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Formation of hierarchical nanospheres of ZnS induced by microwave irradiation: A highlighted assembly mechanism
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Materials Chemistry and Physics 109 (2008) 164–168
Formation of hierarchical nanospheres of ZnS induced by microwave irradiation: A highlighted assembly mechanism Qi-Zhi Yao a , Gu Jin a , Gen-Tao Zhou b,∗ a
School of Chemistry and Materials, University of Science and Technology of China, Hefei 230026, PR China b CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China Received 26 February 2007; received in revised form 8 November 2007; accepted 14 November 2007
Abstract Using sodium thiosulfate (Na2 S2 O3 ) and zinc nitrate hexahydrate as initial materials, ZnS nanospheres were prepared under natural lights at room temperature. The ZnS nanospheres as a preformed building block were further assembled into hierarchical nanospheres by microwave (MW) irradiation in a high pure nitrogen-protected atmosphere. X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectra (XPS) analytical techniques were used to characterize as-prepared products, revealing that the monodisperse ZnS nanoshperes were assembled with the smaller nanoparticles with a mean diameter of 5 nm. As a result, a novel microwave-induced assembly mechanism was proposed and highlighted. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Semiconductors; Microwave-induced assembly; Zinc sulfide
1. Introduction The preparation of nanostructured materials and the fabrication of mesostructured materials with ordered mesoporosity are the foremost fields of materials chemistry owing to their unique chemical and electronic properties, which have potential applications in the fields of catalysis, electronics, optics, solar energy conversion, magnetic materials, etc. The extended interest is now focused on introducing structural hierarchy and chemical functionality to improve the range of properties and applications in a wide range of materials. There are a large number of strategies available for extending the length scale of structural organization in the inorganic materials. One possible strategy that uses the preformed building blocks to assemble complex multilevel structures has been referred to as Nanotectonics [1]. The synthesis of II–VI semiconductor particles and films has been studied for some time due to their technologically important properties. Recently their assembly into hierarchical structures has been widely studied, which could cause a dramatic improvement in their chemical, electronic and optic properties
[2–5]. Among a variety of techniques, template agents, such as various surfactants, liquid crystals must be used, and afterwards the removal of template agents will be unavoidable in order to get hierarchical nanostructures. Common means for the removal of templates are solvent extraction and high-temperature calcinations. However, for sulfides, only solvent extraction is feasible owe to instability of sulfides under calcination. Therefore, if one can develop a novel method that can overcome the drawback of using template and enhance interaction among nanoparticles in order to accomplish the self-assembly of the preformed nanoparticles, this will significantly simplify experimental procedures and enrich Nanotectonics. Microwave (MW) irradiation is an attractive method for synthesis of nanocrystals and various functional materials [6–12]. MW synthetic method has been demonstrated to be straightforward, fast, efficient and environmental friendly, with high selectivity for the preparation of some compounds [6–12]. Herein we report a facile technique, which can induce the preformed nanoparticles to spontaneously assemble into a hierarchical structure without any template agents. 2. Experimental
∗
Corresponding author. Tel.: +86 551 3600533; fax: +86 551 3603554. E-mail address: [email protected] (G.-T. Zhou).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.010
All starting materials were of analytic reagent grade, and deionized water was used as solvent. Sodium thiosulfate (Na2 S2 O3 ) and zinc nitrate hexahydrate
Q.-Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 164–168 (Zn(NO3 )2 ·6H2 O) were purchased from Shanghai Chemical Reagent Company. All chemicals were used directly without further treatment. A typical experimental process is depicted as follows: A 50 ml of solution with 0.5 M Zn(NO3 )2 and 0.5 M Na2 S2 O3 was prepared using the analytical reagent grade Zn(NO3 )2 ·6H2 O and Na2 S2 O3 . This solution was left for 24 h under static conditions at room temperature to form a white sol. Then, this white sol was transformed to a MW reaction apparatus (a modified domestic MW oven with 900 W and a set of conventional reflux system). The reaction system was purged for 15 min with high pure nitrogen, and then irradiated for a given time under flowing nitrogen. In the post-reaction treatment, the product was centrifuged to obtain white precipitate, and washed with deionized water for several times and absolute ethanol once. The product was dried at 80 ◦ C overnight under vacuum. The phase composition and phase structure of the as-prepared products were analyzed by X-ray diffraction (XRD) using a Bruker D8 Advance Power Xray Diffractometer (Cu K␣ λ = 0.15418 nm) operating at 40 kV/40 mA, with a graphite reflected beam monochromator and variable divergence slits. The scanning rate was 0.02◦ s−1 . The morphologies of the products at different irradiation periods were observed by transmission electron microscope (TEM) using a Hitach H-800. The X-ray photoelectron spectra (XPS) were collected on an ESCALAB-250 X-ray photoelectron spectrometer, using monochromatized Al K␣ X-ray as the excitation source. The binding energy (BE) values obtained in the XPS analysis were corrected by referencing the C 1s peak to 284.60 eV.
