Fabrication and Wettability of Nanoporous Silver Film on Copper from

Jul 22, 2010 - Interestingly, self-assembled nanoporous network of Ag films with ligament and channel width in the range of 20−80 nm was formed on t...
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Fabrication and Wettability of Nanoporous Silver Film on Copper from Choline Chloride-Based Deep Eutectic Solvents C. D. Gu,* X. J. Xu, and J. P. Tu Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: June 6, 2010; ReVised Manuscript ReceiVed: June 30, 2010

Nanoporous Ag films on copper alloy substrates were fabricated by the galvanic replacement reaction from the choline chloride-based deep eutectic solvents (DES) containing AgCl. It was found that the morphologies of the films were largely dependent on AgCl concentrations and plating temperatures. Interestingly, selfassembled nanoporous network of Ag films with ligament and channel width in the range of 20-80 nm was formed on the copper alloy substrate from 0.01 M AgCl DES solution either at room temperature or at 50 °C. The as-deposited porous Ag films exhibited hydrophilic properties. On the contrary, after surface modification by a monolayer of n-dodecanethiol, the contact angle of porous Ag film increased largely with plating times, and a superhydrophobic surface was finally obtained with a contact angle of 160 ( 1°. Wenzel and Cassie-Baxter models were used to qualitatively illustrate the importance of the surface roughness of the Ag films to the wetting property. 1. Introduction Intensive research efforts are currently focused on the fabrication of superhydrophobic surfaces because of their significance in both fundamental research and practical applications, including water-repellent coatings, self-cleaning surfaces, and low-drag microfluidics.1-5 Superhydrophobic surfaces, inspired by some plants’ leaves and insects’ shells, share two common features: they are made of (or covered by) hydrophobic materials, and the surfaces are not flat at the micro/nanometer scales but have micro/nanobinary structures.1-5 Galvanic cell reaction has been used extensively for producing micro/ nanobinary structures with hydrophobic and superhydrophobic surfaces because of its facile operation and low cost.2,6,7 Copper and copper alloys are important engineering metallic materials, which have been attracting intensive interest to be used as superhydrophobic substrates.7-18 Micro/nanobinary structure of silver films on copper and copper alloys was fabricated by galvanic exchange reactions from the aqueous AgNO3 solution.6 However, the micro/nanobinary structured silver films usually exhibit hydrophilic property with contact angles less than 50°.6 With the further surface modification process by coating low surface energy materials, such as stearic acid7 and n-dodecanethiol,2,6 the Ag surface was turned from hydrophilicity to hydrophobicity. Room-temperature ionic liquids are superior media for the electrodeposition of metals and semiconductors and have an unprecedented potential to revolutionize electroplating because of their wide electrochemical windows, extremely low vapor pressures, and numerous, only partly understood, cation/anion effects.19 Abbott et al. first described the applications of a relatively new class of ionic liquid on the basis of eutectic mixtures of choline chloride (ChCl) with a hydrogen bond donor species, which is named as deep eutectic solvents (DESs).20 Moreover, unlike the conventional ionic liquids, DESs can be * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86 571 87952573. Fax: +86 571 87952856.

easily prepared at low cost and with high purity. DESs are promising solvents used for the deposition of a range of metal coatings including Zn,21 Cr,22 and Cu23 at high current efficiency and also for metal dissolution processes such as electropolishing.24,25 Recently, Abbott et al. demonstrated that porous metallic silver films can be obtained by electroless deposition from the ChCl-based ionic liquid.26,27 However, the question of whether the porous Ag films from ionic liquids are hydrophobic or hydrophilic has not been explored. In our opinion, the pores and channels in the silver deposit make the silver surface rough with nanostructures, which could confer a potential hydrophobic property to the surface. In this study, porous Ag films on copper alloy substrates with different surface morphologies were fabricated by the galvanic replacement reaction from the DES solution containing ChCl and ethylene glycol (EG) eutectic mixtures. Surface topography and crystal structure of these films were characterized by scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. The wettability of these surfaces was extensively investigated through measuring the contact angle (CA) of water. 2. Experimental Section 2.1. Fabrication of Porous Ag Films on Cu Alloys. ChCl (AR, Aladdin) and ethylene glycol (EG) (AR, Aladdin) were used as received. The DES was prepared according to ref 28, which was formed by stirring the two components in a mole ratio of 1ChCl:2EG at 75 °C until a homogeneous colorless liquid was formed. The used substrate was a commercially available and industry widely used copper alloy foil (cold-rolled alloy C194). Alloy C194 has a nominal composition of 2.4% iron, 0.03% phosphorus, and 0.1% zinc with the balance of copper. A surface cleaning treatment of the copper alloy substrate was conducted to remove the industrial oil from the surface by electrolytic degreasing and deionized water rinsing. Before the plating process, the substrate was dried with flowing air. Porous Ag films on the Cu alloys were obtained by facilely immersing the cleaned copper alloy foils into 0.01 or 0.1 M

