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CHAPTER III

SYNTHESIS AND CHARACTERIZATION OF CuO AND MgO

NANOPARTICLES

3.1 INTRODUCTION

Nanoparticles are generally prepared by wet chemical, mechanical, form-in-

place and gas phase synthesis (Pitkethy 2004) methods. Wet chemical synthesis

route includes sol-gel process, hydrothermal techniques and precipitation methods.

Mechanical preparation techniques include grinding, ball milling and mechanical

alloying. Form-in-place methods include lithography, vacuum deposition

techniques like CVD, PVD and spray coatings. Gas phase synthesis includes flame

pyrolysis, electro-explosion, laser ablation, high temperature evaporation and

plasma synthesis. Several literatures reported various synthetic approaches which

includes sol-gel (Bahnemann et al. 1987), template-free method, microemulsion,

molecular beam epitaxy, thermal decomposition of organic precursor, ultrasonic,

microwave-assisted techniques, solochemical method (Vaezi and Sadrnezhaad

2007), sonochemical preparation, hydrothermal, solvothermal (Hui et al. 2004,

Zhang et al. 2002, Li et al. 2001) and precipitation methods (Zhong, Matijevic 1996

and Lingna, Mamoun 1999). Compared to most other methods, wet chemical

synthesis has been widely adopted due to its simple, cost-effective, versatile and

highly effective for large scale production (Das and Khushalani 2010). This method

offer low temperature growth and scale-up fabrication (Cheng et al. 2006), which

yields colloidal solutions with wide range of particle distributions having improved

reproducibility. Metal oxide nanoparticles are commonly prepared by chemical

synthesis methods.

In this experimental work, chemical precipitation method was adopted to

synthesize Copper Oxide (CuO) and Magnesium Oxide (MgO) nanoparticles.

3.2 MATERIALS AND METHODS

The precursor materials copper (II) chloride (CuCl2•2H2O), Magnesium

chloride [MgCl2.H2O] and sodium hydroxide (NaOH), were purchased from Merck

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and Aldrich. The chemicals used in the experiment al work were of analytical

reagent grade and used without further purification.

3.2.1 Synthesis of CuO Nanoparticles

CuO nanoparticles were synthesized using chemical precipitation method. In

a typical synthesis, 0.5 M of copper (II) chloride (CuCl2•2H2O) was dissolved in

100 ml deionized water (H2O), which was stirred with magnetic stirrer about 30

minutes until the CuCl2 dissolved completely.

CuCl2.2H2O + 2NaOH.H2O → Cu(OH)2 + 2NaCl+ 4H2O

Cu(OH)2 → CuO + H2O↑

Then, sodium hydroxide (NaOH) solution was added drop-wise into the

CuCl2 solution, under constant stirring.

`

Figure 3.1 Preparation processes of CuO nanoparticles

Continuous stirring for 30 minutes

Drying at 70C and Calcination

at 350C

Repeated washing with

double distilled water

Formation of Cu(OH)2 precipitate

0.5 M of CuCl2 solution 0.5 M of NaOH solution

Formation of CuO nanoparticles

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The reaction mixture forms bluish green gel and changed into dark brown

Copper (II) hydroxide (Cu(OH)2) completely. The precipitate of Cu(OH)2 was

washed with distilled water several times and finally centrifuged to remove the

native impurities in the product. Cu(OH)2 precipitate was calcined at 350°C for 2

hours in air to ensure that Cu(OH)2 was turned into CuO completely

(Gopalakrishnan et al. 2014). The schematic representation of CuO nanoparticles

preparation is shown in figure 3.1.

3.2.2 Synthesis of MgO Nanoparticles

In a typical procedure for the preparation of MgO nanoparticles, 0.1 M of

Magnesium chloride (MgCl2) was dissolved in 100 ml de-ionized water and 0.2 M

of aqueous NaOH solution was added dropwise into the above solution under

constant stirring.

