Two architectures of TiO2-based hybrid nanostructures were designed to improve the photocatalytic activity of the bare TiO2. The first one was a metal-semiconductor heterojunction and the second one was a TiO2 hybrid with different carbon allotropes. First, the ultra-fine (2 nm) Ag nanocrystallites-decorated TiO2 hollow sphere heterostructures were fabricated by a two-step hydrothermal method. It showed excellent photodegradation performance of the RhB dye. The efficiency could reach up to 100 % in 20 minutes under simulated sunlight irradiation.

Second, a comparative photocatalytic study of activated carbon-, grapheme-, carbon nanotubes- and fullerene-TiO2 was conducted under visible light irradiation. An enhanced photodegradation of the RhB dye was achieved by using these hybrid nanostructures over that of only using pure TiO2; and the fullerene-TiO2 showed the best photocatalytic performance. The XRD, FE-SEM, TEM, STEM, HRTEM coupled with EDX, UV-visible absorption and PL spectroscopy, XPS, and electrochemical impedance were carried out to correlate their morphology, microstructures, surface properties and electronic band structures to their photocatalytic activities.

In the metal-semiconductor heterostructures, the Ag nanocrystallites were of high crystallinity and distributed evenly on the surface of the TiO2 hollow spheres with compact interfaces. This would reduce the recombination rate of charge carriers and favoured the charge transfer across the interfaces. The increase in number of adsorbed oxygen also facilitated the generation of the hydroxyl radicals; and the reduced surface defects extended the life time of charge carriers upon Ag loading. The Schottky barriers between Ag and illuminated TiO2, upward shift of Fermi level and the increased electron density contributed to the charges separation and transport.

In the architecture of carbon materials-TiO2, the superior adsorption property, favorable chemical bond formation (Ti-O-C), narrower band gap, smaller particle size and effective charge carrier separation had significantly promoted the photocatalytic efficiencies. These two types of heterostructures showed excellent photocatalytic performance in environmental remediation by employing more than one function of components.

The interactions between TiO2 and Ag/carbon had a great effect on the control of morphology, modification of surface properties and electronic band structures. These key factors involved in photocatalysis will be of great importance to the design of functional and effective photocatalysts in future.

Qian Li

The Chinese University of Hong Kong

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