Solid-liquid Contact Electrification Research Articles

A Highly Sensitive Mercury Ion Sensor Based on Solid-Liquid Contact Electrification

In recent years, triboelectric nanogenerators (TENGs) are proved to be as the prime backbone for developing a self-powered sensing system. However, solid-solid contact electrification based nanogenerators suffer mostly due to inefficient contact which poses a major bottleneck for the development of long term durable and stable nanosensors. In this regard, we have reported a strategic methodology to develop a highly sensitive mercury ion sensor based on solid-liquid contact electrification, which has prime importance for the self-powered monitoring of mercury ion due to its high health risk and environmental pollution toxicity. In this work, 3-mercaptopropionic acid (MPA) capped Au nanoparticles (NPs) are employed as the solid friction layer as well as the recognition element for mercury ion detection. In addition, volatile organic solvent acetone is utilized as the contact liquid instead of water. The developed nanosensor exhibits long term stability and contact frequency independent sensing performance compared to previously reported solid-solid triboelectric nanosensors (TENS) for mercury ion detection. The large binding affinity of Hg2+ and the carboxylic groups results in the increase of transferred charges and enhanced surface potential. It is interesting to observe that the work function reduces after the binding of Hg2+ ions onto MPA molecules, which is also favorable for electron transfer during the contact electrification process. The developed nanosensor can provide a wide linear detection range from 10 nM to 1 μM as well as a low detection limit of 10 nM. As a whole, this work demonstrates a novel paradigm for designing a rapid, low-cost, and portable self-powered sensing system for real time highly selective monitoring of mercury ion from complex environmental samples.

Enhanced sensing performance of triboelectric nanosensors by solid-liquid contact electrification

Triboelectric nanogenerators (TENGs) and triboelectric nanosensors (TENSs) are the prime backbones for the realization of environmental mechanical energy harvesting and self-powered sensing applications. However, the low efficiency of concurrent solid-solid contact electrification creates a major bottleneck for the growth of highly promising technologies. To address this problem, herein, we report a strategic protocol to design a TENS by solid-liquid contact electrification for chemical sensing purposes as well as an efficient approach for the chemical enhancement of solid-liquid TENGs. In particular, TiO2 nanosheet arrays and various solvents (including water, ethanol and acetone) are employed as solid triboelectric materials and contact liquids, respectively, for the demonstration of solid-liquid contact electrification for mechanical energy harvesting and catechin detection. As a self-powered sensor, the TiO2 nanosheet array-based TENS provides superior advantages such as long-term stability, frequency-independent output and humidity-insensitive properties compared to previously reported solid-solid TENSs. The chemically enhanced sensing mechanism of the TiO2 nanosheet array-based TENS for catechin detection is further confirmed with the decrease in the work function and can provide a wide linear window (100 nM–100 μM) and a low detection limit (30 nM). All the results support that solid-liquid TENSs pave a new path toward efficient self-powered sensors for environmental and healthcare monitoring.

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

Abstract Contact electrification (CE) is a ubiquitous phenomenon, which occurs at almost any interfaces between any phases, but its mechanism still remains ambiguous, especially for the liquid-solid case. Here, the tribo-voltage and the DC tribo-current are generated by sliding a water droplet over a silicon surface. By varying the doping types and concentrations, the direction of the tribo-voltage and the tribo-current is found highly depending on the built-in electric field at the liquid-solid junction. It is suggested that the electrical output is induced by the triboelectric effect at the water-silicon interface, in which electron-hole pairs are excited at the silicon surface by the energy released at interface during sliding, and further separated by the built-in electric field, which is analogous to that occurs in the photovoltaic effect. The only difference is that the energy in tribovoltaic effect is provided by electron transfer across the sliding interface owing to CE and bonding interaction between liquid molecules and semiconductor surface, rather than by photon. It is demonstrated that the electron transfer exists at the liquid-solid interface, and the “two-step” model for the formation of the electric double-layer is further verified, in which the electron transfer is considered to occur in the first step when the liquid contacts a solid in the very first time, which is then followed by ion chemical/physical adsorption as the second step. The tribovoltaic effect provides a new approach for studying of the mechanism at liquid-solid CE.

