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Kinetically enhanced bubble-exfoliation of graphite towards high-yield preparation of high-quality graphene Peng He, Hongyu Gu, Gang Wang, Siwei Yang, Guqiao Ding, Zhi Liu, and Xiaoming Xie Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02752 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017
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Chemistry of Materials
Kinetically enhanced bubble-exfoliation of graphite towards highyield preparation of high-quality graphene Peng He, , , Hongyu Gu, , Gang Wang, Siwei Yang, ‡§ oming Xie , ,# †‡§
†⊥
∥
‡ ,§
Guqiao Ding,
‡, §,*
Zhi Liu, ,
‡ §,#,*
and Xia-
‡
Center for Excellence in Superconducting Electronics, State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Shanghai 200050, P. R. China. § University of Chinese Academy of Sciences, Beijing, 100049, P. R. China. ⊥ Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramics, Chinese Academy of Sciences. ∥
Department of Microelectronic Science and Engineering, Ningbo University, Ningbo 315211, P. R. China. School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, P. R. China. KEYWORDS: graphene, bubble exfoliation, high-yield, high-quality, kinetic controlling
#
ABSTRACT: Scalable preparation of high-quality graphene with high efficiency is of great significance for many practical applications. Herein, we propose kinetically enhanced bubble exfoliation of graphite to prepare graphene in one pot. Graphene (≤10 atomic layers) of large size (average sheet area of 54.7 µm2) and high quality (average hole mobility up to 554.8 cm2 v-1 s-1) accounts for up to 90.8% of the product without extra screening process, exhibiting evident advantage over conventional bubble-exfoliation methods. Further mechanism analyses highlight the significance of the two-step strategy, i.e., pre-intercalation and subsequent intensified bubble generation manipulated by temperature, for highly efficient bubble exfoliation in one pot.
Graphene, owing to its unique properties, has attracted considerable attention from both fundamental and applied science communities over the past decade.1 High quality graphene often exhibits excellent performance in applications such as energy storage and electronics.2, 3 Therefore, the maximum reservation of graphene structure and properties is essentially important during preparation. Exfoliation of naturally abundant graphite has been demonstrated to be a viable top-down route for highquality graphene.4-6 Among previously reported efforts, exfoliation based on mechanical shearing force and electrochemistry in various liquid systems are considered as two most promising strategies for scalable fabrication in terms of both quality and costs.7, 8 However, these methods are currently limited by the low efficiency of exfoliation and tedious post-separation or screening process.5 While direct mechanical exfoliation gives a maximum yield of only 3.5% through graphite recycling7, electrochemical exfoliation reaches a maximum yield of 75% under optimum conditions8. We deduce that the higher efficiency of electrochemical exfoliation could be attributed to the combination of intercalation and bubble exfoliation. Intercalation can reduce the difficulty of exfoliation by lessening the adhesive forces between the constituent graphene layers. More importantly, bubbles’ generation and expansion between graphene layers provide direct and more effective exfoli-
ation than shearing force, which exerts mainly on the outmost layers of the graphite particles. Nevertheless, electrochemically generated bubbles also break the integrity of the working electrode and hinder the sufficient exfoliation of the detached graphite particles. As a result, there are always very thick graphite particles in the product that require post-screening. Therefore, scalable preparation of high-quality graphene with high efficiency remains a big challenge. To overcome the shortcoming of electrochemical method, bubble generation based on chemical reaction was proposed. Independent of electrical contact, chemical reaction that can generate continuous bubbles is expected to afford ultra-high yield when combined with intercalation. Previous works reported the bubble exfoliation of intercalated graphite based on the catalytic decomposition of hydrogen peroxide (H2O2).4, 6 Preintercalation of catalyst is performed to trigger the next bubble generation and thus the exfoliation. Despite high exfoliation efficiency, the occurrence of intercalation and bubble exfoliation in two different systems (namely two steps in two pots) would increase the complexity of mass production and thus the ultimate costs. Intercalation based on high temperature 4 or toxic intercalant 6 is also undesirable. Recently, J. M. Tour reported the mass production of graphene nanoplatelets with nearly 100% yield.5 This method simplified the procedure to one step in one pot, which is more suitable for
