Identification of the active species in electrocatalysts toward hydrogen evolution reaction (HER) is of great significance for the development of the catalytic industry; however, it is still the subject of considerable controversy. Herein, we applied operando synchrotron X-ray powder diffraction (SXRD) in the NiSe 2 electrocatalyst system, and an in situ phase transformation from cubic NiSe 2 to hexagonal NiSe was revealed. The NiSe phase showed an enhanced catalytic activity. Operando Raman spectroscopy verified the decomposition of NiSe 2 during HER. Theoretical calculations suggested that the charge transfers from the Se site to Ni site during this evolution process, leading to an increased conductivity and a shifting up of d-band center, which is attributed to the enhanced activity. The generated NiSe phase acts as the "real" active species. Our work unravels the underlying phase transition of the electrocatalyst on reductive conditions in alkaline medium and highlights the significance of identifying the intrinsic active sites under realistic reaction conditions.

Work function strongly impacts the surficial charge distribution, especially for metal-support electrocatalysts when a built-in electric field (BEF) is constructed. Therefore, studying the correlation between work function and BEF is crucial for understanding the intrinsic reaction mechanism. Herein, we present a Pt@CoO x electrocatalyst with a large work function difference (ΔΦ) and strong BEF, which shows outstanding hydrogen evolution activity in a neutral medium with a 4.5-fold mass activity higher than 20 % Pt/C. Both experimental and theoretical results confirm the interfacial charge redistribution induced by the strong BEF, thus subtly optimizing hydrogen and hydroxide adsorption energy. This work not only provides fresh insights into the neutral hydrogen evolution mechanism but also proposes new design principles toward efficient electrocatalysts for hydrogen production in a neutral medium.

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such as the strong room-temperature PL and abundant valley-selective excitation of excitonic states in transition metal dichalcogenides (TMDs). [2,[4][5][6][7] Besides, a neutral exciton can capture an additional electron (e) or hole (h) to form a correlated threeparticle state (e-e-h or h-e-h), known as a trion (T), in the presence of excess carriers. [8] It has been demonstrated that trionic effect can strongly affect the PL properties of 2D materials through controlling the excess carriers' population. [9][10][11] However, the small trion binding energy in various TMDs (< 50 meV) greatly hinders the investigation of T emission in 2D systems. Taking the thermal fluctuation and dimensional confinement into account, it is arduous for people to observe T emission in non-monolayer TMDs at room temperature. [12] Recently, the radiative recombination of X 0 and T in black phosphorus (BP) has been studied yet. [13] Robust T emission in monolayer BP was observed at ≈1.63 eV at room temperature with the trion binding energy of ≈100 meV. [14] Although the trion binding energy of BP is considerable, the weak ambient stability, especially for monolayer or bilayer BP, limits its application in future trionic devices. More importantly, subject to the narrow bandgap, the PL emission of BP mainly lies in near-IR range.Violet phosphorus (VP), another allotrope of the elemental phosphorus, was first identified by W. Hittorf in 1865. [15] The subtle van der Waals (vdW) layered phosphorus bi-tubular structure leads to abundant exciton-related phenomena, for example, strong PL emissions in the visible range (from ≈1.67 to ≈2.50 eV) at room temperature. [16][17][18][19][20] Moreover, the thermal stability of VP is higher than that of BP, suggesting that VP and its low-dimensional counterparts are more promising candidates for the research of excitonic physics and optoelectronic applications. [17,21,22] Nevertheless, to the best of our knowledge, the trionic effect in VP have yet to be explored.In this work, we synthesized layered VP crystals by the chemical vapor transport (CVT) method and subsequently studied the trionic effect of the mechanically exfoliated VP flakes comprehensively. We found that a robust T emission contributes to the PL spectra of the VP crystals with a huge trion binding energy of ≈109 meV. Using a mild Ar + plasma post-treatment method, we observed a conversion from T emission to X 0 emission. Trionic effect is a vital excitonic physical phenomenon, which intensively affects the optical and optoelectronic properties of 2D materials. Violet phosphorus (VP) is another allotrope of elemental phosphorus with robust photoluminescence (PL) emission in the visible range. So far, experimental investigations of the excitonic behavior in VP are quite scarce. Herein, the evolution of the PL mechanism in synthesized VP crystals against the Ar + plasma exposure is investigated with emphasis on a conversion from trion to exciton emission. The estimated trion binding energy of VP is ≈109 meV, relatively larger than common la...

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