A Self Powered Solid-Liquid Contact Electrification Based Nanosensor for Chemically Enhanced Detection of Catechin

The harvesting of mechanical energy based on triboelectric effect between solid-solid material and utilizing it for chemical sensing, have been proved to be simple, cost effective and robust. However, it suffers from several drawbacks such as surface instability and drift in output voltage due to several environmental factors like humidity. In this study, a self-powered triboelectric nanosensor (TENS) based on chemically enhanced solid-liquid contact electrification has been demonstrated. The TiO2 nanosheet (NS) array, grown over Ti by two-step hydrothermal process is employed as solid triboelectric material. On the other side, volatile organic solvent acetone is utilized as contact liquid to obtain higher triboelectric performance. A periodic contact separation between TiO2 NS and acetone generates a series of contact frequency independent output voltage cycles with long term stability in comparison to solid-solid contact electrification earlier. When TiO2 NS surface is forced to make a full contact with acetone, the ionization of surface groups on the TiO2 NS surface causes TiO2 NS to be fully positively charged, creating a negatively charged electrical double-layer on the contact surface of acetone to maintain the charge neutrality. As the TiO2 NS surface is moving off the acetone solution, the emerging part of its surface remains positively charged, leaving the negative electrical double-layer in the acetone solution. As a result, a potential difference is developed between TiO2 NS and acetone. Thus, it leads to transfer of electrons from ground to Ti electrode until TiO2 NS is entirely emerged from acetone, reaching an equilibrium state. The chemical modification of TiO2 NS surface with catechin promotes enhanced response voltage due to ligand to metal charge transfer complex between Ti molecules and enediol ligands of catechin, exhibiting 1 nM limit of detection(LOD) and wide linear range (1 µM to 100 µM). The output voltage has been chemically enhanced 1.59 times for 10 mM catechin concentration. Therefore, the study not only demonstrates a self-powered cost-effective nano-sensor but also explores the possibility to improve the electrical performance of solid-liquid based contact electrification through surface chemical environment engineering and through utilizing volatile organic solvents as contact liquid.

Ultrasensitive and Label-Free Detection of Mercury Ions Using a Highly Efficient Solid-Liquid Triboelectric Nanosensor

In the recent years, mercury pollution in the world have increased tremendously due to both natural and anthropogenic reasons. The mercury ions (Hg2+) after being released from the industrial wastes into the environment is transformed into the highly toxic form of methylmercury (CH3Hg+) by the microorganisms present, which further accumulates in the human body via drinking water and food, ultimately causing irreversible damage to reproductive, immune and nervous system. Currently, analytical tools such as ICP-MS, X-ray fluorescence spectroscopy, AES etc are employed for detection of the Hg2+ ions but their utility is overshadowed by certain limitations like complicated sample preparation steps, time-consuming protocol, non-portable nature, requirement of trained professionals etc. Therefore, in this work we propose a highly sensitive and selective sensing method for Hg2+ ions using the concept of solid-liquid contact electrification. This triboelectric nanosensor was developed employing modified Copper(II) oxide nanowires (CuO NWs) as the solid-triboelectric layer and deionized water as the liquid- triboelectric layer. Due to the highly specific reaction between the modified CuO NWs surface and Hg2+ ions, the as-developed sensor can detect the Hg2+ ion concentrations as low as 0.1 nM with a good linear range of 1nM-10 μM. In addition, the unrivalledly flexible nanosensor is coupled with the robotic fingertips for highly automated on-site monitoring of Hg2+ ions, making it possible to track the heavy metal pollution in inaccessible and remote locations. We believe that this work will be a stepping stone towards designing a reusable, portable, low-cost and stand-alone sensing systems which can be adapted into different environmental settings for detection of Hg2+ ion levels.