3. Results and discussion Fig. 1 shows the typical XRD pattern of the ZnS sample irradiated for 30 min. All of the diffraction peaks can be indexed to a face-centered cubic sphalerite with the lattice parameter ˚ and the space group of F-43m (2 1 6) (JCPDF of a = 5.406 A 05-0566). The significant broadening of the (1 1 1), (2 2 0), and (3 1 1) reflections can be seen from Fig. 1. This indicates that the synthesized ZnS is nanosized. The particle size estimated in
165
Fig. 1. Typical XRD pattern of the ZnS hierarchical nanospheres obtained by 30 min MW irradiation.
terms of Scherrer’s equation, d = 0.94λ/B cos θ, on the 2θ = 28.6◦ (1 1 1) peak gives a value of ca 5 nm. Furthermore, the ZnS sample obtained by shortening microwave irradiation (10 min) endowed with a similar XRD pattern to Fig. 1, indicating that the primary particle size is independent of the MW irradiation time. However, the irradiation time strongly affects the secondary particle size and distribution, as discussed below. The purity and composition of synthesized ZnS were examined by XPS analyses. The typical XPS results are shown in Fig. 2, including (a) the survey spectrum, (b) S 2p, (c) Zn 2p3, and (d) Zn LMM. The peaks of Zn and S, together with those of C and O, can be clearly seen in the survey spectra. The weak peaks of O and C might come from H2 O, O2 and CO2 adsorbed
Fig. 2. Typical XPS spectra of the ZnS nanospheres irradiated for 30 min: (a) survey spectrum, (b) S 2p region spectrum, (c) Zn 2p3 region spectrum, and (d) Zn LMM region spectrum.
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Q.-Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 164–168
on the surface of the ZnS nanostructures in air. From the survey spectrum we can conclude that the product was pretty pure. The BEs are 162.20 eV for S 2p and 1021.11 for Zn 2p3, and the kinetic energy for Zn LMM is 989.29 eV. These results are consistent with the BE values reported by Wagner et al. [13]. Quantification of peaks gave a ration of Zn to S as 1.00:0.93, which is close to the stoichiometry of ZnS. The textures of the products at different irradiation periods were observed by TEM. We found that after 10 min irradiation the product exhibits nanospherical structure with the diameters of 60–70 nm, while a 30 min irradiation produces monodisperse nanospheres with a diameter of ca. 120 nm, as shown in Fig. 3A and B. However, the powder XRD patterns of the products all exhibit very broad peaks, which correspond to a particle size of ca. 5 nm either 10 or 30 min MW irradiation. It appears that this is incompatible with the TEM
observations. Selected area electron diffraction (SAED) pattern (Fig. 3C) shows polycrystalline diffraction rings, which can be indexed as cubic sphalerite. Further magnified TEM observations found that the nanospheres consist of much smaller nanoparticles. Based on our experimental results, the chemical reactions can be depicted as follows: hv
S2 O3 2− + H2 O−→SO4 2− + S2− + H+
(1)
S2− + Zn2+ → ZnS
(2)
(primary colloids)
where the dissolved Na2 S2 O3 first disproportionates into SO4 2− and S2− under the natural lights, and then the S2− combines with Zn2+ to form ZnS colloids. Subsequently, MW irradiation induces the primary colloids of ZnS to assemble into hierarchical nanospheres (secondary particles). A plausible assembly pro-
Fig. 3. TEM images of hierarchical nanospheres irradiated by MW for 10 min (A) or 30 min (B), representative SAED pattern (C) of the hierarchical nanospheres, and the deformed morphologies (D) of the hierarchical nanospheres under the electron beam irradiation of the TEM.
