Size can widely affect the surface chemical activities (SCAs) of nanomaterials in chemisorption, catalysis, surface effects, etc., but the underlying electronic nature has long remained mysterious. We report a general electronic principle that drives the origin of size-dependent SCAs by combining experimental probing and theoretical modeling. Using the chemisorption of H2O2 on TiO2 as a model reaction, we experimentally reveal that the central electronic process of surface chemical interactions lies in the competitive redistribution of surface atomic orbitals from energy band states into surface coordination bonds. By defining orbital potential, a site-dependent intrinsic electronic property that determines surface activities, we further establish a mathematical model to uncover the physical nature of how structural factors correlate to SCAs, particularly the roles of size. We discover that the electronic nature of size effect lies in its inverse correlation to orbital potential and amplification effect on other structural factors like defects and coordination numbers.
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