Ordered Honeycomb-Structured Gold Nanoparticle Filmswith 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 AppliedChemistry, 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 goldnanoparticle films by blowing moist air across the surface of the nanoparticle solution. The pore morphologycan be altered from circle to ellipse with tunable aspect ratios by carefully controlling the direction andvelocity of airflow. The formation mechanisms of different surface morphologies have been discussed interms of the surface and interfacial tension.

1. Introduction

Ordered porous materials with pore sizes in themicrometer and sub-micrometer range have elicited muchinterest recentlybecauseof theirapplications inseparationprocesses, catalysis, optoelectronic devices, and so forth.A variety of self-assembled templating methods have beendeveloped to create two-dimensional (2D) and three-dimensional (3D) porous structures, including inverse opaltechniques using colloidal crystal templates,1-6 templatingusing emulsions,7 forming honeycomb structures by rod-coil polymers,8,9 templating self-organized surfactants,10

and forming microphase-separated block copolymers.11-13

Recently, a new method of using water microspheres astemplates has generated great interest in preparingmacroporous films of polymers and nanoparticles.14-27 Inprevious investigations, only circular pores were obtained

in preparing macroporous polymeric and metal nanopar-ticle materials by the use of water microspheres astemplates.

In this work, highly ordered honeycomb-structureddodecanethiol-capped gold nanoparticle (Au-NP) filmswith both circular and elliptic pores were fabricated inthe presence of moist air flowing across the surface of thesolution. The pore morphology can be tuned from circleto ellipse with tunable aspect ratios by carefully controllingthe direction and velocity of airflow. This is the firstexample of a hexagonal array of elliptic pores by the“breath figure” method.

2. Experimental Section

Dodecanethiol (C12H25SH)-stabilized gold nanoparticles weresynthesized at room temperature using a two-phase arrestedgrowth method.28 Initially, 78 mL of a 0.03 M aqueous hydrogentetrachloroaurate(III) trihydrate (HAuCl4) solution and 54 mLof a 0.20 M chloroformic solution of phase transfer catalyst((C8H17)4NBr) were mixed and stirred vigorously for 1 h. Theorganic phase was subsequently collected, and 520 µL ofdodecanethiol was added. After the mixed solution of dodecane-thiol and HAuCl4 was stirred for 15 min, 65 mL of an aqueoussodium borohydride (0.43 M NaBH4) solution was injected. Themixture was stirred for 12 h before the organic/nanoparticle-rich phase was collected. The dispersion was washed three timeswith ethanol to remove the phase transfer catalyst, excess thiol,and reaction byproducts. The dodecanethiol-capped gold nano-particles 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 at20 °C, which is characterized with an Ostwald viscometer.

Dispersions of 0.1-1 wt % of dodecanethiol-capped Au-NPs intoluene were drop-cast onto the cleaned substrates (glass slides,

* To whom correspondence should be addressed. Phone: +86-431-5262175. Fax: +86-431-5262126. E-mail: ychan@ciac.jl.cn.

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10.1021/la047625l CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 01/29/2005

quartz, silicon wafers, etc.). Immediately, moist airflow througha tube with a diameter of 4.0 mm blew across (the velocity of themoist airflow was ∼30-70 m/min) the surface of the dodecane-thiol-capped Au-NP dispersion under ambient conditions (20-25 °C and 60-80% relative humidity) until the solvent wascompletely 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-NPsolution film was characterized by a thermometer. The mercuryprobe of the thermometer was wrapped with a thin layer of lenspaper. After the lens paper fully drank the dodecanethiol-cappedAu-NP solution, the thin layer of lens paper was blown by theairflow for 120 s. The temperature measured could be roughlyregarded as the surface temperature.

Scanning electron microscope (SEM) micrographs were takenusing a Philips XL-30-ESEM-FEG instrument operating at 20kV. The samples for the SEM were coated with a 20-30 Å layerof Au to make them conductive.

3. Results and Discussion

Gold nanoparticles (Au-NPs) are entities of choice todesign functional materials due to their specific size-dependent electronic and optical properties. Especiallythe highly ordered macroporous films of gold nanoparticlesfind applications in photonic crystals, optoelectronicdevices, and surface-enhanced Raman spectroscopy.29-32

Herein, uniformly sized dodecanethiol-capped gold nano-particles with an average diameter of 2.1 nm weresynthesized using a two-phase (organic-water) arrestedgrowth method.28 Due to being protected by the do-decanethiol, the gold nanoparticles are hydrophobic andcan disperse in a variety of organic solvents, such aschloroform and toluene.

