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Green and Mild Oxidation: An Efficient Strategy towards Water-Dispersible Graphene Xiaofei You, Siwei Yang, Jipeng Li, Yuan Deng, Lianqi Dai, Xiong Peng, Haoguang Huang, Jing Sun, Gang Wang, Peng He, Guqiao Ding, and Xiaoming Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13703 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

Green and Mild Oxidation: An Efficient Strategy towards Water-Dispersible Graphene Xiaofei You,#,†,‡ Siwei Yang,#,†,‡ Jipeng Li,§ Yuan Deng,§ Lianqi Dai,& Xiong Peng,& Haoguang Huang,†,‡ Jing Sun,† Gang Wang,† Peng He,*,†,‡ Guqiao Ding,*,†,‡ and Xiaoming Xie†,‡ †

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences, 865 Changning Rd., Shanghai 200050, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Ophthalmology, Shanghai Ninth People’s Hospital, Shanghai Jiaotong University School of Medicine, Shanghai 200011, People’s Republic of China & Zhejiang CHINT Cable Co., LTD., Zhejiang 314006, People’s Republic of China

KEYWORDS: Graphene, Dispersibility, Reduced Graphene Oxide, Hydroxyl Radical, Biomaterial.

ACS Paragon Plus Environment


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ABSTRACT: Scalable fabrication of water-dispersible graphene (W-Gr) is highly desirable yet technically challenging for most practical applications of graphene. Herein, a green and mild oxidation strategy to prepare bulk W-Gr (dispersion, slurry and powder) with high yield was proposed by fully exploiting structure defects of thermally reduced graphene oxide (TRGO) and oxidizing radicals generated from hydrogen peroxide (H2O2). Owing to the increased carboxyl group from the mild oxidation process, the obtained W-Gr can be re-dispersed in low-boiling solvents with a reasonable concentration. Benefiting from the modified surface chemistry, macroscopic samples processed from the W-Gr show good hydrophilicity (water contact angle 55.7o) and excellent bio-compatibility, which is expected to be an alternative biomaterial for bone, vessel and skin regeneration. In addition, the green and mild oxidation strategy is also proven to be effective for dispersing other carbon nanomaterials in water system.


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INTRODUCTION Graphene has attracted tremendous attention from both the experimental and theoretical scientific communities in recent years due to its unique properties.1, 2 It holds great promise for potential applications in nanoelectronics,3 new energy resources4, 5 and biologically active substrates.6, 7 In general, graphene dispersed in various solvents is the prime precursor of most macroscopic graphene-based materials, such as coatings,8 papers,9,

10

tapes11 and functional composites.12

Compared with graphene organic dispersions, graphene aqueous dispersion attracted more attentions owing to the non-toxic, easy-to-remove, and low-cost features of water.13 However, it is noteworthy that both the super-hydrophobic nature and the strong intersheet van der Waals attractions (π-π stacking) of graphene make itself hard to disperse in water.14 Currently, prevailing approaches to address this problem can be categorized into two strategies: (1) chemical modification of graphene surface with hydrophilic groups (such as sulfonate groups15, 16

) and (2) auxiliary dispersing with the aid of extra stabilizers (such as surfactants,17 polymers18

and small molecules19,

20

). For both strategies, graphene oxide (GO) is commonly used as

precursor due to its abundant hydrophilic groups and excellent dispersibility in water. Aqueous graphene dispersion can be obtained directly from well-dispersed GO solution if aggregating could be prevented during the chemical reduction. In chemical modification case, enough modification for dispersing and maximized restore of crystal structure would ensure graphene’s good dispersibility in water as well as other excellent intrinsic properties, electrical conductivity for example. Thus, special attention should be paid to balance the reduction extent and surface modification. Different from chemical modification, auxiliary dispersing maintains the dispersing state during reduction and has little effect on the reduction extent. However, the presence of extra stabilizers will undermine the properties of graphene and are generally undesirable in many practical applications.21, 22 In both cases, post treatments are always required to remove introduced but unreacted chemical reagents after complex treating processes. Thus pollutional and harmful by-products like organic molecules, inorganic salts or waste bases are unavoidable. Recently, we have tried to directly exfoliate hydrophobic graphite into water-dispersible graphene (W-Gr) by controllable edge oxidation23 and electrochemical oxidation24. Though suitable for preparation of high-quality graphene, there methods still suffer from low yield (50-60 %) and small concentration (1-5 g/L). Therefore, facile and scalable preparation of high concentration W-Gr, especially when efficiency and environmental effects are considered, remains a challenging issue. Herein, a green and mild oxidation strategy based on hydrogen peroxide (H2O2) was proposed to enhance hydrophilicity of thermally reduced graphene oxide (TRGO) and fabricate W-Gr in large scale. Unlike the conventional methods, TRGO was used as precursor instead of GO or graphite due to its being dominant commercially available and well-proven potentials for wide applications.25,

26

After 4-15 h of mixing with H2O2 at room temperature, the originally

hydrophobic TRGO was afforded with excellent dispersibility and stability in water. Aqueous



