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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