Effect of curvature on C-F bonding in fluorinated carbons: from fullerene and derivatives to graphite

The Stability of UV-Defluorination-Driven Crosslinked Carbon Nanotubes: A Raman Study

Carbon nanotubes (CNTs) are often regarded as semi-rigid, all-carbon polymers. However, unlike conventional polymers that can form 3D networks such as hydrogels or elastomers through crosslinking in solution, CNTs have long been considered non-crosslinkable under mild conditions. This perception changed with our recent discovery of UV-defluorination-driven direct crosslinking of CNTs in solution. In this study, we further investigate the thermal stability of UV-defluorination-driven crosslinked CNTs, revealing that they are metastable and decompose more readily than either pristine or fluorinated CNTs under Raman laser irradiation. Using Raman spectroscopy under controlled laser power, we examined both single-walled and multi-walled fluorinated CNTs. The results demonstrate that UV-defluorinated CNTs exhibit reduced thermal stability compared to their pristine or untreated fluorinated counterparts. This instability is attributed to the strain on the intertube crosslinking bonds resulting from the curved carbon lattice of the linked CNTs. The metallic CNTs in the crosslinked CNT networks decompose and revert to their pristine state more readily than the semiconducting ones. The inherent instability of crosslinked CNTs leads to combustion at temperatures approximately 100 °C lower than those required for non-crosslinked fluorinated CNTs. This property positions crosslinked CNTs as promising candidates for applications where mechanically robust, lightweight materials are needed, along with feasible post-use removal options.

Fluorinated Carbon Nanohorns as Cathode Materials for Ultra-High Power Li/CFx Batteries

Fluorinated carbon (CFx) has ultrahigh theoretical energy density among cathode materials for lithium primary batteries. CFx, as an active material in the cathode, plays a decisive role in performance. However, the performance of commercialized fluorinated graphite (FG) does not meet this continuously increasing performance demand. One effective way to increase the overall performance is to manipulate carbon-fluorine (C─F) bonds. In this study, carbon nanohorns are first used as a carbon source and are fluorinated at relatively low temperatures to obtain a new type of CFx with semi-ionic C─F bonds. Carbon nanohorns with a high degree of fluorination achieved a specific capacity comparable to that of commercial FG. Density functional theory (DFT) calculations revealed that curvature structure regulated its C─F bond configuration, thermodynamic parameters, and ion diffusion pathway. The dominant semi-ionic C─F bonds guarantee good conductivity, which improves rate performance. Fluorinated carbon nanohorns delivered a power density of 92.5 kW kg-1 at 50 C and an energy density of 707.6 Wh kg-1 . This result demonstrates the effectiveness of tailored C─F bonds and that the carbon nanohorns shorten the Li+ diffusion path. This excellent performance indicates the importance of designing the carbon source and paves new possibilities for future research.

Fluorinated saccharide-derived hard carbon as a cathode material of lithium primary batteries: effect of the polymerization degree of the starting saccharide

Fluorinated hard carbon materials have been considered to be a good candidate of cathode materials of Li/CF_x batteries. However, the effect of the precursor structure of the hard carbon on the structure and electrochemical performance of fluorinated carbon cathode materials has yet to be fully studied. In this paper, a series of fluorinated hard carbon (FHC) materials are prepared by gas phase fluorination using saccharides with different degrees of polymerization as a carbon source, and their structure and electrochemical properties are studied. The experimental results show that the specific surface area, pore structure, and defect degree of hard carbon (HC) are enhanced as the polymerization degree (i.e. molecular weight) of the starting saccharide increases. At the same time, the F/C ratio increases after fluorination at the same temperature, and the contents of electrochemically inactive -CF₂ and -CF₃ groups also become higher. At the fluorination temperature of 500 °C, the obtained fluorinated glucose pyrolytic carbon shows good electrochemical properties, with a specific capacity of 876 mA h g^-1, an energy density of 1872 W kg^-1, and a power density of 3740 W kg^-1. This study provides valuable insights and references for selecting suitable hard carbon precursors to develop high-performance fluorinated carbon cathode materials.