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paste and uniform dispersions in low-boiling solvents were also easily fabricated from the lyophilized W-Gr powder owing to the re-dispersibility. More importantly, by fully utilizing the intrinsic defects to enrich hydrophilic groups and obtain dispersibility, the mild oxidation does not compromise the crystal structure of TRGO seriously, which is demonstrated by the structure characterizations and the reasonable electrical conductivity. Besides, the mild oxidation strategy is much more environmentally friendly since the only by-product is water and the reaction is carried out at room temperature. Accompanied with high batch yield and output, our method is also suitable for scalable fabrication of high concentration W-Gr. Good dispersibility and chemically modified surface enable the preparation of hydrophilic W-Gr sample with excellent hydrophilicity (water contact angle, 55.7o) and excellent bio-compatibility, which is expected to be an alternative biomaterial for bone, vessel and skin regeneration. Expansion of this facile method to prepare aqueous dispersions of other carbon nanomaterials such as activated carbon, carbon nanotube and fullerene was also demonstrated.

EXPERIMENTAL SECTION Materials. Hydrogen peroxide (H2O2, ≥30 wt. %) and manganese sulfate (MnSO4, ≥99 %) were purchased from Aladdin (Shanghai, China) and used as received without further purification. TRGO powder was provided by SIMBATT (Shanghai, China). The preparation process is detailedly described in Supporting Information. The water used throughout all experiments was purified through a Millipore system. Preparation of W-Gr. In a typical experiment, 300 mg TRGO was homogeneously mixed with 50 mL MnSO4 solution (0.8 mg/L) at room temperature. Thereafter, 50 mL 1.0 mol/L H2O2 was added into the mixture and continuous stirring was performed for 4 hours. Then, the mixture was centrifuged and purified to get thick slurry. W-Gr aqueous dispersion was obtained by diluting the slurry with specific amount of deionized water and sonicating for 0.5 h. The slurry can also be lyophilized to obtain W-Gr powders. For large-scale preparation, 50 g TRGO underwent similar process at room temperature and about 54.6 g W-Gr powder can be obtained on every batch. Redispersing of the W-Gr powder results in the bulk fabrication of graphene dispersion. Preparation of Water-Dispersible Active Carbon/Carbon Nanotube/Fullerene. While commercial active carbon was directly used due to its inherent abundant sp3 carbon, carbon nanotube and fullerene were pretreated under oxygen plasma (75 W, 40 sccm, 30 min) in PE-50 series plasma system (Plasma Etch, Inc., USA) to introduce necessary chemically active sites for modification. Then, active carbon/carbon nanotube/fullerene underwent similar process with TRGO at room temperature. Fabrication of W-Gr Papers. Papers prepared in this study were fabricated through vacuum filtration technique. Typically, the dispersions were filtrated through 20 nm AAO membranes (Shanghai Shangmu, China), leading to free-standing papers that could easily be peeled off from


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membranes after drying. Prior to characterization, all papers were dried overnight under 60 °C to remove the remaining water. Biotic

Experiments.

Rat

adiposetissue-derivedstromalcells

(rADSCs)

culture

and

encapsulation rADSCs were obtained from subcutaneous adipose tissue in the inguinal groove of 6-week-old male Sprague Dawley rats (Shanghai Animal Experimental Centre, China) and cultured in F12/DMEM (Dulbecco's Modified Eagle Media: Nutrient Mixture F-12) supplemented with 10 % FBS (Invitrogen) and 100 units/L penicillin-streptomycin (Invitrogen) according to our protocol. Once W-Gr sample was putted on the bottom of a 24-well plate, rADSCs were seeded onto the upper layer of W-Gr sample at a density of 2×104 cells/well. Also, the 24-well plate was added with 1.0 mL F12/DMEM medium each well for culture at 37 oC in an atmosphere with 5 % CO2. Live/dead Assay and CCK-8. A live/dead assay was performed on days 1 and 7. The cell culture medium was removed and the W-Gr sample was washed with phosphate-buffered saline, pH = 7.2. 1.0 mL of medium (without serum) containing 0.001 mM calcein-AM and 0.001 mM ethidium homodimer-1 (Invitrogen) was added to each specimen. The cells were then observed by a Leica TCS SP8 microscope (Leica Microsystems, Germany). Live cells were stained green, dead cells red. Two parameters were measured. First, the percentage of live cells was measured, which was defined as PLive = NLive/(NLive + NDead), where NLive is the number of live cells and NDead is the number of dead cells in the same image. Three specimens of each material were tested (n = 3). Two randomly chosen fields of view were photographed for each specimen for a total of 6 photos per material. The second parameter was live cell morphology. This included rADSCs shape and their distribution. The proliferations of rADSCs were assessed using a cell counting kit-8 (CCK-8). 100 microliters of CCK-8 solution (Dojindo Molecular Technologies, Inc., Japan) was added to each well and incubated with the cells for 4 h at 37 oC. The absorbance was measured at a wavelength of 450 nm. Characterization Methods. Ultraviolet-visible (UV-vis) spectra were recorded on a Carry-100 UV-vis Spectrophotometer. Malvern Zetasizer Nano-ZS90 was used to measure Zeta potential of the nanosheets in the dispersions. Scanning electron microscopy (SEM S4700, Hitachi, Inc.) and transmission electron microscopy (TEM G2 20, FEI Tecnai) were used to image the morphology of the samples. TRGO and W-Gr powders were dispersed in water under 30-minute ultrasonication for SEM test and in alcohol under 1 h ultrasonication for TEM test. Samples for SEM and TEM measurements were prepared by evaporating a drop of water or alcohol dispersion onto a silicon wafer or a carbon-coated copper grid, respectively. Atomic force microscopy (AFM) data were obtained in a Bruker Dimension Icon with a Nanoscope 8.15 in tapping mode. The sample was prepared by dropping a diluted W-Gr ethanol dispersion on a silicon wafer. Raman spectroscopy with the excitation laser line of 532 nm was performed using a ThermoFisher DXR Raman Microscope. The conductivity of TRGO and W-Gr samples was characterized by a four-point-probe method using HALL 5500PC while the density was obtained by measuring the thickness of samples by micrometer. X-ray photoelectron spectroscopy (XPS) measurements were



