O microemulsion

Materials and Design 31 (2010) 1661–1665

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Short Communication

Synthesis of CuS nanocrystal in cationic gemini surfactant W/O microemulsion Lifei Chen a,b, Yazhuo Shang a, Honglai Liu a,*, Ying Hu a a b

State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China Department of Environmental Engineering, Shanghai Second Polytechnic University, Shanghai 201209, China

a r t i c l e

i n f o

Article history: Received 10 September 2008 Accepted 30 May 2009 Available online 3 August 2009

a b s t r a c t In the water-in-oil (W/O) microemulsion stabilized by the cationic gemini surfactant alkanediyl-a, x (dimethydodecyl-ammonium bromide) (12–3-12, 2Br 1), CuS nanorod, tube-like, and star sheet-like structures are successfully prepared. The effect of the concentration ratio of S2 to Cu2+, molar ratio (x0) of water to surfactant in solution, incubation time, and absolute reactant concentration on the morphology and size of CuS nanocrystal have been investigated. The properties of the products and their morphologies are characterized by X-ray powder diffraction (XRD), UV–vis absorption spectrum and transmission electron microscopy (TEM). The growth mechanics of the nanocrystal with different morphology is also supposed. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In recent years semiconductor nanocrystal such as CdS, PbS, ZnS and NiS have received considerable attention because of their unique size-dependent physicochemical, novel optical, and electronic properties that significantly different from those of their bulk counterparts [1,2]. Among these nanomaterials, copper sulfide exhibits its commercial importance as pigment, catalyzer, and colored indicator of nigrosine and so on besides of being an excellent semiconductor [3]. Several methods have been applied to synthesize copper sulfide nanocrystal such as LB film, microemulsions, vesicles, solid crystal, electrochemistry, an organic-assisted hydrothermal process, and thermolysis [4–11]. However, compared with all of the methods mentioned above, microemulsion has been given considerable attention recently for its advantages of relatively simple, mild, and low-cost [12]. The same meaning is that it is easy to handle, demanding no extreme pressure or temperature control, and requiring no special and expensive equipment. Microemulsion consisted of oil, surfactant (usually together with co-surfactant), and water molecule is a thermodynamically stable and isotropic transparent solution. Water-in-oil microemulsion (W/O) is composed of nanometer-sized water droplets dispersed in a continuous oil phase, and stabilized by surfactant molecules. Such thermodynamically stable system offers a stable microenvironment for the chemical reaction to occur when desirable reactant collide each other. The nanoreactors stabilized by surfactant work like the cage-like, winhibiting the excess particles growth and agglomeration when the particles size approach to that of water nanodroplets. Then the desired narrow size distribution * Corresponding author. Tel.: +86 21 64252921. E-mail address: [email protected] (H. Liu). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.05.047

and controlled morphology of the mineralized material can be attained [13–16]. Gemini surfactants are amphiphiles composed of two indentical hydrophobes linked to two head groups with a spacer moiety. The surfactant has attracted many academic and industrial research groups. Their special surface properties were described in several reviews [17–19]. Cationic gemini-based microemulsion was firstly used to polymerize the styrene [20], and recently there are reports on microemulsion and phase behavior of anionic gemini surfactants [21,22]. However, for the best of our knowledge, only limited works have been reported on preparing nanomaterials in geminibased cationic microemulsion. Under this condition, it is worthwhile to have investigation on preparing nanocrystals in a microemulsion stabilized by cationic gemini surfactant. The main objective of our previous work is to study the phase behavior of the n-butanol/n-octane/water/ (12–3-12,2Br 1) system [23] and the microstructure of microemulsion using conductivity measurement, UV–vis absorbance spectra of pyrene probe measurements, and dynamic light scattering (DLS) method [24]. On the base of the previous work, we have successfully synthesized ZnS nanosphere of uniform morphology [25], and in this work we report the preparation of copper sulfide (CuS) nanocrystals in water-in-oil (W/O) microemulsion containing gemini surfactant. The microemulsion is composed of gemini surfactant (12–3-12,2Br 1), n-butanol as co-surfactant, n-octane, and water. All the studies have shown that the size and morphology of CuS can be altered partially by changing molar ratio of S2 to Cu2+, x0 (the molar ratio of water to surfactant), incubation time, and reactant concentration. The resulted nanocrystals were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV absorption spectroscopy.

