Ni Doped CuO Nanoparticles

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Ni Doped CuO Nanoparticles: Structural and Optical Characterizations Sawsan Al-Amri1,2, M. Shahnawaze Ansari1, Saqib Rafique1, Musab Aldhahri1, Sawsan Rahimuddin2, Ameer Azam1 and Adnan Memic1* 1

Center of Nanotechnology, King Abdulaziz University, Jeddah 21589, Saudi Arabia; 2Department of Biochemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia Abstract: In recent years, applications of metal oxide nanoparticles have become increasingly relevant ranging from semiconductor to medical health industries. In work presented here, Ni doped CuO nanoparticles with doping concentrations varying from 0% to 7% have been synthesized via sol-gel combustion route without adding any surfactants or templates. Detailed structural observations by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Raman studies revealed a highly crystalline single phase monoclinic structure for as-synthesized nanoparticles. The decreasing particle size with increasing Ni content was confirmed by field emission scanning electron microscopy (FESEM) as observed by XRD analysis. An obvious increase of band gap estimated by UV-Vis spectrophotometer with increasing Ni dopant concentration was found. Additionally, an increase in the intensity of luminescent emission was observed with photoluminescence (PL) spectra which can be attributed to the defects in the doped samples. These changes in optical properties as a function of Ni doping could present novel strategies leading to tailored metal oxide nanoparticles for desired applications.

Keywords: Luminescence, nanostructured materials, optical properties, sol-gel processes. 1. INTRODUCTION In recent years, fabrication and characterization of nanostructured metal oxide materials have been considered a cornerstone in modern materials science for both fundamental as well as technological reasons [1-4]. Moreover, studying their physical and chemical properties is of special interest, as compared to the bulk materials, due to their widespread applications in different areas [5-7]. Some transition metal oxides (TMOs) such as Fe3O4, SnO2, TiO2, and ZnO were shown to be promising candidates for many applications. Cupric oxide (CuO) is also considered as one of the metal oxides with great practical importance as a p-type semiconductor in electronics and optoelectronic devices; such as gas sensors [8], nano fluid [9], high-Tc super conductors [10], magnetic storage devices [11], catalysts [12] etc. The narrow band gap of CuO at 1.2 eV makes it useful for photoconductive and photothermal applications and as an antibacterial agent [13, 14]. To date, a series of strategies have been exploited to synthesize CuO nanoparticles, including colloidthermal synthesis [15], solid state reaction [16], reflux condensation [17], thermal decomposition of precursors [18], sol-gel [19], sonochemical [20], and hydrothermal [21] etc. Furthermore, the interest in doping of the semiconductor nanoparticles with impurity metal ions is to explore the possibility of tailoring electrical, magnetic and optical properties of the material. Several researchers have reported the improvement in a material’s characteristics using various dopants. Basith et al. have reported that the optical band gap increases from 3.9 eV to 4.3 eV with the increase in Ni *Address correspondence to this author at the Center of Nanotechnology, King Abdulaziz University, PO Box 80216, Jeddah 21589, Saudi Arabia; Tel: +966-56-4975404; Fax: +966-12-6951566; E-mail: [email protected] 1573-4137/15 $58.00+.00

content in CuO matrix. Room temperature ferromagnetism (RFTM) with a saturation magnetization (Ms) of 1.31 x 10-3 emu/g for 2.0 wt% of Ni was also reported [22]. Gülen et al., observed that as Mn content increases in the CuO films, the optical band gap also increases [23]. An obvious blue shift in UV-visible spectrum of Zn doped CuO nanoparticles has been investigated and reported by Rejitha and Krishna [24]. Another study by Rejitha and Krishna found the band gap narrowing for cadmium-doped CuO nanoparticles [25]. In this report, we have synthesized pure and Ni doped CuO nanoparticles using a simple and low cost sol-gel combustion process. The aim of this paper is to evaluate the changes in the structural, morphological and optical properties of CuO nanoparticles brought about through successful doping with Ni as a dopant. 2. EXPERIMENTAL DETAILS High purity nickel nitrate (Ni (NO3 )2.6H2 O from Sigma–Aldrich) and copper nitrate (Cu (NO3)2 .3H2 O from Merk) were used as the sources of copper and nickel respectively. Citric acid was used as a source of fuel. All reagents were directly utilized as received without further purification. Ni doped CuO nanoparticles with doping concentrations ranging from 0% to 7% were prepared by sol-gel combustion route. In a typical synthesis procedure, stoichiometric amounts of Ni (NO3)2 .6H2O, Cu (NO3 )2.3H2 O and citric acid were dissolved in distilled water. The molar ratio of metal nitrates to citric acid was 1:1. The solution was stirred with a magnetic stirrer at 80 °C until the gel was formed (∼1 hour). The gel was washed 2-3 times with ethanol using centrifuge at 19090 rcf to remove any organic impurities. Afterwards, the washed material, in © 2015 Bentham Science Publishers

