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Synthesis and characterization of spinel metal aluminate by a simple microwave assisted green synthesis
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 10664–10671
www.materialstoday.com/proceedings
AMMMT 2016
Synthesis and characterization of spinel metal aluminate by a simple microwave assisted green synthesis R.Yuvasravanaab, P.P.Georgea *, N.Devannab. 0F
a b
Department of chemistry, Madanapalle Institute of Technology and Science, Madanapalle-517325, Andhra Pradesh, India.
Department of chemistry, Jawaharlal Nehru Technological University Anantapur, Ananthapuram – 515002, Andhra Pradesh, India.
Abstract In the current research report we present a simple, eco-friendly, cost and time saving biosynthesis of ZnAl2O4 spinel nano and micro particles using Pomegranate peel extract, that act as the reducing as well as stabilizing agent. Thus synthesized products are then consistently characterized under powder X-ray diffraction (PXRD), scanning electron microscopy (SEM). The powder XRD analysis has confirmed their chemical composition. Optical properties including diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and fourier transform infrared spectroscopy (FTIR) are also investigated. The ZnAl2O4 nanoparticles formation is been confirmed by PXRD, FT-IR and SEM. From XRD the calculated crystalline size of the ZnAl2O4 nanoparticles is in the range of 11.2 nm. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016). Keywords: ZnAl2O4, Pomegranate fruit peel extract, Microwave assistance, powder XRD, Optical properties;
1.
Introduction:
Nanocrystalline spinels are a class of binary transition metal oxide, exhibits interesting mechanical, magnetic, optical and thermal properties compared to that of their bulk materials [1]. Zinc aluminate (ZnAl2O4) is a well-known spinel oxide, and has attracted considerable attention in various interdisciplinary areas. Zinc aluminate is have typical applications in ceramic industries, optical and sensing devises, as catalyst and catalyst supports in manufacture industries, as paints in aerospace industry, dielectrics and electronic applications and these applications * Corresponding author. Tel.: +00919000451689; E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016).
R.Yuvasravana, P.P.George, N.Devanna / Materials Today: Proceedings 4 (2017) 10664–10671
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of Zinc aluminate is because of its unique properties, mechanical, thermal and chemical stability, low surface acidity, excellent optical transparency, wide band-gap energy and high quantum yields [2, 3]. ZnAl2O4 is used in optoelectronic devices operated in the ultraviolet region, photo electronic devices, because of its wide-band gap of 3.8 eV and also used as optical and thermal controlled coating for spacecraft [4]. ZnAl2O4 spinels are good enough for a range of catalytic applications as heterogeneous catalyst in the reactions like cracking, dehydration, hydrogenation and dehydrogenation of organic compounds [5] and they are also used as catalyst support because of less surface acidity, chemical inactiveness and thermal durability [6]. In recent years, ZnAl2O4 is also employed for dehydrogenation of chemicals in chemical and petrochemical industries (Shu Fen Wang et al 2005), degrading the gaseous toluene [7], restoring of ethanol steam [8], trans-esterification of oils of vegetable [9] and iso-butane combustion [10-11]. Several synthesis approaches are reported so far, for the preparation of ZnAl2O4 such as ceramic method, mechano-chemical synthesis [12], solid-state reaction synthesis [13], citrate-gel method [14], hydrothermal [15–18], wet chemical method [19], co-precipitation method [20], sol–gel method [21–24], and microwave assisted-hydrothermal method [25]. In green synthesis method various plant material extracts are used in the preparation of spinal nanoparticles, where these extract of plant materials act as both reducing and stabilizing agent. Opuntia dilenii [26], Sesamum indicum [27] are the two plant extracts explored for the synthesis of ZnAl2O4 nanoparticles. Although there is a good progress in exploring nano sized aluminate spinels, some issues like the preparation route and optical properties of ZnAl2O4 nanoparticles still challenging and needed further study. 2.
Characterization:
Zinc aluminate nanoparticles synthesized through green synthesis route are annealed in muffle furnace at 700 oC for about 3 hours and then characterized by powder-XRD, SEM, FTIR and UV-Vis spectroscopy for its physical and optical properties. The powder-X ray diffraction patterns for the sample is recorded under the Bruker D8 diffractometer instrument with 2θ value ranging from 10o to 80o. The surface morphologies of the metal aluminate is studied under scanning electron microscope (SEM), using a Ziess-SEM instrument, before that the gold sputtering device is used to coat the sample with gold for the better visibility. FT-IR spectra of sample is recorded under Jasco FT/IR-4200 infrared spectrophotometer instrument in transmittance mode and the wavelength ranging from 400 to 4000 nm, the sample is mixed with heat dehydrated potassium bromide powder and made into pellet for the analysis. The optical measurements for the sample is recorded under Jasco UV-670 spectrophotometer instrument for UV-Vis diffuse reflectance spectra with the wavelength range from 200 to 900 nm, at room temperature. And Jasco FP-6300 spectrofluorometer instrument is used to record the photoluminescence measurements of sample at room temperature. 3.
