Shungite Carbon as Unexpected Natural Source of Few-Layer

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Shungite Carbon as Unexpected Natural Source of Few-Layer Graphene Platelets in a Low Oxidation State Emanuela Tamburri,*,† Rocco Carcione,† Sara Politi,† Mariglen Angjellari,† Laura Lazzarini,‡ Lia Emanuela Vanzetti,§ Salvatore Macis,∥ Giancarlo Pepponi,§ and Maria Letizia Terranova† Dipartimento di Scienze e Tecnologie Chimiche and ∥Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via Della Ricerca Scientifica, Rome, Italy ‡ IMEM-CNR, Parco Area delle Scienze 37/A, Località Fontanini, Parma, Italy § MNF, CMM, Fondazione Bruno Kessler, via Sommarive 18, Trento, Italy Downloaded via UNIV OF SUSSEX on July 4, 2018 at 00:55:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

ABSTRACT: The paper reports on the feasibility of obtaining graphene nanomaterials with remarkable structural and chemical features from shungite rocks. The investigation of the composition and structural modifications induced in the pristine, natural C-containing mineraloid by a specifically designed physicochemical purification treatment is performed by a combined use of several techniques (scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopies). The adopted material processing enables efficient extraction of the C phase in the form of thin polycrystalline platelets of a few hundred nanometers sizes, and formed by 6−10 graphene sheets. About 80% of such nanostructures are characterized by a regular sp2 C honeycomb lattice and an ordered stacking of graphene layers with a d-spacing of ∼0.34 nm. The low oxygen content (∼5%), mainly found in the form of hydroxyl functional groups, provides the graphene platelets (GP) with a chemistry strictly close to that of conventional rGO materials. Such a feature is supported by the high conductivity value of 1.041 × 103 S cm−1 found for pelletized GP, which can be considered a valuable active material for a wide spectrum of advanced applications.

1. INTRODUCTION Whether it is because of its amazingly complex physics or its foreseen applicability to practically all high-technology fields, no other material is nowadays so widely investigated as graphene. In the rapidly evolving fields of information technology, energy generation/storage/distribution, lightweight low-emission transportation, advanced healthcare, nanomedicine, and more, graphene and its derivatives are used as building blocks for sophisticated complex systems for electronics and photonics, as paramount additives in multiphase materials or as biomaterial in nanomedicine and biorelated devices.1−14 Graphene materials can be prepared using physical or chemical routes.15−19 The first ones start from an already existing graphite structure and encompass cleaving methodologies as well as liquid phase and thermal exfoliation. Another physical way to produce graphene is the sublimation of Si atoms from SiC. The chemical approaches are based on chemical vapor deposition (CVD) techniques and employ C-containing gaseous mixtures to produce graphene layers mainly on Cu or Ni substrates. © XXXX American Chemical Society

Since graphene materials, and in particular few-layer graphene platelets, are destined to be put into high volume production, nowadays one of the challenges is to develop technologies for scaling up the preparation process while maintaining the properties required for the various applications. In fact, following the methodologies up to now adopted, few-layer graphene structures are generally produced in a not conductive oxidized state (GO) and need a chemical reduction to generate a reduced form (rGO) to be used in up-to-date technologies.20−27 The GO reduction is a rather complex process, driven by a multistep mechanism that varies on the basis of the experimental conditions and the geometrical location (circumference, edges, etc.) from where the oxidized groups are removed.28 With regard to the edges, it is to be noted that in nanographene the local geometry (zigzag or armchair edges) and the chemical termination of C atoms generate different electronic states and that Received: April 27, 2018

A

DOI: 10.1021/acs.inorgchem.8b01164 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

up to obtain a new powder, that from now on will be named as sample T. 2.2. Techniques for Materials Characterization. The morphology of the samples was investigated by means of electron microscopies. SEM analyses were performed by a Cross Beam Workstation ZEISS Auriga equipped with a detector for energy dispersive X-ray spectroscopy (EDX) for elemental analysis. TEM studies were carried out within a JEOL 2200FS field emission microscope operated at 200 kV, with point-to-point resolution of 0.185 nm, equipped with in-column Ω filter, X-ray microanalysis, and two high-angle annular dark field (HAADF) detectors for chemical imaging. The working mechanism of the HAADF-STEM technique is a Z-contrast imaging that, under proper conditions, provides images with intensities that are highly sensitive to variations in the atomic number of atoms in the sample under investigation. The samples for TEM observation were obtained by dropping on holey carbon grids sonicated suspension of the powders dispersed in isopropyl alcohol. The molecular structure of the pristine and processed SH was analyzed by Raman microspectroscopy using a XploRA ONE Raman Microscope (Horiba Jobin Yvon). A 532 nm excitation laser light and 1200 gr/mm diffraction grating coupled with an air-cooled scientific CCD were used for spectra acquisition. All the spectra were fitted using curves with Lorentzian peak shape. X-ray powder diffraction patterns were collected at room temperature by an automated Rigaku RINT2500 rotating anode laboratory diffractometer (50 kV, 200 mA) equipped with the silicon strip Rigaku D/teX Ultra detector. An asymmetric Johansson Ge(111) crystal was used to select the monochromatic Cu Ka1 radiation (λ = 1.54056 Å). The measurement was executed in transmission mode, by introducing the sample in a glass capillary and mounted on the axis of the diffractometer. In order to reduce the effect of possible preferred orientation, the capillary was rotated during measurement to improve the randomization of the orientations of the individual crystallites. XPS analyses were performed using a Kratos Axis UltraDLD instrument (Kratos Analytical, Manchester, U.K.) equipped with a hemispherical analyzer and a monochromatic Al Kα (1486.6 eV) X-ray source. The emission angle between the axis of the analyzer and the sample surface was 90°. The high resolution spectra were obtained with the electron analyzer set at a pass energy of 20 eV, and a step size of 0.05 eV. For each sample, the individual O 1s, and C 1s core lines were collected. The quantification, reported as relative elemental percentage, was performed using the integrated area of the fitted core lines, after Shirley background subtraction, and correcting for the instrument atomic sensitivity factors. Electrical conductivity was evaluated by a transmission current− voltage analysis performed by a Keithley SMU 2400. The metal contacts were made by means of two small metal cylinders between which the samples under investigation were interposed in the form of a pellet.

