"The 'red shift' allows the photocatalyst to absorb visible light by extending the active region of the nanocomposite from UV to the visible," says team member Jian Ku Shang. "As a result, a greater portion of the solar spectrum may be used to drive electronic devices powered by this new material."

Interestingly, the "red shift" in absorption to longer wavelengths runs contrary to the quantum size effect. As semiconductor nanoparticles decrease in size, their excitation energy generally increases, which results in a "blue shift" of their absorption band to a shorter wavelength region. In Shang's work, it appears that this mechanism is disrupted by the interaction between the ultra-fine palladium oxide (PdO) particles and the titanium oxide (TiO2) matrix.

Embedding the ultra-fine semiconductor particles into a thin-film matrix proved to be a challenge. The researchers used an electron beam to evaporate TiO2 and PdO pellets, while at the same time bombarding the growing structure with nitrogen ions. The group found that so-called ion–beam-assisted deposition helped to minimize the size of the embedded PdO particles to just a few nanometers, which gave a highly dispersed matrix.

Satisfied with the production process, Shang is looking ahead. "Our next step is to design sensors and photocatalytic reactors that take advantage of the unique optoelectronic properties of our nitrogen-doped TiO2/PdO material," he explained. "We have filed US and international patent applications and are now discussing technology transfer with industrial partners."

The researchers reported their work in Appl. Phys. Lett.