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Using aqueous foam films as template for the synthesis of zinc sulfide nanoparticles
Materials Chemistry and Physics 106 (2007) 120–125
Using aqueous foam films as template for the synthesis of zinc sulfide nanoparticles Ying Li ∗ , Feng Guo, Xiu-juan He, Guo-Qing Zhao, Jia-jia Wu Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, PR China Received 19 August 2006; received in revised form 13 May 2007; accepted 19 May 2007
Abstract A simple and convenient method for the synthesis of ZnS nanoparticles using liquid foams as template is demonstrated in the paper. Two types of foam formed in a specially designed apparatus from aqueous solution of surfactant, respectively, containing C4 H6 O4 Zn and Na2 S contact each other after foam films drain completely and reach nanometer thickness, S2− and Zn2+ ions entrapped by the surfactant layers at the thin borders between the foam bubbles react resulting in the formation of ZnS nanoparticles. Attempts have also been made to control the size of ZnS nanoparticles via changing experimental conditions such as surfactant types, the solution concentration, etc. The effect of experimental parameters on the ZnS nanoparticles growth was studied in detail to investigate the mechanism leading to the synthesis of ZnS nanoparticles. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; ZnS; Synthesis; Surfactant foams
1. Introduction Nanoparticles have been extensively studied owing to their unique physical and biochemical properties [1]. Many different approaches are used for the generation of nanoparticles in order to obtain the required properties and structures [2], some are particularly useful due to their ability to construct highly organized nanoparticles in a controllable manner [3,4]. Both artificial and natural materials such as Langmuir monolayers [5,6], microemulsions [7], agarose gels [8], animal and plant tissues [9,10], and even biopolymers [11,12], have been used as templates. Their narrow size distribution combined with the inherent steric stabilization makes them ideal template for preparing nano-structured materials. In fact, the technology based on these templates, known as nanotechnology, is possible in theory, but its practical realization still requires the solution of quite challenging issues of applied technology, for example, controlling the geometry, the particle size, the morphology of nanoparticles and their assembly into structures performing specific functions and delivering specific effects [13].
∗
Corresponding author. Tel.: +86 531 88362078; fax: +86 531 88564464. E-mail address: [email protected] (Y. Li).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.044
And now, the exciting and hitherto considerably underexploited dynamic biomimetic template for crystal growth is foam lamellae [14]. The individual gas bubbles in the foam contact immediately after its generation and it results the formation of foam films. During foam generation or immediately after its formation, the liquid starts draining out through the plateau borders following the gravity direction, some stable foam films could thin and reach nanometer thickness, which called nano foam films, and hence offers the possibility for foam film used as locations for template crystallization of nanoparticles. The foam provides a high interfacial area of gas bubbles dispersed in the liquid and we believe that it could help realize scaleup. To develop the technology about foam-based nanoparticles synthesis, we describe herein the formation of spherical ZnS nanoparticles within the liquid lamellae of the foam. Two types of foam formed in a specially designed apparatus from aqueous solution of surfactant, respectively, containing zinc acetate dihydrate and sodium sulfide contact each other after foam films drain completely and reach nanometer thickness, S2− and Zn2+ ions entrapped by the surfactant layers at the thin borders between the foam bubbles react resulting in the formation of ZnS nanoparticles, which were validated later by TEM. Different types of surfactant were used as foaming agents, and different experiment conditions, such as surfactant concentration, inorganic salt concentration, are considered to research the effect of foam
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film template on the morphology and the size of ZnS nanoparti cles. 2. Experimental details 2.1. Chemicals Sodium polyoxyethylene fatty alcohol sulfate (AES,) was obtained from Sinolight Shaoxing Chemicals Co., Ltd., chemically pure. Alkyl polyoxyethylene quaternary ammonium chloride (AEAC) was obtained from Henan Titaning Chemical Technology Co., Ltd., chemically pure. TritonX-100 was purchased from Sigma, analytical pure. Betaine was obtained from Zhejiang jucheng Chemical engineering Co., Ltd., chemically pure. Sodium sulfide (Na2 S·9H2 O) was obtained from Tianjin guangcheng Chemical Plant and zinc acetate dihydrate (C4 H6 O4 Zn·2H2 O) was obtained from Tianjin BASF chemistry Co., Ltd., both are chemically pure.
