Ultrafine TiO_2 powders as rutile and anatase phase were simply precipitated at room temperature for only tens of hours by simply controlling the pH value and Ti~(4+) concentration via aqueous TiCl_4 solution. Under the optimal pH value and Ti~(4+) concentration, the average particle size of powders with rutile phase was 3.7nm, while that of powders with anatase phase was 3.0nm. The average particle size was calculated from the broadening of corresponding X-ray spectral peaks by Scherrer formula. In addition, 3.0 mol.l~(-1) are suggested to be used as concentration of stock solutions instead of the current concentration 2.0 mol.l~(-1).
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