Controllable Nanocarving of Anatase TiO2 Single Crystals with Reactive {001} Facets

Abstract

Owing to its wide applications in photocatalysis, photo/ electrochromics, solar cells, sensors, and smart surface coatings, anatase titanium dioxide (TiO2) has attracted intensive research interests during the past decades. Most available and stable anatase TiO2 single crystals have truncated octahedral-bipyramid shapes, in which the majority of the surface area is normally surrounded by thermodynamically stable {101} facets, rather than the more reactive {001} facets. For a long period, tailored synthesis of {001}-faceted anatase TiO2 single crystals has been a great challenge. Recently, we made a breakthrough in the hydrothermal synthesis of anatase TiO2 single crystals with a large percentage (35–47%) of highly reactive {001} facets by using hydrofluoric acid (HF) as a specific morphology-controlling agent. Later, with the assistant of isopropanol (iPrOH)—a synergistic capping agent together with HF, anatase TiO2 single crystals with a higher ratio of {001} facets were prepared through a water–isopropanol solvothermal synthetic route. During these processes, HF was reported to act as a capping agent by reversing the relative stability of {001} and {101} facets and therefore promoting the growth of {001}faceted surfaces of anatase TiO2. By using similar strategies, a series of studies by research groups worldwide have demonstrated the syntheses and applications of anatase TiO2 with exposed reactive {001} facets. Unfortunately, few researchers have explored the etching effect of HF, which was traditionally considered a strong etching agent for metal oxides. Thus studying the chemical stability of {001}-faceted anatase TiO2 in the presence of HF is quite important and can help to understand the real function of HF during these important chemical processes. Herein, the dual role of HF—as capping agent and etching agent—is revealed through the controllable nanocarving of {001}-faceted anatase TiO2 single crystals. This process was realized through thermal treatment of anatase TiO2 single crystals with 35% of {001} facets by using HF-containing solutions or just by one-step solvothermal reactions of aqueous titanium tetrafluoride (TiF4) solutions in mixed solvents containing HF. In addition, a feasible mechanism is proposed to clarify the etching process involving HF and the growth process of the anatase TiO2 products with diverse morphologies. Figure 1a presents a typical SEM image of anatase TiO2 single crystals with 35% of reactive {001} facets, which was synthesized according to the method described in our earlier