Photocatalytic hydrogen evolution from water holds promise for sustainable hydrogen production. A recent study published in the Journal of the American Chemical Society sheds light on the crucial impact of interfacial water molecules on the reactivity of photocatalysts. By controlling the thickness of adsorbed water layers, researchers observed a direct correlation between H2 formation rate and the structure of H-bond networks. The study led by Toshiki Sugimoto uncovered that the presence of reactive water molecules in overlying layers positively influences hydrogen production, up to a limit of three layers. However, beyond this threshold, the formation rate of H2 decreases significantly due to the strengthening of interfacial H-bond networks, which inhibit proton-coupled hole transfer. The findings emphasize the importance of understanding the physicochemical properties of interfacial water molecules for enhancing photocatalytic efficiency. The research provides insights for the design of optimal interfacial water conditions to improve hydrogen evolution. This study marks a potential shift in the field of photocatalysis by showcasing the advantages of water vapor environments over traditional liquid-phase systems. The discoveries open up new possibilities for engineering innovative photocatalytic systems and advancing renewable energy production.