**2. Materials and methods**

## **2.1. Platinum–Iridium nanowire electrodeposition**

Figure 2 is a schematic detailing the key steps involved in the template synthesis approach used to fabricate our platinum-iridium nanowires and nanowire-template assemblies. Prior to deposition, a conductive thin film layer must be applied to one side of the filtration membrane to 1) seal the base of the pores, thus allowing them to be filled with plating solution, and 2) to provide an electrically conducting base to serve as the working electrode at which metallic ions in solution are reduced and "grown" through the template as a metallic nanowire.

**Figure 2.** Schematic of the fabrication processes of metallic nanowires in AAO nanopores. Drawing not to scale.

Anodisc® nanoporous anodized aluminum filtration membranes (Whatman Inc., UK) were used as substrates for nanowire template synthesis. The templates have an approximate thickness of 60 µm and a diameter of 47 mm. For ease of membrane handling all templates were fitted with a plastic annular ring attached to one side of each membrane.

SEM inspection of both sides of the membranes revealed that the pore apertures sizes were different on one side versus the other. The pore apertures on the side with the plastic ring attached were approximately 20 nm in diameter. On the other side the pore apertures were approximately 200 nm in diameter, randomly distributed and with larger spaces between the pores. Cross-sectional analysis (through the membrane thickness) revealed that the large 200 nm channels continued down the majority of the template thickness, and that in the final 50 nm of thickness, the channels bifurcated into series of smaller finger-like channels with 20 nm diameters. Au thin films (h = 80 nm) were e-beam vapor-deposited on the side of the AAO membranes with the smaller pore apertures to create the sealed, electrical contact base for the working electrode.

Nanowires were electrodeposited using a three-electrode electrochemical cell which contained a larger, vertically-oriented cylindrical channel and a smaller diameter cylindrical chamber were machined into a Teflon® block along with horizontal small via to create a Luggin capillary between the two Figure 3. The larger channel's base was sealed by placing (in the following order, ): an O-ring against the Teflon® block, followed by the AAO membrane with the uncoated side in contact with the O-ring, followed by a thick (*h* = 40 mm) copper plate. These components were fixated in place using a spring clamp. Once assembled, the electrochemical plating solution was filled into the larger chamber and the reference electrode was inserted into the smaller chamber.

**2. Materials and methods**

working electrode.

**2.1. Platinum–Iridium nanowire electrodeposition**

210 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Figure 2 is a schematic detailing the key steps involved in the template synthesis approach used to fabricate our platinum-iridium nanowires and nanowire-template assemblies. Prior to deposition, a conductive thin film layer must be applied to one side of the filtration membrane to 1) seal the base of the pores, thus allowing them to be filled with plating solution, and 2) to provide an electrically conducting base to serve as the working electrode at which metallic ions in solution are reduced and "grown" through the template as a metallic nanowire.

**Figure 2.** Schematic of the fabrication processes of metallic nanowires in AAO nanopores. Drawing not to scale.

were fitted with a plastic annular ring attached to one side of each membrane.

Anodisc® nanoporous anodized aluminum filtration membranes (Whatman Inc., UK) were used as substrates for nanowire template synthesis. The templates have an approximate thickness of 60 µm and a diameter of 47 mm. For ease of membrane handling all templates

SEM inspection of both sides of the membranes revealed that the pore apertures sizes were different on one side versus the other. The pore apertures on the side with the plastic ring attached were approximately 20 nm in diameter. On the other side the pore apertures were approximately 200 nm in diameter, randomly distributed and with larger spaces between the pores. Cross-sectional analysis (through the membrane thickness) revealed that the large 200 nm channels continued down the majority of the template thickness, and that in the final 50 nm of thickness, the channels bifurcated into series of smaller finger-like channels with 20 nm diameters. Au thin films (h = 80 nm) were e-beam vapor-deposited on the side of the AAO membranes with the smaller pore apertures to create the sealed, electrical contact base for the

Nanowires were electrodeposited using a three-electrode electrochemical cell which contained a larger, vertically-oriented cylindrical channel and a smaller diameter cylindrical chamber were machined into a Teflon® block along with horizontal small via to create a Luggin capillary between the two Figure 3. The larger channel's base was sealed by placing (in the following order, ): an O-ring against the Teflon® block, followed by the AAO membrane with the uncoated side in contact with the O-ring, followed by a thick (*h* = 40 mm) copper plate. These

**Figure 3.** Electrochemical cell used for nanowire deposition in nano-channeled aluminum oxide (Al2O3) template.

Electrochemical deposition was performed using a software controlled, programmable potentiostat (Gamry). Electrical contact was made to the base copper plate, via alligator connector, thus making the sputtered Au thin film serve as the working electrode (WE). A silver-silver chloride (Ag/AgCl) electrode was used as the reference electrode (RE) and a spiral wound platinum wire (*ɸ* = 1 mm) was placed in the larger chamber as a counter electrode (CE).

Nanowires were electrochemically deposited from a platinum-iridium plating solution that our group has developed and reported elsewhere [25]. In this work, two key parameters were controlled to affect deposition properties: pH and deposition potential. The pH of the initial plating solution is approximately 1.8 to 2.5, depending on the desired final properties of the deposited alloy, and in this study, was varied from pH = 1.8 to 5.0 by carefully titrating the solution with 3M NaOH (aq) solution. With respect to deposition potential, a potentiodynamic program was used to drive deposition. Previous studies by our group have shown that changing the potential range impacts the compositional ratio of Pt:Ir [25]. Specifically the potential range was cycled over a 150mV potential range, e.g. between U = 0.0 V to -0.15 V vs. Ag/AgCl. The ranges used are listed in Table 1.

### **2.2. Nanowire isolation**

Nanowires were isolated from the AAO templates, Figure 4, for further analysis of the nanowire properties. The electrodeposited AAO templates with embedded nanowires were immersed in an aqueous solution of 3M NaOH(aq) to dissolve the oxide membrane. The nanowire suspension was allowed to stand, to settle the nanowires out of the basic solution. Excess solution (supernatant) was carefully pipetted off and DI water was added to the vial to neutralize the remaining supernatant's pH. This process was repeated three times until a neutral solution was achieved. Nanowires in suspension were then pipetted onto either fresh, un-sputtered AAO filters to capture for SEM or onto TEM mesh grids for TEM analysis.

**Figure 4.** Schematic of the isolation process used to separate electrodeposited nanowires from AAO templates. Draw‐ ing not to scale.
