So far, so good. However, the large bandgap of TiO2 (3.0–3.2 eV) allows photoconversion of only UV radiation, which comprises less than 7% of the solar energy spectrum. Thus, bandgap reduction of TiO2 is a key requirement for effective solar-to-hydrogen conversion.

In a recent study published in Nanotechnology, researchers at the University of Arkansas at Little Rock and the University of Nevada, Reno, developed a process based on nanostructure synthesis and plasma surface modification to enhance the photoelectrochemical conversion efficiency of titania photoanodes.

Titania photoanodes with nanotubular structures were synthesized by electrochemical anodization of titanium thin foils. The photoanode surfaces were then subjected to low-pressure nitrogen plasma. It was found that the plasma treatment significantly enhanced the photoelectrochemical activity of the samples; the photocurrent density of plasma treated material was approximately 80% higher than that of the control electrodes.

The plasma treatment removed surface contaminants, minimized the charge carrier traps and provided n-type doping of the photonaode surface with nitrogen. The increase in photoactivity was ascribed to the surface modifications by plasma treatment and increased absorption of visible light due to nitrogen doping of the photoanode surface, narrowing the bandgap. XPS analysis confirmed doping of nitrogen in the TiO2 surface. Plasma treatments also increased surface roughness and wettabilty, resulting in a higher electrode/electrolyte interfacial contact area for enhancing electrolysis.

While plasma surface doping does not hinder an efficient transport of charge carrier through the bulk material, further advancement of the method is needed to provide effective n-doping over the depth of the depletion layer for efficient light absorption and charge separation.

Based on its results, the group believes that a synergistic combination of nanostructure synthesis of photoanodes and surface structure and chemical modification may advance photoelectrochemical generation of hydrogen using photostable semiconducting electrodes.