A new area of technology: Nanotechnology is emerging, which is discussed as one of the key technologies of the 21st century. Nano-titanium dioxide has comprehensive applications in green chemistry areas such as energy production and wastewater treatment. In this book, pure and lanthanide modified Ttitanium dioxide nanomaterials were synthesized by a sol–gel and applied these nanomaterials for photobiogas/ hydrogen generation and industrial wastewater purification. This book consists of three chapters: The former one gives a general introduction including problematic of the energy and environmental pollution and the role of nanotechnology and nano-semiconductor photocatalysis in solving such problems. A theoretical background about the titanium dioxide as a nano-semiconductor photocatalyst is introduced. A number of applications of titanium dioxide in different fields are discussed. The literature review concerning the titanium dioxide nanomaterials topics is also given. The second chapter demonstrates the experimental techniques and data analysis methods used in this book, and finally, the last chapter includes the presentation and discussion of the obtained results.