Recognized for its high energy density and environmentally friendly characteristics, hydrogen energy is considered one of the most promising alternative energy sources to fossil fuels. Photocatalytic water splitting has emerged as a key technology for hydrogen production, especially under the utilization of semiconductor photocatalysts. Despite significant advancements in the development of various photocatalysts, the rapid recombination of photo-generated electron-hole pairs remains a primary challenge, limiting their hydrogen production efficiency. In this context, developing low-cost, highly efficient photocatalysts that can overcome this challenge is critical. Noble metals such as platinum (Pt) are often employed to enhance photocatalytic hydrogen evolution reaction (HER) performance, but their high cost and limited availability restrict large-scale applications. Consequently, the search for non-noble metal alternatives that can provide comparable or superior photocatalytic activity has become a major research field. Our study focuses on the synthesis of a MoS₂/La₂Ti₂O₇ (MoS₂/LTO) composite photocatalyst as a low-cost and efficient alternative to photocatalytic water splitting. We aim to develop a composite material that improves charge separation and electron transfer efficiency, thereby addressing the limitations of current photocatalysts and contributing to the development of sustainable hydrogen production technologies.

The MoS₂/LTO composite photocatalyst is synthesized via a two-step process. As a perovskite-type material with a three-dimensional step structure, La₂Ti₂O₇ (LTO) is chosen as the base material due to its high stability and suitable conduction band potential for hydrogen evolution. LTO nanosteps are prepared by adopting a molten salt method, which leads to LTO particles with exposed (012) and (010) crystalline planes. These step structures are known to enhance charge separation, which is essential for photocatalytic efficiency. Meanwhile, MoS₂ nanosheets are exfoliated from bulk MoS₂ through lithium intercalation, thereby bringing about the formation of monolayer or few-layer MoS₂. These MoS₂ nanosheets containing both the 1T and 2H phases are then assembled onto the surface of the LTO nanosteps to form the Mix-MoS₂/LTO composite. We anneal the Mix-MoS₂/LTO at 150 ℃ to convert the less stable 1T phase into the more thermodynamically stable 2H phase and thus improve the stability and efficiency of the MoS₂ component. This phase transformation enhances the electronic properties of MoS₂ and improves the interaction between MoS₂ and LTO, promoting efficient charge transfer. The photocatalytic performance of the MoS₂/LTO composite is evaluated under simulated solar light by employing lactic acid as a sacrificial agent. The HER rate is measured, and various characterization techniques including X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), and electrochemical impedance spectroscopy (EIS) are employed to investigate the material’s electronic properties and charge transfer mechanisms. Density functional theory (DFT) calculations are also performed to better understand the charge migration processes at the MoS₂/LTO interface.

XPS analysis reveals that the MoS₂ in the MoS₂/LTO composite is predominantly in the 2H phase after annealing, thus confirming the phase transformation from the 1T to the 2H structure. This phase transformation is critical for improving the stability and catalytic activity of MoS₂. The interaction between MoS₂ and LTO is significantly improved after annealing, as indicated by the shifts in binding energy observed in the Mo 3d and S 2p XPS spectra (Fig. 2). The UV-Vis DRS and Mott?Schottky tests further confirm the formation of a Type-I heterojunction between MoS₂ and LTO, which facilitates the transfer of photo-generated electrons from the conduction band of LTO to MoS₂ (Fig. 3). This electron transfer is essential for improving the separation of electron-hole pairs and enhancing the overall photocatalytic efficiency. The MoS₂/LTO composite exhibits significantly enhanced photocatalytic hydrogen production compared to pure LTO and Pt/LTO in identical experimental conditions. The hydrogen production rate of the MoS₂/LTO composite is found to be 64 times higher than that of pure LTO and 1.9 times higher than that of Pt/LTO (Fig. 4). This significant photocatalytic performance improvement is primarily attributed to the combination of the excellent charge separation ability of LTO and the abundant edge active sites provided by MoS₂. The optimal MoS₂ mass fraction is determined to be 6%, which leads to the highest hydrogen production rate of 4.36 mmol·h^-1·g^-1. At this loading, the MoS₂ nanosheets effectively cover the surface of LTO nanosteps, maximizing the active sites for the HER process. Further increases in MoS₂ loading result in decreased photocatalytic performance, which is possibly due to the over-accumulation of MoS₂ and can block light absorption and hinder charge transfer. In terms of stability, the MoS₂/LTO composite exhibits excellent long-term stability during photocatalytic hydrogen evolution. After 12 h of continuous illumination, there is no significant decrease in hydrogen production, which demonstrates that the composite is highly stable and suitable for long-term utilization in photocatalytic applications. The photocurrent response tests show that the MoS₂/LTO composite exhibits higher photocurrent density than pure LTO, indicating that the composite facilitates more efficient charge separation and transfer during the photocatalytic process. Electrochemical impedance spectroscopy (EIS) is performed to investigate the charge transfer resistance of the MoS₂/LTO composite. The results indicate that the composite has lower charge transfer resistance than pure LTO, further confirming that the MoS₂/LTO heterojunction facilitates charge separation and reduces charge recombination. DFT calculations provide additional insights into the charge migration process. The results demonstrate that photo-generated electrons from LTO can easily migrate to the conduction band of MoS₂ via the built-in electric field at the MoS₂/LTO interface, where they are utilized in the HER at the edge active sites of MoS₂ (Fig. 5). This mechanism significantly enhances the photocatalytic hydrogen production performance of the MoS₂/LTO composite.

The developed MoS₂/LTO composite photocatalyst demonstrates excellent photocatalytic hydrogen production performance due to its unique core-shell structure and enhanced charge separation efficiency. The MoS₂ nanosheets with abundant edge active sites and the LTO nanosteps with effective charge separation properties synergistically enhance the overall photocatalytic activity. The conversion of MoS₂ from the 1T to the 2H phase via annealing plays a crucial role in improving the stability and efficiency of the composite. Our study provides a new approach to designing non-noble metal photocatalysts for efficient hydrogen production via water splitting. By optimizing the structure of the photocatalysts and improving interfacial charge transfer, further improvements in photocatalytic performance can be yielded to facilitate the commercialization of hydrogen energy as a clean and sustainable energy source.