Mechanistic insights into the photocatalytic and electrocatalytic activities of MgNiO2: role of reactive oxygen species and oxygen vacancies†

Granular MgNiO₂ has emerged as a promising catalyst owing to its remarkable electrocatalytic activity and photodegradation efficiency under visible light. In this work, granular surface-engineered MgNiO₂ nanoparticles were synthesized using the precipitation method. The interaction of Mg and Ni, forming Mg–Ni–O structures during high-temperature MgNiO₂ synthesis, was investigated through X-ray photoelectron spectroscopy (XPS) analysis. The presence of Ni³⁺ species in the ionic form indicated charge transfer reactions in the catalyst. The band gaps of the as-prepared MgNiO₂ and NiO were determined to be 2.2 eV and 3.7 eV, respectively. The first-order transverse optical (TO) phonon modes observed at 536 cm⁻¹ indicated the presence of NiO, which was identified as the primary contributor to the Raman peaks. Further, the photocatalytic degradation of caffeine under visible light achieved a removal efficiency of 95.5% within 180 minutes. The intermediate reactive oxidative species (ROS) leading to MgNiO₂ degradation were identified, and their lifetime and diffusion length in the solution were reported. Superoxide (O²⁻˙) and hydroxyl (˙OH) radicals were identified as the main ROS contributing to caffeine degradation. The electrocatalytic oxygen evolution reaction (OER) indicated a high density of oxygen vacancies in MgNiO₂ compared to NiO, suggesting the promoter role of Mg species in the photocatalyst. These insights provide a holistic understanding of MgNiO₂ as a catalyst and its pivotal role in green and efficient caffeine photodegradation and the electrocatalytic OER.