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Thermal exfoliation of electrochemically obtained graphitic materials
Applied Surface Science 481 (2019) 466–472
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Thermal exfoliation of electrochemically obtained graphitic materials ⁎
T
Piotr Krawczyk , Bartosz Gurzęda, Agnieszka Bachar Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, ul. Berdychowo 4, 60-965 Poznań, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphite oxide Reduced graphite oxide Electrochemical oxidation Thermal exfoliation
The present work is devoted to the electrochemical formation and thermal exfoliation of graphite oxide (GO) and reduced graphite oxide (rGO). The main attention was paid on the correlation between cycling processes of GO formation followed by its electrochemical reduction and the stage of their exfoliation. For this purpose a natural graphite was overoxidized in 8 M HClO4 to form GO which was further cathodically reduced yielding rGO. Both processes were performed within one electrochemical process in three-electrode cell using a cyclic voltammetry method. To examine the reversibility of the coupled overoxidation of graphitic matrix and its two-stage reduction the above mentioned operations were three times repeated. The acquired results clearly showed that the properties of GO and rGO significantly change with number of performed cycles. The structure of graphite matrix gradually decreases disenabling the efficient intercalation of perchloric acid during renewed process proceeded during the further cycling. Thermal treatment of electrochemically obtained materials leads to their exfoliation thus contributing to significant modification of structure, morphology and chemical composition. The information on GO properties were provided by the determination of specific surface area, X-ray diffraction analysis (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy analysis (FTIR), transmission electron microscopy (TEM) and scanning electron microscopy observations (SEM).
1. Introduction Graphite oxide is the derivative of graphite that has oxygen atoms intercalated between the graphene layers of graphite lattice [1,2]. The formation of chemicals bonds between the oxygen and carbon atoms of graphene layers cannot proceed directly by oxygenation of graphite. To yield GO the intercalation of oxygen containing intercalate followed by its overoxidation should be performed. Therefore the synthesis of GO involve steps in which graphite intercalation compounds (GIC) is formed that is further transformed into the GO being the most oxidized form of graphite. GO has been prepared by various approaches of strong oxidation realized by chemical [3–6] as well as electrochemical routes [7–13]. Among the commonly used chemical methods of GO preparation are Hummers method and the methods based of its modifications [5,6,14]. Compared to the electrochemical overoxidation of graphite, Hammers methods contributes to the formation of GO of a higher oxidation stage. On the other hand, electrochemical routs are highly controllable and much more attractive from the economical as well as ecological point of view. Contrary to the chemical overoxidation of graphite, electrochemical methods do not require strong oxidants neither necessity of by-products utilization [9–13]. The structure as well as properties of the electrochemical synthesized GO are influenced by the
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employed conditions [7–13]. From the practical point of view the desiring feature of GO is high oxidation degree guaranteeing the greater separation of graphene layers. This effect is highly suitable for the synthesis of graphene materials. One of the commonly used method of graphene preparation is based on the reduction of GO [15–18]. The requirements for appropriated graphene synthesis from GO are the highest possible oxygen release and the preservation of graphene layers from their damaging. GO can be reduced yielding redGO by its thermal [15–18], chemical [19–22] and electrochemical treatment [23–26]. The features of final product are largely dependent upon the particular synthetic route employed. The commonly used thermal reduction proceeds at elevated temperatures (around 1000 °C) under different atmospheres (air, argon, hydrogen). Apart of oxygen release thermal reduction often yield the volume increment due to GO exfoliation. Unfortunately it can be accompanied by the disordering of graphite structure [16,17,27]. However, the above fact does not exclude the use of an electrochemical exfoliation process for obtaining of graphene structures [28–32]. It is known that owing to the electrochemical exfoliation it is possible to yield product of a decreased amount of oxygen and low-defect graphene material. On the other hand chemical reduction of GO requires strong reducers such as hydrazine, borohydride or selected organic acids (e.g.
Corresponding author. E-mail address: [email protected] (P. Krawczyk).
https://doi.org/10.1016/j.apsusc.2019.03.154 Received 15 November 2018; Received in revised form 12 March 2019; Accepted 16 March 2019 Available online 18 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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ascorbic) which residues remain in the reaction mixture as a waste or by-product [19,20]. Another side effect of chemical reduction refers to the replacing of oxygen functionalities by another one that involves other heteroatoms, for example N [19–21]. One of our last work concerning the electrochemical overoxidation of graphite in perchloric acid have showed that it is possible to prepare GO that can be further thermally reduced yielding graphene material of a diminished oxygen content [11]. No information was given about the joined reduction of GO involving electrochemical and thermal process as well as the reversibility of these processes. From our previous investigations it is known that the coupled processes of intercalation and exfoliation can be successfully repeated [33]. The knowledge in this issue related to GO reduction is completed by our present work. The purpose of this work was to study the properties of graphite oxide affected by its cyclic reduction and thermal exfoliation. Electrochemical formation and reduction of GO were carried out in perchloric acid using cyclic voltammetry technique. The electrochemical results obtained for GO, electrochemically reduced GO as well as for exfoliated GOs were discussed in relation to the results acquired from the specific surface measurements as well as XRD, FTIR analysis and SEM observations.
Fig. 1. Cyclic voltammograms recorded during the oxidation of natural graphite in 8 M HClO4. Scan rate 0.01 mV/s, scan range −0.4 ↔ 1.6 V.
investigated using a scanning electron microscopy (S3400 N Hitachi) whereas transmission electron microscopy (Jeol 1200 EX2) was used to examine the size of graphene layers within the GO. Chemical composition of the synthetized GO was estimated on the basis of FTIR measurements (Jasco FTIR-6700 spectrometer) using the KBr technique. The specific surface area of prepared GOs was gained from the N2 adsorption isotherms measured at 77 K with ASAP 2010 apparatus.