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the form of dense precipitate, was burned at 200 °C for 10 minutes. Combustion resulted in the formation of a fluffy mass. Consequently, it was grinded for about half an hour and further annealed at 350 °C to obtain crystalline phase of pure and Ni doped CuO nanoparticles. The structure, crystallite size and phase of Ni doped CuO nanoparticles were analyzed through X-ray diffraction technique (Rigaku, Ultima-IV X-ray diffractometer) using CuKα radiations (λ = 0.154 nm) in the range of 20° ≤ 2θ ≤ 80°. The surface morphology for all the samples was carried out using FESEM (JEOL, JSM-7600F). The compositional analysis of as synthesized nanoparticles was studied using energy dispersive spectroscopy (EDS) (Oxford Instruments) attached with FESEM. The functional groups of molecules and the quality of the prepared nanoparticles was obtained through FT-IR spectrophotometer (Thermo Scientific, Smart iTR). Raman spectra were recorded using Raman microscope (DXR-Thermo scientific) with 532 nm laser as an excitation source at a power of 8 mW. Optical analysis was performrd with UV-Visible spectrophotometer (PerkinElmer 2D Detector Module-Lambda 750). Photoluminescence spectra within the excitation wavelength range 415 to 735 nm were taken under Main Beam Splitter (MBS): invisible MBS-405 using Confocal microscope (Carl Zeiss LSM 780).

substitution of Cu2+ ions by Ni2+ ions in the CuO system. The crystallite size (D) was calculated using DebyeScherrer’s formula [26, 27] given by Eq. (1),

D=

3. RESULTS AND DISCUSSION 3.1. Structural Characterizations: XRD Study The crystallinity and phase of pure and Ni doped CuO nanoparticles were examined by XRD and the results are illustrated in Fig. (1). In the diffraction pattern, all peaks are well indexed to the monoclinic phase of copper oxide (CuO) with space group C2/c that was confirmed from JCPDS card No. 05-0661. The main peaks at 2θ = 35.45 and 38.85 corresponding to (002) and (111) planes are the characteristics peaks for monoclinic phase of pure CuO nanoparticles. No impurities such as Ni metal, Ni oxides or Cu2 O were detected in the XRD pattern, which indicate the purity of the prepared nanoparticles. Furthermore, the sharpness of the peaks in the diffraction pattern indicates the formation of highly crystalline single phase Ni doped CuO nanoparticles. These finding indicate that the crystal structure of the base CuO matrix was not distorted by the Table 1.

Fig. (1). XRD pattern of as synthesized Ni doped CuO nanoparticles.

k! "hkl Cos#

(1)

where k is the shape factor (0.90), λ is the wavelength of Cu-Kα radiation, β is the full-width at half maximum (FWHM), and θ is the diffraction angle. Crystallite size and lattice parameters were calculated using XRD data and are presented in Table 1. The average crystallite size of pure and Ni doped CuO samples were found to be in between the range of 23 to 47 nm. The decrease in lattice parameter and crystallite size can be explained by the lattice distortion induced by Ni doping, which can be attributed in terms of Vegard’s law which states that the change in lattice parameter and crystallite size may be due to the difference between ionic radii of replacing and replaced ions [28]. In the present system, the decrease in lattice constants and crystallite size is because of the smaller ionic radii of Ni2+ (0.69 Å) ions compared to that of Cu2+ (0.73 Å). Therefore, this observation could be possible from Ni2+ replacement of Cu2+ in the CuO lattice structure.

Calculated structural parameters for Ni-doped CuO nanoparticles using XRD data. Sample

Crystallite size (nm)

Lattice Parameter (Å) a

b

c

CuO

47

4.669

3.788

4.947

1% Ni@CuO

46

4.672

3.442

5.126

2% Ni@CuO

43

4.689

3.437

5.134

3% Ni@CuO

41

4.692

3.432

5.139

4% Ni@CuO

37

4.697

3.429

5.142

5% Ni@CuO

32

4.711

3.429

5.152

7% Ni@CuO

23

4.723

3.427

5.168

Ni Doped CuO Nanoparticles: Structural and Optical Characterizations

3.2. Raman Spectroscopy Raman spectroscopy is an important tool to analyze the microstructure, mainly the arrangement of the atoms and the vibrational modes of the nano-sized materials [29, 30]. Fig. (2) shows room temperature Raman spectra of pure and Ni doped CuO nanoparticles synthesized by sol-gel combustion method. Copper oxide belongs to the C62h space group with two molecules per primitive cell. There are twelve zonecenter optical phonon modes with symmetries [31, 32]:

ΓRA = 4Au + 5Bu+Ag + 2Bg

(2)

Where Γ is the degree of vibrational freedom, Au and Bu represents IR modes; Ag and Bg represents Raman modes. There are three acoustic modes (Au + 2Bu), six infrared active modes (3Au + 3Bg), and three Raman active modes (Ag + 2Bg). The position of Raman peaks depends on various factors such as: synthesis technique, crystal structure and geometry [33]. It is evident from Fig. (2) that there are three Raman peaks at 275, 325, and 610 cm−1 corresponding to (Ag + 2Bg) modes. The strong Raman peak at 275 cm−1 is attributed to the Raman active phonon Ag mode of CuO, and the weaker peaks at 325, and 610 cm−1 corresponds to Bg modes, respectively. Raman modes of Cu2O are not present in the system [34], confirming and in line with other results that there was a single phase formation of prepared pure and Ni doped CuO nanoparticles. It can be observed that as Ni content is increasing within the CuO matrix, these fundamental peaks are slightly shifting towards lower wave number which could be attributed to the reduction in particle size. This shift can be explained in the light of Heisenberg uncertainty principle which suggests the relationship between crystallite size and phonon and can be expressed as [35]: ΔxΔp ≥

h2 4

(3)

Where Δx is particle size, Δp is phonon momentum distribution and h is reduced Planck’s constant. According

Fig. (2). Raman spectra of as prepared Ni doped CuO nanoparticles.


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to this principle, as the size of crystallite decreases more and more phonons get confined within the same particle and hence the degree of phonon momentum distribution increases. As a result, the broadening of phonon momentum leads to the broadening of scattered phonon momentum by obeying the law of conservation of momentum. And this phonon dispersion causes asymmetric broadening and leads to the shifting of Raman bands towards lower wave numbers [36]. 3.3. Fourier Transform Infrared (FT-IR) Study FT-IR spectroscopy is a technique which is used to examine the vibrational frequencies of bonds in the molecule and to characterize the surface nature of copper oxide nanoparticles. FT-IR spectra of pure and Ni doped CuO nanoparticles in the range of 400 cm−1 to 1000 cm−1 in powder form are depicted in Fig. (3). Because of higher penetration depth of IR, this technique is more sensitive in comparison to X-ray diffraction (XRD) and Raman spectroscopy for studying phases and lattice distortions [37, 38]. It is clear from Fig. (3) that all samples show bands in the region 400 cm-1 - 600 cm-1 . The strong absorption band at 525 cm-1 is related to the vibration of the Cu–O stretching. The obtained value is in good agreement with the published literature [39-41]. Again, secondary phases such as Cu2 O were not found in the system which confirms the formation of single phase Ni doped CuO nanoparticles [42, 43]. Furthermore, it is observed from Fig. (3) that IR analysis depends on crystallite size. As Ni concentration increases, the intensity and band width of FTIR peaks increases. This may also be explained on the basis of the reduction in the size of the resulting nanoparticles with increased doping levels. Also, the broadening that was observed in Infra-red absorption band may be due to the microstructural changes in the CuO lattice, the change of lattice parameters or change of free electrons present in the system. These assignments are consistent with some literature reported elsewhere [44]. It can also be seen that there is a small shift in all the characteristic peaks which can be interpreted as the difference in the bond length that occurs when Ni ions replace Cu ions, thus confirming the successful doping of Ni ion into CuO lattice.

Fig. (3). FT-IR spectra of Ni doped CuO nanoparticles.

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3.4. Morphological and Compositional Studies

(αhv) = A(hv − Eg)n

The surface morphology and microstructure of pure and Ni doped CuO nanoparticles were investigated by field emission scanning electron microscopy (FESEM). Different concentrations of Ni, Cu and O in the system were examined by energy dispersive spectroscopy (EDS). Fig. (4) shows the surface morphology and elemental composition of the pure, 2% and 4% Ni-doped CuO nanoparticles. As seen in the FESEM images, the crystallites are nearly spherical in shape. It is also evident that the particle size is decreasing with the increase in Ni content into the host CuO matrix, which is consistent with the XRD results calculated by Scherrer’s equation. From the EDS measurement, it was indicated that the nano powders are nearly stoichiometric as shown in Fig. (4). Also, there are no traces of other impurities in the EDS spectra confirming the formation of single phase pure and Ni doped CuO nanoparticles. Thus, the Ni ions were successfully doped in the Cu sites.

