Structural and optical properties of Co- doped ZnS ...

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Mar 19, 2014 - Jamil K. Salem, Talaat M. Hammad,. S. Kuhn, Mohammed Abu Draaz, ..... R. Yousefi, M.R. Muhamad, A.K. Zak, Curr. Appl. Phys. 11,. 767–770 ...

Structural and optical properties of Codoped ZnS nanoparticles synthesized by a capping agent Jamil K. Salem, Talaat M. Hammad, S. Kuhn, Mohammed Abu Draaz, Naser K. Hejazy & R. Hempelmann Journal of Materials Science: Materials in Electronics ISSN 0957-4522 Volume 25 Number 5 J Mater Sci: Mater Electron (2014) 25:2177-2182 DOI 10.1007/s10854-014-1856-8

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Author's personal copy J Mater Sci: Mater Electron (2014) 25:2177–2182 DOI 10.1007/s10854-014-1856-8

Structural and optical properties of Co-doped ZnS nanoparticles synthesized by a capping agent Jamil K. Salem • Talaat M. Hammad • S. Kuhn • Mohammed Abu Draaz • Naser K. Hejazy R. Hempelmann



Received: 28 December 2013 / Accepted: 3 March 2014 / Published online: 19 March 2014 Ó Springer Science+Business Media New York 2014

Abstract Cobalt-doped Zinc sulfide (ZnS) nanoparticles were prepared by a simple chemical method using alkyl hydroxyl ethyl dimethyl ammonium chloride (YH) as capping agent. The structural and optical properties of prepared cobalt-doped ZnS nanoparticles have been characterized. X-ray diffraction patterns and transmission electron microscope images reveal pure cubic ZnS phase with size of about 5–2 nm for all cobalt-doped ZnS nanoparticles. The lattice constant of the samples decreases slightly by the introduction of Co2? The absorption edge of the ZnS:Co2? nanoparticles is blue-shifted as compared with that of bulk ZnS, indicating the quantum confinement effect. The photoluminescence emission band exhibits a blue shift for Co-doped ZnS nanoparticles as compared to the ZnS nanoparticles.

J. K. Salem  M. A. Draaz Chemistry Department, Faculty of Science, Al-Azhar University, P.O. Box 1277, Gaza, Palestine T. M. Hammad (&) Physics Department, Faculty of Science, Al-Azhar University, P.O. Box 1277, Gaza, Palestine e-mail: [email protected] S. Kuhn  R. Hempelmann Department of Physical Chemistry, Saarland University, 66123 Saarbru¨cken, Germany N. K. Hejazy Department of Education, Al-Quds Open University, Gaza Branch, Gaza, GazaStrip, Palestine

1 Introduction Zinc sulfide (ZnS), an important semiconductor compound of the II–VI group, has a wide band gap of 3.7 eV at room temperature and a relatively large exciton binding energy (approximately 40 meV) and has been extensively investigated [1–3]. ZnS has a large energy gap and is able to be doped by most transition and/or rare earth metal ions to obtain remarkable optical properties [4]. ZnS and ZnSbased materials have been applied in the fields of photoluminescence, cathodoluminescence and electroluminescence. There are several reports on the photoluminescence properties of ZnS nanostructures doped by various types of impurities, such as Mn2? [5, 6], Cu? [7], Ag? [8], and Eu3? [9] etc. Undoped ZnS nanoparticles emit in the blue region with broad bandwidth. But doping causes the visible light emission, due to the intermediate transitions under UV light excitation. Recently cobalt has been found to be a prospective doping material in ZnS nanocrystallite. Yung et al. [10] reported that Co3? and Co2? doped ZnS nanoparticles emit visible light. Also ZnS nanoparticles after co-doping with Co2? and Cu2? showed an enhancement in the PL emission [10, 11]. Recently cobalt has been found to be a prospective doping material in ZnS nanocrystallite. These doped ZnS semiconductor materials have a wide range of applications in electroluminescence devices, phosphors, light emitting displays, and optical sensors. Many methods have been explored for the preparation of semiconductor nanoparticles such as the chemical method [12, 13], the mechano–chemical method [14, 15], the hydrothermal process [16], the sol–gel method [17], the electro-spinning technique [18], the ultrasonic radiation method [19], the colloidal chemical treatment method [20], the reverse micelle method [21], the solvothermal method [22] and the thermal evaporation method etc. [23, 24].