A Self Powered Solid-Liquid Contact Electrification Based Nanosensor for Chemically Enhanced Detection of Catechin

In this study, a self-powered triboelectric nanosensor (TENS) based on chemically enhanced solid-liquid contact electrification has been demonstrated. The TiO2 nanosheet (NS) arrays, grown by two step hydrothermal process is employed as solid triboelectric material. On the other side volatile organic solvent acetone is utilized as contact liquid to obtain higher triboelectric performance. The chemical modification of TiO2 NS surface with catechin promotes enhanced response voltage due to ligand to metal charge transfer complex between Ti molecules and enediol ligands of catechin, exhibiting 1 nM LOD and wide linear range (1 µM to 100 µM). The output voltage has been chemically enhanced 1.59 times for 10 mM catechin concentration. The study not only demonstrates a self-powered cost effective nanosensor but also explores the possibility to improve the electrical performance of solid-liquid based contact electrification through surface chemical environment engineering and utilizing volatile organic solvents as contact liquid.

Ultrasensitive and Label-Free Detection of Mercury Ions Using a Highly Efficient Solid-Liquid Triboelectric Nanosensor

The mercury pollution in the world have increased tremendously due to both natural and anthropogenic reasons. The mercury ions (Hg2+) after being released from the industrial wastes into the environment is transformed into the highly toxic form of methylmercury (CH3Hg+) by the microorganisms present, which further accumulates in the human body via drinking water and food, ultimately causing irreversible damage to reproductive, immune and nervous system. Currently, analytical tools such as ICPMS, X-ray fluorescence spectroscopy, AES etc are employed for detection of the Hg2+ions but their utility is overshadowed by certain limitations like complicated sample preparation steps, time-consuming protocol, non-portable nature, requirement of trained professionals etc. Therefore, in this work we propose a highly sensitive and selective sensing method for Hg2+ions using the concept of solid-liquid contact electrification. This triboelectric nanosensor was developed employing modified Copper(II) oxide nanowires (CuO NWs) as the solid-triboelectric layer and deionized water as the liquid-triboelectric layer. Due to the highly specific reaction between the modified CuO NWs surface and Hg2+ions, the as-developed sensor can detect the Hg2+ion concentrations as low as 0.1 nM with a good linear range of 1nM-10 μM. In addition, the unrivalledly flexible nanosensor is coupled with the robotic fingertips for highly automated on-site monitoring of Hg2+ions, making it possible to track the heavy metal pollution in inaccessible and remote locations. We believe that this work will be a stepping stone towards designing a reusable, portable, low-cost and stand-alone sensing systems.

Quantifying electron-transfer in liquid-solid contact electrification

Quantifying electron-transfer in liquid-solid contact electrification

Quantifying electron-transfer in liquid-solid contact electrification and the formation of electric double-layer

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.

Recent advancements in solid-liquid triboelectric nanogenerators for energy harvesting and self-powered applications.

The abundance of water on earth provides a large window to utilize the mechanical energy within river currents and ocean waves. In this regard, hydropower harvesting through solid-liquid contact electrification has received considerable interest in the recent past. Despite advancements in nanotechnology, liquid energy harvesting devices, especially solid-liquid triboelectric nanogenerators (S-L TENGs), require efficient engineering of the interfacial properties of their substrates to transfer liquid mass and momentum rapidly with the effective generation/transfer of surface charges. To face this challenge, several parameters such as the selection of material, surface morphology and surface properties are currently being studied to develop a better system architecture for energy harvesting and self-powered application platforms with three different interacting modes of liquid contact. Moreover, several parameters of the contact solvents such as the ionic activity and polarity have been studied to understand the practical applicability of S-L TENGs to harvest energy from different natural and artificial resources. In addition, the scope of harvesting mechanical energy from other volatile organic compounds has been studied recently. Self-powered applications of S-L TENGs in various fields have also been demonstrated by different research groups. This work reviews recent progress in the development of S-L TENGs for the first time in terms of the different properties of solid and liquid contact materials along with their respective applications. Furthermore, the work concludes with perspectives, future opportunities, and major challenges of fabricating S-L TENGs as an efficient energy harvester.