2.2. Characterization of ZnS nanoparticles The ZnS nanoparticles were characterized by XRD (Model: Rigaku, D/MAX-r A, Japan) with Ni-filtered Cu K␣ radiation, and TEM (Model: JEM-100CX II, Japan). Optical absorbance of the ZnS particles was recorded with an UV–vis spectrophotometer (Model: UV-757, China) in the range 200– 500 nm.
2.3. Preparation of ZnS nanoparticles In a typical experiment, a “M”-shaped vitreous column, 50 cm in height and 8 cm in diameter with sintered ceramic discs embedded in except the middle, was used for generation of the foam [15]. The aqueous mixture of surfactant solution that with C4 H6 O4 Zn and with Na2 S were added to the column, respectively, and then the foam built up by injecting nitrogen gas at a fixed rate at the same time through a porous ceramic disc fixed to the bottom of the foam column. The two types of foam moved up at the same speed. After 10 min, they encountered and gradually collapsed, while the nanoparticles formed. The collapsed foam solution containing the zinc sulfide nanoparticles was collected by a beaker at the bottom of the middle column. The solution was then characterized by different techniques.
3. Results and discussion 3.1. Synthesis of ZnS nanoparticles using anionic and cationic surfactant as foaming agents The anionic surfactant sodium polyoxyethylene fatty alcohol sulfate (AES) and cationic surfactant alkyl polyoxyethylene quaternary ammonium chloride (AEAC) were selected as foaming agents of aqueous solutions, respectively, containing C4 H6 O4 Zn and Na2 S. Fig. 1 shows representative TEM micrograph recorded ZnS nanoparticles that were formed by bubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with 10 × 10−3 mol L−1 C4 H6 O4 Zn and 2 × 10−3 mol L−1 AEAC with 10 × 10−3 mol L−1 Na2 S. Only spherical ZnS crystals are seen in the TEM micrograph, which are fairly of uniform size and shape, from 30 to 40 nm, and no adhesion phenomenon was found. XRD method has been used to characterize the phase purity of the product with the results shown in Fig. 2, which displays the overall phase composition purity of the products. The strong diffraction peaks at 2θ = 28.6◦ , 47.7◦ and 56.7◦ are assigned to (1 1 1), (2 2 0) and (3 1 1) planes of ZnS nanoparticles, in which exhibit pure zinc blende crystal structure [16,17].
Fig. 1. TEM micrograph recorded ZnS nanoparticles that are formed by bubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with 10 × 10−3 mol L−1 C4 H6 O4 Zn and 2 × 10−3 mol L−1 AEAC with 10 × 10−3 mol L−1 Na2 S.
Experiments were carried out by changing the concentration of Zn2+ ions and S2− ions while keeping the concentration of AES (1 × 10−3 mol L−1 ) and AEAC (1 × 10−3 mol L−1 ) constant. The typical TEM micrographs shown in Fig. 3(A–D) demonstrated that fairly uniform spherical nanoparticles were formed and the size of ZnS nanoparticles increases with the increase of concentration of Zn2+ ions and S2− ions, which showed that the nanoparticle size could be conveniently controlled by changing inorganic salt concentration.
Fig. 2. X-ray diffraction pattern of ZnS nanoparticles that is formed by bubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with 10 × 10−3 mol L−1 C4 H6 O4 Zn and 2 × 10−3 mol L−1 AEAC with 10 × 10−3 mol L−1 Na2 S.
