- Email: [email protected]
Preparation of sulfur-doped graphite by solid-state microwave method: The effect of reaction conditions on the sulfur-doping process
Chemical Physics Letters 731 (2019) 136615
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Preparation of sulfur-doped graphite by solid-state microwave method: The effect of reaction conditions on the sulfur-doping process ⁎
Fan Lia, Wanjun Xub, Linyi Lua, Kai Zhouc, , Zhining Xiaa,b,
T
⁎
a
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, China School of Pharmaceutical Science, Chongqing University, Chongqing 401331, China c Analytical and Testing Center, Chongqing University, Chongqing 401331, China b
H I GH L IG H T S
solid-state microwave method was proposed and optimized for the preparation of SGi. • ATherapid (CeSeC) was the preferential conformation of sulfur in SGi. • The thiophene-S • dosage of H SO used in intercalation reaction determined whether S-doping achieve or not. 2
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfur-doping Carbon materials Solid-state microwave Intercalation reaction Large-scale production
The high-quality sulfur-doped graphite (SGi) was prepared from concentrated sulfuric acid-graphite intercalation compounds (H2SO4-GICs) by solid-state microwave irradiation, where H2SO4 was the sulfur (S) source and S-doping occurred within a minute. The preparation of H2SO4-GICs was a key step for S-doping and exfoliation process. Notably, it was found that S-doping was influenced by the dosage of H2SO4, where the lower dosage of H2SO4 was beneficial for the S-doping. The exfoliation efficiency was greatly affected by the temperature and dosage of oxidant, where lower temperature and lower dosage of KMnO4 guaranteed efficient exfoliation.
1. Introduction Sulfur-doped graphene (SG) has attracted a great attention owing to its outstanding properties and potential applications. For example, SG has been demonstrated as potential metal-free catalysts for oxygen reduction reaction which is the key process in fuel cells [1]. Up to now, several typical methods have been proposed for the synthesis of SG, mainly including chemical vapor deposition, solvothermal methods and thermal-annealing graphene oxide (GO) with dopant precursors (such as H2S, SO2, CS2, and L-cystine) under high temperature [2–4]. Among these, chemical vapor deposition methods are usually developed to synthesize large, continuous and defect-free high-quality SG, but they are not suitable for mass production. Although post-treatment of GO is a strategy to realize mass production of SG, the as-prepared products have poor quality because of the high defect density caused by harsh oxidation of graphite. Furthermore, high vacuum or high temperature conditions are usually inevitable among these methods. Thus, these methods are not suitable for the large-scale production of high-quality SG due to the high demand of equipment and low yield of products. As
large-scale production of high-quality SG is the key step to the practical application of SG in many fields, new synthesis method need to be developed. Many efforts are ongoing to largely prepare high-quality graphene and its derivatives. Among these, microwave-assisted methods have been proved to be very efficient methods [5–7]. For example, it was reported that a large quantity of high-quality graphene could be fabricated by solid-state microwave method using graphite as catalyst. During the microwave irradiating, exfoliation and reduction of graphite oxide simultaneously achieved in ambient air [5]. Furthermore, the solid microwave exfoliated graphite oxide was of higher quality than microwave-assisted solvothermal reduced GO, including a much higher exfoliation degree and higher specific surface area, a much larger C/O ratio, and higher conductivity [5,8,9]. As solid-state microwave method do not require expensive instruments, and it can easily increase the temperature of graphite to over 1000 °C with plasma generation which was beneficial for the heteroatom-doping process [5,7], it shows great potential for mass production of high-quality SG. In our previous work, we have successfully synthesized SGi from
⁎ Corresponding authors at: School of Chemistry and Chemical Engineering, School of Pharmaceutical Science, Chongqing University, Chongqing 401331, China (Z. Xia). E-mail addresses: [email protected] (K. Zhou), [email protected] (Z. Xia).
https://doi.org/10.1016/j.cplett.2019.136615 Received 28 May 2019; Received in revised form 12 July 2019; Accepted 16 July 2019 Available online 16 July 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
microwave exfoliation of dried H2SO4-GICs, which could be easily exfoliated to monolayer SG by mild solution-phase sonication [10]. Briefly, the dopant (H2SO4) was intercalated into the interlayer of graphite via a chemical oxidation intercalation reaction thereby forming the stable H2SO4-GICs. During microwave irradiation, SGi formed with an obvious increase in volume due to violent gas-releasing. As H2SO4-GICs was the precursor of graphite oxide, this strategy seems to be similar to the microwave exfoliation of graphite oxide [5,11,12], but for graphite oxide, the S atom would be totally removed when graphite oxide was heated to 900 °C and there was no S atom doping into the backbones of carbon sheet via microwave exfoliation of graphite oxide [13]. Furthermore, in our previous study, we also found that some of H2SO4-GICs can achieve S-doping while others could not, and the reason for this phenomenon remained unclear. Therefore, in this study, the intercalation reaction conditions and microwave irradiation conditions were investigated to explore the factors which affected the S-doping process, as well as the reduction and exfoliation process. The X-ray photoelectron spectrometer (XPS) and X-ray diffractometer (XRD) characterization results of H2SO4-GICs and SGi were mainly selected as the evaluating indicators. The effects of reaction temperatures, dosage of oxidant (KMnO4) and dopant (H2SO4), as well as microwave irradiation time on the exfoliation efficiency and S-doping level were discussed in detail. Figuring out the reason why graphite oxide and some H2SO4-GICs could not achieve S doping during microwave exfoliation was of great significance on other doping methods relating with intercalation process. Furthermore, clarifying the factors which affected the exfoliation efficiency and S-doping process can further deepen the significance of producing SGi and SG in industrial quantities.
Table 1 Influence of intercalation reaction conditions on the exfoliation efficiency.
2. Experiments
The morphology and microstructure of the as-synthesized H2SO4GICs and SGi were investigated by SEM, Raman spectrometer and XRD. The elemental compositions and chemical status of elements in H2SO4GICs and SGi was investigated by XPS. The loading amount of dopants in H2SO4-GICs and the S-doping level was expressed by the content of oxidized-S and thiophene-S (eCeSeCe) measured by XPS, respectively. The expanded volume (EV) was used as an indicator to evaluate the exfoliation efficiency [15] and it was calculated as a quotient of the volume of SGi and the mass of H2SO4-GICs, therefore having a unit of volume/mass (mL/g). All values of EV were collected from three independent measurements for each sample and presented as average and standard error.
#Sample
Mixture ratioa A:B:C
Temperature (°C)
Expanded volume (mL/g)
T1-1 T1-2 T1-3 T1-4 T1-5 T1-6 T1-7 T1-8 T1-9 T1-10 T1-11
1:2:40 1:6:40 1:4:40 1:1:20 1:1:20 1:1:20 1:2:20 1:3:20 1:1.5:10 1:1:10 1:0.5:10
30 17 0 0 17 30 30 30 0 30 17
5 3 2 162 144 118 25 3 171 204 122
a
A-graphite powder (g); B-KMnO4 (g); C-H2SO4 (mL).
Table 2 Influence of intercalation reaction conditions on the S-doping level. #Sample
Mixture ratioa A:B:C
Temperature (°C)
S-doping level (at. %)
T2-1 T2-2 T2-3 T2-4 T2-5 T2-6 T2-7
1:2:20 1:2:16 1:2:12 1:2:8 1:1.5:8 1:1:8 1:0.5:8
0 0 0 0 0 0 0
0.00 0.03 0.51 0.88 0.94 0.90 0.78
a
A-graphite powder (g); B-KMnO4 (g); C-H2SO4 (mL).
2.4. Characterization
2.1. Reagents and apparatus Graphite powder (2000 mesh) was purchased from Aladdin Reagents (Shanghai, China). The rest reagents such as H2SO4, KMnO4, 30% hydrogen peroxide (H2O2) solution and hydrochloric acid (HCl) were of analytical grade and were purchased from KeLong Chemical Co. Ltd. (Sichuan, China). The microwave irradiation was conducted by a microwave oven (LWMC-205, LingJiang Technology Development Co. Ltd., Nanjing, China). The used characterization techniques were scanning electron microscope (SEM) (JSM-7600F, JEOL Ltd., Tokyo, Japan), XPS (ESCALAB250Xi, Thermo Fisher Scientific, Waltham, USA), XRD (X’Pert Powder, PANalytical, Almelo, The Netherlands) and Raman spectrometer (LabRAM HR Evolution, Horiba Jobin Yvon Co. Ltd., Paris, France).
