Interfacial Ce-S bonds enhanced Mo-doped ZnIn₂S₄/oxygen-deficient CeO₂ S-scheme heterojunction for efficient photocatalytic overall water splitting

1.1 Catalyst synthesis and characterization

Mo-ZIS/O_V-CeO₂ is constructed by incorporating Mo doping into ZnIn₂S₄ and combining oxygen-vacancy-rich CeO₂ through a two-step hydrothermal method (Scheme 1). For comparison, pristine ZnIn₂S₄ (ZIS), ZnIn₂S₄ with Mo doping (Mo-ZIS), CeO₂ with O vacancies (O_V-CeO₂), and ZIS/O_V-CeO₂ are also prepared. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) are employed to analyze the structures and morphologies of all catalysts. The results reveal that Mo-ZIS exhibits a dense flower-like morphology similar to that of ZIS, featuring a lattice fringe of 0.32 nm corresponding to the (102) plane of ZnIn₂S₄ (Figure 1A–C and Supporting Information S1: Figure S1).²⁸ The O_V-CeO₂ nanorods have lengths of 100–500 nm and diameters of ∼10 nm, with an interlayer distance of 0.31 nm assigned to the (111) lattice plane of CeO₂ (Figure 1D–F).²⁹ O_V-CeO₂ nanorods are loaded onto the flower-like Mo-ZIS surface, and the coexistence of the (102) plane of ZnIn₂S₄ and the (111) plane of CeO₂ demonstrates a tight contact at the heterogeneous interface in Mo-ZIS/O_V-CeO₂ (Figure 1G–I).^{29, 30} Energy dispersive X-ray (EDX) images of Mo-ZIS/O_V-CeO₂ show uniform distribution of Mo, S, Zn, In, Ce, and O atoms within the structure (Figure 1J and Supporting Information S1: Figure S2).

Raman and X-ray photoelectron spectroscopy (XPS) are also effective methods for characterizing the structures of all samples. In the Raman spectrum of Mo-ZIS, excepted for the LO₁, TO₂, and A_1g characteristic peaks of ZIS, a vibration peak attributed to the Mo-S bonds is detected at 405 cm⁻¹, confirming that the Mo atom is successfully doped and bonds with the S atom (Figure 2A and Supporting Information S1: Figure S3).²⁴ Meanwhile, the Mo 3d spectrum of Mo-ZIS exhibits two XPS peaks corresponding to Mo 3d_5/2 and Mo 3d_3/2, associated with Mo⁵⁺, further demonstrating successful Mo doping in the ZIS structure (Supporting Information S1: Figure S4).³¹ In the S 2p XPS spectra of ZIS, Mo-ZIS, ZIS/O_V-CeO_2, and Mo-ZIS/O_V-CeO₂, the S²⁻ and S₂²⁻ peaks are observed (Figure 2B).²⁸ The positive shift in the S 2p peak of Mo-ZIS compared to ZIS is ascribed to the presence of S vacancies which arise from the loss of S atoms due to the disruption of the original S-metal bonding by doping Mo atoms.³² As for O_V-CeO₂, the Raman peaks at approximately 457 and 605 cm⁻¹ correspond to the F_2g Raman mode of cubic CeO₂ and the presence of Ce³⁺ coupled with O vacancies, respectively, which demonstrate the successful introduction of O vacancies into CeO₂ (Supporting Information S1: Figure S3).³³ Additionally, the O 1s spectrum of O_V-CeO₂ features two XPS peaks at 529.5 and 531.3 eV, corresponding to oxygen atoms within the O_V-CeO₂ lattice (O_latt) and oxygen adsorbed on its surface (O_ads), and the higher ratio of O_ads to O_latt (O_ads/O_latt) in the samples indicates the higher content of O vacancies (Figure 2C).^{34, 35}

