structural characterization of zno, cuo and fe2o3

Structural Characterization of ZnO, CuO and Fe2O3 Nanoparticles:

Evaluation of Rietveld Refinement

A.S. Sathishkumar1*, K. Arun Balasubramanian2, T. Ramkumar3

1*Department of Mechanical Engineering, Sree Sowdambika College of Engineering and

Technology, Aruppukottai, 626134, India.

2Department of Mechanical Engineering, Sethu Institute of Technology, Kariapatti, India.

3Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and

Technology, Pollachi, 642003, India.

1*[email protected], 2 [email protected], [email protected]

Abstract

The goal of the research is to investigate and compare the effects of three different

nanoparticles such as ZnO, CuO and Fe2O3 on the structural properties of high temperature

applications. For this purpose, the ZnO, CuO and Fe2O3 samples were synthesized using sol–

gel method. The as-synthesized nanoparticles were characterized by SEM, XRD and AFM.

Agreeing to the results of XRD, the Rietveld refinement was carried out to obtain the crystal

structure and purity of synthesis were achieved. The crystallite size of mentioned nanoparticles

was achieved based on Williamson-Hall plot and Debye-Scherrer method. The morphology of

as-synthesized nanoparticles was studied by SEM analysis with the mean particle size. The

topography of the as-synthesized nanoparticles was also evaluated using AFM.

Keywords: ZnO; CuO; Fe2O3; sol–gel; Rietveld refinement; Williamson-Hall plot

1. Introduction

Nanoscale materials have attracted much attention in large areas because of their unique physical,

optical, chemical and magnetic properties and their immense application potential. Zinc Oxide (ZnO)

nanoparticle is an inexpensive, large excitation binding energy of 60 MeV and n-type semiconductor

with a wide bandgap (3.37 eV) and. It has greater potential in the form of thin films, nanowires,

nanorods, optical devices, solar cells and soon [1].

Copper oxide (CuO) was one of the potential p-type semiconductors and gains significant attention

because of its excellent, optical, electrical, physical and magnetic properties. CuO with narrow band

gap (1.2 eV) is widely used in various applications. CuO NPs have been shown to be good catalysts for

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various cross-coupling reactions and conversions of solar energy, gas sensor, field emission of soon [2-

4].

Iron oxide (Fe2O3) was a suitable particle for the broad investigation of the polymorphism and

the magnetic and structural phase transitions of nanoparticles. Iron oxide exists in four polymorphs

(alpha, beta, gamma, epsilon). Gamma and epsilon type are ferromagnetic, beta type Fe2O3 is a

paramagnetic material while alpha type is a titled anti ferromagnetic material [5]. On account of

attractive properties, Fe2O3 nanoparticles have great potential in scientific and industrial applications.

In general, nanoparticles have been produced in a variety of ways such as Co-precipitation method,

microwave assisted methods, spray pyrolysis, Sol-gel method and hydrothermal method. However,

CuO are used in semiconductors, solar energy transformation, and high-tech superconductors

applications and Fe2O3 are used as catalyst in drug industries. In contrasts Sol-gel process is a very

popular bottom-up approach to the synthesize of nanoparticles for its simplicity, low cost,

reproducibility, reliability conditions of synthesis. Some researchers [6-7] have prepared nanoparticles

by microwave-assisted synthesis method. Recently Vijay Kumar et al. [8] acquired Zinc (II) acetate

dihydrate as a precursor and dimethylformamide as a solvent for producing ZnO nanoparticles using

microwave assisted methods and the resulting NPs were used for lighting and dye removal applications.

Nirmala Grace et al. [9] used copper acetate as the precursor and polyethylene glycol (PEG) as a

stabilizer in ethanol medium to make CuO NPs and demonstrated an amperometric sensor to detect

glucose. Many researchers [10-11] synthesized nanoparticles using the co-precipitation method. Nittaya

Tamaekonga et al. [10] synthesized ZnO NPs from Zinc sulphateheptahydrate by the modified

precipitation method and the obtained NPs were characterized. Sadraei R et al. [11] prepared ZnO

nanoparticles from Zinc sulphateheptahydrate and ammonium hydroxide using direct precipitation

method. J.N. Hasnidawani et al. [12] prepared ZnO nanostructure of Zinc acetate dihydrate with the

sol-gel method and achieved the nano size in the range of 81.28 nm to 84.98 nm. S.R.Brintha et al. [13]

synthesized ZnO nanoparticles from Zinc acetate and methanol as precursors by sol-gel and

hydrothermal method. Kayani et al. [14] produced copper oxide nanoparticles from copper nitrate and

acetic acid by a simple sol-gel process.

