Formation of metal oxide nanotubes in neutral aqueous solution based on a photocatalytic effect.

Hollow nanotubes are of great interest for use as catalysts and gene delivery vehicles, and in terahertz electronic devices. Diverse approaches to fabricating such hollow nanostructures have been discussed: catalytic growth, transformation of core–shell nanowires into nanotubes by the Kirkendall effect, the use of porous templates, hydrothermal synthesis based on ion-exchange reactions, selfassembly, and gas-phase synthesis. Alternative methods with simple and straightforward procedures may provide new insights into the design of nanomaterials. Here, we present a new concept for forming nanotubes based on a photocatalytic effect. As examples, ZnO–TiO2 and CuO–TiO2 core–shell nanowires undergo a reaction in a neutral aqueous solution under UV light. The resulting oxide nanotubes show a uniform and tunable wall thickness. Mechanisms for the tube formation will be suggested and discussed. The morphology of the ZnO–TiO2 core–shell nanowires was investigated by transmission electron microscopy (TEM). For details on sample preparation see the Experimental Section. Figure 1a shows typical ZnO nanowires coated with a TiO2 shell. A uniform layer of amorphous TiO2 with a thickness of 25 nm on a single-crystalline ZnO nanowire is visible in Figure 1b. Energy-dispersive X-ray (EDX) analysis revealed a characteristic intensity profile of core–shell nanowires where the intensities of Zn and Ti are predominantly in the core and the shell part, respectively (Figure 1e). After exposure of the samples in neutral aqueous solution to UV light for 2 h, hollow TiO2 nanotubes with uniform, smooth walls had formed (Figure 1c; a magnified view of a single nanotube is shown in Figure 1d). ZnO was completely eliminated from the core part of the nanowires, and the reaction resulted in a TiO2 nanotube without any structural modification or damage. EDX line analysis across the nanotube revealed the distinct presence of Ti in the nanotube. The intensity of the Zn signal in the TiO2 shell was below the detection limit (Figure 1 f). We evoke a photocatalytic effect as part of the etching mechanism that leads to the formation of the hollow nanotubes. It is known that the photocatalytic reaction of water with TiO2 produces H + ions. 13] Once the aqueous solution in the vicinity of the TiO2 surface under UV irradiation is saturated with H ions, the ions might diffuse partly to the ZnO core through the nanoshell. The diffusion length of H ions through a TiO2 shell is apparently more than 50 nm since TiO2 nanotubes with a thickness of 50 nm were formed in controlled experiments. Eventually, a chemical reaction can take place continuously at the ZnO–TiO2 interface. To verify the diffusion of H ions through the oxide shell, a controlled Figure 1. a) Typical TEM image of single-crystalline ZnO nanowires coated with a TiO2 layer 25 nm thick by atomic layer deposition; scale bar: 250 nm. b) Magnified view of a ZnO–TiO2 core–shell nanowire; scale bar: 80 nm. c) Typical TEM image of TiO2 nanotubes formed under UV irradiation for 2 h in aqueous solution; scale bar: 650 nm. d) Magnified view of a TiO2 nanotube; scale bar: 100 nm. e) EDX line analysis across a single ZnO–TiO2 core–shell nanowire. f) EDX line analysis across a single TiO2 nanotube.