Electrochemistry at Tungsten Conical Sharp Tip Electrodes
Authors: Bruno-Mota Uriel , Valdez Jesus , Yurtsever Aycan , Tavares Ana C 
Published: May 2020

Most of the electrochemical studies are performed with macroscopic, bulk electrodes. The materials of interest are placed on centimeter sized electrodes and the current originating from large macroscopic areas are measured. Although, this traditional approach produces strong signals that are perfect for quantitative analyses it lacks the capability to resolve individual entities or sub-micron structural heterogeneities, that contribute to the electrochemical signal. It averages-out different structures and hence, it becomes impossible to perform in depth studies of chemical reactions occurring at different sites.
With modern developments in nanofabrication, simulations and computer aided design, as well as in state of the art potentiostats, the doors were open for the rapid development of a highly active field of research in the last years: The electrochemistry at the nanoscale, which has allowed to achieve electrochemical measurements from submicron entities [1], [2], and it’s leading the way to the development of single molecule electrochemistry. In this work we will present our approach to the study of electrochemical reactions on single entities, which includes the fabrication of very sharp conical tips to be used as working electrodes and to achieve high spatial resolution.
Figure A shows a scanning electron microscope image of a tungsten tip prepared by electrochemical etching of a tungsten wire in KOH solution, with a radius of curvature of 100 nm. Two main oxides are formed during the anodic polarization of the tungsten electrodes, passivating their surface [3], [4]. However, cyclic voltammetry studies in acid and neutral electrolytes, revealed a potential window and scan rate where a conductive oxide is formed and thus suitable to be used as electrode material. As a result, the tungsten tips were further characterized electrochemically by cyclic voltammetry in presence of a redox probe and as a function of the exposed surface area in the electrolyte solution. An electrochemical response characteristic of ultramicroelectrodes was recorded for tip lengths below 70 µm, Figure B. It was also found that the electroactive area is higher than the geometric one [5], Figure C. The contribution of the roughness of the electrode, the meniscus effect and a possible field enhancement due to the geometry of the electrodes to the electrochemical surface area will be quantified.
This work represents thus, a step ahead towards a better understanding of the electrochemical processes in increasingly smaller structures to eventually reach the ultimate goal of the use of nanometer-sized electrodes: single molecule electrochemistry.
Acknowledgements
To Consejo Nacional de Ciencia y Tecnología (CONACYT) for the financial support with the scholarship 739820 for graduate studies abroad.
References
[1] L. A. Baker, “Perspective and Prospectus on Single-Entity Electrochemistry,” J. Am. Chem. Soc., vol. 140, pp. 15549–15559, 2018.
[2] Y. Wang, X. Shan, and N. Tao, “Emerging tools for studying single entity electrochemistry,” Faraday Discuss., vol. 193, pp. 9–39, 2016.
[3] M. Anik and K. Osseo-Asare, “Effect of pH on the anodic behavior of tungsten,” J. Electrochem. Soc., vol. 149, no. 6, 2002.
[4] M. Anik, “pH-dependent anodic reaction behavior of tungsten in acidic phosphate solutions,” Electrochim. Acta, vol. 54, no. 15, pp. 3943–3951, 2009.
[5] C. G. Zoski and M. V. Mirkin, “Steady-state limiting currents at finite conical microelectrodes,” Anal. Chem., vol. 74, no. 9, pp. 1986–1992, 2002.

Figure 1