Photocatalysis is considered by many as the most promising solution for energy and environment problems. Among all the photocatalysts available, titanium dioxide remains one of the most studied for decades. Reducing its bandgap via doping is crucial to harvest visible-light, which accounts for 48% of the total solar energy. In addition, it is very important to keep the particle size small to avoid recombination of the photo-generated electrons and holes. In this book, nano-scaled metal and / or non-metal elements have been used as dopants to modify the electronic structure of TiO2. X-ray...