One-step large-scale synthesis of micrometer-sized silver nanosheets by a template-free electrochemical method

We have synthesized micrometer-sized Ag nanosheets via a facile, one-step, template-free electrochemical deposition in an ultra-dilute silver nitrate aqueous electrolyte. The nanosheet growth was revealed to occur in three stages: (1) formation of polygonal Ag nuclei on a substrate, (2) growth of {112}-faceted nanowire from the nuclei, and (3) anisotropic growth of (111)-planar nanosheets, approximately 20 to 50 nm in thickness and 10 μm in width, in the <112>−direction. The vertical growth of the facet nanowire was induced by the strong interface anisotropy between the deposit and electrolyte due to the ultra-dilute concentration of electrolyte and high reduction potential. The thickness of Ag nanosheets was controllable by the adjustment of the reduction/oxidation potential and frequency of the reverse-pulse potentiodynamic mode.

Until now, two-dimensional silver nanostructures have been fabricated using surfactants (capping agent) [6,13], sacrificial materials [14], and hard templates (porous alumina) [15]. Although these methods have the merits of controlling the morphology and size of Ag nanostructures, they are complicated and costly. A chemical route without any surfactants led to the large-scale synthesis of micrometer-sized Ag nanosheets (approximately 15 μm in size and 28 nm in thickness) after the addition of a small quantity of H 2 PdCl 4 as seeds for the growth of Ag nanosheets [16]. With such solution-based methods, colloidal nanosheets were randomly dispersed in a liquid before being used for their purposes. Recently, a simple galvanic reaction was used to synthesize silver nanoplates with thicknesses of 50 to 70 nm and a size of approximately 1 μm on GaAs substrates [9,17]. In an alternative approach, current density of a potentiostatic electrochemical method using poly(vinyl pyrrolidone) was kinetically controlled to synthesize vertically cross-linking Ag nanosheets of several micrometers in width [8,18]. However, there are very limited studies on the facile and large-scale synthesis of Ag nanosheets by an electrochemical deposition without any templates and surfactants.
In this study, we report a facile, large-scale, one-step process of synthesizing Ag nanosheets (tens of micrometers in size and several tens of nanometers in thickness). Our process uses a template-and surfactant-free electrochemical deposition in an ultra-dilute electrolyte of low electrical conductivity (less than 50 μS•cm −1 ). The growth mechanism was revealed by time-dependent growth analyses. The present method is environment friendly and low cost because the precursor concentration of Ag ions is very low (several tens of μM) compared with that (above several mM) used in conventional electrochemical methods.

Preparation of Ag nanosheets
Ag nanosheets were deposited on a substrate by a reversepulse potentiodynamic electrochemical deposition. The aqueous electrolyte was composed of 0.02 mM AgNO 3 (#209139, reagent A.C.S., Sigma-Aldrich, St. Louis, MO, USA) and 1.32 mM NH 4 OH (#13370-0380, Guaranteed Reagent, Junsei Chemical Co., Ltd., Chuo-ku, Tokyo, Japan). The AgNO 3 concentration was varied as 0.2 and 2 mM, respectively, to observe the effects of concentration on the morphologies of Ag deposits. A two-electrode system that comprised a Ag plate (1 mm in thickness and 5 cm in length, 99.9%, Alfa Aesar, Wardhill, MA, USA) as a counter electrode and a Au film-coated Si substrate as a working electrode was used. The exposed area of Au film (90-nm thick) was 0.5 cm × 0.5 cm. The electrolyte was supplied into the rectangular Teflon bath at the constant flow rate of 200 ml/min using a peristaltic pump (# S 600, dslab 24, Gyeonggi-do, Korea). The interdistance between the working and counter electrodes was set at 1 cm. For the reverse-pulse potentiodynamic mode, the reduction potentials (V R ) were set to be 10, 15, and 20 V, and oxidation potentials (V O ) were set to be 0.05, 0.2, and 0.4 V. The deposition time was varied as 20, 40, 70, and 120 min, respectively. The frequency was controlled as 1, 10, 100, and 1,000 Hz, respectively. The reduction period of the reverse-pulse was set at 3%.

