Synthesis, Characterization, and Hierarchical Organization of Tungsten Oxide Nanorods: Spreading Driven by Marangoni Flow

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

Lithium confinement and dynamics in hexagonal and monoclinic tungsten oxide nanocrystals: a 7Li solid state NMR study

Mixed-valence tungsten bronzes A_xWO₃ (A = alkali metal, NH₄⁺, etc.) are a series of compounds with adaptive structural and compositional features that make them a hot research topic in thermoelectrics, electrochromics, catalysis or energy applications in battery electrodes. The mixed hexagonal lithium ammonium bronze Li_x(NH₄)_yWO₃ is a new member of this structural family whose properties are compared to those of the pure hexagonal tungsten bronze (NH₄)_xWO₃. Surface and structural (nanoconfined) Li⁺ cations were characterized by ⁷Li single pulse excitation and ¹H-⁷Li cross-polarization (CP) NMR experiments. CP build-up curves and two-dimensional heteronuclear correlation solid-state NMR techniques provide information about the spatial connectivity between different proton and Li⁺ species. At 500 °C the bronze structurally transforms from the hexagonal to a monoclinic phase, and defects are formed that are characterized through the Li⁺ environment. ⁷Li exchange spectroscopy (EXSY) NMR experiments provide information about the chemical exchange between the lithium species. The measured ⁷Li T₁ and T₂ relaxation time constants and the T₁/T₂ ratio allow characterizing the local strength of Li⁺ binding.

Magneli-type tungsten oxide nanorods as catalysts for the selective oxidation of organic sulfides

Selective oxidation of thioethers is an important reaction to obtain sulfoxides as synthetic intermediates for applications in the chemical industry, medicinal chemistry and biology or the destruction of warfare agents. The reduced Magneli-type tungsten oxide WO_3-x possesses a unique oxidase-like activity which facilitates the oxidation of thioethers to the corresponding sulfoxides. More than 90% of the model system methylphenylsulfide could be converted to the sulfoxide with a selectivity of 98% at room temperature within 30 minutes, whereas oxidation to the corresponding sulfone was on a time scale of days. The concentration of the catalyst had a significant impact on the reaction rate. Reasonable catalytic effects were also observed for the selective oxidation of various organic sulfides with different substituents. The WO_3-x nanocatalysts could be recycled at least 5 times without decrease in activity. We propose a metal oxide-catalyzed route based on the clean oxidant hydrogen peroxide. Compared to other molecular or enzyme catalysts the WO_3-x system is a more robust redox-nanocatalyst, which is not susceptible to decomposition or denaturation under standard conditions. The unique oxidase-like activity of WO_3-x can be used for a wide range of applications in synthetic, environmental or medicinal chemistry.

The contact angle of an evaporating droplet of a binary solution on a super wetting surface

We study the dynamics of the contact angle of a droplet of a binary solution evaporating on a super wetting surface. Recent experiments have shown that although the equilibrium contact angle of such a droplet is zero, the contact angle can show complex time dependence before reaching the equilibrium value. We analyse such phenomena by extending our previous theory for the dynamics of an evaporating single component droplet to a double component droplet. We show that the time dependence of the contact angle can be quite complex. Typically, it first decreases slightly, and then increases and finally decreases again. Under certain conditions, we find that the contact angle remains constant over a certain period of time during evaporation. We study how the plateau or peak contact angle depends on the initial composition and the humidity. This theory explains the experimental results reported previously.

