Here, we demonstrate the formation of porous gold nanowires with diameters <60 nm by a two-step process involving the successive electrodeposition of silver then gold into the pores of a track-etched polycarbonate filtration membrane, followed by treatment with nitric acid. The resulting nanowires possess a unique, highly porous morphology, which yields a very high accessible surface area to volume ratio compared to solid nanowires of the same dimensions. Combined with the high aspect ratio of these particles (which allows for easy isolation from solution), this makes them eminently suitable for use in catalysis and sensing applications. The formation of such porous gold nanostructures was ascertained to result from the low diffusivity of the silver species within the narrow membrane pores.
Gold nanoparticles, discrete ensembles of gold atoms with one or more dimensions on the submicron scale, are materials that have attracted considerable interest in recent decades owing to their unique-size-dependant electronic and optical properties as well as their high unit surface areas [
Typically, these applications utilize small solid nanoparticles ranging between 2–50 nm in diameter, as this size range is where such nanoparticles exhibit their useful properties [
One recent development that promises to help overcome these issues has been the preparation of porous gold nanowires. Owing to their high aspect (length to diameter) ratios, such particles can be easily isolated from solution, while their porosity yields exposed surface features (known as ligaments) with dimensions suitable for use in applications, even when the particles as a whole are relatively large [
Very few methods have been published to date by which porous gold nanowires can be synthesized. Current methods involve the electrodeposition of gold into a nanoporous template; either as a pure gold, followed by galvanic exchange reaction [
The process for synthesizing porous gold nanowires by template electrodeposition with chemical etching: a nanoporous template (a) is coated on one side with a conductive layer (b). Gold and the sacrificial metal are then simultaneously electroplated exclusively into the template pores onto the conductive layer (c). The resulting alloy filled template (d) is then treated to de-alloy the nanowires (and thereby form pores) (e), followed by removal of the template to isolate the nanowires.
To date, however, the vast majority of the literature has only involved large porous nanowires (≥200 nm in diameter) [
Here, we report the synthesis of narrow (diameter ≤60 nm), high-aspect-ratio porous gold nanowires (through electrodeposition into nanoporous templates) which possess a unique porous morphology that is characterized by small ligament dimensions. These nanowires were unexpectedly fabricated using a modified version of an established procedure [
A commercially available radiation track-etched, hydrophilic polycarbonate membrane (Sterlitech, 47 mm diameter, >5
Diagram of the electroplating cell used to deposit metal into the pores of a nanoporous membrane.
Following this, the water is replaced with a silver electroplating solution (Silver Cyless II (silver oxide), Technic). An Ag/AgCl reference electrode and a platinum counter electrode (anodic surface area ~2 cm2) are then suspended in the solution close to the membrane. 100 mC of silver (0.112 mg) is then deposited potentiostatically (−1.0 V versus Ag/AgCl) using a BAS 100 b electrochemical analyzer. Following silver deposition, the cell is thoroughly rinsed three times with nanopure water, and the cell is then filled with a gold plating solution (434 neutral soft gold (gold cyanide), Technic). This electrodeposition and rinsing process is then carried out with the gold solution in the same manner.
The filled membrane is first subjected to a brief (10 sec) treatment with 4 M nitric acid at room temperature in order to selectively dissolve the silver. The membrane is then rinsed with distilled water, blotted dry, and placed into a 10 mL polyethylene centrifuge tube. The tube is then filled with 6 mL of dichloromethane and the membrane is dissolved with brief sonication and shaking. The nanowires are then sedimented into a pellet by centrifuging the suspension at 3200 RPM (1090 g) for 30 min. The supernatant is removed and replaced with 3 mL of pure dichloromethane and the pellet is broken up with sonication. This process is repeated several times to remove the residual polymer from the membrane. This process is then further repeated twice more using ethanol instead of dichloromethane, with the final product being stored in 3 mL of ethanol.
Nanowire samples are imaged by transmission electron microscopy (TEM) using a Phillips CM200 TEM equipped with a Gatan 832 SC1000 CCD camera and energy-dispersive X-ray analysis (EDAX) apparatus at an accelerating voltage of 200 kV. The spot size setting for general imaging is 3, but it is set as small as spot size 7 for collecting elemental composition (EDAX) data at specific locations of the nanowires. Nanowire samples are prepared for TEM by briefly sonicating nanowire suspensions in ethanol, and then placing one drop (~5
TEM images (along with EDAX analysis) of the nanowires that result from the deposition of 25 mC of gold (0.051 mg) into the pores of polycarbonate membranes under the conditions used in this work are shown in Figures
TEM images of porous gold nanowires (25 mC).