where A is a constant, α represents absorption coefficient, hv is the incident photon energy and n equals 1/2 and 2 for direct and indirect band gap respectively. The energy intercept of a plot of (α hv)2 vs hv yields Eg for a direct transition [45]. The energy band gap of as synthesized pure and Ni doped CuO nanoparticles with 1, 2, 3, 4, 5 and 7 wt% of Ni content is calculated to be 2.73, 3.18, 3.25, 3.35, 3.41, 3.47, and 3.80 eV, respectively. It is evident from the Fig. (5) that with increasing Ni content, the energy band gap was slightly increasing. A blue shift of the absorption edge is exhibited for all samples. The energy band gap for pure CuO nanoparticles is found to be (Eg = 2.73 eV), which is higher than that for the bulk CuO (Eg = 1.85 eV) [15]. This increase in the band gap can be attributed to the well-known quantum size effect of semiconductors. In general, when the light is absorbed by a material, the electron jumps to the conduction band by leaving a hole in the valance band. But, if the particle size is small, it behaves like a quantum well and the energy difference between the position of conduction band and a free electron that leads to the quantization of their energy levels. This theory is applicable when the size of the particle lies in the range comparable to the de Broglie wavelength of a charge carrier. The observed band gap values of Ni doped CuO nanoparticles are greater than that of pure CuO nanoparticles which was also reported by Basith et al. for Ni doped CuO nanostructures of dimensions between 20 nm to 30 nm [46]. It can also be understood in terms of particle size which is continuously decreasing by increasing Ni contents in CuO base matrix. The reduction in particle size leads to the quantum size effect which is very well discussed in the above statements. According to the literature, there are various factors such as: particle size, presence of metal ions as an impurity, or lattice defects (e.g. Cu+1 and O vacancies) that are some of the key factors affecting the band gap energy in CuO systems [47].

3.5. Optical Properties UV-visible absorption spectra of as-synthesized pure and Ni doped CuO nanoparticles have been recorded by UVvisible spectrophotometer in the range of wavelength between 190 nm to 500 nm at room temperature. The optical energy band gap was calculated using Tauc relation:

(4)

Photoluminescence is also a useful tool for probing electron-hole surface processes of semiconductor materials, determination of band gap energy, and identification of specific defects for radiative transitions and impurity levels in these materials [25, 48]. Earlier reports have shown that there are

Fig. (4). FESEM images and EDS spectra of (a) pure CuO, (b) 2% and (c) 4% Ni@CuO nanoparticles.

Fig. (5). Plots of (αhν)2 vs hν for direct transitions.

Ni Doped CuO Nanoparticles: Structural and Optical Characterizations

several other factors affecting the emission wavelength of the semiconductor oxide, including the particle’s morphology, size, and concentration of the dopant and excitation wavelength [49]. Fig. (6) shows the PL spectra for assynthesized pure and Ni doped CuO nanoparticles recorded at room temperature using a confocal microscope. The samples were excited using laser excitation source at a wavelength of 405 nm. A sharp emission band that belongs to the green color spectrum was estimated in the visible region at 423 nm. This emission band could be assigned to the defects and oxygen vacancies generated in all samples but not to the band gap emission. Several other researchers have reported that the emission bands at green and red wavelength are related to oxygen vacancies and metal ions sitting at interstitials in the semiconductor oxide materials lattice [50-53]. It is observed from Fig. (6) that there is a direct relationship between the concentration of Ni and the intensity of luminescent emission which can be attributed to the defects such as oxygen interstitials in the doped samples. Also, it is evident that there is a small blue shift in the PL peak which is slightly increasing with the increase in Ni concentration. It


with increasing Ni content from 0 to 7%. Photoluminescence spectra showed an increase in the intensity of luminescent emission with increasing Ni concentration which may be due to the defects and oxygen vacancies generated in Ni doped CuO samples. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS The publication of this paper was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (363-903-1433). The authors, therefore, acknowledge with thanks DSR technical and financial support. REFERENCES [1]

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Fig. (6). PL spectra of Ni doped CuO nanoparticles.

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can be attributed as quantum confinement effect resulting from the decrease in particle size which is due to the lattice distortions raised by introducing Ni ions into CuO matrix.

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CONCLUSIONS

[10]

Pure and Ni doped CuO nanoparticles have been synthesized via sol-gel combustion route. We report on the effect of Ni doping on the structural, morphological, and optical properties of CuO nanparticles. The formation of monoclinic phase of pure and Ni doped CuO were confirmed from the XRD patterns where Ni2+ ions substituted Cu2+ ions. The crystallite size ranged from 23 nm to 47 nm. It was observed that there was a decrease in the crystallite size with increasing Ni concentration. By Raman and FT-IR spectra we further confirmed the purity of the prepared samples. SEM results were consistent with the previous XRD results. All of the doped samples showed a blue shift in the optical band gap, which can be assigned to the quantum confinement effects. There was a significant increase in the energy band gap of Ni doped CuO nanoparticles from 2.73 eV to 3.80 eV

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Received: July 26, 2014

Revised: September 23, 2014

Accepted: October 22, 2014

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[39]

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