Monodisperse Uniform CeO 2 Nanoparticles : Controlled Synthesis and Photocatalytic Property

Ceria nanostructure-based catalysts have attracted much attention in recent years because of their unique physiochemical properties. Herein, we have presented a simple two-phase approach for the synthesis of ceria nanocrystals. Structural and morphological characterization by XRD and TEM showed that the as-synthesized monodisperse CeO 2 nanoparticles (NPs) had cubic fluorite crystal structure with average particle size about 6.75 nm. The effects of hydrothermal temperature, annealing time, and concentration of cerium nitrate on the nanostructures of the products were also investigated and discussed. In addition, the CeO 2 nanocrystals proved to be an effective catalyst for the photodegradation of blue methylene under UV illumination.


Introduction
Ceria, which is semiconducting, abundant, nontoxic, and inexpensive [1][2][3][4][5], is widely applied in heterogeneous catalytic reactions due to the high mobility of oxygen species and reversible transition between Ce 4+ and Ce 3+ oxidation states in CeO 2 crystal [6].Recent researches have revealed that the size, morphology, surface structure, and synergistic interaction dramatically influence the catalytic performance of CeO 2 .Wang et al. [7] found that the spindlelike CeO 2 exhibited the highest DMC yield, followed by nanorods, nanocubes, and nanooctahedrons, because the exposed active (111) planes, defect sites, and a large amount of acid-basic acids were active factors for dimethyl carbonate (DMC) formation.Zhou et al. [8] reported that the CeO 2 nanorod catalyst exhibited higher ethanol catalytic oxidation activity and CO 2 selectivity than the nanocube catalyst because the ceria nanorods had an amorphous structure, which yields more crystal defects.Liu et al. [9] presented that the CuO/CeO 2 catalysts nanorods showed the best activity for NO reduction by CO due to the strong synergetic interactions between copper(II) oxide and ceria nanorods and so forth.Thus, the development of facile approach for synthesis of nanostructured CeO 2 is of great importance to the investigation of its new physicochemical properties.
The synthesis of metal oxides nanostructures with various morphologies has drawn great attention.Two-phase route is a promising method to synthesize uniform nanocrystal with controlled particle size and shape.Compared to the singlephase approach, the two-phase strategy can obtain the products with good crystallinity under relative mild adopted conditions [10].Brust et al. [11] first used a two-phase approach to synthesize gold nanocrystals, which holds a prominent position.Similarly, some noble metal nanostructures such as Ag [12], Pt [13], and Pd [14] have been fabricated by modifying Brust's method.Related progress using two-phase method was extended to the synthesis of varying nanostructured metal oxides.Pan et al. [15] used a hydrothermal two-phase route to synthesize monodisperse TiO 2 nanocrystals in a water-toluene mixture.The ZrO 2 nanostructures with various shapes such as spheres and rodlike were also fabricated by this method [16].Adschiri et al. modified this approach for the fabrication of CeO 2 nanocubes using hydroxide precursor under supercritical water conditions.Nguyen and Do [10] developed two alternative two-phase approaches to prepare oxide nanocrystals of two groups of elements: rare earths, for example, Sm, Ce, Er, Gd, La, and Y, and transition metals, for example, Mn, Cr, Co, and Ni, in a water-toluene mixture using rare earth nitrate salts and rare earth-oleate complexes as starting precursors.Although some progress has been made in fabrication of nanomaterials, this method usually uses metal-surfactant complexes as precursor and through a complex experimental course.Furthermore, to the best of our knowledge, the controllable two-phase synthesis of CeO 2 nanostructures using cerium nitrate as starting precursor has rarely been reported.Thus it is essential to explore simple and inexpensive methods for the synthesis of uniform-sized ceria nanocrystals.
In this work, the novel two-phase approach for controllable synthesis of uniform CeO 2 NPs using metal salt instead of organometallic compounds is reported, which is an environmentally friendly and economical route with mild reaction condition.The results show that monodisperse CeO 2 NPs can be achieved by changing some synthesis conditions such as aging time, hydrothermal temperature, and cerium nitrate concentration.In addition, the enhanced catalytic activity of as-fabricated CeO 2 nanocrystals for photodegradation of methylene blue (MB) under UV illumination was also found.

