One-Step Synthesis and Assembly of Two-Dimensional Arrays of

Aug 26, 2010 - Nirmal Goswami , Anupam Giri , Shantimoy Kar , Megalamane ... John , Paulrajpillai Lourdu Xavier , Thalappil Pradeep , Samir Kumar Pal...
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One-Step Synthesis and Assembly of Two-Dimensional Arrays of Mercury Sulfide Nanocrystal Aggregates at the Air/Water Interface Yan-Gang Yang, Hong-Guo Liu,* Lan-Jun Chen, Kuang-Cai Chen, Hui-Ping Ding, and Jingcheng Hao* Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China Received June 12, 2010. Revised Manuscript Received August 5, 2010 Ordered two-dimensional (2D) arrays of β-HgS nanocrystal aggregates were prepared successfully at the air/water interface through the interfacial reaction between Hg2þ ions in the subphase and H2S in the gaseous phase under the direction of liquid-expanded monolayers of arachidic acid (AA). These 2D arrays are composed of hexagonal or quasihexagonal aggregates with the size of several hundreds of nanometers that consist of several tens of HgS nanocrystals with the size of several nanometers. The formed HgS nanocrystals together with AA molecules self-assembled into round aggregates due to the interactions between the species, and the aggregates self-assembled into 2D arrays further due to the attractions between them. During the self-assembly process, the soft round aggregates transformed into hexagonal or quasi-hexagonal ones. The experimental conditions, especially the phase states of the AA monolayers and temperature, have great influences on the formation of the 2D arrays. To the best of our knowledge, this is the first case to get 2D ordered arrays at the air/water interface through a one-step synthesis and assembly process.

Introduction Nanoparticles have unique optical, electronic, and magnetic properties different from the bulk materials, while the ordered or periodic arrays and superlattices of nanoparticles show more special properties arising from the coupling interaction between them, which lead to important potential applications in optoelectronic nanodevices.1-6 It was expected that the ordered assemblies of quantum dots would be a prerequisite for the future chip generations.7 Several approaches have been developed to fabricate two-dimensional (2D) ordered or periodic arrays of nanoparticles (such as quantum dots, nanorings, etc.) and nanoparticle assemblies. For example, 2D ordered arrays have been prepared via the self-assembly of modified nanoparticles at the air/water interface,8-12 liquid/liquid interface,13 and solid *Corresponding author. Tel þ86-531-88362805; fax: þ86-531-88564464; e-mail: [email protected]; [email protected]. (1) Evanoff, D. D., Jr.; Chumanov, G. ChemPhysChem 2005, 6, 1221. (2) Perez, A.; Dupuis, V.; Tuaillon-Combes, J.; Bardotti, L.; Prevel, B.; Bernstein, E.; Melinon, P.; Favre, L.; Hannour, A.; Jamet, M. Adv. Eng. Mater. 2005, 7, 475. (3) Nawa, N.; Bara, R.; Nakabayashi, S.; Dushkin, C. Nano Lett. 2003, 3, 293. (4) Genov, D. A.; Sarychev, A. K.; Shalaev, V. M.; Wei, A. Nano Lett. 2004, 4, 153. (5) Hossain, M. K.; Shimada, T.; Kitajima, M.; Imura, K.; Okamoto, H. Langmuir 2008, 24, 9241. (6) Neve-Oz, Y.; Golosovsky, M.; Frenkel, A.; Davidov, D. Phys. Status Solidi A 2007, 204, 3878. (7) Schmid, G. In Nanoscale Materials in Chemistry, Klabunde, K. J., Ed.; John Wiley & Sons: New York, 2001. (8) Liljeroth, P.; Vanmaekelbergh, D.; Ruiz, V.; Kontturi, K.; Jiang, H.; Kauppinen, E.; Quinn, B. M. J. Am. Chem. Soc. 2004, 126, 7126. (9) Pang, J.; Xiong, S.; Jaeckel, F.; Sun, Z.; Dunphy, D.; Brinker, C. J. J. Am. Chem. Soc. 2008, 130, 3284. (10) Yang, Y.; Kimura, K. J. Phys. Chem. B 2006, 110, 24442. (11) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Langmuir 2003, 19, 7881. (12) Ma, H.; Hao, J. Chem.—Eur. J. 2010, 16, 655. (13) Li, Y.-J.; Huang, W.-J.; Sun, S.-G. Angew. Chem., Int. Ed. 2006, 45, 2537. (14) Fried, T.; Shemer, G.; Markovich, G. Adv. Mater. 2001, 13, 1158. (15) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620. (16) Nawa, M.; Baba, R.; Nakabayashi, S.; Dushkin, C. Nano Lett. 2003, 3, 293.

