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Synthesis and optical properties of Cr doped ZnS nanoparticles capped by 2-mercaptoethanol
Physica B 406 (2011) 1944–1949
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and optical properties of Cr doped ZnS nanoparticles capped by 2-mercaptoethanol D. Amaranatha Reddy, A. Divya, G. Murali, R.P. Vijayalakshmi n, B.K. Reddy Department of Physics, Sri Venkateswara University, Tirupati 517502, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2010 Received in revised form 23 February 2011 Accepted 24 February 2011 Available online 21 March 2011
Nanoparticles of Zn1 xCrxS (x ¼ 0.00, 0.005, 0.01, 0.02 and 0.03) were prepared by a chemical coprecipitation reaction from homogenous solutions of zinc and chromium salts. These nanoparticles were sterically stabilized using 2-mercaptoethanol. Here a study of the effect of Cr doping on structural, morphological and optical properties of nanoparticles was undertaken. Elemental analysis, morphological, structural and optical properties have been investigated by energy dispersive analysis of X-rays (EDAX), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible spectroscopy .EDAX measurements confirmed the presence of Cr in the ZnS lattice. XRD showed that ZnS:Cr nanoparticles crystallized in zincblende structure with preferential orientation along (1 1 1) plane. The average sizes of the nanoparticles lay in the range of 3–6 nm and ˚ Lattice contraction was observed with an increase of lattice parameters were in the range of 5.2–5.4 A. Cr concentration. The particle size and lattice parameters obtained from TEM and SAED images were in agreement with the XRD results. The absorption edge shifted to lower wavelengths with an increase in Cr concentration as per UV–Vis spectroscopy. The band gap energy values were in the range of 3.85– 4.05 eV. This blueshift is attributed to the quantum confinement effect. & 2011 Elsevier B.V. All rights reserved.
Keywords: ZnS:Cr Chemical co-precipitation 2-Mercaptoethanol Lattice contraction Blueshift
1. Introduction Fabrication of micrometer to nanometer scale inorganic materials with spherical morphologies is of great interest to a material chemist due to their importance in both basic and applied research [1,2]. Since last two decades, wide band gap II–VI compounds with high refractive indices and multi-photon absorption properties have received much interest for applications in high-capacity communication networks, optoelectronics and bio-photonic devices [3–6]. Among all the II–VI compounds ZnS is nontoxic, chemically more stable and is a suitable semiconductor host matrix for a wide variety of dopants on account of its wide energy band gap (3.7 eV at 300 K) [7]. ZnS in nanocrystalline form has been extensively studied due to its unique properties, and it is a potential compound for use as a photocatalyst in environmental contaminant elimination, H2 evolution, CO2 reduction, electroluminescent devices, infrared widows and lasers [8–14]. Research on nanocrystals of ZnS containing Mn, Fe, Ni and Cu has been in full swing as the solubility limit for these transition metals in II–VI host lattice is high [15–18]. Compared to the widely studied Mn-doped ZnS systems, the theoretical and
n
Corresponding author. Tel.: þ91 877 2260211. E-mail address: [email protected] (R.P. Vijayalakshmi).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.02.062
experimental researches on Cr doped ZnS are still limited. However, very recently, it has been realized that Cr based DMS will be potential candidates for possible applications in the area of spintronics, as they are likely to be ferromagnetic at room temperature. Incorporation of magnetic Cr ions in nonmagnetic ZnS allows the generation of diluted magnetic semiconductors (DMS), which can exhibit interesting magnetic and magnetooptical properties. However, literature on the influence of Cr doping on optical and structural properties at room temperature in nanocrystalline ZnS is sparingly available. In addition, up to now physical methods such as laser ablation and molecular beam epitaxy, which require high temperature have only been used to synthesize ZnS:Cr in nanoform [19,20]. Also till now, the synthesis of ZnS:Cr nanoparticles has not been attempted by chemical route inspite of the fact that they are capable of producing high purity materials. Lack of data on Cr doped ZnS formed by chemical synthesis has motivated the present authors to synthesize Cr doped ZnS nanoparticles, using chemical co-precipitation method. At present, the chemical co-precipitation method has been extensively used to obtain various kinds of functional nanoparticles due to its simplicity and low cost. Another advantage with this method is that the resulting semiconducting nanoparticles comprising of organic molecules once formed, do not coalesce into bigger particles either in the solution or after they are recovered as dried powder. For analysis purpose,
D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
particles suspended in the liquid or dry powder can be used for UV–Visible absorption studies. Therefore, the optical properties of Cr doped ZnS are far from being unambiguous. In the present work, Cr doped ZnS nanocrystals are synthesized at room temperature by the chemical co- precipitation method using 2-Mercapto ethanol as the capping agent. More emphasis is laid on the influence of Cr doping on structural and optical properties of ZnS nanocrystals.
