For Immediate Release
Researchers have developed a novel combination of materials that have organic and inorganic properties, with the goal of using them in technologies that convert carbon dioxide from the atmosphere into a liquid fuel.
“Fundamentally, the goal of this project was to engineer a surface that would allow us to efficiently convert atmospheric carbon dioxide into methanol, which is a liquid fuel,” says Gregory Parsons, corresponding author of a paper on the work and Celanese Acetate Professor of Chemical and Biomolecular Engineering at North Carolina State University. “Our hypothesis was that a class of materials called metalcones would be a valuable tool for addressing this challenge. Our work in this paper focuses on the engineering of a metalcone thin film for this application.”
Inorganic materials tend to be solid and have stable characteristics. Organic materials can have spongelike physical properties and tend to be more chemically reactive. Metalcone thin films are both organic and inorganic – and therefore have both organic and inorganic properties.
“We wanted to find a way to create a metalcone thin film that retains the inorganic properties that make it a good interface between a semiconductor material and the liquid environment surrounding it,” Parsons says. “But we also wanted the metalcone to maintain the organic properties that create efficient pathways for electrons to move.”
“The problem is that metalcones face a significant obstacle for practical use in this context,” says Hyuenwoo Yang, first author of the paper and a postdoctoral researcher at NC State. “If you put metalcones in an aqueous solution, the organic properties allow the metalcones to dissolve – making them practically useless. If you anneal the metalcones at high temperatures, they become physically stable, but you lose the attractive electrochemical properties.
“But now we’ve demonstrated an approach that improves a metalcone’s stability and electrochemical properties, making them very promising candidates for use in photoelectric chemical carbon dioxide reduction,” Yang says.
For this work, the researchers used a metalcone called tincone, which is essentially a tin oxide (SnO2) in which the oxygen atoms are replaced by organic oxide components. In other words, in tin oxide materials, it is the oxygen atoms that connect the molecules of tin oxide to each other; in tincone, those tin oxide molecules are connected to each other by a carbon chain.
Because annealing at high temperatures eliminates the attractive electrochemical properties, the researchers decided to try annealing tincone at a range of lower temperatures.
“We found that the sweet spot was a ‘mild’ annealing at 250 degrees Celsius,” Yang says. “This made the tincone substantially more stable in an aqueous electrolyte, which is necessary for potential use in photoelectric chemical carbon dioxide reduction applications. In addition to improving its stability, the mild annealing also improved charge transport, making the electrochemical properties even more desirable for these applications.
“Our next steps involve binding carbon dioxide catalysts to this mild-annealed tincone and incorporating this engineered material into an application to see how efficiently it can convert atmospheric CO2 into methanol.”
The paper, “Mild-Annealed Molecular Layer Deposition (MLD) Tincone Thin Film as Photoelectrochemically Stable and Efficient Electron Transport Layer for Si Photocathodes,” is published in ACS Applied Energy Materials. The paper was co-authored by Christopher Oldham, a senior project manager at NC State; Arun Joshi Reddy, a postdoctoral researcher at NC State; Paul Maggard, a professor of chemistry at NC State; and by Carrie Donley, Renato Sampaio, John Dickenson, Pierpaolo Vecchi and Gerald Meyer of the University of North Carolina at Chapel Hill.
This work was supported as part of the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE), an Energy Innovation Hub funded by the U.S. Department of Energy’s Office of Science under grant number DE-SC0021173.
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Note to Editors: The study abstract follows.
“Mild-Annealed Molecular Layer Deposition (MLD) Tincone Thin Film as Photoelectrochemically Stable and Efficient Electron Transport Layer for Si Photocathodes”
Authors: Hyuenwoo Yang, Christopher J. Oldham, K. Arun Joshi Reddy, Paul A. Maggard and Gregory N. Parsons, North Carolina State University; Carrie L. Donley, Renato N. Sampaio, John C. Dickenson, Pierpaolo Vecchi and Gerald J. Meyer, University of North Carolina at Chapel Hill
Published: Feb. 22, ACS Applied Energy Materials
DOI: 10.1021/acsaem.4c02997
Abstract: Metalcone thin films, composed of inorganic–organic hybrids, are synthesized using molecular layer deposition (MLD) through reactions between organometallic precursors (e.g., Sn, Al, and Ti) and organic reactants (e.g., ethylene glycol and glycerol). Despite their unique properties, metalcones exhibit significant vulnerability to water due to their organic components, limiting their potential in electrochemical applications. This study focuses on enhancing the photoelectrochemical stability of tincone thin films in aqueous electrolyte while preserving their hybrid characteristics through mild annealing in air at 250 °C. As-deposited and vacuum-annealed tincone thin films exhibited significant degradation under these conditions, while high-temperature-annealed (500 °C) tincone thin films offered improved stability with a significant decline in charge transfer efficiency. In contrast, mild annealing in air maintained the C–O bond at half level and improved the stability and charge transport without compromising the unique characteristics of tincone. This was confirmed by ellipsometry, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Mild-annealed tincone deposited on a lightly doped p-type silicon (p-Si) photocathode produced a 20-fold increase in CO volume compared to high-temperature annealed tincone in a CO2-saturated potassium bicarbonate (KHCO3) electrolyte with dispersed graphene oxide–cobalt phthalocyanine (GO-CoPc) under 1 sun illumination at 0.9 V vs reversible hydrogen electrode (RHE), while maintaining the faradaic efficiency for CO and H2. These results suggest that mild-annealed tincone thin films hold significant potential as protective charge transport layers on silicon photocathodes for the aqueous CO2 reduction reaction (CO2RR).
