Characteristics and optical properties of MgO nanowires synthesized by solvothermal method

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Characteristics and optical properties of MgO nanowires synthesized by solvothermal method N.M.A. Hadia a,n, Hussein Abdel-Hafez Mohamed a,b a b

Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt Physics Department, Teachers College, King Saud University, 11148 Riyadh, Saudi Arabia

a r t i c l e i n f o

Pacs: 62.23.Hj 68.37.Hk 68.37.Og 78.55.Et 81.07.Gf Keywords: Nanostructures Chemical synthesis TEM and SEM Photoluminescence spectroscopy

abstract Magnesium oxide (MgO) nanowires were synthesized by solvothermal method using magnesium nitrate hexahydrate and sodium hydroxide. Field emission scanning electron microscopy (FE-SEM) and transmission scanning electron microscopy (TEM) measurements indicate that the product consists of a large quantity of nanowires with average diameter of 20 nm and average length of several micrometers. Explorations of X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), Fourier transformer infrared spectroscopy (FTIR), selected area electronic diffraction (SAED) and high-resolution transmission electron microscope (HRTEM) indicate that the product is high-quality cubic single-crystalline nanowires. The optical properties of the samples are investigated using UV–visible spectroscopy to study the refractive index and optical dielectric constant. The photoluminescence (PL) measurement suggests that the product has an intensive emission centered at 437 nm, showing that the product has potential application in optical devices. The advantages of our method lie in high yield, the easy availability of the starting materials and allowing their large-scale production at low cost. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction Considerable interest is focused on the synthesis of nanocrystalline materials because of their unique physical and chemical properties that distinguish them from bulkphase materials. The particle size and shape of materials significantly affect their properties. Thus, the control of the size and morphology of nanocrystalline materials can lead to the discovery of new physical and chemical properties [1]. One-dimensional (1D) nanostructures, such as nanowires, nanorods, nanobelts, nanoribbons, nanoneedles, and nanotubes, have attracted considerable attention due to their unique and fascinating properties as well as their potential technological applications [2]. In particular, 1D nanostructures

n

Corresponding author. Tel./fax: þ20 1129264358, þ 20 93 4601159. E-mail address: [email protected] (N.M.A. Hadia).

are emerging as powerful building blocks for nanoscale photonic devices such as light-emitting diodes, photodiodes, lasers, active waveguides, and integrated electro-optic modulator structures because of their higher luminescence efficiency [3–5]. Among the metal oxides studied, magnesium oxide (MgO), in particular, has received a large amount of attention. MgO is a typical wide band gap (7.8 eV) insulator. Its electronic and optical properties are very attractive because its low heat capacity and high melting point make it an ideal candidate for insulation applications [6]. MgO nanostructures have also been used as protective layers for dielectrics in AC circuits to improve discharge characteristics and panel lifetime as a result of their antisputtering properties, high transmittance, and secondary electron emission coefficient [7]. Nowadays, various morphologies of MgO nanostructures such as nanoparticles [8], nanotubes [9,10], nanosheets [11–14], nanowires [10,15], whiskers [11,16], nanobelts

http://dx.doi.org/10.1016/j.mssp.2014.03.049 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

