Single Nanoflake Photoelectrochemistry Reveals Intrananoflake Doping Heterogeneity That Explains Ensemble-Level Photoelectrochemical Behavior

Abstract

Transition metal dichalcogenide (TMD) nanoflake thin films are attractive electrode materials for photoelectrochemical (PEC) solar energy conversion and sensing applications, but their photocurrent quantum yields are generally lower than those of bulk TMD electrodes. The poor PEC performance has been primarily attributed to enhanced charge carrier recombination at exposed defect and edge sites introduced by the exfoliation process. Here, a single nanoflake PEC approach reveals how an alternative effect, doping heterogeneity, limits ensemble-level PEC performance. Photocurrent mapping and local photocurrent–potential (i–E) measurements of MoS2 nanoflakes exfoliated from naturally occurring bulk crystals revealed the presence of n- and p-type domains within the same nanoflake. Interestingly, the n- and p-type domains in the natural MoS2 nanoflakes were equally efficient for iodide oxidation and tri-iodide reduction (IQE values exceed 80%). At the single domain-level, the natural MoS2 nanoflakes were nearly as efficient as nanoflakes exfoliated from synthetic n-type MoS2 crystals. Single domain-level i–E measurements explain why natural MoS2 nanoflakes exhibit an n-type to p-type photocurrent switching effect in ensemble-level measurements: the n- and p-type diode currents from individual domains oppose each other upon illuminating the entire nanoflake, resulting in zero photocurrent at the switching potential. The doping heterogeneity effect is likely due to nonideal stoichiometry, where p-type domains are S-rich according to XPS measurements. Although this doping heterogeneity effect limits photoanode or photocathode performance, these findings open the possibility to synthesize efficient TMD nanoflake photocatalysts with well-defined lateral p- and n-type domains for enhanced charge separation.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

“…The advantage of this technique is that it directly measures the rate of the electrochemical reaction of interest at a specific region (as the Faradaic current is proportional to reaction rate). Combining SECM with an illumination source, which may be coupled with the UME for localized illumination, enables scanning photoelectrochemical measurements at semiconductor electrodes. − The spatial resolution of SECM depends primarily on the diameter of the UME. While typical resolutions for this technique are hundreds of nanometers to microns, recent advances in using smaller nanoelectrodes for SECM have provided spatial resolutions of 15 to 55 nm, ,, which is comparable to that obtainable by single-molecule fluorescence imaging.…”

“…Scanning electrochemical microscopy (SECM) maps the rates of heterogeneous charge transfer across electrode surfaces. − An ultramicroelectrode (UME) with a diameter typically ranging from several hundred nanometers to a few micrometers is scanned across the electrochemically active surface to measure current under an applied potential. The advantage of this technique is that it directly measures the rate of the electrochemical reaction of interest at a specific region (as the Faradaic current is proportional to reaction rate).…”

The Hidden Role of the Supporting Electrode for Creating Heterogeneity in Single Entity Electrochemistry

“…Correlated optical microscopy and electrochemistry has been particularly useful in assigning structure-property relationships by extracting quantitative electrochemical information from optical signatures at the single particle level. [4,11,15,[17][18][19][20][21][22] Many of these optical approaches require a transparent and conductive supporting electrode that allows for both the ability to capture an optical signature from the nanoparticle(s) of interest as well as provide control over the substrate potential. In particular, transparent conductive oxide thin films such as tin-doped indium oxide (ITO) are often used as the supporting electrodes due to their high conductivity and excellent transparency in the visible region of light, while also remaining relatively inert towards many electrochemical reactions of interest due to its poor electrocatalytic properties.…”

Direct electrochemical identification of rare microscopic catalytic active sites

“…S12), it is likely attributable to local differences in electronic structure, 17 possibly induced by spatial variations in composition ( e.g. , non-stoichiometric Mo : S ratios 18 ) and/or surface topology ( e.g. , mechanical straining of the BP 19,20 ) on the sub-10 nm scale ( i.e.…”

“…Although the exact cause of the surface contrast in SEM is not known (seen in both secondary electron and backscatter electron detector modes, see ESI, † Fig. S12), it is likely attributable to local differences in electronic structure, 17 possibly induced by spatial variations in composition (e.g., non-stoichiometric Mo : S ratios 18 ) and/or surface topology (e.g., mechanical straining of the BP 19,20 ) on the sub-10 nm scale (i.e., beyond the limit of detection for SECCM, herein). Changes in the local electronic structure may influence the free energy for adsorbed atomic hydrogen (DG H* ), which is a widely-accepted descriptor for understanding the performance and further predicting the activity of HER electrocatalysts (i.e., DG H* E 0 eV for an ''ideal'' HER electrocatalyst).…”