Monolayer NbSe2 Favors Ultralow Friction and Super Wear Resistance.

The urgent demand for atomically thin, superlubricating, and super wear-resistant materials in micro/nanoelectromechanical systems has stimulated the research of friction-reducing and antiwear materials. However, the fabrication of subnanometer-thick films with superlubricating and super wear-resistant properties under ambient conditions remains a huge challenge. Herein, high-quality monolayer (ML) NbSe2 (∼0.8 nm) with ultralow friction and super wear resistance in an atmospheric environment was successfully grown by chemical vapor deposition (CVD) for the first time. Moreover, compared with few-layered (FL) NbSe2, ML NbSe2 has a lower friction coefficient and better wear resistance. On the basis of density function theory (DFT) calculations, the adhesion and the degree of charge transfer between ML NbSe2 and the substrate is larger than that of the topmost layer to the underlying layers of NbSe2 with two or more layers, which can be used to explain that the ML NbSe2 favors ultralow friction and super wear resistance.

High-Output Single-Electrode Droplet Triboelectric Nanogenerator Based on Asymmetrical Distribution Electrostatic Induction Enhancement.

Droplet-based triboelectric nanogenerators (D-TENGs) have recently gained much attention due to their great potential in harvesting energy. However, the output performance of conventional single-electrode droplet-based TENGs is limited owing to low induced electrification efficiency. The asymmetric distribution of electric fields on both sides of the electrode edge enhances the electrostatic induction process and improves the output performance of D-TENG. Herein, an induced electrification-enhanced droplet-based triboelectric nanogenerator (IED-TENG) is developed to effectively enhance the output performance by simultaneously optimizing the electrode structure and the dynamics of the water droplet. One droplet falling from a height of 30 cm results in a -70 V output voltage and -6 µA short-circuit current, which is 70 times and 20 times the full-inductive-electrode mode, respectively. The working principle and the relationship between electric signal and droplet dynamics are analyzed in detail. Moreover, the peak output voltage can reach -110 V, and the peak current can get -140 µA by using the power generation of multiple water droplets. The present protocol provides an easy and reproducibility strategy in energy harvesting and sensing areas.

A New Reversible Thermosensitive Liquid-Solid TENG Based on a P(NIPAM-MMA) Copolymer for Triboelectricity Regulation and Temperature Monitoring.

Intelligent and highly precise control of liquid-solid triboelectricity is of great significance for energy collection and electrostatic prevention. However, most of the traditional methods are irreversible and complex, greatly limiting their applicability. Here, a reversible thermosensitive liquid-solid triboelectric nanogenerator (L-S TENG) is assembled based on P(NIPAM-MMA) (PNM) copolymer for tunable triboelectrification. Through temperature regulation, the conformation between acylamino and isopropyl groups changes with the interfacial wettability and triboelectricity of PNM. When the temperature rises from 20 to 60 °C, the contact angle of PNM rises from 22.49° to 82.08°, and the output of the PNM-based L-S TENG shows a 27-fold increase. In addition, this transformation is reversible and repeatable with excellent durability for up to 60 days. Other organic liquids, such as glycol, exhibit positive response to temperature for this PNM-based L-S TENG. Polymers including polymethylmethacrylic, polytetrafluoroethylene, and polyimide are verified to not have such thermo-sensitivity properties. In addition, a droplet-based wireless warning system based on PNM is designed and actuated for monitoring specific temperature. The introduction of thermal PNM not only provides new material for reversible manipulation of L-S TENG, but also provides a new method for designing highly sensitive temperature warning sensors.

New Method for the Corrosion Resistance of AZ31 Mg Alloy with a Porous Micro-Arc Oxidation Membrane as an Ionic Corrosion Inhibitor Container.

This work introduces a new composite anticorrosion coating for the AZ31 magnesium alloy, based on the synergistic effect of an organic/inorganic composite coating with a micro- and nanoporous micro-arc oxidation (MAO) membrane as the container of ionic corrosion inhibitor (M-16). The surface morphologies and size of the micro/nanocontainers in the porous MAO membrane before and after filling with M-16 corrosion inhibitor are examined by scanning electron microscopy (SEM). The effectiveness of M-16 for corrosion suppression on AZ31 Mg alloy with and without epoxy coating as the top sealing layer is demonstrated by electrochemical impedance spectroscopy (EIS) and salt spray tests. The potentiodynamic polarization and electrochemical impedance spectroscopy measurements show that, compared with the bare AZ31 Mg alloys, the composite coating has superior corrosion resistance with the a lower corrosion current (9.7 × 10-9 A/cm2) and a higher protection efficiency (99.3%) after immersion in 3.5 wt % NaCl solution and, meanwhile, has stronger salt spray resistance within 30 days. The results demonstrate the synergistic effect of the isolation protection of the micro-arc oxidation layer and the inhibition of M-16 and that the epoxy coating contributed to the protection for AZ31 Mg substrate to some extent. Therefore, it is anticipated that the composite coating has a potential application in the protection of metals and their alloys.

