Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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.
1
ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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.
2
ACS Paragon Plus Environment
Page 2 of 24
Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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
