High Concentration Self-crosslinkable Graphene Dispersion

Subscriber access provided by UNIV OF CAMBRIDGE

Article

High Concentration Self-crosslinkable Graphene Dispersion Junshuo Cui, and Shuxue Zhou Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00884 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

High Concentration Self-crosslinkable Graphene Dispersion Junshuo Cui, Shuxue Zhou* Department of Materials Science, State Key Laboratory of Macromolecular Engineering, Advanced Coatings Research Center of Ministry of Education of China, Fudan University, Shanghai 200433, China ABSTRACT: The formation of a homogeneous graphene dispersion is a fundamental prerequisite for the preparation of high-performance graphene-based materials, yet it remains a challenge to obtain, particularly at high concentrations, meanwhile possessing film-forming ability. In the present work, a new TSiPD•+ molecule, consisting of a conjugated core and terminal silanol groups, was synthesized. With the aid of the TSiPD•+, a stable graphene dispersion in water was obtained with a concentration up to 10 mg/mL. More interestingly, the graphene dispersion was able to self-crosslink, after direct casting on substrates, forming highly conductive graphene films with good mechanical strength and excellent solvent resistance. The outstanding performance of the graphene films is owed to the condensation of the silanol groups in the TSiPD•+ molecules. The use of the aqueous graphene as conductive ink for paper, glass, and even polymer films was demonstrated.

dispersion. Secor et al.17 produced a graphene/cellulose film by inkjet printing the ethyl cellulose/graphene dispersion. In the cases mentioned above, PVP or ethyl cellulose functioned not only as a stabilizer of the graphene dispersion, but also as a binder in the composites. The second strategy involves the use of dispersants that possess reactive groups. With the help of these reactive groups, the dispersant-stabilized graphene can chemically interact with the matrix to avoid incompatibilities. For instance, a dispersant containing epoxy groups, reported by both Ding et al.18 and Teng et al.19, and one with vinyl groups, reported by Otieno et al.20, were used to enhance the interfacial interaction of graphene sheets with the epoxy resin and polyurethane matrix, respectively. The drawback of the first strategy is that the polymer dispersants are not cross-linkable, so that the obtained graphene films have poor solvent resistance. Under these conditions, the erosion of the solvents could cause a serious deterioration of the mechanical strength of the graphene-based composite films, and possibly lead to their total breakdown. In the second case, additional cross-linking agents are indispensable, resulting in difficulty in fabrication of composites with a high graphene content, which is essential for electrical conductive applications such as conductive inks or coatings.21,22 Sometimes, the additional cross-linking agent may destroy the graphene dispersion stability.23,24 Therefore, the development of new dispersants with self-crosslinking ability is still in demand for high performance graphene-based films.

INTRODUCTION Owing to its excellent electrical, thermal, and mechanical properties, graphene has been widely studied in various fields, such as electronics, energy, composite materials and bio-applications.1-4 To obtain high performance graphene-based materials, the formation of a homogeneous graphene dispersion is a key issue; however, the nanosheets tend to aggregate due to the existence of strong van der Waals forces and π-π interactions between the large sp2 conjugated structures.5,6 The non-covalent functionalization of graphene, namely, the use of specific polymers, surfactants, or conjugated aromatic molecules as stabilizers, has been extensively adopted to prepare graphene dispersions, owing to the convenience of the process and its low impact on the intrinsic properties of graphene.7-13 Nevertheless, in most cases, these chemicals only work as stabilizers, without a self-crosslinking function. Another polymer has to be added to aid filmformation from the graphene dispersion. This causes both a compatibility problem between the dispersant-attached graphene sheets and the polymer matrix, and it deteriorates the electrical conductivity of the graphene film, due to the increased amount of nonconductive components in the film.6,14 So far, two strategies have generally been adopted to solve the above challenges. The first consists in employing functional polymers that simultaneously favor the filmforming ability and the dispersion of graphene.15-17 For example, Huang et al.15 prepared a composite film by drop-casting a reduced graphene oxide (rGO)/polyvinylpyrrolidone (PVP) aqueous dispersion onto scotch tape. Ali et al.16 deposited a PVP/graphene film on a silicone substrate by electrospraying the