Q.-Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 164–168
Fig. 4. Schematic assembly mechanism of hierarchical ZnS nanospheres induced by MW irradiation.
cess for hierarchical ZnS nanospheres under our experimental conditions is schematically depicted in Fig. 4. Using a low power of 280 W microwave, Zhao et al. [14] directly irradiated the solution of thioacetamide and zinc salt (Zn(Ac)2 , ZnSO4 , or Zn(NO3 )2 ) to, respectively, prepare ZnS nanocrystals. A similar assembly structure to ours was obtained when using Zn(Ac)2 as zinc resource. In the case of ZnSO4 , or Zn(NO3 )2 , however, only nanoparticles were obtained. They explained that the ZnS assembly structure deriving from Zn(Ac)2 can be attributed to preferential crystallization of zinc acetate out of aqueous solution due to its lower solubility in aqueous solution relative to ZnSO4 and Zn(NO3 )2 , and then the crystallized zinc acetate acts as the seeds for the precipitation of ZnS. Simultaneously, the Ac− anions bonded to the surface of the ZnS prevent the growth of the particles, and the hydrogen bonds between Ac− anions make the primary particles aggregate into assembly nanoballs. Yang et al. [15] also synthesized pure and doped ZnS nanoparticles by MW irradiation using the homogeneous precursor solution of Zn(Ac)2 and Na2 S. As a result, their pure ZnS particles occurred with most spheres and moderate agglomerations. In our case, Zn(NO3 )2 as zinc source and Na2 S2 O3 as S source first form ZnS colloids under the natural lights, and then MW irradiation leads to the monodisperse assembled nanoballs. It suggests that the preformed nanosized building blocks before MW irradiation may play crucial role in controlling the formation of hierarchical ZnS nanoballs. Therefore, another assembly mechanism may control the formation of hierarchical ZnS nanoballs. It is well known that a plethora of oxides and a few halides absorb MW energy very efficiently and can be heated up to 1000 K or more [12,16]. Carbon powders and metals are also good MW susceptor and heat up rapidly [12,16,17]. Several chalcogenides, such as Ag2 S, Cu2 S, CuFeS2 , MoS2 , PbS, FeS2 , can strongly couple with microwaves, and quickly are heated to higher temperatures [12]. Some chalcogenides, such as HgS, As2 S3 , ZnS (sphalerite), etc., also interact with microwaves but are not heated so rapidly as those mentioned above [12]. This implies that ZnS colloids can slightly absorb MW irradiation. When these solids that are susceptive to MW are involved in the reaction system, hot spots are locally created on the solid–liquid interfaces, i.e. the effect of hot spots [6]. The formation of the local hot spots may enhance the interactions between the solid particles. It is the weak absorption characteristic of ZnS to MW
167
irradiation that just enhance the interactions among the ZnS nanoparticles, so that the primary colloids of ZnS aggregate into the larger spherical nanostructures. The similar hierarchical nanospheres of Cd1−x Znx Se alloys have also been prepared using MW-assisted polyol method by Gedanken’s group [8], notwithstanding the MW susceptibility of these alloys and their hierarchical nanosphere formation mechanism keep unknown. Liu et al. [18] synthesized radial ZnS nanoribbons by MWassisted solvothermal route, they also highlighted that the unique MW irradiation facilitates the formation of radial spheres of nanoribbons. Furthermore, during conducting TEM analysis, we interestingly observed that the higher magnifications or the prolonging irradiation of electron beam agitated the significant deformation of the assembled nanospheres