Surface Fluorination Research Articles

Dyeable Hydrophilic Surface Modification for PTFE Substrates by Surface Fluorination.

Polytetrafluoroethylene (PTFE) is the most widely used fluoropolymer that has various functionalities such as heat resistance, chemical resistance, abrasion resistance, and non-adhesiveness. However, PTFE is difficult to dye because of its high water repellency. In this study, the PTFE surface was modified by a combination of gold sputtering and surface fluorination to improve dyeability. X-ray photoelectron spectroscopy indicated that, compared with the untreated sample, the gold-sputtered and acid-washed surface of PTFE had a negligible number of C-F terminals. Furthermore, the intensity of the C-C peak increased drastically. The polar groups (C=O and C-Fx) increased after surface fluorination, which enhanced the electronegativity of the surface according to the zeta potential results. Dyeing tests with methylene blue basic dye showed that the dye staining intensity on the surface of fluorinated PTFE samples was superior to other samples. It is due to the increased surface roughness and the negatively charged surface of fluorinated PTFE samples. The modified PTFE substrates may find broad applicability for dyeing, hydrophilic membrane filters, and other adsorption needs.

Two-dimensional H- and F-BX (X = O, S, Se, and Te) photocatalysts with ultrawide bandgap and enhanced photocatalytic performance for water splitting.

We theoretically propose a type of monolayer structure, H- or F-BX (X = As, Sb; Y = P, As), produced by surface hydrogenation or fluorination, with high stability, large band structures and high light absorption for photocatalytic water splitting. Based on first-principles calculations with the HSE06 functional, the electronic properties and optical properties were explored to reveal their potential performance in semiconductor devices. Additionally, owing to the Janus structure and high electronegativity of the monolayers, our calculations showed that surface fluorination can easily create an internal electric field compared with surface hydrogenation, which results in different trends of increasing bandgaps in monolayer H- and F-BX. We also found that the monolayers H- and F-BX have suitable band edges and high solar to hydrogen (STH) efficiency, enabling them to be photocatalysts for water splitting. Our work not only proposes eight monolayer semiconductors for expanding the number of two-dimensional semiconductors, but also provides a guide for how to regulate semiconductors for application in photocatalytic water splitting by using surface hydrogenation and fluorination.

High-throughput calculation of interfacial friction of two-dimensional material

Friction generally occurs in the relative motion or the contact interface with the trend of relative motion, which impedes the relative motion and produces energy loss. Micro-scale friction is different from the macro-scale friction due to surface effects and other factors. It is necessary to study the friction behavior on a nano-scale. First-principles method is an important way to study and understand friction on a nano-scale. Nevertheless, the constructing of nearly a thousand models and the processing of a large number of data are very time consuming. In this paper, we establish a high-throughput computational program based on the first-principles method to study the interfacial friction of two-dimensional materials. The program realizes modeling, submitting computation tasks, multi-task concurrent calculation, data collection and processing, and image rendering of calculation results. All of these are done in batch automatically, which greatly saves researchers’ time. In this work, this program is used to simulate the normal load by changing the distance between layers and calculate the potential energy surface of BN/BN and graphene/graphene bilayer sliding systems at a series of interlayer distances, as well as the interlayer friction forces and friction coefficients. The study finds that with the decrease of the interlayer distance, the averaged friction force at BN/BN interface increases approximately linearly, and the friction coefficient is in a range of 0.11–0.17. The friction force at graphene/graphene interface first increases, then decreases, and increases again. The friction coefficient reaches a minimum value (0.014) under a load of 12 nN, and these results are consistent with the previous results, verifying the reliability of the calculation program. In addition, we investigate the effect of surface hydrogenation and fluorination on the tribological property of the BN bilayer and find that the friction at the fluorinated BN/BN interface decreases, which is attributed to the smaller charge transfer at interface. Although the high-throughput calculation method realizes the automation and high-throughput calculation of tribological property at solid interface, there are still some limitations. Firstly, the effect of interlaminar bending is not considered in the process of interlaminar relative sliding. Secondly, the essence of the calculation result is static friction, rather than dynamic friction. In addition, the method does not consider the influence of temperature.

