Article pubs.acs.org/Langmuir
Surface Capping-Assisted Hydrothermal Growth of GadoliniumDoped CeO2 Nanocrystals Dispersible in Aqueous Solutions Kazuyoshi Sato,*,† Manami Arai,† Jean-Christophe Valmalette,‡,§ and Hiroya Abe∥ †
Division of Environmental Engineering Science, Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan ‡ Université du Sud Toulon Var, BP 20132, F-83957 La Garde Cedex, France § CNRS, IM2NP (UMR 7334), BP 20132, F-83957 La Garde Cedex, France ∥ Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan S Supporting Information *
ABSTRACT: 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 (SSABET), 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 (dXRD) estimated from the XRD patterns and the equivalent particle size (dBET) estimated from the SSABET 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 dXRD and dBET 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. electrolyte in solid oxide fuel cells,6 oxygen sensors,7 and automotive three-way catalysts.8 The fabrication of uniform nanocomposites of doped CeO2 with metals and oxides is the key to enhancing the performance of such catalysts and electrochemical devices. The growth of nanocrystals in an aqueous medium without agglomeration offers great advantages for the fabrication of uniform nanocomposites.9,10 Chemical routes, including sol−gel,11 coprecipitation,12 gel combustion,13 and hydrothermal methods,14 have been developed for the growth of doped CeO 2 nanocrystals. Among them, the hydrothermal method is attractive because of the simultaneous control of the nucleation, crystallization, growth, and dispersion of the particles in one-
1. INTRODUCTION Cerium oxide (CeO2) with a cubic fluorite-type crystallographic symmetry is receiving much attention because of its great potential for use in a variety of applications, including photocatalysts,1 polymer membrane fuel cells,2 catalysts for the water gas shift reaction,3 polishing agents,4 and UV absorbers.5 In addition, the doping of trivalent lanthanides (Ln3+), such as Gd3+ and Sm3+, into the CeO2 lattice leads to the formation of oxygen vacancies (V•• O ) as a consequence of charge compensation, and thus provides conductivity of oxygen ions. This oxygen vacancy formation can be described with the Kröger−Vink notation as follows: CeO2
× Ln2O3 ⎯⎯⎯⎯→ 2Ln•Ce + V O•• + 3OO
(1)
The higher oxide ion conductivity of doped CeO2 compared with that of pure CeO2 and other oxygen ion conducting materials makes it a promising candidate for use as an © 2014 American Chemical Society
Received: July 19, 2014 Revised: September 14, 2014 Published: September 16, 2014 12049
dx.doi.org/10.1021/la502861k | Langmuir 2014, 30, 12049−12056
Langmuir
Article
Figure 1. Strategy for the growth of the GDC nanocrystals dispersible in an aqueous solution. stability of the carbonate complexes and the amount of capping agent. The total cation concentration was 0.2 mol/L. The complex solution was enclosed in a polytetrafluoroethylene-lined stainless steel autoclave, heated at 125−150 °C for 6−24 h in an oven and then cooled to room temperature. The SO42−, NO3‑, excess N(CH3)4+, and unconverted cations into the solid were removed by washing the product solution after passing it through an ultrafiltration system with a molecular weight cutoff of 5000. The washing sequence was continued until the major anion SO42− was removed, which was confirmed using BaCl2 titration (BaCl2 + SO42− → BaSO4 + 2Cl−) of the ultrafiltered solution. A portion of the resultant colloidal solution of GDC nanocrystals was then freeze dried to obtain powdered GDC nanocrystals for subsequent characterization. 2.2. Characterization of the GDC Nanocrystals. The crystalline phase of the powdered GDC nanocrystals was characterized using powder X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.1542 nm). The crystallite size was estimated using the full width of half maxima of a diffraction peak and Sherrer’s formula.21 The Gd3+ doping amount in the nanocrystals was evaluated for as-received and acid leached powder samples via energy dispersive X-ray spectroscopy using a calibration curve; a rhodium target was employed for the excitation of fluorescent X-rays. The acid leaching was performed by immersing the powdered nanocrystals into HCl solution (∼ pH 3) with continuous stirring for 6 h followed by washing with purified water and subsequent drying. The morphology of the GDC nanocrystals was observed using a transmission electron microscope (TEM) with an acceleration voltage of 200 kV. The hydrodynamic diameter of the nanocrystals in the aqueous solution was determined via dynamic light scatting (DLS). The specific surface area (SSABET) was estimated based on the Brunauer−Emmett−Teller (BET) theory22 for N2 gas adsorption isotherms obtained at 77 K. The local structure of the nanocrystals, including the presence of oxygen vacancies and the location of the Gd3+ in the lattice, was investigated using Raman spectroscopy. Continuous waves with wavelengths of 514 and 364 nm were employed as the excitation lines. The yield of the nanocrystals was determined from the mass of the collected freezedried powder compensated by the weight loss during heating up to 800 °C in air using thermogravimetric analysis.
pot in an aqueous medium is possible. Moreover, the aid of thermally unstable organic molecules is available because the growth of crystalline oxides can be achieved below 200 °C. However, the precise doping of guest elements into the host CeO2 lattice remains as a challenging task at such low temperatures due to the different nucleation and growth behaviors of Ln2O3 and CeO2. Although some research groups have reported the growth of organic molecule-capped Ln3+doped CeO 2 nanocrystals using the hydrothermal approach,15,16 the evidence for successful doping of Ln3+ into the CeO2 lattice was insufficient. Furthermore, these nanocrystals were dispersible only in nonaqueous solvents due to the high hydrophobicities of the capping agents. Recently, we successfully grew ZrO217 and SnO218 nanocrystals of
