Nanoscale COMMUNICATION Cite this: DOI: 10.1039/c5nr07145c Received 14th October 2015, Accepted 11th November 2015 DOI: 10.1039/c5nr07145c www.rsc.org/nanoscale Creating nanoporosity in silver nanocolumns by direct exposure to radio-frequency air plasma Abdel-Aziz El Mel,* a Nicolas Stephant, a Jonathan Hamon, a Damien Thiry, a Adrien Chauvin, a Meriem Chettab, a Eric Gautron, a Stephanos Konstantinidis, b Agnès Granier a and Pierre-Yves Tessier a Nanoporous materials are of great importance for a broad range of applications including catalysis, optical sensors and water fil- tration. Although several approaches already exist for the creation of nanoporous materials, the race for the development of versatile methods, more suitable for the nanoelectronics industry, is still ongoing. In this communication we report for the first time on the possibility of generating nanoporosity in silver nanocolumns using a dry approach based on the oxidation of silver by direct exposure to a commercially available radio-frequency air plasma. The silver nanocolumns are created by glancing angle deposition using mag- netron sputtering of a silver target in pure argon plasma. We show that upon exposure to the rf air plasma, the nanocolumns trans- form from solid silver into nanoporous silver oxide. We further show that by tuning the plasma pressure and the exposure dur- ation, the oxidation process can be finely adjusted allowing for precisely controlling the morphology and the nanoporosity of the silver oxide nanocolumns. The generation of porosity within the silver nanocolumns is explained according to a cracking-induced oxidation mechanism based on two repeated events occurring alternately during the oxidation process: (i) oxidation of silver upon exposure to the air plasma and (ii) generation of nanocracks and blisters within the oxide layer due to the high internal stress generated within the material during oxidation. Introduction With remarkable and enhanced properties compared to solid nanomaterials, porous nanostructures are lately being widely explored for various applications including water desalina- tion, 1 Li-ion batteries, 2 catalysis, 3–5 resistive random access memory 6 and optical sensors. 7 In addition to the intrinsic characteristics of the material forming the porous nano- structures, the properties and behavior of such nanomaterials were reported to be dependent on the size and the distribution of the nanopores. 8,9 The way of generating nanoporosity varies from a material to another. For example, to create nanoporous metals, dealloy- ing is the simplest approach that can be used; 10,11 on the other hand, to synthesize nanoporous oxides, anodization is the most frequently employed process. 12 In some particular cases, wet chemistry-based approaches such as anodization and dealloying, are not tolerated in the processing of nano- devices as the device might be damaged due to its exposure to an acidic electrolyte solution during the fabrication phase of the nanoporous material by dealloying. In the case of anodiza- tion, an electrical contact is required to inject the current through the electrolyte making it impossible to apply this process to insulating substrates. For this reason the race towards the development of novel appealing dry approaches to synthesize tailor-made nanoporous materials is ongoing. The oxidation of metals is a fast growing field covering a broad range of research topics. In general, the final product of the oxidation process varies according to the characteristics of the metal in terms of shape, size and intrinsic properties 13–16 as well as the employed oxidation process (e.g., thermal oxidation, 14,16–20 oxidation in a liquid phase 21 as well as low temperature oxidation using cold plasma processes 22 ). For example, thermally oxidizing bulk or thin films of copper at a temperature above 400 °C allows the formation of single crystal CuO nanowires. 16,22–25 On the other hand, by applying thermal oxidation to metal nanostructures, such as nanowires, one can instead transform them into metal oxide nanotubes. 26–28 The oxidation approaches based on cold plasma processes are very promising since they allow design- ing various types of metal oxide nanostructures at very low temperatures. 22,29 a Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 Rue de la Houssinière B.P. 32229, 44322 Nantes cedex 3, France. E-mail: Abdelaziz.elmel@cnrs-imn.fr; Fax: +33 (0)240 373 959; Tel: +33 (0)240 376325 b Chimie des Interactions Plasma-Surface (ChIPS), CIRMAP, Research Institute for Materials Science and Engineering, University of Mons, 23 Place du Parc, B-7000 Mons, Belgium This journal is © The Royal Society of Chemistry 2015 Nanoscale Published on 18 November 2015. Downloaded by UNIVERSITE NANTES on 07/12/2015 13:16:34. View Article Online View Journal