Application of the surface potential data to elucidate interfacial equilibrium at ceria/aqueous electrolyte interface

Interfacial properties of ceria (CeO₂) nanoparticles and highly organized ceria crystal planes {111} and {100} in the aqueous electrolyte solution were studied. It was confirmed by high resolution electron spectroscopy that a primary ceria nanoparticle consists mostly of two crystal planes {111} and {100} with different surface sites exposed to the aqueous electrolyte solution. Interfacial properties of ceria nanoparticles are directly related to the reactivity and surface densities of existing surface sites. However, surface characterization (potentiometric titrations and electrophoretic measurements) provides only some kind of average surface properties i.e. average surface charge densities and surface potentials. The point of zero charge (pH_pzc) of ceria nanoparticles was measured to be between 6.4 and 8.7, depending on the electrolyte concentration, and the isoelectric point at pH_iep = 6.5. With the purpose of understanding ceria nanoparticles surface charging the inner surface potentials of ceria macro crystal planes {111} and {100} were measured for the first time, by means of single crystal electrodes, as a function of pH and ionic strength. The inner surface potential directly affects the state of ionic species bound to a certain surface plane and is thus an essential parameter governing interfacial equilibrium. From the measured Ψ ₀(pH) data and applying the Multi Site Complexation Model the thermodynamic equilibrium constants of doubly-coordinated ≡Ce₂-OH (at the {100} ceria crystal plane) as well as singly-coordinated ≡Ce₁-OH and triply-coordinated ≡Ce₃-OH (at the {111} ceria crystal plane) were evaluated. The Ψ ₀(pH) function differs for two examined ceria planes, however the inner surface potentials of both planes depend on ionic strength having a broad electroneutrality region between pH = 6 and pH = 9.