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2. Experimental 2.1. Chemicals Gemini cationic surfactant trimethylene-1,3-bisdodecyldimethyl ammonium bromide (12–3-12,2Br ) is synthesized in lab (99%). n-Butanol (99%)) and n-octane (95%) are supplied by Shanghai Feida trade company and Shanghai Wulian chemical factory, respectively. Copper nitrate hexahydrate (Cu(NO3)2
7H2O, 99.5%) is purchased from Shanghai Zhenxin reagent factory, and sodium sulfide (Na2S
9H2O, 98%) is provided by Shanghai Lingfeng reagent company. Double distilled and deionized water is used throughout. All the chemicals are of analytical purity and are used as received without further purifications.

The microemulsion is stirred for 30 min and is kept at 30 °C in a thermostatic bath. During this work, x0, reactant concentrations in water, molar ratio of S2 to Cu2+ and incubation time are varied to obtain uniform nanocrystals. The precipitated powders obtained by centrifugation are watered by distilled water and ethanol (analytical purity) at least five times in order to get rid of the surfactant, residual reactants and byproducts. All the products are dried in vacuum oven for 12 h at 50 °C. One drop of the resulting power dispersed by ethanol is placed on a copper grid covered with an amorphous carbon film. The resulted samples are exposed to the air at room temperature for drying until suitable for the subsequent TEM analysis.

2.4. Characterization 2.2. Determination of phase diagram Fix the weight ratio of 12–3-12,2Br (surfactant) to n-butanol (co-surfactant) at 1:1, and the mixture is supersonicated for half an hour and is sealed to be latter use. To weight each constituent according to different samples selected from the pseudo-ternary phase diagram and put it into the graduate tube, then seal the tube and supersonicated it for ten minutes. All the tubes are putted in the thermostatted tank at 30 °C for 58 h. Phase equilibration is achieved depending on whether the visual appearance remained unaltered. Equilibrated samples are examined visually with or without crossed polarisers to determine the number and isotropy of the present phases. Then the pseudo-ternary phase diagram shown in Fig. 1 of quaternary system can be obtained. 2.3. Synthesis of CuS nanocrystals

TEM (JEOL JEM-100CX II electron microscope operating at 100 kV) is used to observe the morphology and size of the products. The XRD is applied to have qualitative analysis of CuS nanocrystal. The UV–vis absorption spectra of the solutions are recorded on a Shimadzu UV-2450 spectrophotometer in the wavelength range of 200–900 nm using a 10-nm quartz cell.

3. Results and discussion The experimental conditions and the morphologies of all the samples are listed in Table 1. Fig. 2 shows the XRD pattern of one sample. All the diffraction peaks can be perfectly indexed to the hexagonal CuS and indicate that the product as prepared consist of pure phase and no other type phase shows incorporation. There is no doubt that the diffrac-

The microemulsion containing sulfide ion is prepared with fixed surfactant + co-surfactant content of 35wt% and varied weight content of water and oil. The aqueous solution of copper ion is added into the above microemulsion slowly with mechanical agitation.

H2O 0.0 1.0 0.2 2I 0.4

O/W 0.8 2 III L.C.

0.6

0.6

2 II

3

0.4

B.C.

0.8

0.2 2 IV

1.0 Oil 0.0

W/O 0.2

0.4

0.6

0.8

0.0 1.0 S+A

Fig. 1. Pseudo-ternary phase diagram of n-butanol/n-octane/water/(12–312,2Br 1) system at 30 °C with S/A = 1/1 (weight ratio) [23].

Fig. 2. XRD pattern of CuS sample prepared in W/O microemulsion (aqueous [Cu2+] = 0.2 mol/L, x0 = 12, [S2 ]/[Zn2+] = 7.5, 30 °C) with incubation time 24 h.

