Among the three major phases in titania, anatase is reported to be a better photocatalytically active phase. Anatase to rutile transformations, under normal conditions, usually occurs at a temperatue range of 600-700°C. Various chemical additives have previously been employed to extend the anatase transition to higher temperatures. The effect of employing various concentrations of formic acid and water on phase transition has systematically been studied by XRD, FTIR, and Raman spectroscopy. A considerably higher anatase phase (41%) has been obtained at 800°C, and 10% anatase composition is retained after annealing the materials at 900°C for the optimized composition. On comparison, a control sample which has been prepared without formic acid showed that the rutile phase formed at a temperature of 600°C, FTIR and Raman studies indicated that the formate group favored a bridging (syn-anti or syn-syn) mode of chelation depending on the reaction conditions. It has been concluded that the resulting syn-anti binding hinders cross-linking of the gel network, resulting in a weakened structure and thus causing the anatase to rutile transformation temperature to occur at a lower temperature than with the syn-syn mode of binding where more ordered gel networks are formed.
Anatase to rutile transition 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 transition temperature. In the current study, the effect of addition of increasing amounts of silver to 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°Cshowed that 5 mol % Ag TiO 2 contained both anatase (46%) and rutile (54%), whereas the undoped sample primarily consists of anatase (95%). At 800°C all silver doped samples converted to 100% rutile and the undoped TiO 2 consisted of both anatase (55%) and rutile (45%). XPS analysis showed that Ag 0 and Ag 2 O has been formed on the surface of the titania formate complex without calcination (>100°C) indicating that photo-oxidation has 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 (Ag 2 O and Ag 0 ) 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 an early rutile formation.
Semiconductor research has recently shown great promise recently in areas such as hydrogen production through photocatalytic water splitting, 1 dye-sensitized solar cells, 2 and photocatalytic remediation of harmful organics from air and water. 3,4 Of the semiconductors investigated, titanium dioxide (TiO 2 ) has received the most attention, because of its excellent stability, nontoxicity, cost, and availability, as well as its ability to produce highly oxidizing radicals. 5,6 Zinc oxide (ZnO) has also shown promise in the areas of solar cells, 7 gas sensors, 8,9 and photocatalysis. 10 The simultaneous synthesis of a ZnO-TiO 2 composite usually results in the formation of one or more of the three known compounds from the ZnO-TiO 2 system: Zn 2 TiO 4 (zinc orthotitanate), with a cubic spinel crystal structure; Zn 2 Ti 3 O 8 , with a cubic defect spinel structure; and ZnTiO 3 (zinc metatitanate), with a rhombohedral ilmenite structure. 11-14 Zn 2 Ti 3 O 8 is the metastable, low-temperature (<820 °C) form of ZnTiO 3 ; 14 however, pure ZnTiO 3 is not easily obtained, because it transforms to Zn 2 TiO 4 and rutile. 12,13 Zinc orthotitanate (Zn 2 TiO 4 ) is an inverse spinel that has been used as a catalyst and a pigment. 15 As a catalyst, ZnTiO 4 is used as a sorbent for the removal of sulfur from coal gasification product gases, in hot gas desulfurization units, at temperatures in the range of 400-700 °C. Zinc orthotitanate can easily withstand these temperatures, but it is also one of the leading regenerable catalysts. [15][16][17][18][19][20][21] Zn 2 TiO 4 may also be used for the photocatalytic
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