the locally created standing waves and edge states influence the chemical reactivity of graphene.29,30 In addition, the reduction processes often present serious drawbacks, since the final nanosized material can exhibit a variable composition (C/H and C/O ratios) with incomplete recovery of the sp2 domains, and also with a strong tendency to aggregation.23,31,32 In the search of an alternative source of not-oxidized fewlayer graphene, we found that a new challenge could be offered by a natural heterogeneous carbon-rich mineraloid of the Precambrian age, namely, shungite (SH), that is widespread mainly over Karelia, Russia.33−35 The scientific importance of SH lies in the dissimilarity and unlikeness of its carbon content with respect to all known carbon material sources. In fact, SH exhibits a very complex microstructure given by a peculiar carbon phase coexisting with other mineral species. A series of advanced characterizations have indeed emphasized how the SH carbon occurs in the form of both a multilevel fractal organization of sp2 C globules, formed by stacks of curved graphene entities, fullerenes, and nanograins characterized by long and relatively uniformly spaced fringes approaching those of graphite.35−41 Moreover, it was recently highlighted how such crystallites are formed by a successive aggregation of reduced graphene oxide sheets.42−44 On the basis of that reported above, we thought it worthwhile to test the feasibility to extract the graphene component from the raw SH material using wet-chemistry processes able to maintain the reduced state and the structure of the graphene phase present in the mineral. We report here on the chemical/ physical characteristics of the pristine shungite, and the characterizations by scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron, and Raman spectroscopies of the graphene materials obtained following purification procedures settled in the course of our research. The developed processing protocol demonstrated the ability to yield graphene platelets formed by a little number of stacked sheets and characterized by a very low oxidation degree, a feature that makes further reduction treatments unnecessary. The achievement of a system similar to the conventional rGO materials was confirmed by the peculiar electrical properties exhibited by the platelets pressed in pellet form. This outcome is of considerable importance in the perspective of using such structures either as individual entities or assembled in more complex systems exploiting the full potential of graphene-based technologies.

2. EXPERIMENTAL SECTION 2.1. Extraction of Shungite Carbon. The SH used for the present experiments was a powder of type-III shungite rock, gently provided by the Geology Institute of the Karelian Research Centre of the Russian Academy of Sciences. It comes from deposits of diapirs which represent organosiliceous rocks with a variable amount of the main C (20−55 wt %) and SiO2 (35−75 wt %) components. A systematic study allowed us to define a protocol for SH processing able to eliminate foreign species and amorphous carbon, limiting at the same time the oxidation degree of the final material. This goal was achieved by adopting the below described procedure, that was settled taking into consideration previously reported strategies to eliminate silicates,45 to remove metals, oxides, and amorphous carbon.46 The as-received SH powder, that henceforth will be named as sample S, was dissolved and kept under power ultrasound for 12 h in an aqueous solution of 20 M HF and 15 M HNO3 in a 1:1 volume ratio. After that, the powder was recovered and dispersed in a 12 M HCl aqueous solution for an overnight treatment under magnetic stirring in a thermostatically controlled water bath. At the end of each treatment, the powder was repeatedly washed and centrifuged until a pH = 6 was reached. The final product was collected and dried

3. RESULTS AND DISCUSSION 3.1. Characterization of Pristine Shungite−Sample S. The pristine SH powder was initially analyzed by electron microscopy to evaluate morphology and elemental composition. Figure 1 shows a SEM image of sample S evidencing the presence of microsized aggregates of clusters that are combined with multilayer structures. The high inhomogeneity exhibited by the sample can be reasonably explained with the presence of many different phases (quartz, metal oxides, silicates, carbon nanostructures) in the pristine rock. The elemental analysis performed by EDX on several batches of sample S, revealed a raw powder mainly composed by C (∼27%), Si (∼18%), and O (∼46%), whereas other elements, such as S, Al, Na, Fe, Mg, and K, are detected only in traces (Table 1) and are not homogeneously distributed inside the material. A detailed view of shungite structure was obtained by means of XRD measurements. Figure 2 reports a typical B


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Inorganic Chemistry

Figure 2. XRD pattern of pristine shungite powder (sample S) with diffraction signals attribution. Figure 1. SEM image of pristine shungite powder - sample S.