2. Experimental 2.1. Material preparation Precursor used for preparation of all investigated samples was natural graphite (purity 99.5%, flake size 170–283 μm). Graphite oxide (GO) and electrochemically reduced GO (redGO-1c and redGO-3c) are the products of successive steps of electrochemical reaction of graphite with 8 M HClO4 (purity 71%) realized by cyclic voltammetry method. Electrochemical investigations were performed with scan rate 0.01 mV/ s within the potential range −0.4 ↔ 1.6 V starting from the rest potential of electrode (ER) towards the higher potentials. In three-electrode cell used for this purpose graphite was a working electrode, platinum wire (purity 99.9%, 1 mm in diameter) was playing a role of counter electrode, whereas Hg/Hg2SO4/1 M H2SO4 served as a reference electrode. During the first cycle when the electrode reached 1.6 V the measurement was interrupted due to formation graphite oxide. Electrochemically reduced graphite oxides were obtained by cyclic anodic oxidation of graphite up to 1.6 V followed by the cathodic reduction spreading to the potential of −0.4 V. All electrochemical measurements were carried out using an Autolab PGSTAT 302N potentiostat/galvanostat. More details on electrode preparation can be found in our previous works [10–13]. After the finishing the electrochemical synthesis the products were prepared for the further investigations by washing with distilled water until filtrate became neutral and finally drying in air. Thermal treatment of graphite oxide (GO), electrochemically reduced GO (redGO-1c and redGO-3c) yielded exfoliated graphite oxide (ex-GO), exfoliated reduced graphite oxides (exredGO-1c and ex-redGO-3c), respectively. Thermal process was performed in a muffle furnace for 1 min at 700 °C. A ceramic crucible filled with the examined sample was promptly inserted into the hot muffle furnace. Owing to this the considered process had a shocked character. After the treatment, the as prepared RGO was rapidly removed from the furnace and cooled in air until the room temperature was reached.
3. Results and discussion 3.1. Electrochemical formation of graphite oxide based materials Cyclic voltammograms depicting the electrochemical formation of the GO as well as its reduced forms redGO-1c and redGO-3c are shown in Fig. 1. After starting the measurement from the rest potential of electrode towards the more positive potentials the process of HClO4 intercalation into the graphite flakes begins. Anodic oxidation of graphite contributes to its oxidation yielding C+. On further polarization ClO4− ions start to penetrate the interlayer space of graphite lattice and participate in formation of ionic bonds with oxidized surface of graphene layers thus forming graphite intercalation compounds with perchloric acid (HClO4-GIC) [11]. During the first cycle the above mentioned process is illustrated by the number of small peaks spreading over the potential range 0.6 and 1.2 V (green line). These peaks depict the successive filling the interlayer spaces with intercalate yielding GIC of a lower stages of intercalation. Stage of intercalation is defining as a number of graphene layers isolating the two neighbored layers of intercalate. The huge anodic peak with maximum at 1.27 V (peak A1) illustrates the overoxidizing of HClO4-GIC yielding graphite oxide (GO). During the experiment in which GO was prepared for analysis process of graphite overoxidation was interrupted at 1.6 V and the obtained material was taken out from the electrochemical cell for analysis. Whereas to obtain a electrochemically reduced graphite (red-GO) the process was continued during the backward scanning. Cathodic reduction of the previously prepared GO resulted in formation a cathodic peak with maximum positioned at 0.06 V (peak C1). This peak is attributed to the transformation of GO into the electrochemically reduced graphite oxide (redGO-1c). The charge density of peak A1 is very closed to that noted for C1 meaning that the charges required for graphite transformation into GO is very similar to that needed to its reduction to form redGO (Table 1). Cathodic reduction of GO ends when the electrode reached the potential of −0.4 V. In order to examine the possibility of cycling reduction of GO the whole process was performed through the 2
2.2. Material characterization The structure of the synthetized graphitic materials was examined by (XRD) analysis which was performer on PANalytical diffractometer using Cu Ka radiation (1.54 Å) with 2Ɵ scan range from 4 to 60° and step size of 0.04°. Structure and the amount of surface defects were evaluated on the basis of Raman spectroscopy. The acquired spectra were recorded on inVia Renishaw micro-Raman system with an argon laser, emitting 514.5 nm wavelength. Morphological properties were 467
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Table 1 Charge density and location of anodic and cathodic peaks assigned to graphite oxide formation and its reduction, respectively. Peak A1 C1 A2 C2 A3 C3
Charge density [C g−1]
Peak maximum [V]
3663.8 3689.2 4829.8 1754.8 4002.3 970.2
1.27 0.06 1.18 0.05 0.92 0.02
Table 2 Raman spectra analysis for graphite, GO, electrochemically reduced GO and exfoliated GO. Sample
Graphite GO exGO redGO-1c ex-redGO-1c redGO-3c ex-redGO-3c
Band position [cm−1]
ID/IG
D band
G band
D′ band
2D band
D + G band
1356 1350 1364 1357 1357 1352 1357
1582 1591 1582 1585 1584 1591 1594
– – 1620 1623 1624 1619 –
2729 2709 2728 2723 2725 2718 2710
– 2933 – 2953 2953 2947 2955
0.27 1.13 0.87 1.36 1.13 1.52 1.44
evidencing that the properties of electrode material are also changed. However the intensity of cathodic peaks attributed to the reduction of GO (peak C2 and C3) markedly decreases but their location remains almost unchanged during second and third cycle. By comparing the peak charges for the successive cycles it is seen that anodic ones are significantly higher thus evidencing the worsening of reaction reversibility. The high discrepancy in charge densities between the anodic and cathodic peaks indicates that the participation of reaction yielding GO within the anodic peaks decreases on cycling. Taking into account assumption that cathodic peaks represent reactions of GO reduction, the abrupt decrease in their charge evidences the less amount of GO being reduced. It means that less amount of graphite oxide is formed during the previous anodic overoxidation of graphite. On the other hand the increased charge of anodic peaks due to cycling overoxidation can be explained by the increased participation of side reactions associated with the oxidation of demanded graphite matrix yielding surface functionalities. It is known that to perform intercalation the ordered structure of the graphitic material is required [33,34]. If graphene layers become damaged and bent, the intercalation as well as process of GO formation can be disturbed. It cannot be excluded that during the process of cycling overoxidation followed by cathodic reduction the structure of graphite is modified.