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Scheme 1. Schematic illustration of the two-step bubble-exfoliation strategy for graphene preparation in one pot. scalable fabrication. However, the resulting powders are mainly nanoplatelets of 10-35 nm in thickness, corresponding to 28-86 atomic layers.9 And its dependence on oleum to enhance exfoliation means extra costs and possible exposure to toxic SO3 fumes. Therefore, scalable preparation of high-quality graphene with high efficiency remains challenging. Herein, we propose a one-pot, two-step chemical method based on sufficient sulphuric acid intercalation and intensified chemical bubble exfoliation. Micron-sized graphene sheets (≤10 layers) of high quality were facilely prepared with a yield of 90.8%. Mechanism analyses further indicate, for the first time, that pre-intercalation and subsequent intensified bubble generation are equally necessary for efficient bubble exfoliation of graphite flakes in one pot. Scheme 1 presents the strategy we propose to prepare high-quality graphene. As an attempt to enhance the exfoliation efficiency, after the addition of 10 g Na2S2O8 to the mixture of 1 g graphite and 90 mL concentrated sulphuric acid, intercalation and bubble exfoliation are separated into two successive stages at 50 °C and 80 °C, respectively. Chemicals and detailed experimental procedure are described in the Supporting Information. The initial intercalation stage at 25 °C is featured by the change of sheets color from black to blue (Figure S1a), which is the indication of successful and sufficient intercalation5. This is further confirmed by the blue shift of the G peak (by 22 cm-1) and the suppression of 2D peak in the Raman spectrum (Figure S1b)5. The disappearance of the characteristic (002) diffraction peak of graphite at 2θ~26.5° (Figure S1c) and new characteristic peaks reveal the intercalation at 25 °C. During the 10-minute intercalation stage, bubbles generates in the system but in a very mild manner. Whereas, when the temperature increases to 80 °C, bubble generation becomes significantly fierce during the initial 5 min (Figure S2a-d) and then slows down with persistent bubble release in the next 30 min (Figure S2e). Bubble generation at 80 °C is initially accompanied by the rapidly volume expansion and formation of greenish-yellow foams and their slow transformation into green aggregates floating on the top layer (Figure S2f). The color transition may be caused by change of sheet thickness and intercalation
stage with the rapid consumption of the oxidant and varied circumstance of all particles. SEM images (Figure S3) of the precursor graphite and resulting powders after intercalation at 25 °C, initial exfoliation at 80 °C and deep exfoliation at 80 °C are presented respectively to well demonstrate the process of slight exfoliation to intensified exfoliation. As can been seen, graphite particles undergo mild and non-uniform exfoliation during the 10-minute intercalation at 25 °C, presumably due to the insufficient bubbles. Whereas, when the temperature is elevated to 80 °C, the intercalated graphite particles quickly expand to worm-like structure composed of thick platelets in one minute and then to expanded particles composed of thin and interconnecting sheets within 30 min. This bubbledependent exfoliation in microscopic scale is consistent with the reaction phenomenon observed and clearly indicate the driving role of bubbles for exfoliation. Also from the observations, we confirm that intercalation is successfully separated from exfoliation by temperature manipulation. In a typical batch, intercalation and exfoliation accomplish within one hour, followed by normal washing and drying to obtain about 0.5 g black powder with an average apparent density of 0.08 g cm-3. The hydrophobic nature of the powder greatly simplifies the extraction and purification process in concentrated sulphuric acid and water. No extra screening step like centrifugation gets involved in the procedure and the production volume is limited only by the size of reactor used. Transmission Electron Microscope (TEM) and Atomic Force Microscope (AFM) are employed to assess the efficiency of the newly proposed bubble exfoliation strategy. Typical TEM image (Figure 1a) and height profile in AFM image (Figure 1c and d) show the presence of graphene sheets with thickness no more than 10 atomic layers. This indicates that the bubble exfoliated and expanded particles can be further exfoliated to isolated graphene sheets through mild sonication (200W, 10 min). Six-fold symmetry in the Selected Area Electron Diffraction (SAED, inset in Figure 1a) pattern is an indication of the preserved sp2 carbon structure after exfoliation. More TEM images of the resulting powders (Figure S4 a-h) show the coexistence of single layer graphene, few-layer graphene and graphene nanoplatelets with layer number in 11-20 range.
