Surface Capping-Assisted Hydrothermal Growth of Gadolinium-Doped CeO₂ Nanocrystals Dispersible in Aqueous Solutions

Nanocrystals of 20 mol % Gd³⁺-doped CeO₂ dispersible in basic aqueous solutions were grown via hydrothermal treatment of anionic Ce­(IV) and Gd­(III) carbonate complexes at 125–150 °C for 6–24 h with N­(CH₃)₄⁺ as a capping agent. The nanocrystals were characterized in detail using dynamic light scattering (DLS), ζ-potential measurements, X-ray diffraction (XRD), specific surface area measurements based on the Brunauer–Emmett–Teller theory (SSA_BET), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy, and Raman spectroscopy. DLS analysis revealed that the highly transparent product solution consisted of nanocrystals approximately 10–20 nm of hydrodynamic diameter with a very narrow size distribution, while the ζ-potential analysis results strongly suggested that the N­(CH₃)₄⁺ capped negatively charged sites on the nanocrystals’ surface and provided sufficient repulsive steric effect to prevent agglomeration. Moreover, the crystallite size (<i>d</i>_XRD) estimated from the XRD patterns and the equivalent particle size (<i>d</i>_BET) estimated from the SSA_BET data were in the range between 5–6 and 4–4.5 nm, respectively, and nearly constant independent of reaction time, indicating suppressed Ostwald ripening due to capping. Good agreement between the values obtained from the <i>d</i>_XRD and <i>d</i>_BET analyses with the size of the primary nanocrystals observed in the TEM image also confirmed that the primary nanocrystals were single crystals and nearly free from aggregation. Furthermore, the gadolinium content in the as-prepared nanocrystals was determined to be very close to 20 mol % and remained unchanged after HCl treatment, indicating successful doping of stoichiometric amount of Gd³⁺ into CeO₂ lattices. Finally, the Raman analysis suggested that only a slightly Gd³⁺-rich phase was present in the nanocrystals grown for shorter reaction times. By increasing the reaction time, even at 125 °C, the Gd³⁺ was homogeneously distributed into the CeO₂ lattices via solid state diffusion

Hydrothermal Growth of Tailored SnO₂ Nanocrystals

We studied the growth of SnO₂ nanocrystals with a tailored structure via surface capping assisted hydrothermal approach with tetramethyl ammonium hydroxide (N­(CH₃)₄OH; TMAH). KOH and NaOH were also used instead of TMAH for comparison. The nanocrystals with a size ranging from 3.2 ± 0.9 to 74 ± 20 nm were grown at 150 °C for 24 h depending on the pH. N­(CH₃)₄⁺ capped the surface of SnO₂ and improved the dispersion of the nanocrystals in basic aqueous solutions. The capping provided nanocrystals finer than those grown with KOH and NaOH because of suppressed Ostwald ripening via a reduction in surface energy. High-resolution transmission electron microscopy revealed that the nanocrystals grown in strong basic solutions with TMAH had cubic morphology terminated by the (001) and (110) faces. This strongly suggests that N­(CH₃)₄⁺ preferentially caps the (001) face with the highest surface energy and decreases its surface energy to be comparable to that of the (110) face with the lowest surface energy. Anisotropic capping promotes the formation of the cubic superstructure via the directed self-assembly of primary cubic nanocrystals in very strong basic solutions. Raman spectroscopy suggested that the SnO₂ nanocubes grown in strong basic solutions with TMAH have less of a surface hydration layer as well as bulk combined water