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[10,17], nanofibers [18], and other morphologies [19,20], have been successfully synthesized by various methods, such as chemical vapor deposition (CVD) [21], domestic microwave oven [22], carbothermal reduction [23], sol-gel [24], dual magnetron sputtering [25], hydrothermal synthesis [26] and thermal evaporation [27]. However, large scale, high-efficiency controlled synthesis of MgO nanostructures still remains as a challenge. Hydrothermal synthesis was favored among researchers due to it being an economical and simple method. Al-Hazmi et al.[26] successfully obtained nanostructures 1D magnesium oxide (MgO) nanowires by microwave hydrothermal process at 180 1C for 30 min. In this work, we successfully synthesize MgO nanowires by solvothermal method using magnesium nitrate hexahydrate and sodium hydroxide. The morphological, structural, optical properties and photoluminescence (PL) of the as-prepared MgO nanowires are reported. 2. Experimental 2.1. Materials Magnesium nitrate hexahydrate [Mg(NO3)2  6H2O] and sodium hydroxide [NaOH] were purchased from SigmaAldrich. All the reagents used in the experiments were of analytical grade and used without further purification. 2.2. Synthesis of MgO nanowires The starting materials used for the synthesis were magnesium nitrate hexahydrate [Mg(NO3)2  6H2O] and sodium hydroxide [NaOH]. Mixed solvent of ethanol and water in equal volume ratio was used as the solvent. 6 g of NaOH were added to 70 ml of the mixed solvent and stirred until it dissolves completely (pH-11). 3.446 g of magnesium nitrate were directly added to the above solution and after stirring for a few minutes, a white precipitate was obtained and transferred to 100 ml autoclave. The closed autoclave was then placed inside a preheated hot-air oven maintained at 180 1C for 10 h, after that it was cooled down to room temperature (RT). The obtained precipitate was filtered, washed with distilled water for several times to remove the nitrates and then with ethanol to reduce the agglomeration, and later dried at 80 1C for 2 h. Finally, the white colored material was calcined at 500 1C for 3 h in an electrical oven.

The morphology of samples was studied by field-emission scanning electron microscopy (FE-SEM), and was performed on a JSM-6100 microscope (JEOL, Japan) with an acceleration voltage of 30 kV. The chemical composition of the synthesized nanostructures was also analyzed using energy dispersive analysis of X-ray (EDAX) unit attached with the FE-SEM. Transmission electron microscopy (TEM) images and the corresponding selected area electron diffraction (SAED) patterns were obtained with a 2000 EX II microscope (JEOL, Japan) at an acceleration voltage of 200 kV. High resolution transmission electron micrographs (HRTEM) were obtained on a JEM-2100F (JEOL, Japan) with an accelerating voltage of 200 kV. For TEM observation, the synthesized products were ultrasonically dispersed in ethanol and a drop of the suspension was placed on a Cu grid coated with carbon film. The optical properties were measured at room temperature using the Perkin Elmer Lambda 900 UV–vis spectroscopy. The room temperature photoluminescence (PL) spectrum of the products was measured using Edinburgh Instruments FLS920 steady-state fluorescence spectrometer (U.K.) with Xe lamp as the excitation light source (with a wavelength of 350 nm). 3. Results and discussion 3.1. Structural studies Fig. 1 shows the XRD pattern of the as-prepared MgO nanowires. It can be seen that the nanowires were highly crystalline in nature with diffraction peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) corresponding to the cubic structure of MgO with a lattice constant of a ¼0.421 nm (JCPDS: 04-0829). The sharp diffraction pattern indicated that the structure possessed good crystallinity. No characteristic peak of impurities was detected in the pattern, indicating the high purity of the obtained product. Therefore, these X-ray diffraction results clearly show that the MgO NWs are pure MgO and well crystallized. This result is similar to several previous reports such as Refs. [28,29]. The crystallite size for the synthesized MgO NWs is calculated by Scherer's formula [30,31]: D¼

0:9 λ β cos θ

ð1Þ

2.3. Characterization techniques The structure of as-prepared samples were characterized by X-ray powder diffraction (XRPD), being the X-ray patterns from 101 and 801 at 2θ collected by a Philips X'Pert PRO MPD (PANalytical, The Netherlands) using graphite-monochromatized CuKα radiation (λ¼ 1.54184 Å), operating at 45 kV and 40 mA. For IR measurements, the films were grown on KBr. The IR measurements were carried out using a fourier transform infrared spectroscopy (FT-IR) spectrophotometer (IRPrestige-21, Shimadzu) in the wave number range 400–4000 cm  1 with 4 cm  1 resolution.

