Surface and grain‐boundary excess of ZnO‐doped SnO₂ nanopowders by the selective lixiviation method

Abstract

The primary characteristic of nanopowders is the high surface area and consequently high fraction of atoms on the interfaces, which changes the energy of the system. The additive distribution in the nanopowder interfaces is a fundamental aspect to control the energy, particle size, and final properties of nanopowders. In this work, the surface excess was determined using a selective lixiviation method, where a low‐water‐soluble oxide, SnO2, was used as the matrix, and a high‐water‐soluble oxide, ZnO, was used as the additive. The X‐ray photoelectron spectroscopy (XPS) analysis confirmed that ZnO segregated on SnO2 surfaces. However, after acid lixiviation the same analysis showed an undetectable surface concentration of ZnO. The evaluation of the nanostructure change and surface composition enables us to calculate the heat of segregation for the grain boundary (ΔHsegnormalgb=−47.2kJ·mol−1) and surface (ΔHsegnormals=−36.4kJ·mol−1) and the interface energy reduction because of segregation. At low‐ZnO concentrations, the additive solubilizes in the bulk and promotes particle growth. However, the segregation to the grain boundary and surface determines the relative stability of each interface, which promotes hard agglomeration and particle size stabilization at intermediated ZnO amounts. At high‐ZnO concentrations, the surface segregation stabilizes the solid‐gas interface and decreases the agglomeration and final particle size.

Li2O‐doped MgAl2O4 nanopowders: Energetics of interface segregation

“…The integration of the heat of segregation obtained for each increment in Li + concentration calculated using the Langmuir–Mclean equation for both interfaces gives the integrated heat of segregation. Combined with the interface excess, this allows the variation of the interface energy to be calculated using Equation . The accuracy of this method has been proven for materials such as BaO‐doped TiO 2 ; surface‐energy calculations performed using the method agreed well with data obtained via the water‐adsorption method …”

“…The segregation energetics for nanoxides have been calculated using the Langmuir‐Mclean approach (Equation ) to determine the heat of segregation, as proposed by Chang et al This calculation employs the surface and GB excess obtained via selective lixiviation, as described previously …”

“…The amount of Li segregated on the surfaces was determined by performing ICP OES on the supernatant obtained after the selective lixiviation of the MgAl 2 O 4 powders. This procedure was previously developed for the quantification of segregated ions based on the differences in solubility between the additive and the matrix. Because MgAl 2 O 4 has poor solubility in water and Li 2 O in contact with water is instantaneously converted into Li(OH), which has a high solubility (0.127 g/mL), distilled water was used as the solvent.…”

“…Nanoceramics can only be manufactured when the precursor powders are nanosized and smaller than the desired final size for the nanoceramic. One strategy that has been successfully used to obtain ceramic nanopowders is the use of additives that are segregated at the interfaces (GBs and surfaces) and reduce the interfaces energy, reducing the driving force for particle growth . If additive segregation at the interfaces occurs in nanoscale crystalline domains, the thermodynamic effects of segregation are more evident than those in coarser materials owing to the higher interface density …”

“…One strategy that has been successfully used to obtain ceramic nanopowders is the use of additives that are segregated at the interfaces (GBs and surfaces) and reduce the interfaces energy, reducing the driving force for particle growth. [12][13][14][15][16] If additive segregation at the interfaces occurs in nanoscale crystalline domains, the thermodynamic effects of segregation are more evident than those in coarser materials owing to the higher interface density. 17 Equation (1) indicates that the main thermodynamic consequence of segregation is a decrease in the interface energy (σ i ), which is related to the solute excess at the interface (Γ i ), the interface energy of the pure solute (σ 0 ), and the enthalpy of segregation (ΔH i seg ).…”