Edge-Oxidation of Graphites by Hydrogen Peroxide - Langmuir (ACS

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Edge oxidation of graphite by hydrogen peroxide Aniello Vittore, Maria Rosaria Acocella, and Gaetano Guerra Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03489 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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Edge oxidation of graphite by hydrogen peroxide Aniello Vittore, Maria Rosaria Acocella, Gaetano Guerra Department of Chemistry and Biology and INSTM Research Unit, Università di Salerno, Fisciano (SA), Italy Abstract A simple and eco-friendly procedure of edge oxidation of high-surface-area graphite, based on hydrogen peroxide treatments at 60°C, is presented. Already short-term treatments lead to O/C weight ratios higher than 0.1, leaving unaltered interlayer spacing and correlation length. This clearly indicates that all oxidized groups are located on exposed sites (mainly on lateral edges) of the graphitic layers. Short-term H2O2 treatments, as expected, increase hydrophilicity and reduce thermal stability with respect to the starting graphite. Long-term treatments, on the contrary, reduce hydrophilicity and increase thermal stability with respect to the starting graphite, mainly due to surface area reduction associated with the oxidation procedure. Exfoliation of a substantial fraction of the obtained edge-oxidized graphite can be achieved by simple procedures of dispersion and sonication in water.

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Introduction Chemical oxidation of graphite by treatments with strong mineral acids and oxidizing agents generally leads to graphite oxide, a layered crystalline material.1-4 Graphite oxide can be easily exfoliated by different procedures leading to graphene oxide and, after reduction, to graphene (more precisely to reduced graphene oxide).1-4 This kind of procedure can lead to high levels of oxidation with oxidized groups being located not only on the lateral terminations of the graphene layers but also on the graphitic planes, with disruption of the aromatic graphene structure. For many applications, mainly for those needing high electrical conductivity, the maintenance of the ordered graphitic structure in micro- or nano-platelets, which are only functionalized on the graphitic edges, is preferred.5-12 Many reports also show edge functionalization of graphite leading to exfoliation toward edge functionalized graphene.13-20 Many studies, mainly from the Baek’s group, use ball-milling of graphite with gases (CO2, N2, F2 and Cl2), liquids (SO3, Br2) and solids (I2, maleic anhydride and elemental sulphur) leading to selective functionalization of graphite edges by different functional groups.6,8,9,20 Different approaches use organic reactions, mainly in solution, such as Friedl-Craft and Diels Alder, which are selective for C-H sp2, being prevailingly present on the edges of graphitic planes.5,10,12 In some recent papers, hydrogen peroxide was used as an eco-friendly reagent, for exfoliation treatments on reduced graphite oxide21 as well as on edge oxidized graphite oxide,22 i.e. on graphite samples already oxidized by stronger oxidizing agents. In this paper, a simple procedure of edge functionalization of high-surface-area graphite (HSAG) based on eco-friendly hydrogen peroxide treatments is presented. The proposed procedure easily leads to edge-oxidized graphite with O/C weight ratios higher than 0.1, but with unaltered crystalline order perpendicular to the graphitic planes. Moreover, exfoliation of a substantial fraction of the edge-oxidized graphite, can be obtained by simple procedures of dispersion and sonication in water.


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Experimental section High surface area graphite (HSAG), with a minimum carbon wt% of 99.5 and a surface area of 330 m2/g, was purchased from Asbury Graphite Mills Inc. HSAG oxidation has been obtained by hydrogen peroxide. 400mg of HSAG and 400mL of H2O2 are introduced in a 1L flask, placed in a thermostat bath at 60 °C and under magnetic stirring. After the indicated time, about a liter of water is added and the whole is filtered under vacuum. At least two washes are carried out with 100 ml of H2O and finally the powders are dried at 60 °C for one night. Surface areas of carbon and oxidized carbon samples were measured by nitrogen adsorption at liquid nitrogen temperature (77 K) with a Nova Quantachrome 4200e instrument. Before adsorption measurements, samples were degassed at 60°C under vacuum for 24 h. The surface area values were determined by using 11-point BET analysis. Elemental analysis was performed with a Thermo FlashEA 1112 Series CHNS-O analyzer, after pretreating samples in an oven at 100 °C for 12 h, by using BBOT as standard. Thermogravimetric analysis (TGA) was carried out on a TA-Instruments Q500, from room temperature up to 800 °C at a heating rate of 10 °C/min, under N2 flow of 60 mL/min. The reported scans refer to samples previously treated in the apparatus at 90°C, hold at that temperature for 10 min, and then cooled to room temperature at a rate of 10 °C/min. FTIR spectra were obtained with a FTIR (BRUKER Vertex70) spectrometer equipped with deuterated triglycine sulfate (DTGS) detector and a KBr beam splitter, at a resolution of 2.0 cm−1. The frequency scale was internally calibrated to 0.01 cm−1 using a He−Ne laser. 32 scans were signal averaged to reduce the noise. Spectra of powder samples have been collected by using KBr pellets. Wide-angle X-ray diffraction (WAXD) patterns were obtained by an automatic Bruker D2 Phaser diffractometer, in reflection, at 35 KV and 40 mA, using the nickel filtered Cu-Kα radiation (1.5418 Å). Correlation lengths (D) were determined by using Scherrer’s equation:

D

K  cos 

(1)

where λ is the wavelength of the incident X-rays and θ the diffraction angle, assuming the Scherrer constant K = 1. Evaluations of loss of amorphous carbon of Table 1 and of degree of exfoliation are conducted by assuming for the baseline the range 15° < 2θ < 40°. XPS measurements were carried out in a SPECS spectrometer (Germany), equipped with a Phoibos 150 MCD-9 detector, by a non-monochromatic X-ray source (Al and Mg) that operates at


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200 W. The samples were submitted to vacuum at 1 × 10–9 mbar. The CASA software was used for spectral deconvolution. Raman spectra were obtained at room temperature with a micro-Raman spectrometer Renishaw inVia with a 514 nm excitation wavelength (laser power 30 mW) in the range (100–3000 cm-1). Aqueous dispersions of HSAG and oxidized HSAG (4mg/ml) were sonicated for different time (2h or 6h) by using an ultrasonic bath (240 W) and subsequently centrifuged (Awel MF20-R) at 2000rpm for 20 min. Supernatant was carefully recovered and dried at 60°C.


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Results and discussion

Oxidation of HSAG by H2O2 Oxidation by hydrogen peroxide of highly ordered graphites is generally poorly effective, leading to O/C weight ratios lower than 0.01. This green oxidation procedure is instead effective with high surface area graphites. Results reported in this paper refer to a high-surface area graphite (HSAG, 330 m2/g),23 whose wide-angle X-ray diffraction (WAXD) pattern is shown by a black curve in Figure 1. The absence of a well defined 101 peak and the low intensity of the 112 peak with respect to the hk0 reflections indicate the occurrence of disorder in the relative position of parallel graphitic layers, not far from the limit disordered turbostratic structure.24,4 Moreover, breadths of diffraction peaks allow evaluating correlation lengths (i.e., crystalline order) being significantly larger in the graphitic planes (D110 ≈30nm) than perpendicular to the graphitic planes (D002 ≈10nm).4

Figure 1. WAXD patterns (normalized on the basis of the height of the 002 reflection) of the starting graphite (black curve) and after different times of treatment by H2O2 at 60°C. Two insets show enlarged regions of the patterns.


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Elemental analyses of HSAG samples after different reaction times, are collected in Table 1. It is apparent that H2O2 treatments lead to a substantial degree of oxidation already for short times (weight ratio O/C ≈ 0.10, for 15 min of treatment). For longer treatments, the O/C ratio more slowly increases, reaching a maximum value of O/C ≈ 0.17, after 9 hours. Further increases of reaction times lead to an unexpected reduction of oxidation degree with O/C leveling off to ≈ 0.10 (6th column of Table 1 and black filled circles and left scale of Figure 2).

Table 1. Weight losses and elemental analysis of HSAG, after different reaction times with H2O2 at 60°C. Weight loss %

Loss of amorphous carbon a wt%

Sample

%C

%N

%H

%O

O/C

HSAG

99.50

0.40

0.10

< 0.05

0.4 nitrogen sorption of the partially exfoliated sample becomes even higher than for the starting high-surface area graphite (HSAG in Figure 6). The observed increases of nitrogen sorption is compatible with the partial exfoliation shown by WAXD patterns of Figure 8.


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Additional information on the mainly exfoliated supernatant fraction is obtained by Raman spectra (Figure 9). Already the starting high surface area graphite (Figure 9a) exhibits a high intensity of peaks associated with disorder or confinement of the graphitic layers 23, 35-37 (the so called D and 2D peaks located at 1350 cm-1 and 2700 cm-1, respectively). The ratios between the intensity of D and 2D peak with respect to the G peak (located at 1590 cm-1 and corresponding to bulk crystalline graphite) are definitely high (ID/IG = 0.42; I2D/IG =0.60), irrespective of the high crystallinity (black pattern in Figure 1) and of negligible oxygen content (Table 1) of the used high-surface-area graphite. These ratios slightly increase as a consequence of H2O2 oxidation (e.g. after 3h of treatment, ID/IG =0.49; I2D/IG =0.63, Figure 9b). An additional increase of these ratios and hence of the disorder is observed for the supernatant fraction of the oxidized sample, as obtained after sonication in water for 2h (ID/IG =0.53; I2D/IG =0.63, Figure 9b).

Figure 9. Raman spectra of the HSAG (blue curve), the edge-oxidized HSAG after H2O2 treatment for 3 h (brown curve) and of the supernatant fraction after sonication in water for 2h (black curve).

It is worth adding that partially exfoliated graphite has been described as more suitable than fully exfoliated reduced graphene oxide samples to get low electrical percolation thresholds in polymer composites.11


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Conclusions A simple and eco-friendly procedure of edge functionalization of high-surface-area graphite treatments is presented. The proposed metal-free and organic solvent-free procedure is based on hydrogen peroxide and is conducted at temperatures as low as 60°C. Already short term treatments (15 min) lead to O/C weight ratios higher than 0.1, associated with unaltered interlayer spacing (d002=0.339 nm) and unaltered correlation length (D002≈ 10nm), which clearly indicate that interlayer oxidation is negligible and that all oxidized groups are located on exposed sites (mainly on lateral edges) of the graphitic layers. An unexpected apparent narrowing of the intense 002 reflection (corresponding to the interlayer distance) is also observed. This phenomenon is rationalized by considering that the H2O2 treatment also removes from starting graphite its amorphous carbon fraction (higher than 30wt%). Short-term H2O2 treatments increase hydrophilicity and reduce thermal stability with respect to the starting graphite. Long-term treatments, on the contrary, reduce hydrophilicity and increase thermal stability with respect to the starting graphite. This latter phenomenon has been rationalized on the basis of removal of amorphous carbon and reduction of surface area associated with H2O2 treatments. Exfoliation of a substantial fraction of the edge-oxidized graphite can be obtained by simple procedures of dispersion and sonication in water. Edge oxidized graphites and graphenes can be useful as nanofillers of polymer based composites easily leading to thermal and electrical percolation, by facilitating filler-filler interactions.11 The presently proposed graphite oxidation procedure provides edge-oxidized graphite as well as prevailingly exfoliated and edge-oxidized graphite, only by using hydrogen peroxide at 60°C, i.e. in conditions definitely milder than those already reported in the literature.

Acknowledgements. We acknowledge Prof. Sergio Navalon of Universitat Politécnica de Valéncia for providing XPS measurements and Prof. Liberata Guadagno and Dr. Regina Raimondo of University of Salerno for providing Raman spectra and for useful discussions. Financial support by Italian Ministry of University and Research (MIUR).


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