Experimental
2.1.Materials.All chemicals were used as received without further purification.Ce(NO 3 ) 3 ⋅6H 2 O, C 17 H 33 COOK, tertbutylamine, and oleic acid were purchased from Merck.Methylene blue, toluene, and ethanol were purchased from Sigma-Aldrich.

CeO 2 Nanoparticles Preparation and Characterization.
Here, the uniform CeO 2 NPs were synthesized based on the previous reported by Nguyen and Do [10] with significant modifications.For synthesis of CeO 2 nanocrystal, a homogeneous ethanol solution (10 mL) containing 3 g C 17 H 33 COOK was mixed with an organic solution of toluene (20 mL) and oleic acid (2 mL).Afterward, this organic phase was mixed with 20 mL of an aqueous solution containing cerium nitrate salt (0.08 M) and tert-butylamine (0.15 mL).The obtained two-phase mixture was transferred to a Teflonlined stainless steel autoclave and heated at 180 ∘ C for 24 h.The system was cooled to room temperature and then the nanocrystal solution was precipitated with the excess volume of ethanol.Finally, the as-synthesis CeO 2 NPs were recovered through filtering, washing, and drying at room temperature.The morphology and crystalline structure of products were characterized using transmission electron microscopy (TEM, Model JEO-3432, Japan) and an X-ray diffractometer (XRD, D8 Advance, Bruker, Germany), respectively.The particle size distribution analysis from TEM images was done by using the ImageJ program.

Photocatalytic Tests.
The photocatalytic activity of CeO 2 nanocrystals was evaluated by the degradation of MB as model contaminant.The reactions were performed under UV light (20 W, 365 nm) at ambient temperature.In a photoreaction experiment, 50 mg of the catalyst was added in a conical flask containing 100 mL of varying concentrations of MB (4, 8, and 12 ppm).Prior to irradiation, the suspension was kept in the dark under stirring for 60 min to ensure the establishing of an adsorption/desorption equilibrium.At certain time intervals, about 5 mL suspensions were withdrawn and centrifuged and then filtered to remove the residual catalyst particulates for analysis.The filtrates were analyzed using UV-Visible spectrophotometer (Jasco V-550 UV-Vis).The concentration of the MB was determined by measuring the absorption intensity at its maximum absorbance wavelength (662 nm) of this dye, where  0 is initial concentration of MB and   is the concentration of dye at any time of photocatalytic process.S2).The formation of CeO 2 NPs involves two steps: nucleation and growth.The effect of reaction temperature on the formation mechanism of CeO 2 NPs can be explained as follows.At low temperatures, the nucleation rate was similar to the growth rate [17].The particles initially formed act as the seeds and become larger as reaction proceeds.At the same time, other small NPs are formed.A moderately elevated temperature is conductive to the fast nucleation.When the nucleation rate is larger than growth rate, small particles with narrow size distribution can be easily obtained.

The Effect of Hydrothermal Time.
In order to understand the particle formation, the effects of reaction time on size and shape of products were investigated.After 12 h of reaction, the average particle size was 5.73 nm (Figure 2(a)).
The average size slightly increases (6.75 nm) for 24 h of reaction (Figure 2(b)).This means that larger particles are growing at the expense of smaller particles.It could be attributed to Ostwald ripening which is growth mechanism by dissolving and depositing of smaller particle on the more stable surface of the larger nanoparticles [18].After 36 h, the size and morphology of the products do not change (Figure 2(c)), suggesting that the crystal growth reaches saturated state at 24 h.

Influence of Salt Concentration.
The size evolution of CeO 2 NPs is found to be a function of cerium nitrate concentrations in the water solution (Figure 3).At a relatively low cerium nitrate concentration ( = 0.04 M), the CeO 2 NPs are formed with an average diameter of 5.43 nm.Their average particle size increases from 5.43 to 6.75 nm when cerium precursor concentration is 0.08 M (Figure 3(b)).By increasing the cerium ion concentration to 0.12 M, the morphology of the product obtained has irregular shape with large particle size distribution.Generally, the size and shape evolution of nanocrystals are strongly dependent on metal monomer concentrations.Low monomer concentrations favor isotropic growth, whereas high monomer concentrations favor anisotropic growth [19].

The Formation Mechanism of CeO 2
Nanoparticles.The phase purity and crystal structure of as-synthesized CeO 2 nanocrystals are characterized by XRD pattern (Figure 4).All the distinguishable peaks correspond to the (111), ( 200), ( 220), (311), (222), and (400) planes of a cubic fluorite structure (space group: Fm3m) of CeO 2 as identified using the standard data JCPDS Card Number 34-0394.The average crystal size of synthesized NPs which is calculated from XRD data using the Debye-Scherrer equation is about 6.34 nm, which is similar to TEM result.The results suggest that the uniform CeO 2 NPs are a single crystal.
The formation mechanism of CeO 2 NPs can be explained in Figure 5.The Ce 3+ ions are easily oxidized to the Ce 4+ ion by the trace oxygen existing in the aqueous solution because the Ce 4+ ions have stable empty 4f 0 subshells [20].The cerium hydroxide is formed under basic solution due to the hydrolysis of tert-butylamine in water to generate OH − ions [10].Upon hydrothermal treatment, cerium hydroxide is dehydrated to oxide nuclei at interfaces.

Photocatalytic Activity of CeO 2
Nanoparticles.The photocatalytic activity of as-synthesized CeO 2 NPs is studied by the degradation of MB solution under UV irradiation.The dye is an N-containing, heteroaromatic, cationic thiazine green colour organic dye with chemical formula C 16 H 18 ClN 3 S used as colouring agent in a variety of industries [21].Figures 6(a)-6(c) show the UV-Vis absorption spectra of the photodegradation for varying MB dye concentrations versus irradiation time.It can be found that intensity of the absorption of MB decreases with time.The time needed for complete degradation of 4, 8, and 12 ppm of MB was 12, 15, and 27 min, respectively.The decomposition rate of MB decreases with the increase in the amount of MB in solution (Figure 6(d)).This phenomenon can be explained on two effects: (i) an increase in the number of dye molecules  adsorbed on the surface of the catalyst leads to a decrease in the number of active sites that generate hydroxyl radicals [22]; (ii) the density of solution increases with the increase in the MB concentration in solution which restricts the penetration of the light into the solution [23,24].The mechanism of MB degradation by CeO 2 NPs under UV light can be explained on the basic of generation of electron/hole pairs.Under the UV light irradiation, the electrons from the valence band (O 2p) of CeO 2 get excited to an empty conduction band (Ce 4f) which generates the positive holes and negative electrons (Ce 4+(e − ) − O 2−(h + ) ) on the surface of the nanocrystal [25].These electrons further react with the dissolved oxygen to generate the superoxide radical anions (O 2 •− ), while h + in valence band reacts with the adsorbed water and produces more OH • radicals [26].These OH • and O 2 •− are reactive species which are mainly responsible for degradation of MB.The mechanism of photocatalysis can be summarized as Figure 7. Generally, the lifetime of  photoexcited electrons is very much less and hence the transfer of e − to longer distance is kinetically prohibited.Therefore, most photocatalysis happens in the near distance [27].The nanostructured materials help to improve the adsorption of photons in the UV-Visible region.This could be improved for the fast response in the decomposition of dyes [28].Thus, the monodisperse uniform CeO 2 NPs are promising photocatalyst for the wastewater treatment.

Conclusions
The monodisperse CeO 2 NPs were successfully synthesized by a simple, easier, and low cost two-phase approach.
The possible mechanism could be described by Ostwald ripening process, and the formation of monodisperse CeO 2 nanocrystals strongly depends on synthesis conditions.The CeO 2 nanocrystals act as good catalyst for photodegradation of MB solution and completion of this dye (5 ⋅ 10 −6 M) at room temperature is achieved in about 27 min under UV illumination, which is critical for practical applications.

Figure 2 :
Figure 2: TEM images of CeO 2 NPs synthesized at different reaction times and corresponding particle size distribution analysis: (a) 12 h, (b) 24 h, and (c) 36 h.

Figure 5 :
Figure 5: The scheme of the formation mechanism of uniform CeO 2 NPs from cerium nitrate.

Figure 6 :Figure 7 :
Figure 6: UV-Vis spectra (a) and photodegradation curves of MB versus time (b) under UV irradiation of CeO 2 nanocrystal catalysts.