Template-Free and Direct Electrochemical Deposition of Hierarchical

Aug 30, 2010 - A diffusion-limited aggregation (DLA) mechanism is used to explain the formation of HDGMs. Typically, the as-synthesized HDGMs exhibite...
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J. Phys. Chem. C 2010, 114, 15617–15624

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Template-Free and Direct Electrochemical Deposition of Hierarchical Dendritic Gold Microstructures: Growth and Their Multiple Applications Weichun Ye, Junfeng Yan, Qian Ye, and Feng Zhou* State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ReceiVed: June 27, 2010; ReVised Manuscript ReceiVed: August 14, 2010

Hierarchical dendritic gold microstrutures (HDGMs) with secondary and tertiary branches are directly electrodeposited on an indium tin oxide (ITO) substrate without the use of any templates, surfactants, or stabilizers. The effects of electrodeposition potential and HAuCl4 concentration on the formation of HDGMs and time-dependent morphological evolution are investigated in detail. A diffusion-limited aggregation (DLA) mechanism is used to explain the formation of HDGMs. Typically, the as-synthesized HDGMs exhibited much higher electrocatalytic activity and enhanced stability toward ethanol electrooxidation compared to bulk gold electrode and also display great Raman enhancement activity with the detection limit of 10-12 M for rhodamine 6G. The surface of HDGMs possesses hydrophobicity even without modification with lowsurface-energy coatings and has a remarkable superhydrophobic property even in corrosive solutions over a wide pH range after the treatment with n-dodecanethiol. In addition, (super)oleophobicity is successfully obtained by modification with 1H,1H,2H,2H-perfluorodecanethiol. Introduction Gold micro/nanostructures are fascinating because of their unique optical, electronic, and chemical properties and multifunctional applications in such areas as nanoelectronics, biomedicine, sensing, catalysis, surface-enhanced Raman scattering (SERS), and self-cleaning functions.1-11 For example, nanosized Au particles exhibit high catalytic activity toward the electrooxidation of alcohols in alkaline solution owing to their large surface-to-volume ratio and the existence of highly active binding sites on the surface of the particles.5,6 Roughened gold films have been typically used as the effective SERS substrates with the estimated enhancement factor of 106-1015.7-9 Rough gold surfaces are also excellent candidates to create superhydrobic surfaces after simple chemical modification.10,11 The intrinsic properties of gold nanostructures largely depend on their size and shape.12-14 Consequently, it is of considerable interest to synthesize specific gold micro/nanostructures. Hierarchical dendritic gold microstructures (HDGMs) are one type of hyperbranched structures which are generally formed by hierarchical self-assembly of freshly generated precursors under nonequilibrium conditions. They have high surface areas, special shapes, and high activity, making them promising candidates for the design and fabrication of new functional nanomaterials.15 In recent years, several approaches have been reported to fabricate HDGMs,16-19 most of which are synthesized in the presence of organic additives or surfactants, or other templates. However, the use of organic additives or surfactants may introduce heterogeneous impurities, and the use of templates may complicate the synthetic procedure, which will limit their applications in practice. Therefore, it could be essential to develop a surfactant-free and template-free route to fabricate HDGMs with high yield. Electrodeposition is a powerful and convenient tool to create micro/nanostructures. In electrodeposition, the growth rate can * To whom correspondence should be addressed. Tel: +86-931-4968366; Fax: +86-931-4968163. E-mail: [email protected].

