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Along with the revolution of the Internet of Things (IoT), smart, flexible, easily fabricated, and highly sensitive gas sensors are essential for many portable electronics. In this study, a wearable NO2 sensor based on a nanomesh structure woven with Ti3C2Tx microflakes and Bi2S3 nanobelts operated at room temperature was successfully designed and prepared. The effects of the mass ratio between two materials on the performance were evaluated. Bi2S3 nanobelts embedded inside accordion-like Ti3C2Tx provide large specific surface areas and abundant active sites for the adsorption of NO2 molecules. Attributed to the high conductivity of Ti3C2Tx, the carriers generated in the sensitive layers are easily transmitted during sensing, thus significantly reducing the response and recovery times. The sensor based on the nanomesh containing 30 wt % Ti3C2Tx exhibits excellent sensing performance, fast response/recovery rates, and high stability even in arbitrary deformed shape conditions. Upon exposure to 1 ppm NO2, the response of the sensor is 12.67, and the response/recovery times are 5/28 s, respectively. A stable sensing performance can be maintained after 1500 cycles of deformation. This work demonstrates a simple but reliable manufacturing strategy of wearable gas sensors for the next generation of IoT monitoring network-linking human activities. [ABSTRACT FROM AUTHOR]

A functionalized carbon nanotube (FCNT)/Ni-(HCO3)2-Ni nanostructures were synthesized by decorating nanoflaky and mesoporous Ni-(HCO3)2 onto FCNT along with in situ formed reduced metallic Ni for the purpose of constructing an energy-storage system with both high specific capacity and cycling stability. Such flaky nanostructures can significantly increase the contact surface areas between the mesoporous and nanoflaky Ni-(HCO3)2-Ni and electrolyte, thus immensely reducing the electrolyte-ion diffusion paths and accelerating the electron-transport rates through the use of a FCNT. The in situ reduced metallic Ni atoms are directly contacted with the electrolytes and quickly converted into NiOOH in the alkaline environment. For this reason, the capacity retention of battery-type electrochemical materials can be significantly enhanced, especially at the beginning of cycles. The obtained flaky FCNT/Ni-(HCO3)2-Ni nanomaterials exhibit excellent electrochemical performances with a high specific capacity of 203 mAh·g–1 at 3 A·g–1 current density for supercapacitor energy-storage systems. An aqueous asymmetric supercapacitor made of FCNT/Ni-(HCO3)2-Ni//graphene oxide achieves a 132 Wh·kg–1 energy density at 2880 W·kg–1 power density and an excellent cycling stability with 84% after 6000 cycles. [ABSTRACT FROM AUTHOR]

Humidity sensing and water molecule monitoring have become hot research topics attributed to their potential applications in monitoring breathing/physiological conditions of humans, air conditioning in greenhouses, and soil moisture in agriculture. However, there is a huge challenge for highly sensitive and precision humidity detection with wireless and fast responsive capabilities. In this work, a hybrid/synergistic strategy was proposed using a LiNbO 3 /SiO 2 /SiC heterostructure to generate shear-horizontal (SH) surface acoustic waves (SAWs) and using a nanocomposite of polyethylenimine-silicon dioxide nanoparticles (PEI-SiO 2 NPs) to form a sensitive layer, thus achieving an ultrahigh sensitivity of SAW humidity sensors. Ultrahigh frequencies (1∼15 GHz) of SAW devices were obtained on a high-velocity heterostructure of LiNbO 3 /SiO 2 /SiC. Among the multimodal wave modes, we selected SH waves for humidity sensing and achieved a high mass-sensitivity of 5383 MHz · mm 2 · μg -1 . With the PEI-SiO 2 NP composite as the sensitive layer, an ultrahigh sensitivity of 2.02 MHz/% RH was obtained, which is two orders of magnitude higher than those of the conventional SAW humidity sensors (∼202.5 MHz frequency) within a humidity range of 20-80% RH.

Machine-learning assisted handwriting recognition is crucial for development of next-generation biometric technologies. However, most of the currently reported handwriting recognition systems are lacking in flexible sensing and machine learning capabilities, both of which are essential for implementation of intelligent systems. Herein, assisted by machine learning, we develop a new handwriting recognition system, which can be applied as both a recognizer for written texts and an encryptor for confidential information. This flexible and intelligent handwriting recognition system combines a printed circuit board with graphene oxide-based hydrogel sensors. It offers fast response and good sensitivity and allows high-precision recognition of handwritten content from a single letter to words and signatures. By analyzing 690 acquired handwritten signatures obtained from seven participants, we successfully demonstrate a fast recognition time (less than 1 s) and a high recognition rate (∼91.30%). Our developed handwriting recognition system has great potential in advanced human-machine interactions, wearable communication devices, soft robotics manipulators, and augmented virtual reality.

Ice accretion on economically valuable and strategically important surfaces poses significant challenges. Current anti-/de-icing techniques often have critical issues regarding their efficiency, convenience, long-term stability, or sustainability. As an emerging ice mitigation strategy, the thin-film surface acoustic wave (SAW) has great potentials due to its high energy efficiency and effective integration on structural surfaces. However, anti-/de-icing processes activated by SAWs involve complex interfacial evolution and phase changes, and it is crucial to understand the nature of dynamic solid-liquid-vapor phase changes and ice nucleation, growth, and melting events under SAW agitation. In this study, we systematically investigated the accretion and removal of porous rime ice from structural surfaces activated by SAWs. We found that icing and de-icing processes are strongly linked with the dynamical interfacial phase and structure changes of rime ice under SAW activation and the acousto-thermally induced localized heating that facilitate the melting of ice crystals. Subsequently, interactions of SAWs with the formed thin water layer at the ice/structure interface result in significant streaming effects that lead to further damage and melting of ice, liquid pumping, jetting, or nebulization.

Transition metal sulfides and oxides with high theoretical capacities have been regarded as promising anode candidates for a sodium-ion battery (SIB); however, they have critical issues including sluggish electrochemical kinetics and poor long-term stability. Herein, a dual carbon design strategy is proposed to integrate with highly active heterojunctions to overcome the above issues. In this new design, CoS 2 /CoO hollow dodecahedron heterojunctions are sandwiched between open framework carbon-spheres (OFCs) and a reduced graphene oxide (rGO) nanomembrane (OFC@CoS 2 /CoO@rGO). The CoS 2 /CoO heterojunctions effectively promote electron transfer on their surface and provide more electrochemical active sites through their hierarchical hollow structures assembled by nanodots. Meanwhile, the dual-carbon framework forms a highly conductive network that enables a better rate capability. More importantly, the dual carbon can greatly buffer volume expansion and stable reaction interfaces of electrode material during the charge/discharge process. Benefitting from their synergistical effects, the OFC@CoS 2 /CoO@rGO electrode achieves a high reversible capacity of 460 mAh g -1 at 0.05 A g -1 and still maintains 205.3 mAh g -1 even when current density is increased by 200 times when used as an anode material for SIBs. Their cycling property is also remarkable with a maintained capacity of 161 mAh g -1 after 3500 charging/discharging cycles at a high current density of 1 A g -1 . The dual-carbon strategy is demonstrated to be effective for enhanced reaction kinetics and long-term cycling property, providing siginificant guidance for preparing other high-performance electrode materials.

Tuning the optical and electrical properties of two-dimensional (2D) hexagonal boron nitride (hBN) is critical for its successful application in optoelectronics. Herein, we report a new methodology to significantly enhance the optoelectronic properties of hBN monolayers by substitutionally doping with sulfur (S) on a molten Au substrate using chemical vapor deposition. The S atoms are more geometrically and energetically favorable to be doped in the N sites than in the B sites of hBN, and the S 3p orbitals hybridize with the B 2p orbitals, forming a new conduction band edge that narrows its band gap. The band edge positions change with the doping concentration of S atoms. The conductivity increases up to 1.5 times and enhances the optoelectronic properties, compared to pristine hBN. A photodetector made of a 2D S-doped hBN film shows an extended wavelength response from 260 to 280 nm and a 50 times increase in its photocurrent and responsivity with light illumination at 280 nm. These enhancements are mainly due to the improved light absorption and increased electrical conductivity through doping with sulfur. This S-doped hBN monolayer film can be used in the next-generation electronics, optoelectronics, and spintronics.