Instruments and characterization
The homemade two-electrode system was composed of a dual DC power supply (Agilent E3620A, Agilent Technologies, Santa Clara, CA, USA) and a function generator (Agilent 33220A). The detailed description can be found in previous work [19]. The microstructures of Ag nanosheets were observed using a field-emission scanning electron microscope (SEM; Hitachi S-4800, Hitachi Ltd., Chiyoda-ku, Japan). The crystal structures were analyzed using a high-resolution transmission electron microscope (HRTEM; FEI Tecnai G2 F30, 300 kV, FEI, Hillsboro, OR, USA). Figure 1 shows the typical SEM images of Ag nanosheets that were electrodeposited in an ultra-dilute electrolyte in the potentiodynamic mode (V R = 15 V, V O = 0.2 V, 100 Hz, and 3%) for 120 min. Ag nanosheets had a width up to approximately 10 μm and a thickness of approximately 30 nm and were grown on the facetted Ag nanowires. In comparison, when the AgNO 3 concentration was 0.2 mM, the facetted granular Ag islands grew with the size of 0.2 to 2 μm, as shown in Figure 2a. With the further increase of AgNO 3 concentration up to 2 mM, the granular islands were densely generated and formed a granular (columnar) layer, as shown in Figure 2b. This indicates that the growth of facetted nanowires and nanosheets shown in Figure 1 was closely related to the dilute concentration.