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substrates.14-16 By using the template-directing assembly technique, 2D ordered arrays of metal and inorganic compound nanoparticles have been fabricated. The templates include soft ones, such as DNA nanogrids17,18 and block copolymer micelles,19,20 and hard ones, such as porous alumina21 and monolayers of colloidal crystals.22-25 In addition, lithographic technique26-29 and nanoskiving method30 have been utilized to form 2D ordered arrays of nanoparticles. These techniques have their respective features. Langmuir monolayer and Langmuir-Blodgett (LB) multilayer techniques have been utilized to synthesize inorganic nanoparticles for two decades. Nanoparticulate films of metal sulfide semiconductors, including CdS,31-35 ZnS,34-36 and CuS,36 and (17) Sharma, J.; Chhabra, R.; Liu, Y.; Ke, Y.; Yan, H. Angew. Chem., Int. Ed. 2006, 45, 730. (18) Zhang, J.; Liu, Y.; Ke, Y.; Yan, H. Nano Lett. 2006, 6, 248. (19) Morin, S. A.; La, Y.-H.; Liu, C.-C.; Streifer, J. A.; Hamers, R. J.; Nealey, P. F.; Jin, S. Angew. Chem., Int. Ed. 2009, 48, 2135. (20) Aizawa, M.; Buriak, J. M. Chem. Mater. 2007, 19, 5090. (21) Hobbs, K. L.; Larson, P. R.; Lian, G. D.; Keay, J. C.; Johnson, M. B. Nano Lett. 2004, 4, 167. (22) Dong, W.; Dong, H.; Wang, Z.; Zhan, P.; Yu, Z.; Zhao, X.; Zhu, Y.; Ming, N. Adv. Mater. 2006, 18, 755. (23) Li, C.; Hong, G.; Wang, P.; Yu, D.; Qi, L. Chem. Mater. 2009, 21, 891. (24) Zheng, Y. B.; Wang, S. J.; Huan, A. C. H.; Wang, Y. H. J. Non-Cryst. Solids 2006, 352, 2532. (25) Li, Y.; Sasaki, T.; Shimizu, Y.; Koshizaki, N. Small 2008, 4, 2286. (26) Ji, R.; Lee, W.; Scholz, R.; Gosele, U.; Nielsch, K. Adv. Mater. 2006, 18, 2593. (27) Komanicky, V.; Iddir, H.; Chang, K.-C.; Menzel, A.; Karapetrov, G.; Hennessy, D.; Zapol, P.; You, H. J. Am. Chem. Soc. 2009, 131, 5732. (28) Kim, S.; Jung, J.-M.; Choi, D.-G.; Jung, H.-T.; Yang, S.-M. Langmuir 2006, 22, 7109. (29) Xia, Q.; Chou, S. Y. Nanotechnology 2009, 20, 285310. (30) Xu, Q.; Rioux, R. M.; Dickey, M. D.; Whitesides, G. M. Acc. Chem. Res. 2008, 41, 1566. (31) Yuan, Y.; Cabasso, I.; Fendler, J. H Chem. Mater. 1990, 2, 226. (32) Yi, K. C.; Fendler, J. H. Langmuir 1990, 6, 1519. (33) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Chem. Mater. 1991, 3, 922. (34) Zhao, X. K.; Fendler, J. H. Chem. Mater. 1991, 3, 168. (35) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (36) Zhao, X. K.; Xu, S.; Fendler, J. H. Langmuir 1991, 7, 520.

