J. Indian Chem. Soc., Vol. 91, January 2014, pp. 47-52 Synthesis and characterization of zirconium oxide nanoparticle prepared by aqueous gelation method Seena Kurakaran*, Abraham George and A. Sreekumaran Nair Department of Chemistry, Mar Ivanios College, University of Kerala, Thiruvananthapuram-695 015, Kerala, India E-mail : seenakurakaran@gmail.com Manuscript received online 18 February 2013, revised 22 March 2013, accepted 29 March 2013 Abstract : Tetragonal zirconium oxide (ZrO 2 ) nanostructure was synthesized by a novel aqueous gelation route. Surface morphology analysis depicts the formation of cylindrical nanocrystals. The structural analysis confi rm that the assynthesized ZrO2 product is mainly of tetragonal phase with minor fraction of monoclinic phase having crystallite size of less than 25 nm. The tetragonal phase of ZrO 2 is strongly evidenced by XRD and Raman analysis. Thermal analysis shows that the gel is continuously dehydrated in the temperature range between room temperature and 500 ºC. The DSC analysis coupled with TG and structural information, indicate that the exothermic processes between 349 ºC and 460 ºC can be attributed to the nucleation process of the formation of tetragonal zirconia, with phase transformation at 460 ºC. The UV-Vis absorption spectrum showed a strong absorption peak at 226 nm and the estimated optical band gap was found to be 5.6 eV. Keywords : Aqueous gelation, ZrO 2 , SAED, HRTEM, tetragonal. Introduction Zirconia nanoparticles find applications in various fields. It has been reported as a better catalyst1 and catalyst support compared to classical materials such as Al2O3, SiO2 and TiO2. The modified versions of zirconium dioxide viz. the sulfated ZrO2, zirconia substituted mixed oxides, various transition metal stabilized zirconia and hydrous zirconium oxide have been reported to be effective for several organic reactions, and gas phase reactions2. The partial substitution of ZrO2 by CeO2 improves the thermal stability3, oxygen storage capacity and redox properties of ceria considerably. The catalytic performance of ZrO2 depends also on the structural, textural properties, surface area, synthesis method adopted and calcination temperatures. ZrO2 nanostructures are of significant current interest in preparing piezoelectric, electrooptic, dielectric and nanocomposite materials4–6. It tends to become more conductive with increasing temperatures. Pure ZrO2 exists in three polymorphic phases at different temperatures : monoclinic, tetragonal and cubic. At very high temperatures (>2370 ºC) the material has a cubic structure. At intermediate temperatures (1150–2370 ºC) it has a tetragonal structure. At low temperatures (below 1150 ºC), the material transforms into the monoclinic structure, which is a thermodynamically stable phase7. The versatility of ZrO2 nanoparticles has prompted researchers to find convenient and cheap methods to prepare the nanoparticles. Different methods have been adopted for the preparation of phase pure ZrO2. The sol gel route using zirconium alkoxide8 has been proved to be one of the best methods to obtain phase pure ZrO2 nanoparticles and composite nanostructures9–12. Aqueous gelation method has turned out to be a simple and excellent route for the preparation of zirconium nanoparticles13. The aqueous gelation method has many advantages viz. a highly homogenous crystalline product can be obtained directly at a relatively lower reaction temperature (< 150 ºC), favors a decrease in agglomeration between particles, narrow particle size distribution, phase homogeneity, uniform composition, high product purity and controlled particle morphology14. Moreover the reaction takes place at ordinary laboratory conditions with less expensive chemicals. There is a wide scope of using this method to produce other oxide materials with required properties for catalysis. 47 J. Indian Chem. Soc., Vol. 91, January 2014 In the present work, we discuss the synthesis of ZrO2 nanoparticles by aqueous gelation method using zirconyl oxychloride as the precursor. We present the synthesis of t-ZrO2 nanoparticles with controlled morphology and high crystallinity. Structural analysis of the nanoparticle was performed by transmission electron microscopy, X-ray diffractometry, Raman and IR and UV-Visible spectroscopies and thermal analysis. Results and discussion Preparation : The aqueous gelation method using sodium acetate can be considered as a highly versatile method. When sodium acetate was added to the zirconyl oxychloride solution, the pH increased from 2 to 4. This pH is conducive for the formation of the gel. The acetate ions function as a good ligand coordinating to Zr controlling its hydrolysis, leading to the formation of ZrO2-x(OH)2x.yH2O15. Infrared spectrum : The IR spectrum of the gel was recorded (Fig. 1a) and shows two prominent peaks. A broad band around 3500 cm–1 corresponds to the stretching vibrations of the O-H bonds. The sharp bands around 1500 cm–1 are due to the water molecules and also the vibrations of the Zr-O bonds. The IR spectra of the ZrO2 nanoparticles (Fig. 1b) essentially show the various stretching frequencies at 500, 572, 740, 1104 and 1187 cm–1 respectively as reported earlier16. The features particularly at 740 cm–1 and 500 cm–1 due to Zr-O-Zr asymmetric and Zr-O stretching modes respectively, confirm the formation of phases. X-Ray diffraction analysis : XRD profile of ZrO2 nanoparticle synthesized at 600 ºC is given in Fig. 2. The diffraction peaks in the spectrum are composed of t-ZrO2 with minor quantities of m-ZrO2. Almost all the diffraction peaks with similar relative intensity as suggested in the JCPDS data are observed in the experimental XRD pattern and are significantly sharp, hence confirming the purity and high crystallinity of the synthesized ZrO2 sample. The XRD of the as-prepared gel after drying was recorded and exhibits no characteristic peaks showing that the dried gel is amorphous. The strongest diffraction peak at 30.1º corresponds to the (111) plane of t-ZrO2. Small diffraction peaks, are attributed to the monoclinic phase (reflections at 28.3º, 48 Fig. 1. (a) IR spectrum of ZrO2-x(OH)2x.yH2O gel and (b) IR spectrum of ZrO2 powder annealed at 600 ºC. indexed as [111] and 55.2º, indexed as [130], PDF # 811314 and PDF # 652357). A particle with tetragonal structure has lower surface energy than that of monoclinic structure when the size is very small. As a result, the smaller particle is preferentially stabilized with the tetragonal structure, and a larger one with the monoclinic structure. Results of Garvie showed that below 30 nm the metastable t-ZrO2 is stable in nanocrystal ZrO2 and above this value m-ZrO2 will be stable17. The results of Shukla reveals that metastable t-ZrO2 can exist in large size (500– 600 nm) at room temperature18 much larger than the critical size reported in literature. For most of the reported Kurakaran et al. : Synthesis and characterization of zirconium oxide nanoparticle prepared etc. Fig. 2. XRD profile of ZrO2 powder synthesized at 600 ºC. ZrO2 samples synthesized by hydrothermal techniques, the as synthesized products are of mainly tetragonal or cubic or mixed phases19–21 and they attain monoclinic phase only after calcinations or annealing at higher temperature (>1000 ºC). In this case, the soft conditions prefer the formation of t-ZrO2. Thermal analysis : The gel prepared as such was subjected to thermal analysis. The TG curve (Fig. 3a) shows, three distinct inflections, corresponding to the loss of adsorbed and embedded water molecules and also the dehydration of the gel leading to the formation of zirconia nanoparticles. The total weight loss is 27.5%. The DSC curve (Fig. 3b) shows one endothermic peak at 110 ºC and an exothermic peak at 460 ºC. The DSC and TG analysis compared with XRD results indicate that the exothermic process between 349 and 460 ºC can be associated to the nucleation of tetragonal phase and the transformation into this phase from amorphous state. The endothermic peak at 110 ºC, corresponds to the dehydration process. Raman spectroscopy : Amorphous, monoclinic, tetragonal and cubic phases have been observed in ZrO2. These phases may be identified from one another by means of Raman scattering. On the basis of experimental results22,23, tetragonal ZrO2 is expected to yield a spectrum consisting of six bands Fig. 3. (a) TG curve during dehydration process of ZrO2-x(OH)2x. yH2O between RT and 800 ºC at a heating rate of 10 ºC min–1 and (b) DSC profile of ZrO2-x(OH)2x.yH2O between RT and 700 ºC at a heating rate of 10 ºC min–1. 49 J. Indian Chem. Soc., Vol. 91, January 2014 with frequencies at about 146, 267, 315, 456, 607 and 645 cm–1. In Fig. 4, the Raman spectrum of the ZrO2 nanoparticles, annealed at 600 ºC, in the 100–800 cm–1 region is presented. At 600 ºC, the tetragonal phase is clearly evidenced with bands at 148, 268, 317, 462 and 647 cm–1. One small band at 187 cm–1 can be observed due to the monoclinic phase. Fig. 4. Raman spectrum of ZrO2 powder annealed at 600 ºC. Crystal structure analysis by TEM : Further, the structural information of the ZrO 2 nanostructure was studied by TEM, high resolution TEM (HRTEM), and selected area electron diffraction (SAED). TEM image of ZrO2 showing tiny cylindrical particles with diameters below 25 nm and particles with regular cylindrical shaped nanostructures. Average aspect ratio of ZrO2 nanopowder is found to be 5 : 15. The SAED pattern and HRTEM image of ZrO 2 nanostructures in the form of cylindrical shaped particles are shown in Fig. 5(a) and (b) with TEM image in the inset respectively. The HRTEM images show well resolved lattice fringes indicating the single crystalline nature and high crystallinity of the synthesized product. The lattice fringes are of equidistance and clear all along the length of the crystal, without any lattice mismatch. These fringes are separated by 0.316 nm, which agrees well with the interplanar spacing corresponding to the (111) plane of t-ZrO2. The equally spaced reflections in the SAED pattern and lattice fringes correspond to the 50 Fig. 5. (a) SAED pattern and (b) HRTEM image of ZrO2 annealed at 600 ºC with TEM image in the inset. lattice planes of t-ZrO2, hence suggesting the purity and high crystallinity of the synthesized ZrO2 product. UV-Visible absorption spectroscopy : UV-Vis absorption spectrum recorded for the ZrO2 nanomaterial, in the wavelength range 200–400 nm is as shown in Fig. 6. UV-Vis spectrum shows a sharp absorption peak centered at around 226 nm. This can arise due to the transition between the valence band and the conduction band24. The optical band gap determined for the ZrO2 nanoparticle from the absorption spectrum is 5.6 eV. Experimental Synthesis of ZrO2 nanoparticles : ZrO2 nanoparticle was synthesized by a simple aqueous gelation technique. Zirconyl oxychloride hexahydrate Kurakaran et al. : Synthesis and characterization of zirconium oxide nanoparticle prepared etc. Fig. 6. UV-Vis spectrum of ZrO2 nanomaterial annealed at 600 ºC. ZrOCl 2.8H2O (E. Merck, India) and sodium acetate (CH3COONa, E. Merck, India) were used as the starting materials for the synthesis. All the chemicals were of analytical grade and used without further purification. ZrOCl2.8H2O (0.01 M) was taken in an autoclavable bottle and was stirred with a magnetic stirrer. An equal volume of CH3COONa (0.01 M) solution was added dropwise to the zirconium salt with constant stirring. The clear homogeneous mixture obtained was then maintained at 110 ºC for 6 h, resulting in the formation of a gel. The gel was centrifuged using a high speed centrifuge and washed several times with distilled water and dried in air. The white powder so obtained was calcined in an alumina crucible in a muffle furnace at 600 ºC for 4 h. Fine white powder of zirconia thus obtained was subjected to instrumental analysis. Structural characterization : The X-ray diffractogram (XRD) was recorded on a Siemens D-5000 X-ray diffractometer with a Cu K radiation (wavelength 1.54 Å). The observed data was compared with JCPDS standards. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were done on a JEOL 3010 machine operating at an accelerating voltage of 300 kV. For TEM analysis, the ZrO2 was ultrasonically dispersed in ethanol and then spread on carbon-coated copper grids and dried under vacuum. UV-Visible spectrum was recorded on a Shimadzu UV-3600 instrument. For the spectroscopic analysis, the nanomaterial was dispersed in ethaJICS-7 nol by sonication. Thermal analysis was done on a Shimadzu DPG-60 in the range of RT to 800 ºC at a heating rate of 10 ºC/min in nitrogen atmosphere at a flow rate of 100 cm3 min–1. The differential scanning calorimetry (DSC) traces were obtained using a Dupont DSC 2010 differential scanning calorimeter attached to Thermal Analyst 2100 data solution. The samples were heated from RT to 700 ºC in N2 atmosphere at a heating rate of 10 ºC/min. The infrared (IR) spectrum of the samples were recorded in the range 400–4000 cm–1 on a Fourier transform infrared (FTIR) spectrometer (model Thermo-Nicolet Avatar 370) using the KBr pellet method. Raman spectroscopy measurements were performed at room temperature on a computerized Spex 1403 double monochromator in combination with the 514.5 nm line on an Argon Laser at a power level of 50 mW at the Laser head. Conclusion ZrO2 nanoparticles were synthesized by aqueous gelation using zirconium oxychloride as the starting material. Surface morphology analysis confirms the formation of cylindrical crystalline nanostructure. XRD, HRTEM and SAED studies show that the synthesized ZrO2 is tetragonal with minor monoclinic phase. Tetragonal phase is clearly evidenced by Raman spectrum. The DSC and TG analysis compared with XRD results indicate that the exothermic process about 349 and 460 ºC can be associated to the nucleation process of tetragonal phase and the transformation to this phase from amorphous state. The transformation of ZrO2-x(OH)2x.yH2O into tetragonal zirconia by aqueous gelation process has been proved. 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