Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

MoSe is a promising earth-abundant electrocatalyst for the hydrogen-evolution reaction (HER), even though it has received much less attention among the layered dichalcogenide (MX ) materials than MoS so far. Here, a novel hydrothermal-synthesis strategy is presented to achieve simultaneous and synergistic modulation of crystal phase and disorder in partially crystallized 1T-MoSe nanosheets to dramatically enhance their HER catalytic activity. Careful structural characterization and defect characterization using positron annihilation lifetime spectroscopy correlated with electrochemical measurements show that the formation of the 1T phase under a large excess of the NaBH reductant during synthesis can effectively improve the intrinsic activity and conductivity, and the disordered structure from a lower reaction temperature can provide abundant unsaturated defects as active sites. Such synergistic effects lead to superior HER catalytic activity with an overpotential of 152 mV versus reversible hydrogen electrode (RHE) for the electrocatalytic current density of j = -10 mA cm , and a Tafel slope of 52 mV dec . This work paves a new pathway for improving the catalytic activity of MoSe and generally MX -based electrocatalysts via a synergistic modulation strategy.

2D transition metal dichalcogenide (TMD) materials have been recognized as active platforms for surface‐enhanced Raman spectroscopy (SERS). Here, the effect of crystal structure (phase) transition is shown, which leads to altered electronic structures of TMD materials, on the Raman enhancement. Using thermally evaporated copper phthalocyanine, solution soaked rhodamine 6G, and crystal violet as typical probe molecules, it is found that a phase transition from 2H‐ to 1T‐phase can significantly increase the Raman enhancement effect on MoX2 (X = S, Se) monolayers through a predominantly chemical mechanism. First‐principle density functional theory calculations indicate that the significant enhancement of the Raman signals on metallic 1T‐MoX2 can be attributed to the facilitated electron transfer from the Fermi energy level of metallic 1T‐MoX2 to the highest occupied molecular orbital level of the probe molecules, which is more efficient than the process from the top of valence band of semiconducting 2H‐MoX2. This study not only reveals the origin of the Raman enhancement and identifies 1T‐MoSe2 and 1T‐MoS2 as potential Raman enhancement substrates, but also paves the way for designing new 2D SERS substrates via phase‐transition engineering.

Carbonyl sulfide (COS) is an air pollutant that causes acid rain, ozonosphere damage, and carbon dioxide (CO) generation. It is a heterocumulene and structural analogue of CO. Relevant to organic synthesis, it is a source of C═O or C═S groups and thus an ideal one-carbon (C1) building block for synthesizing sulfur-containing polymers through the similar route of CO copolymerization. In contrast, traditional synthesis of sulfur-containing polymers often involves the condensation of thiols with phosgene and ring-opening polymerization of cyclic thiocarbonates that are generally derived from thiols and phosgene; thus, COS/epoxide copolymerization is a "greener" route to supplement or supplant current processes for the production of sulfur-containing polymers. This Accounts highlights our efforts on the discovery of the selective formation of poly(monothiocarbonate)s from COS with epoxides via heterogeneous zinc-cobalt double metal cyanide complex (Zn-Co(III) DMCC) and homogeneous (salen)CrX complexes. The catalytic activity and selectivity of Zn-Co(III) DMCC for COS/epoxide copolymerization are similar to those for CO/epoxide copolymerization. (salen)CrX complexes accompanied by onium salts exhibited high activity and selectivity for COS/epoxide copolymerization under mild conditions, affording copolymers with >99% monothiocarbonate units and high tail-to-head content up to 99%. By way of contrast, these catalysts often show moderate or low activity for CO/epoxide copolymerization. Of note, a specialty of COS/epoxide copolymerization is the occurrence of an oxygen-sulfur exchange reaction (O/S ER), which may produce carbonate and dithiocarbonate units. O/S ER, which are induced by the metal-OH bond regenerated by chain transfer reactions, can be kinetically inhibited by changing the reaction conditions. We provide a thorough mechanistic understanding of the electronic/steric effect of the catalysts on the regioselectivity of COS copolymerization. The regioselectivity of the copolymerization originates from the solely nucleophilic attack of the sulfur anion to methylene of the epoxide, and thus, the chiral configuration of the monosubstituted epoxides is retained. COS-based copolymers are highly transparent sulfur-containing polymers with excellent optical properties, such as high refractive index and Abbe number. Thanks to their good solubility and many available epoxides, COS/epoxide copolymers can potentially be a new applicable optical material. Very recently, crystalline COS-based polymers with or without chiral carbons have been synthesized, which may further expand the scope of application of these new materials.

We report the first example of a regioregular and fully alternating poly (propylene monothiocarbonate) (PPMTC) from the well-controlled copolymerization of two asymmetric monomers, carbonyl sulfide and racemic propylene oxide, using (Salen)CrCl in conjunction with bis(triphenylphosphoranylidene)ammonium chloride. The maximum turnover of frequency of this catalyst system was 332 h −1 at 25°C. The contents of monothiocarbonate and tail-to-head linkages of PPMTC were up to 100% (based on 1 H NMR spectra) and 99.0% (based on 13 C NMR spectra), respectively. PPMTC samples have number-average molecular weight (M n ) up to 25.3 kg/mol with polydispersity index of 1.41. The very low decomposition temperature of 137°C and high refractive index of 1.63 of PPMTC make it a potential scarifying optical adhesive.

Spinel-type oxides have been found to be active electrocatalysts for OER. However, their semiconductor character severely limits their catalytic performance. Herein, we report a facile solvothermal pathway for the synthesis of spinel-type Ni x Fe 3−x O 4 oxides/Ni metal nanocomposites. The good electrical contact between the metal and semiconductor oxide interface and welltuned compositions of Ni x Fe 3−x O 4 spinel oxides are crucial to achieve better OER performance. Specifically, the Ni x Fe 3−x O 4 /Ni nanocomposite sample prepared from a metal precursor ratio of y = 0.15 [y = Fe/(Fe + Ni)] that results in an x value of about 0.36 exhibits catalytic activity with an overpotential of 225 mV to achieve an electrocatalytic current density of j = 10 mA cm −2 and a Tafel slope of 44 mV dec −1 in alkaline electrolyte. This study not only provides new perspectives to designing nanocomposite catalysts for OER but also opens a promising avenue for further enhancing electrocatalytic performance via interface and composition engineering.

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The structure evolution and oxidation behavior of ZrB 2 -SiC composites in air from room temperature to ultrahigh temperature were investigated using furnace testing, arc jet testing, and thermal gravimetric analysis (TGA). The oxide structure changed with the increasing temperature. SiC content has no apparent influence on the evolution of structure during the oxidation of ZrB 2 -SiC below 1600°C. However, the evolution of structure for ZrB 2 -SiC above 1800°C was significantly affected by the SiC content. The formation of the SiC depleted layer in the ZrB 2 -SiC system not only depends on the surrounding conditions of pressure and temperature but also on the structure distribution of the SiC in the ZrB 2 matrix. The apparent recrystallization of the ZrO 2 occurred above 1800°C. The SiC content should be controlled at ∼16% in the ZrB 2 -SiC system for the ultrahigh-temperature application. The mechanisms of the structure evolution during oxidation in air were also analyzed.