Tuesday, December 17, 2013

Researchers split water into hydrogen, oxygen using light, nanoparticles

Technology potentially could create a clean, renewable source of energy
December 17, 2013
Transmission electron micrograph of cobalt oxygen nanoparticles (credit: Longb Liao et al./ Nature Nanotechnology)
Researchers from the University of Houston have found a catalyst, cobalt oxide nanoparticles, that can quickly generate hydrogen from water using sunlight, potentially creating a clean and renewable source of energy.
Photocatalytic water-splitting experiments have been tried since the 1970s, but this was the first to use cobalt oxide and the first to use neutral water under visible light at a high energy conversion efficiency without co-catalysts or sacrificial chemicals, said Jiming Bao, lead author of their paper in Nature Nanotechnology and an assistant professor in the Department of Electrical and Computer Engineering at UH.
Researchers prepared the nanoparticles in two ways: using femtosecond laser ablation (removal of material from the surface of an object) and through mechanical ball milling. Different sources of light were used, ranging from a laser to white light simulating the solar spectrum. He said he would expect the reaction to work equally well using natural sunlight.
Once the nanoparticles are added to water and light is applied, the water separates into hydrogen and oxygen almost immediately, producing twice as much hydrogen as oxygen, as expected from the 2:1 hydrogen to oxygen ratio in H2O water molecules, Bao said.
The experiment has potential as a source of renewable fuel, but at a solar-to-hydrogen efficiency rate of around 5 percent, the conversion rate is still too low to be commercially viable. Bao suggested a more feasible efficiency rate would be about 10 percent, meaning that 10 percent of the incident solar energy will be converted to hydrogen chemical energy by the process.
“As most photocatalysts can respond only to ultraviolet light, their solar-to-hydrogen efficiency is not reported frequently,” the researchers state in their paper. “The recently reported efficiency of white-light water-splitting devices is about 0.1%.”
Other issues remain to be resolved, as well, including reducing costs and extending the lifespan of cobalt oxide nanoparticles, which the researchers found became deactivated after about an hour of reaction.
The work, supported by the Welch Foundation, will lead to future research, he said, including the question of why cobalt oxide nanoparticles have such a short lifespan, and questions involving chemical and electronic properties of the material.
Researchers at Sam Houston State University, the Chinese Academy of Sciences, Texas State University, Carl Zeiss Microscopy LLC, and Sichuan University were also involved.

Abstract of Nature Nanotechnology paper
The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously, but the use of photocatalysts to split water into stoichiometric amounts of H2 and O2 (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga1−xZnx)(N1−xOx)) do not exhibit good activity beyond 440 nm and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized from non-active CoO micropowders using two distinct methods (femtosecond laser ablation and mechanical ball milling), and the CoO nanoparticles that result can decompose pure water under visible-light irradiation without any co-catalysts or sacrificial reagents. Using electrochemical impedance spectroscopy, we show that the high photocatalytic activity of the nanoparticles arises from a significant shift in the position of the band edge of the material.

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