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2009, 113, 12927–12929 Published on Web 07/01/2009
On the Rotation of the Janus CuO/CuS Colloids Formed at a Pickering Emulsion Interface Dong Li, Yongjun He,* and Sha Wang Department of Applied Chemistry, Xi’an UniVersity of Science & Technology, Xi’an 710054, China ReceiVed: April 8, 2009; ReVised Manuscript ReceiVed: June 16, 2009
We have prepared Janus CuO/CuS colloids by a precipitation reaction in the water phase of a Pickering emulsion. The relative proportion of CuO to CuS in the Janus colloids can be tailored by variation of the polarity of the oil phase. The Janus CuO/CuS colloids did not rotate at the emulsion interface. Our findings are important to understand the fundamentals of Pickering emulsions and will also help in designing appropriate approaches to synthesize Janus colloids and other new materials in Pickering emulsions. 1. Introduction About a century ago, Ramsden found that when some fine solid powders were mixed with water and an oily solvent, a solid-stabilized emulsion (often referred to as a Pickering emulsion) could be obtained even if no surfactants were used.1,2 The fine solid powders situated at the surface of the droplets formed a spherical shell and impeded the coalescence when two droplets approached.3 In recent years, broad interest has been reawakened in the study of Pickering emulsions because of their potential use in the development of novel nano/microstructures.4-11 Janus colloids having asymmetrical surface regions have wide potential applications in fields such as sensors, electronics, photonics, and drug delivery because of their tunable surface properties.12-17 Recently, Pickering emulsions have been found to be a suitable medium for the synthesis of Janus colloids due to their unique oil/solid/water three-phase circumstance.18-21 In consideration of the possible “rotation” of the colloids at a Pickering emulsion interface during reactions, various methods have been developed to avoid its effect on the formation of Janus colloids. Hong et al. reported a wax solidification method to immobilize silica particles at the emulsion interface.18 Parts of the silica particles are protected by being embedded into wax, and the unprotected parts are then selectively modified with a silane to form Janus colloids. Suzuki et al. prepared Janus colloids by a fast carbodiimidation onto one part of the microgel particles at one liquid phase of a Pickering emulsion.19 Liu et al. synthesized Janus colloids by biphasic grafting at a liquid/ liquid Pickering emulsion interface.20 In this approach, the rotation of colloids is restricted by the in situ formation of Janus colloids with an amphiphilic character. In our previous work, we prepared Janus Cu2(OH)2CO3/CuS colloids via a Pickering emulsion route.21 The Cu2 (OH)2CO3 colloids were fixed at emulsion interfaces by polymerizing the droplets of the Pickering emulsion. These strategies effectively prevent the effect of the potential rotation of the colloids at the interface on the formation of Janus colloids. However, there is rare evidence for the rotation of the colloids at the interface of a Pickering emulsion. It is still a challenge to clarify the rotating behavior of colloids at a Pickering emulsion interface. * To whom correspondence should be addressed. Phone: +86-2985583183. E-mail:
[email protected].
10.1021/jp903262c CCC: $40.75
SCHEME 1: Synthesis of Janus CuO/CuS Colloids by a Precipitation Reaction in the Water Phase of a Pickering Emulsion
Here, we present the formation of CuO/CuS Janus colloids by a precipitation reaction in the water phase of a Pickering emulsion. To investigate the rotating behavior of colloids at the interfaces of Pickering emulsions, we did not take any measures to restrict their rotation. 2. Experimental Details Monoclinic CuO colloids with an average diameter of 4.2 µm were prepared by a surfactant-assisting precipitation reaction (Figures S1 and S2 in Supporting Information). The synthesis of Janus CuO/CuS colloids involves two stages (Scheme 1). (i) A quantity of 0.048 g of CuO colloids is dispersed in 10 mL of deionized water. Then, 2 mL of toluene is mixed with the CuO dispersion. A stable toluene-in-water Pickering emulsion stabilized by CuO colloids is generated via sonication. (ii) A quantity of 0.01 g of thioacetamide is dissolved in 5 mL of water, and this solution is dripped into the Pickering emulsion at a rate of 0.5 mL/h. After reaction for 10 h, the emulsion is destabilized by adding alcohol. The mixture is filtrated, and the residues are washed with water and alcohol three times. The obtained Janus CuO/CuS colloids are dried at room temperature under vacuum. To investigate the relative proportion of CuS to CuO in the Janus colloids, the Janus colloids were immersed in 0.01 mol/L hydrochloric acid under agitation for 2 h to completely dissolve 2009 American Chemical Society
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Figure 1. Optical picture of a droplet of the toluene-in-water Pickering emulsion stabilized by CuO colloids.