Gram-scale selective synthesis of WO3-x nanorods and (NH4)xWO3 ammonium tungsten bronzes with tunable plasmonic properties

Localized surface plasmon resonance properties in unconventional materials like metal oxides or chalcogenide semiconductors have been studied for use in signal detection and analysis in biomedicine and photocatalysis. We devised a selective synthesis of the tungsten oxides WO_3-x and (NH₄)_xWO₃ with tunable plasmonic properties. We selectively synthesized WO_3-x nanorods with different aspect ratios and hexagonal tungsten bronzes (NH₄)_xWO₃ as truncated nanocubes starting from ammonium metatungstate (NH₄)₆H₂W₁₂O₄₀·xH₂O. Both particles form from the same nuclei at temperatures >200 °C; monomer concentration and surfactant ratio are essential variables for phase selection. (NH₄)_xWO₃ was the preferred reaction product only for fast heating rates (25 K min^-1), slow stirring speeds (∼150 rpm) and high precursor concentrations. A proton nuclear magnetic resonance (¹H-NMR) spectroscopic study of the reaction mechanism revealed that oleyl oleamide, formed from oleic acid and oleylamine upon heating, is a key factor for the selective formation of WO_3-x nanorods. Since oleic acid and oleylamine are standard surfactants for the wet chemical synthesis of many metal and oxide nanoparticles, the finding that oleyl oleamide acts as a chemically active reagent above 250 °C may have implications for many nanoparticle syntheses. Oriented attachment of polyoxotungstate anions is proposed as a model to rationalize phase selectivity. Magic angle spinning (MAS) ¹H-NMR and powder X-ray diffraction (PXRD) studies of the bronze after annealing under (non)inert conditions revealed an oxidative phase transition. WO_3-x and (NH₄)_xWO₃ show a strong plasmon absorption for near infra-red light between 800 and 3300 nm. The maxima of the plasmon bands shift systematically with the nanocrystal aspect ratio.

Fabrication and application of macroscopic nanowire aerogels

Assembly of nanowires into three-dimensional macroscopic aerogels not only bridges a gap between nanowires and macroscopic bulk materials but also combines the benefits of two worlds: unique structural features of aerogels and unique physical and chemical properties of nanowires, which has triggered significant progress in the design and fabrication of nanowire-based aerogels for a diverse range of practical applications. This article reviews the methods developed for processing nanowires into three-dimensional monolithic aerogels and the applications of the resultant nanowire aerogels in many emerging fields. Detailed discussions are given on gelation mechanisms involved in every preparation method and the pros and cons of the different methods. Furthermore, we systematically scrutinize the application of nanowire-based aerogels in the fields of thermal management, energy storage and conversion, catalysis, adsorbents, sensors, and solar steam generation. The unique benefits offered by nanowire-based aerogels in every application field are clarified. We also discuss how to improve the performance of nanowire-based aerogels in those fields by engineering the compositions and structures of the aerogels. Finally, we provide our perspectives on future development of nanowire-based aerogels.

In-situ preparation of molybdenum trioxide-silver composites for the improved photothermal catalytic performance of cyclohexane oxidation

The selective catalytic oxidation of cyclohexane has important theoretical and practical application value. However, high conversion rate and high selectivity are difficult to achieve simultaneously by conventional catalytic system. In this work, blue molybdenum trioxide (MoO₃) nanorods with oxygen vacancies were prepared by hydrothermal method using hydrated molybdic acid as a precursor under the reduction of formic acid, and in-situ produced MoO₃-silver (MoO₃-Ag) composites were further used in the photothermal catalytic oxidation of cyclohexane with high conversion and high selectivity using dry air as oxidant. The results showed that the best conversion rate of cyclohexanone and cyclohexanol (KA oil) could reach 8.6% with the selectivity of 99.0%. The excellent catalytic performance of MoO₃-Ag composites can be attributed to the significantly increased visible and near-infrared light absorption caused by the plasma resonance effect of Ag nanoparticles and oxygen vacancies, and the prevented charge recombination by MoO₃-Ag Schottky heterojunction. This work provides new reference solutions for the design and preparation of high-performance photothermal catalysts for the selective oxidation of hydrocarbons.

