Approaching nanoscale oxides: models and theoretical methods

Abstract

This tutorial review deals with the rapidly developing area of modelling oxide materials at the nanoscale. Top-down and bottom-up modelling approaches and currently used theoretical methods are discussed with the help of a selection of case studies. We show that the critical oxide nanoparticle size required to be beyond the scale where every atom counts to where structural and chemical properties are essentially bulk-like (the scalable regime) strongly depends on the structural and chemical parameters of the material under consideration. This oxide-dependent behaviour with respect to size has fundamental implications with respect to their modelling. Strongly ionic materials such as MgO and CeO(2), for example, start to exhibit scalable-to-bulk crystallite-like characteristics for nanoparticles consisting of about 100 ions. For such systems there exists an overlap in nanoparticle size where both top-down and bottom-up theoretical techniques can be applied and the main problem is the choice of the most suitable computational method. However, for more covalent systems such TiO(2) or SiO(2) the onset of the scalable regime is still unclear and for intermediate sized nanoparticles there exists a gap where neither bottom-up nor top-down modelling are fully adequate. In such difficult cases new efforts to design adequate models are required. Further exacerbating these fundamental methodological concerns are oxide nanosystems exhibiting complex electronic and magnetic behaviour. Due to the need for a simultaneous accurate treatment of the atomistic, electronic and spin degrees of freedom for such systems, the top-down vs. bottom-up separation is still large, and only few studies currently exist.

Effect of Size and Structure on the Ground-State and Excited-State Electronic Structure of TiO2 Nanoparticles

“…61 In these cases, the size of the particle is not large enough to build a Madelung field, which is an important component defining the stability of the structure of bulk polymorphs. Small (TiO 2 ) n (i.e.…”

Oxide Nanomaterials: Synthetic Developments, Mechanistic Studies, and Technological Innovations

“…Where do we stand in the fabrication, understanding, and implementation of oxide nanomaterials? There is no doubt that overwhelming breakthroughs in oxide nanochemistry have been achieved [8] and it is impossible to fully appreciate them all within a single Review. Therefore, we provide a survey of selected exemplary works that outline the progress in research on oxide nanomaterials covering the period since 2002.…”

Symmetry and nonstoichiometry as possible origins of ferromagnetism in nanoscale oxides

“…Previously, a number of theoretical calculations on MgO nanoclusters have been performed [12−15] since MgO is often considered as a prototype of ionic oxides. It has been demonstrated that the cubic rock-salt model can be applied to MgO nanoclusters when going beyond the size range of ~50 atoms [15]. However, most of the MgO clusters investigated previously are stoichiometric ones with an even number of atoms [12−15], and, to our knowledge, the electronic structure of the "cubic nonstoichiometric" clusters, i.e., cubic clusters consisting of an odd number of atoms, has not been theoretically examined.…”