Tailoring the Wetting Properties of Surface-Modified Nanostructured

ARTICLE pubs.acs.org/JPCC

Tailoring the Wetting Properties of Surface-Modified Nanostructured Gold Films Jianzhong Zhu,† Giovanni Zangari,*,‡ and Michael L. Reed† †

Department of Electrical Engineering and ‡Department of Materials Science, University of Virginia, Charlottesville, Virginia 22904, United States

bS Supporting Information ABSTRACT: Nanostructured gold films with a dendritic morphology were synthesized by electrodeposition on porous Si surfaces. The films were modified by methyl-terminated self-assembled monolayers, resulting in the formation of superhydrophobic surfaces with a contact angle near to 180° and undetectable hysteresis. Gradual smoothening of the dendritic features of the Au surfaces could be achieved by argon ion etching, resulting in a continuously tunable wetting angle. Below a contact angle of 150°, these surfaces showed increased adhesion and significant contact-angle hysteresis.

’ INTRODUCTION The wetting properties of surfaces have an essential role in tuning the interaction of the system of interest with the external environment. Superhydrophobic surfaces, in particular, are of importance in nature, where they enable self-cleaning,1 and in various microsystems, to decrease stiction or friction as well as to facilitate fluid flow in microfluidic devices.2 To minimize adhesion to a surface, a water drop placed on such surface should form a large contact angle, thus minimizing the contact area. This requirement is satisfied by a surface with a low intrinsic surface energy and a pronounced roughness. Contact-angle hysteresis is also often taken as an indirect measure of adhesion, with a low hysteresis indicating facile drop separation from a solid surface. Such correlation has been observed in several instances3,4 but should be used with caution because it is not completely general. Contact-angle hysteresis may in fact arise for various reasons, including roughness and chemical inhomogeneities, mechanical irreversibility of cohesion/decohesion processes, or molecular rearrangements at the surface.5 Assuming this correlation to be valid, a low contact-angle hysteresis (and therefore low adhesion) may be favored by reducing the solid/liquid contact area through a combination of a suitable topology of the roughness and the attainment of a metastable Cassie
Baxter state for the liquid, where air pockets are present between the liquid drop and the valleys of the rough solid surface.6,7 Numerous approaches to the synthesis of superhydrophobic surfaces have been reported using a wide variety of materials, micro- or nanotextured topographies, and surface modification methods; such techniques have been reported and analyzed in recent reviews.8,9 Whereas superhydrophobic behavior is necessary to implement self-cleaning and to minimize stiction, in some applications, it may be of interest to tune the extent of the liquid/solid interaction; in this Article, we demonstrate control of the surface r 2011 American Chemical Society

Figure 1. Cross-sectional SEM image of a porous Si substrate obtained by chemical etching in a HF/H2O2/colloidal Au solution. Scale bar is 200 nm.

wettability of Au surfaces in a model system by tailoring nanoscale morphology via the electrochemical growth of Au onto nanoporous Si substrates and the successive ion beam etching of the resulting surface, followed by a stabilization and decrease in the intrinsic surface energy using suitable self-assembled monolayers (SAMs). We show that the details of the surface morphology determine the observed contact angle and also affect adhesion and, within the limits discussed above, contact-angle hysteresis. Received: May 28, 2011 Revised: July 26, 2011 Published: July 26, 2011 17097

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Figure 2. Scanning electron micrographs (SEMs) of Au films electrodeposited onto porous Si. (a) Top view of the
1.0 VSSE film. (b) Top view of the
1.4 VSSE film. (c) Cross section of the
1.0 VSSE film. (d) Cross section of the
1.4 VSSE film. Scale bar is 1 μm.