Fig. 2. XRD patterns of graphite, GO, electrochemically reduced GO and exfoliated GO.
3.2. The characterization of structure of GO based materials The structure of GO as well as its reduced form due to cycling treatment of graphite in 8 M HClO4 can be elucidated on the basis of XRD measurements. XRD patterns for all investigated samples are shown in Fig. 2. In order to facilitate the comparison of structural changes Fig. 2 also involves XRD pattern of original graphite with characteristic signal at 26.5° attributed to the graphitic structure. On the pattern of graphite and GO one can observe a diffraction peak at 14.9° evidencing the increment of interlayer space from 3.35 Å for graphite to 5.94 Å. This effect is caused by the incorporation of oxygen functionalities between the graphene layers of graphite matrix [35]. On spectrum for GO also emerges a small signal at 26.5° depicting the residual graphite phase within the formed GO. Thermal treatment of GO led to its transformation into the non-crystalline phase with randomly displaced graphene stacks [3,16,36]. Such a behavior is illustrated by the disappearance of diffraction peaks (sample exGO). Cathodic reduction of the previously formed GO results in further structural changes being associated with the reduction of interlayer space. This effect is illustrated by the formation of diffraction peak at 25.2° (redGO-1c). Thermal treatment of the electrochemically reduced GO contributed to further reduction of average d spacing to 3.59 Å. This is evidenced by the shift of diffraction peak (redGO) to 26.2° (ex-redGO1c). Such a behavior can be explained by the successive release of oxygen from the graphitic matrix which is accompanied by the decrease in interlayer distance of graphite lattice. Taking into account the electrochemical behavior it can be assumed that the process of exfoliation of electrochemically reduced GO acquired during the first cycle (redGO-
Fig. 3. Raman spectra of graphite, GO, electrochemically reduced GO and exfoliated GO.
consecutive cycles and the obtaining products were called as redGO-2 and redGO-3c, respectively. As one can see during the cycling the intensity of recorded peaks significantly decreases. In case of forward scanning, the diminution in activity of GO formation depicted by a lowering the anodic peak is accompanied by its shift towards the less positive potentials. Moreover, for the second and third cycle peaks attributed to GO formation (peak A2 and A3) become much more wider compared to that noted for the first cycle (peak A1). This behavior indicates that the mechanism of anodic overoxidation changes on cycling 468
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Fig. 4. SEM images of GO (a) and electrochemically reduced GO: reGO-1c (b), redGO-3c (c).
Fig. 5. SEM images of exfoliated GO (a) and exfoliated reduced GO: ex-reGO-1c (b), ex-redGO-3c (c).
Fig. 6. TEM images of GO (a) and electrochemically reduced GO: reGO-1c (b), redGO-3c (c).
were also examined using Raman spectroscopy (see Fig. 3). As one can see Raman spectra involve characteristic bands for graphitic materials. The first one, D band, is associated with the defects affected by the presence of sp3 hybridized carbon, oxygen functional groups or edges [37,38]. G band corresponds to the stretching mode of sp2 bonded carbon [39]. Defects of graphite structure are related with the appearance of D′ band [37,40]. Next band denoted as 2D is a second-order of D band. For graphite 2D band consists of two bands [37,39]. D + G band belongs to the second-order of D band and is connected with the presence of disordered structure [41,42]. Raman spectra of GO show that the anodic overoxidation of graphite in HClO4 aqueous solution highly deforms graphitic structure. Intensity of D band increases
1c) most likely differs compared to that for redGO-3c. The main difference is associated with the decrease in GO participation within the product. Contrary, the amount of disordered carbon being the product of multiply electrochemical treatment increases. Therefore during the thermal exfoliation the mentioned products are thermally decomposed thus uncovering the residuals of GO allowing its XRD detection. Sample prepared after cathodic reduction in third cycle (redGO-3c) exhibits non-ordered character, whereas thermal exfoliation of this sample reveals appearance of residual phases of reduced GO (sample ex-redGO3c). Changes in graphitic structure caused by electrochemical treatment of graphite as well as thermal treatment of GO and reduced GO samples 469
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Fig. 7. TEM images of exfoliated GO (a) and exfoliated reduced GO: ex-reGO-1c (b), ex-redGO-3c (c).
consequence, the sp3 hybridized carbon becomes partially rearranged to sp2 hybridized carbon and the ID/IG ratio decreases to 0.87. Along with the increasing number of oxidation-reduction cycles of electrochemical treatment the intensity of D and D′ bands also increases. It can be assumed that despite the cathodic removal of oxygen functionalities from the graphitic structure the process of cyclic electrochemical treatment causes significant deformation and destruction of graphitic structure. As in the case of GO exfoliation, thermal treatment of redGO-1c and redGO-3c causes partial restoration of sp2 hybridized carbon.