Fig. 1. X-ray diffraction pattern of the MgO NWs.

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where λ is the wavelength of the target used, β (in radians) is the full width at half maximum (FWHM) intensity of the diffraction peak located at 2θ, and θ is the Bragg angle. The average crystallite sizes of MgO NWs were calculated to be about 17 nm and matches well with the size obtained from FE-SEM and TEM images in this article. The specific surface area (SA) is one of the important parameters used to characterize powder samples and it depends on particle size, shape, and density of the sample and is given by [32]: SA ¼

6  103 Dρ

ð2Þ

where D is the size of the particles and ρ is the density of MgO (3.58 g/cm3). The computed value of specific surface area is approximately 98.6 m2/g. 3.2. Morphological analysis The general morphology and particle size of synthesized one-dimensional MgO nanowires with different magnifications were examined by observed FE-SEM images as depicted in Fig. 2(a and b). It can be seen that the nanowires are continuous and arranged roughly parallel to each other, and the growth rate of the nanowires is very high; as most of the nanowires are attached to each other through one of their surfaces and exhibit sharp edges with smooth and

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clean surfaces. In addition to that, it is clear that the synthesized MgO grow as nanowires which are formed in large-quantity. The MgO nanowires exhibits diameters in the range of  16–20 nm and average length of several micrometers and is consistent with the above X-ray result. EDAX analysis was carried out to determine the chemical composition of the MgO nanowire shown in Fig. 2(c). The EDAX spectrum indicated that the nanowires were composed of elemental Mg and O and it is found that the nanowires contain 54.6% magnesium and 45.4% oxygen. The small amount of gold signal comes from the using gold layer used as a conductive material for specimen coating. It is note that the Mg/O atomic ratio, approximated from these data is in good agreement with that of the bulk ratio. A slightly lower oxygen atomic ratio compared to that of magnesium is probably high oxygen vacancies in the MgO NWs. To obtain more detailed information of MgO nanowires, TEM and selected area electron diffraction (SAED) were used to characterize the configuration and crystalline structure of the MgO nanowires. TEM samples were prepared by sonicating the substrate in acetone by ultrasonic treatment. A drop of the dispersion solution was then placed on a porous carbon (C) film supported on a copper (Cu) microgrid. Fig. 3(a) is the low magnification TEM image of the product. No spherical droplet or nanoparticle can be seen at tips of the nanowires, agreeing with

Fig. 2. FE-SEM images of MgO NWs with various magnifications of 6500  (a) and 30,000  (b) and EDS spectra (c) of MgO NWs.

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Fig. 3. TEM image ((a), (b)), SAED pattern (c) and HRTEM image (d) of the synthesized MgO NWs.

the SEM images. Fig. 3(b) exhibits the TEM image of a single nanowire with the width of approximately 20 nm. Fig. 3(c) shows an associated selected area electron diffraction (SAED) pattern. The SAED pattern, recorded perpendicular to the nanowire long axis, can be indexed for [0 0 1] zone axis of crystallineMgO. The length direction is supposed to be along the [1 0 0] direction, as shown in Fig. 3(d) is a high resolution TEM (HRTEM) image enlarging an area enclosed by the square in Fig. 3(d), revealing a good crystallinity. The interplanar spacings are about 0.21 nm, corresponding to the (2 0 0) plane of cubic MgO. 3.3. Infrared characteristics Fig. 4 shows the FTIR transmittance spectra, in the wavenumber range 400–4000 cm  1, of MgO NWs. The FTIR study gives information about phase composition as well as the way that the oxygen is bound to the metal ions.

Fig. 4. FTIR spectrum of MgO NWs.

