Stabilization of mixed valencies in Cu, Zn-based oxides
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Stabilization of mixed valencies in Cu, Zn-based oxides Anne Le Nestour 1, Manuel Gaudon1, Mona Tréguer-Delapierre 1, Ronn Andriessen2 and Alain Demourgues 1 1 Institut de Chimie de La Matière Condensée de Bordeaux (ICMCB-CNRS) 87 avenue du Dr Albert Schweitzer, Pessac, Cedex, 33608 France. 2 Agfa-Gevaert N.V. Septestraat 27- B-2640 Morstel, BELGIUM ABSTRACT Zinc aluminate spinels partially doped or fully substituted by copper cations have been synthesized by various routes, leading to homogeneous solid-solutions. UV-Visible-NIR absorption spectra of those compounds show that the colour can be governed either by the inversion rate of the spinel (i.e. the distribution of copper cations in the various available crystallographic sites) or by the valence state of the copper (Cu2+/Cu+). Moreover Cu-doped zinc oxide has been prepared by solid state route as a reference for copper in a tetrahedral environment. Copper oxidation state and the symmetry of Cu environment in aluminate spinels have been correlated to optical absorption properties and discussed considering O-Cu (Zn) charge transfer band (CTB) and intra-atomic transitions. The energy and the optical density of CTB and intra-atomic transitions bands in Zn1-xCuxAl2O4 and Zn1-εCuεO oxides can be easily tuned taking into account the Cu2+/Cu+ ratio, the Cu occupancy in octahedral or tetrahedral environments and finally the Cu-O hybridization.
INTRODUCTION Semiconducting oxides with tunable wide or narrow band gaps are of a great interest for the design of electronic and optical devices. ZnO and Zn-based oxides, because of their strong Zn-O hybridization as well as the occurrence of strong Zn-Zn interactions, exhibit unusual band diagram and dispersion curves, leading to relevant electronic properties when Zn2+ cations or O2are partially substituted by other species into the network. Cu can be mainly stabilized in +III (3d8), +II, (3d9), +I (3d10) oxidation states in oxides, the last one adopting the same electronic configuration as Zn2+ cations (3d10). Spinel network deriving from the rocksalt structure exhibits octahedral (Oh) as well as tetrahedral (Td) cationic sites whereas in zinc oxide, only Td environments are occupied by zinc cations. Then the Cu substitution in ZnO as well in ZnAl2O4 spinels has been investigated by various routes. Depending on the valence state of Cu as well as their environment, optical absorption and band gap will be discussed.
EXPERIMENTAL Compounds (Zn1-xCuxAl2O4, 0≤x≤1) and Cu-doped zinc oxide were prepared by a conventional solid state route, starting from binary oxides of high purity in stoichiometric amounts. The mixtures are heated at 1000°C during 12 hours and slowly cooled down. Several
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grindings and annealings are necessary to obtain pure phases. Chemical compositions have been controlled by Castaing microprobe. Esterification process [1] has been also used for the preparation of Zn1-xCuxAl2O4, (0
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