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Synthesis, structural, and optical properties of type-II ZnOZnS coreshell nanostructure M. Sookhakian a,n , Y.M. Amin a , W.J. Basirun b,c , M.T. Tajabadi b , N. Kamarulzaman d a Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysia b Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia c Nanotechnology & Catalysis Research Centre (NanoCat), Institute of Postgraduate Studies, University Malaya, 50603 Kuala Lumpur, Malaysia d Centre for Nanomaterials Research Institute of Science, Level 3 Block C (Old Engineering Building), Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia article info Article history: Received 11 October 2012 Received in revised form 7 May 2013 Accepted 10 July 2013 Available online 31 July 2013 Keywords: ZnOZnS coreshell Photoluminescence Type II band alignment Hetero-interface abstract We demonstrate a facile one-step method for the preparation of ZnOZnS coreshell type-II nanos- tructures, pure ZnS quantum dots and pure ZnO nanoparticles with different experimental conditions. Treatment with sodium hydroxide as a capping agent is investigated systematically during the synthesis of ZnS quantum dots (QDs). The thickness of the ZnS shell is controlled by the concentration of the sodium sulphide during the synthesis of ZnOZnS coreshell nanostructures. The morphology and structure of samples are veried by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX). The UVvis absorption spectra of the pure ZnS QDs exhibit a blue shift in the absorption edge due to the quantum connement effect. The PL emission spectra of the ZnOZnS coreshell nanostructure are compared with the ZnO nanoparticles. The ZnOZnS coreshell nanostructures show decrease in the UV and green emissions with the appearance of a blue emission, which are not found in the ZnO nanoparticles. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor nanocrystals, particularly quantum dots (QDs), have attracted much attention in recent years due to their unique physical and chemical properties which are different from the bulk materials. They have a wide range of physical applications such as light-emitting diodes, biomedical labelling, photo-catalysis, optical waveguide, photo-conductive devices, solar cells, lasers and sen- sors [112]. The physical and chemical properties of semiconduc- tor nanoparticles (NPs) are controlled by a spatial three dimensional connement of electrons and holes in a small box called quantum connement effect [13]. Due to the quantum connement effect, a signicant increase in the band-gap is observed when the nanoparticle size is close to the exciton Bohr radius and results in a blue shift in the absorption spectra. The effective mass model [14] is used to analyse the size effects on luminescent properties of the nanostructures. Recently, IIVI semiconductors, particularly CdTe and CdS, has been extensively analysed for their size and shape control [15,16]. It was found that the nanoparticles encapsulated with a shell coating layer enhances the optical properties. ZnO is one of the most famous IIVI semiconductor nanoparticles due to a large exciton binding energy of 60 meV and a wide band gap of 3.3 eV at room temperature, and are used for various applications such as optoelectronics [17], eld-effect transistors [18], sensors [19], transparent conducting lms [20], light-emitting diodes [21] and catalysts [22]. It is reported that the optical properties of ZnO NPs could be signi- cantly improved by the encapsulation of the ZnO with wide band- gap nanoparticles [23]. ZnS is a non-toxic semiconductor with a wide and direct band gap which can be observed naturally in two phases: rst is the zinc blend structure with a cubic phase, and second is the wurtzite structure with a hexagonal phase. The band-gap energy of the bulk cubic and hexagonal phases of ZnS is 3.68 eV and 3.80 eV respectively. Zinc blend ZnS is more stable at lower tempera- ture and atmospheric pressure, but transforms to wurtzite ZnS at temperature higher than 1000 1C [24]. Recently, ZnS QDs are reported to show various luminescence properties such as photo-luminescence, electro-luminescence, mechano-lumines- cence, and thermal-luminescence [2528]. Also ZnS QDs is a phos- phor material and are widely used in infrared windows, at-panel displays and LED [29,30] due to their wide exciton binding energy of 40 meV. It is found that, ZnS nanoparticles (NPs) is an excellent shell coating layer on the ZnO, and enhances the optical properties of the ZnOZnS coreshell nanostructures. Li et al. [23] fabricated ZnOZnS coreshell nanowires by a self-assembling mechanism Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.07.032 n Corresponding author. Tel.: +60 146474432. E-mail address: [email protected] (M. Sookhakian). Journal of Luminescence 145 (2014) 244252

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Journal of Luminescence 145 (2014) 244–252

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23http://d

n CorrE-m

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

Synthesis, structural, and optical properties of type-II ZnO–ZnScore–shell nanostructure

M. Sookhakian a,n, Y.M. Amin a, W.J. Basirun b,c, M.T. Tajabadi b, N. Kamarulzaman d

a Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysiab Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysiac Nanotechnology & Catalysis Research Centre (NanoCat), Institute of Postgraduate Studies, University Malaya, 50603 Kuala Lumpur, Malaysiad Centre for Nanomaterials Research Institute of Science, Level 3 Block C (Old Engineering Building), Universiti Teknologi MARA (UiTM), 40450 Shah Alam,Selangor, Malaysia

a r t i c l e i n f o

Article history:Received 11 October 2012Received in revised form7 May 2013Accepted 10 July 2013Available online 31 July 2013

Keywords:ZnO–ZnS core–shellPhotoluminescenceType II band alignmentHetero-interface

