- Email: [email protected]
Preparation and characterization of short length ZnO nanorods and ZnO@ZnS core–shell nanostructures
Nano Communication Networks 3 (2012) 197–202
Contents lists available at SciVerse ScienceDirect
Nano Communication Networks journal homepage: www.elsevier.com/locate/nanocomnet
Preparation and characterization of short length ZnO nanorods and ZnO@ZnS core–shell nanostructures Geeta Rani ∗ , P.D. Sahare Department of Physics and Astrophysics, University of Delhi, Delhi – 110 007, India
article
info
Article history: Received 20 June 2012 Accepted 11 September 2012 Available online 23 September 2012 Keywords: ZnO@ZnS Morphology Band-gap Photoluminescence etc
abstract Short length ZnO nanorods with uniform diameter have been synthesized by a solid state reaction method using Zinc acetate as precursor. This is a larger scale production and inexpensively synthesized method without any templates or additives. ZnO@ZnS core–shell nanostructures were successfully fabricated by sulfidation of ZnO nanorods via a facile chemical synthesis. The as-obtained samples were characterized by X-ray diffraction (XRD), UV–Vis absorption, Transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), fluorescence spectrophotometer (PL) and Fourier Transformed Infra-red spectroscopy (FTIR). The results showed that the pure ZnO nanorods were hexagonal wurtzite crystal structure and the ZnS tube like structure is cubic structure grown on the surface of the ZnO nanorods. The optical band gap was found to be increased with increasing sulfidation process. The measurement of luminescence revealed that ZnO@ZnS core–shell structure integrated the luminescent effect of ZnO and ZnS. The broad blue emission of ZnO@ZnS core–shell was dramatically changed. The FTIR spectra were studied to find the role of organic impurities trapped inside the material. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction A great variety of nano-related studies hold many researchers’ interest and attention because of the amazing properties of nanomaterials and their possible applications in many fields. In studies of nanoscale materials, preparation techniques as the basis of further research and properties as the foundation of applications take very high positions. Recently, considering the properties of materials are greatly affected by their morphologies, a wide range of metal oxides with different morphologies, providing great opportunities for the discovery of new properties and potential uses, have been synthesized via different methods. As an important II–VI semiconductor with wide band gap and large exciton binding energy, ZnO has unique optical and electronic properties. It is regarded as a promising material applied in optoelectronics [30], varistors [28], chemical sensors [23], catalysts [15], field emission displays
∗
Corresponding author. Tel.: +91 9560752969. E-mail address: [email protected] (G. Rani).
1878-7789/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.nancom.2012.09.003
[29] and cosmetic [11]. This wide variety of applications requires the fabrication of morphological and surface functional ZnO nanostructures. Modification of ZnO by impurity incorporation and surface coating can efficiently adjust its electrical, optical, and magnetic properties [1,17,9]. Surface coating of ZnO with wide-band-gap semiconductors to form core–shell nanostructures has been recognized as one of the most advanced and intriguing methods to improve the luminescence properties of the Metal oxide, as demonstrated by ZnO nanorod@CdS nanoparticle core–shell composites [7] and ZnO@CdSe core–shell nanoparticles [14]. ZnS is a well-known luminescent material having prominent applications in flat-panel displays, sensors, and lasers [12], and also has been applied in photo catalysts [6], pigments [10], non linear optical devices [20] and infrared windows [4]. Because the band gap of ZnS is larger than that of ZnO, the luminescent property of ZnO@ZnS core–shell composites could be improved. Li et al. [18] prepared ZnO nanowires by a vapor-phase transport process, and then fabricated ZnO@ZnS core–shell
198
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
nanowires through the reaction of Zn (NO3 )2 and Na2 S on ZnO nanowires in aqueous solutions. They found that the UV emission of nanorods was dramatically enhanced. Sun et al. [22] had successfully synthesized a ZnO@ZnS nanocable through a catalyst-free thermal vapor transport method and investigated its microstructure. In the present work, we first fabricated the short length ZnO nanorods by solid state reaction method and then introduce the ZnS shell onto the ZnO nanorods. To illustrate the effect of the ZnS layer, core–shell ZnS/ZnO nanostructure were designed and produced by a hydrothermal and chemical etching process. The morphology by TEM, crystal structure by XRD and luminescence properties were studied in detail, which show the formation of the ZnS/ZnO core–shell nanostructure. 2. Experimental All chemicals used in this work were of analytical reagent grade and used as received without further purification. All the aqueous solutions were prepared using double distilled water.
2.4. Characterization The structure of the obtained powder samples were identified by X-ray diffraction technique (XRD). The XRD was done on the machine D8-Advance model of Bruker Inc. using monochromatic Cu–Kα line (λ = 1.54056 Å) and well matched with the PCPDF data files to confirm the formation of different phases in the material. The UV–visible spectra of the material were taken in wavelength range, 250–600 nm using Evolution UV–Vis. Spectrophotometer (Thermo Scientific Inc. make) to estimate the band gaps of the core–shell systems. The TEM images were taken to study morphology (shape and size) of the nanocrystalline material and the images and selected area electron diffraction (SAED) pattern were recorded on a transmission electron microscope (FEI TEM model TECNAI G2 T30, U–TWIN) at 300 kV. PL spectra were carried out on a fluorescence spectrophotometer (Fluorolog Horiba JOBIN YVON) using Xe lamp with excitation wavelength 350 nm. The surface chemistry of the samples was analyzed with Fourier Transmission Infrared spectroscopy (FTIR) and performed with a Perkin Elmer Spectrometer Spectrum RX-I (at resolution of 4 cm−1 ).
