… transparent, electrically conducting, and with intriguing surfaces

Some non-transition metal oxides with a full d-shell such as In2O3, ZnO, and SnO2, have a band gap of the order of 3 eV (meaning they are optically transparent) and an s-like conduction band (which implies that they conduct quite well when they are slightly n-doped). The combination of these properties is as rare as it is useful: Transparent conductive oxides (TCOs) are needed in virtually any technology that combines light with electronics. Moreover, the conductivity of these materials changes easily when they are exposed to oxidising or reducing gases, a property which is richly exploited in chemical sensing. And, as is true for most metal oxides, also TCOs play a role in catalysis. Surfaces and interfaces are critical wherever these materials are used and we want to learn more about them.

You are looking through indium oxide now

The most common transparent conductive oxide to date is In2O3. For use in applications, In2O3 is usually doped with Sn and then called ITO (indium tin oxide). Apart from utilising it, e.g., in solar cells, it is also used for transparent electrodes of LCD screens and OLED displays, and as an active material in gas sensors. More recently, applied studies discovered In2O3 to be catalytically active in CO2 reduction reactions, which is also of interest for surface science.

Due to the conductivity of In2O3 all surface science instrumentation can be applied - and we focus on STM, non-contact AFM, TPD and XPS (both laboratory and synchrotron sources).

We are currently investigating the In2O3(111) surface utilising single crystals and crystalline thin films (grown in house with PLD, see "Atomically Controlled Thin-film Growth"). This surface stands out for its large unit cell (1.43 nm, three-fold symmetric) and complexity of the atomic arrangement with a multitude of inequivalent oxygen and indium sites, combined with its remarkable structural stability (non-polar, no reconstruction but a relaxed bulk termination).

 

Water on indium oxide: structure model and ncAFM image

© Margareta Wagner/IAP

One of the ongoing topics is the interaction of this surface with water – from single molecules to the multilayer adsorbed from the gas phase, but (soon) also liquid water. From the adsorption of individual dissociated water molecules we were able to determine the proton affinity of different surface oxygen atoms in a combined experimental (AFM) and theoretical (DFT) approach. Moreover, we found "hydrophobic pockets" in the rather large In2O3(111) unit cell that could be relevant for reactions involving non-polar (bio)molecules in the aqueous phase.

  • M. Wagner, P. Lackner, S. Seiler, A. Brunsch, R. Bliem, S. Gerhold, Z. Wang, J. Osiecki, K. Schulte, L. A. Boatner, M. Schmid, B. Meyer, U. Diebold
    Resolving the structure of a well-ordered hydroxyl overlayer on In2O3(111): Nanomanipulation and theory
    ACS Nano 11, 11531 (2017); doi: 10.1021/acsnano.7b06387

  • M. Wagner, B. Meyer, M. Setvin, M. Schmid, U. Diebold
    Direct assessment of the acidity of individual surface hydroxyls
    Nature 592, 722 (2021); doi: 10.1038/s41586-021-03432-3

  • H. Chen, M. A. Blatnik, C. L. Ritterhoff, I. Sokolović, F. Mirabella, G. Franceschi, M. Riva, M. Schmid, J. Čechal, B. Meyer, U. Diebold, M. Wagner
    Water structures reveal local hydrophobicity on the In2O3(111) surface
    ACS Nano 16, 21163 (2022); doi: 10.1021/acsnano.2c09115