A New Approach on the Synthesis of Transition Metal Oxides-Carbon Fibers Hybrid Structures By Localized Chemical Vapor Deposition: From a Single Suspended Nanofiber at the Wafer Level to Nanofiber Mats for Large Production

Abstract

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 development of the LCVD technique to coat carbon nanofibers (CNF) mats with WO3-x for large scale catalysis. The CNFs mats are prepared by far-field electrospinning technique of Polyacrylonitrile/Dimethylformamide (PAN/DMF) solution followed by stabilization and carbonization process. The CNFs mat is placed into a custom glass reactor with sealed electrical outlets to apply Joule Heating. The reactor is divided into the CVD chamber, where the sample is heated up and a preheating chamber containing the precursor solution. With the precursor flowing into the CVD chamber, an increasing voltage is applied to the sample , generating Joule heating, and the current is measured. The deposited WO3-x at the deposition temperatures decreases the resistance of the coated fibers promoting a uniform deposition along all the CNF mat. In Figure 1, a single suspended GCW and section of a carbon mat are shown before and after the LCVD process.Ongoing work is on the analysis of the electrocatalytic HER (Hydrogen evolution reaction) activity of the coated CNF mats compared to commercial Pt catalysts.[1] R. Wu, J. Zhang, Y. Shi, D. Liu, B. Zhang, Metallic WO2-carbon mesoporous nanowires as highly efficient electrocatalysts for hydrogen evolution reaction, J. Am. Chem. Soc. 137 (22) (2015) 6983-6986.[2] J. Chen, D. Yu, W. Liao, M. Zheng, L. Xiao, H. Zhu, M. Zhang, M. Du, J. Yao, WO3-x nanoplates grown on carbon nanofibers for an efficient electrocatalytic hydrogen evolution reaction, ACS Appl. Mater. Interfaces 8 (28) (2016) 18132-18139[3]Y. Lim, S. Kim, Y.M. Kwon, J.M. Baik, H. Shin, A highly sensitive gas-sensing platform based on a metal-oxide nanowire forest grown on a suspended carbon nanowire fabricated at a wafer level, Sens. Actuators B Chem. 260 (2018) 55-62.[4] Cisquella-Serra, A., Gamero-Castaño, M., Ferrer-Argemi, L., Wardini, J. & Madou, M. Controlled joule-heating of suspended glassy carbon wires for localized chemical vapor deposition. Carbon 156, 329–338 (2020)[5] L. Ferrer-Argemi, E.S. Aliabadi, A. Cisquella-Serra, A. Salazar, M. Madou, J. Lee, Size dependent electrical and thermal conductivities of electro-mechanically-spun glassy carbon wires, Carbon 130 (2018) 87-93. Figure 1