Mesoporous properties of nanosized anatase titania powders prepared by urea hydrolysis with PEG dispersant

Abstract

Titania has three polymorphs: anatase, rutile and brookite. Nanosized titania has received extensive investigation because of its specific physico-chemical properties and wide applications in photoconductor, highly pigments, filler, coatings, dielectric materials and catalysts support [1–4]. The studies on the preparation, morphology, particle size and crystalline phase transformation of the nanosized titania powders are very extensive, but the textural properties and pore characterization of the nanosized anatase titania with mesostructure are little investigated. In this paper, we report the preparation of this material by homogeneous precipitation of titanium sulfate (Ti(SO4)2) and urea in aqueous solution of polyethylene glycol (PEG). The pore characterizations of as-synthesized and calcined powders were investigated. Ti(SO4)2 and urea were used as starting materials. PEG (average molecular weight from 200 to 19 000) was used as dispersant to give static hindrance to the precipitating particles. The urea and Ti(SO4)2 with controlled mole ratio between 2.67 and 10.0 were dissolved in deionized water at room temperature to obtain Ti(SO4)2 solution in concentration of 0.4 mol/L. To this solution, different dosage of PEG was added and then the mixture was heated with reflux condenser to reach a constant temperature (104 ◦C). Urea hydrolyzed gradually till no gas could be detected. The resulting precipitates, washed with deionized water to eliminate SO2− 4 ions, and then washed with absolute ethanol for several times, were dried at room temperature in air. The calcination of the powders was carried out at 360 ◦C in air for 5 h. The precipitate morphology and electron diffraction patterns were examined by transmission electron microscopy (TEM) in a Jeol JEM2010 operating at 200 kV. The crystalline phase identification was performed by X-ray diffraction (XRD) (Model D/max-2400, Rigaku, Japan) method using Cu Kα radiation. The reflection of the plane (101) was used for the crystallite size determination by Scherrer formula. The specific surface areas of as-synthesized and calcined powders were measured by Brunauer-Emmett-Teller (BET) method using nitrogen adsorption apparatus (Model ASAP 2000, Micromeritics). When the starting solution was heated at average rate of 2 ◦C/min for about 50 min, the reaction temperature could be fixed at 104 ◦C because the reaction was carried out on the Florence flask with reflux condenser. Urea hydrolyzed gradually above 60 ◦C bringing turbid solution, which showed precipitates were produced. Throughout this process, the pH value of solution increased with reaction time. Fig. 1 shows the pH value during the prepared process as a function of reaction time at different urea/Ti4+ mole ratio. At the time of pH value staying at a fixed value, the urea hydrolysis was over. Fig. 2 shows the pH value during the prepared process with different molecular weight of PEG as a function of reaction time. From these two figures, it can be seen that the urea concentration affects pH value of the solution, while the molecular weight of PEG has little significant effect on the pH value. Increasing