An investigation was carried out about the gold nanotube synthesis via a galvanic replacement reaction. The progress of the gold nanotube synthesis was investigated using electron microscopy and UV-Vis spectroscopy. In addition, the reaction rates of gold nanotube formation in the early stage of the reaction were studied. The chlorine ion concentration linearly increased with the gold precursor concentration but deviated from the stoichiometric amounts. This deviation was probably due to AgCl precipitates formed by the reaction of chlorine ions with dissolved silver ions. The replacement reaction was promoted with increased temperature and was nonlinearly proportional to the gold ion concentration. The outcomes of this research will enhance the current understanding of the galvanic replacement reaction.
Metal nanotubes attract much interest due to their characteristic structure with a high aspect ratio, physical durability, and chemical activity. Compared to carbon nanotubes, metal nanotubes have enhanced reactivities toward chemicals and higher electroconductivities. These properties make metal nanotubes useful for applications in electrical devices, sensors, and catalysts among other engineering applications [
For the practical production of gold nanotubes, a facile and benign synthesis method with well-understood fundamentals should be utilized. Various methods to synthesize metal nanotubes have been proposed in a number of studies. Metal nanotubes have been grown by regrowth of metal precursor on surface-modified silicon nanowires [
In this paper, we studied the synthesis of gold nanotubes from sacrificial silver nanorod templates and the related reaction rates. Even though the galvanic replacement reaction takes place voluntarily due to the reduction potential difference, in practice, the reaction rate is influenced by the reaction temperature and the reactant concentrations. We investigated the effects of these factors on the replacement reaction to further understand gold nanotube formation via galvanic replacement.
For the synthesis of silver nanorods, anhydrous ethylene glycol (99.8%), silver nitrate (AgNO3, 99.9%), and polyvinyl pyrrolidone (PVP, Mw 55,000) were purchased from Aldrich. Gold(III) chloride trihydrate (HAuCl4
The polyol process was used to synthesize sacrificial silver nanorods which were prepared via the growth of seed silver nanoparticles. In ethylene glycol, AgNO3 was added to generate seed silver nanocrystals. These seeds were further grown into nanorods by the capping agent. A detailed protocol for the synthesis of Ag nanorods has been presented elsewhere [
To synthesize gold nanorods, the suspending medium of ethylene glycol was replaced with water. A 10 mL suspension of the synthesized Ag nanorods was centrifuged to collect the solid nanorods. The supernatant solution was removed, and then the nanorod aggregates were washed with acetone to remove any surface-adsorbed PVP. Removal of ethylene glycol and PVP is required to prevent the additional growth of gold [
The synthesized silver nanorods and gold nanotubes were investigated using field emission scanning electron microscopy (FE-SEM, Hitachi S-4200, Nissei Sangyo Co., 30 kV) and energy-filtering transmission electron microscopy (EF-TEM, LIBRA 120, Zeiss Co., 120 kV). The chemical composition of the nanoparticles was confirmed by energy dispersive X-ray analysis (EDX). UV-Vis absorption spectra were obtained (UV-Vis, 1601 PC, Shimadzu Co.) before and after the replacement reaction. The concentration of the remaining gold ions was determined by measuring the UV-Vis absorbance intensity at a wavelength of 310 nm after preparing a standard curve [
The sacrificial silver nanorod templates were prepared via the polyol process in which silver nanoparticles were used as seeds for nanorod growth. To direct the c-axial growth of the seed nanoparticles, PVP acted as a capping molecule directing the silver growth [
Characteristics of the sacrificial Ag nanorods. (a) TEM image of the seed silver nanoparticles (scale bar: 100 nm), (b) TEM image of the synthesized sacrificial Ag nanorods (scale bar: 1
From the synthesized sacrificial Ag nanorods, the Au nanotubes were prepared. Preparation of the Au nanotubes was performed via the galvanic replacement reaction using a 10 mM HAuCl4 solution. The reaction is a voluntarily reaction in which the substitution of the Ag(s) core with Au(s) is driven by the difference in their standard reduction potentials. The standard reduction potentials of Ag and Au ions at 25
The higher standard reduction potential of the gold ion enables the replacement reaction to proceed spontaneously such that the following chemical reaction is possible:
The solid silver rod is the source of electrons similar to other galvanic replacement reactions, and a gold nanotube template is formed simultaneously.
Images of the synthesized porous Au nanotubes are shown in Figures
Characteristics of synthesized gold nanotubes. (a, b) TEM and SEM images of the prepared gold nanotubes (scale bar: 200 nm), (c) magnified image of the gold nanotubes (scale bar: 200 nm), and (d) EDXA peaks of the porous gold nanotubes.
The replacement reaction using the gold precursor was promoted by increasing the gold precursor concentration. In Figure
UV-Vis spectra of the gold nanorods prepared at different gold precursor concentrations.
To investigate the replacement reaction in detail, the concentration of dissolved Ag ions after the galvanic replacement reaction was monitored. In addition, the degree of galvanic replacement was determined while varying the gold precursor ion concentration. The silver ion concentration in the aqueous solution was monitored after completion of replacement with gold. The concentration of dissolved silver ions increased linearly with the gold precursor concentration, as seen in Figure
Concentrations of silver ions dissolved from the solid silver nanorods along with the gold precursor concentration.
To investigate the reaction rate, the progress of the replacement reaction was monitored by measuring the gold precursor concentration. In Figure
(a) Gold ion concentration as a function of time at different reaction temperatures (lines are guide for eye), (b) the fitted line of the reaction rate equation determined from the data points.
The reaction rate of the galvanic replacement is influenced by the concentrations of both the solid Ag nanorods and the
The above rate equation is valid when the concentration of silver nanorods is much higher than that of
The
Reaction rate constant,
Temperature [ | 25 | 40 | 60 |
---|---|---|---|
0.780 | 0.943 | 1.225 |
The galvanic replacement method was applied for the synthesis of gold nanotubes from sacrificial silver nanorods. The progress of the replacement reaction was monitored by obtaining UV-Vis spectra and by measuring the silver and gold ion concentrations. The replacement reaction was promoted by increased gold precursor concentration as well as by increased reaction temperature. The dissolved silver concentration increased linearly with the gold ion concentration. The replacement reaction was promoted with increased temperature, which enhances the reaction rate constant. The reaction rate depends on the gold precursor concentration nonlinearly. This study determined kinetic information about gold replacement with sacrificial silver nanorod templates through the galvanic replacement reaction.
This work was supported by Korea Science and Engineering Foundation (KOSEF M10755020001-08N5502-00110). S. Kwon and H. Dong contributed equally to the paper.