diffraction pattern acquired on the sample along with signals attribution. A preliminary analysis of the spectrum indicates that the principal bulk crystalline material is quartz, which gives rise to the most intense peaks at about 26 and 21 deg.47,48 Unfortunately, the signals related to quartz cover those of carbon materials, so that graphite or fullerenes phases main signals are barely observable. The peaks visible at 37 and 46 deg can be attributed to silico-alluminates and iron silicide, respectively.49 Such heterogeneous composition makes clear how the setting up of an efficient purification procedure was a fundamental task in order to obtain a pure graphene-like carbon phase from the natural material. 3.2. Characterization of Shungite Carbon−Sample T. Sample T was subjected to a variety of characterization techniques to determine the morphological and structural features of the phase extracted from the SH rock. To investigate the morphology of the purified SH, a local analysis at the nanoscale was performed by means of transmission electron microscopy techniques. In Figure 3 a typical HAADF image of sample T is reported. Thanks to the different levels of intensity, the image is able to point out the layered nature of the sample, showing how it is mainly composed by stacks of sheets forming platelets with planar sizes of a few hundred nanometers. An elemental microanalysis allowed assessing the effects of the physicochemical treatments on such nanostructures and determining the extraction yield of the carbon phase. The data, obtained by several EDX measurements, were compared with those achieved for sample S (Table 1). By observing the elemental composition of the sample before and after the purification, it is evident that the treatment was very efficient in completely removing all silicates and metal oxide species from the raw powder. In fact, the purified sample T is essentially composed by carbon (∼96%) with a very low percentage of oxygen (∼4%) corresponding to a C extraction yield of 85%. Residual traces of chlorine from HCl are also found. The high increase of the C/O atomic ratio from ∼0.6

Figure 3. Scanning TEM HAADF image of purified shungite powder sample T.

(before purification) to more than 24 (after purification) suggests obtaining of a carbon phase with a very low oxidation grade. Raman spectroscopy and XRD analyses were therefore employed to identify both the molecular structure and crystallinity of the platelets extracted from the rock. In Figure 4 the Raman spectra of samples S and T are shown. The disappearance of signals related to silicates and metal oxides in the spectrum of sample T confirms the effective removal of the noncarbonaceous species by means of the purification procedure. An in-depth analysis was carried out on the deconvolved spectra recorded in two distinct spectral regions corresponding to 1100−1800 cm−1 (Figure 5a) and 2550−3100 cm−1 (Figure 5b) range. The spectral features that appear in the 1100−1800 cm−1 region can be attributed to those of the first order Raman spectrum of graphitic carbon structures (Figure 5a). Both the spectra of samples S and T are characterized by the presence of the G band related to the in-plane stretching mode E22g of

Table 1. Elemental Composition of Pristine (S) and Purified (T) SH Rock sample

C (%)

O (%)

Si (%)

S (%)

Al (%)

Na (%)

Fe (%)

Mg (%)

K (%)

S T

26.8 95.6

46.3 3.9

17.6

0.2

2.5

1.2

4.7

0.3

0.5

Cl (%) 0.5

C


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for samples S and T we used the equation developed in ref 60 that holds as follows: 560 ij ID yz × jj zz E l4 k IG {

−1

La (nm) =

(1)

where El is the excitation energy and ID/IG is the ratio between the integrated intensities of D and G bands. La values of (8 ± 5%) nm and (15 ± 10%) nm were obtained for S and T, respectively. The larger crystallite mean sizes found for the purified material T is an interesting result considering the absence of any annealing treatment of the sample. Further information can be gained considering the region between 2550 and 3100 cm−1, where appear the second order graphite Raman signals (Figure 5b). In particular, the most distinctive features are the two-dimensional (2-D) band at ∼2700 cm−1, historically known as the G′ band, and the D + G band at ∼2900 cm−1. The G′ band is the second order of zone boundary phonons, and it is found to be very sensitive to the stacking of graphene sheets along the c axis.62 In particular, this band exhibits an interesting dependence of shape, intensity, and frequency on the modification of the electronic bands with the number of stacked graphene layers.63−65 For a single layer graphene the G′ signal is given by a single, sharp peak at ∼2680 cm−1 that splits into different subpeaks with increasing the number of graphene sheets. Specifically, for bilayer graphene the G′ band is constituted by four components, whereas in the case of a few-layer graphene only two components, i.e., G′(3D)A, G′(3D)B, are observed. Consequently, the Raman spectrum of more than 5,6 graphene layers becomes hardly distinguishable from that of bulk three-dimensional (3-D) crystalline graphite. On the other hand it should be recalled that also graphite without a periodic AB stacking, the so-called turbostratic graphite, shows a single G′ peak, namely, the G′(2D). However, such a peak is found upshifted of 20 cm−1 with respect to that of single-layer graphene and has an almost doubled fwhm.63,66 In a first approximation, this kind of disordered graphite, in view of the weak interactions between adjacent graphene planes and of the poor interlayer alignment, is considered a 2-D graphitic material.62 All the above considerations make thus clear that

Figure 4. Raman spectra of pristine (S) and purified (T) SH samples.

graphite, and of the D and D′ bands due to graphite phonon modes active in a reduced translational symmetry. These bands, arising from impurities, edges and finite size effects, are commonly known as “disordered graphite bands”.50−54 Table 2 reports the values of position, full width at half-maximum (fwhm), and integrated intensity of all the bands, obtained by averaging over several spectra acquired on each sample. The presence of D, G, and D′ Raman signals is a clear manifestation of how the rock is naturally constituted by graphite grains characterized by nanoscale dimensions, as evidenced by the upward shift of the G peak for both S (1598 cm−1) and T (1586 cm−1) samples.55 In particular, the greater blue-shift of the G band for sample S, associated with the lower integrated intensity and to the larger fwhm, can be rationalized considering a higher degree of disorder for the C phase present in the rock, and a consequent larger dispersion in the average sizes of the nanographitic grains. A widely used approach to estimate the mean crystallite sizes (La) of polycrystalline nanostructured graphitic materials from Raman spectra is to consider the ratio of the D and G band intensities (ID/IG). The first equation for such relationship was initially elaborated by Tuinstra and Koenig,56,57 and later generalized by other researchers to take into account Raman experiments performed with different excitation laser energies.51,58−61 To evaluate the La parameter