3. Results and discussion 3.1. Characterization of H2SO4-GICs and SGi
2.2. Preparation of H2SO4-GICs using different intercalation reaction conditions
Fig. 1 displayed the SEM images of H2SO4-GICs and SGi, respectively. For H2SO4-GICs (Fig. 1a), the carbon sheets were in a stacking structure, indicating that the interlayer spacing of graphite did not change obviously during the intercalation reaction [16]. In contrast, for SGi, a loose porous network was demonstrated as shown in Fig. 1b, which hinted that the stacking carbon sheets were expanded by releasing gaseous species during microwave irradiating H2SO4-GICs. This loose porous network was beneficial for the preparation of monolayer SG [15]. The XPS spectra of H2SO4-GICs and SGi (Fig. 2a) both showed the presence of C, O and S atoms at ~284.6, ~164.2 and ~533.2 eV, but the relative content of them were different. For H2SO4-GICs, the content of S and O were ~1.85 at.% and ~18.7 at.%, indicating that H2SO4 molecules has been intercalated into graphite, as well as that many oxygen-containing functional groups has been introduced into the bulk graphite. However, for SGi, both of the content of S and O decreased, which was due to the thermal decomposition and releasing of H2SO4, as
The preparation of H2SO4-GICs was conducted by an oxidant-assisted intercalation reaction [14], and H2SO4-GICs with different microstructures were prepared using different dosage of KMnO4 and H2SO4, as well as different reaction temperatures. Tables 1 and 2 detailed the intercalation reaction conditions, and the detailed operations were given in Supporting Information. 2.3. Preparation of SGi via different microwave irradiation time The as-prepared dried H2SO4-GICs were placed in an open reactor located in a microwave oven. The SGi was obtained by treating the H2SO4-GICs at 800 W microwave irradiation for several seconds (5 s/ pulse, 1–10 cycles). Pulsed irradiation (5 s/pulse) was applied to investigate the change of the content and chemical status of each element in SGi at different irradiation time. 2
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
Fig. 1. The SEM images of (a) H2SO4-GICs and (b) SGi.
to that oxidized-S could be transformed into thiophene-S at high temperature [4]. Thus, S atoms could incorporate into carbon network through the solid-state microwave irradiation. The high-resolution O1s spectrum (Fig. 2d) of H2SO4-GICs showed a main peak at 531.6 eV corresponding to hydroxyl, while the O1s spectrum of SGi could be fitted with two main peaks at ~531.7 and ~533.9 eV corresponding to hydroxyl and adsorbed water, respectively [20]. The appearance of the peak at ~533.9 eV was attributed to the formation of H2O during the process of the H2SO4 decomposition when H2SO4-GICs was treated by microwave irradiation [21]. The Raman spectrum of graphite exhibited a highly ordered graphitic Raman feature with a sharp and symmetrical G-band (~1569 cm−1) and a nearly absent D-band (~1344 cm−1) [6], while a new D′-band appeared at ~1619 cm−1 in the Raman spectrum of
well as the microwave-induced deoxygenation [5]. The high-resolution C1s spectra of H2SO4-GICs (Fig. 2b) showed higher intensity of peaks at ~286.7 eV and ~288.1 eV compared with that of SGi, indicating that the H2SO4-GICs contained abundant oxygencontaining groups [17]. In the high-resolution S2p spectra (Fig. 2c), H2SO4-GICs showed only two peaks at ~167.9 and ~169.0 eV, which were corresponding to the spin-orbit coupling of oxidized-S (CeSOxeC) [18]. The oxidized-S was attributed to the intercalated sulfate species (ISSs) (H2SO4 or HSO4−) [19]. Therefore, it demonstrated that S atoms could not incorporate into carbon network during the intercalation reaction. For SGi, two new peaks appeared at ~162.9 and ~164.2 eV, which were attributed to the spin-orbit coupling of thiophene-S (CeSeC) [18], and the peak intensity of oxidized-S decreased. This phenomenon was due
Fig. 2. (a) The XPS spectra of H2SO4-GICs and SGi; (b)–(d) The high-resolution C1s, S2p, O1s spectra of H2SO4-GICs and SGi. 3
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
exfoliation process involved the thermal decomposition of the ISSs, subsequently gas generation and violently gas releasing [11]. Nearly all of the in H2SO4-GICs could thermally decompose into gaseous species (SO2, O2 and H2O) [21]. Thus, the quantity of gaseous species was directly correlated with the loading amount of ISSs, which means that the higher S loading amount in H2SO4-GICs would produce larger volume of gas, consequently resulting in higher EV. Therefore, the S loading amount of H2SO4-GICs was also a factor influenced the exfoliation. However, the correlation between the S loading amount and the EV showed a weak linearity (R2 = 0.5195), which might be due to the fact that the decomposition of oxygen-containing functional groups would also produce abundant gaseous species [25]. Moreover, the exceptions existing in both Fig. 4b and c need to be discussed in detail. As shown in Fig. 4b, the C/O ratio of #T1-11 was as high as ~5.8, but its EV (~120 mL/g) was lower than that of #T1-10 (~200 mL/g), whose the C/O ratio was ~4.5. This phenomenon violated the rule that EV was positive correlated with the C/O ratio. However, from the point of view in S loading amount, #T1-11 (~0.70 at.%) had much lower S loading amount than #T1-10 (~1.75 at.%), which was conformed to the general relationship between EV and S loading amount. Thus, this exception in Fig. 4b indicated that when the C/O ratio was high enough, the S loading amount was the main factor determined the exfoliation efficiency. In Fig. 4c, exceptions came from these H2SO4-GICs with C/O ratio less than 2.38. The S loading amount of them was high over ~1.8 at.%, but their EV sharply decrease. This result demonstrated that when the H2SO4-GICs was intensively oxidized and lost the intact layered structure needed for microwave exfoliation, the S loading amount no longer worked for the exfoliation efficiency. Additionally, Fig. 4d showed that the S loading amount was inversely correlation with value of C/O ratio. This result was due to that higher content of oxygen-functional groups was beneficial to broaden the spacing of carbon layers, resulting in more sulfate species moving cross and binding on the sheets. At the same time, when more sulfate species intercalated in the carbon sheets, the interlayer spacing expanded further, and consequently more oxidant came into the interlayers, resulting in excessive oxidation [19]. In other words, the C/O ratio and S loading amount were not two independent factors, and they mutually restricted each other. Therefore, to balance the relationship between C/O ratio and S loading amount would be a very important step for efficient exfoliation. In summary, the exfoliation efficiency was influenced by not only the loading amount of ISSs but also the integrity of the layered structure (C/O ratio). Considering the relationship between experimental results and reaction conditions (Table 1), it could be roughly concluded that the lower reaction temperature was beneficial for the exfoliation efficiency, and 0.5 < KMnO4:graphite < 1.5 could achieve efficient exfoliation at these investigated reaction temperatures.
Fig. 3. The Raman spectra of graphite, H2SO4-GICs and SGi.
H2SO4-GICs, which was attributed to “boundary layer” in graphite planes adjacent to layer of the intercalant [22] (Fig. 3). Thus, the appearance of D′-band suggested the successful intercalation of H2SO4. After the microwave treatment of H2SO4-GICs, the D′-band disappeared, suggesting that the intercalant has been eliminated. The increase of ID/IG ratio in H2SO4 and SGi demonstrated the increase of disordered domains [23], suggesting that defect could be formed during the oxidant-assisted intercalation reaction and microwave treatment [19]. 3.2. Effects of reaction conditions on the exfoliation, reduction and Sdoping process The preparation of SGi involved two steps. The first step is the preparation of H2SO4-GICs via an oxidant-assisted intercalation reaction. In this step, the dosage of KMnO4 and H2SO4 as well as the reaction temperature would influence the properties of H2SO4-GICs, which tremendously related to the properties of subsequent product (SGi). The second step is microwave irradiating H2SO4-GICs to form SGi, where exfoliation, reduction and S-doping could achieve simultaneously. During microwave irradiation, H2SO4-GICs with different properties would become different SGi with various C/O ratio, exfoliation efficiency and S-doping level. In this part, we investigated various reaction conditions to figure out how they influenced the properties of products. 3.2.1. Effect on the exfoliation efficiency During microwave irradiation, the Van der Waals forces between carbon sheets would be weaken by violently gas releasing, resulting in a remarkable volume increase of H2SO4-GICs. The EV was used as an indicator to evaluate the exfoliation efficiency. As shown in Fig. 4b, there was a positive correlation between EV and C/O ratio of H2SO4GICs, except for sample #T1-11. Generally, the EV increased with the increase of C/O ratio, suggesting that the less oxygen-functional groups on carbon sheets, the higher EV would be reached. Furthermore, when the C/O ratio of H2SO4-GICs were below ~2.38 (Fig. 4a), these H2SO4GICs could not be efficiently exfoliated. These inefficient exfoliations were due to the excessive oxidant and high temperature in the intercalation reaction which caused the intensive oxidation and tearingdown on the layered structure of graphite [24]. Therefore, a relatively intact layered structure (higher C/O ratio) would be needed to ensure the efficient microwave exfoliation. The relationship between EV and S loading amount of H2SO4-GICs was demonstrated in Fig. 4c. By eliminating those H2SO4-GICs whose C/O ratio was below ~2.38, there was a positive correlation between the EV and S loading amount. The reasons were analyzed as follows: the
3.2.2. Effects of microwave irradiation time During microwave irradiation, the exfoliation, reduction and Sdoping occurred simultaneously. In this part, the effects of microwave irradiation time were investigated. Firstly, the EV of H2SO4-GICs showed an apparent increase within the former two cycles of pulsed microwave irradiation, and in the subsequent cycles of irradiation, the EV was no obvious change. During the ten cycles of irradiation, the content and chemical status of S in H2SO4-GICs were changing. In Fig. 5a, for the initial H2SO4-GICs, all of the S bonding configurations were oxidized-S, owing to the sorption of ISSs on the carbon sheets [18]. After microwave irradiation for only two cycles, the peaks of oxidized-S obviously decreased and new peaks corresponding to thiophene-S appeared. With the increase of irradiation cycles, the peak corresponding to oxidized-S decreased gradually, and finally the oxidized-S preserved only ~15% of the total S content after 10 cycles (Fig. 5b). In Fig. 5b, the content of thiophene-S had a sharp increase within the former two cycles and then slightly 4
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
Fig. 4. (a) The EV, C/O ratio and S loading amount of H2SO4-GICs listed in Table 1; (b) The relationship between EV and C/O ratio; (c) The relationship between EV and S loading amount; (d) The relationship between S loading amount and C/O ratio.
