Anion-Assisted Synthesis of TiO2 Nanocrystals with Tunable Crystal Forms and Crystal Facets and Their Photocatalytic Redox Activities in Organic Reactions

In this work, we develop a facile and general strategy to control the crystal forms and crystal facets of TiO2 nanocrystals. Ti(OH)4 was used as the precursor and different anions were used as capping agents without any other organic surfactants. These anions can selectively adsorb on the specific crystal facets of anatase, inducing the transformation of conventional {101} facets to unconventional {001} facets and {100} facets or even phase transformation to rutile and brookite. Rutile and brookite TiO2 nanocrystals as well as anatase TiO2 nanocrystals with different facets ({101}, {001}, and {100}) exposed are obtained. Photocatalytic selective reduction of nitrobenzene and selective oxidation of benzyl alcohol are employed as a probe reaction to test the redox properties of the as-prepared TiO2 nanocrystals. The results show that the photocatalytic redox properties of TiO2 NCs are dependent on their crystal forms and crystal facets. Specially the photocatalytic activities of different anatase crystal facets show different orders in reduction and oxidation reactions, respectively. The reduction ability of different anatase crystal facets can be ranked as {101} > {001} > {100}. While the oxidation ability of different facets can be ranked as {101} ≈ {001} ≈ {100}. Surface and electronic structures should be the origin that account for their different activity orders in different reactions. Based on the results in the two model reactions, one important principle should be pointed out. When we discuss the crystal-facet-dependent catalytic activities of TiO2 nanocrystals, we should analyze the results based on specific reactions.