Figure 5. Deconvolved (a) first and (b) second order Raman spectra of pristine (S) and purified (T) SH samples along with signals attribution. D


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a detailed analysis of the G′ band actually permits to distinguish between graphite systems with a small and controlled number of graphene sheets, and to identify the structural organization of graphene-based nanostructures. Table 3 reports the values of position, integrated intensity, and fwhm of the bands originated from the curve fitting of the second-order Raman spectra (Figure 5b). Also in this case the deconvolution was performed on a spectrum obtained by averaging over several spectra acquired on samples S and T. For both the samples the fitting procedure enables us to discriminate three components for the G′ band, identified as the G′(3D)A, G′(3D)B, and G′(2D) peaks. This means that the graphite phase occurring in the rock is substantially given by variable amounts of 2-D and three-dimensional (3-D) arrangements. An estimation of the percentage of the two structures can be obtained by taking into account the integrated intensities of each component. Specifically, the relative quantity of ordered 3-D graphite (Y%) can be achieved by the following ratio:

28 ± 25% 24 ± 13% 1626 ± 0.1% 1623 ± 0.4%

2.41 × 105 ± 20% 1.88 × 105 ± 24%

fwhm (cm−1) position (cm )

−1

D′-band

intensity (a.u.)

Inorganic Chemistry

1.50 × 10 ± 11% 2.64 × 106 ± 10% 56 ± 8% 46 ± 8% 1598 ± 0.2% 1586 ± 0.3% 2.25 × 10 ± 18% 2.14 × 106 ± 13%

6

fwhm (cm ) position (cm ) fwhm (cm )

74 ± 7% 72 ± 11%

position (cm )

1359 ± 0.2% 1354 ± 0.3%

sample

S T

IG′(3D)A + IG′(3D)B IG′(3D)A + IG′(3D)B + IG′(2D)

× 100 (2)

where IG′(3D)A, IG′(3D)B, and IG′(2D)) represent the integrated intensity values of the G′(3D)A, G′(3D)B, and G′(2D) peaks, respectively. In addition, some studies67−69 demonstrated that the ratio between the integrated intensity values of G′-derived peaks and D + G band can be used as a powerful indicator of the π-conjugation of the graphitic materials. As highlighted in refs 67 and 69, a high value for such ratio, in association with well resolved G′ and (D + G) bands, suggests a large extension of in-plane regular networks of conjugated CC bonds.67,69 The conjugation extent can be indicated by the parameter C, calculated as C=

IG′ I(D + G)

=

IG′(3D)A + IG′(3D)B + IG′(2D) I(D + G)

(3)

The values of Y% and C parameters obtained by applying eqs 2 and 3 are as follows: Y (%): S = 32%; T = 84%. C: S = 1.2; T = 2.9. As one can observe, sample T exhibits the higher values for both the parameters. This indicates that the carbon platelets extracted by the rock are mainly made up of an ordered 3-D assembly of graphene layers with a limited number of defects in the π-conjugated network. All the main outcomes obtained from the Raman analysis are plotted as 3-D histogram in Figure 6. Comparing the values of La, C, and Y (%) as for the raw and purified rock powders, we can definitively assess that the adopted purification procedure is able to isolate from the rock polycrystalline graphene platelets (GP) with nanometric crystallite sizes and extensive π-conjugation domains. This result can be explained by admitting that the physicochemical process removed the small and highly defective sp2 C entities enveloping the larger and crystalline 3-D graphitic grains. The purification treatment, therefore, succeeded not only in eliminating all the noncarbon phases, but also in better defining the ordered graphitic nanodomains from the complex matrix of the rock. The structure of the graphitic phase of both S and T samples was studied in a greater detail by X-ray diffraction. Typical XRD spectra, deconvolved with pseudovoigt profiles, are shown in Figure 7.

6

intensity (a.u.)

−1

G-band

−1

D-band

−1 −1

Table 2. Position, FWHM, and Integrated Intensity of D, G, and D′ bands for samples S and T

intensity (a.u.)

Y% =

E


Article 3.63 × 105 ± 14% 126 ± 11%

Figure 6. 3-D histogram plot depicting La, C, and Y parameters. Red axis: La from eq 1; black axis: Y (%) from eq 2; blue axis: C parameter from eq 3.

Figure 7. Deconvolved XRD spectra of (a) sample S with a magnified image within the range of 20−50° (2θ) in the inset, and (b) sample T with indication of crystal plane reflections.

1.50 × 105 ± 21% 2700 ± 0.1%

36 ± 27%

2727 ± 0.2%

62 ± 16%

6.70 × 105 ± 26%

2953 ± 0.4%

intensity (a.u.)

