Conversion from ZnO nanospindles into ZnO/ZnS core/shell composites and ZnS microspindles

Cryst. Res. Technol. 44, No. 4, 402 – 408 (2009) / DOI 10.1002/crat.200800574

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Conversion from ZnO nanospindles into ZnO/ZnS core/shell

composites and ZnS microspindles

Fei Li*1,2

, Xueqin Liu1, Tao Kong

1, Zhen Li

1, and Xintang Huang

2

1 Faculty of Materials Science and Chemical Engineering, China University of Geosciences, Wuhan, 430074,

P. R. China 2 Center of Nano-Science and Technology, Department of Physics, Central China Normal University,

Wuhan, 430079, P. R. China

Received 28 November 2008, revised 20 December 2008, accepted 7 January 2009

Published online 30 January 2009

Key words core/shell, ZnS, hydrothermal, luminescence.

PACS 81.07.Bc, 81.05.Dz, 81.16.Be

The formation process of ZnO/ZnS core/shell microcomposites and ZnS microspindles prepared by the

reaction of ZnO colloids and thioacetamide under hydrothermal conditions was investigated in detail by X-

ray powder diffraction, field emission scanning electron microscopy , transmission electron microscopy and

selected-area electron diffraction techniques. The precursors of spindlelike ZnO colloids were prepared by a

hydrothermal method with the help of a surfactant. A growth mechanism was proposed to account for the

formation of ZnO/ZnS core/shell microcomposites and ZnS microspindles. Luminescence measurement

revealed that ZnO/ZnS core/shell microcomposites integrated the luminescence effect of ZnO and ZnS. The

blue and green emissions were dramatically enhanced, while the orange emission disappeared. The results

provide a good approach to tune the visible emission of the ZnO nanostructures by ZnS coating.

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

The ability to design, obtain, control, manipulate, and modify structures at the nanometer scale is of great scientific and technological interest to material scientists [1]. Recently, core/shell composites have been paid much attention due to their interesting properties and potential applications, such as increasing the solubility and stability [2, 3], drug delivery [4] and altering the emission properties [5,6]. Various kinds of core/shell composites have been designed and fabricated, such as metal/metal [7], metal/semiconductor [8], semiconductor/metal [9] and semiconductor/semiconductor [10]. The structure, size and composition of core/shell composites can be easily altered in a controllable way to tailor their magnetic [11], optical [12], magnetic-optical [13], optical-electrical [14], electrical-magnetic [15], mechanical [16] and catalytic properties [17].

As an important II-VI semiconductor with a wide band gap, ZnO possesses unique optical and electronic properties. It is regarded as the promising material applied in optoelectronics [18], varistors [19], chemical sensors [20], catalysts [21] and field emission displays [22]. On the other hand, ZnS is also a kind of important semiconductor, having the largest band gap among II-VI compounds. It is a well-known luminescent material, having prominent applications in flat-panel displays [23], electroluminescent devices [24], sensors [25], lasers [26] and photocatalysts [27].

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 ____________________

* Corresponding author: e-mail: [email protected]

Cryst. Res. Technol. 44, No. 4 (2009) 403

www.crt-journal.org © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

metal oxide, as demonstrated by ZnO nanorod/CdS nanoparticle core/shell composites [28] and ZnO/CdSe core/shell nanoparticles [29]. Because the band gap of ZnS is larger than that of ZnO, the luminescent property of ZnO/ZnS core/shell composites could be improved. Chaudhuri prepared ZnO/ZnS core/shell nanorods through a catalyst-free thermal vapor transport method via the reaction of ZnO nanorod films and gaseous H2S [30]. Xue synthesized ZnO/ZnS nanocables by hydrothermal method through the reaction of Na2S and ZnO with the assistance of HSCH2COOH [31].

In the present contribution, we reported here a facile hydrothermal route to the large-scale synthesis of ZnO/ZnS core/shell composites and ZnS microspindles through the sulfidation of pure ZnO nanospindles. The conversion ratio of ZnO/ZnS core/shell composites can be easily controlled by the amount of thioacetamide (TAA, C2H5NS). The crystal structure, morphology and luminescence properties of the samples are studied in detail and the growth mechanisms of these structures are preliminarily discussed.