2.1. Preparation of ZnO nanorods 3. Results and discussion ZnO nanorods were synthesized by solid-state reaction method. A homogeneous solution of 1M Zinc acetate dehydrate (Zn(CH3 COO)2 .2H2 O) in deionized water was prepared. The solution was oven dried and the dried precursor material was ground in agate mortar and mixed thoroughly, packed in a quartz boat and fired at 500 °C in air. High-purity quartz boat was used without any metallic contamination was used for firing the sample in the furnace. 2.2. Preparation of tubular ZnO/ZnS core/shell nanostructures The as-prepared ZnO nanorods (0.324 g) and thioacetamide (TAA, 0.3 g respectively) dissolved in 30 ml deionized water were put in to a Teflon-lined stainless steel autoclave of 100 ml capacity with the addition of 20 ml water so that the volume of the solution is about 80% volume of the autoclave. The solution was stirred vigorously for 10 min then sealed and maintained at 100 °C for 6 h, and then cooled down to room temperature. The resulting precipitate was collected by filtration and washed with absolute ethanol and distilled water in sequence for several times. The final product was dried in a vacuum box at 50 °C for 4 h. 2.3. Preparation of ZnS nanotubes Tubular ZnO/ZnS core/shell nanostructures (0.3 g) were soaked in 20 ml KOH aq. solution (4 mol/l) under sonication for 1 h at room temperature. Finally, the obtained products were collected by centrifugation and washed with distilled water repeatedly and then dried in vacuum oven at 50 °C for 4 h.
Fig. 1 shows the XRD patterns of the obtained samples. All the peaks of Fig. 1(a) can be identified as hexagonal wurtzite ZnO with lattice constants of a = 3.250 Å, and c = 5.207 Å, which is consistent with the literature data of JCPDS 36-1451. No impurity peaks were detected showing that the products are pure phase. When we add TAA to ZnO nanorods both wurtzite ZnO and cubic ZnS (JCPDS card No. 65-5476, a = 5.404 Å) are found to exist in the synthesized products, which can be seen in Fig. 1(b,c). It reveals that the products have two phases of ZnO and ZnS. The Nanorods share the characteristics of both ZnO and ZnS because the peaks of (220)_ZnS and (311)_ZnS are very adjacent to the peaks of (102)_ZnO and (110)_ZnO, therefore, the peaks of the (220)_ZnS and (102)_ZnO are overlapped together, so are the peaks of the (311)_ZnS and (110)_ZnO which can be clearly seen in Fig. 1(b). The average size of ZnO is calculated about 29 nm according to Scherrer’s formula. Similarly the average size of the ZnO@ZnS nanorods is 19 nm, so is the average size of the ZnO of the ZnO/ZnS nanostructures. This implies only a small part of ZnO is converted into ZnS. Fig. 2 presents the UV–Vis spectra of the ZnO nanorods and ZnO@ZnS core/shell nanostructures. They show an obvious change in optical absorption behaviors, after deposition of the ZnS nanoparticles on the surface of the ZnO nanorods. The band edge of the bare ZnO nanorods locates at around 380 nm, which is close to the value of bulk ZnO (Eg , ZnO = 3.37 eV) [21]. The bare ZnO nanorods show no absorption in the visible region. A significant red shift in the absorption edge is observed in the UV-vis spectra of the ZnO/ZnS core/shell nanorods because of deposition of the ZnS, and the photo absorption in the visible region at the range of 400–700 nm was obviously enhanced. For the
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
199
Fig. 1. XRD patterns of (a) ZnO (b) ZnO@ZnS (c) ZnS.
Fig. 2. (α hν)2 versus hν plot and Inset shows the Optical absorption spectrum of (a) ZnO (b) ZnO@ZnS (c) ZnS.
allowed direct transition, the variation of α with photon energy (hν ) obey Tauc’s plot that form [24].
(α hν)2 = A(hν − Eg )
(1)
where A is a constant, Eg is band gap, h is the Planck constant and α is the absorption coefficient. Fig. 2 illustrates the (α hν)2 versus hν plot used for the evaluation of the band gap of nanostructures. It revealed from the figure that the band gap of the prepared samples increase from 3.31 to 3.64 eV with change in phase. The qualitative information about the band gap evaluated from Fig. 2 (inset) are well corroborated with the quantitative results of Fig. 2. The morphologies of the samples were investigated by TEM as shown in Fig. 3. The TEM images of the ZnO nanorods and ZnO@ZnS core/shell nanorods shows that a ZnO core and ZnS shell are formed with ZnS nanoparticles covering the surface of the ZnO nanorods. The sharp interface between the core and shell clearly shows that the ZnO nanorods are fully sheathed by a ZnS layer along the entire length. The average width and length of nanorods
are about 17 nm and 53 nm based on TEM images, respectively. It can be seen that ZnS nanoparticles grow on the surface of ZnO nanorods as shown in Fig. 3(b) which is in good agreement with the result of XRD analysis. The selected area electron diffraction (SAED) pattern manifests the crystallinity of the samples. In order to investigate the detailed construction of the ZnO/ZnS nanostructures, EDS measurement is conducted on the surface of the core–shell and found that Zn, O and S are the primary detected elements implies that the obtained core–shell is composed of ZnO and ZnS shown in Fig. 3(d). Fig. 4(a,b) showed the PL spectra of the pure ZnO nanorods and the ZnO@ZnS core–shell nanostructures, respectively. For a comparison, the PL spectra of pure ZnS nanoparticles were also shown in Fig. 4(c). Two emitting bands, including a strong broad blue emission centered at around 505 nm and a weak orange band centered at around 383 nm, has been observed in ZnO nanorods. The origin of the broad blue emission could be attributed to the defect related emissions [2,3]. The orange
200
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
Fig. 3. TEM images and their corresponding SAED patterns of (a) ZnO (b) ZnO@ZnS (c) ZnS and (d) EDS of ZnO@ZnS core shell nanostructure.
