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 SnO 2 is synthesized atop the hexagonal CoS template (CoSS) via direct air calcination of as‐synthesized Co‐doped SnS 2 (CoS) nanosheets. The structural evolution facilitated the emergence of Co 2+ and Co 3+ states, complemented by surface‐adsorbed sulfur oxyanions (SO 4 2− , HSO 3 ‐ , SO 3 2− ). 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 Co 3+ states serve as catalytically active sites that enhance OER, while the synergistic interaction between Co 3+ and the sulfur oxyanions promotes HER activities. Density functional theory (DFT) calculations revealed an upshifted d‐band centre (ɛ d ) 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 V O ‐CoSS has more unfilled electronic states near the Fermi level (ɛ F ) 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.