Interfaces between a liquid and a solid (L-S) are the most important surface science in chemistry, catalysis, energy, and even biology. Formation of an electric double layer (EDL) at the L-S interface has been attributed due to the adsorption of a layer of ions at the solid surface, which causes the ions in the liquid to redistribute. Although the existence of a layer of charges on a solid surface is always assumed, the origin of the charges is not extensively explored. Recent studies of contact electrification (CE) between a liquid and a solid suggest that electron transfer plays a dominant role at the initial stage for forming the charge layer at the L-S interface. Here, we review the recent works about electron transfer in liquid-solid CE, including scenerios such as liquid-insulator, liquid-semiconductor, and liquid-metal. Formation of the EDL is revisited considering the existence of electron transfer at the L-S interface. Furthermore, the triboelectric nanogenerator (TENG) technique based on the liquid-solid CE is introduced, which can be used not only for harvesting mechanical energy from a liquid but also as a probe for probing the charge transfer at liquid-solid interfaces.

Contact Electrification at the Liquid-Solid Interface.

By combining the effects of contact electrification and electrostatic induction, triboelectric nanogenerators (TENGs) can effectively convert mechanical energy into electric power or signals. Over the past decade, TENG development has progressed rapidly, from fundamental scientific understanding to advanced technologies and applications. This Primer gives a brief overview of TENGs, including the mechanisms of contact electrification and electrodynamics, applications, future opportunities and limitations. As an interdisciplinary field, advances are expected in both theoretical and experimental aspects of TENGs. For example, technologies based on Maxwell’s equations for a mechano-driven, slow-moving system can be coupled with a theoretical understanding of physical laws and concepts. From this, general simulation models can be established and corresponding experiments designed to optimize TENGs for a range of applications. Triboelectric nanogenerators (TENGs) convert mechanical energy into electric power by combining contact electrification and electrostatic induction. This Primer introduces the theoretical background of TENGs, gives an overview of fabrication methods and discusses how they can be applied as energy harvesting devices and self-powered systems. 

Triboelectric nanogenerators

DOI: 10.1002/aenm.201903713 power supply has become miniaturized, discretized, environmentally friendly, and sustainable, which poses new challenges for energy harvesting from ambient environment.[1–4] As a new energy technology, triboelectric nanogenerators (TENGs) have the characteristics of light weight, small size, wide selection of materials, and high energy conversion efficiency, which can meet the energy demand of the information society.[2,5,6] Compared with electromagnetic generators, piezoelectric generators, and electrostatic generators, TENGs have enormous advantages as a killer-level application to harvest the lowfrequency and small-amplitude abundant mechanical energy in the environment.[7,8] Based on the triboelectric effect and electrostatic induction effect, the traditional four modes of TENGs (contactseparation mode, lateral sliding mode, single-electrode mode, and freestandingtriboelectric-layer mode) can provide alternating pulses to the external circuit through the change of contact area or separation distance.[9] However, conventional TENGs usually use organic polymer insulation materials, and the charges generated by friction tends to accumulate on the surface of the material, resulting in an alternating output.[10–13] Recently, semiconductor materials began to attract researchers’ interest, and a series of work has made great progress: the tribotunneling nanogenerator based on The triboelectric nanogenerator (TENG) is a new energy technology that is enabled by coupled contact electrification and electrostatic induction. The conventional TENGs are usually based on organic polymer insulator materials, which have the limitations and disadvantages of high impedance and alternating output current. Here, a tribovoltaic effect based metal– semiconductor direct-current triboelectric nanogenerator (MSDC-TENG) is reported. The tribovoltaic effect is facilitated by direct voltage and current by rubbing a metal/semiconductor on another semiconductor. The frictional energy released by the forming atomic bonds excites nonequilibrium carriers, which are directionally separated to form a current under the built-in electric field. The continuous average open-circuit voltage (10–20 mV), short-circuit directcurrent output (10–20 μA), and low impedance characteristic (0.55–5 kΩ) of the MSDC-TENG can be observed during relative sliding of the metal and silicon. The working parameters are systematically studied for electric output and impedance characteristics. The results reveal that faster velocity, larger pressure, and smaller area can improve the maximum power density. The internal resistance is mainly determined by the velocity and the electrical resistance of semiconductor. This work not only expands the material candidates of TENGs from organic polymers to semiconductors, but also demonstrates a tribovoltaic effect based electric energy conversion mechanism.