Many literatures not been attentive to produce ZnO, CuO & Fe2O3 materials using sol-gel

method. But there has also not been any attempt made to compare the structural and morphological

properties of these materials. Moreover, literature is also not very informative about these materials

fabricate through sol-gel method. It was therefore planned to carry out to investigate three different

nanoparticles ZnO, CuO & Fe2O3 were synthesized exploit by sol-gel route technique and also to

analyse the micro-structural, topographical and morphological properties of prepared nanoparticles

were investigated after annealing. Furthermore, the crystalline size of the nanoparticles was determined

by the Debye-Scherer and Williamson-Hall plot (W-H) method.

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2. Synthesis of nanoparticles

2.1 ZnO nano particles

Zinc Oxide was prepared from Zinc acetate dihydrate based on a simple Sol-gel technique. Zinc

acetate dihydrate (2.195 g, 0.01M) was dissolved in 20ml of methanol and the solution was stirred at

room temperature. A clear and transparent sol without precipitation and turbidity was obtained. In

addition, 0.1N (20 ml) NaOH was added and stirred vigorously for 60 mins. to maintain the pH value

7. The resulting sol was kept undisturbed until white precipitates settled [2-4]. The settled precipitate

was collected by filtration and washed with methanol to remove the by-products. Further the precipitate

is transferred to microscope slide and held in an air oven at 105°C for 1 hr time period. After cooling,

the dried powder was collected and transferred to a clean silica crucible. Moreover, the powder was

calcinised in the muffle furnace at 700°C for 3 hrs time period.

2.2 Copper Oxide nano particles

Copper Oxide was prepared from Copper Nitrate based on simple Sol-gel method. Copper

Nitrate (2.41 g, 0.01M) and citric acid (4g) were dissolved in 20 ml and 10 ml of distilled water

respectively. The prepared solutions were mixed and stirred vigorously at the room temperature for 2-

3 minutes. The ammonia solution is added dropwise to raise the pH of the solution at about 7. The

prepared sol was heated to 90°C on a hot plate and stirred until the gel occurred [5]. During the heating,

5ml of Diethanolamine (DEA) are added to the gel to obtain the dark powder. The resulting dark powder

is transferred to a silica crucible and calcinised in a muffle furnace at 700°C for 3 hrs.

2.3 Iron Oxide nano particles

Iron Oxide was prepared from iron nitrate based on the simple Sol-gel method. Iron Nitrate (4g,

0.01M) was dissolved in 50 ml of distilled water. To this solution is added slowly 4g of citric acid and

stirred until the dissolution takes place. The Liquid ammonia solution is added dropwise to increase the

pH of the solution at about 7. The prepared sol was heated on a hot plate at 80°C for 1 hour until gel

formation takes place [6]. The resulting gel is transferred to clean silica crucible and is calcined in a

muffle furnace at 700°C for 3 hrs. The fine reddish-brown ash powder of iron oxide was collected.

2.4 Characterization

The structural properties of as-synthesized nanoparticles including structure and crystallite size

were determined by X-ray diffractometer. The XRD patterns were obtained from PANalytical X’Pert

PRO powder X-ray Diffractometer using CuKα radiation (λ=1.54056Å) at 40kv and 30mA over the range

of 2θ= 20°-80° and the average size of the crystallites was calculated by Debye-Scherrer equation and

Williamson-Hall plot. The surface morphology and particle size of the synthesized powder samples

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were analysed using a SEM VEGA3 TESCAN Model at 10kV. The topography of the powder samples

was examined by the Atomic Force Microscope XE-70, South Korea.