The Effect of C/Si Ratio and Fluorine Doping on the Gas Permeation Properties of Pendant-Type and Bridged-Type Organosilica Membranes

A series of pendant-type alkoxysilane structures with various carbon numbers (C₁-C₈) were used to fabricate sol-gel derived organosilica membranes to evaluate the effects of the C/Si ratio and fluorine doping. Initially, this investigation was focused on the effect that carbon-linking (pendant-type) units exert on a microporous structure and how this affects the gas-permeation properties of pendant-type organosilica membranes. Gas permeation results were compared with those of bridged-type organosilica membranes (C₁-C₈). Network pore size evaluation was conducted based on the selectivity of H₂/N₂ and the activation energy (E_p) of H₂ permeation. Consequently, E_p (H₂) was increased as the C/Si ratio increased from C₁ to C₈, which could have been due to the aggregation of pendant side chains that occupied the available micropore channel space and resulted in the reduced pore size. By comparison, these permeation results indicate that pendant-type organosilica membranes showed a somewhat loose network structure in comparison with bridged-type organosilica membranes by following the lower values of activation energies (E_p). Subsequently, we also evaluated the effect that fluorine doping (NH₄F) exerts on pendant-type [methytriethoxysilane (MTES), propyltrimethoxysilane (PTMS)] and bridged-type [1,2-bis(triethoxysilyl)methane (BTESM) bis(triethoxysilyl)propane (BTESP)] organosilica structures with similar carbon numbers (C₁ and C₃). The gas-permeation properties of F-doped pendant network structures revealed values for pore size, H₂/N₂ selectivity, and E_p (H₂) that were comparable to those of pristine organosilica membranes. This could be ascribed to the pendant side chains, which might have hindered the effectiveness of fluorine in pendant-type organosilica structures. The F-doped bridged-type organosilica (BTESM and BTESP) membranes, on the other hand, exhibited a looser network formation as the fluorine concentration increased.

Nanotube Functionalization: Investigation, Methods and Demonstrated Applications

This review presents an update on nanotube functionalization, including an investigation of their methods and applications. The review starts with the discussion of microscopy and spectroscopy investigations of functionalized carbon nanotubes (CNTs). The results of transmission electron microscopy and scanning tunnelling microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, Raman spectroscopy and resistivity measurements are summarized. The update on the methods of the functionalization of CNTs, such as covalent and non-covalent modification or the substitution of carbon atoms, is presented. The demonstrated applications of functionalized CNTs in nanoelectronics, composites, electrochemical energy storage, electrode materials, sensors and biomedicine are discussed.

Thermal stability of C-F/C(-F)2 bonds in fluorinated graphene detected by in situ heating infrared spectroscopy

The thermal stability of fluorinated graphene (FG) plays an important role in its application and research, and thus it is necessary to conduct in-depth research on the thermal stability of the C-F bond in FG. Herein, FG with different types and distributions of C-F/C(-F)₂ bonds were synthesized, and the correlation between the C-F/C(-F)₂ bonds and thermal stability of these FG samples was monitored via in situ heating infrared spectroscopy (in situ FTIR). The stability of the different types and distributions of C-F/C(-F)₂ bonds in FG and the temperatures at which these C-F/C(-F)₂ bonds were eliminated were determined. In terms of C-F bonds in FG, the most stable type is that in C(-F)₂ of perfluorinated FG, followed by the C-F bonds in perfluorinated FG. The thermal stability of isolated C-F bonds and C(-F)₂ bonds adjacent to the conjugated structure was the worst, which would be detached from FG at low temperature (≤82 °C). Furthermore, the evolution of the conjugated structures in FG during thermal annealing was also affected by the type and distribution of the C-F bonds.

Modes of Disorder in Poly(carbon monofluoride)

Poly(carbon monofluoride), or (CF)_n, is a layered fluorinated graphite material consisting of nanosized platelets. Here, we present experimental multidimensional solid-state NMR spectra of (CF)_n, supported by density functional theory (DFT) calculations of NMR parameters, which overhauls our understanding of structure and bonding in the material by elucidating many ways in which disorder manifests. We observe strong ¹⁹F NMR signals conventionally assigned to elongated or "semi-ionic" C-F bonds and find that these signals are in fact due to domains where the framework locally adopts boat-like cyclohexane conformations. We calculate that C-F bonds are weakened but are not elongated by this conformational disorder. Exchange NMR suggests that conformational disorder avoids platelet edges. We also use a new J-resolved NMR method for disordered solids, which provides molecular-level resolution of highly fluorinated edge states. The strings of consecutive difluoromethylene groups at edges are relatively mobile. Topologically distinct edge features, including zigzag edges, crenellated edges, and coves, are resolved in our samples by solid-state NMR. Disorder should be controllable in a manner dependent on synthesis, affording new opportunities for tuning the properties of graphite fluorides.

