Anatase TiO2 Crystal Facet Growth: Mechanistic Role of Hydrofluoric Acid and Photoelectrocatalytic Activity

This work reports a facile hydrothermal approach to directly grow anatase TiO2 crystals with exposed {001} facets on titanium foil substrate by controlling pH of HF solution. The mechanistic role of HF for control growth of the crystal facet of anatase TiO2 crystals has been investigated. The results demonstrate that controlling solution pH controls the extent of surface fluorination of anatase TiO2, hence the size, shape, morphology, and {001} faceted surface area of TiO2 crystals. The theoretical calculations reveal that {001) faceted surface fluorination of anatase TiO2 can merely occur via dissociative adsorption of HF molecules under acidic conditions while the adsorption of Na+F- is thermodynamically prohibited. This confirms that the presence of molecular form of HF but riot F- is essential for preservation of exposed {001} facets of anatase TiO2. Anatase TiO2 crystals with exposed {001} facets can be directly fabricated on titanium foil by controlling the solution pH <= 5.8. When pH is increased to near neutral and beyond (e.g., pH >= 6.6), the insufficient concentration of HF ([HF] <= 0.04%) dramatically reduces the extent of surface fluorination, leading to the formation of anatase TiO2 crystals with {101} facets and titanate nanorods/nanosheets. The anatase TiO2 nanocrystals with exposed {001} facets exhibits a superior photoelectrocatalytic activity toward water oxidation. The findings of this work clarify the mechanistic role of HF for controlling the crystal facet growth, providing a facile means for massive production of desired nanostructures with high reactive facets on solid substrates for other metal oxides.