10.1021/jp105182y  2010 American Chemical Society Published on Web 07/22/2010

Fabrication of Nanoporous Silver Film on Copper

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Figure 1. Surface morphologies of as-deposited Ag films on Cu alloys from 0.01 M AgCl DES solution at RT, where the insets are the corresponding high resolution images. The deposition time is (a) 1, (b) 5, and (c) 10 min. (d) The EDS mapping analysis corresponding to c. The peaks of Cu and Fe in Figure 1d are from the Cu alloy substrate.

AgCl (99.8%, Aladdin) DES solution without stirring at room temperature (RT, 22 ( 2 °C) or at 50 °C. After a certain deposition time, the samples were sequentially rinsed with dichloromethane and methanol. Suface modification of the as-prepared silver surfaces was performed by immersing the silver film/substrate in an ethanol solution of n-dodecanethiol (1 × 10-3 M) for about 20 h, then by washing with ethanol and water, and finally by drying with air. 2.2. Characterization. Structure and morphology of asdeposited silver films were investigated by X-ray diffraction (XRD, PANalytical X’Pert PRO diffractometer and Cu KR radiation (λ ) 1.54056 Å)) and by field emission scanning electron microscopy (FESEM, Hitachi S-4800), respectively. Chemical compositions were analyzed with an energy-dispersive X-ray spectrometer (EDS) attached to the SEM. CA measurements were carried out by using a commercial instrument (SL200B, Solon Tech., Shanghai). The indicator drop images were stored via a monochrome video camera by using a PCbased control acquisition and data process. DI water droplets each having a volume of 1 µL were used in the wetting tests. The static CA values presented in the text were reproducible within (1° unless otherwise stated. 3. Results and Discussion Because of the thermodynamics of Ag+ reduction and active metal (Cu, Fe, and Zn in the Cu alloy substrates) dissolutions, the deposition of Ag onto the substrates would occur.6 In a previous study, Ag films composed of grains and dendrites were formed on the same Cu alloy substrates when dipping the substrates into AgNO3 aqueous solutions.6 In this case, different morphologies of Ag films were formed when dipping the same substrates into AgCl DES solutions, which might be attributed to the different viscosities and speciations between the AgNO3 aqueous and AgCl DES solutions.

Figure 1 shows the surface morphologies of as-deposited Ag films from 0.01 M AgCl DES solution at RT as a function of deposition times. It can be seen that nanostructured Ag is formed where the grain sizes are in the range of 20-40 nm with a deposition time of 1 min. When prolonging the deposition time to 5 min, grains are readily connected with each other as shown in Figure 1b. Interestingly, it is found that the connected Ag grains with a kind of self-assembly produce nanosized pores between them, which constructs an open, bicontinuous highly porous network with ligament and pore size in the range of 15-55 nm. The morphology of the nanoporous Ag films is very similar to the reported nanoporous Au fabricated by the wellknown dealloying strategy.29-32 At the deposition time of 10 min (Figure 1c), the average ligament size increases from ∼37 nm to ∼40 nm and the average pore size decreases from ∼32 nm to ∼30 nm with the comparison of Figure 1b and 1c. Moreover, some larger Ag grains with the size of 100 nm are grown on the nanoporous Ag films. The corresponding distributions of ligament and pore sizes are given in Figure 3. Figure 1d gives the EDS mapping analysis corresponding to Figure 1c, which indicates that the film is composed of pure Ag. It was reported that the viscosity of the DES changed significantly as a function of temperature and that increasing the temperature would decrease the viscosity of DES.20 Here, we also changed the plating temperature to see the temperature effects on the plating process. Figure 2 gives the surface morphologies of as-deposited Ag films from 0.01 M AgCl DES solution at 50 °C as a function of deposition time. Obviously, the deposition rate is significantly increased at the higher plating temperature. After the deposition time of 1 min, self-assembled nanoporous Ag films are formed on the Cu alloy substrate as shown in Figure 2a. As summarized in Figure 3, the average ligament size of porous Ag is 35 nm with a size range of 20-45 nm. When further increasing the deposition time to about 30 min, it is found that the average ligament size of porous Ag

