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Materials Chemistry and Physics 148 (2014) 1064e1070

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Morphology, mechanism and optical properties of nanometer-sizedMgO 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

* Corresponding author.E-mail addresses: avanish.aks555@gma

(A.K. Srivastava).

http://dx.doi.org/10.1016/j.matchemphys.2014.09.0180254-0584/© 2014 Elsevier B.V. All rights reserved.

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

� Nanometer-sized MgO was synthe-sized by wet chemical process.

� The synthesized MgO nanocrystalshave cubic symmetry and a sphericalgrain-like morphology.

� Size of obtained MgO nanocrystals isabout 50 nm and an optical band gapvalue of 5.91 eV.

� Resulting nano-sized MgO demon-strates blue emission band at about421 nm.

� Our approach is simple, economicand suitable for high-yield produc-tion of MgO nanostructures.

a r t i c l e i n f o

Article history:Received 5 February 2014Received in revised form11 September 2014Accepted 22 September 2014Available online 5 October 2014

Keywords:OxidesInsulatorsElectron microscopyOptical propertiesPhotoluminescence spectroscopy

a b s t r a c t

In the present study, uniform sized magnesium oxide (MgO) nanostructures with high yield were suc-cessively synthesized via a simple wet chemical process under calcination temperatures of 500 and800 �C. The structure analysis was conducted and pure phase formation of MgO was identified byemploying X-ray diffractometry. Both SEM and HRTEM measurements were performed to characterizethe morphology and particle size of the MgO nanocrystals. The nanosized MgO exhibits an optical bandgap value of 5.91 eV as obtained from the UV-visible absorption spectrum using the Tauc equation. TheMgO samples produced an intense blue emission at 421 nm upon 300 nm excitation which is closelyrelated to oxygen vacancy defect centers. A plausible mechanism is proposed to understand the for-mation 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 optoelectronicnanodevices.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

It is known that the nanostructures have great potential ap-plications in science and technology because of their interestingstructural and optical properties [1e3]. Recently, oxide based

il.com, aks@nplindia.org

nanostructures [4,5] have received considerable attention inresearch 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 awayfrom the other elements thus inducing substantial electric field atthe interatomic scale [10]. Metal oxides are especially used asadsorbents, catalyst supports, optical sensors, and they are alsoused in biocompatibility, bioimaging [11,12] and many more due totheir exceptional nanoscale structures, superior chemical and

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

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e1070 1065

thermal stability [13], size-, shape-dependent, tunablemorphology and optical band characteristics [14e18]. Amongvarious promising metal oxides, large band gap materials, such asAl2O3, HfO2, MgO and CaO, have broad technological significance[19e25]. Their simple crystal structures, strong ionic bondingbetween cations and anions, smooth surface features and highmelting points make them suitable candidates for the applicationsin 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 typestructure and also a ubiquitous material is attracting both funda-mental and application studies [4,15,16,27e31]. MgO is a highlyefficient spin injector [32], high insulating material [4,15,28] andone of the most important alkaline earth metal oxides which isused for applications in metal oxide semiconductor gate-controlled devices, as a refractory material, for water purifica-tion, in paints and nanooptics [15e17,21,33,34]. Because of its eco-friendliness and low cost, MgO is an excellent functional oxide,exploited as unreactive substrates [15,31,35e37]. NanostructuredMgO has high surface area and therefore acts as coating agentwhich helps in increasing the energy conversion efficiency [38].More interestingly, MgO nanostructures not only exhibit the bluephotoluminescence emission at room temperature but they arealso used as indicators and photon sources. Origin of the blueemission characteristics is still a subject of discussion [39,40]. Ithas been believed that the blue photoluminescence in MgO isessential because of the presence of surface defects, such as F-centers (F0, Fþ1 and Fþ2, corresponding to the oxygen ion vacancieswith 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 andintrinsic/extrinsic defects which are created during various syn-thetic methods may form luminescent centers on the insulatingsurface of materials [43].

Various synthetic approaches have been adopted to fabricatedifferent kinds of MgO nanostructures like nano-particles, sheets,rods, wires, tubes, cages and belts by exploiting the techniques ofdiverse physical and/or chemical routes [11,15,16,30,34,35,37,44e47]. Most of these methods are expensive andinvolve toxic organic reagents for the synthesis. Moreover, thesemethods need sophisticated instrumentation with complicatedreaction conditions to procure even a low-yield of the material [47].Alternatively, awet chemical method is an easy way to fabricate thefascinating material in short reaction time, using hazardless start-ing ingredients. Moreover, this approach is advantageous comparedto other techniques as it is simple, reliable and inexpensive for thefeasibly scale-up in high yield with high purity production ofmaterial.

