A PHOTOEMISSION STUDY OF THE ELECTRONIC STRUCTURE INDUCED BY POTASSIUM ADSORPTION ON TiO₂(110)

Abstract

Electronic structure effects induced by potassium adsorption up to one monolayer (ML) on a nearly stoichiometric TiO 2(110) surface has been studied by means of angle-resolved photoemission spectroscopy (ARUPS and ARXPS) from valence states and core levels. In agreement with the observations on K/TiO 2(100) [P.J. Hardman et al., Surf. Sci.269/270, 677 (1992)], potassium adsorption at room temperature leads—due to K-to-substrate charge transfer—to the reduction of surface Ti ions (to nominally Ti 3+ ions), evidenced by lowered Ti 2p core-level binding energy (ΔBE=–1.6 eV ) and occupation of Ti 3d-like band-gap states centered at 0.9 eV BE. The gap-state intensity exhibits a pronounced maximum at 0.37 ML coverage, where the work function has a weak minimum. This behavior is in agreement with a ionic-to-neutral transition of the K-substrate bonding with increasing K coverage, as suggested recently [Souda et al., Surf. Sci.285, 265 (1993)]. Annealing of a surface precovered with 0.27 ML potassium up to 1000 K results in metallization of the surface, evidenced by (i) the occupation of a second gap-state centered at 0.4 BE and with a considerable state-density at the Fermi energy, and (ii) Ti 2p core-levels lowered by 3.2 eV in BE (nominally “ Ti 2+” ions). This dramatic reduction of the surface is healed out with complete desorption of potassium. A discussion in terms of desorption of KO x species and oxygen diffusion from the bulk to the surface is given.

First-principles study of potassium adsorption onTiO2surfaces

“…This state is associated with a new peak in the electronic DOS about 1 eV above the VBM. States have been observed about 3 eV above the VBM in TiO 2 samples reduced by either adsorption of alkali metals [22][23][24][25]28,55 or by the creation of oxygen vacancies. 14,7,28 Spin-polarized states at a similar energy with respect to the VBM were found in recent theoretical investigations of potassium adsorption on the ͑110͒ surface 52 and on oxygen deficient ͑110͒ surfaces in which both DFT and HF approaches were used.…”

“…1, [22][23][24][25][26][27][28] Low energy electron diffraction ͑LEED͒ data indicate that K adsorption on the ͑100͒ surface at half-monolayer coverage results in an ordered c(2ϫ2) geometry. 22 This is confirmed by surface x-ray absorption fine structure ͑SEXAFS͒ studies which also probe the local geometry of the adsorption site.…”

“…Similar states have been reported for K-adsorption on the TiO 2 ͑110͒ surface. 28 Such states are indicative of a reduced surface 7 and in this context are apparently populated by charge transferred from the K overlayer.…”

Theoretical Analysis of K Adsorption on TiO2(110) Rutile Surface

“…The stoichiometric surface has no occupied gap states and is inert to chemisorption of a wide variety of molecules ,− Both of them induce occupied gap states placed around 1 eV below the conduction band edge. − Different spectroscopic evidences − have shown that these states are predominantly Ti 3d in character. These results reveal the presence of reduced Ti atoms in the system.…”

An improved catalytic effect of nitrogen-doped TiO2 nanofibers for rechargeable Li–O2 batteries; the role of oxidation states and vacancies on the surface

“…These studies have shown that alkali metal adsorption induces a reduction of Ti 4+ to Ti 3+ on the surface through the electron transfer from the adsorbate (alkali metal ion) to Ti cations mediated by surface oxygen nally forming a surface layer in which adsorbate atoms are oxidized. [31][32][33][34][35][36] When more exposed oxygen sites are coupled with electrons from nitrogen doping, it is natural that the reaction energies and the corresponding potentials during ORR are decreased.…”