High-Performance Single-Crystal Lithium-Rich Layered Oxides Cathode Materials via Na2WO4-Assisted Sintering.

Single-crystal lithium-rich layered oxides (LLOs) with excellent mechanical properties can enhance their crystal structure stability. However, the conventional methods for preparing single-crystal LLOs, require large amounts of molten salt additives, involve complicated washing steps, and increase the difficulty of large-scale production. In this study, a sodium tungstate (Na2WO4)-assisted sintering method is proposed to fabricate high-performance single-crystal LLOs cathode materials without large amounts of additives and additional washing steps. During the sintering process, Na2WO4 promotes particle growth and forms a protective coating on the surface of LLOs particles, effectively suppressing the side reactions at the cathode/electrolyte interface. Additionally, trace amounts of Na and W atoms are doped into the LLOs lattice via gradient doping. Experimental results and theoretical calculations indicate that Na and W doping stabilizes the crystal structure and enhances the Li+ ions diffusion rate. The prepared single-crystal LLOs exhibit outstanding capacity retention of 82.7% (compared to 65.0%, after 200 cycles at 1 C) and a low voltage decay rate of 0.76 mV per cycle (compared to 1.80 mV per cycle). This strategy provides a novel pathway for designing the next-generation high-performance cathode materials for Lithium-ion batteries (LIBs).

Direct Write Printing of Ultraviolet-Curable Bulk Superhydrophobic Ink Material.

Although superhydrophobic surfaces have various promising applications, their fabrication methods are often limited to 2D plane surfaces that are vulnerable to abrasion and have limited adhesion to the substrate. Herein, an ultraviolet (UV) curable ink with bulk superhydrophobicity, consisting of poly(dimethylsiloxane) (PDMS) resins, hydrophobic silica, and solvent (porogen), was successfully developed for UV-assisted direct write printing processing. After UV curing of the ink followed by solvent evaporation, the generated porous structure cooperates with silica particles to form a self-similar and hierarchical structure throughout the bulk material, which can keep its original morphology even after cyclic abrasion (over 1000 times) and thus exhibits durable superhydrophobicity. With this unique ink, UV-assisted direct write printing can not only create 2D superhydrophobic surfaces on various substrates (e.g., paper and wire mesh) but also fabricate self-supporting 3D superhydrophobic objects for various applications such as waterproofing and oil-water separation. The printed objects exhibited a stable superhydrophobicity against liquid corrosion and mechanical damage. In addition, the 3D printing approach can be used to optimize the oil-water separation performance of the superhydrophobic porous materials by tuning the pore size, thus presenting promising applications.

Plasmon-Driven Photochemical Reduction Reaction on Silver Nanostructures for Green Fabrication of Superhydrophobic Surface.

Green fabrication of superhydrophobic surface by water-based processing is still challenging, because introduction of the substances with hydrophilic moieties compromises its superhydrophobicity. Herein, a plasmon-driven photochemical reduction reaction under ultraviolet light (UVA) irradiation is first discovered and is applied to deoxygenation of hydrophilic organic adsorbates on rough nano-Ag coating for the formation of stable superhydrophobic surface. A nano-Ag coating with strong localized surface plasmon resonance in the UVA region is prepared by a water-based silver mirror reaction and results in a unique chemical reduction reaction on its surface. Consequently, the low residual hydrophilic functionalities and the formed cross-linked structure of the adsorbate on Ag nanoparticles (NPs) enables the coating to exhibit stable superhydrophobicity against to both air and water. The superhydrophobic Ag NP-coated sandpaper can also be used as a surface-enhanced Raman scattering (SERS) substrate to concentrate aqueous analytes for trace detection.

Anion-Cation Dual-Ion Multisite Doping Stabilizes the Crystal Structure of Li-Rich Layered Oxides.

