Copper cobalt hydroxysalts and oxysalts: bulk and surface characterization

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 NaHCO 3; 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-xCo xCO 3(OH) 2, while for Cu:Co equal to 0:100 and 15:85 the products are CoCO 3, spherocobaltite, and Cu-containing spherocobaltite Co 0.85Cu 0.15CO 3, respectively. From both reflectance spectroscopy and magnetic susceptibility measurements the presence of only Cu 2+ and Co 2+ 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 Co 2+ ions, in spite of the larger Co 2+ ion radius with respect to that of Cu 2+, indicates that distortion of the MO 6 polyhedra at the cation site of the malachite lattice is less when Cu 2+ is replaced by Co 2+. The presence of Cu 2+ in the spherocobaltite trigonal lattice shows a small cell volume decrease as expected from ionic radii difference. XPS has confirmed the presence of only Cu 2+ and Co 2+ 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.