Materials Chemistry and Physics 106 (2007) 120–125
Using aqueous foam films as template for the synthesisof zinc sulfide nanoparticles
Ying Li ∗, Feng Guo, Xiu-juan He, Guo-Qing Zhao, Jia-jia WuKey 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
bstract
A simple and convenient method for the synthesis of ZnS nanoparticles using liquid foams as template is demonstrated in the paper. Two types ofoam formed in a specially designed apparatus from aqueous solution of surfactant, respectively, containing C4H6O4Zn and Na2S contact each otherfter foam films drain completely and reach nanometer thickness, S2− and Zn2+ ions entrapped by the surfactant layers at the thin borders between
he foam bubbles react resulting in the formation of ZnS nanoparticles. Attempts have also been made to control the size of ZnS nanoparticlesia changing experimental conditions such as surfactant types, the solution concentration, etc. The effect of experimental parameters on the ZnSanoparticles growth was studied in detail to investigate the mechanism leading to the synthesis of ZnS nanoparticles. 2007 Elsevier B.V. All rights reserved.
Nanoparticles have been extensively studied owing to theirnique physical and biochemical properties [1]. Many differ-nt approaches are used for the generation of nanoparticles inrder to obtain the required properties and structures [2], somere particularly useful due to their ability to construct highlyrganized nanoparticles in a controllable manner [3,4]. Both arti-cial and natural materials such as Langmuir monolayers [5,6],icroemulsions [7], agarose gels [8], animal and plant tissues
9,10], and even biopolymers [11,12], have been used as tem-lates. Their narrow size distribution combined with the inherentteric stabilization makes them ideal template for preparingano-structured materials. In fact, the technology based on theseemplates, known as nanotechnology, is possible in theory, butts practical realization still requires the solution of quite chal-
enging issues of applied technology, for example, controllinghe geometry, the particle size, the morphology of nanoparticlesnd their assembly into structures performing specific functionsnd delivering specific effects [13].
And now, the exciting and hitherto considerably underex-loited dynamic biomimetic template for crystal growth is foamamellae [14]. The individual gas bubbles in the foam contactmmediately after its generation and it results the formation ofoam films. During foam generation or immediately after itsormation, the liquid starts draining out through the plateau bor-ers following the gravity direction, some stable foam filmsould thin and reach nanometer thickness, which called nanooam films, and hence offers the possibility for foam film useds locations for template crystallization of nanoparticles. Theoam provides a high interfacial area of gas bubbles dispersedn the liquid and we believe that it could help realize scale-p.
To develop the technology about foam-based nanoparticlesynthesis, we describe herein the formation of spherical ZnSanoparticles within the liquid lamellae of the foam. Two typesf foam formed in a specially designed apparatus from aqueousolution of surfactant, respectively, containing zinc acetate dihy-rate and sodium sulfide contact each other after foam films drainompletely and reach nanometer thickness, S2− and Zn2+ ionsntrapped by the surfactant layers at the thin borders between theoam bubbles react resulting in the formation of ZnS nanopar-
icles, which were validated later by TEM. Different types ofurfactant were used as foaming agents, and different exper-ment conditions, such as surfactant concentration, inorganicalt concentration, are considered to research the effect of foam
ene quaternary ammonium chloride (AEAC) was obtained from Henan Titaninghemical Technology Co., Ltd., chemically pure. TritonX-100 was purchased
rom Sigma, analytical pure. Betaine was obtained from Zhejiang jucheng Chem-cal engineering Co., Ltd., chemically pure. Sodium sulfide (Na2S·9H2O) wasbtained from Tianjin guangcheng Chemical Plant and zinc acetate dihydrateC4H6O4Zn·2H2O) was obtained from Tianjin BASF chemistry Co., Ltd., bothre chemically pure.
.2. Characterization of ZnS nanoparticles
The ZnS nanoparticles were characterized by XRD (Model: Rigaku,/MAX-r A, Japan) with Ni-filtered Cu K� radiation, and TEM (Model:
EM-100CX II, Japan). Optical absorbance of the ZnS particles was recordedith an UV–vis spectrophotometer (Model: UV-757, China) in the range 200–00 nm.
.3. Preparation of ZnS nanoparticles
In a typical experiment, a “M”-shaped vitreous column, 50 cm in height andcm in diameter with sintered ceramic discs embedded in except the middle, wassed for generation of the foam [15]. The aqueous mixture of surfactant solutionhat with C4H6O4Zn and with Na2S were added to the column, respectively, andhen the foam built up by injecting nitrogen gas at a fixed rate at the same timehrough a porous ceramic disc fixed to the bottom of the foam column. Thewo types of foam moved up at the same speed. After 10 min, they encounterednd gradually collapsed, while the nanoparticles formed. The collapsed foamolution containing the zinc sulfide nanoparticles was collected by a beaker at theottom of the middle column. The solution was then characterized by differentechniques.
