Role of aluminum, cobalt co-doping on the band gap tuning of TiO2 thin film deposited by spray pyrolysis

Abstract

Titanium dioxide (TiO2) thin films were prepared by a spray pyrolysis technique onto a glass substrate. The film properties are characterized by Field-emission scanning electron microscopy, Energy-dispersive X-ray spectroscopy (EDX), Atomic force microscopy (AFM), X-ray diffraction (XRD) technique, and four-probe point method. The Scanning electron microscope and Atomic force microscope images of pristine samples have shown particle shape. The pore diameters are found to be in between 6 nm to 9 nm for Al doped, spherical and slender shaped particles are ranging from 42 nm to 50 nm for co-doped sample observed by field emission scanning electron microscope. The EDX spectra show that all studied samples are stoichiometric. AFM images show that the surface roughness are varying from 34.72 nm to 89.83 nm. The XRD pattern shows that the brookite phase is present in pure, doped and co-doped samples. The cell parameters have been analyzed by FULLPROF package. The lattice parameters are found in good agreements with the experimental and theoretical results. For different compositions of as-deposited studied films, the optical band gap energy varies from 3.52 eV to 3.70 eV for doped and 3.52 eV to 3.57 eV for co-doped samples. The optical properties show that the films are highly transparent in the visible region. The (Al, Co) samples transparency decrease with increasing co-doping as compared to pristine one and reverse phenomena occurred in absorption spectra. The resistivity gradually decreases with an increase of temperature, which confirms that the samples exhibit semiconducting behavior. Experimental studied structural parameters were used in theoretical calculation via Material studio 8.0. The mechanical properties indicated that the optimized sample is ductile and possesses a low bulk modulus. Pristine and Al-doped and (Al, Co) co-doped samples are found to have an indirect electrical band gap, mechanical stability. The electrical band gap of the samples varies from 3.11 eV to 4.39 eV for doped and 2.38 eV to 3.11 eV for co-doped samples. The absorbance edge is shifted to the lower energy region after (Al, Co) co-doped (red shift). The charge density map indicated that the covalent bond is present in all samples. A combined analysis of the structural, morphological, electronic, and optical properties of the compound suggests that the studied sample is a potential candidate for gas sensing and optoelectronic applications.