Open Access

Synthesis and Characterization of Highly Intercalated Graphite Bisulfate

  • Marcella Salvatore1, 2,
  • Gianfranco Carotenuto1,
  • Sergio De Nicola3Email author,
  • Carlo Camerlingo3,
  • Veronica Ambrogi1, 2 and
  • Cosimo Carfagna1, 2
Nanoscale Research Letters201712:167

https://doi.org/10.1186/s11671-017-1930-2

Received: 23 November 2016

Accepted: 18 February 2017

Published: 6 March 2017

Abstract

Different chemical formulations for the synthesis of highly intercalated graphite bisulfate have been tested. In particular, nitric acid, potassium nitrate, potassium dichromate, potassium permanganate, sodium periodate, sodium chlorate, and hydrogen peroxide have been used in this synthesis scheme as the auxiliary reagent (oxidizing agent). In order to evaluate the presence of delamination, and pre-expansion phenomena, and the achieved intercalation degree in the prepared samples, the obtained graphite intercalation compounds have been characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), infrared spectroscopy (FT-IR), micro-Raman spectroscopy (μ-RS), and thermal analysis (TGA). Delamination and pre-expansion phenomena were observed only for nitric acid, sodium chlorate, and hydrogen peroxide, while the presence of strong oxidizers (KMnO4, K2Cr2O7) led to stable graphite intercalation compounds. The largest content of intercalated bisulfate is achieved in the intercalated compounds obtained from NaIO4 and NaClO3.

Keywords

Graphite intercalation compounds Graphite bisulfate Nano graphite Micro-Raman spectroscopy Fourier transform IR spectroscopy

Background

Graphite intercalation compounds (GICs) are technologically useful functional materials made of graphite flakes uniformly embedding small molecules or metal ions between the graphene sheets [15]. Such materials have been intensively studied because of the “staging phenomenon” [6] and the manifold anomalous physico-chemical behaviors [312]. In addition, GICs are very important in graphite manufacturing [13] and potentially useful in other industrial fields like that of superconductors [14, 15], heterogeneous catalysts [16], anode materials [17], and pyrophoric reactants stabilization [17]. Currently, GICs are used in the preparation of expandable graphite, graphite nanoplatelets (GNP), and single-layer graphene [1821]. Graphite nitrate and graphite bisulfate have the ability to significantly expand by thermal heating. Owing to the intercalation/expansion processes, the π- π interactions, acting along the graphite c-crystallographic direction, are significantly weakened. Therefore, graphite fragmentations in separated nanocrystals take place by simply applying some sono-acustic energy. The resulting graphite nanocrystal thickness is depending on the GICs intercalation degree, usually classified in terms of “staging” index m, i.e., the number of graphite layers between two intercalant layers [2]; and consequently, the highest “staging index” is required in the graphene preparation. Depending on nature of the intercalating agent, the type of oxidant, and the experimental conditions, it is possible to obtain GICs as “stage I”, “stage II”, and “stage III” [14]. In particular, in a stage I compound, a single layer of graphene is alternated regularly with intercalated species. In a second stage (third stage, etc.), compound with two (three, etc.) graphene layers are separated by layers of intercalation [22]. According to the literature [23, 24], expandable graphite can be prepared by using various types of GICs hosting Brønsted acids (e.g., sulfuric acid, nitric acid, and acetic acid), metal chlorides (FeCl3, CuCl3, and ZnCl2) [25], gold [26], metals (Na), and alkali metals (Li, Cs, and K) in tetrahydrofuran (THF) [27, 28]. Firstly, C. Schafhäutl [29] and B. Brodie in 1851 [30] described the graphite bisulfate synthesis based on the following reaction scheme [23]:
$$ \begin{aligned} 24n\text{C} &+ {\text{O}_{x}}^{z} + m\mathrm{H}_{2}\text{SO}_{4}\rightarrow {{\mathrm{C}_{24}}^{+}}\cdot {\text{HSO}_{4}}^{-}\\ &\times(m-1) \mathrm{H}_{2}\text{SO}_{4} + {\text{HO}_{x}}^{\left(z-1\right)} \end{aligned} $$
(1)

where O x is the oxidizing agent and C is the carbon atoms in the graphite. Therefore, graphite bisulfate compounds consist of graphite layers intercalated by HSO\(_{4}^{-}\) and H2SO4 molecules [23]. The stage and kinetics of bisulfate formation depend on the sulfuric acid concentration and on the type of oxidizing agent involved in the reactive system [23, 31]. At that time, very limited structural information were given in the literature concerning these systems, since X-ray diffraction was one of the few available characterization approaches. Here, graphite bisulfate has been prepared by classical liquid-phase synthesis techniques and some new reaction scheme based on never investigated oxidizing agents. The achieved materials have been fully characterized regarding their morphology and structure by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), infrared spectroscopy (FT-IR), micro-Raman spectroscopy (μ-RS), and thermogravimetric analysis (TGA).

Methods

The different graphite bisulfates were synthesized by treating graphite flakes with an oxidizing agent/sulfuric acid mixture (see Table 1). In particular, a glass flask placed in a thermostatic bath was used as reactor. Slow air-bubbling was applied to homogenize the system during the reaction. The reaction time was 1 h, and a 9:1 by volume H2SO4/oxidizing agent ratio was selected for all compositions. Such a small amount of oxidizing agent is enough to induce intercalation. The reactions were performed in isothermal condition at the temperatures indicated in Table 1. Constant quantities of graphite flakes (2 g, Aldrich, > 100 mesh), sulfuric acid (40 ml), and oxidizing agent molar amounts were used. Cold deionized water was added to reactive mixture to end the reaction. The selected temperatures were different since oxidizers have a different reactivity.
Table 1

Oxidizing agent used in the reactive mixtures with H2SO4 and experimental conditions

Agent

HNO3

KNO 3

H2O2

KMnO4

Temperature reaction (°C)

40

30

40

30

Agent

K2Cr2O7

NaIO4

NaClO3

 

Temperature reaction (°C)

30

40

30

 

FT-IR and micro-Raman (μ-RS) spectroscopies were used to analyze the chemical structure of pure graphite and “as prepared” graphite bisulfates. FT-IR were recorded with a PerkinElmer Frontier FT-IR spectrometer, in the range 4000–800 cm −1 with 4 cm −1 resolution and 6 scans. The KBr pressed disc technique (1 mg of sample and 160 mg of KBr) was used. The KBr was first heated in a furnace overnight at 120 °C to minimize the amount of the adsorbed water. A Jobin-Yvon system from Horiba ISA, with a Triax 180 monochromator, equipped with a liquid nitrogen cooled charge-coupled detector was used for the μ-RS measurements. The grating of 1800 grooves/mm allows a final spectral resolution of 4 cm −1. The spectra were recorded in air at room temperature using a 17 mW He-Ne laser source (wavelength 632.8 nm). The spectrum accumulation time was 120 s. The laser light was focused to a 2 μm spot size on the sample through an Olympus microscope with ×100 optical objective. The spectra obtained were analyzed in terms of convoluted Lorentzian functions by using a best-fitting routine of GRAMS/AI (2001, Thermo Electron) program, which is based on the Levenberg-Marquardt nonlinear least-square methods. Wide-angle X-ray powder diffraction (XRD) measurements were performed using a Philips XPW diffractometer with Cu K α radiation (1.542 Å) filtered by nickel. The scanning rate was 0.02°/s, and the scanning angle was from 5 to 45°. A SEM (Philips model XL20) was used to investigate the morphology. EDS analysis (model Inca Oxford 250) was carried out to confirm the presence of the intercalating agent between graphite layers. The thermal expansion threshold of the different intercalation compounds was measured by thermogravimetric analysis (TGA), using a TA Q5000 instrument equipped with an infrared furnace. TGA measurements were performed using about 1.0/1.8 mg of the sample inserted in an alumina crucible placed in a platinum pan. We adopted such method in order to prevent the leakage of the sample during the expansion phenomenon. The experiments were performed under fluxing nitrogen (flow rate of 25 mL/min).