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The spectra consist of two main bands are in the range of 400–1000 cm  1, in which, the high frequency band of MgO stretching (ν1) is in the range of 550–580 cm  1 and the low frequency stretching band (ν2) is the range of 410–450 cm  1 [31,33]. The bands below 1000 cm  1 are related to the Mg–O absorption [34]. A high absorption band, which appeared at 1635 cm  1, is also observed in the MgO spectrum. This band is related to bending vibration of absorbed water and surface hydroxyl (  OH) [14,15]. It is interesting to note that the 3441 cm  1 and 3699 cm  1 bands on the FTIR spectrum of the dried sample were completely disappeared after calcination at 500 1C, which is due to the formation of MgO [31]. 3.4. Growth mechanism of MgO nanowires The morphology of a single crystalline nano/microstructure is often determined by the intrinsic symmetry of the corresponding lattices. The shape of a crystal can also be considered in terms of the growth kinetics, by which the fastest growing plane should dominant leaving behind the slowest growing plane as the facets of the sample. In general, the growth rate of a face will be controlled by a combination of internal, structurally related factors (intermolecular bonding preferences or dislocations), and external factors (supersaturation, temperature, solvents and impurities) [35]. The formation mechanism of MgO nanowires can be explained based on buffer action of magnesium ions. The Magnesium nitrate hexahydrate reacts with the OH  ions in the solvent to form hydrated magnesium ions (n[Mg (H2O)p]2 þ ) at temperature of 180 1C for 10 h. It is widely believed that hydrated magnesium ions become solid magnesium hydroxide ([Mgn(OH)2n]) through stepwise coordination of hydroxyl ions and the subsequent condensation of hydroxy l groups bound to individual magnesium ions. The growth process can be elaborated as follows [36]. MgðNO3 Þ2 U6H2 O þ 2NaOH -MgðOHÞ2 ðsÞ þ 2NaNO3 2þ

Mg

ðaqÞ þ 2OH



ðaqÞ-MgðOHÞ2 ðsÞ

ð3Þ ð4Þ

The magnesium hydroxide transformed into magnesium oxide through dehydration (at 500 1C) followed by the equation: MgðOHÞ2 -MgO þH2 O

ð5Þ

3.5. Optical properties It is advantageous to study the optical properties of nanostructures that are important for different industrial applications. UV–vis absorption spectroscopy technique has been used to study the optical properties of many materials as powders or thin films. Experimentally, the following equation has usually been used to estimate the band gap energy Eg of various materials of direct energy gap [37]. ðαhvÞ2 ¼ Bðhv Eg Þ

ð6Þ

Fig. 5. Absorbance spectra (a) and) plot of (αhυ)2 vs (hυ) (b) for MgO NWs.

where α is the absorption coefficient, h is Planck's constant, v is the photon frequency, B is a material-dependent constant and Eg is the band gap energy between the conduction (CB) and valance band (VB) of the material. The value of the absorption coefficient can be determined by using the following equation [38]: p¼

1 It A d ln ¼  2:33 d A I O d log e

ð7Þ

where d is the thickness of the used cuvette (the sample thickness), I0 and It are the intensities of incident and transmitted light, respectively, A is the absorbance. Fig. 5(a) shows the typical optical absorption spectra of MgO NWs (λmax ¼276 nm). Relatively clear absorption edges of the MgO nanowires were observed clearly. In order to calculate the optical band gap energy Eg of the asprepared MgO NWs, we fit the absorption data to Eq. (6) by extrapolating of the linear region of the plot of (αhν)2 on the y-axis against photon energy (E¼hν) on the x-axis. The energy gap is obtained by the x-axis intercept of the extrapolated linear part of the graph (Fig. 5(b)). The correct values of the optical gap calculated from the figure 4.51 eV for the MgO NWs. Clearly, this value is considered greater than the values that were estimated by other literatures [39,40] and in the same range of other [41]. But, interestingly these observed band gap energies of MgO NWs are invariably lower than the band gap value of bulk MgO 7.8 eV. Such a lower band gap has also been reported by