Herein, we report a new efficient graphene dispersant that can not only stabilize high concentration graphene (10 mg/mL) in water, but also self-crosslink when cast on

1 ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. (a) Synthesis of TSiPD•+, (b) molecular structure of TCNQ•- and TMPD•+.

a substrate. The molecular structure of the dispersant (TSiPD•+), as shown in Scheme 1a, was easily synthesized from the cheap, commercially available chemicals, (3glycidyloxypropyl)trimethoxysilane (GPTMS) and pphenylenediamine (PDA). Conductive composites with high graphene content were obtained by directly drying the TSiPD•+-stabilized dispersion. The composites have good mechanical strength and are durable in water, hydrochloric acid, and various organic solvents. More importantly, the conductivity of the composite was less influenced by the presence of TSiPD•+, benefiting from the presence of tightly stacked graphene sheets.

Dispersion of graphene in water. The TSiPD•+ solution (1.0 wt%) was first prepared by hydrolyzation of TSiPD in an HCOOH solution (0.1 M) for 72 h at 60°C, and was diluted to the desired concentration before use. Known amounts of graphene powders were added to the TSiPD•+ solutions, and subsequently tip-sonicated in an ice bath using a JYD-650 L (Shanghai Precision Instrument Co., Ltd.) ultrasonic cells crushing homogenizer (cycle: 3 s on and 3 s off, power: 350 W.) For graphene concentrations below 2.5 mg/mL, the sonication time was 30 min; whereas for the 5 and 10 mg/mL graphene solutions, the sonication times were 1 and 2 h, respectively.

EXPERIMENTAL SECTION

Preparation of self-crosslinking graphene films. The “Graphene” pattern on glass slide, the cartoons and Chinese character on polyethylene terephthalate (PET) films, and the molecular structure on printer paper were obtained by directly writing with a 10 mg/mL TSiPD•+stabilized graphene dispersion on the corresponding substrates. A plastic pipette tip (1000 μL) was used to contain the graphene dispersion employed. The graphene-PVP and graphene-TSiPD•+ films for electrical conductivity measurement and mechanical strength test were prepared by drop-casting the corresponding dispersion on glass slides and drying them at 60°C in an oven.

Materials. PDA, anhydrous methanol and formic acid (HCOOH, 88%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). GPTMS, (97%) and N,N,N’,N’-tetramethyl-p-phenylenediamine dihydrochloride (TMPD, 98%) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). All the chemicals were used as received without further purification. Graphene powders (XF001W, thermally reduced, lateral size of 0.5–5 microns, 7.0–7.5 at.% oxygen, BET surface area of 500–1000 m2/g, and electrical resistivity ≤ 0.30 Ω cm) were purchased from Nanjing XFNANO Materials Co., Ltd. (Nanjing, China).

Characterization. NMR experiments were conducted on a Bruker AVANCE III spectrometer (Switzerland). FTIR spectra were obtained from a Bruker VERTEX 70 spectrometer (Madison, WI, USA), using KBr pellets as supports. Mass spectrometry (MS) data was obtained from a Waters 3100 Mass Detector (Milford, MA, USA). UV/Vis absorption spectra were measured using a Hitachi U-4100 spectrophotometer (Tokyo, Japan) in a quartz cell of path-length 10 mm. The electronic paramagnetic resonance (EPR) was recorded at room temperature using a Bruker EMX-8/2.7 (Germany), with 9866 MHz frequency and 6.373 mW power. Zeta-potential and sizedistribution of the graphene dispersion were obtained from a zetasizer Nano ZS90 instrument (Marlvern, UK). Prior to measurement, each sample was diluted to 0.01 mg/mL by gentle shaking. The surface and cross-sectional morphology of the composites was characterized by scanning electron microscopy (SEM, VEGA 3 XMU,