of C/O ratio among other graphene and its derivatives prepared by post-treatment approaches [5,8,27]. The highest value of C/O ratio might be due to the preservation of the relatively intact π-system in H2SO4-GICs compared with graphite oxide [19], and the large unoxidized graphitic region in H2SO4-GICs can act as the microwave acceptor to initiate the microwave-induced deoxygenation [5,28]. Therefore, this result suggested that solid-state microwave irradiating H2SO4-GICs had potential to prepare high-quality SGi.
increased in the residual cycles. Notably, the total content of S decreased during microwave irradiation (Fig. 5b), which means that the number of S atoms was not conserved, suggesting that nearly 50% of S atoms escaped from H2SO4-GICs during the microwave irradiation. After ten cycles of microwave irradiation, oxidized-S was nearly vanished and thiophene-S was the main bonding configurations of S in SGi. As thiophene-S was known as one of the most effective active sites in the electrocatalytic reaction, the final products might probably exhibit enhanced electrocatalyst performance [20,26]. Although the microwave-induced deoxygenation has been reported by other researches [5], it was worthy to note that the C/O ratio of the final SGi (10 cycles), was as high as 21.2, which was the highest value
3.2.3. Effect on the S-doping process The S-doping level of SGi resulting from various dosage of H2SO4 and KMnO4 were demonstrated in Fig. 6 and Table 2. When the dosage
Fig. 5. (a) The high-resolution S2p spectra of H2SO4-GICs treated by different cycles of microwave irradiation; (b) The effect of microwave irradiation time on the content and bonding configurations of S. 5
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
Fig. 6. The C/O ratio and S loading amount of H2SO4-GICs listed in Table 2 as well as their S-doping levels after microwave irradiation. (a) The effect of the dosage of H2SO4; (b) The effect of the dosage of KMnO4.
Fig. 7. (a) The high-resolution S2p spectra and (b) high-resolution C1s spectra of H2SO4-GICs treated by different dosage of H2SO4.
3.3. Mechanism of S-doping
of KMnO4 was the same, the S-doping level increased with the decrease of dosage of H2SO4 (Fig. 6a). In the high-resolution S2p spectrum of #T2-1 SGi (Fig. 7(a), top), no peaks appeared at the binding energies from 164 to 170 eV, indicating that no S atoms retained after microwave irradiation. Therefore, H2SO4-GICs treated by 20 mL H2SO4 (#T21) could not dope S atoms into the carbon planes. In contrast, when the dosage of H2SO4 decreased to 8 mL, the peaks at ~165.4 and ~164.2 eV corresponding to the spin-orbit coupling of thiophene-S appeared (Fig. 7(a), bottom), suggesting that lower dosage of H2SO4 was beneficial for the S-doping process. Smaller dosage of H2SO4 was not further investigated, because lower volume of H2SO4 was not large enough to immerse the graphite powder. Above all, the dosage of H2SO4 was a key factor to determine whether the S-doping achieve or not and lower dosage of H2SO4 was beneficial for the high S-doping level. Additionally, the S-doping level also affected by the dosage of KMnO4. As shown in Fig. 6b, while keeping the dosage of H2SO4 at a same level, the S-doping level first increased and then decreased with the decrease of KMnO4. The first tendency of increase might be due to preventing the adverse effect of relative higher degree of oxidization, where the hole-like defects might lead to the easy loss of dopants. In contrast, the subsequent decrease of S-doping level was due to the decrease of ISSs loading amount, which caused by the decrease of KMnO4 [19]. However, the S-doping level changed very slightly with different dosage of KMnO4, suggesting that the dosage of KMnO4 was not a major factor for S-doping, compared with the dosage of H2SO4.
According to the discussion above, the S-doping was greatly affected by the dosage of H2SO4, but how it influenced the S-doping was still unknown. In this part, we aimed to figure out the mechanism of Sdoping, which might have referential value to other heteroatom-doping methods involving intercalation reaction. In Fig. 6a, the dosage of KMnO4 remained unchanged, however, the C/O ratio increased with the decrease of the dosage of H2SO4. The highresolution C1s spectra of these H2SO4-GICs also exhibited that high dosage of H2SO4 can promote the oxidation of carbon sheets. In Fig. 7(b), two intense bands at 284.7 and 286.9 eV were corresponding to the graphitic C]C species and CeO species [8] and the CeO/C]C ratio, calculated from the quotient of their peak area, decrease with the decrease of the dosage of H2SO4. As the low dosage of H2SO4 caused low CeO/C]C ratio and high Sdoping level, it seems that low oxidation degree (high C/O ratio) guaranteed the success of S-doping, but that was not the case. Because the C/O ratio of #T1-4, #T1-5, #T1-6 were higher than that of #T2-4 (3.5–4.5 v.s. 2.9), but no S atoms doped into #T1-4, #T1-5, #T1-6. Thus, the degree of oxidation was not the fundamental factor, and the dosage of H2SO4 must influence other properties of H2SO4-GICs, which determined the achievement of S-doping. XRD was applied to investigate the microstructures of H2SO4-GICs, which might affect the S-doping process. The XRD pattern corresponding to #T2-1 H2SO4-GICs was different from that of #T2-4 H2SO46
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
where lower temperature (0 °C) and moderate dosage of KMnO4 (0.5 < KMnO4:graphite < 1.5) guaranteed efficient exfoliation. In addition, the study also found that the exfoliation and sulfur doping were almost achieved within the former two cycles of pulsed microwave irradiation (about 10 s), while the deoxygenation and reduction process of H2SO4-GICs continued with the increase of microwave irradiation time. The C/O ratio of SGi after 10 cycles of microwave irradiation reached 21.2, which was higher than that of other graphene or its derivatives reported by post-treatment approaches at present. Conflicts of interest There are no conflicts to declare. Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgements
Fig. 8. The XRD patterns for H2SO4-GICs treated by different dosage of H2SO4.
We gratefully acknowledge the project supported by Scientific and Technological Resaerch Program of Chongqing Municipal Education Commission (KJQN201800113).
