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.
Polymers are commonly used to fabricate triboelectric nanogenerators (TENGs). Here, several polymer films with similar main chains but different functional groups on the side chain are employed to ...clarify the contributions of each functional group to contact electrification (CE). The results show that the electron‐withdrawing (EW) ability and density of these functional groups on the main chain can determine both the polarity and density of CE‐induced surface charges. Similar results are obtained for CE in both the polymer–polymer and polymer–liquid modes. A theoretical mechanism involving electron cloud overlap is proposed to explain all of these results. More importantly, the unsaturated groups on poly(tetrafluoroethylene) molecular chain are proved to have a much stronger EW ability than the saturated groups. The density of these unsaturated groups can be increased using a sputtering technique, suggesting that this is a facile and effective method of enhancing the performance of TENGs. These results clarify the correlation between the molecular structure and macroscopic electrification behavior of polymers.
A series of polymer films with different functional groups on the side chain are employed for clarifying the contribution of functional groups to their contact electrification (CE) processes with both solids and liquids. The unsaturated groups of PTFE can enhance the electronegativity of the whole functional group.
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.
As a well‐known phenomenon, contact electrification (CE) has been studied for decades. Although recent studies have proven that CE between two solids is primarily due to electron transfer, the ...mechanism for CE between liquid and solid remains controversial. The CE process between different liquids and polytetrafluoroethylene (PTFE) film is systematically studied to clarify the electrification mechanism of the solid–liquid interface. The CE between deionized water and PTFE can produce a surface charges density in the scale of 1 nC cm−2, which is ten times higher than the calculation based on the pure ion‐transfer model. Hence, electron transfer is likely the dominating effect for this liquid–solid electrification process. Meanwhile, as ion concentration increases, the ion adsorption on the PTFE hinders electron transfer and results in the suppression of the transferred charge amount. Furthermore, there is an obvious charge transfer between oil and PTFE, which further confirms the presence of electron transfer between liquid and solid, simply because there are no ions in oil droplets. It is demonstrated that electron transfer plays the dominant role during CE between liquids and solids, which directly impacts the traditional understanding of the formation of an electric double layer (EDL) at a liquid–solid interface in physical chemistry.
The mechanism behind the contact electrification between liquid and solid remains controversial. According to experimental results and theoretical calculations, the contact electrification of ionic liquids and solids is caused by both electron transfer and ion transfer. Furthermore, the contact electrification between nonionic liquids and solids is mainly due to electron transfer.
Abstract
Electron transfer has been proven the dominant charge carrier during contact electrification at the liquid–solid interface. However, the effect of electron spin in contact electrification ...remains to be investigated. This study examines the charge transfer between different liquids and ferrimagnetic solids in a magnetic field, focusing on the contribution of O
2
molecules to the liquid–solid contact electrification. The findings reveal that magnetic fields promote electron transfer at the O
2
-containing liquid–solid interfaces. Moreover, magnetic field-induced electron transfer increases at higher O
2
concentrations in the liquids and decreases at elevated temperatures. The results indicate spin-selected electron transfer at liquid–solid interface. External magnetic fields can modulate the spin conversion of the radical pairs at the O
2
-containing liquid and ferrimagnetic solid interfaces due to the Zeeman interaction, promoting electron transfer. A spin-selected electron transfer model for liquid–solid contact electrification is further proposed based on the radical pair mechanism, in which the HO
2
molecules and the free unpaired electrons from the ferrimagnetic solids are considered radical pairs. The spin conversion of the HO
2
• •e
−
pairs is affected by magnetic fields, rendering the electron transfer magnetic field-sensitive.
