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Microwave-assisted synthesis and characterization of CaO nanoparticles
Article in International Journal of Nanoscience · September 2012
DOI: 10.1142/S0219581X12500275
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MICROWAVE-ASSISTED SYNTHESIS AND CHARACTERIZATION
OF CaO NANOPARTICLES
ARUP ROY*and JAYANTA BHATTACHARYAy Department of Mining Engineering
Indian Institute of Technology Kharagpur 721302, India
Received 23 April 2010 Accepted 5 August 2010
Calcium oxide (CaO) is an important inorganic compound, which is used across various industries as catalyst, toxic-waste remediation agent, adsorbent, etc. CaO nanoparticles were obtained by the microwave irradiation technique, usingCaðNO3Þ24H2Oand NaOH as starting materials. The formation of monocrystalline CaO nanoparticles was con¯rmed by the XRD (X-ray di®raction) and TEM (transmission electron microscopy) as well as by SAED (selected area electron di®raction) analysis. The structure of the CaO nanocrystal was found to be cubic with particle size, 24 nm and surface area, 74 m2=g.
Keywords: Microwave technique; nanoparticles; CaO; transmission electron microscopy;
monocrystalline.
1. Introduction
Calcium oxide (CaO) is an exceptionally important industrial compound, which is used as catalyst, toxic-waste remediation agent, an additive in refractory, in paint as well as for other fundamental applications.1Ultra¯ne metal oxide particles can be used as bactericide and adsorbent. Particularly CaO has also shown great promise as a destructive adsorbent for toxic chemical agent.2
Few literature are found on the preparation of nano-CaO. There are mainly two methods on the preparation of nano-CaO according to the litera- ture. One is thermal decomposition.3,4 The other is solgel.1CaO nanoparticles can be obtained up to about 14 nm size through solgel method, the cost
of which is very high. What is more, the process is very complicated and time-consuming. So it is very di±cult to apply solgel method in industries.
Thermal decomposition method has some advan- tages such as simple process, low cost, ease of obtaining high purity product, etc. So it is a promising and prospective method to be applied into industry. In thermal decomposition method, CaO is often obtained directly through calcining CaCO3. High calcination temperature is needed. By this process, it is very di±cult to get nanoscale CaO, but getting micrometer CaO (above 100 nm), directly through calciningCaCO3is easily possible.5 The microwave-assisted route is yet another method for the synthesis of metal oxides and has
†Correspondening author.
Vol. 10, No. 3 (2011) 413418
#.c World Scienti¯c Publishing Company DOI:10.1142/S0219581X11008150
413
been gaining signi¯cance in the synthesis of oxide nanomaterials.6,7 Microwaves have been used to accelerate organic chemical reactions for some time, because the method is generally fast, simple, energy- e±cient, and less time-consuming.8,9Unfortunately, the exact nature of microwave interactions with reactant during the synthesis of materials is some- what unclear and speculative. However, energy transfer from microwaves to the material is believed to occur either through resonance or relaxation, which results in rapid heating. Clark and Sutton10 have reviewed various aspects of microwave applications reported in the literature. Several recent reports have appeared which explain the microwave synthetic route.1118Many microwave preparations reported in the literature have been made on the laboratory scale of only a few grams.1921However, the use of higher power levels for synthesis of ceramics for specialized applications has also been reported.22,23
The purpose of this study is to investigate a simple and rapid synthesis method using calcium nitrate and sodium hydroxide as starting materials for the growth of the CaO nanoparticles. Advan- tages of this method are as follows:
(a) The nanoparticles of CaO is formed by domestic microwave oven.
(b) The preparation time is short.
(c) Less expensive method.
The characterization of the prepared metal oxides was undertaken by employing structural (X-ray dif- fraction (XRD) and spectroscopic (Fourier trans- form infrared (FT-IR)) and surface morphological (scanning electron microscopy (SEM) and trans- mission electron microscopy (TEM)) techniques.
2. Experimental Part 2.1. Materials
CaðNO3Þ24H2O(Marck), sodium hydroxide (Marck), and deionized water were used; other reagents were obtained from local supplies.
2.2. Preparation of CaO nanoparticles In a typical procedure (Fig. 1), 0.5 M CaðNO3Þ2 4H2Oand 0.7 M of NaOH were separately dissolved in 50 mL deionized water and mixed to form 100 mL mixture (solutions).
The mixture was stirred for 10 min at room temperature and it turned into white gel. These were then irradiated by microwave energy using domestic microwave oven having frequency 2.45 GHz (maximum power 800 W) in ambient at- mosphere for 10 min. After microwave processing, the solution was allowed to cool to reach room temperature, naturally. The resulting precipitate was collected by vacuum ¯ltration and washed with deionized water and absolute ethanol, and dried in vacuum at 80C for 1 h.
2.3. Characterizations
The phase analysis and crystal structure of CaO was studied by X-ray di®ractograms (XRD). The XRD pattern was recorded using a di®ractometer of Philips X-pert Pro X-ray di®ractometer (HRXRD) with 0.1789 nm CoKradiation. Particle sizes and morphology were studied with a ¯eld emission scanning electron microscope (FESEM) of Carl Zeiss model Supra-40 (with an accelerated voltage 1020 kV) and a high-resolution transmission elec- tron microscope (HRTEM) of JEOL (JEM-2100), with an operating voltage 200 kV. The average compositional analysis for CaO was studied with in situ EDS in conjunction with the FESEM ima- ging of selective regions. In sample preparation for HRTEM studies, a small portion of CaO was dis- persed in acetone and sonicated for 30 min. Part of this dispersion was dropped over a carbon ¯lm supported by a copper grid and dried in vacuum before imaging. The IR spectra of thin pellets of nanoparticles in the form of powder in a KBr matrix were studied. A NexusTM 870 FT-IR (Thermo Nicolet, USA) spectrophotometer equipped with a deuterated triglycine sulfate thermoelectric cool (DTGS-TEC) detector collected the data over a range of 5004000 cm1.
