Pulsed laser deposition of STO thin films on rGO-protected silicon photocathodes for enhanced photoelectrochemical water splitting

Abstract

Epitaxial strontium titanate (STO) thin films were investigated as functional coatings for silicon (Si)-based photoelectrochemical (PEC) water splitting, providing both surface protection and enhanced photoactivity. In this study, ~10 nm-thick STO films were deposited by pulsed laser deposition (PLD) onto bare Si and Si substrates buffered with reduced graphene oxide (rGO). Interface engineering involved SrO-assisted surface deoxidation and controlled spin-coating of one to three graphene oxide layers, achieving ~50-100% surface coverage. The films were deposited at two temperatures, 515 ℃ and 700 ℃ and characterized using 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). Both AFM and XRR analyses showed smoother surfaces and reduced roughness for STO films grown on rGO-buffered Si substrate. XRD results revealed that deposition at 700 ℃ led to the formation of textured films on all substrates, while growth at 515 ℃ on SrO/rGO-treated Si yielded highly crystalline STO with a dominant (002) out-of-plane orientation. These structural improvements were confirmed by RHEED patterns, which displayed sharp streaks on rGO-buffered samples, indicating superior epitaxial quality. Electrochemical characterization showed that STO/rGO photocathodes achieved superior PEC performance, with a reduced onset potential (0.24 V vs RHE), a high photocurrent density (-27.78 mA cm-2), and improved stability during operation. In contrast, non-epitaxial samples with silicate or silicide interfacial layers, especially those formed at 700 ℃ exhibited reduced activity and durability, that can be attributed to increase charge transfer resistance, as indicated by electrochemical impedance spectroscopy. These findings emphasize the crucial role of interface design and growth temperature on the structural and functional properties of oxide/Si heterostructures for PEC water splitting.