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Cite this:DOI: 10.1039/c4cp02228a

Facile synthesis of Ag–ZnO hybrid nanospindlesfor highly efficient photocatalytic degradation ofmethyl orange

Sini Kuriakose,a Vandana Choudhary,a Biswarup Satpatib andSatyabrata Mohapatra*a

Highly photocatalytically active Ag nanoparticle decorated ZnO nanospindles were synthesized by

a facile wet chemical method. The structural and optical properties of the as-synthesized materials were

characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM),

transmission electron microscopy (TEM) and high resolution TEM (HRTEM) with energy dispersive X-ray

spectroscopy, UV-visible absorption spectroscopy and Raman spectroscopy. The photocatalytic activity

of these nanostructures was evaluated by analyzing sunlight driven degradation of methyl orange (MO)

dye and it was observed that Ag nanoparticle modified ZnO nanospindles show significantly enhanced

photocatalytic activity for degradation of MO, as compared to ZnO nanospindles. We attribute the

observed enhanced photocatalytic activity of Ag nanoparticle decorated ZnO nanospindles to their

improved sunlight utilization efficiency and the efficient suppression of recombination of

photogenerated charge carriers due to the electron scavenging action of Ag nanoparticles and the

interfacial electron transfers due to the Schottky junction between Ag nanoparticles and ZnO

nanospindles.

1. Introduction

Water pollution caused by toxic chemicals and organic dyes inindustrial waste water has prompted significant research inter-ests for sustained development by environmental remediation.Methyl orange (MO) is one of the azo dyes widely used in textile,paper and leather industries, which causes serious healthhazards.1 Semiconductor photocatalysis has emerged as oneof the most promising processes for waste water treatment ascompared to other conventional techniques.2–4 NanostructuredZnO with its wide band gap and large exciton binding energy5

is suitable for a wide range of applications6–18 includingUV lasers,10 dye sensitized solar cells,11–15 gas sensors16,17 andUV sensors.18 ZnO nanostructures with different morphology suchas nanoparticles, nanorods, nanospindles, nano/micro needles andnanoflowers19–26 have attracted increasing attention as promisingphotocatalysts. However, the rapid electron–hole recombination rateand inefficient utilization of sunlight by ZnO nanostructures limittheir photocatalytic activity.27,28 The photocatalytic efficiency of ZnOnanostructures can be enhanced by their surface modification withnoble metal nanoparticles.29 This has been attributed to the Schottky

barrier created at the metal–ZnO interface, which facilitates theseparation of the photogenerated charge carriers.30 In addition,the surface plasmon resonance (SPR) of noble metal nanoparticlesleads to significant enhancement in the visible light absorptioncapacity of the nanostructured ZnO photocatalysts.31 Noble metalssuch as Ag and Au have been used for surface modification of ZnOnanostructures to improve their photocatalytic efficiency.32–35 Silvernanoparticle decorated ZnO nanostructures have been found toexhibit enhanced photocatalytic activity towards degradation oforganic dyes.29,31,34–39 Xie et al.34 have shown that Ag decoratedZnO nanostructures exhibit improved photostability and enhancedphotocatalytic activity for degradation of crystal violet dye upon80 minutes of UV irradiation. Liu et al.35 have synthesized worm-like Ag–ZnO core–shell heterostructures with improved photo-catalytic activity as compared to pure ZnO nanostructures.

In this paper, we report the synthesis of Ag nanoparticledecorated ZnO nanospindles by a facile wet chemical method.The as-prepared Ag nanoparticle decorated ZnO nanospindlesshow enhanced photocatalytic activity for sunlight drivendegradation of MO dye taking only 40 minutes to degrade99.5% of the dye, as compared to ZnO nanospindles whichdegrade only 55% of the dye for the same duration. We havedemonstrated that the photocatalytic activity of ZnO nanospin-dles gets significantly enhanced upon decoration with Agnanoparticles and increases with the extent of Ag loading.

a School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University,

Dwarka, New Delhi 110078, India. E-mail: smiuac@gmail.comb Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India

Received 21st May 2014,Accepted 3rd July 2014

DOI: 10.1039/c4cp02228a

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2. Experimental sectionMaterials

Zinc nitrate hexahydrate (Zn(NO3)2�6H2O) (Merck, Germany),potassium hydroxide (KOH) (SRL, India), trisodium citrate(CDH, India) and silver nitrate (AgNO3) (Spectrochem, India)were used as the precursors for synthesis and methyl orange(MO) (SRL, India) was used as the model dye for photocatalyticstudies. All the reagents were of analytical grade and usedwithout any further purification.

