Dual behavior or coexistence of trapped and free states in reducible rutile TiO2

Abstract

Excess electron transport is explored at the reduced TiO2(110) rutile surface through High-Resolution Electron Energy-Loss Spectroscopy. Changes in lineshape, intensity and energy position of elastic and phonon peaks are associated to changes in charge carrier and band gap state excitations. The dielectric modeling points to a defined inward charge profile and to distinct bulk and surface transport properties. The one-to-one correlation between band gap state intensity and carrier density favors a dual polaronic behavior of charge carriers over the coexistence of trapped and free-states.

Point Defects and Related Excess Electrons in the Dielectric Profile of the Reduced TiO2(110) Surface

“…The lorentzian ts match the relative area and FWHMs of phonon loss peaks calculated from tabulated loss function of stoichiometric rutile 43,49 (Figure 5); however, the phonon frequencies are shifted by fractions of meV as a result of the combination of opposite screenings of BGS and carrier excitations, 49 which proves the existence of both in line with the dual polaronic behavior of excess electrons. 32,49 Finally, in agreement with the dielectric model, QEP prole and QEP/phonon ratio much depend on the scattering geometry (Figure 2-a,c,e) but not BGS (Figure 2-b,d,f). The interband transitions are orientation-dependent (Figure 2-d,f), an unexpected phenomenon (see supporting information (SI) of Reference 49 vs Figure S1-b,d) likely due to the breakdown of the dielectric assumption caused by the surface sensitivity at the energies considered.…”

“…HREELS is a relevant method for discussing the issue. Since it reconciles transport and spectroscopy, it can distinguish the different entities that contribute to dielectric behavior, provided that it goes beyond the limit of previous work which characterized plasmon, phonon and BGS excitations by means of a single t. 32,49 We require the direct knowledge of the individual ngerprints of each entity contributing to the electronic status of the reduced TiO 2 surface [R-TiO 2 (110)] with a particular focus on the vividly debated charge carrier excitations and Ti int . We plan to meet the challenge by a step-by-step loss analysis of R-TiO 2 (110) exposed to oxygen which, because of the speci-cities of its reactivity, is expected to allow a cartography of the defective titania surface.…”

“…38,47,48 Contributions of phonons, interband transitions and defect-related excitations (plasmon and BGS) 49 were dened to reinvestigate the case via the semiclassical dielectric theory of EELS. Fits demonstrate (i) distinct surface and bulk transport properties and (ii) a dual behavior of excess electrons (BGS oscillator strength clearly correlates with plasmon frequency 32 ) which, due to their polaronic behavior, appear as either deeply bound or highly mobile depending on excitation frequency.…”

“…The issue prompted us to develop innovative methods 14,32 to isolate the TiO 2 (110) point defects and to revisit the modeling of High-Resolution Electron Energy Loss spectra (HREELS) of titania on the basis of the dielectric theory. 3338 As a powerful probe for surface excitations, HREELS parallels optics while covering a wider energy range with a variable surface/bulk sensitivity.…”

Carboxylate Adsorption on Rutile TiO2(100): Role of Coulomb Repulsion, Relaxation, and Steric Hindrance

“…The effect of excess electrons on the overlayer symmetry is not thought to be significant because the density of excess electrons is only 0.03 per surface unit cell based on that measured for the (110) surface. 55 Moreover, a recent study evidences little modification of the surface polaron density on acetic acid dissociative adsorption. 56 As for the influence of subsurface hydrogen, there are numerous studies that suggest that the protons remain at the surface in the form of bridging hydroxyls following dissociative adsorption of carboxylic acids.…”