Ice accumulation causes great risks to aircraft, electric power lines, and wind-turbine blades. For the ice accumulation on structural surfaces, ice adhesion force is a crucial factor, which generally has two main sources, for exampple, electrostatic force and mechanical interlocking. Herein, we present that surface acoustic waves (SAWs) can be applied to minimize ice adhesion by simultaneously reducing electrostatic force and mechanical interlocking, and generating interface heating effect. A theoretical model of ice adhesion considering the effect of SAWs is first established. Experimental studies proved that the combination of nanoscale vibration and interface heating effects lead to the reduction of ice adhesion on the substrate. With the increase of SAW power, the electrostatic force decreases due to the increase of dipole spacings, which is mainly attributed to the SAW induced nanoscale surface vibration. The interface heating effect leads to the transition of the locally interfacial contact phase from solid-solid to solid-liquid, hence reducing the mechanical interlocking of ice. This study presents a strategy of using SAWs device for ice adhesion reduction, and results show a considerable potential for application in deicing.

Timely detection and elimination of surface condensation is crucial for diverse applications in agriculture, automotive, oil and gas industries, and respiratory monitoring. In this paper, a smart patch based on a ZnO/aluminum (∼5 μm/50 μm thick) flexible Lamb wave device has been proposed to detect, prevent, and eliminate condensation, which can be realized using both of its surfaces. The patch is operated using a machine-learning algorithm which consists of data preprocessing (feature selection and optimization) and model training by a random forest algorithm. It has been tested in six cases, and the results show good detection performance with average precision = 94.40% and average F 1 score = 93.23%. The principle of accelerating evaporation is investigated to understand the elimination and prevention functions for surface condensation. Results show that both dielectric heating and acoustothermal effect have their contributions, whereas the former is found more dominant. Furthermore, the functional relationship between the evaporation rate and the input power is calibrated, showing a high linearity ( R 2 = 97.64%) with a slope of ∼3.6 × 10 -5 1/(s·mW). With an input power of ∼0.6 W, the flexible device has been proven effective in the prevention of condensation.

Surface acoustic wave (SAW) devices are widely used for physical, chemical, and biological sensing applications, and their sensing mechanisms are generally based on frequency changes due to mass-loading effects at the acoustic wave propagation area between two interdigitated transducers (IDTs). In this paper, a new sensing mechanism has been proposed based on a significantly enhanced mass-loading effect generated directly on Au IDT electrodes, which enables significantly enhanced sensitivity, compared with that of conventional SAW devices. The fabricated ultrahigh-frequency SAW devices show a significant mass-loading effect on the electrodes. When the Au-electrode thickness increased from 12 to 25 nm, the Rayleigh mode resonant frequency decreased from 7.77 to 5.93 GHz, while that of the higher longitudinal leaky SAW decreased from 11.87 to 9.83 GHz. The corresponding mass sensitivity of 7309 MHz·mm 2 ·μg -1 (Rayleigh mode) is ∼8.9 × 10 11 times larger than that of a conventional quartz crystal balance (with a frequency of 5 MHz) and ∼1000 times higher than that of conventional SAW devices (with a frequency of 978 MHz). Trinitrotoluene concentration as low as 4.4 × 10 -9 M (mol·L -1 ) can be detected using the fabricated SAW sensor, proving its giant mass-loading effect and ultrahigh sensitivity.