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Raj and his co-workers [42], which may be due to varied extent of non-stoichiometry of the structures. In our work, after finished the synthesis process may generate various structural defects, e.g. oxygen vacancies due to the calcinations at 500 1C for 3 h in an electrical oven. These oxygen vacancies would induce the formation of new energy levels in the bandgap of the MgO nanostructures and leading to a smaller band gap compared to bulk. The lower band gap also caused of nanosize effect, density of the defects and the short-range repulsion energy between atoms [42]. The refractive index n is a very important physical parameter related to the microscopic atomic interactions. From theoretical view point, there are basically two different approaches of viewing this subject: the refractive index will be related to the density and the local polarizability of these entities [43]. Consequently, many attempts have been made in order to relate the refractive index and the energy gap Eg through simple relationships [44–46]. However, these relations of n are independent of temperature and incident photon energy. Here the various relations between n and Eg will be reviewed. Ravindra et al. [46] had been presented a linear form of n as a function of Eg: ð8Þ

n ¼ αþ βEg

Table 1 Calculated refractive indices for different MgO nanostructures using Ravindra et al. [46], Herve and Vandamme [47] and Ghosh et al. [48] models corresponding to optical dielectric constant. n

ε1

Mg(OH)2 nanostructure

a

1.878 1.282b 1.258c

3.526a 1.643b 1.582c

MgO nanowire

2.210a 1.035b 1.990c 1.736d 1.74e 1.99n

4.884a 1.071b 3.960c 3.0f 3.1g 3.98n

MgO/Mg(OH)2 nanocomposite

2.192a 1.966b 1.958c

4.804a 3.865b 3.833c

n

This work. Ref. [46]. Ref. [47]. c Ref. [48]. d Ref. [51] experimental. e Ref. [52] theoretical. f Ref. [53] experimental. g Ref. [54] theoretical. a

b

1

where α¼ 4.048 and β¼ 0.62 eV . Light refraction and dispersion will be inspired. Herve and Vandamme [47] proposed an empirical relation as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 A n ¼ 1þ ð9Þ Eg þ B where A¼13.6 eV and B¼3.4 eV. For group-IV semiconductors, Ghosh et al. [48] have published an empirical relationship based on the band structure and quantum dielectric considerations of Penn [49] and Van Vechten [50]: n2  1 ¼

A ðEg þBÞ2

ð10Þ

where A¼8.2Eg þ134, B¼0.225Eg þ2.25 and (Eg þ B) refers to an appropriate average energy gap of the material. The calculated refractive indices of the end-point compounds are listed in Table 1. This is verified by the calculation of the optical dielectric constant ε1 which depends on the refractive index. Note that ε1 ¼n2 [54]. It is clear that the investigated n using the model of Ghosh et al. [48] is important for nanowires, while Herve and Vandamme model [47] is appropriate for nanostructures and nanocomposites for enhancing the efficiency of dyesensitized solar cells. It means high absorption and low reflection may be attributed to increase solar cell efficiency. Although MgO is a typical wide band gap insulator, the PL properties of its nanocrystals have been studied because of the presence of defects [55,56]. Fig. 6 shows the photoluminescence spectrum (PL) of MgO NWs that are measured at room temperature using the excitation wavelength of 350 nm. An intensive emission peak at around 437 nm can be observed in the PL spectrum. Clearly, the PL peak is not the band gap emission, and can be ascribed to crystal defects or defect levels associated with oxygen vacancies that

Fig. 6. PL spectra of MgO NWs.

have been formed during the calcinations process, which is similar to the literatures [55,56]. This makes the synthesized MgO nanowires are very useful in plasma display panel applications. 4. Conclusions Magnesium oxide (MgO) nanowires were synthesized by solvothermal method using magnesium nitrate hexahydrate and sodium hydroxide. The diameter of nanowires is in the range of 16–20 nm and average length of several micrometers. XRD, FTIR, FE-SEM, TEM, SAED and HRTEM analyses coincidentally indicate that the as-synthesized MgO nanowires are highly crystallized single crystals with cubic structure and grow along the [1 0 0] direction. The UV–vis spectrum for MgO nanowires shows an enhanced