CuO. Then, the mixture was filtrated, and the residues were washed with water and alcohol three times. The obtained ringent CuS shells were dried at room temperature under vacuum. For comparison, two other experiments were conducted under the same conditions but using different oily solvents (n-octanol and styrene) as the oil phase. The SEM images of samples were collected by a Hitachi S-2700 scanning electron microscope equipped with an energy dispersion spectrum (EDS) unit. XRD patterns were obtained by a Rigaku D/MAX-3C X-ray diffraction meter using CuKR radiation with 40 kV and 20 mA at a 0.2° scan rate (in 2θ). The optical photos of Pickering emulsions were taken by a Shangguang PX1 microscope. 3. Results and Discussion Figure 1 shows a typical optical microscope picture of a droplet of the toluene-in-water Pickering emulsion stabilized by CuO colloids. The CuO colloids were situated at the oil/ water interface of the emulsion and formed an inconsecutive layer. Figure 2a displays the SEM images of the as-prepared CuO/ CuS colloids. It can be seen that the CuO/CuS colloids had the same morphology as their CuO precursors (Figure S1 in Supporting Information). This can be interpreted as follows. In the water phase of the Pickering emulsion, the hydrolysis of thioacetamide and CuO generated sulfide ions and cupric ions, respectively. As the thioacetamide solution was slowly dripped into the Pickering emulsion, the saturation degree of the water phase was controlled, and a homogeneous nucleation of CuS was avoided. Therefore, CuS was formed via a heterogeneous nucleation mechanism and attached at the surfaces of the CuO colloids. The CuO colloids served as the source of cupric ions as well as the template for the formation of CuS.
Letters XRD analyses confirmed that the CuO/CuS colloids consisted of hexagonal CuS and monoclinic CuO (Figure S3 in Supporting Information). Energy dispersion spectrum (EDS) analyses showed that there was no signal of sulfur inside of a circular region at the surfaces of the CuO/CuS colloids (Figures S4 and S5 in Supporting Information), confirming that only part of the surface of the CuO precursors was covered by CuS, namely, the CuO/CuS colloids were Janus particles. However, the relative proportion of CuO to CuS in the CuO/CuS colloids could not be clearly distinguished in the SEM images. After the CuO/CuS colloids were immersed in 0.01 mol/L hydrochloric acid for 2 h, the CuO part of the colloids was completely dissolved. The residues were ringent shells (Figure 2b). XRD results showed that the ringent shells were made up of pure hexagonal CuS (Figure S6 in Supporting Information). When the CuO/CuS colloids were immersed in hydrochloric acid for only 5 min, CuO was partially dissolved (Figure 2c). From Figure 2a-c, it also can be seen that the synthesized CuO/ CuS colloids were Janus particles. As CuS was more philophobic than CuO (see Measurement of the Wettability of CuS and CuO Colloids in the Supporting Information), the Janus CuO/CuS colloids were amphiphilic. The CuS part of the Janus colloids was initially formed from the water phase of the emulsion. If the Janus CuO/CuS colloids rotated at the interface of the Pickering emulsion, the CuS region of the Janus colloids would turn to the oil phase,20,22 and the CuO region would face the water phase. Then, the CuO region would be covered by subsequently formed CuS, and finally CuO/ CuS core/shell colloids would be formed. In other words, Janus CuO/CuS colloids would not be obtained. This implies that the Janus CuO/CuS colloids did not rotate during the precipitation reaction. To verify the above deduction, two comparison experiments were conducted under the same conditions but using different oily solvents as the oil phase. When n-octanol was used as the oil phase, the ringent CuS shells with more shallow depth were obtained (Figure 3a). As the polarity of n-octanol was higher than that of toluene, more proportion of the CuO colloids entered the n-octanol phase of the Pickering emulsion and was protected. CuS was formed and attached at the surfaces of the CuO colloids from the water phase, and finally, the Janus CuO/CuS colloids with less CuS proportion were formed. After the CuO part in the Janus colloids was dissolved by dilute hydrochloric acid, the ringent CuS shells with more shallow depth were formed. When styrene was used as the oil phase, the depth of the obtained ringent CuS shells (Figure 3b) was deeper than that by using n-octanol as the oil phase, which also can be interpreted by the difference in the polarity of styrene from n-octanol. It is clear that the CuS region of the Janus CuO/CuS colloids was
Figure 2. SEM images of (a) Janus CuO/CuS colloids, (b) ringent CuS shells, and (c) Janus CuO/CuS colloids immersed into dilute hydrochloric acid for 5 min.
Letters
J. Phys. Chem. C, Vol. 113, No. 30, 2009 12929 the fundamentals of Pickering emulsions and will also help in designing appropriate approaches to synthesize Janus colloids and other new materials in Pickering emulsions. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50572088), the Natural Science Foundation of Shaanxi Province (2006B29), and the Natural Science Foundation of Shaanxi Provincial Education Office (07JK313).