to form hollow nanospherical structures, as shown in Fig. 3D. This indicates that the formed hierarchical nanospherical structure is loose, rather than a compact structure, so that significant deformation occurs under the strong electron beam irradiation. Such loose secondary nanostructure formation may be attributed to the weak susceptibility of sphalerite ZnS to MW irradiation, otherwise the stronger hot spots or hot surfaces of ZnS particles would lead to tightly assembled nanostructures. As to the formation of the spherical morphology of the hierarchical nanostructures, we speculate that it might be related to the MW mode. The dominant MW mode in a rectangular or circular waveguide is the TE01 - or TE10 -mode, respectively. The electric and magnetic fields associated with TE01 - or TE10 -wave mode form closed loops, and the loops are uniformly separated from adjacent ones [19–21], these closed loops have a cage-like shape and might take the effect of a template [21]. The entire electromagnetic field pattern supplied an ideal environment for the formation of hierarchical nanospheres. As a result, the primary ZnS nanospheres assemble into larger hierarchical nanospheres under the MW irradiation. Although the exact nature of MW interaction with reactants remains somewhat unclear, it is believed that MW heating behavior of reaction system is affected by such the factors as the MW irradiation power, material composition, physical state, dielectric constant, dielectric loss, conduction loss, and so on [9–12,22]. An extensively accepted fact is that MW thermogenesis originates from intercoupling of the dielectric dipoles present in dielectric materials with the applied electric field of microwaves. In contrast to the conventional conducting heating that depends on extra heating source, the thermal of MW heating is produced in the interior of the heated materials, i.e. volumetric heating, and therefore resulting in the rapid heating up of the irradiated materials. Moreover, in semiconductor materials, the conduction loss is dominated by the electronic transport, which increases with both increasing temperature and decreasing band gap [12]. So it can be expected that the ZnS colloids have higher degree of MW absorption than the reaction media, leading to colloidal particles aggregation to form hierarchical structures, whereas primary colloidal size keeps nearly constant even a prolonging irradiation (30 min). As a result, XRD and TEM analyses indicate that the size of primary colloid is independent of the irradiation time.
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4. Conclusion In short, based on the weak susceptibility of sphalerite ZnS to MW irradiation, a facile MW-induced assembly process was developed for the fabrication of hierarchical nanospheres of ZnS. A novel assembly mechanism is proposed. We anticipate that such assembly strategy may be applied to other hierarchical nanostructure assembly of some MW susceptive materials. The extended work is in progress. Acknowledgements This work was supported from the Natural Science Foundation of Anhui (Project No. 050440701) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] S.A. Davis, M. Breulmann, K.H. Rhodes, B.J. Zhang, S. Mann, Chem. Mater. 13 (2001) 3218. [2] R.K. Rana, L.Z. Zhang, J.C. Yu, Y. Mastai, A. Gedanken, Langmuir 19 (2003) 5904. [3] M. Wachhold, K.K. Rangan, S.J.L. Billinge, V. Petkoy, J. Heising, M.G. Kanatzidis, Adv. Mater. 12 (2000) 85. [4] Y.A. Vlasov, N. Yao, D.J. Norris, Adv. Mater. 11 (1999) 165. [5] J.Q. Li, H. Kessler, M. Soulard, L. Khouch, M.H. Tuilier, Adv. Mater. 10 (1998) 946.
