An intriguing case of morphology control and phase transitions in TiO2 nanostructures with enhanced photocatalytic activity

Abstract

The control over crystal structure and morphology is crucial to design an active catalyst, particularly where several accessible morphologies and phases are possible. This work presents a strategy of how TiO2 transforms from anatase phase, passing through mixed phase transitions, to pure rutile phase. The XRD result show phase transformations, that are accompanied with the morphology changes from anatase particulates to well-defined rod-shaped rutile characterized through TEM. While the steady state photoluminance (SSPL) spectra indicate the mixed phase display enhanced photoluminescence (PL) as compared to individual anatase or rutile phase. The Time-resolved photoluminescence (TRPL) measurements also demonstrate enhanced PL lifetime in anatase-rutile mixed phase sample, whereas valence band XPS results confirm the existence of energy differences in band edge positions of pure anatase, (3.31 eV) anatase-rutile phase heterojunction (3.15 eV) and rutile TiO2 (3.08 eV). These differences are evident as photocatalytic activity results reveal that the mixed phase sample exhibits superior performance (8 mmol g−1h−1 of hydrogen evolution from water under UV illumination) than pristine phases. We attribute this higher hydrogen evolution rate to the built-in electric field (type-II heterojunctions) that facilitates charge transfer across the interface in mixed phase sample. This example highlights the scope of fine-tuning structure-property relationships in a myriad of materials in a similar fashion.