text: Photocatalysts must conform to several key principles in order to be considered effective at water splitting. A key principle is that  and  evolution should occur in a stoichiometric 2:1 ratio; significant deviation could be due to a flaw in the experimental setup and/or a side reaction, neither of which indicate a reliable photocatalyst for water splitting. The prime measure of photocatalyst effectiveness is quantum yield (QY), which is:
  : QY (%) = (Photochemical reaction rate) / (Photon absorption rate) × 100%
  To assist in comparison, the rate of gas evolution can also be used. A photocatalyst that has a high quantum yield and gives a high rate of gas evolution is a better catalyst.
  The other important factor for a photocatalyst is the range of light that is effective for operation. For example, a photocatalyst is more desirable to use visible photons than UV photons.