Table 3 BET surface area calculated for GO based materials. Sample Graphite GO redGO-1c redGO-3c exGO ex-redGO-1c ex-redGO-3c
Specific surface area [m2 g−1] 0.3 1.2 7.7 25.3 677.8 185.0 28.5
drastically, hence ratio of D band intensity to intensity of G band (ID/IG) also increases as compared to the starting graphite (see Table 2). This effect is undeniably caused by the formation of oxygen functionalities during the electrochemical treatment [10]. Thermal exfoliation of GO leads to the abrupt removal of oxygen functional groups. In
3.3. SEM and TEM observations of graphite based materials The comparison of SEM images recorded for samples GO, redGO-1c and redGO-3c (Fig. 4) allows knowing the changes in morphology caused by electrochemical reduction, while the observation of images
Fig. 8. FTIR spectra of graphite, GO, electrochemically reduced GO and exfoliated GO. 470
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evaluated on the basis of FTIR analysis. The infrared spectra recorded for all investigated samples are shown in Fig. 8. As one can see the modification of GO structure due to its electrochemical reduction and subsequent thermal exfoliation is accompanied by the slight alteration of its chemical composition. From the literature data, it is known that thermal treatment of GO performed in an air atmosphere commonly reduces the amount of epoxy (CeOeC) and alkoxy (CeO) groups [15,36]. In case of the present investigations signals arising from both type of groups disappear only due to thermal treatment of electrochemically reduced GO. For exfoliated GO, the wide band with maximum at 1240 cm−1 is still observed revealing the appearance of CeOeC bonds [15,31]. On the other hand, spectra recorded for electrochemically reduced GO (samples redGO-1c and redGO-3c) involve signal at around 1089 cm−1 that are attributed to CeO bonds of alkoxy groups [15,36]. From the FTIR investigations it is evidenced that electrochemical reduction up to −0.4 V as well as thermal exfoliation of GO materials noticeably reduces the amount of oxygen on within the graphite matrix. The scale of this effect strongly depends on method performed for the GO reduction. Cyclic cathodic treatment of GO decreases mainly epoxy groups whereas thermal exfoliation impacts on the diminution in alkoxy functionalities. As one can see the well shaped peak at 1240 cm−1 evidencing appearance of CeOeC bonds can be observed only for exfoliated GO (sample not treated cathodically). For the rest samples the mentioned peak becomes very small (redGO-1c and redGO-3c) or completely disappear (ex-redGO-1c and ex-redGO-3c). Such a behavior indicates that thermal exfoliation contribute to the removal of epoxy groups from the graphite matrix, whereas electrochemical reduction results only in partial decomposition of the above mentioned functionalities. The observed discrepancy well correlates with the result of XRD investigations. Signals at wavenumber of 850 cm−1 can be associated with the decomposition of graphitic matrix due to electrochemical and/or thermal treatment.
for samples exGO, ex-redGO-1c and ex-redGO-3c provide information on the role of thermal exfoliation in creation of morphological properties (Fig. 5). As can be seen the information provided from the SEM images coincide with that acquired from the electrochemical as well as XRD measurements. The multi repeated electrochemical treatment of GO composed of its overoxidation and reduction modify its structural and morphological properties. SEM image of GO gathered after its cathodic reduction in third cycle have revealed significant decomposition of its structure (Fig. 4c). The damaged graphene layers pertaining to electrochemically reduced GO are randomly positioned. Moreover, the concentration of surface defects and edges is markedly increased. A shock thermal treatment of GO and reduced GO results in their exfoliation accompanied by the huge deformation of graphene layers (see Fig. 5) due to abrupt eruption of gaseous products (CO, CO2) of thermal decomposition of oxygen functionalities pertaining to GO. By comparing the images of exfoliated GO it can be assumed that the scale of surface deformation depends on the sample subjected to thermal treatment. The highest effects of exfoliation stage are seen for the thermally treated GO (Fig. 5a). The further analysis of carbon phases within the electrochemically reduced and thermally exfoliated graphite oxides were performed by TEM observation. Fig. 6 involves TEM images of GO and electrochemically reduced GO, whereas Fig. 7 depicts images for their exfoliated forms. The presented TEM images confirm the SEM observations that the structure of GO underwent electrochemical reduction is worsened. Carbon layer becomes shorter. On the other hand the transparency of graphene layers is improved compared to GO (Fig. 6a) which may indicate their partial delamination. It seems that much higher level of delamination of graphene layers is yielded by thermal exfoliation (see Fig. 7). Due to shock thermal treatment of graphene layers of exfoliated GO become thinner. The above mentioned effect is accompanied by the increase in concentration of edges and surface defects.
4. Conclusions 3.4. Measurements of BET surface area Our investigations showed that thermal treatment of GO based materials results in significant changes in their structural as well as morphological properties. The scale of these modifications is strongly influenced by the type of material subjected to exfoliation. Taking into account the development of specific surface area as a main criterion of evaluation of structural changes it can be stated that the most pronounced effects of thermal treatment were gathered for exfoliated GO. It is also proved that the repeated cyclic electrochemical treatment involving overoxidation and subsequent cathodic reduction of GO leads to the worsening of structural ordering. It cannot be excluded that during the cycled oxidation-reduction of GO the electrochemical exfoliation occurs. FTIR investigations have revealed that the changes in oxygen functionalities on GO surface depend on the type of applied reduction treatment. The cathodic treatment of GO contributed to modification in the amount of alkoxy functionalities whereas the thermal reduction impacts on the appearance epoxy functionalities.
From the results of calculations of BET surface area (Table 3) it is clear that the visible development of specific surface area is taking place due to cyclic electrochemical treatment of GO. The BET surface area of sample after its repeated overoxidation and cathodic reduction (reGO-3c) is very closed to that noted after thermal exfoliation (exredGO-3c). This result is in coincidence with the observation of SEM images evidencing the advanced decomposition of graphene layer within the cathodically reduced GO (see Fig. 4c). It justify the assumption that during the multistep electrochemical treatment of GO in HClO4 the electrochemical exfoliation likely occurs. It is worth to note that the specific surface area of redGO-3c is very closed to that commonly noted for exfoliated graphites [33]. On the other hand, the most pronounced development of surface area is gathered for thermally exfoliated GO. BET surface area of exGO over 3.5 times exceeds that calculated for the exfoliated samples beforehand subjected to cathodic reduction (ex-redGO-1c). This effect can be explained by the fact that during the successive cycles of electrochemical treatment the highly ordered graphitic structure becomes worsened. The gradually decrease in GO phase within the multi reduced material results in diminution in exfoliation stage. It is widely known that the exfoliation effects is strongly dependent on the amount as well as distribution of intercalate [33]. The results of BET calculations confirm the statement of electrochemical investigations.
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Full length article
Thermal exfoliation of electrochemically obtained graphitic materials ⁎
T
Piotr Krawczyk , Bartosz Gurzęda, Agnieszka Bachar Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, ul. Berdychowo 4, 60-965 Poznań, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphite oxide Reduced graphite oxide Electrochemical oxidation Thermal exfoliation
The present work is devoted to the electrochemical formation and thermal exfoliation of graphite oxide (GO) and reduced graphite oxide (rGO). The main attention was paid on the correlation between cycling processes of GO formation followed by its electrochemical reduction and the stage of their exfoliation. For this purpose a natural graphite was overoxidized in 8 M HClO4 to form GO which was further cathodically reduced yielding rGO. Both processes were performed within one electrochemical process in three-electrode cell using a cyclic voltammetry method. To examine the reversibility of the coupled overoxidation of graphitic matrix and its two-stage reduction the above mentioned operations were three times repeated. The acquired results clearly showed that the properties of GO and rGO significantly change with number of performed cycles. The structure of graphite matrix gradually decreases disenabling the efficient intercalation of perchloric acid during renewed process proceeded during the further cycling. Thermal treatment of electrochemically obtained materials leads to their exfoliation thus contributing to significant modification of structure, morphology and chemical composition. The information on GO properties were provided by the determination of specific surface area, X-ray diffraction analysis (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy analysis (FTIR), transmission electron microscopy (TEM) and scanning electron microscopy observations (SEM).
1. Introduction Graphite oxide is the derivative of graphite that has oxygen atoms intercalated between the graphene layers of graphite lattice [1,2]. The formation of chemicals bonds between the oxygen and carbon atoms of graphene layers cannot proceed directly by oxygenation of graphite. To yield GO the intercalation of oxygen containing intercalate followed by its overoxidation should be performed. Therefore the synthesis of GO involve steps in which graphite intercalation compounds (GIC) is formed that is further transformed into the GO being the most oxidized form of graphite. GO has been prepared by various approaches of strong oxidation realized by chemical [3–6] as well as electrochemical routes [7–13]. Among the commonly used chemical methods of GO preparation are Hummers method and the methods based of its modifications [5,6,14]. Compared to the electrochemical overoxidation of graphite, Hammers methods contributes to the formation of GO of a higher oxidation stage. On the other hand, electrochemical routs are highly controllable and much more attractive from the economical as well as ecological point of view. Contrary to the chemical overoxidation of graphite, electrochemical methods do not require strong oxidants neither necessity of by-products utilization [9–13]. The structure as well as properties of the electrochemical synthesized GO are influenced by the
⁎
employed conditions [7–13]. From the practical point of view the desiring feature of GO is high oxidation degree guaranteeing the greater separation of graphene layers. This effect is highly suitable for the synthesis of graphene materials. One of the commonly used method of graphene preparation is based on the reduction of GO [15–18]. The requirements for appropriated graphene synthesis from GO are the highest possible oxygen release and the preservation of graphene layers from their damaging. GO can be reduced yielding redGO by its thermal [15–18], chemical [19–22] and electrochemical treatment [23–26]. The features of final product are largely dependent upon the particular synthetic route employed. The commonly used thermal reduction proceeds at elevated temperatures (around 1000 °C) under different atmospheres (air, argon, hydrogen). Apart of oxygen release thermal reduction often yield the volume increment due to GO exfoliation. Unfortunately it can be accompanied by the disordering of graphite structure [16,17,27]. However, the above fact does not exclude the use of an electrochemical exfoliation process for obtaining of graphene structures [28–32]. It is known that owing to the electrochemical exfoliation it is possible to yield product of a decreased amount of oxygen and low-defect graphene material. On the other hand chemical reduction of GO requires strong reducers such as hydrazine, borohydride or selected organic acids (e.g.
Corresponding author. E-mail address: [email protected] (P. Krawczyk).
https://doi.org/10.1016/j.apsusc.2019.03.154 Received 15 November 2018; Received in revised form 12 March 2019; Accepted 16 March 2019 Available online 18 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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ascorbic) which residues remain in the reaction mixture as a waste or by-product [19,20]. Another side effect of chemical reduction refers to the replacing of oxygen functionalities by another one that involves other heteroatoms, for example N [19–21]. One of our last work concerning the electrochemical overoxidation of graphite in perchloric acid have showed that it is possible to prepare GO that can be further thermally reduced yielding graphene material of a diminished oxygen content [11]. No information was given about the joined reduction of GO involving electrochemical and thermal process as well as the reversibility of these processes. From our previous investigations it is known that the coupled processes of intercalation and exfoliation can be successfully repeated [33]. The knowledge in this issue related to GO reduction is completed by our present work. The purpose of this work was to study the properties of graphite oxide affected by its cyclic reduction and thermal exfoliation. Electrochemical formation and reduction of GO were carried out in perchloric acid using cyclic voltammetry technique. The electrochemical results obtained for GO, electrochemically reduced GO as well as for exfoliated GOs were discussed in relation to the results acquired from the specific surface measurements as well as XRD, FTIR analysis and SEM observations.
Fig. 1. Cyclic voltammograms recorded during the oxidation of natural graphite in 8 M HClO4. Scan rate 0.01 mV/s, scan range −0.4 ↔ 1.6 V.
investigated using a scanning electron microscopy (S3400 N Hitachi) whereas transmission electron microscopy (Jeol 1200 EX2) was used to examine the size of graphene layers within the GO. Chemical composition of the synthetized GO was estimated on the basis of FTIR measurements (Jasco FTIR-6700 spectrometer) using the KBr technique. The specific surface area of prepared GOs was gained from the N2 adsorption isotherms measured at 77 K with ASAP 2010 apparatus.
