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Current Applied Physics 12 (2012) 632e636

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Current Applied Physics

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Synthesis, structural and optical characterization of nanocrystalline ZnS:Cuembedded in silica matrix

Ashish Tiwari a,*, S.A. Khan b, R.S. Kher c

aDepartment of Chemistry, Government E. R. R. PG Science College, Bilaspur 495006, IndiabGovernment College Seepat, 495555, IndiacDepartment of Physics, Government E. R. R. PG Science College, Bilaspur 495006, India

a r t i c l e i n f o

Article history:Received 28 May 2011Received in revised form16 August 2011Accepted 17 September 2011Available online 2 October 2011

Keywords:ZnS:Cu nanoparticlesSol gel growthXRDElectron microscopyOptical properties

* Corresponding author.E-mail address: ashish_chem@yahoo.in (A. Tiwari

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.09.014

a b s t r a c t

The synthesis of Cu doped ZnS nanoparticles inside the pore of an inorganic silica gel matrix is presented.The synthesized nanoparticles were characterized by powder X-ray diffraction (XRD), scanning electronmicroscopy (SEM) and energy dispersive X-ray (EDX). X-ray diffraction pattern reveals the crystallinewurtzite phase of ZnS. The existence of silica gel in modeling morphologies of the nanoparticles wascharacterized using Fourier transform infrared (FTIR) spectrometer. Thickness of the silica shell was alsocalculated. UV- absorption spectrum shows the appearance of an absorption peak at 273 nm whichconfirms the blue shift as compared to that of bulk ZnS. The photoluminescence (PL) emission spectrumof the sample showed a broad band in the range 465e510 nm due to the transition from the conductionband edge of ZnS nanocrystals to the acceptor like t2 state of Cu.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Nanomaterials are the corner stone of nanotechnology and areanticipated to play an important role in the future economy.Remarkable variations in fundamental physical properties occurwhen one changes from a macroscopic solid to a particle consistingof a countable number of atoms. Among the semiconductor nano-particles, doped zinc sulphide is an important IIeVI semiconductor,which has been synthesized and researched extensively because ofits broad spectrum of potential applications such as in catalysts,electronic and optoelectronic nanodevices [1e5]. Doping of semi-conductor nanocrystals manipulate the band structure of thenanocrystals and show intense emissions in a wide range of wave-length depending on the nature and concentration of the dopant. Italso modifies luminescence efficiency and the positions of emissionbands, thus influencing their practical applications. Studies of Cudoping are more challenging as Cu ions does not fit easily into theZnS lattice and for this reason, ZnS:Cu [6,7] hasnotbeen investigatedas widely as ZnS:Mn nanoparticles. Xu et al. [8] found that theintegrated PL intensity from ZnS:Cu nanocrystals is stronger thanthat fromMn-doped ZnS nanocrystalline phosphors.

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A large portion of atoms in nanocrystals is located on or near thesurface, hence the surface properties should have a significanteffect on their structural and optical properties [9]. Moreover tohave enhanced luminescence properties, a good surface modifica-tion is essential. Silica (dielectric constant w 4.5) is a popularmaterial to form core shell particles because of its extraordinaryability against coagulation. Its non-coagulating nature is due tovery low value of Hamaker constant [10], which defines the Van derWaal forces of attraction among the particles and the medium. It isalso chemically inert, optically transparent and does not affect thereactions at core surface. Thus silica coating of the ZnS:Cu nano-particles have several advantages over other capping agents, whichare used for surface passivation of nanoparticles such as (i) thesurface passivated “dead layers” generated due to the presence ofthe unsaturated dangling bonds on the surface are removed, (ii)organically passivated nanoparticles cannot withstand high pro-cessing temperature, (iii) the growth of the nanoparticles is limitedto the size of the pore, which results in narrow particle sizedistribution.

Cu-doped ZnS bulk phosphors are one of the well-studiedluminescent materials and have attracted much attention in pasttherefore an extension of studies toward nanocrystalline form isdesirable. In the present paper, we adopted a coordination chem-istry method to dope Cu ions in the preparation of ZnS:Cu @ SiO2nanoparticles and the optical properties were studied.

A. Tiwari et al. / Current Applied Physics 12 (2012) 632e636 633

2. Materials and methods

2.1. Preparation of ZnS:Cu @ SiO2 nanoparticles

All reagents were analytically pure and used without furtherpurification. A silica sol served as the precursor for the host ZnS:Cunanoparticles. It was prepared by dissolving tetraethyl orthosili-cate, Si(OC2H5)4 (TEOS) in ethanol and then adding distilled waterin it. Nitric acid (2 N) was used as a catalyst. The volume ratio forabove reaction was set to 10.4:14:8:5 for TEOS:ethanol:wa-ter:HNO3. A solution of zinc acetate, Zn(CH3COO)2. 2H2O(0.0197 mol), copper acetate, Cu(CH3COO)2. H2O and thiourea,NH2CSNH2 (0.02 mol) was prepared in ethanol and distilled water(volume ratio of ethanol to water was 2:1). Molar ratio of Zn:Cu:Swas fixed to 1: 0.1: 1. The solution containing Zn and Cu is addedand S precursor was slowly added into the silica sol under vigorousstirring. The stirring was continued for a further 1e2 h after thecompletion of the mixing to obtain the final sol. Finally the sol waskept at room temperature until its complete gelation. The gel wasdried at 100e150 �C.