The electrochemical performance of fluorinated ketjenblack as a cathode for lithium/fluorinated carbon batteries

The inferior rate capacity of lithium/fluorinated carbon (Li/CF x ) batteries limits their application in the field, requiring large discharge current and high power density. Herein, we report a novel type of fluorinated carbon with superior performance through gas-phase fluorination of ketjenblack. The investigation shows that the F/C ratio of the fluorinated ketjenblack (FKB) increases with the fluorination temperature, whereas the discharge voltage decreases due to the lowered content of semi-ionic C-F bonds. Accordingly, a suitable fluorination temperature of 520 °C was selected, under which the product exhibits the largest specific capacity of 924.6 mA h g-1 with discharge potential exceeding 3.1 V (vs. Li/Li+) and the highest energy density of 2544 W h kg-1 with power density of 27 493 W kg-1. This energy density is higher than the theoretical energy density of commercial fluorinated graphite (2180 W h kg-1). In addition, the sample delivers good rate capability demonstrated by a specific capacity retention ratio of 79.5% even at a current density of 20C. Therefore, the FKB material may have very promising practical applications in lithium primary batteries.

Surface specificity and mechanistic pathway of de-fluorination of C60F48 on coinage metals

We provide experimental and theoretical understanding on fundamental processes taking place at room temperature when a fluorinated fullerene dopant gets close to a metal surface. By employing scanning tunneling microscopy and photoelectron spectroscopies, we demonstrate that the on-surface integrity of C60F48 depends on the interaction with the particular metal it approaches. Whereas on Au(111) the molecule preserves its chemical structure, on more reactive surfaces such as Cu(111) and Ni(111), molecules interacting with the bare metal surface lose the halogen atoms and transform to C60. Though fluorine-metal bonding can be detected depending on the molecular surface density, no ordered fluorine structures are observed. We show the implications of the metal-dependent de-fluorination in the electronic structure of the molecules and the energy alignment at the molecule-metal interface. Molecular dynamics simulations with ReaxFF reactive force field corroborate the experimental facts and provide a detailed mechanistic picture of the surface-induced de-fluorination, which involves the rotation of the molecule on the surface. Outstandingly, a thermodynamic analysis indicates that the effect of the metal surface is lowering and diminishing the energy barrier for C-F cleave, demonstrating the catalytic role of the surface. The present study contributes to in-depth knowledge of the mechanisms that affect the degree of stability of chemical species on surfaces, which is essential to advance our understanding of the chemical reactivity of metals and their role in on-surface chemical reactions.

Double Beneficial Role of Fluorinated Fullerene Dopants on Organic Thin-Film Transistors: Structural Stability and Improved Performance

The present work assesses improved carrier injection in organic field-effect transistors by contact doping and provides fundamental insight into the multiple impacts that the dopant/semiconductor interface details have on the long-term and thermal stability of devices. We investigate donor [1]benzothieno[3,2-b]-[1]benzothiophene (BTBT) derivatives with one and two octyl side chains attached to the core, therefore constituting asymmetric (BTBT-C8) and symmetric (C8-BTBT-C8) molecules, respectively. Our results reveal that films formed out of the asymmetric BTBT-C8 expose the same alkyl-terminated surface as the C8-BTBT-C8 films do. In both cases, the consequence of depositing fluorinated fullerene (C₆₀F₄₈) as a molecular p-dopant is the formation of C₆₀F₄₈ crystalline islands decorating the step edges of the underlying semiconductor film surface. We demonstrate that local work function changes along with a peculiar nanomorphology lead to the double beneficial effect of lowering the contact resistance and providing long-term and enhanced thermal stability of the devices.

