Morphology, mechanism and optical properties of nanometer-sized MgO synthesized via facile wet chemical method

Materials Chemistry and Physics 148 (2014) 1064e1070

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Morphology, mechanism and optical properties of nanometer-sized MgO synthesized via facile wet chemical method Rajni Verma, Kusha Kumar Naik, Jitendra Gangwar, Avanish Kumar Srivastava* CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Nanometer-sized MgO was synthesized by wet chemical process.
The synthesized MgO nanocrystals have cubic symmetry and a spherical grain-like morphology.
Size of obtained MgO nanocrystals is about 50 nm and an optical band gap value of 5.91 eV.
Resulting nano-sized MgO demonstrates blue emission band at about 421 nm.
Our approach is simple, economic and suitable for high-yield production of MgO nanostructures.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 February 2014 Received in revised form 11 September 2014 Accepted 22 September 2014 Available online 5 October 2014

In the present study, uniform sized magnesium oxide (MgO) nanostructures with high yield were successively synthesized via a simple wet chemical process under calcination temperatures of 500 and 800  C. The structure analysis was conducted and pure phase formation of MgO was identified by employing X-ray diffractometry. Both SEM and HRTEM measurements were performed to characterize the morphology and particle size of the MgO nanocrystals. The nanosized MgO exhibits an optical band gap value of 5.91 eV as obtained from the UV-visible absorption spectrum using the Tauc equation. The MgO samples produced an intense blue emission at 421 nm upon 300 nm excitation which is closely related to oxygen vacancy defect centers. A plausible mechanism is proposed to understand the formation of the observed MgO nanocrystals on the basis of experimental observations and interpretations. The optical properties of MgO suggest that it could be an exceptional choice for optoelectronic nanodevices. © 2014 Elsevier B.V. All rights reserved.

Keywords: Oxides Insulators Electron microscopy Optical properties Photoluminescence spectroscopy

1. Introduction It is known that the nanostructures have great potential applications in science and technology because of their interesting structural and optical properties [1e3]. Recently, oxide based

* Corresponding author. E-mail addresses: (A.K. Srivastava).

[email protected],

http://dx.doi.org/10.1016/j.matchemphys.2014.09.018 0254-0584/© 2014 Elsevier B.V. All rights reserved.

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nanostructures [4,5] have received considerable attention in research fields of material science, physics and chemistry [6e9] due to the presence of oxygen, a highly electronegative element, which tends to pull the bonding electrons towards itself and away from the other elements thus inducing substantial electric field at the interatomic scale [10]. Metal oxides are especially used as adsorbents, catalyst supports, optical sensors, and they are also used in biocompatibility, bioimaging [11,12] and many more due to their exceptional nanoscale structures, superior chemical and

