氨水处理SI

Electronic Supplementary Material

Microwave Synthesis of Large Few-Layer Graphene Sheets in Aqueous Solution of Ammonia

Izabela Janowska1 ( ), Kambiz Chizari1, Ovidiu Ersen2, Spyridon Zafeiratos1, Driss Soubane1, Victor Da Costa2, Virginie Speisser2, Christine Boeglin2, Matthieu Houllé1, Dominique Bégin1, Dominique Plee3, Marc-Jacques Ledoux1, and Cuong Pham-Huu1

1 Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse (LMSPC), UMR7515 CNRS-Université de Strasbourg 25, rue Becquerel, 67087 Strasbourg Cedex 08, France

2Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR7504 CNRS-Université de Strasbourg 23, rue du Loess, 67037 Strasbourg Cedex 02, France

3 Arkema, Groupement de Recherche de Lacq (GRL), BP 34, 64170 LACQ, France

Supporting information to DOI 10.1007/s12274-010-1017-1

S-1. Wetting behaviour of the expanded graphite treated by water and aqueous solution of ammonia

The wetting behaviour of the expanded graphite (a) in pure water and (b) in concentrated ammonia solution (33 wt.%) is presented in Fig. S-1. The increased amount of solid settled at the bottom in (b) shows that wetting is significantly increased with increasing ammonia concentration.

Figure S-1Optical photos showing the wetting behaviour of the expanded graphite with (a) pure water and (b) ammonia solution (33 wt.%). The ammonia solution results in better wetting of the expanded graphite compared to water which could explain the higher exfoliation of the expanded graphite as a function of increasing ammonia concentration

Address correspondence to janowskai@unistra.fr

According to Hernandez et al. [1], exfoliation is favored when the solvent surface tension is high enough to overcome the adhesion force between two graphite layers. The surface tension of ammonia is 61.05 mJ/m2 (34.5 wt.%, 18 °C) which is close to that of acetone (25 mJ/m2) and thus could be sufficient to participate in the exfoliation process [2].

S-2. Morphology of the expanded graphite

The morphology of the pristine expanded graphite was analyzed by SEM and the results are presented in Fig. S-2.

Figure S-2SEM micrographs of the expanded graphite after sonication (before microwaves heating): (a) and (b) in water; (c) and (d) in the 33 wt.% aqueous ammonia solution. The morphologies of both samples are the same before the microwaves irradiation experiment

S-3. Morphological stability of the graphene sheet

TEM observations (Figs. S-3(a)–S-3(c)) indicate that in the absence of an underlying carbon membrane on the TEM grid, the large graphene sheets rapidly rolled up to form an entangled structure due to their high inability to self-maintain a high aspect ratio 2-D structure [3, 4]. The FLG sheets also tended to exhibit a strong adhesion to any surface in order to maintain their planar morphology (Figs. S-3(d) and S-3(e)), although some folding can be observed by TEM in the middle of the sheets.

Figure S-3(a) and (b) Low-magnification TEM images of the scrolled graphene sheets when deposited on a carbon membrane-free TEM grid. The scrolling can be attributed to the fact that the high aspect ratio of the material makes it extremely unfavorable to maintain the planar structure. (c) Medium-resolution TEM image showing the typical border of a scrolled graphene sheet with cross-overlapping of several graphene layers. (d) Bright-field TEM image of a single graphene sheet deposited on a carbon membrane and partly folded onto itself, in an area where the membrane is not present below the sheet. The apparent buckling is marked by a black arrow on the figure. (e) TEM image of a graphene flake suspended on a carbon membrane showing the strong adhesion of the sheet to the substrate

S-4. Evidence of the presence of graphene monolayers by selected area electron diffraction (SAED) Confirmation of the presence of graphene monolayers can be obtained using the method proposed by Hernandez et al. [1]and by Meyer et al. [5]. Starting from the diffraction patterns characteristic of graphene/graphite showing sixfold symmetry, they compared the intensities of the {1100} spots relative to those of the {2110} spots. Computational studies have shown that, if the multilayer retains the Bernal (AB) stacking of the source material (graphite), the ratio of the intensity of the {1100} peaks relative to that of the {2110} peaks gives an unambiguous local identification of monolayer or multilayer configuration. An example of such a characterization of the graphene synthesized by exfoliation of expanded graphite in aqueous ammonia solution is presented

below. Low magnification TEM micrographs of the analyzed border are presented in Figs. S-4(a) and S-4(b). A

ratio > 1 is a signature of monolayer graphene (Fig. S-4(c)), whereas a ratio < 1 suggests the presence of few-layer graphene (Fig. S-4(e)). From a statistical TEM analysis, we have also observed that mono- and few-layer graphene sheets have relatively well-defined edges, whereas thicker flakes generally have edges that are more rounded. Similar observations have also been reported by Hernandez et al. [1] during their TEM investigation of the graphene sheets obtained from liquid-phase exfoliation of graphite.

Figure S-4(a) Representative TEM image of a typical graphene flake. We can observe a well-defined rolling up at the edge of the graphene sheet. (b) Closer TEM view of the area marked by a white circle in the previous image. The contrast inhomogeneity in the area suggests that it is not completely flat and presents a microscopic roughness that appears to be intrinsic to graphene membranes. (c) SAED pattern of the previous image which allows the presence of only one layer of carbon to be identified, as well as a line profile (d) corresponding to the diffraction intensity along the line marked on the pattern. (e) SAED pattern of a multilayer graphene sheet (TEM image not shown here). (f) Dark field image of the area corresponding to (b) obtained by selecting the (-1010) peak with the objective aperture. This imaging mode is much more sensitive to the spatial orientation of the lattice and allows a better visualisation of the disoriented areas. The periodic waves visible at the top of the image correspond to the regions where the scrolling of the graphene starts

S-5. Influence of the liquid nature on the graphene morphology and thickness

TEM micrographs obtained from the graphene recovered after microwave exfoliation of the expanded graphite in water are presented in Fig. S-5. The graphene-like material obtained is present as round-shaped flakes with micrometer size (Fig. S-5(a)). The flakes were thick enough to allow high resolution investigation of the material microstructure, which consists of a periodic superstructure across the material (Fig. S-5(b)). According to the observed results, one can conclude that exfoliation in water of expanded graphite mostly leads to the formation of graphene flakes, whereas FLG was observed on exfoliation in aqueous solution of ammonia. Synthesis in water also leads to the formation of a significant amount of oxygenated functional groups on the graphene sheet border or on the basal plane, as evidenced by XPS (see Fig. 4 in the main text). The presence of these oxygenated functional groups could result in considerable modification of the electronic properties of the as-synthesized carbon sheets [6, 7].

Figure S-5(a) Low-magnification TEM micrograph of the micrometer size graphene sheets synthesized by microwave exfoliation of expanded graphite in water. (b) High-resolution TEM showing the periodic arrangement of the carbon microstructure

S-6. ATR–FTIR spectroscopy

The different degrees of oxidation of the graphene sheets obtained from exfoliation in water and ammonia solution observed by XPS analysis were also confirmed by attenuated total reflectance–Fourier transform infrared (ATR–FTIR) spectroscopy. ATR–FTIR spectra of the graphene sheets film obtained from exfoliation in water (top) and 33 wt.% ammonia solution (bottom) are presented in Fig. S-6. Similar to the FT–IR spectra of other oxidized carbon materials such as carbon fiber or activated carbon, the broad band between 3000 and 3700 cm–1 corresponds to the presence of the oxygenated groups [8]. Based on the IR spectra of graphite oxide (GO), the three main peaks observed in this region can be related to the free hydroxyl group (3544 cm–1), hydrogen bonded hydroxyl groups (3400 cm–1), and water molecules (3152 cm–1) [9]. The bands in the region 3000–3700 cm–1 for the graphene sheets obtained in water are much more intense than those of graphene obtained in ammonia solution. The intensities of other bands assigned to the vibrations of O–H, C–OH, and C–O groups also vary significantly for the two samples. In the case of the more oxidized graphene sheets prepared by exfoliation in water, two strong bands at 1401 cm–1 (O–H) and 1352 cm–1 (C–OH) are observed, whereas for the sample obtained from ammonia solution, an intense band at 1109 cm–1 (C–O vibration) and only a weak band at 1396 cm–1 (O-H) are present.

Figure S-6ATR–FTIR spectra of the few-layer graphene materials synthesized in water (top) and in an ammonia solution (bottom) References

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