Improving TiO₂ Anatase Nanostability Via Interface Segregation: The Role of the Ionic Radius

Abstract

Titanium dioxide nanoparticles are of particular interest in photocatalysis and artificial photosynthesis studies. Their properties are dependent on parameters such as the crystal polymorph, for example, anatase/rutile phases, the specific surface area, and the capacity to adsorb CO2 and H2O gas molecules. A cause-effect relationship exists between those parameters, the surface thermochemistry, and the performance in application. However, the lack of systematic thermodynamic data has hindered a more comprehensive understanding to enable control. This work investigates the relationship between the surface thermochemistry of alkaline earth metals doped TiO2 and the associated micro and nanostructural features. TiO2 nanoparticles doped with Mg2+, Ca2+, Sr2+, or Ba2+ showed spontaneous segregation of dopants to the nanoparticles’ interfaces, leading to incremental surface energy reduction as directly measured by microcalorimetry of adsorption. The phenomenon was a direct function of ionic radius and led to the improved thermal stability of the nanoparticles against coarsening, stabilizing the anatase polymorph, and increasing the specific surface area.

Intrinsic defects generated by iodine during TiO2 crystallization and its relationship with electrical conductivity and photoactivity

“…The iodine concentration was measured using a HI 4111 iodide ionselective electrode (ISE) connected to a pH Analyzer 300 M. The samples were dissolved in an aqueous sulfuric acid solution (50.0% in volume) and the pH was neutralized with sodium hydroxide. The selective lixiviation method 4,5 was also employed to try to detect iodine on the surface of the samples. The iodine concentration in samples was also visualized by energy-dispersive X-ray spectroscopy (EDS) using an FEI Inspect F50 FEG scanning electron microscope (SEM), operated at 20 kV and 122.7 eV resolution.…”

Interface segregation of iron sintering aid in gadolinium-doped ceria

“…39,42,55 Such an effect is still unclear, but it was attributed to a structural disorder related to a low-density superficial layer in nanoparticles. 38 Since the surface of nanocrystalline oxides represents a high percentage of the total volume of the particle, surfaces with a high number of defects are likely to induce a decrease in the overall density of the material. This effect is more pronounced in the Fe-doped samples 1Fe-CGO and 5Fe-CGO, which have measured densities of 5.366 g cm −3 and 4.746 g cm −3 , respectively.…”

“…36 The enthalpy of interface segregation (Δ H seg ) (eqn (1)) can be associated with different parameters such as the difference in interface energy between the dopant and the host (Δ H σ ), the elastic solute strain energy owing to the size difference between the dopant and host ions (Δ H ε ), the solute–solvent interaction (Δ H ω ), and the electrostatic potential/charge compensation (Δ H ϕ ). 37,38 Δ H seg = Δ H σ + Δ H ε + Δ H ω + Δ H ϕ The additive is expected to segregate at the interface with higher energy to minimize the system's overall energy. 39 Thus, if the dopant preferably segregates at the grain boundary, it will decrease the grain boundary energy, favouring densification.…”

“…36 The enthalpy of interface segregation (ΔH seg ) (eqn (1)) can be associated with different parameters such as the difference in interface energy between the dopant and the host (ΔH σ ), the elastic solute strain energy owing to the size difference between the dopant and host ions (ΔH ε ), the solute-solvent interaction (ΔH ω ), and the electrostatic potential/charge compensation (ΔH ϕ ). 37,38…”

Solid‐state NMR for the analysis of interface excesses in Li‐doped MgAl2O4 nanocrystals

“…The quantification of the distribution of the dopant across the bulk and interfaces is challenging due to the difficulty of accessing and chemically analyzing each individual interface. Although the total amount of dopants is easily quantified by common techniques, such as X‐ray fluorescence spectroscopy (XRF), the spatial distribution relies on extensive electron microscopy analyses associated with spectroscopic tools, such as EDS and EELS 18 . Recently, dopant excess on the surface of nano‐oxides has been successfully quantified by a convenient technique called selective lixiviation 8,19–21 .…”