Effect of Agglomerated State in Mesoporous TiO2 on the Morphology of Photodeposited Pt and Photocatalytic Activity

Abstract

Two mesoporous TiO2 samples (M1-TiO2 and M2-TiO2) with different morphologies were synthesized, and the photocatalytic and photoelectrochemical properties of both TiO2 and their photoplatinized counterparts (0.05, 0.1, and 1.0 wt % of Pt) were systematically investigated. Electron microscopic analysis showed that M1-TiO2 consists of densely packed nanoparticles forming spherical secondary particles (0.5 to 1.0 mu m), whereas M2-TiO2 is made up of loosely agglomerated nanoparticles. Subsequently, this morphological difference led to the formation of different Pt clusters (photodeposited on them): large Pt nanopartides on M1-TiO2 versus well-dispersed smaller Pt nanoparticles on M2-TiO2. The photocatalytic activities of platinized M1-TiO2 and M2-TiO2 were investigated for H-2 production and 4-chlorophenol degradation. Whereas M1-TiO2 exhibited the highest photoactivity With 0.1 wt % Pt loading, the activity of M2-TiO2 increased with increasing Pt loading (up to 1.0 wt %). The critical role of surface Pt morphology on the photocatalytic behavior of M1-TiO2 and M2-TiO2 was investigated using electrochemical impedance spectroscopy and photocurrent measurements. In the case of M1-TiO2, an increase in Pt cluster size enhanced the charge-transfer resistance and reduced the interfacial electron transfer efficiency, whereas the same loading of Pt on M2-TiO2 effectively enhanced the interfacial charge transfer. This dissimilar interfacial charge-transfer kinetics for M1-TiO2 and M2-TiO2 indicates that the TiO2 microstructure controls the photodeposited Pt morphology, which subsequently affects the photocatalytic activity. This study reveals that the agglomerated state of TiO2 nanoparticles can be an important parameter in determining the photocatalytic activity in both the suspension and film states.