2.2. Reaction mechanism for the formation of ZnS:Cu @ SiO2

nanoparticles

The reaction mechanism is explained in Equations (1 and 2). Thefirst step is hydrolysis in which ethoxy groups are replaced by OHgroups.

In the second step silicic acid undergoes polycondensationprocess to form SiO2.

SiðOC2H5Þ4þ4H2O/SiðOHÞ4þ4C2H5OH (1)

nSiðOHÞ4/nSiO2 þ 2n H2O (2)

The solubility of CuS is less than that of ZnS hence it seems thatZn2þ and Cu2þ cannot be coprecipitated with S2� ions but thioureaminimize the solubility difference between CuS and ZnS by coor-dinatingwith Cu2þ and thus the possibility of coprecipitation of ZnSand CuS can be greatly improved during the synthesis and thismake it possible to obtain ZnS:Cu nanoparticles, inwhich Cu2þ ionsreplace the Zn2þ ions in the lattice [11].

2.3. Characterization

The synthesized specimen was characterized by powdered XRDusing RigakuMiniflex with Cu Ka (1.5406 Å) radiation in 2q range of

Fig. 1. X-ray diffraction pattern of SiO

20�e70� at step size of 0.02� (2q). The scanning electron micros-copy (SEM) measurements were recorded on a FEI Quanta 200microscope equipped with EDX unit. The FTIR spectrum wasrecorded on SHIMADSU FTIR 8400S spectrophotometer. A UVeVISspectrum was recorded on SHIMADSU UV 117 spectrophotometer.The PL of the sample was recorded at room temperature usingSHIMADZU RF 5301 PC Spectrofluorophotometer.

3. Results and discussion

3.1. XRD analysis

It is known that ZnS adopts two structural polymorphs, i.e.hexagonal wurtzite and cubic sphalerite (zinc blende). Consid-ering that wurtzite ZnS is much more desirable for its opticalproperties than the sphalerite phase, the low temperaturesynthesis and stabilization of wurtzite ZnS nanostructures isextremely practical. Fig. 1 shows the XRD pattern of SiO2 cappedZnS:Cu nanoparticles. Broad diffraction peaks were obtained, inaccordance with the characteristics of nanosized materials. Itreveals the hexagonal structure with major diffraction peakscorresponding to the (100), (002) and (101) planes. It can be seenthat all peaks were in good agreement with wurtzite (JCPDS # 36-1450) except some additional peaks which may be due toformation of Zn2SiO4 which is not unlikely as the final productwas sintered in air at 100e150 �C. Crystallite size of cappedZnS:Cu was calculated by following Scherrer’s equation [12] asshown in Eq. (3):

D ¼ Kl=ß cosq (3)

In this equation D is the mean crystallite size, K the constant(shape factor, about 1), l the X-ray wavelength (1.54056 Å), b thefull width at half maximum (FWHM) of diffraction peak and q thediffraction angle.

The average size calculated from the above formula is 7.5 nm.

3.2. Microstructural study

Fig. 2 a and b shows the SEM micrograph and EDX spectrum ofthe as prepared SiO2 capped ZnS:Cu nanoparticles. Themorphologyof the particles is nearly spherical. The smaller particles can beeasily seen clinging to the surface of the big particles. Because SEMcan often only determine the particle size of secondary particles, itis assumed that the secondary particles of ZnS:Cu consists of theprimary particles which are in the nanometer range, as estimated

2 capped ZnS:Cu nanoparticles.

Fig. 2. a and b: SEM micrograph and EDX spectrum of SiO2 capped ZnS:Cu nanoparticles respectively.

A. Tiwari et al. / Current Applied Physics 12 (2012) 632e636634

by Scherrer’s equation. The EDX spectra show peaks correspondingto Zn, S, Si, O, and Cu thus confirming the presence of theseelements in as prepared ZnS:Cu @ SiO2 nanoparticles.

3.3. Thickness of silica layer

The thickness of the silica layer was theoretically calculated byfollowing method. Assuming particles are perfectly spherical thevolume of core and shell is given by Eqs. (4)e(6).

VZnS ¼ 4=3ðd=2Þ3 (4)

Vshell ¼n4=3ðd=2þ LÞ3-4=3ðd=2Þ3

o(5)

where L is the thickness of the shell and d is the average diameter ofthe particles. The molar ratio of zinc acetate and TEOS was taken tobe 0.0197 mol and 0.046 mol respectively. Considering that if all ofthe zinc acetate and TEOS are converted to ZnS and silica respec-tively then volume ratio between ZnS and silica can be determinedas shown in Table 1.