Results and Discussion:
The reaction mechanism of spinel preparation can be described as follows and the schematic synthesis procedure is illustrated in Figure-1. below, in mechanism nitrogen dioxide and oxygen is liberated from nitrate salts and carbon dioxide and water vapour from peel extract solution. Zn(NO3)2+ 2Al(NO3)3+ P-Extract → ZnAl2O4+ NO2↑+ 2O2↑ + CO2↑ + H2O↑. 3.1. Powder X-ray Diffraction (PXRD): The powder X-ray diffraction patterns for the ZnAl2O4 sample is recorded under the Bruker D8 diffractometer instrument with 2θ value ranging from 10o to 80o. The PXRD measurements are taken to analyze the crystal purity and phase recognition. The PXRD measurements obtained for the spinel sample is presented in Figure-2(a). The diffraction pattern for ZnAl2O4 are observed at 2θ values at 31.4°, 36.9°, 45.5°, 55.8°, 59.5° and 65.4°, that can be correlated to (220), (311), (400), (422), (511), and (440) planes respectively, and these planes are associated with the cubic phase of ZnAl2O4 with JCPDS card No. 73-1961 (space group Fd3m). The peaks of any other phase are not detected, that suggests the complete development of ZnAl2O4 spinel under applied experimental condition in the present work and no other phases or elements are identified in the sample.
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The diffraction pattern had sharp peaks, indicating the good crystallinity and purity of the spinel sample. The average particle size of thus synthesised spinel samples is calculated with the help of Debye Scherrer formula, as demonstrated below
Where L is the average crystal particle size (Ao), λ is the wavelength of X-ray used (0.154 nm), θ is the diffraction angle, and β the peak position and the FWHM (full-width at half maximum). The average particle size is found to be 11.2 nm.
Figure-1: Schematic representation of synthesis of ZnAl2O4 spinels.
3.2. Scanning electron microscopy (SEM) studies: The SEM images provide illustration of surface morphology of the ZnAl2O4 spinels, and are shown in Figure-2(b). It can be seen from SEM images that the presence of nearly spherical like agglomerated nanoparticles that have the diameter range from 0.1 to 0.6 µm. The nano and micro particles are evenly scattered in the form of small particles and large agglomerates having nearly spherical like structure. The agglomerated structures of these samples are due to the solid state welding of fine particles that may be formed at the time of annealing at 700 oC. The size of agglomerated ZnAl2O4 nanoparticles prepared in this work are comparatively similar to that of the ZnAl2O4 nanoparticles synthesized by microwave method using Sesamum (Sesamum indicum L.) extract by C. Ragupathi and his co-workers [27]. C. Ragupathi’s procedure involves microwave heating for 10 minutes using Sesamum (Sesamum indicum L.) extract. Whereas the current microwave assistance method involves the use of pomegranate fruit peel extract and are irradiated under microwaves for five minutes. Precursor solution containing fruit peel extract and metal nitrates react to form complex matrix on microwave irradiation, followed with the formation of aluminate samples at the end, this confirms that addition of fruit peel extract may have influenced the size of nanoparticles, as we know that the structure of the samples changes accordingly with respect to the synthesis procedure. Metal aluminates are excellent microwave adsorptive materials, hence can be readily heated to the higher temperatures with in short time. And hence the microwave application in the field of nanoparticles synthesis is more advantageous and significant in the reduction of time of synthesis, growth and morphology of nanoparticle, at the specified stable reaction conditions. The estimated particle size from XRD is observed to be slightly different from that of SEM images, that can be explained by structural disorder resulted due to the lattice strain caused by different ionic radii or clustering of nanoparticles.
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Figure-2: (a) XRD of ZnAl2O4 spinels. (b) SEM image of ZnAl2O4 spinels.
3.3. Fourier transform infrared (FT-IR): The FT-IR spectrum of ZnAl2O4 is shown in Figure-3 was recorded in the frequency range from 400 – 4000 cm−1, respectively. In the spectra, the broad bands near 3432 and 1626 cm−1 are referred to the –OH stretching and deformative vibrations of water molecules, the water molecules may be absorbed by sample from atmosphere at the time of sample preparation in the presence of atmosphere. The peaks at 1411 cm−1 can be attributed to the Al–O stretching vibrations. In the spectra, the metal–oxygen stretching frequencies are related to the peaks that appear in the range 400–1000 cm−1 and are assigned to the vibrations of Zn–O, Al–O and Zn–O–Al bonds. From figure 3 FTIR spectra of ZnAl2O4 exhibits two sharp peaks centered at 599 and 443 cm−1 respectively and they are related to the metal-oxygen stretching and bending frequencies like Al–O–Zn, Al–O and Zn-O stretching and bending vibration. C. Ragupathi and his co-workers reported the similar IR spectra of ZnAl2O4 with bands observed at 650 and 490 cm−1 for ZnAl2O4 [27]. The characteristic absorption band at 924 cm−1 represented by IR spectra of ZnAl2O4 of C. Ragupathi’s work can also be seen at 1025 cm−1 in the IR spectra in the current work. The peaks at 2922 and 2855 cm−1 in the IR spectra are may be due to the presence of CO32- ion. The existence of CO2 in the sample can be explained by two reasons that are, mixing of aerial CO2 that occurred at the time of sample preparation and the presence of CO2 inside the powders sample that may be formed due to annealing of plant material.