Electrostatic Self-Assembly of Au Nanoparticles onto Thermosensitive Magnetic Core-Shell Microgels for Thermally Tunable and Magnetically Recyclable Catalysis.

A facile route to fabricate a nanocomposite of Fe3O4@poly[N-isopropylacrylamide (NIPAM)-co-2-(dimethylamino)ethyl methacrylate (DMAEMA)]@Au (Fe3O4@PND@Au) is developed for magnetically recyclable and thermally tunable catalysis. The negatively charged Au nanoparticles with an average diameter of 10 nm are homogeneously loaded onto positively charged thermoresponsive magnetic core-shell microgels of Fe3O4@poly(NIPAM-co-DMAEMA) (Fe3O4@PND) through electrostatic self-assembly. This type of attachment offers perspectives for using charged polymeric shell on a broad variety of nanoparticles to immobilize the opposite-charged nanoparticles. The thermosensitive PND shell with swollen or collapsed properties can be as a retractable Au carrier, thereby tuning the aggregation or dispersion of Au nanoparticles, which leads to an increase or decrease of catalytic activity. Therefore, the catalytic activity of Fe3O4@PND@Au can be modulated by the volume transition of thermosensitive microgel shells. Importantly, the mode of tuning the aggregation or dispersion of Au nanoparticles using a thermosensitive carrier offers a novel strategy to adjust and control the catalytic activity, which is completely different with the traditional regulation mode of controlling the diffusion of reactants toward the catalytic Au core using the thermosensitive poly(N-isopropylacrylamide) network as a nanogate. Concurrent with the thermally tunable catalysis, the magnetic susceptibility of magnetic cores enables the Fe3O4@PND@Au nanocomposites to be capable of serving as smart nanoreactors for thermally tunable and magnetically recyclable catalysis.

Gecko-inspired but chemically switched friction and adhesion on nanofibrillar surfaces.

Chemically switched friction nano-fibrillar surfaces (SiNWAs-PSPMA & SiNWAs-PMAA arrays) can be constructed by finely decorating ordered Si nanowire arrays with responsive polymer brushes. As expected, these surfaces sense humidity or pH smartly and show reversible friction switching, based on swelling and shrinking of the polymer brushes, which is successfully monitored by AFM in liquid media.

A high-performance triboelectric nanogenerator with dual nanostructure for remote control of switching circuit.

Preparing nanostructured surfaces has been considered an effective method to improve the output of triboelectric nanogenerators (TENGs), but how to quickly prepare materials with a nanostructured surface for TENGs has always been a challenge. Here, polypropylene nanowires and electrospun nylon 11 nanofibers were successfully prepared through a simple and time-saving method with a high success rate. Compared with a flat TENG, the output performance of a dual nanostructured TENG is enhanced by more than 5 times. After 1H,1H,2H,2H-perfluorooctyl trichlorosilane was assembled on the surface of the polypropylene film, the dual nanostructured TENG achieved the maximum output with the short-circuit current, output voltage, and charge density of 63.3 µA, 1135 V and 161.5 µC m-2, respectively. Compared with a planar structured TENG, the short-circuit current and output voltage were enhanced by about 18 times, and the charge density was increased by about 36 times. In addition, the TENG showed good working stability with almost no decrease in output after continuous operation for 193 000 cycles. The electricity generated by this TENG can successfully light up 1280 LEDs and continuously power a multi-functional electronic watch. Finally, the triboelectric signal generated by this TENG was used to control an optocoupler switch, indicating good application prospects in a remote control switching circuit.

High-Output Triboelectric Nanogenerator Achieved through Conductive Layer Strategy for Motion Step Sensing.