Synthesis of TSiPD. PDA (1.08 g, 10 mmol) was first dissolved in anhydrous methanol (25 mL) under sonication, and GPTMS (9.75 g, 40 mmol) was subsequently added to the solution. The mixture was then left to react at the reflux temperature for 48 h in a dark and anhydrous environment (the condenser was equipped with a drying tube). Finally, methanol was removed by vacuum rotary evaporation. TSiPD was obtained as a brown liquid after drying in a vacuum oven for 24 h at 40°C. Yield: 97.5% (calculated from the 1H NMR spectra integral area ratio of residual GPTMS in the product). 1H NMR (400 MHz, CD3Cl, δ): 6.79-6.68 (d, br, 4H), 4.19-3.98 (t, br, 4H), 3.57 (s, 36H), 3.53-3.11 (m, br, 24H, overlapped), 1.72 (s, 8H), 0.68 (s, 8H). 13C NMR (100 MHz, CD3Cl, δ): 141.50, (116.77, 115.31), 73.36, 72.71, 68.74, (58.19, 56.65), 50.52, 22.75, 5.38. MS (ESI+, m/z): calculated for C42H88N2O20Si4 + H+: 1053.50; found: 1053.51.


Page 2 of 9

Page 3 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


•+

Figure 1. Illustration of the graphene dispersion stabilized by TSiPD and its self-crosslinking mechanism.

stabilizing graphene dispersions.29-31 Furthermore, it was demonstrated that a para-quinone parent with an extended conjugate structure effectively interacts with graphene. On the basis of these observations, we surmise that a similar structure, a radical cation form of TMPD (TMPD•+) (Scheme 1b), could also interact with graphene via π-π interactions since the electrons are delocalized within the central benzene ring and the two nitrogen atoms. TMPD•+ is a stable radical cation that can be easily obtained from one-electron oxidization of TMPD.32,33 Because its derivatives can be easily synthesized from the cheap PDA and other reactive agents, we chose the asTMPD•+ structure as the affinity site for graphene.

TESCAN, Czech Republic). Transmission electron microscopy (TEM) images of the graphene sheets were obtained using a Tecnai G2 20 TWIN microscope (Hillsboro, OR, USA) operated at an accelerated voltage of 200 kV. Atomic force microscopy (AFM) was performed with a Bruker Dimension Edge scanning probe microscope (Santa Barbara, CA, USA) in tapping mode under ambient conditions; the sample was prepared by spin-coating a drop of graphene dispersion (0.5 mg/mL, graphene: TSiPD•+ = 1:0.8) on freshly cleaved mica. Electrical conductivities were measured with a RTS-9 four-point probe meter (Guangzhou, China). Mechanical strength test: Graphene dispersions (2 mL, 2 mg/mL) with different TSiPD•+ contents were first deposited on glass slides (25 mm × 35 mm) and dried at 60°C in an oven. The mechanical strength of the obtained coating films was tested by a pencil hardness tester (BY type, Shanghai Pushen Chemical Machinery Co., Ltd, China) equipped with pencils having different hardness (CHUNG HWA 101, 6B-6H). Before the test, the lead of the pencil was grinded with sandpaper until it became flat and was held firmly against the coating film at a 45° angle. The mass loaded on the pencil tips was 1000 g, and it was pushed away from the operator for no less than 10 mm.

GPTMS was chose to react with PDA to form a TMPD derivative. The residual trimethoxysilyl groups can hydrolyze in water, forming the hydrophilic silanol groups (Si-OH) and thus making the molecule soluble in water and some polar organic solvents. More importantly, the Si-OH groups can form strong Si-O-Si bonds through inter/intramolecular dehydration condensation, so as to endow the graphene dispersion with self-crosslinking ability and the high mechanical strength of the finally obtained graphene films. The multifunctional character of the TSiPD•+ dispersant is illustrated in Figure 1.