Figure-3: FTIR of ZnAl2O4 spinels.
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3.4. Optical properties: Zinc aluminate [ZnAl2O4] nanoparticles are widely used for the photo electronic devices because of its large band gap [27]. The optical study for Zinc aluminates was performed using a Jasco UV-670, a UV-VIS-NIR diffuse reflectance spectrophotometer and the reflectance spectra are recorded at room temperature. Metal aluminates exhibit colour due to d-d electron transitions, where as ZnAl2O4 a normal spinal is colourless compound that is because Zn+2 ion is tetrahedrally coordinated and have completely filled d-orbitals. Figure-4(a) shows the typical UV–Vis absorbance spectrum of ZnAl2O4 nanoparticles and shows UV-absorption bands peaked at 218, 262, 304 and 370 nm respectively. UV–Vis reflectance spectrum is recorded to estimate the optical band gap of ZnAl2O4 nanoparticles and is shown in Figure-4(b). The optical band gap for a semiconductor is estimate by using the following equation: α(ν) = A(ħν/2 – Eg)m/2, The band gap is determined by relating diffuse-reflectance (R) of the samples with the Kubelka–Munk function F(R) in the equation F(R) = (1-R)2 /2R and the energy intercept plot of (F(R)*hν)2 versus hν is plotted which yields the Eg, dir for a direct-allowed transition, when the linear regions are extrapolated to the zero ordinate, one can get the band gap of the sample. Hence by this method, the band gap is calculated for ZnAl2O4 nanoparticles and the band gap is found to be 4.1 eV for ZnAl2O4 and it is shown in Figure 5(a). The decrease in nanoparticles size leads to the increase in optical band gap, because of the quantum confinement the decrease in crystal size, band gap energy exhibits an increasing trend. Thus the results from UV–Vis reflectance spectra are in good agreement with that of XRD results.
Figure-4: (a) UV-Vis absorbance spectrum (b) UV-Vis reflectance spectrum.
The oxygen vacancy and defect related transitions are can be determined by using Photoluminescence (PL) spectrum. The surface states and quantity of defects can be varied with reference to the crystallite size and morphology and synthesis conditions [27]. The PL spectra of ZnAl2O4 nanoparticles synthesised by microwave assistance method are shown in Figure-5(b). The emission spectra of ZnAl2O4 nanoparticles are recorded in wavelength ranges from 220 to 600 nm by exiting them at 210 nm wavelength. The PL emission spectra of neat ZnAl2O4 nanoparticles, demonstrate the emission bands peaked at three wavelength regions 335 nm, 422 nm and 472 nm respectively. The PL emission bands for the ZnAl2O4 nanoparticles, synthesised by microwave reduction using sesamum plant extract showed the broad emission bands at 492, 530 and 545 nm [27]. Whereas the ZnAl2O4 nanoparticles synthesised by microwave assistance using pomegranate fruit peel extract showed the broad emission bands at 335, 422 and 472 nm, this blue shift is due to the quantum size effect, and a slight difference in the average crystallite size and intra-band gap errors like oxygen vacancies because of the use of pomegranate fruit peel extract in the synthesis procedure. A broad UV band and weak visible bands in the region confirmed the good crystallinity of the as synthesised spinel sample by microwave assistance method.
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Fruit peel extract of pomegranate consists of polyphenols such as gallic acid, punicalagin and ellagic acid as reported by Wenjuan Qu et.al. [28]. The polyphenol molecules exhibit antioxidant property, similar molecular mechanism can be anticipated to be responsible for the formation of nano ZnAl2O4 spinels. These fruit peels are naturally available, non-polluting and non-toxic, thus fruit peel extract as active ingredient, enables synthesis of nanoparticles at low cost. In microwave assistance, precursor materials are thermally decomposed with the help of microwave radiation, to synthesis the spinel nanoparticle, thus it is economical, rapid, and accessible nowadays. In the current study, the microwave-assisted green synthesis technique, metal nitrates are dissolved and mixed well with fruit peel extract and the solution is placed in household microwave for combustion process. Microwave radiation, dehydrates and decomposes the solution into flake like residue, washed and annealed to give ZnAl2O4 nano spinals. Fruit peel material is completely decomposed and removed in the form of carbon related by-products, either in microwave or at the annealing process, resulting in pure and 85% yield of zinc aluminates nano spinels.
Figure-5: (a) Band gap energy plot of ZnAl2O4 spinels. (b) PL spectra of ZnAl2O4 spinels.
4.
