Comparison of photocatalytic performance of anatase TiO₂ prepared by low and high temperature route

Abstract

Anatase TiO₂ was prepared by a facile sol–gel method at low temperature through tailoring the pH of sol–gel without calcination. As a control, anatase TiO₂ was also synthesized by the conventional sol–gel process, in which calcination at 500 °C was required to transform the amorphous oxide into highly crystalline anatase. As-prepared samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). Their photocatalytic activities were evaluated by degradation of methyl orange under UV light irradiation. On the basis of experiment results, it could be concluded that TiO₂ prepared by low temperature route showed more advantages in small particle size, highly dispersion nature, abundance of surface hydroxyl groups, strong PL signal, and high photocatalytic activity over TiO₂ obtained by the conventional sol–gel process. Furthermore, the reason of the former possessing higher photocatalytic activity was discussed.

Introduction

TiO₂ is well-known for its applications in high refractive optics, oxide semiconductors, oxygen sensors, photovoltaics, photocatalysis, and pigments [1], [2], [3]. The applications of TiO₂ are strongly dependent on its parameters such as crystal phase, particle size and surface structure (like surface hydroxyl, oxygen vacancy, etc.) [4], whereas these parameters are closely related to the preparation method. Sol–gel was often employed to prepare TiO₂ because of its simplicity and low equipment requirement. The conventional sol–gel process usually involved uncontrollable fast hydrolysis and condensation, and therefore could result in the formation of amorphous TiO₂. A subsequent high temperature thermal treatment, generally calcination at 500 °C, was required to transform the amorphous oxide into highly crystalline anatase [5]. The high temperature would seriously affect the particle size and surface structure, and even would result in a collapse of the mesoporous structure [6]. It is thus necessary to synthesize nanocrystalline anatase under mild conditions.

Up to date, there have been some reports on the synthesis of nanocrystalline anatase under mild conditions. For instance, Chen et al. prepared nanostructure anatase by heating the mixture of Ti and NH₄Cl powders in air at 300 °C [7]. Using a simple soft chemistry method, Melghit et al. synthesized TiO₂·xH₂O gel through the reaction of TiCl₃ and NH₄OH and H₂O₂, followed by treatment at 300 °C to transfer the gel to anatase structure with nanospherical shape [8]. Zhu et al. obtained the anatase TiO₂ by a nonhydrolytic sol–gel reaction of TiCl₄ and benzyl alcohol at low temperature, followed by subsequent 400 °C calcination to form the high nanocrystalline anatase TiO₂ [9]. Li et al. adopted a novel simple method to prepare nanoanatase TiO₂ of high crystalline at a low temperature of 100 °C using diethyl ether anhydrous as the solvent [10]. Besides these, the most general method for synthesizing nanocrystalline titania at low temperature, usually below 150 °C, was hydrothermal synthesis [11], [12] and supercritical CO₂ technique [13], in which high pressure and special equipment were often required.

Recently, we have synthesized successfully nanocrystalline anatase TiO₂ at 65 °C by a facile sol–gel method via the control of pH [14]. This method avoided the high temperature treatment, which guarantees the product particles obtained in the nanosize range. Furthermore, the prepared TiO₂, which was rich in surface hydroxyl groups, was found to exhibit high dispersibility [14]. It is anticipated that thus synthesized anatase TiO₂ should exhibit high photocatalytic activities. In this work, anatase TiO₂ was prepared by above mentioned sol–gel method at low temperature. Photocatalytic performance of sample was investigated by photodegradation of methyl orange (MO) under UV light irradiation. As a control, anatase TiO₂ was also synthesized by the conventional sol–gel process, in which calcination at 500 °C was required to transform the amorphous oxide into highly crystalline anatase. Interestingly, anatase TiO₂ prepared by the low temperature route showed far higher activity in the photodegradation of MO, compared to anatase TiO₂ prepared by the conventional sol–gel process. Based on the characterizations, the reason of better performance of the former was discussed.