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carried out using a Thermo ESCALAB 250Xi spectrometer. A non-linear least squares curve fitting program (XPSPEAK41 software) with a Gaussian-Lorentzian mix function and Shirley background subtraction was used to deconvolve the XPS peaks. The peak constraints for fitting were used. Atomic ratios of different oxygen-containing functional groups were calculated from the XPS spectra after correcting the relative peak areas. Fourier transform infrared spectroscopy (FTIR) was recorded by a Bruker Vertex 70v FT-IR spectrometer under vacuum (< 100 Pa) in KBr pellet. Water contact angle (θc) measurements were performed with FTA200 Goniometer (First Ten Angstroms, Inc., USA). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were characterized using a thermal analyzer (STA449F3, NETZSCH, Germany) under a flow of argon kept at 20 mL/min. TRGO and W-Gr samples were tested over a temperature range of 30-600 oC with a heating rate of 10 oC/min. Conductivity, biotic and water contact angle measurement samples were prepared by compressing powders to various degrees in stainless steel die (Φ 30 mm). Fluorescent emission and excitation spectra were recorded on a PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at room temperature (25 °C) in aqueous solution.


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RESULTS AND DISCUSSION

Figure 1. (a) Roadmap of the green and mild oxidation strategy for dispersions, slurries and powders of W-Gr. (b) Sedimentation behavior of W-Gr aqueous dispersion; (insets) as-prepared W-Gr aqueous dispersion and dispersion after 60-day standing in 10×10 mm quartz cuvettes by a red laser beam displaying evident Tyndall effect. (c) Zeta potential of TRGO and W-Gr, verifying the highly negatively charged surface of W-Gr. Mild Oxidation for W-Gr. Currently, TRGO is the prevailing graphene available in market and its aqueous dispersion is of great importance for further applications. To obtain stable, high concentration aqueous dispersion from this intrinsically hydrophobic graphene, chemical



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modification is a better choice considering that the presence of extra stabilizers is undesirable for their possible detrimental effects on the terminal properties. From the perspective of commercialization, greenness and scalability, modification approach that excludes involvement of any hard-to-remove reagent should be taken into consideration. Bearing these in mind, we propose the H2O2 based mild oxidation strategy to enrich hydrophilic groups of TRGO and afford it dispersibility in water. This proposal stems mainly from two considerations (1) Oxidation represent a general way to graft oxygen-containing groups on carbon material;10 (2) H2O2 is a well-known green oxidant, releasing free radicals (such as ·OH and ·OOH) of reasonable oxidability while generating water as the only by-product.27 The oxidation process, as detailed in the Experimental Section, starts with the mixture of TRGO, minimum catalyst (MnSO4) and diluted H2O2 solution (1.0 mol/L). Finally, 300 mg lyophilized W-Gr powder completely dispersed into 200 mL deionized water with no obvious aggregates observed after 2-month-storage (Figure S1, Supporting Information). This was quite different from the control experiment result without the presence of H2O2, in which TRGO powder floats on the water surface. The H2O2 treated W-Gr aqueous dispersion, as depicted in Figure 1a, can be either diluted into dispersions with good flowability or distilled into thick paste of high apparent viscosity (depending on the content of graphene). Excellent stability without macroscopic aggregations or phase separation was remained during 2-month-storage of these mixtures. Subsequent lyophilisation of those mixtures resulted in the powders that can be easily re-dispersed in water with the aid of mild sonication. This lyophilize-redisperse-process is necessary when the downstream application is sensitive to H2O2. Furthermore, powders are more economical and convenient for storage and transport than solutions in practical application. It should be mentioned that, for a certain amount of H2O2 and the same treatment period, excessive TRGO would lead to poor stability or even aggregates. Further study showed that controlling of the TRGO used and extending time of the H2O2 treatment can solve this problem. When initial content of TRGO ranges from 5-10 g/L, an oxidation time of 8-15 h is required. We ascribe the poor stability for short period of treatment to the inherently aggregated microstructure of the commercial TRGO, as shown in SEM images of the original powder (Figure S2a, b, Supporting Information). So, prolonged stirring time is required to break the original aggregates and achieve adequate treatment during the oxidation. To quantitatively assess the effectivity of this method, we characterized the stability and Zeta potential of the diluted aqueous dispersion (1.5 g/L) fabricated from W-Gr powders. W-Gr powders derived from a typical experiment (4 h of H2O2 treatment) were chosen to ensure the representativeness. The stability of the W-Gr aqueous dispersion was monitored by UV-vis spectroscopy. Obviously, W-Gr dispersion shows absorption peak at 276 nm (Figure S3, Supporting Information), suggesting the presence of sp2 carbon structure.28 According to previous report,21 the change of absorbance at 660 nm against time was collected to reflect the change of graphene content against storing time in the dispersion. The sedimentation of dispersion, causing