The coexistence of the Raman characteristic peaks of ZIS and O_V-CeO₂, along with the presence of Mo-S stretching vibration peaks, confirms the successful formation of the Mo-ZIS/O_V-CeO₂ heterojunction (Figure 2A). Meanwhile, the X-ray diffraction patterns indicate that Mo-ZIS/O_V-CeO₂ exhibits diffraction peaks that correspond to ZnIn₂S₄ (JCPDS card No. 65-2023) and CeO₂ (JCPDS card No. 34-0394), confirming that the Mo-ZIS/O_V-CeO₂ heterojunction was successfully synthesized, consistent with the Raman results (Supporting Information S1: Figure S5).^{29, 36} By loading O_V-CeO₂ on the surface of Mo-ZIS, the Raman vibrational peaks and XPS peaks of Mo-ZIS/O_V-CeO₂ exhibit significant shifts compared to O_V-CeO₂, implying the intense interfacial interaction between Mo-ZIS and O_V-CeO₂ (Figure 2B–D, Supporting Information S1: Figures S6, and S7).^{28, 37} Moreover, the XPS peaks exhibit more significant shifts in Mo-ZIS/O_V-CeO₂ (S 2p (0.31 eV shift) and Ce 3d (0.64 eV shift)) compared to ZIS/O_V-CeO₂ (S 2p (0.16 eV shift) and Ce 3d (0.42 eV shift)), which suggests the stronger interfacial interaction intensity and more substantial electron transfer in Mo-ZIS/O_V-CeO₂. Notably, in addition to the XPS peaks attributed to Ce 3d_3/2 and Ce 3d_5/2, Mo-ZIS/O_V-CeO₂ exhibits an additional peak at 886.4 eV associated with the formation of Ce-S bonds, absent in ZIS/O_V-CeO₂, indicating that strong substantial electron transfer induces the formation of a robust interfacial Ce-S bond between Mo-ZIS and O_V-CeO₂ (Figure 2D).³⁸ Additionally, the Fourier transform infrared (FTIR) spectrum of Mo-ZIS/O_V-CeO₂ also exhibits an extra Ce-S peak at 493 cm⁻¹, further confirming the successful construction of Ce-S bonds at the heterogeneous interface (Figure 2E).³⁹ These results indicate that Mo atom doping into ZIS can facilitate the construction of the Ce-S bonds between Mo-ZIS and O_V-CeO₂.

To elucidate the Ce-S bond formation mechanism, we utilize electron paramagnetic resonance spectra (EPR) to analyze the electron structures within the samples. After doping Mo atom into ZIS, an EPR signal at g = 2.002 appears in Mo-ZIS, demonstrating that the Mo atom incorporation introduces a significant number of unsaturated S atoms with strong electron-withdrawing capability (Figure 2F,G, and Supporting Information S1: Figure S8).^{11, 24, 26} O_V-CeO₂ displays the strongest EPR signal at g = 1.96, attributed to the presence of abundant isolated electrons in O_V-CeO₂.^{40, 41} The abundance of O vacancies creates an electron-rich environment for Ce atoms which can readily combine with unsaturated S atoms. Notably, along with interaction between Mo-ZIS and O_V-CeO₂, the EPR signal corresponding to the isolated electron of O_V-CeO₂ decreases and the O_ads/O_latt ratio associated with the O vacancies reduces from 1.85 to 0.43, indicating depletion of isolated electrons in the Mo-ZIS/O_V-CeO₂ structure (Figure 2C,F and Supporting Information S1: Note 1).³⁵ Based on the XPS, FTIR, and EPR results, we demonstrate that the unsaturated S atoms introduced by Mo doping in Mo-ZIS readily capture isolated electrons on the Ce atom in O_V-CeO₂, leading to the formation of Ce-S bonds, as depicted in Figure 2G.

1.2 Photocatalytic performance of Mo-ZIS/O_V-CeO₂

We evaluated the photocatalytic OWS performance of the samples in pure water under Xe lamp (AM 1.5 G) irradiation, using Pt and CoO_x as H₂ and O₂ evolution co-catalysts, respectively (Supporting Information S1: Figure S9). The Mo-ZIS/O_V-CeO₂ photocatalyst can split water into H₂ and O₂ at a molar ratio of 2:1 (Figure 3A). After the doping of Mo atoms, Mo-ZIS exhibits H₂ and O₂ evolution rates with a value of 16.6 and 7.9 μmol g⁻¹ h⁻¹, which is 2.1 times higher than ZIS (7.7 μmol g⁻¹ h⁻¹ of H₂ and 3.3 μmol g⁻¹ h⁻¹ of O₂). Notably, by introducing O_V-CeO₂ and optimizing the mass ratio between Mo-ZIS and O_V-CeO₂, Mo-ZIS/O_V-CeO₂ exhibited the highest photocatalytic OWS activity, achieving H₂ and O₂ evolution rates of 512.7 and 256.3 μmol g⁻¹ h⁻¹, respectively (Figure 3A,B, and Supporting Information S1: S10). The apparent quantum yield (AQY) and STH efficiency are important parameters for evaluating photocatalytic performance. The AQY values of Mo-ZIS/O_V-CeO₂ obtained at various wavelengths align with its absorption spectrum, confirming that the photocatalytic process is driven by photons (Figure 3C and Supporting Information S1: Table S1). Additionally, Mo-ZIS/O_V-CeO₂ exhibited the highest AQY at 380 nm, achieving a value of 3.01%. The STH average value of Mo-ZIS/O_V-CeO₂ is approximately 0.14%, surpassing that of most ZnIn₂S₄-based heterojunctions (Figure 3D, Supporting Information S1: Figure S11, and Table S2).^{15, 16, 22, 23, 42-46}