Surface fluorine preservation dependence of Ti3C2Tx MXene for high electrochemical properties in ionic liquid electrolytes.

We identified the contribution of -CF3 terminations to the Ti3C2Tx surface structure when ethanol and water were used as solvents during delamination through experimental and computational studies. Ethanol-treated -CF3-terminated Ti3C2Tx achieves better prevention of nanoflake aggregation, hydrophobicity, and small size, enabling enhanced capacitive properties in ionic liquid compared to water-treated Ti3C2Tx in aqueous and ionic liquid electrolytes.

Fabrication of Transferable and Micro/Nanostructured Superhydrophobic Surfaces Using Demolding and iCVD Processes

Superhydrophobic surfaces possess enormous potential in various applications on account of their versatile functionalities. However, artificial superhydrophobic surfaces with ultralow solid/liquid adhesion often require complicated structure fabrication and surface fluorination processes. Here, we designed a superhydrophobic surface possessed of micro/nanoscale structures by employing facile and low-cost demolding and initiated chemical vapor deposition (iCVD) processes. The achieved micro/nanostructured superhydrophobic surface has a maximum static contact angle of ∼170°, a roll-off angle and contact angle hysteresis below 1°, ultralow solid/liquid adhesion for water droplets, and maintains excellent superhydrophobicity after exposure to strongly corrosive species, like strong acid/base and salt solutions, for 60 h. This reasonability-designed method of creating the superhydrophobic surface could provide valuable guidelines for the manufacture of transferable superhydrophobic surfaces and facilitate potential applications extending from optoelectronic devices to self-cleaning materials, such as solar cells, windows, and electronic displays.

Surface Smoothing by Atomic Layer Deposition and Etching for the Fabrication of Nanodevices.

In many nano(opto)electronic devices, the roughness at surfaces and interfaces is of increasing importance, with roughness often contributing toward losses and defects, which can lead to device failure. Consequently, approaches that either limit roughness or smoothen surfaces are required to minimize surface roughness during fabrication. The atomic-scale processing techniques atomic layer deposition (ALD) and atomic layer etching (ALE) have experimentally been shown to smoothen surfaces, with the added benefit of offering uniform and conformal processing and precise thickness control. However, the mechanisms which drive smoothing during ALD and ALE have not been investigated in detail. In this work, smoothing of surfaces by ALD and ALE is studied using finite difference simulations that describe deposition/etching as a front propagating uniformly and perpendicular to the surface at every point. This uniform front propagation model was validated by performing ALD of amorphous Al2O3 using the TMA/O2 plasma. ALE from the TMA/SF6 plasma was also studied and resulted in faster smoothing than predicted by purely considering uniform front propagation. Correspondingly, it was found that for such an ALE process, a second mechanism contributes to the smoothing, hypothesized to be related to curvature-dependent surface fluorination. Individually, the atomic-scale processing techniques enable smoothing; however, ALD and ALE will need to be combined to achieve thin and smooth films, as is demonstrated and discussed in this work for multiple applications.

Customizing defect location in MWCNTs/Fe3O4 composites by direct fluorination for enhancing microwave absorption performance