As an emerging high-efficiency energy conversion device, improving the output of triboelectric nanogenerators (TENGs) is still a key method to promote practical application of TENGs. This paper systematically investigated the influence of component composition, thickness, and surface morphology of the metal conducting layer on the performance of triboelectric nanogenerators. It has been established that these three factors have a significant influence on the output performance of TENGs. Among the four common metals Au, Pt, Ag, and Cu, the triboelectric nanogenerator achieves its maximum output when utilizing Ag as the conducting layer, with optimal performance observed at a thickness of 278 nm. TENGs with nanostructured conducting layers have better output as the nanostructure amplifies the induction charging area, thereby effectively augmenting the performance of TENGs. In particular, when contrasted with a triboelectric nanogenerator utilizing copper foil as the conducting layer alongside poly(vinylidene difluoride) and Nylon-11 as friction layers in the common work, the short-circuit current of the triboelectric nanogenerator increased by 2.3 times, and the maximum short-circuit current reached 149 µA when the conducting layer was replaced with Ag, and the enhanced triboelectric nanogenerator successfully illuminated 1536 commercial LEDs. In addition, the TENG-based smart insoles combined with pedometers can realize signal sensing and the real-time recording of steps during exercise. This research provides a new simple and reliable method to further improve the output of the TENG.

Contact-electro-catalytic CO2 reduction from ambient air.

Traditional catalytic techniques often encounter obstacles in the search for sustainable solutions for converting CO2 into value-added products because of their high energy consumption and expensive catalysts. Here, we introduce a contact-electro-catalysis approach for CO2 reduction reaction, achieving a CO Faradaic efficiency of 96.24%. The contact-electro-catalysis is driven by a triboelectric nanogenerator consisting of electrospun polyvinylidene fluoride loaded with single Cu atoms-anchored polymeric carbon nitride (Cu-PCN) catalysts and quaternized cellulose nanofibers (CNF). Mechanistic investigation reveals that the single Cu atoms on Cu-PCN can effectively enrich electrons during contact electrification, facilitating electron transfer upon their contact with CO2 adsorbed on quaternized CNF. Furthermore, the strong adsorption of CO2 on quaternized CNF allows efficient CO2 capture at low concentrations, thus enabling the CO2 reduction reaction in the ambient air. Compared to the state-of-the-art air-based CO2 reduction technologies, contact-electro-catalysis achieves a superior CO yield of 33 µmol g-1 h-1. This technique provides a solution for reducing airborne CO2 emissions while advancing chemical sustainability strategy.

Revamping Triboelectric Output by Deep Trap Construction.

High output performance is critical for building triboelectric nanogenerators (TENGs) for future multifunctional applications. Unfortunately, the high triboelectric charge dissipation rate has a significant negative impact on its electrical output performance. Herein, a new tribolayer is designed through introducing self-assembled molecules with large energy gaps on commercial PET fibric to form carrier deep traps, which improve charge retention while decreasing dissipation rates. The deep trap density of the PET increases by two orders of magnitude, resulting in an 86% reduction in the rate of charge dissipation and a significant increase in the charge density that can be accumulated on tribolayer during physical contact. The key explanation is that increasing the density of deep traps improves the dielectric's ability to store charges, making it more difficult for the triboelectric charges trapped by the tribolayer to escape from the deep traps, lowering the rate of charge dissipation. This TENG has a 1300% increase in output power density as a result of altering the deep trap density, demonstrating a significant improvement. This work describes a simple yet efficient method for building TENGs with ultra-high electrical output and promotes their practical implementation in the sphere of the Internet of Things.

Novel three-dimensional nanoporous alumina as a template for hierarchical TiO2 nanotube arrays.

Hierarchical micro-/nano-structures made easy. By using extremely rough, chemically etched microstructured aluminium foils, anodization in phosphoric acid under very harsh conditions, e.g., 10 wt% phosphoric acid and room temperature, can be repeatedly accomplished without suffering from breakdown. As a result, an alumina membrane with a three-dimensionally distributed nanopore structure is formed, which can be used as a general template to fabricate hierarchical micro-/nano-structures.

Core/Shell photocatalyst with spatially separated co-catalysts for efficient reduction and oxidation of water.

A simple method allows the preparation of core/shell photocatalysts with spatially separated co-catalysts for efficient water splitting. The high activity was attributed to the core/shell structure and separated co-catalysts that assisted separation and collection of the electrons and holes at the respective co-catalysts, owing to active rectification of electron and hole transport (see picture; Eg =2.1 eV).

Visualization of Charge Dynamics when Water Droplets Bounce on a Hydrophobic Surface.

