Decoupling the Surface and Bulk Reactivities of MXenes and Catalytic Activity Tuning through Surface Chemistry Modification

Abstract

MXenes, a family of two-dimensional (2D) materials based on transition metal carbides and nitrides, show great promise as electrocatalysts for electrochemical reactions but suffer from a lack of fundamental understanding of their electrocatalytic mechanisms. Decoupling the surface and bulk reactivities are key to understanding the electrocatalytic mechanisms of these emerging nanomaterials. Herein, we use Raman spectroscopy to decouple the surface and bulk reactivities of the Ti2NTx MXene. By attenuating the Raman intensity, we were able to control the penetration depth of the incident light. We show that with 5–10% laser power, we can restrict the access to the surface of the Ti2NTx MXene, which shows predominantly the oxide phase from the passivation layer. With 50–100% laser power, we can penetrate the bulk of the MXene material, as evidenced by the Ti–N bond network characteristics. By using this approach, we can study and understand the surface reactivity of MXenes, which has not been understood. To expand these findings to electrochemical reactions, we have analyzed the surface reactivities of the pristine and NaOH treated Ti2NTx MXenes using Raman spectroscopy and hydrogen evolution reaction (HER). The surface chemistry of the NaOH treated Ti2NTx MXene was different from that of the pristine ones and showed predominantly −OH termination groups and no −F. This resulted in an excellent HER activity of ∼100 mV overpotential at 10 mA cm–2 and an impressive Tafel slope of 98 mV dec–1. These results are comparable to those for the benchmark Pt/C in the alkaline electrolyte. To summarize, we propose an approach for decoupling the surface and bulk reactivities of MXenes and a simple method to tune their HER activity to match those for noble metals. This approach can be applied to other materials and systems to accelerate the discovery of electrocatalysts and electrode materials.