2. Experimental 2.1. Material preparation Precursor used for preparation of all investigated samples was natural graphite (purity 99.5%, flake size 170–283 μm). Graphite oxide (GO) and electrochemically reduced GO (redGO-1c and redGO-3c) are the products of successive steps of electrochemical reaction of graphite with 8 M HClO4 (purity 71%) realized by cyclic voltammetry method. Electrochemical investigations were performed with scan rate 0.01 mV/ s within the potential range −0.4 ↔ 1.6 V starting from the rest potential of electrode (ER) towards the higher potentials. In three-electrode cell used for this purpose graphite was a working electrode, platinum wire (purity 99.9%, 1 mm in diameter) was playing a role of counter electrode, whereas Hg/Hg2SO4/1 M H2SO4 served as a reference electrode. During the first cycle when the electrode reached 1.6 V the measurement was interrupted due to formation graphite oxide. Electrochemically reduced graphite oxides were obtained by cyclic anodic oxidation of graphite up to 1.6 V followed by the cathodic reduction spreading to the potential of −0.4 V. All electrochemical measurements were carried out using an Autolab PGSTAT 302N potentiostat/galvanostat. More details on electrode preparation can be found in our previous works [10–13]. After the finishing the electrochemical synthesis the products were prepared for the further investigations by washing with distilled water until filtrate became neutral and finally drying in air. Thermal treatment of graphite oxide (GO), electrochemically reduced GO (redGO-1c and redGO-3c) yielded exfoliated graphite oxide (ex-GO), exfoliated reduced graphite oxides (exredGO-1c and ex-redGO-3c), respectively. Thermal process was performed in a muffle furnace for 1 min at 700 °C. A ceramic crucible filled with the examined sample was promptly inserted into the hot muffle furnace. Owing to this the considered process had a shocked character. After the treatment, the as prepared RGO was rapidly removed from the furnace and cooled in air until the room temperature was reached.
3. Results and discussion 3.1. Electrochemical formation of graphite oxide based materials Cyclic voltammograms depicting the electrochemical formation of the GO as well as its reduced forms redGO-1c and redGO-3c are shown in Fig. 1. After starting the measurement from the rest potential of electrode towards the more positive potentials the process of HClO4 intercalation into the graphite flakes begins. Anodic oxidation of graphite contributes to its oxidation yielding C+. On further polarization ClO4− ions start to penetrate the interlayer space of graphite lattice and participate in formation of ionic bonds with oxidized surface of graphene layers thus forming graphite intercalation compounds with perchloric acid (HClO4-GIC) [11]. During the first cycle the above mentioned process is illustrated by the number of small peaks spreading over the potential range 0.6 and 1.2 V (green line). These peaks depict the successive filling the interlayer spaces with intercalate yielding GIC of a lower stages of intercalation. Stage of intercalation is defining as a number of graphene layers isolating the two neighbored layers of intercalate. The huge anodic peak with maximum at 1.27 V (peak A1) illustrates the overoxidizing of HClO4-GIC yielding graphite oxide (GO). During the experiment in which GO was prepared for analysis process of graphite overoxidation was interrupted at 1.6 V and the obtained material was taken out from the electrochemical cell for analysis. Whereas to obtain a electrochemically reduced graphite (red-GO) the process was continued during the backward scanning. Cathodic reduction of the previously prepared GO resulted in formation a cathodic peak with maximum positioned at 0.06 V (peak C1). This peak is attributed to the transformation of GO into the electrochemically reduced graphite oxide (redGO-1c). The charge density of peak A1 is very closed to that noted for C1 meaning that the charges required for graphite transformation into GO is very similar to that needed to its reduction to form redGO (Table 1). Cathodic reduction of GO ends when the electrode reached the potential of −0.4 V. In order to examine the possibility of cycling reduction of GO the whole process was performed through the 2
2.2. Material characterization The structure of the synthetized graphitic materials was examined by (XRD) analysis which was performer on PANalytical diffractometer using Cu Ka radiation (1.54 Å) with 2Ɵ scan range from 4 to 60° and step size of 0.04°. Structure and the amount of surface defects were evaluated on the basis of Raman spectroscopy. The acquired spectra were recorded on inVia Renishaw micro-Raman system with an argon laser, emitting 514.5 nm wavelength. Morphological properties were 467
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Table 1 Charge density and location of anodic and cathodic peaks assigned to graphite oxide formation and its reduction, respectively. Peak A1 C1 A2 C2 A3 C3
Charge density [C g−1]
Peak maximum [V]
3663.8 3689.2 4829.8 1754.8 4002.3 970.2
1.27 0.06 1.18 0.05 0.92 0.02
Table 2 Raman spectra analysis for graphite, GO, electrochemically reduced GO and exfoliated GO. Sample
Graphite GO exGO redGO-1c ex-redGO-1c redGO-3c ex-redGO-3c
Band position [cm−1]
ID/IG
D band
G band
D′ band
2D band
D + G band
1356 1350 1364 1357 1357 1352 1357
1582 1591 1582 1585 1584 1591 1594
– – 1620 1623 1624 1619 –
2729 2709 2728 2723 2725 2718 2710
– 2933 – 2953 2953 2947 2955
0.27 1.13 0.87 1.36 1.13 1.52 1.44
evidencing that the properties of electrode material are also changed. However the intensity of cathodic peaks attributed to the reduction of GO (peak C2 and C3) markedly decreases but their location remains almost unchanged during second and third cycle. By comparing the peak charges for the successive cycles it is seen that anodic ones are significantly higher thus evidencing the worsening of reaction reversibility. The high discrepancy in charge densities between the anodic and cathodic peaks indicates that the participation of reaction yielding GO within the anodic peaks decreases on cycling. Taking into account assumption that cathodic peaks represent reactions of GO reduction, the abrupt decrease in their charge evidences the less amount of GO being reduced. It means that less amount of graphite oxide is formed during the previous anodic overoxidation of graphite. On the other hand the increased charge of anodic peaks due to cycling overoxidation can be explained by the increased participation of side reactions associated with the oxidation of demanded graphite matrix yielding surface functionalities. It is known that to perform intercalation the ordered structure of the graphitic material is required [33,34]. If graphene layers become damaged and bent, the intercalation as well as process of GO formation can be disturbed. It cannot be excluded that during the process of cycling overoxidation followed by cathodic reduction the structure of graphite is modified.
Fig. 2. XRD patterns of graphite, GO, electrochemically reduced GO and exfoliated GO.