3. Results

3.1 XRD analysis: Phase Identification

3.1.1 ZnO nano particles

Figure 1(a) illustrates the XRD patterns of ZnO nanoparticles. The sharp peaks of XRD indicate

that the nanoparticles as-synthesized are well crystallized. The diffraction peaks at 32.06°, 34.72°,

36.54°, 47.83°, 56.88°, 63.14°, 66.64°, 68.21° and 69.35° are indexed to (100), (002), (101), (102),

(110), (103), (200), (112) and (201) respectively. The peak positions and relative intensities are quite

well matched with the hexagonal phase with the Wurtize structure with space group (P63mc), and unit

cell parameters a=b=0.3220 nm and c=0.5200 nm. Figure 1(b) shows the identified phases between

prepared ZnO and reference pattern ICDD card No.01-075-1526.

Figure 1(a). XRD patterns of ZnO nanoparticles & Figure 1(b). Standard reference

pattern of ZnO

3.1.2 CuO nano particles

Figure 2(a) shows the XRD patterns of CuO nanoparticles. The noticeable peaks of XRD

indicate that the nanoparticles as-synthesized are in crystalline form. The diffraction peaks at 32.55°,

35.58°, 38.75°, 48.84°, 53.50°, 58.36°, 61.58°, 68.09° are indexed to (110), (-111), (111), (-202), (020),

(202), (-113) and (220) respectively. The peak positions and relative intensities are matched well to

Monoclinic crystal system with space group (C2/c), ICDD card No. 01-89-5896 and unit cell parameters

(a)

(b)

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a=0.4683nm, b=0.3424nm and c=0.5129 nm. Figure 2(b) shows that there is an identified phases

between prepared CuO and reference pattern ICDD card No.01-089-5896

Figure 2(a). XRD patterns of CuO nanoparticles & Figure 2(b). Standard

reference pattern of CuO

3.1.3 Fe2O3 nano particles

Figure 3 (a) shows the XRD patterns of nanoparticles of Fe2O3 as present in the synthesis. The

detectable peaks of XRD indicate that the crystalline nature is present. The diffraction peaks at 24.21°,

33.24°, 35.74°, 40.98°, 49.60°, 54.21°, 62.63° and 64.21° are indexed to (012), (104), (110), (113),

(024), (116), (214) and (300) respectively. The peak positions and relative intensities are well matched

to the rhombohedral crystal system with the space group (R-3c) and unit cell parameters a=b=0.50206

nm and c=1.37196 nm. Figure 3(b) shows the identified phases between prepared CuO and reference

pattern ICDD card No.01-089-8103.

(a)

(b)

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Figure 3(a). XRD patterns of ZnO nanoparticles & Figure 3(b). Standard reference

pattern of Fe2O3

3.2 Crystallite Size Calculation

3.2.1 Debye-Scherrer Method

The crystallite sizes of the nanoparticles are calculated from a full-width half maximum of the

diffracted peaks. The Bragg peak breadth is a combination of both instrument and sample dependent

effects. To decouple these aberrations, the line broadening is collected from the standard material such

as silicon to find out the instrumental broadening. The instrumental broadening corresponding to the

diffraction peak of ZnO, CuO and Fe2O3 nanoparticles were estimated using the relation [15].

Bhkl= [(Bhkl)2measured-(Bhkl)2

instrumental] (1)

D = 𝑘𝜆

𝐵ℎ𝑘𝑙 cos 𝜃 (2)

The peak broadening at a lower angle is more meaningful for the calculation of crystallite size;

therefore, the crystallite size of nanoparticles as-synthesized were calculated using high-intensity peak

(101) at 2θ value 36.54° for ZnO nanoparticles. The calculated crystallite size was 35.57 nm. For CuO,

the maximum intensity peak (-111) occurred at 2θ value 35.58°. The calculated crystallite size was

42.11nm. For Fe2O3 the peak with the highest intensity at 33.74 ° was obtained and the crystallite size

was 43.41nm.

(a)

(b)

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3.2.2 Williamson-Hall Method

It is well-known that the line broadening in the XRD pattern is influenced by the crystallite size

and the internal strain. The crystallite size (D) was attained by fitting the following Williamson-Hall

equation using XRD data of as-synthesized ZnO, CuO and Fe2O3 nanoparticles [16].