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Fig. 3. TEM images of ZnS nanoparticles grown in foams formed by bubbling nitrogen gas in an aqueous mixture with different concentration of Zn2+ ions and S2− ions. The inorganic salt concentration: (A) 5 × 10−3 mol L−1 ; (B) 10 × 10−3 mol L−1 ; (C) 15 × 10−3 mol L−1 ; (D) 20 × 10−3 mol L−1 .
3.2. Using one type of surfactant as foaming agents Different from the above experiment, only one type of surfactant was used to be foam stabilizer of both aqueous solution containing C4 H6 O4 Zn and Na2 S. Several types of surfactant are applied to this experiment as foaming agent for the synthesis of ZnS nanoparticles, such as anionic surfactant AES, cationic surfactant AEAC, nonionic surfactant TritonX-100 and amphoteric surfactant Betaine. It is interesting to note that spherical ZnS nanoparticle can be obtained using these surfactants as foaming agents, too. Fig. 4 shows the relation of nanoparticles size distribution and surfactant types, the TEM images were not shown for brevity. From the figure we can learn that at the same surfactant concentration and salt concentration, the particle size is the biggest when using anionic surfactant stabilized foams as template, while it is the smallest when cationic surfactant stabilized foams are used. The particles size is almost equal when nonionic surfactant and amphoteric surfactant are used.
Fig. 4. Relation of nanoparticles size distribution and surfactant type. The surfactant solution concentration is 2 × 10−3 mol L−1 and the salt solution concentration is 10 × 10−3 mol L−1 .
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Fig. 5. UV–vis absorption spectra of ZnS nanoparticles using (A) AEAC, (B) Betaine, (C) AES, and (D) TritonX-100 stabilized foams as template.
UV–vis absorption spectra of ZnS nanoparticles obtained from these four types of surfactant stabilized foams have been presented in Fig. 5A–D, respectively. An exciting observation is that the absorbance of nanoparticles changes with the size of ZnS nanoparticles got from foams formed by different types of surfactant, and has a well parallelism. The absorption peaks all appear at about 310 nm and have a modest blue-shift (about 35 nm) compared to corresponding peak from bulk ZnS (345 nm) [18,19], which could be attributed to quantum size effects of the ZnS nanoparticles [20,21]. 3.3. Synthesis of smaller ZnS nanoparticles with size less than 10 nm using foam film template In this experiment, anionic surfactant sodium polyoxyethylene fatty alcohol sulfate (AES) was chosen to be foaming agents. Fig. 6 shows representative TEM micrograph of ZnS nanoparticles that were formed by bubbling nitrogen gas in an aqueous mixture aqueous mixture of 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 C4 H6 O4 Zn and 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 Na2 S. Only spherical ZnS crystals are seen in the micrograph and the particles size are about 8 nm. So smaller nano particles with size less than 10 nm could be easily synthesized using foams as template. 3.4. Effect of surfactant concentration on the nanoparticle size Experiments have been done to investigate the effect of surfactant concentration on the nanoparticle size. Similar results have been got for foams formed by different types of surfactant, as shown in Fig. 7(A–D), which are, respectively, the TEM micrographs of ZnS nanoparticles formed at different surfactant solution of AES and AEAC. Also, only spherical ZnS crystals
Fig. 6. TEM micrograph of ZnS nanoparticles that are formed by bubbling nitrogen gas in an aqueous mixture of 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 C4 H6 O4 Zn and 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 Na2 S.