In the present work, we observed the effect of calcinationtemperatures on the microstructural and optical properties ofMgO produced via a simple wet solution method in a large scalewith affordable cost. With the assistance of characterizationtechniques, such as XRD, SEM and HRTEM, it was noted that thereaction conditions and calcination temperatures have an influ-ence on crystalline structure and both, morphological and opticalcharacteristics of the synthesized MgO nanocrystals. On the basisof experimental observations, a plausible mechanism and thetransformation process from Mg(OH)2 to MgO is proposed to un-derstand the formation of fascinating MgO nanostructures. Theoptical transitions, both absorption and emission, in MgO nano-crystals suggest the potential application of MgO in optoelectronicdevices.

2. Experimental

MgO nanocrystals were obtained by a simple wet chemicalmethod and subsequent calcination. Magnesium nitrate hexahy-drate [Mg(NO3)2.6H2O] (99%, Qualigens) and sodium hydroxide[NaOH] (98%, Sisco Research Laboratory) were used for the syn-thesis of MgO. Millipore water (18 MU) was used during all theexperimental processes and other characterizations. In a typicalexperiment, 50 ml of 1 M NaOH solution was added dropwise into50 ml of 0.2 M aqueous solution of Mg(NO3)2.6H2O under constantstirring at room temperature. This procedure was carried out untilthe pH of the solution reached 12.0 and a dense white coloredprecipitate appeared indicating the formation of hydroxide pre-cursor Mg(OH)2. The resultant was filtered off and washedrepeatedly by millipore water and methanol to remove unreactedanions such as nitrate (NO3

�). Subsequently the dried white powderat room temperature was collected and calcined at high tempera-ture to produce crystalline MgO nanostructures. To obtain the finalproduct, the as-synthesized Mg(OH)2 precursor was heated in themuffle furnace with a constant heating rate of 10 �C/min andcalcined at two different temperatures of 500 and 800 �C for 2 h.Two temperatures were used in the calcination process to deter-mine the effect of calcination temperature on the microstructuraland optical properties.

The crystallinity and the purity of the samples were measuredby X-ray diffraction (XRD) pattern by using a Rigaku bench top X-ray diffractometer equipped with a monochromatic Cu-Ka radia-tion (l ¼ 1.541 Å) as X-ray source and scanning in 2q range from 10to 80�. The morphology and the size of powder particles wereanalyzed by a scanning electron microscope (Zeiss EVO MA-10SEM operating at 10.0 keV). The high-resolution transmissionelectron microscopy and fast Fourier transform analysis wereperformed by using a HRTEM (FEI Tecnai G2 F30 STWIN operatingat 300 keV) and the attached GATAN digital-micrograph software.The UV-vis absorption spectra of the MgO products were recordedusing a UVevis spectrometer (UV-2401 PC, Shimadzu CorporationJapan). The room temperature photoluminescence (PL) in-vestigations were performed using a Perkin Elmer LS-55 fluores-cence spectrophotometer with a Xenon (Xe) lamp as the source ofexcitation.

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e10701066

3. Results and discussion

3.1. Crystallographic phase transition, phase identification andphase purity

The crystal structure, phase identification and phase purity ofthe as-synthesized Mg(OH)2 and MgO products were obtained byusing XRD, as shown in Fig. 1. The lower segment of Fig. 1 dem-onstrates the XRD pattern of the as-synthesized Mg(OH)2 whitepowder. All the diffraction peaks, (001), (100), (101), (102), (110),(111), (013), and (201), obtained from the sample can be indexed toa hexagonal crystal structure of Mg(OH)2 with space group P3m1,possessing lattice parameters of a¼ b¼ 0.313 nm and c¼ 0.475 nm(standard JCPDF card no. 75e1527). The position of the diffractionpeaks and the relative intensity of the peaks were identified ascharacteristic of the X-ray diffraction pattern of Mg(OH)2. Therewere no peaks corresponding to impurities or the remnant ofMg(NO3)2, signifying the successful completion of the hydrolysisprocess leading to the formation of Mg(OH)2. The peak centered at37.82� with FWHM (obtained by Gauss fit to the relevant peak) 1.20was used for calculation of average crystallite size of Mg(OH)2sample. The estimated average crystallite size by the Scherrer'srelation was 7 nm for Mg(OH)2 sample.