GICs (Fig. 8). Both of these H2SO4-GICs showed a peak at ~26.61° 2θ, which was corresponding to the 002 diffraction angle of graphite (~3.35 Å), suggested the preservation of graphitic domains [29]. The broadening of the graphitic peak was a reflection of the disorder of graphite crystals after the intercalation process [30]. For #T2-1 H2SO4-GICs, there was another intense peak at ~11.06° 2θ, and the interlayer distance (di) corresponding to this peak was 7.98 Å, which was assigned to the stage-1 H2SO4-GICs [31]. Furthermore, the diffraction angles lined at ~22.17° and ~45.15° 2θ were the 0 0 2 and 0 0 4 signals of the stage-1 H2SO4-GIC, respectively [32]. Therefore, #T2-1 H2SO4-GICs mainly contained two phase in the crystal, graphitic domains and stage-1 structure. For #T2-4 H2SO4-GICs, there was a weak peak at ~9.79° 2θ, and the corresponding interlayer distance (di) was 9.02 Å, which can be assigned to the stage-4 H2SO4-GIC [33]. Furthermore, the peak at 26.61° 2θ in #T2-4 was broader than that in #T2-1, showing an obviously shoulder peak at ~24.64° 2θ, which can be assigned to 005 diffraction angle of stage-4 H2SO4-GIC [32]. Therefore, graphitic domains and stage-4 structure were the major crystal existing in #T2-4 H2SO4-GICs. In conclusion, when the dosage of H2SO4 was 8 mL, stage-4 GIC was the preferential microstructure, which was due to the lack of H2SO4 molecules for intercalation [34]. In contrast, when the dosage of H2SO4 was as high as 20 mL, the preferential microstructure of H2SO4-GICs was stage-1 structure [19]. By comparing the S-doping level of #T2-1 and #T2-4, the low stage structure of H2SO4-GICs (stage-1 GICs) was undesirable for the S-doping process. A possible explanation was proposed as follows: effective collision was necessary for chemicals to react [35]. The stage-4 structure, in which the ISSs were surrounded by multilayer graphite sheets, might be beneficial for the effective collision, because the constraining force of multilayer sheets would increase the probability of effective collision. On the contrary, the constraining of single layer graphite sheets on dopants was weak, where the dopants would gasify quickly and escape easily from the graphite bulk [16]. Thus, high stage structure of H2SO4-GICs might be a key condition for Sdoping.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136615. References [1] S. Yang, L. Zhi, K. Tang, X. Feng, J. Maier, K. Müllen, Efficient synthesis of heteroatom (N or S)-doped graphene based on ultrathin graphene oxide-porous silica sheets for oxygen reduction reactions, Adv. Funct. Mater. 22 (2012) 3634–3640. [2] G. Hui, L. Zheng, S. Li, G. Wenhua, G. Wei, C. Lijie, R. Amrita, Q. Weijin, V. Robert, M.A. Pulickel, Synthesis of S-doped graphene by liquid precursor, Nanotechnology 23 (2012) 275605. [3] L. Zhao, B. Yu, F. Xue, J. Xie, X. Zhang, R. Wu, R. Wang, Z. Hu, S.T. Yang, J. Luo, Facile hydrothermal preparation of recyclable S-doped graphene sponge for Cu2+ adsorption, J. Hazard. Mater. 286 (2015) 449–456. [4] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X.A. Chen, S. Huang, Sulfurdoped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano 6 (2012) 205–211. [5] R. Liu, Y. Zhang, Z. Ning, Y. Xu, A catalytic microwave process for superfast preparation of high quality reduced graphene oxide, Angew. Chem. Int. Ed. 49 (2017) 15677–15682. [6] D. Voiry, J. Yang, J. Kupferberg, R. Fullon, C. Lee, H.Y. Jeong, H.S. Shin, M. Chhowalla, High-quality graphene via microwave reduction of solution-exfoliated graphene oxide, Science 353 (2016) 1413. [7] J. Zheng, H.T. Liu, B. Wu, C.A. Di, Y.L. Guo, T. Wu, G. Yu, Y.Q. Liu, D.B. Zhu, Production of graphite chloride and bromide using microwave sparks, Sci. Rep. 2 (2012) 662. [8] W. Chen, L. Yan, P.R. Bangal, Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves, Carbon 48 (2010) 1146–1152. [9] X. Yuan, Z. Wu, H. Zhong, H. Wang, X. Chen, L. Leng, L. Jiang, Z. Xiao, G. Zeng, Fast removal of tetracycline from wastewater by reduced graphene oxide prepared via microwave-assisted ethylenediamine-N, N'-disuccinic acid induction method, Environ. Sci. Pollut. Res. Int. 23 (2016) 18657–18671. [10] F. Li, L. Lu, D. Gao, M. Wang, D. Wang, Z. Xia, Rapid synthesis of three-dimensional sulfur-doped porous graphene via solid-state microwave irradiation for protein removal in plasma sample pretreatment, Talanta 185 (2018) 528–536. [11] B. Tryba, A.W. Morawski, M. Inagaki, Preparation of exfoliated graphite by microwave irradiation, Carbon 43 (2005) 2417–2419. [12] Y. Zhu, S. Murali, M.D. Stoller, A. Velamakanni, R.D. Piner, R.S. Ruoff, Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors, Carbon 48 (2010) 2118–2122. [13] A. Dimiev, D.V. Kosynkin, L.B. Alemany, P. Chaguine, J.M. Tour, Pristine graphite oxide, J. Am. Chem. Soc. 134 (2012) 2815–2822. [14] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339-1339. [15] L. Zhu, X. Zhao, Y. Li, X. Yu, C. Li, Q. Zhang, High-quality production of graphene by liquid-phase exfoliation of expanded graphite, Mater. Chem. Phys. 137 (2013) 984–990. [16] G. Carotenuto, A. Longo, L. Nicolais, S. De Nicola, E. Pugliese, M. Ciofini, M. Locatelli, A. Lapucci, R. Meucci, Laser-induced thermal expansion of H2SO4-
4. Conclusion The SGi was successfully synthesized via microwave irradiating H2SO4-GICs and the S-doping level of SGi can reach to 0.94 at.% among the tested reaction conditions. Moreover, it was found that the sulfur atoms doping or not was only influenced by the dosage of H2SO4 used in the oxidant-assisted intercalation reaction, and the relatively lower dosage of H2SO4 was beneficial for the S-doping. The exfoliation efficiency was greatly affected by the temperature and dosage of oxidant, 7
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
intercalated graphite lattice, J. Phys. Chem. C 119 (2015) 15942–15947. [17] S. Pei, J. Zhao, J. Du, W. Ren, H.-M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (2010) 4466–4474. [18] M. Li, C. Liu, H. Zhao, H. An, H. Cao, Y. Zhang, Z. Fan, Tuning sulfur doping in graphene for highly sensitive dopamine biosensors, Carbon 86 (2015) 197–206. [19] A.M. Dimiev, J.M. Tour, Mechanism of graphene oxide formation, ACS Nano 8 (2014) 3060–3068. [20] Z. Lin, G. Waller, Y. Liu, M. Liu, C.-P. Wong, Facile synthesis of nitrogen-doped graphene via pyrolysis of graphene oxide and urea, and its electrocatalytic activity toward the oxygen-reduction reaction, Adv. Energy Mater. 2 (2012) 884–888. [21] F. Guittonneau, A. Abdelouas, B. Grambow, HTR fuel waste management: TRISO separation and acid-graphite intercalation compounds preparation, J. Nucl. Mater. 407 (2010) 71–77. [22] D.C. Alsmeyer, R.L. Mccreery, In situ Raman monitoring of electrochemical graphite intercalation and lattice damage in mild aqueous acids, Anal. Chem. 64 (1992) 1528–1533. [23] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nanotech. 8 (2013) 235–246. [24] S. Park, J. Kim, K.-J. Jeon, S.-H. Yoon, Characterization on the expanding nature of graphite in microwave-irradiated exfoliation, J. Nanosci. Nanotechno. 16 (2016) 4450–4455. [25] B. Tryba, A.W. Morawski, K. Kałucki, Trace analyses of gaseous products formed during heat treatment of high stage H2SO4-GICs and expanded graphite, J. Phys. Chem. Solids 65 (2004) 165–169. [26] J.M. You, M.S. Ahmed, H.S. Han, J.E. Choe, Z. Ustundag, S. Jeon, New approach of nitrogen and sulfur-doped graphene synthesis using dipyrrolemethane and their
[27]
[28] [29]
[30] [31]
[32] [33]
[34]
[35]
8
electrocatalytic activity for oxygen reduction in alkaline media, J. Power Sources 275 (2015) 73–79. Y. Liu, Y. Ma, Y. Jin, G. Chen, X. Zhang, Microwave-assisted solvothermal synthesis of sulfur-doped graphene for electrochemical sensing, J. Electroanal. Chem. 739 (2015) 172–177. H. Hu, Z. Zhao, Q. Zhou, Y. Gogotsi, J. Qiu, The role of microwave absorption on formation of graphene from graphite oxide, Carbon 50 (2012) 3267–3273. S. Dubin, S. Gilje, K. Wang, V.C. Tung, K. Cha, A.S. Hall, J. Farrar, R. Varshneya, Y. Yang, R.B. Kaner, A one-step, solvothermal reduction method for producing reduced graphene oxide dispersions in organic solvents, ACS Nano 4 (2010) 3845–3852. J. Li, L. Vaisman, G. Marom, J.-K. Kim, Br treated graphite nanoplatelets for improved electrical conductivity of polymer composites, Carbon 45 (2007) 744–750. A.M. Dimiev, S.M. Bachilo, R. Saito, J.M. Tour, Reversible formation of ammonium persulfate/sulfuric acid graphite intercalation compounds and their peculiar Raman spectra, ACS Nano 6 (2012) 7842–7849. F. Kang, Y. Leng, T.-Y. Zhang, Influences of H2O2 on synthesis of H2SO4-GICs, J. Phys. Chem. Solids 57 (1996) 889–892. N.E. Sorokina, M.A. Khaskov, V.V. Avdeev, I.V. Nikol’skaya, Reaction of graphite with sulfuric acid in the presence of KMnO4, Russ. J. Gen. Chem. 75 (2005) 162–168. R. Levi-Setti, G. Crow, Y.L. Wang, N.W. Parker, R. Mittleman, D.M. Hwang, Highresolution scanning-ion-microprobe study of graphite and its intercalation compounds, Phys. Rev. Lett. 54 (1985) 2615. K. Takayanagi, Vibrational and rotational transitions in molecular collisions, Prog. Theor. Phys. Supp. 25 (1963) 1–98.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Preparation of sulfur-doped graphite by solid-state microwave method: The effect of reaction conditions on the sulfur-doping process ⁎
Fan Lia, Wanjun Xub, Linyi Lua, Kai Zhouc, , Zhining Xiaa,b,
T
⁎
a
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, China School of Pharmaceutical Science, Chongqing University, Chongqing 401331, China c Analytical and Testing Center, Chongqing University, Chongqing 401331, China b
H I GH L IG H T S
solid-state microwave method was proposed and optimized for the preparation of SGi. • ATherapid (CeSeC) was the preferential conformation of sulfur in SGi. • The thiophene-S • dosage of H SO used in intercalation reaction determined whether S-doping achieve or not. 2
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfur-doping Carbon materials Solid-state microwave Intercalation reaction Large-scale production
The high-quality sulfur-doped graphite (SGi) was prepared from concentrated sulfuric acid-graphite intercalation compounds (H2SO4-GICs) by solid-state microwave irradiation, where H2SO4 was the sulfur (S) source and S-doping occurred within a minute. The preparation of H2SO4-GICs was a key step for S-doping and exfoliation process. Notably, it was found that S-doping was influenced by the dosage of H2SO4, where the lower dosage of H2SO4 was beneficial for the S-doping. The exfoliation efficiency was greatly affected by the temperature and dosage of oxidant, where lower temperature and lower dosage of KMnO4 guaranteed efficient exfoliation.
