Large-scale controlled fabrication of highly roughened flower-like silver nanostructures in liquid crystalline phase

Large-scale controllable fabrication of highly roughened flower-like silver nanostructures is demonstrated experimentally via electrodeposition in the liquid crystalline phase. Different sizes of silver flowers are fabricated by adjusting the deposition time and the concentration of the silver nitrate solution. The density of the silver flowers in the sample is also controllable in this work. The flower-like silver nanostructures can serve as effective surface-enhanced Raman scattering and surface-enhanced fluorescence substrates because of their local surface plasmon resonance, and they may have applications in photoluminescence and catalysis. This liquid crystalline phase is used as a soft template for fabricating flower-like silver nanostructures for the first time, and this approach is suitable for large-scale uniform fabrication up to several centimetres.

Morphology and constituent characterization. Several electrodepositions for different growth times and AgNO 3 concentrations are performed. Different sizes of flower-like silver nanostructures, ranging from 250 nm to 1.5 μ m, are obtained. The field emission scanning electron microscope (FESEM) is used to characterize the size, shape and other morphological properties of the silver nanostructures. Figure 1 shows a FESEM image of the silver flower-like nanostructures that was obtained from the electrodeposition process.
In Fig. 1, we can observe the silver flowers. The AgNO 3 concentration in the electrodeposition process is 0.1 mol/L and the growth time is 60 minutes. Figure 1(a) presents the FESEM image with low magnification and indicates that the electrodeposited silver nanostructures are quite uniform on the surface of the sample. In fact, the silver flowers are uniform over the entire working electrode in our experiment, and we believe that such uniform silver flowers can be achieved at an even larger scale. Figure 1(b) is a magnified image of (a) and shows that the sizes of each silver flower are also uniform. Different flowers have nearly the same size which indicates the same growth process for each flower. Figure 1(c) is the image of a single silver flower, where the morphology and size are clearly demonstrated. The size of the flower is approximately 1.5 μ m. The shape is quite like a rose and is composed of a high density of petals. The thickness of the petals is approximately 25 nm to 100 nm. There are many horns on the petals and thin gaps between adjacent petals. Figure 1(d) is the side view of the silver flowers. The height is approximately 700 nm and is uniform for different flowers. From the image, we can also observe that there are no obvious differences in morphology (petal density, petal thickness, flower width) between the bottom and top of each flower. The elemental constitution of the silver flowers is analyzed using the energy-dispersive spectrum (EDS). The EDS pattern is shown in Fig. 2, from which only peaks of silver, carbon and sulfur are observed. The peaks of carbon and sulfur can be attributed to the residual AOT in the sample; because of the nanoscale space between the petals and flowers, the template molecule cannot be completely removed from the sample by the washing process. Thus, we believe that the flowers are composed primarily of silver.
Growth mechanism and growth control. The growth mechanism of such flower-like nanostructures in the liquid crystalline phase can be attributed to the synergistic soft template mechanism 27,28 , which refers to the cooperative effect of the liquid crystalline phase soft template and the self-assembly of Ag. The AOT is a type of amphiphilic copolymer whose molecular structure contains a hydrophilic head and two hydrophobic chains, as Fig. 3(a) shows. When the AOT, p-xylene and water are mixed together, the hydrophilic heads of the amphiphilic copolymer aggregate to form a micelle with water inside and leave the hydrophobic tails outside in the p-xylene, which indicates that the process of microphase separation has started. When the phase equilibrium is achieved, the microphase-separated nanostructures in the liquid crystalline phase are many water pores surrounded by p-xylene 26 , as Fig. 3(b) shows. The Ag ions are encapsulated in the water pores. The initial nucleation of silver takes place in such pores only when deposition starts, as Fig. 3(c) illustrates. As the deposition process proceeds, the aggregated silver structures break the liquid crystalline phase template and become flowerlike due to the self-assembly effect of Ag, as shown in Fig. 3(d). The liquid crystalline phase acts as a soft template. The exact mechanism for the growth of such metal nanostructures in the liquid crystalline phase template remains a challenging research question. However, we can still use this method to fabricate such silver flower-like nanostructures conveniently.
The size and morphology of the silver flower-like nanostructures depend primarily on the deposition time and the AgNO 3 concentration. Figure 4 shows the FESEM images of silver flowers for different deposition times. From Fig. 4, we can observe different size flowers for different growth time.  Fig. 4(d), we can observe that the petals of the flower are very dense and thin. The average petal thickness is approximately 10 nm, which indicates a large surface to mass ratio (up to 15 m 2 /g, roughly estimated). As Fig. 4 demonstrates, we can control the size of silver flowers by controlling the deposition time.
To describe the density of the electrodeposited silver flowers, the coverage ratio is used, which refers to the ratio of the area covered by silver flowers to the total area of the working electrode. Figure 5 shows the FESEM images of different coverage ratio samples for different growth times. Figure 5(a) is an image for a 5-minute deposition, where the flowers are very small and sparse with a coverage ratio of only 1.8%. Figure 5(b) is for 10 minutes of deposition, and the coverage ratio is approximately 9.7%. Figure 5 Figure 6 illustrates the relationship between the coverage ratio and the deposition time. From Fig. 6, we can see that the coverage ratio of silver flowers increases with the deposition time. When the deposition time is less than 5 minutes, the silver nanostructures are small discrete dots instead of flowers, and the coverage ratio is rather low (not show in this work). As the deposition time increases, the coverage ratio increases continually, but the speed of increasing slows down noticeably.
The AgNO 3 concentration also plays an important role in the formation of the silver flower nanostructures. In this work, we also used other concentrations AgNO 3 solution to perform electrodepositions.      Fig. 4 shows, and the growth speed is faster, which can be observed in Fig. 7 (i)-(p). A high silver ion concentration is favourable for the formation of complicated structures. Generally, the increase of the silver ion concentration will also increase the growth speed of silver nanostructures during the electrodeposition process. Consequently, we can obtain more complicated flower-like silver structures quickly by increasing the AgNO 3 concentration.

