Synthesis of TiO₂ nanoparticles by hydrolysis of TEOT and decrease of particle size using a two-stage mixed method

Abstract

A new preparation method of particles was introduced, which consisted of a semibatch–batch two-stage reaction to decrease the size of TiO₂ fine particles. Using a semibatch process as a first stage, the particle size grew to a certain level (132 nm). Using a batch process as a second stage, however, the particle size decreased about 42 nm. The particles prepared by using a two-stage (semibatch–batch) method had a smaller mean particle size and smaller standard deviation than those obtained from the single-stage process. In this work, the statistical experimental method was also used to compare the various properties of the TiO₂ particles according to the six parameters [concentration of TEOT during 1st and 2nd stages, reaction temperature during 1st and 2nd stages, the amount of hydroxypropyl cellulose (HPC) during 1st and 2nd stages]. It was found that the optimum conditions for the maximum reduction of TiO₂ particle size using this method were as follows: (1st TEOT (M): 0.091, 2nd TEOT (M): 0.146, 1st temperature (°C): 20.0, 2nd temperature (°C): 38.2, 1st HPC (×10⁻² g/cm³): 0.018, 2nd HPC (×10⁻² g/cm³): 0.290). The experimental results measured by using the above optimum conditions were in agreement with calculated results and produced the smallest size (∼42 nm in diameter) with the value of DPS (−90 nm). In addition, we found that the main parameters affecting the decrease of particle size were concentration of TEOT and the reaction temperature during second stage.

Introduction

Metal oxide fine particles such as Al₂O₃ [1], SiO₂ [2], Ta₂O₅ [3], TiO₂ [4], [5], and ZrO₂ [6] are widely used in industrial applications as catalysts, ceramics, pigments, and so on [7]. Of those metal oxide particles, TiO₂ particles are used industrially in pigments, catalyst supports, electronic devices, mechanical materials, etc. [8]. TiO₂ nanoparticles less than 100 nm in diameter, especially, possess interesting optical, dielectric, and photocatalytic properties. Therefore, many efforts have sought to produce TiO₂ nanoparticles with controlled size, shape, and porosity for use in thin films, ceramics, composites, and catalysts. In addition, ultrafine-grained compacts of TiO₂, so-called nanophase TiO₂, are expected to have high mechanical strength and low sintering temperature. These nanometer-sized effects are caused by the large surface-to-volume ratio, resulting in more atoms along the grain boundaries than in the bulk material. Ultrafine TiO₂ agglomerates fuse at lower temperature than other ceramics; accordingly, they can be used as sintering assisting material.

The most common procedure for the preparation of TiO₂ particles reported in literature is based on the hydrolysis of acidic solutions of Ti(IV) salts. In addition, gas-phase oxidation reactions of TiCl₄ [9], [10], [11] and hydrolysis reaction of titanium alkoxides [12], [13] have been employed to generate finely divided, high-purity TiO₂ powders. To prevent agglomeration, Jean and Ring [14] have used hydroxypropyl cellulose to provide steric stabilization during the precipitation of TiO₂ particles and synthesized monodispersed powders. Nanophase ceramics with an average grain size of less than 100 nm have generated considerable scientific interest recently because of the improvements in a variety of properties that are expected to result from grain size reduction to the nanometer scale. Many efforts have been directed toward the synthesis of submicron particles with a narrow size distribution for the preparation of nanophase ceramic materials [15], [16], [17] that have improved properties [18], [19]. In most cases, the nanoparticles are synthesized in a vacuum or under low pressure by evaporation of a bulk source material. TiO₂ particles have also been made by reaction in aerosols [20], [21].

Generally, TiO₂ fine particles were prepared by the controlled hydrolysis and condensation of titanium tetra ethoxide (TEOT) in the dilute alcohol solution using a batch precipitation technique on a little scale, in which the hydrolysis was carried out using a beaker and a magnetic mixer under the nitrogen atmosphere. The precipitation parameters such as a reactant concentration, a concentration ratio (R) of water and TEOT, reaction temperature and ageing conditions had an effect on the particle size, size distribution, morphology, and state of agglomeration. However, TiO₂ fine particles through a batch process were not enough to obtain the nanophase particles with a narrow size distribution. In addition, Ring [22] has indicated several problems in the batch process: (1) high operating cost, (2) low productivity and (3) batch-to-batch product variation. Accordingly, this study prepared TiO₂ nanoparticles initially using a semibatch process, a system in which TEOT reactants are fed into the reactor with a constant flow rate, which is easier than a batch process in controlling the size, shape, and size distribution because of short nucleation and a slow hydrolysis rate, followed by the batch method. These nanophase materials have several technical applications like catalysis, lowering the sintering temperature, increasing the sintering rate, controlling the microstructure in high performance ceramic materials and magnetic applications. In this respect, we have made an attempt to produce TiO₂ nanoparticles using a semibatch–batch two-stage mixed reaction.

The objectives of this work are: (1) to suggest a new method for the synthesis of TiO₂ nanoparticles, (2) to establish the optimum conditions for preparing nanoparticles and for decreasing the TiO₂ particle size, and (3) to find the main parameters affecting the properties of TiO₂ nanoparticles. Furthermore, the agreement between the theoretical particle size calculated by the statistical experimental method and the experimental result is compared.