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anisotropic nanoparticles, such as triangular nanoparticles of PbS37-39 and nanorods of CdS,40 were prepared and studied systematically by Fendler and his co-workers. These nanoparticulate films and nanoparticles were formed at the air/water interface through interfacial reactions between metal ions in the subphases and H2S in the gaseous phase under the direction of liquid-condensed or solid monolayers of long-chain fatty acids. This technique has been developed to be a useful tool to synthesize metal sulfide nanoparticles at the air/water interface.41-44 On the other hand, quantum dots (QDs) of metal sulfides, including CdS,45-47 PbS,47,48 ZnS,48 and HgS49,50 embedded in layered structures, were synthesized successfully by treating the deposited LB multilayers of metal alkanoates or metal alkanoates/ thioalkanoates with dry H2S gas. It should be noted that the Langmuir monolayers used in these studies are liquid-condensed or solid ones that enrich metal ions from the subphases and/or direct the formation of nanoparticles during the reaction process. In order to preserve the layered structure and prevent the formed QDs from aggregating, the LB multilayers were fabricated by depositing the Langmuir monolayers at high surface pressure. Recently, researchers have tried to prepare ordered arrays of metal sulfide nanoparticles in a simpler and more convenient one-step synthesis and assembly process at the air/water interface through interfacial reactions between metal ions and H2S gas under the templating of Langmuir monolayers. For example, one-dimensional (1D) arrays of metal sulfide nanoparticles, including CdS, Ag2S, and PbS, were produced at the air/water interface under the condensed Langmuir monolayers of a linear polymer,51-54 and 1D chains of ZnS nanoparticles55 were prepared at the air/water interface under the condensed Langmuir monolayers of an amphiphilic porphyrin derivative by making use of the templating effect of the linear supermolecules formed in the monolayers. Herein, we described a simple, novel, and inexpensive way to prepare large areas of highly ordered 2D arrays of HgS nanocrystal aggregates at the air/water interface via an interfacial reaction and assembly process. Our strategy is to make use of the nucleation and growth of HgS nanocrystals at the air/water interface via the reaction between Hg2þ ions in the subphase and H2S in the gaseous phase under liquid-expanded Langmuir monolayers of arachidic acid (AA) molecules, and to make use of the self-assembly of the (37) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (38) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5505. (39) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (40) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (41) Yu, W. L.; Huang, W.; Zhu, B. Y.; Zhao, G. X. Mater. Lett. 1997, 33, 221. (42) Pan, Z. Y.; Liu, X. J.; Zhang, S. Y.; Zhang, L. G.; Lu, Z. H.; Liu, J. Z. J. Phys. Chem. B 1997, 101, 9703. (43) Wang, C.-W.; Liu, H.-G.; Bai, X.-T.; Xue, Q.; Chen, X.; Lee, Y.-I.; Hao, J.; Jiang, J. Cryst. Growth Des. 2008, 8, 2660. (44) Xin, G.-Q.; Ding, H.-P.; Yang, Y.-G.; Shen, S.-L.; Xiong, Z.-C.; Chen, X.; Hao, J.; Liu, H.-G. Cryst. Growth Des. 2009, 9, 2008. (45) Urquhart, R. S.; Furlong, D. N.; Mansur, H.; Grieser, F.; Tanaka, K.; Okahatat, Y. Langmuir 1994, 10, 899. (46) Guo, S.; Konopny, L.; Popovitz-Biro, R.; Cohen, H.; Porteanu, H.; Lifshitz, E.; Lahav, M. J. Am. Chem. Soc. 1999, 121, 9589. (47) Konopny, L.; Berfeld, M.; Popovitz-Biro, R.; Weissbuch, I. E.; Leiserowitz, L.; Lahav, M. Adv. Mater. 2001, 13, 580. (48) Moriguchi, I.; Nii, H.; Hanai, K.; Nagaoka, H.; Teraoka, Y.; Kagawa, S. Colloids Surf. A 1995, 103, 173. (49) Elliot, D. J.; Furlong, D. N.; Grieser, F. Colloids Surf. A 1999, 155, 101. (50) Elliot, D. J.; Furlong, D. N.; Gengenbach, T. R.; Grieser, F. Colloids Surf. A 1995, 102, 45. (51) Berman, A.; Charych, D. Adv. Mater. 1999, 11, 296. (52) Berman, A.; Belman, N.; Galan, Y. Langmuir 2003, 19, 10962. (53) Belman, N.; Berman, A.; Ezersky, V.; Lifshiz, Y.; Golan, Y. Nanotechnology 2004, 15, S316. (54) Belman, N.; Golan, Y.; Berman, A. Cryst. Growth Des. 2005, 5, 439. (55) Xiao, F.; Liu, H.-G.; Wang, C.-W.; Lee, Y.-I.; Xue, Q.; Chen, X.; Hao, J.; Jiang, J. Nanotechnology 2007, 18, 435603.

