Fabrication of Self-Standing Silver Nanoplate Arrays by Seed-Decorated Electrochemical Route and Their Structure-Induced Properties

We present an electrochemical route to synthesize silver nanoplates on seed-decorated Indium tin oxide (ITO) glass substrate. e nanoplates are several tens of to several hundred nanometers in dimension. e density of nanoplates covered on the substrate can be controlled well by adjusting the amounts of seed. All the nanoplates are standing on the substrate uniformly even at very high density. Silver nanoplate arrays displayed an extraordinary superhydrophobicity a�er chemical modi�cation and can serve as highly active surface-enhanced Raman scattering (SERS) substrates for microdetection. e arrays can also be used as electrodes for electrochemical capacitor with high power density.


Introduction
Surface morphology and assembly behavior of nanostructured materials decide their performances; therefore, their investigations have attracted much interest in recent years originated from their widespread applications in optics, electronics, optoelectronics, information storage, self-cleaning, catalysis biological, chemical sensing, surface-enhanced Raman scattering (SERS) effect, and so forth [1][2][3][4][5][6][7][8][9][10].Among these, SERS is a typical instance, which is mainly determined by the surface morphology and structure of noble metal materials [11].It means a dramatic enhancement of intensity of the Raman spectra of the molecule absorbed on the surface of metal [12][13][14][15][16][17].Some reports have revealed that sharp protrusion and nanogaps in highly roughed metallic surface, so called active "hot" spots, are responsible for SERS, because of larger electromagnetic enhancement caused by local surface plasma resonance at these sites [18,19].In addition, other properties, that is, self-cleaning surface, use as electrode for electrochemical capacitor, are also determined by the surface morphology [20][21][22][23][24][25][26][27][28].Self-cleaning surface should have static contact angles (CA) of water droplets larger than 150 ∘ and the slid angle (SA) smaller than 10 ∘ , which can be prepared by a combination of reducing the surface energy and enhancing the surface roughness [29][30][31][32][33][34].e self-cleaning performance can be widely used for preventing the adhesion of water or snow on the surface of some objects.In electrochemical capacitor, electrochemical reaction occurred at the interface between electrode and electrolyte, so a large electrochemical interface of electrode is favorable to the enhancement of the power density [35].In whole, assembly of nanostructured material with high rough surface is the key factor in the above applications.
Previously, we reported a spin-coated Ag seed growth method to fabricate standing silver nanoplate arrays with high rough surfaces [36].But through this method, the obtained silver nanoplates are not sufficiently tightly anchored on indium tin oxide (ITO) substrate.Additionally, these nanoplates are not uniform in size and distribution when we need nanoplate array with a more high distribution density covered on the substrate for some applications.e main reason is poor uniform distribution of Ag seeds with a high density by spin-coating method.In this paper, we report a simple electrochemical approach decorated by Ag seeds to synthesize silver nanoplates with controllable density (roughness) covered on the ITO substrate.e obtained silver nanoplates are covered on the substrate uniformly with very high density.e whole procedure was shown in Figure 1.Firstly, we decorated a layer of 3-aminopropyltrimethoxysilane (APTMS) on clean ITO (Figure 1(a)), and then this substrate was immersed into as-prepared Ag colloid solution for decorating Ag seeds on the substrate (Figure 1(b)), followed growth of Ag nanoplates by electrochemical deposition route on the substrate directly.e as-prepared silver nanoplate arrays demonstrated structured enhanced SERS activity and a good superhydrophobic property aer chemical modi�cation with n-dodecanethiol, which has important potential application in �uidic device, self-cleaning surfaces, organic molecule detection, and so forth.In addition, the arrays can also be used as electrode for electrochemical capacitor with high power density.

Experiment
2.1.Decorating Seeds.e details of preparing silver seeds have been reported previously [36�.Brie�y, 60 mM AgNO 3 of 0.5 mL and 35 mM sodium citrate of 1 mL were added into 98 mL distilled water.e mixed solution was stirred for �ve minutes.en, an aqueous 20 mM NaBH 4 solution of 0.5 mL, which had been aged at room temperature for 2 h, was added quickly, stirred for 1 h, and then aged at room temperature for 24 h before use.ITO substrates were ultrasonically cleaned in acetone and then in ethanol for 1 h.Cleaned ITO substrates were placed into a dilute solution of 3-aminopropyltrimethoxysilane (APTMS) (0.3 mL of APTMS in 3 mL of methanol) for 12 hours and rinsed with methanol to remove redundant APTMS.e polymer-coated substrate was immersed in above Ag seeds colloid solution for different time, 4 h, 8 h, and 10 h respectively.en, the substrates were taken out and dried with high-purity �owing nitrogen.