. Results and discussion
.1. Synthesis of ZnS nanoparticles using anionic andationic surfactant as foaming agents
The anionic surfactant sodium polyoxyethylene fatty alcoholulfate (AES) and cationic surfactant alkyl polyoxyethylene qua-ernary ammonium chloride (AEAC) were selected as foaminggents of aqueous solutions, respectively, containing C4H6O4Znnd Na2S. Fig. 1 shows representative TEM micrographecorded ZnS nanoparticles that were formed by bubbling nitro-en gas in an aqueous mixture of 2 × 10−3 mol L−1 AES with0 × 10−3 mol L−1 C4H6O4Zn and 2 × 10−3 mol L−1 AEACith 10 × 10−3 mol L−1 Na2S. Only spherical ZnS crystals are
een in the TEM micrograph, which are fairly of uniform sizend shape, from 30 to 40 nm, and no adhesion phenomenonas found. XRD method has been used to characterize thehase purity of the product with the results shown in Fig. 2,hich displays the overall phase composition purity of the
roducts. The strong diffraction peaks at 2θ = 28.6◦, 47.7◦ and6.7◦ are assigned to (1 1 1), (2 2 0) and (3 1 1) planes of ZnSanoparticles, in which exhibit pure zinc blende crystal structure16,17].
Experiments were carried out by changing the concentrationf Zn2+ ions and S2− ions while keeping the concentration ofES (1 × 10−3 mol L−1) and AEAC (1 × 10−3 mol L−1) con-
tant. The typical TEM micrographs shown in Fig. 3(A–D)emonstrated that fairly uniform spherical nanoparticles wereormed and the size of ZnS nanoparticles increases withhe increase of concentration of Zn2+ ions and S2− ions,hich showed that the nanoparticle size could be con-
ig. 2. X-ray diffraction pattern of ZnS nanoparticles that is formed byubbling nitrogen gas in an aqueous mixture of 2 × 10−3 mol L−1 AESith 10 × 10−3 mol L−1 C4H6O4Zn and 2 × 10−3 mol L−1 AEAC with0 × 10−3 mol L−1 Na2S.
122 Y. Li et al. / Materials Chemistry and Physics 106 (2007) 120–125
F itrogen gas in an aqueous mixture with different concentration of Zn2+ ions and S2−i l L−1; (C) 15 × 10−3mol L−1; (D) 20 × 10−3mol L−1.
3
fcaZfsnatffttbn
ig. 3. TEM images of ZnS nanoparticles grown in foams formed by bubbling nons. The inorganic salt concentration: (A) 5 × 10−3mol L−1; (B) 10 × 10−3mo
.2. Using one type of surfactant as foaming agents
Different from the above experiment, only one type of sur-actant was used to be foam stabilizer of both aqueous solutionontaining C4H6O4Zn and Na2S. Several types of surfactant arepplied to this experiment as foaming agent for the synthesis ofnS nanoparticles, such as anionic surfactant AES, cationic sur-
actant AEAC, nonionic surfactant TritonX-100 and amphotericurfactant Betaine. It is interesting to note that spherical ZnSanoparticle can be obtained using these surfactants as foaminggents, too. Fig. 4 shows the relation of nanoparticles size dis-ribution and surfactant types, the TEM images were not shownor brevity. From the figure we can learn that at the same sur-actant concentration and salt concentration, the particle size is
he biggest when using anionic surfactant stabilized foams asemplate, while it is the smallest when cationic surfactant sta-ilized foams are used. The particles size is almost equal whenonionic surfactant and amphoteric surfactant are used.
Fig. 4. Relation of nanoparticles size distribution and surfactant type. Thesurfactant solution concentration is 2 × 10−3 mol L−1 and the salt solution con-centration is 10 × 10−3 mol L−1.
Y. Li et al. / Materials Chemistry and Physics 106 (2007) 120–125 123
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Fig. 6. TEM micrograph of ZnS nanoparticles that are formed by bub-b −3 −1
w2
ah
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octitmoobclotiwasaoelectric interaction, which could be both important factors used
ig. 5. UV–vis absorption spectra of ZnS nanoparticles using (A) AEAC, (B)etaine, (C) AES, and (D) TritonX-100 stabilized foams as template.