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species at the air/water interface, including AA molecules and the formed HgS nanocrystals, to organize them into 2D ordered arrays. This method combines synthesis and assembly into a onestep process. The key factors that affect the formation of the 2D ordered arrays are temperature, molecular density of AA molecules at the air/water interface, and the concentrations of Hg2þ in the subphase and H2S in the gaseous phase. Besides the 2D ordered arrays, round aggregates of the nanocrystals were observed.

Experimental Section Materials. AA (>99%) and mercury acetate (>98.0%) were purchased from Shanghai Chem. Co. (Shanghai, China) and used as received. Sodium sulfide, sulfuric acid, and chloroform are analytical reagents. The water used is highly purified with the resistivity of g18.0 MΩ cm. Preparation and Characterization of 2D Ordered Arrays. Aqueous solutions of mercury acetate with the concentrations of 1  10-3, 1  10-4, 5  10-5, and 1  10-5 mol L-1 were used as subphases, and a chloroform solution of AA with the concentration of 0.1 mg mL-1 was used as the spreading solution. Ten milliliters of the aqueous solution of mercury acetate was poured into a Petri dish with the inner diameter of 5.7 cm to form an air/ water interface. Then, a certain volume of AA chloroform solution was spread onto the air/water interface by using a microsyringe. After the organic solvent evaporated for 10 min, the Petri dish was placed in a container. Then, a beaker containing a certain volume of sodium sulfide aqueous solution with the concentration of 4.464  10-4 mol L-1 was placed in this container. The amount of H2S produced was controlled by the volume of Na2S aqueous solution used. For example, when 10 mL of the mercury acetate aqueous solution with the concentration of 1  10-4 mol L-1 was used, the volume of the Na2S solution should be 3.0 mL. Then, the molar ratio of H2S/Hg2þ was controlled to be 4:3. After excess sulfuric acid was added into the sodium sulfide solution, the container was sealed immediately, because H2S is a toxic gas and the amount of H2S produced should be controlled. The volume of the spreading solution was 15, 28, or 66 μL; the corresponding mean area per AA molecule was calculated to be 0.88, 0.47, or 0.20 nm2; and the corresponding surface pressure was 0, 12-14, or 32-40 mN m-1, respectively; according to the π-A isotherms obtained by using a NIMA 611 trough (Coventry, Great Britain). The temperature was controlled at 14, 16, 18, 20, and 24 °C, respectively. One hour later, the products formed at the air/water interface were transferred onto carboncoated copper grids and hydrophobized quartz slides, respectively, by using Langmuir-Schaefer method.56,57 These samples were investigated by using a transmission electron microscope (TEM; JEM-100CXII, JEOL Ltd., Tokyo, Japan) with the accelerating voltage of 100 kV and a high-resolution TEM (HR-TEM; JEOL-2010, JEOL Ltd., Tokyo, Japan) with the accelerating voltage of 200 kV. The selected-area electron diffraction (SAED) pattern was obtained by using the TEM (JEM100CXII) with a camera length of 55 cm. The samples deposited on quartz slides were investigated by using an X-ray diffractometer (XRD, Rigaku D/Max 2200PC).

