The study presents the fundamental scientific understanding of electron transfer in contact electrification in solid–solid and liquid–solid cases and a newly revised model for the formation of ...electric double layer. The potential revolutionary impacts of triboelectric nanogenerators as energy sources and sensors are presented in the fields of health care, environmental science, wearable electronics, internet of things, human–machine interfacing, robotics, and artificial intelligence.
This essay presents the fundamental scientific understanding of electron transfer in contact electrification in solid–solid and liquid–solid cases and a newly revised model for the formation of electric double layer. The potential revolutionary impacts of triboelectric nanogenerators as energy sources and sensors are presented in the fields of health care, environmental science, wearable electronics, internet of things, human–machine interfacing, robotics, and artificial intelligence.
Nanogenerators (NGs) are a field that uses Maxwell's displacement current as the driving force for effectively converting mechanical energy into electric power/signal, which have broad applications ...in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics and artificial intelligence. NGs are usually based on three effects: piezoelectricity, triboelectricity (contact electrification), and pyroelectricity. In this paper, a formal theory for NGs is presented starting from Maxwell's equations. Besides the general expression for displacement vector D = εE used for deriving classical electromagnetic dynamics, we added an additional term Ps in D, which represents the polarization created by the electrostatic surface charges owing to piezoelectricity and/or triboelectricity as a result of mechanical triggering in NG. In contrast to P that is resulted from the electric field induced medium polarization and vanishes if E = 0, Ps remains even when there is no external electric field. We reformulated the Maxwell equations that include both the medium polarizations due to electric field (P) and non-electric field (such as strain) (Ps) induced polarization terms, from which, the output power, electromagnetic behavior and current transport equation for a NG are systematically derived. A general solution is presented for the modified Maxwell equations, and analytical solutions about the output potential are provided for a few cases. The displacement current arising from ε∂E/∂t is responsible for the electromagnetic waves, while the newly added term ∂Ps/∂t is the application of Maxwell's equations in energy and sensors. This work sets the first principle theory for quantifying the performance and electromagnetic behavior of a nanogenerator in general.
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•Nanogenerators (NGs) are a field that uses Maxwell's displacement current as the driving force for effectively converting mechanical energy into electric power/signal.•A formal theory for NGs is presented starting from Maxwell's equations.•Expanding the applications of Maxwell's equations in energy and sensors.
Entropy was first introduced in thermodynamics for describing the statistical behavior of molecules. In this paper, we use the entropy theory to describe the distribution and powering of small ...electronics in the era of internet of things. We refer the power transmitted from power plants to utility units as “ordered” energy for fixed sites, while refer the energy harvested from environment as “random” energy. Our conclusion is that the “ordered” energy is able to solve part of the power need for distributed electronics for IoTs, but the remaining part has to be supplied by the “random” energy harvested from our living environment using resources such as solar, vibration, motion and even thermal. The entropy idea defines the role played by energy harvesting as it will become increasingly important owing to the ever expanding usage of distributed electronics at the time of intelligence.
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•The entropy theory is applied to describe the distribution and powering of small electronics in the era of internet of things.•The “ordered” energy is able to solve part of the power need for IoTs, but the remaining part has to be supplied by the “random” energy harvested from environment.•The entropy idea defines the role played by energy harvesting as it will become increasingly important owing to the ever expanding usage of distributed electronics.
Integration of advanced nanogenerator technology with conventional textile processes fosters the emergence of textile‐based nanogenerators (NGs), which will inevitably promote the rapid development ...and widespread applications of next‐generation wearable electronics and multifaceted artificial intelligence systems. NGs endow smart textiles with mechanical energy harvesting and multifunctional self‐powered sensing capabilities, while textiles provide a versatile flexible design carrier and extensive wearable application platform for their development. However, due to the lack of an effective interactive platform and communication channel between researchers specializing in NGs and those good at textiles, it is rather difficult to achieve fiber/fabric‐based NGs with both excellent electrical output properties and outstanding textile‐related performances. To this end, a critical review is presented on the current state of the arts of wearable fiber/fabric‐based piezoelectric nanogenerators and triboelectric nanogenerators with respect to basic classifications, material selections, fabrication techniques, structural designs, and working principles, as well as potential applications. Furthermore, the potential difficulties and tough challenges that can impede their large‐scale commercial applications are summarized and discussed. It is hoped that this review will not only deepen the ties between smart textiles and wearable NGs, but also push forward further research and applications of future wearable fiber/fabric‐based NGs.
Combining the advantages of smart textiles and mechanical energy harvesting technology, fiber/fabric‐based piezoelectric and triboelectric nanogenerators will play an increasing role in wearable electronics and artificial intelligences. In view of their current research status and development trends, a comprehensive, systematic, and multiperspective review is presented to provide better understanding and beneficial guidance for future research and product design.
Contact electrification (CE) has been known for more than 2600 years but the nature of charge carriers and their transfer mechanisms still remain poorly understood, especially for the cases of ...liquid-solid CE. Here, we study the CE between liquids and solids and investigate the decay of CE charges on the solid surfaces after liquid-solid CE at different thermal conditions. The contribution of electron transfer is distinguished from that of ion transfer on the charged surfaces by using the theory of electron thermionic emission. Our study shows that there are both electron transfer and ion transfer in the liquid-solid CE. We reveal that solutes in the solution, pH value of the solution and the hydrophilicity of the solid affect the ratio of electron transfers to ion transfers. Further, we propose a two-step model of electron or/and ion transfer and demonstrate the formation of electric double-layer in liquid-solid CE.
