Wetting Phenomenon in Nanoporous Gold Films E

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M. Utzc, and M. L. Reeda a Department of ... independent of the size of the droplet. Non-porous gold films ... morphologies. Scale bar is the same for all images.
ECS Transactions, 6 (11) 83-89 (2007) 10.1149/1.2790417, © The Electrochemical Society

Wetting Phenomenon in Nanoporous Gold Films E. Sekera, T. Tauerb, J. Zhua, M. Begleya,b,c, H. Bart-Smithc, G. Zangarib, R. G. Kellyb, M. Utzc, and M. L. Reeda a

Department of Electrical and Computer Engineering Department of Materials Science and Engineering c Department of Mechanical and Aerospace Engineering University of Virginia, Charlottesville, VA 22904 b

We have fabricated nanoporous gold (np-Au) films on silicon wafers by dealloying films formed by simultaneous sputter deposition of gold and silver. We report an interesting wetting phenomenon observed after placing a droplet of liquid on the npAu film surface. As a droplet was dispensed onto the surface, wetting equilibrium was quickly reached and a “wetting halo” appeared with its perimeter equidistant from the perimeter of the droplet. Non-porous gold control samples did not display such behavior. This phenomenon was systematically studied by dispensing liquid droplets with different surface tensions and then measuring the width of the wetting halo. It was observed that halo width increased with surface tension. Experiments were extended to observe the effects of film thickness and porosity on the halo width. The width increased as film thickness increased. The relationship between the width and porosity was inconclusive due to large data scatter. Introduction There has been growing interest in nanoporous gold (np-Au) due to its biocompatibility and large surface-to-volume ratio. Sensor and catalyst applications of this material have been investigated (1,2). One way to produce np-Au is to selectively dissolve (dealloy) the less noble constituent of a gold-based alloy. The mechanisms of dealloying have been previously studied using bulk alloys and thin foils (3-10). Meanwhile several groups have researched the structure-property relationship in np-Au (11-16), as others focused on its sensor applications (17-19). Following the recent history of studies on np-Au structures (i.e., films bonded on a substrate and bulk materials), our group has developed a new technique to fabricate freestanding np-Au structures using conventional microfabrication methods (20). We observed very different stress and morphology evolution in our thermal treatment studies on freestanding beams (21) compared to bonded films or bulk materials. Further studies on thermal treatment of gold-silver alloy and np-Au led us to even more interesting observations (22). In this paper, we will direct our attention to a non-mechanical aspect of np-Au. We will report on an interesting wetting phenomenon that was coincidentally observed during

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ECS Transactions, 6 (11) 83-89 (2007)

our experiments on mechanical properties of np-Au. As a drop of liquid is dispensed onto a flat nanoporous gold film, a wetting halo appears, as seen in Fig. 1. When the drop is placed onto the surface, the wetting equilibrium is very rapidly reached, and the perimeter of the resulting halo is equidistant from the perimeter of the drop. The halo is due to liquid being pulled into pores via capillary action. The width of the halo is independent of the size of the droplet. Non-porous gold films do not exhibit this behavior, that is, no halo forms and only a droplet is seen.

Figure 1. Wetting behavior on blanket np-Au film on a silicon wafer. This behavior was systematically studied to investigate how the halo width varied with (i) film thickness, (ii) film porosity, and (iii) pore radius. Experiment This section will summarize the experimental procedure for sample preparation and characterization methods. Sample Preparation 50 mm-diameter silicon wafers with p-type doping and were used as the substrate. Nine ~1.5 mm-wide and ~40 mm-long rectangles of silver-gold alloy were patterned on 50 mm silicon wafers. The np-Au film is obtained by dealloying a sputtered gold-silver alloy. However, in order to prevent the films from peeling during dealloying a ~100 nmthick adhesion layer of chrome and gold was sputtered prior to depositing gold-silver alloy. The atomic composition of the alloy was determined to be 40%Au-60%Ag via energy dispersive spectroscopy. Three wafers were coated with different film thicknesses by varying the deposition time. Wafers were then scribed into six equal pieces with a dicing saw, but not broken into individual pieces yet. All three wafers were dealloyed in HNO3 at 95°C for ~10 minutes, rinsed in deionized (DI) water, dried with nitrogen, and manually broken into quarters. The morphology of np-Au films can be easily coarsened via thermal treatment (9,21). Two additional morphologies were created by annealing some samples at 150°C and 200°C for 10 minutes. Thermal treatment was performed with an AXIC Inc. As-One rapid thermal processor (RTP) in nitrogen at atmospheric pressure. The temperature ramping rate was 20°C/sec, and the sample was removed from the RTP after 4 minutes of cooling when the temperature dropped below 50°C. At the end of the annealing step, two identical sets of nine samples (three half-rectangles each) combining three different film thicknesses and three different pore morphologies were made.

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ECS Transactions, 6 (11) 83-89 (2007)

Characterization Methods Film thickness was measured using a Dektak 8 stylus-based profilometer. In order not to damage the surface, a very small stylus load of 1 mg was used. Each of the three rectangles per sample was measured, and the thickness was averaged. The pore morphology of the samples was observed with a Zeiss FESEM SUPRA 40 scanning electron microscope at 50,000X magnifications as seen in Fig. 2. Captured images were then digitally processed using WCIF ImageJ software (23) to obtain percentage porosity and average pore radius of each sample. Percent porosity is the area of ‘dark’ portions of the image with respect to the total area. Each isolated dark region was considered a pore, and the pore radius was calculated with the assumption that the pores were circular. For details of the image processing procedure, please see ref. (21). Again, each rectangle was separately observed, and the data were averaged.