be readily and independently controlled by deposition potential without changing the concentration of reactants. As a result, electrodeposition can be manipulated to some extent by altering the interplay between the crystal growth rate and the mass transport rate, which makes it an ideal method to systematically study the dendritic growth of inorganic materials. For example, Dong et al. obtained hierarchical Au microstructures using square wave voltammetric method.20-22 Flower-like gold nanostructures were fabricated by using electrochemical deposition.23,24 Ohsaka and co-workers reported the electrochemical deposition of gold nanoparticles with controllable crystallographic orientations such as enriched Au [111] facet25 and Au [110] facet.26,27 In this work, a facile and template-free approach is demonstrated to fabricate HDGMs on a surface of indium tin oxide (ITO) substrate by constant potential electrolysis. The size and morphology of HDGMs can be easily controlled by HAuCl4 concentration, electrodeposition time, or potential. On the basis of the experimental results, a growth mechanism of HDGMs is proposed. The as-prepared gold dendrites are very “clean” and highly active. The high electrochemical activities toward ethanol oxidation and strong SERS enhancement are reported. The HDGMs surface becomes superhydrophobic after further chemisorption of a self-assembled monolayer of n-dodecanethiol, and (super)oleophobicity after modification with 1H,1H,2H,2Hperfluorodecanethiol. Experimental Section Reagents and Materials. HAuCl4 was purchased from Beijing Chemical Factory (Beijing, China). Rhodamine 6G (R6G), n-dodecanethiol and 1H,1H,2H,2H-perfluorodecanethiol were obtained from Sigma. ITO was purchased from Shenzhen Hivac Vacuum Photoelectronics Co., Ltd. (Shenzhen, China). Electrochemical Deposition of HDGMs. For a typical electrochemical fabrication of HDGMs, constant potential electrolysis was conducted at E ) -0.6 V. The electrolyte was the solution of 0.1 M Na2SO4 and 30 mM HAuCl4. ITO was used as working electrode. Before being used, ITO was cleaned

10.1021/jp105929b  2010 American Chemical Society Published on Web 08/30/2010

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Figure 1. SEM images of Au dendrites with different magnification (A-C), which are electrodeposited on ITO under constant potential electrolysis at -0.6 V for 600 s in the solution of 0.1 M Na2SO4 + 30 mM HAuCl4; (D) EDX spectrum; and (E) XRD pattern, (*) peaks from the ITO glass.

by sonicating sequentially for 10 min each in acetone, 10% NaOH in ethanol and distilled water. All electrochemical experiments were carried out with a CHI660A electrochemical workstation (Shanghai, China). A clean platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. The asprepared gold films were used to test the electrocatalytical activity toward the oxidation of 1 M ethanol in 1 M NaOH. SEM and XRD Analysis. The sample surfaces were observed by field emission scanning electron microscopy (FESEM, JEOL JSM-6701F). XRD analysis was performed with an X-ray diffraction analyzer (Rigaku D/max-2400, Cu KR radiation, λ)0.1541 nm). SERS and UV-vis Measurements. The Raman scattering measurements were performed at room temperature on a Raman system (JY-HR800) with confocal microscopy. The solid-state diode laser (532 nm) was used as an excitation source. The laser power on the samples was kept with 1 mW and the typical spectrum acquisition time was 30 s. The probed area was about 1 µm in diameter with a 100× microscope objective lens. The spectra were recorded after immersing the sample into a diluted R6G ethanolic solution overnight, rinsing with ethanol and drying in air. The absorption spectrum was recorded with a Perkin-Elmer Lambda UV-vis 800 spectrophotometer in air. Wettability Tests. To test the wettability, the ITO substrate coated with HDGMs was immersed into the ethanol solutions of 1 mM n-dodecanethiol or 1 mM 1H,1H,2H,2H-perfluorodecanethiol overnight, then taken out and washed repeatedly with ethanol, and finally dried in air. Contact angles were measured with an optical contact angle meter at ambient

temperature. Water droplets (5 µL) or liquid droplets (5 µL) were dropped carefully on to the sample surfaces and the average value of four measurements at different positions of the sample was adopted as the contact angle. Results and Discussion Characterization of HDGMs. Figure 1A is the panoramic SEM image of the gold deposits, which were electrodeposited under constant potential electrolysis of -0.6 V for 600 s. It is clearly seen that the ITO substrate is covered with a thick layer of elegant gold dendritic microstructures with overall lengths of 10-20 µm. Further detailed examinations (Figure 1B and C) reveal that well-defined dendritic structures are arranged along an elongated central backbone with secondary or tertiary branches. The secondary branches grow at a fixed angle with respect to the backbone, of which the length is around several micrometers while the diameter varies in a range of 200-300 nm. The EDX spectrum (Figure 1D) proves the 100% purity of HDGMs. A typical XRD pattern is shown in Figure 1E. The peaks located at 39.8°, 46.4°, 64.6°, 77.6°, and 81.8° are assigned to [111], [200], [220], [311], and [222] Au facecentered-cube (fcc) crystal diffractions (JCPDF No.04-0802), respectively. Gold dendrites have a higher [111]/[200] ratio than quasi-spherical gold nanopaticles (3.54 vs 2.37), indicating that gold dendrites are preferentially dominated by Au [111] facets. The peaks at 35.2°, 50.5°, and 60.2° are assigned to the ITO substrate. Effects of the Reaction Conditions on Morphology. For electrodeposition technique, applied potential is an important