Methods

For the synthesis of ZnO nanospindles, Zn(NO3)2�6H2O andKOH were used as the starting materials. In a typical synthesis,200 ml aqueous solution of Zn(NO3)�6H2O (0.1 M) was preparedand aqueous KOH (2 M) solution was added drop wise underconstant stirring till pH B 12 was reached. The white precipitateso obtained was stirred for 2 hours and aged overnight at roomtemperature. The precipitate obtained by filtration was repeatedlywashed with deionized water and then dried in an oven at 80 1Cfor 20 hours. For the preparation of Ag nanoparticle decoratedZnO nanospindles, the synthesized ZnO nanoparticles wereultrasonically dispersed in 100 ml of deionized water. Differentamounts of TSC for different TSC concentrations (0.2 mM,0.5 mM, 1 mM, 5 mM) were added to these 100 ml aqueoussolutions of as-synthesized ZnO nanoparticles and magneticallystirred overnight. For the photodeposition of Ag nanoparticles ontothe surface of ZnO nanospindles, different amounts of AgNO3 forconcentrations (0.2 mM, 0.5 mM, 1 mM, 1 mM) were added to theabove solutions, respectively, and stirred for 30 minutes in the darkbefore irradiating the suspension with sunlight for 2 hours.Depending on the Ag concentration, the colour of the suspensionchanged from white to pale yellow to grey. The final products wereobtained by centrifugation and repeated washing with deionizedwater and then dried in an oven at 80 1C for 20 hours. The pristineZnO and Ag–ZnO nanostructured samples prepared with differentTSC concentrations of 0.2 mM, 0.5 mM, 1 mM, and 5 mM arehereafter referred to as PZ, AZ1, AZ2, AZ3 and AZ4, respectively.

Characterization

Room temperature powder X-ray diffraction (XRD) measure-ments were carried out using a Panalytical X’pert Pro diffracto-meter with Cu Ka radiation (l = 0.1542 nm). The morphologyof ZnO and Ag–ZnO samples was studied by field emissionscanning electron microscopy (FESEM). Transmission electronmicroscopy (TEM) investigations were carried out using a FEI,TECNAI G2 F30, S-TWIN microscope operating at 300 kV. Thesamples ultrasonically dispersed in ethanol were coated ontocarbon coated Cu grids and used for TEM studies. UV-visibleabsorption spectroscopy was carried out using a dual beamspectrophotometer (HITACHI U3300) in the wavelength range of200–800 nm. For this the powder samples were dispersedin deionized water by sonication and deionized water was used asthe reference medium. Micro-Raman spectra have been recordedfrom 100 cm�1 to 800 cm�1 using a Horiba Jobin Yvon LabRAMspectrometer using an argon laser (488 nm) of spot size 1 mm.

Photocatalytic measurements

The photocatalytic activity of ZnO nanospindles and Ag nanoparticledecorated ZnO nanospindles was studied using MO as a model dye.For the photocatalytic studies, 5 mg of the nanostructured ZnO orAg–ZnO photocatalysts was ultrasonically dispersed in 5 ml ofdeionized water. Aqueous 10 mM MO was added into the solutionswith different photocatalysts and thoroughly mixed and kept in thedark for 1 hour for reaching adsorption saturation. The mixturescontaining different nanostructured photocatalysts (ZnO andAg–ZnO) and 10 mM MO dye were irradiated with sunlight fordifferent times (10, 20, 40 and 60 minutes) with intermittentshaking for uniform mixing. After the sunlight exposure, thephotocatalysts were removed from the dispersion by centrifugationand the final concentration of MO dye in the solution was monitoredby UV-visible absorption spectroscopy in the wavelength range of200–800 nm with deionized water as the reference medium.