Electrochemical Deposition.
Silver nitrate (AgNO 3 , 99+%), and poly(vinyl pyrrolidone) (PVP, MW = 30 000) were purchased from Shanghai Chemical Reagent Co.All chemicals were analytically graded and were used without further puri�cation.Typically, 0.05 g AgNO 3 and 0.25 g PVP were added to 50 mL of water followed by stirring till complete dissolution.Such aqueous solution was used as electrolyte for electrodeposition.e electrochemical deposition was carried out under current density 5 A/cm 2 at room temperature.A graphite �ake was used as anode and the Ag seed-decorated ITO substrate as cathode.Aer deposition for about 5 h, the substrate with products was taken out, cleaned with distilled water for several times and dried with high-purity �owing nitrogen for characterization and property study.
2.3.Characterization.e as-prepared samples were characterized by X-ray diffraction (XRD) (Philips X ′ pert-PRO, Cu K (0.15418 nm) radiation), �eld emission scanning electronic microscope (FESEM, sirion 200 FEG).Before contact angle (CA) measurement, the substrate was immersed in ethanol solution of n-dodecanethiol (1 × 10 −3 M) for about 10 h and washed in turn with ethanol and water and dried with nitrogen to coat a self-assembled monolayer of low free energy materials.Distilled water droplets with a volume of 5 L were used in contact angle test.
For Raman spectral measurement, the samples were immersed in 4-aminothiophenol (4-ATP) solution with different concentration for 30 min, 10 −6 mol/L, 10 −8 mol/L, 10 −10 mol/L, and 10 −12 mol/L, respectively.Aer that, the substrates were taken out from the solution and rinsed with de-ionized water, and then dried with high-purity �owing nitrogen.e Raman spectra were recorded on a macroscopic confocal Raman spectrometer, using a laser beam with an excitation wavelength of 532 nm.
For electrochemical measurements, a typical threeelectrode cell was employed.e electrolyte was 5 M KOH solution.An Hg/HgO electrode and a platinum electrode were used as the reference electrode and the counter electrode.

Results and Discussion
Aer electrodeposition on the ITO substrate coated with the Ag-seeds, the samples were characterized by XRD �rstly, as shown in Figure 2(A) (curve (a), (b), (c) corresponding to samples obtained preimmersed Ag seeds colloid solution for 4 h, 8 h, 10 h), and the intensity of diffraction increase gradually with increasing immersing time.Four diffraction peaks (2) at 38.2, 44.3, 64.4, and 77.3 ∘ were observed, corresponding to the (111), ( 200), (220), and (311) of the face-centered cubic (fcc) structure of silver with space group Fm3m (JCPDS), respectively.
Figures 2(B)-2(D) display SEM image of as-prepared samples.We can see that these units are nanoplates with the size in several tens to several hundred nanometers.Obviously, the densities of coverage of the nanoplates become dense with enlarging time of polymer-decorated ITO substrate into Ag colloid solution, and the density of coverage is about 7.2 × 10 7 plates/cm 2 , 4.0 × 10 8 plates/cm 2 , and 3.5 × 10 9 plates/cm 2 , respectively.When the nanoplates are very dense, all nanoplates are standing on the ITO substrate uniformly and cross-linking together with each other, leading to a very rough surface (see Figure 2(D)), while some nanoplates are gathered together at high density fabricated by our previous method.But when the nanoplates are sparse, most of them are lying on the ITO substrate, and hence the surface roughness is low (Figure 2(B)).It demonstrates that we can also control the density of coverage of Ag nanoplates through decorating Ag seed layer method.at is, the roughness of the surface can be tuned.

The Application of Ag Nanoplate Arrays
where   and  are the contact angles of a rough (textured) surface and a �at surface, respectively,  1 and  2 are the fractional interfacial areas of the Ag nanoplates arrays and the air interspaces among the Ag nanoplates arrays ( 1 +  2 = 1).From this equation, it is easy to conclude that increasing the value of  2 can increase CA.For a �at surface modi�ed with the n-dodecanethiol, the CA is 108 ∘ , so  2 is calculated to be 0.91.is means that the larger fraction of air ( 2 ) among the interspaces of standing Ag nanoplate arrays with higher density leads to superhydrophobicity of the surface.Another feature of a water-repellent surface from the dynamic viewpoint is a low sliding angle, which enables a water drop to easily roll down the surface.is means that these silver nanoplate arrays with a rough surface trap a large amount of air among nanoplate structures, leading to a very good superhydrophobicity with self-cleaning effect.5).is evidence indicated that this rough surface built with Ag nanoplates arrays had an excellent SERS performance.We attribute this enhancement to the many protrusions of standing Ag nanoplates and interstitials or nanogaps is formed among these nanoplates, which provide many hot "spots" for SERS [38,39].Obviously, with increasing the density of Ag nanoplates, the "hot spots" increase correspondingly.