To meet future needs for clean and sustainable energy, tremendous progress has been achieved in development for scavenging wind energy. The most classical approach is to use the electromagnetic ...effect based wind turbine with a diameter of larger than 50 m and a weight of larger than 50 ton, and each of them could cost more than $0.5 M, which can only be installed in remote areas. Alternatively, triboelectric nanogenerators based on coupling of contact‐electrification and electrostatic induction effects have been utilized to scavenge wind energy, which takes the advantages of high voltage, low cost, and small size. Here, the development of a wind‐driven triboelectric nanogenerator by focusing on triboelectric materials optimization, structure improvement, and hybridization with other types of energy harvesting techniques is reviewed. Moreover, the major applications are summarized and the challenges that are needed to be addressed and development direction for scavenging wind energy in future are highlighted.
Triboelectric nanogenerators have been utilized to scavenge wind energy by coupling contact‐electrification and electrostatic induction effects, where these devices have advantages of high voltage, low cost, and small size as compared with conventional electromagnetic effect‐based wind turbines.
Piezopotential‐assisted catalysis is of great significance for low cost and efficient catalysis processes. Here, Aux/BaTiO3 plasmonic photocatalysts are fabricated by precipitating Au nanoparticles ...on piezoelectric BaTiO3 nanocubes through a chemical approach. The Au nanoparticles (<8 nm) are decorated uniformly on the surface of BaTiO3, which endows the heterostructure with a wide light absorption from 300 to 600 nm. The photocatalytic properties of the heterostructures are investigated in detail toward methyl orange (MO) degradation. The Au content, piezoelectric potential of the BaTiO3 substrate, and surface plasmon resonance (SPR) are confirmed to be vital to the photocatalytic activity. The Au4/BaTiO3 shows an optimum photocatalytic performance for a complete degradation of MO in 75 min under full spectrum light irradiation with auxiliary ultrasonic excitation. The piezoelectric field originating from the deformation of BaTiO3 further enhances the separation of photon‐generated carriers induced by SPR and promotes the formation of hydroxyl radicals, which results in a strong oxidizing ability of organic dyes. This work introduces the piezotronic effect to enhance plasmonic photocatalysis with Aux/BaTiO3 heterostructures, which is ready to extend to other catalytic systems and offers a new option to design high‐performance catalysts for pollutant treatment.
The piezotronic effect is introduced to enhance plasmonic photocatalysis by fabricating Aux/BaTiO3 nano‐heterostructures. Piezoelectric polarization of the BaTiO3 nanocrystal upon sonication suppresses the recombination of photogenerated hot electron–hole pairs to enhance the localized surface plasmon resonance of Au nanoparticles to improve the photocatalysis process.
The fundamental principle of piezotronics and piezo‐phototronics were introduced by Wang in 2007 and 2010, respectively. Due to the polarization of ions in a crystal that has non‐central symmetry in ...materials such as the wurtzite structured ZnO, GaN and InN, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. Owing to the simultaneous possession of piezoelectricity and semiconductor properties, the piezopotential created in the crystal has a strong effect on the carrier transport at the interface/junction. Piezotronics is about the devices fabricated using the piezopotential as a “gate” voltage to tune/control charge carrier transport at a contact or junction. The piezo‐phototronic effect is to use the piezopotential to control the carrier generation, transport, separation and/or recombination for improving the performance of optoelectronic devices, such as photon detector, solar cell and LED. This manuscript reviews the updated progress in the two new fields. A perspective is given about their potential applications in sensors, human‐silicon technology interfacing, MEMS, nanorobotics and energy sciences.
Piezotronics is about devices fabricated using the piezopotential as a “gate” voltage to tune/control charge carrier transport at a contact or junction. The piezo‐phototronic effect is to use the piezopotential to control the carrier generation, transport, separation and/or recombination for improving the performance of optoelectronic devices, such as photon detectors, solar cells and LEDs. This manuscript reviews progress in these two new fields. A perspective is given about their potential applications in sensors, human‐silicon technology interfacing, MEMS, nanorobotics, and energy sciences.
The Coronavirus Disease 2019 (COVID-19) pandemic continues to have a devastating effect on the health and well-being of the global population. A critical step in the fight against COVID-19 is ...effective screening of infected patients, with one of the key screening approaches being radiology examination using chest radiography. It was found in early studies that patients present abnormalities in chest radiography images that are characteristic of those infected with COVID-19. Motivated by this and inspired by the open source efforts of the research community, in this study we introduce COVID-Net, a deep convolutional neural network design tailored for the detection of COVID-19 cases from chest X-ray (CXR) images that is open source and available to the general public. To the best of the authors' knowledge, COVID-Net is one of the first open source network designs for COVID-19 detection from CXR images at the time of initial release. We also introduce COVIDx, an open access benchmark dataset that we generated comprising of 13,975 CXR images across 13,870 patient patient cases, with the largest number of publicly available COVID-19 positive cases to the best of the authors' knowledge. Furthermore, we investigate how COVID-Net makes predictions using an explainability method in an attempt to not only gain deeper insights into critical factors associated with COVID cases, which can aid clinicians in improved screening, but also audit COVID-Net in a responsible and transparent manner to validate that it is making decisions based on relevant information from the CXR images. By no means a production-ready solution, the hope is that the open access COVID-Net, along with the description on constructing the open source COVIDx dataset, will be leveraged and build upon by both researchers and citizen data scientists alike to accelerate the development of highly accurate yet practical deep learning solutions for detecting COVID-19 cases and accelerate treatment of those who need it the most.
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.
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