Ordered Honeycomb-Structured Gold Nanoparticle Films with

Langmuir 2005, 21, 2017-2021

2017

Ordered Honeycomb-Structured Gold Nanoparticle Films with Changeable Pore Morphology: From Circle to Ellipse Jian Li, Juan Peng, Weihuan Huang, Yang Wu, Jun Fu, Yang Cong, Longjian Xue, and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China Received September 24, 2004. In Final Form: December 9, 2004 Two-dimensionally ordered honeycomb structures have been prepared on dodecanethiol-capped gold nanoparticle films by blowing moist air across the surface of the nanoparticle solution. The pore morphology can be altered from circle to ellipse with tunable aspect ratios by carefully controlling the direction and velocity of airflow. The formation mechanisms of different surface morphologies have been discussed in terms of the surface and interfacial tension.

1. Introduction Ordered porous materials with pore sizes in the micrometer and sub-micrometer range have elicited much interest recently because of their applications in separation processes, catalysis, optoelectronic devices, and so forth. A variety of self-assembled templating methods have been developed to create two-dimensional (2D) and threedimensional (3D) porous structures, including inverse opal techniques using colloidal crystal templates,1-6 templating using emulsions,7 forming honeycomb structures by rodcoil polymers,8,9 templating self-organized surfactants,10 and forming microphase-separated block copolymers.11-13 Recently, a new method of using water microspheres as templates has generated great interest in preparing macroporous films of polymers and nanoparticles.14-27 In previous investigations, only circular pores were obtained * To whom correspondence should be addressed. Phone: +86431-5262175. Fax: +86-431-5262126. E-mail: [email protected]. (1) Velev, O. D.; Jede, T. A.; Lobo, R. E.; Lenhoff, A. M. Nature 1997, 389, 447. (2) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (3) Kulinowski, K. M.; Jiang, P.; Vaswani, H.; Colvin, V. L. Adv. Mater. 2000, 12, 833. (4) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630. (5) Park, S. H.; Xia, Y. Chem. Mater. 1998, 10, 1745. (6) Deutsh, M.; Vlasov, Y. A.; Norris, D. J. Adv. Mater. 2000, 12, 1176. (7) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (8) Widawski, G.; Franc¸ ois, B. Nature 1994, 369, 387. (9) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (10) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (11) Jeong, U.; Kim, H.-C.; Rodriguez, R. L.; Tsai, I. Y.; Stafford, C. M.; Kim, J. K.; Hawker, C. J.; Russell, T. P. Adv. Mater. 2002, 14, 274. (12) Park, C.; Cheng, J. Y.; Fasolka, M. J.; Mayes, A. M.; Ross, C. A.; Thomas, E. L. Appl. Phys. Lett. 2001, 79, 848. (13) Mansky, P.; Harrison, C. K.; Chaikin, P. M. Appl. Phys. Lett. 1996, 68, 2586. (14) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387. (15) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225. (16) Franc¸ ois, B.; Pitois, O.; Franc¸ ois, J. Adv. Mater. 1995, 7, 1041. (17) Govor, L. V.; Bashmakov, I. A.; Kiebooms, R.; Dyakonov, V.; Parisi, J. Adv. Mater. 2001, 13, 588. (18) Govor, L. V.; Bashmakov, I. A.; Kaputski, F. N.; Pientka, M.; Parisi, J. Macromol. Chem. Phys. 2000, 201, 2721. (19) Stenzel-Rosenbaum, M. H.; Davis, T. P.; Fane, A. G.; Chen, V. Angew. Chem., Int. Ed. 2001, 40, 3428. (20) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071. (21) Peng, J.; Han, Y. C.; Fu, J.; Yang, Y. M.; Li, B. Y. Macromol. Chem. Phys. 2003, 204, 125.