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Figure 3. Typical SEM images of gold structures electrodeposited under constant potential electrolysis at -0.6 V for 600 s in the solution of 0.1 M Na2SO4 containing 15 mM HAuCl4 (A) or 5 mM HAuCl4 (C). Panels (B) and (D) show high-magnification images of panels (A) and (C), respectively.

Figure 2. Typical SEM images of gold nanostructures electrodeposited in the solution of 0.1 M Na2SO4 + 30 mM HAuCl4 for 600 s: (A) E ) 0.2 V; (C) E ) -0.2 V; (E) E ) -1.0 V. Panels (B), (D), and (F) show high-magnification images of panels (A), (C), and (E), respectively.

factor for controlling the morphologies of gold micro/nanostructures. The morphological changes of dendrites obtained under different potentials were characterized by SEM (Figure 2). Under the potential of 0.2 V, the as-synthesized product consists of a large quantity of well dispersed and uniform microstructures with the diameter of ∼2 µm from lowmagnification image (Figure 2A). A higher-magnification image (Figure 2B) reveals that these microstructures are flowerlike and are built with many nanoflakes as building blocks that merge in a staggered way. When applying a potential of -0.2 V (Figure 2C,D), the morphology of the gold deposits changes greatly, exhibiting stem-type tectorum-like microstructures having a stem with the length of several micrometers nanosheets of about 50-100 nm thickness surrounding them. Decreasing the electrodeposition potential to -1.0 V, a layer of dendritic structured gold film is obtained in high density (Figure 2E); however, a higher-magnification image (Figure 2F) demonstrates that the morphology is somewhat different compared with the images of Figure 1A, the branches are larger, coarser, and denser. These results obviously provide solid evidence that the applied potential plays a very important role in the fabrication of HDGMs. For electrochemical nucleation and growth, two limiting cases can be distinguished, i.e., instantaneous and progressive nucleation.28-30 For instantaneous nucleation, the nuclei number rapidly reaches a value that is constant. In the case of progressive nucleation, new nuclei are formed continuously. And the nucleation mechanism of Au depends strongly on the deposition potential, which would result in different morphologies.31 Furthermore, the HAuCl4 concentration shows a considerable effect on the formation of HDGMs (Figure 3). When the HAuCl4 concentrations are decreased from 30 mM to 15 mM, the dendritic structures can be obtained but are sparsely distributed on the ITO surface and a great deal of particles are formed.