Li-rich layered oxide (LLOs) cathode materials are gaining increasing attention as lithium-ion batteries (LIBs) pursue greater energy density. However, LLOs still suffer from severe capacity fading and voltage decay due to their unstable crystal structure. Hence, the anion-cation dual-ion multisite doping strategy based on Mg and S atoms is used to stabilize the crystal structures of LLOs. Mg substitutes Li atoms in the Li and transition-metal (TM) layers, while S atoms occupy tetrahedral interstitial sites and lattice O sites, all of which contribute to the crystal structure stability of LLOs. Theoretical calculations show that Mg/S dual-ion multisite doping successfully reduces the energy levels of the TM 3d-O 2p and isolated O 2p orbitals, which effectively stabilizes the lattice oxygen. Therefore, multisite-doped samples exhibit promising electrochemical performance. This strategy provides a new approach to enhance the crystal structure stability of LLOs for the design of high-energy-density Li-ion batteries.

Mechanoluminescent-Triboelectric Bimodal Sensors for Self-Powered Sensing and Intelligent Control.

Self-powered flexible devices with skin-like multiple sensing ability have attracted great attentions due to their broad applications in the Internet of Things (IoT). Various methods have been proposed to enhance mechano-optic or electric performance of the flexible devices; however, it remains challenging to realize the display and accurate recognition of motion trajectories for intelligent control. Here, we present a fully self-powered mechanoluminescent-triboelectric bimodal sensor based on micro-nanostructured mechanoluminescent elastomer, which can patterned-display the force trajectories. The deformable liquid metals used as stretchable electrode make the stress transfer stable through overall device to achieve outstanding mechanoluminescence (with a gray value of 107 under a stimulus force as low as 0.3 N and more than 2000 cycles reproducibility). Moreover, a microstructured surface is constructed which endows the resulted composite with significantly improved triboelectric performances (voltage increases from 8 to 24 V). Based on the excellent bimodal sensing performances and durability of the obtained composite, a highly reliable intelligent control system by machine learning has been developed for controlling trolley, providing an approach for advanced visual interaction devices and smart wearable electronics in the future IoT era.

Lunar Dust-Mitigation Behavior of Aluminum Surfaces with Multiscale Roughness Prepared by a Composite Etching Method.

Reducing lunar dust adhesion to various material surfaces is important for protecting equipment from damage during lunar exploration missions. In this study, we investigate the lunar dust-mitigation ability and dust adhesion force of aluminum (Al) substrates prepared using different etching methods. Among them, composite etching methods (combining chemical and electrochemical steps) can result in multiscale structures with micro- and nanoroughness, reducing the contact area between the substrate and thus decreasing lunar dust adhesion. After composite etching, the dust adhesion force of the Al substrate was significantly reduced by 80% from 45.53 to 8.89 nN. The dust adhesion force of Al substrates dominates their dust-mitigation performance in floating dust environments. The lunar dust coverage (2.19%) of the Al substrate modified by composite etching (placed with a tilt angle of 90°) was 4-fold lower than that of the pristine Al substrate (9.11%), indicating excellent lunar-dust repellence. In addition, other factors such as tilt angle of the substrate and dust loading significantly affect dust-mitigation performance of the modified Al substrates. The Al substrate with an excellent dust-mitigation ability highlights good potential for lunar exploration missions.

Phenotyping Bacteria through a Black-Box Approach: Amplifying Surface-Enhanced Raman Spectroscopy Spectral Differences among Bacteria by Inputting Appropriate Environmental Stress.

Surface-enhanced Raman spectroscopy (SERS) stands out in the field of microbial analysis due to its rich molecular information, fast analysis speed, and high sensitivity. However, achieving strain-level differentiation is still challenging because numerous bacterial species inevitably have very similar SERS profiles. Here, a method inspired by the black-box theory was proposed to boost the spectral differences, where the undifferentiated bacteria was considered as a type of black-box, external environmental stress was used as the input, and the SERS spectra of bacteria exposed to the same stress was output. For proof of the concept, three types of environmental stress were explored, i.e., ethanol, ultraviolet light (UV), and ultrasound. Enterococcus faecalis (E. faecalis) and three types of Escherichia coli (E. coli) were all subjected to the stimuli (stress) before SERS measurement. Then the collected spectra were processed only by simple principal component analysis (PCA) to achieve differentiation. The results showed that appropriate stress was beneficial to increase the differences in bacterial SERS spectra. When sonication at 490 W for 60 s was used as the input, the optimal differentiation of bacteria at the species (E. faecalis and E. coli) and strain-level (three E. coli) can be achieved.

