Comparison on the TiO2 crystalline phases deposited via dip and spin coating using green sol–gel route

Abstract

TiO2 exist in three polymorphs known as anatase, rutile and brookite. Amongst these polymorphs, brookite are less reported and mostly produced as a by-product. In this work, two deposition methods which are dip and spin coating are selected to compare the effect of deposition methods on TiO2 crystalline phase formation, particularly on identifying the brookite presence. The sol used was made free from solvent as an attempt for a green sol–gel route. The heat treatment temperature is varied at 200 °C, 300 °C, 400 °C and 500 °C for 3 h. The produced thin films are then characterized by X-ray diffraction (XRD), Raman spectroscopy (RS) and transmission electron microscopy (TEM). Crystallite size was calculated using Scherrer's equation. The cross sectional morphology of the thin films was examined with the scanning electron microscope (SEM). Results show that the deposition methods influence the phase formation and crystallinity. TiO2 thin films produced via dip coating composed of anatase and rutile despite the temperatures variation. In contrast, spin coating produced single brookite (1 1 1) with Raman spectra of 319 cm−1 and 320 cm−1 at 200 °C and 300 °C. The brookite crystallite size is 47.9 nm at 200 °C and 58.4 nm at 300 °C. TEM results had confirmed the brookite presence with the lattice fringes of 0.28 nm. However, at 400 °C and 500 °C, XRD pattern reveals no formation of TiO2 phases. Therefore, in this green sol–gel route, it is found that spin coating deposition at low temperature is preferable for brookite formation whereas dip coating is more suitable for anatase and rutile. Keywords: Brookite, Crystallite size, TiO2 thin film, Spin coating, Dip coating