M A T E R I A L S C H A R A C T E R I Z A T I O N 6 8 ( 2 0 1 2 ) 7 7 – 8 1

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Synthesis and characterization of NiO nanoparticles by thermaldecomposition of nickel linoleate and their optical properties

Abul Kalama,⁎, Abdullah G. Al-Sehemia, Ayed S. Al-Shihria, Gaohui Dub, Tokeer Ahmadc

aDepartment of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box 9004, Saudi ArabiabZhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University,Jinhua 321004, ChinacNanochemistry Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +966 558634606E-mail address: abul_k33@yahoo.com (A.

1044-5803/$ – see front matter © 2012 Elseviedoi:10.1016/j.matchar.2012.03.011

A B S T R A C T

Article history:Received 9 November 2011Received in revised form20 March 2012Accepted 21 March 2012

Well dispersed nickel oxide nanoparticles have been synthesized successfully by directcalcination of nickel linoleate. The structure, morphology and properties of the nanoparticleswere characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanningelectronmicroscopy, transmission electronmicroscopy, high-resolution transmission electronmicroscopy and ultraviolet–visible spectroscopy. Transmission electron microscopic studiesshow that nickel oxide nanoparticles are uniformwith an average size of 14–20 nm. The opticalband gap of 3.8 eV is obtained using UV–Visible spectroscopy which exhibits the red shift com-pared with the bulk counterpart.

© 2012 Elsevier Inc. All rights reserved.

Keywords:NiO nanomaterialsScanning electron microscopyTransmission electron microscopyX-ray diffractionOptical properties

1. Introduction

Nanoparticles (1–100 nm) have attracted extensive researchinterest in the past decades, due to the finite size effect atnano-dimension, surface effect, and macroscopic quantumtunneling effect [1–3]. Nanostructured NiO has a wide rangeof applications as a p-type semiconductor with a wide bandgap (3.6–4.0 eV), however bulk NiO is an antiferromagnetic in-sulator [4,5]. In the recent past, nano-sized NiO is also used aselectrode materials [6], magnetic materials [7,8], fabrication ofp–n heterojunctions [9], catalysis [10–12], optical materials[13], gas sensors [14,15], electro-chromic films [16], fuel cellelectrodes [17], battery [18], and electrochemical capacitors[19]. In view of the wide range applications of nickel oxidenanoparticles, various methods have been applied for thesynthesis of nano-structured NiO, such as sol–gel techniques[20–22], co-precipitation method [23–25], microemulsionmethod [8,26,27], ultrasonic radiation [28], solvothermal syn-

.Kalam).

r Inc. All rights reserved.

thesis [29,30], anodic arc plasma method [31] and microwaveirradiation [32]. Hollow NiO nanostructures can also be pre-pared by the oxidation of Ni nanoparticles [33,34]. The ther-mal decomposition approach to inorganic precursors isconsidered to be a very important method in preparingmetal oxide nanoparticles [35,36]. Niasari et al. have preparedNiO nanoparticles from nickel phthalate complexes using asolid-state thermal decomposition route. They studied the ef-fect of calcination temperature on the particle size, which wasfound to be 29 nm [37].

However, to the best of our knowledge, most of the reportedexperimental techniques for the synthesis of nano-powders arestill limited in laboratory scale due to some unresolved prob-lems, such as special conditions, tedious procedures, complexapparatus, low yield, and high cost. From a practical point ofview, it is vital to develop a way to manufacture high-qualitynano-powders at high yield and low cost with homogeneoussize distribution.

Fig. 1 – FTIR spectrum of (a) as-synthesized nickel linoleateand (b) NiO nanoparticles in KBr pellet.

Fig. 2 – XRD patterns of NiO spherical nanoparticles obtainedat 400 °C.

78 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 8 ( 2 0 1 2 ) 7 7 – 8 1

In this paper, we report the fabrication of NiO nanoparti-cles via the formation of nickel linoleate and the subsequentthermal decomposition of the precursor, and discuss the crys-tal structure, morphology and optical properties. There aremany advantages of the present method as it uses a greenchemistry solvent technique, simple approach, saves time,uses cost effective chemicals, and provides smaller crystallinenanoparticles of NiO.

2. Experimental

All chemicals, solvents and reagents are of analytical gradeand used without further purification. To prepare the metalcomplexes, 0.01 mol (0.75 g) Ni(NO3)2.6H2O and 0.02 mol(1.5 ml) linolenic acid were dissolved in 25 ml ethanol at80 °C. Then, 0.02 mol (0.7 ml) triethylamine was slowly addeddrop wise into the solution with vigorous stirring. Green pre-cipitate was instantly appeared into the solution. After beingfiltered and washed with ethanol, the precipitate was driedat 100 °C for 12 h. Finally, the product nickel linoleate was cal-cined in a muffle furnace at 400 °C for 2 h in air.

