Enhanced photocatalytic activity of mesoporous TiO2 aggregates by embedding carbon nanotubes as electron-transfer channel

Abstract

Mesoporous multiwalled carbon nanotubes/titanium dioxide (CNTs/TiO₂) nanocomposites with low loading amounts (0–0.5 wt%) of CNTs embedded inside mesoporous TiO₂ aggregates has been prepared by a simple one-pot hydrothermal method using titanium sulfate as titanium source. The as-prepared CNTs/TiO₂ samples are carefully characterized, analyzed and discussed. In contrast to previous reports with high CNT loading, our results indicate that a low CNT loading slightly influences the textural properties (including crystallite size, degree of crystallinity, specific surface areas, and pore volume etc.) and UV-light absorption of the mesoporous TiO₂ aggregates. The SEM and TEM results demonstrate that the CNTs are mostly embedded in the mesoporous TiO₂ aggregates. Moreover, chemical bonds are formed at the interface between CNTs and TiO₂, which is confirmed by the Raman, IR and XPS analyses. Significantly, we point out that PL analysis in terms of intensity of PL signals seems to not be a reliable way to monitor the recombination rate in the CNTs/TiO₂ composite, due to the quenching effect of CNTs. Instead, the analysis of transient photocurrent responses is introduced, which definitely reflects CNTs as fast electron transfer channels in chemically-bonded CNTs/TiO₂ composites with low CNT loading. Notably, the positive synergy effects of CNTs and TiO₂ depend on both the CNT loading amount and the state of interfacial contacts. In our study, only these chemically bonded CNTs/TiO₂ nanocomposites with appropriate loading amounts (<0.1 wt%) favor the separation of photogenerated electron-hole pairs and decrease their recombination rate and thus display significantly enhanced photocatalytic activity for degrading acetone in air under UV irradiation, as compared with pristine TiO₂ counterparts and commercial P25 photocatalyst. In contrast, a high CNT loading (>0.1 wt%) results in a decrease in photocatalytic activity; a simple mechanical mixing of CNTs and TiO₂ without forming chemical bonds at the interface also results in inferior photocatalytic performance.