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.
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are emerging as promising building blocks of high-performance photocatalysts for visible-light-driven water splitting because of their unique physical, chemical, electronic, and optical properties. This review focuses on the fundamentals of 2D TMDC-based mixed-dimensional heterostructures and their unique properties as visible-light-driven photocatalysts from the perspective of dimensionality and interface engineering. First, we discuss the approaches and advantages of surface modification and functionalization of 2D TMDCs for photocatalytic water splitting under visible-light illumination. We then classify the strategies for improving the photocatalytic activity of 2D TMDCs via combination with various low-dimensional nanomaterials to form mixed-dimensional heterostructures. Further, we highlight recent advances in the use of these mixed-dimensional heterostructures as high-efficiency visible-light-driven photocatalysts, particularly focusing on synthesis routes, modification approaches, and physiochemical mechanisms for improving their photoactivity. Finally, we provide our perspectives on future opportunities and challenges in promoting real-world photocatalytic applications of 2D TMDC-based heterostructures.
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