Results and Discussion 2D Arrays. Figure 1 shows the TEM micrographs of the formed 2D ordered arrays of HgS nanocrystal aggregates at different temperatures. Large areas of 2D arrays appeared in the image obtained at 14 °C. The 2D array consists of hexagonal close-packed quasi-hexagonal aggregates with the size of 263 ( 6.9 nm. Each aggregate has a condensed core and a loose shell. Figure 1b is an enlarged TEM micrograph taken from another (56) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (57) Balasubramanian, K. K.; Cammarata, V. Langmuir 1996, 12, 2035.

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Figure 1. TEM images of two-dimensional arrays of aggregates of HgS nanocrystals formed at the air/water interface at 14 (a,b), 16 (c), and 18 °C (d). The mean molecular area of AA is 0.47 nm2, corresponding to the surface pressure of 12 mN m-1. The subphase concentration of Hg2þ is 1  10-4 mol L-1. The molar ratio of H2S: Hg2þ in the reaction systems is 4:3.

area. It can be seen that the aggregates are composed of HgS nanocrystals with the size of 7.9 ( 1.9 nm, and the average number of nanocrystals in one aggregate is found to be about 60. It is hard to observe another nanostructure at this temperature except for 2D arrays, though the sizes of the aggregates differ from each other in the 2D arrays taken from different places. 2D arrays of HgS nanocrystal aggregates can be formed at higher temperatures. Figure 1c,d shows the TEM images of the arrays formed at 16 and 18 °C, respectively. It can be seen that the arrays are composed of hexagonal close-packed hexagonal aggregates with the size of ca. 300 nm, and each aggregate is composed of nanocrystals. Compared with the arrays formed when T = 14 °C, the scale of these arrays decreases, and randomly dispersed aggregates appear. Other Nanostructures. Figure 2 shows the TEM micrographs of the nanostructures formed at higher temperatures. Besides the 2D arrays shown in Figure 1c, two kinds of round aggregates were formed when T = 16 °C, as shown in Figure 2a,b. These aggregates consist of a condensed core, a loose intermediate part, and a condensed ring-like shell composed of nanocrystals. The average sizes of these two kinds of aggregates are 359 ( 13.8 and 152 ( 9.0 nm, respectively. Although the aggregates are monodisperse in size, the sizes of the aggregates formed in different areas differ from each other. It can also be seen from Figure 2b that some horn-like nanoparticles attached on the rings. The different packing manner, different sizes, and different morphologies of the aggregates indicate that the local microenvironments greatly affect the aggregation of the species. Figure 2c,d shows two kinds of other aggregates formed at 18 °C, except for the 2D arrays shown in Figure 1d. The average sizes of these aggregates are 229 ( 6.1 and 192 ( 5.9 nm, respectively, indicating that monodisperse aggregates were formed. The aggregates shown in Figure 2c have a condensed core and a loose shell, while the aggregates in Figure 2d have a condensed core, a loose intermediate part, and a condensed ring-like shell, similar to the aggregates in Figure 2a. These differences also reveal the influence of the microenvironments on the formation of the aggregates. When the temperature increased to 20 and 24 °C, no 2D array or round aggregate appeared. The aggregates were composed of larger nanocrystals with large horns on their peripheries (Figure 2e,f). Langmuir 2010, 26(18), 14879–14884

Figure 2. TEM micrographs of HgS aggregates formed at 16 (a,b), 18 (c,d), 20 (e), and 24 °C (f). The other experimental conditions are the same as described in the caption of Figure 1.

