Synthesis and characterisation of nanostructured neodymium titanium oxides by sol–gel process: Controlling the phase composition, crystal structure and grain size

Abstract

Nanocrystalline neodymium titanium oxide thin films and powders with different phase compositions with mesoporous structure were produced by a straightforward particulate sol–gel route. The sols were prepared in various Nd:Ti molar ratios and they showed a narrow particle size distribution in the range 20–26 nm. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that the powders contained mixtures of Nd 4Ti 9O 24, Nd 2Ti 4O 11, Nd 3Ti 4O 12 for titanium dominant powders (Nd:Ti ≤ 45:60), mixtures of Nd 2TiO 5 and Nd 2O 3 for neodymium dominant powders (Nd:Ti ≥ 75:25) and pure Nd 3Ti 4O 12 phase for equal molar ratio of Nd:Ti, depending on the annealing temperature and Nd:Ti molar ratio. Moreover, it was found that Nd:Ti molar ratio influences the preferable orientation growth of the neodymium titanium oxide compounds. Transmission electron microscope (TEM) images confirmed that the average crystallite size of the powders annealed at 400 °C was in the range 1.0–2.8 nm and a gradual increase was occurred up to 6.7 nm by heat treatment at 1000 °C. The activation energy of crystal growth reduced with a decrease of Nd:Ti molar ratio, calculated in the range 6.90–18.12 kJ mol −1. Low activation energies indicating that the grain size will not change much with increase in temperature. Field emission scanning electron microscope (FE-SEM) analysis revealed that the deposited thin films had uniform, mesoporous and nanocrystalline structure. Moreover, atomic force microscope (AFM) images presented that the thin films had a columnar like morphology with average grain size in the range 17–30 nm at 600 °C and 47–60 nm at 800 °C, depending upon the Nd:Ti molar ratio. Based on Brunauer–Emmett–Taylor (BET) analysis, the synthesized powders showed mesoporous structure containing pores with sphere like shapes. The surface area of the powders was enhanced by increasing Nd:Ti molar ratio and reached as high as 110 m 2 g −1 for the powder containing Nd:Ti = 75:25 (molar ratio) at 400 °C. In addition, the mesoporous structure of the powders was stable at high annealing temperatures up to 900 °C.