"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."
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