Results and Discussion

The morphology of graphite flakes after the oxidation/intercalation treatment has been investigated by comparing SEM micrographs of different GICs to that of starting graphite flakes. The obtained images are reported in Fig. 1. Treatments with H2SO4/oxidant lead to intercalated graphite. In addition to the erosion phenomenon also a delamination and pre-expansion were clearly visible for some samples. The image of a natural graphite single flake, reported in Fig. 1a, shows that the flake layers are really close to each other, and the surface is flat and uniform. Figures 1 b, f show the GICs obtained using HNO3 and K2Cr2O7 as oxidizer agent. From these images, in addition to intercalation, it is possible to observe a “delamination” phenomenon that is probably due to the strong oxidation effect. When KNO3 is used as an oxidizer (Fig. 1 c), only a light intercalation phenomenon occur. In this case, small white particles are observed on the intercalated flakes, probably resulting from the KNO3 crystals not dissolved during the chemical treatment. Furthermore, Fig. 1 d shows an image of the GIC resulting from the reaction with H2O2 as oxidizing agent. In this case, a graphite pre-expansion phenomenon occurred during the intercalation process. Such a phenomenon is probably related to the H2O2 decomposition to H2O and O2 which takes place at room temperature. Figure 1 e, g show the images of GICs obtained from KMnO4 and NaIO4, respectively. In this case, it is possible to observe the erosion of the flake boundary. This morphology is tightly connected with the intercalation process although a layer separation is not evident as in the cases of the other samples. The GIC obtained using NaClO3 as an oxidizing agent, Fig. 1 h, shows a strong intercalation phenomenon. From this image, it is possible to recognize multiple layers forming the flake. EDS microanalysis was carried out on small sample areas to determine nature and percentage of the elements present in the flake. As an example, we report the data of EDS spectrum from graphite and GIC obtained by HNO3. In Fig. 2 a, b are indicated the positions where EDS analysis has been performed on the graphite and on the GIC (HNO3) surface, respectively. The elemental composition for each EDS spectrum is reported in Table 2. In natural graphite, it can be clearly seen only the presence of the element carbon (besides some impurities). The EDS spectra of the other GIC samples detected a significant quantity of sulfur and oxygen due to the oxidation/intercalation process.
Fig. 1

Scanning electron micrographs (SEM) of a starting graphite flakes, GIC from b HNO3, c KNO 3, d H2O2, e KMnO4, f K2Cr2O7, g NaIO4, and h NaClO3

Fig. 2

Positions where EDS analysis has been performed on a starting graphite flakes and b GIC from HNO3. The elemental composition is reported in Table 2

Table 2

EDS analysis: elemental composition. Results in weight %

 

Spectrum

C

O

S

Si

Other

Total

 

Spectrum 1

100.00

100.00

Graphite

Spectrum 2

99.50

0.50

100.00

(Fig. 2 a)

Spectrum 3

99.56

0.44

100.00

GIC (HNO3)

Spectrum 1

75.48

21.83

2.32

0.07

0.30

100.00

(Fig. 2 b)

