Thickness effect on the optical properties of TiO2-anatase thin films prepared by ultrasonic spray pyrolysis: Experimental and ab initio study

Abstract

TiO2 thin films with different thicknesses were deposited on glass substrates by using the Ion Layer Gas Reaction (Spray-ILGAR) technique at a substrate temperature of 460°. The effects of deposition time on the morphology and the optical properties of deposited anatase TiO2 thin films are studied. A wide range of characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), Atomic Force Microscopy (AFM) and Ultraviolet–Visible-Near Infrared spectrum were used to characterize prepared TiO2 thin films. The experimental results show that the morphology of deposited thin film changes from sheet like to pyramid like morphology and the optical band gap decreases with increasing the thickness of TiO2 thin films from 3.41 eV to 2.97 eV for thicknesses ranging respectively from 48 nm to 510 nm. XPS measurements revealed oxygen excess, which can be ascribed to the appearance of oxygen vacancy point defects and/or surface adsorption of different species with oxygen element. The ab initio calculations were conducted based on the full-potential linearized augmented plane wave (FP-LAPW) method, to calculate the indirect band gap of different TiO2 thin film thicknesses. The band gap energy calculated using the modified Becker Johnson (mBJ) potential was 3.19 eV for pure bulk TiO2 and 2.98 eV for bulk TiO2 with 8% oxygen vacancy, which is closer to the experimental band gap value of 2.97 eV measured for 510 nm film thickness.