Bhkl= BDS+ Bε

Where Bε= Cεtan 𝜃

Bhkl =𝑘𝜆

𝐷 cos 𝜃 + 4𝜀 tan 𝜃

where ε is the maximum compressive strain alone which can be calculated from the observed

broadening and C is the constant equals to 4, which depends on the assumptions regarding the nature

of inhomogeneous strain. Figure 4(a) shows the Williamson-Hall plot (4sin θ Vs Bhklcos θ) for ZnO

nanoparticles. From the graph, it reveals that the strain was extracted from the slope and the crystallite

size was elicited from the y-intercept of the linear fit. kλ

D = 0.00422 and the crystallite size of ZnO were

found 34.48nm. Figure 4(b) shows the Williamson-Hall plot for CuO nanoparticles. The strain was

calculated as same procedure as mentioned above. kλ

D = 0.00383 and the crystallite size were found

37.81 nm and be 38.11nm for CuO nano particles. Figure 4(c) shows the Williamson-Hall plot for Fe2O3

nanoparticles and kλ

D = 0.00284, crystallite size of Fe2O3 was found 51nm.

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Figure 4 (a-c). The W-H analysis of ZnO, CuO and α-Fe2O3 nanoparticles. Fit to the

data, the strain is extracted from the slope and the crystalline size is extracted from

the y-intercept of the fit.

3.2.3 Structural Analysis-Rietveld Refinement

Figure 5 shows crystal formation of ZnO, CuO and Fe2O3 and Figure 6 shows Rietveld

refinement plots. The Rietveld refinement has been executed via Fullprof Suite. By using Inorganic

Crystal Structure Database (ICSD) the prime values of atom coordinates, cell parameters and space

group have taken from equivalent reference patterns. Table 1 shows the atomic and structural

parameters were taken from standard ICSD database. The corresponding ICSD reference id for ZnO,

CuO and Fe2O3 are 98-001-7943, 98-005-0444 and 98-005-3823 respectively and taking as starting

models for refinement. The background parameter to be refined was selected using 12-coeffiecients

Fourier-cosine series and the refinement of profile shape was inspected by pseudo-Voigt function [17].

The pseudo-Voigt function was chosen for peaks because it is the definite combination of Lorentzian

curve and Gaussian curve functions. Primarily, the background coefficients and the peak positions were

refined and adjusted for nil shift errors by relating a number of successive refinements. Afterwards the

specifications such as scale factor, unit cell parameters, structure determinant, positional specifications

(c)

(a) (b)

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and occupancy were refined. The matched peak positions and the intensities precisely indicates a good

refinement model. It has been clearly noticed from figure that the detected and calculated patterns are

in good agreement. Table 2 evident that the good agreements of refine structural data. The goodness of

fit (GOF) for peak shape and position, structure and background are measured in terms of profile R-

factors and chi2 (χ2). The reasonably lower values of Rp, Rwp, Rexp and χ2 are considered to be good

profile refinement [18-20]. However, the values of Profile R parameters are only one condition for

referring the excellence of Rietveld fits. The refined structural parameters and profile R-factors have

been itemized in the table. The refined structures were visualized and the bond lengths & bond angles

were analysed by using VESTA software.

Figure 5. crystal formation of ZnO, CuO and Fe2O3 nano particles

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Figure 6. Rietveld refinement plots of ZnO, CuO and Fe2O3 nano particles annealed

at 700°C for 3h.

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Table 1. Atomic and structural parameters from standard ICSD data

Elements X Y Z B Occ Lattice

parameter

Density (g/cm3) /

Cell Volume

(Å3)

ZnO(ICSD reference # 98-001-7943)

Zn 0.33330 0.66670 0.00000 0.5000 1.000 a = 3.2200Å

b = 3.2200Å

c = 5.2000Å

α = 90°

β = 90°

γ = 120°

5.79 /

46.69 O 0.33330 0.66670 0.37500 0.5000 1.000

CuO (ICSD reference # 98-005-0444)

Cu 0.25000 0.25000 0.00000 0.5000 1.000

a = 4.6830Å

b = 3.4240Å

c = 5.1290Å

α = 90°

β = 99.4400°

γ = 90°

6.51 /

81.13

O 0.00000 0.41700 0.25000 0.5000 1.000

Fe2O3(ICSD reference # 98-005-3823)