are seen in these TEM micrographs, and the particles size almost has no change. 3.5. Mechanism discussion The foam may be considered as a temporary dilute dispersion of bubbles in the liquid, and on ageing, the structure gradually changes and the bubbles transform into polyhedral gas cells with thin flat walls formed by surfactant layers. To maintain mechanical equilibrium within the structure, the film walls drain until they meet at 120◦ [22]. In Scheme 1 we illustrate a possible mechanism operated in the foam that could explain the process of ZnS nanoparticles formation. The region ‘A’ in Scheme 1 occurs between two neighboring foams is called the plateau borders while the region ‘P’ where such plateau borders meet is called the plateau junctions. Region ‘A’ shows the liquid lamellae within the plateau borders of the foam wherein the thickness of this region is very small, and region ‘P’ shows a bigger reaction space. It is likely that the substrate ions could be entrapped in both the two regions, which could be called the nanoreactor where ZnS nanoparticles are formed. The thickness of region ‘A’ and the size of region ‘P’ are different when different types of surfactant as foaming agents are used at the same draining time, and the variation of charges of head group of different kinds of surfactant effect the entrapment of the inorganic ions by the electric interaction, which could be both important factors used to control size of the particle. All the above experiment results testified that region ‘P’ might be the main nanoreactor at the occasions using completely drained foam films as template to synthesize nanoparticles.
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Fig. 7. TEM images of ZnS nanoparticles grown in foams formed by bubbling nitrogen gas in an aqueous mixture with different surfactant concentration of AES and AEAC. The surfactant concentration: (A) 2 × 10−3 mol L−1 ; (B) 4 × 10−3 mol L−1 ; (C) 6 × 10−3 mol L−1 ; (D) 8 × 10−3 mol L−1 .
4. Conclusions
Scheme 1. Different regions in the foam to be involved in the synthesis of ZnS nanoparticles.
In conclusion, an extremely simple method that uses foam, stabilized by surfactants, as template for the synthesis of zinc sulfide nanoparticles has been described. This is probably the first report for a completely foam-based synthesis of zinc sulfide nanoparticles at room temperature. The moving up of two types of foam containing C4 H6 O4 Zn and Na2 S and their encounter results in the formation of unidispersed spherical zinc sulfide nanoparticles. The size of nanoparticles could be controlled by changing the concentration of inorganic salt, and smaller ZnS nanoparticles could be easily got using foam films as template. There is no obvious effect on the size of nanoparticles by changing surfactant concentration, and region ‘P’ might be the main nanoreactor at the occasions using completely drained foam films as template to synthesize nanoparticles.
Y. Li et al. / Materials Chemistry and Physics 106 (2007) 120–125
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T. Tsukatani, H. Fujihara, Langmuir 21 (2005) 12093–12095. R.W. Siegel, Chem. Eng. News (Sci./Technol.) 77 (1999) 25. S. Mann, Angew. Chem. Int. Ed. 39 (2000) 3392. W.L. Murphy, D.J. Mooney, J. Am. Chem. Soc. 124 (2002) 1910. E. Loste, E. Diaz-Marti, A. Zarbakhsh, F.C. Meldrum, Langmuir 19 (2003) 2830. D. Rautaray, S.R. Sainkar, N.R. Pavaskar, M. Sastry, Cryst. Eng. Commun. 4 (2002) 626. M. Li, S. Mann, Langmuir 16 (2000) 7088. D. Yang, L. Qi, J. Ma, Chem. Commun. 10 (2003) 1180. W. Ogasawara, W. Shenton, S.A. Davis, S. Mann, Chem. Mater. 12 (2000) 2835. Y. Shin, J. Liu, J.H. Chang, Z. Nie, G.J. Exarhos, Adv. Mater. 13 (2001) 728.
