The influence of alumina dopant on the structural transformation of gel-derived nanometre titania powders

Abstract

Nanometre materials, characterized by an ultrafine grain size, have attracted much attention in the past few years because of their unusual chemical, mechanical, optical, electrical and magnetic properties and their wide applicability [1]. Thermodynamically, however, they are metastable. Under certain conditions, the grains comprising the material may grow to a larger scale, which may consequently result in the disappearance of some unique properties of the nanometre material. Therefore, it is important to investigate the thermal stability of a nanostructural material. For the past two decades, sol-gel routes to ultrafine metallic oxide powders have been widely investigated [2]. Among these oxides, titania is a very important material for its humidity [3], hydrogen[4], and oxygen[5] sensitive properties and some catalysis applications [6]. Usually, titania has three different structures: brookite, anatase and futile. The former two phases are both metastable, and the futile phase is of a thermodynamic stable state. Some properties of titania may strongly depend on its microstructure; for example, many studies have suggested that the anatase phase of titania is the superior support of VzOs/TiO4 catalyst for the selective partial oxidation reaction compared to the rutile phase [6]. It is well known that many properties of ceramic materials can be improved by a small amount of doping. For gas-sensitive oxide materials, doping is often necessary to increase the sensitivity and selectivity [7] of the material. In this letter, nanometre titania powders with and without alumina dopant were prepared by a sol-gel method. The structural development of these powders was studied systematically, and the influence of a small amount of alumina dopant on the structural changes was also investigated. Tetrabutyl titanate and aluminium isopropoxide were used as the precursors of titania and alumina, respectively. In preparing TiO2 sol, Ti (OBu)4 was dissolved in ethanol, and then HC1 + H 2 0 solution was dropped into Ti(OBu)4 solution with continuous stirring for an hour. The molar ratio of these reactants was Ti(OBu)4:EtOH:HCl:H20=l:15: 0.3:1. After a week, a transparent orange gel was obtained. To form the alumina-doped titania sol, a given amount of aluminium isopropoxide was added to the Ti(OBu)4 solution (molar ratio AI(C3H70) to Yi(OBu)4, 0.12) and stirred thoroughly using a magnetic mixer and ultrasonic wave successively before the addition of HC1 + H 2 0 solution. The gelation time for alumina-doped titania sol is also about a week. After drying in a vacuum tube (10 -1 Pa) furnace at 333 K for 5 h, the gels were heat treated at different temperatures for 2 h under oxygen atmosphere (O2 flow rate: 40mlmin-1). Changes in the structures of these powders with temperature were investigated by thermogravity analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) experiments. Both TiO2 dry gels without and with A1203 dopant were confirmed to be of amorphous structure by XRD experiments, as shown in Fig. la and b. For pure titania gel (Fig. la), partial crystallization occurred after annealing at 523 K for 2 h, and the powders annealed at a temperature below 773 K are of anatase structure. A phase transformation from anatase to rutile occurred for annealing temperatures above 823 K and was completed at 1073 K. On the other hand, the alumina-doped TiO2 gel (Fig. lb) remained amorphous after annealing at 623 K for 2 h. When the annealing temperature was 723 K, crystallization began, and the phase transformation from anatase to rutile did not occur until the annealing temperature was elevated to 1073 K. Moreover, a few anatase crystallites still existed at an annealing temperature of 1223 K. This leads to the conclusion that a little alumina addition