in preparing macroporous polymeric and metal nanoparticle materials by the use of water microspheres as templates. In this work, highly ordered honeycomb-structured dodecanethiol-capped gold nanoparticle (Au-NP) films with both circular and elliptic pores were fabricated in the presence of moist air flowing across the surface of the solution. The pore morphology can be tuned from circle to ellipse with tunable aspect ratios by carefully controlling the direction and velocity of airflow. This is the first example of a hexagonal array of elliptic pores by the “breath figure” method. 2. Experimental Section Dodecanethiol (C12H25SH)-stabilized gold nanoparticles were synthesized at room temperature using a two-phase arrested growth method.28 Initially, 78 mL of a 0.03 M aqueous hydrogen tetrachloroaurate(III) trihydrate (HAuCl4) solution and 54 mL of a 0.20 M chloroformic solution of phase transfer catalyst ((C8H17)4NBr) were mixed and stirred vigorously for 1 h. The organic phase was subsequently collected, and 520 µL of dodecanethiol was added. After the mixed solution of dodecanethiol and HAuCl4 was stirred for 15 min, 65 mL of an aqueous sodium borohydride (0.43 M NaBH4) solution was injected. The mixture was stirred for 12 h before the organic/nanoparticlerich phase was collected. The dispersion was washed three times with ethanol to remove the phase transfer catalyst, excess thiol, and reaction byproducts. The dodecanethiol-capped gold nanoparticles were then redispersed in a variety of organic solvents, such as chloroform and toluene. The viscosity of 0.5 wt % dodecanethiol-capped Au-NP solution in toluene is ∼0.71 cP at 20 °C, which is characterized with an Ostwald viscometer. Dispersions of 0.1-1 wt % of dodecanethiol-capped Au-NPs in toluene were drop-cast onto the cleaned substrates (glass slides, (22) de Boer, B.; Stalmach, U.; Nijland, H.; Hadziioannou, G. Adv. Mater. 2000, 12, 1581. (23) de Boer, B.; Stalmach, U.; Melzer, C.; Hadziioannou, G. Synth. Met. 2001, 121, 1541. (24) Song, L.-L.; Bly, R. K.; Wilson, J. N.; Bakbak, S.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. Adv. Mater. 2004, 16, 115. (25) Yonezawa, T.; Onoue, S.-Y.; Kimizuka, N. Adv. Mater. 2001, 13, 140. (26) Shah, P. S.; Sigman, M. B.; Stowell, C. A., Jr.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. Adv. Mater. 2003, 15, 971. (27) Bo¨ker, A.; Lin, Y.; Chiapperini, K.; Horowitz, R.; Thompson, M.; Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A. D.; Emrick, T.; Russell, T. P. Nat. Mater. 2004, 3, 302. (28) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.

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Figure 1. Schemes of the formation of the circular honeycomb structure in the films of dodecanethiol-capped gold nanoparticles. quartz, silicon wafers, etc.). Immediately, moist airflow through a tube with a diameter of 4.0 mm blew across (the velocity of the moist airflow was ∼30-70 m/min) the surface of the dodecanethiol-capped Au-NP dispersion under ambient conditions (2025 °C and 60-80% relative humidity) until the solvent was completely evaporated (Figure 1a). The thickness of the film is ∼460 nm (0.5 wt % dodecanethiol-capped Au-NPs in toluene). The surface temperature of the dodecanethiol-capped Au-NP solution film was characterized by a thermometer. The mercury probe of the thermometer was wrapped with a thin layer of lens paper. After the lens paper fully drank the dodecanethiol-capped Au-NP solution, the thin layer of lens paper was blown by the airflow for 120 s. The temperature measured could be roughly regarded as the surface temperature. Scanning electron microscope (SEM) micrographs were taken using a Philips XL-30-ESEM-FEG instrument operating at 20 kV. The samples for the SEM were coated with a 20-30 Å layer of Au to make them conductive.