Fig. 4. PL spectra of (a) ZnO (b) ZnO@ZnS (c) ZnS measured at room temperature with excitation wavelength are 350 nm.
emission was reported to be due to oxygen interstitials [5], suggesting oxygen excessive in the ZnO nanorods. The PL spectra of ZnO@ZnS core–shell nanostructure showed an enhanced broad blue emission and a disappeared orange emission, comparing to that of pure ZnO nanorods. The enhancement in blue emission could be explained as that ZnS nanoparticle has a higher band gap than ZnO and then it suppressed the tunneling of the charge carriers from the cores to the ZnS nanoshell. As a result, more photo generated electrons and holes were confined inside the ZnO core, giving rise to a high quantum yield. The strongest blue emission of ZnO@ZnS core–shell in the range of 400–650 nm was supposed to origin from the synergic interactions of ZnO core and ZnS nanoshell. However, the
orange emission disappeared because ZnS nanoshell was successfully grown on ZnO nanorods, the concentration of oxygen interstitials was reduced greatly. Consequently, the orange emission disappeared. The above results indicated that the sulfidation process had a great effect on the relative intensity and position of typical PL properties of ZnO nanorods. Therefore, the PL properties of the ZnO nanorods could be tuned by this approach. Compositional observation of all the samples obtained using FTIR spectroscopy as shown in Fig. 5. The observed bands in the present study suggest the presence of acetate group is loosely bound on the surface of ZnO by a dangling bond [19]. For the sample (a), bands near 3417 cm−1 represent the O–H stretching mode,
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
201
Fig. 5. FTIR spectra of (a) ZnO (b)ZnO@ZnS (c) ZnS nanoparticles.
indicative of the presence of hydroxyl ion (OH) in the samples, those at 2838–2914 cm−1 are the C–H stretching mode. The 2358 cm−1 region serves as absorbed CO2, 1597 cm−1 is the C=O stretching mode, 1394 cm−1 is the C–H deformation mode and 954–1180 cm−1 are the C–O stretching modes. The band positioned at 472 cm−1 clearly representing the ZnO band arises from the bonding between Zn–O [8,27,13,26,25,16]. As the sulfidation of ZnO occurs, the organic bands are slightly decreased in ZnO@ZnS nanostructures because of the existence of two phases in the sample as shown in Fig. 5(b). All the bands in ZnS are increased to a great extent but the extent of the band at 472 cm−1 decreased because of the formation of single ZnS phase as shown in Fig. 5(c). 4. Conclusion In summary, two phases could be prepared using the sulfidation technique. This procedure is much simpler has a wide potential application in large scale conversion from metal oxide to metal Sulfide therefore ZnO@ZnS core shell has potential application in the Phase transition. Characteristic transmission from prominent FTIR peaks are also analyzed and assigned. The broad blue emission and a passivated orange emission provides a good indication of tuning the visible emission of the core–shell nanostructures so, this is a promising material applied in the fabrication of nanoscale optical devices. Acknowledgments The authors would like to thank the University Science Instrumentation Centre (USIC), University of Delhi for providing instrumentation facilities and UGC for financial assistance. References [1] S.Y. Bae, H.W. Seo, J. Park, J. Phys. Chem. B 108 (2004) 5206–5210. http://dx.doi.org/10.1021/jp036720k. [2] W.D. Cheng, P. Wu, X.Q. Zou, T. Xiao, J. Appl. Phys. 100 (2006) 054311-1–054311-4. http://dx.doi.org/10.1063/1.2338601. [3] J.P. Cheng, X.B. Zhang, X.Y. Tao, H.M. Hu, Z.Q. Luo, F. Liu, J. Phys. Chem. B 110 (2006) 10348–10353. http://dx.doi.org/10.1021/jp060133s.
