Mar 5, 2009
Photoelectrochemical efficiency of titania photoanodes enhanced
Hydrogen production from sunlight by splitting water using photoelectrochemical electrolysis is the most direct method for solar-to-hydrogen conversion. Looking at the process in more detail, nanotubular titania (TiO2) emerges as one of the most promising photo-anode materials for water splitting using solar radiation thanks to the combination of a band structure that straddles the reduction and oxidation potential of water, a high corrosion resistance in aqueous electrolytes and the material's low cost.
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.
About the author
This work was performed at the University of Arkansas at Little Rock and University of Nevada, Reno, and was supported by the United States Department of Energy and Arkansas Science and Technology Authority. Dr Rajesh Sharma is a Research Faculty at the Graduate Institute of Technology at the University of Arkansas at Little Rock. Prajna P Das and Vishal Mahajan are graduate students at the University of Nevada, Reno. Dr Mano Misra is professor at the Department of Chemical and Metallurgical Engineering at the University of Nevada, Reno. Jacob Bock is an undergraduate student at the University of Arkansas at Little Rock. Dr Steve Trigwell is manager of the Applied Science and Technology Laboratories at ASRC Aerospace, in the Kennedy Space Center, Florida. Dr Alexandru Biris and Dr Malay Mazumder are assistant professor and Emeritus professor respectively at the Applied Science Department at the University of Arkansas at Little Rock.