Contact-electro-catalysis for Direct Synthesis of H2 O2 under Ambient Conditions.

Hydrogen peroxide (H2 O2 ) is an indispensable basic reagent in various industries, such as textile bleach, chemical synthesis, and environmental protection. However, it is challenging to prepare H2 O2 in a green, safe, simple and efficient way under ambient conditions. Here, we found that H2 O2 could be synthesized using a catalytic pathway only by contact charging a two-phase interface at room temperature and normal pressure. Particularly, under the action of mechanical force, electron transfer occurs during physical contact between polytetrafluoroethylene particles and deionized water/O2 interfaces, inducing the generation of reactive free radicals (⋅OH and ⋅O2 - ), and the free radicals could react to form H2 O2 , yielding as high as 313 µmol L-1 h-1 . In addition, the new reaction device could show long-term stable H2 O2 production. This work provides a novel method for the efficient preparation of H2 O2 , which may also stimulate further explorations on contact-electrification-induced chemistry process.

Multifunctional Meta-Tribomaterial Nanogenerators for Energy Harvesting and Active Sensing.

Discovering novel multifunctional metamaterials with energy harvesting and sensing functionalities is likely to be the next technological evolution of the metamaterial science. Here, we introduce a novel concept called self-aware composite mechanical metamaterial (SCMM) that can transform mechanical metamaterials into nanogenerators and active sensing mediums. In pursuit of this goal, we examine new paradigms where finely tailored and seamlessly integrated self-recovering snapping microstructures composed of topologically different triboelectric materials can form self-powering and self-sensing meta-tribomaterial systems. We explore various deformation mechanisms required to induce contact electrification between these snapping microstructures under periodic deformations. The multifunctional meta-tribomaterial systems created under the SCMM concept will act as triboelectric nanogenerators capable of generating electrical signals in response to the applied mechanical excitations. The generated electrical signal can be used for active sensing of the applied force and can be stored for empowering sensors and embedded electronics. We conduct theoretical and experimental studies to understand the mechanical and electrical behavior of the multifunctional SCMM systems. The broad application of the proposed SCMM concept for designing artificial materials with novel properties and functionalities is highlighted via prototyping self-powering and self-sensing blood vessel stents and shock absorbers.

Power generation from the interaction of a liquid droplet and a liquid membrane.

Triboelectric nanogenerators are an energy harvesting technology that relies on the coupling effects of contact electrification and electrostatic induction between two solids or a liquid and a solid. Here, we present a triboelectric nanogenerator that can work based on the interaction between two pure liquids. A liquid-liquid triboelectric nanogenerator is achieved by passing a liquid droplet through a freely suspended liquid membrane. We investigate two kinds of liquid membranes: a grounded membrane and a pre-charged membrane. The falling of a droplet (about 40 µL) can generate a peak power of 137.4 nW by passing through a pre-charged membrane. Moreover, this membrane electrode can also remove and collect electrostatic charges from solid objects, indicating a permeable sensor or charge filter for electronic applications. The liquid-liquid triboelectric nanogenerator can harvest mechanical energy without changing the object motion and it can work for many targets, including raindrops, irrigation currents, microfluidics, and tiny particles.

Piezotronic effect tuned AlGaN/GaN high electron mobility transistor.

The piezotronic effect utilizes strain-induced piezoelectric polarization charges to tune the carrier transportation across the interface/junction. We fabricated a high-performance AlGaN/GaN high electron mobility transistor (HEMT), and the transport property was proven to be enhanced by applying an external stress for the first time. The enhanced source-drain current was also observed at any gate voltage and the maximum enhancement of the saturation current was up to 21% with 15 N applied stress (0.18 GPa at center) at -1 V gate voltage. The physical mechanism of HEMT with/without external compressive stress conditions was carefully illustrated and further confirmed by a self-consistent solution of the Schrödinger-Poisson equations. This study proves the cause-and-effect relationship between the piezoelectric polarization effect and 2D electron gas formation, which provides a tunable solution to enhance the device performance. The strain tuned HEMT has potential applications in human-machine interface and the security control of the power system.