We also evaluated the photocatalytic stability of the samples. After 16 h of photocatalytic tests, Mo-ZIS/O_V-CeO₂ retains 81% of its initial performance, whereas ZIS/O_V-CeO₂ retains only 51% (Figure 3E and Supporting Information S1: Figure S12). To investigate the reason for the improved stability, the S 2p XPS spectrum and structure of Mo-ZIS/O_V-CeO₂ after photocatalytic testing were analyzed. For metal sulfide photocatalysts, the S atoms in the structure are highly susceptible to oxidation by photogenerated holes, resulting in photo-corrosion and destroying its structure.⁶ Benefitted from the robust interfacial Ce-S bond, the S 2p binding energy shift of Mo-ZIS/O_V-CeO₂ after photocatalytic tests is only 0.09 eV, indicating that the oxidation of S atoms is significantly suppressed (Figure 3F). Meanwhile, the structure of Mo-ZIS/O_V-CeO₂ remained mostly unchanged after photocatalytic tests (Supporting Information S1: Figure S13). Based on this, we conclude that the improved stability results from the construction of robust interfacial Ce-S bonds, which accelerate the extraction of photogenerated holes and prevent the oxidation of Mo-ZIS.

1.3 Kinetics of charge separation and transport

The optical and electrical properties of photocatalysts are crucial in determining the photocatalytic performance. Ultraviolet-visible (UV-vis) spectroscopy was conducted to assess the light absorption capabilities of all catalysts.⁴⁷ With the introduction of Mo doping and O_V-CeO₂, light absorption gradually extends into the infrared region and exhibits enhanced absorption intensity along, and Mo-ZIS/O_V-CeO₂ exhibits the highest light absorption capability (Figure 4A). The electrochemical impedance spectroscopy (EIS) results reveal that Mo-ZIS/O_V-CeO₂ exhibits the smallest Nyquist radius size, indicating that the successful construction of the interfacial Ce-S bonds can reduce the interface carrier transport barrier (Supporting Information S1: Figure S14 and Table S3).²⁸ Moreover, we calculate the carrier transport activation energy of various samples using the Arrhenius equation by analyzing EIS plots obtained at different temperatures.⁴⁸ After the construction of interfacial Ce-S bonds, Mo-ZIS/O_V-CeO₂ exhibits a lower carrier transport activation energy (0.17 eV), which is reduced by 43% compared to ZIS/O_V-CeO₂ and beneficial for carrier separation and transfer (Figure 4B). Meanwhile, we calculated the built-in electric field intensity via the surface photovoltage and surface photoelectron density (Supporting Information S1: Figure S15).⁴⁹ Notably, the built-in electric field intensity of Mo-ZIS/O_V-CeO₂ demonstrated a substantial increase, being 2.1-time greater than that of ZIS/O_V-CeO₂ (Figure 4C). This indicates that the introduction of interfacial Ce-S bonds enhances the built-in electric field of Mo-ZIS/O_V-CeO₂ providing a superior driving force for carrier transport.

Photoluminescence (PL) and time-resolved photoluminescence (TRPL) tests are performed to investigate the dynamics of photogenerated carrier separation and transfer.^{11, 50} With the introduction of Ce-S bonds, the PL peak intensity of the samples gradually decreases and the carrier lifetime correspondingly extends, and Mo-ZIS/O_V-CeO₂ exhibits the lowest PL peak intensity and the longest carrier lifetime of 1.76 ns (Figure 4D,E, and Supporting Information S1: Table S4). These results suggest that interfacial Ce-S bonds can effectively enhance the separation efficiency of photogenerated carriers, effectively suppressing recombination.^{51, 52} Benefiting from the enhanced carrier separation and transport capability, Mo-ZIS/O_V-CeO₂ exhibits the highest photocurrent density of 2.45 μA cm⁻², being 10.6, 5.1, 24.5, and 1.8 times greater than ZIS, Mo-ZIS, O_V-CeO₂, and ZIS/O_V-CeO₂, respectively (Figure 4F).^{53, 54} Additionally, the doping of Mo atoms and the introduction of O_V-CeO₂ result in a more loose structure for Mo-ZIS/O_V-CeO₂, exposing the largest specific surface area and electrochemically active surface area compared to other samples, thereby providing more active sites for photocatalytic OWS (Supporting Information S1: Figures S16 and S17, and Table S5).⁵⁵ Meanwhile, EPR radical trapping tests and linear sweep voltammetry (LSV) are crucial methods for assessing the photocatalytic activity of the H₂ evolution reaction (HER) and O₂ evolution reaction (OER).^{56, 57} Compared to ZIS/O_V-CeO₂, Mo-ZIS/O_V-CeO₂ exhibits stronger EPR signals and lower overpotentials, indicating superior photocatalytic HER and OER activity in Mo-ZIS/O_V-CeO₂ (Supporting Information S1: Figures S18 and S19).