RESULTS AND DISCUSSION Design and synthesis of the self-crosslinkable dispersant (TSiPD•+). It is well known that an effective dispersant for graphene must meet two criteria, that is, high affinity towards graphene sheets and good compatibility with the dispersing medium.2 Large conjugated molecules, such as perylene bisimides,12,25 diazaperopyrenium dications,26 pyrenes,6,10 flavin 27,28 mononucleotides, and naphthalene diimides11 have been developed because they can physically attach to the graphene surface via strong π-π interactions. In addition to these polycyclic aromatic structures, we noticed that, the tetracyanoquinodimethane anion (TCNQ•- in Scheme 1b), a small conjugated molecule, is also capable of

•+

Figure 2. (a) UV-Vis absorption spectra of 0.001 M TSiPD •+ and TMPD aqueous solutions, inset: photograph of the •+ •+ TSiPD solution. (b) EPR spectrum of a 0.01 M TSiPD solution.

In our experiment, TSiPD was first obtained by the reaction of PDA with GPTMS, with a 97.5% yield. The detailed characterization spectra are provided in the



Page 4 of 9

•+

Figure 3. Photographs of a 0.5 mg/mL graphene dispersion (a) with and (b) without TSiPD . Typical (c) TEM, and (d) AFM •+ images of the TSiPD -stabilized graphene sheets. (e) DLS measurement results of the 0.5, 3, and 10 mg/mL graphene dispersions, inset: 10 mg/mL graphene dispersion (left) and 1 000 times diluted (right).

Supporting Information. To obtain the aqueous TSiPD•+ solution, TSiPD was first hydrolyzed in deionized water at room temperature. Nevertheless, the hydrolyzation rate was found to be too low to form a homogeneous solution. A 0.1 M formic acid solution was hence used to accelerate the hydrolyzation of the alkoxysilanes.34 TSiPD gradually dissolved in the acidic media and formed a clear solution under stirring conditions, demonstrating the generation of hydrophilic Si-OH groups. Beyond our expectation, the color of the solution simultaneously turned to light blue during the hydrolytic process. The solution displayed strong UV-Vis absorption peaks centered at 576 and 626 nm (Figure 2a), with a band profile is in accordance with that of the TMPD•+ solution. This phenomenon indicated that TSiPD•+ was generated in the hydrolytic process. To further confirm this formation, an EPR measurement was conducted.35,36 A strong EPR signal, with g = 2.0030, was observed (Figure 2b), solidly evidencing the formation of TSiPD•+.

dispersion state of the graphene sheets with TSiPD•+ in water. The presence of sheets consisting of one or few layers was clearly observed. Figure 3d shows a typical AFM image of the graphene-TSiPD•+ hybrid. The sample was prepared by spin-coating the dispersion on a freshly cleaved mica surface. The cross-section analyses indicated that the average thickness of a one-layered grapheneTSiPD•+ hybrid is ~2 nm; the thickness of the two-layered hybrid is ~3.2 nm. Since the dispersant acted on both surfaces of the sheets, we reasonably concluded that the TSiPD•+ thickness on the surface is ~0.4 nm, and the thickness of the graphene sheet is ~1.2 nm, which is consistent with most previous reports.9,11,12,37 The TEM and AFM results demonstrated that the graphene sheets were uniformly dispersed, and that TSiPD•+ effectively prevented the agglomeration of the commercial graphene in water. Dynamic light scattering (DLS) was also used to evaluate the dispersion results. Only a single peak, centered at 157 nm, appeared (Figure 3e), indicating the homogeneity of the dispersion.38,39