2. Experimental ZnS and Zn1 xCrxS nanoparticles with x¼0.005, 0.01, 0.02 and 0.03 were synthesized using a chemical co-precipitation method at room temperature. All the chemicals used are of AR grade (Merck and SD fine chemicals), without further purification. Zinc acetate dehydrate Zn(CH3COO)2 2H2O, sodium sulfide (Na2S) and CrO3 were used as source materials. All these chemical
1945
ingredients were weighed in stoichiometric proportions as per the required dopant concentrations. First, zinc acetate dehydrate was dissolved in distilled water (50 ml) and stirred for 10 min to achieve complete dissolution. Sodium sulfide and chromium oxide were also dissolved in distilled water separately as per stoichiometric ratio. First sodium sulfide solution was added drop by drop to the zinc acetate solution till the color changed to white. Next a chromium oxide solution was added to this. The color of the solution changed to yellow. An appropriate amount of 2-mercaptoethanol (1 ml) was also added to the reaction medium to control the particle size of ZnS:Cr. During the nucleation and growth process of ZnS:Cr, the solution appeared dark red in color. The resulting solution was stirred continuously for 8 h using a magnetic stirrer. The final product was filtered out and washed with methanol followed by de-ionized water several times and then dried at 60 1C for 3 h, to remove both water and organic capping and other byproducts formed during the reaction process. Chemical analysis and morphological studies were carried out
Fig. 1. (a) EDAX spectra of undoped ZnS nanoparticles. (b) EDAX spectra of Zn1 xCrxS (x ¼0.03) nanoparticles.
1946
D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
using scanning electron microscopy (SEM) with EDAX attachment (model CARL-ZEISS EVO MA 15). Structural investigations were done using a ‘Seifert 3003 TT X-ray diffractometer’ with Cu-Ka radiation with a wavelength of 1.542 A˚ .The system was operated at 30 KeV and a scan range of 20–701 was used. Crystal structure, grain size and lattice parameter were obtained from the XRD data. The grain size and structure confirmations were done by transmission electron microscopy (TEM) and Model Phillips TECHNAI FE 12. Band gap studies were carried out by recording optical absorption spectra using a Carey-5E UV–Vis–NIR lambda 950 spectrometer in the wavelength range 200–800 nm.
3.2. Morphological studies Scanning electron microscopy (SEM) is a powerful tool to study the surface morphology especially by observing the top and the cross-sectional views. SEM images of pure ZnS nanoparticles and Cr doped ZnS nanoparticles are shown in Fig. 2(a)–(c). The SEM images depict considerable agglomeration of particles with more or less spherical morphology. It is also evident that with increasing Cr content the extent of agglomeration of particles decreased. 3.3. Structural analysis
3. Results and discussion 3.1. Elemental analysis Chemical compositions of Zn, S and Cr in ZnS:Cr nanoparticles were estimated from EDAX spectra. Fig. 1(a) and (b) shows EDAX spectra of pure ZnS and Zn1 xCrxS (x¼0.03), respectively. The EDAX spectra indicate the presence of Zn, S and Cr elements. From these spectra it is obvious that with increasing Cr concentration the relative intensity of Cr lines and Zn lines varies. The estimated atomic percentages of Zn, S and Cr are close to the nominal (target) values. The observed deviations in the estimated compositions are small and are within 73% by weight.
