Room‐temperature Synthesis of Amorphous Molybdenum Oxide Nanodots with Tunable Localized Surface Plasmon Resonances

Two‐dimensional (2D) semiconductors have recently emerged as a remarkable class of plasmonic alternative to conventional noble metals. However, tuning of their plasmonic resonances towards different wavelengths in the visible‐light region with physical or chemical methods still remains challenging. In this work, we design a simple room‐temperature chemical reaction route to synthesize amorphous molybdenum oxide (MoO_3−x ) nanodots that exhibit strong localized surface plasmon resonances (LSPR) in the visible and near‐infrared region. Moreover, tunable plasmon resonances can be achieved in a wide range with the changing surrounding solvent, and accordingly the photoelectrocatalytic activity can be optimized with the varying LSPR peaks. This work boosts the light–matter interaction at the nanoscale and could enable photodetectors, sensors, and photovoltaic devices in the future.     

Surfactant‐Free Nonaqueous Synthesis of Plasmonic Molybdenum Oxide Nanosheets with Enhanced Catalytic Activity for Hydrogen Generation from Ammonia Borane under Visible Light

Plasmonic materials have drawn emerging interest, especially in nontraditional semiconductor nanostructures with earth‐abundant elements and low resistive loss. However, the actualization of highly efficient catalysis in plasmonic semiconductor nanostructures is still a challenge, owing to the presence of surface‐capping agents in their synthetic procedures. To fulfill this, a facile non‐aqueous procedure was employed to prepare well‐defined molybdenum oxide nanosheets in the absence of surfactants. The obtained MoO_3‐x nanosheets display intense absorption in a wide range attributed to the localized surface plasmon resonances, which can be tuned from the visible to the near‐infrared region. Herein, we demonstrate that such plasmonic semiconductor nanostructures could be used as highly efficient catalysts that dramatically enhance the hydrogen‐generation activity of ammonia borane under visible light irradiation.     

Room‐Temperature and Aqueous‐Phase Synthesis of Plasmonic Molybdenum Oxide Nanoparticles for Visible‐Light‐Enhanced Hydrogen Generation

A straightforward aqueous synthesis of MoO_3?x nanoparticles at room temperature was developed by using (NH₄)₆Mo₇O₂₄?4 H₂O and MoCl₅ as precursors in the absence of reductants, inert gas, and organic solvents. SEM and TEM images indicate the as‐prepared products are nanoparticles with diameters of 90–180 nm. The diffuse reflectance UV‐visible‐near‐IR spectra of the samples indicate localized surface plasmon resonance (LSPR) properties generated by the introduction of oxygen vacancies. Owing to its strong plasmonic absorption in the visible‐light and near‐infrared region, such nanostructures exhibit an enhancement of activity toward visible‐light catalytic hydrogen generation. MoO_3?x nanoparticles synthesized with a molar ratio of Mo^VI/Mo^V 1:1 show the highest yield of H₂ evolution. The cycling catalytic performance has been investigated to indicate the structural and chemical stability of the as‐prepared plasmonic MoO_3?x nanoparticles, which reveals its potential application in visible‐light catalytic hydrogen production.     

Direct Monitoring of Cell Membrane Vesiculation with 2D AuNP@MnO2 Nanosheet Supraparticles at the Single‐Particle Level

We herein demonstrate robust two‐dimensional (2D) UFO‐shaped plasmonic supraparticles made of gold nanoparticles (AuNPs) and MnO₂ nanosheets (denoted as AMNS‐SPs) for directly monitoring cell membrane vesiculation at the single‐particle level. Because the decorated MnO₂ nanosheets are ultrathin (4.2 nm) and have large diameters (230 nm), they are flexible enough for deformation and folding for parceling of the AuNPs during the endocytosis process. Correspondingly, the surrounding refractive index of the AuNPs increases dramatically, which results in a distinct red‐shift of the localized surface plasmon resonance (LSPR). Such LSPR modulation provides a convenient and accurate means for directly monitoring the dynamic interactions between 2D nanomaterials and cell membranes. Furthermore, for the endocytosed AMNS‐SPs, the subsequent LSPR blue‐shift induced by etching effects of reducing molecules is promising for exploring the local environment redox states at the single‐cell level.     