Table 1 Experimental conditions and the morphologies of all the samples. Number

[S2 ]/[Cu2+]

x0

Time (h)

[Cu2+] (mol/L)

Morphology

Crystal size (nm)

1 2 3 4 5 6 7 8

7.5 15 30 30 30 30 30 30

12 12 12 20 30 30 30 30

24 24 24 24 24 12 48 24

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4

Needle and rod-like Straight rod Straight rod and star sheet-like Tube-like Rod and star sheet-like Star sheet-like Rod and star sheet-like Rod and star sheet-like

(14–25)(250–950) (37–50)(500–1450) 50(1–2)um and 1um  2um 55(1.8–3)um 71(2.8–5.1)um and 700 800–1000 50  3um and 1um 200  7um and 900

L. Chen et al. / Materials and Design 31 (2010) 1661–1665

tion peaks are broader than the corresponding standard pattern and this is resulted by the small size of the product. Fig. 3 shows the morphology and size of the samples prepared with the typical experimental procedure. In which, the composing of the microemulsion is fixed, and only the molar ratio of sulfide ion to copper ion is increased. The rod-like and needle-like CuS nanocrystal given in Fig. 3A with diameters of 14–25 nm and lengths up to 250–950 nm can be obtained when the molar ratio is 7.5. With the molar ratio increasing to 15 the straight nanorod with uniform diameter can be obtained and is shown in Fig. 3B. The rod diameter is about 50 nm and the most length is at least 1.45 um. When the molar ratio reaches to 30 the CuS nanocrystal with new star sheet morphology, in addition to the normal nanorod, appears. All the morphologies are given in Fig. 3C. In order to get the uniform star sheet morphology of CuS nanocrystal, firstly we fix the molar ratio to be 30 and change the x0 (the molar ratio of water to surfactant) from 12 to 20 and further to 30. Tube-like and scrappy sheet-like CuS nanocrystal form when the w0 is 20. Their morphologies are given in Fig. 4A. As shown in the Fig. 4B, if the w0 is 30 there is still rod-like CuS nanocrystal in the resulted samples. And the ratio of the length to the diameter increases evidently with increasing x0. Only by changing the x0 the uniform star sheet nanocrystal can not be obtained, but when the x0 is 30 the star sheet-like nanocrystal morphology is relative ideal. During the nanocrystal growth the incubation time is also an

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important factor, so the molar ratio and the x0 are fixed at 30 and the incubation time is changed from 24 to 12 and further to 48 h. The morphology of the latter two resulted samples is shown in Figs. 5A and 4B. It is apparent that when the incubation time is 12 h the uniform star sheet-like CuS nanocrystal can be obtained. But if the incubation time is increased to 48 h the rod-like or tube-like CuS nanocrystal also can form. And the diameter and the length of which are bigger than that of the sample obtained when the incubation time is 24 h. At the same time the size of

Fig. 6. TEM images of CuS samples prepared in W/O microemulsion ([S2 ]/ [Cu2+] = 30, x0 = 30, 30 °C) for 24 h with aqueous [Cu2+] = 0.4 mol/L.

Fig. 3. TEM images of CuS samples prepared in W/O microemulsion (aqueous [Cu2+] = 0.2 mol/L, x0 = 12, 30 °C) for 24 h with [S2 ]/[Cu2+] (A) 7.5; (B) 15; (C) 30.

Fig. 4. TEM images of CuS samples prepared in W/O microemulsion (aqueous [Cu2+] = 0.2 mol/L, [S2 ]/[Cu2+] = 30, 30 °C) for 24 h with x0 (A) 20; (B) 30.

Fig. 5. TEM images of CuS samples prepared in W/O microemulsion (aqueous [Cu2+] = 0.2 mol/L, [S2 ]/[Cu2+] = 30, x0 = 30, 30 °C) with incubation time (A) 12 h; (B) 48 h.

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Fig. 7. Sketch map for the growth process of tube-like and star sheet-like CuS nanocrystals (red dots designate CuS particles).

ti-sheet CuS nanocrystal can form (the sketch wiew is shown in Fig. 7C). The adjacent and same orientation sheet nanocrystal also can curl into the multi-homocentric tube-like. The UV–vis absorption spectra (Fig. 8) of CuS sample dispersed in alcohol shows an absorption band with a peak maximum at 225 nm. The UV–vis absorption spectrum shows a blue-shift when compared to that of bulk CuS [27], indicating the presence of the size quantization effect. There is long tail in the curve, which may be attributed to the very high aspect ratio of the as-prepared nanorods.