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e1070

thermal stability [13], size-, shape-dependent, tunable morphology and optical band characteristics [14e18]. Among various promising metal oxides, large band gap materials, such as Al2O3, HfO2, MgO and CaO, have broad technological significance [19e25]. Their simple crystal structures, strong ionic bonding between cations and anions, smooth surface features and high melting points make them suitable candidates for the applications in the areas of electronics, advanced ceramics and optical devices [15,16,22,26]. Among these oxide nanostructures, magnesium oxide (MgO; magnesia) which is a binary ionic compound with a rock-salt type structure and also a ubiquitous material is attracting both fundamental and application studies [4,15,16,27e31]. MgO is a highly efficient spin injector [32], high insulating material [4,15,28] and one of the most important alkaline earth metal oxides which is used for applications in metal oxide semiconductor gatecontrolled devices, as a refractory material, for water purification, in paints and nanooptics [15e17,21,33,34]. Because of its ecofriendliness and low cost, MgO is an excellent functional oxide, exploited as unreactive substrates [15,31,35e37]. Nanostructured MgO has high surface area and therefore acts as coating agent which helps in increasing the energy conversion efficiency [38]. More interestingly, MgO nanostructures not only exhibit the blue photoluminescence emission at room temperature but they are also used as indicators and photon sources. Origin of the blue emission characteristics is still a subject of discussion [39,40]. It has been believed that the blue photoluminescence in MgO is essential because of the presence of surface defects, such as Fcenters (F0, Fþ1 and Fþ2, corresponding to the oxygen ion vacancies with two, one and zero electrons, respectively) [19,28,41,42]. Indeed, a defected surface usually plays a decisive role in catalysis, reactivity, optical and transport properties at oxide surfaces [28,31]. Generally defects, such as point defects, F-centers and intrinsic/extrinsic defects which are created during various synthetic methods may form luminescent centers on the insulating surface of materials [43]. Various synthetic approaches have been adopted to fabricate different kinds of MgO nanostructures like nano-particles, sheets, rods, wires, tubes, cages and belts by exploiting the techniques of diverse physical and/or chemical routes [11,15,16,30, 34,35,37,44e47]. Most of these methods are expensive and involve toxic organic reagents for the synthesis. Moreover, these methods need sophisticated instrumentation with complicated reaction conditions to procure even a low-yield of the material [47]. Alternatively, a wet chemical method is an easy way to fabricate the fascinating material in short reaction time, using hazardless starting ingredients. Moreover, this approach is advantageous compared to other techniques as it is simple, reliable and inexpensive for the feasibly scale-up in high yield with high purity production of material. In the present work, we observed the effect of calcination temperatures on the microstructural and optical properties of MgO produced via a simple wet solution method in a large scale with affordable cost. With the assistance of characterization techniques, such as XRD, SEM and HRTEM, it was noted that the reaction conditions and calcination temperatures have an influence on crystalline structure and both, morphological and optical characteristics of the synthesized MgO nanocrystals. On the basis of experimental observations, a plausible mechanism and the transformation process from Mg(OH)2 to MgO is proposed to understand the formation of fascinating MgO nanostructures. The optical transitions, both absorption and emission, in MgO nanocrystals suggest the potential application of MgO in optoelectronic devices.

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2. Experimental MgO nanocrystals were obtained by a simple wet chemical method and subsequent calcination. Magnesium nitrate hexahydrate [Mg(NO3)2.6H2O] (99%, Qualigens) and sodium hydroxide [NaOH] (98%, Sisco Research Laboratory) were used for the synthesis of MgO. Millipore water (18 MU) was used during all the experimental processes and other characterizations. In a typical experiment, 50 ml of 1 M NaOH solution was added dropwise into 50 ml of 0.2 M aqueous solution of Mg(NO3)2.6H2O under constant stirring at room temperature. This procedure was carried out until the pH of the solution reached 12.0 and a dense white colored precipitate appeared indicating the formation of hydroxide precursor Mg(OH)2. The resultant was filtered off and washed repeatedly by millipore water and methanol to remove unreacted anions such as nitrate (NO 3 ). Subsequently the dried white powder at room temperature was collected and calcined at high temperature to produce crystalline MgO nanostructures. To obtain the final product, the as-synthesized Mg(OH)2 precursor was heated in the muffle furnace with a constant heating rate of 10  C/min and calcined at two different temperatures of 500 and 800  C for 2 h. Two temperatures were used in the calcination process to determine the effect of calcination temperature on the microstructural and optical properties. The crystallinity and the purity of the samples were measured by X-ray diffraction (XRD) pattern by using a Rigaku bench top Xray diffractometer equipped with a monochromatic Cu-Ka radiation (l ¼ 1.541 Å) as X-ray source and scanning in 2q range from 10 to 80 . The morphology and the size of powder particles were analyzed by a scanning electron microscope (Zeiss EVO MA-10 SEM operating at 10.0 keV). The high-resolution transmission electron microscopy and fast Fourier transform analysis were performed by using a HRTEM (FEI Tecnai G2 F30 STWIN operating at 300 keV) and the attached GATAN digital-micrograph software. The UV-vis absorption spectra of the MgO products were recorded using a UVevis spectrometer (UV-2401 PC, Shimadzu Corporation Japan). The room temperature photoluminescence (PL) investigations were performed using a Perkin Elmer LS-55 fluorescence spectrophotometer with a Xenon (Xe) lamp as the source of excitation.

Fig. 1. X-ray diffraction patterns of the as-synthesized Mg(OH)2 (lower segment) and the MgO (upper segment) synthesized at two different calcination temperatures 500 and 800  C.

