**2. Experimental**

A stack of two polymer films, a polyimide (PI) film (Kapton, DuPont-Toray Co., Ltd., Tokyo, Japan) with a thickness of 12 µm on a polycarbonate (PC) film (Panlite, Teijin Ltd., Tokyo, Japan) of a 50 µm thickness, was irradiated with <sup>40</sup>Ar ions at an energy of 150 MeV using the azimuthally varying field (AVF) cyclotron at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA). Figure 1 depicts the depth profile of the linear energy transfer (LET) of the <sup>40</sup>Ar ion calculated by the Stopping and Range of Ions in Matter (SRIM) code [14], together with the configuration for the irradiation of the first 12 µm-thick PI and the second 50 µm-thick PC films. The stopping depth was 54 µm from the front surface, meaning that the impinging ion penetrated through the first PI film and stopped in the second PC film. Consequently, we employed the present irradiation configuration. The track etching of the PC film irradiated in this way started from one side, yielding non-penetrating conical pores. The number of irradiated ions per 1 cm<sup>2</sup> , namely the ion fluence, was fixed at 3.0 <sup>×</sup> <sup>10</sup><sup>7</sup> and 1.0 <sup>×</sup> <sup>10</sup><sup>8</sup> ions/cm<sup>2</sup> .

Figure 2 shows the preparation procedure, which involved filling the non-penetrating pores of track-etched membranes with electroplated platinum. After the <sup>40</sup>Ar irradiation, the PC films were etched in 2.0, 4.0 and 6.0 mol/dm<sup>3</sup> aqueous NaOH solutions at 60 ◦C. The etched films were then washed with water and dried at room temperature. The surface and cross-section of the track-etched membranes were observed by field emission scanning electron microscopy (FE-SEM) (JSM-6700F, JEOL Ltd., Tokyo, Japan). Before the observation, a thin gold layer was deposited on the sample surface to improve its

conductivity. In order to create the fine cross-section of the etched pores, the membranes were embrittled by exposure to UV light [10]. *Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 3 of 13

**Figure 1.** Depth profile of the LET of 150 MeV <sup>40</sup>Ar ions for a 12μm-thick PI overlaid on 50μm-thick PC, which was calculated by our SRIM simulation. **Figure 1.** Depth profile of the LET of 150 MeV <sup>40</sup>Ar ions for a 12 µm-thick PI overlaid on 50 µm-thick PC, which was calculated by our SRIM simulation.

Figure 2 shows the preparation procedure, which involved filling the non-penetrating pores of track-etched membranes with electroplated platinum. After the 40Ar irradiation, the PC films were etched in 2.0, 4.0 and 6.0 mol/dm<sup>3</sup> aqueous NaOH solutions at 60 °C. The etched films were then washed with water and dried at room temperature. The surface and cross-section of the track-etched membranes were observed by field emission scanning electron microscopy (FE-SEM) (JSM-6700F, JEOL Ltd., Tokyo, Japan). Before the observation, a thin gold layer was deposited on the sample surface to improve its conductivity. In order to create the fine cross-section of the etched pores, the membranes were embrittled by exposure to UV light [10]. For the electrodeposition, the track-etched membrane was physically coated with a thin platinum layer from its pore-open side; the current and time of the vapor deposition were 20 mA and 520 s, respectively. This layer reached a thickness of approximately 40 nm and worked as a cathode for the electrodeposition of the platinum cones in a commercial platinum electrolyte (PRECIOUSFAB Pt3000, Tanaka Kikinzoku Kogyo K.K., Tokyo, Japan) at 50 ◦C. The applied voltage was repeatedly pulsed at 1.8 V for 1 s and at 0 V for 2 s; the repetition number was 200 with a total deposition time of 10 min. After filling the non-penetrating pores with electroplated platinum from the bottom and reinforcement with a thick electroplated gold substrate, the track-etched membranes were dissolved in a 6.0 mol/dm<sup>3</sup> aqueous NaOH solution at 60 ◦C, leaving the free-standing platinum cone array.

The obtained platinum cones were observed by FE-SEM and transmission electron microscopy (TEM) (JEM-2100F, JEOL Ltd., Tokyo, Japan). The elemental composition was investigated using an energy-dispersive X-ray (EDX) analyzer (X-Max, HORIBA Ltd., Kyoto, Japan). The electrochemical measurements were performed using a three-electrode cell at room temperature. The platinum cones were used directly as the working electrodes. A platinum foil and Ag/AgCl (KCl sat.) were used as the counter and reference electrodes, respectively. All of the potentials were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.197. The electrochemically active surface area (ECSA) of the platinum was measured by an established procedure using cyclic voltammetry (CV) [15]. The potential of the working electrode was scanned from 0.02 to 1.17 V vs. RHE at a scan rate of 50 mV/s in a N2 saturated 0.5 mol/dm<sup>3</sup> aqueous H2SO<sup>4</sup> solution. The coulombic charge corresponding to the adsorption peak of atomic hydrogen, *Q*Pt-H, was estimated by integrating the CV curve in the hydrogen underpotential deposition region. Assuming a coulombic charge of

0.21 mC/cm<sup>2</sup> for hydrogen adsorption on a smooth polycrystalline platinum surface, we used the equation ECSA (cm<sup>2</sup> ) = *Q*Pt-H/0.21. Subsequently, in order to measure the ethanol oxidation reaction activity, the CV was performed in a N2-saturated aqueous solution containing 0.5 mol/dm<sup>3</sup> ethanol and 0.5 mol/dm<sup>3</sup> H2SO4. The electrode was cycled in the potential range from 0 to 1.2 V vs. RHE at a scan rate of 20 mV/s. *Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 4 of 13

**Figure 2.** (**a**) Metal cones prepared using the track-etched membranes with penetrating pores as a template, as was the case in most of the previous studies. The conventional method involves sputter-coating with a thin metal layer on one side of track-etched membranes in order to prepare the cathode for electrodeposition. (**b**) Schematics of the main steps in the fabrication of the platinum cones based on a combination of ion-track etching and electrodeposition techniques: (**i**) ion irradiation of a PC film to obtain non-penetrating tracks; (**ii**) asymmetric etching to form non-penetrating conical pores; (**iii**) vapor deposition of a thin platinum layer from the pore-open side as a cathode; (**iv**) electrochemical deposition of platinum from the bottom of the non-penetrating pores, and from the surface of the track-etched membranes; (**v**) freestanding platinum cone array after reinforcement with a thick electroplated gold substrate and the removal of the template. **Figure 2.** (**a**) Metal cones prepared using the track-etched membranes with penetrating pores as a template, as was the case in most of the previous studies. The conventional method involves sputtercoating with a thin metal layer on one side of track-etched membranes in order to prepare the cathode for electrodeposition. (**b**) Schematics of the main steps in the fabrication of the platinum cones based on a combination of ion-track etching and electrodeposition techniques: (**i**) ion irradiation of a PC film to obtain non-penetrating tracks; (**ii**) asymmetric etching to form non-penetrating conical pores; (**iii**) vapor deposition of a thin platinum layer from the pore-open side as a cathode; (**iv**) electrochemical deposition of platinum from the bottom of the non-penetrating pores, and from the surface of the track-etched membranes; (**v**) free-standing platinum cone array after reinforcement with a thick electroplated gold substrate and the removal of the template.
