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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multi-electron H2O and CO2 activation, but<br />
methods are lacking that allow coupling <strong>of</strong> these<br />
components to electron/hole conducting moieties<br />
in 3-D frameworks. Molecular-type linkages<br />
need to be developed <strong>for</strong> efficient charge<br />
conduction between catalytic sites and<br />
photoactive components embedded in the<br />
assembly.<br />
Integrated Time-resolved Probes<br />
Current research on self-assembly has been<br />
limited to observation <strong>of</strong> ordered structures using<br />
conventional techniques such as X-ray and<br />
electron diffraction, transmission electron<br />
microscopy, and atomic <strong>for</strong>ce microscopy.<br />
Moving self-assembly science <strong>for</strong>ward requires<br />
an experimental window that reveals the three-dimensional structural nature, and time scales <strong>of</strong><br />
the “embryonic nuclei” that trigger self-assembling processes as they cross from the nanoscale to<br />
microscopic and macroscopic dimensions. It is equally critical to observe in real time and space<br />
the trans<strong>for</strong>mations and intermediate states that assemblies go through be<strong>for</strong>e reaching their final<br />
<strong>for</strong>m. This in<strong>for</strong>mation is not presently accessible, and requires invention <strong>of</strong> “integrated” timeresolved<br />
probes that record in real time the evolution <strong>of</strong> the system across length scales. These<br />
might be presently unknown hybrids <strong>of</strong> scanning probe techniques, near-field strategies, confocal<br />
microscopy, magnetic resonance imaging, tomographic techniques, vibrational spectroscopies,<br />
and others. Opening this spatial and temporal window on self-assembling systems will allow us<br />
to direct systems externally (e.g., through solvent, temperature, external fields, and photons) into<br />
the desired targets. A grand challenge is to develop such probes <strong>for</strong> “self-assembly dynamics”<br />
that tolerate compositionally controlled atmospheres, liquid phases, variable temperature, and<br />
variable pressure.<br />
New Computational Approaches<br />
New computational approaches are needed to integrate simulations across disparate time and<br />
length scales that are important <strong>for</strong> assembly <strong>of</strong> solar fuel/energy producing systems (see<br />
Figure 65). For example, modeling has traditionally been carried out separately <strong>for</strong> increments <strong>of</strong><br />
length scales using quantum mechanics (0.1–10 nm), statistical mechanics (1–1,000 nm),<br />
mesoscale (0.1–100 µm), and continuum mechanics (1 mm–10 m). Time scales range from<br />
quantum mechanical methods (10 −15 s) to continuum methods (1–10 5 s). There is a critical need<br />
<strong>for</strong> theoretical modeling and simulation (TMS) to span all these length and time scales<br />
seamlessly to meet the needs <strong>of</strong> solar research, to provide insight into the <strong>for</strong>ces and processes<br />
that control the organization <strong>of</strong> functional elements over all length and time scales; to understand<br />
quantitatively the kinetics <strong>of</strong> catalyzed photochemical energy conversion reactions over many<br />
length scales in complex, hybrid systems; to identify active sites on nanostructured surfaces, etc.<br />
176<br />
Figure 64 Hierarchical assembly <strong>of</strong><br />
mesoporous oxide within nanoscopic channels<br />
<strong>of</strong> porous alumina membrane. This example<br />
illustrates principles <strong>of</strong> multiple length<br />
ordering.