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the loss of absorbance, is generally more evident during the initial stage of storing,20, 29 So the absorbance change of the as-prepared dispersion (1.5 g/L) in the first sessions is monitored. As shown in Figure 1b, the loss of absorbance at 660 nm is only about 2% after 60 h, which is in accord with our experimental observation. The absorbance-time curve of W-Gr aqueous dispersion indicates a very stable system. Insets in Figure 1b show the as-prepared W-Gr aqueous dispersion and dispersion after 60-day standing in 10×10 mm quartz cuvettes irradiated by a red laser beam. The Tyndall effect suggests the colloidal nature and homogeneous dispersing state. Besides, W-Gr aqueous dispersion also shows good stability at high temperature (Figure S4, Supporting Information). For a 1.5 g/L W-Gr aqueous dispersion, increase of the storing temperature indeed leads to deterioration of the dispersing stability, but only a concentration of approx. 6 % was lost during 120 h of standing at 80 oC. This minimum loss of graphene content at elevated temperature would facilitate the transportation and long-term storage. Meanwhile, Zeta potential measurement (Figure 1c) shows the obvious increase of the average value from -3.7 mV (TRGO aqueous suspension) to -49.0 mV (W-Gr aqueous dispersion). Therefore, the good stability during 2-month storage can be explained by highly negatively charged surface of the dispersed graphene. Long-time stability is predictable since stable dispersing of general colloidal system is characterized by Zeta potential more negative than -30 mV.30 The significant change of surface charge also indicates the successful modification of TRGO though mild oxidation. Table 1. A comparison among various approaches for W-Gr. Methods

Chemical modification

H2O2 treatment Acid treatment Alkali treatment

Auxiliary dispersing

Polymer surfactant

Modifiers

T(oC)

By-produ cts

Yield (%)

Output (g/Batch)

H2O2

RT

H2O

109.2

54.6

This work

Sulfanilic acid, N2H4

0, 100

IS

73.2

0.060

16

KOH

PHT

60.0

0.030

31

NaOH, KOH

160

N2H4, NaOH Acrylamide, N2H4 QAP, N2H4 PEG, N2H4

Ref.

42.2

0.310

32

80

KOH, SOM KOH, NaOH IS

60.0

1.200

33

PHT

SOM

52.1

0.005

34

95 70

SOM SOM

56.3 63.0

0.051 0.002

35 36

T: Reaction temperature, QAP: Quaternary ammonium pendant, PEG: polyethylene glycol, RT: Room temperature, PHT: Partial high temperature, IS: Inorganic salts, SOM: Small organic molecules. Up to date, various methods based on different precursors and strategies have been developed to prepare water-dispersible graphene. A comparison of current dominant methods is necessary to assess the proposed mild oxidation strategy. Typical methods were compared from the point of greenness and scalability (Table 1). The agents and by-products involved, the temperature required, the batch yield and output are emphasized here for the sake of scalable fabrication. For traditional



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approaches, strong reductants (such as: N2H4), strong bases (such as: NaOH and KOH) and polymer surfactants (such as: QAP and PEG) are commonly used and extreme rigorous temperature conditions are always needed (heat or refrigeration to control the reaction). Thus pollutional and harmful by-products like organic molecules, inorganic salts or waste bases are unavoidable. In comparison, with water being the only by-product, our H2O2 based mild oxidation strategy is much more environmentally friendly. Moreover, the whole process is carried out at room temperature, which is suitable for large scale preparation. Moreover, the yield and batch output of our approach is higher than that of previously reported approaches by two orders of magnitude. In terms of greenness and scalable fabrication, we can clearly identify the advantages of our method in practical applications over conventional methods. Characterization of W-Gr. Further characterizations were carried out to confirm the microstructure and composition of the product after H2O2 treatment. As-prepared samples were used together with the untreated TRGO for comparison. Typical microstructure depicted by the SEM (Figure S5, Supporting Information) and TEM images (Figure 2a, b) clearly indicate that the powders comprise of micro-sized and freely outstretched sheets of few-atomic-layer graphene. The densely distributed wrinkles (characteristic of TRGO) on the sheets are similar to that of the sheets before treatment (Figure S2c, Supporting Information). Statistical analysis (Figure 2c) based on TEM images of isolated sheets (Typical TEM image is shown in Figure S6a, Supporting Information) show the distributions of the sheet thickness and the lateral size, which is averaged at 3 layers and 5.2 µm2, respectively. As another method to determine thickness and lateral size, AFM image gives dimension feature (Figure S6b) consistent with the TEM-based statistical results. TRGO provided by SIMBATT is comprised of graphene sheets with small fraction of single-layer graphene sheet. This scalable graphene product was used because we aimed at the development of the new technology to obtain W-Gr that suitable for bulk production. Different with deep oxidation of traditional modification methods that may reduce the thickness of graphene into single layer, our mild oxidation occurs mainly on the defect sites of TRGO (as detailed in the Oxidation Mechanism section) and is unlikely to change the atomic layer. Hence, the resulting W-Gr would inherit the thickness of the staring materials. With the similar structure, we think other graphene products are also viable and higher fraction of single-layer W-Gr is also possible if we rationally choose the staring graphene. Selected-area electron diffraction (SAED) taken from different positions on individual sheets were shown in Figure 2d. Well-deﬁned diﬀraction spots in site 1 and 2 together with diffuse rings in site 3 and 4 illustrate the existence of both sp2 crystalline domains and defects in the H2O2 treated sheets.37 It should be mentioned that the co-presence of crystalline domains and defects is also the structure feature of the untreated TRGO as reflected by SAED (Figure S2c insert). All these results suggest that the morphology of the graphene undergoes no obvious change during the H2O2 treatment.


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Figure 2. (a, b) High-resolution TEM (HRTEM) and TEM images of W-Gr. (c) Thickness and area distribution histogram of W-Gr with the black Gaussian fitting curve. (d) SAED images of W-Gr suggesting good in-plane carbon crystalline structure of the W-Gr sheets. (e) Raman spectra of TRGO and W-Gr showing ID/IG of 0.907 and 0.923 respectively. (f) C 1s XPS spectra and (g) O 1s XPS spectra of TRGO and W-Gr revealing the different surface chemistry. Raman spectroscopy and XPS further revealed the structure and surface chemistry at the molecular level. It is well known that the G peak (1586 cm-1) in Raman spectrum related to the vibration of sp2 bonded carbon atoms while the D peak (1354 cm-1) gives evidences for the presence of either edges or topological defects in the graphene sheet.38 Moreover, the intensity ratio of D peak and G peak (ID/IG) is an important basis to estimate the defect level of graphene.39 Therefore, the D and G peaks in Raman results (Figure 2e) further confirm the presence of both



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crystalline and defects in treated W-Gr sheets. The similar curve shape and slightly increased ID/IG (from an average value of 0.907 to 0.923) compared with the untreated TRGO samples indicate no significant increase of defect content after H2O2 treatment. In combination with the TEM results, Raman curves also confirm that the powders undergone H2O2 treatment preserved the main structure of TRGO. Actually, from the relationship between density (ρ) and conductivity (κ) (Figure S7, Supporting Information), we can come to the conclusion that the mild oxidation treatment did not destroy the crystalline structure of TRGO heavily. Though with no significant change in defect content, considerable variation of surface chemistry was detected by XPS. Firstly, XPS results show that the oxygen content increased from 8.3 at. % to 15.8 at. %, indicating mild degree of oxidation during H2O2 treatment. Secondly, the composition of oxygen-containing groups underwent obvious change. As seen in Figure 2f, g, both C 1s and O 1s spectra show that oxygen exists mainly in the form of oxygen-containing groups. The C 1s spectra of TRGO and W-Gr both can be divided into four peaks around 284.73, 286.03, 287.13 and 289.15 eV, corresponding to the signals of C=C/C-C, C-OH/C-O-C, C=O and O-C=O groups, respectively.20,

40

This indicates TRGO and W-Gr have the same type of

oxygen-containing groups. However, the content of O-C=O group, i.e. the carboxyl, in W-Gr (12.7 %) is far higher than that in TRGO (3.2 %). Furthermore, the O 1s spectra of TRGO and W-Gr can be divided into two peaks around 531.63 and 533.55 eV, corresponding to the signals of C=O/O-C=O and C-OH/C-O-C.41,

