpOs displays lower binding energies for its 4f7/2 and 4f5/2 peaks, measured at 50.2 eV and 52.9 eV, respectively. The binding energy shift is attributed to the enhanced electron-donating capacity of the triazole ligand. It is worth noting that the binding energy of the [Os] complex deposited on the silicon electrode differs from that of electrodeposited Os metal reported in the literature59. Fig. 4c presents the high-resolution XPS spectra of F 1 s, where the peak at 687.6 eV is attributed to the spin-orbit of F 1 s in CF3 group. Furthermore, Fig. 4d–f exhibit the high-resolution XPS spectra of C 1 s, N 1 s, and O 1 s, respectively, confirming the existence of C, N, O elements from the [Os] complexes. Following PEC measurements, the Si/TiO2/trzpOs electrode demonstrates comparable binding energies and peak intensities for Os, F, C, N, and O elements (Fig. 4b–f). These results indicate that the trzpOs complex exhibits excellent tolerance for CO2 reduction.Fig. 4: Elemental composition and oxidation state of the electrode before and after PEC test.XPS survey spectra (a) and high-resolution XPS spectra of Os 4 f (b), F 1 s (c), C 1 s (d), N 1 s (e) and O 1 s (f) for Si/TiO2/przpOs and Si/TiO2/trzpOs before and after PEC measurements.Catalytic mechanismDensity functional theory (DFT) calculations were performed to investigate the structure-property relationship. The absorption spectra and photoluminescence spectra of przpOs and trzpOs were simulated. As shown in Fig. S10, przpOs and trzpOs exhibit maximum absorbance centered (λabs) at 423 and 393 nm, respectively, with corresponding maximum emission peaks at 845 and 821 nm. These calculated results align well with the experimental spectra, indicating that trzpOs possesses a wider bandgap than przpOs. Energy level calculations depicted in Fig. S11 reveal that przpOs and trzpOs have LUMO values of −2.16 eV and −2.39 eV, and HOMO values of −4.67 eV and −4.99 eV, respectively. Furthermore, trzpOs demonstrates a larger dipole moment (μ) of 18.23 D compared to 12.77 D for przpOs.The frontier orbital distributions of the ground (S0) and excited state (T1) of the Os complexes are explored by DFT calculations (see Supplementary Fig. S12). In the S0 state, the HOMO orbits of przpOs and trzpOs predominantly reside on the 3,3′-bipyrazole and 3,3′-bi(1,2,4-triazole) ligands, while the LUMO orbits are predominantly situated on their phenanthroline ligands. Notably, trzpOs exhibits extensively delocalized HOMO compared to przpOs. Upon excitation, an electron transitions from the original HOMO orbit to the original LUMO orbit, maintaining the LUMO orbital distribution in the excited state, while the HOMO orbital becomes further dispersed. Remarkably, in both the ground and excited states, trzpOs displays more dispersed HOMO orbitals than przpOs, with the same N atom at position 4 exhibiting a distinct electron cloud distribution, making it a favorable binding site for CO2 reactions. Furthermore, electrostatic potential distributions reveal negative charge accumulations on the same N4 atom of przpOs and trzpOs (see Supplementary Fig. S12), indicating a conducive site for the nucleophilic attack reaction with CO2 substrate.The conversion pathway of CO2 initial from the ground state (S0) or triplet excited state (T1) of przpOs and trzpOs photocatalysts was further studied by DFT calculations18. As shown in Figs. S13–S20, the optimized adsorption configurations of [Os] complexes are shown for each intermediate, such as *CO2, *COOH, *CO, *CHO, *CH2O, *CH3, and *CH4, with C atoms binding to the electron deficient N atoms on the bipyrazole and triazole ligands of przpOs and trzpOs, respectively. Notably, when CO2 adsorption occurs in the ground state of przpOs and trzpOs, the Gibbs free energy changes (∆G) are +0.20 and +0.08 eV, respectively. Subsequently, converting the adsorbed *CO2 in ground state (S0) to *COOH is an endothermic reaction that must overcome an energy barrier of +2.03 and 1.91 eV for przpOs and trzpOs catalysts, respectively (Fig. 5c). Oppositely, when [Os] complex is in T1 state, according to its long luminescence lifetime as shown in Fig. 1d, the Gibbs free energy for CO2 adsorption decreases to −0.07 and −0.04 eV for przpOs and trzpOs, respectively. Correspondingly, the required energy for the conversion of *CO2 to *COOH is dramatically decreased to +0.34 and +0.25 eV, respectively. These results suggest that the formation of *COOH intermediates is the rate-limiting step for further hydrogenation processes60, and [Os] complexes tend to interact with CO2 substrates through excited states and drive the subsequent conversion process.Fig. 5: DFT calculations of the CO2 conversion pathway and the transition states of *CO2 protonation.Calculated adsorption configurations of CO2 and reactive intermediates on przpOs (a) and trzpOs (b). a Gibbs free energy diagrams for CO2 adsorption and CO2 reduction to CH4 by przpOs and trzpOs catalysts. Transition states of przpOs (b) and trzpOs (c) via directly protonation (Pathway I) and via nitrogen heteroatom ferrying (Pathway II).The subsequent CO2 hydrogenations, for instance, the kinetic protonation process of adsorbed *CO2, are the crucial steps of the CO2R reaction. Since the CO2R reaction was carried out in aqueous, hydrated proton is