The performance of TSiPD•+ as graphene dispersant. To verify the capability of TSiPD•+ as a stabilizer for graphene in water, a 0.5 mg/mL dispersion was first tested by tip-sonication of graphene powders in this solution. The weight ratio between graphene and TSiPD•+ was 1:0.8. It should be noted that we hypothesized that the methoxyl groups were completely hydrolyzed, but without condensation between the silanol groups. As expected, a stable and homogenous dispersion was obtained simply by carrying out tip-sonication for 30 min (Figure 3a). After either settling for one month or centrifuging at 2000 rpm for 30 min, only very small amounts of sediments appeared at the bottom. By contrast, graphene sheets without TSiPD•+ settled completely within a short time (Figure 3b). The zetapotential of the dispersion was found to be +35.7 mV and explains the excellent stability of the dispersion. TEM images in Figure 3c provide direct evidence for the

When further increasing the graphene content to 2.5 mg/mL, a stable and homogeneous dispersion was still obtained with a graphene-to-TSiPD•+ weight ratio of 1:0.8. On the other hand, the dispersion became metastable when the graphene concentration was increased to 3 mg/mL. Agglomeration took place a few hours after the freshly-prepared dispersion had settled. Several works previous reported in the literature have demonstrated that the dispersion concentration of graphene depends on the stabilizer concentration.6,7 We thus tried to increase the amount of TSiPD•+ to obtain a stable dispersion. If the weight ratio of graphene-to-TSiPD•+ was increased to 1:1.2, the 3 mg/mL graphene dispersion could be stored stably. Even after having been stored for one month, the DLS measurement continued to show a single peak (Figure 3e) with size distribution being similar to that of 0.5 mg/mL graphene dispersion. With a graphene-to-TSiPD•+ weight


Page 5 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Surface (left) and cross-sectional (right) SEM images of the “Graphene” pattern deposited on a glass slide (inset). •+ (b) Pencil hardness test results on graphene-TSiPD composite films at different weight ratios, inset: the testing apparatus. (c) Optical microscope images of the 1:1.2 composite after stroking by 3H and 4H pencils. (d) The “Graphene” pattern soaked in •+ different solvents for 24 h. (e) A graphene-PVP and graphene-TSiPD composite line deposited on a glass slide (left), and sonication for 5 s after dipping in water (right).

ratio of 1:1.2, stable graphene dispersions could be obtained for graphene concentrations up to 10 mg/mL. A small peak occurred for large particle size (Figure 3e), but its integrated intensity is negligible relative to that of the peak at small size. The 10 mg/mL dispersion (Figure 3e, inset: left) can transform into a translucent dispersion without the formation of any visible agglomerates after being diluted by 1000 times merely by gentle shaking (Figure 3e, inset: right), in accordance with the DLS result. It should be noted that the concentration of 10 mg/mL is much higher than that using conventional surfactant or polymer as stabilizer, where it is usually below 1 mg/mL.17,37,40,41 The excellent stability of the TSiPD•+containing graphene dispersion confirmed our initial hypothesis that the conjugated as-TMPD•+ structure could effectively interact with graphene.

Pencil hardness tests were adopted to determine the mechanical properties of the graphene-TSiPD•+ composite coatings. The graphene-TSiPD•+ dispersion was first coated on glass slides and dried at 60°C, and a pencil hardness tester (the inset in Figure 4b) was then used to perform the experiments. The results are shown in Figure 4b. When the weight ratio of graphene-to-TSiPD•+ was 1:0.8, the coating film hardness was 2B, indicating a moderate mechanical strength. When the ratio was changed to 1:1.2, the film hardness dramatically increased to 3H. The film surfaces obtained after stroking by a 3H or 4H pencil were observed using an optical microscope, as shown in Figure 4c. Although the mass loaded on the pencil tips was as high as 1000 g, the surface was still intact, with only slight scratches on it, demonstrating the high mechanical strength of the graphene-TSiPD•+ composite film. This may be due to the formation of an integrated film with high TSiPD•+ content. An additional increase in the TSiPD•+ content did not bring appreciable improvement in the hardness, which remained at 3H for weight ratios becoming 1:1.6. Only when the grapheneTSiPD•+ ratio was down to 1:2.4, the mechanical strength increased to 4H.

Features of the composites prepared from the selfcrosslinkable graphene dispersion. Graphene composites were fabricated by directly casting the prepared dispersion on several substrates. As shown in Figure 4a, a “Graphene” pattern was deposited on a glass slide using the 10 mg/mL graphene dispersion. From the surface and cross-sectional SEM images, we found that the graphene sheets were uniformly stacked in a layer-bylayer form. This was mainly attributed to the homogeneous dispersion of graphene, which made the nanosheets extend fully. Since TSiPD•+ has plenty of SiOH groups, dehydration condensation occurred between these groups during solvent evaporation. The high-energy Si-O-Si bonds were thus formed across the entire composite, resulting in the graphene sheets being tightly stacked and exhibiting good mechanical strength.

The graphene-TSiPD•+ composites also exhibited good solvent resistance. Figure 4d shows the “Graphene” pattern soaked in either water, ethanol, acetone, tetrahydrofuran (THF), dichloromethane (CH2Cl2), dimethylformamide (DMF) or 2 M HCl solutions for 24 h. The pattern remained intact even after soaking followed by 20 min sonication. Only the pattern immersed in the 2 M NaOH solution for 12 h was gradually deteriorated and partially peeled off from the glass substrate. This was mainly due to the breaking, in the alkaline media, of the



Figure 5. A 10 mg/mL graphene dispersion directly written on (a) printer paper and (b) PET films. (c) Electrical conductivities of •+ graphene composites at different weight ratios of graphene to TSiPD (or PVP-K30). Cross-sectional SEM images of (d) •+ graphene-TSiPD and (e) graphene-PVP films. (f, g) The corresponding surface SEM images.

TSiPD•+ weight ratio of 1:0.8, the graphene-TSiPD•+ film still exhibited a high conductivity (531 S/m), equal to about 45.4% of that of the pure graphene film (1169 S/m). In contrast, the conductive value of the corresponding graphene-PVP film was 102 S/m, only 8.7% of that of the pure graphene film. When the graphene-to-dispersant weight ratio was 1:2.4, the conductivity of the grapheneTSiPD•+ film was 128 S/m, about 10.5 times higher than that of the graphene-PVP film (11.1 S/m). These results indicated that the electrical conductivity of the graphene film is less impacted by the presence of the TSiPD•+ dispersant compared to PVP-K30.

Si-O-Si bond that existed within the composite and at the interfaces between graphene and the glass substrate. A PVP-stabilized graphene film on glass substrate was used as comparison. Because no covalent bond existed, the film broke down within 5 s after soaking and sonication in water (Figure 4e). Therefore, the self-crosslinkable TSiPD•+ dispersant was proved to efficiently improve the solvent resistance of graphene films. The TSiPD•+-stabilized graphene dispersion can be used directly as conductive ink on printer paper. As shown in Figure 5a, the molecular structure of TSiPD•+ on paper was composed of graphene, of which the connected part is conductive, with a resistance of 3.42 kΩ. Due to the reaction of the Si-OH of TSiPD•+ with the hydroxyl groups of the paper cellulose fiber, the pattern on paper is so strong that it cannot be eliminated by moderate erasing. Its resistance was not noticeably changed even after repeated abrasion. Figure 5b shows a hand-drawn Chinese character and two cartoon animals on flexible PET films using the graphene dispersion as ink. Because the graphene sheets could be uniformly stacked on the smooth substrate, the pattern has a much lower resistance, dropping to 0.458 kΩ. More interestingly, the resistance did not vary even when the film was bended arbitrarily, suggesting the potential application of the graphene dispersion in flexible circuits.

The great difference in the conductivities of these two composites is most likely due to the different packing density of graphene. When using TSiPD•+ as dispersant, dehydration condensation takes place between the Si-OH groups, resulting in a weight loss of about 12%. This weight loss caused the formation of a denser composite film, as revealed from the film thickness shown in Figure 5d and 5e. At the same graphene-to-dispersant ratio of 1:2.4, the graphene-TSiPD•+ film has a thickness of ~9.3 μm, while the thickness of the graphene-PVP film is ~11 μm. This suggests that in the former film the graphene volume fraction is higher than in the second. Since the electrical conductivity of composite materials (σ) is related to the volume fraction (φ) of the conductive filler by the power-law expression,42,43 σ = σ0 (φ − φc)t, it is reasonable for the graphene-TSiPD•+ film to possess a higher electrical conductivity.