Superhydrophobic and Recyclable Cellulose-Fiber-Based Composites for High-Efficiency Passive Radiative Cooling.

Passive daytime radiative cooling (PDRC) involves cooling down an object by simultaneously reflecting sunlight and thermally radiating heat to the cold outer space through the Earth's atmospheric window. However, for practical applications, current PDRC materials are facing unprecedented challenges such as complicated and expensive fabrication approaches and performance degradation arising from surface contamination. Herein, we develop scalable cellulose-fiber-based composites with excellent self-cleaning and self-cooling capabilities, through air-spraying ethanolic poly(tetrafluoroethylene) (PTFE) microparticle suspensions embedded partially within the microsized pores of the cellulose fiber to form a dual-layered structure with PTFE particles atop the paper. The formed superhydrophobic PTFE coating not only protects the cellulose-fiber-based paper from water wetting and dust contamination for real-life applications but also reinforces its solar reflectivity by sunlight backscattering. It results in a subambient cooling performance of 5 °C under a solar irradiance of 834 W/m2 and a radiative cooling power of 104 W/m2 under a solar intensity of 671 W/m2. The self-cleaning surface of composites maintains their good cooling performance for outdoor applications, and the recyclability of the composites extends their life span after one life cycle. Additionally, dyed cellulose-fiber-based paper can absorb appropriate visible wavelengths to display specific colors and effectively reflect near-infrared lights to reduce solar heating, which synchronously achieves effective radiative cooling and esthetic varieties.

SnO-Sn3O4 heterostructural gas sensor with high response and selectivity to parts-per-billion-level NO2 at low operating temperature.

Considering the harmfulness of nitrogen dioxide (NO2), it is important to develop NO2 sensors with high responses and low limits of detection. In this study, we synthesize a novel SnO-Sn3O4 heterostructure through a one-step solvothermal method, which is used for the first time as an NO2 sensor. The material exhibits three-dimensional flower-like microparticles assembled by two-dimensional nanosheets, in situ-formed SnO-Sn3O4 heterostructures, and large specific surface area. Gas sensing measurements show that the responses of the SnO-Sn3O4 heterostructure to 500 ppb NO2 are as high as 657.4 and 63.4 while its limits of detection are as low as 2.5 and 10 parts per billion at 75 °C and ambient temperature, respectively. In addition, the SnO-Sn3O4 heterostructure has an excellent selectivity to NO2, even if exposed to mixture gases containing interferential part with high concentration. The superior sensing properties can be attributed to the in situ formation of SnO-Sn3O4 p-n heterojunctions and large specific surface area. Therefore, the SnO-Sn3O4 heterostructure having excellent NO2 sensing performances is very promising for applications as an NO2 sensor or alarm operated at a low operating temperature.

Crack growth-driven wettability transition on carbon black/polybutadiene nanocomposite coatings via stretching.

Ordered topography patterns with a mechanical response are usually designed to achieve wettability switching by geometric parameter changes through mechanical stimuli. However, their fabrication often needs expensive and complicated micro/nano-fabrication processing (e.g. photolithography and ion etching). In this study, a nano-carbon black (CB)/polybutadiene (PB) coating with a Wenzel superhydrophobic state was prepared on a rubber substrate by a facile method combining solution mixing and spraying coating. By stretching the composite coating, the generated cracks divided the continuous coating into new micro-nano mastoids, resulting in the formation of new hierarchical roughness for Cassie superhydrophobicity. The Wenzel-to-Cassie transition behavior was dependent on the CB loading in the coating. During stretching, the cracks propagated more rapidly in the coating with higher CB loading and induced the desired hierarchical structure to consequently enable the Wenzel-to-Cassie transition earlier at a lower stretching strain. The stretched coating presented good anti-wetting (a sliding angle of 5°) and low water adhesion. After releasing, the coating returned to its original Wenzel state by structure recovery. Thus, the switchable wettability of the coating can be adopted for no-loss water droplet transfer by controlling the droplet adhesion through cyclic stretching-releasing, and exhibits good potential for microfluidic and biomedical applications.