Also, the growth of dendritic structures is not sufficient with the existence of very short branches (Figure 3A and B). When lowering the HAuCl4 concentration to 5 mM, only particles or clusters with the diameter of 100-300 nm are formed instead of dendritic structures (Figure 3C and D). According to previous studies,32-34 the structure direction is mainly determined by the competition between the thermodynamic and kinetic factors for the growth of crystals of a cubic symmetry. Under kinetic control, the 〈100〉 direction is favored and benefits dendritic growth (symmetric branched). Conversely, when the thermodynamic factor dominates the reaction, the 〈110〉 direction is favored and leads to the formation of fractal structure (randomly ramified). Xia et al.35 proved that, for an fcc noble metal, the thermodynamically favorable shapes are multiple twinned particles or truncated nanocubes. In this work, a lower HAuCl4 concentration would lead to a reaction condition closer to the thermodynamic balance and the domination of the 〈110〉 growth. So, only particles or clusters were obtained when adding 5 mM HAuCl4 into the electrolyte. Morphological Evolution and Time-Dependent Optical Properties. Figure 4 shows the time-dependent morphological evolution of HDGMs, which were electrodeposited at -0.6 V in the solution of 0.1 M Na2SO4 + 30 mM HAuCl4. At 30 s, gold nanoparticles and nanoclusters of 50-200 nm were formed on the ITO surface, and many nanoflower-like crystals of about 300 nm in size appeared, which are built with several small particles (Figure 4A,B). When the deposition time was prolonged to 180 s, long stems with unclear “feathers” were observed (Figure 4C,D), indicating further growth from the original nanoflower-like crystals. Obviously, the dendritic microstructures stay at the evolutional stage of growth. After 300 s of electrodepsoition, a complex morphology with HDGMs containing primary, secondary, and tertiary branches was basically formed, as shown in Figure 4E,F. Gold particles normally exhibit plasmonic absorption determined by the growth density and their sizes. Figure 5 shows the absorption spectra of gold products electrodeposited on ITO substrates for different times, which were recorded ranging from 400 to 800 nm in the air. A pronounced absorption peak with the entirely declined absorption was not observed in the visible light region for the sample electrodeposited for 30 s. With prolonged electrodeposition time, the surface plasmonic absorp-

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Figure 6. Tafel plots in 1 M NaOH +1 M ethanol aqueous solution on bulk gold electrode and HDGMs electrodes corresponding to different electrodeposition time.

TABLE 1: Tafel Parameters for the Oxidation of Ethanol (1 M) in 1 M NaOH Solution on Different Gold Electrodes

Figure 4. Time-dependent morphological evolution of HDGMs with the electro- deposition time of 30 s (A,B), 180 s (C,D), and 300 s (E,F), which were electrodeposited at -0.6 V in the solution of 0.1 M Na2SO4 + 30 mM HAuCl4.

Figure 5. Absorption spectra for gold films on ITO substrates, electrodeposited at -0.6 V in 0.1 M Na2SO4 + 30 mM HAuCl4 for different time.

tion broad peak appeared at about 570 nm. Particularly, increasing the electrodeposition time from 180 to 600 s, the absorption peak intensity increased while the maximum absorption wavelength red-shifted, confirming an increase in size.36 These UV-vis absorption measurements are consistent with the SEM observations. The absorption peaks at about 440 nm are ascribed to residual interference fringes of the ITO film.37 The formation of HDGMs can be explained as a sequence growth process of nucleation-adsorption-growth-branchinggrowth. It is believed that the growth mechanism of the dendritic or tree-like microstructures follows a diffusion-limited aggregation (DLA) mode.38 Cluster formation occurs by sticking of particles in a random way to a selected seed in contact, allowing the particles to form. The reaction conditions (including electrodeposition potential, HAuCl4 concentration, and electrodeposition time) are of great significance to control over the nucleation and directional aggregation that gives rise to a

electrode

bulk gold

gold (30 s)

gold (180 s)

gold (300 s)

gold (600 s)

E0 (V) J0 (µA cm-2) Tafel slope (mV decade-1)

-0.238 0.69 491

-0.287 0.82 507

-0.339 1.20 678

-0.363 1.52 688

-0.530 1.54 849

nonequilibrium system and thereby favors the formation of dendrites. Dendritic growth occurs at the tips and stems of branches. As the stem grows in length, new shorter branches are continuously formed at the tips. Electrocatalytical Activity. Figure 6 compares the Tafel plots of the HDGMs electrodes obtained for different electrodeposition time with bulk gold electrode in 1 M NaOH +1 M ethanol aqueous solution. The Tafel plots were recorded at a scan rate of 1 mV s-1 between -1.0 and 0.6 V (versus SCE) and the Tafel parameters are listed in Table 1. The values of the exchange current density (j0) are calculated according to a Tafel extrapolation method. It is observed that the j0 value even on 30-s electrodeposited gold film is 1.19× larger than that on bulk gold electrode, and the equilibrium potential (E0) decreases obviously from -0.238 to -0.287 V. Moreover, E0 shifts negatively while the values of j0 increase with increasing electrodeposition time. This indicates that the gold deposits formed by hierarchical dendritic microstructures significantly accelerate the electrooxidation of ethanol. However, the calculated Tafel slopes also increase as the gold deposition time increases, which implies that the electrocatalytic activities toward ethanol oxidation was enhanced with the electrodeposition time, probably due to increased surface area. Cyclic voltammetry was further used to compare the electrocatalytic activity toward ethanol oxidation between HDGMs electrode (600 s deposition) and bulk gold electrode. Figure 7A shows the cyclic voltammograms obtained from the HDGMs electrode (curve a) and bulk gold electrode (curve b) in 1 M NaOH aqueous solution. A broad oxidation wave located at about 0.38 V and a reduction wave located at 0.0 V, corresponding to oxidation and reductions of gold.39,40 From Figure 7A, it is found that the peak current density of the HDGMs electrode is much larger than that of bulk gold electrode and also the peaks for the redox pair (0.38/0.0 V) of gold oxide monolayer appear at more negative potentials, revealing the high surface area and high activity of HDGMs electrode due to the dendritic microhierarchical structure. The typical cyclic voltammograms obtained from ethanol oxidation on HDGMs electrode (curve a) and bulk gold electrode (curve b) are shown in

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Figure 7. Cyclic voltammograms in the solutions at 50 mV s-1: (A) 1 M NaOH solution and (B) 1 M NaOH +1 M ethanol aqueous solution. The insert figure of (B) are i-t curves obtained for ethanol oxidation at 0.2 V. Curves a and b represent the HDGMs electrode and bulk gold electrode, respectively.

TABLE 2: Comparison of the Electrochemical Performances of Ethanol Oxidation in 1 M NaOH + 1 M Ethanol Solution on HDGMs Electrode and Bulk Gold Electrode electrode

ES (V)

EP (V)

HDGMs electrode -0.04 0.16 bulk gold electrode -0.06 0.18

jpf jpr (mA cm-2) (mA cm-2) jpr/jpf 10.88 0.59

2.95 0.38

0.27 0.64

Figure 7B. The HDGMs electrode performs much better electrocatalytic capability and the onset potential of ethanol oxidation at 0.16 V is more negative than the bulk gold electrode. Additionally, the gold oxidation peak appears at a more positive potential (0.40 V). The peak current density (jpf) in the forward potential scan and the peak current density (jpr) in the reverse scan are summarized in Table 2. The oxidation peak in the reverse scan corresponds to the electrochemical oxidation CO and other adsorbed species.41,42 So the smaller the jpr/jpf value, the better the poisoning-resistance of the electrodes for ethanol oxidation becomes. The value for jpr/jpf on the HDGMs electrode is 0.27, which is much smaller than that on the bulk gold electrode (0.64), showing that the HDGMs have better poisoning-resistance and steady-state behavior for ethanol oxidation. The stability of ethanol oxidation on the HDGMs electrode and bulk gold electrode was investigated (at 0.2 V) and shown in the insert figure of Figure 7B. The rapid current decay indicates the reliable antipoisoning ability of the electrode as the electrocatalyst. The enhanced peak current density and better stability are attributed to the larger surface area of HDGMs. SERS Measurements. The micro/nanostructures and amount of metal materials are two important parameters related to the detection limit in SERS applications.43 Here, we study the effect of the electrodeposition time on the SERS enhancements for the detection of 10-5 M R6G in ethanolic solution, in which the gold deposits were obtained by electrodeposition at -0.6 V in the solution of 0.1 M Na2SO4 + 30 mM HAuCl4. Strong Raman signals are clearly observed in Figure 8. Pronounced peaks at 1650, 1576, 1510, 1364, 1314, 1185, and 778 cm-1 are assigned to the xanthene ring stretch, ethylamine group wag, and carbon-oxygen stretch of R6G.44 These characteristic Raman peaks are in agreement with previous work.45-47 To further demonstrate the variation in SERS efficiency with different deposition times, the Raman peak of 1364 cm-1 was chosen as the identification position. The SERS intensities corresponding to the gold films deposited on ITO for 600 s

Figure 8. SERS spectra acquired from R6G of 10-5 M absorbed on the gold films electrodeposited at -0.6 V in 0.1 M Na2SO4 + 30 mM HAuCl4 for 600 (a), 300 (b), 180 (a), and 30 (d) s.