The influence of TSiPD•+ on the electrical conductivity of graphene was investigated. PVP-K30, the most commonly used polymer dispersant for graphene, was chosen for comparison. Figure 5c displays the conductivities of the graphene composites measured by the four-point probe method. It is obvious that all conductivities decreased when increasing the amount of dispersant, since it blocked the connection between graphene sheets. Nevertheless, even with a graphene-to-

Different stacking of the graphene sheets is another reason responsible for the difference in conductivity. As seen from Figure 5f, no large embossment was observed on the surface of the graphene-TSiPD•+ film. In contrast, the surface of the graphene-PVP film was quite rough. These micrometer scale embossments (Figure 5g) could


Page 6 of 9

Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials This work was financially supported by the National Key Research and Development Program of China (2017YFA0204600), and the National Natural Science Foundation of China (51673047).

remarkably impact the electron transfer in the composite, and thus result in a lower conductivity. The morphology difference is mainly due to the different dispersion mechanisms. In the case of TSiPD•+, most of its molecules could uniformly attach to the graphene surface in the dispersion via π-π interactions, and graphene sheets could crosslink in a flat form after condensation. On the other hand, for PVP-K30, the long polymer chains could not be adsorbed on the graphene surface as uniformly as the small molecules, and a slight phase separation may have taken place during the drying process, resulting in an uneven stacking of graphene.

REFERENCES 1. Georgakilas, V.; Tiwari, J. N.; Kemp, K. C.; Perman, J. A.; Bourlinos, A. B.; Kim, K. S.; Zboril, R. Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications. Chem. Rev. 2016, 116, 5464-5519. 2. Ciesielski, A.; Haar, S.; El Gemayel, M.; Yang, H.; Clough, J.; Melinte, G.; Gobbi, M.; Orgiu, E.; Nardi, M. V.; Ligorio, G.; Palermo, V.; Koch, N.; Ersen, O.; Casiraghi, C.; Samorì, P. Harnessing the Liquid-Phase Exfoliation of Graphene Using Aliphatic Compounds: A Supramolecular Approach. Angew. Chem., Int. Ed. 2014, 53, 10355-10361. 3. Liu, W.; Niu, H.; Yang, J.; Cheng, K.; Ye, K.; Zhu, K.; Wang, G.; Cao, D.; Yan, J. Ternary Transition Metal Sulfides Embedded in Graphene Nanosheets as Both the Anode and Cathode for HighPerformance Asymmetric Supercapacitors. Chem. Mater. 2018, 30, 1055-1068. 4. Cui, J.; Zhou, S. Facile fabrication of highly conductive polystyrene/nanocarbon composites with robust interconnected network via electrostatic attraction strategy. J. Mater. Chem. C 2018, 6, 550-557. 5. Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156-6214. 6. Parviz, D.; Das, S.; Ahmed, H. S. T.; Irin, F.; Bhattacharia, S.; Green, M. J. Dispersions of Non-Covalently Functionalized Graphene with Minimal Stabilizer. ACS Nano 2012, 6, 8857-8867. 7. Wajid, A. S.; Das, S.; Irin, F.; Ahmed, H. S. T.; Shelburne, J. L.; Parviz, D.; Fullerton, R. J.; Jankowski, A. F.; Hedden, R. C.; Green, M. J. Polymer-stabilized graphene dispersions at high concentrations in organic solvents for composite production. Carbon 2012, 50, 526-534. 8. Buzaglo, M.; Ruse, E.; Levy, I.; Nadiv, R.; Reuveni, G.; Shtein, M.; Regev, O. Top-Down, Scalable Graphene Sheets Production: It Is All about the Precipitate. Chem. Mater. 2017, 29, 9998-10006. 9. Liang, Y.; Wu, D.; Feng, X.; Müllen, K. Dispersion of Graphene Sheets in Organic Solvent Supported by Ionic Interactions. Adv. Mater. 2009, 21, 1679-1683. 10. An, X.; Simmons, T.; Shah, R.; Wolfe, C.; Lewis, K. M.; Washington, M.; Nayak, S. K.; Talapatra, S.; Kar, S. Stable Aqueous Dispersions of Noncovalently Functionalized Graphene from Graphite and their Multifunctional High-Performance Applications. Nano Lett. 2010, 10, 4295-4301. 11. Zhang, L.; Zhang, Z.; He, C.; Dai, L.; Liu, J.; Wang, L. Rationally Designed Surfactants for Few-Layered Graphene Exfoliation: Ionic Groups Attached to Electron-Deficient πConjugated Unit through Alkyl Spacers. ACS Nano 2014, 8, 66636670. 12. Englert, J. M.; Röhrl, J.; Schmidt, C. D.; Graupner, R.; Hundhausen, M.; Hauke, F.; Hirsch, A. Soluble Graphene: Generation of Aqueous Graphene Solutions Aided by a Perylenebisimide-Based Bolaamphiphile. Adv. Mate. 2009, 21, 4265-4269. 13. Sreeprasad, T. S.; Berry, V. How Do the Electrical Properties of Graphene Change with its Functionalization? Small 2013, 9, 341350. 14. Bepete, G.; Anglaret, E.; Ortolani, L.; Morandi, V.; Huang, K.; Pénicaud, A.; Drummond, C. Surfactant-free single-layer graphene in water. Nat. Chem. 2016, 9, 347.

