Mar 5, 2009
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.Nano Team Increases Efficiency of Sun-to-Fuel Process
NANOTECHNOLOGY, FUEL CELLS SOLAR CONVERSION
Contact InformationAvailable for logged-in reporters only
DescriptionResearchers find great promise in a process that could use solar energy to use hydrogen, the third most abundant element on earth's surface, as the ultimate alternative to fossil fuels. This process increase dramatically the efficiency of titania photoanodes used to convert solar energy into hydrogen in fuel cells.
Newswise — Researchers at UALR -- the University of Arkansas at Little Rock -- said they have developed a process involving nanostructure that shows great promise in boosting the efficiency of titania photoanodes used to convert solar energy into hydrogen in fuel cells.
Hydrogen, the third most abundant element on earth’s surface, has long been recognized as the ultimate alternative to fossil fuels as an energy carrier. Automobiles using hydrogen directly or in fuel cells have already been developed, but the biggest challenge has been how to produce hydrogen using renewable sources of energy.
Scientists in Japan discovered in 1970 that semiconductor oxide photoanodes can harness the photons from solar radiation and used them to split a water molecule into hydrogen and oxygen, but process was too inefficient to be viable.
The UALR team, working with researchers at the University of Nevada, Reno, and supported by the U.S. Department of Energy and the Arkansas Science and Technology Authority (ASTA), has reported an 80 percent increase in efficiency with a new process.
The new process has been outlined in a recent study published in the journal Nanotechnology and also reported on the website Nanotechweb.org.
Electrochemical methods were utilized to synthesize titania photoanodes with nanotubular structures. The photoanode surfaces were then subjected to low-pressure nitrogen plasma to modify their surface properties. The plasma treatment increased the light absorption by the photoanode surface. It also removed surface impurities that are detrimental for photoelectrochemical hydrogen production.
“The plasma treatment significantly enhanced the photo electrochemical activity of the samples,” said Dr. Rajesh Sharma, assistant research professor in applied science in UALR’s Donaghey College of Engineering and Information Technology (EIT). “The photocurrent density of plasma treated material was approximately 80 percent higher than that of the control electrodes.”
Sharma’s highly interdisciplinary research interests encompass materials science, electrostatics, and particulate technology. He developed an atmospheric pressure plasma reactor for surface modification of materials in a variety of applications.
In addition to his work on nanostructured materials for photoelectrochemical processes, he is also working on development of an electrodynamic screen for dust mitigation application for future Mars and Lunar missions.
In addition to Sharma, the project team includes Drs. Alexandru Biris, assistant professor in applied science and chief science officer of Nanotechnology Center at UALR; UALR Professor-emeritus Malay Mazumder, and UALR undergraduate student Jacob Bock of Cabot.
Team members in Nevada include Dr. Mano Misra in the Department of Chemical and Metallurgical Engineering at UNR, and graduate students Prajna P. Das and Vishal Mahajan at the UNR.Dr. Steve Trigwell, manager of the Applied Science and Technology Laboratories at the Kennedy Space Center in Florida, also participated in the research.
Now - as to Applied Nanotech (APNT) - is Yaniv et al's TiO2 work relevant?:
United States Patent 7,300,634
Yaniv , et al. November 27, 2007
- I envisage an energy source like that proposed above creating H2 from H2O using titanium oxide WITH APNT's ASSISTANCE AND INVOLVEMENT. Perhaps I am expecting too much...but I'd still like to see it. It's pretty important and would make all our hopes and desires come to fruition. We helped replace oil with something better, cheaper and from Texas, too. ;-)