3. Results and Discussion Gold nanoparticles (Au-NPs) are entities of choice to design functional materials due to their specific sizedependent electronic and optical properties. Especially the highly ordered macroporous films of gold nanoparticles find applications in photonic crystals, optoelectronic devices, and surface-enhanced Raman spectroscopy.29-32 Herein, uniformly sized dodecanethiol-capped gold nanoparticles with an average diameter of 2.1 nm were synthesized using a two-phase (organic-water) arrested growth method.28 Due to being protected by the dodecanethiol, the gold nanoparticles are hydrophobic and can disperse in a variety of organic solvents, such as chloroform and toluene. (29) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (30) Kulinowski, K. M.; Jiang, P.; Vaswani, H.; Colvin, V. L. Adv. Mater. 2000, 12, 833. (31) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (32) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W. Adv. Mater. 2001, 13, 396.

Highly ordered honeycomb structures were readily formed by blowing an airflow across the solution surface in a moist atmosphere (humidity 60-80%) after a 0.1-1 wt % dodecanethiol-capped gold nanoparticle dispersion in toluene was drop-cast on a glass slide (Figure 1). When a moist airflow blows across the toluene solution of dodecanethiol-capped gold nanoparticles, rapid evaporation of the solution increases the superficial concentration and induces a rapid cooling of the solution surface due to the volatile solvent (e.g., toluene). When moist air contacts the cold solution surface, moisture condenses on it and produces sub-micrometer- or micrometer-sized water droplets with a narrow size distribution.34-38 Convection currents induced by temperature gradients and capillary forces between the water droplets favor a regular stacking of the water spheres. After toluene has evaporated completely, the water droplets have become immobilized in the dodecanethiol-capped gold nanoparticle film. The sample then warmed, and the encapsulated water droplets subsequently evaporated. As a result of the evaporation of the water droplet templates, a highly ordered honeycomb-structured film of dodecanethiol-capped Au-NPs with two-dimensional, hexagonal, and close-packed air holes formed (Figure 2). A top view SEM image of the dodecanethiol-capped gold nanoparticle film clearly displays that the pores are circular with a diameter of 1-3 µm. The inset fast Fourier transform images indicate that the porous film has a two-dimensional, long-range periodic structure and a hexagonal array of holes. We repeated the experiment in the absence of moisture in the atmo(33) Pitois, O.; Franc¸ ois, B. Colloid Polym. Sci. 1999, 277, 574. (34) Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Phys. Rev. Lett. 1996, 76, 3762. (35) Family, F.; Meakin, P. Phys. Rev. Lett. 1988, 61, 428. (36) Peng, J.; Han, Y. C.; Yang, Y. M.; Li, B. Y. Polymer 2004, 45, 447. (37) Steyer, A.; Guenoun, P.; Beysens, D.; Knobler, C. M. Phys. Rev. B 1990, 42, 1086. (38) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79.

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Figure 2. SEM images of the highly ordered circular honeycomb-structured films of dodecanethiol-capped gold nanoparticles. The inset shows two-dimensional fast Fourier transforms (FFTs) of the topography, illustrating the long-range order of the array.