103 ± 11% 2947 ± 0.4% 3.76 × 105 ± 18%

The fitting operation on the spectrum of sample S points out only the (002) main peak of hexagonal graphite at 2ϑ ≈ 26.1°, because the intense quartz reflections completely obscure the signals related to the C phase. In the spectrum of sample T, instead, four broad signals are clearly detected at 26.44°, 43.80°, 53.90°, and 77.90°, ascribable to the (002), (101), (004), and (110) hexagonal graphite reflections (JCPDS card no. 41-1487). The crystal plane corresponding to every peak is reported in the diffractogram. As is well-known, the (002) and (004) reflections of graphite are associated with the periodicity of graphene sheets stacking along the perpendicular direction (c-axis), whereas the (101) and (110) reflections arise from periodicity within the graphene layers (honeycomb lattice of a single graphene sheet). Interestingly, despite the use of a strong oxidant agent for the purification, for sample T an interlayer distance (d002) among graphene sheets of 0.339 nm was obtained. Such a value is very similar to the distance between adjacent graphene planes in 3-D graphite (0.335 nm),

58 ± 14% 2680 ± 0.2% T

1.10 × 105 ± 18%

55 ± 25% 2673 ± 0.1% S

2.50 × 104 ± 28%

2699 ± 0.1%

70 ± 16%

2732 ± 0.7%

86 ± 31%

1.51 × 105 ± 28%

fwhm (cm−1) position (cm−1) intensity (a.u.) fwhm (cm−1) sample

position (cm−1)

intensity (a.u.)

position (cm−1)

fwhm (cm−1)

position (cm−1)

fwhm (cm−1)

intensity (a.u.)

D+G G′(3D)B G′(2D) G′(3D)A

G′

Table 3. Position, FWHM and Integrated Intensity of G′-Derived Peaks and D + G Band for the S and T Samples

4.55 × 105 ± 19%

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F


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Inorganic Chemistry

Figure 8. (a) Bright Field HR-TEM image of a platelet. The red squared inset shows a high magnification evidencing the polycrystalline structure of the platelet. (b) Fast Fourier transform of (a) showing the typical ring pattern of polycrystalline materials. (c) Cross-section of a platelet wrapped edge. In the red square 7 graphite layers can be counted.

and undoubtedly lower than the values typically found for the oxidized graphene after reduction, i.e., rGO.70 A more comprehensive structural study of the sample was then performed by analyzing the two main peaks related to both parallel and perpendicular periodicity of graphene sheets. Specifically, the mean dimensions of crystallites along the crystallographic a and c axes, named La and Lc, were obtained from the (002) and (110) peaks respectively by means of the Scherrer equation.36,71 The calculated values are La = (13 ± 14%) nm and Lc = (2.5 ± 12%) nm. It is interesting to note that, within the margin of error, there is a complete agreement between the values of the La parameter obtained by XRD and Raman spectroscopy. Such a result made us confident about the employed characterization methodology, and enabled us to delineate with high accuracy the structural features of the graphene platelets produced. Moreover, this supported us in evaluating the number of graphene layers orderly stacked in the graphitic grains. In fact, from the values of the mean thickness (Lc) and interlayer spacing (d002) of graphene packages, an average number of layers between 6 and 10 was estimated. As a whole, the diffraction analysis confirms that the C material extracted from the rock consists of graphene platelets characterized by a small number of regularly stacked graphene sheets and a very low interlayer expansion. The bright field high-resolution TEM analysis shown in Figure 8 allows us to gather more information on the crystalline structure of the platelets. Figure 8a shows a typical plan-view HRTEM image of a platelet. The HRTEM pattern shows a polycrystalline system, with crystallite average dimensions on the order of 10 nm. The red squared inset in Figure 8a represents a magnification of the platelet region below the inset itself, where the hexagonal atomic arrangement typical of the basal plane of graphite is clearly observed. In Figure 8b, the ring pattern of the Fast Fourier transform of the picture reported in Figure 8a attests to the polycrystalline character of the platelet with graphitic nanodomains lacking any preferred orientation. The spots on

the ring can be ascribed to both (101) and (100) type reflections of the graphite lattice, having lattice spacing of 0.20 and 0.21 nm, respectively. Finally, the image in Figure 8c permits the estimation of the number of graphene layers from an edge of a platelet. In particular, as shown in the red square, it is possible to count seven layers with an interlayer spacing distance of 0.34 nm. Overall, the combined use of Raman spectroscopy, XRD, and HRTEM investigations confirms the structural features of the extracted carbon phase. We can state that it consists of orderly stacked and periodically arranged few-layer graphene systems giving some hundred nanometer platelets. At this stage, it becomes particularly interesting to investigate the oxidation degree of such polycrystalline graphene platelets. A qualitative and quantitative definition of the chemical state was therefore performed using XPS spectroscopy. The XPS survey spectra taken from both the samples S and T are reported in Figure 9.

Figure 9. Survey XPS spectra of pristine (S) and purified (T) SH samples. G


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Inorganic Chemistry Both the spectra show visible peaks at 284.4 and 530.0 eV, ascribable to carbon and oxygen, respectively. Signals related to silicon, aluminum, and iron can be observed only in the spectrum of the pristine rock. A quite weak signal corresponding to chlorine (Cl 2p) is instead present in the spectrum of sample T. The presence in sample S of Si, Al, and Fe, and the concomitant absence of such elements in sample T, confirm the effectiveness of the purification procedure. The extremely low amount of chlorine, detected only on the sample T, can be likely explained as a residual of the HCl treatment. In agreement with what was highlighted by EDX, the analysis of the survey spectra evidences a significant change in the C/O ratio. Precisely, for the pristine rock rich in oxygen due to the great amount of silicates and metal oxidizes species, a C/O atomic ratio less than unity is observed. After the purification treatment, the C contribution reaches a value of about 95%, with a corresponding C/O atomic ratio of ∼21. The low oxygen amount detected in sample T can be explained with the presence of some oxygenated functional groups, reasonably introduced on the graphene network during the processing of the raw rock.