4. Conclusion

Fig. 8. Room-temperature UV–vis absorption spectra of CuS sample prepared in W/ O microemulsion (aqueous [Cu2+] = 0.2 mol/L, x0 = 12, [S2 ]/[Zn2+] = 7.5, 30 °C) with incubation time 24 h.

the star sheet CuS nanocrystal increases with the increasing incubation time. Furthermore the influence of reaction concentration is also investigated. Fig. 6 gives the morphology including the rodlike and star sheet-like nanocrystals synthesized when the reaction concentration of the copper ion is change to 0.4 mol/L. Compared with sample 5, it is clear that increasing the copper ion concentration can make the rod-like nanocrystal grow rapidly. From studies of all these TEM images, we tried to propose the formation process of the rod-like, star sheet-like, and tube-like nanocrystals. The microemulsion microstructure, in which the nanorod generate, may be spherical droplet dispersing in the continue oil phase. With copper ion addition the CuS particles form and act as seeds for the later born particles to grow on. Due to the fact that the particles are confined in the microreactors offered by the microemulsion, shape of the particles is tuned according to the template [26]. The CuS particles formed latter are continually absorbed on the nuclei. Then the particles aggregate into larger ones along a certain orientation under the direction of micelles. In the micelles further growth of the larger particles result in rod-like shapes. The 12–3-12, 2Br 1 is ionic surfactant and can generate copper bromine. The more sulfide ions is released the easier formation of the copper sulfide will be. The aggregation of copper sulfide can be accelerated. It is the reason that the uniform rodlike CuS nanocrystal can form in the sample 2 rather than the sample 1. With increasing x0 and molar ratio of sulfide ion to copper ion the microemulsion may lie in the bicontinuous region. In which it may be that the net of water-tube disperse in the oil array (the partial sketch map is shown in Fig. 7A). The growth and aggregation of the CuS particles may along the inside of the water-tube (the cutaway view is shown in Fig. 7B), and the tube-like CuS nanocrystal can generate. In the water-tube net there are many decussations. The decussations overlap, and with the CuS particals growth and aggregation along a certain orientation the star mul-

In summary, we have demonstrated that the new type cationic gemini surfactant (12–3-12, 2Br-1) can be successfully used in the synthesis of CuS nanocrystals. A facile and effective water-in-oil microemulsion approach for the synthesis of pure hexagonalphase CuS was obtained. In addition, by carefully adjusting the synthetic conditions such as the concentration ratio of S2 to Cu2+, molar ratio (x0) of water to surfactant in solution, incubation time, and absolute reactant concentration, the resultant CuS nanostructures can be selectively controlled to be nanorod, tube-like, and new star sheet-like structures with various dimensions given by TEM. Furthermore, UV–vis absorption spectra blue-shift of CuS indicates the presence of the size quantization effect. The possible growth process of the CuS nanocrystal with different morphology is also supposed.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Projects No. 20490204) and Shanghai Municipal Education Commission of China.

References [1] Hu J, Odom TW, Lieber CM. Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Acc Chem Res 1999;32:435–45. [2] Dekker C. Carbon nanotubes as molecular quantum wires. Phys Today 1999;52:22–8. [3] Gao L, Wang EB, Lian SY, Kang ZH, Lan Y, Wu D. Microemulsion directed synthesis of different CuS nanocrystals. Solid State Commun 2004;130:309–12. [4] Eldering CA, Kowel ST, Knoesen A, Anderson BL, Higgins BG. Characterization of modulated spin–coated and langmuir-blodgett thin film etalons. Thin Solid Films 1989;179:535–42. [5] Haram SK, Mahadeshwar AR, Dixit SG. Synthesis and characterization of copper sulfide nanoparticles in Triton X-100 water- in-oil microemulsions. J Phys Chem 1996;100:5868–73. [6] Igumenova TI, Vasil’tsova OV, Parmon VN. Formation and photocatalytic properties of Q sized nanoparticles of various transition metal sulfides in the inner cavities of lecithin vesicles modified with sodium dodecylsulfate. J Photochem Photobiol A 1996;94:205–13. [7] Wang W, Chen X, Efrima S. Fabrication of semiconductor nanoparticles in a three-dimensional organic-layered solid crystal. Chem Mater 1999;11:1883–9. [8] Hwu S-J, Li H, Mackay R, Kuo Y-K, Skove MJ, Mahapatro M, et al. Bench-top synthesis of solid-state copper(I) sulfides, KCu7-XS4 (x = 0.0, 0.12, 0.34), via nonaqueous electrochemistry. Chem Mater 1998;10:6–9.

