Structure and optical absorption properties of NiTiO3 nanocrystallites

Nickel titanate (NiTiO3) nanocrystallites are synthesized via a solid-state reaction from a precursor prepared by a homogeneous precipitation method. Ilmenite-structural NiTiO3 consists of alternating layers of NiO6 and TiO6 octahedra. It not only strongly absorbs ultraviolet light (wavelength <360 nm) but also selectively absorbs visible light mainly in a wavelength range of 420-540 nm and above 700 nm. The synthetic NiTiO3 is a direct-gap semiconducting material with a band gap of 3.00 eV and has obvious absorbance peaks at 448, 502, and 743 nm, corresponding to the photon energies of 2.77, 2.47, and 1.67 eV, respectively. Nevertheless, NiTiO3 does not exhibit obvious photocatalytic activity in the degradation of methylene blue in water. It is proposed that the visible light absorbance peaks of NiTiO3 mainly originate from the Ni: d -> d charge-transfer transitions within its valence band. NiTiO3 has wide energy gaps from the hybridized Ni 3d and O 2p orbitals to the Ti 3d orbitals, which block both Ni2+ -> Ti4+ and O2- -> Ti4+ charge-transfer transitions between valence band and conduction band, and thus baffle its photocatalytic performance.