From Figures 1 and 2, one can see that the temperature has a great effect on the formation of the 2D arrays and aggregates. Structure Characterization. Figure 3a shows the SAED pattern of HgS nanocrystals corresponding to Figure 2d. This pattern clearly shows the characteristic of cubic HgS (β-HgS) polycrystallite,58-61 in which the diffraction rings can be indexed to (111), (200), (220), (311), (222), (400), (331), (422), and (333), respectively. It can be seen that this pattern is obviously different from the ED patterns of hexagonal HgS (R-HgS).61,62 The sample was further investigated by using X-ray diffractometry. Figure 3b exhibits the XRD pattern of HgS nanocrystals deposited on a quartz slide. The broad peak centered at about 22° is assigned to the quartz slide. Two weak peaks appeared at 26.9° and 52.4°, which correspond to the diffraction of (111) and (311) planes of β-HgS. The broad peaks suggest that the particles are nanosized ones,63 and the low intensity of the peaks suggests that the formed particles are nanocrystalline,64 or lower amount of the deposited nanoparticle on the quartz slide. (58) Szuszkiewicz, W.; Dynowska, E.; Dluzewski, P.; Paszkowicz, W.; Szczepanska, A.; Witkowska, B. Phys. Status Solidi B 2002, 229, 73. (59) Wang, H.; Zhang, J.-R.; Zhu, J.-J. J. Cryst. Growth 2001, 233, 829. (60) Shao, M.; Kong, L.; Li, Q.; Yu, W.; Qian, Y. Inorg. Chem. Commun. 2003, 6, 737. (61) Wang, H.; Zhu, J.-J. Ultrasonics Sonochem. 2004, 11, 293. (62) Ren, T.; Xu, S.; Zhao, W.-B.; Zhu, J.-J. J. Photochem. Photobiol. A 2005, 173, 93. (63) Green, M.; Prince, P.; Gardener, M.; Steed, J. Adv. Mater. 2004, 16, 994. (64) Patil, R. S.; Gujar, T. P.; Lokhande, C. D.; Mane, R. S.; Han, S.-H. Solar Energy 2007, 81, 648.

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Figure 3. SAED and XRD patterns of the formed nanostructures corresponding to Figure 2d.

Figure 4 gives the HRTEM micrograph of the formed HgS nanocrystals. The two-dimensional lattice image appears clearly in the HRTEM micrograph. The interplanar distances were measured to be 0.338, 0.280, and 0.200 nm from Figure 4d, closing to 0.338, 0.293, and 0.207 nm, the spacing between (111), (200), and (220) facets of β-HgS with zinc blende crystal structure (PDF No. 6-261), respectively, further confirming the formation of β-HgS. It can be seen that these nanoparticles have the same lattice images, indicating that they are highly oriented at the air/water interface with (110) face as the basal plane.58 Influences of Other Experimental Conditions on the Nanostructures. Besides temperature, other conditions, such as the interfacial densities of AA molecules, the concentrations of the subphases, and the molar ratios of H2S:Hg2þ in the reaction systems have great influence on the formation of 2D arrays. Figure 5a,b shows the TEM micrographs of HgS nanostructures formed at 16 °C by spreading different amounts of AA molecules. The mean molecular areas of AA molecules are 0.88 and 0.20 nm2, respectively, corresponding to the surface pressure of about 0 and 32-40 mN m-1 according to the π-A isotherm shown in Figure 6. It can be seen that dendrites and irregularly aggregated nanoparticles other than 2D arrays and round aggregates appeared, respectively, indicating that the states of AA monolayer have great influences on the formation of the nanostructures. Although round aggregates appeared in Figure 5c,d, they do not organize into an ordered array. This means that lower H2S:Hg2þ ratios go against the formation of 2D arrays. When the subphase concentration increased to 1 10-3 mol L-1, randomly dispersed 14882 DOI: 10.1021/la102407s

Figure 4. HRTEM micrographs of HgS nanocrystals corresponding to Figure 1b.

