We use chemical vapour deposition (CVD) for the synthesis of pure and doped MgO nanoparticles [1]. In this presentation we demonstrate how the admixture of a second metal component alters the electronic surface properties of MgO nanocrystals. When MgO nanoparticles are doped with monovalent Li-ions smallest amounts already induce dramatic changes in the optical and chemical surface properties [2]. Isovalent Ca2+ cations can be distributed homogeneously in MgO nanocrystals although CaMgO mixtures are thermodynamically forbidden on a macroscopic scale. Subsequent thermal activation leads to calcium ion segregation into the MgO surface. These CaMgO nanocrystals represent novel materials with enhanced surface basicity. Furthermore, unexpected optical surface properties of those mixtures are reflected by dramatic enhancement in photoluminescence emission which is also red-shifted with respect to pure MgO [3].
Whereas it is known that mixing MgO (band gap 7,8 eV) with ZnO (band gap 3,4 eV) represents a powerful approach for band gap engineering [4] the effect of ZnO admixture on the surface electronic structure of MgO is so far unexplored. After vacuum annealing, binary ZnxMgx-1O nanoparticles produced by CVD adopt an average particle size of 15 nm and retain the cubic shape characteristic of pure MgO nanocrystals. UV/Vis absorption and luminescence spectroscopy indicate surface enrichment with Zn-ions. A FT-IR investigation of H2-chemisorption confirms enhanced hydrogen adsorption as compared to undoped MgO nanocubes.
All examples will underline that CVD synthesis and thermal processing of MgO-based nanocrystals provide efficient means for manipulating the chemical and optical surface properties of insulating oxides.
[1] S. Stankic et al. Angew. Chem. Int. Ed. 44 (2005) 4917.
[2] T. Berger et al. J. Cat. 247 (2007) 61.
[3] S. Stankic et al. Nano Lett. 5 (2005) 1889.
[4] I. Takeuchi et al. J. Appl. Phys. 94 (2003) 7336.
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