(curve a), 300 s (curve b), and 180 s (curve c), respectively, exhibit 3.9×, 2.7×, and 1.6× as strong as the film deposited for 30 s (curve d). It has been widely accepted that there are two primary enhancement mechanisms in SERS: one being electromagnetic (EM) enhancement that arises from the extremely high local fields due to surface plasmon resonance (SPR) and the other being chemical enhancement due to resonance Raman-like interaction between the substrate and the adsorbate. Theoretical investigations indicate that the most intensive amplitudes of SPR appear at the tips of the dendrites and then extend and decrease along their sides; the weakest amplitudes appear at their stems.48 Accordingly, the SERS hot spots are considered to be the dendrite branches and localized by optical focusing on the surface. This explains the experimental results that enhancing the SERS activity with the deposition time can be attributed to the increasing amounts of gold dendrites. As discussed above, the optimized Au dendritic surfaces may be excellent candidates for SERS-based ultrasensitive molecular sensing. Figure 9A shows the SERS spectra of R6G with various concentrations from 1.0 × 10-7 M to 1.0 × 10-13 M. It can be seen that the Raman signals strongly decrease in intensity with a decrease in the concentration of R6G. The signal-to-noise (S/ N) ratio of SERS peak at 1364 cm-1 is 3 at 10-12 M and the SERS peak is barely detected at 10-13 M. To calculate the SERS enhancement factor (EF) for R6G adsorbed on the surface od gold dendrites, the Raman spectrum of 10-2 M R6G dispersed on blank glass is obtained under the same experimental conditions (Figure 9B). According to the method developed by

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Figure 9. (A) SERS spectra of R6G with different concentrations adsorbed on HDGMs. (B) shows the Raman spectrum of 10-2 M R6G dispersed on blank glass.

Tian et al,49 the EF value can be calculated by the following equation:

EF ) (ISERS/Ibulk)(Nbulk/NSurface) where ISERS and Ibulk denote the integrated intensities for the 1364 cm-1 band of the 1 nM R6G adsorbed on the surface of the gold dendrites and 10-2 M R6G on glass, respectively; whereas NSurface and Nbulk represent the corresponding number of R6G molecules excited by the laser beam. Herein, the calculated EF value is evaluated to be 2.8 × 106, which is close to the values of ZnO/Au nanoneedle and nanorod arrays.50 Wettability Test. The as-prepared gold film in Figure 1A resembles the hierarchical dendritic microstructures in nature. So, it would be interesting to explore these structures as waterrepellent surfaces. Then the wettability of HDGMs on the ITO surface electrodeposited at -0.6 V was investigated with different electrodeposition time. As shown in Figure 10 (left), the gold film is hydrophilic with a CA 55° after electrodeposited in 0.1 M Na2SO4 + 30 mM HAuCl4 for 30 s. However, its wettability switches from hydrophilic to hydrophobic with increasing the electrodeposition time. The CAs are 100°, 105°, and 132°, corresponding to 180, 300, and 600 s deposition times, respectively. Coating a monolayer of n-dodecanethiol to the gold films can confer the metal films with a low surface energy, and so changes their wetting properties greatly. The gold films were immersed in 1 mM n-dodecanethiol ethanolic solution overnight. The CAs increased greatly, as shown in Figure 10 (right). For 30 s electrodeposition, the gold film exhibits hydrophobicity, giving a CA of 120°. With extended electrodeposition time, the films become superhydrophobic and the CA increased obviously after the same surface modification. After 600 s deposition, the gold film has a remarkable superhydrophobic property with a CA of 166°. A sequence of photographs of a water droplet lowered onto HDGMs with n-dodecanethiol modification is shown in Figure 11. In this sequence, a clear comparison of droplet shape is displayed for the initial state due to gravity, the exact contact state, the tight and severe contacting states under pressure, and the final state when removed from the substrate. When the droplet just touches the surface, it does not change its initial shape. A similar phenomenon is observed even when the droplet is squashed severely (Figure 11c); when the substrate is removed from the droplet, no water was left on surface (Figure 11f). This observation proves that the surface