’ EXPERIMENTAL DETAILS We used 200 n-type Si (110) wafers with resistivity 15 Ω 3 cm as substrates. A 200 nm thick layer of nanoporous Si was formed by immersion of the wafers in a HF/H2O2 mixture containing a colloidal suspension of Au nanoparticles, as reported in detail by Zhu et al.10 Au electrodeposition (ECD) was performed onto the resulting porous Si substrates under potential control at room temperature using a saturated sulfate (SSE) reference electrode (0 VSSE = 0.615 VSHE) and a Pt-coated mesh as a counter electrode. In the following, all potential values will be referred to the SSE. The solution for Au ECD was a commercial electrolyte for soft gold, TECHNI GOLD 25 ES (Technic, Cranston RI), diluted with deionized (DI) water (resistivity 18.2 MΩ 3 cm, Millipore) to half its initial concentration. A smooth as-plated Au surface was hydrophilic but gradually turned to hydrophobic upon adsorption of hydrocarbons in ambient air. To avoid this time-dependent behavior and to tailor suitably the surface energy, we modified the Au surfaces by coating with 1-undecanethiol (1-UDT, HS-(CH2)10-CH3, Aldrich) to form a dense SAM with a methyl termination, which lowers the surface energy and enhances hydrophobicity. Prior to the formation of the SAM of 1-UDT, the Au surfaces were cleaned in an O2 plasma to remove any surface contamination; then, they were immediately immersed into solutions containing 1 mM of the 1-UDT in 99% ethanol for 24 h. The samples were finally extracted and rinsed with copious amounts of ethanol to remove the residual thiols, then blown with nitrogen to dry.

The surface morphology of the as-deposited dendritic gold films was progressively flattened by Ar sputtering in increasing 2 min intervals, using a Trion ICP-RIE tool. The ICP (inductively coupled plasma) power was 100 W, the RIE (reactive ion etching) power was 100 W, the chamber pressure was 10 mTorr, and the Ar gas flow rate was 30 sccm. The etch rate of planar Au under these conditions was 32.2 nm/min. After RIE, the Au surface was coated with the 1-UDT SAM. The porous Si surface and the Au films were imaged by scanning electron microscopy (SEM, Carl Zeiss 982). Surface roughness of the Au films was analyzed quantitatively using an atomic force microscope (AFM, Dimension 3100 Veeco) in tapping mode. The contact angle was measured using a stereo microscope; a 5 μL DI water drop was placed on the SAMmodified Au dendritic structure, and side images of the drop were digitized and processed by using the ImageJ software (National Institute of Health).

’ RESULTS AND DISCUSSION Immersion of the Si wafers in the HF/H2O2/Au etching solution for 5 min leads to the formation of a porous Si layer with a thickness of ∼200 nm, consisting of a roughened surface region and of elongated pores (∼20 nm diameter) crossing the entire porous layer thickness (Figure 1). ECD of Au was performed on these substrates, using a constant applied potential between
1.0 and
1.4 VSSE. The total charge passed during ECD was kept constant at 0.5 C/cm2, resulting in an equivalent 17098

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Figure 3. (a) SEM images of Au films electrochemically deposited at various applied potentials. Inset: water droplets placed on the corresponding Au surfaces after coating with hydrophobic SAMs. Scale bars represent 200 nm. (b) Contact angles of a 5 mm water drop on Au films versus applied potentials.

thickness of 350 nm. Films grown at
1.0 VSSE exhibit a compact surface (Figure 2a,c); on the contrary, higher overpotentials result in the formation of dendritic films (Figure 2b,d). Nucleation and growth of Au occurs at isolated sites of the porous Si surface, leading to a large Au reduction current at these locations. Under such conditions, the local current density at these growth sites can become very large and lead over time to depletion of Au ions in the proximity of the electrode and to ECD under diffusion control, a growth mode that favors growth instabilities and dendrite formation.11 The main stem of the dendrites has a length of ∼1 μm, whereas the side branches are ∼100 nm long; the distance between nearby dendrites is of the same order of magnitude. These films are characterized by a high void volume, which can be visually estimated at around 60
70%. Gold surfaces grown at different potentials were modified with the 1-UDT SAM. The contact angle for compact Au surfaces grown at the lowest overpotential (V =
1.0 V) is ∼125° (Figure 3), slightly larger than the contact angle observed on a smooth Au surface (103 ( 2°).12 The contact angle on the other surfaces, all exhibiting dendritic features, however, is very close to 180° (Figure 3). The contact-angle hysteresis for these superhydrophobic surfaces was impossible to determine; it was in fact very difficult to force the water drop to adhere to these surfaces (see movie in the Supporting Information); this suggests a very facile roll-off of the drop, implying negligible contact-angle hysteresis as well as limited adhesion. The superhydrophobicity of these surfaces is probably a combined result of the low intrinsic surface energy induced by the SAM coating and the complex topography of the dendritic gold films. The low adhesion and the negligible contact-angle hysteresis are due to the low solid
liquid contact area, originated by the unique dendritic morphology (Figure 2d). It has been reported that the Cassie
Baxter configuration is metastable,13
18 but it is particularly robust when the surface features generating superhydrophobic behavior are smaller, that is, on the 100 nm scale.19 Whereas the presence of many secondary protrusions in the Au dendrites generates numerous regions of potential pinning of the liquid/air