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E. Braun, Y. Eichen, U. Sivan, G. BenYoseph, Nature 391 (1998) 775. J.J. Storhoff, C.A. Mirkin, Chem. Rev. 99 (1999) 1849. G. Bognolo, Adv. Colloid Interface Sci. 106 (2003) 169. B.-D. Dong, J.J. Cilliers, R.J. Davey, J. Garside, E.T. Woodburn, J. Am. Chem. Soc. 120 (1998) 1625. F. Guo, Y. Li, H.-X. Xu, G.-Q. Zhao, X.-J. He, Mater. Lett. (available online 28 March 2007), in press. S.H. Yu, Y.S. Wu, J. Yang, et al., Chem. Mater. 9 (1998) 2312. S. Chen, W. Liu, Mater. Res. Bull. 36 (2001) 137. C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1986. C. Jiang, W. Zhang, G. Zou, W. Yu, Y. Qian, Mater. Chem. Phys. 103 (2007) 24–27. T. Trindade, P. O’Brien, Chem. Mater. 9 (1997) 523. M.K. Naskar, A. Patra, M. Chatterjee, J. Colloid Interface Sci. 297 (2006) 271–275. R.J. Pugh, Adv. Colloid Interface Sci. (2005) 114–115, 239–251.
Using aqueous foam films as template for the synthesis of zinc sulfide nanoparticles Ying Li ∗ , Feng Guo, Xiu-juan He, Guo-Qing Zhao, Jia-jia Wu Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, PR China Received 19 August 2006; received in revised form 13 May 2007; accepted 19 May 2007
Abstract A simple and convenient method for the synthesis of ZnS nanoparticles using liquid foams as template is demonstrated in the paper. Two types of foam formed in a specially designed apparatus from aqueous solution of surfactant, respectively, containing C4 H6 O4 Zn and Na2 S contact each other after foam films drain completely and reach nanometer thickness, S2− and Zn2+ ions entrapped by the surfactant layers at the thin borders between the foam bubbles react resulting in the formation of ZnS nanoparticles. Attempts have also been made to control the size of ZnS nanoparticles via changing experimental conditions such as surfactant types, the solution concentration, etc. The effect of experimental parameters on the ZnS nanoparticles growth was studied in detail to investigate the mechanism leading to the synthesis of ZnS nanoparticles. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; ZnS; Synthesis; Surfactant foams
1. Introduction Nanoparticles have been extensively studied owing to their unique physical and biochemical properties [1]. Many different approaches are used for the generation of nanoparticles in order to obtain the required properties and structures [2], some are particularly useful due to their ability to construct highly organized nanoparticles in a controllable manner [3,4]. Both artificial and natural materials such as Langmuir monolayers [5,6], microemulsions [7], agarose gels [8], animal and plant tissues [9,10], and even biopolymers [11,12], have been used as templates. Their narrow size distribution combined with the inherent steric stabilization makes them ideal template for preparing nano-structured materials. In fact, the technology based on these templates, known as nanotechnology, is possible in theory, but its practical realization still requires the solution of quite challenging issues of applied technology, for example, controlling the geometry, the particle size, the morphology of nanoparticles and their assembly into structures performing specific functions and delivering specific effects [13].
∗
Corresponding author. Tel.: +86 531 88362078; fax: +86 531 88564464. E-mail address: [email protected] (Y. Li).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.044
And now, the exciting and hitherto considerably underexploited dynamic biomimetic template for crystal growth is foam lamellae [14]. The individual gas bubbles in the foam contact immediately after its generation and it results the formation of foam films. During foam generation or immediately after its formation, the liquid starts draining out through the plateau borders following the gravity direction, some stable foam films could thin and reach nanometer thickness, which called nano foam films, and hence offers the possibility for foam film used as locations for template crystallization of nanoparticles. The foam provides a high interfacial area of gas bubbles dispersed in the liquid and we believe that it could help realize scaleup. To develop the technology about foam-based nanoparticles synthesis, we describe herein the formation of spherical ZnS nanoparticles within the liquid lamellae of the foam. Two types of foam formed in a specially designed apparatus from aqueous solution of surfactant, respectively, containing zinc acetate dihydrate and sodium sulfide contact each other after foam films drain completely and reach nanometer thickness, S2− and Zn2+ ions entrapped by the surfactant layers at the thin borders between the foam bubbles react resulting in the formation of ZnS nanoparticles, which were validated later by TEM. Different types of surfactant were used as foaming agents, and different experiment conditions, such as surfactant concentration, inorganic salt concentration, are considered to research the effect of foam
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film template on the morphology and the size of ZnS nanoparti cles. 2. Experimental details 2.1. Chemicals Sodium polyoxyethylene fatty alcohol sulfate (AES,) was obtained from Sinolight Shaoxing Chemicals Co., Ltd., chemically pure. Alkyl polyoxyethylene quaternary ammonium chloride (AEAC) was obtained from Henan Titaning Chemical Technology Co., Ltd., chemically pure. TritonX-100 was purchased from Sigma, analytical pure. Betaine was obtained from Zhejiang jucheng Chemical engineering Co., Ltd., chemically pure. Sodium sulfide (Na2 S·9H2 O) was obtained from Tianjin guangcheng Chemical Plant and zinc acetate dihydrate (C4 H6 O4 Zn·2H2 O) was obtained from Tianjin BASF chemistry Co., Ltd., both are chemically pure.
