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Electro- and photocatalytic reduction of N2 to NH3 – the nitrogen reduction reaction (NRR) – is an environmentally- and energy-friendly alternative to the Haber-Bosch process for ammonia production. There is a great demand for the development of novel semiconductor-based electrocatalysts with high efficiency and stability to perform this process under visible light irradiation and ambient conditions. Herein we report electro-, and photocatalytic NRR with transition metal dichalcogenides (TMDCs), viz MoS2 and WS2. Improved acid treatment of bulk TMDCs yields exfoliated TMDCs (exTMDCs) only a few layers thick with ~10 % S vacancies. Linear scan voltammograms on exMoS2 and exWS2 electrodes reveal significant NRR activity for exTMDC-modified electrodes, which is greatly enhanced by visible light illumination. Spectral measurements confirm ammonia as the main reaction product of electrocatalytic and photocatalytic NRR, and the absence of hydrazine byproduct. Femtosecond-resolved transient absorption studies provide direct evidence of interaction between photo-generated excitons/trions with N2 adsorbed at S vacancies. DFT calculations corroborate N2 binding to exMoS2 at S-vacancies, with substantial pi-backbonding to activate dinitrogen. Our findings suggest that chemically functionalized exTMDC materials could fulfill the need for highly-desired, inexpensive catalysts for the sustainable production of NH3 using sunlight under neutral pH conditions.

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