Comparison of nickel titanate (NiTiO3) powders synthesized by sol–gel and solid state reaction

This work was concerned with the synthesis of NiTiO3 powders by the sol–gel and the solid state reaction methods, and the investigation of their structural, electronic and optical properties required for their application in photocatalysis. The main goal was to determine which of these two synthesis methods was better for obtaining a pure NiTiO3 crystalline phase. Synthesized powders were characterized by EDS, XRD, Raman and UV–vis diffuse reflectance spectroscopies. An analysis of the results shows that the sol–gel route with an annealing step at 1350 °C for 2 h in air atmosphere leads to a highly pure nickel titanate powder without any further purification step. A band gap of 2.2 eV was determined for this sample from diffuse reflectance spectroscopy. The solid state reaction route also allows the preparation of NiTiO3 powders, but with a reduced purity (95%) as some TiO2 and NiO phases are found in the powders, after the same thermal treatment as for the sol–gel samples. The EDS and XRD results were consistent between themselves for all samples, showing that the powders obtained by the sol–gel route and annealed at 1350 °C, during 2 h, are pure polycrystalline NiTiO3 with the Ilmenite structure (hexagonal symmetry with lattice parameters around a=b=0.5 nm and c=1.38 nm). In addition, the observed 10 active Raman modes confirmed the presence of the Ilmenite phase of NiTiO3. Finally, the absorption spectra as determined from UV–vis diffuse reflectance shows the presence of at least 3 absorption bands that could be explained by the main (HOMO–LUMO) transitions plus two other bands related to charge transfer from O2− to Ti4+ and Ni2+ to Ti4+. It can be expected that these photo-activated charge transfer phenomena will provide a high photo-catalytic activity to this material. All of the above results show that the sol–gel route allows the preparation of pure nano-crystalline NiTiO3 powders.