The ability to actuate liquids remains a fundamental challenge in smart microsystems, such as those for soft robotics, where devices often need to conform to either natural or three-dimensional solid shapes, in various orientations. Here, we propose a hierarchical nanotexturing of piezoelectric films as active microfluidic actuators, exploiting a unique combination of both topographical and chemical properties on flexible surfaces, while also introducing design concepts of shear hydrophobicity and tensile hydrophilicity. In doing so, we create nanostructured surfaces that are, at the same time, both slippery (low in-plane pinning) and sticky (high normal-to-plane liquid adhesion). By enabling fluid transportation on such arbitrarily shaped surfaces, we demonstrate efficient fluid motions on inclined, vertical, inverted, or even flexible geometries in three dimensions. Such surfaces can also be deformed and then reformed into their original shapes, thereby paving the way for advanced microfluidic applications.

Nickel dichalcogenides have received extensive attention as promising noble-metal-free nanocatalysts for a hydrogen evolution reaction. Nonetheless, their catalytic performance is restricted by the sluggish reaction kinetics, limited exposed active sites, and poor conductivity. In this work, we report on an effective strategy to solve those problems by using an as-designed new porous-C/Ni 2 SeS nanocatalyst with the Ni 2 SeS nanostubs anchored on with porous-carbon skeletons process. On the basis of three advantages, as the enhancement of the intrinsic activity using the ternary sulfoselenide, increased number of exposed active sites due to the 3D hollow substrate, and increased conductivity caused by porous-carbon skeletons, the resulting porous-C/Ni 2 SeS requires an overpotential of only 121 mV at a current density of 10 mA cm -2 with a Tafel slope of 78 mV dec -1 for hydrogen evolution in acidic media and a good long-term stability. Density functional theory calculations also show that the Gibbs free energy of hydrogen adsorption of the Ni 2 SeS was -0.23 eV, which not only is close to the ideal value (0 eV) and Pt reference (-0.09 eV) but also is lower than those of NiS 2 and NiSe 2 ; large electrical states exist in the vicinity of the Fermi level, which further improves its electrocatalytic performance. This work provides new insights into the rational design of ternary dichalcogenides and hollow structure materials for practical applications in HER catalysis and energy fields.

A nanostructural catalyst with long-term durability under harsh conditions is very important for an outstanding catalytic performance. Herein, a new ultrastable PtCo/Co 3 O 4 -SiO 2 nanocatalyst was explored to improve the catalytic performance of carbon monoxide (CO) oxidation by virtue of the surface active lattice oxygen derived from strong metal-support interactions. Such a structure can overcome the issues of Co 3 O 4 -SiO 2 inactivation by water vapor and the Pt inferior activity at low temperature. Further, Co 3 O 4 -SiO 2 nanosheets endow superior structure stability under high temperatures of up to 800 °C, which gives long-term catalytic cyclability of PtCo/Co 3 O 4 -SiO 2 nanocomposites for CO oxidation. Moreover, the large specific surface areas (294 m 2 g -1 ) of the nanosheet structure can expose abundant surface active lattice oxygen, which significantly enhanced the catalytic activity of CO oxidation at 50 °C over 30 days without apparent aggregation of PtCo nanoparticles after 20 cycles from 50 to 400 °C. It can be expected to be a promising candidate as an ultrastable efficient catalyst.

Recent discovery of piezoelectricity that existed in two-dimensional (2D) layered materials represents a key milestone for flexible electronics and miniaturized and wearable devices. However, so far the reported piezoelectricity in these 2D layered materials is too weak to be used for any practical applications. In this work, we discovered that grain boundaries (GBs) in monolayer MoS 2 can significantly enhance its piezoelectric property. The output power of piezoelectric devices made of the butterfly-shaped monolayer MoS 2 was improved about 50% by the GB-induced piezoelectric effect. The enhanced piezoelectricity is attributed to the additional piezoelectric effect induced by the existence of deformable GBs which can promote polarization and generates spontaneous polarization with different piezoelectric coefficients along various directions. We further made a flexible piezoelectric device based on the 2D MoS 2 with the GBs and demonstrated its potential application in self-powered precision sensors for in situ detecting pressure changes in human blood for health monitoring.

