Deep and Shallow TiO2 Gap States on Cleaved Anatase Single Crystal (101) Surfaces, Nanocrystalline Anatase Films, and ALD Titania Ante and Post Annealing

Abstract

Using photoelectron spectroscopy (PES), deep (around 1.2 eV below Fermi level) and shallow (around 0.2 eV below Fermi level) gap states are investigated in differently prepared TiO2 samples: In situ cleaved single anatase crystal TiO2 (101) surface, sintered slurry of nanocrystalline anatase, amorphous atomic layer deposited (ALD) titania, and nanocrystalline anatase transformed by annealing from ALD titania. Deep gap states are generally attributed to under-coordinated Ti atoms due to oxygen defects. The origin of shallow gap states is unclear. PES on in situ cleaved anatase TiO2 (101) surfaces show in part no or weak emission from deep, but always weak emission from shallow gap states. Amorphous ALD titania initially is free of gap states, but deep gap states are easily induced by exposure to synchrotron radiation, while shallow gap states do not form. Exposure to synchrotron radiation also induced deep gap states in in situ cleaved single crystal (101) surfaces and in the nanoporous anatase films, wher...