engleski

Diffuse Reflectance Spectroscopy analysis of sol-gel derived cobalt-doped gahnite

Majority of blue ceramic pigments are based on cobalt ion, one of most common crystalline structures used is cobalt aluminate (CoAl2O4). In addition to being scarce and thus expensive, cobalt and its salts are widely considered to be toxic and environmentally hazardous substances. In order to minimize the use of cobalt in the preparation of blue ceramic pigments, the zinc aluminate spinel (or gahnite), ZnAl2O4, prove to be convenient host lattice allowing the development of intense blue colors with small amounts of cobalt. Both zinc and cobalt aluminate have spinel structure of the general formula AIIBIII2O4. Thus, the zinc by cobalt replacement in (Zn, Co)Al2O4 solid solutions enables production of blue pigment with reduced negative health, environmental sustainability and economic considerations. Different chemical processing methods are used for the preparation of spinels, whereas sol-gel yielded nanocrystalline spinels powders are pure, homogeneous, ultrafine and produced on lower temperatures and with shorter calcinations times compared to other techniques. In this work the nano-particles of (ZnCo)Al2O4 spinel pigment were prepared by sol-gel method using aluminium-sec-butoxide [Al(OsBu)3], zinc nitrate hexahydrate Zn(NO3)2 × 6H2O and cobalt nitrate hexahydrate Co(NO3)2 × 6H2O as a starting materials and ethyl-acetoacetate (C6H10O3) as a chelating agent. Samples with 4, 8 and 12 at.% of zinc replaced with cobalt in the gahnite lattice were prepared. Thermal evolution of prepared samples, beside X-ray diffraction (XRD) which is common method for such task, was characterized by UV–VIS diffuse reflectance spectroscopy (DRS). In the course of thermal treatment at lower temperatures a partial oxidation of Co2+ to Co3+ occurs. The Co3+ ions are accommodated in octahedral sites causing the green appearance of samples. The temperature induced spectrum evolution could be interpreted as a progressive reduction of Co3+ ions to Co2+ ions i.e. as the green to blue color change due to a electronic transitions in 540–630 nm range, characteristic of Co2+ ions with 3d7 configuration in tetrahedral coordination. It has been determined that the appearance of Co3+ and Co2+ ions, as well the distribution of Co2+ cations in tetrahedral and octahedral sites of zinc cobalt aluminate crystal lattice depend on Co loading and calcinations temperature. The blue ceramic pigment based on Co-doped gahnite could be obtained with low Co content at temperature of 800°C.

pigment; cobalt; gahnite; sol-gel

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