Phase‐Controlled Synthesis of Monolayer W1−xRexS2 Alloy with Improved Photoresponse Performance

Tuning bandgap and phases in the ternary 2D transition metal dichalcogenides (TMDs) alloys has opened up unexpected opportunities to engineer optoelectronic properties and explore potential applications. In this work, a salt‐assisted chemical deposition vapor (CVD) growth strategy is reported for the creation of high‐quality monolayer W_1?xRe_xS₂ alloys to fulfill a readily phase control from 1H to DT by changing the ratio of Re and W precursors. The structures and chemical compositions of doping alloys are confirmed by combining atomic resolution scanning transmission electron microscopy‐annular dark field imaging with energy dispersive X‐ray spectroscopy (EDS) and X‐ray photoelectron spectroscopy, matching well with the calculated results. The field‐effect transistors (FETs) devices fabricated based on 1H‐W_0.9Re_0.1S₂ monolayer exhibit a n‐type semiconducting behavior with the mobility of 0.4 cm² V^?1 s^?1. More importantly, the FETs show high‐performance responsivity with a value of 17 µA W^?1 in air, which is superior to that of monolayer CVD‐grown WS₂. This work paves the way toward synthesizing monolayer ternary alloys with controlled phases for potential optoelectronic applications.     

H+‐Insertion Boosted α‐MnO2 for an Aqueous Zn‐Ion Battery

Rechargeable Zn/MnO₂ batteries using mild aqueous electrolytes are attracting extensive attention due to their low cost, high safety, and environmental friendliness. However, the charge‐storage mechanism involved remains a topic of controversy so far. Also, the practical energy density and cycling stability are still major issues for their applications. Herein, a free‐standing α‐MnO₂ cathode for aqueous zinc‐ion batteries (ZIBs) is directly constructed with ultralong nanowires, leading to a rather high energy density of 384 mWh g^?1 for the entire electrode. Greatly, the H⁺/Zn²⁺ coinsertion mechanism of α‐MnO₂ cathode for aqueous ZIBs is confirmed by a combined analysis of in situ X‐ray diffractometry, ex situ transmission electron microscopy, and electrochemical methods. More interestingly, the Zn²⁺‐insertion is found to be less reversible than H⁺‐insertion in view of the dramatic capacity fading occurring in the Zn²⁺‐insertion step, which is further evidenced by the discovery of an irreversible ZnMn₂O₄ layer at the surface of α‐MnO₂. Hence, the H⁺‐insertion process actually plays a crucial role in maintaining the cycling performance of the aqueous Zn/α‐MnO₂ battery. This work is believed to provide an insight into the charge‐storage mechanism of α‐MnO₂ in aqueous systems and paves the way for designing aqueous ZIBs with high energy density and long‐term cycling ability.     

Self‐Formed Electronic/Ionic Conductive Fe3S4 @ S @ 0.9Na3SbS4⋅0.1NaI Composite for High‐Performance Room‐Temperature All‐Solid‐State Sodium–Sulfur Battery

Fe₃S₄ @ S @ 0.9Na₃SbS₄?0.1NaI composite cathode is prepared through one‐step wet‐mechanochemical milling procedure. During milling process, ionic conduction pathway is self‐formed in the composite due to the formation of 0.9Na₃SbS₄?0.1NaI electrolyte without further annealing treatment. Meanwhile, the introduction of Fe₃S₄ can increase the electronic conductivity of the composite cathode by one order of magnitude and nearly double enhance the ionic conductivities. Besides, the aggregation of sulfur is effectively suppressed in the obtained Fe₃S₄ @ S @ 0.9Na₃SbS₄?0.1NaI composite, which will enhance the contact between sulfur and 0.9Na₃SbS₄?0.1NaI electrolyte, leading to a decreased interfacial resistance and improving the electrochemical kinetics of sulfur. Therefore, the resultant all‐solid‐state sodium–sulfur battery employing Fe₃S₄ @ S @ 0.9Na₃SbS₄?0.1NaI composite cathode shows discharge capacity of 808.7 mAh g^?1 based on Fe₃S₄@S and a normalized discharge capacity of 1040.5 mAh g^?1 for element S at 100 mA g^?1 for 30 cycles at room temperature. Moreover, the battery also exhibits excellent cycling stability with a reversible capacity of 410 mAh g^?1 at 500 mA g^?1 for 50 cycles, and superior rate capability with capacities of 952.4, 796.7, 513.7, and 445.6 mAh g^?1 at 50, 100, 200, and 500 mA g^?1, respectively. This facile strategy for sulfur‐based composite cathode is attractive for achieving room‐temperature sodium–sulfur batteries with superior electrochemical performance.     