A Two-dimensional Amorphous Plasmonic Heterostructure of Pd/MoO3-x for Enhanced Photoelectrochemical Water Splitting Performance

Two-dimensional (2D) heterostructures based on localized surface plasmon resonance (LSPR) have a great potential for solar energy harvesting applications. Exploring 2D amorphous plasmonic heterostructures with high light absorption and catalytic activity is desirable yet challenging. Herein, 2D Pd/MoO_3-x amorphous heterostructures can be obtained by immobilizing Pd single atoms in unsaturated coordination sites of amorphous MoO_3-x, because of strong metal-support interactions, and it reaches a current density of 50 μA cm⁻² for photoelectrochemical response with good durability, and exhibits a high incident-photon-to-current-conversion efficiency (IPCE) of 14.8% at 460 nm. Such an enhanced catalytic effects are contributed to the enhanced light absorption in visible region and change of electronic structure owing to enhanced electron transfer through dominant Pd−O bonds, which facilitate water splitting. This work moves a step closer to the expansion of photovoltaic device with the high conversion efficiency for visible light for amorphous heterostructures.     

Selective and Tunable Near‐Infrared and Visible Light Transmittance of MoO3−x Nanocomposites with Different Crystallinity

In this Communication, we report MoO_3−x nanocomposites in which the near‐infrared and visible light transmittance can be selectively modulated through the crystallinity. The MoO_3−x nanocomposites were fabricated by a hydrothermal method, and their optical properties were characterized by UV‐Vis spectrometer. The obtained results proved the possibility to tune the nanocomposite's optical properties in the UV/Visible spectral region: crystalline MoO₃ mainly regulates the near‐infrared range (800–2600 nm), and amorphous MoO_3−x mainly changes the visible range from 350 nm to 800 nm and MoO_3−x , with semi‐crystalline structures mainly modulating around 800–1000 nm. These kinds of optical modulations could be attributed to small polar absorption, free electron absorption and plasmon absorption according to different crystallinity. Our work may create new possibilities for future applications such as photochromism, photocatalysis, and electrochromism.     

The Fundamentals of Real‐Time Surface Plasmon Resonance/Electrogenerated Chemiluminescence

We report the integration of surface plasmon resonance (SPR), cyclic voltammetry and electrochemiluminescence (ECL) responses to survey the interfacial adsorption and energy transfer processes involved in ECL on a plasmonic substrate. It was observed that a Tween 80/tripropylamine nonionic layer formed on the gold electrode of the SPR sensor, while enhancing the ECL emission process, affects the electron transfer process to the luminophore, Ru(bpy)₃²⁺, which in turn has an impact on the plasmon resonance. Concomitantly, the surface plasmon modulated the ECL intensity, which decreased by about 40 %, due to an interaction between the excited state of Ru(bpy)₃²⁺ and the plasmon. This occurred only when the plasmon was excited, demonstrating that the optically excited surface plasmon leads to lower plasmon‐mediated luminescence and that the plasmon interacts with the excited state of Ru(bpy)₃²⁺ within a very thin layer.     

Plasmonic Nickel–TiO2 Heterostructures for Visible‐Light‐Driven Photochemical Reactions

Plasmon‐mediated carrier transfer (PMCT) at metal–semiconductor heterojunctions has been extensively exploited to drive photochemical reactions, offering intriguing opportunities for solar photocatalysis. However, to date, most studies have been conducted using noble metals. Inexpensive materials capable of generating and transferring hot carriers for photocatalysis via PMCT have been rarely explored. Here, we demonstrate that the plasmon excitation of nickel induces the transfer of both hot electrons and holes from Ni to TiO₂ in a rationally designed Ni–TiO₂ heterostructure. Furthermore, it is discovered that the transferred hot electrons either occupy oxygen vacancies (V_O) or produce Ti³⁺ on TiO₂, while the transferred hot holes are located on surface oxygens at TiO₂. Moreover, the transferred hot electrons are identified to play a primary role in driving the degradation of methylene blue (MB). Taken together, our results validate Ni as a promising low‐cost plasmonic material for prompting visible‐light photochemical reactions.     