[6] M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa, T. Tsuji, Chem. Eur. J. 11 (2005) 440. [7] J.C. Yu, X.L. Hu, Q. Li, L.Z. Zhang, Chem. Commun. (2005) 2704. [8] H. Grisaru, O. Palchik, A. Gedanken, V. Palchik, M.A. Slifkin, A.M. Weiss, J. Mater. Chem. 12 (2002) 339. [9] G.T. Zhou, O. Palchik, I. Nowic, E. Sominski, Y. Koltypin, A. Gedanken, J. Solid State Chem. 177 (2004) 361. [10] G.T. Zhou, O. Palchik, I. Nowic, R. Herber, Y. Koltypin, A. Gedanken, J. Solid State Chem. 177 (2004) 3014. [11] G.T. Zhou, O. Palchik, V.G. Pol, E. Sominski, Y. Koltypin, A. Gedanken, J. Mater. Chem. 13 (2003) 2607. [12] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882. [13] C.D. Wagner, W.W. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin-Elmer Corporation, Wellesley, MA, 1979. [14] Y. Zhao, J.M. Hong, J.J. Zhu, J. Cryst. Growth 270 (2004) 438. [15] H.M. Yang, C.H. Huang, X.H. Su, A.D. Tang, J. Alloys Compd. 402 (2005) 274. [16] A.G. Whittaker, D.M.P. Mingos, J. Microwave Power Electromagn. Energy 29 (1994) 195. [17] D.R. Baghurst, D.M.P. Mingos, J. Chem. Soc. Chem. Commun. (1988) 829. [18] X.Y. Liu, B.Z. Tian, C.Z. Yu, B. Tu, D.Y. Zhao, Chem. Lett. 33 (2004) 522. [19] O.P. Gandhi, Microwave Engineering and Applications, Pergamon Press, Oxford, 1981, pp. 73–100. [20] M.W. Shao, F. Xu, Y.Y. Pen, J. Wu, Q. Li, S.Y. Zhang, Y.T. Qian, New J. Chem. 26 (2002) 1440. [21] M.W. Shao, Q. Li, L.F. Kong, W.C. Yu, Y.T. Qian, J. Phys. Chem. Solids 64 (2003) 1147. [22] G. yang, K. Kong, W.H. Hou, W.J. Yan, J. Phys. Chem. B 109 (2005) 1371.
Materials Chemistry and Physics 109 (2008) 164–168
Formation of hierarchical nanospheres of ZnS induced by microwave irradiation: A highlighted assembly mechanism Qi-Zhi Yao a , Gu Jin a , Gen-Tao Zhou b,∗ a
School of Chemistry and Materials, University of Science and Technology of China, Hefei 230026, PR China b CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China Received 26 February 2007; received in revised form 8 November 2007; accepted 14 November 2007
Abstract Using sodium thiosulfate (Na2 S2 O3 ) and zinc nitrate hexahydrate as initial materials, ZnS nanospheres were prepared under natural lights at room temperature. The ZnS nanospheres as a preformed building block were further assembled into hierarchical nanospheres by microwave (MW) irradiation in a high pure nitrogen-protected atmosphere. X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectra (XPS) analytical techniques were used to characterize as-prepared products, revealing that the monodisperse ZnS nanoshperes were assembled with the smaller nanoparticles with a mean diameter of 5 nm. As a result, a novel microwave-induced assembly mechanism was proposed and highlighted. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Semiconductors; Microwave-induced assembly; Zinc sulfide
1. Introduction The preparation of nanostructured materials and the fabrication of mesostructured materials with ordered mesoporosity are the foremost fields of materials chemistry owing to their unique chemical and electronic properties, which have potential applications in the fields of catalysis, electronics, optics, solar energy conversion, magnetic materials, etc. The extended interest is now focused on introducing structural hierarchy and chemical functionality to improve the range of properties and applications in a wide range of materials. There are a large number of strategies available for extending the length scale of structural organization in the inorganic materials. One possible strategy that uses the preformed building blocks to assemble complex multilevel structures has been referred to as Nanotectonics [1]. The synthesis of II–VI semiconductor particles and films has been studied for some time due to their technologically important properties. Recently their assembly into hierarchical structures has been widely studied, which could cause a dramatic improvement in their chemical, electronic and optic properties