2.4. Calculation of crystallite size from X-ray di®raction
Detailed knowledge of crystallite size, shape, and strain in a ¯nely divided powder often helps to correlate many physical properties of a system
Fig. 1. Experimental procedure for synthesis of CaO nanocrystals.
414 A. Roy & J. Bhattacharya
undergoing transformation in a solid-state reaction.
X-ray line broadening analysis provides a method of
¯nding bulk average size of coherently di®racting domains and r.m.s. strain. The average crystallite size (D) from X-ray line broadening has been cal- culated using the Scherrer equation.24,25 The instrumental broadening was corrected using quartz as an internal standard:
D¼ 0:9 1=2cos ;
whereis the wavelength of the X-ray beam,1=2is the angular width at the half-maximum intensity, andis the Bragg angle.
2.5. Density evaluation from X-ray data
The X-ray density of the samples has been com- puted from the values of lattice parameters using the formula26
d¼4 M Na3 ;
where 4 represents the number of molecules in a unit cell of a spinel lattice,Mthe molecular weight of the sample,Nthe Avogadro's number, andathe lattice parameter of the sample.
The lattice constant for the structure was calculated using the equation
d¼ a
ðh2þk2þl2Þ1=2 :
3. Results and Discussion 3.1. X-ray di®raction studies
The XRD patterns of the synthesized calcium oxide (CaO) are shown in Fig.2.
Thed-spacing values of the sample matched well with the standard PDF database (JCPDS ¯le 77- 2376). Unit cell parameters were obtained by least- square re¯nement of the powder XRD data. XRD
study revealed that the products are monophasic cubic calcium oxide (CaO) with lattice constant a= 4.801Å (Fm3m space group) having nanosized particles; the values in the parenthesis indicate respective Miller indices. The characteristic peaks were higher in intensity and narrower in spectral width, indicating that the products were of good crystallinity. No peaks corresponding to impurities were detected, showing that the ¯nal are high- quality CaO. The crystallite size, density, and volume of the samples calculated from XRD data are given in Table1.
3.2. Microstructure studies
FESEM micrographs in Fig. 3 have nearly cubic shapes of synthesized CaO nanoparticles with30 nm average particle size on the cross sections.
These micrographs were taken from powder coated with gold on a double-sided carbon tape.
Compositional analysis carried out with EDX ana- lyzer (in conjunction with FESEM) showed no sig- ni¯cant impurity present in the sample (Fig.4). The observed peaks in Fig. 4 except Ca and O could come from carbon tape.
Fig. 2. The XRD pattern of CaO nanoparticles immediately after synthesis.
Table 1. A comparison of typical structural parameters in nanopowder and bulk CaO.
Parameter a Average size (D) (nm) Volume (V) (nm3) Density () (g/cm3) Surface area (m2/g)
Bulk CaO 4.808 — 0.11114 3.34 —
Synthesized CaO 4.801 24 0.11061 3.23 74.46
Structural characterization through HRTEM was rather a direct way that provides visual dem- onstration to estimate particle size exactly. Figure5 shows the typical HRTEM image of the synthesized CaO nanocrystals.
Bright ¯eld images of the sample indicate that the sample was made of dispersive single-crystal particles having cubic shapes. The selected area electron di®raction (SAED) pattern is shown in the
inset of Fig. 5. The SAED pattern recorded on samples of nanoparticles indicates that they were crystalline in nature. The SAED pattern had ¯ve re°ections (111), (200), (220), (311), and (222) at 0.2782 nm, 0.2403 nm, 0.1700 nm, 0.1451 nm, and 0.1389 nm, dhkl values in agreement to the XRD values 0.2771 nm, 0.2400 nm, 0.1697 nm, 0.1447 nm, and 0.1385 nm, respectively. The images showed an abundance of particles whose size distribution was given by the histogram shown in the inset of Fig.6.
The histogram was obtained by analyzing several
Fig. 3. FESEM micrograph of synthesized CaO nanoparticles.
Fig. 4. EDX spectrum of the synthesized CaO particles revealing the presence of calcium and oxygen.
Fig. 5. HRTEM micrographs of synthesized CaðOHÞ2 nano- particles with a selected area di®raction pattern (SAED) in the inset.
Fig. 6. Histogram showing the particle size distribution of synthesized CaO nanoparticles.
416 A. Roy & J. Bhattacharya
frames of similar bright ¯eld images using IMAGE J software.
The particles have an average size of 25 nm, which is in close agreement with an averageD-value 23 nm, determined from the XRD peak broad- ening. Such particles (separated) show an average surface area () of 74m2=g.
3.3. Infrared spectroscopy studies
To verify chemical purity, CaO powders were ana- lyzed by FTIR. The bands due to hydroxyl and carbonate are distinctly displayed in the spectrum (Fig.7).
The strong band at 3643cm1corresponds to the OH bonds from the remaining hydroxide.27Bands at 1417cm1 and 866cm1correspond to the CO bond. The wide and strong bands at around 427cm1 and 553cm1 correspond to the CaO bonds (Table2).
4. Conclusions
Nanostructured CaO with cubic morphology was successfully produced fromCaðNO3Þ2and NaOH by
using a microwave radiation in ambient atmos- phere. XRD, FESEM, EDX, TEM, SAED, and FTIR analyses revealed the presence of face-centered cubic structured CaO nanoparticles with 24 nm sizes.
It is a simple and e±cient method to produce CaO nanoparticles with regular shape, small size, narrow size distribution, and high purity.
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