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For n ¼ 2 and n ¼ 3, the water molecules binding sites are similar, whereas for n ¼ 4, the binding sites are significantly different and the complex is very stable even at room temperature [48].

3. Results and discussion 3.1. Crystallographic phase transition, phase identification and phase purity The crystal structure, phase identification and phase purity of the as-synthesized Mg(OH)2 and MgO products were obtained by using XRD, as shown in Fig. 1. The lower segment of Fig. 1 demonstrates the XRD pattern of the as-synthesized Mg(OH)2 white powder. All the diffraction peaks, (001), (100), (101), (102), (110), (111), (013), and (201), obtained from the sample can be indexed to a hexagonal crystal structure of Mg(OH)2 with space group P3m1, possessing lattice parameters of a ¼ b ¼ 0.313 nm and c ¼ 0.475 nm (standard JCPDF card no. 75e1527). The position of the diffraction peaks and the relative intensity of the peaks were identified as characteristic of the X-ray diffraction pattern of Mg(OH)2. There were no peaks corresponding to impurities or the remnant of Mg(NO3)2, signifying the successful completion of the hydrolysis process leading to the formation of Mg(OH)2. The peak centered at 37.82 with FWHM (obtained by Gauss fit to the relevant peak) 1.20 was used for calculation of average crystallite size of Mg(OH)2 sample. The estimated average crystallite size by the Scherrer's relation was 7 nm for Mg(OH)2 sample. The synthesis procedure to produce nanostructured MgO was based on the precipitation reaction occurred between the Mg2þ ions and OH ions during stirring which results in the formation of Mg(OH)2 and subsequent thermal decomposition of Mg(OH)2 as the precursor at 500 and 800  C for 2 h:

MgðNO3 Þ2 $6H2 O þ 2NaOH/MgðOHÞ2 /MgO

(1)

The upper segment of Fig. 1 displays the powder XRD patterns of MgO samples calcined at 500 and 800  C for 2 h. All the diffraction peaks, (111), (200), (220), (311), and (222) and the relative intensity of the peaks observed from the samples correspond well to a pure cubic crystal structure of MgO with space group Fm3m, possessing lattice parameters of a ¼ b ¼ c ¼ 0.419 nm (standard JCPDF card no. 75e1525). There were no peaks arising from Mg(OH)2 and impurities, which indicates a complete transformation of Mg(OH)2 / MgO at 500  C and thus the pure MgO crystals have been successively synthesized. Interestingly, it can also be observed that the width of diffraction peaks is considerably broadened, indicating a small domain size of crystallites. The broadening diminishes and the peaks become more intense with the increase of calcination temperature to 800  C, revealing the growth of the MgO crystallinity and crystallite sizes. The peak centered at 42.92 and 42.90 with FWHM 0.50 and 0.34, respectively for the MgO samples obtained at the calcination temperatures of 500 and 800  C were used for calculation. The obtained average crystallite size was 17 and 25 nm for MgO samples calcined at temperatures of 500 and 800  C, respectively.

3.2. Mechanism of formation and crystal structure of MgO In this synthesis system, a complex [Mg(NO3)(H2O)n]þ is formed during the preparation of magnesium hydroxide precipitate from magnesium nitrate hexahydrate. The consequent reaction series in the formation of MgO are as follows: dissolution

MgðNO3 Þ2 $6H2 Oƒƒƒƒƒ! MgðNO3 Þ2 þ 6H2 O

(2)


þ MgðNO3 Þ2 þ nH2 O/ Mg NO3 ðH2 OÞn þ NO 3

(3)

where n ¼ 1e4.


