Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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Self-organized Hierarchical Structures<br />
Biological systems employ a hierarchical organization to carry out many functions, including<br />
those <strong>of</strong> photosynthesis. Chemical processes such as microphase separation in block copolymers,<br />
template-directed sol-gel synthesis <strong>of</strong> porous materials, layer-by-layer synthesis, and<br />
nanoparticle self- and directed-assembly (see Figure 63) have opened the door to a vast variety<br />
<strong>of</strong> hierarchical structures that are organized on several length scales. The challenge is to map<br />
these new synthesis techniques onto the demands <strong>of</strong> artificial photosynthesis in order to better<br />
control light-harvesting; charge separation; traffic control <strong>of</strong> holes, electrons, and molecules;<br />
catalytic reactions; and permanent separation <strong>of</strong> the photogenerated fuel and oxidant. A detailed<br />
understanding <strong>of</strong> the kinetics <strong>of</strong> the processes in complex multicomponent systems (e.g., selfassembled<br />
polymer cells, quantum dot sensitized solar cells, organic-inorganic hybrid cells, and<br />
solar-fuel conversion systems) is essential to their rational design and utilization in efficient<br />
photochemical energy conversion.<br />
Figure 63 Self-assembling organic nanoribbons (Source: S.<br />
Stupp, Northwestern University, unpublished)<br />
For example, visible light-driven water-splitting or CO2 reduction with high efficiency is<br />
currently achieved only in the presence <strong>of</strong> sacrificial reagents. The design <strong>of</strong> new photocatalysts<br />
that obviate the need <strong>for</strong> sacrificial reagents is imperative <strong>for</strong> achieving efficient solar fuel<br />
producing assemblies. Structured assemblies need to be developed that allow organization <strong>of</strong> the<br />
active units (e.g., light-harvesting, charge conduction, chemical transport, and selective chemical<br />
trans<strong>for</strong>mation) <strong>for</strong> optimum coupling <strong>for</strong> efficient fuel production. One class <strong>of</strong> such assemblies<br />
are 3-D high-surface-area inert supports that allow precise spatial arrangement <strong>of</strong> the active<br />
components in a predetermined way <strong>for</strong> optimum coupling and protection from undesired<br />
chemistries. These supports (see Figure 64) should have structural elements (walls, membranes)<br />
that allow separation <strong>of</strong> primary redox products on the nanometer scale to prevent undesired<br />
cross-reactions and facilitate prompt escape <strong>of</strong> the products from the fuel <strong>for</strong>ming sites. Catalytic<br />
sites should be separated in such a way that energy-rich products, such as hydrogen and oxygen,<br />
cannot recombine thermally. A few molecular catalytic components are currently available <strong>for</strong><br />
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