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ISSN 2040-3364

PAPER Qian Wang et al. TiC2: a new two-dimensional sheet beyond MXenes

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DOI: 10.1039/C6NR10004J

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Stephen A. Hodge,a David J. Buckley,b Hin Chun Yau,a Neal T. Skipper,b Christopher A. Howard,b and Milo S. P. Shaffer*a Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

Chemical and electrochemical reduction methods allow the dispersion, processing, and/or functionalization of discrete sp 2hybridised nanocarbons, including fullerenes, nanotubes and graphenes. Electron transfer to the nanocarbon raises the Fermi energy creating nanocarbon anions, thereby activating an array of possible covalent reactions. The Fermi level may then be partially or fully lowered by intended functionalization reactions, but in general, techniques are required to remove excess charge without inadvertent covalent reactions that potentially degrade the nanocarbon properties of interest. Here, simple and effective chemical discharging routes are demonstrated for graphenide polyelectrolytes and are expected to apply to other systems, particularly nanotubides. The discharging process is inherently linked to the reduction potentials of such chemical discharging agents and the unusual fundamental chemistry of charged nanocarbons.

Introduction A variety of chemical and electrochemical methods of producing reduced fullerene, carbon nanotube and graphene polyelectrolyte salts, have been developed in recent years; the species are known as fullerides, nanotubides and graphenides, respectively.1–11 These salts are attractive as they allow the nanocarbons to dissolve spontaneously in a variety of polar, aprotic solvents (such as N,N-dimethylformamide [DMF], Nmethyl-2-pyrrolidone [NMP], dimethylacetamide [DMAc], dimethylsulfoxide [DMSO]) without the requirement for typical destructive high-shear methods (ultrasonication12,13 or shear mixing14) or repetitive ultracentrifugation steps.15 The ability to tune the charge/carbon stoichiometry also allows the purification of as-produced nanocarbon powders without the necessity for aggressive oxidative treatments. 5,16 However, in the reduced state, the nanocarbon salts are highly reactive and must be handled under inert atmosphere. The reactivity can be beneficial as it allows the individualised nanocarbons to be covalently-functionalized, using a wide range of electrophilic, radical and redox based reagents.7,9,17–20 Compared to other functionalization strategies, such as the common use of strong acids to prepare carboxylated nanotubes21 or graphene oxide,22 the reductive approach is appealing, as the connectivity of the carbon framework is maintained, and damage minimised. On the other hand, even low level covalent functionalization 23 (160 µm). KC8 was prepared by the vapour transport method56 generating the characteristic gold coloured stage 1 GIC. For each experiment, 30 mL, NMP (anhydrous grade, 99.5%, further dried by 4 Å molecular sieves, both purchased from Sigma-Aldrich, UK), was added to 10 mg KC8 in a 100 mL Young’s tap Schlenk tube inside a MBraun LabmasterSP glove box with levels of H2O and O2 below 1 ppm at all times. The sample tube was removed from the glove box, mildly sonicated for 30 min (ultrasonic cleaner, VWR, UK) and returned to the glove box for discharging reactions. Chemical discharging reactions. Iodine (anhydrous beads, −10 mesh, 99.999%) and trityl chloride (97%) were purchased from Sigma-Aldrich. C60 powder (99.5%) was obtained from SES Research, Houston, US. Chemical discharging reagents were added in a three times excess relative to potassium. In the case

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KC8 vacuum annealing. The KC8 powder was put into a stainless steel KF25 tube (0.5 m length) and baked at 700 oC, 10-6 mbar for 24 h. For the 1000 oC anneal, the 700 oC annealed KC8 was exposed to air for ~5 seconds for loading into a sealed graphite crucible inside a graphite furnace. The crucible was baked at 1000 oC at 10-6 mbar for 24 h. All temperature ramps were controlled to 5 oC/min.

Scanning Electron Microscopy-Energy DispersiveView X-ray (SEMArticle Online DOI: 10.1039/C6NR10004J EDX) spectroscopy. SEM-EDX was performed using a high resolution field emission gun scanning electron microscope (FEGSEM) Leo Gemini 1525, with a built-in energy dispersive and wavelength dispersive X-ray spectrometer (EDX) (INCA, using INCA suite software V4.15, 2009, Oxford Instruments Plc., UK). Graphitic samples for SEM/EDX spectroscopy were contacted to aluminium stubs using silver dag (All SEM preparation products purchased from Agar Scientific, UK).

Results and Discussion

Characterisation methods

Graphenide exposure to ambient air

X-ray photoelectron spectroscopy (XPS). XPS spectra were recorded using a Thermo Scientific K-Alpha instrument using focused (400 μm spot) monochromatic Al-Kα radiation at a pass energy of 40 eV. The binding energies were referenced to the sp2 C 1s peak of graphite at 284 eV.

To confirm the product of the hypothesised oxygen reduction reaction (ORR), KC8 dispersions were discharged by exposure to ambient air. Exposure of graphenide/NMP solution to the atmosphere resulted in the slow precipitation of graphene (over 72 hours) presumed to be sufficient to reach complete discharge. The resulting material was filtered through a 100 nm pore size PTFE membrane and washed with NMP, water, chloroform and ethanol (100 mL of each) to remove the potassium (hydr)oxide salts. The starting graphite material had only a low oxygen content (C/O ~30; shown in Fig. 4), which increased significantly for air discharged KC8 (C/O ~9). This ratio in the product is consistent with mildly oxidised graphene oxide (GO) or reduced graphene oxide (RGO).57 High-resolution XPS (ESI Fig. S1) revealed a broad O 1s signal that was fitted with two components (532.1 eV and 531.2 eV) with an approximate 60:40 peak area ratio that suggests the presence of both C=O and C-O bonds, respectively. Also, XPS revealed the presence of ~2-6 at.% potassium remaining even after washing. This potassium could be associated with the formation of potassium carboxylates, residual KOH, or trapped potassium between graphene layers. The observation of absorbed and intercalated potassium hydroxides/alkoxides upon reaction of GICs with water or alcohols was previously reported, even following hot aqueous/acid and ethanol washing steps.27 The discharging of a piece of KC8 powder in air was also monitored under a microscope (SI Movie 1) showing rapid formation of these potassium (hydr)oxide salts on the surface of the deintercalating graphite powder. As reported previously, the quantification of specific oxygencontaining moieties directly by XPS is challenging due to the prevalence of oxygen-related contamination and the difficulty of reliably fitting low concentration peaks in the carbon shakeup tail. An interesting improvement can be obtained by tagging each functional group with three fluorine atoms using chemically-selective reagents. Thus, for more detailed quantification, derivitization reactions of the hydroxyl, carboxyl and carbonyl groups were performed using trifluoroacetic anhydride (TFAA), trifluoroethanol (TFE) and trifluoroethylhydrazine (TFH), respectively (Fig. 2a), using a methodology described by Buono et al.58

Thermogravimetric analysis-mass spectrometry (TGA-MS). TGA-MS was performed using a Mettler Toledo TGA/DSC 1 with a GC200 flow controller. The TGA was coupled to a mass spectrometer (Hiden MS fitted with a 200 a.u. quadrupole sensor). Samples were heated from 30 to 100 °C at 35 °C/min, and then held isothermally at 100 °C for 30 min in an inert atmosphere (N2, 60 mL/min) to remove residual moisture. The temperature was then ramped to 800 °C at 10 °C/min. Oxidative TGA of as-received natural flake graphite and vacuum annealed KC8 at 700 °C and 1000 °C were performed in air (60 mL/min) with a temperature ramp from 100-1100 °C. Raman Spectroscopy. Raman spectroscopy was performed using a Renishaw InVia micro-Raman Spectrometer using a 532 nm laser using < 1mW laser power and 1800 gr/mm grating. At least 30 spectra were measured for each sample over the range 1250-3250 cm-1. UV-vis absorption spectroscopy. UV-vis spectroscopy was performed using a Perkin Elmer Lambda 950 spectrophotometer Samples were dissolved in toluene and spectra were recorded from 200 to 800 nm with a resolution of 1 nm, using a 10 mm path length UV quartz cuvette. Nuclear Magnetic Resonance (NMR) spectroscopy. NMR spectra were acquired using a Bruker AM 400 spectrometer operating at 9.4 T. Samples were dissolved in CDCl3 and all spectra (for 1H and 13C) were recorded with 16 scans and D1 = 1 s. Distortionless enhancement polarisation transfer (135 DEPT) NMR spectra was acquired with 386 scans with D1 = 1 s. All chemical shifts (δ) are given in ppm, where the residual CHCl 3 peak was used as an internal reference for 1H NMR (δH = 7.28 ppm), and for 13C NMR (δC = 78.23 ppm).

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of iodine discharging, a stock solution of iodine in NMP (11.3 g/L) was prepared in advance to ensure that the sublimation of iodine pellets did not contaminate the glove box atmosphere.


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TFH

The concentrations of oxygen present as carboxyl, View carbonyl and Article Online 10.1039/C6NR10004J hydroxyl groups equate to 1.4, 6.5 and DOI: 4.6 at.%, respectively. While this oxygen content is slightly higher than that expected from the original untagged XPS (~10 at.%), there is clear indication that hydroxyl and carbonyl (ketone) groups comprise the majority of functionalities present on the air discharged graphenide material. Except for the TFH case, in which nitrogen is explicitly grafted, the N signals are small (