Rising atmospheric levels of carbon dioxide and depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply.¹  Therefore, the searches for renewable energy sources and sustainable environmental technologies are some of the foremost challenges for maintaining our quality of life.²  Honda and Fujishima discovered in 1972 the use of TiO₂ electrodes for the photoelectrochemical (PEC) splitting of water,³  opening up a great opportunity for the use of photocatalysis in the utilization of solar energy and environmental remediation.^4,5  Utilizing abundant solar energy to convert water and carbon dioxide into chemical fuels, so-called “artificial photosynthesis”, could address the energy crisis and global climate change simultaneously.⁴  Artificial photosynthesis is a chemical process, as its name suggests, which mimics nature’s photosynthesis to produce chemical fuels from sunlight and Earth-abundant materials such as water, carbon dioxide and many more, as shown in Figure 4.1a. The most important advantages of this process are the use of solar energy and its ecological purity, which offer the possibility of accomplishing energy cycles without environmental pollution or additional warming of Earth.⁶  Since Earth receives more solar energy than what is required to meet the human energy demands, the production of so-called “solar fuels” using photocatalysis is extremely relevant for a future portfolio of sustainable energy sources.⁷  For example, if the solar fuel produced is hydrogen, this is a very useful energy carrier that is convertible to electrical power without generating by-products that are harmful to the environment.⁸  Other solar fuels such as synthesis gas (syngas; CO + H₂) and light hydrocarbons would also be extremely useful as sustainable feedstocks in the chemical and energy industries.⁹