稀有金属(英文版) 2015,34(01),6-11

收稿日期：18 November 2013

Synthesis of CeO₂ nanoparticles using flame-assisted spray pyrolysis and solid state diffusion routes

K. R. Nemade S. A. Waghuley

Department of Physics, Sant Gadge Baba Amravati University

Abstract：

The stable and crystalline phase of pure nanostructured CeO2 was directly synthesized by flame-assisted spray pyrolysis and solid state diffusion route. Different characterization techniques, including X-ray diffraction(XRD), scanning electron microscopy(SEM), Fourier transform infrared spectroscopy(FTIR), ultraviolet–visible(UVVis), and thermo gravimetric analysis(TGA) were employed to examine the structural, morphological, optical, and thermal properties of the final product. Similarly, the comparative carbon dioxide(CO2)-sensing response of as-synthesized Ce O2 nanoparticles by both routes was also reported. The CeO2 nanoparticles synthesized by solid state diffusion method exhibit good sensitivity(3.38 %) at room temperature,low operating temperature(398 K), fast response time(32 s),and recovery time(36 s) along with good stability.

Keyword：

CO2 gas sensing; Flame-assisted spray pyrolysis; Solid state diffusion;

Author： S. A. Waghuley,e-mail: sandeepwaghuley@sgbau.ac.in;

Received： 18 November 2013

1 Introduction

Nanotechnology oriented research is advancing at a fastpace due to their exceptional properties, such as tunableelectrical conductivity, luminescent efficiency of semiconductors, and formability of ceramics [1]. Ceriumdioxide (Ce O₂) is solitary important rare-earth oxides [2].The perse capabilities of cerium oxide (Ce O₂) nanoparticles encouraged researchers to study its various modernapplications such as sensors [3], solar cell [4], and fuel cell[5] by various synthesis methods. In particular, Ce O₂nanoparticles were considered as most interesting materialsfor their application in therapy [6].

The nanostructured Ce O₂with various morphologieswas directly synthesized by a new, simple, and greenchemical precipitation method reported by Li et al. [7]. Huet al. [8] developed a new method to synthesize singlecrystalline Ce O₂nanoparticles with the advantages of rapidsynthesis, normal atmosphere, 100 % productive ratio, andlow cost. Yan et al. [9] reported the nanosized Ce O₂particles with a diameter of *200 nm, loaded successfully onactivated carbon via a single-step hydrothermal process byconsidering various synthetic parameters, includinghydrothermal temperature, precursor concentration, andreaction time. Li et al. [10] demonstrated the synthesis ofCe O₂nanoparticles by a simple co-precipitation methodand studied the structural and magnetic properties. Zhanget al. [11] reported the shape-controlled synthesis of Ce O₂nanoparticles for catalytic application. The results of workclearly show that Ce O₂is an effective component in catalytic reactions and their catalytic activities show morphology dependence. Li et al. [12] reported an alternativetechnique that is mechanochemical reaction for the synthesis of Ce O₂nanoparticles. Zhang et al. [13] reported thesynthesis of Cu-doped Ce O₂spheres by a simple hydrothermal method, which shows the excellent catalytic performance. Araujo et al. [14] studied the catalytic efficiencyof Ce O₂nanoparticles depending on its morphology, synthesized by a microwave-assisted hydrothermal method. Inthis study, Ce O₂plays a vital role in emerging technologiesfor environmental and energy-related applications. Zhanget al. [15] demonstrated the size-controlled synthesis ofCe O₂nanospheres via a simple solvothermal treatment andalso studied the size-dependent catalytic activity. Pan et al.[16] reported the synthesis route for Ce O₂nanoplates by ahydrothermalreactionassistedbyhexadecyltrimethylammonium bromide. Zhang et al. [17] synthesized the Ce O₂nanospindles by employing a simpletemplate-free solvothermal route for investigation of catalytic activity. Ce O₂-based ceramics are potential candidatesfor their broad range of applications such as catalyst [18],abrasives [19], and pigments [20]. Kaspar et al. [21]demonstrated the Ce O₂use as three-way catalytic converters because of its unique redox properties and highoxygen storage capacity. This rare-earth oxide plays vitalrole as it supports the water gas reaction, disperses preciousmetal, and inhibits the sintering of alumina supports.

Inspiring from above discussion, this work was plannedto synthesize Ce O₂nanoparticles by two different routesthat were flame-assisted spray pyrolysis and solid statediffusion technique. It was also explored comparative CO₂sensing properties of as-synthesized Ce O₂nanoparticles.

2 Experimental

2.1 Synthesis of Ce O2by flame-assisted spraypyrolysis route

Cerium chloride (Ce Cl₃ 7H₂O) and sodium hydroxide(Na OH) used in this work were of analytical grade, withoutfurther purification. Ce O₂nanoparticles were prepared viaflame-assisted spray pyrolysis route. In the typical procedure, a stock solution of 1 mol L^-1solution of Ce Cl₃ 7H₂Oand Na OH was prepared by dissolving suitable quantity indistilled water of resistivity not less than 18.2 MX cm^-1separately. The prepared solution was mixed under magnetic stirring for 10 min at room temperature. After thisstep, prepared solution was loaded in chamber of spraypyrolysis, which was made of a capillary tube with an outerdiameter of 1 mm (inner diameter of 0.6 mm) and anopening of 1.2 mm in diameter (Labtronics, India). Thespray was evaporated by a supporting flamelets maintainedat 573 K. The flow rate was controlled by a flow controller.The product was collected on a Si O₂substrate.

