Frei and Jiao used mesoporous silica as their scaffold, growing their cobalt nanocrystals within the naturally parallel nanoscale channels of the silica via a technique known as "wet impregnation." The best performers were rod-shaped crystals measuring 8 nanometers in diameter and 50 nanometers in length, which were interconnected by short bridges to form bundled clusters. The bundles were shaped like a sphere with a diameter of 35 nanometers. While the catalytic efficiency of the cobalt metal itself was important, Frei said the major factor behind the enhanced efficiency and speed of the bundles was their size.
"We suspect that the comparatively very large internal area of these 35 nanometer bundles (where catalysis takes place) was the main factor behind their increased efficiency," he said, "because when we produced larger bundles (65 nanometer diameters), the internal area was reduced and the bundles lost much of that efficiency gain."
Frei and Jiao will be conducting further studies to gain a better understanding of why their cobalt oxide nanocrystal clusters are such efficient and high-speed photocatalysts and also looking into other metal oxide catalysts. The next big step, however, will be to integrate the water oxidation half reaction with the carbon dioxide reduction step in an artificial leaf type system.
"The efficiency, speed and size of our cobalt oxide nanocrystal clusters are comparable to Photosystem II," said Frei. "When you factor in the abundance of cobalt oxide, the stability of the nanoclusters under use, the modest overpotential and mild pH and temperature conditions, we believe we have a promising catalytic component for developing a viable integrated solar fuel conversion system. This is the next important challenge in the field of artificial photosynthesis for fuel production."
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory