Versatile Functionalization of Carbon Nanomaterials by Ferrate(VI)

Highlights Various forms of carbon nanomaterials are selected as substrates to clear the mist in understanding the reactivity/utility of ferrate(VI) in oxidizing carbon nanomaterials. It unravels a modest reactivity of ferrate(VI) in liquid phase that only oxidizes the active defects on carbon surface and a powerful oxidizing ability in solid state that can open the inert C=C bonds in carbon lattice. Respective benefit and limitation of the wet and dry approaches using ferrate(VI) in functionalizing carbon nanomaterials are discussed. Electronic supplementary material The online version of this article (10.1007/s40820-019-0353-2) contains supplementary material, which is available to authorized users.


Versatile Functionalization of Carbon Nanomaterials by Ferrate(VI)
Ying Zhou 1 , Zhao-Yang Zhang 1 Table   Table S1 Literature results on K2FeO4 oxidation of carbon materials in liquid phase Refs. carbon materials oxidant solvents reaction conditions d002 [a] (nm) O/C ratio [b] (%) ID/IG [c] [S1] 1 g graphite Note: cSA (concentrated sulfuric acid), cPA (concentrated phosphoric acid), RT (room temperature). Data of the raw materials are shown in parentheses.

S1 Supplementary
[a] d002 represents the interlayer spacing of graphite, tested by XRD; [b] O/C ratio was tested by XPS; [c] ID/IG was tested by Raman spectra.

S2.1.2 Synthesis and Purification Procedure
Preparation procedure of K2FeO4 was adapted from literature [7]. 205 mL of 37% HCl was slowly added dropwise to 33. The synthesized K2FeO4 was further purified by recrystallization as described in our previous work [S8].

S2.2.2 Liquid-phase K2FeO4 Oxidation
Oxidation of DCNT 100 mg of DCNTs was slowly added to 20.0 mL of sulfuric acid (95%-98%) in a 50 mL two-necked flask under argon atmosphere, and the dispersion was sonicated for 30 min. Then 2.5 g of K2FeO4 was slowly added to the flask under argon flow at 0°C, and the reaction mixture was stirred at 60 °C for 2 h. The resulting dispersion was diluted in 500 mL of cold water and settled for 30 min. The solid was obtained by centrifugation (5000 rpm, 10 min), followed by washing with 2 M HCl (several times to remove Fe 3+ ). The obtained solid was then dispersed in water under brief sonication, followed by filtration (0.22 µm PTFE membrane) and extensive washing with water to become neutral. Water-dispersible portions of the DCNTs were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 5000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.2.3 Oxidation of GCNT
100 mg of GCNTs was slowly added to 20.0 mL of sulfuric acid (95%-98%) in a 50 mL two-necked flask under argon atmosphere, and the dispersion was sonicated for 30 min. Then, 2.5 g of K2FeO4 was slowly added to the flask under argon flow at 0 °C, and the reaction mixture was stirred at 60 °C for 2 or 8 h. The resulting dispersion was diluted in 500 mL of cold water and settled for 30 min. The solid was obtained by centrifugation (5000 rpm, 10 min), followed by washing with 2 M HCl (several times to remove Fe 3+ ). The obtained solid was then dispersed in water under brief sonication, followed by filtration (0.22 µm PTFE membrane) and extensive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.2.4 Oxidation of DCNF
100 mg of DCNFs was slowly added to 20.0 mL of sulfuric acid (95%-98%) in a 50 mL two-necked flask under argon atmosphere. After that, 2.5 g of K2FeO4 was slowly added to the flask under argon flow at 0 °C, and the reaction mixture was stirred at 60 °C for 2 h. The resulting dispersion was diluted in 500 mL of cold water and settled for 30 min. The solid was obtained by centrifugation (5000 rpm, 10 min), followed by washing with 2 M HCl (several times to remove Fe 3+ ). The obtained solid was then dispersed in water under brief sonication, followed by filtration (0.22 µm PTFE membrane) and extensive washing with water to become neutral. Water-dispersible portions of the DCNFs were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 3000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.2.5 Oxidation of GCNF
100 mg of GCNFs was slowly added to 20.0 mL of sulfuric acid (95%-98%) in a 50 mL two-necked flask under argon atmosphere. After that, 2.5 g of K2FeO4 was slowly added to the flask under argon flow at 0 °C, and the reaction mixture was stirred at 60 °C for 2 or 8 h. The resulting dispersion was diluted in 500 mL of cold water and settled for 30 min. The solid was obtained by centrifugation (5000 rpm, 10 min), followed by washing with 2 M HCl (several times to remove Fe 3+ ). The obtained solid was then dispersed in water under brief sonication, followed by filtration (0.22 µm PTFE membrane) and extensive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.2.6 Oxidation of Nanographite
The nanographite used in this experiment was purified from the commercial nanographite. The commercial nanographite was dispersed in DMSO and sonicated for 30 min to form a dispersion of 1 mg mL -1 . Then, the black supernatant was collected by centrifuging at 4000 rpm for 15 min, while the bulk precipitates were discarded. The dispersed nanographite in the supernatant was collected by centrifuging at 12,000 rpm for 5 min. The obtained sample was finally dried at 60 °C in a vacuum oven.
100 mg of purified nanographite was slowly added to 20.0 mL of sulfuric acid (95%-98%) in a 50 mL two-necked flask under argon atmosphere. After that, 2.5 g of K2FeO4 was slowly added to the flask under argon flow at 0°C. Next, the dispersion was stirred at 60 °C for 2 h or 8 h or 12 h. The resulting dispersion was diluted in 500 mL of cold water and settled for 30 min. The solid was obtained by centrifugation (5000 rpm, 10 min), followed by washing with 2 M HCl (several times to remove Fe 3+ ). The obtained solid was then dispersed in water under brief sonication, followed by filtration (0.22 µm PTFE membrane) and extensive washing with water to become neutral. Waterdispersible portions of the nanographite were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 5000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.3.1 Oxidation of DCNT
mixture was then introduced into a 50 mL stainless milling jar together with 26 g of 5 mm-diameter stainless steel balls (ball-to-powder weight ratio 10:1). Ball milling was performed at a rotational speed of 250 or 300 rpm for 2 h in a horizontal planetary ball milling (WXQM-2L, Tecan Powder). The jar was opened every 30 min to break up the mixture materials if they were agglomerated or adhered to the sidewall during milling process. The resulting mixture was slowly added to 50 mL of 2 M HCl and settled for 2 h, followed by centrifugation at 5000 rpm for 10 min to get the solid. The acid washing process was repeated for 3 times to completely remove the ferric irons. The obtained solid was filtrated and washed by water to become neutral. Water-dispersible portions of the DCNTs were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 5000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60°C in a vacuum oven.

S2.3.2 Oxidation of GCNT
100 mg of GCNTs and 2.5 g of K2FeO4 were mixed together by brief grinding in an agate mortar. The mixture was then introduced into a 50 mL stainless milling jar together with 26 g of 5 mm-diameter balls (stainless steel balls). Ball milling was performed at a rotation speed of 250 or 300 rpm for different reaction time (2, 8 or 12 h). The jar was opened every 1 h to break up the mixture materials if they were agglomerated or adhered to the sidewall during milling process. The resulting mixture was slowly added to 50 mL of 2 M HCl and settled for 2 h, followed by centrifugation at 5000 rpm for 10 min to get the solid. The acid washing process was repeated for 3 times to completely remove the ferric irons. The obtained solid was filtrated and washed by water to become neutral. If applicable, water-dispersible portions of the GCNTs were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 5000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.3.3 Oxidation of DCNF
100 mg of DCNFs and 2.5 g of K2FeO4 were mixed together by brief grinding in an agate. The mixture was then introduced into a 50 mL stainless milling jar together with 26 g of 5 mm-diameter balls (stainless steel balls or agate balls). Ball milling was performed at different rotational speeds including 100, 150, and 250 rpm for 2 h. The jar was opened every 30 min to break up the mixture materials if they were agglomerated or adhered to the sidewall during milling process. The resulting mixture was slowly added to 50 mL of 2 M HCl and settled for 2 h, followed by centrifugation at 5000 rpm for 10 min to get the solid. The acid washing process was repeated for 3 times to completely remove the ferric irons. The obtained solid was filtrated and washed by water to become neutral. Water-dispersible portions of the DCNFs were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 3000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60 °C in a vacuum oven.

S2.3.4 Oxidation of GCNF
100 mg of GCNF and 2.5 g of K2FeO4 were mixed together by brief grinding in an agate mortar. The mixture was then introduced into a 50 mL stainless milling jar together with 26 g of 5 mm-diameter balls (stainless steel balls or agate balls). Ball milling was performed at a rotational speed of 100 or 250 rpm for different reaction time including 2, 8, and 12 h. The jar was opened every 1 h to break up the mixture materials if they were agglomerated or adhered to the sidewall during milling process. The resulting mixture was slowly added to 50 mL of 2 M HCl and settled for 2 h, followed by centrifugation at 5000 rpm for 10 min to get the solid. The acid washing process was repeated for 3 times to completely remove the ferric irons. The obtained solid was filtrated and washed by water to become neutral. If applicable, water-dispersible portions of the GCNFs were isolated as follows. The products were redispersed in water under sonication for 10 min, followed by centrifugation at 3000 rpm for 10 min to get the supernatant. Then the sediment was collected together to repeat the process of redispersion/centrifugation till the supernatant became clear. After that, the collected black supernatant was precipitated by 1 M HCl, followed by filtration and successive washing with water, alcohol and diethyl ether. The obtained product was finally dried at 60°C in a vacuum oven.

S2.4 Characterization
For MALDI-FT ICR MS analysis, a Bruker 7.0 T SolariX FTICR hybrid quadrupole-FT ICR mass spectrometer (Bruker Daltonics, Bremen, Germany), equipped with an ESI/APCI/MALDI ion source, and external ion accumulation was used in positive ion mode over a mass range of m/z 150.48-3000 in broadband detection mode. This MALDI source has a pulsed smart beam-II UV laser with an attenuator that allows fine adjustment of laser fluence (Azura Laser AG, Berlin, Germany). The MS instrument was tuned and calibrated with sodium trifluoroacetate in ESI source. The laser beam focus was set at the small value. The detector plate was 210 V, plate offset was 80 V, laser power was 25%-35%, frequency was 500 Hz; laser shots were 250, time of light was 0.9 ms. 1 H NMR (400 MHz) spectroscopic data were recorded on an AVANCE III HD 400 spectrometer with chemical shift of the residual solvent as an internal reference. Raman spectra were recorded with a 633 nm excitation laser by using a DXR Raman microscope (Thermo Fisher Scientific). XPS measurements were performed by ESCALAB 250Xi X-ray photoelectron spectroscopy (Thermal Scientific) using Al Kα X-rays and the binding energies were calibrated with respect to C 1s peak at 284.6 eV. TG was measured on a Pyris 1 thermogravimetric analyzer (TA), in which the temperature was increased to 700 °C at a heating rate of 10 °C /min under nitrogen. SEM images were acquired using a MIRA3 instrument (TESCAN). TEM was conducted on Tecnai G2 spirit Biotwin (FEI) and TALOS F200X (FEI) transmission electron microscopes.

S2.5 Quantification of Cage-opened C60
The product of C60 was dissolved in toluene under sonication for 5 min, followed by centrifugation at 12000 rpm for 3 min to remove the unreacted C60 in the supernatant. Then the obtained sediment was subjected to repeated process of dissolution/centrifugation till the supernatant became colorless. The sample was then dried at 40 °C in a vacuum oven and The sample were tested by MALDI-FT ICR MS and relative content of the cage-opened products was quantified by Eq. S1: where µ is the relative content of cage-opened products, m1 is the mass of raw C60, m2 is the mass of C60 product after reaction, m3 is the mass of sediment after wash by toluene, α represents the relative percentage of cage-opened products in MALDI-FT ICR MS spectrum.

S3.1 Purity Analysis of K2FeO4
Our K2FeO4 has a purity of 95%, as determined by spectrophotometry. The XRD pattern showed diffraction peaks that matches well with the standard PDF card of K2FeO4 crystals (Fig. S1), confirming the single-phase character of the sample without any crystalline impurities (e.g., KCl, KNO3, and K2CO3·1.5H2O). The purity of ferrate(Ⅵ) was also determined by the 57 Fe Mössbauer spectrum. As shown in Fig. S2, the value of the isomer shift parameter at room temperature (-0.89 mm s -1 ) corresponds well with the previously reported data for K2FeO4 [S10, S11] and the relative content of Fe(Ⅵ) was up to 97.6%, proving the high purity of our potassium ferrate(VI) again.

S3.3.1 Liquid-phase Oxidation
We note that the amount of K2FeO4 have an influence on the oxidation performance. In liquid phase oxidation, the dosage of 1.5 g and 2.5 g respectively led to 28.6% and 37.4% yield of the watersoluble DCNT product.
2.5 g dosage per 100 mg of carbon materials were used throughout our experiments.
As shown in Fig. S5, the morphology of the nanocarbons was not affected by the liquid-phase oxidation. Large amounts of long individual tubes or fibers were clearly observed in the products (even with a long reaction time), which was consistent with the pristine materials. Therefore, K2FeO4 in H2SO4 environment is mild enough for protection of the carbon structures.

Fig. S5 (a) SEM and (b) TEM images of four typical nanocarbons before and after liquid-phase reaction
The commercial nanographite consisted of a number of different-sized platelets and many of them were in several microns, as shown in Figure S6a. We have extracted the smaller platelets with lateral sizes well below 1 µm (Fig. S6b), which provide much higher ratio of edge-to-plane sites for our experiment.

S3.3.2 Solid-state Oxidation
The strength of milling force can be evaluated by stress energy SE, given by Eq. S2 where vt is the circumferential velocity, ρ and d is the density and diameter of the milling ball, respectively [S12]. In our experiments, the diameter of the milling ball was fixed at 5 mm, and the influence factors were rotation 1speed and the density/type of the milling ball.
As shown in Fig. S8, raw DCNTs exhibited tangled network consisted of long tubes. After 2 h-ballmilling treatment, the morphology of DCNTs remained similar features at the speed of 250 rpm. However, when the SE was increased by 1.44-fold (300 rpm), the structural integrity of DCNTs were destroyed, resulting in shortened and curled tubes.

Figure S8
SEM images of DCNTs before and after milling under different conditions (stainless steel balls were used) Similar phenomena occurred for GCNTs (Fig. S9). No obvious damage was observed at the speed of 250 rpm, indicating that 250 rpm provide mild milling strength for GCNTs. When the input energy was stronger at 300 rpm, severe structural damage was observed. For fragile GCNFs (Fig. S10), we suggested the use of low-density agate balls, mild milling speed (100 rpm) and short reaction time (2 h) to protect the integrity of the fibers. When the milling time was increased, GCNFs were damaged gradually under these mild conditions. On the other hand, the fibers were quickly destroyed when high-density stainless steel balls were used with a higher milling speed (250 rpm).

Fig. S10 SEM images of GCNFs before and after milling under different conditions
For DCNFs that were even more fragile (Fig. S11), mild energy input (100 rpm× 2 h, agate balls) could also cause structural damage. Any further increase of the milling speed or the density of milling balls could make the situations worse: most of the fibers were cut short and even became fragments. The HRTEM images of nanocarbons in Fig. S12 further confirmed that the structures of these carbon materials remained intact by liquid-phase reaction and the mild mechanochemical conditions.  After K2FeO4 oxidation, obvious increase in ID/IG was observed due to the opening of C=C bonds. It also led to large upshifts in D and G bands, arising from the "doping" effect by oxygenated groups [S13, S14]. Compared to K2SO4, K2CrO4 only led to slight changes in the Raman spectra, because its ability to open C=C bonds is much weaker than that of K2FeO4.

Fig. S13
Comparison of the Raman spectra of 12 h products. To compare the overall defect degree, water-dispersible portions of the products were not isolated, thus lower ID/IG ratios were recorded than those in Figure 3d for the K2FeO4 samples. [a] 250 rpm using steel balls for DCNT and GCNT; 100 rpm using agate balls for DCNF and GCNF