2.2. Characterization of ZnS nanoparticles The ZnS nanoparticles were characterized by XRD (Model: Rigaku, D/MAX-r A, Japan) with Ni-filtered Cu K␣ radiation, and TEM (Model: JEM-100CX II, Japan). Optical absorbance of the ZnS particles was recorded with an UV–vis spectrophotometer (Model: UV-757, China) in the range 200– 500 nm.
2.3. Preparation of ZnS nanoparticles In a typical experiment, a “M”-shaped vitreous column, 50 cm in height and 8 cm in diameter with sintered ceramic discs embedded in except the middle, was used for generation of the foam [15]. The aqueous mixture of surfactant solution that with C4 H6 O4 Zn and with Na2 S were added to the column, respectively, and then the foam built up by injecting nitrogen gas at a fixed rate at the same time through a porous ceramic disc fixed to the bottom of the foam column. The two types of foam moved up at the same speed. After 10 min, they encountered and gradually collapsed, while the nanoparticles formed. The collapsed foam solution containing the zinc sulfide nanoparticles was collected by a beaker at the bottom of the middle column. The solution was then characterized by different techniques.
3. Results and discussion 3.1. Synthesis of ZnS nanoparticles using anionic and cationic surfactant as foaming agents The anionic surfactant sodium polyoxyethylene fatty alcohol sulfate (AES) and cationic surfactant alkyl polyoxyethylene quaternary ammonium chloride (AEAC) were selected as foaming agents of aqueous solutions, respectively, containing C4 H6 O4 Zn and Na2 S. Fig. 1 shows representative TEM micrograph recorded ZnS nanoparticles that were formed by bubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with 10 × 10−3 mol L−1 C4 H6 O4 Zn and 2 × 10−3 mol L−1 AEAC with 10 × 10−3 mol L−1 Na2 S. Only spherical ZnS crystals are seen in the TEM micrograph, which are fairly of uniform size and shape, from 30 to 40 nm, and no adhesion phenomenon was found. XRD method has been used to characterize the phase purity of the product with the results shown in Fig. 2, which displays the overall phase composition purity of the products. The strong diffraction peaks at 2θ = 28.6◦ , 47.7◦ and 56.7◦ are assigned to (1 1 1), (2 2 0) and (3 1 1) planes of ZnS nanoparticles, in which exhibit pure zinc blende crystal structure [16,17].
Fig. 1. TEM micrograph recorded ZnS nanoparticles that are formed by bubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with 10 × 10−3 mol L−1 C4 H6 O4 Zn and 2 × 10−3 mol L−1 AEAC with 10 × 10−3 mol L−1 Na2 S.
Experiments were carried out by changing the concentration of Zn2+ ions and S2− ions while keeping the concentration of AES (1 × 10−3 mol L−1 ) and AEAC (1 × 10−3 mol L−1 ) constant. The typical TEM micrographs shown in Fig. 3(A–D) demonstrated that fairly uniform spherical nanoparticles were formed and the size of ZnS nanoparticles increases with the increase of concentration of Zn2+ ions and S2− ions, which showed that the nanoparticle size could be conveniently controlled by changing inorganic salt concentration.