HgS nanoparticles formed after the reaction, and dendritic nanostructures formed when the subphase concentration decreased to 5  10-5 or 1  10-5 mol L-1. In both cases, no round aggregate was found. Formation Mechanism. After the organic solvent evaporated, AA molecules were dispersed at the air/water interface as single molecules or small aggregates. Due to their amphiphilicity, the carboxyl groups attached on the water surface to bond Hg2þ ions from the subphase and the alkyl chains extended to the air. So, Hg2þ ions were enriched in the interfacial phase. When H2S gas diffused to the interface, the reaction between H2S and Hg2þ took place, resulting in the nucleation and growth of HgS nanocrystals and the formation of hybrid nanoclusters of AA molecules and HgS nanocrystals due to the electrostatic or coordinate interactions between the carboxylic groups and Hg(II) ions. AA molecules adsorbed on the upper plane of the formed HgS nanocrystal in a hybrid nanocluster, the alkyl chains Langmuir 2010, 26(18), 14879–14884

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Figure 5. TEM micrographs of HgS nanostructures formed at the air/water interface at 16 °C. The subphase concentration is 1  10-4 mol L-1. The mean molecular areas of AA are 0.88 (a), 0.20 (b), and 0.47 nm2 (c,d), respectively. The molar ratios of H2S:Hg2þ in the reaction systems are 4:3 (a, b), 4:5 (c), and 2:5 (d), respectively.

Figure 6. π-A isotherms of AA monolayers formed at the air/ mercury acetate aqueous solution interface at different temperatures.

extended to air, and the other planes of the HgS nanocrystal come into contact with the subphase. The hybrid nanoclusters floated at the air/water interface and attached to each other to form aggregates due to the interactions between the hybrid species, including the attraction between the alkyl chains and capillary force between the HgS nanocrystals and the repulsive force (electrostatic and/or hydration repulsive force) between the nanocrystals. At the initial stage of the reaction, bigger nanoparticles formed due to the sufficient supply of Hg2þ ions enriched in the interfacial layer. They gathered to form the condensed core of the aggregates. Hg2þ ions in the interfacial layer were consumed rapidly as the reaction proceeded. Then, smaller hybrids formed with the slower and continued supply of Hg2þ ions that diffused from the subphase to the interface. The smaller hybrids together with smaller AA clusters adsorbed to the core to form a loose shell surrounding the core. It is reasonable to postulate that the formed aggregates are round ones according to the principle of interfacial free energy minimum, as indicated in Figure 2. The 2D arrays would be the result of a self-assembly process of the round aggregates. The aggregates came close to one another during the process, probably due to the attraction between the alkyl chains of AA molecules in the adjacent round aggregates. The small HgS particles packed loosely in the peripheric parts of the aggregates in the 2D arrays, indicating that aggregates are flexible Langmuir 2010, 26(18), 14879–14884

