Synthesis and Characterization of CeO 2 Nanocrystals by Solvothermal Route

Cerium dioxide nanoparticles were prepared by solvothermal technique. The structural analysis was carried out using X-ray diffraction. It showed that the cerium dioxide nanoparticles exhibited cubic structure. Grain sizes were estimated from High Resolution Transmission Electron Microscopy images. The size of the nanoparticles is around 20 nm. The surface morphological studies from Scanning Electron Microscope (SEM) and HRTEM depicted spherical particles with formation of clusters. Thermal and electrical Insulating behaviors were determined.


Introduction
Cerium dioxide or ceria (CeO 2 ) is an important rare-earth oxide, which has multiple applications such as electrolyte materials of solid oxide fuel cells 1 , ultraviolet blocking materials 2 , catalysts 3 , Chemical Mechanical Polishing (CMP) 4 and oxygen gas sensors 5 .It has a fluorite-like cubic structure in which each cerium site is surrounded by eight oxygen sites in face-centered cubic (FCC) arrangement and each oxygen site has a tetrahedron cerium site.Recently, a variety of methods based on wet chemical routes have been extensively employed to synthesize of CeO 2 nanoparticles like precipitation [6][7][8][9] , hydrothermal 10,11 , sol-gel method 12 , and microemulsion method 13 .Among these preparation methods, microwave-assisted solution method is a simple and inexpensive method to prepare nanomaterials.Microwave is electromagnetic radiation with frequency range of 0.3-300 GHz and corresponding wavelength from 1 mm to 1000 mm.In the microwave irradiation region, the frequency of the applied irradiation is low enough so that the dipoles have time to respond to the alternating electric field and therefore respond to rotation.This method has been successfully applied for the preparation of a variety of nano-sized inorganic materials.The microwave synthesis, which is generally quite fast, simple, and energy efficient, has been developed, and widely used for TiO 2 nanoparticles 14 , metal (Cu, Hg, Zn, Bi, Pb) sulfide nanoparticles 15 , uniform and stable polymer-stabilized colloidal clusters of Pt, Ir, Rh, Pd, Au and Ru 16 , CuO 17 etc.Compared with conventional heating, microwave heating has an advantage of high efficiency and rapid formation of nanoparticles with a nano-size distribution and less agglomeration.The fundamental mechanism of microwave heating involves agitation of polar molecules or ions that oscillate under the effect of an oscillating electric field.In the presence of an oscillating field, particles try to orient themselves.This constant re-orientation creates friction and collisions between molecules, thus producing heat 18 .
In the present study, we employed the microwave method to prepare the nanometer-sized cerium oxide ultrafine particles using ethylene glycol as a capping agent.The structural features, electrical, thermal and optical properties of the ceria nanoparticles were determined in depth with X-ray powder diffraction (XRD), high resolution transmission electron microscope (HRTEM), scanning electron microscope (SEM), FTIR spectroscopy, UV-vis absorption spectroscopy, thermo gravimetric and differential thermal analysis (TG/DTA) and impedance analysis studies.

Materials
The ammonium Ce (IV) nitrate, [(NH 4 ) 2 Ce (NO 3 ) 6 ] (AR grade MERCK), Sodium hydroxide [NaOH] (AR grade MERCK), ethylene glycol (AR grade MERCK) and ammonia solution 30% of GR (AR grade MERCK) were used to synthesize the nanoparticles of this work.Water used in this investigation was de-ionized.

Synthesis
In a typical synthesis of ceria (CeO 2 ) nanoparticles were prepared as follows: the precursors like ammonium Ce(IV) nitrate and sodium hydroxide (NaOH) were taken in 1:4 molar ratios and dissolved completely in de-ionized water.Here, the pH value of the solution was adjusted to be 12.The mixture solution was stirred well using a magnetic stirrer for about 1 hour with a stirring rate of 1000 rpm.Then the prepared mixture solution was kept in the microwave Materials Research oven (900 W, 2450 MHz, Onida, India) at a temperature of 50 °C for about 30 minutes.When ammonium ceric nitrate is treated with sodium hydroxide, the hydrolysis process took place and the products like sodium nitrate, ammonium hydroxide, and cerium hydroxide were formed.During the reaction, one proton (H + ) is removed from cerium hydroxide due to polar nature of water and this lead to the formation of hydrated CeO 2 .Synthesized pale-yellow precipitate was filtered and washed with de-ionized water twice.Annealing of the synthesized powder at 800 °C in air for 6 hours will result in the formation of CeO 2 nanoparticles.

Instrumentation
Powder X-ray diffraction pattern of the nanoparticles was obtained using a powder X-ray Diffractometer (PAN analytical Model, Nickel filtered Cu Kα radiations with λ = 1.54056Å at 35 kV, 10 mA).The sample was scanned over the required range for 2Ө values (10-80°).The particle size analysis for the sample was carried out using the particle size analyzer (Nanophox, model: 0102 P).The FTIR spectrum of the sample was recorded using a Shimadzu 8400S spectrometer by the KBr pellet technique in the range 400-4500 cm -1 .The optical spectra of sample have been recorded in the region 190-1100 nm using a UV-1800 series spectrophotometer in the absorption mode.The SEM image of the synthesized cerium dioxide nanoparticles was recorded using a Hitachi Scanning Electron Microscope.The size and shape of nanoparticles was obtained by high resolution transmission electron microscopy (HRTEM) and HRTEM measurements were carried out on a JOEL JEM 2100.TG/DTA of the nanoparticles of the present work were carried out simultaneously using Seiko thermal analyzer in air atmosphere at a heating rate of 20 °C per minute for a temperature range of 20-1000 °C.Alumina was taken as the reference material and the alumina crucible was used.Electrical measurements were performed using an impedance bridge (Zahner IM6) for the frequency range 10 µHz to 8 MHz.

Results and Discussion
Figure 1a, b shows the XRD patterns of as prepared and annealed at 800 °C/6 h.Comparing the result with XRD pattern of the bulk CeO 2 crystallizing in the cubic fluorite structure, it has been observed that the positions of peaks of the nanocrystal samples are in agreement with those of the bulk CeO 2 , demonstrating the formation of nanocrystals with cubic fluorite structure with lattice parameters matching the bulk.No additional peaks were observed that would correspond to any secondary phase in the sample annealed at 800 °C/6h.It is observed that the reflection peaks become sharper and narrower while increasing the annealing temperature indicating improvement of crystallinity 19 .The reflection peaks of the two XRD patterns were indexed using the software 'INDEXING'.The 'hkl' values are compared with the standard JCPDS file (PCPDF 34-0394) 20 .The exhibited XRD peaks correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) of a cubic fluorite structure of CeO 2 is identified using the standard data 21,22 .The unit cell parameters of this sample were found using the software 'UNITCELL' and the observed values are a = b = c = 5.407 Å and α = β = γ = 90°.The grain sizes estimated using full width at half-maximum (FWHM) of the peaks, using the Debye-Scherer's equation where D is the crystallite size in nm, λ is the wavelength of the X-rays (1.5406 Å), β is the full width at half maximum and θ is the diffraction peak angle.The grains are observed to be nearly same and all are in nearly spherical in shape, as observed in the SEM image.It is observed that the size of maximum number of particles is around 20 nm.The obtained values in this work are in good agreement with the values reported by other research group [23][24][25][26][27][28] .The particle size distribution is presented in the Figure 1c and it is observed that the size of particles is varies from 10 and 75 nm.It is in good agreement with the particle size observed on the HRTEM image.
From SEM and HRTEM images of CeO 2 nanoparticles, the morphology of nanoparticles is observed to be nearly spherical with slight agglomeration.Figure 1d indicates that the nanoparticles were nearly spherical and slightly elongated and it confirms the possibility of different crystallographic planes of different atomic density.Figure 1e demonstrate that the nanoparticles range from 10-20 nm, which are approximately the sizes reported by Chen and Chang 29 .These images also provide evidence for the formation of larger aggregates (supra-aggregates) as observed in the HRTEM image.These supraaggregates are likely to be the stable form of these nanoparticle suspensions.We could also observe some smaller particles of 5-6 nm in size, which are combined together to grow to the irregular particle with the size about 20 nm like a single particle observed by HRTEM 30 . Figure 1f shows the SAED pattern of Ceria nanoparticles.Since the CeO 2 particles tend to lower the surface energy, we could observe the agglomeration of nanoparticles like spherical shape with the size between 20 and 60 nm in the SEM image.
The FT-IR spectrum of the nanoparticles of CeO 2 annealed at 800 °C is shown in Figure 2a.The broad absorption band located around 3448 cm -1 corresponds to the O-H stretching vibration of residual water and hydroxyl groups, while the absorption band at 1647 cm -1 is due to the scissor bending mode of associated water.The existence of CH 2 vibrations at 2453.3 and 2939.3cm−1 indicates that the surfactant is not present in the as-synthesized sample.The bands at 3382.9 and 1647.1 cm −1 can be attributed to the O-H vibration in absorbed water on the sample surface 31 .In addition to the bands in the 850-1600 and 2800-3000 cm −1 , the band due to the stretching frequency of Ce-O can be seen below 700 cm −1 .The FT-IR peaks at about 1515, 1265, 1130, 1064, 952 and 862 cm −1 are similar to those of commercial CeO 2 powders 32 and CeO 2 nanoparticles 33 .The band at 862 cm -1 and 819 cm -1 corresponds to (Ce-O) metal-oxygen bond and the corresponding functional groups are given in tables 3.5.The assignments for the peaks/bands of the FT-IR spectra of the samples have been given in accordance with the data reported in the literature 34 .UV-visible absorption spectral study may be assisted in understanding electronic structure of the optical band gap of the material.Absorption in the near ultraviolet region arises from electronic transitions associated within the sample.The size-dependent quantum size effect and the optical quality of the ceria nanoparticles, optical absorption studies are carried out by ultrasonically dispersing the samples in spectroscopic grade toluene.The Figure 2c shows a well-defined absorption peak located at 321 nm.The sharp and strong absorption spectrum with clear excitonic feature is in good accordance with the narrow size distribution of ceria nanocrystal.In comparison with UV-visible absorption spectrum of CeO 2 nanoparticles reported in the literature 35 , the peaks at 321 nm and band/peak in the spectrum located at around 500-700 nm are observed to be shifted towards lower wavelength side, which clearly shows the blue shift.It indicates the absorption positions depend on the morphologies and sizes of CeO 2 .The UV absorption ability of CeO 2 is related with band gap energy.The UV-absorption edge provides a reliable estimate of the band gap of any system.From the plots of hν against (αhν) 2 , band gap energy can be found accurately.Here α is the absorption coefficient and h and ν have the usual meanings.The absorption coefficient (α) was determined by using the relation α = 2.303 log (abs)/t where't' is the size of the particle in nm and 'abs' is the absorbance.The relation between the absorption coefficient (α) and the incident photon energy hν can be written as α h ν = A (h ν -E g ) 1/2 [36] , where A is a constant and E g is the band gap energy of the material.When the allowed direct transition is pre-assumed, as usually done by other researchers 37,38 , band gap energy may be obtained from extrapolation of the straight line portion of the hν vs (αhν) 2 plot to α = 0.The band gap energy was calculated by plotting the optical energy hν against (αhν) 2 and is shown in Figure 2d.The absorption of CeO 2 in the UV range originates from the charge-transfer between the O 2p and Ce 4f states in O 2-and Ce 4+ , which is much stronger than the 4f 1 -5d 1 transition from the Ce 3+ species in the mixed valence ceria system.From the intersection of the extrapolated linear portion, the E g value of the CeO 2 samples was determined as 3.22 eV.It is found that the band gap of the CeO 2 nanoparticles in the present study is 3.22 eV and this value is found to be greater than that of bulk CeO 2 nanoparticles.The bulk band gap of CeO 2 is 3.19 eV [39] .It implies that the band gap energy of nanocrystalline CeO 2 increases by 0.03 eV when compared to that of bulk CeO 2 and this increase in the band gap energy is due to qunantum confinement of charge carriers in the CeO 2 nanosystem.As a result, the absorption band is shifted to the range of higher energy and blue shift is observed.
The thermo gravimetric and differential thermal analyses (TG/DTA) are important to check the thermal stability and to identify various transitions (exothermic and endothermic) of a substance.Figure 2b shows the thermograms (TG/DTA curves) of as-prepared CeO 2 nanoparticles.The weight loss indicates the decomposition of hydrated oxide, i.e.CeO 2 •H 2 O to CeO 2 .The endothermic peak at 95 °C corresponds to the removal of adsorbed water and the amount of weight loss in TG curve noticed here is low (0.42 mg).The exothermic peak (band) observed in between next stage happened in between 120-600 °C, which could be ascribed to removal of chemisorbed water and the release of organic residues that are trapped inside the pores.The next stage was observed in between 600-800 °C, which was likely due to the desorptions of the hydrated oxide on the CeO 2 nanoparticles.Thus, the decrease in the weight occurs in a rather wide range of temperature 40 .The final stage is ascribed to the combustion decomposition of the organic matters of the samples.
Impedance spectroscopy has been a useful tool for probing the electrical/dielectric properties and operation mechanisms of organic and inorganic semiconductors/devices [41][42][43][44] .Complex impedance plane plots of Z' verses Z", where Z' and Z" are the real and imaginary parts of the complex impedance respectively.They are useful for determining the dominant resistance of the sample [45][46][47][48][49] .Figure 3a, b shows the complex impedance plots of the CeO 2 nanoparticles at 303 K and 373 K respectively.It shows that the data with the help of XRD pattern and HRTEM images is 20 nm with cubic fluorite structure.Agglomeration of particles for the sample prepared by solvothermal method is inferred from SEM measurements.The Ceria nanoparticle showed good thermal stability and a strong UV-vis absorption below 400 nm with a well-defined absorption peak at 310 nm, the direct band gap was found to be 3.22 eV.The shift of the band gap absorption in the UV-vis spectrum agrees closely.The Cole-Cole plots of the above materials reveal the insulating (dielectric) nature of the materials.From the data of AC conductivity of CeO 2 sample, the insulating nature of the sample is confirmed and also it is noticed that there is a slight increase of AC conductivity when the temperature of the sample increases.points lie on the single semicircle where center lies on axis off the real axis and increase like a straight line.It is observed that the CeO 2 nanoparticles are insulators in lower temperature and at higher temperatures its conductivity increases.The Figures 3c, d show initially the samples having semiconducting nature at lower frequencies and dielectric nature at higher frequencies.From the data of AC conductivity of CeO 2 sample, the insulating nature of the sample is confirmed.The exponential increase of the AC conductivity value with frequency at 303 K and 373 K is similar.It is observed that in the temperature range 303-373 K, there is no remarkable change of conductivity since the sample is a metal-oxide (CeO 2 ) and also it is noticed that there is a slight increase of AC conductivity when the temperature of the sample increases up to 373 K from 303 K 50 .

Conclusion
Microwave-assisted solvothermal method has been successfully established for the preparation of nanocrystalline CeO 2 particles.The method is found to be convenient, rapid, and efficient also for the possibility to control the morphological and structural properties.Single phase Ceria nanocrystals were obtained after proper annealing.The average particle size of samples calculated

Figure 3 .
Figure 3. (a) and (b) cole -cole plot of ceria nanoparticles at 303 K and 373 K. (c) and (d) AC conductivity spectra of ceria nanoparticles at 303 K and 373 K.