High-Power-Density, High-Energy-Density Fluorinated Graphene for Primary Lithium Batteries

Li/CF_x is one of the highest-energy-density primary batteries; however, poor rate capability hinders its practical applications in high-power devices. Here we report a preparation of fluorinated graphene (GF_x) with superior performance through a direct gas fluorination method. We find that the so-called “semi-ionic” C-F bond content in all C-F bonds presents a more critical impact on rate performance of the GF_x in comparison with sp² C content in the GF_x, morphology, structure, and specific surface area of the materials. The rate capability remains excellent before the semi-ionic C-F bond proportion in the GF_x decreases. Thus, by optimizing semi-ionic C-F content in our GF_x, we obtain the optimal x of 0.8, with which the GF_0.8 exhibits a very high energy density of 1,073 Wh kg⁻¹ and an excellent power density of 21,460 W kg⁻¹ at a high current density of 10 A g⁻¹. More importantly, our approach opens a new avenue to obtain fluorinated carbon with high energy densities without compromising high power densities.

Radical mechanism of a nucleophilic reaction depending on a two-dimensional structure

The mechanism of nucleophilic substitution deserves more investigation to include more reaction systems such as two-dimensional (2D) materials. In this study, we used fluorinated graphene (FG) as a representative 2D material to reveal the in-depth mechanism of its defluorination and nucleophilic substitution reaction under attack of common nucleophiles to explore the chemistry of 2D materials and enrich the research on the nucleophilic substitution reaction. DFT calculations and electron paramagnetic resonance spectroscopy (EPR) demonstrated that defluorination of FG occurred via a radical mechanism after a single electron transfer (SET) reaction between the nucleophile and C-F bond, and a spin center was generated on the nanosheet and fluorine anion. Moreover, neither the S_N1 nor S_N2 mechanism was suggested to be appropriate for the substitution reaction of FG with a 2D structure due to the corresponding kinetics or thermodynamics disadvantage; hence, its nucleophilic substitution was proved to occur via a radical mechanism initiated by the defluorination step. The proposed substitution mechanism of FG demonstrates that nucleophilic substitution via a radical mechanism can also be applied to the attacking process of common nucleophiles without any particular conditions. Furthermore, it has been discovered that triethylamine without active hydrogen can be covalently attached to graphene nanosheets via a nucleophilic substitution reaction with FG; this further indicates a radical process for the nucleophilic substitution of FG rather than an S_N1 or S_N2 mechanism. The detailed process of the nucleophilic substitution reaction of FG was revealed to occur via a radical mechanism depending on the 2D structure of FG, which could also represent the typical characteristic of 2D chemistry.

Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency

Through introducing six fluorine atoms onto quinoxaline (Qx), a new electron acceptor unit-hexafluoroquinoxaline (HFQx) is first synthesized. On the basis of this unit, we synthesize a new donor-acceptor (D-A) copolymer (HFQx-T), which is composed of a benzodithiophene (BDT) derivative donor block and an HFQx accepting block. The strong electron-withdrawing properties of fluorine atoms increase significantly the open-circuit voltage (V_oc) by tuning the highest occupied molecular orbital (HOMO) energy level. In addition, fluorine atoms enhance the absorption coefficient of the conjugated copolymer and change the film morphology, which implies an increase of the short-circuit current density (J_sc) and fill factor (FF). Indeed, the HFQx-T:ITIC blended film achieves an impressive power conversion efficiency (PCE) of 9.4% with large short-current density (J_sc) of 15.60 mA/cm², high V_oc of 0.92 V, and FF of 65% via two step annealing (thermal annealing (TA) and solvent vapor annealing (SVA) treatments). The excellent results obtained show that the new copolymer HFQx-T synthesized could be a promising candidate for organic photovoltaics.

Defluorination and covalent grafting of fluorinated graphene with TEMPO in a radical mechanism

The work innovatively reveals the radical mechanism of derivative reactions of fluorinated graphene including its defluorination and covalent grafting, meanwhile first confirming the destination of deciduous fluorine atoms after defluorination.