13/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jlumin.2013.07.032

esponding author. Tel.: +60 146474432.ail address: [email protected] (M. Sook

a b s t r a c t

We demonstrate a facile one-step method for the preparation of ZnO–ZnS core–shell type-II nanos-tructures, pure ZnS quantum dots and pure ZnO nanoparticles with different experimental conditions.Treatment with sodium hydroxide as a capping agent is investigated systematically during the synthesisof ZnS quantum dots (QDs). The thickness of the ZnS shell is controlled by the concentration of thesodium sulphide during the synthesis of ZnO–ZnS core–shell nanostructures. The morphology andstructure of samples are verified by X-ray diffraction (XRD), high resolution transmission electronmicroscopy (HRTEM) and energy dispersive X-ray analysis (EDX). The UV–vis absorption spectra of thepure ZnS QDs exhibit a blue shift in the absorption edge due to the quantum confinement effect. The PLemission spectra of the ZnO–ZnS core–shell nanostructure are compared with the ZnO nanoparticles. TheZnO–ZnS core–shell nanostructures show decrease in the UV and green emissions with the appearanceof a blue emission, which are not found in the ZnO nanoparticles.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanocrystals, particularly quantum dots (QDs),have attracted much attention in recent years due to their uniquephysical and chemical properties which are different from the bulkmaterials. They have a wide range of physical applications such aslight-emitting diodes, biomedical labelling, photo-catalysis, opticalwaveguide, photo-conductive devices, solar cells, lasers and sen-sors [1–12]. The physical and chemical properties of semiconduc-tor nanoparticles (NPs) are controlled by a spatial threedimensional confinement of electrons and holes in a small boxcalled quantum confinement effect [13]. Due to the quantumconfinement effect, a significant increase in the band-gap isobserved when the nanoparticle size is close to the exciton Bohrradius and results in a blue shift in the absorption spectra. Theeffective mass model [14] is used to analyse the size effects onluminescent properties of the nanostructures. Recently, II–VIsemiconductors, particularly CdTe and CdS, has been extensivelyanalysed for their size and shape control [15,16]. It was found thatthe nanoparticles encapsulated with a shell coating layer enhancesthe optical properties. ZnO is one of the most famous II–VI

ll rights reserved.

hakian).

semiconductor nanoparticles due to a large exciton binding energyof 60 meV and a wide band gap of 3.3 eV at room temperature, andare used for various applications such as optoelectronics [17],field-effect transistors [18], sensors [19], transparent conductingfilms [20], light-emitting diodes [21] and catalysts [22]. It isreported that the optical properties of ZnO NPs could be signifi-cantly improved by the encapsulation of the ZnO with wide band-gap nanoparticles [23].

ZnS is a non-toxic semiconductor with a wide and direct bandgap which can be observed naturally in two phases: first is the zincblend structure with a cubic phase, and second is the wurtzitestructure with a hexagonal phase. The band-gap energy of thebulk cubic and hexagonal phases of ZnS is 3.68 eV and 3.80 eVrespectively. Zinc blend ZnS is more stable at lower tempera-ture and atmospheric pressure, but transforms to wurtziteZnS at temperature higher than 1000 1C [24]. Recently, ZnS QDsare reported to show various luminescence properties such asphoto-luminescence, electro-luminescence, mechano-lumines-cence, and thermal-luminescence [25–28]. Also ZnS QDs is a phos-phor material and are widely used in infrared windows, flat-paneldisplays and LED [29,30] due to their wide exciton binding energyof 40 meV. It is found that, ZnS nanoparticles (NPs) is an excellentshell coating layer on the ZnO, and enhances the optical propertiesof the ZnO–ZnS core–shell nanostructures. Li et al. [23] fabricatedZnO–ZnS core–shell nanowires by a self-assembling mechanism

M. Sookhakian et al. / Journal of Luminescence 145 (2014) 244–252 245

and reported a significant improvement in the ultraviolet emis-sion. Zhu et al. [31] reported the enhancement in ultravioletemission for ZnO–ZnS core–shell microspheres synthesised by aspherical template via the solution method.

In this study, we have synthesised pure ZnS QDs, ZnO NPs,ZnS–ZnO nanocomposites and ZnO–ZnS core–shell nanostructureswith flexible shell thickness. The effect of the thickness on theoptical properties of ZnO–ZnS core–shell nanostructures is inves-tigated. The control of the shell thickness and optical properties ofthe ZnO–ZnS core–shell nanostructures was carried out by reg-ulating the concentration of Na2S, which is the sulphur source inthe hydrothermal process. The NaOH treatment as a capping agentfor the synthesis of ZnS QDs is also systematically investigated inthis work.

2. Experimental

2.1. Sample preparation

All chemicals were purchased from the Merck Co., are ofanalytical purity and used without further purification. All experi-ments were carried out in atmospheric pressure.

2.2. Synthesis of ZnS QDs and ZnS–ZnO nanocomposite

The synthesis of ZnS QDs were carried out by mixing 50 mlsodium sulphide (Na2S) solution with 50 ml zinc acetate dehydrate(Zn(CH3COO)2 �2H2O) solution in a 1:1 M ratio followed by vigor-ous stirring for 30 min. When the starting materials were dis-solved in double distilled water, S2� combines with Zn2+ to formZnS quantum dots, therefore, a seeding solution was initiated. Tostudy the effect of NaOH, four different samples were prepared.For sample S (1), the seeding solution was heated at 90 1C for30 min. For the second sample S (2), 20 ml 0.5 M NaOH was addeddrop-wise with vigorous stirring at 50 1C for 30 min and thesolution pH was 5.5. Finally the solution was heated at 90 1C for30 min. For the third and fourth samples S (3) and S (4), 30 ml and50 ml 0.5 M NaOH were added respectively, under the sameconditions. The pH of the solutions was 6 and 6.5 respectively.The final products were left to cool at room temperature, filteredand washed several times with acetone followed by doublydistilled water to remove traces of impurities. In the final step,all products were dried in a vacuum oven at 80 1C for 24 h.