. Fig. 3 shows the XRD patterns of pure ZnS and Cr doped ZnS nanoparticles. Patterns of all the samples show a strong (1 1 1) diffraction peak; mixed diffraction peaks corresponding to CrO, CrS, ZnO etc., were not observed. This indicates the absence of impurities and also that Cr has entered the ZnS host lattice as a substituent. The d-spacing values and relative intensities of the peaks coincide with the JCPDS data (80-0020) of ZnS for the cubic structure. Ichino et al. [19] reported about the zincblende structure in Zn1–xCrxS (x o0.015) thin films grown on GaP substrates by molecular beam epitaxy. Martyshkin et al. [20] reported mixed structure of wurtzite and zincblende in Cr doped ZnS nanocrystals formed by laser ablation. In the present work we observed that the diffraction peak of (1 1 1) is slightly shifted to higher angles as the Cr concentration increases. It may be due to the smaller radius
Fig. 2. SEM images of Zn1 xCrxS (x ¼0.00, 0.01 and 0.03) nanoparticles.
D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
1947
5.45
(111) (220)
5.40
(311) Lattice parameter (Å)
X=0.03
Intensity (a.u)
X=0.02
X=0.01
5.35
5.30
5.25
5.20 X=0.005 5.15 0.000
30
40 50 2θ (Degrees)
60
0.010 0.015 0.020 Composition (%)
0.025
0.030
Fig. 5. Lattice parameter (a) as a function of composition (x).
X=0.00
20
0.005
70
Fig. 3. XRD spectra of Zn1 xCrxS(x ¼0.00, 0.005, 0.01 0.02 and 0.03) nanoparticles.
5.0
Grainsize (nm)
4.5
4.0
3.5
Fig. 6. TEM micrograph of Zn1 xCrxS (x¼ 0.01) nanoparticles.
3.0 0.000
0.005
0.010 0.015 0.020 Composition (%)
0.025
0.030
Fig. 4. Grain size (D) as a function of composition (x).
˚ than that of Zn2 þ (0.74 A); ˚ hence the chromium of Cr3 þ (0.63 A) ions have entered the ZnS lattice substitutionally. The crystallite size of ZnS sample can be calculated according to the Scherrer equation (D ¼ 0.89l/b Cos y), and the average size is in the range 3–6 nm. The full width at half maximum (FWHM) of diffraction peaks in Cr doped ZnS nanoparticles is larger compared to that of pure ZnS, and also the intensity of the planes (2 2 0) and (3 1 1) decreases with increase in Cr concentration, which illustrates that Cr doping can appreciably influence the ZnS crystallinity. The grain size as a function of Cr doping is shown in Fig. 4. The lattice parameter as a function of Cr doping is shown in Fig. 5. It is interesting to note that the values of ‘a’ decrease as Cr concentration is increased. The decrease in lattice parameters in a narrow
range of 5.2–5.4 A˚ is quite obvious as Cr3 þ ions have smaller ionic radius than Zn ions (Zn2 þ ). In a few nanometer size range of particles the lattice parameter decreases considerably as the grain size decreases. Thus the observed lattice contraction may be due to the decrease in grain size as Cr content is increased [Fig. 4]. The linearity of the lattice parameter plot shows that Vegard’s law [21] is valid for this system, confirming the substitutional nature of the present system. Ichino et al. [19] also reported a definite decrease in lattice constant lying in a narrow range of 5.36–5.41 A˚ with an irregular variation with increasing Cr concentration in MBE grown Zn1 xCrxS thin films for x ¼0–0.04. Bhargava et al. [22] reported a similar decrease in lattice parameter with increase in Cr concentration in Cr doped ZnO nanoparticles. No data on lattice parameter of Cr doped ZnS nanoparticles is available for comparison. Figs. 6 and 7 show typical TEM and SAED images of Zn1 xCrxS (x¼0.01), respectively. The lattice parameter values are in agreement with the values obtained from XRD patterns. TEM image shows that the particles are few nm (o 10 nm) in size.
