Photopatterning and solar energy conversion using nanocrystalline metal oxide films
Smith, Anthony R.
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This dissertation will focus on three topics involving photochemistry at nanostructured inorganic interfaces: (1) the use of photochemistry to achieve the programmable assembly of nanoparticles and the site-selective deposition of nanoparticles onto surfaces, (2) controlling the fundamental photochemistry of anthracene on surfaces and (3) the optimization of dye-sensitized solar cells (DSSCs) incorporating aggregating organic dyes. All three topics, which are reviewed in Chapter 1, use transparent nanocrystalline metal oxide films as the support medium. Chapter 2 reports how interfacial photodimerization reactions between adsorbed molecules were exploited to achieve the organization of nanoscale components into ordered assemblies in three dimensions and on surfaces. The combination of top-down and bottom-up assembly methods have proven useful for the patterned deposition of nanoparticles onto surfaces. However, the direct photoinduced attachment of nanoparticles to surfaces may enable the preparation of composite nanomaterials via a single photochemical reaction. This part of the dissertation will focus on surface functionalization chemistry, photoinduced reactions in solution and on surfaces, photochemically triggered materials assembly processes, and the characterization of composite nanomaterials. The use of an interfacial photodimerization reaction to trigger materials assembly processes may represent an attractive fabrication strategy, in which patterned nanostructured materials, potentially including ternary and higher-order materials, are formed in one step. In Chapter 2, the use of anthracene dimerization to trigger the assembly of nanoparticles to a surface is shown. The primary interest is the interfacial photodimerization between anthracene molecules on separate particles and how this is limited by side reactions. Isolating and limiting the side reactions can increase the probability of the desired mechanism taking place. In Chapter 3 the intra -monolayer dimerization is isolated and attempted to be controlled through the use of co-adsorbants and co-capping agents both on the surface and in suspension. Through studying the fundamentals of the photodimerization of anthracene the desired mechanisms can be isolated and used in further applications. Chapter 4 of the dissertation will focus on DSCCs incorporating chalcogenoxanthylium dyes that undergo controlled aggregation on surfaces. The orientation and aggregation state of dyes were controlled systematically by varying the position of the surface-attachment group relative to the xanthylium core. Six types of dyes were analyzed. The first underwent H-aggregation on TiO 2 surfaces; the second adsorbed in amorphous monolayers; the third adsorbed through amino groups and underwent J-aggregation on TiO 2 surfaces; the fourth, fifth, and sixth dyes vary in the position of the surface-attachment group between the ortho-, meta-, and para- position. Aggregated dyes exhibited broader absorption bands and increased light-harvesting efficiencies relative to dyes that adsorbed in amorphous monolayers. Surface-attachment position provided insight into the benefit of conjugation on photocurrent efficiencies. The photoelectrochemical performance followed similar trends. Our findings reveal that the light-harvesting efficiency, IPCE, and absorbed photon-to-current efficiency (APCE) of DSSCs with organic dyes can be optimized by systematically varying the structure of the dyes and their orientation and aggregation state on the surface.