Photoexciting nanostructured metals and building a device
that splits water into hydrogen and oxygen is a radically new way to convert
sunlight directly into fuel.
Converting sunlight into electricity is a mature science and
a lucrative business, normally carried out in photodiodes based on
semiconductor materials. Artificial photosynthesis—using sunlight to power the
autonomous conversion of reactants to higher free-energy products, mimicking
the way plants and phytoplankton convert carbon dioxide (CO2) and water to
sugars—has not yet evolved sufficiently to be commercialized. This is because
an autonomous, light-driven device that splits water into dihydrogen (H2) and
dioxygen (O2), as first reported by Fujishima and Honda,1 that is robust enough
to function unattended for many months is normally based on a wide-bandgap
semiconductor. Such a device makes use only of the UV part of the sun's
spectrum and is consequently very inefficient.