PLD-grown thin films of STO on silicon photocathodes for photoelectrochemical hydrogen evolution reaction

Abstract

Strontium titanate (STO) thin films epitaxially grown on silicon (Si) substrates act as both protective and electroactive layers in photoelectrochemical (PEC) water splitting. To investi- gate the influence of crystallinity and interface quality on hydrogen evolution reaction, ~10 nm-thick STO films were deposited via pulsed laser deposition (PLD) onto bare and reduced graphene oxide (rGO)-buffered Si substrates. The integration STO with Si was facilitated using SrO-assisted deoxidation and precise control the Si surface coverage with spin-coating of one to three graphene oxide layers (~50 - 100 % surface coverage). The STO films were grown at 515 and 700 °C, and characterized by reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), and X-ray photoelectron spectroscopy (XPS). AFM and XRR revealed smoother morphology and lower roughness for STO films grown on rGO-buffered Si. XRD showed that films grown at 700 °C developed a textured structure on both substrate types, while those grown at 515 °C on SrO/rGO-treated Si exhibited high crystallinity with strong (002) out-of-plane orientation. These results were supported by RHEED, which showed sharp streaks indicative the improved structural order on rGO-buffered substrates. Electrochemical measurements demonstrated that the epitaxial STO/rGO photocathodes had superior performance compared to non-epitaxial ones, with a lower onset potential (0.24 V vs. RHE) and a much higher photocurrent density (-28.78 mA cm-2 ), with improved long-term stability as confirmed by chronoamperometry (CA). In contrast, non-epitaxial samples and those with silicate/silicide interfacial layers, particularly at 700 °C, exhibited reduced activity and stability, as shown by electrochemical impedance spectroscopy (EIS). These results highlight the critical role of interfacial design, crystalline orientation, and growth temperature in optimizing PEC performance.