Robust Working Mechanism of Water Droplet‐Driven Triboelectric Nanogenerator: Triboelectric Output versus Dynamic Motion of Water Droplet

AbstractHarvesting energy from water flow based on liquid–solid contact electrification has been receiving intensive attention recently. However, due to the high deformability of water, challenges remain on exploring the dynamic motion of water in relation to the electrical output characteristic of water‐based triboelectric nanogenerator (TENG). Herein, by measuring the electrical output of a water droplet‐driven TENG (Wd‐TENG) along with observing the dynamics of the droplets by a high‐speed camera, a robust working mechanism of the Wd‐TENG is proposed, considering droplet spread, rebound, slide, and coalescence on the solid surface during the triboelectric cycle. It is demonstrated that the spread–rebound motion of droplet upon impact on the triboelectric layer surface results in one positive–negative current peak and the detachment of the droplet from the surface leads to another negative current peak. Droplet coalescence is also observed and found to cause larger negative peak when the merged droplet leaves the surface. The proposed mechanism is sufficiently proved by the results under different droplet dripping rates, sizes, surface wetting properties, and types of liquid. These findings provide an extended understanding of the liquid–solid contact electrification, which is significant in both fundamental and applied science of liquid–solid TENG.

Self-powered Flexible PDMS Channel Assisted Discrete Liquid Column Motion Based Triboelectric Nanogenerator (DLC-TENG) as Mechanical Transducer

Fluid flow in small scale channel as droplet, continuous or discrete volume of liquid has been used in the multidisciplinary research field to be utilized in wide variety of applications in chemistry, biology, medical technologies. Recently, energy scavenging technique that merges the microfluidics with liquid–solid contact electrification charging from the small flows has become a new area of investigation to be applied as sensory module. In this article, a flexible fluid based mechanical sensing element using the triboelectrification of the oscillatory discrete volume of liquid flowing inside the hydrophobic mini channel over PTFE polymeric film is described. To achieve the high output efficiency and to reduce the wettability of the channel, micropatterned PDMS surface was implied for smooth operation and constant power output of the device. Pressure gradient was applied for the forward–backward motion of the liquid column over the free-standing mode porous PTFE thin film-based electrode and a maximum of 10 nA of short circuit current and 20 mV of open circuit voltage was achieved at 40 mbar pressure and frequency of 1 Hz. Finally, the flexible DLC-TENG shows better performance as a transducer to be effectively used in measuring the frequency response to the angular displacement of the rotary actuator consequently pressure fluctuation sensing. The obtained results are promising whereas counting nominal deflection of pressure such as in the robotic manipulator or actuator is regarded as an essential factor.

A Nonencapsulative Pendulum‐Like Paper–Based Hybrid Nanogenerator for Energy Harvesting

AbstractThe newly invented triboelectric nanogenerator (TENG) is deemed to be a more efficient strategy than an electromagnetic generator (EMG) in harvesting low‐frequency (<2 Hz) water wave energy. Various TENGs with different structures and functions for blue energy have been developed, which can be roughly divided into two types: liquid–solid contact electrification TENGs and fully enclosed solid–solid contact electrification TENGs. Robustness and packaging are critical factors in the development of TENGs toward practical applications. Furthermore, for fully enclosed TENGs, the requirements and costs of packaging are very high, and they can difficult to disassemble after enclosed, if there is something wrong with the devices. Herein, a nonencapsulative pendulum‐like paper based hybrid nanogenerator for energy harvesting is designed, which mainly consists of three parts, one solar panel, two paper based zigzag multilayered TENGs, and three EMG units. This unique structure reveals the superior robustness and a maximum peak power of zigzag multilayered TENGs up to 22.5 mW is realized. Moreover, the device can be used to collect the mechanical energy of human motion in hand shaking. This work presents a new platform of hybrid generators toward energy harvesting as a portable practical power source, which has potential applications in navigation and lighting.