The efficiency of the photocatalysts for photocatalytic degrada-tion of MO dye was calculated using the following formula:

Z = (C0 � C)/C0 (1)

where C0 is the concentration of aqueous MO dye beforeaddition of any photocatalyst and C is the concentration ofMO in the reaction suspension with photocatalyst followingsunlight exposure for time t.

3. Results and discussionMorphology and crystal structure

FESEM images of the as-synthesized samples PZ, AZ1, AZ3 andAZ4 are shown in Fig. 1. The presence of nanospindle likestructures can be clearly seen in the FESEM image (Fig. 1(a)) ofthe pristine ZnO sample PZ. It can be seen that the nanospindles

Fig. 1 (a–d) FESEM images of samples PZ, AZ1, AZ3 and AZ4 showing thepresence of nanospindle like structures.

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consist of smaller nanoparticles and are formed mainly due tooriented attachment of smaller ZnO nanoparticles. Fig. 1(b–d)show FESEM images of Ag nanoparticle decorated ZnO nano-spindles prepared using different concentrations of TSC andAgNO3. It can be seen that as the citrate concentration increases,there is increased aggregation of the nanoparticles into nano-spindles of complex morphology. The presence of small nano-particles decorating the surface of these ZnO nanospindles andthe complex aggregated nanospindle structures can be clearlyseen. In an earlier work40 we have shown that the concentrationof trisodium citrate plays an important role in deciding the finalmorphology of the ZnO nanostructures. Citrate ions bind to theZnO(0001) surface of the nanostructures through –COOH and–OH groups and hence lead to increased aggregation of nano-spindles into complex morphology at higher concentrations.

Structural information was further obtained using TEM studies.From a low magnification TEM image (Fig. 2(a)) of sample PZ, thepresence of ZnO nanostructures of anisotropic shapes can beclearly seen. Higher magnification images revealed that theseanisotropic nanostructures are nanospindles, made up of smallernanoparticles. Fig. 2(b) shows the TEM image revealing secondarygrowth of smaller nanospindles on ZnO nanospindles. The highresolution TEM image of a ZnO nanospindle in Fig. 2(c) clearlyshows lattice fringes and the measured lattice spacing is 2.81 Åwhich corresponds to the (100) interplanar spacing (d-spacing) ofZnO (d100 for ZnO is 2.81 Å [JCPDS # 36-1451]). Energy dispersiveX-ray spectroscopy (EDX) data from regions marked by area 1 inFig. 2(d) are plotted in Fig. 2(e) which clearly shows the presence ofZn and O. Carbon and Cu signals in EDX spectra are due to theC-coated Cu-grid used for the TEM study.

TEM micrographs of ZnO nanospindles decorated with Agnanoparticles are presented in Fig. 3. TEM images of the AZ4sample revealed the presence of anisotropic nanostructures

decorated with nanoparticles. The SAD pattern from a regionmarked by a dotted circle is shown in Fig. 3(c). The distinct spotpattern, which can be seen from this SAD pattern, is a result ofthree–four small crystals and suggests the single crystallinenature of these heterostructures. The SAD pattern furtherconfirms the formation of the crystalline hexagonal phase ofZnO. The SAD pattern was indexed for a single particle usinghexagonal ZnO [JCPDS # 36-1451] marked by red circles.The spots marked by a yellow circle in the SAD pattern withd-spacing 1.98 Å could be (102) of another ZnO or (002) of fccAg. (d102 for ZnO is 1.91 Å and d002 for Ag is 2.04 Å.) The HRTEMstudy of these decorating nanoparticles confirmed them to beof Ag. The high resolution TEM image of ZnO nanospindle inFig. 3(d) clearly shows lattice fringes and the measured latticespacing is 2.79 Å and that of Ag in Fig. 3(e) is 2.34 Å, whichcorrespond to the (100) and (111) interplanar spacing of ZnOand Ag, respectively.

The chemical composition of the hybrid nanostructures wasinvestigated by performing STEM-HAADF-EDX analysis. Fig. 4(a)depicts the STEM-HAADF image of Ag–ZnO nanostructures.Energy dispersive X-ray spectroscopy (EDX) data from regionsmarked by area 1 in Fig. 4(a) are plotted in Fig. 4(b) which clearlyshows Ag, Zn and O. Carbon and Cu signals in EDX spectra aredue to the C-coated Cu-grid. The spatial distributions of theatomic contents across the Ag–ZnO nanostructures wereobtained using drift corrected EDX imaging. The STEM-HAADFimage and the corresponding chemical maps for Zn, O and Agfrom the area marked in Fig. 4(a) are presented in Fig. 4(c–g).EDX maps confirmed the decoration of ZnO nanospindles withAg nanoparticles.