Tribovoltaic Effect on Metal–Semiconductor Interface for Direct-Current Low-Impedance Triboelectric Nanogenerators

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

The tribovoltaic effect and electron transfer at a liquid-semiconductor interface

Summary Although perovskite has been widely explored in optoelectronic devices, it has been rarely explored as a Schottky junction. Here, we demonstrate a light- and mechanical-energy co-harvesting generator based on a dynamic metal/perovskite Schottky junction. In dark conditions, by moving Al over perovskite film (conductivity: 70.0 S/cm), output voltage and current density of −0.70 V and −41.1 A/m2, respectively, are achieved. As defects eliminate the photo-generated carriers in the static metal/perovskite junction under light illumination, the photoresponse is negligible. Strikingly, the output current of dynamic metal/perovskite junction under light illumination is enhanced by 3-fold compared with the dynamic junction in the dark condition. This nonlinear photo-enhanced effect is proposed as a result of strong interaction between the photo-generated and bound back carriers in the dynamic Schottky junction as well as the high light-absorption coefficient of perovskite. A flexible Al/perovskite generator has been demonstrated as a wearable energy-harvesting device.

Co-harvesting Light and Mechanical Energy Based on Dynamic Metal/Perovskite Schottky Junction

Direct-Current Generator Based on Dynamic PN Junctions with the Designed Voltage Output.

The overloaded energy cost has become the main concern of the now fast developing society, which make novel energy devices with high power density of critical importance to the sustainable development of human society. Herein, a dynamic Schottky diode based generator with ultrahigh power density of 1262.0 W m-2 for sliding Fe tip on rough p-type silicon is reported. Intriguingly, the increased surface states after rough treatment lead to an extremely enhanced current density up to 2.7 × 105 A m-2, as the charged surface states can effectively accelerate the carriers through large atomic electric field, while the reflecting directions are regulated by the built-in electric field of the Schottky barrier. This research provides an open avenue for utilizing the surface states in semiconductors in a subversive way, which can co-utilize the atomic electric field and built-in electric field to harvest energy from the mechanical movements, especially for achieving an ultrahigh current density power source.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201901925

Surface States Enhanced Dynamic Schottky Diode Generator with Extremely High Power Density Over 1000 W m-2.

After the electromagnetic generator, searching for novel electric generators without strong magnetic field is highly demanded. The generator without strong magnetic field calls for a physical picture distinct from the traditional generators. As the counterpart of the static PN junction has been widely used in the integrated circuits, we develop an electric generator named dynamic PN generator with a high current density and voltage output, which converts mechanical energy into electricity by sliding two semiconductors with different Fermi level. A dynamic N-GaAs/SiO2/P-Si generator with the open-circuit voltage of 3.1 V and short-circuit density of 1.0 A/m2 have been achieved. The physical mechanism of the dynamic PN generator is proposed based on the built-in electric field bounding back diffusing carriers in dynamic PN junctions, which breaks the equilibrium between drift and diffusion current in the PN junction. Moreover, the dynamic MoS2/AlN/Si generator with the open-circuit voltage of 5.1 V and short-circuit density of 112 A/m2 (11.2 mA/cm2) have also been achieved, which can effectively output a direct-current and light up a blue light-emitting diode directly. This dynamic MoS2/AlN/Si generator can continuously work for hours without obvious degradation, demonstrating its unique mechanism and potential applications in many fields where the mechanical energy is available.

High Performance Direct-Current Generator Based on Dynamic PN Junctions

Dynamic Schottky Diode Direct-Current Generator under Extremely Low Temperature

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