Mechanically Active Transducing Element Based on Solid–Liquid Triboelectric Nanogenerator for Self-Powered Sensing

As power generation from the electronically active materials has been made remarkable progress to scavenge the ambient mechanical energy, harnessing energy from fluidics with liquid–solid contact electrification charging from the flow has become new capabilities to be applied as the sensory module. In this paper, a mechanical fluidic nanogenerator based on solid–liquid contact triboelectrification is demonstrated in which liquid electricity signal can be transferred into from the mechanical motion. This simply designed device is composed of Polytetrafluoroethylene (PTFE) tube and copper electrodes at both ends. To evaluate the effectiveness of the device, deionized water (DI) is poured into the tube and the movement is controlled by air flow from the internal volume difference of the rotating mechanism. The peak current value of the mechanical active transducing element based triboelectric nanogenerator (MAT-TENG) signal is used for analyzing movement speed. Speed gradient was applied for back and forth motion of the liquid column with a stable peak of short circuit current and open circuit voltage was about 0.23 µA and 2.31 V, respectively, at the length of the motion amplitude of 23 mm and frequency of 1 Hz. Finally, the flexible MAT-TENG has shown distinct response with the change of the motion length and speed of the liquid and the results have been achieved with better linearity ~ R2 = 0.99. Therefore, the velocity transducing can be very effective in the practical applications of the TENGs based for self-powered sensing.

Solid–Liquid Triboelectrification Control and Antistatic Materials Design Based on Interface Wettability Control

AbstractSolid–liquid contact triboelectrification has been a hot and important topic due to its severe electrostatic hazards in the petroleum industry and its positive potential application in energy harvesting. Here, the influence of surface structure, composition, and interfacial wettability on the solid–liquid triboelectrification basing on the friction of polypropylene (PP) films and deionized water is systematically studied. PP films with different nanowire structures are prepared by a hot pressing method with varied anodic aluminum oxide as templates. The PP films are surface functionalized with three different functional groups by a surface self‐assembly technology. The results show that compared with the surface structure, the solid–liquid triboelectrification is more sensitive to the surface components. While from the macroscopic view the interfacial wettability plays a dominant role in its electrical output, with the failure of the hydrophobic surface of the PP film, the triboelectrical output continues to deteriorate. Moreover, the increase or decrease of solid–liquid triboelectrification can be achieved by controlling the structure or composition. This study demonstrates the possibility of adjusting the solid–liquid contact electrification in a controllable manner and provides a basis for the utilization or the prevention of solid–liquid friction charges.

(Invited) Biomimetic Approaches for the Practical Utilization of Discrete Liquid-Solid Contact Electrification

The concept of “discrete liquid-solid contact electrification”, which is result of sequential process of the liquid contact on the solid surface and successive detachment of contact liquid from the solid surface, was reported. After the first report on discrete liquid-solid contact electrification inside the conventional pipet tip surface, the phenomenon is deeply investigated and explored with the help of utilization on a hydrophobic surfaces, which enables easy detachment of liquid from its surface. As a result, the phenomenon can be intentionally enhanced. Recently, discrete liquid-solid contact electrification has been utilized as a fundamental operation mechanism of the energy harvester, called triboelectric nanogenerator (TENG), which utilizes the contact electrification phenomenon with electrical induction to efficiently convert ambient mechanical energy to electrical energy. In this presentation, the history from the discovery of discrete liquid-solid contact electrification to its application as a TENG is introduced. Especially, the biomimetic approaches (mimicking natural lotus leaf and moth's eye) in the fabrication the TENG are introduced not only to enhance the electrical output as an energy harvester, but also to give an additional function for practical utilization. The present study would broaden the applications of discrete liquid-solid contact electrification, which is ubiquitous but previously inconspicuous in our daily life.

Harvest of ocean energy by triboelectric generator technology

Water wave energy widely distributed in the globe is one of the most promising renewable energy sources. However, it has not been effectively exploited by current energy harvesting technologies which primarily rely on electromagnetic generator (EMG). EMG's have various limitations, especially when operating in environment with irregular and/or low frequencies ( < 5 Hz) wave motions. Triboelectric nanogenerators (TENGs) exhibit obvious advantages over EMG in harvesting energy from low-frequency water wave motions. The networking of TENGs has been regarded as a potential method towards large-scale blue energy harvesting. In this review, recent progress of the TENG technology for blue energy harvesting is presented, including a comparison between TENG and EMG in physics and engineering design, and the fundamental mechanism of nanogenerator based on Maxwell's displacement currents is systematically introduced. The review of hydrodynamic TENG, liquid-solid contact electrification TENG, hybrid (dual-modes) TENG, fully enclosed TENG, and TENG network for blue energy harvesting is discussed. The TENG networks are expected to harvest large-scale blue energy from the ocean, which will be a feasible approach for realizing the blue energy dream. Moreover, the energy harvested by TENG from various sources, such as human motion and vibration, is not only new energy, but more importantly, energy using for the new era-the era of internet of things.At the request of the authors, this article is being retracted effective 15 May 2019.