Fig. 1. (a) XRD pattern of pure ZnS in the absence of NaOH and in the presence ofdifferent amounts of 0.5 M NaOH (20 ml, 30 ml and 50 ml) and (b) XRD patterns ofthe pure ZnO and ZnO–ZnS core–shell nanostructures with different concentrationof Na2S (0.01 M, 0.1 M and 0.2 M).

2.3. Synthesis of ZnO and ZnO–ZnS core–shell nanostructures

In a typical process, 0.04 mol zinc acetate dehydrate dissolvedin 50 ml doubly distilled water was mixed with 50 ml 0.5 M NaOHat 60 1C with vigorous stirring for 30 min. The mixed solution washeated at temperature above the boiling point of water until allthe solvent was evaporated leaving the ZnO powder. The powderwas collected, washed several times with acetone and doublydistilled water and finally dried in an oven at 120 1C for 12 h toobtain S (A). We used the same solutions to synthesise the ZnO–ZnS core–shell nanostructures. Then 50 ml 0.01, 0.1 and 0.2 Msodium sulphide were added drop-wise to the above solution toobtain three different samples of S (B), S (C) and S (D) respectively.After 30 min of stirring, the solution was heated at temperatureabove the boiling point of water until all the solvent wasevaporated. The powders were collected, washed several timeswith acetone and doubly distilled water and finally dried at 80 1Cin the oven for 24 h.

3. Characterisation

The phase and crystallite size of the nanostructured ZnS QDs,ZnO NPs, ZnS–ZnO nanocomposite and ZnO–ZnS core–shell nanos-tructures were characterised using an automated X-ray powderdiffractometer (XRD, PANalytical's Empyrean) with a monochro-mated CuKα radiation (λ¼1.54056 A1). The particle size andstructural characterisation of the as-synthesised product wereperformed using a high resolution transmission electron micro-scopy (HRTEM-FEIG-4020, 500 kV) and high resolution field emis-sion scanning electron microscopy (FESEM- FEI Quanta 200 F). Thesamples were ultrasonicated in acetone before the HRTEM and FE-SEM characterisation. Energy dispersive X-ray analysis (EDAX)using EDX-System (Hitachi, S-4800) instrument was attached tothe FE-SEM instrument to investigate the elemental compositionof the samples. Optical reflectance and absorption spectra of thepowders were recorded with a double beam UV–VIS–NIR spectro-photometer (Hitachi, U-3500). Room temperature photo-luminescence (PL) spectra were obtained using a helium–cad-mium laser (Hitachi; FL 3000) with a wavelength of 325 nm.

4. Results and discussion

4.1. Crystalline structure

Fig. 1(a) shows the XRD patterns of the pure ZnS QDs anddifferent ZnS–ZnO nanocomposites prepared by different molefractions of sodium hydroxide. For sample S (1) and sample S (2),

M. Sookhakian et al. / Journal of Luminescence 145 (2014) 244–252246

all the XRD peaks can be attributed to the cubic crystalline phaseof ZnS which is very close to the standard JCPDS data (Card no: 01-0792) for zinc blend. For sample S (1), the three broad and strongpeaks with 2θ values of (28.55970.001)1, (47.51670.001)1 and(56.29170.001)1 correspond to the crystal planes of (1 1 1),(2 2 0) and (3 1 1) respectively, is due to zinc blend ZnS. Fromthe Rietveld method, the calculated lattice parameter a¼(5.365470.0031) Å is less than the expected bulk lattice para-meter of (5.406070.0001) Å. The reason for this difference is dueto the tetragonal deformity and distortion of the nanoparticles unitcell [32]. The XRD diffraction peaks are completely broad, which isdue to the small size of the crystalline ZnS QDs. For both samples S(1) and S (2), the crystallite sizes are estimated from the FWHM ofXRD diffraction peak (111) using the Debye-Scherrer equation,D¼kλ/β cos θ. The parameter D is the crystallite size of nanopar-ticle, k is the shape factor is 0.9; λ is the X-ray wavelength is 1.54 Å,β is the full width at half maximum of the XRD diffraction peak inradian and θ is the Bragg diffraction angle. From the Scherrer'sequation, the estimated crystallite sizes of the nanoparticles are(4.3570.05) nm and (3.4570.05) nm for S (1) and S (2) respec-tively. It can be observed that there is a slight difference betweenthe XRD patterns of S (1) and S (2) with the addition of NaOH. Thepeak profile analysis of ZnS (1 1 1) crystal plane illustrates that theFWHM of XRD diffraction peak (1 1 1) increases gradually but thepeak shift of (1 1 1) is negligible (see Table 1) with the addition ofNaOH. These changes are due to the addition of NaOH and areattributed to the decrease of the lattice parameters of ZnS QDs.Therefore, NaOH as a capping agent influences the crystallite sizeduring the growth but does not influence the crystalline structure.To illustrate this phenomenon, the lattice parameter values aregiven in Table 1. With the presence of NaOH, the crystallite sizedecreases but the crystal structure remains unchanged. The mainreason for this effect is the reaction of zinc acetate with NaOH inaqueous solution to yield zinc hydroxide Zn(OH)2 [33] which is awell known surfactant [34]. Therefore to investigate the cappingeffect, NaOH treatment was analysed for both samples S (1) and S(2). On the other hand, it is known that the presence of NaOHincreases the pH of the aqueous media [35]. Therefore the increaseof NaOH mole fraction facilitates the growth of ZnO. With theaddition of 30 ml NaOH in colloidal solution, the pH reached 6 andhexagonal ZnO NPs starts to form. With further increase of NaOHmole fraction, the pH exceeded 6 and both cubic ZnS andhexagonal ZnO were found to exist as the ZnS–ZnO nanocompositemixture. The XRD pattern in Fig. 1 shows the hexagonal ZnOcharacteristic planes of 2θ¼(31.82070.001)1, (34.46770.001)1,(36.19170.001)1, (47.56970.001)1, (56.78370.001)1, (62.72870.001)1, (66.22970.001)1, (67.86170.001)1, (68.99970.001)1,(72.67570.001)1 and (76.80970.001)1 which correspond to thecrystal planes of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3),(2 0 0), (1 1 2), (2 0 1), (4 0 0) and (2 0 2) respectively. Theseresults agrees well with the JCPDS data (Card no: 003-0888),which belongs to hexagonal wurtzite ZnO NPs.

