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. 2022 Feb 16:2022:9861463.
doi: 10.34133/2022/9861463. eCollection 2022.

Understanding Contact Electrification at Water/Polymer Interface

Yang Nan  1   2 , Jiajia Shao  1   2 , Morten Willatzen  1   2 , Zhong Lin Wang  1   2   3   4
Affiliations

Understanding Contact Electrification at Water/Polymer Interface

Yang Nan et al. Research (Wash D C). .

Abstract

Contact electrification (CE) involves a complex interplay of physical interactions in realistic material systems. For this reason, scientific consensus on the qualitative and quantitative importance of different physical mechanisms on CE remains a formidable task. The CE mechanism at a water/polymer interface is a crucial challenge owing to the poor understanding of charge transfer at the atomic level. First-principle density functional theory (DFT), used in the present work, proposes a new paradigm to address CE. Our results indicate that CE follows the same trend as the gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of polymers. Electron transfer occurs at the outmost atomic layer of the water/polymer interface and is closely linked to the functional groups and atom locations. When the polymer chains are parallel to the water layer, most electrons are transferred; conversely, if they are perpendicular to each other, the transfer of charges can be ignored. We demonstrate that a decrease in the interface distance between water and the polymer chains leads to CE in quantitative agreement with the electron cloud overlap model. We finally use DFT calculations to predict the properties of CE materials and their potential for triboelectric nanogenerator energy harvesting devices.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
Sketch of seven polymers. Polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene difluoride (PVDF), polydimethylsiloxane (PDMS), Nylon 66, polyimide (Kapton), polyethylene terephthalate (PET), and water. The dashed areas depict their monomers.
Figure 2
Figure 2
The effect of the number of water layers and the length of the chain on charge transfer. (a) The side view of a PTFE with four C atoms after CE with four layers of water. The distance between two water layers is 3.2 Å while the distance between a water layer and the PTFE is 2.5 Å. (b) Sketches of a PTFE chain with four C atoms in contact with different number of water layers and the corresponding transferred charges. (c) The CDD of CE between a PTFE chain with four C atoms and water with four layers. (d) Configurations of different length of PTFE in contact with one water layer in parallel and (e) vertical ordering. (f) The relationship between the charge transfer and different length of PTFE in contact with water at forms of parallel and vertical ordering, respectively.
Figure 3
Figure 3
The effect of saturation on charge transfer. The charge transfer between PTFE and water in parallel ordering when the PTFE is (a) saturated and (b) unsaturated. The charge transfer between PTFE and water in vertical ordering when the PTFE is (c) saturated and (d) unsaturated. The corresponding total charge transfer for atoms of C and F (e) in parallel ordering and (f) vertical ordering, respectively.
Figure 4
Figure 4
Charge transfer of different atoms and CDD for (a) PP, (b) PVDF, (c) PDMS, (d) Nylon 66, (e) Kapton, and (f) PET. Note that the pillar represents the total transferred charges for an atom in polymers. Charge differences of selected atoms (in the dashed circle) for (g) PTFE, (h) PP, and (i) PVDF after contact with water, respectively. And their contact configurations are shown in Figure 3(a) and (a, b), respectively.
Figure 5
Figure 5
(a) The average electrostatic potential along the z axis before and after PVDF contacting with water. (b) The difference between the vacuum level and the Fermi level of PVDF after contacting with water. (c) The comparison of work function differences of polymers in this study after contacting with water. Inset shows the electrostatic potential mapping of PVDF. (d) The relationship between contacting distance and the system energy and the corresponding charge transfer of PVDF. (e) The DOS at the determined state of the change of contacting distance. (f) The variations in HOMO and LUMO levels as function of the contacting distance.
Figure 6
Figure 6
(a) The configuration of amorphous PTFE in contact with water layer, and the average distance between them is 2.5 Å. (b) The relationship between charge transfer and the HOMO-LUMO gap of amorphous polymers. (c) The HOMO and LUMO levels of each isolated polymer. The order of polymers is arranged according to their ability to accept electrons in (b). Schematics of level occupations of water and polymers (d) before contact, (e) in contact, and (f) after contact. Here, Evac represents the vacuum energy level.
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