nanostructure design, and development of nanoscale pore architectures that steer reaction
intermediates to desired fuel products. Such assemblies could be developed in the form of
nanoporous membranes, in effect producing an artificial “leaf.”
Multi-junction solar cells convert light to electricity across the solar spectrum, and are the
highest-efficiency solar conversion devices known. In these devices, high quantum efficiency is
achieved only with epitaxially grown single crystal heterojunctions, which are prohibitively
expensive to produce. The analogous nanocrystal heterojunction devices either do not exist, or
have not yet been tested as solar photoconversion devices. However, nanomaterials offer many
potential advantages for solar cells, such as the low cost of single crystal synthesis, tolerance for
lattice mismatch in junctions, and the ability to control three-dimensional architecture through
shape-controlled growth, microphase separation, and layer-by-layer synthesis. Novel
architectures such as branched nanocrystals, nanowires, nanoribbons, and nanotubes provide
useful building blocks for coupling of light-harvesting and photocatalytic components into
functioning photocatalytic assemblies. The challenge is to design these assemblies to drive
energetically demanding reactions, such as water-splitting, by using visible and near-infrared
light.
NEW SCIENTIFIC OPPORTUNITIES
The design and preparation of an integrated, molecule-based system that will convert sunlight
into useful fuels is a challenging goal. However, natural photosynthesis has already achieved this
goal within the context of the biological world. By understanding the natural process and
exploiting it in artificial constructs, it will be possible to construct artificial photosynthetic
systems maximized for production of fuels useful to human society.
Understand the Dependence of Excitation Energy and Charge Flow on Molecular
Structure and Intermolecular Boundaries from the Molecular to the Device Scale
A major scientific challenge is to develop a complete understanding of how weak, non-covalent,
associative interactions, such as hydrogen bonds and π-π interactions, promote or inhibit energy
and charge flow across molecular boundaries. This is critical to achieving an integrated artificial
photosynthetic system because formation of a functional system by self-assembly of building
blocks requires controlled energy and charge flow across the weak associative points of
molecular contact. Studies are also needed on nanostructured and self-assembling junctions
(e.g., at semiconductor nanocrystal/polymer and polymer/polymer interfaces) to understand the
effects of composition, dimensionality, and overall architecture on the dynamics of excitons and
charge carriers. In addition, to design better interfacial catalysts for water oxidation and fuel
formation, the detailed molecular understanding that is being developed for molecular catalysts
needs to be translated to surface-bound and colloidal catalysts. This requires the development of
time-resolved structure-specific spectroscopic tools (vibrational, X-ray Absorption Fine
Structure [XAFS], etc.) with very high sensitivity to identify transient intermediates and catalyst
structural changes under reaction conditions. Theory and computational tools must also be
developed to assist experimental studies with the goal of identifying active sites on surfaces with
atomic precision.