Single Nanoflake Photoelectrochemistry Reveals Champion and

perimeter edge. c) Plot of EQE versus r for the nanoflake in (a). The small red circles represent data from every pixel within the nanoflake contour a...
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C: Energy Conversion and Storage; Energy and Charge Transport

Single Nanoflake Photoelectrochemistry Reveals Champion and Spectator Flakes in Exfoliated MoSe Films 2

Michael Aaron Todt, Allan E Isenberg, Sanjini U. Nanayakkara, Elisa M. Miller, and Justin B. Sambur J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12715 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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The Journal of Physical Chemistry

Single Nanoflake Photoelectrochemistry Reveals Champion and Spectator Flakes in Exfoliated MoSe2 Films Michael A. Todt,† Allan E. Isenberg,† Sanjini U. Nanayakkara,‡ Elisa M. Miller,‡ and Justin B. Sambur∗,¶ †Department of Chemistry, Colorado State University, Fort Collins CO 80523,USA ‡National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA ¶Department of Chemistry, Colorado State University, Fort Collins CO 80523, USA E-mail: [email protected] Phone: (970) 491-3096

Abstract Semiconducting transition metal dichalcogenide (TMD) nanoflake thin films are promising large-area electrodes for photoelectrochemical solar energy conversion applications. However, their energy conversion efficiencies are typically much lower than 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 similar energy conversion efficiencies to 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 MoSe2 samples studied herein, 7% of nanoflakes are highly active champions whose photocurrent efficiency exceed that of the bulk crystal.

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However, 66% of nanoflakes are inactive spectators that 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 ensemblelevel measurements, reveal underlying performance issues of exfoliated TMD electrodes for photoelectrochemical energy conversion applications.

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The Journal of Physical Chemistry

Introduction Transition metal dichalcogenides (TMDs) such as MoX2 and WX2 (where X= S or Se) are highly efficient and stable electrode materials for photoelectrochemical solar energy conversion to electricity 1–4 and chemical fuels. 5–9 For example, seminal work in the 1980s demonstrated >10% solar-to-electrical energy conversion efficiency using bulk n-WSe2 /I− ,I− 3 /Pt solar cells. 10–12 However, these cells employed small 10% of illuminated areas on 4/59 or 7% of these nanoflakes produce EQE values that exceed the bulk crystal mean. This ”champion” population, which is hidden in ensemble-average measurements, reveals that some isolated nanoflakes can achieve similar photocurrent efficiencies to bulk single crystals. The second major observation from Figure 2b is that there is a large, ”spectator” nanoflake population (40/59, or 68% of nanoflakes) whose mean EQE values are equal to bare ITO regions. These spectator nanoflakes are detrimental to device performance because they absorb a significant fraction of incident photons but they do not produce photocurrent. We note that these inactive flakes are electrically connected to the ITO substrate because they produce a measurable steady state photocurrent under much higher illumination intensities (e.g., >10 kW/cm2 ), but the resulting EQE values are