Silver nanoparticles were electrodeposited from 0.3 M oxalic acid electrolyte on a pure aluminum working electrode under silver ion concentration-limited condition. A silver wire was held in a glass tube containing 1.0 M KCl solution as the counter electrode. Ion exchange between the glass tube and the main electrodeposition bath through a capillary was driven by the overpotentials as high as 10 V supplied by an electrochemical workstation. Due to the reaction between chlorine anion and silver cation to form AgCl solid at the Ag/AgCl electrode, the silver ion concentration-limited condition holds in the electrolyte. It is found that silver grows at the aluminum working electrode to form nanoparticles with an average size of about 52.4 ± 13.6 nm. With the increasing of the deposition time, the silver nanoparticles aggregate into clusters. The silver particle clusters are separated with approximately 112.6 ± 19.7 nm due to the hydrogen bubble-induced self-assembling, which is shown by the confined deposition of silver on a gold coating. The surface roughness of the aluminum substrate leads to the reduced uniformity of silver nanoparticle nucleation and growth.

## Introduction

As one of the most widely used nanomaterials, silver nanoparticle has antibiotic and catalytic functions. It can also enhance the absorption of solar energy. Silver nanoparticles can be prepared through various methods including thermal evaporation [1], laser ablation [2], electromagnetic levitation gas condensation [3], radio-frequency sputtering [4], chemical reaction [5], phase transformation [6], self-assembly [7], and plasma electrochemical reduction [8]. Traditional electrodeposition as a relatively simple approach was also used to make silver nanoparticles [9,10]. However, nanoparticles tend to agglomerate and form large sized particle clusters [11] or continuous film at the cathode.

In order to reduce particle agglomeration, controlled electrodeposition of Ag nanoparticles has been proposed. For example, a quaternary ionic liquid microemulsion was used to synthesize silver nanoparticles in the size range of 2–13 nm without agglomeration [12]. The ionic liquid microemulsion was used as a soft template and cosurfactant for the deposition. It also served as an electron carrier from the cathode to the aqueous phase, which allows the occurrence of electrochemical reactions happened in microemulsions. Capping silver nanoparticles with organic molecules can form a stable organosol by using a stabilizing agent [13]. The resultant sol with high density of nanoparticles is stable. Ag agglomeration can be effectively prevented.

Electrodeposition of silver nanoparticles doped with chitosan hydrogel was performed and the silver nanoparticles were uniformly distributed in the chitosan compound [14]. Hard templates were applied in preparing well-dispersed silver nanoparticles. Silver particles with the size of 5 nm diameter were deposited from AgNO3 solution on TiO2 nanotube arrays [9,15,16]. The TiO2 nanotube arrays were prepared by the anode oxidization method [1721]. Ag nanoparticles were deposited onto both inner wall and outer wall of the nanotubes, which is helpful for higher separation efficiency of electrons and holes. Carbon-based materials including pure graphite sheets (PGSs), active carbons, and glassy carbon electrodes were used as substrate materials for silver electrodeposition [2230]. Due to the lamellar structure of PGSs, both surface deposition of Ag particles and some extent of intercalation were observed. It is also shown that the deposition of Ag on just deposited silver is easier than the formation of new particles on the bare carbon surface [31]. Indium tin oxide was used as the substrate for silver nanoparticle deposition [3234]. ZnO nanotube [35], ZnO film [36], and the semiconducting substrate GaN [37] were also considered as cathode materials. For conductive metal electrodes, the noble metal, Pt, was used for silver nanoparticle electrodeposition [38].

In view of why it is necessary to deposit silver nanoparticles on metals, there are various reasons that the process is interesting for scientific study and the generated product is valuable for commercial enterprise as well. First, silver film plating on metals has been traditionally used to generate a less expensive versions of household items that would otherwise be made of solid silver. Second, silver nanoparticle or coating have been applied to various metal surface including aluminum, copper, and nickel to further increase the conductivity of these metals. Specifically, silver layer deposited on aluminum facilitates the joining of aluminum to other alloys. For example, when joining 6061 aluminum to AISI 4340 alloy steel, electroplated silver at the 6061 aluminum alloy can prevent the formation of the detrimental Fe–Al intermetallic compound due to the Ag–Al phase formation at the interface of the weldment [39]. Most recently, silver coating has been electroplated on orthodontic brackets made by stainless steel and Ni–Ti alloy to examine the friction reduction effect [40].

