Titanium dioxide (TiO2), also known as titania, is a strong photocatalyst that breaks down organic compounds when exposed to light. It is widely used as a coating material for preventing the build-up of dirt, bacteria and organic pollutants on outdoor surfaces. Self-cleaning glass, for example, typically has a thin coating of TiO2.
Of all the different forms of TiO2 coating, TiO2 nanotube arrays have probably the highest photocatalytic activity. The nanotubes in a TiO2 nanotube array are hollow, vertically aligned and densely packed; the ultrathin wall thickness of TiO2 nanotubes and the unique architecture of TiO2 nanotube arrays are some of the major factors contributing to the superior photocatalytic activity. Unfortunately, fabrication of TiO2 nanotube arrays on surfaces is still a significant challenge.
Gao Han and co-workers at the A*STAR Institute of Materials Research and Engineering1 have now fabricated a transparent palladium-functionalized TiO2 nanotube array on a glass substrate. They studied how the photocatalytic activity of the TiO2 nanotube array changes as the wall thickness or height of the nanotubes was adjusted, and identified the nanotube configuration that gave the highest photocatalytic activity.
To grow the TiO2 nanotube array, the researchers deposited TiO2 atom-by-atom onto a glass substrate masked with a porous alumina template. By sequentially passing titanium- and oxygen-bearing gases over the masked substrate, TiO2 nanotubes grew up through the pores. They then removed the template by selective etching, and added a dusting of palladium nanoparticles by ‘sputtering’ of atomic palladium from a high-purity source.
The researchers could control the shape, size and height of the pores in the template by varying its preparation conditions. They could also control the wall thickness of the nanotubes by changing the number of gas exposure cycles, resulting in nanotubes with walls thick enough to promote high photocatalytic activity but thin enough to provide good transparency.
Tests showed that TiO2 nanotube arrays with a wall thickness of 15 nanometers and height of 200 nanometers had the highest photocatalytic activity, degrading 76% of methylene blue — a model pollutant — in aqueous solution within four hours. The TiO2 nanotube arrays were also shown to degrade another model pollutant, stearic acid.
The researchers believe their fabrication technique is useful not only for making better self-cleaning glass (Fig. 1), but also for making self-cleaning ceramics and plastics. “Transparent TiO2 nanotube arrays grown on glass are not only important in self-cleaning applications, but also highly sought after in solar-cell applications,” says Gao. “We have demonstrated an easy and simple method to grow them in a controllable way.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering