Effect of germanium oxide (GeO2) additive on the anatase-to-rutile phase transition

The effect of germanium oxide (GeO2) addition on the anatase-to-rutile phase transition was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). TiO2 xerogels, containing up to 20 mol% GeO2 were prepared by refluxing and hydrolyzing titanium tetraisopropoxide (TTIP) and germanium butoxide (GB) using nitric acid as a catalyst. The following occurred with increasing amounts of GeO2 in the xerogels: (i) the crystallization temperature of anatase increased from 410degrees to 565degreesC and the DTA temperature of the anatase-to-rutile phase transition increased from 676degrees to 977degreesC in 20 mol% GeO2-containing xerogel; (ii) the crystallite size of anatase became smaller; (iii) the lattice a-parameter of the anatase showed little change, but the c-parameter decreased up to 20 mol% GeO2; (iv) both the lattice a- and c-parameters of the rutile decreased monotonically. From these results, the added GeO2 is considered to become incorporated into the anatase structure. The following occurred with increasing anatase heating temperature: (i) the lattice c-parameter of the anatase increased gradually and approached the value of pure anatase; and (ii) the chemical composition of the xerogel surfaces, measured by XPS, showed an increase in GeO2 content, indicating the expulsion of GeO2 from the anatase to form an amorphous surface layer. The formation of this amorphous GeO2 surface layer is thought to play an important role in retarding the anatase-to-rutile phase transition by suppressing diffusion between the anatase particles in direct contact, and limiting their ability to act as surface nucleation sites for rutile, as in the case of SiO2 additions. However, GeO2 addition is less effective than SiO2 in retarding the phase transition.