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Figure 2. Surface morphologies of as-deposited Ag films on Cu alloys from 0.01 M AgCl DES solution at 50 °C. The deposition time is (a) 1, (b) 5, and (c) 30 min. (d) Typical cross section SEM image of as-deposited porous Ag films. The sample shown here is obtained from 0.01 M AgCl DES solution at 50 °C for about 30 min.

Figure 3. Distributions of ligament and pore sizes in the nanoporous Ag films as shown in Figures 1 and 2.

increased to about 75 nm as shown in Figures 2c and 3. Furthermore, the inset of Figure 2c indicates that plenty of Ag grains with sizes of about 10 nm were formed on the ligaments of porous Ag. However, the pores between the ligaments of Ag still exist with a size range of about 15-35 nm. The nanoporous metals made by dealloying, such as Ag and Au, generally possess three-dimensional porous structures and have been extensively investigated because of their great potential

for applications in heterogeneous catalysis, electrocatalysis, fuel cell technologies, biomolecular sensing, surface-enhanced Raman scattering, and plasmonics.30-33 Figure 2d gives a typical cross section SEM image of the porous Ag films, which shows that there are some nanoscaled pores in the films as indicated by arrows. Clearly, the density of pores in our case is less than the reported nanoporous metals made by dealloying.30-33 In our case, it is the porous structures that facilitate the sustainable deposition of Ag on Cu substrates. Significantly, it is the first time to report the fabrication of nanoporous Ag films by galvanic replacement reactions. Here, we suggest that it might be the soft template effect of the viscous DES that is responsible for the formation of nanoporous Ag. At the beginning of the depositions, Ag grains are formed on the substrate as shown in Figure 1a. The DES fills the interspaces of Ag grains or pores between the Ag ligaments. On one hand, the DES could supply Ag+ for the sustainable deposition. On the other hand, the DES could restrict the further growth of Ag ligaments to a certain extent thus forming the nanoporous structures of Ag. Considering that the galvanic replacement reaction is facile to operate compared with the metallurgy-dealloying process, it would be more promising to perform the catalytic and optical applications with our nanoporous Ag films. The Ag+ concentration in the DES solution also influences the morphology of Ag deposits. Figure 4 is the SEM images of as-deposited Ag films on Cu alloys from 0.1 M AgCl DES solution at RT, which is much distinguished from those obtained from 0.01 M AgCl DES solution at RT as shown in Figure 1. After the deposition time of 20 s, plenty of Ag grains are randomly distributed on the substrate, which exhibits a bimodal grain size distribution where 10-20 nm sized grains are embedded in the 200-300 nm sized grains as shown in Figure 4a. In this case, no self-assembled porous network of Ag, the same as the ones shown in Figures 1 and 2, is formed. Moreover, some of the Ag grains are faceted indicating that the crystal-

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Figure 4. Surface morphologies of as-deposited Ag films on Cu alloys from 0.1 M AgCl DES solution at RT. The deposition time is (a) 20 s, (b) 1 min, and (c) 5 min.

