Mixed-Anion Band Engineered Cobalt Oxynitride for Efficient Solar Water Splitting and Nitrogen Fixation

This study presents a strategic band structure engineering approach for synthesizing of cobalt oxynitride (CON) using cobalt nitride (CN) as a precursor. Comprehensive structural characterization, employing X-ray diffraction with Rietveld refinement, micro-Raman spectroscopy, and high-resolution transmission electron microscopy, confirmed the presence of Co–O and Co–N bonding within the single-phase CON structure, highlighting the integrated coordination environment of cobalt in the system. Notably, DFT calculations reveal that body-diagonal positioning of nitrogen atoms in oxynitride structure offers superior thermodynamic stability due to minimized anion repulsion, symmetric Co–N bonding, and reduced lattice strain. Optical and physicochemical analyses revealed enhanced light absorption and a reduced bandgap of 2.41 eV for CON compared to 2.62 eV for the cobalt oxide (CO) system. This improvement arises from the effective hybridization of O 2p and N 2p orbitals, leading to a hybrid phase that denotes a solid-solution-type Co–O–N lattice with local bonding heterogeneity, rather than a separate multiphase structure. The CON system demonstrated superior photocatalytic performance, achieving a dye degradation efficiency of ∼92%, a hydrogen (H2) evolution rate of 1748.6 μmol g–1 h–1, and an ammonia (NH3) production rate of 411.2 μmol g–1 h–1, significantly outperforming both oxide (CO: ∼62%, 512.5, 187.3 μmol g–1 h–1) and nitride (CN: ∼39%, 152.4, 78.6 μmol g–1 h–1) systems. These enhancements arise from synergistic interactions within the mixed-anion oxynitride lattice, which optimize charge separation and transfer dynamics during the photocatalytic reactions. This work highlights cobalt oxynitrides as a promising class of materials for energy conversion and environmental remediation, offering a new paradigm in the functional tuning of conventional metal oxides.