[2–5]. Among a variety of techniques, template agents, such as various surfactants, liquid crystals must be used, and afterwards the removal of template agents will be unavoidable in order to get hierarchical nanostructures. Common means for the removal of templates are solvent extraction and high-temperature calcinations. However, for sulfides, only solvent extraction is feasible owe to instability of sulfides under calcination. Therefore, if one can develop a novel method that can overcome the drawback of using template and enhance interaction among nanoparticles in order to accomplish the self-assembly of the preformed nanoparticles, this will significantly simplify experimental procedures and enrich Nanotectonics. Microwave (MW) irradiation is an attractive method for synthesis of nanocrystals and various functional materials [6–12]. MW synthetic method has been demonstrated to be straightforward, fast, efficient and environmental friendly, with high selectivity for the preparation of some compounds [6–12]. Herein we report a facile technique, which can induce the preformed nanoparticles to spontaneously assemble into a hierarchical structure without any template agents. 2. Experimental
∗
Corresponding author. Tel.: +86 551 3600533; fax: +86 551 3603554. E-mail address: [email protected] (G.-T. Zhou).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.010
All starting materials were of analytic reagent grade, and deionized water was used as solvent. Sodium thiosulfate (Na2 S2 O3 ) and zinc nitrate hexahydrate
Q.-Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 164–168 (Zn(NO3 )2 ·6H2 O) were purchased from Shanghai Chemical Reagent Company. All chemicals were used directly without further treatment. A typical experimental process is depicted as follows: A 50 ml of solution with 0.5 M Zn(NO3 )2 and 0.5 M Na2 S2 O3 was prepared using the analytical reagent grade Zn(NO3 )2 ·6H2 O and Na2 S2 O3 . This solution was left for 24 h under static conditions at room temperature to form a white sol. Then, this white sol was transformed to a MW reaction apparatus (a modified domestic MW oven with 900 W and a set of conventional reflux system). The reaction system was purged for 15 min with high pure nitrogen, and then irradiated for a given time under flowing nitrogen. In the post-reaction treatment, the product was centrifuged to obtain white precipitate, and washed with deionized water for several times and absolute ethanol once. The product was dried at 80 ◦ C overnight under vacuum. The phase composition and phase structure of the as-prepared products were analyzed by X-ray diffraction (XRD) using a Bruker D8 Advance Power Xray Diffractometer (Cu K␣ λ = 0.15418 nm) operating at 40 kV/40 mA, with a graphite reflected beam monochromator and variable divergence slits. The scanning rate was 0.02◦ s−1 . The morphologies of the products at different irradiation periods were observed by transmission electron microscope (TEM) using a Hitach H-800. The X-ray photoelectron spectra (XPS) were collected on an ESCALAB-250 X-ray photoelectron spectrometer, using monochromatized Al K␣ X-ray as the excitation source. The binding energy (BE) values obtained in the XPS analysis were corrected by referencing the C 1s peak to 284.60 eV.
3. Results and discussion Fig. 1 shows the typical XRD pattern of the ZnS sample irradiated for 30 min. All of the diffraction peaks can be indexed to a face-centered cubic sphalerite with the lattice parameter ˚ and the space group of F-43m (2 1 6) (JCPDF of a = 5.406 A 05-0566). The significant broadening of the (1 1 1), (2 2 0), and (3 1 1) reflections can be seen from Fig. 1. This indicates that the synthesized ZnS is nanosized. The particle size estimated in
165
Fig. 1. Typical XRD pattern of the ZnS hierarchical nanospheres obtained by 30 min MW irradiation.
terms of Scherrer’s equation, d = 0.94λ/B cos θ, on the 2θ = 28.6◦ (1 1 1) peak gives a value of ca 5 nm. Furthermore, the ZnS sample obtained by shortening microwave irradiation (10 min) endowed with a similar XRD pattern to Fig. 1, indicating that the primary particle size is independent of the MW irradiation time. However, the irradiation time strongly affects the secondary particle size and distribution, as discussed below. The purity and composition of synthesized ZnS were examined by XPS analyses. The typical XPS results are shown in Fig. 2, including (a) the survey spectrum, (b) S 2p, (c) Zn 2p3, and (d) Zn LMM. The peaks of Zn and S, together with those of C and O, can be clearly seen in the survey spectra. The weak peaks of O and C might come from H2 O, O2 and CO2 adsorbed
Fig. 2. Typical XPS spectra of the ZnS nanospheres irradiated for 30 min: (a) survey spectrum, (b) S 2p region spectrum, (c) Zn 2p3 region spectrum, and (d) Zn LMM region spectrum.