Fourier transform infrared spectroscopy spectra of nickellinoleate and calcined product were recorded by JASCO 460plus at room temperature in the range from 400 to 4000 cm−1

by using the KBr pellet technique. The specimens werespurred by BOC Edwards gold coating machine. The morphol-ogy of NiO nanoparticles was analyzed by using JEOL JSM-6390LV Scanning electron microscopy. The microstructures were

Table 1 – IR data of a and b.

S.N. IR frequenciesexperimental

Assigned functionalgroup

1 2930 and 2847 cm−1 υas (C\H) and υs (C\H) of CH2

2 3423 cm−1 υ (O\H)3 1725 cm−1 υ (C_O)4 1579 and 1453 cm−1 υas (COO−) and υs (COO−)5 455 cm−1 υ (Ni\O)

investigated using transmission electron microscopy andhigh-resolution transmission electron microscopy performedon a JEOL 2100F instrument at an acceleration voltage of200 kV. The sample for transmission electron microscopyanalysis was prepared by dispersing the final sample in etha-nol; this dispersing was then dropped on a copper gridcovered with an amorphous carbon film. The products werealso characterized by X-ray diffractometer on a ShimadzuX-ray diffractometer XRD-6000 with CuKα (λ=1.54178 Å) radi-ation for phase identification. X-ray diffraction pattern wascollected at room temperature using a continuous scan overan angular range of 2θ=20°–80° with step size of 0.02°and scan rate of 2°min−1. UV–Visible absorption spectrawere recorded using Shimadzu 1601PC ultraviolet–visibledouble-beam spectrophotometer with a wavelength range of235–800 nm.

3. Results and Discussions

Fourier transform infrared spectroscopy is the useful tool toshow the presence of any functional group in organiccompounds. Fig. 1 shows the FT-IR spectra of nickel linoleateand the calcined products. The main absorption bands at~3423, ~2930, ~2847, ~1725, ~1579 and ~1453 cm−1 corroborateswith the presence of O\H, C\H, symmetric and asymmetricC_O stretching vibrations of nickel linoleate, respectively(Fig. 1a). Compared with the bands in Fig. 1a, the band at ~2930, ~2847, ~1579 and ~1453 cm−1 disappeared as shown in

Table 2 – XRD data of NiO.

S.N. d valueexperimental

2θexperimental

d valuestandard

Millerindices

1. 2.4276 37.0 2.4127 1112. 2.0989 43.06 2.0895 2003. 1.4814 62.66 1.4775 2204. 1.2619 75.24 1.2600 3115. 1.2082 79.22 1.2063 222

Fig. 3 – SEM images of NiO nanoparticles obtained by thermal decomposition of nickel linoleate at 400 °C with differentmagnifications.

79M A T E R I A L S C H A R A C T E R I Z A T I O N 6 8 ( 2 0 1 2 ) 7 7 – 8 1

Fig. 1b after calcination at 400 °C for 2 h. The intensity of thepeak at ~3423 cm−1 is significantly decreased in Fig. 1b, indicat-ing thedehydration andsubsequent eliminationof theC\HandC_O bonds. A new band appears at ~455 cm−1 (Fig. 1b), whichindicates that nickel linoleate is completely decomposed at400 °C. The absorption band at ~455 cm−1 is the characteristicband of Ni\O stretching vibrational mode, that shows the for-mation of nanoparticles as compared to the bulk form of NiO(Table 1) in which the bands which lies between 390 and403 cm−1 [38].

Fig. 4 – (a) TEM images of NiO, (b) HRTEM image showing the (11histogram.

Fig. 2 shows the X-ray diffraction pattern of the productobtained from the decomposition of the precursor at 400 °C.The observed diffraction peaks are well assigned to the cubicstructure of NiO (space group Fm3/m; a=4.176 Å; JCPDS78–0423). Moreover, no other peak is observed belonging toany adsorbed impurities or phase such as Ni(OH)2, NiO2,NiCO3. Thus the X-ray diffraction studies confirm the absolutetransformation to NiO (Table 2). The XRD result is also inagreement with the results as obtained from FT-IR studies. Itcan be seen from Fig. 2 that the diffraction peaks are markedly

1) planes of NiO nanoparticles and (c) size distribution

Fig. 5 – (a) UV–vis absorption spectrum and (b) plots of (Ahυ)n

as a function of photon energy (hυ) for NiO nanoparticles.

