Copper–cobalt hydroxysalts and oxysalts: bulk and surface characterization

Abstract

Copper–cobalt oxysalts of various Cu : Co atomic ratios (100 : 0. 92 : 8, 85 : 15, 77 : 23, 67 : 33, 15 : 85, 0 : 100) have been prepared by coprecipitation, at constant pH, from solutions of copper and cobalt nitrates added to a solution of NaHCO₃; the structure and chemical nature of the Cu—Co oxysalts have been investigated by several complementary techniques such as X-ray diffraction (XRD), magnetic susceptibility, reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS) to obtain information both on their bulk and on their surface properties.

In the range of Cu : Co atomic ratios from 100 : 0 to 67 : 33 the materials consist of Co-containing malachite, Cu_{2 –x}Co_xCO₃(OH)₂, while for Cu : Co equal to 0 : 100 and 15 : 85 the products are CoCO₃, spherocobaltite, and Cu-containing spherocobaltite Co_0.85Cu_0.15CO₃, respectively. From both reflectance spectroscopy and magnetic susceptibility measurements the presence of only Cu²⁺ and Co²⁺ species has been ascertained in all materials. Antiferromagnetic interactions are strong at high copper content and become weaker at increasing cobalt loading. The observed decrease in volume of the malachite monoclinic unit cell with incorporation of Co²⁺ ions, in spite of the larger Co²⁺ ion radius with respect to that of Cu²⁺, indicates that distortion of the MO₆ polyhedra at the cation site of the malachite lattice is less when Cu²⁺ is replaced by Co²⁺. The presence of Cu²⁺ in the spherocobaltite trigonal lattice shows a small cell volume decrease as expected from ionic radii difference. XPS has confirmed the presence of only Cu²⁺ and Co²⁺ species at the surface of the materials. It is also shown that the local nature of the chemical bond is essentially determined by the nature of the first-neighbour ligands. At higher copper content, where the materials are cobalt-containing malachite, the surface is enriched in cobalt.