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 TiO 2 samples: In situ cleaved single anatase crystal TiO 2 (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 TiO 2 (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, whereas emission from shallow gap states stays constant. Amorphous ALD films transform to anatase nanocrystals by annealing as shown by XRD and AFM and show deep and shallow gap state emissions similar to the nanoporous anatase films prepared by sintering spin coated anatase slurry. Atomic force microscopy (AFM) reveals the presence of terraces on the cleaved (101) surfaces. The comparison of the data collected on the differently prepared anatase and amorphous titania samples suggest assigning shallow gap states to under-coordinated Ti atoms located at edge sites. Conformal amorphous films that show no edges in AFM, accordingly have no shallow gap states.

Roles of Vanadium and Nitrogen in Photocatalytic Activity of VN‐Codoped TiO2 Photocatalyst

“…In addition to these impurity levels, the hybridization between V‐N, also confirm by XPS causes further narrowing in band gap of V–N‐codoped TiO 2 . In xV4N, the increase in V concentration creates the oxygen vacancies, as confirmed by XPS and XRD results, which are known to form deep levels at 1.0–1.2 eV below the CBM in band gap . Indeed, this oxygen vacancy is responsible for reduction of band gap in addition to the isolated V 4+ ‐based impurities state and being more efficient with increasing concentration of V in codoped TiO 2 .…”

The Work Function of TiO2

“…UP spectra are usually more sensitive to surface defects than XP spectra, which is mostly due to the higher surface sensitivity of UPS and the higher intensities. It is accepted that the so-called band gap surface states would form due to occupied Ti 3d states (Ti 3+ ) near the O vacancy sites on the surface, but also due to Ti 3+ interstitials in the subsurface region [4,11,18,63,64]. It is clear from Figure 4 that all sputtered TiO 2 surfaces show the well-known Ti 3d 1 emission lines in the band gap region.…”

“…In 2008 Yang et al [7] synthesized anatase single crystals with 47% of the minority (001) facet via a hydrothermal route employing fluoric acid. This successful synthesis has surged further fundamental investigations of both anatase (101) and (001) facets [8][9][10][11]. However, although the electronic structure of anatase bulk and surfaces including band gaps and surface states are well known due a wide range of experimental and theoretical studies using complementary techniques, particularly using electron spectroscopy (see e.g., [11][12][13][14][15][16][17][18][19][20][21]), the surface potentials-Fermi level position, work function and ionization potential-have not yet been studied systematically.…”

Strategies for stable water splitting via protected photoelectrodes

“…However there are still some unresolved issues with Leaky TiO 2 such as exactly how charge conducts through the TiO 2 . Both DFT calculations 160 and experimental results 161 have shown that oxygen vacancies can allow for deep level trap sites as those found in the Leaky TiO 2 . However calculations by Lewerenz show that the charge transfer was probably not due to a Poole-Frenkel type mechanism.…”