RESUMO
A pigment of your imagination: A range of polycrystalline solid solutions of a zinc-rich Zn(x-1)Mn(x)O system (see figure) have been prepared and studied in terms of their colour, diffuse reflectance spectra, Mn valence state and electronic structure. The intense optical absorption arises from Mn(2+) doping and is thought to be due to forbidden or partially forbidden transitions between the valence and the conduction band.We report an investigation of zinc-rich polycrystalline solid solutions of the Zn(1-x)Mn(x)O system concerning the colour, the diffuse reflectance spectra, the valence state of manganese and the electronic structure. Samples were prepared by a chemical-vapour-transport-assisted route and optimized with respect to colour strength. In agreement with previous experimental results, EPR studies showed that manganese is in the divalent charge state. The nature of the very intense optical absorption, which is caused by Mn(2+) doping and determines the colour of the material, is discussed. It is argued that the Mn(2+)-induced optical absorption is due to forbidden or partially forbidden transitions between the valence and the conduction band that involve Mn admixed states. This assignment is also confirmed by quantum chemical calculations using the semiempirical molecular orbital method MSINDO.
RESUMO
The work presents experimental results of an in situ investigation of optical absorption of LiNbO(3) : MgO during reducing (95%Ar + 5%H(2)) and oxidizing (O(2)) high-temperature treatments in the temperature range from room temperature to 1073 K. The absorption spectra measured at in situ conditions at high temperatures in reducing/oxidizing atmospheres as well as the kinetics recorded at fixed wavelengths during rapid replacement of gas atmospheres have been analyzed. The origin of the changes in optical absorption caused by the reducing/oxidizing treatments is discussed in terms of hydrogen and oxygen ion diffusion and the point defect structure of the material.
RESUMO
Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.
RESUMO
Titanium dioxide (TiO2) is widely used for applications in heterogeneous photocatalysis. We prepared nanocrystalline powders of the anatase as well as the rutile modification by high-energy ball milling of the coarse grained source materials for up to 4 h. The resulting average grain size was about 20 nm. The morphology of the powders was investigated with transmission electron microscopy, X-ray powder diffraction, and BET surface area determination. Measurements of the catalytic activity reveal a maximum as a function of the milling time at about 40 min. This maximum could be explained by a superposition of two counteracting effects. The first one is the increase of the specific surface area resulting in an increase of the catalytic activity, and the second one is a change of the electronic structure at the surface of the TiO2 particles corresponding to a reduction of the surface. The latter one was confirmed by light absorption experiments, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy.
