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.