Effects of Mn Doping on Surface Chemistry, Phase Stability, and Physical Properties of NiO Synthesized via Green Methods

This study investigates the effect of manganese (Mn) doping (1–5 wt. %) on the phase transition of nickel hydroxide (Ni(OH)$_2$) to nickel oxide nanoparticles (NiO NPs) synthesized via a green route. Phase transition, structural, optical, and magnetic properties of Mn-doped NiO are thoroughly investigated. Thermogravimetric analysis (TGA) reveals the influence of Mn on the thermal decomposition and the stability of Ni(OH)$_2$ and the organic compounds from olive leaf extract under inert calcination conditions. X-ray photoelectron spectroscopy (XPS) provides insights into surface chemistry modifications before and after calcination, as well as in the presence of Mn. X-ray diffraction (XRD) confirms Mn incorporation and lattice distortion within the NiO structure. HRTEM and BET analyses show that 2 wt. % Mn is a critical concentration, yielding the smallest spherical particle size (6 nm) and the highest surface area. The calculated work function from ultraviolet photoelectron spectroscopy (UPS) reveals a decrease from 6.0 eV (0 wt. % Mn) to 5.4 eV (5 wt. % Mn). Analysis of the valence band maximum (VBM) region further indicates a bandgap widening with Mn incorporation. ... mehrRaman spectra reveal the appearance of the two magnon (2M) vibrational mode, suggesting modified magnetic behavior. Vibrating sample magnetometry (VSM) indicates a ferromagnetic response, with the appearance of a Néel temperature at 5 wt. % Mn, indicating an antiferromagnetic to paramagnetic transition. This study offers key insights into the design of stable, high-performance materials for photovoltaic and magnetic applications.