Surfactant‐Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible‐Light‐Enhanced Hydrogen Generation from Ammonia Borane

WO_3?x nanowires were successfully synthesized through a simple surfactant‐free solvothermal method. These nanowires exhibit strong plasmonic absorption in the visible and near‐infrared region owing to the abundant oxygen vacancies. The plasmon excitation of these WO_3?x nanowires provide five times enhancement on the hydrogen generation from ammonia borane.     

Boosting Electrocatalytic Hydrogen Evolution over Metal–Organic Frameworks by Plasmon‐Induced Hot‐Electron Injection

Efficient hydrogen evolution via electrocatalytic water splitting holds great promise in modern energy devices. Herein, we demonstrate that the localized surface plasmon resonance (LSPR) excitation of Au nanorods (NRs) dramatically improves the electrocatalytic hydrogen evolution activity of CoFe‐metal–organic framework nanosheets (CoFe‐MOFNs), leading to a more than 4‐fold increase of current density at ?0.236 V (vs. RHE) for Au/CoFe‐MOFNs composite under light irradiation versus in dark. Mechanistic investigations reveal that the hydrogen evolution enhancement can be largely attributed to the injection of hot electrons from AuNRs to CoFe‐MOFNs, raising the Fermi level of CoFe‐MOFNs, facilitating the reduction of H₂O and affording decreased activation energy for HER. This study highlights the superiority of plasmonic excitation on improving electrocatalytic efficiency of MOFs and provides a novel avenue towards the design of highly efficient water‐splitting systems under light irradiation.     

Synthesis of Plasmonic Group‐4 Nitride Nanocrystals by Solid‐State Metathesis

Ceramic nanoparticles that exhibit a plasmonic response are promising next‐generation photonic materials. In this contribution, a solid‐state metathesis method has been reported for the synthesis of Group 4 nitride (TiN, ZrN, and HfN) nanocrystals. A high‐temperature (1000 °C) reaction between Group 4 metal oxide (TiO₂, ZrO₂, and HfO₂) nanoparticles and magnesium nitride powder yielded nitride nanocrystals that were dispersible in water. A localized surface plasmonic resonance was observed in the near‐infrared region for TiN and in the visible region of light for ZrN and HfN nanocrystals. The frequency of the plasmon resonance was dependent on the refractive index of the solvent and the nanocrystal size.     

Synergistic Doping and Intercalation: Realizing Deep Phase Modulation on MoS2 Arrays for High‐Efficiency Hydrogen Evolution Reaction

A synergistic N doping plus PO₄^3? intercalation strategy is used to induce high conversion (ca. 41 %) of 2H‐MoS₂ into 1T‐MoS₂, which is much higher than single N doping (ca. 28 %) or single PO₄^3? intercalation (ca. 10 %). A scattering mechanism is proposed to illustrate the synergistic phase transformation from the 2H to the 1T phase, which was confirmed by synchrotron radiation and spherical aberration TEM. To further enhance reaction kinetics, the designed (N,PO₄^3?)‐MoS₂ nanosheets are combined with conductive vertical graphene (VG) skeleton forming binder‐free arrays for high‐efficiency hydrogen evolution reaction (HER). Owing to the decreased band gap, lower d‐band center, and smaller hydrogen adsorption/desorption energy, the designed (N,PO₄^3?)‐MoS₂/VG electrode shows excellent HER performance with a lower Tafel slope and overpotential than N‐MoS₂/VG, PO₄^3?‐MoS₂/VG counterparts, and other Mo‐base catalysts in the literature.     