lographic planes with low surface energy are preferentially exposed. When increasing the deposition time to 1 min, the size of the faceted Ag grains increases to 300-500 nm, and about 10 nm sized Ag grains also could be found at the gap of larger grains (see Figure 4b). Furthermore, multiply twined grains with a 5-fold symmetry also appeared at this deposition time, which is highlighted by white lines in Figure 4b. The multiply twined grains are thermodynamically more stable, often in the decahedral shape, and are bound almost entirely by the lower energy {111} facets.34,35 We also find that several multiply twined grains are coalesced together by sharing common crystallographic orientations, which should be attributed to the oriented attachment growth mechanism.4,36 The substrate has been fully covered by the Ag grains after the deposition time of 1 min. However, the randomly distributed Ag grains with different sizes bring abundant voids in the film, which could make Ag+ ions penetrate the Ag film and react with the Cu alloy substrate thus facilitating the sustainable deposition of Ag.27 Figure 4c gives the surface morphology of as-deposited Ag films from 0.1 M AgCl DES solution at RT for 5 min deposition. The major difference between Figure 4b and 4c is that some nodular structures with the size of about 10 nm are formed on the larger Ag grains thus making the faceted Ag grains lose their regular geometries. More importantly, several nanometric voids also exist around the Ag grains even after the deposition time of 5 min. Thus, the films obtained from 0.1 M AgCl DES solution at RT can also be called porous Ag. Figure 5 shows XRD patterns of as-deposited Ag films on Cu alloys from 0.01 M AgCl DES solution at RT for 10 min and at 50° for 30 min and 0.1 M AgCl DES solution at RT for 10 min. In Figure 5, the peaks indexed by filled squares and filled cycles are assigned to the face-centered cubic (fcc) pure silver and fcc Cu. The silver lattice constant calculated from the XRD pattern is about 4.081 Å, a value in agreement with the reported a ) 4.086 Å from JCPDS 04-0783. Both the surface roughness and the low surface energy coating are prerequisites for synthesizing superhydrophobicity.2,4,6 The

Figure 5. XRD patterns of as-deposited Ag films on Cu alloys from (a) 0.01 M AgCl DES solution at RT for 10 min and (b) at 50° for 30 min and from (c) 0.1 M AgCl DES solution at RT for 10 min.

wetting properties of the Cu alloy substrate and the porous Ag films before and after surface modification were characterized by water CA tests, and the results are summarized in Figure 6 and Table 1. It is believed that the surface modification process by immersing the substrate in an ethanol solution of ndodecanethiol (1 × 10-3 M) overnight could confer the surface a low surface energy coating which is a self-assembled monolayer of surface-active molecule. As shown in Figure 6, the Cu alloy surface after modification with n-dodecanethiol has a CA value of about 91 ( 1°, which is similar to that before surface modification. However, as for the as-deposited Ag films, the CA values decrease with the deposition times. On the contrary, after the surface modification process, the CA values increase with the deposition time. It further gives the evidence that both the surface roughness and the low surface energy coating play dominant roles in controlling the wetting properties of the solid surfaces. Under the same deposition time of 5 min, the samples of porous Ag film obtained from the three plating

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Gu et al. interface.38,39 It is therefore expected that the distinct surface wetting for Ag films with nanosized grains observed above can be described using the well-known Young’s equation, cos θ ) (γSG - γSL)/γLG, by considering the size dependences of γSG and γSL.37 In this equation, γ represents the surface tensions involved in the system, θ is the contact angle, S is the solid, L is the liquid, and G is the gaseous phase. However, it is not feasible since this equation is only applicable to a liquid droplet on a flat surface,5 while the surface is rough in our case. Instead, the water CA on rough surfaces should be explained with the roughness in light of the other two models, namely, Wenzel40 and Cassie-Baxter41 models. In the Wenzel model, the CA is given by

cos θw ) r · cos θ

(1)

where r is the roughness factor defined as the ratio of the actual area of a rough surface to the projected area, θ is the CA on the corresponding smooth surface, and θw is that of the rough surface. Since r is greater than 1 because of roughness, there are two situations for this model: for θ < 90°, θw< θ; for θ > 90°, θw > θ. On the other hand, assuming a water CA of 180° for air and θ for a flat surface of solid, the modified Cassie-Baxter model42 derived the nominal contact angle θc as

cos θc ) fcos θ + (1 - f)cos 1800 ) f (1 + cos θ) - 1 (2)

Figure 6. (a) Variation of water CA with depositon time. Solid and open symbols denote the CA values of Ag films at as-deposited states and at surface-modified states, respectively. The CA of Cu substrate holds the stable value of 91 ( 1° as indicated by the shadow region. (b) CA hysteresis of the surface-modified sample obtained from 0.1 M AgCl DES solution at RT.

