Influence of the Nanocrystal Surface or Interface on the Lattice Parameter

Several experiments have shown a relationship between the adsorption state of nanocrystal surfaces and the change in the lattice parameter. Hence, for nanocrystals in ceramic powders like BaTiO₃ or SrTiO₃, the presence of water molecules and OH⁻ ions on the surface causes an increase in the lattice parameter. After desorption of the molecules by a suitable heat treatment, the values for the bulk solid are recovered.

Figure 1 shows how one may adjust the lattice parameter in γ-Fe₂O₃ by varying the surface energy of the nanocrystal. There are two clearly distinct regimes: first a contraction of the nanocrystal, and then an expansion. The first regime corresponds to a phase in which OH⁻ and H₂O are chemisorbed on the surface, and an initial water monolayer is formed, with water vapor pressures below a certain critical value. The second regime, on the other hand, corresponds to the formation of water multilayers by physisorption, and the relaxation of the oxide by strengthening of the bonds between the water layers. The existence of two distinct regimes has also been demonstrated for iron nanoparticles coated with a thin layer of γ-Fe₂O₃ , and also when oxygen is adsorbed on carbon nanotubes .

Fig.1. Dependence of the lattice parameter for a nanometric   γ-Fe₂O₃ powder on the state of adsorption of water vapour on the  powder.   For water vapour  pressures below a critical value (around P/P₀ = 0.1),  the formation of a monolayer appears to compress the oxide,  whilst for  high  pressures,  the formation of multilayers of water would  appear  to relax the oxide by strengthening the bonds between water layers  [32].  (1) 630 kJ/mol: chemisorption of OH²⁻ and physisorption of H₂O  (monolayer on γ-Fe₂O₃). (2)  45 kJ/mol:  physisorption of H₂O  (multilayer on H₂O)