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Q.-Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 164–168
on the surface of the ZnS nanostructures in air. From the survey spectrum we can conclude that the product was pretty pure. The BEs are 162.20 eV for S 2p and 1021.11 for Zn 2p3, and the kinetic energy for Zn LMM is 989.29 eV. These results are consistent with the BE values reported by Wagner et al. [13]. Quantification of peaks gave a ration of Zn to S as 1.00:0.93, which is close to the stoichiometry of ZnS. The textures of the products at different irradiation periods were observed by TEM. We found that after 10 min irradiation the product exhibits nanospherical structure with the diameters of 60–70 nm, while a 30 min irradiation produces monodisperse nanospheres with a diameter of ca. 120 nm, as shown in Fig. 3A and B. However, the powder XRD patterns of the products all exhibit very broad peaks, which correspond to a particle size of ca. 5 nm either 10 or 30 min MW irradiation. It appears that this is incompatible with the TEM
observations. Selected area electron diffraction (SAED) pattern (Fig. 3C) shows polycrystalline diffraction rings, which can be indexed as cubic sphalerite. Further magnified TEM observations found that the nanospheres consist of much smaller nanoparticles. Based on our experimental results, the chemical reactions can be depicted as follows: hv
S2 O3 2− + H2 O−→SO4 2− + S2− + H+
(1)
S2− + Zn2+ → ZnS
(2)
(primary colloids)
where the dissolved Na2 S2 O3 first disproportionates into SO4 2− and S2− under the natural lights, and then the S2− combines with Zn2+ to form ZnS colloids. Subsequently, MW irradiation induces the primary colloids of ZnS to assemble into hierarchical nanospheres (secondary particles). A plausible assembly pro-
Fig. 3. TEM images of hierarchical nanospheres irradiated by MW for 10 min (A) or 30 min (B), representative SAED pattern (C) of the hierarchical nanospheres, and the deformed morphologies (D) of the hierarchical nanospheres under the electron beam irradiation of the TEM.
Q.-Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 164–168
Fig. 4. Schematic assembly mechanism of hierarchical ZnS nanospheres induced by MW irradiation.
cess for hierarchical ZnS nanospheres under our experimental conditions is schematically depicted in Fig. 4. Using a low power of 280 W microwave, Zhao et al. [14] directly irradiated the solution of thioacetamide and zinc salt (Zn(Ac)2 , ZnSO4 , or Zn(NO3 )2 ) to, respectively, prepare ZnS nanocrystals. A similar assembly structure to ours was obtained when using Zn(Ac)2 as zinc resource. In the case of ZnSO4 , or Zn(NO3 )2 , however, only nanoparticles were obtained. They explained that the ZnS assembly structure deriving from Zn(Ac)2 can be attributed to preferential crystallization of zinc acetate out of aqueous solution due to its lower solubility in aqueous solution relative to ZnSO4 and Zn(NO3 )2 , and then the crystallized zinc acetate acts as the seeds for the precipitation of ZnS. Simultaneously, the Ac− anions bonded to the surface of the ZnS prevent the growth of the particles, and the hydrogen bonds between Ac− anions make the primary particles aggregate into assembly nanoballs. Yang et al. [15] also synthesized pure and doped ZnS nanoparticles by MW irradiation using the homogeneous precursor solution of Zn(Ac)2 and Na2 S. As a result, their pure ZnS particles occurred with most spheres and moderate agglomerations. In our case, Zn(NO3 )2 as zinc source and Na2 S2 O3 as S source first form ZnS colloids under the natural lights, and then MW irradiation leads to the monodisperse assembled nanoballs. It suggests that the preformed nanosized building blocks before MW irradiation may play crucial role in controlling the formation of hierarchical ZnS nanoballs. Therefore, another assembly mechanism may control the formation of hierarchical ZnS nanoballs. It is well known that a plethora of oxides and a few halides absorb MW energy very efficiently and can be heated up to 1000 K or more [12,16]. Carbon powders and metals are also good MW susceptor and heat up rapidly [12,16,17]. Several chalcogenides, such as