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D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
4.10
4.05
Bandgap (eV)
4.00
3.95
3.90
3.85 0.000
0.005
0.010 0.015 0.020 Composition (X)
0.025
0.030
Fig. 9. Variation of bandgap (Eg) with Cr content (x).
increase in Cr concentration is attributed to size quantization effect due to the small size of the particles. Fig. 7. SAED pattern of the Zn1 xCrxS (x ¼0.01) nanoparticles.
5. Conclusions Zn1 xCrxS (x¼0.00, 0.005, 0.01, 0.02 and 0.03) nanoparticles of grain size 3–6 nm were successfully synthesized for the first time by chemical co-precipitation using 2-mercaptoethanol as a stabilizing agent. Samples of all compositions exhibited zincblende structure. Lattice contraction was observed with increasing Cr concentration. From optical studies it was observed that the bandgap increased with increasing Cr concentration.
X=0.03
Absorbance (arb.units)
X=0.02
X=0.01 Acknowledgements
X=0.005 The authors are highly grateful to the University Grants Commission, New Delhi, India, for providing financial support.
X=0.00 References
200
300
400
500 600 Wavelength (nm)
700
800
Fig. 8. Optical absorption spectra of Zn1 xCrxS (x¼ 0.00, 0.005, 0.01, 0.02 and 0.03) nanoparticles.
4. Optical absorption Fig. 8 shows the absorbance versus wavelength (l) traces of the undoped ZnS and different Cr doped ZnS nanoparticles in the wavelength range of 200–800 nm. The absorption edge is observed in the range of 320–305 nm, which is blueshifted compared to bulk ZnS due to quantum confinement effects. As the Cr concentration increases the absorption edge shifts to lower wavelength side and intensity also increases with increasing Cr concentration compared to undoped ZnS. The calculated band gap values lie in the range of 3.85–4.05 eV. The band gap as a function of Cr doping is shown in Fig. 9. The increase in bandgap with
[1] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [2] A.P. Alivisatos, Science 271 (1996) 933. [3] Y. Gao, A. Tonizzo, A. Walser, M. Potasek, R. Dorsinville, Appl. Phys. Lett. 92 (2008) 033106. [4] J. He, G.D. Scholes, Y.L. Ang, W. Ji, C.W.J. Beh, W.S. Chin, Appl. Phys. Lett. 92 (2008) 1311114. [5] R.A. Ganeev, M. Baba, M. Morita, D. Rau, H. Fujii, A.I. Ryasnyansky, N. Ishizawa, M. Suzuki, H. Kuroda, J. Opt. A Pure Appl. Opt. 6 (2004) 447. [6] M. Chattopadhyay, P. Kumbhakar, C.S. Tiwary, R. Sarkar, A.K. Mitra, U. Chatterjee, J. Appl. Phys. 105 (2009) 024313. [7] N. Kumbhojkar, V.V. Nikesh, A. Kshirsagar, S. Mahamuni, J. Appl. Phys. 88 (2000) 6260. [8] J.S. Hu, L.L. Ren, Y.G. Guo, H.P. Liang, A.M. Cao, L.J. Wan, C.L. Bai, Angew. Chem. Int. Ed. 44 (2005) 1269. [9] Claudia L. Torres-Martı´nez, Richard Kho, Omar I. Mian, Rajesh K. Mehra, J. Colloid Interface Sci. 240 (2001) 525. [10] A. Kudo, M. Sekizawa, Chem. Commun. (2000) 1371. [11] H. Fujiwara, H. Hosokawa, K. Murakoshi, Y. Wada, S. Yanagida, Langmuir 14 (1998) 5154. [12] M. Bredol, J. Merichi, J. Mater. Sci. 33 (1998) 471. [13] P. Calandra, M. Goffredi, V.T. Liveri, Colloids Surf. A 160 (1999) 9. [14] T.V. Prevenslik, J. Lumin. 1210 (2000) 87. [15] H.C. Warada, S.C. Ghosha, B. Hemtanona, C. Thanachayanontb, J. Dutta, Sci. Technol. Adv. Mater. 6 (2005) 296. [16] P. Yang, M. Lu, D. Xu, D. Yuan, J. Chang, G. Zhou, M. Pan, Appl. Phys. A 74 (2002) 257. [17] S. Sambasivam, D. Paul Joseph, D. Raja Reddy, B.K. Reddy, C.K. Jayasankar, Mater. Sci. Eng. B 150 (2008) 125.
