Manifesting p-d Orbital Hybridization Through Strategic D-Band Engineering: A Pathway to Boosted Bifunctional HER/OER Electrochemical Performance in Self-Templated Co-SnO2 Grown Over Co-SnS2 Nanosheets

Abstract

In this study, Co-doped SnO2 is synthesized atop the hexagonal CoS template (CoSS) via direct air calcination of as-synthesized Co-doped SnS2 (CoS) nanosheets. The structural evolution facilitated the emergence of Co2+ and Co3+ states, complemented by surface-adsorbed sulfur oxyanions (SO42−, HSO3-, SO32−). CoSS deposited over carbon cloth (CoSS/CC) exhibited superior bifunctional HER and OER, demonstrating higher stability and efficiency than their CoS/CC counterparts. Notably, CoSS/CC||CoSS/CC shows the overall water splitting at a minimum cell voltage of 1.5 V, significantly lower than CoS/CC||CoS/CC. Mechanistically, the Co3+ states serve as catalytically active sites that enhance OER, while the synergistic interaction between Co3+ and the sulfur oxyanions promotes HER activities. Density functional theory (DFT) calculations revealed an upshifted d-band centre (ɛd) and enhanced metal-oxygen covalency (Δ) in CoSS, with superior charge transfer and p-d hybridization. ATR-FTIR, Raman, and XPS investigations confirmed surface reconstruction of CoSS/CC electrodes with enhanced electrical conductivity. It is related that a highly strained system of VO-CoSS has more unfilled electronic states near the Fermi level (ɛF) to facilitate a stable interaction with HER/OER intermediates. Overall, this study underscores the superior bifunctional electrocatalytic efficiency of CoSS/CC over CoS/CC, establishing it as a promising candidate for efficient overall water splitting.