Synthesis and characterization of Zn-incorporated TiO2 thin films: impact of crystallite size on X-ray line broadening and bandgap tuning

In the present work, the synthesis of pure and Zn-doped TiO2 thin films is reported using a simple and cost-effective thermal spray pyrolysis technique (SPT) with an aim to investigate the influence of Zn doping on the structural, morphological, optical and the electrical properties of TiO2 nanoparticles. The resulted TiO2 thin films have been deposited on commercial glass slides at substrate temperature of 450 °C using titanium(IV) butoxide as a precursor. The samples are structurally characterized by X-ray diffraction (XRD) method. Surface morphology, optical and electrical properties of the sample are elucidated by field emission scanning electron microscopy (FESEM), UV–visible spectroscopy and four-point probe method, respectively. X-ray diffractograms show the formation of pure anatase TiO2 phase and after Zn doping anatase phase of TiO2 remains unaltered. The crystallite size is found using the Scherrer method and also Williamson–Hall (W–H), Halder–Wagner (H–W) and Wagner–Agua (W–A) methods are employed to estimate the crystallite sizes and the strain from X-ray peak-broadening analysis. The obtained results indicate that the crystallite size and the lattice strain evaluated from Scherrer method, W–H, H–W and W–A analyses are highly intercorrelated. The characterization results of FESEM illustrate the existence of homogenous and well-dispersed spherical grains with the average diameter of 22–8 nm and agglomerated grains are observed as the Zn is inserted into TiO2 lattice as dopant. The estimated value of optical bandgap of TiO2 nanoparticles is red shifted from 3.86 to 3.78 eV and 4.12 to 4.04 eV in Tauc relation and K–M function, respectively. From four-point probe setup, it is found that the pure film deposited at 450 °C has manifested lowest resistivity 4.25 × 10−3 Ω-cm and then decreased further with increase of Zn contents.