In a recent interview with Nanowerk, Ayusman Sen, a professor at the Department of Chemistry at Penn State, explained how he uses titanium dioxide to convert optical energy to mechanical energy via photocatalysis. The team’s findings were published recently in Advanced Functional Materials.

“The whole system consists only of titania, water, sometimes organics, and light input. The system is very forgiving, requiring no careful control of substrate concentration or catalyst conditioning, and is easily controllable by external light,” says Sen.

Via Nanowerk:

“There are two categories of autonomous movement associated with titanium dioxide: the photo-induced motility of titanium dioxide particles (ranging from 0.2 to 2.5 µm) and the photo-induced reversible ‘microfireworks’ where silicon dioxide particles, which tend to gather around titanium dioxide particles, were shown to immediately move away from the titanium dioxide particles upon exposure to UV light, thereby creating an exclusion zone cleared of particles around each individual titanium dioxide particle. When the UV source is removed, the tracer particles pull back toward the titanium dioxide particle and form the aggregates again.”

Sen explains that the photoactivity of titanium dioxide comes from its hole–electron separation triggered by photons of energy equal to or higher than its bandgap.

“The reactions produce more product molecules than the reactants consumed, making it possible to propel a titanium dioxide particle by the mechanism of osmotic propulsion,” he says.

Sen claims that the titanium dioxide-based motor system is highly active, inexpensive, clean and simple.

Self-propelled motion of synthetic materials can be useful in applications such as bottom-up assembly of structures, pattern formation and drug delivery at specific locations.

The video below shows the photoactivity of a large titania particle in 1M methanol. The surrounding tracer particles are silica.