2.2 Synthesis of Ce O2by solid state diffusion route

CeO₂nanoparticles were prepared by the solid state diffusion route using the starting chemicals, Ce Cl₃ 7H₂O andNa OH. The chemicals were taken in 1:1 amounts. Thestarting materials were mixed thoroughly for 1 h using theagate mortar pestle. The crushed samples were placed in asilica crucible and heated at 1,073 K in muffle furnace for3 h. Then, they were removed, crushed in the mortar pestleagain and heated at 1,073 K for 8 h. Finally, the samples inthe furnace were allowed to cool down to roomtemperature.

2.3 Material characterizations

The X-ray diffraction (XRD) analysis was carried out on aRigaku miniflex-II X-ray diffractometer to reveal thestructure of materials. The particle size distributions of assynthesized powders were determined using Horiba particle size analyzer (LA-960) with doubly distilled water as amedium. Scanning electron microscopy (SEM) studies ofthe Ce O₂as-synthesized samples were carried out on JEOLJSM-7500F. The Fourier transform infrared (FTIR) spectrum of the samples was acquired on Shimadzu FTIRspectrometer of model:8400S. Ultraviolet–visible (UV–Vis) analysis performed on Perkin Elmer UV spectrophotometer. The thermo gravimetric analysis (TGA) wasobtained with a Shimadzu DTG-60h thermal analyserunder nitrogen atmosphere.

2.4 Chemiresistor fabrication and gas sensingmeasurements

The chemiresistors of as-prepared Ce O₂samples werefabricated by screen-printing technique on glass substrateof size 30 mm 9 30 mm using temporary binder (composed of butyl carbitol and ethyl cellulose). The ohmiccontact was made on the deposited film, which wasachieved using high-conducting silver paint and annealingthe chemiresistors at 373 K for 30 min in argon. Theaverage thickness of screen-printed layer was about 12 lm,measured by digimatic outside micrometer (Series-293,Japan). The gas-sensing characteristics of chemiresistorswere studied by specially designed sensing unit. Thetemperature and humidity within the chamber were specifically controlled. The sensing response was studiedusing air as background gas with H₂O lower than3 9 10^-6. The fixed volume of the CO₂was inserted intothe gas chamber to keep required concentration inside thechamber. The reading of chemiresistor was acquired afterperiod of 15 s. The voltage drop method was employed tomeasure resistance change [22]. The sensing response ofchemiresistor is defined as [23]:

where R_aand R_gare the resistance of chemiresistor in airand gas, respectively.

3 Results and discussion

The XRD pattern of Ce O₂nanoparticles synthesized fromflame-assisted spray pyrolysis and solid state diffusion isnearly the same and possibly indexed to the standard PDF-00057-0401 data card of CeO 2with cubic fluorite structure, andspace group Fm3m (225) is shown in Fig. 1a, b. The diffraction peaks appears at various 2h positions, exactly indexed tothe formation of this compound. The lattice parameter calculated from the (111) plane of the CeO 2nanoparticles sampleis found to be a = b = c = 0.5513 nm. The average crystallite sizes of Ce O₂nanoparticles synthesized using flameassisted spray pyrolysis and solid state diffusion method wereestimated from the Scherer’s equation, which are found to be17.6 and 15.2 nm, respectively.

Fig. 1 XRD patterns of Ce O2synthesized by a flame-assisted spraypyrolysis and b solid state diffusion method

Fig. 2 FESEM images of Ce O2synthesized by a flame-assisted spraypyrolysis and b solid state diffusion method

Figure 2a, b displays the FESEM images of Ce O₂nanoparticles derived using flame-assisted spray pyrolysisand solid state diffusion method, respectively. The carefulanalysis of SEM images shows that morphologies obtainedfor both synthesis techniques are nearly similar. Theaverage crystallite size for Ce O₂nanoparticles obtained inboth synthesis techniques ranges between 15 and 22 nm.The SEM images of both samples reflect the presence ofagglomeration.

The particle size distribution patterns of the as-synthesized Ce O₂samples are shown in Fig. 3a, b. The green lineshows the cumulative Gaussian distribution of percentagefrequency versus particle size. The cumulative distributionvalue of Ce O₂synthesized by flame-assisted spray pyrolysis is found to be 16.9 nm, while this value for solid statediffusion route is found to be 15.3 nm. The obtained valuesof particle size distribution are in good agreement withXRD and FESEM analysis.

The FTIR spectrum of as-prepared Ce O₂nanoparticlesusing flame-assisted spray pyrolysis and solid state diffusion technique are shown in Fig. 4. The broadband in thelower energy region (2,200–4,000 cm^-1) is attributed tothe presence of Ce–O [24]. The two intense bands ataround 723 and 496 cm^-1are assigned to the antisymmetric Ce–O–Ce stretching mode and surface bridging ofoxide formed due to condensation of surface hydroxylgroups [25]. The signal at 1,635 cm^-1is assigned to thebending frequency of molecular H₂O (H–O–H) [26].

