КОЛЛОИДНЫЙ ЖУРНАЛ, 2014, том 7б, № 5, с. 607-613

УДК 541.18.05


© 2014 г. Xueshuang Zheng, Li Liu, Xingping Zhou1

College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, P. R. China Поступила в редакцию 01.11.2013 г.

Uniform hydrophobic cerium oxide (CeO2) nanoparticles in a cubic structure with an average size of 4.6 nm were obtained by a novel oil—water interface method in the presence of 0.40 M NaOH. Effects of reactants concentration, oxidation reaction temperature, and the type of surfactants on the final products were investigated. The products were characterized by X-ray diffraction, transmission electron microscopy, UV-visible spectroscopy, room-temperature photoluminescence spectroscopy and contact angle measurements. The products exhibited high luminescence and strong hydrophobicity. The data suggest that Ce(OH)x x (x < 4) is a precursor complex for the formation of CeO2 nanoparticles in liquid phase and its concentration controls the size of CeO2 particles. The adsorption of the surfactant influences the formation of the hydrophobic particles of CeO2 by the oil—water interface method.

DOI: 10.7868/S0023291214050188


Recently, nanosized cerium oxide (CeO2) particles have attracted an intense interest, owing to its superior ability in storage of oxygen and diffusion oxygen vacancy in high temperature. As a new functional material, cerium oxide is widely used in catalysts [1, 2], UV absorbing materials [3], fuel cells [4], polishing powders [5], etc.

In past decades, numerous methods have been developed to synthesize nanosized CeO2 particles, such as homogeneous precipitation method [6, 7], hydrothermal [8, 9] and solvothermal processes [10, 11], and microemulsion method [12, 13]. Among them, homogeneous precipitation method is a common and simple strategy to synthesize nanomaterials. However, this method also has some disadvantages, such as poor dispersion and wide size distribution of products, as well as high pressure and high temperature conditions of the synthesis. The microemulsion method allows the fabrication of well-dispersed products; however, the yield is commonly low. There are only few reports on the synthesis of hydrophobic CeO2 nanoparticles with enhanced monodispersity [14, 15] and high yield of the product.

The novel method of synthesis at the oil—water interface was employed to prepare CdS [16]. For this method, the reactants present in different phases, and the nucleation and growth of nanoparticles take place at the oil—water interface. The particles are immobilized at this interface under complex action of the interfacial tensions of oil—solid, water—solid, and oil-water interfaces [17]. Once the proper adsorption of

1 Corresponding author. E-mail: xpzhou@dhu.edu.cn.

surfactants on the growing particles is achieved, the Gibbs surface energy of these particles decreases. The particles spontaneously migrate into the oil phase that stops their growth and results in the formation of monodisperse system. In comparison to microemulsion, hydrothermal and solvothermal processes, the oil-water interface method offers several advantages such as mild reaction conditions, low cost, high yield, simple operation, and narrow size-distribution of particles.

Our group has prepared nearly monodisperse ZnS [18], NaYF4 [19] and TiO2 [20] nanoparticles by the improved oil-water interface method. Herein, we utilized the oil-water interface method to prepare stable monodisperse hydrophobic CeO2 nanoparticles. The effects of concentration of the reactant and surfactant and oxidation reaction temperature on the size of CeO2 particles were investigated. The optical properties of CeO2 nanoparticles were also studied.

2. MATERIALS AND METHOD 2.1. Materials

Cerium(III) nitrate hexahydrate (AR), sodium oleate (AR), sodium hydroxide (AR), cyclohexane (AR), hexadecyl trimethyl ammonium bromide (CTAB, AR), Tween-80 (AR), and ethanol (AR) were purchased from Sinopharm Chemical Reagent Shanghai Co., Ltd.

2.2. Synthesis

The standard conditions for fabricating CeO2 nanoparticles by oil—water interface method were es-

Fig. 1. (a) TEM and (b) HRTEM images of CeO2 and (c) XRD pattern of CeO2.

tablished as follows: 7.50 mmol sodium oleate and 2.50 mmol cerium nitrate hexahydrate were dissolved in 10 mL and 5.0 mL of deionized water, respectively, and then transferred into a three-necked flask. Under vigorous stirring, 25 mL cyclohexane was added into the system and then heated at 40°C for 30 min. After that, 10 mL of 0.40 M sodium hydroxide was added into this mixture and kept at the same temperature for 10 h. Hereafter, the resulting product was transferred to a 100 mL Teflon-lined stainless-steel autoclave and aged at 180°C for 24 h. Finally, the mixture was cooled down to room temperature and about 20 mL of etha-nol was added into the upper oil phase to precipitate the nanoparticles. The powder was collected by cen-trifugation and then washed for three times, using eth-anol and deionized water. The product was dried at 50°C for 12 h in a vacuum.