Fig. 1(b) shows the XRD pattern of the pure ZnO NPs and ZnO–ZnS core–shell nanostructures. It can be shown that these XRDpatterns consist of two sets of diffraction peaks; first is thehexagonal wurtzite ZnO NPs and second is the cubic zinc blend

Table 1Calculated structural parameters and crystallite size from XRD pattern.

Sample code 2θ [º] FWHM [º] Reference parameter (from

a (Å) V (Å)3

S (1) (28.55970.001) (1.944470.0001) (5.40670.001) (157.9S (2) (28.58770.001) (2.456170.0001) (5.40070.001) (157.4

ZnS QDs. The XRD pattern of S (A) belongs to the pure hexagonalZnO NPs with lattice parameters of a¼b¼(3.249070.0001) Å andc¼(5.205070.0001) Å, and agrees well with the JCPDS data (Cardno: 00-0664) of pure ZnO. The XRD patterns of S (B), S (C) and S(D) belong to the as-synthesised ZnO–ZnS core–shell nanostruc-tures, obtained by the addition of different concentrations of Na2Sin the hydrothermal method. From Fig. 1(b), the XRD diffractionintensity of ZnS (1 1 1) crystal plane increases gradually with theincrease of Na2S concentration, which indicates a steady growth ofthe ZnS nanoparticles. From the XRD patterns in Fig. 1(b), theformation of the ZnO–ZnS core–shell nanostructures can beattributed to the overlap of the peaks of (1 0 2) and (1 1 0) crystalplanes of ZnO with the peaks of (2 2 0) and (3 1 1) crystal planes ofZnS. Also the inter-planar spacing for the strongest peak intensityof the pure wurtzite ZnO (d101) and zinc blend ZnS (d111) changesconsiderably (d101 ¼ 2:476 Å, d111 ¼ 3:120 Å) for the ZnO–ZnScore–shell nanostructures which indicates that the ZnS QDs isattached to the ZnO NPs. Furthermore, the diffraction planes ofZnS become dominant when the Na2S concentration increases. Itshows that the ZnO NPs are completely adjoined with the ZnS QDs.The crystallite sizes of the ZnO core and the ZnS shell can beestimated from the Debye–Scherrer equation and are(22.6570.05) nm and (6.9570.05) nm respectively.

4.2. Morphology and chemical composition

The FE-SEM images of the pure ZnS QDs without NaOH andpure ZnO NPs are shown in Fig. 2(a) and (b) respectively. As can beseen, both samples are significantly agglomerated due to theabsence of capping agent. All aggregated particles are composedof different number of smaller crystallites. The inset of Fig. 2(b) shows a typical TEM image of the pure ZnO NPs. The imagesshow that the particles have almost hexagonal shapes and belongto hexagonal wurtzite ZnO NPs, with an approximate particle sizeof 25 nm. This is due to the large surface to volume ratio of thenanoparticles, where the atoms located close the surface areas,and has a functional role in the optical and electrical properties.Atoms on the nanoparticle surface are bonded by weaker forcesdue to the absence of neighbouring atoms, which is one of themain reasons of their increased surface reactivity. Because of theirstrong surface reactivity most nanoparticles undergo agglomera-tion. To overcome the agglomeration effect, NaOH has been usedto obtain smaller and narrower particle size distribution and alsoto control the morphology of the ZnS QDs final product. Therefore,to investigate the effect of NaOH on the smaller particles, we haveperformed TEM and HRTEM studies of the as-synthesised ZnS QDsand ZnS–ZnO nanocomposite. The TEM images of the pure ZnSQDs without and with the NaOH are shown in Fig. 3(a) and(b) respectively. Fig. 3(a) illustrates that the nanoparticles are anagglomeration of smaller particles. As can be shown in Fig. 3(b) and (c), the presence of NaOH affects the size distribution ofthe nanoparticles. Also, NaOH as a capping agent is able toseparate the nanoparticles and prevent their agglomeration. NaOHindeed increases the separation between the single particles andrelaxes the single particles from agglomeration. However thepresence of the nanoparticles is still visible and the separation of

JCPDS) Calculated parameters (from Rietveld) Crystallite size

a (Å) V (Å)3 Sherrer method (nm)