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(111) (220)

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The present work focuses on synthesizing Co-doped ZnS powder by a simple chemical method and using C16H36NOCl (Praepagen HY, R = 12–14) as capping agent. The synthesized samples were characterized using X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDX), Transmission electron microscope (TEM), FTIR spectroscopy, UV–Vis absorption and Photoluminescence (PL) techniques. The optical and structural properties of the synthesized material are evaluated systematically as a function of dopant (Co?2) concentration.

J Mater Sci: Mater Electron (2014) 25:2177–2182

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2 Experimental To synthesize Co-doped ZnS, the following materials were used. Zinc acetate (Zn(CH3COO)2 .2H2O), cobalt acetate (Co(CH3COO)2 .4H2O) and sodium sulfide (Na2S. xH2O) obtained from Merck were used as precursors, C16H36NOCl (Praepagen HY, R = 12–14) was purchased from Clariant. The chemical reagents were of analytical reagent grade and used without further purification. All the glass wares used in this experimental work were acid washed. Distilled water was used for all dilutions and sample preparations. In a typical synthesis, 0.1 M of Zn(CH3COO)2 .2H2O and the respective amounts (0.1–1 %) of cobalt acetate in 25 ml distilled water were dissolved and another solution of 0.1 M Na2S and appropriate amount of HY solution was prepared. HY was used as capping agent. The Na2S solution was added dropwise to the zinc acetate solution under vigorous stirring. For each experiment, the molar amounts of Zn(CH3COO)2 and Na2S used were equal. During the whole reaction process, the reactants were vigorously stirred under air atmosphere at 30 °C. The formed nanocrystals were separated from the solution by centrifugation. Then, the sample was washed several times with distilled water and ethanol and dried in an hot air oven at 80 °C for 24 h. The undoped ZnS nanoparticle was also synthesized by following the same procedure without doping material. UV–vis absorption spectra were collected using a UV– vis spectrophotometer (Shimadzu, UV-2400) in the wavelength range from 200 to 700 nm. PL spectra were recorded with a spectrofluorometer (JASCO, FP-6500); the extinction wavelength was selected to be 270 nm. The X-ray diffraction (XRD) patterns of the dried as-prepared and classified samples were obtained using an X-ray diffractometer PANalytical X0 pert (PANalytical) with Cu Ka radiation (0.154 nm wavelength) under 40 kV and 200 mA. The TEM analysis was done with JEM2010 (JEOL) transmission electron microscope with energy dispersive X-Ray Spectrometer INCA (Oxford Instruments). FTIR spectroscopy were recorded using a Fourier transform infrared spectrophotometer (Frontier (Perkin

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Fig. 1 X-ray diffraction patterns of Co-doped ZnS nanoparticles a undoped ZnS, b 0.1 %Co, c 0.5 % Co and d 1 % Co

Elmer); The samples are measured on a Zink Selenide Crystal, it is working as a Multiple Reflection ATR system.

3 Results and discussion Figure 1 shows the XRD patterns of the Co-doped ZnS nanoparticles (a) pure ZnS (b) 0.1 %Co, (c) 0.5 %Co and (d) 1 %Co. XRD pattern of the prepared ZnS and ZnS:Co samples were obtained and these are shown in Fig. 1. This figure shows the three diffraction peaks at 2h values 29.04, 48.06 and 57.11. The peaks are appearing due to reflection from the (111), (220) and (311) planes of the cubic phase of the ZnS. The XRD pattern of the prepared samples are well matched with the standard cubic ZnS (JCPDSCardno.5566). No other phases have been observed. ZnS nanoparticles have been grown with cubic phase. On comparing with the standard sample (JCPDS card No. 050566), the X-ray diffractogram and 2h values of ZnS were found to be in fairly good agreement, thus confirming the zinc blende crystal structure. No diffraction peaks corresponding to Co precipitates or Co-related impurity phase were detected, which further confirmed the formation of ZnS:Co2? solid solution instead of Co precipitation or second phase. The lattice constant a was estimated using the following formula [25], qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k 2 a¼ ð1Þ h2 þ k2 þl 2 sin h ˚ ), h is the where k is the X-ray wavelength (1.54056 A Bragg angle, and the Miller index of the crystal plane is ˚ , which is (hkl). For x = 0, a was found to be 5.4024 A close to the reported value of cubic blende ZnS (JCPDS

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radius than Zn?2 (0.074 nm) [26]. Moreover, this also suggests that some Zn2? sites were substituted by Co2?. The average particle sizes of the samples were estimated from the broadening of the diffraction peaks using the Debye–Scherrer equation

