Mesoporous and nanocrystalline sol-gel derived NiTiO₃ at the low temperature: Controlling the structure, size and surface area by Ni:Ti molar ratio

Nanocrystalline nickel titanate (NiTiO₃) thin films and powders with mesoporous structure were produced at the low temperature of 500 °C by a straightforward particulate sol-gel route. The sols were prepared in various Ni:Ti molar ratios. X-ray diffraction and Fourier transform infrared spectroscopy revealed that the powders contained mixtures of the NiTiO ₃ and NiO phases, as well as the anatase-TiO₂ and the rutile-TiO₂ depending on the annealing temperature and Ni:Ti molar ratio. Moreover, it was found that Ni:Ti molar ratio influences the preferable orientation growth of the nickel titanate, being on (202) planes for the nickel dominant powders (Ni:Ti ≥ 75:25) and on (104) planes for the rest of the powders (Ni:Ti: ≤ 50:50). The average crystallite size of the powders annealed at 500 °C was in the range 1.5-2.4 nm and a gradual increase occurred up to 8 nm by heat treatment at 800 °C. The activation energy of crystal growth decreased with an increase of Ni:Ti molar ratio, calculated in the range 24.93-37.17 kJ/mol. Field emission scanning electron microscope analysis revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 20-35 nm. Moreover, atomic force microscope images presented that the thin films had a hill-valley like morphology with roughness mean square in the range 41-57 nm. Based on Brunauer-Emmett-Taylor analysis, the synthesized powders showed mesoporous structure containing pores with needle and plate like shapes. The mesoporous structure of the powders was stable at high annealing temperatures and one of the highest surface areas (i.e., 156 m²/g) reported in the literature was obtained for the powder containing Ni:Ti = 50:50 at 500 °C.