Conclusions:
In this work ZnAl2O4 nanoparticles are synthesised under microwave assistance by using pomegranate fruit peel extract as reducing and stabilising agent and obtained anticipated results. At room temperature the pomegranate fruit peel extract promotes the formation of nanoparticles with a fast kinetics and no other harmful chemicals are used except that of metal nitrate salts and fruit peel extract. This microwave assistance method is more reliable, can be performed in simple and easy steps and gets the results within a short time. The formation of ZnAl2O4 nanoparticles is been supported by PXRD, FTIR and SEM and the crystalline sizes of ZnAl2O4 nanoparticles is in the range of 11.2 nm, as calculated from PXRD measurements. The band gap values calculated from reflectance spectra of UV-VIS-NIR diffuse reflectance spectrophotometer, ranges between 3.9 – 4.0 eV which explains the nano dimensions of ZnAl2O4 nanoparticles. In microwave combustion process, with in short duration of time the reaction mixture in the oven gets heated to higher temperature spontaneously by takes energy from the microwave field. Hence by, this work presents a smooth and comfortable method for the preparation of bimetal oxide spinels like zinc aluminates with a promising and good crystalline structure. This fabrication procedure can be further applied in the preparation of other bimetal oxides.
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5.
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Experimental:
5.1. Materials: Zinc and aluminium nitrates are the precursor material, (97% purity Merck chemicals, India) and are used as obtained without any further refinement. The pomegranate (Punica granatum.) fruits were collected from local fruit market, Madanapalle, Andhrapradesh. 5.2. Preparation of the Plant Extract: The pomegranate (Punica granatum.) fresh peels are washed thoroughly with distilled water to remove unwanted dust and they are dried in shade to make sure that they look fresh at the time of extract preparation. 20g of pomegranate fruit peel was weighed, cut into slices and added to 100 ml distilled water in a 250 ml Erlenmeyer flask and boiled on a gas burner for 15 minutes, cooled and filtered using filter paper, the filtrate thus obtained is used as aqueous fruit peel extract for synthesis of nanoparticles. 5.3. Synthesis of Metal Aluminates: Stoichiometric mass proportions of Zinc nitrate and Aluminium nitrates in 1:2 ratio are dissolved in 50 ml distilled water in 250 ml beakers. Then 5 ml of fruit peel extract was added to the nitrate solution, and stirred for 15 minutes using a magnetic stirrer. The solution is then irradiated under microwave radiation using a house hold microwave oven for about 5 minutes, with break at every 60 sec to avoid wastage of the solution by spilling. Initially solution underwent boiling, dehydration and decomposition by releasing gases leaving dry flakes like solid behind. This dry solid is washed several times with water and ethanol for its purity and annealed in muffle furnace at 700 oC for about 3 hours. Acknowledgements: Authors thanks the Management, Madanapalle Institute of Technology and Science, Angallu, Madanapalle, Andhra Pradesh, for encouraging to complete this research work. Dr. P. P. George is thankful to the UGC Major Research project (UGC-MRP- Major Chem. - 2013- 25694). R. Yuvasravana is thankful to VIT, Vellore, for PXRD measurements, SV University, Tirupathi, for SEM measurements, BITS-Pilani, Hyderabad campus for FTIR, UV-Vis and PL spectroscopy respectively. References [1]
S. Guangzhuang, S. Guangai, Z. Mian, W. Shifa, Z. Xiaotao, and X. Xia, (2016), Russian Journal of Physical Chemistry A, Vol. 90, No. 3, pp. 691–699.
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F.M. Stringhini, E. L. Foletto, D. Sallet, D.A. Bertuol, F.O. Chiavone and C.A.O. Nascimento, (2014), Journal of Alloys and Compounds, 588, 305-309.
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A. Manikandan, M. Durka, M. Amutha Selvi, and S. Arul Antony, (2016), Journal of Nanoscience and Nanotechnology, Vol. 16, 357–373.
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X. Li, Z. Zhu, Q. Zhao and L. Wang, (2011), Journal of Hazardous Materials, 186, 2089-2096.
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A.E. Galetti, M.F. Gomez, L.A. Arrúa and M.C. Abello, (2010), Applied Catalysis A: General., 380, 40-47.
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V. Pugnet, S. Maury, V. Coupard, A. Dandeu, A.A. Quoineaud, J.L. Bonneau, (2010), Applied Catalysis A: General., 374, 71-78.
[10] W. Staszak, M. Zawadzki and J. Okal, (2010), Journal of Alloys and Compounds., 492, 500-507. [11] Suellen Battistona, Caroline Rigoa, Eric da Cruz Severoa, Marcio Antonio Mazuttia, Raquel Cristine Kuhna, Andre Gundelb, Edson Luiz Folettoa, (2014), Materials Research, 17, 3, 734-738. [12] A.D. Ballarini, S.A. Bocanegra, A.A. Castro, S.R. de Miguel, O.A. Scelza, (2009), Catal. Lett., 129, 293.
