Strongly Bound Surface Water Affects the Shape Evolution of Cerium Oxide Nanoparticles

Abstract

The surface structure and composition of functional materials are well known to be critically important factors controlling the surface reactivity. However, when doped the surface composition will change and the challenge is to identify its impact on important surface processes and nanoparticle morphologies. We have begun to address this by using a combination of density functional theory and potential-based methods to investigate the effect of surface dopants on water adsorption and morphology of the technologically important material, CeO 2 , which finds application as electrolyte in SOFCs, catalyst in soot combustion, and enzyme mimetic agent in biomedicine. We show that by mapping CeO 2 surface phase diagrams we can predict nanoparticle morphologies as a function of dopant, temperature and water partial pressure. Our results show that a low temperature, un-doped CeO 2 nanocubes with active {100} surface sites are thermodynamically stable. But at the typical high temperature operating conditions favours polyhedra where {100} surfaces are replaced by less active {111} surfaces by surface ion migration. However, on doping with trivalent cations, such as Gd 3+ , will increase binding of water on the {100} surfaces and hence act to preserve the cuboidal architecture, by capping the active surfaces. As surfaces tend to be decorated by impurities and dopants it is clear that their role should receive more attention and the approach we describe can be routinely applied to nanomaterials, morphologies and associated active/inactive surfaces.

Thermodynamic Evolution of Cerium Oxide Nanoparticle Morphology Using Carbon Dioxide

“…17,19 The lowest energy {111} surfaces form an octahedron and thus the nano-octahedron is the thermodynamically most stable form. [20][21][22] In practice, different synthesis conditions access other morphologies. [23][24][25][26][27][28][29] These include nano-cubes comprised of {100} surfaces [30][31] and nano-rods comprised of a mixture of surfaces (the actual surface structure of rods has been the subject of considerable debate 32 ).…”

“…The {100} and {110} surfaces are higher in energy than the {111} and thus nano-cubes and nano-rods are nominally less stable than nano-octahedra. As a result, nano-cubes and nano-rods might degrade over time and form less reactive nano-octahedra (Figure 1); 21 surface faceting of ceria nanopowder and nanocubes has been demonstrated at 730°C 33 and 600°C 34 respectively. Clearly, having synthesised particles expressing particularly facets it is also useful to examine how to protect these shapes from degradation.…”

“…Some molecules, such as water, and hydroxyl groups when adsorbed at the surface of CeO2 nano-cubes can stabilise the reactive {100} surfaces and thus, in theory, prevent the degradation of nano-cubes to nano-octahedra. [20][21]30 However, despite the many studies on the adsorption of small molecules on ceria, key questions remain. For example, how do molecules present during synthesis, post-synthetic treatments or during catalytic processes, modify the expression and evolution of specific surfaces?…”

Influence of Carbon Dioxide on the Energetics of Cerium Oxide Surfaces and Nanoparticle Morphology

“…A U value of 5 eV is applied to Ce f states as this value has been utilized successfully in other studies. 30,41,42 The structures were optimized until the residual forces on each atom were less than 10 meV Å -1 . All calculations were spin polarized and an initial ferromagnetic ordering was used throughout, which was shown to produce no difference in the energetic of CeO 2 systems.…”

“…were calculated with the addition of temperature as follows 41,42,49 γ CO 2 ,T,p = γ Bare + (C(E ads,T − RT ln( p…”