þ Mg NO3 ðH2 OÞ3 #Mg2þ þ NO 3 þ 3H2 O

(4)

Mg2þ þ 2OH/MgðOHÞ2 Y

(5)

MgðOHÞ2 /MgO þ H2 O

(6)

The formation of magnesium oxide nanocrystals from magnesium nitrate hexahydrate involves three steps, (i) dissolution, (ii) formation of an intermediate, and (iii) transformation into MgO. In dissolution, the solid powder of magnesium nitrate hexahydrate undergoes solvation to release magnesium nitrate, given by Eq. (2). This magnesium nitrate reacts with sodium hydroxide solution and forms a monovalent cation intermediate complex by the reaction Eq. (3). In the complex [Mg(NO3)(H2O)3]þ, there occurs bidentate binding motif of the nitrate to the Mg bivalent cation and thus Mg is five-fold coordinated. So being an unstable complex, it breaks to give magnesium bivalent cation according to the reaction Eq. (4). This magnesium bivalent cation reacts with hydroxyl species of sodium hydroxide to give white precipitate of magnesium hydroxide using the reaction Eq. (5). The obtained precursor of Mg(OH)2 (hexagonal structure) produces six fold coordinated MgO (cubic structure) when calcined at 500 and 800  C in accordance with the reaction Eq. (6). The obtained MgO has rocksalt (NaCl, B1) type structure. The high ionic interaction between Mg2þ (1s22s22p6) and O2 (1s22s22p6) provides it high thermal stability [2,16,27]. The strong attraction between two oppositely charged ions and the formation of divalent cation of magnesium and divalent anion of oxygen leads to the formation of MgO. On the basis of above experimental observations and analysis, we proposed a plausible formation mechanism for the observed MgO (schematic in Fig. 2a). Initially, the nucleation of the complex [Mg(NO3)(H2O)n]þ occurs and subsequently the precursor Mg(OH)2 is obtained when NaOH solution was added dropwise. The pure, uniform and high yield product of MgO nanocrystals was obtained during the calcination process of the precursor. This Mg(OH)2 precursor with hexagonal crystal symmetry (Fig. 2b) was transformed into MgO nanocrystals with the cubic structure (Fig. 2c). During the decomposition/dehydration process, the precursor Mg(OH)2 loses water molecules and transforms into MgO by an oxolation mechanism [49], while the morphology remains unchanged. A pictorial representation for the transformation process from Mg(OH)2 to MgO has been depicted in Scheme 1. 3.3. Topography and morphological characteristics Scanning electron microscopy (SEM) analysis was performed to evaluate the fine-scaled topological features of the samples. SEM micrographs at different magnifications for the samples of assynthesized Mg(OH)2 and calcined MgO at two different temperatures of 500 and 800  C were depicted in Fig. 3. It was observed that the microstructure of all the samples was uniform without any significant change in homogeneity. Fig. 3a revealed the Mg(OH)2 particles consist mainly of coarse crystals with sizes larger than 300 nm, while the nanostructured MgO obtained by calcining at 500  C (Fig. 3b) contains many nanocrystals with sizes smaller than 100 nm. After calcining Mg(OH)2 at higher temperature of 800  C (Fig. 3c), a spherical grain-like morphology is inherited with appearance of furthermost density and the structure of MgO get transformed into small uniform nanoparticles within the size range

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Fig. 2. (a) Schematic illustration of the plausible mechanism for the formation of the MgO nanocrystals obtained by calcining the as-synthesized Mg(OH)2 product. Crystal structure models of (b) h-Mg(OH)2 hexagonal phase and (c) c-MgO cubic phase. Atoms for magnesium, oxygen and hydrogen are represented by Mg, O and H symbols, respectively. In both models, the intense lattice planes (110) for h-Mg(OH)2 and (200) for c-MgO obtained from XRD patterns are indicated by the gray rectangles. Unit cells of both models are represented by gray solid lines.

Scheme 1. Pictorial representation for the Mg(OH)2 / MgO transformation process from starting material Mg(NO3)2; blue, pink and green balls represent the oxygen (O), magnesium (Mg) and hydrogen (H) ions, respectively. The symbols rþ and r provide the ionic radius of cations and anions, respectively. The loss of water (H2O) molecules from the Mg(OH)2 precursor is performed by the black dotted rectangles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3. High-magnification SEM images of (a) as-synthesized Mg(OH)2, (b) MgO obtained at calcination temperature 500  C, (c) MgO obtained at calcination temperature 800  C. Insets in (a)e(c) correspond to lower magnification micrographs.