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absorption intensity in the low wavelength region and an optical energy gap of 4.51 eV determined from the absorption spectra, which can be expected to hold promise for the design of optical devices owing to their strong PL emission centered at 437 nm and also may have significant scientific and technological applications as building blocks for many other functional devices due to the new nanostructures. It is proven that Ghosh et al. model is appropriated for solar cell applications. We expect that it would be a promising route to synthesize some other metal oxides with various nanostructures. Acknowledgment The authors would like to thank the Deanship of scientific research, King Saud University,Riyadh, Saudi Arabia, for funding and supporting this research. We would like to thank Prof. Dr. Blanca Hernando Grande (Department of Physics, University of Oviedo, C/Clavo Sotelo s7n, 33007 Oviedo, Spain) for helping us. References [1] X. Wang, W. Liu, H. Yang, X. Li, N. Li, R. Shi, H. Zhao, J. Yu, Acta Mater. 59 (2011) 1291–1299. [2] G. Hodes, Adv. Mater. 19 (2007) 639–655. [3] S. Noda, A. Chutinan, M. Imada, Nature 407, 608–610. [4] S. Noda, N. Yamamoto, M. Imada, H. Kobayashi, M. Okano, J. Lightwave Technol. 17 (1999) 1948–1955. [5] W. Qingqing, X. Gang, H. Gaorong, J. Solid State Chem. 178 (2005) 2680–2685. [6] M.C. Wu, J.S. Corneille, C.A. Estrada, J.W. He, D.W. Goodman, Chem. Phys. Lett. 182 (1991) 472–478. [7] Y.W. Choi, J. Kim, Thin Solid Films 460 (2004) 295–299. [8] J.Y. Park, Y.J. Lee, K.W. Jun, J.O. Baeg, D.J. Yim, J. Ind. Eng. Chem. 12 (2006) 882–887. [9] H.-B. Lu, L. Liao, H. Li, D.-F. Wang, Y. Tian, J.-C. Li, Q. Fu, B.-P. Zhu, Y. Wu, Eur. J. Inorg. Chem. 2008 (2008) 2727–2732. [10] Y. Yan, L. Zhou, J. Zhang, H. Zeng, Y. Zhang, L. Zhang, J. Phys. Chem. C 112 (2008) 10412–10417. [11] C. Yongjun, L. Jianbao, H. Yongsheng, Y. Xiaozhan, D. Jinhui, J. Cryst. Growth 245 (2002) 163–170. [12] M.A. Shah, A. Qurashi, J. Alloys Compd. 482 (2009) 548–551. [13] T. Selvamani, A. Sinhamahapatra, D. Bhattacharjya, I. Mukhopadhyay, Mater. Chem. Phys. 129 (2011) 853–861. [14] L. Sun, H. He, C. Liu, Z. Ye, Appl. Surf. Sci. 257 (2011) 3607–3611. [15] Y.F. Hao, G.W. Meng, C.H. Ye, X.R. Zhang, L.D. Zhang, J. Phys. Chem. B 109 (2005) 11204–11208. [16] Q. Shi, Y. Liu, Z. Gao, Q. Zhao, J. Mater. Sci. 43 (2008) 1438–1443. [17] R.Z. Ma, Y. Bando, Chem. Phys. Lett. 370 (2003) 770–773. [18] C. Shao, H. Guan, Y. Liu, R. Mu, J. Mater. Sci. 41 (2006) 3821–3824.

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Please cite this article as: N.M.A. Hadia, H.A.-H. Mohamed, Materials Science in Semiconductor Processing (2014), http: //dx.doi.org/10.1016/j.mssp.2014.03.049i