1. Introduction Sulfur-doped graphene (SG) has attracted a great attention owing to its outstanding properties and potential applications. For example, SG has been demonstrated as potential metal-free catalysts for oxygen reduction reaction which is the key process in fuel cells [1]. Up to now, several typical methods have been proposed for the synthesis of SG, mainly including chemical vapor deposition, solvothermal methods and thermal-annealing graphene oxide (GO) with dopant precursors (such as H2S, SO2, CS2, and L-cystine) under high temperature [2–4]. Among these, chemical vapor deposition methods are usually developed to synthesize large, continuous and defect-free high-quality SG, but they are not suitable for mass production. Although post-treatment of GO is a strategy to realize mass production of SG, the as-prepared products have poor quality because of the high defect density caused by harsh oxidation of graphite. Furthermore, high vacuum or high temperature conditions are usually inevitable among these methods. Thus, these methods are not suitable for the large-scale production of high-quality SG due to the high demand of equipment and low yield of products. As
large-scale production of high-quality SG is the key step to the practical application of SG in many fields, new synthesis method need to be developed. Many efforts are ongoing to largely prepare high-quality graphene and its derivatives. Among these, microwave-assisted methods have been proved to be very efficient methods [5–7]. For example, it was reported that a large quantity of high-quality graphene could be fabricated by solid-state microwave method using graphite as catalyst. During the microwave irradiating, exfoliation and reduction of graphite oxide simultaneously achieved in ambient air [5]. Furthermore, the solid microwave exfoliated graphite oxide was of higher quality than microwave-assisted solvothermal reduced GO, including a much higher exfoliation degree and higher specific surface area, a much larger C/O ratio, and higher conductivity [5,8,9]. As solid-state microwave method do not require expensive instruments, and it can easily increase the temperature of graphite to over 1000 °C with plasma generation which was beneficial for the heteroatom-doping process [5,7], it shows great potential for mass production of high-quality SG. In our previous work, we have successfully synthesized SGi from
⁎ Corresponding authors at: School of Chemistry and Chemical Engineering, School of Pharmaceutical Science, Chongqing University, Chongqing 401331, China (Z. Xia). E-mail addresses: [email protected] (K. Zhou), [email protected] (Z. Xia).
https://doi.org/10.1016/j.cplett.2019.136615 Received 28 May 2019; Received in revised form 12 July 2019; Accepted 16 July 2019 Available online 16 July 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
microwave exfoliation of dried H2SO4-GICs, which could be easily exfoliated to monolayer SG by mild solution-phase sonication [10]. Briefly, the dopant (H2SO4) was intercalated into the interlayer of graphite via a chemical oxidation intercalation reaction thereby forming the stable H2SO4-GICs. During microwave irradiation, SGi formed with an obvious increase in volume due to violent gas-releasing. As H2SO4-GICs was the precursor of graphite oxide, this strategy seems to be similar to the microwave exfoliation of graphite oxide [5,11,12], but for graphite oxide, the S atom would be totally removed when graphite oxide was heated to 900 °C and there was no S atom doping into the backbones of carbon sheet via microwave exfoliation of graphite oxide [13]. Furthermore, in our previous study, we also found that some of H2SO4-GICs can achieve S-doping while others could not, and the reason for this phenomenon remained unclear. Therefore, in this study, the intercalation reaction conditions and microwave irradiation conditions were investigated to explore the factors which affected the S-doping process, as well as the reduction and exfoliation process. The X-ray photoelectron spectrometer (XPS) and X-ray diffractometer (XRD) characterization results of H2SO4-GICs and SGi were mainly selected as the evaluating indicators. The effects of reaction temperatures, dosage of oxidant (KMnO4) and dopant (H2SO4), as well as microwave irradiation time on the exfoliation efficiency and S-doping level were discussed in detail. Figuring out the reason why graphite oxide and some H2SO4-GICs could not achieve S doping during microwave exfoliation was of great significance on other doping methods relating with intercalation process. Furthermore, clarifying the factors which affected the exfoliation efficiency and S-doping process can further deepen the significance of producing SGi and SG in industrial quantities.
Table 1 Influence of intercalation reaction conditions on the exfoliation efficiency.
2. Experiments
The morphology and microstructure of the as-synthesized H2SO4GICs and SGi were investigated by SEM, Raman spectrometer and XRD. The elemental compositions and chemical status of elements in H2SO4GICs and SGi was investigated by XPS. The loading amount of dopants in H2SO4-GICs and the S-doping level was expressed by the content of oxidized-S and thiophene-S (eCeSeCe) measured by XPS, respectively. The expanded volume (EV) was used as an indicator to evaluate the exfoliation efficiency [15] and it was calculated as a quotient of the volume of SGi and the mass of H2SO4-GICs, therefore having a unit of volume/mass (mL/g). All values of EV were collected from three independent measurements for each sample and presented as average and standard error.
#Sample
Mixture ratioa A:B:C
Temperature (°C)
Expanded volume (mL/g)
T1-1 T1-2 T1-3 T1-4 T1-5 T1-6 T1-7 T1-8 T1-9 T1-10 T1-11
1:2:40 1:6:40 1:4:40 1:1:20 1:1:20 1:1:20 1:2:20 1:3:20 1:1.5:10 1:1:10 1:0.5:10
30 17 0 0 17 30 30 30 0 30 17
5 3 2 162 144 118 25 3 171 204 122
a
A-graphite powder (g); B-KMnO4 (g); C-H2SO4 (mL).
Table 2 Influence of intercalation reaction conditions on the S-doping level. #Sample
Mixture ratioa A:B:C
Temperature (°C)
S-doping level (at. %)
T2-1 T2-2 T2-3 T2-4 T2-5 T2-6 T2-7
1:2:20 1:2:16 1:2:12 1:2:8 1:1.5:8 1:1:8 1:0.5:8
0 0 0 0 0 0 0
0.00 0.03 0.51 0.88 0.94 0.90 0.78
a
A-graphite powder (g); B-KMnO4 (g); C-H2SO4 (mL).
2.4. Characterization
2.1. Reagents and apparatus Graphite powder (2000 mesh) was purchased from Aladdin Reagents (Shanghai, China). The rest reagents such as H2SO4, KMnO4, 30% hydrogen peroxide (H2O2) solution and hydrochloric acid (HCl) were of analytical grade and were purchased from KeLong Chemical Co. Ltd. (Sichuan, China). The microwave irradiation was conducted by a microwave oven (LWMC-205, LingJiang Technology Development Co. Ltd., Nanjing, China). The used characterization techniques were scanning electron microscope (SEM) (JSM-7600F, JEOL Ltd., Tokyo, Japan), XPS (ESCALAB250Xi, Thermo Fisher Scientific, Waltham, USA), XRD (X’Pert Powder, PANalytical, Almelo, The Netherlands) and Raman spectrometer (LabRAM HR Evolution, Horiba Jobin Yvon Co. Ltd., Paris, France).