Fe 0.00000 0.00000 0.35530 0.5000 1.000 a = 5.0230Å

b = 5.0230Å

c = 13.7080Å

α = 90°

β = 90°

γ = 120°

5.31 /

299.52 O 0.69400 0.00000 0.25000 0.5000 1.000

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Table 2. Refined structural parameters data

Composition

Crystal

Structure/Space

group

Density

(g/cm3)

Lattice

Parameters

Cell

Volume(Å3)

R-Factors

(%)

ZnO Hexagonal /

P 63 m c 5.471

a = 3.2480Å

b = 3.2480Å

c = 5.2030Å

α = 90°

β = 90°

γ = 120°

47.535

Rp= 6.58

Rwp= 8.90

Rexp= 4.72

χ2= 3.56

GOF= 1.9

CuO Monoclinic /

C 1 2/c 1 6.517

a = 4.6832Å

b = 3.4225Å

c = 5.1278

Å

α = 90°

β = 99.438°

γ = 90°

81.0770

Rp= 7.19

Rwp= 9.23

Rexp= 7.05

χ2= 1.71

GOF= 1.3

Fe2O3 Rhombohedral/

R -3 c 7.115

a = 5.0352Å

b = 5.0352Å

c = 13.750Å

α = 90°

β = 90°

γ = 120°

301.9026

Rp= 6.04

Rwp= 7.67

Rexp= 7.14

χ2= 1.16

GOF= 1.1

3.2.4 Microstructural studies

The morphology of the as-synthesized ZnO, CuO and Fe2O3 nanoparticles was analysed using

an SEM micrograph and the obtained images are shown in figure 7 (a-c). The SEM image of ZnO (Refer

figure 7(a)) reveals that the morphology of nanoparticles is spherical in nature with little agglomeration

and that a crystal size was found in the 35nm range [21-22]. The SEM image of CuO (Refer figure 7(b))

clearly reveals that the morphology of nanoparticles is in non-spherical crystals with some

agglomeration and indicates that the range of 41nm. The SEM image of Fe2O3 (Refer figure 7(c)) expose

that NPs are non-spherical in shape and have agglomerations, and indicate the crystal size of 47 nm.

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Figure 7. SEM Micrographs of a) ZnO b) CuO and c) Fe2O3

3.2.5 AFM analysis

Fig. 8 (a-c) the AFM surface texture images of ZnO, CuO and Fe2O3 particles in respective of

3D image. the average roughness (Ra) is used to study changes in the time of creation of a new surface

as well as spatial differences when examining the surface feature using different scales. This was due

to the parameter is more sensitive to large deviations with respect to the mean line [23-25]. The average

roughness (Ra) value of ZnO, CuO and Fe2O3 particles are 80 nm, 50 nm and 80 nm respectively.

(a) (b)

(c)

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Figure 8. AFM 3D Images of a) ZnO b) CuO and c) Fe2O3

4. Conclusion

By using sol-gel route ZnO, CuO and Fe2O3 were developed with a normal crystallite size in

the nanometre level by varying concentration of preliminary precursors. The XRD and Rietveld results

confirmed the development of a single hexagonal, monoclinic and rhombohedral phase of ZnO, CuO

and Fe2O3 respectively. It was clearly noted that the perceived and calculated patterns are in good

agreement from the refinement model and matched peak positions. The goodness of fit for peak shape

and position, structure and contextual is measured by profile R factors and chi2 (χ2). The profile

parameters values such as Rp, Rwp, RB, RF, χ2 are less than 10% are relatively low values and thereby

directed the goodness of Rietveld analysis. This broadening was analysed by the Scherrer formula, and

the W-H method. From the results, it was observed that the elongation value decreased but the particle

size increased. The SEM images of ZnO, CuO and Fe2O3 revealed a particle size of approximately 35

± 5, 41± 5 and 42± 5 nm. The SEM results were in good agreement with the results of the W-H method.

The AFM results also demonstrated that the particle sizes of ZnO, CuO and Fe2O3 are 40 nm, 80 nm

and 80 nm respectively.

(a)

(c)

(b)

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structural characterization of zno, cuo and fe2o3

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