Fig. 3 Particle size distribution pattern of Ce O2synthesized by a flame-assisted spray pyrolysis and b solid state diffusion method

Fig. 4 FTIR spectra of Ce O2synthesized by flame-assisted spraypyrolysis and solid state diffusion method

Fig. 5 UV–Vis spectra of as-synthesized Ce O2synthesized

Figure 5 shows the UV–Vis absorption spectra of Ce O₂nanoparticles synthesized using flame-assisted spraypyrolysis and solid state diffusion method. The spectrum ofboth samples shows a strong UV absorption band around308 nm with marginal difference in absorption, whichreflects the presence of weak quantum confinement [27].This weak quantum confinement is attributed to electron–phonon-coupling system in Ce O₂[28].

Figure 6 shows the TGA curve of the as-synthesizedCe O₂nanoparticles. Both TGA traces show weight loss up to383 K. This weight loss is assigned to desorption of water.Beyond 383 K, Ce O₂synthesized by flame-assisted spraypyrolysis shows continuous weight loss up to 730 K. Thisshows that Ce O₂synthesized by flame-assisted spray pyrolysis is thermally unstable. While, the Ce O₂synthesized bysolid state diffusion route does not show significant weightloss up to 530 K. This shows that Ce O₂synthesized by solidstate diffusion route is thermally stable in temperature rangeof 383–530 K. But, beyond 530 K, Ce O₂becomes thermallyunstable and shows significant weight loss. This weight lossin both samples at higher temperature is attributed to thecomplete removal of inorganic residues and the formation ofhighly crystalline Ce O₂.

Fig. 6 TGA traces of as-received Ce O2

Fig. 7 Variation of sensing response of Ce O2chemiresistors withconcentration of CO2at room temperature

Figure 7 shows the sensitivity response to various CO₂concentrations at room temperature (303 K). The sensitivity response of the chemiresistors to CO₂is almost linearand shows good dependances on concentration CO₂. Theresistance of chemiresistor increases with concentration ofCO₂increasing, because CO₂is oxidizing gas havingstrong electron withdrawing power [29]. Therefore, as theconcentration of CO₂increases, more and more electronsinject from the surface of Ce O₂chemiresistor, whichresults in the increase of resistance. The CeO₂synthesizedby solid state diffusion shows the highest value of sensitivity (3.38 %) at 10,000 9 10^-6.

Figure 8 shows the operating temperature dependenceof the sensitivity to 5,000 9 10^-6CO₂. In the measuredtemperaturedomain,thevalueofsensitivityto5,000 9 10^-6CO₂increases at first, undergoes a maximum, and finally drops. The sensitivity values start to dropfrom certain temperature. This decrease in sensitivity isattributed to desorption of atmospheric oxygen ions fromsensing surface due to thermal vibrations [30, 31]. Thehighest value of the sensitivity (3.16 %) is found at 398 Kfor Ce O₂synthesized by solid state diffusion route.Therefore, for CO₂gas detection, the operating temperature of the chemiresistor can be 398 K.

Fig. 8 Ce O2chemiresistors response as a function of operatingtemperature to 5,000 9 10-6CO2

Fig. 9 StabilitycharacteristicsofCe O2chemiresistorsto5,000 9 10-6CO2

Fig. 10 Transient response of Ce O2chemiresistors to 5,000 9 10-6CO2

Figure 9 shows the stability characteristics of Ce O₂chemiresistors at room temperature to 5,000 9 10^-6CO₂.In order to verify the stability of chemiresistor, its responsewas measured for 60 days, at an interval of 10 days. Boththe chemiresistors have roughly constant sensing response.This point out that the stability of both chemiresistors isgood against CO₂.

The transient characteristic of Ce O₂chemiresistors to5,000 9 10^-6CO₂studied at room temperature is displayed in Fig. 10. For this, gas was introduced in thechamber, and resistance of chemiresistor was measured inair and in presence of gas. The Ce O₂synthesized by solidstate diffusion route shows fast response time toward theCO₂of the order 32 s. For measuring the recovery time,chemiresistor was uncovered to air. The chemiresistorattains fast recovery in 36 s.

4 Conclusion

Nanocrystalline Ce O₂was successfully synthesized viaflame-assisted spray pyrolysis and solid state diffusionroute. XRD analysis clearly shows that both routes cansuccessfully synthesize pure form of Ce O₂. The morphologies of final products studied through FE-SEM are similarfor the both synthesis route. FTIR study reveals that thepurity of Ce O₂nanoparticles are obtained as final productvia both synthesis routes. The strong UV absorption bandaround 308 nm shows the presence of quantum confinement. TGA study was successfully employed for the analysis of thermal stability of as-synthesized Ce O₂nanoparticles. Gas-sensing study of both chemiresistorsshows that Ce O₂synthesized by solid state diffusion routeis more sensitive than Ce O₂synthesized by flame-assistedspray pyrolysis route.