Nanostructured titanium dioxide (TiO2) films were synthesized with controlled morphology and thickness in an ambient pressure single-step flame aerosol reactor (FLAR) for use in water splitting photocells and dye-sensitized solar cells. Two different morphologies were studied:  a granular morphology and a highly crystalline columnar morphology. The granular morphology consisted of nanoparticles, approximately 10 nm in diameter, aggregated into fractal structures on the substrate. The granular morphology contained a large number of grain boundaries and other interfacial defects. The columnar morphology consisted of single-crystal structures, approximately 85 nm in width, oriented normal to the substrate. The well controlled deposition process was used to deposit films with thicknesses in the range from 98 nm to 12 μm to establish the relationship to water splitting and dye-sensitized solar cell performance. It was found that for watersplitting there was an optimum thickness (∼1.5 μm), which was a tradeoff between light absorption and electron transport losses, where the conversion efficiency was maximized. Due to differences in electron transport and lifetime in the TiO2 film, for both applications the columnar morphology outperformed the granular morphology, achieving a uv-light to hydrogen conversion efficiency of 11% for water splitting and a visible light to electricity conversion efficiency of 6.0% for the dye-sensitized solar cell.

Electrical conductivities of thin films of silicon nanocrystals (4-6 nm) exhibit high sensitivity to water vapor. Specifically, water adsorption on the surface of silicon nanocrystal (NC) films increases their electrical conductivity by a factor of four at room temperature and an order of magnitude at 175 K. The increase in conductivity is reversible and can manifest as peaks or hysteresis loops in temperature-dependent conductivity measurements even when the measurements are conducted under vacuum at 10-5 Torr and in the presence of only residual amounts of water vapor. Hydrogenterminated silicon nanocrystals are easily oxidized to form submonolayer to monolayer of chemically bound oxygen on their surfaces when annealed at 300°C in a glovebox with 0.1 ppm of water vapor. Annealing under vacuum at 300°C retains H-passivation without oxidation. The electrical conductivity of films made from hydrogen-terminated silicon nanocrystals is 200 times higher than the electrical conductivity of films made from silicon nanocrystals with a monolayer of chemically bound oxygen. However, the conductivities of both types of films increase upon adsorption of water on the nanocrystal surfaces. These findings underscore the importance of controlling silicon nanocrystal surfaces in determining the electrical properties of their thin films.

Titanium dioxide films are a critical component of many next-generation low cost solar cells. Film morphology has been identified as an efficiency-limiting property. A gas phase, single-step, rapid, atmospheric-pressure process to synthesize TiO 2 films with controlled morphology is reported. The process is based on a flame aerosol reactor (FLAR). Two different morphologies were synthesized for this report, granular and columnar. The granular morphology consists of nanoparticles aggregated into fractal structures on the substrate, and is characterized by high surface area and poor electronic properties. The columnar morphology is highly crystalline; composed of 1D structures oriented normal to the substrate, characterized by lower surface area and superior electronic properties. Films with both morphologies are applied to a hydrogen-producing photo-watersplitting cell and a photovoltaic dye-sensitized solar cell. For watersplitting, the columnar morphology outperforms the granular by almost 2 orders of magnitude, achieving a uv-light to hydrogen conversion efficiency of about 11%. In contrast, for the dye-sensitized solar cell, the granular morphology outperforms the columnar, due to enhanced dye absorption arising from the larger TiO 2 surface area.