Adsorption mechanism of different organic chemicals on fluorinated carbon nanotubes

Multi-walled carbon nanotubes (MC) were fluorinated by a solid-phase reaction method using polytetrafluoroethylene (PTFE). The surface alteration of carbon nanotubes after fluorination (MC-F) was confirmed based on surface elemental analysis, TEM and SEM. The incorporation of F on MC surface was discussed as F incorporation on carbon defects, replacement of carboxyl groups, as well as surface coating of PTFE. The adsorption performance and mechanisms of MC-F for five kinds of representative organic compounds: sulfamethoxazole (SMX), ofloxacin (OFL), norfloxacin (NOR), bisphenol a (BPA) and phenanthrene (PHE) were investigated. Although BET-N2 surface area of the investigated CNTs decreased after fluorination, the adsorption of all five chemicals increased. Because of the glassification of MC-F surface coating during BET-N2 surface area measurement, the accessible surface area of MC-F was underestimated. Desorption hysteresis was generally observed in all the sorption systems in this study, and the desorption hysteresis of MC-F were stronger than the pristine CNTs. The enhanced adsorption of MC-F may be attributed the pores generated on the coated PTFE and the dispersed CNT aggregates due to the increased electrostatic repulsion after fluorination. The rearrangement of the bundles or diffusion of the adsorbates in MC-F inner pores were the likely reason for the strong desorption hysteresis of MC-F. The butterfly structure of BPA resulted in its high sorption and strong desorption hysteresis. The exothermic sorption character of OFL on CNTs resulted in its strong desorption hysteresis.

Reduction and transformation of fluorinated graphene induced by ultraviolet irradiation

The effect of ultraviolet irradiation on fluorinated graphene (FG) dispersed in toluene was investigated for the first time. The chemical and physical characteristics of FG before and after ultraviolet irradiation were analyzed by UV-vis, FTIR, XPS,EDS, oxygen flask combustion (OFC), XRD, TGA, Raman, SEM, TEM and fluorescence spectroscopy. It is found that the F/C ratio initially decreases rapidly and then slowly with irradiation time, finally to 0.179 after irradiation for 48 h. The nature of partial C-F bonds transforms from covalent to "semi-covalent" bonding in the process of irradiation. The restoration of new sp(2) clusters is fast at the early stage within 6 h of irradiation, promoting the structural rearrangement. The morphology of irradiated fluorinated graphene (iFG) is not significantly destroyed by ultraviolet while more overlapped sheets are formed due to quick defluorination. Photoluminescence (PL) properties show that "blue emission" located at 432 nm is enhanced due to the recovery of sp(2) domains. In particular, compared to non-aromatic solvents, there is a "synergistic effect" between aromatic solvents and ultraviolet in the defluorination process. FG is unstable and shows some structural transformations under ultraviolet irradiation, which can be used to tune its structure and properties.

Chemical Makeup and Hydrophilic Behavior of Graphene Oxide Nanoribbons after Low-Temperature Fluorination

Here we investigated the fluorination of graphene oxide nanoribbons (GONRs) using H2 and F2 gases at low temperature, below 200 °C, with the purpose of elucidating their structure and predicting a fluorination mechanism. The importance of this study is the understanding of how fluorine functional groups are incorporated in complex structures, such as GONRs, as a function of temperature. The insight provided herein can potentially help engineer application-oriented materials for several research and industrial sectors. Direct (13)C pulse magic angle spinning (MAS) nuclear magnetic resonance (NMR) confirmed the presence of epoxy, hydroxyl, ester and ketone carbonyl, tertiary alkyl fluorides, as well as graphitic sp(2)-hybridized carbon. Moreover, (19)F-(13)C cross-polarization MAS NMR with (1)H and (19)F decoupling confirmed the presence of secondary alkyl fluoride (CF2) groups in the fluorinated graphene oxide nanoribbon (FGONR) structures fluorinated above 50 °C. First-principles density functional theory calculations gained insight into the atomic arrangement of the most dominant chemical groups. The fluorinated GONRs present atomic fluorine percentages in the range of 6-35. Interestingly, the FGONRs synthesized up to 100 °C, with 6-19% of atomic fluorine, exhibit colloidal similar stability in aqueous environments when compared to GONRs. This colloidal stability is important because it is not common for materials with up to 19% fluorine to have a high degree of hydrophilicity.