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[13] N.J. van der Laag, M.D. Snel, P.C.M.M. Magusin, G. de with, (2004) J. Eur. Ceram. Soc., 24, 2417. [14] R.K. Singh, A. Yadav, A. Narayan, M. Chandra, R. K. Verma, (2012), J. Therm. Anal. Calor., 107, 205. [15] X.Y. Chen, C. Ma, Z.J. Zhang, B.N. Wang, (2008), Mater. Sci. Eng., 151, 224. [16] M. Zawadzki, J. Wrzyszcz, (2000), Mater. Res. Bull., 35, 109. [17] C.C. Yang, S.Y. Chen, S.Y. Cheng, (2004), Powder Technol., 148, 1, 3. [18] Z. Chen, E. Shi, Y. Zheng, W. Li, N. Wu, W. Zhong, (2002), Mater. Lett., 56, 601. [19] Z. Zhu, X. Li, Q. Zhao, S. Liu, X. Hu, G. Chen, (2011), Mater. Lett., 65, 194. [20] S. Farhadi, S. Panahandehjoo, (2010), Appl. Catal. A., 382, 293. [21] S. Kurajica, E. Tkalcec, J. Sipusic, G. Matijasic, I. Brnardic, I. Simcic, (2008), J. Sol–Gel Sci. Technol., 46, 152. [22] X. Duan, D. Yuan, X. Wang, H. Xu, (2005), J. Sol–Gel Sci. Technol., 35, 221. [23] A.R. Phani, M. Passacantando, S. Santucci, (2001), Mater. Chem. Phys., 68, 66. [24] Y. Wu, J. Du, K.L. Choy, L.L. Hench, J. Guo, (2005), Thin. Solid. Films, 472, 150. [25] M. Zawadzki, (2006), Solid. State. Sci., 8, 14. [26] C. Ragupathi, J. Judith Vijaya, S. Narayanan, L. John Kennedy, Seeram Ramakrishna.,(2013), Chinese Journal of Catalysis, 34, 10, 1951– 1958. [27] C. Ragupathi, J. Judith Vijaya, A. Manikandan, and L. John Kennedy., (2013), Journal of Nanoscience and Nanotechnology, Vol. 13, 8298–8306. [28] Wenjuan Qu, Pingping Li, Jihua Hong, Zhiling Liu, Yufang Chen, Andrew P Breksa III and Zhongli Pan, (2013), J Sci Food Agric, 2014, 94, 1005–1012.
ScienceDirect Materials Today: Proceedings 4 (2017) 10664–10671
www.materialstoday.com/proceedings
AMMMT 2016
Synthesis and characterization of spinel metal aluminate by a simple microwave assisted green synthesis R.Yuvasravanaab, P.P.Georgea *, N.Devannab. 0F
a b
Department of chemistry, Madanapalle Institute of Technology and Science, Madanapalle-517325, Andhra Pradesh, India.
Department of chemistry, Jawaharlal Nehru Technological University Anantapur, Ananthapuram – 515002, Andhra Pradesh, India.
Abstract In the current research report we present a simple, eco-friendly, cost and time saving biosynthesis of ZnAl2O4 spinel nano and micro particles using Pomegranate peel extract, that act as the reducing as well as stabilizing agent. Thus synthesized products are then consistently characterized under powder X-ray diffraction (PXRD), scanning electron microscopy (SEM). The powder XRD analysis has confirmed their chemical composition. Optical properties including diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and fourier transform infrared spectroscopy (FTIR) are also investigated. The ZnAl2O4 nanoparticles formation is been confirmed by PXRD, FT-IR and SEM. From XRD the calculated crystalline size of the ZnAl2O4 nanoparticles is in the range of 11.2 nm. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016). Keywords: ZnAl2O4, Pomegranate fruit peel extract, Microwave assistance, powder XRD, Optical properties;
1.
Introduction:
Nanocrystalline spinels are a class of binary transition metal oxide, exhibits interesting mechanical, magnetic, optical and thermal properties compared to that of their bulk materials [1]. Zinc aluminate (ZnAl2O4) is a well-known spinel oxide, and has attracted considerable attention in various interdisciplinary areas. Zinc aluminate is have typical applications in ceramic industries, optical and sensing devises, as catalyst and catalyst supports in manufacture industries, as paints in aerospace industry, dielectrics and electronic applications and these applications * Corresponding author. Tel.: +00919000451689; E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016).
R.Yuvasravana, P.P.George, N.Devanna / Materials Today: Proceedings 4 (2017) 10664–10671
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of Zinc aluminate is because of its unique properties, mechanical, thermal and chemical stability, low surface acidity, excellent optical transparency, wide band-gap energy and high quantum yields [2, 3]. ZnAl2O4 is used in optoelectronic devices operated in the ultraviolet region, photo electronic devices, because of its wide-band gap of 3.8 eV and also used as optical and thermal controlled coating for spacecraft [4]. ZnAl2O4 spinels are good enough for a range of catalytic applications as heterogeneous catalyst in the reactions like cracking, dehydration, hydrogenation and dehydrogenation of organic compounds [5] and they are also used as catalyst support because of less surface acidity, chemical inactiveness and thermal durability [6]. In recent years, ZnAl2O4 is also employed for dehydrogenation of chemicals in chemical and petrochemical industries (Shu Fen Wang et al 2005), degrading the gaseous toluene [7], restoring of ethanol steam [8], trans-esterification of oils of vegetable [9] and iso-butane combustion [10-11]. Several synthesis approaches are reported so far, for the preparation of ZnAl2O4 such as ceramic method, mechano-chemical synthesis [12], solid-state reaction synthesis [13], citrate-gel method [14], hydrothermal [15–18], wet chemical method [19], co-precipitation method [20], sol–gel method [21–24], and microwave assisted-hydrothermal method [25]. In green synthesis method various plant material extracts are used in the preparation of spinal nanoparticles, where these extract of plant materials act as both reducing and stabilizing agent. Opuntia dilenii [26], Sesamum indicum [27] are the two plant extracts explored for the synthesis of ZnAl2O4 nanoparticles. Although there is a good progress in exploring nano sized aluminate spinels, some issues like the preparation route and optical properties of ZnAl2O4 nanoparticles still challenging and needed further study. 2.
