Engineering Charge Transfer in PLD-Grown STO Thin Films on Si(001) by an rGO Interlayer for Photoelectrochemical Hydrogen Evolution

Abstract

In order to explore the influence of the crystalline and interfacial properties of the protective layer on the photoelectrochemical hydrogen evolution (HER) by a Si photocathode, a ∼10 nm-thick strontium titanate (STO) film was grown using the pulsed laser deposition (PLD) method on both bare and silicon substrates buffered with reduced graphene oxide (rGO) at 515 and 700 °C. The integration of STO involved a combination of spin coating of graphene oxide and SrO-assisted deoxidation of the silicon substrate. Postgrowth analysis revealed a smoother morphology and reduced roughness of the STO films grown on rGO-buffered regions, in line with the XRR results. The RHEED method revealed textured STO films grown at 700 °C on both bare and rGO-buffered silicon substrates. The best crystallinity of STO was observed for rGO/SrO-deoxidized silicon surfaces at 515 °C, showing sharp RHEED streaks, indicating a well-ordered surface structure and confirming the preferential (002) out-of-plane orientation observed in the XRD measurements. Linear sweep voltammetry (LSV) and chronoamperometry (CA) measurements demonstrated that the epitaxially protected photocathode exhibited superior enhancement of the HER compared to its nonepitaxial counterpart. The epitaxial photocathode exhibited a more positive onset potential and higher photocurrent density. The CA results of the epitaxial STO/rGO/Si sample revealed better stability over prolonged operation compared to the nonepitaxial photocathode, which showed a significant decline of current. The EIS results show that the presence of silicate/silicides hinders HER significantly, especially in the case of samples prepared at 700 °C, where the integrity of the layers may be affected by the solid-state reaction between STO and Si. The obtained results illustrate how the crystalline and interface structures of the STO films influence the HER and the corresponding charge transfer, thus contributing to the engineering of more stable and efficient PEC devices in the future.