The synthesis procedure to produce nanostructured MgO wasbased on the precipitation reaction occurred between the Mg2þ

ions and OH� ions during stirring which results in the formation ofMg(OH)2 and subsequent thermal decomposition of Mg(OH)2 asthe precursor at 500 and 800 �C for 2 h:

MgðNO3Þ2$6H2Oþ 2NaOH/MgðOHÞ2/MgO (1)

The upper segment of Fig.1 displays the powder XRD patterns ofMgO samples calcined at 500 and 800 �C for 2 h. All the diffractionpeaks, (111), (200), (220), (311), and (222) and the relative intensityof the peaks observed from the samples correspond well to a purecubic crystal structure of MgO with space group Fm3m, possessinglattice 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 impu-rities, which indicates a complete transformation ofMg(OH)2 / MgO at 500 �C and thus the pure MgO crystals havebeen successively synthesized. Interestingly, it can also be observedthat the width of diffraction peaks is considerably broadened,indicating a small domain size of crystallites. The broadening di-minishes and the peaks become more intense with the increase ofcalcination temperature to 800 �C, revealing the growth of theMgOcrystallinity and crystallite sizes. The peak centered at 42.92� and42.90� with FWHM0.50 and 0.34, respectively for theMgO samplesobtained at the calcination temperatures of 500 and 800 �C wereused for calculation. The obtained average crystallite size was 17and 25 nm for MgO samples calcined at temperatures of 500 and800 �C, respectively.

3.2. Mechanism of formation and crystal structure of MgO

In this synthesis system, a complex [Mg(NO3)(H2O)n]þ is formedduring the preparation of magnesium hydroxide precipitate frommagnesium nitrate hexahydrate. The consequent reaction series inthe formation of MgO are as follows:

MgðNO3Þ2$6H2O�����!dissolutionMgðNO3Þ2 þ 6H2O (2)

MgðNO3Þ2 þ nH2O/�Mg

�NO3

�ðH2OÞn�þ þ NO�

3 (3)

where n ¼ 1e4.

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

�Mg

�NO3

�ðH2OÞ3�þ

#Mg2þ þ NO�3 þ 3H2O (4)

Mg2þ þ 2OH/MgðOHÞ2Y (5)

MgðOHÞ2/MgOþ H2O (6)

The formation of magnesium oxide nanocrystals from magne-sium nitrate hexahydrate involves three steps, (i) dissolution, (ii)formation of an intermediate, and (iii) transformation into MgO. Indissolution, the solid powder of magnesium nitrate hexahydrateundergoes solvation to release magnesium nitrate, given by Eq. (2).This magnesium nitrate reacts with sodium hydroxide solution andforms a monovalent cation intermediate complex by the reactionEq. (3). In the complex [Mg(NO3)(H2O)3]þ, there occurs bidentatebindingmotif of the nitrate to theMg bivalent cation and thusMg isfive-fold coordinated. So being an unstable complex, it breaks togive magnesium bivalent cation according to the reaction Eq. (4).This magnesium bivalent cation reacts with hydroxyl species ofsodium hydroxide to give white precipitate of magnesium hy-droxide using the reaction Eq. (5). The obtained precursor ofMg(OH)2 (hexagonal structure) produces six fold coordinated MgO(cubic structure) when calcined at 500 and 800 �C in accordancewith 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 chargedions and the formation of divalent cation of magnesium and diva-lent 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 observedMgO (schematic in Fig. 2a). Initially, the nucleation of the complex[Mg(NO3)(H2O)n]þ occurs and subsequently the precursor Mg(OH)2is obtained when NaOH solution was added dropwise. The pure,uniform and high yield product of MgO nanocrystals was obtainedduring the calcination process of the precursor. This Mg(OH)2precursor with hexagonal crystal symmetry (Fig. 2b) was trans-formed into MgO nanocrystals with the cubic structure (Fig. 2c).During the decomposition/dehydration process, the precursorMg(OH)2 loses water molecules and transforms into MgO by anoxolation mechanism [49], while the morphology remains un-changed. A pictorial representation for the transformation processfrom Mg(OH)2 to MgO has been depicted in Scheme 1.

3.3. Topography and morphological characteristics

Scanning electron microscopy (SEM) analysis was performed toevaluate the fine-scaled topological features of the samples. SEMmicrographs at different magnifications for the samples of as-synthesized Mg(OH)2 and calcined MgO at two different tempera-tures of 500 and 800 �Cwere depicted in Fig. 3. It was observed thatthe microstructure of all the samples was uniform without anysignificant change in homogeneity. Fig. 3a revealed the Mg(OH)2particles consist mainly of coarse crystals with sizes larger than300 nm, while the nanostructured MgO obtained by calcining at500 �C (Fig. 3b) contains many nanocrystals with sizes smaller than100 nm. After calcining Mg(OH)2 at higher temperature of 800 �C(Fig. 3c), a spherical grain-like morphology is inherited withappearance of furthermost density and the structure of MgO gettransformed into small uniform nanoparticles within the size range

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 structuremodels 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 bothmodels, 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 rep-resented 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 theMg(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 thisarticle.)