[4] Y. Ding, X.D. Wang, Z.L. Wang, Chem. Phys. Lett. 398 (2004) 32–36. http://dx.doi.org/10.1016/j.cplett.2004.09.031. [5] A.B. Djurisic, Y.H. Leung, K.H. Tam, L. Ding, W.K. Ge, H.Y. Chen, S. Gwo, Appl. Phys. Lett. 88 (2006) 103107-1–103107-3. http://dx.doi.org/ 10.1063/1.2182096. [6] H. Fujiwara, H. Hosokawa, K. Murakoshi, Y. Wada, S. Yanagida, Langmuir 14 (1998) 5154–5159. http://dx.doi.org/10.1021/la9801561. [7] T. Gao, Q.H. Li, T.H. Wang, Chem. Mater. 17 (2005) 887–892. http://dx.doi.org/10.1021/cm0485456. [8] D. Geetha, T. Thilagavathi, Digest J. Nanomaterials and Biostructures 5 (2010) 297–301. [9] B.Y. Geng, G.Z. Wang, Z. Jiang, T. Xie, S.H. Sun, G.W. Meng, L.D. Zhang, Appl. Phys. Lett. 82 (2003) 4791–4793. http://dx.doi.org/10.1063/1. 1588735. [10] A.E. Gorshteyn, A. Robbat, Ind. Eng. Chem. Res. 41 (2002) 2646–2651. http://dx.doi.org/10.1021/ie010831t. [11] T. Iwasaki, M. Satoh, T. Masuda, T. Fujita, J. Mater. Sci. 35 (2000) 4025–4029. http://dx.doi.org/10.1023/A:1004826002503. [12] Y. Jiang, X.M. Meng, J. Liu, Z.Y. Xie, C.S. Lee, S.T. Lee, Adv. Mater. 15 (2003) 323–327. http://dx.doi.org/10.1002/adma.200390079. [13] K. Kakiuchi, E. Hosono, T. Kimura, H. Imai, S. Fujihara, J Sol-Gel Sci Technol. 39 (2006) 63–72. http://dx.doi.org/10.1007/s10971-0066321-6. [14] J.Y. Kim, F.E. Osterloh, J. Am. Chem. Soc. 127 (2005) 10152–10153. http://dx.doi.org/10.1021/ja052735f. [15] T.J. Kuo, C.N. Lin, C.L. Kuo, M.H. Huang, Chem. Mater. 19 (2007) 5143–5147. http://dx.doi.org/10.1021/cm071568a. [16] Y.J. Kwon, K.H. Kima, C.S. Limb, K.B. Shim, Journal of Ceramic Processing Research 3 (2002) 146. [17] J.J. Liu, M.H. Yu, W.L. Zhou, Appl. Phys. Lett. 87 (2005) 1725051–172505-3. http://dx.doi.org/10.1063/1.2084321. [18] J.H. Li, D.X. Zhao, X.Q. Meng, Z.Z. Zhang, J.Y. Zhang, D.Z. Shen, Y.M. Lu, X.W. Fan, J. Phys. Chem. B 110 (2006) 14685–14687. http://dx.doi.org/10.1021/jp061563l. [19] R. Mishra, Raghvendra S. Yadav, A.C. Pandey, Sharda S. Sanjay, C. Dar, J. Luminescence 130 (2010) 365–373. http://dx.doi.org/10.1016/j. jlumin.2009.09.022. [20] F. Ramtollli, G. Saccinni, Cryst. Res. Tech. 37 (2002) 1325–1330. http://dx.doi.org/10.1002/crat.200290009. [21] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447–5451. http://dx. doi.org/10.1063/1.367375. [22] C.W. Sun, J.S. Jeong, J.Y. Lee, J. Cryst. Growth 294 (2006) 162–167. http://dx.doi.org/10.1016/j.jcrysgro.2006.06.023. [23] Z.P. Sun, L. Liu, L. Zhang, D.Z. Jia, Nanotechnology 17 (2006) 2266–2270. http://dx.doi.org/10.1088/0957-4484/17/9/032. [24] J. Tauc, A. Menthe, J. Non-Cryst. Sol. 8–10 (1972) 569–585. http://dx.doi.org/10.1016/0022-3093 72 90194-9. [25] I. Uslu, B. Başer, A. Yayli, M.L. Aksu, e-Polymers (ISSN: 1618-7229) 145 (2007) http://www.e-polymers.org. [26] H.M. Xiong, R.Z. Ma, S.F. Wang, Y.Y. Xia, J. Mater. Chem. 21 (2011) 3178. http://dx.doi.org/10.1039/C0JM02577A. [27] G. Xiong, U. Pal, J.G. Serrano, K.B. Ucer, R.T. Williams, Phys. Stat. Sol.(c) 3 (2006) 3577. http://dx.doi.org/10.1002/pssc.200672164. [28] J.C. Zhang, S.X. Cao, R.Y. Zhang, L.M. Yu, C. Jing, Curr. Appl. Phys. 5 (2005) 381–386. http://dx.doi.org/10.1016/j.cap.2004.03.004.
202
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
[29] Z.X. Zhang, H.J. Yuan, J.J. Zhou, D.F. Liu, S.D. Luo, Y.M. Miao, Y. Gao, J.X. Wang, L.F. Liu, L. Song, Y.J. Xiang, X.W. Zhao, W.Y. Zhou, S.S. Xie, J. Phys. Chem. B. 110 (2006) 8566–8569. http://dx.doi.org/10.1021/ jp0568632. [30] J. Zhong, H. Chen, G. Saraf, Y. Lu, C.K. Choi, J.J. Song, D.M. Mackie, H. Shen, Appl. Phys. Lett. 90 (2007) 203515-1–203515-3. http://dx.doi. org/10.1063/1.2741052. Geeta Rani is a Science Graduate and Postgraduate from the University of Delhi, Delhi and is also doing her Ph.D. from Delhi University (Physics). She carried out all her major research work in the University Science Instrumentation Centre (USIC) of the University of Delhi, Delhi. Her research interests include synthesis of phosphors, luminescent materials and semiconductors nanoparticles and characterization of materials using the XRD, PL- spectrophotometer, UV–Vis, FTIR, TG-DTA, SEM, TEM, EDS and Thermoluminescence(TL) Techniques.
P.D. Sahare is presently working as a faculty member in the Department of Physics and Astrophysics, University of Delhi, India. He was also a faculty member at Nagpur University and the University of Pune, India. He obtained his Master and Doctorate from RTM Nagpur University, Nagpur. He did his post-doctorate degree from the Department of polymer science and Engineering, University of Massachusetts, Amherst, USA. He has published more than 100 research papers in international peer reviewed journals of repute. He was instrumental in organizing many national and international conferences. He is recipient of IAAM Advanced Materials Scientist Award-2011. Professor Sahare has obtained various prestigious fellowships including National Overseas Scholarship UGC and CSIR Research Associateships. He is actively engaged as reviewer, editor and member of scientific bodies around the world. He has close association with Lebedev Physical institute, Moscow and JINR, Dubna (Russia). His area of interest include nanomaterials, luminescent phosphor, radiation dosimetry, organic dye laser, gas detector and optical sensors.