Characterization:
Zinc aluminate nanoparticles synthesized through green synthesis route are annealed in muffle furnace at 700 oC for about 3 hours and then characterized by powder-XRD, SEM, FTIR and UV-Vis spectroscopy for its physical and optical properties. The powder-X ray diffraction patterns for the sample is recorded under the Bruker D8 diffractometer instrument with 2θ value ranging from 10o to 80o. The surface morphologies of the metal aluminate is studied under scanning electron microscope (SEM), using a Ziess-SEM instrument, before that the gold sputtering device is used to coat the sample with gold for the better visibility. FT-IR spectra of sample is recorded under Jasco FT/IR-4200 infrared spectrophotometer instrument in transmittance mode and the wavelength ranging from 400 to 4000 nm, the sample is mixed with heat dehydrated potassium bromide powder and made into pellet for the analysis. The optical measurements for the sample is recorded under Jasco UV-670 spectrophotometer instrument for UV-Vis diffuse reflectance spectra with the wavelength range from 200 to 900 nm, at room temperature. And Jasco FP-6300 spectrofluorometer instrument is used to record the photoluminescence measurements of sample at room temperature. 3.
Results and Discussion:
The reaction mechanism of spinel preparation can be described as follows and the schematic synthesis procedure is illustrated in Figure-1. below, in mechanism nitrogen dioxide and oxygen is liberated from nitrate salts and carbon dioxide and water vapour from peel extract solution. Zn(NO3)2+ 2Al(NO3)3+ P-Extract → ZnAl2O4+ NO2↑+ 2O2↑ + CO2↑ + H2O↑. 3.1. Powder X-ray Diffraction (PXRD): The powder X-ray diffraction patterns for the ZnAl2O4 sample is recorded under the Bruker D8 diffractometer instrument with 2θ value ranging from 10o to 80o. The PXRD measurements are taken to analyze the crystal purity and phase recognition. The PXRD measurements obtained for the spinel sample is presented in Figure-2(a). The diffraction pattern for ZnAl2O4 are observed at 2θ values at 31.4°, 36.9°, 45.5°, 55.8°, 59.5° and 65.4°, that can be correlated to (220), (311), (400), (422), (511), and (440) planes respectively, and these planes are associated with the cubic phase of ZnAl2O4 with JCPDS card No. 73-1961 (space group Fd3m). The peaks of any other phase are not detected, that suggests the complete development of ZnAl2O4 spinel under applied experimental condition in the present work and no other phases or elements are identified in the sample.
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R.Yuvasravana, P.P.George, N.Devanna / Materials Today: Proceedings 4 (2017) 10664–10671
The diffraction pattern had sharp peaks, indicating the good crystallinity and purity of the spinel sample. The average particle size of thus synthesised spinel samples is calculated with the help of Debye Scherrer formula, as demonstrated below
Where L is the average crystal particle size (Ao), λ is the wavelength of X-ray used (0.154 nm), θ is the diffraction angle, and β the peak position and the FWHM (full-width at half maximum). The average particle size is found to be 11.2 nm.
Figure-1: Schematic representation of synthesis of ZnAl2O4 spinels.
3.2. Scanning electron microscopy (SEM) studies: The SEM images provide illustration of surface morphology of the ZnAl2O4 spinels, and are shown in Figure-2(b). It can be seen from SEM images that the presence of nearly spherical like agglomerated nanoparticles that have the diameter range from 0.1 to 0.6 µm. The nano and micro particles are evenly scattered in the form of small particles and large agglomerates having nearly spherical like structure. The agglomerated structures of these samples are due to the solid state welding of fine particles that may be formed at the time of annealing at 700 oC. The size of agglomerated ZnAl2O4 nanoparticles prepared in this work are comparatively similar to that of the ZnAl2O4 nanoparticles synthesized by microwave method using Sesamum (Sesamum indicum L.) extract by C. Ragupathi and his co-workers [27]. C. Ragupathi’s procedure involves microwave heating for 10 minutes using Sesamum (Sesamum indicum L.) extract. Whereas the current microwave assistance method involves the use of pomegranate fruit peel extract and are irradiated under microwaves for five minutes. Precursor solution containing fruit peel extract and metal nitrates react to form complex matrix on microwave irradiation, followed with the formation of aluminate samples at the end, this confirms that addition of fruit peel extract may have influenced the size of nanoparticles, as we know that the structure of the samples changes accordingly with respect to the synthesis procedure. Metal aluminates are excellent microwave adsorptive materials, hence can be readily heated to the higher temperatures with in short time. And hence the microwave application in the field of nanoparticles synthesis is more advantageous and significant in the reduction of time of synthesis, growth and morphology of nanoparticle, at the specified stable reaction conditions. The estimated particle size from XRD is observed to be slightly different from that of SEM images, that can be explained by structural disorder resulted due to the lattice strain caused by different ionic radii or clustering of nanoparticles.
