Solar energy is a vast potential source of renewable energy. However, we still lack efficient and robust devices for solar energy conversion. Structure-function relationships of existing materials provide important insights for designing new photocatalysts which convert solar to chemical energy. Titanium dioxide (TiO2) nanocomposites show enhanced photocatalytic activities in environmental applications due to interfacial charge separation. Heterogeneous arrays, such as molecular photocatalysts supported on nanoparticle surfaces, are promising candidates for high-efficiency solar fuel production. The structures and photochemistry of TiO2-based materials can be investigated with a variety of techniques including electron paramagnetic resonance (EPR) spectroscopy. The potential of catalysis and nanoscience in the context of sustainability research will be discussed.