Porous silver-coated pNIPAM-co-AAc hydrogel nanocapsules

This paper describes the preparation and characterization of a new type of core-shell nanoparticle in which the structure consists of a hydrogel core encapsulated within a porous silver shell. The thermo-responsive hydrogel cores were prepared by surfactant-free emulsion polymerization of a selected mixture of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc). The hydrogel cores were then encased within either a porous or complete silver shell for which the localized surface plasmon resonance (LSPR) extends from visible to near-infrared (NIR) wavelengths (i.e., λ_max varies from 550 to 1050 nm, depending on the porosity), allowing for reversible contraction and swelling of the hydrogel via photothermal heating of the surrounding silver shell. Given that NIR light can pass through tissue, and the silver shell is porous, this system can serve as a platform for the smart delivery of payloads stored within the hydrogel core. The morphology and composition of the composite nanoparticles were characterized by SEM, TEM, and FTIR, respectively. UV-vis spectroscopy was used to characterize the optical properties.

The Effect of Agglomeration Reduction on the Tribological Behavior of WS2 and MoS2 Nanoparticle Additives in the Boundary Lubrication Regime

This study investigates the impact of different surfactants and dispersion techniques on the friction and wear behavior of WS2 and MoS2 nanoparticles additives in a Polyalphaolefin (PAO) base oil under boundary lubrication conditions. The nanoparticles were dispersed using Oleic acid (OA) and Polyvinylpyrrolidone (PVP) to investigate their impact on particle agglomeration. The size distribution of the dispersed nanoparticles in PAO was measured by dynamic light scattering. The nanoparticles treated using PVP resulted in the most stable particle size. Friction studies showed that nanoparticle agglomeration reduction and the homogeneity of the suspension did not significantly impact the friction reduction behavior of the lubricant. Reciprocating wear experiments showed that, for our test conditions, both WS2 and MoS2 nano additives exhibited maximum wear depth reduction (45%) when using the PVP surface treatment compared to base oil. The wear results confirmed the significance of minimizing agglomeration and promoting high dispersion in promoting favorable wear resistance under boundary lubricant conditions. Analysis of the wear surfaces showed that a tribofilm formation was the primary wear reduction mechanism for WS2 particles treated by PVP while, in the case of MoS2 treated by PVP, the mechanism was load sharing via particles rolling and/or sliding at the interface.

Transforming Monolayer Transition-Metal Dichalcogenide Nanosheets into One-Dimensional Nanoscrolls with High Photosensitivity

One-dimensional (1D) nanoscrolls derived from two-dimensional (2D) nanosheets own unusual physical and chemical properties that arise from the spiraled 1D morphology and the atomic thin 2D building blocks. Unfortunately, preparation of large-sized nanoscrolls of transition-metal dichalcogenides (TMDCs) remains a big challenge, which greatly restricts the fabrication of single-scroll devices for their fundamental studies and further applications. In this work, we report a universal and facile method, by making use of the evaporation process of volatile organic solvent, to prepare TMDC (e.g., MoS₂ and WS₂) nanoscrolls with lengths of several tens to one hundred micrometers from their 2D precursors presynthesized by chemical vapor deposition on Si/SiO₂. Both atomic force microscopy and electron microscopy characterizations confirmed the spirally rolledup structure in the resulting nanoscrolls. An interlayer spacing of as small as ∼0.65 nm was observed, suggesting the strong coupling between adjacent layers, which was further evidenced by the emergence of new breathing mode peaks in the ultralow frequency Raman spectrum. Importantly, compared with the photodetector fabricated from a monolayer MoS₂ or WS₂ nanosheet, the device based on an MoS₂ or WS₂ nanoscroll showed the much enhanced performance, respectively, with the photosensitivity greatly increased up to 2 orders of magnitude. Our work suggests that turning 2D TMDCs into 1D scrolls is promising in achieving high performances in various electronic/optoelectronic applications, and our general method can be extended to the preparation of large-sized nanoscrolls of other kinds of 2D materials that may bring about new properties and phenomena.