3. Results and discussion 3.1. Characterization of H2SO4-GICs and SGi
2.2. Preparation of H2SO4-GICs using different intercalation reaction conditions
Fig. 1 displayed the SEM images of H2SO4-GICs and SGi, respectively. For H2SO4-GICs (Fig. 1a), the carbon sheets were in a stacking structure, indicating that the interlayer spacing of graphite did not change obviously during the intercalation reaction [16]. In contrast, for SGi, a loose porous network was demonstrated as shown in Fig. 1b, which hinted that the stacking carbon sheets were expanded by releasing gaseous species during microwave irradiating H2SO4-GICs. This loose porous network was beneficial for the preparation of monolayer SG [15]. The XPS spectra of H2SO4-GICs and SGi (Fig. 2a) both showed the presence of C, O and S atoms at ~284.6, ~164.2 and ~533.2 eV, but the relative content of them were different. For H2SO4-GICs, the content of S and O were ~1.85 at.% and ~18.7 at.%, indicating that H2SO4 molecules has been intercalated into graphite, as well as that many oxygen-containing functional groups has been introduced into the bulk graphite. However, for SGi, both of the content of S and O decreased, which was due to the thermal decomposition and releasing of H2SO4, as
The preparation of H2SO4-GICs was conducted by an oxidant-assisted intercalation reaction [14], and H2SO4-GICs with different microstructures were prepared using different dosage of KMnO4 and H2SO4, as well as different reaction temperatures. Tables 1 and 2 detailed the intercalation reaction conditions, and the detailed operations were given in Supporting Information. 2.3. Preparation of SGi via different microwave irradiation time The as-prepared dried H2SO4-GICs were placed in an open reactor located in a microwave oven. The SGi was obtained by treating the H2SO4-GICs at 800 W microwave irradiation for several seconds (5 s/ pulse, 1–10 cycles). Pulsed irradiation (5 s/pulse) was applied to investigate the change of the content and chemical status of each element in SGi at different irradiation time. 2
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
Fig. 1. The SEM images of (a) H2SO4-GICs and (b) SGi.
to that oxidized-S could be transformed into thiophene-S at high temperature [4]. Thus, S atoms could incorporate into carbon network through the solid-state microwave irradiation. The high-resolution O1s spectrum (Fig. 2d) of H2SO4-GICs showed a main peak at 531.6 eV corresponding to hydroxyl, while the O1s spectrum of SGi could be fitted with two main peaks at ~531.7 and ~533.9 eV corresponding to hydroxyl and adsorbed water, respectively [20]. The appearance of the peak at ~533.9 eV was attributed to the formation of H2O during the process of the H2SO4 decomposition when H2SO4-GICs was treated by microwave irradiation [21]. The Raman spectrum of graphite exhibited a highly ordered graphitic Raman feature with a sharp and symmetrical G-band (~1569 cm−1) and a nearly absent D-band (~1344 cm−1) [6], while a new D′-band appeared at ~1619 cm−1 in the Raman spectrum of
well as the microwave-induced deoxygenation [5]. The high-resolution C1s spectra of H2SO4-GICs (Fig. 2b) showed higher intensity of peaks at ~286.7 eV and ~288.1 eV compared with that of SGi, indicating that the H2SO4-GICs contained abundant oxygencontaining groups [17]. In the high-resolution S2p spectra (Fig. 2c), H2SO4-GICs showed only two peaks at ~167.9 and ~169.0 eV, which were corresponding to the spin-orbit coupling of oxidized-S (CeSOxeC) [18]. The oxidized-S was attributed to the intercalated sulfate species (ISSs) (H2SO4 or HSO4−) [19]. Therefore, it demonstrated that S atoms could not incorporate into carbon network during the intercalation reaction. For SGi, two new peaks appeared at ~162.9 and ~164.2 eV, which were attributed to the spin-orbit coupling of thiophene-S (CeSeC) [18], and the peak intensity of oxidized-S decreased. This phenomenon was due
Fig. 2. (a) The XPS spectra of H2SO4-GICs and SGi; (b)–(d) The high-resolution C1s, S2p, O1s spectra of H2SO4-GICs and SGi. 3
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
exfoliation process involved the thermal decomposition of the ISSs, subsequently gas generation and violently gas releasing [11]. Nearly all of the in H2SO4-GICs could thermally decompose into gaseous species (SO2, O2 and H2O) [21]. Thus, the quantity of gaseous species was directly correlated with the loading amount of ISSs, which means that the higher S loading amount in H2SO4-GICs would produce larger volume of gas, consequently resulting in higher EV. Therefore, the S loading amount of H2SO4-GICs was also a factor influenced the exfoliation. However, the correlation between the S loading amount and the EV showed a weak linearity (R2 = 0.5195), which might be due to the fact that the decomposition of oxygen-containing functional groups would also produce abundant gaseous species [25]. Moreover, the exceptions existing in both Fig. 4b and c need to be discussed in detail. As shown in Fig. 4b, the C/O ratio of #T1-11 was as high as ~5.8, but its EV (~120 mL/g) was lower than that of #T1-10 (~200 mL/g), whose the C/O ratio was ~4.5. This phenomenon violated the rule that EV was positive correlated with the C/O ratio. However, from the point of view in S loading amount, #T1-11 (~0.70 at.%) had much lower S loading amount than #T1-10 (~1.75 at.%), which was conformed to the general relationship between EV and S loading amount. Thus, this exception in Fig. 4b indicated that when the C/O ratio was high enough, the S loading amount was the main factor determined the exfoliation efficiency. In Fig. 4c, exceptions came from these H2SO4-GICs with C/O ratio less than 2.38. The S loading amount of them was high over ~1.8 at.%, but their EV sharply decrease. This result demonstrated that when the H2SO4-GICs was intensively oxidized and lost the intact layered structure needed for microwave exfoliation, the S loading amount no longer worked for the exfoliation efficiency. Additionally, Fig. 4d showed that the S loading amount was inversely correlation with value of C/O ratio. This result was due to that higher content of oxygen-functional groups was beneficial to broaden the spacing of carbon layers, resulting in more sulfate species moving cross and binding on the sheets. At the same time, when more sulfate species intercalated in the carbon sheets, the interlayer spacing expanded further, and consequently more oxidant came into the interlayers, resulting in excessive oxidation [19]. In other words, the C/O ratio and S loading amount were not two independent factors, and they mutually restricted each other. Therefore, to balance the relationship between C/O ratio and S loading amount would be a very important step for efficient exfoliation. In summary, the exfoliation efficiency was influenced by not only the loading amount of ISSs but also the integrity of the layered structure (C/O ratio). Considering the relationship between experimental results and reaction conditions (Table 1), it could be roughly concluded that the lower reaction temperature was beneficial for the exfoliation efficiency, and 0.5 < KMnO4:graphite < 1.5 could achieve efficient exfoliation at these investigated reaction temperatures.
Fig. 3. The Raman spectra of graphite, H2SO4-GICs and SGi.
H2SO4-GICs, which was attributed to “boundary layer” in graphite planes adjacent to layer of the intercalant [22] (Fig. 3). Thus, the appearance of D′-band suggested the successful intercalation of H2SO4. After the microwave treatment of H2SO4-GICs, the D′-band disappeared, suggesting that the intercalant has been eliminated. The increase of ID/IG ratio in H2SO4 and SGi demonstrated the increase of disordered domains [23], suggesting that defect could be formed during the oxidant-assisted intercalation reaction and microwave treatment [19]. 3.2. Effects of reaction conditions on the exfoliation, reduction and Sdoping process The preparation of SGi involved two steps. The first step is the preparation of H2SO4-GICs via an oxidant-assisted intercalation reaction. In this step, the dosage of KMnO4 and H2SO4 as well as the reaction temperature would influence the properties of H2SO4-GICs, which tremendously related to the properties of subsequent product (SGi). The second step is microwave irradiating H2SO4-GICs to form SGi, where exfoliation, reduction and S-doping could achieve simultaneously. During microwave irradiation, H2SO4-GICs with different properties would become different SGi with various C/O ratio, exfoliation efficiency and S-doping level. In this part, we investigated various reaction conditions to figure out how they influenced the properties of products. 3.2.1. Effect on the exfoliation efficiency During microwave irradiation, the Van der Waals forces between carbon sheets would be weaken by violently gas releasing, resulting in a remarkable volume increase of H2SO4-GICs. The EV was used as an indicator to evaluate the exfoliation efficiency. As shown in Fig. 4b, there was a positive correlation between EV and C/O ratio of H2SO4GICs, except for sample #T1-11. Generally, the EV increased with the increase of C/O ratio, suggesting that the less oxygen-functional groups on carbon sheets, the higher EV would be reached. Furthermore, when the C/O ratio of H2SO4-GICs were below ~2.38 (Fig. 4a), these H2SO4GICs could not be efficiently exfoliated. These inefficient exfoliations were due to the excessive oxidant and high temperature in the intercalation reaction which caused the intensive oxidation and tearingdown on the layered structure of graphite [24]. Therefore, a relatively intact layered structure (higher C/O ratio) would be needed to ensure the efficient microwave exfoliation. The relationship between EV and S loading amount of H2SO4-GICs was demonstrated in Fig. 4c. By eliminating those H2SO4-GICs whose C/O ratio was below ~2.38, there was a positive correlation between the EV and S loading amount. The reasons were analyzed as follows: the
3.2.2. Effects of microwave irradiation time During microwave irradiation, the exfoliation, reduction and Sdoping occurred simultaneously. In this part, the effects of microwave irradiation time were investigated. Firstly, the EV of H2SO4-GICs showed an apparent increase within the former two cycles of pulsed microwave irradiation, and in the subsequent cycles of irradiation, the EV was no obvious change. During the ten cycles of irradiation, the content and chemical status of S in H2SO4-GICs were changing. In Fig. 5a, for the initial H2SO4-GICs, all of the S bonding configurations were oxidized-S, owing to the sorption of ISSs on the carbon sheets [18]. After microwave irradiation for only two cycles, the peaks of oxidized-S obviously decreased and new peaks corresponding to thiophene-S appeared. With the increase of irradiation cycles, the peak corresponding to oxidized-S decreased gradually, and finally the oxidized-S preserved only ~15% of the total S content after 10 cycles (Fig. 5b). In Fig. 5b, the content of thiophene-S had a sharp increase within the former two cycles and then slightly 4
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
Fig. 4. (a) The EV, C/O ratio and S loading amount of H2SO4-GICs listed in Table 1; (b) The relationship between EV and C/O ratio; (c) The relationship between EV and S loading amount; (d) The relationship between S loading amount and C/O ratio.