Figure 10. Contact angle measurements of a water droplet (5 µL) on Au films without (left) and with (right) n-dodecanethiol modification. The Au films were electrodeposited at -0.6 V in 0.1 M Na2SO4 + 30 mM HAuCl4 for different time.

of HDGMs is superhydrophobic and nonadhesive to water droplet. According to the Cassie model,51 a rough hydrophobic surface can be considered as a kind of porous or hierarchical medium at which the penetration of the liquid is not favorable. Thus, air pockets remain trapped below the liquid, which sits above a patchwork of solid and air. As can be seen from the HDGMs (shown in Figure 1A), this kind of highly roughened surface is mainly composed of air, which eventually leads to a strong reduction of contact angle hysteresis. It can be concluded that such a strong superhydrophobicity is due to the higher surface roughness of HDGMs and the subsequent surface treatment. The environment in which the superhydrophobic surfaces could be applied is a fairly important factor for microelectromechanical systems (MEMS), microfluidic devices, and other

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Figure 11. Sequential photographs of the water droplet (5 µL) were recorded after the water droplet made contact with the n-dodecanethiol modified HDGMs. (a) Initial state; (b) exact contact; (c and d) severe contact; (e) tight contact; and (f) final state.

surface tension, the CAs of which are above 160°. Even for peanut oil and hexadecane having very low surface tension, the CAs of their liquids are shown to be close to 150°, which also exhibit oleophobicity. Conclusions

Figure 12. The relationship between pH value and contact angle on the HDGMs modified with n-dodecanethiol.

applications.52 Thus, the CA values in corrosive conditions were measured to test the applicability of the prepared superhydrophobic surface. Figure 12 shows the relationship between pH value and CA on the as-prepared gold dendritic surface (corresponding to Figure 1A) modified with n-dodecanethiol. The CA is almost unchanged with varying pH values from 1 to 13. Such observation indicates that the substrate has good stability, which is very important for real applications. In addition, the wetting properties of the HDGMs were also evaluated using glycerol, glycol, peanut oil, and hexadecane after modification of 1H,1H,2H,2H-perfluorodecanethiol, a chemical with even lower surface energy. Figure 13 provides the photographs of respective droplets and CAs. The surface displays super-repellencies toward glycerol and glycol with high

Well-fined HDGMs with secondary and tertiary branches were successfully synthesized on ITO substrates under constant potential electrolysis, without surfactant or template. The applied potential showed a significant effect on the morphologies of HDGMs; interestingly, unusual flowerlike and stem-type tectorum-like microstructures were obtained at higher electrodeposition potentials (0.2 and -0.2 V, respectively). Moreover, the HAuCl4 concentration considerably influenced the growth of HDGMs; at a lower HAuCl4 concentration (5 mM), only particles or clusters were obtained. On the basis of the timedependent morphological evolution which was monitored by SEM and UV-vis spectra, a DLA mode is used to explain the formation mechanism of HDGMs. The as-synthesized HDGMs exhibited multifunctional applications: (i) much higher electrocatalytic activity and enhanced stability toward ethanol electrooxidation compared to a commercial gold electrode; (ii) great Raman enhancement activity with the detection limit of 10-12 M for R6G; (iii) a remarkable superhydrophobic property over a wide range of pH after the treatment with n-dodecanethiol; (iv) (super)oleophobicity obtained by modifying with 1H,1H,2H,2H-perfluorodecanethiol. This synthetic strategy may open new and practical routes to the synthesis of inorganic micro/ nanostructures with hierarchical architectures as well as multifunctional devices.

Figure 13. Photographs of liquid droplets on the dentritic gold surfaces modified with 1H, 1H, 2H, 2H-perfluorodecanethiol.

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