Facile size-controllable synthesis process, bandgap blue shift, and enhanced photocatalytic performances of [111]-faceted anatase TiO2 nanocrystals

Abstract

The facet exposed on a nanocrystal surface strongly affects the physicochemical properties of the crystal surface but it is not easy to control in the synthesis process. Herein we demonstrate a facile synthesis process for size-controllable [111]-faceted anatase TiO₂ nanocrystals by hydrothermal treatment of a mixed solution of tetramethylammonium hydroxide solution and titanium isopropoxide. The [111]-faceted cubic anatase single nanocrystals are formed by a topochemical transformation reaction of nanosized titanate nanosheets to anatase nanofragments and Ostwald ripening crystal growth of the nanofragments. By using the size-controlled anatase nanocrystals, we have unveiled for the first time that the bandgap blue shift with reducing crystal size is dependent on the crystal-facet, and increases in the order of non-facet < [111]-facet < {010}-facet. The quantitative relationships between the bandgap and the surface area are given for the non-faceted, [111]-faceted, and {010}-faceted anatase nanocrystals. The photocatalytic studies on the [111]-faceted anatase TiO₂ nanocrystals, a commercial P25 sample, and a non-faceted ST-20 anatase nanocrystal sample reveal that the increasing photocatalytic activity order matches the increasing bandgap order.