Fig. 5 shows XRD patterns of ZnO and Ag–ZnO samplesrevealing the hexagonal wurtzite crystal structure of ZnO [JCPDSno. 36-1451] and face centred cubic structure of Ag [JCPDS card

Fig. 2 (a) Low magnification TEM image of ZnO nanospindles. (b) High magnification TEM image of one ZnO nanospindle. (c) HRTEM image showinglattice fringes. (d) STEM-HAADF image. (e) EDX spectra from a region marked by area 1 in (d).

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no. 04-0783]. No extra peaks related to silver oxides wereobserved, but there is one peak related to body centeredZn(OH)2. The average crystallite size for ZnO was estimated tovary from 19 nm to 22 nm and for Ag the estimated crystallitesize varied from 8 nm to 18 nm in different samples.

Growth mechanism of nanospindles

A schematic of the growth mechanism of ZnO nanospindles isdepicted in Fig. 6. Formation of ZnO nanoparticles from

aqueous solutions of zinc nitrate and KOH involves the follow-ing reactions:41

Zn2+ + 2OH� - Zn(OH)2 (2)

Zn(OH)2 - ZnO + H2O (3)

Zn(OH)2 + 2OH� 2 [Zn(OH)4]2� (4)

ZnO + H2O + 2OH� 2 [Zn(OH)4]2� (5)

Fig. 4 (a) STEM-HAADF image of Ag–ZnO nanospindles. (b) EDX spectra from a region marked by area 1 in (a). (c–g) STEM-HAADF-EDX images takenfrom the area marked by an alloy orange box indicating the locations of different atoms across the structure. (c) STEM-HAADF image. (d) Zn–K map(green). (e) O–K map (orange). (f) Ag–L map (magenta). (g) Composite map of Ag (orange) and ZnO (green).

Fig. 3 (a) Low magnification TEM image of Ag–ZnO nanospindles. (b) High magnification TEM image of Ag–ZnO nanospindles. (c) Selected areadiffraction pattern from Ag–ZnO nanospindles. (d) HRTEM image showing (111) lattice fringes of the hexagonal wurtzite crystal structure of ZnO. (e)HRTEM image showing (111) lattice fringes of fcc Ag.

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White precipitates of Zn(OH)2 are formed by the addition ofaqueous KOH into Zn nitrate solution which decompose to formZnO nuclei. ZnO nuclei grow into nanoparticles depending on theZn2+ concentration and the synthesis conditions. [Zn(OH)4]2� ionsform in the presence of excess OH� ions, which helps in theformation of aggregates of ZnO nanoparticles. It is known that themorphology of the final product depends on the amount of reactantspresent. With the increase in the amount of reactants, orientedgrowth of the nanoparticles can produce larger nanostructures with

different shapes.24 In our case the ZnO nanospindles formed due tooriented attachment of ZnO nanoparticles, as evident fromboth FESEM and TEM results. Initially formed ZnO nanoparticlesaggregate in a specific direction to form a spindle like structure inorder to reduce the surface energy. This process is known as orientedattachment which has been shown to result in the formation ofvarious anisotropic nanostructures.20,23,24,42–44 In our case, overnightstirring with TSC leads to surface functionalization of ZnOnanospindles with citrate ions. Photoreduction of Ag ions uponsunlight irradiation leads to deposition of Ag nanoparticles ontoZnO nanospindles leading to the formation of Ag–ZnO hybridnanospindles.

Optical properties

The UV-visible absorption spectra of pristine ZnO and Ag–ZnOnanospindle samples with varying Ag concentration are shownin Fig. 7(a). The band gap of Ag–ZnO hybrid nanospindles withdifferent Ag nanoparticle loading is slightly blue shifted ascompared to that of the pristine ZnO nanospindles. It can alsobe clearly seen that the modification of ZnO nanospindles withAg nanoparticles results in enhanced absorption of light in thenear UV-visible region thereby improving their sunlight utiliza-tion efficiency. Since the concentration of decorating Ag nano-particles is small, as evident from the TEM results, the SPRpeak corresponding to Ag nanoparticles could not be clearlyobserved in the absorption spectra. The plasmonic response of

Fig. 6 Growth mechanism of Ag nanoparticle decorated ZnO nanospindles.

