Metal nanoclusters are used in photosynthesis to generate energy. Taking this as inspiration, Feng Jiao and Heinz Frei (Lawrence Berkeley National Laboratory, California) have prepared rugged metal nanoclusters that, in conjunction with a sensitizer, generate oxygen from water under visible light and mild conditions.
The catalyst and sensitizer.
The oxygen-evolving catalyst is comprised of cobalt oxide nanoclusters embedded in porous silica (for stability). The sensitizer (a metal complex that converts light into an electrical current) is ruthenium bipyridine persulfate.
At mildly acidic pH (to avoid degradation of the ruthenium complex), room temperature, under blue (476 nanometer) 240 milliwatt light, the catalyst generates oxygen from water (as measured through mass spectrometry) for approximately 1 hour. After that, little oxygen is produced.
Termination of catalytic activity is due to consumption of persulfate, not degradation of the metal nanoclusters. Confirmation of this hypothesis was obtained by imaging the metal nanoclusters and determining their oxidation state, before and after use, and determining that the metal nanoclusters still exhibit the same properties as before the experiments.
Efficiency of the catalyst and sensitizer.
How efficiently do the metal nanoclusters convert water to oxygen? The turnover frequency (the number of oxygen molecules generated per unit time), for 8% catalyst loading in silica, is 3450 per second per metal nanocluster.
This makes efficient use of the light to generate oxygen. Metal nanoclusters are far more efficient catalysts (by a factor of 16) and have a greater surface area than micrometer-scale metal clusters, rendering them more suitable for commercial applications.
The efficiency of the ruthenium bipyridine persulfate sensitizer is 18% (in the case of 4% catalyst loading in silica). This is dependent on many factors, such as electron and charge transfer between all of the species involved, and the amount of light absorbed by the sensitizer in the first place.
Overall evaluation.
Feng Jian and Heinz Frei have developed a metal nanocluster catalyst that generates oxygen from water, powered by a ruthenium bipyridine persulfate sensitizer that converts light into an electrical current. These nanoclusters are stable, cobalt oxide is a common material, the required conditions are mild, and light is efficiently generated, all of which suggest future practical utility in solar cells.
for more information:
Jiao, F.; Frei, H.
Nanostructured cobalt oxide clusters in mesoporous silica
as efficient oxygen-evolving catalysts.
Angew. Chem. Int. Ed. 2009, 48, 1841-1844.