The phase transition of multilayer MoS2 nanosheets from semiconducting 2H to metallic 1T (2H/1T) has been realized mainly by chemical methods (e.g., Li intercalation). Here, we develop a simple yet effective method, cyclic voltammetry, to successfully tune the 2H/1T phase transition of multilayer MoS2 nanosheets without using intercalation species. The phase transition is triggered by the electrochemical incorporation of S vacancies (obtained by electrochemical etching), which on the one hand injects electrons into the framework of S–Mo–S and on the other hand facilitates the sliding of S planes. Density functional theory calculations show that O doping in the framework of S–Mo–S decreases the energy barrier for forming S vacancies and stabilizes the 1T-phase by occupying the 4d orbital of Mo. Our calculations further show that the presence of S vacancies and O incorporation not only reduces the bandgap of MoS2, indicating an increased conductivity, but also decreases the hydrogen adsorption free energy, implying significant improvement of hydrogen evolution reaction (HER) activity. Indeed, the overpotential and Tafel plot of the electrochemically treated MoS2 nanosheets are decreased respectively by 174 mV and 25 mV/dec at a cathodic current density of 10 mA cm–2 compared with pristine 2H-MoS2 nanosheets. The HER experiment also reveals the order of catalytical activity for the studied phases and structural defects: 1T-MoS2 > S vacancies > O doping >2H-MoS2. Our study has provided a new route to control the phase transition of multilayer MoS2 nanosheets with promising applications potentially in catalysis and optoelectronics.

Plasmon-induced hot carriers have vast potential for light-triggered highefficiency carrier generation and extraction, which can overcome the optical band gap limit of conventional semiconductor-based optoelectronic devices. Here, it is demonstrated that Au/TiO 2 dumbbell nanostructures assembled on a thin Au film serve as an efficient optical absorber and a hot-carrier generator in the visible region. Upon excitation of localized surface plasmons in such coupled particle-on-film nanocavities, the energetic conduction electrons in Au can be injected over the Au/TiO 2 Schottky barrier and migrated to TiO 2 , participating in the chemical reaction occurring at the TiO 2 surface. Compared with the same dumbbell nanostructures on an indium tin oxide (ITO) film, such nanocavities exhibit remarkable enhancement in both photocurrent amplitude and reaction rate that arise from increased light absorption and near-field amplification in the presence of the Au film. The incidentwavelength-dependent photocurrent and reaction rate measurements jointly reveal that Au-film-mediated near-field localization facilitates more efficient electron-hole separation and transport in the dumbbells and also promotes strong d-band optical transitions in the Au film for generation of extra hot electrons. Such nanocavities provide a new plasmonic platform for effective photoexcitation and extraction of hot carriers and also better understanding of their fundamental science and technological implications in solar energy harvesting.

The interplay between local field enhancement and backaction determines plasmonic chiral responses and leads to widely tunable geometry-dependent optical activities.

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Plasmon-induced hot carriers have vast potential for light-triggered highefficiency carrier generation and extraction, which can overcome the optical band gap limit of conventional semiconductor-based optoelectronic devices. Here, it is demonstrated that Au/TiO 2 dumbbell nanostructures assembled on a thin Au film serve as an efficient optical absorber and a hot-carrier generator in the visible region. Upon excitation of localized surface plasmons in such coupled particle-on-film nanocavities, the energetic conduction electrons in Au can be injected over the Au/TiO 2 Schottky barrier and migrated to TiO 2 , participating in the chemical reaction occurring at the TiO 2 surface. Compared with the same dumbbell nanostructures on an indium tin oxide (ITO) film, such nanocavities exhibit remarkable enhancement in both photocurrent amplitude and reaction rate that arise from increased light absorption and near-field amplification in the presence of the Au film. The incidentwavelength-dependent photocurrent and reaction rate measurements jointly reveal that Au-film-mediated near-field localization facilitates more efficient electron-hole separation and transport in the dumbbells and also promotes strong d-band optical transitions in the Au film for generation of extra hot electrons. Such nanocavities provide a new plasmonic platform for effective photoexcitation and extraction of hot carriers and also better understanding of their fundamental science and technological implications in solar energy harvesting.