2 Experimental

All of the chemical reagents (Shanghai Chemicals Co. Lt) used in the experiments were analytical grade without further purification and treatment. First, Polyvinylpyrrolidone (PVP K30, 2g) and Zn(CH3COO)2.2H2O (1 g) were dissolved in 50 mL double distilled water. The resulting reaction mixture was stirred for several minutes, followed by the addition of 2 mol/l KOH solution under continuous stirring. The pH value of the whole solution was adjusted to 13 and white flocculent precipitate immediately appeared. The above turbid solution was loaded into a 100 mL teflon-lined autoclave with a filling capacity of about 80%, sealed and maintained at 160 °C for 24 h, and then cooled naturally to room temperature. After that, the precipitates were filtered and washed with distilled water and ethanol several times to remove the impurities, then the precipitates were dried under vacuum at 60 °C for 3 h and the as-prepared ZnO nanospindles were obtained. Afterward, about 0.2 g of as-obtained ZnO colloid particles were dispersed into double distilled water and some TAA was added into the solution, and then the solution was transferred into a teflon-lined autoclave. Hydrothermal growth was performed at the temperature of 130 °C for 18 h. Finally, the as-obtained products were centrifuged, washed with ethanol and water several times and dried at 60 °C under vacuum for 3 h. To investigate the effect of TAA on the formation of ZnO/ZnS core/shell microspindles and ZnS microspindles in our synthetic method, the amount of TAA was varied with the other conditions unchanged to get molar ratios of S/Zn of 1:5, 1:2, 1:1 and 1.2:1, respectively.

The phase and composition of the products were determined by a Dmax-3β diffractometer equipped with monochromatic high-intensity Cu Kα radiation (λ= 1.54178 Å). The size and morphology of the samples were observed by FESEM (JEOL JSM-6700F) and TEM (Tecnai F20, FEI Corp.). TEM also was used to provide selected area electron diffraction (SAED) images and energy dispersive X-ray (EDX) spectra. The PL spectra of the samples were measured by a fluorescence spectrophotometer (LS-55) using a 450 W monochromatized xenon lamp with excitation wavelength of 250 nm.

3 Results and discussion

Figure 1 shows the XRD patterns of the as-obtained samples. All the peaks of figure 1a 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 with the molar ratios of S/Zn of 1:5, 1:2, 1:1, both wurtzite ZnO and sphalerite ZnS (JCPDS card No. 65-5476, a=5.404 Å) are found to exist in the synthesized products, which can be seen in figure 1 b, c and d. It reveals that the products have two phases of ZnO and ZnS. Because the peaks of (220)_ZnS and (311)_ZnS are very adjacent to the peaks of (102)_ZnO and (110)_ZnO, respectively, therefore, the peaks of the (220)_ZnS and

(102)_ZnO are overlapped together, so are the peaks of the (311)_ZnS and (110)_ZnO. With the increase of TAA, the diffraction intensity of ZnS crystal becomes strong gradually and the peak of (200) crystal plane of

404 Fei Li et al.: Conversion from ZnO nanospindles into composites and microspindles

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

ZnS begins to appear, which indicates that the crystallization of ZnS gets more and more perfect. Furthermore, the diffraction intensity of ZnO fades away. When TAA gets to the highest amount with the molar ratio of S:Zn=1.2:1, all of the diffraction peaks correspond to sphalerite ZnS. It implies that hexagonal ZnO crystals have been converted into sphalerite ZnS crystals completely. We can also find that the full width at half-maximum of the (111) diffraction peak of ZnS becomes more and more narrow with the increase of TAA, which indicates that the ZnS nanoparticles gets larger and larger gradually.

Figure 2 presents the EDX spectra of ZnO/ZnS core/shell microcomposites and ZnS crystals. With the increase of TAA, the peak intensity for element S becomes stronger and stronger and the peak for element O gets weaker and weaker. Finally, the products turn into ZnS crystals completely. This result is in accordance with that of XRD analysis.

Fig. 1 XRD patterns of the products synthesized using

different molar ratios of S:Zn. (a) pure ZnO; (b) S:Zn=1:5;

(c) S:Zn=1:2; (d) S:Zn=1:1; (e) S:Zn=1.2:1.

Fig. 2 EDX spectra of the products synthesized using

different molar ratios of S:Zn. (a) S:Zn=1:5; (b) S:Zn=1:2;

(c) S:Zn=1:1; (d) S:Zn=1.2:1.

Fig. 3 Typical TEM images

of pure ZnO nanospindles.

The inset presents the corres-

ponding SAED pattern.

Figure 3 clearly reveals highly monodisperse distribution spindlelike nanostructures with a mean length of 400 nm and a mean width of 240 nm at the center, 30 nm at the tip. A few partly broken ZnO nanospindles can also be observed. The inset SAED pattern of an individual ZnO nanospindle was recorded with the electron beam along the [002] long axis direction, indicating the single crystalline nature of the ZnO nanospindle.

