Preparation of ultrathin chromium oxide films on Cu(110) investigated by XPS and LEED

doi:10.1016/S0039-6028(98)00324-0

Surface Science

Volume 411, Issues 1–2, 11 August 1998, Pages 35-45

https://doi.org/10.1016/S0039-6028(98)00324-0 Get rights and content

Abstract

Ultra-thin chromium oxide films grown on Cu(110) have been investigated using XPS and LEED. The films are prepared by deposition of chromium atoms, exposure to oxygen and heating at 673 K in vacuum. The first layer of the oxide shows a surface structure with hexagonal symmetry. The major chromium component of the oxide was Cr²⁺ and the surface structure is ascribed to CrO(111)-like oxide. At the oxide coverage more than two layers, the LEED pattern transforms to a ( $3 × 3) R 30°$ structure with respect to CrO(111) followed by the oxidation of the chromium atoms to Cr³⁺. The surface oxide structure corresponds to the Cr₂O₃(111) surface. Both surface oxides are not stable to oxygen exposure (∼1.3×10⁻⁵ Pa) at above 473 K and a copper oxide layer with Cu⁺ (one layer at maximum at 773 K) segregates to the top of the chromium oxide as characterized by XPS and X-AES. Most of the chromium atoms are reduced to Cr²⁺. The CrO(111)-like LEED pattern become sharp.

Introduction

Information on the surface structure and reactivity of transition metal oxides is important in research relating to catalysts, corrosion etc. Ordered ultra-thin films of metal oxide on another metal substrate have been investigated in connection with surface science studies of bulk metal oxide surfaces 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Moreover, thin films may have a new surface structure and chemical reactivity. Increased attention has been paid to ultra-thin metal oxide films.

The Cu(100) surface has been used as a substrate for the growth of thin oxide films such as vanadium oxide [12]and iron oxide 6, 7. These oxide films give a surface structure with hexagonal symmetry, being different from the atomic arrangement of the substrate. Hexagonal vanadium oxides are also formed on Ni(110) with two-fold symmetry and a corrugated surface structure [13]. In the present studies the growth of the chromium oxide was investigated on Cu(110) to compare the difference in structure and chemical states from the ultra-thin vanadium and iron oxide films.

Chromium oxides are widely used materials as catalysts and anti-corrosive material etc. Surface science studies of the structure and chemical reactivity of Cr₂O₃ surfaces have been carried out by preparing the oxide on a Cr(110) surface 14, 15, 16, 17, 18, 19. The growth and structure of chromium oxides have been investigated on Pt(111) 8, 9. Oxides such as Cr₃O₄- and Cr₂O₃-like oxides have been prepared. The effect of the difference in metal substrates is the other interest in the present study on the growth of the ultra-thin chromium oxides on Cu(110). The ultra-thin chromium oxide surfaces were characterized by XPS and LEED. In contrast with the preparation of the oxides of vanadium and iron, a segregation of the copper oxide with Cu⁺ to the topmost layer was observed after exposure to O₂ at elevated temperatures.

Section snippets

Experimental

The experiments were performed in an ultrahigh vacuum chamber (a base pressure of 2.6×10⁻⁷ Pa) equipped with XPS (VSW) and LEED (Omicron) systems. XPS (excited by Mg K radiation) were recorded at an electron take-off angle of 30°, defined as the angle between the plane of the sample and the trajectory of the outgoing photoelectron. The core electron binding energies were calibrated with respect to the Cu 2p_3/2 peak (932.7 eV) of the clean Cu(110) substrate.

The surface of the Cu(110) crystal (from

Growth and structure

The chromium atoms were deposited onto the clean Cu(110) surface in successive increments. After each increment the chromium atoms were exposed to O₂ (20 L) and heated at 673 K in vacuum. The Cu 2p_3/2, O 1s and Cr 2p_3/2 peak intensities were plotted as a function of the increments, Fig. 1. The Cr 2p_3/2 and O 1s peak intensities increased linearly and showed a change of the slope around 29 min, suggesting that the chromium oxide grew mostly as two-dimensional islands at least up to the break point. At

Growth and structure of chromium oxide on Cu(110)

A lattice constant of 3.01 Å was estimated from the hexagonal LEED pattern (A) observed at the chromium oxide coverage up to ∼1.0θ. The constant does not resemble any constant of the surfaces of Cr₃O₄ and Cr₂O₃. The pattern can be discussed by correlating it with results for iron oxide surfaces intensively studied on Pt(111) and Pt(100) 1, 2, 3, 4, 5since iron is an element which is very close to chromium in terms of bond radii and the structures of oxides. Three kinds of iron oxide surfaces

Conclusion

Chromium oxide films grown on Cu(110) have been characterized by means of XPS and LEED. The oxide prepared by deposition of chromium atoms, exposure to O₂ (20 L) and heating at 673–773 K in vacuum grows with two types of structures depending on the coverage. Up to ∼1.0θ coverage a chromium oxide consisting mostly of CrO(111) containing Cr²⁺ is formed, showing a hexagonal LEED pattern. The oxide containing Cr³⁺ exists at the same time. At a coverage of around 2.0θ the chromium oxide giving the

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