Modification of the Chemisorption Properties of Epitaxial Delafossite CuFeO₂ Thin Films by Substituting Fe for Ga in the Crystal Structure

Films of CuFeO₂ and CuFe_0.75Ga_0.25O₂ were grown over sapphire substrates in high vacuum using a pulsed laser deposition technique. The films grew with rhombohedral delafossite structure and highly epitaxial in the c-direction. Samples were characterized by X-ray diffraction, Raman spectroscopy and atomic force microscopy. Surface of the films were inspected with X-ray and UV photoelectron spectroscopy. Adsorption of CO₂ and H₂O was studied by a thermal program desorption technique. In both films Cu and Fe were exposed at the surface–gas interface. X-ray photoelectron data indicated that CO₂ adsorbs preferentially at Cu sites forming a similar coordination to CuCO₃. The energy for desorption of CO₂ and H₂O was estimated to be 30 kcal mol⁻¹ (1.3 eV atom⁻¹) for CuFeO₂ and 36 kcal mol⁻¹ (1.6 eV mol⁻¹) for CuFe_0.75Ga_0.25O₂. UV photoelectron spectroscopy showed that the valence band of the CuFeO₂ delafossite oxides is modified with the substitution of Fe by Ga in the crystal lattice. The semiconductor band gap of CuFeO₂ delafossite oxides also increased from 1.2 to 1.5 eV due to the substitution of Fe by Ga in the crystal lattice.