Figure 4 shows the SEM and TEM images of the ZnO/ZnS core/shell microspindles synthesized by hydrothermal method via the reaction of ZnO nanospindles and various amount of TAA. The amount of TAA



is denoted by the molar ratio of S:Zn, where the amount of ZnO is constant. On the whole, all the ZnO/ZnS microcomposites prepared at various molar ratios of S:Zn have a uniform spindlelike shape with a mean length of 1.5 µm and a mean width of 500 nm at the center, 200 nm at the end. The aspect ratio of the ZnO/ZnS core/shell microspindles is in the range of 3~5. A dense nanoshell, which is composed of ZnS nanoparticles, is successfully coated on the ZnO nanospindles. It can be clearly seen that the ZnS nanoparticles grows larger and larger with the increase of TAA. When the molar ratio of S:Zn is 1:5, the ZnS nanoparticles are very small with a mean size of 20nm. When the proportion is raised to 1:2 and 1:1, the ZnO nanoparticles have an average width of 40nm and 80nm, respectively. The SAED pattern displays a spotted pattern that corresponds to the single crystal of ZnO, and a set of diffraction rings which fits to the polycrystalline ZnS nanoparticles. We also find that the spotted pattern of ZnO single crystal is weaker and weaker and the diffraction rings of ZnS polycrystal are stronger and stronger with the increase of TAA. This is in good agreement to the XRD results. Furthermore, we also find a very interesting phenomenon that the ZnO cores of the ZnO/ZnS microspindles are made up of two ZnO nanospindles on the whole. As a result, ZnO/ZnS core/shell microspindles are much larger than pure ZnO nanospindles.

Fig. 4 SEM and TEM images of the ZnO/ZnS core/shell microspindles prepared at various molar ratios of

S:Zn. a) S:Zn=1:5; (b) S:Zn=1:2; (c) S:Zn=1:1. The insets show the corresponding SAED patterns.

Figure 5 gives the SEM and TEM images of pure ZnS microspindles, which are completely converted from ZnO nanospindles. The ZnS microspindles are made up of many ZnS nanoparticles with a mean size of about



100nm. On the whole, the SAED pattern (inset in figure 5) is made up of diffraction rings corresponding to ZnS polycrystal, which proves the XRD results of pure ZnS crystal.

Fig. 5 SEM and TEM images of the pure ZnS microspindles. The inset shows the corresponding SAED

pattern.

It is of great significance to understand the growth mechanism in order to control and design tailored structures. It is believed that the selective adsorption of PVP on various crystallographic planes of the nanocrystals plays a vital role in controlling the product morphologies [32, 33]. As for ZnO nanospindles, the synergic interactions between ZnO nanocrystal and PVP led to the formation of the interesting morphology. After TAA was added into the ZnO colloid solution, TAA hydrolyzed and releases H2S immediately under the hydrothermal condition. The chemical reaction of the hydrolysis process can be expressed as follows:

CH3CSNH2+H2O → CH3CONH2+H2S (1)

H2S is easy to react with ZnO at the interface of the ZnO nanospindles to produce ZnS nuclei, which is a neutralization reaction and can be formulated as follows:

ZnO(s)+H2S → ZnS(s)+H2O (2)

Accordingly, heterogenous nucleation takes place and a large number of ZnS nuclei clinging to the ZnO nanospindles are produced. Due to the continuous reaction of (2), the ZnS nuclei grow larger and larger to form ZnS nanoparticles and ZnO/ZnS core/shell nanospindles are formed. The size of the ZnS nanoparticles is determined by the amount of TAA. The more of TAA, the larger of the ZnS nanoparticles. Finally, ZnS nanoparticles attached to two different ZnO/ZnS core/shell nanospindles aggregate in a certain way, most probably by electrostatic gravitation, and ZnO/ZnS core/shell microspindles takes shape, which is composed of two ZnO/ZnS core/shell nanospindles. It is because of this kind of interaction that ZnO/ZnS core/shell microspindles are much larger than bare ZnO nanospindles. When the amount of TAA is sufficient, such as the molar ratio of S:Zn=1.2:1, ZnO/ZnS core/shell micospindles will converted into pure ZnS microspindles completely by consuming the remained ZnO core. The whole process is illustrated in figure 6.

