  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.
