Abstract
Changes in chemical and physical properties resulting from water adsorption play an important role in the characterization and performance of device-relevant materials. Studies of model oxides with well-characterized surfaces can provide detailed information that is vital for a general understanding of water–oxide interactions. In this work, we study single crystals of indium oxide, the prototypical transparent contact material that is heavily used in a wide range of applications and most prominently in optoelectronic technologies. Water adsorbs dissociatively already at temperatures as low as 100 K, as confirmed by scanning tunneling microscopy (STM), photoelectron spectroscopy, and density functional theory. This dissociation takes place on lattice sites of the defect-free surface. While the In2O3(111)-(1 × 1) surface offers four types of surface oxygen atoms (12 atoms per unit cell in total), water dissociation happens exclusively at one of them together with a neighboring pair of 5-fold coordinated In atoms. These O–In groups are symmetrically arranged around the 6-fold coordinated In atoms at the surface. At room temperature, the In2O3(111) surface thus saturates at three dissociated water molecules per unit cell, leading to a well-ordered hydroxylated surface with (1 × 1) symmetry, where the three water OWH groups plus the surface OSH groups are imaged together as one bright triangle in STM. Manipulations with the STM tip by means of voltage pulses preferentially remove the H atom of one surface OSH group per triangle. The change in contrast due to strong local band bending provides insights into the internal structure of these bright triangles. The experimental results are further confirmed by quantitative simulations of the STM image corrugation.
Highlights
The interaction of oxide surfaces with water vapor under ambient conditions plays an important role in thin-film device technology, as water adsorption or hydroxylation of the surface influence, for example, the catalytic reactivity and electronic properties
The present investigations were carried out using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), as well as density functional theory (DFT) calculations
Exposing the In2O3(111) surface to water at room temperature (RT) quickly leads to saturation, with several different features observed in STM along the way
Summary
The interaction of oxide surfaces with water vapor under ambient conditions plays an important role in thin-film device technology, as water adsorption or hydroxylation of the surface influence, for example, the catalytic reactivity and electronic properties. ITO is currently widely used as a contact electrode in liquid crystal displays and solar cells, and recently, it is of increasing interest as a sensor and catalytic material.. ITO is currently widely used as a contact electrode in liquid crystal displays and solar cells, and recently, it is of increasing interest as a sensor and catalytic material.17,18 It seems that indium oxide will remain the TCO material of choice for the foreseeable future. Reducing the surface results in single In adatoms; the reduction can either be achieved thermally, by heating in ultrahigh vacuum (UHV), or chemically, by doping with an impurity atom.30 These adatoms adsorb preferentially at one specific site of the unit cell, forming an ordered array.
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