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Fig. 2 EDX of 0.1 % Co-doped ZnS nanoparticles

˚ ). Similarly, the lattice card, No. 03-0570, a = 5.4000 A constant a for x = 0.1, 0.5 and 1 % of Co was found to be ˚ , respectively. It is clear that 5. 2942, 5.2445 and 5.1908 A the lattice constant of the Co-doped ZnS nanoparticles decreases with the increase of the Co?2 content. This can be explained by the fact that Co?2 (0.072 nm) has a smaller

kk ðB cos hÞ

ð2Þ

where D is the particle size, k a fixed number of 0.9, k the X-ray wavelength, h the Bragg’s angle in radians, and B the full width at half maximum of the peak in radians. The average particle size of pure, 0.1, 0.5 and 1 % of Co concentration were found to be 4.8, 3.8, 3 and 2.4 nm respectively. Typical EDX spectra of a 1 % Co-doped ZnS sample are displayed in Fig. 2. The spectra reveals that only three elements, Zn and O, and Co exist in Co-doped ZnS nanoparticles. No traces of other elements were found in the spectrum confirm the purity of the samples. Figure 3 demonstrates the typical TEM image and the corresponding size distribution diagram of Co-doped ZnS nanoparticles. The result shows that the as-synthesized Codoped ZnS nanoparticles are in narrow size distribution, the shape of the particles is approximately spherical. The average particle size of samples was found to be 4–2.5 nm,

Fig. 3 TEM micrographs of Co-doped ZnS nanoparticles a undoped ZnS, b 0.1 %Co, c 0.5 % Co and d 1 % Co

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Fig. 5 Structures of alkyl hydroxyl ethyl dimethyl ammonium chloride, C16H36NOCl (Praepagen HY, R = 12–14)

which is in good agreement with the average particle size obtained using XRD results. Both XRD and the TEM measurements suggest that most of the Co?2 atoms have substituted into the ZnS lattice sites. FTIR spectra of ZnS and Co doped ZnS in the presence of a capping agent (HY) are shown in Fig. 4. The molecular structure of HY is shown in Fig. 5, where the hydrophilic ammonium group acts as polar head and the hydrocarbon chain acts as non-polar tail. The formation of new bands and the shift in existing bands in adsorbed HY can be taken as an indicative of their binding mode. A broad peak at 3,400–3,430 cm-1 due to –O–H stretching has been observed in all the samples because the surfactant contains OH group. The typical bands in

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Fig. 7 Optical band gap spectra of Co-doped ZnS nanoparticles

3,000–2,800 cm-1 range due to –C–H symmetric and asymmetric stretching vibrations modes of alkyl chains experience negligible shifts. This indicates free alkyl chain of surfactants that have not been adsorbed over NPs surface. The peaks in 1,400–1,500 cm-1 range due to –C–H bending vibrations of –N–CH3 moiety have been either shifted or completely suppressed in the presence of ZnS nanoparticles. The C–N stretching absorption occurs in the region 1,134 cm-1. The strong band in 1,013 cm-1 is due to single bond stretching vibration of C–O. By comparing

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Fig. 8 PL spectra of Co-doped ZnS nanoparticles

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Conducation band CB Excited state

h+ Anarrow emission band near band edge position

IZn and S Blue emission 2.64 eV at wavelength 469nm

Violet emission 3.52 eV at wavelength 353 nm

Excitation at 270 nm

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Valence band VB Ground state Brod emission band at longer wavelength

Fig. 9 Schematic energy level diagram showing the emission mechanism in the ZnS:Co nanoparticles. Here,Vs stands for sulfur vacancy, VZn for zinc vacancy, IZn for sulfur interstitials and IZn for zinc interstitials

these spectra with that of ZnS undoped, it was found that there has been a significant shift in the peaks due to –C–N stretching and –C–H bending vibrations of –N–CH3 moiety. It is clear that the peaks due to ammonium head group of HY are shifted without any significant shift in hydrocarbon tail region. These results confirmed the adsorption of capping agent (HY) on the surface of ZnS nanoparticles through the interaction of positively charged ammonium group of HY to the surface of ZnS nanoparticles. A UV–vis spectroscopy study is a powerful method for investigating the effects of impurity doping on the optical properties of ZnS nanostructures, because doped ZnS nanostructures are expected to have different optical

