Manifesting p‐d Orbital Hybridization Through Strategic <i>D</i> ‐Band Engineering: A Pathway to Boosted Bifunctional HER/OER Electrochemical Performance in Self‐Templated Co‐SnO ₂ Grown Over Co‐SnS ₂ Nanosheets

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 (SO₄ ^2-, HSO₃ ^-, SO₃ ^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 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 (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.