of 40e60 nm. The size of the product was found to be smaller than that of the as-synthesized and calcined sample at low temperature (500  C). The low-magnification SEM images were displayed in the inset of Fig. 3aec of the corresponding samples. It can be observed that the calcined MgO samples and as-synthesized Mg(OH)2 sample have identical morphology. The low-magnification SEM micrograph (inset in Fig. 3a) shows that the Mg(OH)2 comprises of densely packed uniform sized particles with sizes in the range of 350 nm to 1 mm, indicating the bursting of spherical structure due to the removal of water molecules. The morphology of the grains is uniform in each region (as represented by white dotted circles in main image Fig. 3aec) and the sizes of the grains reduced on increase of calcination temperatures, signifying the elimination of OH ions from the surface of Mg(OH)2 and the complete transformation into MgO at even low calcination temperature of 500  C and different crystalline quality obtained at high calcination temperature of 800  C. Electron microscopy data revealed that the particle size at 800  C is smaller than the particle size at 500  C. However the crystallite size has shown a reverse trend. Although the trend is peculiar, but probably on increasing the calcination temperature the individual particles are decomposing into finer size possibly due to thermal-stability reasons. However, since the crystal structure remains the same, the corresponding crystallites keep growing at higher temperatures of calcination. To further obtain more structural information and to characterize the sample in real and reciprocal space, HRTEM was performed (Fig. 4). The MgO sample corresponding to the calcination temperature of 800  C was chosen for this study. A typical brightfield TEM image at high magnification of the MgO nanostructure (Fig. 4a) exhibits the fine size grains about 45 nm in width and 75 nm in length. The upper right inset (in Fig. 4a) shows the bright-

Fig. 4. (a) bright-field TEM micrograph and (b) high-resolution TEM image showing the lattice fringe scaling of the MgO sample corresponds to calcination temperature of 800  C. Insets illustrate a corresponding lower magnification image (inset in a) and fast Fourier-transformed pattern (inset in b). Another inset (inset part of a) provides the particle size distribution of the corresponding nanocrystals.

field TEM image of the corresponding sample at low magnification. This image reveals that the yield, uniformity and purity are high, which is the focal point of our present investigation. The particle size distribution (inset in Fig. 4a inset), obtained from TEM results, shows the grain-size of about 50 nm and approximately spherical nanocrystals. However, the crystal structure analysis of MgO nanostructure can be achieved based on lattice fringe image which is displayed in Fig. 4b. The lattice resolved high-resolution TEM (HRTEM) image, consisting of well organized lattice fringes of spacing d ¼ 0.21 nm, represents the d spacing between the (200) planes of a face-centered cubic MgO. No evidence for imperfections at the lattice scale on the (200) planes is observed and the stacking of (200) planes is atomically clean within the particle. A corresponding FFT pattern (displayed in inset Fig. 4b) in reciprocal space exhibits a spot like diffraction pattern of (200) planes of the cubic phase MgO, in agreement with the XRD pattern. 3.4. Optical properties: absorption and emission It is generally accepted that the optical transitions arise when a photon is absorbed or emitted by the defect [28]. Therefore, optical absorption and luminescence emission were studied to know the existence of intrinsic point defects, in particular oxygen vacancies, in the synthesized MgO nanocrystals. Fig. 5a illustrates the UV-vis absorption spectra of MgO nanocrystals calcined at 500 and 800  C. The absorption maximum for MgO at 500  C was found to be at about 262 nm, which can be attributed to the electronic excitations of 3-coordinated surface anions at the corners of the crystallites. Inset in Fig. 5a demonstrates a plot of (ahn)2 versus hn for MgO at different calcination temperatures. The corresponding optical band gap value calculated from the UV-vis spectra using the Tauc equation (shown as the dotted lines in Fig. 5a inset) was 5.72 and 5.91 eV for calcination temperatures 500 and 800  C, respectively. The lower band gap value of MgO sample corresponding to calcination temperature of 500  C is due to presence of the 3coordinated surface anions at the corners, whereas the existence of 4-coordinated surface anions at the edges is responsible for the higher band gap value for the MgO sample corresponding to calcination temperature of 800  C. We infer that owing to the occurrence of 6-coordinated surface anions at the terrace in the perfect MgO surface, MgO possess the optical band gap of 7.8 eV [11]. To investigate the presence of defects or oxygen vacancies on the surface of MgO, room-temperature photoluminescence (PL) spectroscopy was carried out. PL is a direct optical tool to describe the surface defects and electronic energy band structure. Generally, the intrinsic band edge structure, due to excitonic recombination and other internal/external factors (intrinsic or extrinsic defects), is because of photogenerated hole and electrons, and is responsible for the luminescence characteristics in metal oxides [43,50]. It has