2.3. Instruments and Characterization

X-ray diffraction (XRD) patterns were recorded by using a D8 Focus diffractometer (Bruker) with CuZa (k = 0.15406 nm) radiation. Transmission electron microscopy (TEM) images were taken on a Hitachi H-800 transmission electron microscope operating at 200 kV. The UV-vis spectra were measured with a UV-visible spectrophotometer JASCO V-530 with a light length of 1 cm. Photoluminescence (PL) spectra were detected by a Hitachi F-4500 luminescence spectrometer at room temperature. The contact angles were measured using video contact angle measuring instrument, 0CA40 Micro.


3.1. Analysis of Morphology and Structure

Figure 1 shows TEM images and XRD profile of CeO2 nanoparticles fabricated by the oil—water interface method under the standard conditions. Uniform and well-dispersed nanoparticles were obtained with an average size of 4.60 nm, as calculated from images in Fig. 1a. The high resolution TEM image shown in Fig. 1b indicates high crystallinity of the structure, the d spacing of lattice fringes of 0.32 nm is indexed to the (111) plane of CeO2. From Fig. 1c, all the diffraction peaks were indexed as those of pure cubic fluorite CeO2, with no other crystalline impurities.

3.2. Factors Influencing on the Formation

of CeO2 Nanoparticles

3.2.1. Effect of reactants concentration. TEM images of CeO2 synthesized at different concentrations of reactants with the OH-/Ce3+ molar ratio of 12 : 1 are shown in Fig. 2. The sizes of the CeO2 nanoparticles calculated from the TEM images were 3.1 and 3.2 nm for the concentrations of Ce3+ being equal to 0.01 M and 0.10 M, respectively. As evidenced by Fig. 2, the size and morphology of CeO2 remained unaltered upon an increase of the reactant concentration by 9 times at the same molar ratio. This stability is advantageous with respect to hydrothermal and solvothermal syntheses [9] and, thus, provide an opportunity of high-yield fabrication of nanoparticles.

3.2.2. Effect of reaction temperature. XRD profiles of products obtained in a course of the oxidation showed that nucleation took place just after the oxidation of Ce(OH)3 to Ce(OH)4. Figure 3 shows the images of samples obtained at different oxidation reac-


Fig. 2. TEM images of CeO2 nanoparticles synthesized at different cerium(III) salt concentrations: (a) 0.01 M and (b) 0.10 M.

Fig. 3. TEM images of CeO2 nanoparticles synthesized at different temperatures: (a) 40°C and (b) 60°C.

tion temperatures under standard conditions. The size of CeO2 nanoparticles was 4.6 nm for the oxidation temperature of 40°C. However, the size decreased to 3.4 nm upon increasing temperature to 60°C, that adversely influenced the quality of dispersion. This effect may originates from endothermic transformation of Ce(OH)4 to CeO2. High temperature favors the nucle-ation of CeO2, resulting in a wide size distribution of small nanoparticles.

3.2.3. Effect of NaOH concentration and hydro-complex precursor. TEM images of CeO2 nanoparticles obtained with different concentrations of NaOH

are shown in Fig. 4. For concentrations of 0.10, 0.40, 0.60, 0.80 and 1.20 M, the sizes of CeO2 nanoparticles calculated from TEM images were 3.0, 4.6, 4.8, 4.9, and 3.2 nm, respectively. The results suggest that the size of CeO2 nanoparticles is maximal when the OH/Ce3+ molar ratio reaches the stoichiometric ratio of Ce(OH)4 equal to 4 or more. However, when OH- ions were in a large excess, we observed an unexpected decrease of the size of CeO2 nanoparticles.

To explain this effect, we have to understand the influence of pH on the formation of metal oxide from

Fig. 4. TEM images of CeO2 nanoparticles synthesized at different NaOH concentrations: (a) 0.10 M, (b) 0.40 M, (c) 0.60 M, (d) 0.80 M, and (e) 1.20 M.

Diameter, nm 5.0

0.6 0.8 1.0 1.2 1.4 Concentration of NaOH, М

Fig. 5. Dependence of the size of CeO2 nanoparticles on concentration of NaOH.

metal hydroxide. In case of of TiO2 and ZrO2 nanoparticles, Ti(OH)+ and Zr(OH)- have been confirmed as their precursor complexes [21—23], respectively. Moreover, the size of nanoparticles has been found to increase for TiO2 and to decrease for ZrO2 upon the increase of pH in their hydroxide suspensions.

In general, metal hydro-complexes usually play a key role in the formation of metal oxide from metal hydroxide in aqueous solution [24, 25]. According to the Eq. (1), the concentration of nucleation precursor

Ti(OH)+ of TiO2 decreased and then the size of the resulting TiO2 particles increased in more basic solutions. In contrast to TiO2, the concentration of nucle-

ation precursor Zr(OH)5 of ZrO2 increased with increasing concentration of OH-, that results in the decrease of the size of ZrO2 particles (Eq. (2)). That is, the metal complex f

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