Piezoelectric two-dimensional (2D) van der Waals (vdWs) materials are highly desirable for applications in miniaturized and flexible/wearable devices. However, the reverse-polarization between adjacent layers in current 2D layered materials results in decreasing their in-plane piezoelectric coefficients with layer number, which limits their practical applications. Here, we report a class of 2D layered materials with an identical orientation of in-plane polarization. Their piezoelectric coefficients ( e 22 ) increase with layer number, thereby allowing for the fabrication of flexible piezotronic devices with large piezoelectric responsivity and excellent mechanical durability. The piezoelectric outputs can reach up to 0.363 V for a 7-layer α-In 2 Se 3 device, with a current responsivity of 598.1 pA for 1% strain, which is 1 order of magnitude higher than the values of the reported 2D piezoelectrics. The self-powered piezoelectric sensors made of these newly developed 2D layered materials have been successfully used for real-time health monitoring, proving their suitability for the fabrication of flexible piezotronic devices due to their large piezoelectric responses and excellent mechanical durability.

Ultrafast response/recovery and high selectivity of gas sensors are critical for real-time and online monitoring of hazardous gases. In this work, α-Fe 2 O 3 nano-ellipsoids were synthesized using a facile one-step hydrothermal method and investigated as highly sensitive H 2 S-sensing materials. The nano-ellipsoids have an average long-axis diameter of 275 nm and an average short-axis diameter of 125 nm. H 2 S gas sensors fabricated using the α-Fe 2 O 3 nano-ellipsoids showed excellent H 2 S-sensing performance at an optimum working temperature of 260 °C. The response and recovery times were 0.8 s/2.2 s for H 2 S gas with a concentration of 50 ppm, which are much faster than those of H 2 S gas sensors reported in the literature. The α-Fe 2 O 3 nano-ellipsoid-based sensors also showed high selectivity to H 2 S compared to other commonly investigated gases including NH 3 , CO, NO 2 , H 2 , CH 2 Cl 2 , and ethanol. In addition, the sensors exhibited high-response values to different concentrations of H 2 S with a detection limit as low as 100 ppb, as well as excellent repeatability and long-term stability.

Developing high-active and low-cost bifunctional materials for catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) holds a pivotal role in water splitting. Therefore, we present a new strategy to form NiS/Ni 2 P heterostructures. The as-obtained NiS/Ni 2 P/carbon cloth (CC) requires overpotentials of 111 mV for the HER and 265 mV for the OER to reach a current density of 20 mA cm -2 , outperforming their counterparts such as NiS and Ni 2 P under the same conditions. Additionally, the NiS/Ni 2 P/CC electrode requires a 1.67 V cell voltage to deliver 10 mA cm -2 in a two-electrode electrolysis system, which is comparable to the cell using the benchmark Pt/C||RuO 2 electrode. Detailed characterizations reveal that strong electronic interactions between NiS and Ni 2 P, abundant active sites, and smaller charge-transfer resistance contribute to the improved HER and OER activity.

Nanowire lithography (NWL) uses nanowires (NWs), grown and assembled by chemical methods, as etch masks to transfer their one-dimensional morphology to an underlying substrate. Here, we show that SiO2 NWs are a simple and compatible system to implement NWL on crystalline silicon and fabricate a wide range of architectures and devices. Planar field-effect transistors made of a single SOI-NW channel exhibit a contact resistance below 20 kOmega and scale with the channel width. Further, we assess the electrical response of NW networks obtained using a mask of SiO2 NWs ink-jetted from solution. The resulting conformal network etched into the underlying wafer is monolithic, with single-crystalline bulk junctions; thus no difference in conductivity is seen between a direct NW bridge and a percolating network. We also extend the potential of NWL into the third dimension, by using a periodic undercutting that produces an array of vertically stacked NWs from a single NW mask.