Vshell=VZnS ¼n4=3ðd=2þLÞ3-4=3ðd=2Þ3

o.n4=3ðd=2Þ3

o2:23 ¼

n4=3ðd=2þLÞ3-4=3ðd=2Þ3

o.n4=3ðd=2Þ3

o (6)

Putting the value of d ¼ 7.5 nm (calculated from XRD data) weget L ¼ 1.8 nm.

3.4. FTIR spectra

In the FTIR spectra of the as prepared sample (Fig. 3), the broadfeature between 3376 cm�1

e 3402 cm�1 is due to free SieOHgroup. SieOeSi bend appears at 477.88 cm�1 and 772 cm�1. The

Table 1Calculation of thickness of silica shell.

Compound mole Density (g/l) Mw M (mg) Vol. (ml)

ZnS 0.0197 4.09 97.47 1920 469.5SiO2 0.046 2.64 60.00 2760 1045

peak at 1090.3 cm�1 corresponds to SieO stretching. The bandbetween 954 cm�1e1090 cm�1 is very intense and corresponds tothe formation of SiO2 network. The new band at 1638 cm�1 may bedue to ZneOeSi vibration.

3.5. Optical studies

Optical excitation of electrons across the band gap is stronglyallowed transition, producing an abrupt increase in absorptivityat the wavelength corresponding to the gap energy. UVabsorption spectrum of the sample is shown in Fig. 4. It can beseen that the strongest absorption peak of the prepared sampleappears at around 273 nm (lp) which is fairly blue-shifted fromthe absorption edge (334 nm) of the bulk ZnS [13] as shown inthe inset of Fig. 4. Semiconductor crystallites in the diameterrange of a few nanometers show a three dimensional quantumsize effect in their electronic structure. UV absorption spectrumprovides information relating to the band gap and size of theparticle Effective mass approximation (EMA) [14] has been usedto explain the change of energy gap as function of particle size.The particle size can be calculated, by using the followingequation [15].

Fig. 3. FTIR spectrum of SiO2 capped ZnS:Cu nanoparticles.

Fig. 4. Absorption spectrum of SiO2 capped ZnS:Cu nanoparticles. Inset is theabsorption spectrum of bulk ZnS.

Fig. 5. a: Photoluminescence excitation spectrum (PLE) of SiO2 capped ZnS:Cu nano-particles (lem ¼ 485 nm). b: Photoluminescence emission spectrum of SiO2 cappedZnS:Cu nanoparticles (lex ¼ 380 nm). Inset is the PL spectrum of enlarged UV range.

A. Tiwari et al. / Current Applied Physics 12 (2012) 632e636 635

rðnmÞ ¼�-0:2963

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�-40:1970þ13620=lp

���-7:34þ2481:6=lp

�q ð7Þ

The particle size (d¼ 2r) calculated was 3.2 nm. The particle sizeis not in good agreement with the crystallite size calculated fromXRD data. It may be due to the broadening of the (100) peak due toSiO2 matrix where as the calculation of particle size by opticalabsorption studies involves parameters related to ZnS irrespectiveof the matrix.

In order to clearly understand the nature of the emissionbands, we have recorded PLE spectra at room temperature. Fig. 5ashows photoluminescence excitation (PLE) spectrum ofZnS:Cu@SiO2 nanocrystals with 0.1 mol % Cu doping. The exci-tation spectrum is composed of a strong and broad band in therange of 350e395 nmwith a peak maximum around 380 nm. ThePLE spectrum reveals that the excitation originates primarilyfrom absorption of light by the ZnS host, as expected. Fig. 5b shows the room temperature PL emission spectra of silicacapped ZnS:Cu nanocrystals. It showed a broad emission band inthe range of 465e510 nm with a peak maximum around 485 nm.Because of the interaction between Cu ions and ZnS we candeduce that the PL spectrum of ZnS:Cu was attributed to thetransition between bottom of the conduction band of ZnS and thelevels of Cu impurities which acted as luminescent center. Unlikethe case of ZnS:Mn, so far no uniform model exists that comprisesthe sum of main luminescent properties of ZnS:Cu. In the presentwork, the PL peak obtained in the blue region is attributed to thetransition from the conduction band edge of ZnS nanocrystals tothe acceptor like t2 state of Cu [16]. This observation is quitesimilar to the results reported by Jayanti et al. [17] and Corradoet al. [18]. The inset in Fig. 5b shows the magnified spectra of UVregion with a weak and asymmetric emission band at 355 nmthat can be related to trap state emission or some other donor-to-acceptor level transitions [19,20].

The UVeVis studies as well as the photoluminescence stronglysuggest that we have succeeded in synthesizing Cu ion doped ZnSnanoparticles embedded in SiO2 matrix.

4. Conclusion

It has been shown that inorganic photoluminescent coatingscan be produced by the solegel process. The photoluminescencestrongly suggest that the incorporation of Cu ions in ZnS hassucceeded as revealed by the emission peak around 485 nm due to3 d9 4 s1 4 3 d10 transition of Cu ions. The blue shift of theabsorption peak directly reflects the effect of quantum confine-ment. The optical properties of the silica capped ZnS:Cu nano-particles may be very interesting for further application onluminescent devices.

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