Nanocrystalline ceramic powders with crystallites of 1-10 nm in dimension form a new class of material with unique hybrid properties characteristic of neither the molecular nor the bulk solid state limits. UV optical absorption, below the bulk exciton energy, is often interpreted in terms of low coordinated surface features such as corners and edges[1,2]. However, in powders there are also a large number of interfaces between nanocrystallites, therefore the photon energies required to excite at such places have been calculated using time dependent Density Functional Theory. A large number of interface geometries that are typical of real powders have been considered.
Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of interfaces features can be exited with photon energies less than 5 eV – an energy usually thought to selectively excite low coordinated surface sites. We have also studied the ability of interfaces to trap charge, which is an important issue for applications such as catalysis, semiconductor technologies, tribology and space exploration. We find that delocalised holes can be transiently trapped at a large number of interfaces within a powder. These trapped holes must overcome a barrier in order to escape to more favourable locations, such as low coordinated surface anions, via terrace and edges of nanocrystallites. On the contrary there are far fewer interfaces that can trap electrons.
The calculations are performed using an embedded cluster methodology[3], which combines quantum-mechanical and classical levels of approximation self-consistently enabling large systems to be studied. A region near the interface feature is treated quantum-mechanically using the B3LYP density functional and a 6-31G basis set.
[1] Garrone et al., Phil. Mag. B 42, 683 (1980)
[2] S. Stankic et al, Nano Letters 5, 1889-1893 (2005)
[3] P. V. Sushko, A. L. Shluger, C. R. A. Catlow, Surf. Sci., 450, 153 (2000)
[4] K. P. McKenna, P. V. Sushko and A. L. Shluger, (Submitted to JACS) |