Visualizing the motion of water droplets and understanding their electrification behavior holds significance for applications related to droplet transport, self-cleaning, and anti-icing/deicing and for providing a comprehensive explanation of the solid-liquid triboelectrification mechanism. Here, by constructing microcolumnar structures on the polytetrafluoroethylene surface, a water droplet-based single electrode triboelectric nanogenerator was fabricated for visualizing charge dynamics when a water droplet bounces on a hydrophobic surface. The motion state of the water droplet is closely linked to its electrification behavior through the integration of a high-speed camera and an ammeter. The electrification behavior stemming from the bounce of the water droplet is dynamically captured in real-time. The results show that the magnitude and polarity of the electrical signal have strong dependence on the motion state of the water droplet. For instance, when a water droplet approaches or moves away from the substrate in a single direction, a unipolar electrical signal is generated. However, when the water droplet reaches its limit in the initial motion direction, it signifies a static equilibrium state, resulting in the electrical signal being at zero. Furthermore, we examine the impact of factors such as impact speed, drop contact area, contact line spreading/retraction speed, and impact angle on electrification. Finally, based on the close relationship between poly(ethylene oxide) (PEO) droplet bounce dynamics and electrical signals, the bouncing details of PEO droplets with different concentrations are tracked by electrical signals. This study digitally presents the whole process of droplet bounce in situ and provides a means for monitoring and tracking droplet movement.

Macroscale Superlubricity with Ultralow Wear and Ultrashort Running-In Period (∼1 s) through Phytic Acid-Based Complex Green Liquid Lubricants.

Liquid superlubricity has attracted much attention, due to its ability to significantly reduce friction on the macroscale. However, the severe wear caused by the long running-in period is still one of the bottlenecks restricting the practical application of liquid superlubricating materials. In this work, the obtained polyethylene glycol-phytic acid (PEG-PA) composite liquid lubricants showed outstanding superlubricating properties (µ ≈ 0.006) for Si3N4/glass friction pairs with an ultrashort running-in period (∼1 s) under high Hertzian contact pressure of ∼758 MPa. More importantly, even after up to 12 h (∼700 m of travel), only about 100 nm deep wear scars were found on the surface of the glass sheet (wear rate = 2.51× 10-9 mm3 N-1 m-1). From the molecular point of view, the water molecules anchored between the two friction pairs have extremely low shear force during the friction process, and the strong hydrogen bond interaction between PEG and PA greatly improves the bearing capacity of the lubricant. This work addresses the challenge of liquid superlubricant simultaneously exhibiting low shear force and high load-carrying capacity and makes it possible to obtain liquid superlubrication performance with an extremely short running-in time.

Deep Trap Boosted Ultrahigh Triboelectric Charge Density in Nanofibrous Cellulose-Based Triboelectric Nanogenerators.

For their use in self-powered implantable or wearable electronics, cellulose nanofiber (CNF)-based triboelectric nanogenerators (TENGs) have drawn a lot of attention. However, the low triboelectric charge density (TECD) hinders its further application as a tribolayer for TENGs. In this work, a sulfonated cellulose nanofiber was prepared as an electropositive tribolayer for TENGs to obtain ultrahigh electrical output performance. Since the introduction of sulfonic acid effectively increased the dielectric properties and hole deep trap density of the CNF film, the triboelectric charge storage capacity of the CNF-SO3Na film was improved. The results showed that the TECD of the CNF-SO3Na film increased by 460% compared with the pristine CNF film. Furthermore, the dielectric constant and deep trap density of the CNF-SO3Na film increased by 2.4 times and 8.1 times. This work encourages the use of TENGs in real-world wireless transmission applications by outlining an easy and effective method for building high-performance TENGs.

Spontaneous phase and morphology transformations of anodized titania nanotubes induced by water at room temperature.

We report a spontaneous phase transformation of titania nanotubes induced by water at room temperature, which enables the as-anodized amorphous nanotubes to be crystallized into anatase mesoporous nanowires without any other post-treatments. These mesoporous TiO(2) nanomaterials have a markedly improved surface area, about 5.5 times than that of the as-anodized TiO(2) nanotubes, resulting in a pronounced enhanced photocatalytic activity. The present approach not only allows a flexible control over the morphology of TiO(2) nanostructures but can fundamentally eliminate the need for high temperature operations for crystallizing amorphous TiO(2).

A New Self-Healing Triboelectric Nanogenerator Based on Polyurethane Coating and Its Application for Self-Powered Cathodic Protection.