42

The obviously increased content of C=O/O-C=O groups

again confirmed the C 1s spectra results. FT-IR spectra (Figure S8, Supporting Information) show similar result. The spectra of TRGO and W-Gr reveal the presence of O-H stretching around 3428 cm-1, C=O stretching around 1720 cm-1, and C-O stretching around 1087 cm-1.43, 44 Remarkably, the peak around 1720 cm-1 of carboxyl has been enhanced relatively, indicating that the content of carboxyl increases in oxygen-containing groups after the oxidation process. Therefore, we can conclude that the increased carboxyl content is mainly responsible for the increase of oxygen level during H2O2 treatment and this delicate change of surface chemistry might contribute to the dispersibility of W-Gr. TGA and DSC results also confirm the increased content of oxygen-containing groups, which lead to the improvement of graphene’s hydrophilicity. As is shown in Figure S9, different from TRGO, a sharp mass loss (~14.4 %) of W-Gr takes place around 100-300 oC, while the DSC curve of W-Gr also shows a sharp endothermic peak correspondingly. According to previous reports,45-48 this mass loss is ascribed to the decomposition of hydrophilic but labile oxygen-containing groups, including carboxylic, alcohol and epoxy groups, introduced during the H2O2 treatment. In comparison, TGA and DSC curves of TRGO show similar feature but with less mass loss from the start to 600 oC than W-Gr. It is not difficult to understand this difference considering the high temperature treatment (450 °C) during the preparation process of TRGO. These results are consistent with XPS results, and further confirms the mild oxidation process. Actually, like in GO aqueous dispersions, the electrostatic repulsive forces generated by oxygen-containing groups (especially carboxyl with higher ionization degree


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in water) between graphene sheets are mainly responsible for the formation of well-dispersed graphene aqueous dispersion.10

Figure 3 (a) Photograph shows the flexible graphene paper with metallic luster. (b, c) Frontal and cross-section SEM images of W-Gr paper. (d) θc of TRGO and W-Gr sample measured to be 97.1° and 55.7°, respectively. (e) Statistical results of θc of W-Gr sample in this work and some other graphene related materials. (f) CCK-8 assay for the rADSCs proliferation on the blank experiment, TRGO sample and W-Gr sample. (g) Representative fluorescence images showing the cell viability of rADSCs on W-Gr sample after 1 and 7 day of incubation, visualized by staining with a LIVE/DEAD® Cell Viability Kit (Thermo Fisher Scientific). The live cells appear green while the dead ones are red. Stable W-Gr aqueous dispersions enable the fabrication of macroscopic graphene materials compared with their powder counterparts. We then prepared graphene paper via the filtration technique (Figure S10, Supporting Information). Typically, 10.0 mL of 1.5 g/L W-Gr dispersion was filtrated through a 20 nm AAO membranes (Shanghai Shangmu, China). A free-standing, smooth-faced and bendable paper (Figure 3a) was obtained after dried and peeled off from the membrane. Frontal (Figure 3b) and high-resolution cross-section (Figure 3c) SEM images of the



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paper confirm W-Gr sheets were completely disengaged and freely outstretched. The well-aligned graphene sheets along the plane of the paper indicate the parallel deposition of W-Gr sheets on the filter membrane, just as GO and chemical converted graphene behave during filtration.49 This indirectly reflects the dispersing state of individual sheets in water. This behaviour, together with the layered structure and uniform thickness (7.4 µm), demonstrated the outstretched and homogeneously dispersing state of graphene in water. It should be mentioned that the whole filtration process completed within 5 h, which is much efficient than the filtration of equal volume of GO and chemical converted graphene. This might be ascribed to the wrinkles of the sheets as we observed in the TEM images. Relatively short processing time will also facilitate the applications of W-Gr. The change of surface chemistry is also embodied in the enhancement of the hydrophilicity. As seen in Figure 3d, samples assembled from TRGO and W-Gr have apparently different water contact angle (θc). While TRGO sample exhibits hydrophobic nature (θc = 97.1°) of the untreated sheets, W-Gr composed sample possess a θc value of 55.7°, which lies in the range of hydrophilic surface. We should emphasize that the hydrophilicity of W-Gr is comparable to that of the GO reported by previous experimental researches (θc ~30-70°).50, 51 Also, this hydrophilicity is better than most graphene related materials reported before (Figure 3e).50, 52, 53 The W-Gr sample holds potential in many applications where hydrophilicity is desirable, such as biologically active substrate. The rADSCs were selected to evaluate the biocompatibility of W-Gr sample thus formed. Cells proliferation was assessed using CCK-8 assay. As shown in Figure 3f, the absorbance (OD) for rADSCs on the W-Gr sample is 0.29, 0.61, 1.17 and 1.67 at day 1, 3, 5 and 7, respectively. Remarkable proliferation of rADSCs can be observed after 1 day of incubation. By contrast, the blank experiment shows the absorbance for rADSCs is 0.24, 0.46, 0.78 and 1.02 at day 1, 3, 5 and 7, respectively. This indicates the W-Gr sample is suitable for cell proliferation. Moreover, the absorbance for rADSCs on TRGO sample is 0.13, 0.20, 0.24 and 0.26 at day 1, 3, 5 and 7, respectively. The poor biocompatibility of TRGO sample indicates that the good hydrophilicity is the key factor for tissue repair materials. We further show the excellent biocompatibility of W-Gr sample intuitively by bio-fluorescence imaging. As shown in Figure 3g, rADSCs seeded on W-Gr sample exhibited excellent viability. The rADSCs became elongated and exhibited a typical fibroblast-like shape, illustrating that the W-Gr sample surface provides a well microenvironment for the cells adhesion and proliferation. Cells viabilities kept above 95 % within the whole incubation progress and even increased during the prolonged culture time. In day 7, cells viabilities were all over 99.5 % and dead cells were barely seen. That means W-Gr sample provided a well living environment for cells and did not affect its bio-functions at all. The rADSCs gathering in a cluster of living demonstrates that those cells on the W-Gr sample having a favourable stem cell pluripotency and multiple differentiation