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Fig. 5. (a) UV-vis spectra of the aqueous dispersion of the MgO nanocrystals obtained at calcination temperatures 500 and 800  C. Inset (in a) demonstrates the Tauc plots for the MgO sample corresponding to calcination temperature of 500 and 800  C. (b) Room temperature photoluminescence emission spectra of the MgO sample recorded under excitation at 300 nm showing intense blue emission. The peak positions, numbered as 1, 2 and 3 are shown in nanometer. (c) The possible absorption (left) and emission (right) process in MgO.

been shown that the cubic-MgO is a wide-band gap energy material and normally does not demonstrate the PL characteristic [40]. In the MgO crystalline structure, some structural defects are pre2þ sent, such as magnesium vacancies (Mg2þ 3c ; Mg4c ) and oxygen va2 2 cancies (O3c ; O4c ) [41]. There are three active sites or surface anions (oxygen, O2) present; (i) 3-coordinated at the corner (O2 3c ), (ii) 4-coordinated at edges (O2 4c ) and (iii) 5-coordinated at the planar site (O2 5c ) in the MgO nanocrystals, which belong to surface defects at the stepped and kinked surfaces [11,19,41,51]. In the present investigation, we measured the PL spectra of MgO samples calcined at 500 and 800  C. Fig. 5b provides the PL emission spectra (excitation wavelength: 300 nm, 4.13 eV) of the MgO samples. We observed a wide range of emissions at 325e690 nm (1.79e3.81 eV). The spectra reveal one intense emission peak and two accompanying shoulder peaks for both the samples. The occurrence of the major blue PL emission peak at about 421 nm (2.94 eV) is due to the presence of oxygen vacancy with lowcoordinate oxygen anions (O2 3c ) in the MgO [21]. The various structural defects in their crystalline structure and high2 coordinated oxygen anions (O2 4c and O5c ) are responsible for the presence of almost suppressed shoulder emissions in the PL spectra of the MgO at about 447 nm (2.77 eV) and 483 nm (2.56 eV). It was interesting to see from Fig. 5b that on increasing the calcination temperature of the MgO samples the observed PL spectrum decreases drastically in intensity without appearance of new emission band. We infer that the change in the intensity may be because of the change in crystallite size of the MgO nanocrystals concluding that the increase in nanocrystallite size with increase of calcination temperature is responsible for reducing the luminescence intensity. As the crystallite size increases with increase of calcination

temperature, the density of the defects gets reduced and moreover the surface to volume ratio also decreases. Because of these alterations in the microstructure at nano-scale, the intensity of the PL emission decreases with increase of calcination temperature. PL is an intrinsic property, which is in agreement with the results revealing that the defect centers in MgO nanocrystals are oxygendeficient [51,52]. In our synthesized MgO samples, the origin of PL emission can be attributed to the existence of oxygen vacancies/ intrinsic or extrinsic point defects on the metal oxide surface during the transformation of Mg(OH)2 to MgO. The existence of such intrinsic point defects on the surface of MgO nanocrystals are confirmed by PL studies, which act as an invisible agent and offers an inexpensive alternative for the promising electronic and optical applications in nanodevices [21,44]. The optical transitions (absorption and emission process) observed from the UV-vis absorption and photoluminescence spectra are illustrated in Fig. 5c. 4. Conclusions We employed a wet chemical method to fabricate MgO nanocrystals with varying calcination temperatures and without the presence of solvent, template and surfactant. The reaction conditions are very simple, reliable, and cost-effective as well as require easily available starting material which allows the production of MgO nanocrystals in high-yield so that the material can easily be scaled-up for industrial production. The calcination temperature maintained during the synthesis of MgO with cubic crystal structure was found to have prominent impact on the crystallite sizes of the particles. Increasing the calcination temperatures from 500 to 800  C, improved the optical band gap of MgO nanocrystals. The