With the increasing demand for carbon neutrality, the development of renewable and recycle green energy has attracted wide attention from researchers. A novel self-healing triboelectric nanogenerator (TENG) was constructed by applying a linear silicone-modified polyurethane (PU) coating as a triboelectric layer, which was obtained by reacting hydroxypropyl silicone oil and hexamethylene diisocyanate under the catalysis of Sn. The linear self-healing coating as the friction electrode could effectively alleviate the damages of TENG devices during long-term energy harvesting. When the triboelectric layer of the TENG device shows abrasion, the broken silicone-modified polyurethane polymer chains would gradually be cross-linked again through hydrogen bonding to achieve a self-healing effect. The entire self-healing process of the friction coating could be completed in 30 min at room temperature. The PU-based self-healing TENG exhibits an evident and stable output performance with a short-circuit current of 31.9 µA and output voltage of 517.5 V after multiple cutting-healing cycles, which could light 480 commercial LEDs. Besides, a self-powered cathodic protection system supplied by the self-healing TENG was constructed, which could transfer negative triboelectric charges to the protected metal surface to achieve an anti-corrosion effect by harvesting mechanical energy. Due to the self-healing characteristics of the TENG device as the power supply part, this intelligent system possesses great application potential in the long-term corrosion protection of multiple metal application industries, such as the marine industry.

Quantifying Wetting Dynamics with Triboelectrification.

Wetting is often perceived as an intrinsic surface property of materials, but determining its evolution is complicated by its complex dependence on roughness across the scales. The Wenzel (W) state, where liquids have intimate contact with the rough surfaces, and the Cassie-Baxter (CB) state, where liquids sit onto air pockets formed between asperities, are only two states among the plethora of wetting behaviors. Furthermore, transitions from the CB to the Wenzel state dictate completely different surface performance, such as anti-contamination, anti-icing, drag reduction etc.; however, little is known about how transition occurs during time between the several wetting modes. In this paper, wetting dynamics can be accurately quantified and tracked using solid-liquid triboelectrification. Theoretical underpinning reveals how surface micro-/nano-geometries regulate stability/infiltration, also demonstrating the generality of the authors' theoretical approach in understanding wetting transitions. It can clarify the functioning behavior of materials in real environment.

Control of triboelectrification on Al-metal surfaces through microstructural design.

The instantaneous discharge of accumulated static charge due to contact electrification can cause irreversible damage to electrostatic-sensitive systems. Despite major advances in reducing tribo-charges, the problem remains intractable. Here, four alumina microstructures are fabricated on aluminum (Al) by combining chemical etching and anodic oxidation, and the effects of surface composition and structure on the triboelectric performance are studied by assembling them with a polytetrafluoroethylene membrane into a solid-solid triboelectric nanogenerator. The results show that the short-circuit current of the hierarchical nanoporous anodic aluminum oxide (micro/nano-AAO) modified Al is 8.77 times smaller than that of pristine Al, which is attributed to the reduced contact area and presence of an oxide film on the surface of the modified metal. By regulating the diameter of alumina nanotubes, a positive correlation between the contact area and the measured charge density is theoretically demonstrated, which establishes the size of the contact area as the main factor affecting triboelectric outputs. In addition, the micro/nano-AAO based phone shell could provide more effective electrostatic protection than that based on an acrylic coating. This novel regulation of the triboelectric output by microstructural design provides a new direction for the development of antistatic materials in a vacuum and non-grounded environment.

Gas-liquid two-phase flow-based triboelectric nanogenerator with ultrahigh output power.

Solid-liquid triboelectric nanogenerators (SL-TENGs) have shown promising prospects in energy harvesting and application from water resources. However, the low contact separation speed, small contact area, and long contacting time during solid-liquid electrification severely limit their output properties and further applications. Here, by leveraging the rheological properties of gas-liquid two-phase flow and the Venturi-like design, we circumvent these limitations and develop a previously unknown gas-liquid two-phase flow-based TENG (GL-TENG) that can achieve ultrahigh voltage and volumetric charge density of 3789 volts and 859 millicoulombs per cubic meter, respectively. With a high-power output of 143.6 kilowatts per cubic meter, a 24-watt commercial lamp can be directly lighted by a continuous-flow GL-TENG device. The high performance displayed SL-TENGs in this work provides a promising strategy for the practical application of solid-liquid TENGs in energy harvesting and sensing applications.