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potentials. The cell density for rADSCs on the W-Gr sample is 45 and 544 cells/mm2 at day 1 and 7, respectively. The cell density for rADSCs on the W-Gr sample is 45, 103, 211, 305, 381, 460 and 544 cells/mm2 at day 1, 2, 3, 4, 5, 6 and 7, respectively. By contrast, the cell density for rADSCs on the TRGO is 33, 36, 41, 39, 45, 47 and 51 cells/mm2 at day 1, 2, 3, 4, 5, 6 and 7, respectively (Figure S11). All above results indicate the W-Gr sample can be used as a biologically active substrate with good biocompatibility and cytocompatibility that can be attributed to its good hydrophilicity property.6 Those features of W-Gr make itself capable for potentially attractive biomedical applications, especially as a new biomaterial for bone, vessel and skin regeneration.

Figure 4. (a)Scheme shows the chemical pathway for H2O2 decomposition catalyzed by Mn ions. (b) The PL intensity of the mixed solution of TA, TRGO and H2O2 aqueous solution (red curve) and TA, TRGO without H2O2 (black curve). (c) The generated highly active hydroxyl radicals in the mixture oxidize the TRGO sheets effectively. (d) The content of different oxygen-containing



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functional groups in W-Gr decreases along with the growth of m0, resulting the decrease of oxygen content (atom %). (e) The saturation concentration of W-Gr dispersed in commonly used low-boiling solvents. Oxidation Mechanism. Successful dispersing of graphene in water demonstrates that H2O2 is a reasonable agent for surface modification. Understanding the underlying mechanism is necessary for controlling this method according to practical requirements. Based on XPS and FT-IR results above, H2O2 indeed acts as an oxidant that increase the oxygen content of TRGO. More importantly, as indicated before, the oxidation is featured by the change in relative content of oxygen-containing groups and nearly unchanged crystal domain of initial TRGO structure. Thus, we deduce that the oxidation mainly occur on the so-called defect sites (namely sp3 carbon) on the graphene,54 including the oxygen-containing groups and carbon-hydrogen bond. Rational chose of H2O2 as a moderate oxidant is the key point of this method. It is well-known that catalytic decomposition of H2O2 by minimum Mn ions can generate highly active radicals (such as: ·OH and ·OOH, as shown in Figure 4a) at room temperature.55, 56 To confirm that, terephthalic acid (TA) was used as a fluorescent probe for ·OH tracking since it could capture ·OH and generate 2-hydroxy terephthalic acid (TAOH), emitting obvious fluorescence around 435 nm when irradiated by laser (315 nm).57, 58 The red curve in Figure 4b shows the photoluminescence (PL) intensity in the mixed solution of TA, TRGO and H2O2 after 10 h. Compared with control experiment (TA and TRGO, black curve in Figure 4b), remarkable PL peak at 435 nm indicates the presence of ·OH. Thus all possible oxidation process can actually be considered as the reactions between ·OH and TRGO sheets that carry various oxygen-containing groups (including epoxy and hydroxyl groups on the basal plane, while other groups, e.g. carboxyl and carbonyl, present at the edges).59 It should be noted that the oxidation states of carbon in those groups are different and the transformation from lower oxidation states to higher ones should be taken into consideration. Actually, that transformation is experimentally confirmed by significantly increased content of carboxyl when crystalline domains of TRGO are largely remained. In fact, oxidation from alcohol, aldehyde and ketone to carboxylic acid is thermodynamically favourable when ·OH is used as oxidant (as calculated in Supporting Information). Oxidizing edgy sp2 carbon atoms of crystal domain to phenolic hydroxyl group is also energetically possible (Figure S12, Supporting Information). Like carboxyl group, phenolic hydroxyl can contribute to the dispersing of W-Gr by generating repulsive solvation forces between graphene sheets after ionization.10 The oxidation of edgy sp2 carbon is responsible for the mildly compromised electrical conductivity. However, inner sp2 carbon atoms constituting the crystalline domains are inert since oxidation of pristine graphene generally requires harsh condition.60 Therefore, as shown in Figure 4c, the whole modification of TRGO can be interpreted as a mild oxidation process that occurs between the inherent oxygen-containing groups and H2O2-derived ·OH. This oxidation is featured by increased content of carboxyl groups and almost unchanged crystal domains of TRGO.


