An etching–redeposition isomerization process for the shape control of anatase TiO2 nanocrystals

Shape control of inorganic nanocrystals (NCs) is critical for tuning their physical and chemical properties and consequently has great impact on their performance in applications. Most of the previous studies on the shape control of NCs have been focused on strategies toward controlling the growth process. Strategies and theories toward the reverse process, e.g., post-synthetic chemical transformation, are yet to be systematically investigated and established. Here we report that structural control of anatase TiO2 NCs can be achieved by a controllable etching and re-deposition process. Fluorides were employed as both a chemical etchant and a shape directing agent in a nonaqueous sol–gel synthesis reaction. The trace amount of hydrofluoric acid formed at increased temperature is responsible for providing the driving force for the shape transformation of TiO2 NCs. Accordingly, the shape can be continuously tuned from rods to truncated octahedra. The as-prepared TiO2 NCs with different shapes showed photocatalytic activities with a clear trend, which proved the effectiveness of this shape control strategy for finding TiO2 photocatalysts with optimized performance. This post-synthetic chemical transformation method can be developed into a general strategy for different types of NCs with improved performances in various applications.