Ag2 S, Cu2 S, CuFeS2 , MoS2 , PbS, FeS2 , can strongly couple with microwaves, and quickly are heated to higher temperatures [12]. Some chalcogenides, such as HgS, As2 S3 , ZnS (sphalerite), etc., also interact with microwaves but are not heated so rapidly as those mentioned above [12]. This implies that ZnS colloids can slightly absorb MW irradiation. When these solids that are susceptive to MW are involved in the reaction system, hot spots are locally created on the solid–liquid interfaces, i.e. the effect of hot spots [6]. The formation of the local hot spots may enhance the interactions between the solid particles. It is the weak absorption characteristic of ZnS to MW
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irradiation that just enhance the interactions among the ZnS nanoparticles, so that the primary colloids of ZnS aggregate into the larger spherical nanostructures. The similar hierarchical nanospheres of Cd1−x Znx Se alloys have also been prepared using MW-assisted polyol method by Gedanken’s group [8], notwithstanding the MW susceptibility of these alloys and their hierarchical nanosphere formation mechanism keep unknown. Liu et al. [18] synthesized radial ZnS nanoribbons by MWassisted solvothermal route, they also highlighted that the unique MW irradiation facilitates the formation of radial spheres of nanoribbons. Furthermore, during conducting TEM analysis, we interestingly observed that the higher magnifications or the prolonging irradiation of electron beam agitated the significant deformation of the assembled nanospheres to form hollow nanospherical structures, as shown in Fig. 3D. This indicates that the formed hierarchical nanospherical structure is loose, rather than a compact structure, so that significant deformation occurs under the strong electron beam irradiation. Such loose secondary nanostructure formation may be attributed to the weak susceptibility of sphalerite ZnS to MW irradiation, otherwise the stronger hot spots or hot surfaces of ZnS particles would lead to tightly assembled nanostructures. As to the formation of the spherical morphology of the hierarchical nanostructures, we speculate that it might be related to the MW mode. The dominant MW mode in a rectangular or circular waveguide is the TE01 - or TE10 -mode, respectively. The electric and magnetic fields associated with TE01 - or TE10 -wave mode form closed loops, and the loops are uniformly separated from adjacent ones [19–21], these closed loops have a cage-like shape and might take the effect of a template [21]. The entire electromagnetic field pattern supplied an ideal environment for the formation of hierarchical nanospheres. As a result, the primary ZnS nanospheres assemble into larger hierarchical nanospheres under the MW irradiation. Although the exact nature of MW interaction with reactants remains somewhat unclear, it is believed that MW heating behavior of reaction system is affected by such the factors as the MW irradiation power, material composition, physical state, dielectric constant, dielectric loss, conduction loss, and so on [9–12,22]. An extensively accepted fact is that MW thermogenesis originates from intercoupling of the dielectric dipoles present in dielectric materials with the applied electric field of microwaves. In contrast to the conventional conducting heating that depends on extra heating source, the thermal of MW heating is produced in the interior of the heated materials, i.e. volumetric heating, and therefore resulting in the rapid heating up of the irradiated materials. Moreover, in semiconductor materials, the conduction loss is dominated by the electronic transport, which increases with both increasing temperature and decreasing band gap [12]. So it can be expected that the ZnS colloids have higher degree of MW absorption than the reaction media, leading to colloidal particles aggregation to form hierarchical structures, whereas primary colloidal size keeps nearly constant even a prolonging irradiation (30 min). As a result, XRD and TEM analyses indicate that the size of primary colloid is independent of the irradiation time.
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