Nanogenerator-based dual-functional and self-powered thin patch loudspeaker or microphone for flexible electronics.

Ferroelectret nanogenerators were recently introduced as a promising alternative technology for harvesting kinetic energy. Here we report the device's intrinsic properties that allow for the bidirectional conversion of energy between electrical and mechanical domains; thus extending its potential use in wearable electronics beyond the power generation realm. This electromechanical coupling, combined with their flexibility and thin film-like form, bestows dual-functional transducing capabilities to the device that are used in this work to demonstrate its use as a thin, wearable and self-powered loudspeaker or microphone patch. To determine the device's performance and applicability, sound pressure level is characterized in both space and frequency domains for three different configurations. The confirmed device's high performance is further validated through its integration in three different systems: a music-playing flag, a sound recording film and a flexible microphone for security applications.

Enhancing Photoresponsivity of Self-Aligned MoS2 Field-Effect Transistors by Piezo-Phototronic Effect from GaN Nanowires.

We report high-performance self-aligned MoS2 field-effect transistors (FETs) with enhanced photoresponsivity by the piezo-phototronic effect. The FETs are fabricated based on monolayer MoS2 with a piezoelectric GaN nanowire (NW) as the local gate, and a self-aligned process is employed to define the source/drain electrodes. The fabrication method allows the preservation of the intrinsic property of MoS2 and suppresses the scattering center density in the MoS2/GaN interface, which results in high electrical and photoelectric performances. MoS2 FETs with channel lengths of ∼200 nm have been fabricated with a small subthreshold slope of 64 mV/dec. The photoresponsivity is 443.3 A·W(-1), with a fast response and recovery time of ∼5 ms under 550 nm light illumination. When strain is introduced into the GaN NW, the photoresponsivity is further enhanced to 734.5 A·W(-1) and maintains consistent response and recovery time, which is comparable with that of the mechanical exfoliation of MoS2 transistors. The approach presented here opens an avenue to high-performance top-gated piezo-enhanced MoS2 photodetectors.

A self-powered AC magnetic sensor based on piezoelectric nanogenerator.

An AC magnetic field, which is a carrier of information, is distributed everywhere and is continuous. How to use and detect this field has been an ongoing topic over the past few decades. Conventional magnetic sensors are usually based on the Hall Effect, the fluxgate, a superconductor quantum interface or magnetoelectric or magnetoresistive sensing. Here, a flexible, simple, low-cost and self-powered active piezoelectric nanogenerator (NG) is successfully demonstrated as an AC magnetic field sensor at room temperature. The amplitude and frequency of a magnetic field can both be accurately sensed by the NG. The output voltage of the NG has a good linearity with a measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10(-7) tesla, which is 400 times greater than a commercial magnetic sensor that uses the Hall Effect. In comparison to the existing technologies, an NG is a room-temperature self-powered active sensor that is very simple and very cheap for practical applications.

PVDF-PZT nanocomposite film based self-charging power cell.

A novel PVDF-PZT nanocomposite film has been proposed and used as a piezoseparator in self-charging power cells (SCPCs). The structure, composed of poly(vinylidene fluoride) (PVDF) and lead zirconate titanate (PZT), provides a high piezoelectric output, because PZT in this nanocomposite film can improve the piezopotential compared to the pure PVDF film. The SCPC based on this nanocomposite film can be efficiently charged up by the mechanical deformation in the absence of an external power source. The charge capacity of the PVDF-PZT nanocomposite film based SCPC in 240 s is ∼0.010 µA h, higher than that of a pure PVDF film based SCPC (∼0.004 µA h). This is the first demonstration of using PVDF-PZT nanocomposite film as a piezoseparator for SCPC, and is an important step for the practical applications of SCPC for harvesting and storing mechanical energy.