Up to now, there is few work on Ag nanoparticle deposition on active metals. The objective of this work is to explore the silver nanoparticle formation at the surface of an active metal substrate through spatially confined growth under the silver ion concentration-limited condition. Pure aluminum was used first to examine the silver nanoparticle initiation and early growth stages. Then, a gold-coated silicon wafer was used as the working electrode to show the hydrogen give-off confined growth of the silver nanoparticles. The surface roughness on the uniformity of silver nanoparticle distribution was also studied.

## Materials and Experimental

All the chemicals including the oxalic acid, ethanol, acetone, and aluminum foil are in ACS purity and supplied by Alfar Aesar, a Johnsons Company, Ward Hill, MA. The silver anode was soaked into a 1.0 M KCl solution held in a glass tube. This anode was purchased from CH Instruments, Austin, TX. To remove the residual stress on the active metal substrate, the pure aluminum foil was annealed in vacuum at 300 °C for 2 hrs using an OTF-1200X-S-UL split tube furnace manufactured by MTI Corporation, Inc., Richmond, CA. A simple two-electrode system was set up for the electrodeposition study. The working electrode is the pure aluminum foil with the dimension of 50 mm × 5 mm × 0.25 mm. The counter electrode is the silver wire which was coated with silver chloride and soaked in 1.0 M KCl solution. A CHI 440 C electrochemical analyzer was used to collect current (I) and voltage (V) data associated with the silver nanoparticle deposition process. The data were plotted to show the I–V behavior. After the electrodeposition of silver nanoparticles, the aluminum electrode was observed under scanning electron microscope (SEM) to reveal the morphology and determine the composition. Particle size distribution was determined by analyzing the SEM images. The Jeol JSM-6010PLUS/LA in-touch SEM with energy dispersive spectrum function was used for the microstructure observation and elemental composition analysis. The acceleration voltage used for the energy dispersive X-ray spectroscopy (EDS or EDX) analysis was 20 kV. A silicon drift X-ray detector with the resolution of 133 eV was used. The thin window can detect from B to U. Active mapping was performed in the scanning area of 25 × 25 (μm). The spectrum acquisition was displayed on the multitouch screen and recorded in a computer with the installed software for both qualitative and quantitative analyses. The distance between different nanoparticles was measured by analyzing the SEM images.

## Results and Discussion

### Current Responses to Scan Potentials.

Cyclic voltammetry technique was used to examine the electrochemical responses of the working electrode in a potential range from 0 V to 1.75 V (versus the silver/silver chloride electrode). The scan rate was 0.01 V/s. Figure 1(a) shows the results from the test. From which we can see that if the scanning voltage changes from 0 V to 1.25 V, the surface of the working electrode demonstrates anodic behavior. Oxidation reaction is confirmed since the current density takes negative numbers as shown by the solid line segment in Fig. 1. When the scan potential increases from 1.25 V to 1.75 V, the working electrode becomes the cathode. The silver deposition becomes the major reaction as the current increases in the positive range from 0 A/m2 to 20 A/m2. The dotted line represents the voltage–current relation associated with the silver deposition.

Next, the scanning potential was increased to a much wider range. It was from 0 V to 10 V and negative polarity was used to examine the cathodic behavior of the working electrode. Again cyclic voltammetry technique was used. The scan rate was kept the same as 0.01 V/s. Figure 1(b) shows the results. A forward scan followed by a backward scan was demonstrated. Obviously, in the small potential range (from 0 V to 1.75 V), the same behavior as seen from Fig. 1(a) was found. In the higher potential range (from 1.75 V to 10 V), there are two distinct stages. The first stage (potential changing from 1.75 V to 3.75 V) represents the electrochemical reaction-limiting behavior. The cathodic current is due to silver deposition and hydrogen generation. In the potential range from 3.75 V to 10 V, the concentration-limiting condition holds. Both hydrogen generation and silver deposition happened in this range. But both reactions are controlled by the mass transfer rates as the current does not increase much with the further increasing in the scan potential.

In the backward scan, we can clearly see the crossover of the curve, which shows a higher current density than the forward scan. This is due to the excessive hydrogen giving off in the potential range far away from the equilibrium potential. This crossover behavior disappears when the overpotential is low. For example, when the scan potential decreases from 1.5 V to 0 V, there is no crossover behavior being found.

### Morphological Features and Elemental Analysis Results.

After the cyclic voltammetry test, the aluminum cathode deposited with silver was rinsed in de-ionized water. Then, it was washed using ethanol. Before it was put into the SEM, it was clean with acetone and air-dried. Figure 2(a) shows the SEM image and Fig. 2(b) reveals the elemental analysis result as obtained by the EDS or EDX. From Fig. 2(a), it can be seen that the silver was deposited at the cathode with good uniformity. This is indicating that the silver seeds are separated evenly. With the progress of electrodeposition, the nanoparticle clusters formed as observed in the SEM image.

In the energy dispersive X-ray spectrum as shown in Fig. 2(b), the major elements, Ag and Al, are found. This confirms that the silver particles are setting on the aluminum foil. The signal from C is due to the adsorption of oxalic acid on the aluminum cathode.

Figures 3(a) and 3(b) present the results of the particle size and spacing. The average size of the nanoparticles is about 52.4 ± 13.6 nm based on the size measurement of 100 randomly chosen nanoparticles in the SEM image as shown in Fig. 2(a). Each particle cluster consists of eight particles approximately. The mean separation distance of the nanoparticle cluster is about 112.6 ± 19.7 nm, based on the average value of 100 measurement results. Earlier study [41] showed that the low concentration of silver ion is a key factor to ensure the uniformity of the silver nanoparticles at high nucleation potential. In our observation, it is found that the hydrogen give-off also contribute to the uniformity of the silver nanoparticles and the separation of the nanoparticle clusters as will be discussed in further details in Sec. 3.3 about the nanoparticle formation mechanisms.

### Reaction Analysis and Nanoparticle Formation Mechanisms.

Why the silver particles form well-separated clusters instead of a continuous film? The explanations to this behavior are due to two reasons. One of the reasons is that the concentration of silver ion is rather low in the solution because of the heterogeneous reactions in the region close to the Ag/AgCl electrode. The heterogeneous reactions in the region close to the Ag/AgCl electrode as the anode follow Eqs. (1) and (2):
$Ag(s)+Cl−(1.0 M)→AgCl (s)+e−$
(1)

$AgCl (s)→ Ag++Cl−$
(2)

The solubility-product constant of AgCl in acidic environment, ksp, is about 1.8 × 10−10 using the method as described in Ref. [42]. If the concentration of (Cl) is 1.0 M, then the calculated (Ag+) is just in the order of 10−10 M. Such a low concentration would generate significant overpotential and the silver particle nucleation rate should be much higher than the growth rate. Therefore, the silver particles are very fine and no continuous film forms.

The electrode overpotential for silver deposition as calculated by the Nernst potential should be
$ΔE=E−E0=−RTFlg[Ag+]$
(3)
Substituting the concentration of chlorine ion into Eq. (3) yields
(4)

The difference between the experiment result as shown in Fig. 1(a) and the calculated value is the contribution from the concentration polarization. In Fig. 1(a), the potential for silver starts depositing is 1.2 V. Thus, the potential due to the concentration polarization is about 0.692 V.

The second reason for the silver nanoparticle clusters' formation and distribute regularly at the aluminum surface is that the intensive hydrogen generation in the high potential range prevents the silver particles to form continuous films. Hydrogen evolution caused the self-assembling of the silver nanoparticles as schematically shown in Fig. 4(a).

In order to show that the hydrogen evolution generates the spatial barrier for the silver growth and causes the self-assembling of the silver nanoparticles, we further investigated the confined electrodeposition of silver under diffusion-controlled conditions. In this experiment, only two-dimensional (2D) confinement is considered. The electrodeposition cell consists of two pieces of glass slides, a gold-coated silicon wafer as the working electrode (cathode) and a pure silver plate serves as the counter electrode (anode). The electrolyte is a silver nitrate solution with 0.1 M concentration. When the overpotential increased from 0 V to 10 V at the rate of 0.01 V/s, initially, silver nanoparticle formed. Then, hydrogen evolution started and the hydrogen bubbles prevented the transverse connection of the nanoparticles to form a continuous film. With the increasing in the scan potential, the hydrogen bubbles blocked the replenishment of silver ion solution and the morphology of the deposited metals is dendriticlike as shown by the optical image of Fig. 4(b). Such a 2D physical model indirectly tests the hypothesis that hydrogen evolution prevents the connection of silver nanoparticles to form continuous films.