Highly ordered honeycomb structures were readilyformed by blowing an airflow across the solution surfacein a moist atmosphere (humidity 60-80%) after a 0.1-1wt % dodecanethiol-capped gold nanoparticle dispersionin toluene was drop-cast on a glass slide (Figure 1). Whena moist airflow blows across the toluene solution ofdodecanethiol-capped gold nanoparticles, rapid evapora-tion of the solution increases the superficial concentrationand induces a rapid cooling of the solution surface due tothe volatile solvent (e.g., toluene). When moist air contactsthe cold solution surface, moisture condenses on it andproduces sub-micrometer- or micrometer-sized waterdroplets with a narrow size distribution.34-38 Convectioncurrents induced by temperature gradients and capillaryforces between the water droplets favor a regular stackingof the water spheres. After toluene has evaporatedcompletely, the water droplets have become immobilizedin the dodecanethiol-capped gold nanoparticle film. Thesample then warmed, and the encapsulated water dropletssubsequently evaporated. As a result of the evaporationof the water droplet templates, a highly ordered honey-comb-structured film of dodecanethiol-capped Au-NPswith two-dimensional, hexagonal, and close-packed airholes formed (Figure 2). A top view SEM image of thedodecanethiol-capped gold nanoparticle film clearly dis-plays that the pores are circular with a diameter of 1-3µm. The inset fast Fourier transform images indicate thatthe porous film has a two-dimensional, long-range periodicstructure and a hexagonal array of holes. We repeatedthe experiment in the absence of moisture in the atmo-

(29) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature1999, 401, 548.

(30) Kulinowski, K. M.; Jiang, P.; Vaswani, H.; Colvin, V. L. Adv.Mater. 2000, 12, 833.

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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,

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Figure 1. Schemes of the formation of the circular honeycomb structure in the films of dodecanethiol-capped gold nanoparticles.

2018 Langmuir, Vol. 21, No. 5, 2005 Li et al.

sphere, and only a solid dodecanethiol-capped gold nano-particle film was left without the ordered arrays of holes.Itprovedthatmoisturewas responsible for thehoneycomb-pattern formation, where the monodisperse water dropletsarrange in a hexagonal arrangement and act as a templatearound which the dodecanethiol-capped gold nanoparticlesassemble.15,26,33

The nucleation, growth, and ordered packing of waterdroplets as templates may be the key parameters in thefabrication of a highly ordered array of holes by the breathfigure method. Due to the high vapor pressure of thesolvent and the velocity of moisture across the solutionsurface, the solvent evaporation leads to the rapid coolingof the surface. The surface was measured to reach aminimum temperature of ∼5 °C. This cooling results inthe nucleation and growth of water droplets that grow asa function of time.33 Water droplet condensation on thecold solid surfaces, known as “breath figures”, has beenstudied for over a century, starting with the early worksof Lord Rayleigh,39,40 Baker,41 and Aitken42 and morerecently by the works of Knobler, Beysens, and co-workers.43,44 Droplet-droplet attraction and convectivecurrents in the evaporating solvent can enhance the waterdroplet self-assembly. When the condensed water dropletsdeposit into the solution, a thin organic liquid filmsurrounds the droplets and the dodecanethiol-capped goldnanoparticles adsorb and precipitate at the solvent-waterinterface.15,26 The water droplets encapsulated with ananoparticle layer can behave as hard spheres and providea weak repulsive capillary force that inhibits coalescence.Then, the water droplets can self-organize to form ahexagonal array by capillary interactions and convectioncurrents and preserve an ordered honeycomb structureon the nanoparticle film during drying.

Through changing the direction and velocity of airflowin the moist atmosphere, interestingly, highly orderedhexagonal arrays of elliptic pores with different aspectratios were first fabricated (Figure 3). SEM images inFigure 3 exhibit that the elliptic holes have a narrow size

distribution. The formation processes of the hexagonalarray of elliptic pores are similar to the circular pores,except for the airflow direction. Once the hexagonal arrayof spherical water droplets formed and the viscosity of thenanoparticle solution increased due to the evaporation ofthe solvent, the spherical water droplets would deformand become ellipsoidal under the action of an additionalshear. Finally, the elliptic pores left on the nanoparticlefilms after the solvent and the water evaporated.

