Controllable Synthesis of Mesoporous TiO2 Hollow Shells: Toward an Efficient Photocatalyst

TiO2 hollow shells with well-controlled crystallinity, phase, and porosity are desirable in many applications. In photocatalysis in particular, they can provide high active surface area, reduced diffusion resistance, and improved accessibility to reactants. Here, the results from studies of the causes for the failure of a prior etching and calcination scheme to make such shells and on a newly-developed simple yet robust process for producing uniform mesoporous TiO2 shells with precisely controllable crystallinity and phase are reported. The key finding is that base etching of the SiO2@TiO2 core-shell particles leads to the formation of sodium titanate species, which, if not removed, promote substantial crystal growth during calcination and destroy the structural integrity of the TiO2 shells. A simple acid treatment of the base-etched samples may convert the sodium titanates into protonated titanates, which not only prevent the formation of the impurity phases, but also help to maintain the structural integrity of the shell and allow precise control of the TiO2 phase and crystallinity. This new development affords convenient optimization of the structure of the hollow TiO2 shells toward efficient photocatalysts, which outperform the commercial P25-TiO2 in the photocatalytic decomposition of organic dye molecules.