1.4 Mechanism of Mo-ZIS/O_V-CeO₂ photocatalytic OWS

The energy band structure of ZIS, Mo-ZIS, and O_V-CeO₂ are determined using the Kubelka-Munk equation and ultraviolet photoelectron spectroscopy (UPS) (Supporting Information S1: Figure S20).²⁹ The UPS results reveal that the Fermi levels of ZIS, Mo-ZIS, and O_V-CeO₂ are −4.17, −3.94, and −4.39 V (vs. vacuum), respectively. The detailed energy band structures of ZIS, Mo-ZIS, and O_V-CeO₂ are constructed by combining the bandgap and band edge potentials, as shown in Supporting Information S1: Figure S21. In situ XPS is an efficient technique for identifying the direction of carrier migration and explaining the mechanism of photocatalysis.^{11, 58} Under illumination, the XPS peaks of S 2p, In 3d, Zn 2p, and Mo 3d in Mo-ZIS/O_V-CeO₂ and ZIS/O_V-CeO₂ shift toward lower binding energies, while those of Ce 3d and O 1s shift to higher binding energies, demonstrating that photogenerated electrons transfer from O_V-CeO₂ to Mo-ZIS or ZIS during illumination (Figure 5A, Supporting Information S1: Figures S22, and S23). Based on the energy band structure and the direction of the photogenerated electron migration, it can be concluded that both Mo-ZIS/O_V-CeO₂ and ZIS/O_V-CeO₂ follow the S-scheme carrier transfer pathway.⁵⁹

More strikingly, after doping with Mo atoms, the Fermi level of Mo-ZIS shifts upward, increasing the Fermi level difference between the different structures of ZIS and O_V-CeO₂ (Supporting Information S1: Figure S21). This enhanced Fermi level difference and the formation of interfacial Ce-S bonds in Mo-ZIS/O_V-CeO₂ lead to an increased built-in electric field intensity compared to ZIS/O_V-CeO₂. These results reduce the interfacial resistance and strengthen the driving force, thereby facilitating the swifter migration of photogenerated carriers at the interface in Mo-ZIS/O_V-CeO₂ (Figure 5B). Impressively, Mo-ZIS/O_V-CeO₂ exhibits a greater shift in binding energy (S 2p (0.36 eV shift) and Ce 3d (0.43 eV shift)) compared to ZIS/O_V-CeO₂ (S 2p (0.18 eV shift) and Ce 3d (0.21 eV shift)) under illumination, indicating a higher quantity of photogenerated carriers are separated, accumulated, and participate in the photocatalytic OWS in Mo-ZIS/O_V-CeO₂ (Figure 5A).

Based on the previous analysis, the OWS mechanism for the Mo-ZIS/O_V-CeO₂ heterostructure photocatalyst was proposed (Figure 5B and Supporting Information S1: Figure S24). Upon illumination, Mo-ZIS/O_V-CeO₂ is excited, generating photoexcited electrons and holes in the conduction band (CB) and valence band (VB), respectively, of Mo-ZIS and O_V-CeO₂.⁶⁰ The photogenerated electrons in the CB of O_V-CeO₂ combine with photogenerated holes in the VB of Mo-ZIS through the low interfacial barrier Ce-S bonds, while the photoinduced electrons in the CB of Mo-ZIS and photoinduced holes in the VB of O_V-CeO₂ are retained. Under the combined effects of the interfacial Ce-S bonds and enhanced internal electric field, efficient charge separation is achieved and the S-scheme carrier transfer pathway is accelerated. Consequently, more photogenerated electrons in the CB of Mo-ZIS are transferred to the Pt sites to participate in HER, while more photogenerated holes in the VB of O_V-CeO₂ are separated to the CoO_x sites for OER.