CO2-Induced Reversible Dispersion of Graphene by a Melamine

Research Institute, Sichuan University, Chengdu 610065, P. R. China. § University of the Chinese Academy of Sciences, Beijing 100049, P. R. China...
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CO2‑Induced Reversible Dispersion of Graphene by a Melamine Derivative Hongyao Yin,†,§ Hanbin Liu,†,§ Wei Wang,†,§ and Yujun Feng*,†,‡ †

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, P. R. China State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, P. R. China § University of the Chinese Academy of Sciences, Beijing 100049, P. R. China ‡

S Supporting Information *

ABSTRACT: Smart graphene with stimuli-responsive dispersity has great potential for applications in medical and biochemical fields. Nevertheless, reversible dispersion/aggregation of graphene in water with biocompatible and removable trigger still represents a crucial challenge. Here, we report CO2-induced reversible graphene dispersion by noncovalent functionalization of reduced graphene oxide with N2,N4,N6-tris(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (MET). It was demonstrated that MET can be strongly adsorbed on graphene surface through van der Waals interaction to facilitate dispersing graphene in water. Moreover, reversible aggregation/dispersion of graphene can be achieved simply by alternately bubbling CO2 and N2 to control the desorption/adsorption of MET on graphene surface.

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properties of graphene.4,12 Normally, foreign stabilizers, such as surfactants and amphiphilic polymers, are often introduced to disperse graphene in water via noncovalent functionalization. While the hydrophobic part of these stabilizers interacts with graphene surface through nondestructive hydrophobic and/or π−π interactions, the hydrophilic moiety provides steric hindrance or electrostatic repulsion to offset the van der Waals attraction force between individual graphene sheets, thus affording stable graphene aqueous dispersion.4,13 Recently, functionalization of the graphene dispersion, especially reversible splitting/aggregation of graphene with environmental-stimuli triggers, has been appealed owing to its potential in medical and biological applications such as drug delivery, diagnostics, and sensing.14,15 To this end, several stimuliresponsive dispersants have been developed to furnish graphene with tunable dispersibility, but the overwhelming majority of the triggers to stimulate the hybrids are only confined to pH,16 temperature,17 and ionic strength.14

INTRODUCTION Owing to its unique mechanical, thermal, electric, and optical properties, graphene as one-atom-thick two-dimensional layers of sp2-bonded carbon has been regarded as one of the most promising materials for many potential high-performance applications.1−3 Although a lot of research has been conducted on graphene, there still remain some technical problems to be solved before its practical applications,4,5 one of which lies in the fact that the as-produced graphene sheets tend to aggregate together due to the van der Waals interactions between themselves and thus are difficult to be dispersed in solvents, imposing troubles in their processing and incorporation into devices.6−8 Also, the aggregation of graphene deteriorates the unusual properties intrinsic to the individual nanosheets and thus declines graphene performances in end uses. Therefore, improvement of graphene dispersibility in solvents, especially in water due to its nontoxicity, easy handling, and potentials in biomedicine, has become a hot topic of both fundamental and practical significance.9−11 Both covalent and noncovalent approaches have been developed to promote dispersion of graphene, but the latter is more preferable because it avoids compromising the inherent © XXXX American Chemical Society

Received: July 30, 2015 Revised: September 30, 2015

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DOI: 10.1021/acs.langmuir.5b02831 Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Molecular Structures of MET and Its Reversible Protonation−Deprotonation by Bubbling CO2 and N2 in the Presence of Water

centrifuge (Shanghai Medical Instruments Co. Ltd., China) to give a homogeneous MET-rGO dispersion. Gas treatment for MET-rGO dispersion: CO2 at the flow rate of 3 mL min−1 is bubbled into 5 mL of as-prepared dispersion for 10 min; then graphene sheets gradually aggregate and precipitate. N2 at the flow rate of 10 mL min−1 is streamed to purge CO2 for 40 min followed by 30 min sonication which will lead to stable and homogeneous dispersion again. To prepare MET-rGO composite for thermogravimetry analysis (TGA), the as-prepared dispersion was filtered through a PTFE microporous membrane (0.22 μm). The MET-rGO hybrids left on the membrane were then washed with deionized water repeatedly and vacuum-dried for 24 h. When preparing the MET-rGO−CO2 composite for TGA characterization, CO2 was bubbled into the as-prepared dispersion at the flow rate of 3 mL min−1 for 10 min at room temperature, and the dispersion was kept for 3 days. Then the mixture was filtered through the aforementioned membrane, and the residues left on the membrane were then washed with deionized water and vacuum-dried for 24 h to remove the remaining water and CO2. Characterization. 1H and 13C NMR spectra were registered at 25 °C on a Bruker AV300 NMR spectrometer at 300 and 75 MHz, respectively. Chemical shifts (δ) were reported in parts per million (ppm) with reference to the internal standard protons of tetramethylsilane (TMS). For NMR analysis, MET-rGO dispersion was prepared in D2O and injected into the NMR tube and then recorded. The NMR spectra of the CO2-treated dispersion were obtained by directly bubbling CO2 at the flow rate of 3 mL min−1 into the tube for 10 min and then measured. UV−vis spectra were recorded on a computer-manipulated doublebeam UV−vis spectrophotometer (UV-4802, Unico, China) operated at a resolution of 1 nm at 25 °C over the wavelength range of 190− 1000 nm. Raman spectra were obtained on LabRAM HR (HORIBA Scientific, France) Raman spectrophotometer with 633 nm wavelength incident laser light. Transmission electron microscopy (TEM) observation was performed on a JEM-100CX TEM instrument (JEOL Ltd., Japan) with an accelerating voltage of 80 kV. The specimens were prepared by placing a drop of dispersion on copper grids coated with carbon and dried at room temperature. Thermogravimetry analysis (TGA) was conducted on a 299-F1 thermal analysis system (NETZSCH, Germany). Samples were heated in flowing N2 (50 mL min−1) from 30 to 800 °C at a heating rate of 10 °C min−1. XPS characterization was performed on a Kratos XSAM800 XPS system (Kratos Ltd.). The parameters used are as follows: analyzer mode, FAT; energy range, X1; exciting source: A1.

However, these stimuli have limitations while smart graphene is utilized in biologically related applications. For example, changes in temperature might be limited in some targeted sites,18 and introduced triggers based on acids, bases, or salts will result in accumulation of side products and then contaminate the system.19 Moreover, reversible change of temperature, pH, and ionic strength might cause damages to biological tissues. Therefore, novel dispersant together with biocompatible and removable trigger for fabricating smart graphene without damaging its inherent structure is in a pressing need. As a readily available, water-soluble, biocompatible, and environmental-benign gas, CO2 is easy to introduce and remove without accumulation and contamination,20 thereby showing potentials in wide range of areas, including biology and related fields, which makes it an ideal trigger for regulating graphene dispersity in water. In fact, owing to its merits, CO2 has been widely employed to reversibly switch surfactants,19 solvents, 21 polymers, 22,23 ionic liquids, 24 and SWNTs hybrids25,26 in recent years; nevertheless, no example has been revealed to tune graphene dispersion state using CO2responsive dispersant. In this paper, we report CO2-induced reversible aggregation/ dispersion of reduced graphene oxide by noncovalent functionalization with N 2 ,N 4 ,N 6 -tris(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (MET, Scheme 1). The dispersant, MET, can be strongly adsorbed on graphene surface via van der Waals interactions to afford stable graphene dispersion. Furthermore, the reversible aggregation/dispersion state of graphene can be achieved through alternately bubbling CO2 and N2 to control the desorption/adsorption of MET on graphene surface. To the best of our knowledge, this is the first example of gas-triggered aggregation/dispersion of graphene through controllable adsorption behavior of dispersant on graphene surface.

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EXPERIMENTAL SECTION

Materials. Reduced graphene oxide (rGO, purity >95 wt %; thickness 0.55−3.74 nm; diameter 0.5−3 μm; specific surface area 554.364 m2 g−1) was kindly provided by Timesnano (Chengdu, China). Cyanuric chloride (99%) and 3-(dimethylamino)-1-propylamine (99%) were purchased from Aldrich and used as received. Water that was triply distilled by a quartz purification system was used throughout this study. CO2 (≥99.998%) and N2 (99.998%) were used as received. The other reagents and solvents with analytical grade were obtained from Shanghai Chemical Reagent Co., Ltd. Sample Preparation. 2.0 mg of rGO powder was added to 2.0 mL of MET aqueous solution (17.5 mg mL−1), and then the mixture was sonicated for 2 h (100 W and 40 kHz) in a bath sonicator (KQ-100, Kunshan Ultrasound Instrument Company, China) at room temperature, followed by 10 min of centrifugation at 1500 rpm with a 80-2

