Field Effect Modulation of Ultrathin 2D Electrodes: New Approaches to Electrocatalysis

The electrochemistry of semiconductors is central to the development of photoelectrochemical conversion systems and to understanding the geophotochemistry of light-absorbing minerals in water, for example. The recent advent of ultrathin two dimensional (2D) semiconductor materials prepared either by exfoliation or thin film growth methods opens up new opportunities to examine the role of dimensionality in semiconductor electrochemistry.

A particularly intriguing possibility is to exploit the transverse field effect—so central to silicon CMOS technology—to modulate the carrier density in the valence (VB) and conduction bands (CB) of a 2D semiconductor that simultaneously serves as the working electrode in an electrochemical cell. This can be accomplished by placing the (grounded) 2D material on a metal gate/dielectric stack, where application of voltage on the gate shifts the VB and CB edges with respect to the Fermi-level; this phenomenon is the transverse field effect. The potential significance of such a field effect modulated 2D working electrode architecture is two-fold: (1) because of the extreme thinness of the 2D material, the charge induced in the semiconductor by the back gate is accessible to an electrolyte solution on the opposite (front) face; (2) this charge is separate from, but in addition to, double-layer charge induced by the independently controlled working electrode potential with respect to the reference.

This talk will show how field effect modulation can control both outer-sphere and inner-sphere (electrocatalytic) electron transfer at 2D semiconductor electrodes. For electrocatalysis, the specific example will be H₂ evolution at 2D MoS₂ back-gated electrodes. Gated electrodes provide a new platform for fundamental investigation of electronic effects in electrocatalysis; these architectures may also be scalable for practical applications in water splitting, for example.