To investigate the potential luminescence properties of the ZnO/ZnS core/shell microspindles, their PL spectra are measured. For a comparison, the luminescence of pure ZnO nanospindles and ZnS microspindles are also studied under the same conditions. Four emitting bands, including a strong blue band emission at around 436 nm, two weak green emissions at 485 nm and 530nm, as well as a weak orange band centered at around 638 nm, have been observed in pure ZnO nanospindles, which can be seen in figure 7a. The origin of the blue emission can be attributed to the electron transition from the energy level of Zn interstitial atoms to the valence band edge [34]. It has been suggested that the green band emission corresponds to the singly ionized oxygen vacancy in ZnO and results from the recombination of a photogenerated hole with the single ionized charged state of the defect [35].The orange emission was reported to be due to oxygen interstitials [36], suggesting oxygen excessive in the ZnO nanospindles. As for the emission of pure ZnS microspindles, it is generally accepted that the blue emission at 420nm (Fig. 7e) can be ascribed to the surface defects of ZnS nanoparticles [37,38]. The luminescence spectra of ZnO/ZnS core/shell micospindles not only integrate the emission feature of pure ZnO nanospindles and pure ZnS microspindles, but also show some difference.



Fig. 6 Illustration of growth process of

ZnO/ZnS core/shell microspindles and

ZnS microspindles.

Fig. 7 Photoluminescence spectra of pure ZnO

nanospindles (a), ZnO/ZnS core/shell microspindles

(b, c, d) and ZnS microspindles (e).

Comparing to the luminescence spectra of pure ZnO nanospindles, the spectra of ZnO/ZnS core/shell microspindles show enhanced blue, green emissions and a disappeared orange emission. As we know, ZnS nanoparticles have a larger band gap than ZnO and suppress the tunneling of the charge carriers from the ZnO core to the ZnS nanoshell. As a result, more photogenerated electrons and holes are confined inside the ZnO core, giving rise to a high quantum yield. The increased PL quantum yield was also reported in CdSe/CdS/ZnCdS/ZnS nanostructure [39]. The strongest blue emission in the range of 412 nm~422 nm is supposed to origin from the synergic interactions of ZnO core and ZnS nanoshell. However, the orange emission peak disappears. We think that oxygen interstitials are reduced greatly because ZnS nanoshell successfully grows on ZnO nanospindles. The remarkable decrease of oxygen interstitials leads to the disappearance of the orange emission. Furthermore, we also find that the amount of TAA has some effect on the emission intensity of ZnO/ZnS core/shell micospindles. When the molar ratio of S:Zn=1:2, the emission is the strongest. This result is possibly because the proportion of the ZnS nanoshell and the ZnO core reaches a best value, which is conducive to the emission enhancement. The above results indicate that the sulfidation process has a great effect on the relative intensity and position of typical PL properties of ZnO nanospindles. Therefore, the luminescence properties of the ZnO nanospindles can be tuned by this approach.



4 Conclusions

In summary, ZnO/ZnS core/shell composites and pure ZnS microspindles have been fabricated by a low-temperature hydrothermal growth via the reaction of ZnO nanospindles and TAA. We study the effect of TAA’s amount on the conversion process of ZnO nanospindles into ZnO/ZnS core/shell composites and ZnS microspindles for the first time. The present study provides a good indication of tuning the visible emission of the ZnO nanostructures by ZnS coating on the surface of ZnO. The ZnO/ZnS core/shell composites have possible applications in the fields of luminescence, electronics, and sensors.

Acknowledgements We thank the Research Foundation for Outstanding Young Teachers, China University of

Geosciences (Wuhan, No.CUGQNL0632). The financial support is gratefully appreciated.

References

[1] F. Caruso, Adv. Mater. 13, 11 (2001).

[2] S. Liu, Z. Zhang, and M. Han, Anal. Chem. 77, 2595 (2005).

[3] Y. Chan, J. P. Zimmer, M. Stroh, J. S. Steckel, R. K. Jain, and M. G. Bawendi, Adv. Mater. 16, 2092 (2004).

[4] Y. Zhu, J. Shi, W. Shen, X. Dong, J. Feng, M. Ruan, and Y. Li, Angew. Chem. Int. Ed. 44, 5083 (2005).

[5] Y. Deng, C. Wang, X. Shen, W. Yang, L. Jin, H. Gao, and S. Fu, Chem. Eur. J. 11, 6006 (2005).

[6] J. H. Song, T. Atay, S. Shi, H. Urabe, and A.V. Nurmikko, Nano. Lett. 5, 1557 (2005).

[7] Y. W. Cao, R. Jin, and C. A. Mirkin, J. Am. Chem. Soc. 123, 7961 (2001).

[8] X. Y. Kong, Y. Ding, and Z. L. Wang, J. Phys. Chem. B 108, 570 (2004).

[9] X. Wang, X. Kong, Y. Yu, and H. Zhang, J. Phys. Chem. C 111, 3836 (2007).

