The project involved researchers from UH, along with those from Sam Houston State University, the Chinese Academy of Sciences, Texas State University, Carl Zeiss Microscopy LLC, and Sichuan University.

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 (Ga_1-xZn_x)(N_1-xO)) 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.

—Liao et al.

Despite some differences between the two types of nanoparticles, both worked equally well.

Different sources of light were used, ranging from a laser to white light simulating the solar spectrum. Bao said he would expect the reaction to work equally well using natural sunlight.

Once the nanoparticles are added and light 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.

Even with an improved solar-to-hydrogen efficiency rate of around 5%, the conversion rate is still too low to be commercially viable. Bao suggested a more feasible efficiency rate would be about 10%.

Other issues also remain to be resolved, 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, Bao said, including the question of why cobalt oxide nanoparticles have such a short lifespan, and questions involving chemical and electronic properties of the material.

Resources

• Longb Liao, Qiuhui Zhang, Zhihua Su, Zhongzheng Zhao, Yanan Wang, Yang Li, Xiaoxiang Lu, et al. (n.d.) (2013) “Efficient solar water-splitting using a nanocrystalline CoO photocatalyst.” Nature Nanotechnology. doi: 10.1038/nnano.2013.272