DENSITY FUNCTIONAL STUDY OF STRUCTURAL AND ELECTRONIC PROPERTIES OF Ca AND Mg DOPED TIO2

Abstract

The scarcity of affordable and environment-friendly sources of energy has led to emergence of photo catalysis to mitigate this problem. This is so if the photo catalysts are active in solar energy spectrum. Environment-friendly, non-toxic and economical photo catalysts would find application in water treatment, providing another solution to the problem of clean water for domestic use. Of many materials available for photo catalysis is Titanium (IV) oxide (TiO2). It possesses many merits such as low cost, high photocatalytic activity, non-toxicity and high availability. However, drawbacks that limit its application include its optical absorption that falls in the ultraviolet part of the electromagnetic spectrum. To extend the optical absorption to a wider region of the solar energy spectrum, various dopants have been added to the oxide to improve its solar efficiency. TiO2 has rapid electron-hole recombination which leads to low rates of the desired chemical transformations in energy absorption. This research modified the structure and electronic properties of pure rutile TiO2 by doping it with alkaline earth metals Ca and Mg for improved photocatalytic application. The objectives were structural optimization of rutile TiO2 and alkaline metal (Mg, Ca) doped rutile using Density Functional Theory (DFT) and determination of electronic structure of rutile TiO2 and alkaline metal (Mg, Ca) doped rutile using DFT. The DFT method as implemented in the Quantum ESPRESSO simulation package was used. The exchange correlation potential was treated with the Generalized Gradient Approximation. Total energy and ionic relaxation calculations were carried out after k-point and ECUT convergence tests. An ECUT energy of 40 Ry and 4x4x7 k-points were used for the total energy calculations. The optimized cell parameters for pure rutile crystal system are a = b = 4.603A c = 2.992A . There is an expansion of the crystal structure and its volume slightly increased. The calculated band gap of undoped rutile is 1.8 eV, which reduces doping with Mg and Ca. There is a shift of the valence band edge to higher energies and introduction of intraband dopant states. The changes in electronic structure are favorable for absorbance of a wider spectrum of solar energy and reduction of charge recombination during photo catalysis. Alkaline earth metal doping of TiO2 rutile modifies the structural and electronic properties of rutile TiO2 in a manner that would make it a more efficient photo catalyst. Co-doping with the alkaline metal dopants could be attempted to investigate the combined effect.