Fig. 7 (a) UV-visible absorption spectra of samples PZ, AZ1, AZ2, AZ3 and AZ4 with varying Ag concentrations. (b) Typical Raman spectra of the Ag–ZnOnanospindle (AZ4) sample showing the characteristic peaks.

Fig. 5 XRD patterns of as-synthesized ZnO and Ag–ZnO samples pre-pared with varying Ag concentration.

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Ag nanoparticles appears as a very broad hump contributing tothe enhanced light absorption in the visible region, which canbe clearly seen from the optical absorption results.

Typical Raman spectra of Ag–ZnO nanospindle sample AZ4 areshown in Fig. 7(b). ZnO with wurtzite structure belongs to the C4

6v

space group with the optic modes A1 + 2B1 + E1 + 2E2.45 In our casethere is an existence of 5 peaks belonging to 2E (low)

2 (134 cm�1),2TA (220 cm�1), B (low)

1 (241 cm�1), E (high)2 (442 cm�1), and

A(low)1 (572 cm�1).46 The E (high)

2 peak is the characteristic Raman peakfor ZnO and is associated with vibration of the oxygen atom.

Photocatalytic studies

Fig. 8(a–e) show the UV-visible absorption spectra of aqueous10 mM MO solutions with different photocatalysts PZ, AZ1, AZ2,

AZ3 and AZ4 following irradiation with sunlight for differentdurations of time. The characteristic absorption peak of MOhas been monitored as a function of sunlight exposure time.From Fig. 8(a–e) it can be clearly seen that the absorption peakdrastically diminishes after 20 min of irradiation and almostcompletely disappears after 40 min of irradiation with thephotocatalysts AZ2, AZ3 and AZ4 with Ag–ZnO nanospindles.It must be pointed out here that adsorption of MO dye onto theZnO and Ag–ZnO nanospindles in the dark prior to sunlightexposure did not result in any significant change in theabsorption spectrum of MO. The photocatalytic degradationefficiency of different photocatalysts is shown in Fig. 8(f).All the Ag–ZnO samples have higher photocatalytic efficiencythan pristine ZnO. It can be clearly seen that sample AZ4 with

Fig. 8 (a–e) UV-visible absorption spectra showing temporal evolution of degradation of MO upon sunlight irradiation using samples PZ, AZ1, AZ2, AZ3 and AZ4as photocatalysts. (f) Variation of photocatalytic efficiency of ZnO nanospindles and Ag–ZnO nanospindles with different Ag loading for degradation of MO.

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maximum Ag nanoparticle loading onto ZnO nanospindles isthe most efficient photocatalyst which degrades 88% of MOfollowing 20 minutes of sunlight exposure, whereas pristineZnO nanospindles degrade only 33% of MO for the sameexposure time. The repetitive test for the photocatalytic degra-dation of MO was also performed and Fig. 9 shows the resultsof four runs of photodegradation of MO by AZ4.

The enhanced photocatalytic activity of Ag–ZnO nanospindlescan be understood as follows. When the ZnO nanospindle absorbsphotons of energy greater than or equal to its band gap, electronsjump to the conduction band, creating an equal number of holes inthe valence band. Electrons can flow from ZnO to Ag nanoparticles,since the energy level of the conduction band of ZnO is higher thanthe Fermi level of Ag–ZnO. Due to this Ag nanoparticles can trapthe photogenerated electrons, preventing their recombination withholes. Photogenerated electrons can produce superoxide anionradicals while holes in the valence band can react with H2O toproduce hydroxyl radicals, both of which cause degradation of MOdye. The reactions27,28 can be summarized as follows and themechanism is shown in Fig. 10.