Silver-nanoparticles-loaded chitosan foam as a flexible SERS substrate for active collecting analytes from both solid surface and solution.

Here, we report a dual-use surface-enhanced Raman scattering (SERS) substrate based on a flexible three-dimensional (3D) chitosan foam, onto which silver nanoparticles (Ag NPs) are firmly immobilized through amino groups from chitosan chains. The SERS substrate can actively collect analytes either on solid surface by swabbing or in solution by adsorption. The compressible characteristic of chitosan foam enables easy removal of solvent through gentle pressing, which can achieve fast pre-concentrating of analytes before measurements. In addition, the substrate is shape adaptable and thus is suitable for sampling contaminants on solid surfaces. The SERS substrates exhibit acceptable reproducibility (16.4% in relative standard deviation). Furthermore, it detects Raman probe Nile Blue A down to 5 pg by swabbing solid surface and Rhodamine 6G down to 10 ppb by adsorbing analyte in the solution. Three pesticide samples (triazophos, methidathion, and isocarbophos) can also be detected down to µg level with the substrate. It is believed that such a versatile SERS substrate may find great opportunity in realistic sensing applications.

Robust Succinonitrile-Based Gel Polymer Electrolyte for Lithium-Ion Batteries Withstanding Mechanical Folding and High Temperature.

Fabrication of a gel polymer electrolyte containing succinonitrile (GPE-SN) with high mechanical strength is quite challenging because the SN electrolyte always suppresses the formation of polymer networks during in situ polymerization. In this work, a mechanically robust GPE-SN was successfully prepared by using a solution immersion method. During fabrication, the paste-like SN electrolyte was transformed into a liquid SN electrolyte with low viscosity by heating at 50 °C and then infiltrated into the UV-cured highly cross-linked polyurethane acrylate (PUA) skeleton. The resulted GPE-SN film exhibits superior tensile strength (6.5 MPa) compared to the one (0.5 MPa) prepared by in situ polymerization (GPE-SN-IN). The high mechanical strength of the GPE-SN-IM film enables the LiCoO2/Li4Ti5O12 film battery to withstand 100-cycle folding without electrolyte damage and capacity loss. Besides, the GPE-SN presents a high ionic conductivity (1.63 × 10-3 S·cm-1 at 25 °C), which is comparable to GPE with a commercial liquid electrolyte (GPE-LE). Because of good thermal stability of the GPE-SN, the LiCoO2/Li cell with this electrolyte shows better charge-discharge cycling stability than that with GPE-LE at high temperature (55 °C). Thus, the GPE-SN prepared by our method could be a promising polymer electrolyte offering better safety and reliability for lithium-ion batteries.

Patternable Poly(chloro-p-xylylene) Film with Tunable Surface Wettability Prepared by Temperature and Humidity Treatment on a Polydimethylsiloxane/Silica Coating.

Poly(chloro-p-xylylene) (PPXC) film has a water contact angle (WCA) of only about 84°. It is necessary to improve its hydrophobicity to prevent liquid water droplets from corroding or electrically shorting metallic circuits of semiconductor devices, sensors, microelectronics, and so on. Herein, we reported a facile approach to improve its surface hydrophobicity by varying surface pattern structures under different temperature and relative humidity (RH) conditions on a thermal curable polydimethylsiloxane (PDMS) and hydrophobic silica (SiO2) nanoparticle coating. Three distinct large-scale surface patterns were obtained mainly depending on the contents of SiO2 nanoparticles. The regularity of patterns was mainly controlled by the temperature and RH conditions. By changing the pattern structures, the surface wettability of PPXC film could be improved and its WCA was increased from 84° to 168°, displaying a superhydrophobic state. Meanwhile, it could be observed that water droplets on PPXC film with superhydrophobicity were transited from a "Wenzel" state to a "Cassie" state. The PPXC film with different surface patterns of 200 µm × 200 µm and the improved surface hydrophobicity showed wide application potentials in self-cleaning, electronic engineering, micro-contact printing, cell biology, and tissue engineering.

Highly Stretchable Superhydrophobic Composite Coating Based on Self-Adaptive Deformation of Hierarchical Structures.

With the rapid development of stretchable electronics, functional textiles, and flexible sensors, water-proof protection materials are required to be built on various highly flexible substrates. However, maintaining the antiwetting of superhydrophobic surface under stretching is still a big challenge since the hierarchical structures at hybridized micro-nanoscales are easily damaged following large deformation of the substrates. This study reports a highly stretchable and mechanically stable superhydrophobic surface prepared by a facile spray coating of carbon black/polybutadiene elastomeric composite on a rubber substrate followed by thermal curing. The resulting composite coating can maintain its superhydrophobic property (water contact angle ≈170° and sliding angle <4°) at an extremely large stretching strain of up to 1000% and can withstand 1000 stretching-releasing cycles without losing its superhydrophobic property. Furthermore, the experimental observation and modeling analysis reveal that the stable superhydrophobic properties of the composite coating are attributed to the unique self-adaptive deformation ability of 3D hierarchical roughness of the composite coating, which delays the Cassie-Wenzel transition of surface wetting. In addition, it is first observed that the damaged coating can automatically recover its superhydrophobicity via a simple stretching treatment without incorporating additional hydrophobic materials.

Cross-Linking Poly(lactic acid) Film Surface by Neutral Hyperthermal Hydrogen Molecule Bombardment.

Constructing a dense cross-linking layer on a polymer film surface is a good way to improve the water resistance of poly(lactic acid) (PLA). However, conventional plasma treatments have failed to achieve the aim as a result of the unavoidable surface damage arising from the charged species caused by the uncontrolled high energy coming from colliding ions and electrons. In this work, we report a modified plasma method called hyperthermal hydrogen-induced cross-linking (HHIC) technology to construct a dense cross-linking layer on PLA film surfaces. This method produces energy-controlled neutral hyperthermal hydrogen, which selectively cleaves C-H bonds by molecule collision from the PLA film without breaking other bonds (e.g., C-C bonds in the polymer backbone), and results in subsequent cross-linking of the carbon radicals generated from the organic molecules. The formation of a dense cross-linking layer can serve as a barrier layer to significantly improve both the hydrophobicity and water vapor barrier property of the PLA film. Because of the advantage of selective cleavage of C-H bonds by HHIC treatment, the original physical properties (e.g., mechanical strength and light transmittance) of the PLA films are well-preserved.

Cross-Linking the Surface of Cured Polydimethylsiloxane via Hyperthemal Hydrogen Projectile Bombardment.

Cross-linking of polydimethylsiloxane (PDMS) is increasingly important with recent focus on its top surface stiffness. In this paper, we demonstrate that hyperthermal hydrogen projectile bombardment, a surface sensitive cross-linking technology, is superior in enhancing the mechanical properties of a cured PDMS surface without significantly degrading its hydrophobicity. Both water contact angle measurements and time-of-flight secondary ion mass spectrometry are used to investigate the variations in surface chemistry and structure upon cross-linking. Using nanoindentation and atomic force microscopy, we confirm that the thickness of the cross-linked PDMS is controllable by the bombardment time, which opens opportunities for tuning cross-linking degree in compliance with arising requirements from the practice.

SERS optrode as a "fishing rod" to direct pre-concentrate analytes from superhydrophobic surfaces.