Fig. 2. X-ray diffraction pattern of ZnS nanoparticles that is formed by bubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with 10 × 10−3 mol L−1 C4 H6 O4 Zn and 2 × 10−3 mol L−1 AEAC with 10 × 10−3 mol L−1 Na2 S.
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Fig. 3. TEM images of ZnS nanoparticles grown in foams formed by bubbling nitrogen gas in an aqueous mixture with different concentration of Zn2+ ions and S2− ions. The inorganic salt concentration: (A) 5 × 10−3 mol L−1 ; (B) 10 × 10−3 mol L−1 ; (C) 15 × 10−3 mol L−1 ; (D) 20 × 10−3 mol L−1 .
3.2. Using one type of surfactant as foaming agents Different from the above experiment, only one type of surfactant was used to be foam stabilizer of both aqueous solution containing C4 H6 O4 Zn and Na2 S. Several types of surfactant are applied to this experiment as foaming agent for the synthesis of ZnS nanoparticles, such as anionic surfactant AES, cationic surfactant AEAC, nonionic surfactant TritonX-100 and amphoteric surfactant Betaine. It is interesting to note that spherical ZnS nanoparticle can be obtained using these surfactants as foaming agents, too. Fig. 4 shows the relation of nanoparticles size distribution and surfactant types, the TEM images were not shown for brevity. From the figure we can learn that at the same surfactant concentration and salt concentration, the particle size is the biggest when using anionic surfactant stabilized foams as template, while it is the smallest when cationic surfactant stabilized foams are used. The particles size is almost equal when nonionic surfactant and amphoteric surfactant are used.
Fig. 4. Relation of nanoparticles size distribution and surfactant type. The surfactant solution concentration is 2 × 10−3 mol L−1 and the salt solution concentration is 10 × 10−3 mol L−1 .
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Fig. 5. UV–vis absorption spectra of ZnS nanoparticles using (A) AEAC, (B) Betaine, (C) AES, and (D) TritonX-100 stabilized foams as template.
UV–vis absorption spectra of ZnS nanoparticles obtained from these four types of surfactant stabilized foams have been presented in Fig. 5A–D, respectively. An exciting observation is that the absorbance of nanoparticles changes with the size of ZnS nanoparticles got from foams formed by different types of surfactant, and has a well parallelism. The absorption peaks all appear at about 310 nm and have a modest blue-shift (about 35 nm) compared to corresponding peak from bulk ZnS (345 nm) [18,19], which could be attributed to quantum size effects of the ZnS nanoparticles [20,21]. 3.3. Synthesis of smaller ZnS nanoparticles with size less than 10 nm using foam film template In this experiment, anionic surfactant sodium polyoxyethylene fatty alcohol sulfate (AES) was chosen to be foaming agents. Fig. 6 shows representative TEM micrograph of ZnS nanoparticles that were formed by bubbling nitrogen gas in an aqueous mixture aqueous mixture of 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 C4 H6 O4 Zn and 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 Na2 S. Only spherical ZnS crystals are seen in the micrograph and the particles size are about 8 nm. So smaller nano particles with size less than 10 nm could be easily synthesized using foams as template. 3.4. Effect of surfactant concentration on the nanoparticle size Experiments have been done to investigate the effect of surfactant concentration on the nanoparticle size. Similar results have been got for foams formed by different types of surfactant, as shown in Fig. 7(A–D), which are, respectively, the TEM micrographs of ZnS nanoparticles formed at different surfactant solution of AES and AEAC. Also, only spherical ZnS crystals
Fig. 6. TEM micrograph of ZnS nanoparticles that are formed by bubbling nitrogen gas in an aqueous mixture of 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 C4 H6 O4 Zn and 10 × 10−3 mol L−1 AES with 2 × 10−3 mol L−1 Na2 S.
