Effect of Ni doping on the structural and optical properties of TiO₂ nanoparticles prepared by co-precipitation method

In this study, Ni-doped TiO₂ nanoparticles with different dopant concentrations were fabricated using coprecipitation. The chemical compositions, and structural and optical properties were characterized using X-ray diffraction (XRD), electron dispersive X-ray spectroscopy (EDS), and UV-VIS diffuse reflectance spectroscopy. Identification of paramagnetic species was conducted using electron spin resonance (ESR). The XRD pattern shows that only a single phase of anatase appeared because Ni²⁺ has a similar ionic radius to that of Ti⁴⁺ and replaced some of the Ti⁴⁺ ions in the TiO₂ lattice. The crystallite size decreased with increasing Ni content. This phenomenon probably arises from the introduction of Ni²⁺ ions which decreased crystalline growth. The UV-VIS diffuse reflectance spectra showed that the UV absorption moved to a longer wavelength (red shift) and the band gap energy decreased. This caused the doped Ni atoms to form a localized energy state below the conduction band of the TiO₂ lattice. The ESR studies on the Ni-doped TiO₂ nanoparticles revealed the presence of Ni²⁺, Ti³⁺, and oxygen defects in all samples.