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Moreover, the degree of oxidation can be tuned by varying the initial content of TRGO precursor in H2O2 (m0). As shown in Figure 4d, the content of oxygen-containing groups increases along with the decrease of m0. The specific content data of different oxygen-containing functional groups in W-Gr with different oxidation degree is listed in Table S2 and the original XPS spectra after curve fitting process is shown in Figure S13 (Supporting Information). Notably, the content of carboxyl (OO-C=O/W-Grtotal) drastically decreases from 0.169 (m0 = 1 g/L) to 0.072 (m0 = 5 g/L) with the decreased quantity of introduced oxidant. That reconfirms the role the mild oxidation plays in increasing the content of carboxyl, ultimately leading to the improvement of W-Gr’s dispersibility. In addition, the W-Gr shows reasonable dispersibility in other low-boiling solvents (methanol, 65 oC; ethanol, 78 oC; acetone, 57 oC; ethyl acetate, 77 oC) of different polarities (represented by dielectric constant, ε), which is more desirable on some occasions like spray deposition and inkjet printing where rapid removal of solvent is required. As is presented in Figure 4e, carboxyl content and polarity codetermine the maximum concentration of W-Gr in different solvents, indicating the role of carboxyl’s ionization plays in dispersing. Due to the supressed ionization, W-Gr gives much lower concentrations in weaker polar solvents (0.005-0.45 g/L) than in water (2.10-10.25 g/L). However, this is proven to be an effective method to fabricate graphene dispersions in various low-boiling solvents. Besides, the proposed strategy is also applicable to other carbon nanomaterials such as activated carbon, carbon nanotube and fullerene. While commercial active carbon was directly used due to its inherent abundant sp3 carbon, carbon nanotube and fullerene were pretreated under oxygen plasma (75 W, 40 sccm, 30 min) to introduce necessary chemically active sites for modification. Analogous to previous oxidation procedure, 50 mL H2O2 (1.0 mol/L) was subsequently added to the mixture of 300 mg carbon precursor (active carbon, carbon nanotube or fullerene) and 50 mL MnSO4 solution (0.8 mg/ L) under continuous stirring at room temperature for 12 hours. After freeze-drying, all of the resultant powders can be re-dispersed in water with a typical concentration of approx. 6.5 g/L for active carbon, 1.5 g/L for carbon nanotube and 0.5 g/L for fullerene, demonstrating the feasibility of this green and mild oxidation as a potential method to obtain water-dispersible carbon nanomaterials. CONCLUSIONS In summary, we developed a green and mild oxidation strategy based on H2O2 to enhance hydrophilicity of TRGO in large scale. By fully utilizing the intrinsic defects of TRGO, the mild oxidation does not evidently compromise the crystal structure of TRGO. Aqueous paste as well as uniform dispersions in various low-boiling solvents can be easily fabricated from the lyophilized W-Gr powders owing to its re-dispersibility. Good dispersibility and chemically modified surface enable the preparation of hydrophilic W-Gr sample (θc, 55.7o) with excellent bio-compatibility, which is expected to be an alternative biomaterial for bone, vessel and skin regeneration. Expansion of this facile method is also proceeded to prepare aqueous dispersions of other carbon



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nanomaterials such as activated carbon, carbon nanotube and fullerene. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Preparation process of TRGO; TRGO aqueous suspension and W-Gr aqueous dispersion; SEM and TEM images of TRGO; UV-vis absorbance spectrum of W-Gr aqueous dispersion; W-Gr aqueous dispersions stability at different temperature; SEM image of W-Gr; TEM and AFM image of isolated W-Gr sheets; Relationship between density (ρ) and conductivity (κ) of TRGO and W-Gr; TGA and DSC curves of TRGO and W-Gr; FT-IR spectra of TRGO and W-Gr; Vacuum filtration pump to fabrication W-Gr papers; Cell density for rADSCs on the W-Gr and TRGO; Thermodynamic calculation; Schematic representation of the oxidization process by hydroxyl radical, ·OH; Original content data of different oxygen-containing functional groups in W-Gr; XPS fitting spectra of W-Gr. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contributions #

These authors contributed equally

Notes The authors declare no competing financial interest. ACKONWLEDGMENTS This work was supported by projects from the Chinese Academy of Sciences (KGZD-EW-303 and XDA02040000), Natural Science Foundation of China (81501605) and Science and Technology Commission of Shanghai Municipality (15YF1406800). REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. (2) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Poperties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385-388. (3) Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S. Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes. Nano Lett. 2009, 9, 4359-4363. (4) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537-1541. (5) Liu, F.; Song, S.; Xue, D.; Zhang, H. Folded Structured Graphene Paper for High Performance Electrode Materials. Adv. Mater. 2012, 24, 1089-1094.


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Green and Mild Oxidation: An Efficient Strategy ... - ACS Publications

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