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RESULTS AND DISCUSSION Recently, theoretic calculation27 revealed that 1,3,5-triazine has unusually strong affinity for graphite, and the affinity dramatically increases when amino groups are connected to the aromatic core, which shows the existence of synergistic effect between the triazine core and amino groups of producing B

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Since rGO was used as starting material in this work and some oxygen-functionalized groups may remain inside to affect the interaction of graphene and dispersant, XPS was employed to examine the amount of oxygen-containing groups. The peak of O element is observed at 534 eV from the XPS spectrum (Figure S1, Supporting Information), which implies that not all the oxygen-containing groups of graphene oxide were reduced. The relative amounts of C and O are calculated to be 89.44% and 10.56%, respectively, demonstrating the ratio C/O is about 9. Then the dispersity of this rGO was investigated. As shown in Figure 2a, rGO cannot be dispersed in distilled water alone

affinity for graphite. This is because the higher electronegativity of nitrogen effectively reduces the net charge on the hydrogen atoms of amino groups, thereby allowing them to accept additional density contributed by graphene. Further, Vazquez and co-workers28 found that melamine can be adsorbed on graphite surface through mutual interactions, but it can be washed away by water, indicative of the affinity between melamine and graphite is not strong enough to withstand solvation effect. Therefore, to disperse graphene in water using 1,3,5-triazine derivatives, more amino groups, particularly the tertiary amino group,27 should be introduced to increase the affinity. Keeping this in mind, MET, which contains three secondary and three tertiary amino groups, was prepared and utilized to disperse graphene in this work. MET was synthesized from the reaction of cyanuric chloride and 3-(dimethylamino)-1-propylamine following a previously reported procedure (details of synthesis are given in the Supporting Information).29 This melamine derivative shows good water solubility owing to the nine nitrogen atoms that can form hydrogen bond with H2O. Furthermore, the pKaH (pKa of the protonated species) of tertiary amino groups of MET is 8.7, measured by the pH titration method, implying MET is a moderate organobase with CO2 switchability. Our previous study30 has proven that MET is sensitive to CO2, which can react with CO2 in the presence of water to produce corresponding bicarbonate salt, METH+ (Scheme 1). Figure 1 gives the 1H NMR spectra of MET in D2O with and without CO2. The peak at 2.12 ppm in the 1H NMR spectrum

Figure 2. (a) Snapshots of rGO dispersed in distilled water and MET aqueous solution. (b) TEM image of rGO dispersed in MET aqueous solution.

even with long-time sonication; however, it was dispersed when MET was added and followed by sonication for 2 h. The resultant suspension was then centrifuged to give a homogeneous dispersion, which appeared as dark black and can stand stable without any noticeable sedimentation and aggregation over 2 weeks. TEM observation shows that most of the graphene sheets have been well dispersed, and no obvious aggregation was found. Figure 2b shows a typical image of dispersed graphene sheets with an average size around 1 μm2. TEM analysis demonstrates that MET is capable of well dispersing graphene in water. The maximum solubility of this rGO in MET aqueous solution is 0.27 mg mL−1, measured by UV−vis spectra of the dispersion following the procedure reported previously (see Supporting Information for details).31 As mentioned above, an effective dispersant should first adsorb onto graphene surface forming a complex via mutual interactions and then prevent graphene sheets from aggregating through steric hindrance or electrostatic repulsive force. If one moiety of dispersant interacts with carbon nanomaterials, the NMR signals of the corresponding groups will be changed.32 Thus, both 1H NMR and 13C NMR spectra of MET-rGO dispersion were examined to investigate the interactions between MET and rGO. The corresponding spectra of MET solution without rGO were employed as references. As depicted in Figure 3a, all the 1H NMR signals assigned to MET except the peak 4 shift to a significantly lower magnetic

Figure 1. Comparison of 1H NMR spectra of MET in D2O with and without CO2.

of MET is attributed to the gemini terminal −CH3 groups (marked as 1). Two triplets are observed at 2.35 and 3.25 ppm, which are assigned to protons 2 and 4 in the methylene group, respectively, whereas the peak at 1.60 ppm is attributed to the protons 3 in the methylene group. After bubbling CO2, obvious changes in the chemical shift of the spectrum were observed, where the peaks of 1 and 2 from the tertiary amino groups shifted downfield by around 0.7 ppm while the peaks of 3 and 4 only experienced 0.2 and 0.1 ppm downfield shift, respectively, indicating the tertiary amino groups of MET are protonated. Moreover, the original signals of MET were recovered when N2 was streamed to displace CO2, indicative of deprotonation of METH+. Therefore, MET, a CO2-switchable and water-soluble melamine derivative, shows the potential to fabricate smart graphene aqueous dispersions.

Figure 3. (a) 1H NMR and (b) 13C NMR spectra of MET and METrGO in D2O. C

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Langmuir field upon complexing with rGO in the dispersion. The peaks of 1 and 2 from tertiary amino groups shift downfield by around 0.12 ppm, whereas the peak of 3 only experiences 0.07 ppm shift, indicative of strong interactions between tertiary amino groups and rGO. Meanwhile, the 13C NMR signal attributed to the carbon atom in triazine ring of MET-rGO dispersion also shows a downfield shift in comparison with the corresponding signal of MET solution, from 165.30 ppm shifts to 165.52 ppm (Figure 3b), suggesting that the triazine core undergoes interactions with rGO in the mixtures. To gain further insight into the rGO dispersion mechanism from MET, absorption spectroscopy was employed to compare the absorption band from MET solution and MET-rGO dispersion, as the spectrum is known to be highly sensitive to the mutual interactions between graphene and dispersant.33 As exhibited in Figure 4, the absorption maximum of MET in the

from the defect sites in the hexagonal framework of the graphite materials and reflects the disordered graphite structure,34 whereas the peak at ∼1596 cm−1 is ascribed to the G band which results from the structure of the sp2-hybridized carbon atoms.35 The extent of the defect sites in graphite materials can be evaluated by the intensity ratio of the two bands (ID/IG), which can also be used to characterize the degree of chemical functionalization in carbon materials as the intensity of D band can be reduced after the shielding of the defect sites by conjugated structure.36,37 For example, Kuang and co-workers36 used a pyrene-containing dispersant to functionalize rGO through noncovalent modification. While the ID/IG ratio of the untreated rGO is as high as 1.46, the ratio of modified graphene decreases to 1.12, owing to the shielding of the defect sites by pyrene moiety. In this work, the ID/IG ratio for the original graphene is 1.05, suggesting the existence of some defect sites which probably derive from the manufacturing process. Nevertheless, the ratio declines to 0.94 upon complexing with MET in dispersion. Such a drop of ID/IG ratio of MET-rGO dispersion indicates that MET has been adsorbed on rGO surface and covered the defect sites in rGO. In addition, TGA was employed to see whether MET has been modified onto rGO and to calculate the content of MET in the MET-rGO complex. To this end, MET-rGO composite was prepared from MET-rGO dispersion and then measured. MET and original rGO were also examined in control experiments. From the curves depicted in Figure 6, one can

Figure 4. UV−vis spectra of MET solution and MET-rGO dispersion.

MET-rGO dispersion at 218 nm showed to be red-shifted together with the broader absorption band with respect to that of the MET solution in the absence of rGO, indicative of extended π-conjugation of the triazine moiety in the presence of rGO, again suggesting the interactions between triazine segment of MET and rGO. Moreover, Raman spectroscopy was also employed as an essential technique to understand the mechanism of forming MET-rGO dispersion as it can provide the evidence of modification degree for graphene. As shown in Figure 5, the peak at ∼1340 cm−1 is assigned to the D band which stems

Figure 6. TGA profiles showing weight loss upon heating for MET, untreated rGO, and MET-rGO composites prepared from MET-rGO dispersion in the presence and absence of CO2.

find that MET becomes unstable with increasing temperature. Its main weight loss takes place at 200−400 °C, and it almost fully decomposes at around 800 °C. In comparison with the original rGO, MET-rGO composite experienced more weight loss as temperature went up. 41% of its initial weight was lost in MET-rGO composite by 800 °C, whereas only 23% was lost in the original rGO. Compared with MET curve, the extra weight loss of the composites is caused by the decomposition of MET inside, which is a direct evidence of the complexation between MET and rGO. According to the weight loss, the content of MET in the MET-rGO composite is calculated to be 23.4%. Thus, it is clear that the combination of the interactions between graphene and both tertiary amino groups and the triazine ring causes MET to be strongly adsorbed onto graphene surface in aqueous solution. Meanwhile, the amino groups of MET form hydrogen bonds with H2O30 to create steric hindrance to counteract the van der Waals interactions

Figure 5. Comparative Raman spectra of untreated rGO and METrGO dispersion in the presence and absence of CO2. D

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Langmuir between graphene sheets, thus resulting in stable MET-rGO dispersion. It is worth noting that tertiary amino groups of MET will be protonated while CO2 is introduced in the presence of water, which might alter the interactions between MET and graphene and further regulate graphene dispersity. Therefore, it is natural to examine whether CO2 is capable of controlling the dispersion/aggregation state of graphene in water. As shown in Figure 7, when CO2 was bubbled into the asprepared MET-rGO dispersion, aggregation of graphene was

solution, indicative of the desorption of triazine segment from graphene surface. This would be also caused by the protonation of tertiary amino groups. The interaction between tertiary amino groups and graphene is broken after bubbling CO2, so that the residual interaction between triazine segment and graphene is not strong enough to support the whole molecule to be adsorbed on graphene surface; as a result, METH+ is desorbed from the complex, and then graphene sheets begin to aggregate. The inset in Figure 4b shows the appearance of MET-rGO dispersion in NMR tubes with and without the treatment of CO2 after 3 days; one can find that the dispersion in the presence of CO2 became clear and most of the graphene precipitated at the bottom, whereas the dispersion without CO2 was still dark black. Raman spectrum of CO2-treated MET-rGO dispersion was examined to confirm the removal of the dispersant from the complex. As shown by Figure 5, the ID/IG ratio experienced a significant recovery, reaching 1.02, which provides the direct evidence of the removal of METH+. Since MET can be removed from MET-rGO complex in the presence of CO2, TGA was employed to precisely calculate how much MET can be removed. As displayed in Figure 6, in comparison with MET-rGO composite, MET-rGO−CO2 composite shows less weight loss while temperature increased. It only lost 29% weight when the temperature reached 800 °C, meaning that 7.8% of dispersant still remained in the composite. Namely, more than two-thirds of dispersant has been removed from the MET-rGO complex after the treatment of CO2. It is worth noting that the homogeneous dispersion can be recovered from the aggregation when CO2 is purged by bubbling N2 followed by sonication for 30 min. Furthermore, the introduction of CO2 will lead to aggregation again. Such a procedure is still effective beyond three cycles of bubbling CO2 and N2, suggesting the aggregation/dispersion state of METrGO hybrids can be reversibly controlled. On the basis of the above results, it is reasonable to speculate the mechanism of this gas-controlled dispersion/aggregation of graphene based on MET. As illustrated in Scheme 2, MET contains nine amino groups which can form hydrogen bond with H2O, thereby showing good water solubility. Owing to the interactions between graphene and both tertiary amino groups and triazine moiety, MET can be strongly adsorbed on graphene surface in water. Meanwhile, the hydrogen bonds between MET and H2O create steric hindrance to prevent graphene sheets aggregation, thus affording stable MET-rGO

Figure 7. TEM image of aggregated graphene sheets after bubbling CO2, and the inset is a corresponding picture of precipitated graphene.

not observed immediately, but graphene sheets were found to fully precipitate at the bottom of the vial 1 week later. Unlike the TEM observation of well-dispersed graphene (Figure 2b), only closely and disorderly stacked graphene sheets were found in the TEM image of CO2-treated graphene dispersion, clearly demonstrating graphene sheets aggregated together in the presence of CO2. NMR spectra of MET-rGO dispersion treated with CO2 were recorded to reveal the mechanism of forming graphene aggregation. Since MET will be converted into METH+ in the presence of CO2 and water, the corresponding NMR spectra of METH+ were employed as references. As exhibited in Figure 8a, compared the 1H NMR signals of MET-rGO−CO2 dispersion with METH+ solution, almost no change is observed which demonstrates no interaction between the protonated tertiary amino groups and graphene, suggesting the protonation breaks the mutual interaction between graphene and tertiary amino groups. Then 13C NMR spectra were employed to confirm whether the interaction between the triazine core and graphene still exists in this case. As shown in Figure 8b, it is clear that no shift between the peak of carbon atom in the triazine ring from MET-rGO−CO2 dispersion and METH+

Figure 8. (a) 1H NMR spectra and (b) 13C NMR spectra of METH+ and MET-rGO−CO2 in D2O. The inset is MET-rGO dispersion in NMR tubes with and without the treatment of CO2 after 3 days. E

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easily switched “off” and “on” through introduction and removal of CO2, making it a removable dispersant for graphene. The removability of dispersant may display advantages in practical uses of graphene. For example, when graphene dispersion is utilized in devices, the remained dispersant might exert negative influence on the thermal, electric, and mechanical properties. As in carbon nanotubes, when employed for transistor-related applications, the sorted semiconducting nanotubes should not be covered with residual dispersing agent; otherwise, the resistance will be high and thus lowers the device performance.38 Moreover, MET is prepared from one-pot reaction of cyanuric chloride and 3-(dimethylamino)propylamine. Both cyanuric chloride and 3-(dimethylamino)propylamine are extensively used in industry production as chemistry materials, and the solvents used in this process are common organic solvents, including toluene, dichloromethane, and ethyl acetate (see Supporting Information for details). The inexpensive starting materials and simple preparation procedure largely reduce the production cost of MET and also make its largescale production possible, which shows the prospect of commercial application.

dispersion. However, MET is converted to METH+ upon CO2 treatment, which breaks the interaction between tertiary amino groups and graphene. Then the residual affinity between triazine core of METH+ and graphene is not sufficient enough to support the whole molecule to be adsorbed on graphene surface, resulting in removal of METH+ from the complex and subsequent aggregation of graphene. When N2 is streamed to displace CO2, METH+ is deprotonated to produce MET which is readsorbed onto graphene surface to afford stable dispersion again. It is the reversible interconversion between METH+ and MET triggered by alternately bubbling CO2 and N2 that gives rise to the desorption/adsorption of dispersant on graphene surface and subsequent aggregation/dispersion of graphene. It should be pointed out that similar compounds with the main structure of MET, including triazine core and aminosubstituted groups, are expected to have the same capability of MET to realize the reversible dispersion and aggregation of graphene in water. The unique mechanism of this reversible graphene dispersion is different from previously reported smart graphene dispersion. In earlier studies,16,17 aromatic molecules, such as pyrene, are often used as adsorption group to be incorporated into stimuliresponsive compounds or polymers to construct smart graphene systems. While the conjugated moiety is attached onto graphene surface through π−π stacking, the environmental-sensitive segment undergoes reversible hydrophilicity upon external stimulation, so that confers the graphene controllable dispersity. Normally, the interaction between multiring aromatizing structure and graphene is strong and cannot be regulated along with the variation of environment. In contrast, the interaction between MET and graphene can be

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CONCLUSION In summary, we have demonstrated CO2-induced reversible dispersion/aggregation of reduced graphene oxide with a melamine derivative. To the best of our knowledge, this is the first example of employing a “green” gas as trigger to reversibly tune graphene dispersity in water, so it would be desirable for the use of graphene in medical and biological F

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applications. Moreover, in comparison with previously reported stimuli-responsive graphene dispersions,14,16,17,39 the reversible dispersity of this graphene depends on the controllable adsorption/desorption of dispersant on graphene surface, which may show potentials in smart sensors and graphene purification. The unique mechanism also provides a new idea to design drugs with controlled released behavior on graphene surface. Such a particular graphene hybrids may open a new way to construct smart nanomaterials with reversible surface properties.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02831. Experimental details; Scheme S1 and Figures S1, S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21273223, 21173207) and Distinguished Youth Fund from Science and Technology Department of Sichuan Province (2010JQ0029). We thank Dr. Shuai He and Dr. Zanru Guo for the help and suggestions with the TEM, TGA, and Raman analysis.

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DOI: 10.1021/acs.langmuir.5b02831 Langmuir XXXX, XXX, XXX−XXX