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[18] S. Sambasivam, B. Sathyaseelan, D. Raja Reddy, B.K. Reddy, C.K. Jayasankar, Spectrochim. Acta Part A 71 (2008) 1503. [19] Kunio Ichino, Yasuharu Morimoto, Hiroshi Kobayashi, Phys. Status Solidi C 3 (2006) 776. [20] D.V. Martyshkin, V.V. Fedorov, C. Kim, I.S. Moskalev, S.B. Mirov, J. Opt. 12 (2010) 024005.
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[21] J.K. Furdyna, J. Kossut, Semiconductors and Semimetals, vol. 25, Academic Press, New York, 1988. [22] Richa Bhargava, Prashant K. Sharma, Sanjeev Kumar, Avinash C. Pandey, Naresh Kumar, J. Solid State Chem. 183 (2010) 1400.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and optical properties of Cr doped ZnS nanoparticles capped by 2-mercaptoethanol D. Amaranatha Reddy, A. Divya, G. Murali, R.P. Vijayalakshmi n, B.K. Reddy Department of Physics, Sri Venkateswara University, Tirupati 517502, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2010 Received in revised form 23 February 2011 Accepted 24 February 2011 Available online 21 March 2011
Nanoparticles of Zn1 xCrxS (x ¼ 0.00, 0.005, 0.01, 0.02 and 0.03) were prepared by a chemical coprecipitation reaction from homogenous solutions of zinc and chromium salts. These nanoparticles were sterically stabilized using 2-mercaptoethanol. Here a study of the effect of Cr doping on structural, morphological and optical properties of nanoparticles was undertaken. Elemental analysis, morphological, structural and optical properties have been investigated by energy dispersive analysis of X-rays (EDAX), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible spectroscopy .EDAX measurements confirmed the presence of Cr in the ZnS lattice. XRD showed that ZnS:Cr nanoparticles crystallized in zincblende structure with preferential orientation along (1 1 1) plane. The average sizes of the nanoparticles lay in the range of 3–6 nm and ˚ Lattice contraction was observed with an increase of lattice parameters were in the range of 5.2–5.4 A. Cr concentration. The particle size and lattice parameters obtained from TEM and SAED images were in agreement with the XRD results. The absorption edge shifted to lower wavelengths with an increase in Cr concentration as per UV–Vis spectroscopy. The band gap energy values were in the range of 3.85– 4.05 eV. This blueshift is attributed to the quantum confinement effect. & 2011 Elsevier B.V. All rights reserved.
