The influence of dopants on anatase-rutile phase transition

Abstract

Titanium dioxide exists in three different crystalline forms: anatase, rutile, and brookite. It is well known that on heating, anatase and brookite can be easily transformed to rutile which is considered as the most stable phase [1]. The aim of this study was to investigate the influence of different dopants on anatase-rutile phase transition. Doped TiO2 samples (TiO2-M, M = V, Mn, and Cu) containing 5 at% of the dopant were prepared by mixing anatase and appropriate oxide (V2O5, MnO2, and CuO) in agate mortar for 30 min. In order to determine the heat treatment conditions, TG/DTA analysis of the samples was performed. Finally, mixed powders as well as the pure anatase phase (TiO2) were heat treated at 700 °C for 3 h. XRD analysis was performed to estimate the phase composition, unit cell parameters, and crystallite sizes. Rutile was formed in all samples: 2.8 wt% in TiO2, 25.5 wt% in TiO2-Mn, 75.8 wt% in TiO2-V, and 95.2 wt% in TiO2-Cu. In TiO2- Mn, TiO2-V, and TiO2, anatase was present beside rutile, while in the case of TiO2-Cu 4.8 wt% of the unreacted CuO was found. Obtained results revealed that all the dopants accelerated anatase-rutile phase transition in the following order: Cu2+ > V5+ > Mn4+. It is well known that defects are the driving force for the anatase-rutile phase transition and since Ti4+ and Mn4+ are isovalent, no new defects were formed by incorporating Mn4+ ions into TiO2 lattice. This resulted in the least amount of rutile in TiO2-Mn comparing to TiO2-V and TiO2-Cu where new defects were probably formed. As no initial oxides were found in TiO2-Mn and TiO2-V, it can be concluded that Mn4+ and V5+ ions were incorporated into the anatase lattice. On the other hand, in the case of TiO2-Cu, even 4.8 wt% of initial CuO was found. The detected residue of CuO can be explained by the fact that the ionic radius of Cu2+ for an octahedral environment (0.870 Å) is much larger than that of Ti4+ (0.745 Å), unlike those for Mn4+ (0.670 Å) and V5+ (0.680 Å). Although almost all introduced quantity of CuO, i.e. 96 %, was found in TiO2-Cu, a small amount was surely necessary to cause the anatase-rutile phase transition. According to calculated crystallite sizes which were in the range of 55 – 90 nm, nanocrystalline samples were prepared. 1. P. I. Gouma, M. J. Mills J. Am. Ceram. Soc. 2001, 84 (3) 619–622.