Electrochemical Properties of Silver Nanoplates Arrays.
e cyclic voltammetry at different scan rate using silver nanoplates arrays with high density (Figure 2 in 5 M KOH solution was shown in Figure 6.It can be noted that the shape of CV curves have no change and the redox peak is at the same position with different scan rates, which illustrate that the silver nanoplates electrodes has a good reversibility.Figure 7 shows the charge and discharge curve.Aer 25 cycles, the efficiency of charge/discharge is not dropped, which exhibits that the silver nanoplates arrays have high stability using as electrode.In addition, the speci�c capacitance values can be calculated according to the following equation: Here,   is speci�c capacitance,  is charge/discharge current,  is the discharge time within the voltage difference (Δ), and  is the mass of silver nanoplates.From Figure 7, we can calculate that the speci�c capacitance is 1270 F g −1 .is value is higher compared with other similar electrode [35].We think that the increase in speci�c capacitance origins from high touching surface of standing Ag nanoplates with KOH solution.

Conclusion
In summary, we prepared Ag nanoplates on ITO substrate by a seed-decorated electrochemical deposition approach.e density of Ag nanoplates can be controlled through the amount of Ag seed predecorated.e Ag nanoplates arrays with highest density can be dispersed uniformly on the substrate, which had high roughened surface and displayed an extraordinary superhydrophobicity aer a chemical mod-i�cation.Such Ag nanostructured array shows high SERS activity and hence is a good material for the microdetection device based on SERS effect.In addition, the Ag nanoplates arrays can be used as electrode for electrochemical capacitor with high power density.

F 1 :
Schematic illustration of formation of nanoplate arrays: (a); a layer of 3-aminopropyltrimethoxysilane (APTMS) absorbed on clean ITO (b) Ag seeds decorated on clean ITO; (c) growth of Ag nanoplate arrays by electrochemical method.

F 2 :
XRD of the as-prepared sample with the polymer-coated substrate immersing in Ag seeds colloid solution for different time, 4 h (curve a), 8 h (curve b), and 10 h (curve c), respectively; FESEM images of the samples prepared with the polymer-coated substrate immersing in Ag seeds colloid solution for different time, 4 h (B), 8 h (C), 10 h (D).

4. 1 .
Superhydrophobicity. e water droplet shapes on the surface of three samples (Figures 2(B)-2(D)) with different density is shown in Figure 3. e water CAs were 132 ∘ , 150 ∘ and 160 ∘ respectively, (Figures 3(a)-3(c)) and the SA is about 7 ∘ for nanoplates arrays with highest density (Figure 2(D)).It is obviously that surface roughness is directly related to the CA.With increasing of density of Ag nanoplates, the surface roughness is increasing, and the CA turned lager accordingly.e spherical water droplet (Figure 3(d)) also can further con�rm the superhydrophobicity.We describe the CA in terms of the Cassie equation [37]:

F 3 :
Photo images of water droplets on the surface of Ag nanoplates arrays with the polymer-coated substrate immersing in Ag seeds colloid solution for different time, 4 h (a), 8 h (b), and 10 h (c), respectively, and the water CAs were 132 ∘ , 150 ∘ , and 160 ∘ ; (d) photograph of (c).

4. 2 .
Surface-Enhanced Raman Scattering Effect.We use these three samples with different density of Ag nanoplates covered as substrates to study the SERS activity, as shown in Figure4corresponding to the Raman spectra of 4-ATP molecular (aer immersion in 10 −6 mol/L 4-ATP solution for 10 min).We can �nd the SERS activity raises with increase of the distribution density of Ag nanoplates arrays (integrating time 1 s).In order to further probe into the detection limit of SERS property of the substrate with highest density Ag nanoplate arrays (shown in Figure2(D)), three different concentrations (10 −8 M, 10 −10 M, 10 −12 M) of 4-ATP were examined.e Raman spectral intensity of 4-ATP turned weaker with decrease of the concentration of 4-ATP in a certain range (integrating time 1 s).But, even when the concentration of 4-ATP was decreased to 10 −12 M, we still can distinguish Raman peaks of 4-ATP (see curve (c) in Figure

F 4 :
Raman spectra of the 4-ATP on different surface of Ag nanoplates arrays (Figure 2(D)) with the polymer-coated substrate immersing in Ag seeds colloid solution for different time, 4 h (a), 8 h (b), 10 h (c), respectively.

F 6 :F 7 :
e cyclic voltammetry at different scan rate with silver nanoplates arrays (Figure 2(D)) as electrode in 5 M KOH solution.e charge and discharge curve using Ag nanoplates arrays (Figure 2(D)) as electrode.Aer 25 cycles, the efficiency of charge/discharge is not dropped.Science Funds for Distinguished Young Scholar (Grant no.1108085J20), and the National Basic Research Program of China (973 Program, Grant no.2011CB302103).