Seeding effect in hydrothermal synthesis of nanosize yttria

Abstract

Nanoscale particles are of interest in fundamental as well as applied research because many properties of materials change drastically when the crystallite size reaches the nano=submicrometer range [1]. Since nanoscale particles have relatively large surface areas, they tend to agglomerate to minimize the total surface energy. Agglomeration adversely affects their properties. One approach to make nanoscale particles without agglomeration involves the use of surfactant molecules during the synthesis stage [2]. The surfactant molecules adsorb on the surface of particles and stabilize the particle against agglomeration by either electrostatic repulsion or steric force [3]. This is one of the chemical approaches to control the particle size. It has been demonstrated that the physical properties, especially particle size and surface area of ceramic powders can be altered by adopting a combination of two processes, namely coprecipitation and hydrothermal. Beidellite Na0:4 Al2(Si3:6Al0:4)O10(OH)2 4H2O has been synthesized by a combination of sol-gel and hydrothermal processes, in which an ideal beidellite composition was prepared from either tetra ethyl orthosilicate (TEOS) [4] or colloidal silica [5]. Yanazisawa et al. [6] used the combination of coprecipitation and hydrothermal methods to obtain beidellite at a lower temperature without agglomeration. Recently, it has been reported that the formation of many transition metal oxides, e.g., Fe3O4, Fe2O3 and a-Al2O3 may be controlled by nucleating or ` seeding'' with, e.g., CuO, SiO2 and a-Al2O3, respectively [7±9]. Applying the seeding effect in order to enhance transformation kinetics and to control the development of a desired phase has been successfully investigated [9±11]. However, to our knowledge it has not been applied to yttria or doped yttria. The aim of this paper is to report the preparation of nanoscale yttria by a combined method of coprecipitation and hydrothermal processes in the presence of a seed. The coprecipitation method was employed to produce a gel in the presence of modi®er (a mixture of tween-80 and â-alanine). Seeds of Y2O3 were added to the coprecipitated gel. The gel was then subjected to a hydrothermal treatment. This study showed that the seeding not only represents a unique method to obtain nanosize yttria but also prevents the formation of hydroxide during the course of hydrothermal treatment. However, the seeding effect was found to be selective of the precursor of yttria. A precursor solution was prepared by dissolving a known amount of precursor salt (Y(NO3)3 5H2O or YCl3 4H2O) in water and stirring for 2 h. The modi®er solution was prepared by dissolving 10% wt=wt of the surface modi®er with respect to Y2O3 in 50 ml of an aqueous ammonium hydroxide solution (pH . 10) and stirring for an hour at room temperature. The precursor solution was then added to the modi®er solution drop-by-drop through a burette at a controlled rate (10 drops per minute) with vigorous stirring. Seeds of 4% wt=wt of Y2O3 particles were added to the gel. The hydrothermal treatment of the gel was carried out at 190 8C for 1, 2 and 4 h in a 300 cm stainless steel autoclave lined with Te on (Bergohof GmbH, Labortechnik; DAH904, Germany). The pressure inside the autoclave was not controlled. However, the autoclave was tight and it is felt that the pressure corresponded to the autogenous water vapour pressure. The resulting powder was washed by 20% acetic acid solution in water and dried in an oven at 60 8C for 24 h. The bulk density of the material was measured using a Micromeritics densitometer (Accu Pyc 1330). The infrared spectra of the samples were recorded on a Fourier transform infrared spectrometer (Brucker, IFS 25). The surface area was measured by applying multipoint Brunauer±Emmett±Teller (BET) for all samples with a minimum of 5 points on a Micrometrics Unit (ASAP 2400). Powders were degassed at 150 8C before surface area measurements. The crystalline phase was determined by powder X-ray diffraction (XRD) on a D-500 Siemens powder diffractometer. The microstructure of particles was investigated in a high resolution transmission electron microscope (HRTEM) (Philips, CM 200, FEG) operated at 200 kV with a line resolution of 0.14 nm. For the preparation of the TEM specimen, the powder was heavily diluted with acetone, partially deagglomerated by ultrasonic