Titanium dioxide (TiO2) is a wide-bandgap semiconductor compound that crystallizes in different polymorphic forms. The best known of these are rutile and anatase crystal structures that have bandgaps in the UV-region of the electromagnetic spectrum (3.0 eV for the rutile and 3.2 eV for the anatase phase). TiO2 is also a photocatalyst and its photocatalytic properties can be improved by nanostructuring the material – that is, by increasing the surface-to-volume ratio as the geometric dimension of a sample is reduced. Nanotubes made from TiO2 can be used in devices to purify air, for example, to make self-cleaning surfaces, in photovoltaics, sensors and in biomedicine.

Now, a team led by Ion Tiginyanu of the Academy of Sciences and Technical University of Moldova and Oliver Schmidt at the Institute for Integrative Nanosciences in Dresden has succeeded in making light-driven micro-engines from TiO2 nanotubes. The researchers studied the process of electrochemical oxidation of titanium sheets and developed a technique to fabricate nanotubes with inner diameters that vary between 50–120 nm from one end of a tube to another. The nanotubes produced have a fixed outer diameter and are connected to each other by bridging nanomembranes that make the network of tubes mechanically strong.

Making the engines move

When placed in hydrogen peroxide (H2O2) solution and illuminated with UV light, the nanotubes begin to move through the solution. “The H2O2 solution photocatalytically decomposes into oxygen and water and produces bubbles in the confined space inside the nanotubes,” explains Tiginyanu. “The oxygen bubbles are ejected through the wider end of the nanotubes, thus generating the driving force for the nanotubes to move.”

“UV-light induced motion of the micro-engines also occurs in pure water,” he adds. In this case it occurs thanks to diffusiophoresis, which is related to the concentration gradient of the chemical species O2-, H+, and OH- photocatalytically generated at the inner surfaces of the nanotube microarrays.

Two-fold action

“We found that upon UV-irradiation, the micro-engines capture micron-sized particles at the narrower ends of the TiO2 nanotubes and transport these particles,” he tells nanotechweb.org. “In fact, the narrower ends of the tubes serve to absorb the fuel (the H2O2 solution) and the wider ends act as an exhaust nozzle. The micro-engines can also carry a cargo thanks to their two-fold action: the narrower ends of the tubes pick up and retain the cargo particles, while the wider ends of most of the tubes continue to act as an engine, allowing the device to move through solution.”

The cargo can be released when the UV light is switched off, he says.

The researchers, reporting their work in Small DOI: 10.1002/smll.201601680, say that they are now busy trying to make micro-engines or micro-submarines that can move or rotate along pre-defined trajectories.