Indium oxide, a transparent conductive oxide (TCO), is widely used in semiconductor industry but it also displays promising performance in electro- and photo catalytic reactions. In all applications, surrounding water molecules in gaseous or liquid state may modify the surface and thereby influence chemical processes at the atomic scale. Understanding the interaction of water with In₂O₃ surfaces at the atomic level is of significant importance for the exact interpretation of reaction mechanisms.
We focus on the hexagonal In₂O₃(111) surface, which has an intrinsically large unit cell (1.43 nm lattice constant; panel a) composed of three distinct areas of reactivity, strongly hydrophilic (B), medium (C), and strongly hydrophobic (A).
In this work we test the reactivity of these areas by unravelling the interfacial water structures on the In₂O₃(111) surface for the whole range of water coverages in UHV, from single dissociated molecules (panel b) to multilayers. Even at high coverages (panel c) we clearly see hydrophilicity and hydrophobicity within the unit cell, both in experiments and calculations. Strong effects of local accumulation/depletion of water is confirmed by DFT and MD for ordered structures consisting of up to 18 water molecules/unit cell. Going beyond UHV, our MD simulations of a liquid layer show the robustness of the strongly hydrophobic area in the unit cell.

(a) Atomic model of In<sub>2</sub>O<sub>3</sub>(111) highlighting the hydrophilic (B) and hydrophobic (C) areas of the unit cell, and the area in-between (A). (b) Three dissociated water molecules adsorbed symmetrically around B (nc-AFM image). (c) At high coverages (18 H<sub>2</sub>O/unit cell) terminating water molecules (bright features) are located in B (nc-AFM image).

(a) Atomic model of In₂O₃(111) highlighting the hydrophilic (B) and hydrophobic (C) areas of the unit cell, and the area in-between (A). (b) Three dissociated water molecules adsorbed symmetrically around B (nc-AFM image). (c) At high coverages (18 H₂O/unit cell) terminating water molecules (bright features) are located in B (nc-AFM image).

Bernd Meyer

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