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Figure 1. Characterization of the graphene. (a)Typical TEM image and corresponding SAED (inset); (b) Layer distribution determined by TEM statistics; Typical AFM image (c) and the height profile (d) along the blue line in c; (e) Raman spectra and (f) high-resolution C 1s XPS spectra.
Thicker nano-platelets (4.5 nm corresponding to 13 layers9) are also detected by the AFM measurement as seen in Figure S5. Thickness statistics based on the TEM images (Figure 1b) indicate that graphene sheets (≤10 layers) accounts for up to 90.8% of the powder sample. Though with a relatively low fraction of single layer graphene (approx. 8%), we should emphasize that the thickness distribution demonstrates a much higher exfoliation efficiency than previously reported one-step chemical bubbleexfoliation routes.5, 10 The lateral size are revealed to be several to tens of microns by the TEM and AFM results, which is obviously larger than that mechanically exfoliated in liquid phases (300-800 nm)7. Statistical areal distribution (Figure S6) shows that about 70 % of sheets exhibit areal sizes range from 1 to 50 µm2 and the maximum areal size can be up to 250 µm2. Larger average sheet area (54.7 µm2) than the sheets (5.1 µm2 on average) prepared by latest electrochemical method 11 and other intercalationexfoliation method (maximum at 50 µm2) 12 is also an evident advantage of our method. Besides, we should also pay attention to the Moiré fringes13, overlapping and folding of graphene sheets revealed by the TEM images (Figure S4 a-g). We extract the exfoliated graphene from concentrated sulphuric acid and water. From the perspective of surface energy, it is not difficult to understand that hydrophobic graphene sheets tend to re-stack together in water. X-ray Diffraction (XRD, Figure S7) also reveals the exfoliated yet stacking state of the graphene powder. However, combined the AFM data with and TEM results, we deduce the stacking sheets can easily isolate from each other when dispersing in alcohol and other solvents. Comparative Raman and XPS spectra enable directly probe of the structure and composite change during the two-step preparation. Quite different from the starting graphite (Figure 1e), an almost symmetrical 2D peak cen-
tered at 2703 cm-1 demonstrates the presence of few-layer graphene. The relative broader 2D peak (FWHM ~80 cm-1) and its weaker intensity (I2D/IG=0.42) than single-layer graphene suggest layer number more than five.14 Notably, the minimal D peak (ID/IG~0.03) located at 1348 cm-1 indicates high quality of the obtained graphene powder. Compared with the Raman signal of liquid-exfoliation based graphene sheets, which generally have an evident D peak owing to the small size (generally ≤5 μm, and typically ≤1 μm) and large fraction of edge defection, the much weaker D signal also confirms the large lateral size of graphene and intact structure of the basal plane. XPS survey spectra (Figure S8) of the graphene powder show a prominent C 1s peak at 284.8 eV with small amount of O (6.3 at.%) while the spectra of the precursor graphite shows a trace amount of O (1.8 at.%). Figure 1f show high resolution XPS scans for C 1s peaks, which can be fitted with three different peaks with binding energy at 284.7, 285.5, and 286.6 eV, assigned to C=C, C-C, and C-O bonds15, respectively. The obvious peak area change of C-C and C-O evidences that the increased C-O is mainly derived from the dangling bonding at the edge. Considering the restored sp2 structure reflected by Raman results, edge oxidation to hydroxyl is responsible for the slight increase of oxygen content. To further investigate the microscopic structure, the powder is characterized by Scanning Tunneling Microscopy (STM). As shown by the magnified image (inset of Figure S9), a typical honeycomb lattice structure is observed. The STM images show few to no defects in a range greater than 100 nm2, indicating the high crystalline quality of obtained sheets. This intact sp2 structure inducing strong π-π interaction is also a key factor to aggravate sheet re-stacking after exfoliation. Comparable bulk conductivity (3.38×104 S m-1 for compacts and 1.01×105 S m-1 for films, Table S1 and Figure S10) and higher mobility (554.8 cm2 v-1 s-1 on average ) of isolat-