Marangoni Contraction of Evaporating Sessile Droplets of Binary Mixtures

The Marangoni contraction of sessile drops of a binary mixture of a volatile and a nonvolatile liquid has been investigated experimentally and theoretically. The origin of the contraction is the locally inhomogeneous evaporation rate of sessile drops. This leads to surface tension gradients and thus to a Marangoni flow. Simulations show that the interplay of Marangoni flow, capillary flow, diffusive transport, and evaporative losses can establish a quasistationary drop profile with an apparent nonzero contact angle even if both liquid components individually wet the substrate completely. Experiments with different solvents, initial mass fractions, and gaseous environments reveal a previously unknown universal power-law relation between the apparent contact angle and the relative undersaturation of the ambient atmosphere: θ_app ∼ (RH_eq - RH)^1/3. This experimentally observed power law is in quantitative agreement with simulation results. The exponent can also be inferred from a scaling analysis of the hydrodynamic-evaporative evolution equations of a binary mixture of liquids with different volatilities.

Solution Growth of Modified Ultrathin W18 O49 Nanobelts with Enhanced Chemical Activity against Alkylamine Radicals

Group IVB ions (Ti⁴⁺ and Zr⁴⁺ ) are employed to modify solution growth of W₁₈ O₄₉ nanocrystals. In the presence of group IVB ions, the morphology of the produced monoclinic W₁₈ O₄₉ nanocrystals can be modulated from inhomogeneous shapes to uniform an ultrathin nanobelt with the (1‾ 01) lattice plane being the main exposure surface. Interestingly, the ultrathin W₁₈ O₄₉ nanobelts exhibit a unique chemical activity against alkylamine radicals, probably originated from the highly increased exposure of the (1‾ 01) plane with extra tungsten and oxygen atoms inserted in the regular lattice, which makes this nanomaterial an active photocatalyst for the N-de-ethylation reaction of the rhodamine B molecules, as well as a promising gas sensor matrix with selective responses to alkylamine molecules, including diethylamine, dimethylamine, triethylamine and trimethylamine. These results could provide useful inspiration for developing other functional nanomaterials with technical significance via modification of their morphologies and chemical properties.

Colloidal metal oxide nanocrystals as charge transporting layers for solution-processed light-emitting diodes and solar cells

This review bridges the chemistry of colloidal oxide nanocrystals and their application as charge transporting interlayers in solution-processed optoelectronics.

Facile Preparation of Multifunctional WS2 /WOx Nanodots for Chelator-Free 89 Zr-Labeling and In Vivo PET Imaging

While position emission tomography (PET) is an important molecular imaging technique for both preclinical research and clinical disease diagnosis/prognosis, chelator-free radiolabeling has emerged as a promising alternative approach to label biomolecules or nanoprobes in a facile way. Herein, starting from bottom-up synthesized WS2 nanoflakes, this study fabricates a unique type of WS2 /WOx nanodots, which can function as inherent hard oxygen donor for stable radiolabeling with Zirconium-89 isotope (89 Zr). Upon simply mixing, 89 Zr can be anchored on the surface of polyethylene glycol (PEG) modified WS2 /WOx (WS2 /WOx -PEG) nanodots via a chelator-free method with surprisingly high labeling yield and great stability. A higher degree of oxidation in the WS2 /WOx -PEG sample (WS2 /WOx (0.4)) produces more electron pairs, which would be beneficial for chelator-free labeling of 89 Zr with higher yields, suggesting the importance of surface chemistry and particle composition to the efficiency of chelator-free radiolabeling. Such 89 Zr-WS2 /WOx (0.4)-PEG nanodots are found to be an excellent PET contrast agent for in vivo imaging of tumors upon intravenous administration, or mapping of draining lymph nodes after local injection.

