The preparation of brookite particles was tried by self-hydrolysis of TiOCl₂ with HCl addition. When 0.1 M TiOCl₂ solution was heated at 100°C by reflux method, the precipitate was obtained within 15 min and was identified as brookite and anatase. By addition of HCl, the starting time of precipitation was delayed and small brookite formed beside rutile large rod particles. These results suggest that deprotonation at the early stage of reaction was suppressed but crystal growth at the late stage was promoted because of an increase in the solubility of TiO₂ sols.
When the TiOCl₂ solution was heated at 100°C for 24 h in autoclave, rutile large particles and brookite small particles formed separately. The small brookite particles could be separated by centrifugation. The photocatalytic activity of synthesized brookite particles was lower compared to commercial brookite particles. When hydrothermal treatment was carried out for synthesized brookite, the photocatalytic activity was enhanced. This was due to improvement of crystallinity of brookite.