Temperature-Dependent Electronic Structure of TiO2 Thin Film Deposited by the Radio Frequency Reactive Magnetron Sputtering Technique: X ray Absorption Near-Edge Structure and X ray Photoelectron Spectroscopy

TiO2 thin films were prepared by the reactive magnetron cosputtering technique on the n-type silicon substrates and annealed at different temperatures ranging from 110 to 250 degrees C. The X-ray diffraction, transmission electron microscopy, and Raman spectroscopy measurements show the formation of TiO2 thin films at 250 degrees C in the anatase phase (101) with a tetragonal structure. The modification in the electronic structure with the increase of annealing temperatures is studied by the X-ray absorption near-edge structure (XANES) at O K- and Ti L-3,L-2-edges showing the splitting of pre-edge spectral features into t(2g) (Ti 3d + O 2p(pi)) and e(g) (Ti 3d + O 2p(sigma)) symmetry bands in the structural matrix. The intensities of the O K-edge and Ti L-3,L-2-edge increase with the annealing temperature because of induced structural disorder/distortion which could be correlated to the modification in unoccupancies that are associated with the hybridization of O-2p and Ti-3d states. The doublet Ti 2p(3/2) and Ti 2p(1/2) in Ti 2p X-ray photoelectron spectroscopy (XPS) are revealed with the t(2g) and e(g) symmetry bands along with the formation of Ti-O/Ti-OH and/or Ti-O-C bonds observed in O 1s XPS spectra. The estimated work functions (ionization potential) obtained from ultraviolet photoelectron spectroscopy were significantly reduced from approximate to 4.21 eV (approximate to 7.69 eV) to approximate to 3.64 eV (approximate to 7.01 eV) when the annealing temperature of the prepared TiO2 thin films is increased from 110 to 250 degrees C.