sphere, and only a solid dodecanethiol-capped gold nanoparticle film was left without the ordered arrays of holes. It proved that moisture was responsible for the honeycombpattern formation, where the monodisperse water droplets arrange in a hexagonal arrangement and act as a template around which the dodecanethiol-capped gold nanoparticles assemble.15,26,33 The nucleation, growth, and ordered packing of water droplets as templates may be the key parameters in the fabrication of a highly ordered array of holes by the breath figure method. Due to the high vapor pressure of the solvent and the velocity of moisture across the solution surface, the solvent evaporation leads to the rapid cooling of the surface. The surface was measured to reach a minimum temperature of ∼5 °C. This cooling results in the nucleation and growth of water droplets that grow as a function of time.33 Water droplet condensation on the cold solid surfaces, known as “breath figures”, has been studied for over a century, starting with the early works of Lord Rayleigh,39,40 Baker,41 and Aitken42 and more recently by the works of Knobler, Beysens, and coworkers.43,44 Droplet-droplet attraction and convective currents in the evaporating solvent can enhance the water droplet self-assembly. When the condensed water droplets deposit into the solution, a thin organic liquid film surrounds the droplets and the dodecanethiol-capped gold nanoparticles adsorb and precipitate at the solvent-water interface.15,26 The water droplets encapsulated with a nanoparticle layer can behave as hard spheres and provide a weak repulsive capillary force that inhibits coalescence. Then, the water droplets can self-organize to form a hexagonal array by capillary interactions and convection currents and preserve an ordered honeycomb structure on the nanoparticle film during drying. Through changing the direction and velocity of airflow in the moist atmosphere, interestingly, highly ordered hexagonal arrays of elliptic pores with different aspect ratios were first fabricated (Figure 3). SEM images in Figure 3 exhibit that the elliptic holes have a narrow size (39) Rayleigh, Lord. Nature 1911, 86, 416. (40) Rayleigh, Lord. Nature 1912, 90, 436. (41) Baker, J. T. Philos. Mag. 1922, 56, 752. (42) Aitken, J. Nature 1911, 86, 516. (43) Beysens, D.; Steyer, A.; Guenoun, P.; Fritter, D.; Knobler, C. M. Phase Transitions 1991, 31, 219. (44) Beysens, D.; Knobler, C. M. Phys. Rev. Lett. 1986, 57, 1433.

distribution. The formation processes of the hexagonal array of elliptic pores are similar to the circular pores, except for the airflow direction. Once the hexagonal array of spherical water droplets formed and the viscosity of the nanoparticle solution increased due to the evaporation of the solvent, the spherical water droplets would deform and become ellipsoidal under the action of an additional shear. Finally, the elliptic pores left on the nanoparticle films after the solvent and the water evaporated. The formation mechanisms of different morphologic pores are revealed in terms of the surface and interfacial tension (Figure 4). When a water droplet condenses on the surface of the nanoparticle solution, it will receive three forces, including two surface tensions and one interfacial tension, at the static state without airflow.17,18 We assume that the dodecanethiol-capped Au-NPs do not affect the interfacial tension between water and toluene (σW/sol ) 36.1 mN/m). The surface tensions between toluene and air and between water and air are σsol/air ) 28.4 mN/m and σW/air ) 72.8 mN/m, respectively. The three tensions try to make the water droplet keep sphericity on the surface of the solution, as shown in Figure 4a. When airflow blows across the solution surface, the water droplet will receive an additional force (Fairflow) originating from the airflow along the normal of the solution surface. The Young equations relate the interfacial surface tensions and the contact angles for the projections in the nanoparticle solution:17,18

σsol/air ) σW/air cos R + σW/sol cos β

(1)

σW/air sin R ) σW/sol sin β + Fairflow

(2)

However, the shape and the hexagonal packing pattern of the water droplets in Figure 4a are not affected by the force of Fairflow. Correspondingly, the resulting honeycombstructured film with circular cavities is exhibited in Figure 2. When the airflow blows across the solution surface along the direction having a small angle (θ) with respect to the normal of the solution surface, the water droplet will receive an additional shear (F′airflow), the vector of which is along the same direction as the airflow (Figure 4b). From the condition that gives the balance of the surface tensions and the shear, we can describe the new equi-

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Figure 3. SEM images of the highly ordered elliptic pores with a hexagonal array. The airflow across the solution surface is along the direction at an angle of 15° with respect to the normal of the solution surface. The velocity of airflow is (a) 32 m/min, (b) 40 m/min, (c) 48 m/min, and (d) 64 m/min. The aspect ratio of the elliptic pores is (a) 1.50, (b) 1.68, (c) 2.15, and (d) 2.55, respectively.