970.01) (5.365470.0031) (154.45670.291) (4.3570.05)670.01) (5.312970.0024) (149.96670.243) (3.4570.05)

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the nanoparticles to larger distances of more than 2–3 nm is notpossible. Fig. 3(a1) shows the HRTEM image of ZnS QDs with theabsence of NaOH. As can be seen, the lattice planes are quiteparallel and the average distance between the neighbouring latticeplanes is (0.3170.01) nm, which corresponds to the (1 1 1) planeof zinc blend ZnS. Also, Fig. 3(a2) shows a typical HRTEM image ofan individual dot where the ZnS QDs has approximately aspherical shape with an average particle size of (571) nm. TheTEM and HRTEM images of the ZnS–ZnO nanocomposite areshown in Fig. 4(a) and (b). It should be mentioned that the ZnSQDs and ZnO NPs were mixed together in solution to produce theZnS–ZnO nanocomposite, therefore the separation between theZnS QDs and ZnO NPs is inevitable because they are formedseparately. Since the capping agent was not used in this solution,the ZnS QDs and ZnO NPs are agglomerated together.

Fig. 5(a) shows the TEM image of ZnO–ZnS core–shell nanos-tructures synthesised with 0.1 M Na2S. The morphology of thecore–shell structure is a mixture of round and rectangular shapeobjects. The rectangular structures are about 70�100 nm whilethe spherical shapes are about 200 nm in diameter. The TEMbright field (BF) clearly shows that ZnO is encapsulated by ZnSwith the appearance of different phase contrast. Furthermore theZnS shell with an estimated thickness of (1871) nm is clearlyobserved, which implies that the encapsulation of ZnO by ZnS wassuccessfully. Fig. 5(b) and (c) are the high resolution TEM (HRTEM)images of the ZnO–ZnS core–shell. The lattice of the ZnO and ZnSis clearly different and they show some degree of crystallinity.

Fig. 2. FESEM images of (a) ZnS QDs and (b) ZnO NPs. Inset of (b) TEM image ofhexagonal ZnO NPs.

Electron diffraction patterns obtained from the fast Fourier trans-form of the high resolution images of the ZnO–ZnS core–shellsamples can be seen in the inset of Fig. 5(b) and (c). Since thesamples were briefly heated, the crystallinity of the ZnO–ZnScore–shell is rather poor. The (0 0 2) lattice of the hexagonal ZnOand the (1 1 1) lattice of the cubic ZnS are marked in the inset ofFig. 5(b) and (c) respectively. The ring patterns show that thissample is polycrystalline in nature. The high spatial resolutiondiffraction patterns unmistakably prove that the sample containsnano-structured ZnO encapsulated by the ZnS outer shell. Inaddition, the presence of NaOH and Na2S plays an important rolein the synthesis of ZnO–ZnS core–shell nanostructures or ZnS–ZnOnanocomposites. Recently, Saravanan et al. [36] reported thatexcess Na2S promotes the size reduction of the ZnS NPs. Liuet al. [37] also explained that excess Na2S has a significant effecton the optoelectronic properties of the CdTe NPs. They illustratedthat, cadmium cations in CdTe can react with Na2S and results inthe agglomeration of CdS. It is believed that the Na2S decomposesand hydrolyses when the system is further heated until all thesolvent evaporates. Therefore, a small amount of H2S and sulphurare released during the heating process. The presence of sulphur isdetected by EDAX analysis (see Fig. 5(d)). Furthermore, the H2Srelease can be easily felt from the rotten egg odour during thesynthesis process. So when the solution reaches a certain tem-perature, Na2S is hydrolysed and releases the H2S (S2�) under thehydrothermal condition. When the H2S reacts to form the initialZnS nuclei, Zn2+ slowly dissolves from the surface of the as-grownZnO, which is a type of neutralisation reaction. Therefore, theinitial ZnS shell is formed by the ion exchange reaction betweenthe H2S and Zn2+ of the ZnO surface. Yi et al. [38] reported that,very small regions which are known as the intermediate gap anddiffusion bridge are formed between the ZnS shell and ZnO coreduring the neutralisation process. The oxygen ions and thedissolved zinc ions can migrate between core and shell in theseregions. However, it should be mentioned that the thickness of theZnS shell depends significantly on the amount of sulphur ions.When the amount of sulphur decreases, the dissolved zinc ionscould not establish a bond with the sulphur ions. Therefore thezinc ions recombine again on the surface of the ZnO core. As canbe seen in Fig. 6(a), eventually a thin ZnS shell and hetero-interface in the ZnO–ZnS core–shell is formed.

4.3. Optical properties

Fig. 7(a) shows the UV–vis absorption spectra of ZnS QDs, S (1) andS (2) powder synthesised without and with NaOH respectively. As canbe seen, the absorption spectra shift to shorter wavelengths whenNaOH is added into the solution. For both samples S (1) and S (2) abroad shoulder appears between 280 nm and 320 nmwith a long tailextending to longer wavelengths. No absorption is observed atwavelengths longer than 370 nm in the UV–vis region for the as-synthesised ZnS QDs. Therefore the as-synthesised ZnS QDs could beused as optically transparent materials for the transmission of infraredand visible light without any attenuation. Fig. 7(b) shows the UV–visreflectance spectra of the as-synthesised ZnS QDs. As can be seen,infrared and visible light can be reflected completely but the UV lightis absorbed (Fig. 7(a)), therefore the as-synthesised ZnS QDs can act asa filter against the UV light entirely. A steep increase in the opticalabsorbance shown in Fig. 7(a) occur around 330 nm and 340 nm for S(1) and S (2) samples respectively. This steep increase in the opticalabsorbance could be due to the electronic transition between thevalence band and conduction band of the ZnS QDs. In addition, Fig. 7(a) indicates that a blue shift of the absorption edge occurs with theaddition of NaOH, which arises from the quantum confinement effectcaused by the photo-generated electron–hole pairs. It is known thatthe optical properties of semiconductors become size dependent