To in-depth investigate the chemistry of GP, signals coming from the C 1s and O 1s core levels were acquired at a higher energy resolution on different points of sample T. The assignment of the C 1s and O 1s components were based on theoretical predictions of core level shifts, and on spectra reported in the literature of carbon systems containing oxygen-related functional groups. For the data analysis, a Shirley background subtraction was initially performed, followed by a curve fitting using Voigt functions with both Gaussian and Lorentzian character. The deconvolution procedure allowed to determine the values of the binding energy (BE), area, and fwhm for each peak. The deconvolved XPS spectra of C and O 1s core level for sample T are shown in Figure 10, panels a and b, respectively. It can be observed that GP are characterized by a C 1s spectrum (Figure 10a) having a broad asymmetric tail toward the higher BE. Such a feature is commonly noted in XPS spectra of samples with a high concentration of sp2 hybridized carbon. The analysis of the spectrum was limited to the BE range from 282 to 296 eV, where the deconvolution identifies five peaks corresponding to the chemical states of carbon in ordered and defective graphite as well as in oxygenated carbon species. Table 4 reports BE, area, fwhm, chemical shift, and percentage contribution of every signal. In particular, the C relative concentration in the various chemical states was evaluated from the ratio between the area of each peak to the C 1s spectrum total area. Taking into consideration the Scofield sensitivity factors72 the ratio σO 1s/σC 1s = 2.85 was used to estimate the areas of the oxygenated functional groups. The data reported in Table 4 were averaged over several spectra acquired on the sample, and the error was calculated by the percentage of the ratio between the standard deviation and the average. The remarkable principal peak found at 284.5 eV can be ascribed to the regular honeycomb network of sp2 carbon atoms. The near peak at 285.0 eV is instead the so-called “defects peak”, correlated to structural defects of the C hexagonal lattice but not to the presence of heteroatoms.73 The two signals located at 286.3 and 287.4 eV can be reasonably attributed to C atoms singly (C−O) or doubly (CO) bonded to oxygen,74 and are the only signals to be well distinguishable in graphite materials with a high C/O ratio. In fact, as reported in refs 75 and 76, only in the case of a high oxygen content a significant discrimination between hydroxyl (C−OH), epoxy (C−O−C), carbonyl (>CO), and carboxylic (HO−CO) oxygenated groups is viable in C 1s spectra of graphitic systems. In this context, the main contribution of the C−O peak, that shows twice the area of the CO peak, is a further confirmation of the low levels of oxidation of the platelets.74 Finally, the broad peak located at 291.0 eV with a shift of about 6 eV with respect to the sp2 C parent peak, can be associated with the shakeup process. This satellite peak, that comes from π−π* transitions excited by the exiting photoelectrons, is a typical feature of carbon in aromatic compounds.77,78

Figure 10. Deconvolved high resolution XPS spectra of (a) C 1s and (b) O 1s core levels of sample T.

Table 4. Binding Energy, Area, FWHM, Chemical Shift and Percentage of C in the Various Configurations signal attribution 2

sp C in honeycomb network defects peak C−O CO π−π*

binding energy (%) 284.5 285.0 286.3 287.4 291.0

± ± ± ± ±

0.01 0.01 0.01 0.02 0.04

area (%) 17589 3828 1144 527 5932

± ± ± ± ±

8 5 8 6 19 H

fwhm (%)

chemical shift

± ± ± ± ±

0.5 1.8 2.9 6.5

0.7 1.1 1.7 2.3 2.9

1 3 4 9 19

percentage contribution (%) 62 12 4 2 20

± ± ± ± ±

6 9 7 4 16


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Inorganic Chemistry

respect to all the oxygen-related functionalities definitively demonstrate that the rock can be considered an effective source of GP with a chemistry strictly close to that of rGO materials.74 To further validate this statement, an electrical characterization by I−V measurements was performed Sample T was thus pelletized by means of a hydraulic press operating at 100 bar, and current vs voltage plots were recorded in the bias range of (−4 to +4) V at RT. The reproducibility of the electrical behavior was tested by multiple measurements. In Figure 11 typical I−V curve measured on GP is shown.

Summarizing, the XPS profile just described provides a clear depiction of the T graphene system, which turns out to be constituted by low defective sp2 networks in a low oxidation state. In order to have a quantitative estimation of the phase having a regular graphene lattice (%G), the ratio between the sum of the areas of the graphite line and its π−π* satellite peak to the total area of the C 1s spectrum was calculated as follows: %G =

Agraphite line + Aπ − π * A sp2C + Adefects + A C − O + A CO + Aπ − π *

× 100 (4)

2

It is found that in GP the content of sp C in a regular graphene lattice configuration is ∼80%. This value states the low presence of both defects and functional groups in the platelets, and highlights a highly delocalized π electrons system. The high degree of electronic conjugation is also confirmed by the high intensity of the satellite peak with respect to that of sp2 C peak. For aromatic compounds the intensity of the satellite peak is typically about 5−10% of the one measured for the honeycomb C peak.79 For the GP the integrated area of the shakeup line is about 30% of total sp2 C peak area, evidencing the high extent of the π conjugation of such nanostructures. With the aim to better identify the contribution of the various O-containing functional groups, the high resolution O 1s spectrum was also thoroughly analyzed. The deconvolution produced five main peaks at 530.6, 531.2, 532.1, 533.2, and 535.5 eV (Figure 10b), assigned to the COOH, CO, C−O−C, and C−OH groups and to chemisorbed oxygen, respectively.80−82 BE, area, fwhm, and percentage of the various O species are reported in Table 5. The relative concentrations of

Figure 11. Typical I−V curve of pelletized GP powder (sample T) measured between −4 and 4 V at RT.

The curve exhibits a rather ohmic characteristic in the range of investigated voltage. The conductivity σ obtained from such measurements was (1.041 ± 0.087) × 103 S cm−1, a value similar to that of polycrystalline graphite (1.250 × 103 S cm−1).86 For rGO thin films, comparable values of conductivity can be achieved depending on the fraction and sizes of sp2 regular domains of the graphene layers.84 In particular, significant electrical performances were found for a sp2 content of ∼85% and domain sizes of 6 nm.87,88 In this context, it is to be noted that the GP extracted from SH are characterized by a few layers with a 80% regular graphene lattice having a mean size of ∼13−15 nm. These features account for the high σ value found, although it has been obtained from bulk measurements on the GP pellet and not on the individual platelets. Such a result therefore suggests that an improved carrier transport can be reasonably expected on the isolated GP entities.

Table 5. Binding Energy, Area, FWHM, Chemical Shift, and Percentage of O in the Various Configurations signal attribution COOH CO C−O−C C−OH O chemisorbed

binding energy (eV) (%) 530.6 531.2 532.1 533.2 535.5

± ± ± ± ±

0.01 0.01 0.01 0.01 0.02

area (%) 406 817 918 1514 319

± ± ± ± ±

8 4 14 3 21

content fwhm (%) percentage (%) 1.9 1.8 1.8 2.0 4.1

± ± ± ± ±

17 13 19 6 20

10 21 23 38 8

± ± ± ± ±

6 8 9 4 24

the functional groups and the chemisorbed oxygen were determined from the areas of each deconvolved peak divided by the O 1s spectrum total area. The data reported in Table 5 were averaged over several spectra acquired on the sample, and the error was calculated by the percentage of the ratio between the standard deviation and the average. In agreement with the results obtained from the analysis of the C 1s spectrum, the ratio between the integrated areas of the peaks related to C atoms singly (C−OH and C−O−C components) and doubly (CO and COOH groups) bonded to oxygen gives a value of 1.9. Moreover, between the less oxidized functional groups, there is a clear prevalence of the C−OH moiety, that overcomes 1.7 times the C−O−C one. The high ratio of hydroxyl-to-epoxy groups is a further evident confirmation of the low oxidation of the GP.83−85 Overall the XPS results point out how the GP extracted from the SH rock are given by graphene sheets with a very low oxygen content (∼5%), and how that majority of oxygen species consist of C−OH groups. The impressive high C/O ratio and the predominant presence of hydroxyl groups with

4. CONCLUSIONS The research herein reported originated from the intent to extract from shungite rocks the graphene units that recent detailed investigations evidenced inside the raw material. Starting from a heterogeneous natural mineraloid, an extensive experimental work was directed toward the development of a protocol able to eliminate the silicates and the metal oxide species present in the pristine rock, leaving a pure carbon material with a 85% yield. Specifically, it was demonstrated that from a raw shungite powder constituted by a ∼ 27% carbon content, the proposed purification procedure is able to extract a homogeneous phase given by ∼96% of C. The combined use of Raman spectroscopy, XRD, and HRTEM analyses evidenced that this C phase is in form of thin platelets with planar sizes of a few I


