In-situ visualization of hydrogen evolution sites on helium ion treated molybdenum dichalcogenides under reaction conditions

Abstract

Nanostructured 2D transition metal dichalcogenides play an increasingly important role in heterogeneous catalysis. These materials are abundant (co-)catalysts with tunable properties to catalyze a number of key reactions related to energy provision, for instance the hydrogen evolution reaction (HER). It is vital to understand which surface sites are active in order to maximize their number and to improve the overall (photo-)catalytic behavior of those materials. Here, we visualize these active sites under HER conditions at the surface of molybdenum dichalcogenides (MoX 2 , X = Se, S) with lateral resolution on the nanometer scale by means of electrochemical scanning tunneling microscopy. The edges of single MoX 2 flakes show high catalytic activity, whereas their terraces are inactive. We demonstrate how the inert basal planes of these materials can be activated towards the HER with the help of a focused beam of a He-ion microscope. Our findings demonstrate that the He-ion induced defects contribute at lower overpotentials to the HER, while the activity of the edges exceeds the activity of the basal defects for sufficiently high overpotentials. Given the lithographic resolution of the helium ion microscope, our results show the possibility to generate active sites in transition metal dichalcogenides with a spatial resolution below a few nanometers.

In Situ Quantification of the Local Electrocatalytic Activity via Electrochemical Scanning Tunneling Microscopy

“…In order to avoid contamination of the sample surface, a Pt wire was employed as a quasi‐reference electrode, which has already been demonstrated as a reliable option for EC‐STM purposes, although a direct transfer to reversible hydrogen electrode (RHE) scale is not practicable. [ 19–22 ] Figure A shows the current density j WE of the working electrode (WE) against different potentials in the ORR region. The curve indicates a steady increase in the current from 0.00 to –0.25 V versus Pt.…”

“…[ 17–19 ] Recently, EC‐STM has been proven capable of visualizing active sites of several model systems by monitoring the noise level in the STM signal (n‐EC‐STM). [ 19–22 ] Due to a local increase in the relative noise level at active compared to non‐active sites, this technique is able to render an activity map of the electrode surface. Moreover, different levels of activity can be distinguished by the extent of the recorded noise.…”

Engineered 2D Transition Metal Dichalcogenides—A Vision of Viable Hydrogen Evolution Reaction Catalysis

“…Upon the chemical doping at the atomic‐level, the slight change of the catalyst may not be detectable by the frequently used “long‐term” (e.g., hours or days) polarization curves of the power samples and ex situ characterizations. Other in situ observations of the catalyst may need to be involved by integration with the conceptual device, such as the electrochemical scanning tunneling microscope (ECSTM), [ 189 ] the scanning transmission electron microscopy (STEM), [ 190 ] the tip‐enhanced Raman‐SPM/AFM (SPM: scanning Probe Microscopy) combined microscopy and beyond. [ 191 ]…”

Atomistic Positioning of Defects in Helium Ion Treated Single-Layer MoS2

“…13,39 Furthermore, HIM induced defects activate the inert basal plane of MoS 2 towards the catalytic hydrogen evolution reaction. 40 So far, the exact nature of these observed effects is not clear. However, oxygen passivated sulfur vacancies (defect B) cannot be responsible for the observed emission or the catalytic activity, as they are also present in pristine MoS 2 .…”