SERS optrodes were used to "fish" aqueous drops from superhydrophobic surfaces. The technique led to an improvement of 2-3 orders of magnitude in the lowest detectable amount of the Raman probe nile blue A, reaching 25 fg (34 attomoles). Further tests run on samples containing pesticide revealed that 20 pg of triazophos could be clearly detected from a single drop.

A silver nanoparticle embedded hydrogel as a substrate for surface contamination analysis by surface-enhanced Raman scattering.

A surface enhanced Raman scattering (SERS) substrate, capable of extracting small amounts of organic species from surfaces of different types of materials with variable roughness, has been fabricated. The substrate consists of Ag NPs encapsulated in poly(vinyl alcohol) (PVA) hydrogels, commonly known as PVA "slime". Unlike traditional SERS substrates, such as colloidal suspensions, the resulting PVA slime SERS substrate presents good viscoelasticity, allowing it to conform to the surface of various materials of arbitrary roughness. Surfaces of different materials, including sandpapers, cotton, metal, and wood, previously contaminated with nile blue A (NBA) were analyzed with the PVA slime SERS substrate. Limits of detection (LOD) as low as 100 ppb (0.79 ng in a total amount on an area of ∼3 cm(2)) were achieved for all surfaces tested. Pesticides and Sudan red III on the glass surface have also been detected, with a LOD of 1.6 ng per ∼3 cm(2).

Conductive polymer nanocomposites with hierarchical multi-scale structures via self-assembly of carbon-nanotubes on graphene on polymer-microspheres.

A novel and highly conductive 3-dimensional (3D) hierarchical multi-scale structure is formed by a new, simple, facile, and water-based method that enables practical production of conductive carbon nanofiller/polymer composites. More specifically, the π-π interaction between CNTs and graphene oxide (GO) is exploited to disperse conductive but non-polar CNTs with amphiphilic GO sheets to form a stable aqueous colloidal solution. Aqueous-dispersible latex-polystyrene microspheres are then added to enable the self-assembly processes of anchoring CNTs on GO and wrapping microspheres with GO-stabilized CNTs for the formation of an intriguing 3D hierarchical multi-scale structure. During this process, GO is reduced to conductive reduced-graphene oxide (RGO). The resultant RGO sheets act as "nano-walls" to prevent CNTs from randomly diffusing into the polymer bulk during thermal pressing of RGO-CNT/microspheres, which results in the formation of a 3D foam-like network of RGO-CNTs with high quality. The resultant composite with such a structure gives an ultra-low percolation threshold (0.03 vol% RGO-CNTs) and a reasonably high conductivity (153 S m(-1) at 4 vol% RGO-CNTs), which could satisfy various applications requiring both transparency and electrical conduction characteristics (e.g. transparent antistatic coatings, capacitive touch-screens, and transparent electronic devices).

High ion conducting polymer nanocomposite electrolytes using hybrid nanofillers.

There is a growing shift from liquid electrolytes toward solid polymer electrolytes, in energy storage devices, due to the many advantages of the latter such as enhanced safety, flexibility, and manufacturability. The main issue with polymer electrolytes is their lower ionic conductivity compared to that of liquid electrolytes. Nanoscale fillers such as silica and alumina nanoparticles are known to enhance the ionic conductivity of polymer electrolytes. Although carbon nanotubes have been used as fillers for polymers in various applications, they have not yet been used in polymer electrolytes as they are conductive and can pose the risk of electrical shorting. In this study, we show that nanotubes can be packaged within insulating clay layers to form effective 3D nanofillers. We show that such hybrid nanofillers increase the lithium ion conductivity of PEO electrolyte by almost 2 orders of magnitude. Furthermore, significant improvement in mechanical properties were observed where only 5 wt % addition of the filler led to 160% increase in the tensile strength of the polymer. This new approach of embedding conducting-insulating hybrid nanofillers could lead to the development of a new generation of polymer nanocomposite electrolytes with high ion conductivity and improved mechanical properties.