These days, our society needs to change its energy supply from fossil fuels towards a renewable source. Harvesting sunlight and storing this energy in the form of hydrogen gas is an environmentally friendly approach to address this task. I design new metal oxide based semiconductors and investigate their properties regarding their use for photoelectrochemical water splitting. Furthermore, nanostructuring and band gap engineering are powerful ways to improve their performance. We have a wide range of characterization techniques available to study the physical properties of these new materials. Finally, these devices are tested to quantify their efficiency splitting water to hydrogen and oxygen by the use of solar energy.