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e1070 1067

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.

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e10701068

of 40e60 nm. The size of the product was found to be smaller thanthat of the as-synthesized and calcined sample at low temperature(500 �C). The low-magnification SEM images were displayed in theinset of Fig. 3aec of the corresponding samples. It can be observedthat the calcined MgO samples and as-synthesized Mg(OH)2 sam-ple have identical morphology. The low-magnification SEMmicrograph (inset in Fig. 3a) shows that the Mg(OH)2 comprises ofdensely packed uniform sized particles with sizes in the range of350 nm to 1 mm, indicating the bursting of spherical structure dueto the removal of water molecules. The morphology of the grains isuniform in each region (as represented by white dotted circles inmain image Fig. 3aec) and the sizes of the grains reduced on in-crease of calcination temperatures, signifying the elimination ofOH� ions from the surface of Mg(OH)2 and the complete trans-formation into MgO at even low calcination temperature of 500 �Cand different crystalline quality obtained at high calcination tem-perature of 800 �C. Electron microscopy data revealed that theparticle size at 800 �C is smaller than the particle size at 500 �C.However the crystallite size has shown a reverse trend. Althoughthe trend is peculiar, but probably on increasing the calcinationtemperature the individual particles are decomposing into finersize possibly due to thermal-stability reasons. However, since thecrystal structure remains the same, the corresponding crystalliteskeep growing at higher temperatures of calcination.

To further obtain more structural information and to charac-terize the sample in real and reciprocal space, HRTEM was per-formed (Fig. 4). The MgO sample corresponding to the calcinationtemperature of 800 �C was chosen for this study. A typical bright-field TEM image at high magnification of the MgO nanostructure(Fig. 4a) exhibits the fine size grains about 45 nm in width and75 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 showingthe lattice fringe scaling of the MgO sample corresponds to calcination temperature of800 �C. Insets illustrate a corresponding lower magnification image (inset in a) and fastFourier-transformed pattern (inset in b). Another inset (inset part of a) provides theparticle size distribution of the corresponding nanocrystals.

field TEM image of the corresponding sample at lowmagnification.This image reveals that the yield, uniformity and purity are high,which is the focal point of our present investigation. The particlesize distribution (inset in Fig. 4a inset), obtained from TEM results,shows the grain-size of about 50 nm and approximately sphericalnanocrystals. However, the crystal structure analysis of MgOnanostructure can be achieved based on lattice fringe image whichis displayed in Fig. 4b. The lattice resolved high-resolution TEM(HRTEM) image, consisting of well organized lattice fringes ofspacing d ¼ 0.21 nm, represents the d spacing between the (200)planes of a face-centered cubic MgO. No evidence for imperfectionsat the lattice scale on the (200) planes is observed and the stackingof (200) planes is atomically clean within the particle. A corre-sponding FFT pattern (displayed in inset Fig. 4b) in reciprocal spaceexhibits a spot like diffraction pattern of (200) planes of the cubicphase MgO, in agreement with the XRD pattern.

3.4. Optical properties: absorption and emission

It is generally accepted that the optical transitions arise when aphoton is absorbed or emitted by the defect [28]. Therefore, opticalabsorption and luminescence emission were studied to know theexistence of intrinsic point defects, in particular oxygen vacancies,in the synthesized MgO nanocrystals. Fig. 5a illustrates the UV-visabsorption spectra of MgO nanocrystals calcined at 500 and800 �C. The absorption maximum for MgO at 500 �C was found tobe at about 262 nm, which can be attributed to the electronic ex-citations of 3-coordinated surface anions at the corners of thecrystallites. Inset in Fig. 5a demonstrates a plot of (ahn)2 versus hnfor MgO at different calcination temperatures. The correspondingoptical band gap value calculated from the UV-vis spectra using theTauc equation (shown as the dotted lines in Fig. 5a inset) was 5.72and 5.91 eV for calcination temperatures 500 and 800 �C, respec-tively. The lower band gap value of MgO sample corresponding tocalcination temperature of 500 �C is due to presence of the 3-coordinated surface anions at the corners, whereas the existenceof 4-coordinated surface anions at the edges is responsible for thehigher band gap value for the MgO sample corresponding tocalcination temperature of 800 �C. We infer that owing to theoccurrence of 6-coordinated surface anions at the terrace in theperfect MgO surface, MgO possess the optical band gap of 7.8 eV[11].

