The procedure of the photocatalyst dispersion preparation starts with weighing the sample in a vial and addition of the desired volume of water to obtain the initial photocatalyst dispersion. It is crucial to consider the appropriate optical density of the dispersion (vide infra, section 2.2), which is determined by the photocatalyst concentration and its optical properties. A reasonable starting point is typically 1 -2 mg/mL, which may then be lowered by diluting the sample after complete dispersion has been achieved.