L. Chen et al. / Materials and Design 31 (2010) 1661–1665 [9] Lu Q, Gao F, Zhao D. One step synthesis and assembly of copper sulfide nanoparticles to nanowires, nanotubes and nanovesicles through an organic amine-assisted hydrothermal process. Nano Lett 2002;2:725–8. [10] Larsen TH, Sigman M, Ghezelbash A, Doty RC, Korgel BA. Solventless synthesis of copper sulfide nanorods by thermolysis of a single source thiolate-derived precursor. J Am Chem Soc 2003;125:5638–9. [11] Tanori J, Pileni MP. Change in the shape of copper nanoparticles in ordered phases. J Adv Mater 1995;7:862–4. [12] Xu J, Li YD. Formation of zinc sulfide nanorods and nanoparticles in ternary W/ O microemulsions. J Colloid Interf Sci 2003;259:275–81. [13] Cizeron J, Pileni MP. Solid solution of CdyZn1-yS nanosized particles: photophysical properties. J Phys Chem B 1997;101:8887–91. [14] Tanori J, Gulik-Krzywicki T, Pilieni MP. Phase diagram of copper(II) bis(2ethylhexyl) sulfosuccinate, Cu(AOT)2-isooctanewater. Langmuir 1997;13:632–8. [15] Estroff LA, Hamilton AD. At the interface of organic of composite materials. Chem Mater 2001;13:3227–35. [16] Walsh D, Lebeau B, Mann S. Morphosynthesis of calcium carbonate (vaterite) microsponges. Adv Mater 1999;11:324–8. [17] Zana R, Benrraou M, Rueff R. Alkanediyl-a,w-bis (dimethylalkylammonium bromide) surfactants. 1. Effect of the spacer chain length on the critical micelle concentration and micelle ionization degree. Langmuir 1991;7:1072–5. [18] Danino D, Talmon Y, Zana R. Alkanediyl-a,x-bis (dimethylalkylammonium bromide) surfactants (dimeric surfactants). 5. Aggregation and microstructure in aqueous solutions. Langmuir 1995;11:1448–56. [19] Zana R, Levy H, Kwetkat K. Mixed micellization of dimeric (gemini) surfactants and conventional surfactants. I. Mixtures of an anionic dimeric surfactant and

[20]

[21]

[22]

[23] [24]

[25]

[26]

[27]

1665

of the nonionic surfactants C12 E5 and C12 E8. J Colloid Inter Sci 1998;197:370–6. Dreja M, Tieka B, Ber J. Polymerization of styrene in microemulsions stabilized by cationic gemini surfactants. Bunsen-gesellschaft-Phys Chem Chem Phys 1998;102-111:1705–9. Kunieda H, Masuda N, Tsubone K. Comparison between phase behavior of anionic dimeric (gemini-type) and monomeric surfactants in water and wateroil. Langmuir 2000;16:6438–44. Magdassi S, Ben Moshe M, Talmon Y, Danino D. Microemulsions based on anionic gemini surfactants. Colloids and sSurf A: Physicochem Eng Aspects 2003;212:1–7. Chen LF, Shang YZ, Liu HL, Hu Y. Phase behavior of butanol/octane/water/ cationic gemi-ni surfactant system. J Dispers Sci Techn 2006;27:317–23. Chen LF, Shang YZ, Liu HL, Hu Y. Synthesis of ZnS nanospheres in microemulsion contain-ing cationic gemini surfactant. J Dispers Sci Techn 2006;27:839–42. Chen LF, Shang YZ, Liu HL, Hu Y. Effect of the spacer group of cationic gemini surfactant on microemulsion phase behavior. J Colloid Interf Sci 2006;301:644–50. Jana NR, Gearheart L, Murphy C. Seed-mediated growth approach for shapecontrolled synthesis of spheroidal and rodlike gold nanoparticles using a surfactant template. J Adv Mater 2001;13:1389–93. Liu JK, Wu QS, Ding YP, Yu Y. Active membrane templating synthesis and photics properties of Ag2 S and CuS nanospheres. J Funct Mate 2004;35:739–41.