Contents lists available at SciVerse ScienceDirect
Nano Communication Networks journal homepage: www.elsevier.com/locate/nanocomnet
Preparation and characterization of short length ZnO nanorods and ZnO@ZnS core–shell nanostructures Geeta Rani ∗ , P.D. Sahare Department of Physics and Astrophysics, University of Delhi, Delhi – 110 007, India
article
info
Article history: Received 20 June 2012 Accepted 11 September 2012 Available online 23 September 2012 Keywords: ZnO@ZnS Morphology Band-gap Photoluminescence etc
abstract Short length ZnO nanorods with uniform diameter have been synthesized by a solid state reaction method using Zinc acetate as precursor. This is a larger scale production and inexpensively synthesized method without any templates or additives. ZnO@ZnS core–shell nanostructures were successfully fabricated by sulfidation of ZnO nanorods via a facile chemical synthesis. The as-obtained samples were characterized by X-ray diffraction (XRD), UV–Vis absorption, Transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), fluorescence spectrophotometer (PL) and Fourier Transformed Infra-red spectroscopy (FTIR). The results showed that the pure ZnO nanorods were hexagonal wurtzite crystal structure and the ZnS tube like structure is cubic structure grown on the surface of the ZnO nanorods. The optical band gap was found to be increased with increasing sulfidation process. The measurement of luminescence revealed that ZnO@ZnS core–shell structure integrated the luminescent effect of ZnO and ZnS. The broad blue emission of ZnO@ZnS core–shell was dramatically changed. The FTIR spectra were studied to find the role of organic impurities trapped inside the material. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction A great variety of nano-related studies hold many researchers’ interest and attention because of the amazing properties of nanomaterials and their possible applications in many fields. In studies of nanoscale materials, preparation techniques as the basis of further research and properties as the foundation of applications take very high positions. Recently, considering the properties of materials are greatly affected by their morphologies, a wide range of metal oxides with different morphologies, providing great opportunities for the discovery of new properties and potential uses, have been synthesized via different methods. As an important II–VI semiconductor with wide band gap and large exciton binding energy, ZnO has unique optical and electronic properties. It is regarded as a promising material applied in optoelectronics [30], varistors [28], chemical sensors [23], catalysts [15], field emission displays
∗
Corresponding author. Tel.: +91 9560752969. E-mail address: [email protected] (G. Rani).
1878-7789/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.nancom.2012.09.003
[29] and cosmetic [11]. This wide variety of applications requires the fabrication of morphological and surface functional ZnO nanostructures. Modification of ZnO by impurity incorporation and surface coating can efficiently adjust its electrical, optical, and magnetic properties [1,17,9]. Surface coating of ZnO with wide-band-gap semiconductors to form core–shell nanostructures has been recognized as one of the most advanced and intriguing methods to improve the luminescence properties of the Metal oxide, as demonstrated by ZnO nanorod@CdS nanoparticle core–shell composites [7] and ZnO@CdSe core–shell nanoparticles [14]. ZnS is a well-known luminescent material having prominent applications in flat-panel displays, sensors, and lasers [12], and also has been applied in photo catalysts [6], pigments [10], non linear optical devices [20] and infrared windows [4]. Because the band gap of ZnS is larger than that of ZnO, the luminescent property of ZnO@ZnS core–shell composites could be improved. Li et al. [18] prepared ZnO nanowires by a vapor-phase transport process, and then fabricated ZnO@ZnS core–shell
198
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
nanowires through the reaction of Zn (NO3 )2 and Na2 S on ZnO nanowires in aqueous solutions. They found that the UV emission of nanorods was dramatically enhanced. Sun et al. [22] had successfully synthesized a ZnO@ZnS nanocable through a catalyst-free thermal vapor transport method and investigated its microstructure. In the present work, we first fabricated the short length ZnO nanorods by solid state reaction method and then introduce the ZnS shell onto the ZnO nanorods. To illustrate the effect of the ZnS layer, core–shell ZnS/ZnO nanostructure were designed and produced by a hydrothermal and chemical etching process. The morphology by TEM, crystal structure by XRD and luminescence properties were studied in detail, which show the formation of the ZnS/ZnO core–shell nanostructure. 2. Experimental All chemicals used in this work were of analytical reagent grade and used as received without further purification. All the aqueous solutions were prepared using double distilled water.
2.4. Characterization The structure of the obtained powder samples were identified by X-ray diffraction technique (XRD). The XRD was done on the machine D8-Advance model of Bruker Inc. using monochromatic Cu–Kα line (λ = 1.54056 Å) and well matched with the PCPDF data files to confirm the formation of different phases in the material. The UV–visible spectra of the material were taken in wavelength range, 250–600 nm using Evolution UV–Vis. Spectrophotometer (Thermo Scientific Inc. make) to estimate the band gaps of the core–shell systems. The TEM images were taken to study morphology (shape and size) of the nanocrystalline material and the images and selected area electron diffraction (SAED) pattern were recorded on a transmission electron microscope (FEI TEM model TECNAI G2 T30, U–TWIN) at 300 kV. PL spectra were carried out on a fluorescence spectrophotometer (Fluorolog Horiba JOBIN YVON) using Xe lamp with excitation wavelength 350 nm. The surface chemistry of the samples was analyzed with Fourier Transmission Infrared spectroscopy (FTIR) and performed with a Perkin Elmer Spectrometer Spectrum RX-I (at resolution of 4 cm−1 ).
