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Large-Area CVD MoS2/WS2 Heterojunctions as a Photoelectrocatalyst for Salt-Water Oxidation Peter C. Sherrell,† Pawel Palczynski,† Maria S. Sokolikova, Francesco Reale, Federico M. Pesci, Mauro Och, and Cecilia Mattevi* Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom

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S Supporting Information *

ABSTRACT: Splitting salt water via sunlight into molecular oxygen and hydrogen for use as fuel or as an energy carrier is a clear pathway toward renewable energy. Monolayer MoS2 and WS2 are promising materials for the energetically demanding water oxidation reaction, absorbing ∼10% of incident light in the visible spectrum and possessing chemical stability and band edges more positive than the oxidation potential of water. A heterostructure of MoS2/WS2 forms a type-II heterojunction, supporting fast separation of the photogenerated charge carriers across the junction. Here, we show the role played by defects in determining the efficiency of the photon-driven oxidation process. By reducing the defects in this material system, it is possible to obtain an incident photon-to-current conversion efficiency (IPCE) of ∼1.6% and a visible-light-driven photocurrent density of 1.7 mA/cm2 for water oxidation. The efficiency is one order of magnitude higher than that of photoelectrocatalytic hydrogen reduction and water oxidation supported by liquid-phase exfoliated transition-metal dichalcogenides (TMDs). This result has been achieved with chemically vapor deposited (CVD) MoS2/WS2 heterojunctions, in the form of 100 μm large flakes assembled to form thin films. The large flakes sizes, compared to liquid-phase exfoliated materials (normally 1.18 V vs RHE (Figure 3 a, b and Figure S8). An early onset of measurable photocurrent at +0.48 V vs RHE was observed with the maximum on/off ratio at +1.01 V vs RHE (Figure S8a). A bare gold foil, which had been subjected to the CVD process C

DOI: 10.1021/acsaem.9b01008 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

Figure 3. Photoelectrochemical characterization of MoS2/WS2 heterostructure on gold in 3.5% w/v NaCl in DI water. (a) Photograph of the active electrode at +0.75 V vs Ag/AgCl (left: no-illumination; right: illuminated) showing gas bubble formation. (b) Linear sweep voltammetry at 2.5 mV/s from +0.8 to −0.8 V vs Ag/AgCl, demonstrating the complete protection of the Au surface as minimal noble-metal hydrogen evolution is observed and a negligible contribution of the substrate to either dark or photocurrent in the water oxidation region occurred. (c) Chronoamperometry at +0.75 V vs Ag/AgCl with chopped illumination at 0.2 Hz. (d) Photocurrent dependence on monochromatic light at +0.75 V vs Ag/AgCl and corresponding absorbance spectra of the heterostructures dispersed via sonication in ethanol.

the scalability of the synthesis process. This approach renders scalable CVD a compelling approach for the synthesis of TMDs, which can overcome the widely used solution-based exfoliation methods. This approach can be extended to different TMD heterojunctions, promoting photoreduction and photo-oxidation reactions including H2 evolution, CO2 conversion, and more.

structure and lead to the large grains overlapping with grain boundaries offset from one another. This offset allows for photogenerated charges to move laterally as well as vertically through the heterojunction with minimal recombination sites. These factors are proposed to minimize loss during charge transfer between MoS2 and WS2 layers. Finally, the relatively high volatility of utilized H2WO4 (and oxyhalide intermediates) ensures enriched WS2 at the surface of the heterostructure (Figure S2) in contact with the electrolyte, providing the ability for photogenerated holes, transferred in the WS2, to directly interact with water molecules. Additionally, the observed kinetics of charge carrier recombination has similar time-scales to those reported by Yu et al. using WSe2 (and WSe2/Pt heterostructures) as photocathodes for the hydrogen evolution reaction.14,15,31



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b01008. Experimental details; supporting materials characterization, including SEM, Raman, STEM, EDS, and reflectance spectra; and supporting electrochemical characterization, including stability testing, CA at 1 sun, LSV, CA, and O2 measurements and IPCE, VRHE, and FE calculations (PDF)



CONCLUSION In conclusion, we have demonstrated promising photocatalytic performance for water oxidation in salt-water conditions based on large area 2D MoS2/WS2 heterojunctions. Minimization of recombination sites enables fast charge carrier kinetics, with a photocurrent density of up to 1.7 mA/cm2 (at 1.19 V vs RHE) for 80 nm thick materials. The epitaxial interface of heterostructures fabricated by CVD enable minimization of atomic defects and nanosheets edge density while preserving



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peter C. Sherrell: 0000-0003-4644-6238 D

DOI: 10.1021/acsaem.9b01008 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

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Federico M. Pesci: 0000-0003-2558-2222 Cecilia Mattevi: 0000-0003-0005-0633 Author Contributions †

These authors contributed equally to the work.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors would like to thank Dr. Ecaterina Ware for the preparation of the TEM lamella via FIB. The authors acknowledge the use of the characterization facilities within the Harvey Flower Electron Microscopy Suite and the X-Ray Diffraction Suite of the Department of Materials, Imperial College London. M.S.S. would like to acknowledge the President’s Ph.D. Scholarship programme at the Imperial College London for financial support. C.M. would like to acknowledge the EPSRC awards, EP/K01658X/1, EP/ K016792/1, EP/M022250/1, the EPSRC-Royal Society Fellowship Engagement Grant EP/L003481/1, the Research Fellows Enhancement Award 2017 RGF\EA\180090, and the award of a Royal Society University Research Fellowship by the U.K. Royal Society.



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DOI: 10.1021/acsaem.9b01008 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX