A systematic study of the effect of silver on the chelation of formic acid to a titanium precursor and the resulting effect on the anatase to rutile transformation of TiO₂

Anatase to rutile transformation in an unmodified synthetic titania usually occurs at a temperature range of 600-700 °C. Various methods such as addition of metallic and nonmetallic dopants and modifying the precursor have previously been reported to influence the anatase to rutile transformation temperature. In the current study, the effect of addition of increasing amounts of silver on the extent of chelation of a formate group to a titanium precursor and the resulting effects on the transformation of anatase to rutile has been studied. The addition of silver (0, 1, 3, and 5 mol %) on the anatase to rutile transformation temperature has been systematically followed by Fourier transform infrared (FTIR), Raman, X-ray diffraction (XRD), differential scanning calorimetry, and X-ray photoelectron spectroscopy (XPS) studies. From the FTIR and Raman spectroscopy studies it was observed that the incorporation of silver caused a reduction in the intensity of the COO^- stretches indicating that the titania formate bridging complex is becoming weaker in the presence of silver. XRD studies indicated an early rutile formation for the silver-doped samples. XRD of the samples calcined at 700 °C showed that 5 mol % Ag TiO₂ contained both anatase (46%) and rutile (54%), whereas the undoped sample primarily consisted of anatase (95%). At 800 °C all silver doped samples converted to 100% rutile and the undoped TiO₂ consisted of both anatase (55%) and rutile (45%). XPS analysis showed that Ag⁰ and Ag₂O formed on the surface of the titania formate complex without calcination (>100 °C) indicating that photo-oxidation occurred. FTIR, Raman, and XPS studies confirmed that the presence of silver in the xerogel before calcination may be responsible for the reduction of the titanium formate bridge. It was concluded that the presence of silver (Ag₂O and Ag⁰) hindered bridging ligands, which resulted in a weakened titanium gel network. This structurally weakened gel network could easily be collapsed during calcination, and it favors early rutile formation.