Discussion
The silver flower-like nanostructures described above can be used as a substrate for SERS and SEF. SERS spectroscopy is a powerful and extremely sensitive spectroscopic tool that can provide a spectral fingerprint of a molecule. Generally, two different mechanisms are thought to cause the SERS phenomenon. One is the enhancement of local electromagnetic fields due to the surface plasmon resonance in metals, and the other is the chemical enhancement that originates from the charge transfer between the molecule and the metal surface. The surface plasmon resonance is generally believed to be the main contributor to SERS and to be several orders stronger than the charge transfer. Because the silver flowers described above have so many horns and thin gaps between the flowers and petals, we believe that a strong Raman enhancement effect will be achieved. Such nanostructures also can be used as a surface-enhanced fluorescence substrate. The enhancement mechanism of SEF is also understood in terms of hot spots, which are related to the excitation of the surface plasmon. The existing studies have shown that the localized field can achieve large fluorescence enhancements by factors ranging up to a few hundred 13 . This material may also have applications in chemical catalysis because of its large surface to mass ratio. Studies concerning SERS and SEF are still ongoing in our group; the main focus of this paper is fabrication. The novel fabrication method, which is very simple, inexpensive and suitable for large-scale fabrications, will make it easier to apply such flower-like silver nanostructures as SERS and SEF substrates.
In summary, we have demonstrated a novel method for the large-scale uniform fabrication of flower-like silver nanostructures. This novel fabrication method, which is quite simple and inexpensive, performs electrodeposition primarily in a ternary liquid crystalline phase. Different sizes and different coverage ratios of silver flowers are obtained by controlling the growth time and the concentration of silver nitrate. These silver flower nanostructures, which possess properties such as hot spots and large surface-to-mass ratios, can serve as surface-enhanced Raman scattering and surface-enhanced fluorescence substrates and may also have applications in photoluminescence and catalysis.

Methods
Materials and preparation of liquid crystalline phase. In this work, we use the liquid crystalline phase as a soft template to perform the electrodeposition process. The liquid crystalline phase is prepared according to the ternary phase diagram 26 , which consists of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), the oil phase p-xylene, and water(AgNO 3 aqueous solution in this work). The purities of AOT, p-xylene and silver nitrate are 98.0 wt%, 99.0 wt% and 99.9 wt%, respectively. All chemical materials are used directly from the manufacturer without further purification. Deionized water with 18 MΩ ·cm resistivity is used as a solvent. In our experiment, we use an aqueous Electrodeposition of flower-like silver nanostructures. To keep the surface clean, the electrodeposition process is prepared and carried out in a cleanroom. The liquid crystalline phase is used as the electrolyte. The anode is a one-millimetre-thick polished silver plate. Indium tin oxide glass (ITO glass) is used as the cathode, which is also the working electrode to collect the flower-like silver nanostructures. The size of both electrodes is 20 mm × 20 mm. Before electrodeposition, the electrodes are cleaned completely with ethanol and deionized water. These procedures can guarantee that only the silver dissolution reaction occurs on the anode and that the liquid crystalline phase is not disturbed during the electrodeposition process. The 2.0 V static potential is applied to the electrodes by the DF1761 potentiostat. The distance between cathode and anode is approximately 0.5 mm, which is determined by the spacers between them. The current density increased gradually from approximately 10 μ A/cm 2 to 120 μ A/cm 2 with the deposition time due to the increase of the surface area. As the growth process goes on, more and more flowers appear, the size of flowers increases and the morphology becomes more complicated. In this work, we perform the deposition process for 5, 10, 15, 30, and 60 minutes to obtain different sizes and coverage ratios for the flower-like nanostructures. After deposition, the working electrode with silver flower-like nanostructures is washed by ethanol and deionized water to remove the liquid crystalline phase left on the electrode.