are seen in these TEM micrographs, and the particles size almost has no change. 3.5. Mechanism discussion The foam may be considered as a temporary dilute dispersion of bubbles in the liquid, and on ageing, the structure gradually changes and the bubbles transform into polyhedral gas cells with thin flat walls formed by surfactant layers. To maintain mechanical equilibrium within the structure, the film walls drain until they meet at 120◦ [22]. In Scheme 1 we illustrate a possible mechanism operated in the foam that could explain the process of ZnS nanoparticles formation. The region ‘A’ in Scheme 1 occurs between two neighboring foams is called the plateau borders while the region ‘P’ where such plateau borders meet is called the plateau junctions. Region ‘A’ shows the liquid lamellae within the plateau borders of the foam wherein the thickness of this region is very small, and region ‘P’ shows a bigger reaction space. It is likely that the substrate ions could be entrapped in both the two regions, which could be called the nanoreactor where ZnS nanoparticles are formed. The thickness of region ‘A’ and the size of region ‘P’ are different when different types of surfactant as foaming agents are used at the same draining time, and the variation of charges of head group of different kinds of surfactant effect the entrapment of the inorganic ions by the electric interaction, which could be both important factors used to control size of the particle. All the above experiment results testified that region ‘P’ might be the main nanoreactor at the occasions using completely drained foam films as template to synthesize nanoparticles.
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Fig. 7. TEM images of ZnS nanoparticles grown in foams formed by bubbling nitrogen gas in an aqueous mixture with different surfactant concentration of AES and AEAC. The surfactant concentration: (A) 2 × 10−3 mol L−1 ; (B) 4 × 10−3 mol L−1 ; (C) 6 × 10−3 mol L−1 ; (D) 8 × 10−3 mol L−1 .
4. Conclusions
Scheme 1. Different regions in the foam to be involved in the synthesis of ZnS nanoparticles.
In conclusion, an extremely simple method that uses foam, stabilized by surfactants, as template for the synthesis of zinc sulfide nanoparticles has been described. This is probably the first report for a completely foam-based synthesis of zinc sulfide nanoparticles at room temperature. The moving up of two types of foam containing C4 H6 O4 Zn and Na2 S and their encounter results in the formation of unidispersed spherical zinc sulfide nanoparticles. The size of nanoparticles could be controlled by changing the concentration of inorganic salt, and smaller ZnS nanoparticles could be easily got using foam films as template. There is no obvious effect on the size of nanoparticles by changing surfactant concentration, and region ‘P’ might be the main nanoreactor at the occasions using completely drained foam films as template to synthesize nanoparticles.
Y. Li et al. / Materials Chemistry and Physics 106 (2007) 120–125
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T. Tsukatani, H. Fujihara, Langmuir 21 (2005) 12093–12095. R.W. Siegel, Chem. Eng. News (Sci./Technol.) 77 (1999) 25. S. Mann, Angew. Chem. Int. Ed. 39 (2000) 3392. W.L. Murphy, D.J. Mooney, J. Am. Chem. Soc. 124 (2002) 1910. E. Loste, E. Diaz-Marti, A. Zarbakhsh, F.C. Meldrum, Langmuir 19 (2003) 2830. D. Rautaray, S.R. Sainkar, N.R. Pavaskar, M. Sastry, Cryst. Eng. Commun. 4 (2002) 626. M. Li, S. Mann, Langmuir 16 (2000) 7088. D. Yang, L. Qi, J. Ma, Chem. Commun. 10 (2003) 1180. W. Ogasawara, W. Shenton, S.A. Davis, S. Mann, Chem. Mater. 12 (2000) 2835. Y. Shin, J. Liu, J.H. Chang, Z. Nie, G.J. Exarhos, Adv. Mater. 13 (2001) 728.
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