of C/O ratio among other graphene and its derivatives prepared by post-treatment approaches [5,8,27]. The highest value of C/O ratio might be due to the preservation of the relatively intact π-system in H2SO4-GICs compared with graphite oxide [19], and the large unoxidized graphitic region in H2SO4-GICs can act as the microwave acceptor to initiate the microwave-induced deoxygenation [5,28]. Therefore, this result suggested that solid-state microwave irradiating H2SO4-GICs had potential to prepare high-quality SGi.
increased in the residual cycles. Notably, the total content of S decreased during microwave irradiation (Fig. 5b), which means that the number of S atoms was not conserved, suggesting that nearly 50% of S atoms escaped from H2SO4-GICs during the microwave irradiation. After ten cycles of microwave irradiation, oxidized-S was nearly vanished and thiophene-S was the main bonding configurations of S in SGi. As thiophene-S was known as one of the most effective active sites in the electrocatalytic reaction, the final products might probably exhibit enhanced electrocatalyst performance [20,26]. Although the microwave-induced deoxygenation has been reported by other researches [5], it was worthy to note that the C/O ratio of the final SGi (10 cycles), was as high as 21.2, which was the highest value
3.2.3. Effect on the S-doping process The S-doping level of SGi resulting from various dosage of H2SO4 and KMnO4 were demonstrated in Fig. 6 and Table 2. When the dosage
Fig. 5. (a) The high-resolution S2p spectra of H2SO4-GICs treated by different cycles of microwave irradiation; (b) The effect of microwave irradiation time on the content and bonding configurations of S. 5
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
Fig. 6. The C/O ratio and S loading amount of H2SO4-GICs listed in Table 2 as well as their S-doping levels after microwave irradiation. (a) The effect of the dosage of H2SO4; (b) The effect of the dosage of KMnO4.
Fig. 7. (a) The high-resolution S2p spectra and (b) high-resolution C1s spectra of H2SO4-GICs treated by different dosage of H2SO4.
3.3. Mechanism of S-doping
of KMnO4 was the same, the S-doping level increased with the decrease of dosage of H2SO4 (Fig. 6a). In the high-resolution S2p spectrum of #T2-1 SGi (Fig. 7(a), top), no peaks appeared at the binding energies from 164 to 170 eV, indicating that no S atoms retained after microwave irradiation. Therefore, H2SO4-GICs treated by 20 mL H2SO4 (#T21) could not dope S atoms into the carbon planes. In contrast, when the dosage of H2SO4 decreased to 8 mL, the peaks at ~165.4 and ~164.2 eV corresponding to the spin-orbit coupling of thiophene-S appeared (Fig. 7(a), bottom), suggesting that lower dosage of H2SO4 was beneficial for the S-doping process. Smaller dosage of H2SO4 was not further investigated, because lower volume of H2SO4 was not large enough to immerse the graphite powder. Above all, the dosage of H2SO4 was a key factor to determine whether the S-doping achieve or not and lower dosage of H2SO4 was beneficial for the high S-doping level. Additionally, the S-doping level also affected by the dosage of KMnO4. As shown in Fig. 6b, while keeping the dosage of H2SO4 at a same level, the S-doping level first increased and then decreased with the decrease of KMnO4. The first tendency of increase might be due to preventing the adverse effect of relative higher degree of oxidization, where the hole-like defects might lead to the easy loss of dopants. In contrast, the subsequent decrease of S-doping level was due to the decrease of ISSs loading amount, which caused by the decrease of KMnO4 [19]. However, the S-doping level changed very slightly with different dosage of KMnO4, suggesting that the dosage of KMnO4 was not a major factor for S-doping, compared with the dosage of H2SO4.
According to the discussion above, the S-doping was greatly affected by the dosage of H2SO4, but how it influenced the S-doping was still unknown. In this part, we aimed to figure out the mechanism of Sdoping, which might have referential value to other heteroatom-doping methods involving intercalation reaction. In Fig. 6a, the dosage of KMnO4 remained unchanged, however, the C/O ratio increased with the decrease of the dosage of H2SO4. The highresolution C1s spectra of these H2SO4-GICs also exhibited that high dosage of H2SO4 can promote the oxidation of carbon sheets. In Fig. 7(b), two intense bands at 284.7 and 286.9 eV were corresponding to the graphitic C]C species and CeO species [8] and the CeO/C]C ratio, calculated from the quotient of their peak area, decrease with the decrease of the dosage of H2SO4. As the low dosage of H2SO4 caused low CeO/C]C ratio and high Sdoping level, it seems that low oxidation degree (high C/O ratio) guaranteed the success of S-doping, but that was not the case. Because the C/O ratio of #T1-4, #T1-5, #T1-6 were higher than that of #T2-4 (3.5–4.5 v.s. 2.9), but no S atoms doped into #T1-4, #T1-5, #T1-6. Thus, the degree of oxidation was not the fundamental factor, and the dosage of H2SO4 must influence other properties of H2SO4-GICs, which determined the achievement of S-doping. XRD was applied to investigate the microstructures of H2SO4-GICs, which might affect the S-doping process. The XRD pattern corresponding to #T2-1 H2SO4-GICs was different from that of #T2-4 H2SO46
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
where lower temperature (0 °C) and moderate dosage of KMnO4 (0.5 < KMnO4:graphite < 1.5) guaranteed efficient exfoliation. In addition, the study also found that the exfoliation and sulfur doping were almost achieved within the former two cycles of pulsed microwave irradiation (about 10 s), while the deoxygenation and reduction process of H2SO4-GICs continued with the increase of microwave irradiation time. The C/O ratio of SGi after 10 cycles of microwave irradiation reached 21.2, which was higher than that of other graphene or its derivatives reported by post-treatment approaches at present. Conflicts of interest There are no conflicts to declare. Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgements
Fig. 8. The XRD patterns for H2SO4-GICs treated by different dosage of H2SO4.
We gratefully acknowledge the project supported by Scientific and Technological Resaerch Program of Chongqing Municipal Education Commission (KJQN201800113).