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ed sheets (Figure S11) compared with previous reports 4, 6, 11 provides another evidence for the high crystal quality of the as-prepared graphene. It is interesting that the pristine structure of the graphene can be preserved in a mixture of concentrated sulphuric acid and oxidant. We ascribe this to the relatively weak oxidant of Na2S2O8 compared with K2MnO8 and its rapid depletion at 80 °C as described in the following discussion. The above results undoubtedly indicate the proposed method is highly effective for preparing high-quality graphene. A comparison of various bubble-exfoliation methods in Table S2 confirms the advantages of the new methods in terms of the exfoliation efficiency and the potential for mass production. Different from the two-pot methods, intercalation and bubble exfoliation, as observed during our preparation and shared by other one-pot methods, are two main processes occurring in a system composed of persulfate, concentrated sulphuric acid and graphite. We make big progress in the exfoliation efficiency simply through a two-step strategy based on temperature regulation and the underlying mechanism deserves further exploration. Intercalation is a well-known process that takes place when graphite is added to the mixture of concentrated sulphuric acid and proper oxidant, 5, 16 e.g., Na2S2O8 in our system. However, as demonstrated in the two-step, twopot methods4, 6, intercalation alone does not necessarily lead to exfoliation of graphite. Bubble generation and expansion provide essential and dominant driving force for exfoliation in the high-viscosity fluid. This is also consist with the morphology evolution of graphite during the whole preparation process describe above. Since intercalation is identical to previous reports, the enhanced exfoliation in our work must originate from the change of bubble process. To understand the origin of bubbles, we start by investigating the chemical behaviors of Na2S2O8 in concentrated sulphuric acid in the absence of graphite. We found that Na2S2O8 exhibits poor dissolution in concentrated sulphuric acid both at 25 °C and at 80 °C. Upon the addition of 0.5 g Na2S2O8 to 45 mL concentrated sulphuric acid at 80 °C, gas bubbles emerge and coexist with white precipitate in the system (Figure S12a-c) and last for about 40 minutes. Bubbles generate on the surface of the white participate and finally disappear with the depletion of the white participate. Similar phenomenon is observed at 25 °C but with a much longer process of bubble generation and precipitate depletion. The tail gas is collected and confirmed to be pure oxygen (O2) by the Gas Chromatography (GC) as presented in Figure S12d. It is clearly that the reaction of Na2S2O8 leads to the formation of O2 and some products that can dissolved in concentrated sulphuric acid. We extract white solid from the system by adding alcohol after complete reaction, but fail to determine the crystal structure through X-ray Diffraction (XRD) measurement (Figure S12e). Considering the instability of persulfates, we deduce that decomposition of Na2S2O8 is responsible for O2 and corresponding byproduct(s) in concentrated sulphuric acid. Further quantitative analysis (Figure 2) of the gas generation rate using the homemade set-up (Figure S12f) indicates that the bubble generation reaction is not only influenced by temperature,
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but also graphite dependent. Graphite can significantly boost the bubble generation rate both at 25 °C and at 80 °C but has little effect on the total gas volume. This can be ascribed to the catalytic activity of defective graphite edge areas toward Na2S2O8 decomposition, which may be endowed with by edge functionalization in concentrated sulphuric acid.17 It should be pointed out that in-situ and continuous bubble formation at the graphite edge is the precondition for effective exfoliation. This mechanism of bubble generation is, to some extent, similar to the twopot methods, in which pre-intercalated interlayer catalyst initiates and boosts the H2O2 decomposition to produce O2 at graphite edges.4, 6 Agitation is found to be critical for enhanced exfoliation and accelerated bubble generation (as seen in Figure S13). This can be explained by the promotion of contact between graphite particles and Na2S2O8 through agitation to enable catalytic decomposition reactions. Temperature is another factor should be given enough priority. With the presence of graphite (Figure 2a and b), maximum bubble generation rate increases by about 55 times when we change the temperature from 25 °C to 80 °C. The one-step exfoliation shows distinct features at the two temperatures as suggested by the SEM images (Figure S14a-f). Isolated and very thick platelets are obtained at low temperature while uniformly expanded particles constitute the high-temperature formed powder. Bubble exfoliation at high temperature exhibits evident advantage in exfoliation efficiency. However, in both cases, we can easily find very thick platelets exceeding 40 atomic layers when we carry out the TEM measurements (Figure S14g-h), indicating lower exfoliation efficiency of one-step route. Volume of the obtained powders from 0.5 g graphite through different routes (Figure S14i) also reflects the higher efficiency exfoliation of two-step method. These results undoubtable highlight the importance of both pre-intercalation at low temperature and intensified
Figure 2. Kinetic analyses on the dependence of bubble generation on temperature and graphite. Gas volume and generation rate at 25 °C (a) and 80 °C (b) as a function of reaction time for 5 g Na2S2O8 in 45 mL concentrated H2SO4 with or without the presence of 0.5 g graphite.