M. Sookhakian et al. / Journal of Luminescence 145 (2014) 244–252248

when the crystallite size decreases and becomes closer to the excitonBohr radius. The typical exciton Bohr radius of ZnS QDs is 3 nm [39],therefore the optical absorption edge of ZnS QDs shifts to shorterwavelengths with the presence of NaOH compared to the absence ofNaOH, because the crystallite size of the nanoparticles decrease from(4.3570.05) nm to (3.4570.05) nm. The estimated band gap for bothS (1) and S (2) samples are calculated using the following equation[40]

αhν¼ Aðhν�EgÞn

where h is the Plank's constant, ν is the photo-frequency, A is aconstant, Eg is the band-gap and n is the index which depends on thetype of transition. The n value for a direct band-gap semiconductor is

Fig. 3. TEM images of ZnS QDs (a) with the absence of NaOH; (b) and (c) with the presHRTEM image of individual dot.

Fig. 4. (a) TEM and (b) HRTEM images

1/2, and is 2 for an indirect band-gap semiconductor. ZnS is a directband-gap semiconductor, therefore n¼1/2. Thus, the estimated band-gap can be obtained from the plot of (αhν)2 vs. hν as shown in the insetof Fig. 7(a). The calculated optical band-gap values are (3.8670.01) eVand (3.7870.01) eV for S (1) and S (2), respectively, which aresignificantly higher compared to the bulk ZnS (3.65 eV) due to thequantum confinement effect in ZnS QDs. The calculated band-gap forthe as-synthesised ZnS QDs in the presence of NaOH is slightly higherthan the ZnS QDs. Generally, it can be said that the band-gap valueincrease with the decrease in the particle size.

Fig. 7(c) shows the UV–vis absorption spectra for the pure ZnONPs and ZnO–ZnS core–shell NPs synthesised with different Na2Sconcentrations. As can be seen, the observed band edge of the

ence of NaOH in different magnifications. (a1) HRTEM image of ZnS QDs, and (a2)

of mixed ZnS–ZnO nanocomposite.

M. Sookhakian et al. / Journal of Luminescence 145 (2014) 244–252 249

pure ZnO NPs is around 360 nm, which is significantly lower thanthe value of the bulk ZnO (λ¼375 nm, Eg¼3.37 eV) [41], thereforethe presence of a strong blue shift absorption edge can be seen forthe pure ZnO NPs compared to the bulk ZnO. However, an obviouschange has occurred in the optical absorption spectra, with thecoating of ZnS QDs on ZnO NPs. It can be seen that there is asignificant red shift to longer wavelength in the absorption edge ofthe ZnO–ZnS core–shell when the ZnO core is encapsulated by theZnS shell. Due to the deposition of the ZnS shell, photo-absorptionin the range of 380–420 nm is enhanced and the absorptionintensity of the ZnO–ZnS core–shell nanostructure becomes stron-ger with further increase in the Na2S concentration. In addition, aspectral red shift of 50 nm occurs due to the leakage of excitonsfrom the ZnO core to the ZnS shell [42].

Fig. 7(d) shows the room temperature PL spectra of the pureZnS synthesised without and with the NaOH, measured at 325

Fig. 5. (a) TEM and (b and c) HRTEM images of ZnO–ZnS core–shell nanostructures, Electshell. Inset of (b) the diffraction patterns of the ZnO area, and inset of (c) diffraction patt0.1 M Na2S.

Fig. 6. (a) Schematic diagram of the mechanism of formation of the hetero-interface ofZnO–ZnS core–shell nanostructure.

excitation wavelength. In previous reports the PL emission spectraof ZnS nanostructures can be classified into five different types ofemission mechanisms[43–47]:

(i)

ron derns o

ZnO–

near ultraviolet emission with a wavelength range between330 and 370 nm [43] is due to the band to band transitions,where the band gap of bulk ZnS (3.6 eV) is 330 nm,

(ii)

violet emission with a wavelength range between 380 and400 nm [44] is due to the deep level states such as disloca-tions, interstitials and Zn2+ vacancies,

(iii)

blue emission with a wavelength range between 430 and470 nm [45] is associated with trapped luminescencearising from the Zn2+ vacancies, S2� vacancies and surfacestates,

(iv)

green emission with a wavelength range between 510 and550 nm [46] is due to dopants and impurity atoms where

iffraction from the Fast Fourier transform of the HRTEM of the ZnO–ZnS core–f the ZnS area (d) EDAX spectrum of ZnO–ZnS core–shell nanostructures with

ZnS core–shell nanostructures (b) The schematic diagram of the band-gap of

Fig. 7reflecnano

Fig. 8inten

M. Sookhakian et al. / Journal of Luminescence 145 (2014) 244–252250

transitions occur from the conduction band of ZnS to thedifferent excited levels of the impurity atoms and dopants inthe ZnS band gap, and finally, and

(v)

orange emission with a wavelength range between 600 and630 nm [47], is also associated with the deep level states.