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Figure-2: (a) XRD of ZnAl2O4 spinels. (b) SEM image of ZnAl2O4 spinels.
3.3. Fourier transform infrared (FT-IR): The FT-IR spectrum of ZnAl2O4 is shown in Figure-3 was recorded in the frequency range from 400 – 4000 cm−1, respectively. In the spectra, the broad bands near 3432 and 1626 cm−1 are referred to the –OH stretching and deformative vibrations of water molecules, the water molecules may be absorbed by sample from atmosphere at the time of sample preparation in the presence of atmosphere. The peaks at 1411 cm−1 can be attributed to the Al–O stretching vibrations. In the spectra, the metal–oxygen stretching frequencies are related to the peaks that appear in the range 400–1000 cm−1 and are assigned to the vibrations of Zn–O, Al–O and Zn–O–Al bonds. From figure 3 FTIR spectra of ZnAl2O4 exhibits two sharp peaks centered at 599 and 443 cm−1 respectively and they are related to the metal-oxygen stretching and bending frequencies like Al–O–Zn, Al–O and Zn-O stretching and bending vibration. C. Ragupathi and his co-workers reported the similar IR spectra of ZnAl2O4 with bands observed at 650 and 490 cm−1 for ZnAl2O4 [27]. The characteristic absorption band at 924 cm−1 represented by IR spectra of ZnAl2O4 of C. Ragupathi’s work can also be seen at 1025 cm−1 in the IR spectra in the current work. The peaks at 2922 and 2855 cm−1 in the IR spectra are may be due to the presence of CO32- ion. The existence of CO2 in the sample can be explained by two reasons that are, mixing of aerial CO2 that occurred at the time of sample preparation and the presence of CO2 inside the powders sample that may be formed due to annealing of plant material.
Figure-3: FTIR of ZnAl2O4 spinels.
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3.4. Optical properties: Zinc aluminate [ZnAl2O4] nanoparticles are widely used for the photo electronic devices because of its large band gap [27]. The optical study for Zinc aluminates was performed using a Jasco UV-670, a UV-VIS-NIR diffuse reflectance spectrophotometer and the reflectance spectra are recorded at room temperature. Metal aluminates exhibit colour due to d-d electron transitions, where as ZnAl2O4 a normal spinal is colourless compound that is because Zn+2 ion is tetrahedrally coordinated and have completely filled d-orbitals. Figure-4(a) shows the typical UV–Vis absorbance spectrum of ZnAl2O4 nanoparticles and shows UV-absorption bands peaked at 218, 262, 304 and 370 nm respectively. UV–Vis reflectance spectrum is recorded to estimate the optical band gap of ZnAl2O4 nanoparticles and is shown in Figure-4(b). The optical band gap for a semiconductor is estimate by using the following equation: α(ν) = A(ħν/2 – Eg)m/2, The band gap is determined by relating diffuse-reflectance (R) of the samples with the Kubelka–Munk function F(R) in the equation F(R) = (1-R)2 /2R and the energy intercept plot of (F(R)*hν)2 versus hν is plotted which yields the Eg, dir for a direct-allowed transition, when the linear regions are extrapolated to the zero ordinate, one can get the band gap of the sample. Hence by this method, the band gap is calculated for ZnAl2O4 nanoparticles and the band gap is found to be 4.1 eV for ZnAl2O4 and it is shown in Figure 5(a). The decrease in nanoparticles size leads to the increase in optical band gap, because of the quantum confinement the decrease in crystal size, band gap energy exhibits an increasing trend. Thus the results from UV–Vis reflectance spectra are in good agreement with that of XRD results.
Figure-4: (a) UV-Vis absorbance spectrum (b) UV-Vis reflectance spectrum.
The oxygen vacancy and defect related transitions are can be determined by using Photoluminescence (PL) spectrum. The surface states and quantity of defects can be varied with reference to the crystallite size and morphology and synthesis conditions [27]. The PL spectra of ZnAl2O4 nanoparticles synthesised by microwave assistance method are shown in Figure-5(b). The emission spectra of ZnAl2O4 nanoparticles are recorded in wavelength ranges from 220 to 600 nm by exiting them at 210 nm wavelength. The PL emission spectra of neat ZnAl2O4 nanoparticles, demonstrate the emission bands peaked at three wavelength regions 335 nm, 422 nm and 472 nm respectively. The PL emission bands for the ZnAl2O4 nanoparticles, synthesised by microwave reduction using sesamum plant extract showed the broad emission bands at 492, 530 and 545 nm [27]. Whereas the ZnAl2O4 nanoparticles synthesised by microwave assistance using pomegranate fruit peel extract showed the broad emission bands at 335, 422 and 472 nm, this blue shift is due to the quantum size effect, and a slight difference in the average crystallite size and intra-band gap errors like oxygen vacancies because of the use of pomegranate fruit peel extract in the synthesis procedure. A broad UV band and weak visible bands in the region confirmed the good crystallinity of the as synthesised spinel sample by microwave assistance method.