Figure 3. SEM images of the ringent CuS shells prepared by using (a) n-octanol and (b) styrene as the oil phases.
always facing the water phase during reaction. This also means that the Janus CuO/CuS colloids did not rotate at the interfaces of the Pickering emulsions. To further verify the above result, 0.048 g of the resultant Janus CuO/CuS colloids (prepared using toluene as the oil phase) was redispersed in 10 mL of water, and 2 mL of toluene was mixed with the dispersion. A stable toluene-in-water emulsion was formed after sonication. Then, the same procedure as that for the preparation of the Janus colloids was repeated, and the product was immersed in 0.01 mol/L hydrochloric acid under agitation for 2 h. Intact spherical CuS colloids (Figures S8 and S9 in Supporting Information) instead of ringent CuS shells were obtained. This confirmed that the CuO part of the Janus CuO/CuS colloids was facing the water phase when the emulsion was formed and that the colloids did not rotate during the precipitation reaction. It can be seen that it is thermodynamically favorable for the CuS region of the Janus CuO/CuS colloids to turn to the oil phase.20,22 Apparently, an energy barrier existed for the rotation of the Janus CuO/CuS colloids formed at the interfaces of the Pickering emulsions, which was probably ascribed to the forces (including the van der Waals force) between the colloids and the two liquid phases. However, the details for the behavior of the colloids at a Pickering emulsion interface remain to be further addressed. 4. Conclusions We have prepared Janus CuO/CuS colloids by a precipitation reaction in the water phase of a Pickering emulsion. The relative proportion of CuO to CuS in the Janus colloids can be tailored by variation of the polarity of the oil phase. The Janus CuO/ CuS colloids did not rotate at the emulsion interface. As the colloids at the interface of a Pickering emulsion are generally thought to be “rotating”, our findings are important to understand
Supporting Information Available: Synthesizing procedure for CuO colloids, SEM images of CuO colloids, Janus CuO/ CuS colloids and intact CuS colloids, XRD patterns of CuO colloids, Janus CuO/CuS colloids, ringent CuS shells and intact spherical CuS colloids, EDS traces of Janus CuO/CuS colloids, and picture of the tube for measuring the wettability of CuO and CuS colloids. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ramsden, W. Proc. R. Soc. London 1903, 72, 156. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (3) Aveyard, R.; Binks, B. P.; Clint, J. H. AdV. Colloid Interface Sci. 2003, 100-102, 503. (4) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (5) Binks, B. P. AdV. Mater. 2002, 14, 1824. (6) Golemanov, K.; Tcholakova, S.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2006, 22, 4968. (7) Voorn, D. J.; Ming, W.; Herk, A. M. Macromolecules 2006, 39, 2137. (8) Duan, H.; Wang, D.; Sobal, N. S.; Giersig, M.; Kurth, D. G.; Mohwald, H. Nano Lett. 2005, 5, 949. (9) Noble, P.; Cayre, O.; Alargova, R.; Velev, O.; Paunov, O. J. Am. Chem. Soc. 2004, 126, 8092. (10) He, Y. Mater. Chem. Phys. 2005, 92, 134. (11) He, Y. Mater. Chem. Phys. 2005, 92, 609. (12) Sacanna, S.; Philips, A. AdV. Mater. 2007, 19, 3824. (13) Paunov, V. N.; Cayre, O. J. AdV. Mater. 2004, 16, 788. (14) Correa-Duarte, M. A.; Salgueirino-Maceira, V.; Rodriguez-Gonzalez, B.; Liz-Marzan, L. M.; Kosiorek, A.; Kandulski, W.; Giersig, M. AdV. Mater. 2005, 17, 2014. (15) Sardar, R.; Heap, T. B.; Shumaker-Parry, J. S. J. Am. Chem. Soc. 2007, 129, 5356. (16) Biker, A.; He, J.; Emrick, T.; Russel, T. P. Soft Matter 2007, 3, 1231. (17) Nie, L.; Liu, S.; Shen, W.; Chen, D.; Jiang, M. Angew. Chem., Int. Ed. 2007, 46, 6321. (18) Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495. (19) Suzuki, D.; Tsuji, S.; Kawaguchi, H. J. Am. Chem. Soc. 2007, 129, 8088. (20) Liu, B.; Wei, W.; Qu, X.; Yang, Z. Angew. Chem., Int. Ed. 2008, 47, 3973. (21) He, Y.; Li, K. J. Colloid Interface Sci. 2007, 306, 296. (22) Binks, B. P.; Fletecher, P. I. Langmuir 2001, 17, 4708–4710.
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