Figure 4. Cross-sectional SEM images of the
1.13 V sample as deposited (a) and after prolonged Ar sputtering (b). Scale bar is 1 μm.

interface,20 small energy fluctuations are sufficient to transfer the pinning line across the dendritic pattern when the liquid/solid contact area is small.21 The superhydrophobic properties of nanostructured surfaces are strongly dependent on the details of the surface morphology. An obvious way to tailor superhydrophobicity would therefore involve controlled modification of the surface features. This is, however, difficult to achieve in our samples by changing the applied potential during growth; the growth instabilities leading to dendrite formation occur in parallel with the onset of diffusion limitations,11 a growth mode for which no dependence of deposition rate on potential is observed. The relevant dendrite features (aspect ratio, degree of branching) therefore have negligible dependence on the applied potential. This effect is evident in the films shown in Figure 3, where morphology is observed to change discontinuously from compact to dendritic in 17099

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Figure 5. (a) Surface roughness Rrms of a
1.13 VSSE Au film over various sampling length after Ar sputter etching. The solid lines are linear trend lines fitted using the least-squares method. (b) Contact angles measured on surface-modified Au surfaces after various doses of Ar sputter etching.

correspondence with the onset of diffusion-limiting growth conditions, and correspondingly, the contact angle jumps from ∼125° to near 180°, becoming practically independent of applied potential. On the contrary, it is possible to gradually modify the Au surface morphology by starting from a dendritic Au film grown on Si, in this case at V =
1.13 V (Figure 4a), and progressively smoothen the protruding features of the dendrites by Ar ion etching (Figure 4b). The SEM images of Figure 4 show that the etching process results in the formation of sturdier mesas with relatively flat tops, suggesting that the surface morphology evolves via combined material removal and successive redeposition. A semiquantitative estimate of film roughness as a function of etching time was performed by collecting AFM topography data of these surfaces. After etching, the surface topography of each sample was measured over a 5 μm  5 μm area, and the rms roughness Rrms(l) = ()1/2 was calculated, where h is the position-dependent surface height and the symbol < > indicates an average calculated over the size l of the region being measured. Figure 5a displays Rrms calculated over three different length scales, ranging from l ≈ 200 nm, the typical size of a large dendrite before etching, to l ≈ 1300 nm, the size of a large mound after etching. Because of tip convolution on surfaces with pronounced topography, AFM data may not fully capture the true roughness of the surfaces, but they are still capable to monitor trends in the surface features. The roughness calculated over the length scales investigated (Figure 5a) increases with the sampling length, as expected. Rrms tends to increase after 2 or 4 min etching; this may correspond to the transient formation of particularly rough features; however, the general trend over time shows a decrease in roughness, which is more apparent on small scales than on large ones. Correspondingly, the wetting angle decreases gradually (Figure 5b), suggesting a direct relationship between the contact angle and the sharpness of the surface features. The datum after 2 min of etching does not follow this trend, suggesting that indeed short time etching may increase the roughness of dendritic gold films. At longer times, however, the decrease in contact angle is clear; the smoother features observed in Figure 4b after etching, in particular, would exhibit a larger solid/liquid contact area, which would increase the capillary adhesive force. For etching times above 8 min, when the measured contact angle decreased below 150°, it was observed that the drops could be placed more easily on the surface. In addition, a sizable contact-angle hysteresis, equal to the difference between the advancing and receding contact angle, Δθ = θA
θR was observed, which reached the value Δθ ≈ 10° on a sample etched for 16 min (Figure 6). This suggests that superhydrophobicity and negligible adhesion of surface-modified,

