Disorder-induced electron and hole trapping in amorphous TiO2

Abstract

Thin films of amorphous (a)-TiO 2 are ubiquitous as photocatalysts, protective coatings, photo-anodes, and in memory applications, where they are exposed to excess electrons and holes. We investigate trapping of excess electrons and holes in the bulk of pure amorphous titanium dioxide, a-TiO 2 , using hybrid density-functional theory (h-DFT) calculations. Fifty 270-atom a-TiO 2 structures were produced using classical molecular dynamics and their geometries fully optimized using h-DFT simulations. They have the density, distribution of atomic coordination numbers, and radial pair-distribution functions in agreement with experiments. The calculated average a-TiO 2 band gap is 3.25 eV with no states splitting into the band gap. Trapping of excess electrons and holes in a-TiO 2 is predicted at precursor sites, such as elongated Ti-O bonds. Single electron and hole polarons have average trapping energies (E T) of −0.4 eV and −0.8 eV, respectively. We also identify several types of electron and hole bipolaron states and discuss their stability. These results can be used for understanding the mechanisms of photo-catalysis and improving the performance of electronic devices employing a-TiO 2 films.

Hole Polaron Migration in Bulk Phases of TiO2 Using Hybrid Density Functional Theory

“…There have been numerous DFT investigations into the stability and electronic properties of hole polarons in TiO 2 in perfect and amorphous crystals as well as at extended defects. ,,,− Standard local or semi-local exchange correlation functionals fail to describe hole polarons correctly due to significant residual self-interaction error (SIE), which tends to favor delocalized electronic states. Therefore, most of the above studies have been performed using modifications to standard DFT such as DFT + U or hybrid functionals incorporating some proportion of Hartree–Fock (HF) exchange (hybrid DFT).…”

Oxygen-Vacancy-Driven Orbital Reconstruction at the Surface of TiO2 Core–Shell Nanostructures

“…In the rutile structure of TiO 2 , excess electrons can localize at any lattice Ti atoms and form a small polaron, whereas in the anatase structure of TiO 2 the electrons prefer a free carrier-state and they can only be trapped near oxygen vacancies . Indeed, trapping of the electrons has been observed in the TiO 2 nanoparticles by picosecond and femtosecond hard X-ray absorption spectroscopy experiments and supported by hybrid density-functional theory calculations . Similarly, the redistribution of the electron density also modifies the geometric structure of the Ti 3d–O 2p hybridized states and alters the nature of the chemical bonding. , This modified surface chemistry enhances materials design and advances their reliability in applications (see refs , − , and references therein).…”

“…11 Indeed, trapping of the electrons has been observed in the TiO 2 nanoparticles by picosecond 15 and femtosecond 16 hard X-ray absorption spectroscopy experiments and supported by hybrid density-functional theory calculations. 17 Similarly, the redistribution of the electron density also modifies the geometric structure of the Ti 3d−O 2p hybridized states and alters the nature of the chemical bonding. 18,19 This modified surface chemistry enhances materials design and advances their reliability in applications (see refs 14 Herein, by employing a combination of element selective hard and soft X-ray absorption spectroscopy (XAS) experiments and theoretical calculations, we present a systematic study at the atomic level understanding of the Ti 3d symmetry, and orbital reconstruction at the surface of TiO 2 anatase nanoparticles (nanoanatase).…”

Inherent electron and hole trapping in amorphous phase-change memory materials: Ge2Sb2Te5

“…This can affect the calculated trapping energies, since a larger difference in the degree of spatial localization, before and after the geometry relaxation of the glass structure with an extra electron/hole, results in a higher kinetic-energy cost upon localization. 2,3,8,9 Considering the electron and hole mobility edges, which are located deeper in the bands with respect to the conductionband minimum and valence-band maximum, respectively, this would reduce the computed values of the trapping energies. Such an effect would strengthen the argument of possible charge release at room temperature and/or higher temperatures.…”

“…3 Intrinsic electron and hole trapping in amorphous oxide semiconductors has been demonstrated by theoretical calculations. [4][5][6][7][8][9] Usually, shallow electronic states near the bottom of the conduction band or the top of the valence band can serve as charge-trapping centres, while deeptrap states have been reported, as well, in some amorphous models. 10,11 Phase-change memory materials based on chalcogenide alloys encode stored digital binary data as metastable structural states of the material.…”