Large-scale synthesis of ultrathin tungsten oxide nanowire networks: an efficient catalyst for aerobic oxidation of toluene to benzaldehyde under visible light

As a very important chemical raw material, the selective formation of benzaldehyde from toluene at preparative or industrial levels requires the use of highly corrosive chlorine and high reaction temperatures, which severely corrodes equipment, pollutes the environment, and consumes a lot of energy. Herein, we report a robust and highly active catalyst for the benzaldehyde evolution reaction that is constructed by the surfactant-free growth of oxygen vacancy-rich W18O49 ultrathin nanowire networks. Under atmospheric pressure and visible-light irradiation, the new catalyst can selectively (92% selectivity) catalyze the aerobic oxidation of toluene to benzaldehyde with yields of above 95%.

Evaporation-Induced Self-Assembly of Ultrathin Tungsten Oxide Nanowires over a Large Scale for Ultraviolet Photodetector

Self-assembly of inorganic nanowires on a large scale directly on a substrate represents a great challenge. Starting from colloidally stable dispersions of ultrathin tungsten oxide nanowires, we successfully assemble the nanowires on a centimeter scale on flat or patterned substrates by a simple evaporation-induced self-assembly method. The capillary flow generated during the evaporation is responsible for the assembly of the nanowires. The concentration of the nanowire dispersion has a significant influence on the self-assembly behavior. Well-aligned tungsten oxide nanowire thin films are achieved when the concentration of the dispersion is in the range from 0.5 to 3.0 mg/mL. While at higher concentrations disordered nanowire thin films with cracks are formed, lower concentrations do not result in the formation of a continuous thin film. A macroscopic device based on the self-assembled tungsten oxide nanowires is fabricated, exhibiting good performance for UV light detection. Our results may pave the road for integrating aligned ultrathin semiconductor nanowires into macroscopic devices for optoelectronic applications.

Self-supported tungsten/tungsten dioxide nanowires array as an efficient electrocatalyst in the hydrogen evolution reaction

A tungsten/tungsten dioxide nanowires array was constructed on a carbon paper through the thermal annealing of tungsten trioxide, and was proven to be an efficient hydrogen evolution cathode with strong durability in acidic solutions.

Morphology-controlled synthesis of WO2.72 nanostructures and their photocatalytic properties

WO_2.72 nanowires and urchin-like WO_2.72 nanostructures exhibited enhanced photocatalytic activities for organic pollutants degradation compared to commercial nanostructured tungsten oxide.

Ligand-free and size-controlled synthesis of oxygen vacancy-rich WO3−x quantum dots for efficient room-temperature formaldehyde gas sensing

The present work provides an effective synthetic route for oxygen vacancy-rich WO_3−x QDs. More importantly, the WO_3−x QDs displayed high formaldehyde sensitivity with a detection limit of 1.5 ppm at room temperature.

Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy

The conversion, storage, and utilization of renewable energy have all become more important than ever before as a response to ever-growing energy and environment concerns. The performance of energy-related technologies strongly relies on the structure and property of the material used. The earth-abundant family of tungsten oxides (WOx ≤3 ) receives considerable attention in photocatalysis, electrochemistry, and phototherapy due to their highly tunable structures and unique physicochemical properties. Great breakthroughs have been made in enhancing the optical absorption, charge separation, redox capability, and electrical conductivity of WOx ≤3 through control of the composition, crystal structure, morphology, and construction of composite structures with other materials, which significantly promotes the efficiency of processes and devices based on this material. Herein, the properties and synthesis of WOx ≤3 family are reviewed, and then their energy-related applications are highlighted, including solar-light-driven water splitting, CO2 reduction, and pollutant removal, electrochromism, supercapacitors, lithium batteries, solar and fuel cells, non-volatile memory devices, gas sensors, and cancer therapy, from the aspect of function-oriented structure design and control.

