Single-Nanoflake Photo-Electrochemistry Reveals Champion and Spectator Flakes in Exfoliated MoSe₂ Films

Abstract

Semiconducting transition-metal dichalcogenide (TMD) nanoflake thin films are promising large-area electrodes for photo-electrochemical solar energy conversion applications. However, their energy conversion efficiencies are typically much lower than those of bulk electrodes. It is unclear to what extent this efficiency gap stems from differences among nanoflakes (e.g., area, thickness, and surface structural features). It is also unclear whether individual exfoliated nanoflakes can achieve energy conversion efficiencies similar to those of bulk crystals. Here, we use a single-nanoflake photoelectrochemical approach to show that there are both highly active and completely inactive nanoflakes within a film. For the exfoliated MoSe 2 samples studied herein, 7% of nanoflakes are highly active champions, whose photocurrent efficiency exceeds that of the bulk crystal. However, 66% of nanoflakes are inactive spectators, which are mostly responsible for the overall lower photocurrent efficiency compared to the bulk crystal. The photocurrent collection efficiency increases with nanoflake area and decreases more at perimeter edges than at interior step edges. These observations, which are hidden in ensemble-level measurements, reveal the underlying performance issues of exfoliated TMD electrodes for photo-electrochemical energy conversion applications.

Directly visualizing carrier transport and recombination at individual defects within 2D semiconductors

“…Clearly distinguishing the behavior of defects from that of the bulk material will require high-resolution imaging techniques capable of probing carrier transport and recombination, and a variety of experimental strategies along these lines have been demonstrated. Techniques such as scanning photocurrent microscopy, [22][23][24][25][26][27][28][29][30] scanning near-field optical microscopy, 31,32 electron beam induced current measurements, [33][34][35] or transient absorption microscopy [36][37][38][39][40] have been employed to generate valuable insights into the transport and recombination of carriers within 2DSCs. However, these experiments are often limited in terms of the complexity of the generated response or by the need for carriers to exhibit a strong spectroscopic signature.…”

Nanoscale redox mapping at the MoS2-liquid interface

“…The research of recent years established thin film MoS 2 as a notably potent material for applications with a deep impact of edge and vacancy states 39 , 40 , thickness 41 , domain sizes 15 , 16 , electron transfer kinetics 18 etc. on device performances, individually identified, although highly intertwined 8 , 9 , 15 , 16 , 40 , 41 . The route towards highly efficient materials is the study and discussion of application relevant properties in conjunction with the full bandwidth of physical properties in different environments and their dependencies among each other 17 , 18 .…”

Structural Features Dictate the Photoelectrochemical Activities of Two-Dimensional MoSe2 and WSe2 Nanostructures

“…The trend is also explained by the increasing number of flakes and their decreasing lateral size. 32 …”

“…WSe 2 nanoflakes have been investigated by scanning photocurrent microscopy in a flow electrochemical cell recently. 32 The internal photon-to-electron conversion efficiency (IPCE) increased with the nanoflake area and decreased at the edges of sheets. Further works examined the effect of Au nanoparticles and laser annealing on the PEC activity of liquid phase exfoliated (LPE) WSe 2 and MoSe 2 layers.…”