ZnO + hn - e� (CB) + h+ (VB) (6)

e� + O2 - �O2� (7)

Ag+ + e� (CB) - Ag (8)

h+ + OH� - �OH (9)

�OH + organic dye - degradation products (10)

�O2� + organic dye - degradation products (11)

The photocatalytic efficiency of Ag nanoparticle modifiedZnO nanospindles is better than that of pristine ZnO nanospindlesmainly due to the following reasons. Firstly, the formation of theSchottky junction at the Ag–ZnO interface improves the separationof photogenerated charge carriers, thus decreasing theirrecombination rate.20 Secondly, the SPR of Ag nanoparticlesleads to increased absorption of light in the near UV-visibleregion thereby improving sunlight utilization efficiency. TheSPR absorption of Ag nanoparticles can be tuned by engineeringtheir size, shape, inter-particle spacing and the surroundingmedium.47 In our case the concentration of decorating Agnanoparticles is small, as can be clearly seen from TEM results.Therefore, a sharp SPR peak corresponding to Ag nanoparticlescould not be observed in the absorption spectra. The plasmonicresponse appears as a very broad hump contributing to theenhanced light absorption in the visible region. Decoration withan increased number density of Ag nanoparticles will result in astronger SPR peak and hence improved visible light utilizationleading to stronger enhancement in the photocatalytic activity.Chen et al.1 have studied the photocatalytic degradation of MOin the presence of Ag–ZnO heterostructures which showedenhancement in efficiency upon Ag loading. Gao et al.48 studiedthe photocatalytic properties of Ag–ZnO nanocomposites synthesisedby a biomolecule assisted hydrothermal method and inferred thatAg nanoparticles improve the separation of electrons and holes byacting as electron sinks. In our case, the Ag–ZnO nanospindlesample AZ4 with the highest Ag nanoparticle loading onto ZnOnanospindles shows the highest photocatalytic activity for degrada-tion of MO. As can be seen from FESEM and TEM images sampleAZ4 consists of Ag nanoparticle decorated ZnO nanospindles, whichhave a larger surface area for adsorption of MO dye as compared toZnO nanoparticles. The observed enhanced photocatalytic activity ofAg nanoparticle decorated ZnO nanospindles is attributed to theirimproved sunlight utilization efficiency due to the plasmonicresponse of Ag nanoparticles and the efficient suppression of

Fig. 9 Efficiency of sunlight driven degradation of MO by AZ4 for four runs.

Fig. 10 Schematic band diagram of Ag–ZnO hybrid nanostructures showing the processes leading to photocatalytic degradation of MO dye. Efa: Fermienergy of Ag, Efz: Fermi energy of ZnO, Ef: Fermi energy due to band bending, VB: valence band, CB: conduction band.

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recombination of photogenerated charge carriers due to Agnanoparticles and the Schottky junction between Ag nano-particles and ZnO nanospindles.

4. Conclusions

We have synthesized Ag–ZnO hybrid nanospindles with highphotocatalytic efficiency for sunlight driven degradation of methylorange. ZnO nanospindles were prepared by a facile wet chemicalmethod, involving oriented attachment of ZnO nanoparticles.Decoration of ZnO nanospindles with Ag nanoparticles was doneby citrate assisted photoreduction of Ag ions. We have shown thatthe photocatalytic degradation efficiency of ZnO nanospindles getssignificantly enhanced upon decoration with Ag nanoparticles andthe efficiency increases with an increase in Ag nanoparticle loading.The observed enhanced photocatalytic activity is attributed to theimproved sunlight utilization efficiency due to the plasmonicresponse of Ag nanoparticles and suppression of the electron–holerecombination rate due to the electron scavenging action of Agnanoparticles and electron transfer across the Schottky junctionbetween Ag nanoparticles and ZnO nanospindles.

Acknowledgements

The authors are thankful to Dr Ankush Vij, Dr Saif A. Khan andSrikanth for their help in Raman spectroscopy, FESEM andXRD measurements, respectively. SM is thankful to UniversityGrants Commission (UGC), New Delhi, for providing financialsupport under Major Research Project (F.No: 41-865/2012 (SR)).SM is thankful to the Department of Science and Technology(DST), New Delhi, for providing XRD facility at Guru GobindSingh Indraprastha University, New Delhi, and FESEM facilityat IUAC, New Delhi, under the Nano Mission programme. SK isthankful to Guru Gobind Singh Indraprastha University, NewDelhi, for providing financial assistance through IP Fellowship.

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