To investigate the presence of defects or oxygen vacancies onthe surface of MgO, room-temperature photoluminescence (PL)spectroscopy was carried out. PL is a direct optical tool to describethe surface defects and electronic energy band structure. Generally,the intrinsic band edge structure, due to excitonic recombinationand other internal/external factors (intrinsic or extrinsic defects), isbecause of photogenerated hole and electrons, and is responsiblefor the luminescence characteristics in metal oxides [43,50]. It has

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 theMgO sample corresponding to calcination temperature of 500 and 800 �C. (b) Room temperature photoluminescence emission spectra of the MgO sample recorded under excitationat 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 inMgO.

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e1070 1069

been shown that the cubic-MgO is a wide-band gap energy mate-rial and normally does not demonstrate the PL characteristic [40].In the MgO crystalline structure, some structural defects are pre-sent, such as magnesium vacancies (Mg2þ3c ;Mg2þ4c ) and oxygen va-cancies (O2�

3c ;O2�4c ) [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, wemeasured the PL spectra of MgO

samples calcined at 500 and 800 �C. Fig. 5b provides the PL emis-sion spectra (excitation wavelength: 300 nm, 4.13 eV) of the MgOsamples. We observed a wide range of emissions at 325e690 nm(1.79e3.81 eV). The spectra reveal one intense emission peak andtwo accompanying shoulder peaks for both the samples. Theoccurrence of the major blue PL emission peak at about 421 nm(2.94 eV) is due to the presence of oxygen vacancy with low-coordinate oxygen anions (O2�

3c ) in the MgO [21]. The variousstructural defects in their crystalline structure and high-coordinated oxygen anions (O2�

4c and O2�5c ) are responsible for the

presence of almost suppressed shoulder emissions in the PL spectraof the MgO at about 447 nm (2.77 eV) and 483 nm (2.56 eV). It wasinteresting to see from Fig. 5b that on increasing the calcinationtemperature of the MgO samples the observed PL spectrum de-creases drastically in intensity without appearance of new emissionband. We infer that the change in the intensity may be because ofthe change in crystallite size of the MgO nanocrystals concludingthat the increase in nanocrystallite size with increase of calcinationtemperature 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 moreoverthe surface to volume ratio also decreases. Because of these alter-ations in the microstructure at nano-scale, the intensity of the PLemission decreases with increase of calcination temperature. PL isan intrinsic property, which is in agreement with the resultsrevealing that the defect centers in MgO nanocrystals are oxygen-deficient [51,52]. In our synthesized MgO samples, the origin ofPL emission can be attributed to the existence of oxygen vacancies/intrinsic or extrinsic point defects on the metal oxide surface dur-ing the transformation of Mg(OH)2 to MgO. The existence of suchintrinsic point defects on the surface of MgO nanocrystals areconfirmed by PL studies, which act as an invisible agent and offersan inexpensive alternative for the promising electronic and opticalapplications in nanodevices [21,44]. The optical transitions (ab-sorption and emission process) observed from the UV-vis absorp-tion and photoluminescence spectra are illustrated in Fig. 5c.

4. Conclusions

We employed a wet chemical method to fabricate MgO nano-crystals with varying calcination temperatures and without thepresence of solvent, template and surfactant. The reaction condi-tions are very simple, reliable, and cost-effective as well as requireeasily available starting material which allows the production ofMgO nanocrystals in high-yield so that the material can easily bescaled-up for industrial production. The calcination temperaturemaintained during the synthesis of MgO with cubic crystal struc-ture was found to have prominent impact on the crystallite sizes ofthe particles. Increasing the calcination temperatures from 500 to800 �C, improved the optical band gap of MgO nanocrystals. The

R. Verma et al. / Materials Chemistry and Physics 148 (2014) 1064e10701070

photoluminescence characteristics of nanometer-sized MgO indi-cate that oxygen vacancy defect centers generated at the surface ofthe oxide are responsible for the observed intense blue emissionspeak at about 421 nm upon 300 nm excitation. We suggest that theefficient preparation and the detailed investigations of MgOnanocrystals may open up new simple route to fabricate fine par-ticles with high optical performances, to shed new insight forfuture developing nano-devices.

Acknowledgments

We thank the Director, NPL New Delhi, India for providing thenecessary experimental facilities. Dr. Ajay Dhar, Mr. K. N. Sood, Mr. J.S. Tawale, Dr. Ritu Srivastava and Mr. Praveen are gratefullyacknowledged for providing the necessary instrumentation facil-ities for XRD, SEM, UV-vis and PL. Nano-SHE project (BSC-0112) isgratefully acknowledged.

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