Blue CoAl2O4 spinel via complexation method
Introduction
The cobalt-blue pigment has been known since Middle Ages. In 1777, Gahn and Wenzel discovered CoAl2O4 and 30 years later L.J. Thénard succeeded to reproduce in laboratory “the blue colour” by the reaction of cobalt arsenate/cobalt phosphate with alumina, at high temperature. For this reason, L.J. Thénard has been considered the inventor of “the bright blue” cobalt aluminate [1].
Cobalt-aluminate spinel is an idiochromatic (self-coloured) ceramic pigment, widely used for the coloration of plastics, paint, fibers, paper, rubber, glass, cement, glazes and porcelain enamels [2], [3], [4]. It is, also, used as humidity sensor [5] and ceramic catalyst in the CO2 reforming of methane [6].
The normal spinel-CoAl2O4 has been conventionally synthesized using solid-state reactions at temperatures above 1000 °C, for long periods of time [7], [8], [9], [10]. It was classified as ceramics-grade pigment. Since 1980, wet-chemical methods have been extensively applied to synthesize ultrafine CoAl2O4, among them: coprecipitation method [4], [11], [12], [13], hydrothermal synthesis [14], sol–gel route [3], [5], [15], [16], [17]. Considerable efforts have been made to develop “soft chemistry” routes in which precursors were polynuclear coordination compounds, such as: low temperature combustion synthesis (LCS) [2], [18], [19], complexation method [20], [21].
In this paper, we report on the synthesis of cobalt-aluminate spinels, CoAl2O4, obtained by thermal decomposition of polynuclear multimetallic compounds containing tartarate and gluconate anions as ligands, respectively.
Section snippets
Synthesis of the complex precursors
All chemicals: Al(NO3)3·9H2O, Co(NO3)2·6H2O, tartaric acid and δ-gluconolactone were of reagent quality (Merck) and were used without further purifications.
The polynuclear coordination precursors were prepared as follows: aluminium(III) nitrate and cobalt(II) nitrate were dissolved in the minimum amount of water and mixed with an aqueous solution of carboxylic acid in a 2:1:4 (Al(III):Co(II):tartaric acid) ratio and 2:1:8 (Al(III):Co(II):δ-gluconolactone) ratio, respectively (δ-gluconolactone
Characterization of the complex precursors
The successful application of the complexation method for obtaining aluminate spinel oxides requires two important steps: (1) a detailed study on the formation of the polynuclear complex compounds to establish the parameters that influence the synthesis, such as combination ratio of elements, nature of the ligand, pH of the reaction medium, temperature, etc.; (2) a study of the thermal decomposition mechanism accompanied by the characterization of the mixed oxides.
The polynuclear coordination
Conclusions
From the study carried out in this work the following conclusions can be withdrawn. The complexation method has been found to be suitable for the synthesis of CoAl2O4 spinel. In general we can say that both tartarate and gluconate routes lead to the formation of the single-phase CoAl2O4 spinel. The UV–vis spectrum for CoAl2O4 confirmed tetrahedral coordination of Co2+ (d7) ion which is responsible for the blue bright colour of spinel oxide (Thenard's blue). Luminescence bands observed in the
Acknowledgement
Financial support from the PNII-IDEAS project no. 158/2007.
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