18 August 2010
A simple and easy way to make mixed zinc-cadmium sulfide materials that produce hydrogen by splitting water under visible light has been developed by scientists in the US and China. The mixed materials can harvest a wider range of wavelengths than conventional materials, making them more efficient.
Photocatalytic conversion of sunlight to chemical energy, for example by producing hydrogen is an attractive alternative energy source and a feasible way to tackle the global energy and environmental pollution crises. Conventional photocatalysts, such as TiO2, CdS or ZnS, possess excellent activity and stability but only absorb near-ultraviolet light - which accounts for only 4 per cent of the solar spectrum. Expensive noble metal co-catalysts, such as platinum can be added to increase their absorption range but this increases their cost.
Now, Mietek Jaroniec from Kent State university, Ohio, and Jiaguo Yu from Wuhan University of Technology, have made mixed zinc-cadmium sulfide complexes doped with cadmium sulfide quantum dots (CdS QDs) that show high photocatalytic activity under visible light, without the need for noble metal additives.
CdS quantum dots increase the absorption range of the photocatalyst
'The high H2-production activity of the CdS quantum dot-sensitised material under visible light can be attributed to the facilitated electron transfer from CdS QDs,' says Jaroniec. The team made the mixed solid solution using a simple hydrothermal method to combine ZnS nanoparticles and Cd(NO3)2 salt. Followed by the thermodynamically favourable replacement of Zn2+ ions by Cd2+ ions using cation exchange.
Quantitative analysis shows that the photocatalytic H2-production of the new material is more than 50 times greater than CdS on its own, as well as being significantly better than platinum-doped ZnS under UV and visible light.
Max Lu, an expert in clean energy and environmental technologies at the University of Queensland, Australia, says, 'the results are quite exciting, and the CdS quantum dots are shown to be powerful in facilitating photocatalytic water splitting even without the use of Pt. If the stability is proven to be good, this system should offer opportunity to substantially lift the rate of hydrogen production under visible light irradiation.' Next, the team plan to find other quantum dot-based materials, which could be used to enhance hydrogen generation.