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Fruit peel extract of pomegranate consists of polyphenols such as gallic acid, punicalagin and ellagic acid as reported by Wenjuan Qu et.al. [28]. The polyphenol molecules exhibit antioxidant property, similar molecular mechanism can be anticipated to be responsible for the formation of nano ZnAl2O4 spinels. These fruit peels are naturally available, non-polluting and non-toxic, thus fruit peel extract as active ingredient, enables synthesis of nanoparticles at low cost. In microwave assistance, precursor materials are thermally decomposed with the help of microwave radiation, to synthesis the spinel nanoparticle, thus it is economical, rapid, and accessible nowadays. In the current study, the microwave-assisted green synthesis technique, metal nitrates are dissolved and mixed well with fruit peel extract and the solution is placed in household microwave for combustion process. Microwave radiation, dehydrates and decomposes the solution into flake like residue, washed and annealed to give ZnAl2O4 nano spinals. Fruit peel material is completely decomposed and removed in the form of carbon related by-products, either in microwave or at the annealing process, resulting in pure and 85% yield of zinc aluminates nano spinels.
Figure-5: (a) Band gap energy plot of ZnAl2O4 spinels. (b) PL spectra of ZnAl2O4 spinels.
4.
Conclusions:
In this work ZnAl2O4 nanoparticles are synthesised under microwave assistance by using pomegranate fruit peel extract as reducing and stabilising agent and obtained anticipated results. At room temperature the pomegranate fruit peel extract promotes the formation of nanoparticles with a fast kinetics and no other harmful chemicals are used except that of metal nitrate salts and fruit peel extract. This microwave assistance method is more reliable, can be performed in simple and easy steps and gets the results within a short time. The formation of ZnAl2O4 nanoparticles is been supported by PXRD, FTIR and SEM and the crystalline sizes of ZnAl2O4 nanoparticles is in the range of 11.2 nm, as calculated from PXRD measurements. The band gap values calculated from reflectance spectra of UV-VIS-NIR diffuse reflectance spectrophotometer, ranges between 3.9 – 4.0 eV which explains the nano dimensions of ZnAl2O4 nanoparticles. In microwave combustion process, with in short duration of time the reaction mixture in the oven gets heated to higher temperature spontaneously by takes energy from the microwave field. Hence by, this work presents a smooth and comfortable method for the preparation of bimetal oxide spinels like zinc aluminates with a promising and good crystalline structure. This fabrication procedure can be further applied in the preparation of other bimetal oxides.
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Experimental:
5.1. Materials: Zinc and aluminium nitrates are the precursor material, (97% purity Merck chemicals, India) and are used as obtained without any further refinement. The pomegranate (Punica granatum.) fruits were collected from local fruit market, Madanapalle, Andhrapradesh. 5.2. Preparation of the Plant Extract: The pomegranate (Punica granatum.) fresh peels are washed thoroughly with distilled water to remove unwanted dust and they are dried in shade to make sure that they look fresh at the time of extract preparation. 20g of pomegranate fruit peel was weighed, cut into slices and added to 100 ml distilled water in a 250 ml Erlenmeyer flask and boiled on a gas burner for 15 minutes, cooled and filtered using filter paper, the filtrate thus obtained is used as aqueous fruit peel extract for synthesis of nanoparticles. 5.3. Synthesis of Metal Aluminates: Stoichiometric mass proportions of Zinc nitrate and Aluminium nitrates in 1:2 ratio are dissolved in 50 ml distilled water in 250 ml beakers. Then 5 ml of fruit peel extract was added to the nitrate solution, and stirred for 15 minutes using a magnetic stirrer. The solution is then irradiated under microwave radiation using a house hold microwave oven for about 5 minutes, with break at every 60 sec to avoid wastage of the solution by spilling. Initially solution underwent boiling, dehydration and decomposition by releasing gases leaving dry flakes like solid behind. This dry solid is washed several times with water and ethanol for its purity and annealed in muffle furnace at 700 oC for about 3 hours. Acknowledgements: Authors thanks the Management, Madanapalle Institute of Technology and Science, Angallu, Madanapalle, Andhra Pradesh, for encouraging to complete this research work. Dr. P. P. George is thankful to the UGC Major Research project (UGC-MRP- Major Chem. - 2013- 25694). R. Yuvasravana is thankful to VIT, Vellore, for PXRD measurements, SV University, Tirupathi, for SEM measurements, BITS-Pilani, Hyderabad campus for FTIR, UV-Vis and PL spectroscopy respectively. References [1]
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