UV–vis absorption spectra of ZnS nanoparticles obtainedrom these four types of surfactant stabilized foams have beenresented in Fig. 5A–D, respectively. An exciting observa-ion is that the absorbance of nanoparticles changes with theize of ZnS nanoparticles got from foams formed by dif-erent types of surfactant, and has a well parallelism. Thebsorption peaks all appear at about 310 nm and have aodest blue-shift (about 35 nm) compared to correspond-
ng peak from bulk ZnS (345 nm) [18,19], which could bettributed to quantum size effects of the ZnS nanoparticles20,21].
.3. Synthesis of smaller ZnS nanoparticles with size lesshan 10 nm using foam film template
In this experiment, anionic surfactant sodium polyoxyethy-ene fatty alcohol sulfate (AES) was chosen to be foaminggents. Fig. 6 shows representative TEM micrograph of ZnSanoparticles that were formed by bubbling nitrogen gas inn aqueous mixture aqueous mixture of 10 × 10−3 mol L−1
ES with 2 × 10−3 mol L−1 C4H6O4Zn and 10 × 10−3 mol L−1
ES with 2 × 10−3 mol L−1 Na2S. Only spherical ZnS crys-als are seen in the micrograph and the particles size arebout 8 nm. So smaller nano particles with size less than0 nm could be easily synthesized using foams as temp-ate.
.4. Effect of surfactant concentration on the nanoparticleize
Experiments have been done to investigate the effect of sur-actant concentration on the nanoparticle size. Similar results
ave been got for foams formed by different types of surfac-ant, as shown in Fig. 7(A–D), which are, respectively, the TEM
icrographs of ZnS nanoparticles formed at different surfactantolution of AES and AEAC. Also, only spherical ZnS crystals
ttos
ling nitrogen gas in an aqueous mixture of 10 × 10 mol L AESith 2 × 10−3 mol L−1 C4H6O4Zn and 10 × 10−3 mol L−1 AES with× 10−3 mol L−1 Na2S.
re seen in these TEM micrographs, and the particles size almostas no change.
.5. Mechanism discussion
The foam may be considered as a temporary dilute dispersionf bubbles in the liquid, and on ageing, the structure graduallyhanges and the bubbles transform into polyhedral gas cells withhin flat walls formed by surfactant layers. To maintain mechan-cal equilibrium within the structure, the film walls drain untilhey meet at 120◦ [22]. In Scheme 1 we illustrate a possible
echanism operated in the foam that could explain the processf ZnS nanoparticles formation. The region ‘A’ in Scheme 1ccurs between two neighboring foams is called the plateauorders while the region ‘P’ where such plateau borders meet isalled the plateau junctions. Region ‘A’ shows the liquid lamel-ae within the plateau borders of the foam wherein the thicknessf this region is very small, and region ‘P’ shows a bigger reac-ion space. It is likely that the substrate ions could be entrappedn both the two regions, which could be called the nanoreactorhere ZnS nanoparticles are formed. The thickness of region ‘A’
nd the size of region ‘P’ are different when different types ofurfactant as foaming agents are used at the same draining time,nd the variation of charges of head group of different kindsf surfactant effect the entrapment of the inorganic ions by the
o control size of the particle. All the above experiment resultsestified that region ‘P’ might be the main nanoreactor at theccasions using completely drained foam films as template toynthesize nanoparticles.
124 Y. Li et al. / Materials Chemistry and Physics 106 (2007) 120–125
Fig. 7. TEM images of ZnS nanoparticles grown in foams formed by bubbling nitrogen gas in an aqueous mixture with different surfactant concentration of AESand AEAC. The surfactant concentration: (A) 2 × 10−3 mol L−1; (B) 4 × 10−3 mol L
Scheme 1. Different regions in the foam to be involved in the synthesis of ZnSnanoparticles.
4
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−1; (C) 6 × 10−3 mol L−1; (D) 8 × 10−3 mol L−1.
. Conclusions
In conclusion, an extremely simple method that uses foam,tabilized by surfactants, as template for the synthesis of zinculfide nanoparticles has been described. This is probably therst report for a completely foam-based synthesis of zinc sulfideanoparticles at room temperature. The moving up of two typesf foam containing C4H6O4Zn and Na2S and their encounteresults in the formation of unidispersed spherical zinc sulfideanoparticles. The size of nanoparticles could be controlled byhanging the concentration of inorganic salt, and smaller ZnSanoparticles could be easily got using foam films as template.
here is no obvious effect on the size of nanoparticles by chang-
ng surfactant concentration, and region ‘P’ might be the mainanoreactor at the occasions using completely drained foamlms as template to synthesize nanoparticles.
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