  Nanomaterial surface states can effectively modify or even dominate their
physical and chemical properties due to large surface-to-volume ratios. Such
surface effects are highly dependent on particle size and ligand coverage, yet
the underlying electronic-level mechanism still remains unknown. Using TiO2
nanosheet as a model system, we reveal the electronic nature of
coverage-dependent nanosurface effects through varying ligand coverage and
probing the modified surface bonding and electronic band structures with
near-edge X-ray absorption fine structure. We discover experimentally that
surface ligands can competitively polarize the 3d orbitals of surface Ti atoms
into chemisorption states, which is cooperative with increased ligand
coverages. Such coverage-dependent cooperative orbital redistribution accounts
for various nanosurface effects on regulating the electronic structure, surface
reactivity, optical property, and chemisorption of nanomaterials.