[10] Y. Yang, O. Chen, A. Angerhofer, and Y. C. Cao, J. Am. Chem. Soc. 128, 12428 (2006).

[11] T. Seto, H. Akinaga, F. Takano, K. Koga, T. Orii, and M. Hirasawa, J. Phys. Chem. B 109, 13403 (2005).

[12] R. T. Tom, A. S. Nair, N. Singh, M. Aslam, C. L. Nagendra, R. Philip, K. Vijayamohanan, and T. Pradeep,

Langmuir. 19, 3439 (2003).

[13] H. Kim, M. Achermann, L. P. Balet, J. A. Hollingsworth, and V. I. Klimov, J. Am. Chem. Soc. 127, 544 (2005).

[14] A. G. Jim, W. B. Robert, P. James, G. Jay, H. K. Thomas, and K. Masaru, J. Am. Chem. Soc. 130, 14822 (2008).

[15] Z. Xu, Y. Hou, and S. Sun, J. Am. Chem. Soc. 129, 8698 (2007).

[16] A. Aguiar, S. V. Gonzalez, M. Rabelero, E. Mendizabal, J. E. Puig, J. M. Dominguez, and I. Katime, Macromol.

32, 6767 (1999).

[17] M. Graeser, E. Pippel, A. Greiner, and J. H. Wendorff, Macromol. 40, 6032 (2007).

[18] J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song, D. M. Mackie, and H. Shen, Appl. Phys. Lett. 90, 3515

(2007).

[19] S. C. Pillai, J. M. Kelly, D. E. McCormack, and R. J. Ramesh, Mater. Sci. Technol. 14, 1572 (2004).

[20] Z. P. Sun, L. Liu, L. Zhang, and D. Z. Jia, Nanotechnol. 17, 2266 (2006).

[21] T. J. Kuo, C. N. Lin, C. L. Kuo, and M. H. Huang, Chem. Mater. 19, 5143 (2007).

[22] K. Yu, Y. S. Zhang, R. L. Xu, D. S. Jiang, L. Q. Luo, Q. Li, Z. Q. Zhu, and W. Lu, Solid State Comm. 133, 43

(2005).

[23] M. Bredol and J. Merikhi, J. Mater. Sci. 33, 471 (1998).

[24] G. Sharma, S. D. Han, J. D. Kim, S. P. Khatkar, and Y. W. Rhee, Mater. Sci. Eng. B 131, 271 (2006).

[25] P. T. Snee, R. C. Somers, G. Nair, J. P. Zimmer, M. G. Bawendi, and D. G. Nocera, J. Am. Chem. Soc. 128, 13320

(2006).

[26] T. V. Prevenslik, J. Lumin. 87-89, 1210 (2000).

[27] J. S. Hu, L. L. Ren, Y.G. Guo, H. P. Liang, A. M. Cao, L. J. Wan, and C. L. Bai, Angew. Chem. Int. Ed. 44, 1269

(2005).

[28] T. Gao, Q. H. Li, and T. H. Wang, Chem. Mater. 17, 887 (2005).

[29] J. Y. Kim and F. E. Osterloh, J. Am. Chem. Soc. 127, 10152 (2005).

[30] S. K. Panda, A. Dev, and S. Chaudhuri, J. Phys. Chem. C 111, 5039 (2007).

[31] C. L. Yan and D. F. Xue, J. Phys. Chem. B 110, 25850 (2006).

[32] Y. Sun and Y. Xia, Science 298, 2176 (2002).

[33] D. L. Tao, W. Z. Qian, Y. Huang, and F. Wei, J. Cryst. Growth 271, 353 (2004).

[34] S. B. Zhang, S. H. Wei, and A. Zunger, Phys. Rev. B 63, 5205 (2001).

[35] X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, Appl. Phys. Lett. 78, 2285 (2001).

[36] A. B. Djurisic, Y. H. Leung, K. H. Tam, L. Ding, W. K. Ge, H.Y. Chen, and S. Gwo, Appl. Phys. Lett. 88, 103107

(2006).

[37] J. T. Hu, G. Z. Wang, C. X. Guo, D. P. Li, L. L. Zhang, and J. J. Zhao, J. Lumin. 122-123, 172 (2007).

[38] Y. W. Wang, L. D. Zhang, C. H. Liang, G. Z. Wang, and X. S. Peng, Chem. Phys. Lett. 357, 314 (2002).

[39] R. G. Xie, U. Kolb, J. X. Li, T. Basche, and A. Mews, J. Am. Chem. Soc. 127, 7480 (2005).

Documents

Conversion from ZnO nanospindles into ZnO/ZnS core/shell composites and ZnS microspindles