Keywords: ZnS:Cr Chemical co-precipitation 2-Mercaptoethanol Lattice contraction Blueshift
1. Introduction Fabrication of micrometer to nanometer scale inorganic materials with spherical morphologies is of great interest to a material chemist due to their importance in both basic and applied research [1,2]. Since last two decades, wide band gap II–VI compounds with high refractive indices and multi-photon absorption properties have received much interest for applications in high-capacity communication networks, optoelectronics and bio-photonic devices [3–6]. Among all the II–VI compounds ZnS is nontoxic, chemically more stable and is a suitable semiconductor host matrix for a wide variety of dopants on account of its wide energy band gap (3.7 eV at 300 K) [7]. ZnS in nanocrystalline form has been extensively studied due to its unique properties, and it is a potential compound for use as a photocatalyst in environmental contaminant elimination, H2 evolution, CO2 reduction, electroluminescent devices, infrared widows and lasers [8–14]. Research on nanocrystals of ZnS containing Mn, Fe, Ni and Cu has been in full swing as the solubility limit for these transition metals in II–VI host lattice is high [15–18]. Compared to the widely studied Mn-doped ZnS systems, the theoretical and
n
Corresponding author. Tel.: þ91 877 2260211. E-mail address: [email protected] (R.P. Vijayalakshmi).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.02.062
experimental researches on Cr doped ZnS are still limited. However, very recently, it has been realized that Cr based DMS will be potential candidates for possible applications in the area of spintronics, as they are likely to be ferromagnetic at room temperature. Incorporation of magnetic Cr ions in nonmagnetic ZnS allows the generation of diluted magnetic semiconductors (DMS), which can exhibit interesting magnetic and magnetooptical properties. However, literature on the influence of Cr doping on optical and structural properties at room temperature in nanocrystalline ZnS is sparingly available. In addition, up to now physical methods such as laser ablation and molecular beam epitaxy, which require high temperature have only been used to synthesize ZnS:Cr in nanoform [19,20]. Also till now, the synthesis of ZnS:Cr nanoparticles has not been attempted by chemical route inspite of the fact that they are capable of producing high purity materials. Lack of data on Cr doped ZnS formed by chemical synthesis has motivated the present authors to synthesize Cr doped ZnS nanoparticles, using chemical co-precipitation method. At present, the chemical co-precipitation method has been extensively used to obtain various kinds of functional nanoparticles due to its simplicity and low cost. Another advantage with this method is that the resulting semiconducting nanoparticles comprising of organic molecules once formed, do not coalesce into bigger particles either in the solution or after they are recovered as dried powder. For analysis purpose,
D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
particles suspended in the liquid or dry powder can be used for UV–Visible absorption studies. Therefore, the optical properties of Cr doped ZnS are far from being unambiguous. In the present work, Cr doped ZnS nanocrystals are synthesized at room temperature by the chemical co- precipitation method using 2-Mercapto ethanol as the capping agent. More emphasis is laid on the influence of Cr doping on structural and optical properties of ZnS nanocrystals.
2. Experimental ZnS and Zn1 xCrxS nanoparticles with x¼0.005, 0.01, 0.02 and 0.03 were synthesized using a chemical co-precipitation method at room temperature. All the chemicals used are of AR grade (Merck and SD fine chemicals), without further purification. Zinc acetate dehydrate Zn(CH3COO)2 2H2O, sodium sulfide (Na2S) and CrO3 were used as source materials. All these chemical
1945
ingredients were weighed in stoichiometric proportions as per the required dopant concentrations. First, zinc acetate dehydrate was dissolved in distilled water (50 ml) and stirred for 10 min to achieve complete dissolution. Sodium sulfide and chromium oxide were also dissolved in distilled water separately as per stoichiometric ratio. First sodium sulfide solution was added drop by drop to the zinc acetate solution till the color changed to white. Next a chromium oxide solution was added to this. The color of the solution changed to yellow. An appropriate amount of 2-mercaptoethanol (1 ml) was also added to the reaction medium to control the particle size of ZnS:Cr. During the nucleation and growth process of ZnS:Cr, the solution appeared dark red in color. The resulting solution was stirred continuously for 8 h using a magnetic stirrer. The final product was filtered out and washed with methanol followed by de-ionized water several times and then dried at 60 1C for 3 h, to remove both water and organic capping and other byproducts formed during the reaction process. Chemical analysis and morphological studies were carried out
Fig. 1. (a) EDAX spectra of undoped ZnS nanoparticles. (b) EDAX spectra of Zn1 xCrxS (x ¼0.03) nanoparticles.