Hybrid Fibers Made of Molybdenum Disulfide,Reduced Graphene Oxide,and Multi‐Walled Carbon Nanotubes for Solid‐State,Flexible, Asymmetric Supercapacitors

One of challenges existing in fiber‐based supercapacitors is how to achieve high energy density without compromising their rate stability. Owing to their unique physical, electronic, and electrochemical properties, two‐dimensional (2D) nanomaterials, e.g., molybdenum disulfide (MoS₂) and graphene, have attracted increasing research interest and been utilized as electrode materials in energy‐related applications. Herein, by incorporating MoS₂ and reduced graphene oxide (rGO) nanosheets into a well‐aligned multi‐walled carbon nanotube (MWCNT) sheet followed by twisting, MoS₂‐rGO/MWCNT and rGO/MWCNT fibers are fabricated, which can be used as the anode and cathode, respectively, for solid‐state, flexible, asymmetric supercapacitors. This fiber‐based asymmetric supercapacitor can operate in a wide potential window of 1.4 V with high Coulombic efficiency, good rate and cycling stability, and improved energy density.     

Preparation of Ultrathin Two‐Dimensional TixTa1−xSyOz Nanosheets as Highly Efficient Photothermal Agents

Although two‐dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high‐yield preparation of solution‐processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, Ti_x Ta_1−x S_y O_z (x =0.71, 0.49, and 0.30), from Ti_x Ta_1−x S₂ precursors. The nanosheet exhibits strong absorbance in the near‐infrared region, giving a large extinction coefficient of 54.1 L g⁻¹ cm⁻¹ at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid‐conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets.     

Water Transport with Ultralow Friction through Partially Exfoliated g‐C3N4 Nanosheet Membranes with Self‐Supporting Spacers

Two‐dimensional (2D) graphitic carbon nitride (g‐C₃N₄) nanosheets show brilliant application potential in numerous fields. Herein, a membrane with artificial nanopores and self‐supporting spacers was fabricated by assembly of 2D g‐C₃N₄ nanosheets in a stack with elaborate structures. In water purification the g‐C₃N₄ membrane shows a better separation performance than commercial membranes. The g‐C₃N₄ membrane has a water permeance of 29 L m⁻² h⁻¹ bar⁻¹ and a rejection rate of 87 % for 3 nm molecules with a membrane thickness of 160 nm. The artificial nanopores in the g‐C₃N₄ nanosheets and the spacers between the partially exfoliated g‐C₃N₄ nanosheets provide nanochannels for water transport while bigger molecules are retained. The self‐supported nanochannels in the g‐C₃N₄ membrane are very stable and rigid enough to resist environmental challenges, such as changes to pH and pressure conditions. Permeation experiments and molecular dynamics simulations indicate that a novel nanofluidics phenomenon takes place, whereby water transport through the g‐C₃N₄ nanosheet membrane occurs with ultralow friction. The findings provide new understanding of fluidics in nanochannels and illuminate a fabrication method by which rigid nanochannels may be obtained for applications in complex or harsh environments.     

Low‐Temperature Atomic Layer Deposition of MoS2 Films

Wet chemical screening reveals the very high reactivity of Mo(NMe₂)₄ with H₂S for the low‐temperature synthesis of MoS₂. This observation motivated an investigation of Mo(NMe₂)₄ as a volatile precursor for the atomic layer deposition (ALD) of MoS₂ thin films. Herein we report that Mo(NMe₂)₄ enables MoS₂ film growth at record low temperatures—as low as 60 °C. The as‐deposited films are amorphous but can be readily crystallized by annealing. Importantly, the low ALD growth temperature is compatible with photolithographic and lift‐off patterning for the straightforward fabrication of diverse device structures.     

Template Removal via Boudouard Equilibrium Allows for Synthesis of Mesostructured Molybdenum Compounds

Oxidative thermal removal of the polymeric templates is not trivial for molybdenum oxides and hampers mesostructuring of this material. At ambient oxygen fugacity, Mo^VI is the thermodynamically stable oxidation state and sublimation of MoO₃ leads to a quick loss of the mesostructure through Oswald ripening. Taking advantage of the Boudouard equilibrium allows to fix the oxygen fugacity at a level where non‐volatile MoO_2−x is stable while carbonaceous material may be oxidized by CO₂. Mesostructured MoO_2−x can be chemically converted into MoO₃ or MoN under retention of the mesostructure.     

