**1. Introduction**

Micro/nano-sized wire and cone structures of platinum directly integrated with a conducting substrate have several advantages for electrocatalytic performance [1–3]. First, the large surface areas accelerate the surface reaction. Second, the open spaces among the nanostructures lead to efficient mass transfer. Third, the direct contact with the electrolyte as well as with the substrate makes mixing with a polymer binder unnecessary in electrode production. The resulting binder-free architecture is expected to maintain a high electric conductivity and effectively avoid blocking the active sites. Among various nanostructures, the cone shape enhances the mechanical stability because it has not only a fine tip but also a large base.

One of the methods to prepare metal micro/nanocones is the deposition of metal in a template with conically shaped pores; different techniques using electrodeposition or electroless deposition can be employed. Electroless deposition does not require conducting substrates; however, the surface must be pretreated with Sn2+, Ag, or Pd via a sensitization and activation processes [4]. Consequently, these metal ions and atoms are left as contaminants on the surface of the cones and thus affect their electrocatalytic performance [5,6] On the other hand, electrodeposition requires no pretreatment steps and therefore enables the fabrication of metal-contaminant-free platinum cones.

**Citation:** Sato, Y.; Koshikawa, H.; Yamamoto, S.; Sugimoto, M.; Sawada, S.-i.; Yamaki, T. Fabrication of Sizeand Shape-Controlled Platinum Cones by Ion-Track Etching and Electrodeposition Techniques for Electrocatalytic Applications. *Quantum Beam Sci.* **2021**, *5*, 21. https://doi.org/10.3390/qubs5030021

Academic Editors: Akihiro Iwase and Alberto Bacci

Received: 6 April 2021 Accepted: 24 June 2021 Published: 1 July 2021

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The most commonly used templates are anodic porous alumina and track-etched membranes [7,8]. Our particular interest is focused on track-etched membranes with nanometer-to-micrometer pores, which are prepared by the swift-heavy-ion irradiation of polymer films followed by chemical etching. This is because the ion-track technique offers flexible porous templates with large areas and enables the adjustment of the shape, size, orientation and density of the pores independently by varying the conditions of the irradiation and chemical etching.

To date, studies have been conducted on the fabrication of track-etched membranes with various shapes and configurations. For example, Rauber et al. reported a modified template fabrication method for the preparation of 13–44.2-nm-thick platinum wire networks, which involved ion-irradiation in several steps from different directions [9]. Cylindrical, conical and double-cone-shaped pores were prepared with diameters of approximately 500–1500 nm by a multi-step etching technique, penetrating through a polyethylene terephthalate (PET) membrane of a 12 µm thickness [10]. In order to make the conically shaped pores as a template, chemical etching is usually performed with an etchant on one side of the membrane and a stop solution on the other [11]. For the subsequent electrodeposition of the desired metal, one side of the track-etched membranes is then sputter-coated with a thin metal layer to prepare the cathode [11,12]. In this conventional method using penetrating pores (also called through-holes), the length of the fabricated micro/nanostructures is always equal to the thickness of the track-etched membranes and is never controlled in principle. In contrast, a track-etched membrane with non-penetrating pores can be used for the preparation of cones with a controlled length, thereby enhancing the degree of freedom in the electrocatalyst's design and production. Until the present time, gold cones were fabricated by electroless deposition using a track-etched membrane template with non-penetrating pores, which had diameters and depths in the range of 1–8 µm and 1–11 µm, respectively [13].

In this study, platinum cones were fabricated by an electrodeposition method using track-etched membranes with non-penetrating pores. The size and shape of the template pores were controlled by the etching time and etchant concentration, thereby enabling the preparation of platinum cones with different base diameters and lengths. Finally, we demonstrated their higher electrocatalytic activity by the electro-oxidation of ethanol.