properties in comparison with undoped ZnS. The absorption spectra of ZnS:Co2? nanoparticles in the UV-light region are illustrated in Fig. 6. The absorption edge at 315 nm is assigned to the characteristic absorption band edge of ZnS nanoparticles, which is blue shifted as compared with the corresponding bulk band gap (3.90 eV) of ZnS due to quantum confinement effect. However, the absorption edge for the Co-doped ZnS nanoparticles is slightly shifted to shorter wavelengths (blue shift) compared to undoped ZnS. The fundamental absorption, which corresponds to electron excitation from the valance band to conduction band, can be used to determine the value of the optical band gap of the synthesized ZnS nanoparticles. The relation between the incident photon energy (hm) and the absorption coefficient (a) is given by the following relation:   1 ðahmÞn ¼ A hm  Eg ð3Þ where A is the constant and Eg is the band gap energy of the material and the exponent n depends on the type of transition. For direct allowed transition n = 1/2, for indirect allowed transition n = 2, for direct forbidden n = 3/2 and for indirect forbidden n = 3. Direct band gap of the samples are calculated by plotting (ahm)2 versus hm and then extrapolating the straight portion of the curve on the hm axis at a = 0 as shown in Fig. 7. The straight lines imply that the Co-doped ZnS samples have direct energy band gap and the band gap was increased from 3.95 to 4.08 eV with the increase of the Co2? concentration. The absorption edge shifts towards the lower value of wavelength (higher energy) with a increase in concentration of Co. It is clear that band gap increases with doping concentration slightly. Therefore, introduction of Co systematically shifts the absorbance maxima towards shorter wavelength thereby increasing the band gap. The trend of the observed band gap variation upon doping in present study is similar to that reported earlier [27, 28]. The photoluminescence (PL) emission is one of the most important physical properties in ZnS nanoparticles and depends upon synthesis conditions, shape, size and energetic position of the surface states [29–31]. The PL of the synthesized ZnS and Co doped ZnS nanoparticles is studied at room temperature to further investigate the optical properties (Fig. 8). An intense peaks in the violet region at 352, 360 and 365 nm are observed due to the defect sites (absence of Zn?2 or S-2 ions in ZnS lattice sites) in all the samples. Due to the defect sites, the radiative recombination processes between electrons (in conduction band) and holes (in valance band) became intense resulting in a sharp intense peak [32]. Another feature of PL spectra is the weak emission bands in the blue region at 468 nm. This may be due to sulfur vacancy and interstitial sulfur lattice

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defects [33]. From the XRD study it has been observed that both of these emission bands were characteristic features of cubic phase pure ZnS NPs. In the beginning, when no capping agent was present, the ZnS NPs possess a cubic close packed structure which is confirmed by XRD. PL spectra of aqueous surfactant solution without nanoparticles were also recorded and showed no emission at same excitation. This result indicates that the photophysical properties of ZnS nanoparticles depend up on the size and surface passivation. The photoluminescence spectra of Co doped ZnS nanoparticles in aqueous micellar solution of HY are presented in Fig. 8. On doping from 0.1 – 1.0 % Co ions the peaks positions appear at 360,350and 348 nm and the intensity of both of these bands decreases and showed a blue shift as compared to the undoped ZnS nanoparticles. From XRD measurements it is seen that the particle size is decreased at higher dopant percentages which confirm the blue shift at these concentrations of the Co dopant. Although it is still not certain that this blue shift of the emission peak is due to the quantum confinement effect of the nanocrystals, it is believed that this blue shift is related to small particle size, narrow size distribution, and/or surface defects. According to Borse et al. this behavior was due to the collection of positive charges of Co?2 at the defect sites which intervened the radiative processes [34]. The schematic energy level diagram of the Co-doped ZnS nanoparticles is depicted in Fig. 9. This figure explains the emission mechanism of Co-doped ZnS nanocrystals and supports above discussion.

4 Conclusion Cobalt-doped ZnS nanoparticles were prepared by a simple chemical method and functionlized HY. Based on the XRD patterns, the samples are single-phase with cubic structures. XRD and TEM micrographs indicate the average particle size of samples to be in the 5–2 nm range for the nearly spherical particles. The lattice constant of the samples decreases slightly by the introduction of Co2?. The absorption edge of the Co-doped ZnS nanoparticles is blueshifted as compared with that of bulk ZnS. Room temperature photoluminescence (PL) measurements show that the overall PL intensity is quenched with increasing Cobalt concentration.

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