Results and discussion
The time-dependent growth of the Ag nanosheets was examined by varying the deposition time as 20, 40, 70, and 120 min, respectively, as shown in Figure 3a,b,c,d. The growth occurred in three stages. First, the nucleation of polygonal islands on a substrate occurred, as shown in Figure 3a. The polygonal nuclei were randomly generated on the whole surface of substrate. Second, one-dimensional growth was driven in a specific direction by strong interface anisotropy between the polygonal islands and the electrolyte, which resulted in the facetted Ag nanowires shown in Figure 3b. In the previous work, it was shown that the interface anisotropy becomes stronger due to the field enhancement at the top of the hemispherical islands in an ultra-dilute electrolyte of low electrical conductivity [20]. Third, planar growth on one of the facet planes was initiated and planar nanostructure grew further, forming a facetted nanosheet (Figure 3c). The nanosheets, which were attached to the facetted nanowires, grew wider (up to approximately 10 μm) with increasing deposition time, as shown in Figure 3d. Figure 3e shows the enlarged top-view SEM image of the nanosheet on the specimen shown in Figure 3c. The growth of hexagonal nanosheet can be described, as shown in Figure 3f. After the planar growth (i) on one facet plane of the facetted nanowire, another planar growth occurs on the other facet plane (ii), as shown in Figure 3e. The nanosheet grows further with deposition time and finally forms a hexagonal nanostructure (iv). The whole surface of the substrate was covered with Ag nanosheets that grew along the facetted nanowires, as shown in Figure 1a. The nanosheets attached to the facetted nanowires could easily be detached from the substrate and dispersed into an aqueous solution via sonication for several seconds, which enabled us to easily prepare TEM samples.
As shown in Figure 4, the thickness of the nanosheet depended on the thickness of the facetted nanowires that grew over the islands nucleated on the substrate. Therefore, the thickness of Ag nanosheets could be controlled by varying the island size. In the previous work, the island size was controlled by the deposition frequency  and reduction/oxidation potentials of the reverse-pulse potentiodynamic mode [20]. When the deposition frequency was varied in the range of 1 to 1,000 Hz under the same deposition parameters (V O , V R , and duty) for the sample shown in Figure 1, the thickness and size of Ag nanosheets were controlled in the range of 20 to 50 nm and 3 to 10 μm in size, respectively (Figure 4). At the low frequency of 1 Hz, the deposit was composed of irregular Ag nanosheets shown in Figure 4a. With increase of the frequency from 10 to 1,000 Hz, the planar Ag grew and the thickness decreased from 50 to 20 nm, approximately. Also, the nanosheet size increased with the frequency increasing, as shown in Figure 4. It is noted that the facetted nanowires became thinner with the frequency increasing in the range. It is presumed that the nucleation size became smaller with the shorter period of reduction process. We investigated the effects of the reduction/ oxidation potentials on the growth of Ag nanosheets, as shown in Figure 5. At the reduction potential of −10 V (Figure 5a), the deposit grew so slowly comparing to that shown in Figure 1. It seems that the reduction potential should be applied over V R = −10 V. At the higher reduction potential of −20 V, a lot of nanosheets were deposited and extra nanoparticles grew on the nanosheet surface, as shown in Figure 5b. This was due to the fact that the higher reduction potential leads to higher nucleation and growth rates in electrochemistry. Also, when the oxidation potential was decreased to 0.05 V comparing with the samples (V O = 0.2 V) shown in Figure 1, nanosheets of several micrometers in size grew, and small nanoparticles were deposited on the surface of the nanosheets, as shown in Figure 5c. At the higher V O of 0.4 V, nanosheets grew without nanoparticles on their surface, but the amount of nanosheets decreased much, as shown in Figure 5d. As the oxidation potential played a role of dissolving Ag atoms into the electrolyte, the lower oxidation potential resulted in the higher growth rate, and vice versa [20]. Therefore, the morphology of Ag  nanosheets shown in Figure 5c was similar to that of Ag nanosheets which were deposited at the higher reduction potential of −20 V. Figure 6a shows a bright field (BF) TEM image of Ag nanosheet that was selected from the sample shown in Figure 1a. Ag nanosheet grew along the facetted nanowire, which agreed with the SEM observation. Figure 6b Figure 6b,c, the inner set of spots might originate from the 1/3{422} planes normally forbidden by an fcc crystal structure. The forbidden 1/3{422} reflections were observed in the nanoplate morphology of Ag or Au due to the stacking faults extending parallel to the {111} planes through the entire nanoplates [9,21,22]. The outer spots were partially indexed to {220} Bragg reflections. The planar surfaces of Ag nanosheet were bounded by {111} planes and the edges were bounded by {112} planes. TEM analyses indicated that the Ag nanosheet was single crystal with {111} planar surfaces bounded by {112} edge planes. The FFT images of the facetted nanowire and the nanosheet showed the same crystallographic direction. This indicated that the nanosheet grew coherently along the facet plane of the nanowire. The present results are similar to the previous results in that gold nanobelts and nanocombs, synthesized in the presence of various organic molecules or surfactants, had grown along the <110> and <211> directions because the mixed surfactants induced anisotropic growth by being adsorbed on specific crystal planes [23,24]. In this study, the filamentary effect in the ultra-dilute concentration, as discussed in the previous work [20], might have induced the strong interface anisotropy needed for the anisotropic planar growth. As the ultra-dilute concentration of electrolyte could bring about a thick double layer between the deposit and the electrolyte [25], the slow transportation of Ag ions to the deposit was being controlled by the reduction potential to enable the facet growth to occur. In addition, different surface energies of each lattice plane might contribute to the anisotropic growth. As the surface energies of {111}, {112}, and {110} planes are known to be 1.6055, 1.8642, and 1.9342 J/m 2 [24,26], it appears that the {111}-planar surface is more favorable thermodynamically.

Conclusions
We developed a facile, one-step, low-cost, and large-scale method of fabricating single-crystalline Ag nanosheets with controllable thickness without any templates, capping agents, or sacrificial seed materials. The growth of nanosheets occurred in three stages: polygonal island formation, facetted nanowire growth, and planar growth of nanosheet coherent with the facetted nanowire. The nanosheets with {111}-planar surfaces and {112}-edge planes had a controllable thickness depending upon the deposition frequency and reduction/oxidation potentials. The present method is expected to contribute to the development of environment-friendly and low-cost electrochemical synthesis of nanomaterials.