<p>Covalent semiconductors of the carbon nitride family are among the most promising systems to realize “artificial photosynthesis”, that is exploiting synthetic materials which use sunlight as an energy source to split water into its elements or converting CO₂ into added value chemicals. However, the role of surface interactions and electronic properties on the reaction mechanism remain still elusive. Here, we use in-situ spectroscopic techniques that enable monitoring surface interactions in carbon nitride under artificial photosynthetic conditions. We show that the water adsorption and light illumination cause changes of the surface electron density, which activate the photocatalyst and enable the water splitting process. Our results reveal critical details on the photocatalytic mechanism, which proceeds through proton-coupled electron transfer, and provide key information to design more efficient photocatalyst for artificial photosynthesis.</p>