Hydrogen is a highly versatile fuel that may become one of the key solutions to face our future energy challenges. Among all these methods for hydrogen production, solar water splitting offers possible advantages regarding components integration, stability and costs. The key component for photoelectrochemical (PEC) water splitting is the semiconductor photoelectrode, which requires many material requirements in a single component, such as light absorption, charge separation, charge transport, H2 or O2 evolution kinetics at surface and stability for wide pH range. One of the materials of interest as photoanode for water splitting is hematite (alpha-Fe2O3) because of its suitable bandgap, low-cost and good resistance to corrosion. Considerable effort has been devoted to improving the efficiency of hematite but a complete understanding is still necessary for further application. In this thesis, I focused on investigation of correlation between defects and photoelectrochemical properties of hematite. Several strategies were proposed to modify the defects in hematite bulk, at hematite/substrate interface or on hematite surface, and their impacts on the PEC performance of hematite were studied. In addition, an in-situ operando cell was designed for ambient pressure X-ray spectroscopy to study photoelectrochemical processes occurring at hematite/H2O interface with different bias voltages applied.