conditions studied in this paper exhibit very distinctive wettability as shown in Table 1. Significantly, after the surface modification process, the sample obtained from 0.1 M AgCl DES solution at RT for about 5 min exhibits a superhydrophobic surface with a CA of 160 ( 1° as shown in Figure 6 and Table 1. The CA hysteresis (∆θ) is defined as the difference between advancing (θa) and receding angles (θr), which is about 30° for this case, as shown in Figure 6b. For nanoscaled materials, the surface or interface energy is found to decrease as the size declines,37 which can be explained by the atomic coordination imperfection at the surface or

where f is the area fraction of the liquid-solid contact to the nominal contact area. Equation 2 indicates obviously that θc increases with decreasing f meaning that θc increases with the increasing fraction of air trapped in the film surface during the contact tests. The smooth surface of Ag was prepared by polishing bulk Ag to a mirrorlike finish surface. The surface wettability of the as-polished smooth Ag is studied by CA measurement, which gives a CA value of 79 ( 1°. This value is much the same as the reported one by Safaee et al.7 The corresponding image of a water droplet of about 1 µL on the smooth silver surface is given in Figure 7. Moreover, after the same surface modification process as the porous Ag films, the smooth silver surface still holds its original wetting property. In the present study, Wenzel (eq 1) and Cassie-Baxter (eq 2) models are used to qualitatively illustrate the importance of the surface roughness of the Ag films at as-deposited states and at surface-modified states, respectively. With eqs 1 and 2 and θ ) 79°, the values of r and f are calculated for the samples shown in Table 1. According to the

TABLE 1: Comparison of CA Values of the Ag Film Surfaces before and after the Surface Modification with n-Dodecanethiola

a

The Ag films were obtained from 0.01 M AgCl DES solution at RT and at 50° and from 0.1 M AgCl DES solution at RT. All samples were obtained for the deposition time of 5 min. The corresponding r and f values are calculated from eqs 1 and 2, respectively.

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Figure 7. Image of a water droplet of about 1 µL on the smooth silver surface with water CA of 79 ( 1°.

definition of r, it can be implied that higher r values should correspond to the rougher surfaces. Similarly, according to the definition of f, a smaller f value should indicate that a larger fraction of air is trapped between the Ag films and the water drop. Clearly, the calculated results are consistent with the surface topographies of the films by SEM observations (Figures 1b, 2b, and 4c). It can be concluded that without the low surface energy coating on the nanoporous Ag films, the CA value decreases as the surface roughness increases, which should be due to the two- or three-dimensional capillary effect of the many nanosized pores of the films.42 With the low surface energy coating on the nanoporous Ag films, the CA value increases with increasing the surface roughness, which is attributed to the many more air pockets that repel water droplets offered by a larger surface roughness.6 4. Conclusions Porous Ag films with fcc structures on copper alloy substrates were fabricated by the galvanic replacement reaction from the AgCl DES solutions. It was interestingly found that a selfassembled nanoporous network of Ag films with ligament and channel width in the range of 20-80 nm was formed on the Cu alloy substrate from 0.01 M AgCl DES solution either at RT or at 50 °C. However, by increasing the concentration of AgCl to 0.1 M, Ag films constructed by randomly distributed Ag grains and abundant voids were formed on the Cu alloy substrates. It was suggested that the soft template effect of the viscous DES should be responsible for the formation of nanoporous Ag. Water CA values of the as-deposited Ag films were decreased with increasing the surface roughness of the films. However, after surface modification by a monolayer of n-dodecanethiol, the CA values of porous Ag film increased largely with the plating time. As for the sample obtained from 0.1 M AgCl DES solution for about 5 min deposition, a superhydrophobic surface was finally obtained with a contact angle of 160 ( 1°. With the analysis by Wenzel and Cassie-Baxter models, it was implied that the surface roughness plays an important role on the wetting property of the Ag films. Acknowledgment. This work was supported by the Fundamental Research Funds for the Central Universities (grant no. 2009QNA4006) and by the Research Foundation of Education Bureau of Zhejiang Province under grant no. Y200906938.

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