2.1. Preparation of ZnO nanorods 3. Results and discussion ZnO nanorods were synthesized by solid-state reaction method. A homogeneous solution of 1M Zinc acetate dehydrate (Zn(CH3 COO)2 .2H2 O) in deionized water was prepared. The solution was oven dried and the dried precursor material was ground in agate mortar and mixed thoroughly, packed in a quartz boat and fired at 500 °C in air. High-purity quartz boat was used without any metallic contamination was used for firing the sample in the furnace. 2.2. Preparation of tubular ZnO/ZnS core/shell nanostructures The as-prepared ZnO nanorods (0.324 g) and thioacetamide (TAA, 0.3 g respectively) dissolved in 30 ml deionized water were put in to a Teflon-lined stainless steel autoclave of 100 ml capacity with the addition of 20 ml water so that the volume of the solution is about 80% volume of the autoclave. The solution was stirred vigorously for 10 min then sealed and maintained at 100 °C for 6 h, and then cooled down to room temperature. The resulting precipitate was collected by filtration and washed with absolute ethanol and distilled water in sequence for several times. The final product was dried in a vacuum box at 50 °C for 4 h. 2.3. Preparation of ZnS nanotubes Tubular ZnO/ZnS core/shell nanostructures (0.3 g) were soaked in 20 ml KOH aq. solution (4 mol/l) under sonication for 1 h at room temperature. Finally, the obtained products were collected by centrifugation and washed with distilled water repeatedly and then dried in vacuum oven at 50 °C for 4 h.
Fig. 1 shows the XRD patterns of the obtained samples. All the peaks of Fig. 1(a) can be identified as hexagonal wurtzite ZnO with lattice constants of a = 3.250 Å, and c = 5.207 Å, which is consistent with the literature data of JCPDS 36-1451. No impurity peaks were detected showing that the products are pure phase. When we add TAA to ZnO nanorods both wurtzite ZnO and cubic ZnS (JCPDS card No. 65-5476, a = 5.404 Å) are found to exist in the synthesized products, which can be seen in Fig. 1(b,c). It reveals that the products have two phases of ZnO and ZnS. The Nanorods share the characteristics of both ZnO and ZnS because the peaks of (220)_ZnS and (311)_ZnS are very adjacent to the peaks of (102)_ZnO and (110)_ZnO, therefore, the peaks of the (220)_ZnS and (102)_ZnO are overlapped together, so are the peaks of the (311)_ZnS and (110)_ZnO which can be clearly seen in Fig. 1(b). The average size of ZnO is calculated about 29 nm according to Scherrer’s formula. Similarly the average size of the ZnO@ZnS nanorods is 19 nm, so is the average size of the ZnO of the ZnO/ZnS nanostructures. This implies only a small part of ZnO is converted into ZnS. Fig. 2 presents the UV–Vis spectra of the ZnO nanorods and ZnO@ZnS core/shell nanostructures. They show an obvious change in optical absorption behaviors, after deposition of the ZnS nanoparticles on the surface of the ZnO nanorods. The band edge of the bare ZnO nanorods locates at around 380 nm, which is close to the value of bulk ZnO (Eg , ZnO = 3.37 eV) [21]. The bare ZnO nanorods show no absorption in the visible region. A significant red shift in the absorption edge is observed in the UV-vis spectra of the ZnO/ZnS core/shell nanorods because of deposition of the ZnS, and the photo absorption in the visible region at the range of 400–700 nm was obviously enhanced. For the
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
199
Fig. 1. XRD patterns of (a) ZnO (b) ZnO@ZnS (c) ZnS.
Fig. 2. (α hν)2 versus hν plot and Inset shows the Optical absorption spectrum of (a) ZnO (b) ZnO@ZnS (c) ZnS.
allowed direct transition, the variation of α with photon energy (hν ) obey Tauc’s plot that form [24].
(α hν)2 = A(hν − Eg )
(1)
where A is a constant, Eg is band gap, h is the Planck constant and α is the absorption coefficient. Fig. 2 illustrates the (α hν)2 versus hν plot used for the evaluation of the band gap of nanostructures. It revealed from the figure that the band gap of the prepared samples increase from 3.31 to 3.64 eV with change in phase. The qualitative information about the band gap evaluated from Fig. 2 (inset) are well corroborated with the quantitative results of Fig. 2. The morphologies of the samples were investigated by TEM as shown in Fig. 3. The TEM images of the ZnO nanorods and ZnO@ZnS core/shell nanorods shows that a ZnO core and ZnS shell are formed with ZnS nanoparticles covering the surface of the ZnO nanorods. The sharp interface between the core and shell clearly shows that the ZnO nanorods are fully sheathed by a ZnS layer along the entire length. The average width and length of nanorods
are about 17 nm and 53 nm based on TEM images, respectively. It can be seen that ZnS nanoparticles grow on the surface of ZnO nanorods as shown in Fig. 3(b) which is in good agreement with the result of XRD analysis. The selected area electron diffraction (SAED) pattern manifests the crystallinity of the samples. In order to investigate the detailed construction of the ZnO/ZnS nanostructures, EDS measurement is conducted on the surface of the core–shell and found that Zn, O and S are the primary detected elements implies that the obtained core–shell is composed of ZnO and ZnS shown in Fig. 3(d). Fig. 4(a,b) showed the PL spectra of the pure ZnO nanorods and the ZnO@ZnS core–shell nanostructures, respectively. For a comparison, the PL spectra of pure ZnS nanoparticles were also shown in Fig. 4(c). Two emitting bands, including a strong broad blue emission centered at around 505 nm and a weak orange band centered at around 383 nm, has been observed in ZnO nanorods. The origin of the broad blue emission could be attributed to the defect related emissions [2,3]. The orange
200
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
Fig. 3. TEM images and their corresponding SAED patterns of (a) ZnO (b) ZnO@ZnS (c) ZnS and (d) EDS of ZnO@ZnS core shell nanostructure.