Constructing magnetic-dielectric composites is an effective approach to prepare high-performance multi-walled carbon nanotubes (MWCNTs)-based microwave absorbers. The current composite design strategies mainly focus on the regulation of magnetic materials, while the effect of MWCNTs structure on the microwave absorption (MA) performance is still unclear. Herein, F-MWCNTs with different fluorinated depths, namely surface fluorination (S-F-MWCNTs) and deep fluorination (D-F-MWCNTs) were prepared by molecular-fluorination and atomic-fluorination, respectively. The fluorine atoms on F-MWCNTs were defluorinated in the next loading process, achieving customization of defect location in MWCNTs/Fe3O4 composites. In relation to the fluorination depth of F-MWCNTs, the defects of S-F-MWCNTs/Fe3O4 were mainly distributed on the outer tube of MWCNTs, while they were on both inner and outer tubes of MWCNTs for D-F-MWCNTs/Fe3O4. Under the electromagnetic field, the inner tube defects of D-F-MWCNTs/Fe3O4 were excited to produce enhanced polarization loss and conductive loss. As a result, compared with S-F-MWCNTs/Fe3O4 with only outer tube defects, D-F-MWCNTs/Fe3O4 with both inner and outer tube defects exhibited better MA performance. Further regulating the fluorine content of D-F-MWCNTs, we can adjust the optimal absorption band shifting from high frequency to low frequency. Our work illustrates the importance of inner tube modification of MWCNTs to prepare high-performance MWCNTs-based microwave absorbers.

Surface diffusion enhancement of titania by surface fluorine modification to boost energy storage capacities

Supercapacitors based on environmentally friendly electrode materials have attracted great interest for energy storage devices with wide applications in high-power systems and miniaturized electronic devices, owing to the fast charge/discharge capability and excellent cycling stability. It is highly desired yet challenging to effectively improve the capacitances of TiO2, especially via a facile route. Herein, we demonstrate a surface fluorine modification strategy to greatly improve the capacitance of anatase TiO2 up to 6.4 times. Density functional theory calculations show that the surface F modification can transform the surface storage sites and the diffusion paths for lithium ions. Thus, the diffusion barrier is reduced from 1.46 eV to 0.39 eV, which is confirmed by the experimental results of the significant enhance in diffusion coefficient. This work could provide a new avenue to the design of high-performance electrode materials for electrochemical energy storage and other surface-diffusion-related fields.

Breaking through water-splitting bottlenecks over carbon nitride with fluorination

Graphitic carbon nitride has long been considered incapable of splitting water molecules into hydrogen and oxygen without adding small molecule organics despite the fact that the visible-light response and proper band structure fulfills the proper energy requirements to evolve oxygen. Herein, through in-situ observations of a collective C = O bonding, we identify the long-hidden bottleneck of photocatalytic overall water splitting on a single-phased g-C3N4 catalyst via fluorination. As carbon sites are occupied with surface fluorine atoms, intermediate C=O bonding is vastly minimized on the surface and an order-of-magnitude improved H2 evolution rate compared to the pristine g-C3N4 catalyst and continuous O2 evolution is achieved. Density functional theory calculations suggest an optimized oxygen evolution reaction pathway on neighboring N atoms by C–F interaction, which effectively avoids the excessively strong C-O interaction or weak N-O interaction on the pristine g-C3N4.

Significantly suppressing metal particle-induced surface charge accumulation of spacers in DC gas-insulated power transmission lines

In this letter, we report functions of surface roughening and fluorination on suppressing linear metal particle-induced spacer surface charge accumulation. An appropriate increase in spacer surface conductivity by short-term fluorination and roughening not only increases the metal particle lifting voltage, but also weakens the particle activation. The spacer surface charge shows reduced charge density in roughened spacer, while fluorination modification significantly suppresses the charge density on the spacer surface. For roughened and fluorinated samples, the decrease of surface charge density and the intrinsic lower electric field (due to an increase in conductivity) near the triple junction both contribute to a higher particle lifting voltage. The content in this letter provides an approach to effectively suppress the charge accumulation induced by linear metal particles.

Robust and Highly Transparent Photocurable Fluorinated Polyurethane Coating Prepared via Thiol-Click Reactions and What Essentially Influences Omniphobic Coating’s Anti-Graffiti Properties