3.2. The characterization of structure of GO based materials The structure of GO as well as its reduced form due to cycling treatment of graphite in 8 M HClO4 can be elucidated on the basis of XRD measurements. XRD patterns for all investigated samples are shown in Fig. 2. In order to facilitate the comparison of structural changes Fig. 2 also involves XRD pattern of original graphite with characteristic signal at 26.5° attributed to the graphitic structure. On the pattern of graphite and GO one can observe a diffraction peak at 14.9° evidencing the increment of interlayer space from 3.35 Å for graphite to 5.94 Å. This effect is caused by the incorporation of oxygen functionalities between the graphene layers of graphite matrix [35]. On spectrum for GO also emerges a small signal at 26.5° depicting the residual graphite phase within the formed GO. Thermal treatment of GO led to its transformation into the non-crystalline phase with randomly displaced graphene stacks [3,16,36]. Such a behavior is illustrated by the disappearance of diffraction peaks (sample exGO). Cathodic reduction of the previously formed GO results in further structural changes being associated with the reduction of interlayer space. This effect is illustrated by the formation of diffraction peak at 25.2° (redGO-1c). Thermal treatment of the electrochemically reduced GO contributed to further reduction of average d spacing to 3.59 Å. This is evidenced by the shift of diffraction peak (redGO) to 26.2° (ex-redGO1c). Such a behavior can be explained by the successive release of oxygen from the graphitic matrix which is accompanied by the decrease in interlayer distance of graphite lattice. Taking into account the electrochemical behavior it can be assumed that the process of exfoliation of electrochemically reduced GO acquired during the first cycle (redGO-
Fig. 3. Raman spectra of graphite, GO, electrochemically reduced GO and exfoliated GO.
consecutive cycles and the obtaining products were called as redGO-2 and redGO-3c, respectively. As one can see during the cycling the intensity of recorded peaks significantly decreases. In case of forward scanning, the diminution in activity of GO formation depicted by a lowering the anodic peak is accompanied by its shift towards the less positive potentials. Moreover, for the second and third cycle peaks attributed to GO formation (peak A2 and A3) become much more wider compared to that noted for the first cycle (peak A1). This behavior indicates that the mechanism of anodic overoxidation changes on cycling 468
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Fig. 4. SEM images of GO (a) and electrochemically reduced GO: reGO-1c (b), redGO-3c (c).
Fig. 5. SEM images of exfoliated GO (a) and exfoliated reduced GO: ex-reGO-1c (b), ex-redGO-3c (c).
Fig. 6. TEM images of GO (a) and electrochemically reduced GO: reGO-1c (b), redGO-3c (c).
were also examined using Raman spectroscopy (see Fig. 3). As one can see Raman spectra involve characteristic bands for graphitic materials. The first one, D band, is associated with the defects affected by the presence of sp3 hybridized carbon, oxygen functional groups or edges [37,38]. G band corresponds to the stretching mode of sp2 bonded carbon [39]. Defects of graphite structure are related with the appearance of D′ band [37,40]. Next band denoted as 2D is a second-order of D band. For graphite 2D band consists of two bands [37,39]. D + G band belongs to the second-order of D band and is connected with the presence of disordered structure [41,42]. Raman spectra of GO show that the anodic overoxidation of graphite in HClO4 aqueous solution highly deforms graphitic structure. Intensity of D band increases
1c) most likely differs compared to that for redGO-3c. The main difference is associated with the decrease in GO participation within the product. Contrary, the amount of disordered carbon being the product of multiply electrochemical treatment increases. Therefore during the thermal exfoliation the mentioned products are thermally decomposed thus uncovering the residuals of GO allowing its XRD detection. Sample prepared after cathodic reduction in third cycle (redGO-3c) exhibits non-ordered character, whereas thermal exfoliation of this sample reveals appearance of residual phases of reduced GO (sample ex-redGO3c). Changes in graphitic structure caused by electrochemical treatment of graphite as well as thermal treatment of GO and reduced GO samples 469
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Fig. 7. TEM images of exfoliated GO (a) and exfoliated reduced GO: ex-reGO-1c (b), ex-redGO-3c (c).
consequence, the sp3 hybridized carbon becomes partially rearranged to sp2 hybridized carbon and the ID/IG ratio decreases to 0.87. Along with the increasing number of oxidation-reduction cycles of electrochemical treatment the intensity of D and D′ bands also increases. It can be assumed that despite the cathodic removal of oxygen functionalities from the graphitic structure the process of cyclic electrochemical treatment causes significant deformation and destruction of graphitic structure. As in the case of GO exfoliation, thermal treatment of redGO-1c and redGO-3c causes partial restoration of sp2 hybridized carbon.