The formation mechanisms of different morphologicpores are revealed in terms of the surface and interfacialtension (Figure 4). When a water droplet condenses onthe surface of the nanoparticle solution, it will receivethree forces, including two surface tensions and oneinterfacial tension, at the static state without airflow.17,18

We assume that the dodecanethiol-capped Au-NPs do notaffect the interfacial tension between water and toluene(σW/sol ) 36.1 mN/m). The surface tensions between tolueneand air and between water and air are σsol/air ) 28.4 mN/mand σW/air ) 72.8 mN/m, respectively. The three tensionstry to make the water droplet keep sphericity on thesurface of the solution, as shown in Figure 4a. Whenairflow blows across the solution surface, the water dropletwill receive an additional force (Fairflow) originating fromthe airflow along the normal of the solution surface. TheYoung equations relate the interfacial surface tensionsand the contact angles for the projections in the nano-particle solution:17,18

However, the shape and the hexagonal packing patternof the water droplets in Figure 4a are not affected by theforce of Fairflow. Correspondingly, the resulting honeycomb-structured film with circular cavities is exhibited in Figure2.

When the airflow blows across the solution surface alongthe direction having a small angle (θ) with respect to thenormal of the solution surface, the water droplet willreceive an additional shear (F′airflow), the vector of whichis along the same direction as the airflow (Figure 4b).From the condition that gives the balance of the surfacetensions and the shear, we can describe the new equi-

(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.

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

σsol/air ) σW/air cos R + σW/sol cos â (1)

σW/air sin R ) σW/sol sin â + Fairflow (2)

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librium state at the edge circumference of the contactbetween the water drop and the nanoparticle solution bythe following two equations:

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

to decrease so that the composite force of σW/sol and σW/airon the horizontal equates the composite force of σsol/air andF′airflow on the horizontal. As a result of having receivedthe additional shear (F′airflow), the spherical water dropletwill be distorted and oriented to be ellipsoidal, as shownin Figure 4b. The aspect ratio of the ellipsoidal waterdroplet will not increase until the resultant force of σW/soland 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 ahexagonally ordered array. After the solvent evaporatescompletely, the ellipsoidal water droplet is captured andfixed in the nanoparticle films. When the oriented waterdropletsevaporatecompletely, thepores in thehoneycomb-structured film also turn into an ellipse, as shown in Figure3.

The force of F′airflow can increase with the enhancementof the velocity of airflow. The morphology of the ellipticpores could be tuned via carefully controlling the velocityof airflow. The larger the shear of F′airflow is, the smallerthe angles R′ and â′ become. Here, an airflow that blowsacross the solution surface along an angle of 15° withrespect to the normal of the solution surface was used.Thereafter, theaspect ratiosof theellipticpores canchangefrom 1.50 to 2.55 with an increase of the velocity of airflowfrom 32 to 64 m/min (Figure 3).

Because the ellipsoidal water droplets were difficult tocatch, the angles R′ and â′ in the equations could not beobtained. However, we provided an estimate for the forcedue to the airflow and compared it to the capillary forces

Figure 3. SEM images of the highly ordered elliptic pores with a hexagonal array. The airflow across the solution surface is alongthe 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) 40m/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 offorming (a) circular pores and (b) elliptic pores.

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

σW/air sin R′ ) σW/sol sin â′ + F′airflow cos θ (4)

2020 Langmuir, Vol. 21, No. 5, 2005 Li et al.

that are acting on the individual droplet.17,18 The waterdrop is assumed to have the same diameter as the porediameter, which is ∼2 µm from the SEM image in Figure2. When the velocity of airflow was 32, 40, 48, and 64m/min, the component force of F′airflow that acted on theindividual 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 dropand air is approximately πR2, the capillary force on theindividual 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 thecapillary pressure, which results in the spherical shapeof the droplet. The force on the individual water dropletfrom 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 compositeforce of F′airflow and Fsol/air was larger than that of FW/sol andFW/air. Thus, the water droplets were elongated and theangles R′ and â′ became smaller until the composite forceof F′airflow and Fsol/air equaled that of FW/sol and FW/air, whichresulted from the increase of the contact area of the waterdroplets with the air and the solution. The force F′airflow

played a key role in deforming the water droplets andforming the elliptic pores.

4. ConclusionsIn summary, we provided the first example of a two-

dimensional, hexagonal array of elliptic pores on do-decanethiol-capped gold nanoparticle films by the breathfigure method. The hole morphology can be altered fromcircle to ellipse with tunable aspect ratios by carefullycontrolling the direction and velocity of airflow. Theformation mechanisms of different surface morphologieshave been discussed in terms of the surface and interfacialtension.

Acknowledgment. This work is subsidized bythe National Natural Science Foundation of China(50125311, 20334010, 20274050, 50390090, 50373041,20490220, 20474065, 50403007), the Ministry of Scienceand Technology of China (2003CB615601), the ChineseAcademy of Sciences (Distinguished Talents Program,KJCX2-SW-H07), and the Jilin Distinguished YoungScholars Program (20010101).

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