Spectrum 2

70.37

21.46

7.93

0.07

0.24

100.00

The structural characteristics of the obtained GICs can be evaluated by comparing XRD diffractograms of pure graphite and GICs. In Fig. 3 a, we report the diffractogram of pure graphite, and in Fig. 3 b, a portion of the same diffractogram compared with those of the GICs. As visible, the signal of GICs is significantly different from that of natural graphite. The (002) signal, which is the only one visible in the XRD pattern of natural graphite, represents reflections in the perpendicular direction (c-axis) of the graphite hexagonal planes. The (002) peak in the GICs X-ray diffractograms is clearly broadened compared to that of the pure graphite. Furthermore, the (002) peak results shifted to lower angles for all studied GICs compared to the pure graphite case. This result is probably due to the presence of defects in the GICs crystal lattice. This effect depends on the obtained intercalation and on the kind of oxidizing agents used in the reaction. In particular, the X-ray diffractograms of GICs obtained using NaClO3, KNO3, and K2Cr2O7 as oxidizing agents show the maximum 2θ shift of the (002) peak. The XRD spectra of GICs by HNO3 and H2O2 exhibit maximum broadening of the (002) peak. As it can be seen from the corresponding SEM images (Fig. 1 b, d), the samples appear to be better expanded compared to the other GICs. We ascribe the better expansion to the instability of the HNO3 and H2O2 oxidants which results in gas formation during the intercalation reactions. The oxidation of GIC samples was investigated by FT-IR spectroscopy. Fig. 4 shows FT-IR spectra of prepared samples; as visible, the GIC spectra are remarkably different from that of natural graphite. They exhibit an oxidation signal due to the presence of hydroxyl groups (−OH) at 3500 cm −1. The natural graphite also presents a light oxidation. The oxidation degree was depending on the type of oxidizer used and has a maximum for NaClO3 and a minimum for the KNO3 oxidizer. All spectra also featured a clear signal at 1650 cm −1 ascribed to the C=C double bond stretching vibration and C–O bond stretching vibration at 1200 cm −1. The μ-RS spectra obtained from the graphite intercalated compounds are reported in Fig. 5 and compared with spectrum of pure graphite. In Fig. 5 a, the spectrum region in the wavenumber range of 1200–1850 cm −1 is considered, where a prominent peak (designated as G mode) is generated by graphite at about 1582 cm −1. This peak is clearly visible in the spectrum of graphite, reported in the low part of the figure, and in all other spectra, even if evident differences occur in the observed Raman response. The spectra in Fig. 5 are sorted in increasing degree of differentiation with respect to that of graphite. The spectra of the samples obtained from KNO3, KMnO4, and H2O2 look rather similar to that of graphite even if the higher intensity of Raman mode visible at about 1332 cm −1 (D mode) manifests the occurrence of a relatively large amount of defects in the lattice [32]. For the samples obtained from KNO3, H2O2, and KMnO4, the Raman G mode can be successfully fitted by a single Lorentzian function centered at about 1580 cm −1. This is not the case of the other spectra (related to K2Cr2O7, HNO3, NaIO4, and NaClO3), where the fit require two distinct components, represented by Lorentzian functions centered at about 1580 cm −1 and 1600 cm −1, respectively. Two dotted lines indicate these positions in Fig. 5 a. The first component has a center value close to the G mode position of pristine graphite and is assigned to block of not intercalated graphite layers, while the 1600 cm −1 mode is assigned to graphene layers next to an intercalant layer. The ratio of the intensity of this second component with respect to the intensity of 1580 cm −1 mode, evaluated by the fit procedure, gradually increases from the value of 0.32 for sample K2Cr2O7 to the value of 1.33 of sample NaClO3, indicating a significant intercalation degree of the samples, even if in different stage configurations.
Fig. 3

XRD of graphite and GICs. XRD (002) peak of pure graphite (a) and GICs (b).The curves in b are shifted arbitrarily along the y-axis

Fig. 4

FT-IR spectroscopy. FT-IR spectra of pure graphite and GICs. The spectra are shifted arbitrarily along the y-axis

Fig. 5

μ-RS of GICs in the spectral range of 1200–1850 cm −1 (a) and 2500–2800 cm −1 (b). The spectra are shifted arbitrarily along the y-axis. The dotted linesindicate the spectral position of the main components observed for G (a) and 2D mode (b), respectively

For these compounds, the intensity of the D mode (at 1332 cm −1) is also clearly observable in the Raman spectra (see Fig. 5 a), and it is relatively larger than in the case of GICs before considered (KNO3, KMnO4, and H2O2), indicating the occurrence of structural defects. This is also evidenced by the slight increase of both the position and the width of the G peak, that is a signature of increasing lattice defects [33]. It is worth to note that in the case of doping-like defects the increase of position of G mode with impurity degree is instead accompanied by an increasing stiffness [33].

This general behavior is confirmed by the Raman spectra in the wavenumber range of 2500–2800 cm −1, reported in Fig. 5 b. In this range, graphite is characterized by a broad peak (designed as 2D mode) centered at about 2685 cm −1. Differently from the G mode, the position of this peak depends on the laser excitation wavelength λ (in our case λ = 632.8 nm) [3335]. The differences with respect to graphite become gradually more significant when spectra of the sample obtained from KNO3 to that one derived from NaClO3 are considered, in the order indicated in Fig. 5 b. Beyond the pristine graphite 2D mode (at 2685 cm −1), the fit of the experimental data requires an additional component at about 2648 cm −1 with increasing intensity. This mode becomes the prevalent one in the samples obtained by using NaIO4 and NaClO3 as oxidizers. Two dotted lines represent the position of these two modes in Fig. 5 b. The observed shift of the 2D mode position to lower wavenumber values with respect to pristine graphite is a signature of presence of graphene sheets, indicating the formation of low stage GICs [19]. A large intercalation configuration in the NaIO4 and NaClO3 systems is inferred because the Raman signal results generated by almost monolayers of graphene (low stage number). In the remaining samples, intercalation process is also obtained but with relative thicker layer blocks of graphite (high stage number) interposed to intercalated layers. The thermogravimetric behavior of different GICs was evaluated by thermogravimetric analysis (TGA) (Fig. 6). TGA runs from 25 to 800 °C at heating rate of 10 °C min −1. During the thermal treatment, an expansion phenomenon of GICs occur [13]. Intercalating agent and graphite react according to the following scheme [36, 37].
$$ \mathrm{C} + 2\mathrm{H}_{2}\text{SO}_{4}\rightarrow \text{CO}_{2} + 2\text{SO}_{2} + 2\mathrm{H}_{2}\mathrm{O} $$
(2)
Fig. 6

TGA results for graphite and graphite intercalated compounds

According to this scheme, there is a release of gases during the GIC’s expansion: carbon dioxide, sulfur dioxide, and water vapor. The weight loss, due to the gas released, can be clearly seen for all the considered samples. The weight loss ranged from 3%, for KNO3, up to 52% for NaClO3 (Table 3). The order of data in Table 3, in increasing degree of weight loss, is the same as that used to tabulate μ-RS results in Fig. 5, and it clearly indicates that highest intercalation degree corresponds to higher weight loss.
Table 3

Weight loss percentage of pure graphite flakes and GICs

Sample

Weight loss (%)

Natural graphite

0

KNO3

3

KMnO4

7

H2O2

10

HNO3

17

K2Cr2O7

18

NaIO4

30

NaClO3

52

Conclusions

We have presented and described new chemical routes for synthetizing highly intercalated graphite bisulfate. The reaction schemes use different auxiliary reagents (oxidizing agent): nitric acid, potassium nitrate, potassium dichromate, potassium permanganate, sodium periodate, sodium chlorate, and hydrogen peroxide. Micro-Raman spectroscopy analysis of products has shown that samples treated by NaIO4 and NaClO3 lead to final products with the highest intercalation degree, which is consistent with the weight loss from the TGA data. Furthermore, according to FT-IR data, OH is the only oxygen-containing group generated during the intercalation process. EDS elemental analysis allowed to assess the presence of sulfur and oxygen characterizing the oxidation/intercalation process. The SEM micrographs, FT-IR analysis, micro-Raman spectroscopy, and calculated weight loss (from TGA analysis) confirm that the GIC, obtained using KNO3 as oxidizer, is the most similar to natural graphite, whereas GIC based on NaClO3 seems to have a higher degree of intercalation.

Abbreviations

EDS: 

Energy-dispersive X-ray spectroscopy

FT-IR: 

Fourier transform infrared (spectroscopy)

GIC: 

Graphite intercalated compound

SEM: 

Scansion electron microscope

TGA: 

Thermogravimetric analysis

XRD: 

X-ray powder diffraction

μ-RS: 

micro-Raman spectroscopy

Declarations

Acknowledgements

The authors gratefully acknowledge M. C. Del Barone of laboratory “LaMEST” of CNR-IPCB for SEM analysis and M. R. Marcedula and M. De Angioletti of the Laboratory of “Thermal analysis” of CNR-IPCB for FT-IR and thermogravimetric analysis.

Authors’ contributions

MS with GC and VA conceived the experimental design and contributed to fabrication of samples. MS wrote part of the paper. GC, SDN and C. Cam participated to the characterization of samples and helped to draft the manuscript. C. Car. participated in the design of the experiment and coordination. All authors have read and approved the final manuscript.