Progress towards high-power Li/CFx batteries: electrode architectures using carbon nanotubes with CFx

This perspective describes the current status of lithium–carbon monofluoride batteries and highlights the opportunities for the development of high-power Li/CF_x batteries via utilization of carbon nanotubes.

Fluorination of graphene: a spectroscopic and microscopic study

Since the advent of graphene, there has been intense interest in exploring the possibility of incorporating fluorinated graphene (FG), an ultrathin insulator, into graphene electronics as barriers, gate dielectrics, and optoelectronic elements. Here we report on the synthesis of FG from single-layer graphene sheets grown by chemical vapor deposition (CVD) using CF4 plasma. We examine its properties systematically via microscopic and spectroscopic probes. Our studies show that, by controlling the conditions of the plasma, FG of varying fluorine coverage can be produced; however, the resulting material contains a mixture of CFx (x = 1-3) bonds. Existing grain boundaries and lattice defects of CVD graphene play an important role in controlling its rate of fluorination and the damage of the sheet. Combining topography and current mapping, we demonstrate that the spatial distribution of fluorine on CVD graphene is highly inhomogeneous, where multilayer islands and structural features such as folds, wrinkles, and ripples are less fluorinated and consequently form a conductive network through which charge transport occurs. It is the properties of this network that manifest in the electrical transport of FG sheets. Our experiments reveal the many challenges of deriving electronics-quality FG from current CVD graphene while at the same time point to the possible solutions and potential of FG in graphene electronics and optoelectronics.

"In situ" hard mask materials: a new methodology for creation of vertical silicon nanopillar and nanowire arrays

A novel, simple and in situ hard mask technology that can be used to develop high aspect ratio silicon nanopillar and nanowire features on a substrate surface is demonstrated. The technique combines a block copolymer inclusion method that generates nanodot arrays on substrate and an inductively coupled plasma (ICP) etch processing step to fabricate Si nanopillar and nanowire arrays. Iron oxide was found to be an excellent resistant mask over silicon under the selected etching conditions. Features of a very high aspect ratio can be created by this method. The nanopillars have uniform diameter and smooth sidewalls throughout their entire length. The diameter (15-27 nm) and length of the nanopillars can be tuned easily. Different spectroscopic and microscopic techniques were used to examine the morphology and size, surface composition and crystallinity of the resultant patterns. The methodology developed may have important technological applications and provide an inexpensive manufacturing route to nanodimensioned topographical patterns. The high aspect ratio of the features may have importance in the area of photonics and the photoluminescence properties are found to be similar to those of surface-oxidized silicon nanocrystals and porous silicon.

Tuning the electronic transport properties of grapheme through functionalisation with fluorine

We demonstrate the possibility to tune the electronic transport properties of graphene mono-layers and multi-layers by functionalisation with fluorine. For mono-layer samples, with increasing the fluorine content, we observe a transition from electronic transport through Mott variable range hopping (VRH) in two dimensions to Efros-Shklovskii VRH. Multi-layer fluorinated graphene with high concentration of fluorine show two-dimensional Mott VRH transport, whereas CF0.28 multi-layer flakes exhibit thermally activated transport through near neighbour hopping. Our experimental findings demonstrate that the ability to control the degree of functionalisation of graphene is instrumental to engineer different electronic properties in graphene materials.

Using the 19F NMR chemical shift anisotropy tensor to differentiate between the zigzag and chiral forms of fluorinated single-walled carbon nanotubes

The structural characterization of different kinds of zigzag and chiral single-walled carbon nanotubes (SWNTs) has been investigated theoretically using (19)F NMR spectroscopy. The chemical shift anisotropy (CSA) tensor is computed at different levels of theory for the (19)F nuclei in different forms of functionalized fluorinated carbon nanotubes (CNT). A set of fluorine CSA parameters comprising the span, skew, and isotropic chemical shift is computed for each form of the fluoronanotubes and multidimensional CSA parameter correlation maps are constructed. We show that these correlations are able to clearly distinguish between the chiral and zigzag forms of fluorinated carbon nanotubes (F-SWNTs). Implications for solid-state and liquid-state NMR experiments are discussed.