1946
D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
using scanning electron microscopy (SEM) with EDAX attachment (model CARL-ZEISS EVO MA 15). Structural investigations were done using a ‘Seifert 3003 TT X-ray diffractometer’ with Cu-Ka radiation with a wavelength of 1.542 A˚ .The system was operated at 30 KeV and a scan range of 20–701 was used. Crystal structure, grain size and lattice parameter were obtained from the XRD data. The grain size and structure confirmations were done by transmission electron microscopy (TEM) and Model Phillips TECHNAI FE 12. Band gap studies were carried out by recording optical absorption spectra using a Carey-5E UV–Vis–NIR lambda 950 spectrometer in the wavelength range 200–800 nm.
3.2. Morphological studies Scanning electron microscopy (SEM) is a powerful tool to study the surface morphology especially by observing the top and the cross-sectional views. SEM images of pure ZnS nanoparticles and Cr doped ZnS nanoparticles are shown in Fig. 2(a)–(c). The SEM images depict considerable agglomeration of particles with more or less spherical morphology. It is also evident that with increasing Cr content the extent of agglomeration of particles decreased. 3.3. Structural analysis
3. Results and discussion 3.1. Elemental analysis Chemical compositions of Zn, S and Cr in ZnS:Cr nanoparticles were estimated from EDAX spectra. Fig. 1(a) and (b) shows EDAX spectra of pure ZnS and Zn1 xCrxS (x¼0.03), respectively. The EDAX spectra indicate the presence of Zn, S and Cr elements. From these spectra it is obvious that with increasing Cr concentration the relative intensity of Cr lines and Zn lines varies. The estimated atomic percentages of Zn, S and Cr are close to the nominal (target) values. The observed deviations in the estimated compositions are small and are within 73% by weight.
. Fig. 3 shows the XRD patterns of pure ZnS and Cr doped ZnS nanoparticles. Patterns of all the samples show a strong (1 1 1) diffraction peak; mixed diffraction peaks corresponding to CrO, CrS, ZnO etc., were not observed. This indicates the absence of impurities and also that Cr has entered the ZnS host lattice as a substituent. The d-spacing values and relative intensities of the peaks coincide with the JCPDS data (80-0020) of ZnS for the cubic structure. Ichino et al. [19] reported about the zincblende structure in Zn1–xCrxS (x o0.015) thin films grown on GaP substrates by molecular beam epitaxy. Martyshkin et al. [20] reported mixed structure of wurtzite and zincblende in Cr doped ZnS nanocrystals formed by laser ablation. In the present work we observed that the diffraction peak of (1 1 1) is slightly shifted to higher angles as the Cr concentration increases. It may be due to the smaller radius
Fig. 2. SEM images of Zn1 xCrxS (x ¼0.00, 0.01 and 0.03) nanoparticles.
D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
1947
5.45
(111) (220)
5.40
(311) Lattice parameter (Å)
X=0.03
Intensity (a.u)
X=0.02
X=0.01
5.35
5.30
5.25
5.20 X=0.005 5.15 0.000
30
40 50 2θ (Degrees)
60
0.010 0.015 0.020 Composition (%)
0.025
0.030
Fig. 5. Lattice parameter (a) as a function of composition (x).
X=0.00
20
0.005
70
Fig. 3. XRD spectra of Zn1 xCrxS(x ¼0.00, 0.005, 0.01 0.02 and 0.03) nanoparticles.
5.0
Grainsize (nm)
4.5
4.0
3.5
Fig. 6. TEM micrograph of Zn1 xCrxS (x¼ 0.01) nanoparticles.
3.0 0.000
0.005
0.010 0.015 0.020 Composition (%)
0.025
0.030
Fig. 4. Grain size (D) as a function of composition (x).