Figure 2. SEM micrographs of the nine samples with various thicknesses and morphologies. Scale bar is the same for all images. The columns correspond to annealing temperatures (left to right): no anneal, 150°C, and 200°C. The rows correspond to deposition thicknesses (left- to right): 200, 400, and 600 nm. Wetting Halo Width Measurements Two liquids with different surface tensions (ethanol and DI water) were used for the tests. DI water has approximately three times the surface free energy of ethanol. First, ethanol was poured into a shallow petri dish. A sample of three half-rectangles were dipped in the liquid and placed underneath a stereoscopic microscope. A digital video camera was attached to the microscope to record the wetting halo until the entire liquid evaporated. Fig. 3 is a schematic of how the wet sample looked underneath the microscope. The same procedure of immersion and recording was performed on the remaining eight samples, and repeated for DI water with fresh samples.

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ECS Transactions, 6 (11) 83-89 (2007)

Halo width

1600µm

np-Au

Si

Figure 3. Schematic view of the sample following immersion in liquid. The recorded videos were transferred to a computer, and the halo width was measured using a screen-ruler software. For each sample, the halo width appearing on each rectangle was measured multiple times at various locations and video frames. Results and Discussion It was observed that as the thickness of the np-Au film increased, the halo width increased for both ethanol and DI water (Fig. 4). DI water had a relatively larger dependence on film thickness compared to ethanol.

Figure 4. Halo width versus np-Au film thickness for ethanol and DI water. Unfortunately it was not possible to reach a conclusion about the relationship between halo width and pore morphology due to large data scatter (Figs. 5, 6). For thicker films, thermal treatment resulted in cracks to form on the surface, and this resulted in a nonuniform surface, which is speculated to increase data scatter. Surprisingly, no wetting halo appeared for the thick samples treated at 200°C. It should be noted that these samples had the largest cracks (see Fig. 2, lower right corner).

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ECS Transactions, 6 (11) 83-89 (2007)

Figure 5. Halo width versus percentage porosity of np-Au film.

Figure 6. Halo width versus average pore radius of np-Au film. The most obvious observation was the dependence of halo width on the surface tension of liquids. DI water, having a higher surface tension than ethanol, always displayed a larger halo width. A simple thermodynamic model based on thermodynamic equilibrium of surface energies is not sufficient to explain the experimental observations. In such a model, the total surface energy, consisting of the curved liquid-air surface of the droplet and the liquid-gold interface in the nanoporous gold, is minimized. Depending on the contact angle of the liquid on a flat gold surface, this leads to either complete absorption of the liquid into the nanoporous film (super-hydrophilic behavior) or the complete expulsion of the liquid from the film (super-hydrophobic behavior). An intermediate situation similar to the one observed only results if the contact angle is approximately 89° (24). In this case, a droplet of fixed radius is predicted to exist in equilibrium with the absorbed liquid.

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ECS Transactions, 6 (11) 83-89 (2007)

Adding liquid to the system would increase the volume of the absorbed phase, while the droplet would remain constant in size. In this situation, the chemical potential of the liquid in the droplet phase depends on the surface curvature (and hence, the size) of the droplet, while it is independent of the volume in the adsorbed film. Therefore, a simple surface energy equilibrium theory predicts either complete absorption or expulsion, or a fixed droplet in equilibrium with a variable size absorbed phase. The experimental evidence, however, points to an absorbed phase of fixed dimension (halo size) in equilibrium with a droplet phase of arbitrary dimension. Clearly, the simple surface energy equilibrium does not capture the essential physics of the observed phenomenon. It is probable that liquid evaporation at the halo-air interface and inhomogeniety of the porous matrix need to be considered for a more accurate model. Conclusion This paper reported an interesting wetting behavior, and systematically explored the connections between wetting halo width and surface tension, film thickness, and pore morphology. The halo width increased with surface tension and film thickness. It was not possible to reach a conclusion about the dependence of halo width on pore morphology due to data scatter. Currently, we are investigating the effect of evaporation and humidity on the wetting phenomenon, as we suspect it may play a role in maintaining the observed wetting equilibrium. Acknowledgments The authors gratefully acknowledge support of this research by the National Science Foundation through Grant DMI-0507023 References 1. J. F. Huang and I. W. Sun, Adv. Funct. Mater., 15, 989 (2005). 2. N. V. Lavrik, C. A. Tipple, M. J. Sepaniak, P. G. Datskos, Biomed. Devices., 3, 35 (2001). 3. J. Erlebacher, M. J. Aziz, A. Larma, N. Dimitrov, K. Sieradzki, Nature, 410, 450 (2001). 4. J. Erlebacher, J. Electrochem. Soc., 151, C614 (2004). 5. Y. Ding, Y. Kim, J. Erlebacher, Adv. Mater., 16, 1897 (2004). 6. N. A. Senior and R. C. Newman, Nanotech., 17, 2311 (2006). 7. R. G. Kelly, A. J. Young, R. C. Newman, Amer. Soc. Testing and Materials, J. R. Scully, D. Silverman, editors. 1993. p.94 (PA). 8. R. G. Kelly, A. J. Frost, T. Shahrabi, R. C. Newman, Metall. Trans., 22A, 531 (1991). 9. K. Sieradzki, R. R. Corderman, K. Shukla, Philos. Mag. A, 59, 713 (1989). 10. A. Dursun, D. V. Pugh, S. G. Corcoran, J. Electrochem. Soc., 150, B355 (2003). 11. R. Li and K. Sieradzki, Phys. Rev. Lett., 68, 1168 (1992). 12. J. Biener, A. M. Hodge, A. V. Hamza, Appl. Phys. Lett., 68, 121908 (2005).

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