Step Edge Defects HaveNanoscale Impact on the ElectronicStructure in Semiconducting Transition Metal Dichalcogenide Electrocatalysts

This work uses MoTe₂ single crystals to address the challenge in heterogeneous catalysis of identifying active sites and determining the electronic factors responsible for catalytic activity. We find that for semiconducting MoTe₂, spots that fall along step edges show more catalytic activity than spots that fall solely on the basal plane. In contrast, for a semimetallic phase of MoTe₂, there is no measurable difference between the H₂ evolution activity at the step edges and at the basal plane, indicating that the active sites for H₂ evolution catalysis on transition metal dichalcogenides are phase dependent. We additionally find that the local H₂ evolution activity correlates with local potential differences, higher conductance, and faster rates of outer-sphere electron transfer to a redox-active probe in solution. Interestingly, in the semiconducting phase of MoTe₂, these local electronic structure differences were measured both at the edge sites and at basal plane sites next to the edge, extending >50 nm away from the edge. These results indicate that for MoTe₂, basal plane sites near the edge may have stronger free energies of H adsorption, Δ<i>G</i>_H,ads, than basal plane sites farther from the edge. Thus, we propose that edge sites in semiconducting transition metal dichalcogenides are better thought of as defect sites that substantially modulate the electronic structure in the surrounding region rather than solely as undercoordinated active sites. These results provide exciting opportunities to use defects to alter the electronic structure at existing active sites in semiconductor electrocatalysts.