Without a fragile micro/nanostructure, omniphobic coatings have a natural advantage over superomniphobic coatings in mechanical durability, especially when confronted with finger wipes and squeezes in practical use. Many reported omniphobic coatings with lower liquid sliding angles showed better ink trace resistance. However, according to our results, these two properties are not positively related. The respective critical factors for lowering the liquid sliding angle and resisting tiny contaminants, such as ink stroke and fingerprint, remain elusive. Through three steps of the thiol-click reaction, a surface-dried, highly transparent, mechanically and chemically robust, fluorinated polyurethane coating was photocured efficiently. Surfaces that exhibited better sliding performance but worse resistance to ink stroke and fingerprint were reported. Our results indicate that a heavy cross-linked structure was beneficial for the resistance to tiny contaminants, while the liquid sliding performance only depended on the surface fluorine content, especially the surface enrichment of flexible low-surface-tension chains. Interestingly, an unexpected transformation from a turbid dispersion to a transparent solution during the preparation was observed. The efficient thiol-click reaction induces a micelle-like structure to open and imparts high transparency to the final coatings. Therefore, this work provides useful information to facilitate the study of omniphobic coatings, as well as a good candidate in anti-graffiti applications.

Fabrication of a robust superhydrophobic coating from the aqueous self-assembly of perfluoropolyether-based triblock copolymers

Five types of poly(hydroxyethyl methacrylate)m-perfluoropolyether-poly(hydroxyethyl methacrylate)m triblock copolymers (HFHm) with different poly(hydroxyethyl methacrylate) chain lengths were synthesized successfully. The as-synthesized HFHm can self-assemble into micelles in water with perfluoropolyether core surrounded by poly(hydroxyethyl methacrylate) corona under a dilute concentration. By casting the mixture of the HFHm micelles in water, hydrophilic silica nanoparticles and hydrophilic polyisocyanates as a cross-linking agent, a robust superhydrophobic surface can be obtained on smooth substrates. The addition of silica nanoparticles and polyisocyanates enhances the surface roughness and mechanical durability of the coatings. Due to the high surface roughness (Ra = 889 nm) and the high surface fluorine content (41.9 wt%), the cross-linked HFHm superhydrophobic coating can reach a water contact angle of 157.8 ± 0.7° and a water sliding angle of 3.5 ± 0.6°. After 30 abrasion cycles or immersion for 22 h in strong acid (pH 2) and weak alkaline (pH 9) solutions, the coating still maintains its superhydrophobicity. These phenomena indicate that the HFHm can be used as a kind of environment-friendly coatings with superhydrophobicity.

Enhanced photocatalytic performance of (N, F) co-doped TiO2 loaded on coal-based hierarchical porous carbon foam under simulated sunlight

(N, F) co-doped TiO2 was prepared by solvothermal method with urea as N source followed by surface fluorination, and then loaded onto a monolithic coal-based hierarchical porous carbon foam (HPCF). The activity of the synthesized (N, F)–TiO2/HPCF was compared with pure TiO2, N/F mono-doped TiO2 and the unloaded (N, F)–TiO2 for photocatalytic degradation of phenol. Furthermore, the effect of different pore structures of HPCFs on photocatalytic activity was investigated. The results showed that N doping extended the photoresponse of TiO2 to the visible region, while the lattice doping of fluorine greatly improved its UV-light driven activity. The macropores and thin-walled structure of HPCF ensured that the loaded TiO2 could be exposed to the light, and its micro/mesopores enhanced adsorption capacity. As a result, the (N, F) co-doped TiO2 supported on the HPCF with a larger mesoporosity of 21% and a specific surface area of 301.8 m2/g exhibited the highest photocatalytic activity and a stable cyclic usability under simulated sunlight irradiation. This study indicates great potential of the novel (N, F) co-doped TiO2/HPCF composite in photocatalysis for wastewater treatment.