GICs (Fig. 8). Both of these H2SO4-GICs showed a peak at ~26.61° 2θ, which was corresponding to the 002 diffraction angle of graphite (~3.35 Å), suggested the preservation of graphitic domains [29]. The broadening of the graphitic peak was a reflection of the disorder of graphite crystals after the intercalation process [30]. For #T2-1 H2SO4-GICs, there was another intense peak at ~11.06° 2θ, and the interlayer distance (di) corresponding to this peak was 7.98 Å, which was assigned to the stage-1 H2SO4-GICs [31]. Furthermore, the diffraction angles lined at ~22.17° and ~45.15° 2θ were the 0 0 2 and 0 0 4 signals of the stage-1 H2SO4-GIC, respectively [32]. Therefore, #T2-1 H2SO4-GICs mainly contained two phase in the crystal, graphitic domains and stage-1 structure. For #T2-4 H2SO4-GICs, there was a weak peak at ~9.79° 2θ, and the corresponding interlayer distance (di) was 9.02 Å, which can be assigned to the stage-4 H2SO4-GIC [33]. Furthermore, the peak at 26.61° 2θ in #T2-4 was broader than that in #T2-1, showing an obviously shoulder peak at ~24.64° 2θ, which can be assigned to 005 diffraction angle of stage-4 H2SO4-GIC [32]. Therefore, graphitic domains and stage-4 structure were the major crystal existing in #T2-4 H2SO4-GICs. In conclusion, when the dosage of H2SO4 was 8 mL, stage-4 GIC was the preferential microstructure, which was due to the lack of H2SO4 molecules for intercalation [34]. In contrast, when the dosage of H2SO4 was as high as 20 mL, the preferential microstructure of H2SO4-GICs was stage-1 structure [19]. By comparing the S-doping level of #T2-1 and #T2-4, the low stage structure of H2SO4-GICs (stage-1 GICs) was undesirable for the S-doping process. A possible explanation was proposed as follows: effective collision was necessary for chemicals to react [35]. The stage-4 structure, in which the ISSs were surrounded by multilayer graphite sheets, might be beneficial for the effective collision, because the constraining force of multilayer sheets would increase the probability of effective collision. On the contrary, the constraining of single layer graphite sheets on dopants was weak, where the dopants would gasify quickly and escape easily from the graphite bulk [16]. Thus, high stage structure of H2SO4-GICs might be a key condition for Sdoping.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136615. References [1] S. Yang, L. Zhi, K. Tang, X. Feng, J. Maier, K. Müllen, Efficient synthesis of heteroatom (N or S)-doped graphene based on ultrathin graphene oxide-porous silica sheets for oxygen reduction reactions, Adv. Funct. Mater. 22 (2012) 3634–3640. [2] G. Hui, L. Zheng, S. Li, G. Wenhua, G. Wei, C. Lijie, R. Amrita, Q. Weijin, V. Robert, M.A. Pulickel, Synthesis of S-doped graphene by liquid precursor, Nanotechnology 23 (2012) 275605. [3] L. Zhao, B. Yu, F. Xue, J. Xie, X. Zhang, R. Wu, R. Wang, Z. Hu, S.T. Yang, J. Luo, Facile hydrothermal preparation of recyclable S-doped graphene sponge for Cu2+ adsorption, J. Hazard. Mater. 286 (2015) 449–456. [4] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X.A. Chen, S. Huang, Sulfurdoped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano 6 (2012) 205–211. [5] R. Liu, Y. Zhang, Z. Ning, Y. Xu, A catalytic microwave process for superfast preparation of high quality reduced graphene oxide, Angew. Chem. Int. Ed. 49 (2017) 15677–15682. [6] D. Voiry, J. Yang, J. Kupferberg, R. Fullon, C. Lee, H.Y. Jeong, H.S. Shin, M. Chhowalla, High-quality graphene via microwave reduction of solution-exfoliated graphene oxide, Science 353 (2016) 1413. [7] J. Zheng, H.T. Liu, B. Wu, C.A. Di, Y.L. Guo, T. Wu, G. Yu, Y.Q. Liu, D.B. Zhu, Production of graphite chloride and bromide using microwave sparks, Sci. Rep. 2 (2012) 662. [8] W. Chen, L. Yan, P.R. Bangal, Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves, Carbon 48 (2010) 1146–1152. [9] X. Yuan, Z. Wu, H. Zhong, H. Wang, X. Chen, L. Leng, L. Jiang, Z. Xiao, G. Zeng, Fast removal of tetracycline from wastewater by reduced graphene oxide prepared via microwave-assisted ethylenediamine-N, N'-disuccinic acid induction method, Environ. Sci. Pollut. Res. Int. 23 (2016) 18657–18671. [10] F. Li, L. Lu, D. Gao, M. Wang, D. Wang, Z. Xia, Rapid synthesis of three-dimensional sulfur-doped porous graphene via solid-state microwave irradiation for protein removal in plasma sample pretreatment, Talanta 185 (2018) 528–536. [11] B. Tryba, A.W. Morawski, M. Inagaki, Preparation of exfoliated graphite by microwave irradiation, Carbon 43 (2005) 2417–2419. [12] Y. Zhu, S. Murali, M.D. Stoller, A. Velamakanni, R.D. Piner, R.S. Ruoff, Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors, Carbon 48 (2010) 2118–2122. [13] A. Dimiev, D.V. Kosynkin, L.B. Alemany, P. Chaguine, J.M. Tour, Pristine graphite oxide, J. Am. Chem. Soc. 134 (2012) 2815–2822. [14] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339-1339. [15] L. Zhu, X. Zhao, Y. Li, X. Yu, C. Li, Q. Zhang, High-quality production of graphene by liquid-phase exfoliation of expanded graphite, Mater. Chem. Phys. 137 (2013) 984–990. [16] G. Carotenuto, A. Longo, L. Nicolais, S. De Nicola, E. Pugliese, M. Ciofini, M. Locatelli, A. Lapucci, R. Meucci, Laser-induced thermal expansion of H2SO4-
4. Conclusion The SGi was successfully synthesized via microwave irradiating H2SO4-GICs and the S-doping level of SGi can reach to 0.94 at.% among the tested reaction conditions. Moreover, it was found that the sulfur atoms doping or not was only influenced by the dosage of H2SO4 used in the oxidant-assisted intercalation reaction, and the relatively lower dosage of H2SO4 was beneficial for the S-doping. The exfoliation efficiency was greatly affected by the temperature and dosage of oxidant, 7
Chemical Physics Letters 731 (2019) 136615
F. Li, et al.
intercalated graphite lattice, J. Phys. Chem. C 119 (2015) 15942–15947. [17] S. Pei, J. Zhao, J. Du, W. Ren, H.-M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (2010) 4466–4474. [18] M. Li, C. Liu, H. Zhao, H. An, H. Cao, Y. Zhang, Z. Fan, Tuning sulfur doping in graphene for highly sensitive dopamine biosensors, Carbon 86 (2015) 197–206. [19] A.M. Dimiev, J.M. Tour, Mechanism of graphene oxide formation, ACS Nano 8 (2014) 3060–3068. [20] Z. Lin, G. Waller, Y. Liu, M. Liu, C.-P. Wong, Facile synthesis of nitrogen-doped graphene via pyrolysis of graphene oxide and urea, and its electrocatalytic activity toward the oxygen-reduction reaction, Adv. Energy Mater. 2 (2012) 884–888. [21] F. Guittonneau, A. Abdelouas, B. Grambow, HTR fuel waste management: TRISO separation and acid-graphite intercalation compounds preparation, J. Nucl. Mater. 407 (2010) 71–77. [22] D.C. Alsmeyer, R.L. Mccreery, In situ Raman monitoring of electrochemical graphite intercalation and lattice damage in mild aqueous acids, Anal. Chem. 64 (1992) 1528–1533. [23] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nanotech. 8 (2013) 235–246. [24] S. Park, J. Kim, K.-J. Jeon, S.-H. Yoon, Characterization on the expanding nature of graphite in microwave-irradiated exfoliation, J. Nanosci. Nanotechno. 16 (2016) 4450–4455. [25] B. Tryba, A.W. Morawski, K. Kałucki, Trace analyses of gaseous products formed during heat treatment of high stage H2SO4-GICs and expanded graphite, J. Phys. Chem. Solids 65 (2004) 165–169. [26] J.M. You, M.S. Ahmed, H.S. Han, J.E. Choe, Z. Ustundag, S. Jeon, New approach of nitrogen and sulfur-doped graphene synthesis using dipyrrolemethane and their
[27]
[28] [29]
[30] [31]
[32] [33]
[34]
[35]
8
electrocatalytic activity for oxygen reduction in alkaline media, J. Power Sources 275 (2015) 73–79. Y. Liu, Y. Ma, Y. Jin, G. Chen, X. Zhang, Microwave-assisted solvothermal synthesis of sulfur-doped graphene for electrochemical sensing, J. Electroanal. Chem. 739 (2015) 172–177. H. Hu, Z. Zhao, Q. Zhou, Y. Gogotsi, J. Qiu, The role of microwave absorption on formation of graphene from graphite oxide, Carbon 50 (2012) 3267–3273. S. Dubin, S. Gilje, K. Wang, V.C. Tung, K. Cha, A.S. Hall, J. Farrar, R. Varshneya, Y. Yang, R.B. Kaner, A one-step, solvothermal reduction method for producing reduced graphene oxide dispersions in organic solvents, ACS Nano 4 (2010) 3845–3852. J. Li, L. Vaisman, G. Marom, J.-K. Kim, Br treated graphite nanoplatelets for improved electrical conductivity of polymer composites, Carbon 45 (2007) 744–750. A.M. Dimiev, S.M. Bachilo, R. Saito, J.M. Tour, Reversible formation of ammonium persulfate/sulfuric acid graphite intercalation compounds and their peculiar Raman spectra, ACS Nano 6 (2012) 7842–7849. F. Kang, Y. Leng, T.-Y. Zhang, Influences of H2O2 on synthesis of H2SO4-GICs, J. Phys. Chem. Solids 57 (1996) 889–892. N.E. Sorokina, M.A. Khaskov, V.V. Avdeev, I.V. Nikol’skaya, Reaction of graphite with sulfuric acid in the presence of KMnO4, Russ. J. Gen. Chem. 75 (2005) 162–168. R. Levi-Setti, G. Crow, Y.L. Wang, N.W. Parker, R. Mittleman, D.M. Hwang, Highresolution scanning-ion-microprobe study of graphite and its intercalation compounds, Phys. Rev. Lett. 54 (1985) 2615. K. Takayanagi, Vibrational and rotational transitions in molecular collisions, Prog. Theor. Phys. Supp. 25 (1963) 1–98.