Fig. 4. PL spectra of (a) ZnO (b) ZnO@ZnS (c) ZnS measured at room temperature with excitation wavelength are 350 nm.
emission was reported to be due to oxygen interstitials [5], suggesting oxygen excessive in the ZnO nanorods. The PL spectra of ZnO@ZnS core–shell nanostructure showed an enhanced broad blue emission and a disappeared orange emission, comparing to that of pure ZnO nanorods. The enhancement in blue emission could be explained as that ZnS nanoparticle has a higher band gap than ZnO and then it suppressed the tunneling of the charge carriers from the cores to the ZnS nanoshell. As a result, more photo generated electrons and holes were confined inside the ZnO core, giving rise to a high quantum yield. The strongest blue emission of ZnO@ZnS core–shell in the range of 400–650 nm was supposed to origin from the synergic interactions of ZnO core and ZnS nanoshell. However, the
orange emission disappeared because ZnS nanoshell was successfully grown on ZnO nanorods, the concentration of oxygen interstitials was reduced greatly. Consequently, the orange emission disappeared. The above results indicated that the sulfidation process had a great effect on the relative intensity and position of typical PL properties of ZnO nanorods. Therefore, the PL properties of the ZnO nanorods could be tuned by this approach. Compositional observation of all the samples obtained using FTIR spectroscopy as shown in Fig. 5. The observed bands in the present study suggest the presence of acetate group is loosely bound on the surface of ZnO by a dangling bond [19]. For the sample (a), bands near 3417 cm−1 represent the O–H stretching mode,
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
201
Fig. 5. FTIR spectra of (a) ZnO (b)ZnO@ZnS (c) ZnS nanoparticles.
indicative of the presence of hydroxyl ion (OH) in the samples, those at 2838–2914 cm−1 are the C–H stretching mode. The 2358 cm−1 region serves as absorbed CO2, 1597 cm−1 is the C=O stretching mode, 1394 cm−1 is the C–H deformation mode and 954–1180 cm−1 are the C–O stretching modes. The band positioned at 472 cm−1 clearly representing the ZnO band arises from the bonding between Zn–O [8,27,13,26,25,16]. As the sulfidation of ZnO occurs, the organic bands are slightly decreased in ZnO@ZnS nanostructures because of the existence of two phases in the sample as shown in Fig. 5(b). All the bands in ZnS are increased to a great extent but the extent of the band at 472 cm−1 decreased because of the formation of single ZnS phase as shown in Fig. 5(c). 4. Conclusion In summary, two phases could be prepared using the sulfidation technique. This procedure is much simpler has a wide potential application in large scale conversion from metal oxide to metal Sulfide therefore ZnO@ZnS core shell has potential application in the Phase transition. Characteristic transmission from prominent FTIR peaks are also analyzed and assigned. The broad blue emission and a passivated orange emission provides a good indication of tuning the visible emission of the core–shell nanostructures so, this is a promising material applied in the fabrication of nanoscale optical devices. Acknowledgments The authors would like to thank the University Science Instrumentation Centre (USIC), University of Delhi for providing instrumentation facilities and UGC for financial assistance. References [1] S.Y. Bae, H.W. Seo, J. Park, J. Phys. Chem. B 108 (2004) 5206–5210. http://dx.doi.org/10.1021/jp036720k. [2] W.D. Cheng, P. Wu, X.Q. Zou, T. Xiao, J. Appl. Phys. 100 (2006) 054311-1–054311-4. http://dx.doi.org/10.1063/1.2338601. [3] J.P. Cheng, X.B. Zhang, X.Y. Tao, H.M. Hu, Z.Q. Luo, F. Liu, J. Phys. Chem. B 110 (2006) 10348–10353. http://dx.doi.org/10.1021/jp060133s.
[4] Y. Ding, X.D. Wang, Z.L. Wang, Chem. Phys. Lett. 398 (2004) 32–36. http://dx.doi.org/10.1016/j.cplett.2004.09.031. [5] A.B. Djurisic, Y.H. Leung, K.H. Tam, L. Ding, W.K. Ge, H.Y. Chen, S. Gwo, Appl. Phys. Lett. 88 (2006) 103107-1–103107-3. http://dx.doi.org/ 10.1063/1.2182096. [6] H. Fujiwara, H. Hosokawa, K. Murakoshi, Y. Wada, S. Yanagida, Langmuir 14 (1998) 5154–5159. http://dx.doi.org/10.1021/la9801561. [7] T. Gao, Q.H. Li, T.H. Wang, Chem. Mater. 17 (2005) 887–892. http://dx.doi.org/10.1021/cm0485456. [8] D. Geetha, T. Thilagavathi, Digest J. Nanomaterials and Biostructures 5 (2010) 297–301. [9] B.Y. Geng, G.Z. Wang, Z. Jiang, T. Xie, S.H. Sun, G.W. Meng, L.D. Zhang, Appl. Phys. Lett. 82 (2003) 4791–4793. http://dx.doi.org/10.1063/1. 