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UNC Chemists Rethink How Electrocatalysts Work, Paving the Way for Improvements in Clean Energy

UNC Chemists Rethink How Electrocatalysts Work, Paving the Way for Improvements in Clean Energy



The research reveals that step edge defects in semiconducting MoTe₂ significantly alter the electronic structure of the surrounding material, influencing catalytic activity up to 200 nanometers away.

 

 

Dr. Megan Jackson, a co-author of the paper and an assistant professor in the Department of Chemistry.

February 3, 2025 | By Dave DeFusco

In the race to transition from fossil fuels to renewable energy, the ability to efficiently convert electricity into chemical energy—and vice versa—is critical. Devices such as fuel cells rely on electrocatalysts, materials that facilitate chemical reactions on their surfaces. For decades, scientists have sought to identify the specific sites on these materials where catalysis occurs and to understand the electronic properties that make them active.

A study by Ph.D. student Kenneth Ortiz Chua and Dr. Megan Jackson, an assistant professor in the Department of Chemistry at UNC-Chapel Hill, challenges long-standing assumptions in this field and opens new doors for designing more efficient electrocatalysts. Published in the Journal of the American Chemical Society, their study,Step Edge Defects Have Nanoscale Impact on the Electronic Structure in Semiconducting Transition Metal Dichalcogenide Electrocatalysts,” investigates the catalytic behavior of molybdenum ditelluride (MoTe₂), a two-dimensional material from the family of transition metal dichalcogenides (TMDCs).

The research reveals that step edge defects in semiconducting MoTe₂ do more than provide active sites for hydrogen evolution—they significantly alter the electronic structure of the surrounding material, influencing catalytic activity up to 200 nanometers away. Heterogeneous catalysis occurs at the interface of a solid material and a reacting gas or liquid; however, not all surface sites contribute equally to catalytic activity. Traditional electrochemical experiments measure the average activity of all sites, obscuring the role of individual sites. Similarly, many characterization techniques lack the spatial resolution needed to distinguish between active and inactive regions.

Kenneth Ortiz Chua, a co-author of the paper and Ph.D. student.

This limitation has fueled decades of debate over which features of TMDCs drive their catalytic efficiency. A pivotal 2007 study in Science identified edge sites in molybdenum disulfide (MoS₂) as highly active for hydrogen evolution, primarily attributing this activity to differences in hydrogen adsorption energy between edge and basal plane sites. The prevailing view since then has been that edge sites are inherently more active due to their undercoordinated atomic structure. However, semiconductors have complex electronic structures, so focusing solely on the coordination environment of the edge sites may obscure other important contributing factors.

Dr. Jackson’s team sought to test this hypothesis using a different TMDC, MoTe₂, in both its semiconducting and semimetallic phases. By employing cutting-edge techniques, such as scanning electrochemical cell microscopy, atomic force microscopy and Kelvin probe force microscopy, they were able to map catalytic activity and electronic properties at the nanoscale.

The study’s findings challenge the conventional wisdom that edge sites are simply undercoordinated regions with favorable hydrogen adsorption energies. Instead, the researchers discovered that in semiconducting MoTe₂, edge sites act as defect regions that perturb the electronic structure of the surrounding basal plane. This effect extends over distances of 50 to 200 nanometers, far beyond the edge itself.

“Our work shows that edge sites in semiconductor electrocatalysts should not only be thought of as catalytically active undercoordinated sites but also as defect sites that can dramatically tune the electronics of the surrounding region,” said Dr. Jackson. “This ability to modify nanoscale electronic structure with undercoordinated defect sites presents a powerful opportunity for the bottom-up design of semiconducting electrocatalysts.”

Interestingly, the same phenomenon was not observed in the semimetallic phase of MoTe₂. In this phase, there was no measurable difference in catalytic activity between edge and basal plane sites. This contrast underscores the importance of electronic structure in determining catalytic behavior.

The researchers found that the local catalytic activity in MoTe₂ strongly correlated with electronic properties such as surface potential, conductance and rates of electron transfer. Measurements using KPFM and conductive AFM revealed a higher density of states at the Fermi level near edge sites and their surrounding regions. This higher density of states facilitates faster electron transfer, a key factor in catalytic efficiency.

Moreover, the study indicates that basal plane sites near the edge may exhibit stronger hydrogen adsorption than those farther away, effectively creating a gradient of catalytic activity extending away from the edge.

These insights have profound implications for the design of next-generation electrocatalysts. By rethinking edge sites as defect regions that modulate the electronic structure of their surroundings, researchers can explore new strategies for enhancing catalytic performance. For example, intentionally introducing or engineering defects in semiconducting materials could enable precise control over local electronic properties, optimizing the material for specific reactions.

“Thinking of defects in this way provides an exciting opportunity to rationally tune local electronic structures in semiconducting electrocatalysts,” said Chua. “This approach could be applied to a wide range of reactions, from hydrogen evolution to carbon dioxide reduction.”


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