80 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 8 ( 2 0 1 2 ) 7 7 – 8 1

broadened due to the small size of the particles. Theaverage particle size of NiO is calculated from the majordiffraction peak (200) of NiO using the Debye–Scherrerformula [39]

D ¼ 0:89 λ=β cosθ

where D represents the grain size (diameter of spherical parti-cle) in nm, λ is the wavelength of X-ray (1.5418 Å) for CuKα ra-diation, β is the full width at half maximum (FWHM) ofprominent intense peak, and θ is the diffraction angle of thepeak. The average particle size calculated using the most in-tense reflection (200) at 2θ=43.5° is 13.9 nm (Fig. 2).

Fig. 3 illustrates the scanning electron microscopic (SEM) im-ages of nanostructure nickel oxide calcined at 400 °C for 4 h. Themicroscopic study indicated that all the particles were sphericalin shape with low agglomeration.

The size and morphology of NiO nanoparticles have beenestablished by the transmission electronmicroscopic (TEM) stud-ies. TEM analysis (Fig. 4a) suggests the formation of nearly spher-ical NiO nanoparticles and corroborates with the SEM studies.The sizes of these NiO nanoparticles were found to be in therange of 14 to 20 nm. This is in good agreement with the resultcalculated by using the Scherrer formula. On calcinations at400 °C, nearly spherical nanoparticles are obtained due to the de-composition of Nickel linoleate to NiO (Fig. 4a). The size polydis-persity in the NiO nanoparticles (Fig. 4a) could be seen whichmay be explaineddue to the thermal calcinations of nickel linole-ate at 400 °C temperature that is sufficient to provide agglomera-tion because of the high surface energy of the nanoparticles. TheHRTEM studies have also been carried out for the structural anal-ysis of NiO. HRTEM image (Fig. 4b) of NiO shows the well definedlattice fringes which suggest the highly crystalline nature of theparticles. The spacing of the two neighboring plane is about0.24 nm as shown in Fig. 4b, which is consistent with the inter-planar separationof the (111) plane incubic (fcc)NiO.Thesizedis-tribution of the nanoparticles has been measured and shown inthe size histogram (Fig. 4c) which clearly gives the size distribu-tion of the nanoparticles.

Fig. 5a demonstrates the UV–visible spectrum of the NiOnanoparticles suspension as obtained by ultrasonic dispersionin absolute ethanol. A strong absorption peak in the UV regionis observed at wavelength of ~279 nm. This absorption in UV re-gion is attributed to band gap absorption of NiO [36]. The absorp-tion band gap (Eg) is usually achievedwith the aid of the followingequation:

Ahυð Þn ¼ B hυ−Eg� �

where hυ is the photo energy; A is absorbance, B is the constantrelated to the material; and n indicates either 2 or 1/2 for directtransition and indirect transition, respectively [40]. Hence, the op-tical band gap for the absorption peak can be obtained by extrap-olating the linear portion of the (Ahυ)n−hυ curve to zero. Fig. 5bshows the (Ahυ)2 versus hυ curve for the sample. The band gapof the NiO particles is found to be 3.8 eV, which is smaller thanthe bulk NiO (4.0 eV) [41]. Normally, the nanomaterials show ablue shift in the band gap energy, but the synthesized materialshow the red shift. The effect may be attributed to the chemicaldefects or vacancies present in the crystals which shall result inthe electronic states in the band gap to reduce the band gap ener-gy [42]. No linear relationwas found forn=1/2, suggesting that thecalcined NiO nanoparticles are semiconducting with direct tran-sition at this energy [36]. Furthermore, it is well known that theenergy band gap of the semiconductor nanoparticles increaseswith the decrease of the grain size. Hence, there is no quantumconfinement effect due to their small Bohr radii.

4. Conclusions

Nearly spherical nanoparticles (14–20 nm) of nickel oxidecrystallizing in the cubic structure have been synthesizedthrough the thermal decomposition of nickel linoleate precur-sor in air at 400 °C. FT-IR and XRD results show the gradual de-composition of precursor to produce NiOwith high purity. Thegrain size of nickel oxide nanoparticles is in close agreementwith the TEM and XRD studies. UV–visible studies showthe optical band gap of 3.8 eV, which indicates the red shifton size reduction. This method does not require any complexapparatus, catalyst or any surfactants. This method couldalso be extended for the formation of other metal oxidenanoparticles.

Acknowledgments

The authors are thankful to the Chairman, Department ofChemistry, King Khalid University, Abha, KSA, for providingnecessary research and laboratory facilities.

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