α‐Ni(OH)2 Originated from Electro‐Oxidation of NiSe2 Supported by Carbon Nanoarray on Carbon Cloth for Efficient Water Oxidation

Development of effective oxygen evolution reaction (OER) electrocatalysts has been intensively studied to improve water splitting efficiency and cost effectiveness in the last ten years. However, it is a big challenge to obtain highly efficient and durable OER electrocatalysts with overpotentials below 200 mV at 10 mA cm^?2 despite the efforts made to date. In this work, the successful synthesis of supersmall α‐Ni(OH)₂ is reported through electro‐oxidation of NiSe₂ loaded onto carbon nanoarrays. The obtained α‐Ni(OH)₂ shows excellent activity and long‐term stability for OER, with an overpotential of only 190 mV at the current density of 10 mA cm^?2, which represents a highly efficient OER electrocatalyst. The excellent activity could be ascribed to the large electrochemical surface area provided by the carbon nanoarray, as well as the supersmall size (≈10 nm) of α‐Ni(OH)₂ which possess a large number of active sites for the reaction. In addition, the phase evolution of α‐Ni(OH)₂ from NiSe₂ during the electro‐oxidation process was monitored with in situ X‐ray absorption fine structure (XAFS) analysis.     

Probing the Domain Architecture in 2D α‐Mo2C via Polarized Raman Spectroscopy

MXenes are emerging 2D materials with intriguing properties such as excellent stability and high conductivity. Here, a systematic study on the Raman spectra of 2D α‐Mo₂C (molybdenum carbide), a promising member in MXene family, is conducted. Six experimentally observed Raman modes from ultrathin α‐Mo₂C crystal are first assigned with the assistance of phonon dispersion calculated from density functional theory. Angle‐resolved polarized Raman spectroscopy indicates the anisotropy of α‐Mo₂C in the b–c plane. Raman spectroscopy is further used to study the unique domain structures of 2D α‐Mo₂C crystals grown by chemical vapor deposition. A Raman mapping investigation suggests that most of the α‐Mo₂C flakes contain multiple domains and the c‐axes of neighboring domains tend to form a 60° or 120° angle, due to the weak Mo? C bonds in this interstitial carbide and the low formation energy of the carbon chains along three equivalent directions. This study demonstrates that polarized Raman spectroscopy is a powerful and effective way to characterize the domain structures in α‐Mo₂C, which will facilitate the further exploration of the domain‐structure‐related properties and potential applications of α‐Mo₂C.     

Doping-induced microstructural, textural and optical properties of In2Ti1−xVxO5+δ semiconductors and their role in the photocatalytic splitting of water

The physicochemical properties of V-doped indium titanates (In₂Ti_1−xV_xO_5+δ, 0.0 ≤ x ≤ 0.2) were investigated by using XPS, powder XRD, UV–vis, SEM and luminescence spectroscopy techniques. The Rietveld refinement of XRD data revealed that even though the V-containing samples were isostructural with In₂TiO₅ (orthorhombic space group Pnma), a systematic x-dependent variation was noticeable in the Ti–O bond lengths in [TiO₆] octahedral units, cell parameters and in the value of δ. XPS results confirmed the coexistence of V⁵⁺ and V⁴⁺ states, leading thereby to an enhancement in oxygen non-stoichiometry in the doped samples. A loading-dependent progressive shift from 400 to 750 nm was also observed in the onset of the absorption edge, indicating a significant narrowing of the band gap. Furthermore, the samples with higher V-content were comprised of the grain clusters having larger size and an irregular shape. The UV–vis, photoluminescence and thermoluminescence studies indicate that the doping-induced lattice defects may give rise to certain closely spaced acceptor/donor energy levels in between the band gap of host matrix. The indium titanates are found to serve as stable photocatalysts for water splitting under visible light, where oxygen was the major reaction product. The role of microstructural and morphological properties in the photocatalytic activity is discussed.