Oxygen-defective Gd₂O_3−x films with visible-light absorption produced by precisely controlling oxidation of Gd metal foil

Abstract

Introducing oxygen deficiencies into metallic oxides dramatically improves their functional properties, such as their catalytic activity, ion conductivity, visible-light response, ferroelectric behavior, and ferromagnetism. We successfully produced oxygen-defective Gd₂O_3−x films by an oxidation process using Gd metal foil as the starting material and investigated the films’ optical properties and microstructures. A black oxygen-defective Gd₂O_3−x film nearly 1 µm thick was formed on the surface of Gd metal foil annealed at 773 K for 600 s in air. The Gd₂O_3−x film consisted of a cubic-Gd₂O₃ structure with an equiaxed grain size of a few hundred nanometers. The diffuse reflectance of the Gd₂O_3−x film was less than 30 %, indicating the absorption of both visible and near-infrared light. We also produced several-micrometer-thick oxygen-defective Gd₂O_3−x films on the surface of a Gd metal foil by a two-step heat treatment involving annealing at 773 K for 600 s in air and subsequent heating at 1073 K for 3.6 ks under an O₂ partial pressure of 1.01 × 10⁻⁵ Pa. The Gd₂O_3−x film exhibited less than 20 % reflectance in the visible region. Transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy observations revealed that the Gd₂O_3−x film was composed of a periodic structure with a spacing of several nanometers, which originated from the presence and absence of oxygen deficiencies in the cubic-Gd₂O₃ structure; that is, it was composed of a “hyper-ordered structure.”