1588735. [10] A.E. Gorshteyn, A. Robbat, Ind. Eng. Chem. Res. 41 (2002) 2646–2651. http://dx.doi.org/10.1021/ie010831t. [11] T. Iwasaki, M. Satoh, T. Masuda, T. Fujita, J. Mater. Sci. 35 (2000) 4025–4029. http://dx.doi.org/10.1023/A:1004826002503. [12] Y. Jiang, X.M. Meng, J. Liu, Z.Y. Xie, C.S. Lee, S.T. Lee, Adv. Mater. 15 (2003) 323–327. http://dx.doi.org/10.1002/adma.200390079. [13] K. Kakiuchi, E. Hosono, T. Kimura, H. Imai, S. Fujihara, J Sol-Gel Sci Technol. 39 (2006) 63–72. http://dx.doi.org/10.1007/s10971-0066321-6. [14] J.Y. Kim, F.E. Osterloh, J. Am. Chem. Soc. 127 (2005) 10152–10153. http://dx.doi.org/10.1021/ja052735f. [15] T.J. Kuo, C.N. Lin, C.L. Kuo, M.H. Huang, Chem. Mater. 19 (2007) 5143–5147. http://dx.doi.org/10.1021/cm071568a. [16] Y.J. Kwon, K.H. Kima, C.S. Limb, K.B. Shim, Journal of Ceramic Processing Research 3 (2002) 146. [17] J.J. Liu, M.H. Yu, W.L. Zhou, Appl. Phys. Lett. 87 (2005) 1725051–172505-3. http://dx.doi.org/10.1063/1.2084321. [18] J.H. Li, D.X. Zhao, X.Q. Meng, Z.Z. Zhang, J.Y. Zhang, D.Z. Shen, Y.M. Lu, X.W. Fan, J. Phys. Chem. B 110 (2006) 14685–14687. http://dx.doi.org/10.1021/jp061563l. [19] R. Mishra, Raghvendra S. Yadav, A.C. Pandey, Sharda S. Sanjay, C. Dar, J. Luminescence 130 (2010) 365–373. http://dx.doi.org/10.1016/j. jlumin.2009.09.022. [20] F. Ramtollli, G. Saccinni, Cryst. Res. Tech. 37 (2002) 1325–1330. http://dx.doi.org/10.1002/crat.200290009. [21] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447–5451. http://dx. doi.org/10.1063/1.367375. [22] C.W. Sun, J.S. Jeong, J.Y. Lee, J. Cryst. Growth 294 (2006) 162–167. http://dx.doi.org/10.1016/j.jcrysgro.2006.06.023. [23] Z.P. Sun, L. Liu, L. Zhang, D.Z. Jia, Nanotechnology 17 (2006) 2266–2270. http://dx.doi.org/10.1088/0957-4484/17/9/032. [24] J. Tauc, A. Menthe, J. Non-Cryst. Sol. 8–10 (1972) 569–585. http://dx.doi.org/10.1016/0022-3093 72 90194-9. [25] I. Uslu, B. Başer, A. Yayli, M.L. Aksu, e-Polymers (ISSN: 1618-7229) 145 (2007) http://www.e-polymers.org. [26] H.M. Xiong, R.Z. Ma, S.F. Wang, Y.Y. Xia, J. Mater. Chem. 21 (2011) 3178. http://dx.doi.org/10.1039/C0JM02577A. [27] G. Xiong, U. Pal, J.G. Serrano, K.B. Ucer, R.T. Williams, Phys. Stat. Sol.(c) 3 (2006) 3577. http://dx.doi.org/10.1002/pssc.200672164. [28] J.C. Zhang, S.X. Cao, R.Y. Zhang, L.M. Yu, C. Jing, Curr. Appl. Phys. 5 (2005) 381–386. http://dx.doi.org/10.1016/j.cap.2004.03.004.
202
G. Rani, P.D. Sahare / Nano Communication Networks 3 (2012) 197–202
[29] Z.X. Zhang, H.J. Yuan, J.J. Zhou, D.F. Liu, S.D. Luo, Y.M. Miao, Y. Gao, J.X. Wang, L.F. Liu, L. Song, Y.J. Xiang, X.W. Zhao, W.Y. Zhou, S.S. Xie, J. Phys. Chem. B. 110 (2006) 8566–8569. http://dx.doi.org/10.1021/ jp0568632. [30] J. Zhong, H. Chen, G. Saraf, Y. Lu, C.K. Choi, J.J. Song, D.M. Mackie, H. Shen, Appl. Phys. Lett. 90 (2007) 203515-1–203515-3. http://dx.doi. org/10.1063/1.2741052. Geeta Rani is a Science Graduate and Postgraduate from the University of Delhi, Delhi and is also doing her Ph.D. from Delhi University (Physics). She carried out all her major research work in the University Science Instrumentation Centre (USIC) of the University of Delhi, Delhi. Her research interests include synthesis of phosphors, luminescent materials and semiconductors nanoparticles and characterization of materials using the XRD, PL- spectrophotometer, UV–Vis, FTIR, TG-DTA, SEM, TEM, EDS and Thermoluminescence(TL) Techniques.
P.D. Sahare is presently working as a faculty member in the Department of Physics and Astrophysics, University of Delhi, India. He was also a faculty member at Nagpur University and the University of Pune, India. He obtained his Master and Doctorate from RTM Nagpur University, Nagpur. He did his post-doctorate degree from the Department of polymer science and Engineering, University of Massachusetts, Amherst, USA. He has published more than 100 research papers in international peer reviewed journals of repute. He was instrumental in organizing many national and international conferences. He is recipient of IAAM Advanced Materials Scientist Award-2011. Professor Sahare has obtained various prestigious fellowships including National Overseas Scholarship UGC and CSIR Research Associateships. He is actively engaged as reviewer, editor and member of scientific bodies around the world. He has close association with Lebedev Physical institute, Moscow and JINR, Dubna (Russia). His area of interest include nanomaterials, luminescent phosphor, radiation dosimetry, organic dye laser, gas detector and optical sensors.