used as the proton source for the converting *CO2 to *COOH. As shown in Figs. 5b and S21, przpOs adopts a directly protonation of *CO2 (Pathway I) with a ∆G of +0.44 eV. In contrast, trzpOs can protonate the N atom on triazole ligand first and then transfer protons to *CO2, which is a two-step protonation via nitrogen heteroatom ferrying (Pathway II). The transition state energy barrier of two-step protonation of trzpOs is only +0.40 and +0.04 eV, which is more favorable than the direct protonation process (Figs. 5c, S22). These results confirms that the extra nitrogen atom on triazole ligand could serve as a proton-relay to facilitate the proton transfer for the sequential CO2 hydrogenations. Afterwards, the intermediate exhibits a downhill free energy change at present of protons and electrons, e.g., the transformation of *CO to *CH4 and the release of CH4 are thermodynamically exothermic reactions which can be spontaneously carried out by receiving photogenerated electrons from the Si photocathode (Fig. 5a). DFT results indicate that CH4 is the most easily desorbed product from the [Os] complexes, resulting in the high selectivity for CO2-to-CH4 conversion. In contrast, the required energy for the hydrogenation process from *COOH to *CH3 of trzpOs is smaller than that of przpOs.Mott-Schottky plots of the Si, Si/TiO2 and Si/TiO2/[Os] electrodes exhibit a negative slope in the potential window of 0.2–0.6 VRHE due to the p-type Si semiconductors (see Supplementary Fig. S23). By extrapolating the linearly fitted lines of these plots to 0 of 1/C2, the flat potential (Efb) is obtained from the intercept. Compared with the Efb of 0.40 VRHE for the bare black Si electrode, the Efb of Si/TiO2 increases to 0.55 VRHE due to the formation of p-n heterojunction. Additionally, Si/TiO2/przpOs and Si/TiO2/trzpOs exhibit positive-shifted Efb of 0.58 and 0.65 VRHE (see Supplementary Fig. S23), respectively, indicating that the deposited [Os] complex can effectively gather charge from the Si electrode, ensuring efficient charge separation. Moreover, analysis of XPS valence spectra reveals a 0.033 and 0.404 eV difference of valence band (VB) and Fermi level (EVB-EF) for Si/TiO2/przpOs and Si/TiO2/trzpOs, respectively (see Supplementary Fig. S24). The difference between Efb and EF leads to a variation trend of conduction band (CB) and VB levels at the space charge region. Conclusively, the energy level structure diagram of the Si/TiO2/[Os] material was drawn by combining the results of electrochemistry and spectroscopy. Notably, in the absence of [Os] catalysts, the HER predominates at the electrode using electrons from the Si/TiO2 photocathode (Fig. 6a).Fig. 6: Schematic diagram of energy levels of Si-based photocathode.Solar-driven CO2-to-CH4 conversion on the Si/TiO2 electrode in the absence (a) and presence (b) of [Os] complex.In the conversion of CO2 to methane, multiple electron and proton transfer steps involving 8 electrons are necessary. Consequently, the [Os] catalyst must continuously accept electrons from the Si semiconductor. DFT results indicated that the initial reduction of *CO2 to *COOH is an energy-consuming process and serves as the rate-determining step for CO2 reduction60. Leveraging the [Os] complexes as photocatalyst, the excited [Os] catalysts can lower the energy barrier required for the conversion of *CO2 to *COOH, facilitating the crucial first step of the CO2 reduction reaction. As depicted in Fig. 6, the Si/TiO2/[Os] displays a Z-scheme heterojunction between Si/TiO2 semiconductor and [Os] complexes, which enables the CO2R at the LUMO. The HOMO of [Os] complex accepts the photogenerated electrons from the CBs of Si and TiO261. Based on the energy level heterojunction structure, trzpOs exhibits a higher propensity to acquire electrons from Si/TiO2 compared to przpOs, aligning with catalytic activity findings. As the subsequent conversion processes are thermodynamically energy-neutral and the [Os] catalyst intermediates lack sufficient time to capture photons for excited state generation, the [Os] catalyst only accepts electrons from the Si electrode during the intermediate conversion stages rather than functioning as a photosensitizer. Consequently, the generated [Os]-COOH adducts continue to receive electrons from the Si-based electrode, propelling subsequent conversion steps until methane release occurs.To further understand the interfacial charge and mass transfer processes of Si-based photocathode with [Os] catalysts, electrochemical impedance spectroscopy (EIS) measurements were performed under realistic catalysis conditions with the results presented as Nyquist plots (see Supplementary Fig. S25). Si/TiO2/[Os] demonstrated reduced charge-transfer resistance (Rct) and larger interfacial electrochemical double-layer capacitance (Cdl) in comparison with Si/TiO2 photocathode (Table S6). Cdl represents a quantitative parameter to elucidate charge accumulation at the interface. This result indicates the importance of [Os] complexes in accumulating electrons from semiconductor and improving charge transfer at the electrode-electrolyte interface. Furthermore, the Si/TiO2 electrode functionalized with trzpOs exhibited a lower Rct than its przpOs counterpart, aligning with its superior PEC CO2R catalytic efficiency.