High-Resolution Nanopatterning Method of Carbon Nanofibers with Transition Metal Oxides for Electrocatalysis and Gas Sensing Applications Using Joule Heating and Chemical Vapor Deposition
Authors: Cisquella Serra Albert , Gamero-Castaño Manuel 
Published: July 2021

In the past decade, there has been significant research on carbon-transition metal oxides (TMO) hybrid structures and their applications as novel catalysts and sensors[1-3]. Some of these research efforts include i) development of alternatives to platinum-based electrocatalysts to generate hydrogen and ii) design of hybrid nanofiber-based sensors having a high surface to volume ratio. Carbon-TMO-based structures are strong candidates to fulfill these needs owing to their catalytic and sensing performance. However, typical synthesis processes of these structures, such as hydrothermal deposition of TMO on carbon, or combined pyrolysis of carbon and TMO precursors, are challenging to control and reproduce.
Addressing this controllability issue, we recently developed a highly reproducible process at the wafer level to synthesize carbon-TMO structures, consisting of a suspended Glassy Carbon Wire (GCW) coated with WO3-x[4-5]. A monolithic carbon structure featuring a suspended GCW of known diameter and length is microfabricated by first laying a SU-8 fiber on top of a photopatterned SU-8 scaffold, using the nearfield-electrospinning technique, followed by pyrolysis. The WO3-x coating is deposited via Localized Chemical Vapor deposition (LCVD), activated by controlling the temperature profile on the suspended GCW through Joule heating. The length and thickness of the coating are adjusted by manipulating the current through the GCW and monitoring its voltage. The featured suspended GCW is ideal for understanding and describing the LCVD process of different TMOs on carbon. However, a single microstructure is not suitable for a large catalysis process. Therefore, to exploit the catalytic capability of the developed material, a multi-wire carbon-TMO is crucial.
Here we report the LCVD technique's development to coat carbon nanofibers (CNF) mats with WO3-x for large-scale catalysis. We prepare the CNFs mats using the far-field electrospinning technique of Polyacrylonitrile/Dimethylformamide (PAN/DMF) solution followed by stabilization and carbonization. We have studied in detail the Joule heating of the CNFs mat, and we have developed a model allowing us to predict its temperature at a certain value of power dissipation used for a controlled chemical vapor deposition process. The deposited WO3-x films at the CNF mats are uniform, and the stoichiometry of the coating can be tuned by a post oxidation process or annealing in an inert atmosphere, yielding different catalytic behavior.
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