Structural and Optical Modulations of Wide Band Gap Semiconductor: Titanium Dioxide (TiO 2 )

Abstract

Structural and Optical Modulations of Wide Band Gap Semiconductor: Titanium Dioxide (TiO 2 ) Titanium dioxide (TiO 2 ) is a very interesting material due to its number of applications, e.g., high dielectric constant, photocatalytic application. The photocatalytic activity of TiO 2 was first demonstrated in 1972. TiO 2 has a wide band gap (3.0 - 3.2 eV), thus most of the absorption occurs in the UV range, where solar spectrum consists of only 5 - 8%. It is of immense importance to enhance its photocatalytic activity in the visible part of the solar spectrum. This requires band gap tailoring, which can be achieved by doping. However, finding a suitable dopant for TiO 2 is quite critical. In the present work, structural properties and chemical composition of the Nd doped sputter deposited TiO 2 thin films were studied by varying the Nd concentrations from 0 to 2 at. % using RF sputtering. The results suggested that the incorporation of Nd produced stresses in lattice, which shifted diffraction peaks to a lower 2θ values, reduction in crystallite size, elongation of lattice along c-axis and specific texturing along (004) plane. Raman spectroscopy also confirmed asymmetry of bonds. Theoretical modeling of structure was also performed using VESTA code. SEM images confirmed the transformation of faceted to planar surface. XPS showed the presence of O vacancies and non-stoichiometry in films with increased dopant concentration. V doping in the TiO 2 films also displayed interesting results as V existed in many oxidation states with different ionic radii. With the addition of V, preferential growth along (211) plane occurred. The morphology was transformed from faceted to small elongated grains as the concentration of V increased. The substitution of V in place of Ti made the V−O bonds asymmetrical, which created stress to modify the crystallite and grain sizes. XPS showed low binding energy (L.B.E.) shift and an increase in the () B.E. difference of Ti-2p doublet peaks, which was due to increased nonstoichiometry in the films. This was also attributed to the substitution of V in the V 3+ and V 5+ states. Raman spectroscopy confirmed the increase in nonstoichiometry, phonon confinement, and asymmetry in the bonding with the increase in V concentration. This was also evident in the PL spectra, where strong xicontributions came from the states associated with V 5+ and V 3+ and dominated the spectra on oxygen vacancies (OV) and defect associated PL. Finally, the role of oxygen concentration and doping concentration in affecting the structural, phase transformation and chemical composition was studied using pulsed laser deposition (PLD). Oxygen partial pressure was varied from 100 mtorr (O 2 rich) to 0 mtorr (O 2 absent) and three concentrations of Nd were used as 1.0, 1.5 and 2.0 at.%. With the variation of oxygen and dopant concentrations, a sequence of phase transformations was observed. This also reduced the crystallite size and caused the film thickness to reduce. A reduction of Ti (Ti 4+ to Ti 3+ ) was observed and emergence of the O vacancies and consequently generation of nonstoichiometry in TiO 2 lattice, which resulted in the short range ordering of the TiO 2 structure. The present work suggested that the incorporation of Nd inhibits the phase transformation, which can be allowed by increasing the oxygen partial pressures.