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photoluminescence characteristics of nanometer-sized MgO indicate that oxygen vacancy defect centers generated at the surface of the oxide are responsible for the observed intense blue emissions peak at about 421 nm upon 300 nm excitation. We suggest that the efficient preparation and the detailed investigations of MgO nanocrystals may open up new simple route to fabricate fine particles with high optical performances, to shed new insight for future developing nano-devices. Acknowledgments We thank the Director, NPL New Delhi, India for providing the necessary experimental facilities. Dr. Ajay Dhar, Mr. K. N. Sood, Mr. J. S. Tawale, Dr. Ritu Srivastava and Mr. Praveen are gratefully acknowledged for providing the necessary instrumentation facilities for XRD, SEM, UV-vis and PL. Nano-SHE project (BSC-0112) is gratefully acknowledged. References €ker, J. He, K. Sill, H. Xiang, C. Abetz, X. Li, J. Wang, T. Emrick, S. Long, [1] Y. Lin, A. Bo Q. Wang, A. Balazs, T.P. Russell, Nature 434 (2005) 55e59. [2] J. Goniakowski, C. Noguera, L. Giordano, Phys. Rev. Lett. 93 (2004) 215702.  , P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijk, Nat. Mater. 4 [3] A.S. Arico (2005) 366e377. [4] N.A. Richter, S. Sicolo, S.V. Levchenko, J. Sauer, M. Scheffler, Phys. Rev. Lett. 111 (2013) 045502. [5] A. Chanthbouala, R. Matsumoto, J. Grollier, V. Cros, A. Anane, A. Fert, A.V. Khvalkovskiy, K.A. Zvezdin, K. Nishimura, Y. Nagamine, H. Maehara, K. Tsunekawa, A. Fukushima, S. Yuasa, Nat. Phys. 7 (2011) 626e630. [6] H.J. Shin, J. Jung, K. Motobayashi, S. Yanagisawa, Y. Morikawa, Y. Kim, M. Kawai, Nat. Mater. 9 (2010) 442e447. [7] H. Yan, H.S. Choe, S.W. Nam, Y. Hu, S. Das, J.F. Klemic, J.C. Ellenbogen, C.M. Lieber, Nature 470 (2011) 240e244. [8] M. Deepa, N. Bahadur, A.K. Srivastava, P. Chaganti, K.N. Sood, J. Phys. Chem. Solids 70 (2009) 291e297. [9] J. Gangwar, K.K. Dey, S.K. Tripathi, M. Wan, R.R. Yadav, R.K. Singh, Samta, A.K. Srivastava, Nanotechnology 24 (2013) 415705. [10] J. Heber, Nature 459 (2009) 28e30. [11] K. Krishnamoorthy, J.Y. Moon, H.B. Hyun, S.K. Cho, S.J. Kim, J. Mater. Chem. 22 (2012) 24610e24617. [12] M. Deepa, A.K. Srivastava, S.A. Agnihotry, Acta Mater. 54 (2006) 4583e4595. [13] N. Bahadur, K. Jain, A.K. Srivastava, Govind, R. Gakhar, D. Haranath, M.S. Dulat, Mater. Chem. Phys. 124 (2010) 600e608. [14] J. Goniakowski, L. Giordano, C. Noguera, Phys. Rev. B 87 (2013) 035405. [15] Y.G. Zhang, H.Y. He, B.C. Pan, J. Phys. Chem. C 116 (2012) 23130e23135. [16] K. Zhu, W. Hua, W. Deng, R.M. Richards, Eur. J. Inorg. Chem. 2012 (2012) 2869e2876. [17] W. Zhu, L. Zhang, G.L. Tian, R. Wang, H. Zhang, X. Piao, Q. Zhang, Cryst. Eng. Comm. 16 (2014) 308e318. [18] N. Bahadur, A.K. Srivastava, S. Kumar, M. Deepa, B. Nag, Thin Solid Films 518 (2010) 5257e5264.

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