Identification of Lone-Pair Surface States on Indium Oxide

Daniel W. Davies, Aron Walsh, James J. Mudd, Chris F. McConville, Anna Regoutz, J. Matthias Kahk, David J. Payne, Vin R. Dhanak, David Hesp, Katariina Pussi, Tien Lin Lee, Russell G. Egdell, Kelvin H.L. Zhang

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16 Citations (Scopus)


Indium oxide is widely used as a transparent electrode in optoelectronic devices and as a photocatalyst with activity for reduction of CO 2 . However, very little is known about the structural and electronic properties of its surfaces, particularly those prepared under reducing conditions. In this report, directional "lone-pair" surface states associated with filled 5s 2 orbitals have been identified on vacuum-annealed In 2 O 3 (111) through a combination of hard and soft X-ray photoemission spectroscopy and density functional theory calculations. The lone pairs reside on indium ad-atoms in a formal +1 oxidation state, each of which traps two electrons into a localized hybrid orbital protruding away from the surface and lying just above the valence band maximum in photoemission spectra. The third electron associated with the ad-atoms is delocalized into the conduction band, thus producing the surface electron accumulation layer identified previously on vacuum-annealed In 2 O 3 (111) (1 × 1) surfaces. The surface structure is further supported by low-energy electron diffraction, but there is no chemical shift in indium core level X-ray photoelectron spectra between surface In(I) ad-atoms and bulk In(III). The 5s 2 lone pairs confer Lewis basicity on the surface In sites and may have a pronounced impact on the catalytic or photocatalytic activity of reduced In 2 O 3 .

Original languageEnglish
Pages (from-to)1700-1709
Number of pages10
JournalJournal of Physical Chemistry C
Issue number3
Publication statusPublished - 2019 Jan 24

Bibliographical note

Funding Information:
D.W.D. thanks the Engineering and Physical Sciences Research Council (EPSRC) for support via the Centre for Doctoral Training in Sustainable Chemical Technologies (EPL016354/ 1). Calculations were carried out on the Balena HPC cluster at the University of Bath, which is maintained by Bath University Coomputing Services. V.R.D. thanks EPSRC for support (EP/ NO15800/1). K.H.L.Z gratefully acknowledges the funding support of a Clarendon Scholarship at the University of Oxford and the Thousand Youth Talents Program at Xiamen University. K.P. thanks the Academy of Finland (grant 277829) and the CSC-Finnish IT Centre for Science for support.

Publisher Copyright:
© 2018 American Chemical Society.

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films


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