Oxygen vacancy and hole conduction in amorphous TiO₂

Abstract

The amorphous titanium dioxide (a-TiO2) has drawn attention recently due to the finding that it holds promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The mechanism of hole conductivity at a level much higher than the edge of the valence band is still a mystery. In this work, an amorphous TiO2 model is obtained from molecular dynamics employing the "melt-and-quench" technique. The electronic properties, polaronic states and the hole conduction mechanism in amorphous structure were investigated by means of density functional theory with Hubbard's energy correction (DFT + U) and compared to those in crystalline (rutile) TiO2. The formation energy of the oxygen vacancy was found to reduce significantly (by a few eV) upon amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels, which are comparable to experimental observations and could be responsible for hole conduction in the "leaky" TiO2 recently discovered for the photochemical water-splitting applications.

Surface oxygen-vacancy induced photocatalytic activity of La(OH)3 nanorods prepared by a fast and scalable method

“…16,17 Especially, oxygen-vacancy can serve as photoinduced charge traps and modify the band structure of semiconductor photocatalysts, which is conductive to the enhanced photocatalytic activity. [18][19][20] The light absorption range of semiconductor materials can be enhanced or expanded by introducing oxygen vacancies, which results in the extended light absorption spectra and band gap narrowing. 18,21 Recently, oxygen-vacancy associated properties have been widely surveyed in simple semiconductors, such as TiO 2 , [18][19][20]22 Fe 2 O 3 , 23 and ZnO.…”

“…[18][19][20] The light absorption range of semiconductor materials can be enhanced or expanded by introducing oxygen vacancies, which results in the extended light absorption spectra and band gap narrowing. 18,21 Recently, oxygen-vacancy associated properties have been widely surveyed in simple semiconductors, such as TiO 2 , [18][19][20]22 Fe 2 O 3 , 23 and ZnO. [24][25][26] However, to the best of our knowledge, the oxygen vacancies in La(OH) 3 and their influence on the band structure and photocatalytic activity have seldom been investigated.…”

Deep electron and hole polarons and bipolarons in amorphous oxide

“…In amorphous SiO 2 , holes [15] and electrons have been shown [16,17], to localize spontaneously in deep states with well-defined EPR and optical absorption signatures measured experimentally [18]. The hole trapping energies in amorphous TiO 2 were calculated to be much larger than these in rutile [19]. Under-coordinated indium has been suggested to act as a deep intrinsic electron-trap center in amorphous InGaZnO 4 by theoretical calculations [20].…”

“…The polaron states in the gap of a-HfO 2 are close to the position of the bottom of Si conduction band at Si/HfO 2 interface indicating that these states can be populated via direct tunnelling or electron injection into the oxide. Similar states may exist in other amorphous oxide films and nanoparticles, such as Al 2 O 3 , ZrO 2 , and TiO 2 [19]. Finally, the revealed existence of deep intrinsic states of polaronic origin suggests a possible explanation to the abnormally high barriers often found at the interfaces between metals and high-permittivity insulating oxides such as HfO 2 and ZrO 2 [65].…”

Oxygen defects in amorphous Al2O3: A hybrid functional study

“…However, Al 2 O 3 in MOS devices is usually deposited via atomic layer deposition, which results in an amorphous phase with electronic properties differing considerably from those of crystalline phases. [21][22][23][24] Furthermore, the experimental characterization of defect states in a-Al 2 O 3 has led to conflicting interpretations. The oxygen vacancy has been invoked as an electron trap in oxygen-deficient a-Al 2 O 3 films on the basis of electron energy loss, photoluminescence, and capacitance measurements, and this suggestion has received support from theoretical studies based on density-functional-theory calculations.…”