Figure 4. Models of the water droplets as the templates of forming (a) circular pores and (b) elliptic pores.

librium state at the edge circumference of the contact between the water drop and the nanoparticle solution by the following two equations:

σsol/air + F′ sin θ ) σW/air cos R′ + σW/sol cos β′ (3) σW/air sin R′ ) σW/sol sin β′ + F′airflow cos θ

(4)

If the values of the three tensions σW/sol, σsol/air, and σW/air are constant, the shear induces a shift in the force balance on the spherical water droplet. To reach a new balance of the forces on the water droplet, the angles R′ and β′ have

to decrease so that the composite force of σW/sol and σW/air on the horizontal equates the composite force of σsol/air and F′airflow on the horizontal. As a result of having received the additional shear (F′airflow), the spherical water droplet will be distorted and oriented to be ellipsoidal, as shown in Figure 4b. The aspect ratio of the ellipsoidal water droplet will not increase until the resultant force of σW/sol and F′airflow equates the resulting tension of σsol/air and σW/air (i.e., the generalized force of the water droplet is zero). The shape of the water droplets turns into an ellipsoid, and the pattern of the water droplets still keeps a hexagonally ordered array. After the solvent evaporates completely, the ellipsoidal water droplet is captured and fixed in the nanoparticle films. When the oriented water droplets evaporate completely, the pores in the honeycombstructured film also turn into an ellipse, as shown in Figure 3. The force of F′airflow can increase with the enhancement of the velocity of airflow. The morphology of the elliptic pores could be tuned via carefully controlling the velocity of airflow. The larger the shear of F′airflow is, the smaller the angles R′ and β′ become. Here, an airflow that blows across the solution surface along an angle of 15° with respect to the normal of the solution surface was used. Thereafter, the aspect ratios of the elliptic pores can change from 1.50 to 2.55 with an increase of the velocity of airflow from 32 to 64 m/min (Figure 3). Because the ellipsoidal water droplets were difficult to catch, the angles R′ and β′ in the equations could not be obtained. However, we provided an estimate for the force due to the airflow and compared it to the capillary forces

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that are acting on the individual droplet.17,18 The water drop is assumed to have the same diameter as the pore diameter, which is ∼2 µm from the SEM image in Figure 2. When the velocity of airflow was 32, 40, 48, and 64 m/min, the component force of F′airflow that acted on the individual droplets in the horizontal projection was 3.0 × 10-7, 4.72 × 10-7, 6.73 × 10-7, and 1.20 × 10-6 N, respectively. If the contact plane between the water drop and air is approximately πR2, the capillary force on the individual water droplets from the tension σW/air is FW/air ) πR2PL ) πR2(2σW/air/R) ) 2πRσW/air ) 4.50 × 10-7 N, where R is the radius of the water droplet and PL is the capillary pressure, which results in the spherical shape of the droplet. The force on the individual water droplet from the tension σW/sol is FW/sol ) 2πRσW/sol ) 2.26 × 10-7 N. The force from the tension σsol/air is Fsol/air ) 2πRσsol/air ) 1.78 × 10-7 N. In the horizontal projection, the composite force of F′airflow and Fsol/air was larger than that of FW/sol and FW/air. Thus, the water droplets were elongated and the angles R′ and β′ became smaller until the composite force of F′airflow and Fsol/air equaled that of FW/sol and FW/air, which resulted from the increase of the contact area of the water droplets with the air and the solution. The force F′airflow

played a key role in deforming the water droplets and forming the elliptic pores. 4. Conclusions In summary, we provided the first example of a twodimensional, hexagonal array of elliptic pores on dodecanethiol-capped gold nanoparticle films by the breath figure method. The hole morphology can be altered from circle to ellipse with tunable aspect ratios by carefully controlling the direction and velocity of airflow. The formation mechanisms of different surface morphologies have been discussed in terms of the surface and interfacial tension. Acknowledgment. This work is subsidized by the National Natural Science Foundation of China (50125311, 20334010, 20274050, 50390090, 50373041, 20490220, 20474065, 50403007), the Ministry of Science and Technology of China (2003CB615601), the Chinese Academy of Sciences (Distinguished Talents Program, KJCX2-SW-H07), and the Jilin Distinguished Young Scholars Program (20010101). LA047625L