### Effect of Aluminum Surface Condition on Nanoparticle Nucleation and Growth.

The surface condition of aluminum substrate especially the mechanical roughness has significant influence on the nanoparticle nucleation and growth. A smooth surface helps the uniform nucleation and continued growth of the silver nanoparticles. The nanoparticle clusters also tend to distribute evenly on the smooth surface of aluminum substrate as shown in Fig. 2(a). It was observed that increasing the roughness of the surface caused the change in the nucleation sites. The SEM image of Fig. 5(a) shows the pure aluminum rough surface with the features of burrs, scratch grooves, raised islands, and dimples. The silver nanoparticles started nucleation at these locations as marked by the arrows in the SEM image. Figure 5(b) shows the continued growth of silver nanoparticles at the nucleation sites. It can be clearly seen that the silver nanoparticle clusters form at the burr or edges of grooves as indicated by the arrows in the SEM image.

With the increase of silver nanoparticle deposition time, nucleation of the nanoparticles in other areas besides those rough locations happened. However, those rough locations became the preferred sites to form silver nanoparticle clusters or island as revealed in Fig. 6(a). In Fig. 6(b), the silver nanoparticle cluster formation at the sites at the edge of a vertically aligned groove is shown. It is evident that the surface roughness can reduce the uniformity of silver nanoparticle nucleation and growth. The reason for this behavior should be due to the charge accumulation at the extruded locations. As reported before [43], charge tends to concentrate the points. The higher density of negative charges at the rough regions than the smooth area allows the silver ions to accept electrons and reduce to silver nanoparticles more easily. It must be also indicated that the surface roughness could change the transport behavior. Quantitative analysis by solving some mass transport equations may be able to provide more insights into particle growth kinetics. For more in-depth study in the future, it is meaningful to investigate the roughness-dependent nucleation of silver nanoparticles in the scratched zone.

In this study, we used the Ag/AgCl as a source for providing the silver ion; it is because the silver concentration is stable. The surface condition of a pure silver may be used, but its surface can be influenced by the complex formation with the oxalic acid in the solution, and surface oxidation of silver (discoloring) at low temperature prevents constant concentration of silver ion. Cyclic voltammetry technique is important tool for identifying at which potential level the electrochemical reaction of silver reduction begins. We believe that the shape of the wave form does not matter. It is sure that a direct current setup allows silver deposition, and eventually, the direct current setup should be used in large-scale manufacturing. However, there is another electrochemical reaction in competition, i.e., the hydrogen giving off. It is necessary to understand at which level the silver deposits in particle cluster instead of dendritic or needlelike morphology.

## Conclusions

Silver nanoparticles start electrodeposition from 0.3 M oxalic acid electrolyte on a pure aluminum working electrode under concentration-limited condition at the overpotential of about 1.2 V. At low scan potential, the electrochemical reactions are the major controlled processes. When the scan potential is increased in the range from 1.75 V to 3.75 V, the silver deposition and hydrogen generation are the controlled mechanisms. At even higher potential from 3.75 V to 10 V, concentration-limited condition holds. Severe hydrogen formation happened.

Due to the reaction between chlorine anion and silver cation to form AgCl solid at the Ag/AgCl electrode, the silver ion concentration-limited condition sustains in the electrolyte. Ag grows at the Al working electrode to form nanoparticles with an average size of 52.4 ± 13.6 nm. With the increasing of the deposition time, the silver nanoparticles aggregate into clusters. These clusters are separated with approximately equal distance due to the hydrogen bubble-induced self-assembling. Due to the charge concentration at pointy locations, the surface roughness of the Al substrate generates effects on the particle nucleation and growth. Burr and grooves on the surface of the Al substrate lead to the reduced uniformity of Ag nanoparticle nucleation and growth.

## Acknowledgment

This publication was developed under an appointment to the U.S. Department of Homeland Security (DHS) Summer Research Team Program for Minority Serving Institutions, administrated for the DHS by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between DHS and the U.S. Department of Energy (DOE). ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE Contract No. DE-AC05-06OR23100. We also acknowledge the support by the U.S. National Science Foundation under Grant Nos. CMMI-1333044 and DMR-1429674. The two Summer Visiting Student Researchers, GRT and RSG, were sponsored by the Brazilian Scientific Mobility Program and the Institute of International Education (IIE). This document has not been formally reviewed by DHS. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of DHS, DOE, or ORAU/ORISE. DHS, DOE, and ORAU/ORISE do not endorse any products or commercial services mentioned in this publication.

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