MgCl2. H2O + 2NaOH.H2O → Mg(OH)2 + 2NaCl+ 3H2O

Mg(OH)2 → MgO + H2O↑

Figure 3.2 Preparation processes of MgO nanoparticles

Continuous stirring for 30 minutes

Drying at 80C and Calcination

at 500C

Repeated washing with

double distilled water

Mg(OH)2 precipitate formed

0.5 M of MgCl2 solution 0.5 M of NaOH solution

Formation of MgO nanoparticles

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The molar ratio of metal ions to hydroxide ions was maintained at 1:2. The

resultant Mg(OH)2 precipitates were dried at 80oC for 12 h and calcined at 500

oC

for 3 hours in air atmosphere to obtain MgO nanoparticles. The schematic

representation of MgO nanoparticles preparation is shown in figure 3.2.

3.3 CHARACTERIZATION OF CuO AND MgO NANOPARTICLES

3.3.1 Structural Studies

3.3.1.1 X-Ray Diffractometer

X-ray diffraction is one of the most important characterization tools used in

solid state chemistry and materials science. As a non-destructive testing technique,

X-ray diffraction is a powerful tool for the analysis of crystalline structure. The

wavelength of X-ray is comparable to the crystalline lattice constants, hence it can

be used for the accurate measurement of lattice parameter, crystallite size, lattice

strain etc. Each crystalline solid has unique XRD pattern to identify its crystal

structure. Since, the wavelength of X-rays is also comparable to the size of atoms,

they are ideally employed for analyzing the structural arrangement of atoms and

molecules in wide range of materials.

Figure 3.3 Shimadzu XRD-6000 X-ray diffraction unit

3.3.1.2 Bragg's Law

An X-ray which reflects from the surface of a substance has travelled less

distance than an X-ray which reflects from a plane of atoms inside the crystal. The

penetrating X-ray travels down to the internal layer, reflects, and travels back over

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the same distance before being back at the surface. The distance travelled depends

on the separation of the layers and the angle at which the X-ray entered the material.

For this wave to be in phase with the wave which reflected from the surface it needs

to have travelled a whole number of wavelengths while inside the material. The

diffraction peaks are related to the atomic distances. If many atoms are scattering

the X-rays together, scattered waves from all the atoms interface with each other.

Figure 3.4 Scheme of the X-ray diffraction geometry

These directions are governed by the wavelength (λ) of incident X-rays and

the nature of crystalline sample. Bragg‘s law, formulated by W. L. Bragg in 1913,

relates the wavelength of the X-rays to the spacing of the atomic planes (dhkl) as

condition is satisfied:

nλ = 2d sin θ (3.1)

where‘d’ is the lattice spacing of the crystal and ‘θ’ is the angle of incidence, n is

order of diffraction and λ is the wavelength of X-ray.

Although, Bragg's law is used to explain the interference pattern of X-rays

scattered by crystals, diffraction has been developed to study the structure of all

states of matter with any beam, e.g., ions, electrons, neutrons and protons, with a

wavelength similar to the distance between the atomic or molecular structures of

interest. Furthermore, a powder XRD pattern is also used to determine the average

size of the nanoparticles. The particle D’ is particle diameter size can be calculated

by using the Debye-Scherrer formula:

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D =0.9λ/β cos θ (3.2)

Where λ is the wavelength of X-ray and β is the Full Width at Half Maximum

(FWHM) (in radians) of the peak at 2θ.

3.3.1.3 XRD analysis

The structural characterization of powder samples was performed using

standard X-ray diffractometer (Shimadzu XRD-6000) with monochromatic Cu Kα

(λ = 1.542 Å) radiation operated at 40 kV and 20 mA in the range of 10° to 80°

which is shown in figure 3.3. The wavelength of X-rays (0.1 to 1 Å) is equal to the

interatomic distance in crystals. Control, acquisition and preliminary analysis of the

data are performed by the Shimadzu X’pert pro software. X-ray diffraction can be

observed when X-rays interact with crystalline materials. The peaks of the X-ray

diffraction pattern are compared with the available standard JCPDS data to confirm

the crystal structure.

Figure 3.5 XRD spectrums of copper oxide and Magnesium oxide nanopowder

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The XRD spectra (figure 3.5.) show well-defined peaks of CuO and all

diffraction peaks of the obtained product are consistent with the standard value

which corresponds well with the monoclinic phase structure. The diffraction peaks

at 35°, 38° and 48° corresponds to (-1 1 1), (1 1 1) and (-2 0 2) planes, which are in

good agreement with the “JCPDS” (Joint Committee on Powder Diffraction

Standards) card No.80-1916. The nanoparticles obtained via current synthesis

method consist of good quality pure CuO phases. The mean crystalline size is

calculated from the full-width at half-maximum (FWHM) of XRD lines by using

the Debye-Scherrer relation. The average crystallite diameter of the CuO

nanoparticles is around 15 nm. The sharp and intense peaks in the XRD pattern of

the product indicate good crystallinity of CuO nanoparticles (Gopalakrishnan et al.

2014). A typical X-ray diffraction pattern obtained for copper oxide and magnesium

oxide nanoparticles is shown in figure 3.5.

X-ray diffraction pattern shows that MgO nanoparticles are polycrystalline in

nature and the crystal structure is identified to be cubic structure which matches

well with the standard JCPDS card no. 89-4248. The crystalline size along (2 0 0)

plane is estimated to be 22 nm using Debye-Scherrer’s formula Holzwarth et al.

(2011), which indicates the nanocrystalline nature of the particles.

3.3.2 Morphological studies

3.3.2.1 Scanning Electron Microscope (SEM)

Scanning electron microscope (SEM) is one of the most versatile techniques

available for the observation of surface morphology and structural characteristics of

solid particles. SEM uses a focused beam of high-energy electrons to generate a

variety of signals at the surface of solid specimens. The signals that derive from

electron sample interactions reveal information about the sample including external

morphology chemical composition, crystalline structure and orientation of materials

making up the sample. In most applications, data are collected over a selected area

of the surface of the sample and 2-dimensional image is generated that displays

spatial variations in these properties.

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The basic phenomenon of scanning electron microscope is that electron

beams are generated by an appropriate source, particularly a field emission gun or

tungsten filament. The electron beam is moved by higher voltage and pass through

an electromagnetic lenses and an electro optical system of apertures and to generate

thin beam of electrons. This electron beam scans the surface of the specimen by

means of scan coils. The emitted electrons from the sample due to the scanning

beam are then collected by a detector. The image of the specimen can be viewed

through the screen.

Figure 3.6 Scanning electron microscope JEOL JSM 6390 and Schematic

representation of SEM

The electron column consists of an electron gun and two or more electron

lenses, operated in high vacuum. In common, an electron beam is generated at the

specimen surface with a spot size less than 10 nm (100 A) still carrying sufficient

current to form an acceptable image. The electron gun present in the electron

column produces source of electrons and make the electrons to accelerate with

energy range of 1-40 keV. The electron gun produces stable electron beam with

high current, small spot size, adjustable energy and small energy dispersion. There

are several types of electron guns such as tungsten thermionic, lanthanum

hexaboride (LaB6) cathode Guns and field emission sources used in SEM system.

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In order to reduce the diameter of source of electrons and to place a small, focused

electron beam on the specimen, electron lenses are used. These electron lenses are

employed to de-magnify the image of the crossover in the electron gun (d0 ~ 10-50

m) to the final spot size on the specimen (1 nm to1 m). Micrographs are taken at

suitable accelerating voltages for the best possible resolution using the secondary

electron imaging.

3.3.2.2 SEM Analysis

The Scanning electron microscopic images of Copper oxide and magnesium

oxide nanoparticles are taken using JEOL JSM-6390. The nanoparticles appear

translucent to the incident electron beams. Hence, in the SEM images shown in

figures 3.7 (a) and (b), the precipitated CuO nanoparticles appear translucent.

Figure 3.7 (a) and (b) SEM images of copper oxide nanorods with different

magnifications.

Figure 3.8 (a) and (b) SEM images of magnesium oxide nanoparticles with

different magnifications.

(a)

(a) (b)

(b)

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The SEM images show needle or rod like structure and hence TEM

characterization was taken. The nominal diameter of the nanoparticles at the

beginning of the experiment is μm. From the SEM images of MgO nanoparticles

shown in figures 3.8 (a) and (b), it was observed that the nanoparticles are smaller

in size than in the stock solution probably due to attrition and abrasion.

3.3.2.3 Transmission Electron Microscope(TEM)

One of the most powerful tools for the determination of particle size and

morphology is Transmission Electron Microscopy (TEM). TEM is a microscopic

technique in which electron beams are transmitted through an ultra thin specimen.

TEM micrograph reveals detailed micro structural examination such as

morphology, size, shape and arrangement of the particles through high resolution

and high magnification imaging of the specimen on the scale of atomic diameters.

TEM has the advantage of developing real space images for direct

observation. This technique gives particle size, crystallite size and can provide

details of size distribution. In many cases, aggregates of smaller particles can be

discerned. TEM enables one to see things as small as the order of a few angstroms.

If the nanoparticles consist of more than one phase and the phases provide enough

contrast, then the individual phases may also be visible. TEM operates on the same

basic principles as the light microscope but uses electrons as source instead of light.

Transmission electron microscopy uses high energy electrons (up to 300 kV

accelerating voltage) which are accelerated to nearly the speed of light. The electron

beam with wavelength about million times shorter than light waves behaves like a

wave front. The electron beam is accelerated to energy in the range 20 - 1000 keV

in the electron gun source to make them passes though set of condenser lenses with

desired diameter. The heart of the TEM equipment is the objective lens and the

specimen stage system. The interactions between beam and specimen occurs in this

stage and the two fundamental TEM operations, such as the creation of images and

diffraction patterns that are consequently magnified for viewing and image

recording is made possible.

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Figure 3.9 Transmission electron microscopy JEOL JEM 1230 and Schematic

representation of TEM

Fluorescent screen is employed in the TEM system, in which emitted light

when impacted by the transmitted electrons, for real time imaging and adjustments

and a film camera to record permanent high resolution images. The imaging system

converts the radiation into permanent image which can be viewed. The image

capture is made possible with solid-state imaging devices (charge coupled device)

in the recently developed TEM instruments. In this experimental work, transmission

electron microscope (TEM) observations were performed using JEOL JEM-1230

electron microscope at an accelerating voltage of 120 kV to record the images .

3.3.2.4 TEM Analysis

The structures and shapes are preserved even after the CuO structure is

dispersed in ethanol by ultrasonic vibration for 30 minutes before being deposited

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on a carbon-coated copper grid for the TEM observations. Figure 3.10 (a), (b) and

(c) show TEM images of CuO nanorods with different magnifications. A relatively

straight rod-like shape of smooth surfaces is clearly displayed in Figure 3.10 (c) and

(d) confirm the presence of nanorods with relatively uniform diameters. The CuO

nanorods are randomly oriented and this is a trade-off for not using templates for

preparation.

Figure 3.10 (a), (b), (c) and (d) TEM images of copper oxide nanorods

An examination of single nanorod view of Fig. 3.10 (d) shows the average

length of 80 to150nm with the diameter of 12 to17 nm (Gopalakrishnan et al. 2014).

A relatively straight rod-like shape of large amount and smooth surfaces was clearly

displayed in Figure 3.10 (a) (b) and (c).

(a) (b)

(c) (d)

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Figure 3.11 (a) and (b) TEM images of magnesium oxide nanoparticles

The TEM images of typical MgO nanoparticles are shown in figure 3.11(a)

and (b). The MgO nanoparticles are found as several spherical shaped grains and

are agglomerated to form bigger grains as observed in the TEM image. The MgO

nanoparticles are found as several spherical shaped grains which is having less than

100 nm size as observed in the TEM images.

(a) (b)