Self-organized Hierarchical Structures
Biological systems employ a hierarchical organization to carry out many functions, including
those of photosynthesis. Chemical processes such as microphase separation in block copolymers,
template-directed sol-gel synthesis of porous materials, layer-by-layer synthesis, and
nanoparticle self- and directed-assembly (see Figure 63) have opened the door to a vast variety
of hierarchical structures that are organized on several length scales. The challenge is to map
these new synthesis techniques onto the demands of artificial photosynthesis in order to better
control light-harvesting; charge separation; traffic control of holes, electrons, and molecules;
catalytic reactions; and permanent separation of the photogenerated fuel and oxidant. A detailed
understanding of the kinetics of the processes in complex multicomponent systems (e.g., self-
assembled polymer cells, quantum dot sensitized solar cells, organic-inorganic hybrid cells, and
solar-fuel conversion systems) is essential to their rational design and utilization in efficient
photochemical energy conversion.
Figure 63 Self-assembling organic nanoribbons (Source: S.
Stupp, Northwestern University, unpublished)For example, visible light-driven water-splitting or CO 2 reduction with high efficiency is
currently achieved only in the presence of sacrificial reagents. The design of new photocatalysts
that obviate the need for sacrificial reagents is imperative for achieving efficient solar fuel
producing assemblies. Structured assemblies need to be developed that allow organization of the
active units (e.g., light-harvesting, charge conduction, chemical transport, and selective chemical
transformation) for optimum coupling for efficient fuel production. One class of such assemblies
are 3-D high-surface-area inert supports that allow precise spatial arrangement of the active
components in a predetermined way for optimum coupling and protection from undesired
chemistries. These supports (see Figure 64) should have structural elements (walls, membranes)
that allow separation of primary redox products on the nanometer scale to prevent undesired
cross-reactions and facilitate prompt escape of the products from the fuel forming sites. Catalytic
sites should be separated in such a way that energy-rich products, such as hydrogen and oxygen,
cannot recombine thermally. A few molecular catalytic components are currently available for