Aerosol-assisted chemical vapor deposition of tungsten oxide films and nanorods from oxo tungsten(VI) fluoroalkoxide precursors

Aerosol-assisted chemical vapor deposition (AACVD) of WOx was demonstrated using the oxo tungsten(VI) fluoroalkoxide single-source precursors, WO[OCCH3(CF3)2]4 and WO[OC(CH3)2CF3]4. Substoichiometric amorphous tungsten oxide thin films were grown on indium tin oxide (ITO) substrates in nitrogen at low deposition temperature (100-250 °C). At growth temperatures above 300 °C, the W18O49 monoclinic crystalline phase was observed. The surface morphology and roughness, visible light transmittance, electrical conductivity, and work function of the tungsten oxide materials are reported. The solvent and carrier gas minimally affected surface morphology and composition at low deposition temperature; however, material crystallinity varied with solvent choice at higher temperatures. The work function of the tungsten oxide thin films grown between 150 and 250 °C was determined to be in the range 5.0 to 5.7 eV, according to ultraviolet photoelectron spectroscopy (UPS).

Self-assembly of large-scale P3HT patterns by confined evaporation in the capillary tube

Large-scale P3HT stripe patterns based on a controlled evaporation self-assembly method in a capillary tube have been fabricated.

Self-assembly of segmented anisotropic particles: tuning compositional anisotropy to form vertical or horizontal arrays

Columnar arrays of anisotropic nano- and microparticles, in which the long axes of the particles are oriented perpendicular to the substrate, are of interest for photovoltaics and other applications. Array assembly typically requires applied electric or magnetic fields and/or controlled drying, which are challenging over large areas. Here, we describe a scalable approach to self-assemble multicomponent nanowires into columnar arrays. Self-assembly of partially etched nanowires (PENs) occurred spontaneously during sedimentation from suspension, without drying or applied fields. PENs, which have segments that are either gold or "empty" (solvent-filled) surrounded by a silica shell, were produced from striped metal nanowires by first coating with silica and then removing sacrificial segments by acid etching. Electrostatic repulsion between the particles was necessary for array assembly; however, details of PEN surface chemistry were relatively unimportant. The aspect ratio and relative center of mass (COM) of the PENs were important for determining whether the PEN long axes were vertically or horizontally aligned with respect to the underlying substrate. Arrays with predominantly vertically aligned particles were achieved for PENs with a large offset in COM relative to the geometric center, while other types of PENs formed horizontal arrays. Assemblies were formed over >10 cm(2) areas, with over 60% of particles standing. We assessed array uniformity and reproducibility by imaging many positions within each sample and performing multiple assemblies of differently segmented PENs. This work demonstrates the versatility of gravity-driven PEN array assembly and provides a framework for designing other anisotropic particle systems that self-assemble into columnar arrays.

Vertical arrays of anisotropic particles by gravity-driven self-assembly

Anisotropic particles assemble to spontaneously form columnar arrays. Hybrid nanotube/nanowire particles (silica nanotubes partially filled with metallic cores) deposit with their denser metallic ends towards the surface, orienting them vertically. Up to 84% are observed to be standing over a 0.64 cm(2) area within 15 min. Standing percentage is found to be dependent on particle surface concentration.

Single-crystalline α-Fe2O3 oblique nanoparallelepipeds: high-yield synthesis, growth mechanism and structure enhanced gas-sensing properties

In this paper, single-crystalline α-Fe2O3 oblique nanoparallelepipeds are fabricated in high yield via a facile surfactant-free hydrothermal method, which involves oriented aggregation and Ostwald ripening. The obtained nanocrystals have exposed facets of {012}, {01-4} and {-210} with a rhombohedral α-Fe2O3 structure. The gas sensors based on the as-synthesized α-Fe2O3 nanostructures exhibit high sensitivity, short recovery time, and good reproducibility in ethanol and acetone. The superiority of the gas-sensing properties of the obtained nanostructures should be attributed to the surface structure of the nanocrystals. The as-prepared α-Fe2O3 nanocrystals are significant for exploiting their other applications in the future.