Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity

Abstract

Mesoporous hollow TiO₂ shells with controllable crystallinity have been successfully synthesized by using a novel partial etching and re-calcination process. This method involves several sequential preparation steps as follows: 1) Synthesis of SiO₂@TiO₂@SiO₂ colloidal composites through sol–gel processes and crystallization by calcination, 2) partial etching to preferentially remove portions of the SiO₂ layers contacting the TiO₂ surface, and 3) re-calcination to crystallize the TiO₂ and finally etching of the inner and outer SiO₂ to produce mesoporous anatase TiO₂ shells. The partial etching step produces a small gap between SiO₂ and TiO₂ layers which allows space for the TiO₂ to further grow into large crystal grains. The re-calcination process leads to well developed crystalline TiO₂ which maintains the mesoporous shell structure due to the protection of the partially etched outer silica layer. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as-prepared mesoporous TiO₂ shells show significantly enhanced catalytic activity. In particular, TiO₂ shells synthesized with optimal crystallinity by using this approach show higher performance than commercial P25 TiO₂.