˚ than that of Zn2 þ (0.74 A); ˚ hence the chromium of Cr3 þ (0.63 A) ions have entered the ZnS lattice substitutionally. The crystallite size of ZnS sample can be calculated according to the Scherrer equation (D ¼ 0.89l/b Cos y), and the average size is in the range 3–6 nm. The full width at half maximum (FWHM) of diffraction peaks in Cr doped ZnS nanoparticles is larger compared to that of pure ZnS, and also the intensity of the planes (2 2 0) and (3 1 1) decreases with increase in Cr concentration, which illustrates that Cr doping can appreciably influence the ZnS crystallinity. The grain size as a function of Cr doping is shown in Fig. 4. The lattice parameter as a function of Cr doping is shown in Fig. 5. It is interesting to note that the values of ‘a’ decrease as Cr concentration is increased. The decrease in lattice parameters in a narrow
range of 5.2–5.4 A˚ is quite obvious as Cr3 þ ions have smaller ionic radius than Zn ions (Zn2 þ ). In a few nanometer size range of particles the lattice parameter decreases considerably as the grain size decreases. Thus the observed lattice contraction may be due to the decrease in grain size as Cr content is increased [Fig. 4]. The linearity of the lattice parameter plot shows that Vegard’s law [21] is valid for this system, confirming the substitutional nature of the present system. Ichino et al. [19] also reported a definite decrease in lattice constant lying in a narrow range of 5.36–5.41 A˚ with an irregular variation with increasing Cr concentration in MBE grown Zn1 xCrxS thin films for x ¼0–0.04. Bhargava et al. [22] reported a similar decrease in lattice parameter with increase in Cr concentration in Cr doped ZnO nanoparticles. No data on lattice parameter of Cr doped ZnS nanoparticles is available for comparison. Figs. 6 and 7 show typical TEM and SAED images of Zn1 xCrxS (x¼0.01), respectively. The lattice parameter values are in agreement with the values obtained from XRD patterns. TEM image shows that the particles are few nm (o 10 nm) in size.
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D. Amaranatha Reddy et al. / Physica B 406 (2011) 1944–1949
4.10
4.05
Bandgap (eV)
4.00
3.95
3.90
3.85 0.000
0.005
0.010 0.015 0.020 Composition (X)
0.025
0.030
Fig. 9. Variation of bandgap (Eg) with Cr content (x).
increase in Cr concentration is attributed to size quantization effect due to the small size of the particles. Fig. 7. SAED pattern of the Zn1 xCrxS (x ¼0.01) nanoparticles.
5. Conclusions Zn1 xCrxS (x¼0.00, 0.005, 0.01, 0.02 and 0.03) nanoparticles of grain size 3–6 nm were successfully synthesized for the first time by chemical co-precipitation using 2-mercaptoethanol as a stabilizing agent. Samples of all compositions exhibited zincblende structure. Lattice contraction was observed with increasing Cr concentration. From optical studies it was observed that the bandgap increased with increasing Cr concentration.
X=0.03
Absorbance (arb.units)
X=0.02
X=0.01 Acknowledgements
X=0.005 The authors are highly grateful to the University Grants Commission, New Delhi, India, for providing financial support.
X=0.00 References
200
300
400
500 600 Wavelength (nm)
700
800
Fig. 8. Optical absorption spectra of Zn1 xCrxS (x¼ 0.00, 0.005, 0.01, 0.02 and 0.03) nanoparticles.
4. Optical absorption Fig. 8 shows the absorbance versus wavelength (l) traces of the undoped ZnS and different Cr doped ZnS nanoparticles in the wavelength range of 200–800 nm. The absorption edge is observed in the range of 320–305 nm, which is blueshifted compared to bulk ZnS due to quantum confinement effects. As the Cr concentration increases the absorption edge shifts to lower wavelength side and intensity also increases with increasing Cr concentration compared to undoped ZnS. The calculated band gap values lie in the range of 3.85–4.05 eV. The band gap as a function of Cr doping is shown in Fig. 9. The increase in bandgap with
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