Contact electrification efficiency dependence on surface energy at the water-solid interface

Liquid-solid contact electrification is a useful mechanism to harvest wasted micromechanical energy. In this study, we investigate how the surface properties of a solid substrate affect contact electrification efficiency. Substrate surfaces were modified from hydrophilic to hydrophobic by changing the density of self-assembled monolayers (SAMs) on a SiO2 surface. A substrate with a partially-covered SAM exhibited superior performance. The partially-covered SAM substrate is hydrophobic enough to induce quick dewetting of water from the surface and sufficiently electronegative to induce a high charge density on the surface. The quick dewetting results from the aliphatic tail groups of the SAM and -OH groups make the SiO2 surface electronegative; these two competing properties can be simultaneously obtained by optimizing the SAM density. Our findings contribute to the understanding of contact electrification in liquid-solid-type energy-harvesting devices and advance the strategies to maximize the electrification efficiency by optimizing surface geometries and properties.

(Invited) Use of Discrete Liquid-Solid Contact Electrification As Ways of Self-Powered Sensing

In this presentation, the concept of “discrete liquid-solid contact electrification”, which is result of sequential process of the liquid contact on the solid surface and successive detachment of contact liquid from the solid surface, is reported. After the first report on discrete liquid-solid contact electrification inside the conventional pipet tip surface, the phenomenon is deeply investigated and explored with the help of utilization on a hydrophobic surfaces, which enables easy detachment of liquid from its surface. As a result, the phenomenon can be intentionally suppressed and/or enhanced. Recently, discrete liquid-solid contact electrification has been utilized as a fundamental operation mechanism of the self-powered sensor, which utilizes the contact electrification phenomenon with electrical induction to efficiently convert ambient mechanical energy to electrical energy. In this paper, the fabrication of the self-powered sensor utilizing discrete liquid-solid contact electrification is introduced to help its understanding. With the fact that ordinary pipetting procedure always involves spontaneous discrete liquid-solid contact electrification resulting in generation of net electrical charges on the inner surface of the polymeric pipet tip, in this study, a concept of a smart pipet tip is newly proposed to evaluate the electrolyte concentration of the dispensed solution. The smart pipet tip possessing triboelectricity and thermoelectricity detecting modules is advantageous as it performs in situ evaluation of solution characteristics without any subsidiary solution handling process. The spontaneously generated electric signals are intensively investigated with the theoretical analyses. The proof-of-concept demonstration of the present smart pipet tip is shown for in situ prediction of morphology of nanoparticles during their synthetic reaction, which critically determines their catalytic activity. The present study would broaden the applications of discrete liquid-solid contact electrification, which is ubiquitous but previously inconspicuous in our daily life.

A smart pipet tip: Triboelectricity and thermoelectricity assisted in situ evaluation of electrolyte concentration

Pipet tips are commonly utilized laboratory tools to transfer adjustable volume of liquid in various fields of chemistry, biology and physics. Recently, we have reported that the ordinary pipetting procedure always involves spontaneous liquid-solid contact electrification, resulting in generation of net electrical charges on the dispensed solution and the inner surface of the polymeric pipet tip. In this study, a concept of a smart pipet tip is proposed to evaluate the electrolyte concentration of the dispensed solution by use of spontaneously generated electric signals during the ordinary pipetting procedure. The smart pipet tip possessing triboelectricity and thermoelectricity detecting modules is advantageous as it performs in situ evaluation of solution characteristics without any subsidiary solution handling process. The spontaneously generated electric signals are intensively investigated with the theoretical analyses. The proof-of-concept demonstration of the present smart pipet tip is shown for in situ prediction of morphology of nanoparticles during their synthetic reaction, which critically determines their catalytic activity.