High Performance Solution Processed n‐Type OTFTs through Surface Engineered F–F Interactions Using Asymmetric Silicon Phthalocyanines

AbstractTwo novel asymmetric silicon phthalocyanine (SiPc) derivatives consisting of one axially substituted fluorine and one tri‐alkyl silane group are synthesized and characterized in n‐type solution processed bottom‐gate top‐contact organic thin‐film transistors (OTFTs). The effect of surface energy and fluorination are investigated at the dielectric/semiconductor interface by thin‐film X‐ray diffraction, atomic force microscopy, bright field real‐color microscopy, and grazing‐incidence wide‐angle X‐ray scattering to assess alterations in film conformation, microstructure, and morphology. Low surface energy dielectric modification and the presence of fluorine interactions produce films with large area crystalline domains that promote charge carrier transport resulting in high performing OTFTs, with a clear relationship determined between surface energy, fluorination, and OTFT operation. Through modifying deposition solvent and the exploitation of fluorine–fluorine interactions, the asymmetric SiPc derivative, (tri‐n‐hexylsilyl oxide) fluorosilicon phthalocyanine (F‐3HS‐SiPc), leads to high performing n‐type OTFTs with an average field‐effect mobility of 0.13 cm2 V−1 s−1 and a threshold voltage of 26.3 V. These results successfully demonstrate the use of asymmetric axial fluorination as a route to high performance n‐type OTFT devices through controlled self‐assembly by fluorine–fluorine interactions.

A Surface Modification Strategy Towards Reversible Na-ion Intercalation on Graphitic Carbon Using Fluorinated Few-Layer Graphene

Na-ion batteries (NIBs) are proposed as a promising candidate for beyond Li-ion chemistries, however, a key challenge associated with NIBs is the inability to achieve intercalation in graphite anodes. This phenomenon has been investigated and is believed to arise due to the thermodynamic instability of Na-intercalated graphite. We have recently demonstrated theoretical calculations showing it is possible to achieve thermodynamically stable Na-intercalated graphene structures with a fluorine surface modifier. Here, we present experimental evidence that Na+ intercalation is indeed possible in fluorinated few-layer graphene (F-FLG) structures using cyclic voltammetry (CV), ion-sensitive scanning electrochemical microscopy (SECM) and in situ Raman spectroscopy. SECM and Raman spectroscopy confirmed Na+ intercalation in F-FLG, while CV measurements allowed us to quantify Na-intercalated F-FLG stoichiometries around NaC14–18. These stoichiometries are higher than the previously reported values of NaC186 in graphite. Our experiments revealed that reversible Na+ ion intercalation also requires a pre-formed Li-based SEI in addition to the surface fluorination, thereby highlighting the critical role of SEI in controlling ion-transfer kinetics in alkali-ion batteries. In summary, our findings highlight the use of surface modification and careful study of electrode-electrolyte interfaces and interphases as an enabling strategy for NIBs.

Electrooxidation of per- and polyfluoroalkyl substances in chloride-containing water on surface-fluorinated Ti4O7 anodes: Mitigation and elimination of chlorate and perchlorate formation

Electrooxidation (EO) has been shown effective in degrading per- and polyfluoroalkyl substances (PFASs) in water, but concurrent formation of chlorate and perchlorate in the presence of chloride is of concern due to their toxicity. This study examined EO treatment of three representative PFASs, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS), in chloride-containing solutions on pristine and surface-fluorinated Ti4O7 anodes having different percentage of surface fluorination. The experiment results indicate that surface fluorination of Ti4O7 anodes slightly inhibited PFAS degradation, while significantly decreased the formation of chlorate and perchlorate. Further studies with spectroscopic and electrochemical characterizations and density functional theory (DFT) computation reveal the mechanisms of the impact on EO performance by anode fluorination. In particular, chlorate and perchlorate formation were fully inhibited when fluorinated Ti4O7 anode was used in reactive electrochemical membrane (REM) under a proper anodic potential range (<3.0 V vs Standard Hydrogen Electrode), resulting from slower intermediate reaction steps and short residence time of the REM system. The results of this study provide a basis for design and optimization of modified Ti4O7 anodes for efficient EO treatment of PFAS while limiting chlorate and perchlorate formation.