Figure 6. Water drop (5 μL) placed on a dendritic Au surface etched for 16 min and modified with 1-UDT showing a contact-angle hysteresis of ∼10°.

dendritic Au surfaces could only be maintained by limiting Ar ion etching time.

’ CONCLUSIONS ECD of Au films over nanoporous Si substrates results in the formation of compact or dendritic surfaces, depending on the applied overvoltage. After modification with a 1-UDT SAM, the dendritic Au films are methyl-terminated and exhibit superhydrophobic behavior with a contact angle ∼180° and negligible contact-angle hysteresis. The dendritic morphology varied abruptly with the applied voltage during ECD, forbidding tailoring of the contact angle in the ECD stage. However, the contact angle could be progressively decreased by reactive ion etching of dendritic Au surfaces prior to SAM modification. In this case, a decrease in surface roughness could be related to a decrease in contact angle; below a contact angle of 150°, the surfaces showed increased adhesion and contactangle hysteresis of up to 10°. ’ ASSOCIATED CONTENT

bS

Supporting Information. Movie showing the attempts to place a water drop on top of a superhydrophic modified gold surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Tel: +1 434-243-5474. Fax:+1 434-982-5799. E-mail: gz3e@ virginia.edu.

’ ACKNOWLEDGMENT Support of the National Science Foundation through the Award NSF DMI 0507023 is gratefully acknowledged. ’ REFERENCES (1) Bhushan, B.; Jung, Y. C.; Koch, K. Philos. Trans. R. Soc., A 2009, 367, 1631–1672. (2) Franke, T. A.; Wixforth, A. ChemPhysChem 2008, 9, 2140–2156. (3) Extrand, C. W. Langmuir 2004, 20, 4017–4021. (4) Esherbini, A. I.; Jacobi, A. M. J. Colloid Interface Sci. 2006, 299, 841–849. (5) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736–10747. (6) Dorrer, C.; R€uhe, J. Soft Matter 2009, 5, 51–61. (7) Li, X. M.; Reinhoudt, D.; Crego, M. Chem. Soc. Rev. 2007, 36, 1350–1368. (8) Bhushan, B.; Jung, Y. C. Prog. Mater. Sci. 2011, 56, 1–108. (9) Liu, K.; Jiang, L. Nanoscale 2011, 3, 825–838. (10) Zhu, J.; Bart-Smith, H.; Begley, M. R.; Zangari, G.; Reed, M. L. Chem. Mater. 2009, 21, 2721–2726. (11) Barton, J. L.; Bockris, J. O. Proc. R. Soc. London, Ser. A 1962, 268, 485–505. (12) Tehrani-Bagha, A. R.; Holmberg, K. Langmuir 2008, 24, 6140–6145. (13) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633–2637. (14) Zheng, Q.; Yu, Y.; Zhao, Z. Langmuir 2005, 21, 12207–12212. (15) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618–1622. (16) Koishi, T.; Yasuoka, K.; Fujikawa, S.; Ebisuzaki, T.; Zeng, X. C. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8435–8440. (17) Herminghaus, S. Europhys. Lett. 2000, 52, 165–170. (18) Marmur, A. Langmuir 2003, 19, 8343–8348. (19) Quere, D. Annu. Rev. Mater. Res. 2008, 38, 71–99. (20) Nosonovsky, M. Langmuir 2007, 23, 3157–3160. (21) Lai, Y.; Gao, X.; Zhuang, H.; Huang, J.; Lin, C.; Jiang, L. Adv. Mater. 2009, 21, 1–5.

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