The Mott state in 1T-TaS2 is predicted to host quantum spin liquids (QSLs). However, its insulating mechanism is controversial due to complications from interlayer coupling. Here, we study the charge transfer state in monolayer 1T-NbSe2, an electronic analogue to TaS2 exempt from interlayer coupling, using spectroscopic imaging scanning tunneling microscopy and first-principles calculations. Monolayer NbSe2 surprisingly displays two types of star of David (SD) motifs with different charge transfer gap sizes, which are interconvertible via temperature variation. In addition, bilayer 1T-NbSe2 shows a Mott collapse by interlayer coupling. Our calculation unveils that the two types of SDs possess distinct structural distortions, altering the effective Coulomb energies of the central Nb orbital. Our calculation suggests that the charge transfer gap, the same parameter for determining the QSL regime, is tunable with strain. This finding offers a general strategy for manipulating the charge transfer state in related systems, which may be tuned into the potential QSL regime.

sources into fuels and value-added chemicals. [1−3] However, non-ideal catalytic activity primarily caused by the sluggish kinetics has long posed a crucial challenge in restricting the efficiency of electrocatalytic reactions. [4,5] Based on this, enormous research is devoted to enhancing the intrinsic activity of pre-existing active sites. For example, facet control can selectively expose the high-energy facets of catalysts to promote the adsorption of electrolytes, providing higher catalytic performance. [6] However, catalysts with high-energy facets are generally thermodynamic unstable and their preparations remain greatly challenging. Strain regulation can adjust the local coordination environment of active sites, [7] but its application is restricted by the stability of the modified structure with huge strain. Additionally, alloying with metals/nonmetals is also an effective strategy to decrease the reaction barrier for electrocatalytic reactions, [8] while the thermodynamic miscibility among the different elements is a necessary prerequisite. [9] In essence, the reaction kinetics is effectively triggered to promote the catalytic performance by these design approaches, which is ascribed to appropriate electronic structures. [10,11] Nevertheless, as for the existing catalytic materials, a rational design to tailor the optimal electronic structures is currently lacking, which is highly desired.Here, we propose a design principle, namely "dual self-built gating" to greatly boost the hydrogen evolution reaction (HER) performance of catalysts. Taking ReS 2 and WS 2 as an example, the dual self-built gating originated from in-plane ReS 2 -WS 2 covalent bonds and out-plane ReS 2 /WS 2 interlayer interaction induces electrons to directionally transfer from WS 2 to ReS 2 , [12,13] resulting in charge redistribution at the interface. In this case, owing to the tailored electronic structures, dual selfbuilt gating can balance the adsorption of intermediates and the desorption of hydrogen synergistically, leading to a dramatic improvement in reaction kinetics. As demonstrated by density functional theory (DFT) calculations, the dual gating region shows a Gibbs free energy close to zero (0.03 eV), suggesting that the charge redistribution at the interface enhances the intrinsic activity of active sites. More interestingly, on account of the adjustable carrier density, we also confirm the Optimizing the intrinsic activity of active sites is an appealing strategy for accelerating the kinetics of the catalytic process. Here, a design principle, namely "dual self-built gating", is proposed to tailor the electronic structures of catalysts. Catalytic improvement is confirmed in a model catalyst with a ReS 2 -WS 2 /WS 2 hybridized heterostructure. As demonstrated in experimental and theoretical results, the dual gating can bidirectionally guide electron transfer and redistribute at the interface, endowing the model catalyst with an electron-rich region. The tailored electronic structures balance the adsorption of intermediate...

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