Authors’ information

MS is a materials engineer. She received her degree from the University of Naples Federico II, Italy, in 2013. During her thesis, she worked at CNR-ICTP Institute and her research was focused on bio-polymer-based nanocomposites. At present, she is a Ph.D. student in “Industrial Product and Process Engineering” at the University of Naples Federico II (Dept. DICMaPI). She works at the CNR-IPCB Institute, and her scientific interest is focused on carbon chemistry, nanostructures, and 2D materials. G.C. (Dr) is a senior researcher of the CNR at the Institute CNR-IPCB. His present research interests are in the field of advanced functional materials based on polymer-embedded inorganic nanostructures. In particular, his activity concerns the development of new chemical routes for the controlled synthesis of metal and semiconductor clusters in polymeric matrices, the fabrication of devices based on properties of nanoscopic objects (e.g., luminescence of quantum dots and tunable surface plasmon absorption of nano-sized noble metal alloys), and the investigation of mechanisms involved in atomic and molecular cluster formation in polymeric media (nucleation, growth, aggregation, etc.) by optical and luminescence spectroscopy. He has authored 180 research articles published in international journals, 10 patents, and many conference papers. He is the editor of two Wiley books devoted to metal-polymer nanocomposites, and he is a member of the editorial board of different scientific journals. S. De N. received his BS degree in physics from the University of Naples Federico II, Italy, in 1982. From 1983 to 1987, he was a system analyst at Elettronica (Rome) and Alenia (Naples). He has been staff researcher at the Institute of Cybernetics “E. Caianiello” of the CNR and at National Institute of Optics (CNR-INO). Currently, he is senior researcher at the CNR-SPIN Institute. He has been a scientific coordinator of the research project “Imaging techniques for studying and analyzing microstructured materials” of the Dept. of Phys. Sci. and Matter Technologies of CNR. He has been a coordinator of the research unit in the framework of the CNR-FIRB program, Photonic microdevices in lithium niobate. He has contributed to about 300 technical papers in peer-reviewed international journals, book chapters, conference papers, and several patents. His research interests include the development of quantum methodologies to the description of coherent phenomena in many body systems, quantum tomography, theoretical modeling for studying dynamical effects in mesoscopic systems, transport properties in nanostructured polymeric materials and biological media, and optical and electron beam propagation in nonlinear media and plasma. C. Cam. is staff member of CNR since 1984 and at present, is senior researcher at the CNR-SPIN Institute. His research focus is on exploring the electronic and structural properties of superconductor thin films, and electronically strongly correlated materials as iron pnictides and graphene. His research activity includes micro-Raman spectroscopy analysis and development of methods and processes for micro- and nano-structural engineering. V. A. graduated in chemistry in 1997 and got her Ph.D. in materials engineering in 2000 at the University of Naples Federico II, Italy. She is currently an associate professor at the Dept. of Chem., Mat. and Production Engineering of the University of Naples. She is author/co-author of 60 papers in international journals, three book chapters, and one national patent. Her research interests comprise the synthesis, modification, and characterization of thermoplastic and thermosetting polymers, CNT/polymer composites, and stimuli-responsive/active polymer-based systems. C. Car. is a full professor of Chemistry at the University of Naples Federico II. Since 2001, he is director of the Institute of Polymers, Composites and Biomaterials of CNR (CNR-IPCB). He is coordinator of the sponsored project of CNR, Special Materials for Advanced Technologies, and responsible for the research project BRITE-EURAM. He is an author of 170 international papers and 10 patents. Currently, his research interests are focused on polymer chemistry and technology, polymer functionalization, polymer synthesis, liquid crystalline polymers, composites and nanocomposites, functional and smart textiles, recycling and processing of plastics, advanced materials for packaging, use of natural products as additive of polymers, and nutraceuticals for polymers.

Competing interests

The authors declare that they have no competing interests.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
CNR-IPCB, Institute for Polymers, Composites and Biomaterials
(2)
Department of Chemical, Materials and Production Engineering, University of Naples Federico II
(3)
CNR-SPIN, Institute for Superconductors, Innovative Materials and Devices, S.S. Napoli, Complesso Universitario di M.S. Angelo

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