Triboelectric nanogenerators (TENGs) have been widely applied for energy harvesting and self-powered sensing, whereas smart deformable materials can be combined with the TENG to acquire a more ...intelligent and self-adaptive system. Here, based on the vapor-driven actuation material of a perfluorosulfonic acid ionomer (PFSA), we propose a type of humidity-responsive TENG. The integrated TENG array can automatically bend to the desired angles in response to different humidity conditions, and thus, it can effectively collect energy from both wind and rain drops, where the power density can reach 1.6 W m–2 at a wind speed of 25 m s–1 and 230 mW m–2 under rainy conditions. Meanwhile, this TENG array can fully lay down in dry weather, using the reflective surface to reflect sunlight and heat radiation. The vapor absorption process of the PSFA film can also result in the charge accumulation process. Accordingly, relying on the strong absorption capability of PFSA, a TENG-based vapor sensor with high sensitivity has been developed for monitoring chemical vapor leakage and humidity change. This work opens up a promising approach for the application of the humidity-responsive materials in the field of energy harvesting and self-powered sensors. It can also promote the development of TENG toward a more intelligent direction.
Bulk cobalt does not react with water at room temperature, but cobalt nanometals could yield corrosion at ambient conditions. Insights into the cobalt cluster reactions with water and oxygen enable ...us to better understand the interface reactivity of such nanometals. Here we report a comprehensive study on the gas-phase reactions of Co
clusters with water and oxygen. All these Co
clusters were found to react with oxygen, but only anionic cobalt clusters give rise to water dissociation whereas the cationic and neutral ones are limited to water adsorption. We elucidate the influences of charge states, bonding modes and dehydrogenation mechanism of water on typical cobalt clusters. It is unveiled that the additional electron of anionic Co
clusters is not beneficial to H
O adsorption, but allows for thermodynamics- and kinetics-favourable H atom transfer and dehydrogenation reactions. Apart from the charge effect, size effect and spin effect play a subtle role in the reaction process. The synergy of multiple metal sites in Co
clusters reduces the energy barrier of the rate-limiting step enabling hydrogen release. This finding of water dissociation on cobalt clusters put forward new connotations on the activity series of metals, providing new insights into the corrosion mechanism of cobalt nanometals.
Contact between water droplets with hydrophobic surfaces is a common phenomenon at functional interfaces, and it has been extensively studied. However, quantifying the charge transfer between the ...liquid–solid interfacial contacting, especially for the charge density distribution throughout the movement of liquid droplet on a dielectric surface, remains to be investigated. Here, we developed a pixeled droplet triboelectric nanogenerator (pixeled droplet-TENG) array with high-density electrode array as a probe for measuring the charge transfer at a liquid–solid interface when a water drop moves on the hydrophobic surface. To intuitively observe the charge transfer between the liquid–solid interface, we “imaged” the transferred charges along movement trajectory of a water droplet as it slides along a tilted solid surface at a spatial resolution of 0.4 mm and time sensitivity of 0.02 s. Our study shows that the transferred charges are not uniformly distributed along the path, which is possibly due to the two-step model of electron transfer and ion adsorbed on the solid surface, and thus the formation of an electric double layer will inevitably shield the net surface on the solid surface. Our study presents a probe technology with potential applications in surface chemistry, physics, material science, and cell biology.
Contact electrification (CE) at interfaces is sensitive to the functional groups on the solid surface, but its mechanism is poorly understood, especially for the liquid–solid cases. A core ...controversy is the identity of the charge carriers (electrons or/and ions) in the CE between liquids and solids. Here, the CE between SiO2 surfaces with different functional groups and different liquids, including DI water and organic solutions, is systematically studied, and the contribution of electron transfer is distinguished from that of ion transfer according to the charge decay behavior at surfaces at specific temperature, because electron release follows the thermionic emission theory. It is revealed that electron transfer plays an important role in the CE between liquids and functional group modified SiO2. Moreover, the electron transfer between the DI water and the SiO2 is found highly related to the electron affinity of the functional groups on the SiO2 surfaces, while the electron transfer between organic solutions and the SiO2 is independent of the functional groups, due to the limited ability of organic solutions to donate or gain electrons. An energy band model for the electron transfer between liquids and solids is further proposed, in which the effects of functional groups are considered. The discoveries in this work support the “two-step” model about the formation of an electric double-layer (Wang model), in which the electron transfer occurs first when the liquids contact the solids for the very first time.
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