Article

Inorganic Chemistry

(6) Sutter, P. W.; Flege, J. I.; Sutter, E. A. Epitaxial Graphene on Ruthenium. Nat. Mater. 2008, 7, 406−411. (7) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-Alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’Homme, R. K.; Brinson, L. C. Functionalized Graphene Sheets for Polymer Nanocomposites. Nat. Nanotechnol. 2008, 3, 327−331. (8) Sinar, D.; Knopf, G. K.; Nikumb, S. Graphene-based Inkjet Printing of Flexible Bioelectronic Circuits and Sensors. In Micromachining and Microfabrication Process Technology XVIII, Vol. 8612, International Society for Optics and Photonics; Maher, M. A.; Resnick, P. J.; Eds.; SPIE, 2013; p 861204. (9) Zhang, Y.; Nayak, T. R.; Hong, H.; Cai, W. Graphene: a Versatile Nanoplatform for Biomedical Applications. Nanoscale 2012, 4, 3833−3842. (10) Yao, J.; Sun, Y.; Yang, M.; Duan, Y. Chemistry, Physics and Biology of Graphene-based Nanomaterials: New Horizons for Sensing, Imaging and Medicine. J. Mater. Chem. 2012, 22, 14313− 14329. (11) Ferrari, A. C.; Bonaccorso, F.; Fal’ko, V.; Novoselov, K. S.; Roche, S.; Bøggild, P.; Borini, S.; Koppens, F. H. L.; Palermo, V.; Pugno, N.; Garrido, J. A.; Sordan, R.; Bianco, A.; Ballerini, L.; Prato, M.; Lidorikis, E.; Kivioja, J.; Marinelli, C.; Ryhänen, T.; Morpurgo, A.; Coleman, J. N.; Nicolosi, V.; Colombo, L.; Fert, A.; GarciaHernandez, M.; Bachtold, A.; Schneider, G. F.; Guinea, F.; Dekker, C.; Barbone, M.; Sun, Z.; Galiotis, C.; Grigorenko, A. N.; Konstantatos, G.; Kis, A.; Katsnelson, M.; Vandersypen, L.; Loiseau, A.; Morandi, V.; Neumaier, D.; Treossi, E.; Pellegrini, V.; Polini, M.; Tredicucci, A.; Williams, G. M.; Hong, B. H.; Ahn, J.-H.; Kim, J. M.; Zirath, H.; Van Wees, B. J.; Van der Zant, H.; Occhipinti, L.; Di Matteo, A.; Kinloch, I. A.; Seyller, T.; Quesnel, E.; Feng, X.; Teo, K.; Rupesinghe, N.; Hakonen, P.; Neil, S. R. T.; Tannock, Q.; Löfwander, T.; Kinaret, J. Science and Technology Roadmap for Graphene, Related Two-Dimensional Crystals, and Hybrid Systems. Nanoscale 2015, 7, 4598−4810. (12) Jakus, A. E.; Secor, E. B.; Rutz, A. L.; Jordan, S. W.; Hersam, M. C.; Shah, R. N. Three-dimensional Printing of High-content Graphene Scaffolds for Electronic and Biomedical Applications. ACS Nano 2015, 9, 4636−4648. (13) Berman, O. L.; Kezerashvili, R. Y.; Lozovik, Y. E. Graphenebased Photonics and Plasmonics. In Nanoscale Materials and Devices for Electronics, Photonics and Solar Energy; Korkin, A., Goodnick, S., Nemanich, R., Eds; Springer; 2015; pp 93−126. (14) Rowley-Neale, S. J.; Randviir, E. P.; Dena, A. S. A.; Banks, C. E. An Overview of Recent Applications of Reduced Graphene Oxide as a Basis of Electroanalytical Sensing Platforms. Appl. Mater. Today. 2018, 10, 218−226. (15) Tkachev, S. V.; Buslaeva, E. Y.; Gubin, S. P. Graphene: A Novel Carbon Nanomaterial. Inorg. Mater. 2011, 47, 1−10. (16) Avouris, P.; Dimitrakopoulos, C. Graphene: Synthesis and Applications. Mater. Today 2012, 15, 86−97. (17) Zheng, Q.; Kim, J. K. Graphene for Transparent Conductors: Synthesis, Properties and Applications; Springer: New York, 2015. (18) Bhuyan, M. S. A.; Uddin, M. N.; Islam, M. M.; Bipasha, F. A.; Hossain, S. S. Synthesis of Graphene. Int. Nano Lett. 2016, 6, 65−83. (19) Mohan, V. B.; Lau, K. T.; Hui, D.; Bhattacharyya, D. Graphenebased Materials and their Composites: a Review on Production, Applications and Product Limitations. Composites, Part B 2018, 142, 200−220. (20) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (21) Xu, Z.; Bando, Y.; Liu, L.; Wang, W.; Bai, X.; Golberg, D. Electrical Conductivity, Chemistry, and Bonding Alternations under Graphene Oxide to Graphene Transition as Revealed by in Situ TEM. ACS Nano 2011, 5, 4401−4406. (22) Kuila, T.; Bose, S.; Mishra, A. K.; Khanra, P.; Kim, N. H.; Lee, J. H. Chemical Functionalization of Graphene and its Applications. Prog. Mater. Sci. 2012, 57, 1061−1105.

hundred nanometers, and a number of stacked graphene layers ranging from 6 to 10. These thin graphene nanostructures show sp2 honeycomb regular lattice domains extending for 13−15 nm and constituting the 80% of the total C phase, as pointed out by XPS investigations. Moreover, the remarkable high C/O ratio, ranging from 21 to 24, indicates that only a small fraction of the C atoms are bonded to O atoms in the GP. In particular, the oxygen-containing functional groups are found to be mainly C−OH moieties, further evidencing the low oxidation state of the graphene materials. The high degree of π-conjugation highlighted by Raman spectroscopy and XPS analyses contributes to explain the notable electrical properties of the platelets, with a σ value of 1041 S cm−1 found from bulk I − V measurements. This result is particularly important since similar conductivity performances are typically detected for rGO thin films with a regular sp2 fraction higher than 80%. This seems to suggest a highly efficient charge transport within each individual GP. Putting together the results of all the characterization techniques used in the present research, we can conclude that the adopted purification procedure was able to maintain at the best the outstanding structural and chemical properties of the graphene flakes embedded in the complex rock matrix. Such graphene systems can be isolated in the form of thin platelets with a limited number of stacked sp2 C layers and low oxidation state, associated with those of conventional rGO materials. In this context SH rocks are qualified to emerge as a novel attractive source for production of graphene thin films in a reduced state. The present results establish the first strategies on which to base the design of more complex structures, assembling the number of packed planes in 3-D nanostructures and of the in-plane layout.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Emanuela Tamburri: 0000-0003-2643-8249 Funding

R.C. wishes to acknowledge FBK for financial support. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Natalia N. Rozhkova (Institute of Geology, Karelian Center, Russian Federation) for providing shungite samples. The authors are also very grateful to Francesco Baldassarre and Dr. Dritan Siliqi (Istituto di Cristallografia-CNR, Bari, Italy) for X-ray diffraction measurements.

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REFERENCES

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Shungite Carbon as Unexpected Natural Source of Few-Layer

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