Table 3 BET surface area calculated for GO based materials. Sample Graphite GO redGO-1c redGO-3c exGO ex-redGO-1c ex-redGO-3c
Specific surface area [m2 g−1] 0.3 1.2 7.7 25.3 677.8 185.0 28.5
drastically, hence ratio of D band intensity to intensity of G band (ID/IG) also increases as compared to the starting graphite (see Table 2). This effect is undeniably caused by the formation of oxygen functionalities during the electrochemical treatment [10]. Thermal exfoliation of GO leads to the abrupt removal of oxygen functional groups. In
3.3. SEM and TEM observations of graphite based materials The comparison of SEM images recorded for samples GO, redGO-1c and redGO-3c (Fig. 4) allows knowing the changes in morphology caused by electrochemical reduction, while the observation of images
Fig. 8. FTIR spectra of graphite, GO, electrochemically reduced GO and exfoliated GO. 470
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evaluated on the basis of FTIR analysis. The infrared spectra recorded for all investigated samples are shown in Fig. 8. As one can see the modification of GO structure due to its electrochemical reduction and subsequent thermal exfoliation is accompanied by the slight alteration of its chemical composition. From the literature data, it is known that thermal treatment of GO performed in an air atmosphere commonly reduces the amount of epoxy (CeOeC) and alkoxy (CeO) groups [15,36]. In case of the present investigations signals arising from both type of groups disappear only due to thermal treatment of electrochemically reduced GO. For exfoliated GO, the wide band with maximum at 1240 cm−1 is still observed revealing the appearance of CeOeC bonds [15,31]. On the other hand, spectra recorded for electrochemically reduced GO (samples redGO-1c and redGO-3c) involve signal at around 1089 cm−1 that are attributed to CeO bonds of alkoxy groups [15,36]. From the FTIR investigations it is evidenced that electrochemical reduction up to −0.4 V as well as thermal exfoliation of GO materials noticeably reduces the amount of oxygen on within the graphite matrix. The scale of this effect strongly depends on method performed for the GO reduction. Cyclic cathodic treatment of GO decreases mainly epoxy groups whereas thermal exfoliation impacts on the diminution in alkoxy functionalities. As one can see the well shaped peak at 1240 cm−1 evidencing appearance of CeOeC bonds can be observed only for exfoliated GO (sample not treated cathodically). For the rest samples the mentioned peak becomes very small (redGO-1c and redGO-3c) or completely disappear (ex-redGO-1c and ex-redGO-3c). Such a behavior indicates that thermal exfoliation contribute to the removal of epoxy groups from the graphite matrix, whereas electrochemical reduction results only in partial decomposition of the above mentioned functionalities. The observed discrepancy well correlates with the result of XRD investigations. Signals at wavenumber of 850 cm−1 can be associated with the decomposition of graphitic matrix due to electrochemical and/or thermal treatment.
for samples exGO, ex-redGO-1c and ex-redGO-3c provide information on the role of thermal exfoliation in creation of morphological properties (Fig. 5). As can be seen the information provided from the SEM images coincide with that acquired from the electrochemical as well as XRD measurements. The multi repeated electrochemical treatment of GO composed of its overoxidation and reduction modify its structural and morphological properties. SEM image of GO gathered after its cathodic reduction in third cycle have revealed significant decomposition of its structure (Fig. 4c). The damaged graphene layers pertaining to electrochemically reduced GO are randomly positioned. Moreover, the concentration of surface defects and edges is markedly increased. A shock thermal treatment of GO and reduced GO results in their exfoliation accompanied by the huge deformation of graphene layers (see Fig. 5) due to abrupt eruption of gaseous products (CO, CO2) of thermal decomposition of oxygen functionalities pertaining to GO. By comparing the images of exfoliated GO it can be assumed that the scale of surface deformation depends on the sample subjected to thermal treatment. The highest effects of exfoliation stage are seen for the thermally treated GO (Fig. 5a). The further analysis of carbon phases within the electrochemically reduced and thermally exfoliated graphite oxides were performed by TEM observation. Fig. 6 involves TEM images of GO and electrochemically reduced GO, whereas Fig. 7 depicts images for their exfoliated forms. The presented TEM images confirm the SEM observations that the structure of GO underwent electrochemical reduction is worsened. Carbon layer becomes shorter. On the other hand the transparency of graphene layers is improved compared to GO (Fig. 6a) which may indicate their partial delamination. It seems that much higher level of delamination of graphene layers is yielded by thermal exfoliation (see Fig. 7). Due to shock thermal treatment of graphene layers of exfoliated GO become thinner. The above mentioned effect is accompanied by the increase in concentration of edges and surface defects.
4. Conclusions 3.4. Measurements of BET surface area Our investigations showed that thermal treatment of GO based materials results in significant changes in their structural as well as morphological properties. The scale of these modifications is strongly influenced by the type of material subjected to exfoliation. Taking into account the development of specific surface area as a main criterion of evaluation of structural changes it can be stated that the most pronounced effects of thermal treatment were gathered for exfoliated GO. It is also proved that the repeated cyclic electrochemical treatment involving overoxidation and subsequent cathodic reduction of GO leads to the worsening of structural ordering. It cannot be excluded that during the cycled oxidation-reduction of GO the electrochemical exfoliation occurs. FTIR investigations have revealed that the changes in oxygen functionalities on GO surface depend on the type of applied reduction treatment. The cathodic treatment of GO contributed to modification in the amount of alkoxy functionalities whereas the thermal reduction impacts on the appearance epoxy functionalities.
From the results of calculations of BET surface area (Table 3) it is clear that the visible development of specific surface area is taking place due to cyclic electrochemical treatment of GO. The BET surface area of sample after its repeated overoxidation and cathodic reduction (reGO-3c) is very closed to that noted after thermal exfoliation (exredGO-3c). This result is in coincidence with the observation of SEM images evidencing the advanced decomposition of graphene layer within the cathodically reduced GO (see Fig. 4c). It justify the assumption that during the multistep electrochemical treatment of GO in HClO4 the electrochemical exfoliation likely occurs. It is worth to note that the specific surface area of redGO-3c is very closed to that commonly noted for exfoliated graphites [33]. On the other hand, the most pronounced development of surface area is gathered for thermally exfoliated GO. BET surface area of exGO over 3.5 times exceeds that calculated for the exfoliated samples beforehand subjected to cathodic reduction (ex-redGO-1c). This effect can be explained by the fact that during the successive cycles of electrochemical treatment the highly ordered graphitic structure becomes worsened. The gradually decrease in GO phase within the multi reduced material results in diminution in exfoliation stage. It is widely known that the exfoliation effects is strongly dependent on the amount as well as distribution of intercalate [33]. The results of BET calculations confirm the statement of electrochemical investigations.
Acknowledgements This work was financially supported by the National Science Centre of Poland (2015/17/B/ST8/00371). References [1] T. Nakajima, Y. Matsuo, Formation process and structure of graphite oxide, Carbon 32 (1994) 469–475. [2] O.C. Compton, S.T. Nguyen, Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials, Small 6 (2010) 711–723. [3] H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera- Alonso, D. H. Adamson, R. K. Prud'homme, R. Car, D. A. Saville and I. A. Aksay, Functionalized single
3.5. FTIR measurements of graphite based materials The changes in chemical composition of GO surface due to its electrochemical reduction followed by thermal exfoliation can be 471
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