Dinitrogen‐Molybdenum Complex Induces Dinitrogen Cleavage by One‐Electron Oxidation

Reported here is the N₂ cleavage of a one‐electron oxidation reaction using trans‐[Mo(depe)₂(N₂)₂] ( 1 ) (depe=Et₂PCH₂CH₂PEt₂), which is a classical molybdenum(0)‐dinitrogen complex supported by two bidentate phosphine ligands. The molybdenum(IV) terminal nitride complex [Mo(depe)₂N][BArf₄] ( 2 ) (BArf₄=B(3,5‐(CF₃)₂C₆H₃)₄) is synthesized by the one‐electron oxidation of 1 upon addition of a mild oxidant, [Cp₂Fe][BArf₄] (Cp=C₅H₅), and proceeds by N₂ cleavage from a Mo^II‐N=N‐Mo^II structure. In addition, the electrochemical oxidation reaction for 1 also cleaved the N₂ ligand to give 2 . The dimeric Mo complex with a bridging N₂ is detected by in situ resonance Raman and in situ UV‐vis spectroscopies during the electrochemical oxidation reaction for 1 . Density‐functional theory (DFT) calculations reveal that the unstable monomeric oxidized Mo^I species is converted into 2 via the dimeric structure involving a zigzag transition state.     

Two‐Dimensional Semiconductors Grown by Chemical Vapor Transport

Developing controlled approaches for synthesizing high‐quality two‐dimensional (2D) semiconductors is essential for their practical applications in novel electronics. The application of chemical vapor transport (CVT), an old single‐crystal growth technique, has been extended from growing 3D crystals to synthesizing 2D atomic layers by tuning the growth kinetics. Both single crystalline individual flakes and continuous films of 1 L MoS₂ were successfully obtained with CVT approach at low growth temperatures of 300–600 °C. The obtained 1 L MoS₂ exhibits high crystallinity and comparable mobility to mechanically exfoliated samples, as confirmed by both atomic resolution microscopic imaging and electrical transport measurements. Besides MoS₂, this method was also used in the growth of 2D WS₂, MoSe₂, Mo_x W_1−x S₂ alloys, and ReS₂, thus opening up a new way for the controlled synthesis of various 2D semiconductors.     

The First Molybdenum(VI) and Tungsten(VI) Oxoazides MO2(N3)2, MO2(N3)2⋅2 CH3CN, (bipy)MO2(N3)2, and [MO2(N3)4]2− (M=Mo,W)

Molybdenum(VI) and tungsten(VI) dioxodiazide, MO₂(N₃)₂ (M=Mo, W), were prepared through fluoride–azide exchange reactions between MO₂F₂ and Me₃SiN₃ in SO₂ solution. In acetonitrile solution, the fluoride–azide exchange resulted in the isolation of the adducts MO₂(N₃)₂⋅2 CH₃CN. The subsequent reaction of MO₂(N₃)₂ with 2,2′‐bipyridine (bipy) gave the bipyridine adducts (bipy)MO₂(N₃)₂. The hydrolysis of (bipy)MoO₂(N₃)₂ resulted in the formation and isolation of [(bipy)MoO₂N₃]₂O. The tetraazido anions [MO₂(N₃)₄]²⁻ were obtained by the reaction of MO₂(N₃)₂ with two equivalents of ionic azide. Most molybdenum(VI) and tungsten(VI) dioxoazides were fully characterized by their vibrational spectra, impact, friction, and thermal sensitivity data and, in the case of (bipy)MoO₂(N₃)₂, (bipy)WO₂(N₃)₂, [PPh₄]₂[MoO₂(N₃)₄], [PPh₄]₂[WO₂(N₃)₄], and [(bipy)MoO₂N₃]₂O by their X‐ray crystal structures.