Fig. 7(d) shows all the electronic transitions for both S (1) and S (2).For both samples, it can be seen that violet emission with a broadshoulder occurs around 380–395 nm due to the interstitial Zn defects.This shoulder shifts towards longer wavelengths and a broad blueemission peak centred at around 430 nm is observed. This blueemission peak is associated with the trapped luminescence arisingfrom the energy levels of sulphur vacancies with the holes from thevalence band. Also two other broad peaks are observed at around484 nm and 528 nm. The latter peak detected at 528 nm (2.35 eV) canbe attributed to the recombination of electrons and holes from thesulphur vacancies and zinc vacancies respectively. The emission peakobserved at 484 nm can be understood as follows. The sulphurvacancies generate donor states below the conduction band, so theholes and electrons are generated in donor states and conductionbands respectively. This localised charge exerts a potential, which canenhance the trapped electrons. As a result, during the excitation

. UV–vis absorbance spectra of pure ZnS QDs without (S1) and with (S2) 20 ml 0.5 Mtance spectra of the pure ZnS QDs without (S1) and with (S2) 20 ml 0.5 M NaOHstructures (SB, SC, SD) with different concentrations of Na2S (d) Room temperatur

. (a) The room temperature PL spectra of the pure ZnO NPs (SA) and ZnO–ZnS cosity ratios of the green emission to the UV emission.

process, S2� vacancies start to inject electrons into the conductionband. During this process, whenever a captured electron recombineswith a hole in the valence band, a fundamental blue emission occursat 430 nm, but when a captured electron recombines with a hole inother acceptor levels, which are the interstitial sulphur states, thesecond blue emission occurs at 484 nm. Therefore, blue emissions areattributed to the sulphur vacancies and interstitial sulphur latticedefects.

On the other hand, comparison of the PL spectra of both S(1) and S (2) samples indicates that the presence of NaOH affectsthe emission intensity of the PL spectra. This is because theelectronic transitions in the PL emission spectra occur due to thedefect states and surface vacancies. In the previous sections wehave shown that the incorporation of NaOH affects the size of theZnS QDs and consequently affects the defect states. It should bementioned that the defect states can be classified into shallow ordeep defect states. The shallow defect states which lie above thevalence band and below the conduction band tend to undergoradiative recombination while, the deep defect states tend toparticipate in the non-radiative recombination by emitting pho-nons. Both shallow and deep defect states participate in the PLemission. Because of the high surface to volume ratio, the surface

NaOH. The inset indicates the calculation of band gap for both samples (b) UV–vis(c) UV–vis absorbance spectra of the pure ZnO NPs (SA) and ZnO–ZnS core–shelle PL spectra of the pure ZnS QDs without (S1) and with (S2) 20 ml 0.5 M NaOH.

re–shell nanostructures (SB, SC, SD) with different concentrations of Na2S (b) the

M. Sookhakian et al. / Journal of Luminescence 145 (2014) 244–252 251

consists of large number of atoms and this causes the appearanceof defect states due to the unsaturated dangling bonds. Thesedefect states can play an important role for the relocation ofcharge carriers and the creation of excitons. These excitons can beformed between the surface vacancies and defect states. Due tothe small size of the nanoparticles, electrons can move freely inshort distances between the surface vacancies and defect states.Thus, the defect states and surface vacancies are easy connectionlinks to form excitons between the sub-bands and surface states.Therefore the PL emission spectra for the as-synthesised ZnS QDswith the presence of NaOH become more significant because thedecrease in the nanoparticles size affects the increase of thesurface vacancies and defect states, consequently the probabilityof exciton creation increases and the PL intensity is enhanced. Itshould be mentioned that the difference between the band-gapand the energy absorbed by the ZnS QDs is converted to heat dueto the electron–phonon scattering process which is associatedwith the deep level traps.

Fig. 8 shows the PL measurements of the pure ZnO NPs andZnO–ZnS core–shell nanostructures performed at room tempera-ture with 325 nm excitation, where the pure ZnO NPs powder wasused as the reference. Fig. 8(a) shows that the PL spectra of thepure ZnO NPs (SA) powder consist of two types of emission peaks:a strong and narrow UV emission band centred at 385 nm and abroad green emission band centred at 570 nm. The violet emissionoriginates from the bound excitons arising from the excitonicrecombination corresponding to the near band edge emission ofZnO [48,49]. Different mechanisms has been discussed about thegreen emission previously, such as extrinsic defects [50], deeplevels such as oxygen vacancies or surface states [51] and oxideanti-site defects [52], but the precise mechanism for the greenemission is still unclear.