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bubble exfoliation at high temperature. Relatively low exfoliation efficiency at 80 °C in one-step route can be ascribed to the rapid depletion of Na2S2O8, which is the condition for adequate intercalation of H2SO4. Therefore, the mechanism of increased bubble-exfoliation efficiency can be interpreted as following: pre-intercalation can weaken the interlayer Van der Waals' force (the resistance force of exfoliation) and facilitate the exfoliation while rapid generation of bubbles at high temperature increases the driving force to overcome the weakened layer interaction. Temperature regulation to kinetically separate and intensify both intercalation and bubble exfoliation is the key to successful graphite exfoliation. In conclusion, we demonstrate a two-step, one-pot approach to facilely fabricate high-quality graphene. Graphene (≤ 10 atomic layers) of high quality accounts for 90.8% of the product. Adequate pre-intercalation to weaken the resistance force and intensified bubble generation to increase driving force through temperature manipulation is proven to be significant for highly efficient exfoliation of graphite. Independent of very high temperature intercalation, toxic intercalant and dangerous exfoliation-promoting agent, this method would greatly benefit the scalable fabrication.
ASSOCIATED CONTENT Supporting Information. Experimental Section; Evidences of intercalation at 25 °C; Exfoliation of the intercalated graphite at 80 °C for different time; SEM images showing the process of intercalation and bubble exfoliation; AFM image of typical nanoplatelet; TEM characterization of the graphene powder; Areal size statistics of graphene; XRD patterns of the precursor graphite and the bubble-exfoliated graphene; XPS survey spectra of the precursor graphite and the bubble-exfoliated graphene; STM 2 topography (10×10 nm ) of the graphene; Detailed data of the bulk conductivity measurement; Comparison of bulk electrical conductivity between the new method and the latest or best methods toward high-quality graphene; Mobility of isolated graphene sheets; Comparison of different bubbleexfoliation methods for mass fabrication of high-quality graphene; Kinetic analyses on chemical behavior of Na2S2O8 in concentrated sulphuric acid; Influence of agitation on exfoliation; Assessment of one-step bubble exfoliation. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *Email:
[email protected] (Prof. G. Ding) *Email:
[email protected] (Prof. Z. Liu)
Author Contributions †
These authors contributed equally to this work. Peng He conceives and designs the experiments. Peng He, Hongyu Gu and Gang Wang carry out the characterizations and data analysis. Peng He and Hongyu Gu co-write the paper. Guqiao Ding, Zhi Liu and Xiaoming Xie give many suggestions and directions during the experiments and manuscript revision. All authors attend the discussion of this work.
ACKNOWLEDGMENT
The authors thank Fuping Du and Xiuming Bu for measurement help. This research was supported by National Natural Science Foundation of China (Grant No. 11774368 and 11227902) and the Laboratory Foundation of Chinese Academy of Sciences (Grant No.16S085).
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