EDAX analysis of porous gold nanowires. The copper signal is attributed to background scattering from the copper TEM grid.
Examination of the fine structure of these nanowires by high-resolution TEM (Figures
High-resolution TEM image of porous gold nanowires.
High-resolution TEM image of porous gold nanowires that demonstrates their unique morphology.
The formation of such high-porosity gold nanowires, while an improvement over previous work, is noted as being most unusual, as the procedure used in their synthesis involves the deposition of gold under electroplating conditions typically used to form solid gold nanowires rather than porous gold [
Firstly, it was considered that the current density during electrodeposition of the gold may have been high enough that hydrogen gas was evolved at the cathode during the electrodeposition process, with the resulting bubbles remaining trapped in the narrow membrane pores, thereby creating voids in the deposited gold [
To determine if this may be the case, current versus time data for the gold deposition was thus examined (Figure
Current versus time data for the electrodeposition of gold into nanoporous templates.
A second possible explanation is that these structures are in fact incompletely formed gold nanotubes, which can come about due to the reduction of gold species that are preferentially adsorbed on the walls of the membrane pores [
High-resolution TEM image and EDAX analysis of a bi-segmented nanowire comprised of (a) a solid gold segment and (b) a porous gold segment.
(a) Bisegmented gold nanowires resulting from 0.05 C of gold being deposited. (b) Bisegmented gold nanowires resulting from 0.075 C of gold being deposited. (c) Bisegmented gold nanowires resulting from 0.1 C of gold being deposited.
Based on this result, it was then hypothesized that these porous gold structures may in fact be due to codeposition of the gold with residual silver species from the previous electrodeposition, which remain within the pores even following the use of established pore rinsing methods. This explanation would account for the formation of both the porous segments (coarsening of the gold upon chemical removal of the silver from the resulting gold-silver alloy) and the additional solid gold segments upon further gold deposition (residual silver species are eventually depleted yielding only gold deposition). To test this explanation, TEM images were collected of nanowires (50 mC of charge passed) prior to treatment with nitric acid (Figure
TEM image and EDAX analysis of a bisegmented nanowire prior to dealloying.
Further tests were then performed to determine the source of the residual silver species. These tests, deposition of gold preceded by rinsing of the membrane pores between depositions by sonication or by the application of a electric potential, were designed to discriminate between the two most likely candidates: (a) silver species being generated upon introduction of the gold solution to the cell, that is, dissolution/displacement of the deposited silver by the gold cyanide solution (Ag → Ag
TEM image of gold nanowires synthesized with the membrane template being emptied of residual species between metal depositions by sonication.
Likewise, rinsing the membrane pores between electrodeposition steps by applying a potential of −1.0 V versus Ag/AgCl (to reduce the residual silver species) in a 1 mM KOH electrolyte solution (which acts to promote reduction of the residual silver oxide by ensuring it remains soluble and by weakening electro-osmosis effects in these polyvinylpyrrolidone coated pores [
TEM image of gold nanowires synthesized with the membrane template being emptied of residual species between metal depositions by electrodeposition.
Overall, given that gold-silver alloyed nanowires are formed by this procedure, and that porous gold nanostructures are formed following treatment with nitric acid, it is reasonable to conclude that these structures are a result of the coarsening of the gold during chemical removal of the silver.
This result has interesting implications for the current understanding of how such structure form, as previous work on nanoporous gold nanowires established that a hinged morphology resulted due to confinement of the spinodal decomposition to narrow membrane pores [
In summary, it was found that narrow, high-porosity gold nanowires result from the successive electrodeposition of silver and gold into the pores of commercially available track etched polycarbonate filtration membranes. This unexpected nanostructure formation was found to occur due to the entrapment of residual silver species within the narrow membrane pores. This resulted in their codeposition with gold, with subsequent etching of the silver with nitric acid yielding porous gold through a well-known spinodal decomposition mechanism. The method described here for forming porous gold nanowires is advantageous in that it allows for the use of a broad range of plating solutions, while offering an improvement over previously produced porous gold nanowires due to their smaller diameters and ligament sizes, and, therefore, correspondingly higher accessible surface areas. Such nanostructures are anticipated to be useful in a wide range of catalytic and sensing applications.
The authors thank the Australian government for providing funding for this work under the Australian Postgraduate Award (APA), and Mr. Kerry Gascoigne for assistance with electron microscopy.