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and prone to deform under pressure. This should be the reason why the 2D arrays are composed of hexagonal or near-hexagonal aggregates, as shown in Figure 1. This is similar to the formation of polyhedron foams. Besides the condensed core and the loose middle part, some aggregates formed at higher temperatures have a hard ring-like shell, as shown in Figure 2. The formation of the condensed ringlike shell and horn-like structures should be attributed to the continued supply of Hg2þ ions diffused from the subphase and H2S from the gaseous phase. They met at the edges of the preformed round aggregates, because it was difficult for H2S gas to penetrate the aggregates. Then, the produced HgS nanocrystals adsorbed on the edges to form the condensed ring-like shells and horn-like particles. These condensed shells preserved the round shape of the aggregates, attached to one another, and hindered the formation of ordered close-packed 2D arrays. It is apparent that the amounts of the spreading solutions have great influence on the formed nanostructures. It was shown that a dendritic nanostructure was formed when 15 μL of the spreading solution was used; round and hexagonal aggregates and 2D arrays appeared when 28 μL of the spreading solution was used; and randomly dispersed HgS nanoparticles appeared when 66 μL chloroform solution was spread. As we know, the Langmuir monolayer has different phase states that depend on molecular densities at the air/water interface. When 15 μL solution was spread, the mean molecular area was 0.88 nm2, much more than 0.20 nm2, the limited molecular area of AA. The monolayer is a gaseous one. In the initial state of the reaction, a small quantity of nuclei formed, because the amount of Hg2þ ions is not enough. After nucleation, the concentration of Hg2þ ions in the interfacial phase is lower than that in the subphase. With the reaction proceeding, the Hg2þ ions diffused from the bulk phase to the interface and reacted with H2S gas, leading to the growth of the nuclei and the formation of the dendritic structure. It seems that the diffusion-limited aggregation mechanism should be responsible for the formation of the dendrites. When 28 μL of solution was spread, the mean molecular area was calculated to be 0.47 nm2, the corresponding surface pressure was 12-14 mN m-1, the formed monolayer was a liquid-expanded one, as seen in the π-A isotherm. There are some dispersed molecules and small aggregates in the monolayer, and the molecules can move to some extent. At the beginning of the reaction, much more nuclei formed, and the nuclei grew to form nanocrystals gradually. Hybrid nanoparticles were formed from the interaction between the AA molecules and the nanocrystals. In order to reduce the energy of the system, the hybrid nanoparticles tend to aggregate with each other. Two kinds of interaction played a key role in the formation of round aggregates and 2D arrays: the attraction between the alkyl chains and the interaction between the nanocrystals. After a self-organization process, round aggregates were formed and 2D arrays were fabricated. However, when 66 μL of solution was spread, the mean molecular area is 0.20 nm2, suggesting that a liquid-condensed or solid monolayer was formed. In this case, AA molecules were immobilized at the interface. After nucleation and growth, the formed HgS nanocrystals attached under the monolayer and could not move. So, it is impossible for the nanocrystals to self-organize into round aggregates and ordered arrays. Temperature is another key parameter for the formation of the nanostructures. The influence of temperature on the nanostructures lies in two aspects. On one hand, temperature affects the interaction between the hybrids. On the other hand, temperature affects the nucleation and growth of the nancrystals. At lower temperature, though the nucleation and growth rate of the DOI: 10.1021/la102407s

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nanocrystals is lower, the interaction between the hybrids strengthened, because the movement of the hybrids at the interface reduced. So, it is easier to form round aggregates and 2D arrays of the aggregates. With the temperature increasing, the interaction between the hybrid nanoparticles weakened, while the growth rate of the nanocrystals increased, leading to the formation of aggregates composed of bigger nanocrystals.

Conclusion 2D ordered arrays of HgS nanocrystal aggregates were prepared at the air/water interface in a one-step process via interfacial reaction and self-assembly of the formed species. The key factor is the formation of the liquid-expanded monolayer that acts as a

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template to synthesize the nanocrystals and makes the formed species organize into aggregates and arrays. Temperature and other conditions also have great influence on the formation of the nanostructures. This is a simple, convenient, and inexpensive way to prepare 2D ordered arrays. 2D ordered arrays of metal or semiconductor nanocrystals or nanocrystal aggregates may be produced by using this method under appropriate conditions. Acknowledgment. We acknowledge the financial support from the National Natural Science Foundation of China (No. 20873078) and the National Basic Research Program of China (973 Program, No. 2009CB930103).

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