The mechanism of PL emission in the ZnO–ZnS core–shelldepends greatly on the shell structure but remains inconsistentcompared to the PL emission spectra of the pure ZnO NPs. Manyreports show that the PL emission spectra of ZnO–ZnS core–shellis significantly enhanced compared to the ZnO NPs. Li et al. [23]and Yi et al. [38] reported a dramatic increase in the UV emissionof the ZnO–ZnS core–shell PL spectra because the hetero-interfaceof the as-synthesised ZnO–ZnS core–shell is type I. In thesesystems, due to the presence of a type I band alignment, photo-generated charge carriers which are restricted in the ZnO corecause enhancement in the quantum effect of ZnO core and denseZnS shell, resulting in a steep increase in the UV emission.However, many other research groups has also reported a sig-nificant decrease in the UV emission due to the presence of a typeII band alignment in the hetero-interface between the ZnO coreand a thin ZnS shell. Wang et al. [53] reported that the UVemission of ZnO–ZnS core–shell nanowire significantly decreasesdue to the charge separation effect, compared to the ZnO NPs. Inthis work, a considerable decrease in the UV and green emissionand a significant increase in the blue emission are observed,compared to the ZnO NPs. As can be seen in Fig. 8(a), all of theZnO–ZnS core–shell nanostructures synthesised with differentconcentrations of Na2S show considerable decrease in the UVand green emission intensities. The reason for the UV emissiondecrease anomaly can be attributed to the structural formation ofthe ZnS shell and ZnO core where the hetero-interface of the as-synthesised ZnO–ZnS core–shell is type II. To illustrate this, aschematic mechanism for the formation of type II hetero-interfaceof the as-synthesised ZnO–ZnS core–shell is shown in Fig. 6(a). Ascan be seen in Fig. 5, the structure of ZnS shell is thin with lessporosity, therefore the charge carriers are not effectively restrictedin the ZnO core. Therefore, the photo-generated charge carrierswhich are dispatched from the ZnO core can be scattered by theporous structure of the ZnS shell. The second reason for the

decrease in the UV emission of the ZnO–ZnS core–shell is relatedto the nature of ZnO core. Again, the photo-generated chargecarriers which are emitted from the ZnO core are absorbed by theZnO NPs which are formed on the surface of the ZnO core.Although the photo-generated charge carrier emission from theZnO core is very strong, but the ZnO NPs which are grown onthe surface of ZnO core can absorb some of the emission, resultingin the decrease of the UV emission. As can be seen in Fig. 8(a), thetypical broad blue emission of the PL spectra of ZnO–ZnS core–shell (λ¼ 440–470 nm) shows a significant enhancement com-pared to the pure ZnO NPs. The intensity increase of the blueemission is due to the change in band-gap structure. Fig. 6(b) is aschematic diagram of the band-gap structure for the ZnO–ZnScore–shell. It can be seen that the band-gap of ZnS QDs is higherthan ZnO NPs, therefore the charge carriers need a tunnellingphenomena to move between the ZnO core and ZnS shell. There-fore more photo-generated charge carriers are confined in the ZnOcore, thus resulting in a high quantum yield and enhancement ofthe blue emission.

To illustrate the decrease of green emission of ZnO–ZnScore–shell nanostructures, the ratios of the green emission inten-sity to the UV emission intensity was calculated for three samplesS (B), S (C) and S (D) and shown in Fig. 8(b). It can be seen that thisratio decreases considerably for the core–shell nanoparticles.The most important reason for this decrease could be dueto the reduction of oxygen vacancies. The recombination ofelectrons which are close to the conduction band with the trappedholes in the interstitial oxygen vacancies is the reason for thegreen emission. However, these oxygen vacancies are occupiedby oxygen ions, since a significant number of oxygen ions areproduced during the hydrothermal process. Due to the large ionicradius, these oxygen ions cannot diffuse outwards, consequently asignificant number of oxygen ions are trapped in the intermediategap. Therefore oxygen vacancies in the ZnO core are filled byoxygen ions, resulting in the reduction of oxygen vacancies andconsequently decrease in the green emission. From Fig. 8(b), it canbe seen that this ratio has an inverse relationship with theconcentration of Na2S, where this ratio decreases when theconcentration of Na2S increases. Therefore, it can be said thatwhen the concentration of Na2S increases, the ZnS shell thicknessincreases and consequently more oxygen ions are trapped in theintermediate gap, and more oxygen vacancies are occupied byoxygen ions, resulting in the decrease in the green emissionintensity.

5. Conclusions

In summary, pure ZnS QDs with crystallite size of 3 nm and3.86 eV band gap was successfully synthesised in the first step,where NaOH was used to adjust the pH of the reaction medium.When the pHo6, NaOH behaves as a capping agent to decreasethe size of ZnS QDs, but when the pH46, NaOH induces theformation of in situ ZnO NPs and consequently, the formation ofZnS–ZnO mixed nanocomposites. The ZnO–ZnS core–shell type-IIwith controllable ZnS shell was synthesised by adjusting theconcentration of sodium sulphide. The structural properties wereinvestigated with TEM, HRTEM and FE-SEM. The TEM images andHRTEM micrographs show that the particle size of the pure ZnSQDs, pure ZnO NPs and the average ZnS shell in ZnO–ZnSnanostructure were 5 nm, 25 nm and 18 nm respectively. Theoptical properties were investigated with UV–vis absorptionspectra and PL emission spectra. We observed a blue shift in theUV–vis spectra and an enhanced PL emission of the ZnS QDs dueto the size reduction. Compared to the ZnO NPs, the PL emission ofthe ZnO–ZnS nanostructure decreased due to the type-II band

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alignment structure in the hetero-interface. The UV emission inthe PL spectrum decreased due to the thin and porous ZnS shelland the formation of ZnO NPs on the ZnO core. The green emissionin the PL spectrum also decreased due to the decrease in theoxygen vacancies as well as the appearance of the blue emission inthe PL spectrum is due to the conversion in the band-gapstructure.

Acknowledgements

The authors would like to express sincere thanks to Dr M.R.Mahmoudian for his valuable discussions in this work. The authorswould also like to thank MOHE and University of Malaya forproviding financial assistance with grant number PV136-2012A,FP039 2010B and RG181-12SUS for this work.

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