Study on the Influential Mechanism of Fluorination on the Flashover Voltage of Polypropylene Film Under Nanosecond Pulse Voltage

A polypropylene (PP) film is usually employed as the main insulation of the peaking capacitor, which is important in an electromagnetic pulse simulator to generate high-amplitude nanosecond pulse. The flashover voltage of PP films under nanosecond pulse limits the highest working voltage of a peaking capacitor. Surface fluorination was employed to increase the flashover voltage of PP. The flashover voltages of PP samples with different fluorination times (0, 4, and 6 h) were tested in an oven with nanosecond pulse voltage. The results showed that the flashover voltage of the PP film with 6-h surface fluorination was increased about 15% than the untreated film in the air. To clarify the influential mechanism, the physicochemical properties of PP film samples were tested and characterized. The results of thermally stimulated depolarization current (TSDC) showed that the trap density of the PP film increased obviously after fluorination treatment. With the increase in treatment time, the trap depth and density were increased obviously. The surface conductivity of PP films slightly decreased with the increase in fluorination time. The mechanism of surface fluorination on the influence of the flashover performance of the PP film under nanosecond pulse is discussed. It is concluded that surface fluorination significantly increases the carbon–fluorine (C–F) bond of the PP film, resulting in the increase in the depth and density of the surface trap, and then, a large amount of homocharges trapped in the surface trap reduces the electric field near the electrode.

Multi-Layered Branched Surface Fluorination on PVDF Membrane for Anti-Scaling Membrane Distillation.

Membrane distillation (MD) has emerged as a promising technology for hypersaline wastewater treatment. However, membrane scaling is still a critical issue for common hydrophobic MD membranes. Herein, we report a multi-layered surface modification strategy on the commercial polyvinylidene fluoride (PVDF) membrane via plasma treatment and surface fluorination cycles. The repeated plasma treatment process generates more reaction sites for the fluorination reaction, leading to higher fluorination density and more branched structures. MD tests with CaSO4 as the scaling agent show that the modification strategy mentioned above improves the membrane scaling resistance. Notably, the PVDF membrane treated with three cycles of plasma and fluorination treatments exhibits the best anti-scaling performance while maintaining almost the same membrane flux as the unmodified PVDF membrane. This study suggests that a highly branched surface molecular structure with low surface energy benefits the MD process in both membrane flux and scaling resistance. Besides, our research demonstrates a universal and facile approach for membrane treatment to improve membrane scaling resistance.

Cover Feature: Enhancement of Mass and Charge Transfer during Carbon Dioxide Photoreduction by Enhanced Surface Hydrophobicity without a Barrier Layer (Chem. Eur. J. 43/2022)

What you want, where you want it: Fewer H2O and more CO2 molecules can be delivered directly to an in-situ hydrophobic surface for the photocatalytic reduction of CO2 to CH3OH. A traditional hydrophobic surface would hinder charge transfer, while for a hydrophilic surface, the low solubility and slow diffusion rate limit the mass transfer of CO2. Our in-situ surface fluorination strategy overcomes such mass and charge-transfer limitations enhancing the performance of CRR, which is a promising route for the clean utilization of renewable resources. More information can be found in the Research Article by A. Li, E. Kan, and co-workers (DOI: 10.1002/chem.202201034).

Enhancement of Mass and Charge Transfer during Carbon Dioxide Photoreduction by Enhanced Surface Hydrophobicity without a Barrier Layer.

The CO2 reduction reaction (CRR) represents a promising route for the clean utilization of renewable resources. But mass-transfer limitations seriously hinder the forward step. Enhancing the surface hydrophobicity by using polymers has been proved to be one of the most efficient strategies. However, as macromolecular organics, polymers on the surface hinder the transfer of charge carriers from catalysts to reactants. Herein, we describe an in-situ surface fluorination strategy to enhance the surface hydrophobicity of TiO2 without a barrier layer of organics, thus facilitating the mass transfer of CO2 to catalysts and charge transfer. With less obstruction to charge transfer, a higher CO2, and lower H+ surface concentration, the photocatalytic CRR generation rate of methanol (CH3 OH) is greatly enhanced to up to 247.15 μmol g-1 h-1 . Furthermore, we investigated the overall defects; enhancing the surface hydrophobicity of catalysts provides a general and reliable method to improve the competitiveness of CRR.