CONCLUSIONS In summary, we have synthesized a new graphene dispersant, TSiPD•+, using cheap, commercially available chemicals. The radical cation of a tetra-alkyl substituted PDA on the nitrogen atoms was demonstrated for the first time to be effective in interaction with graphene. The dispersant can stabilize graphene in water with a concentration up to 10 mg/mL, being superior to the commonly used surfactants or polymers. More importantly, the dispersant possesses plenty of reactive silanol groups, which endow the graphene dispersion with a self-crosslinking ability via the dehydration condensation among silanol groups. Highly conductive films with good mechanical strength and excellent solvent resistance were obtained by directly casting the graphene dispersion on various substrates. The impact of TSiPD•+ on the electrical conductivity of graphene film is quite small compared with the commonly used PVP. The outstanding performance of the self-crosslinkable graphene dispersion makes it a potential candidate for use in fields including conductive ink and coatings, electrodes, flexible circuits, and printable electronics.

ASSOCIATED CONTENT Supporting Information. 1 13 The H NMR, C NMR, FTIR, MS characterization spectra of TSiPD, and the calculation of the yield. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Shuxue Zhou. E-mails: [email protected].

ORCID Shuxue Zhou: 0000-0003-4725-8001

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no conflict of interest.

ACKNOWLEDGMENT



15. Huang, L.; Li, C.; Shi, G. High-performance and flexible electrochemical capacitors based on graphene/polymer composite films. J. Mater. Chem. A 2014, 2, 968-974. 16. Ali, A.; Ali, K.; Dang, H. W.; Mahmoud, K. A.; Choi, K. H. Different approaches to PVP/graphene composite film fabrication using electrohydrodynamic atomization technique. J. Mater. Sci.: Mater. Electron. 2015, 26, 2039-2044. 17. Secor, E. B.; Ahn, B. Y.; Gao, T. Z.; Lewis, J. A.; Hersam, M. C. Rapid and Versatile Photonic Annealing of Graphene Inks for Flexible Printed Electronics. Adv. Mater. 2015, 27, 6683-6688. 18. Ding, J.; Rahman, O. u.; Wang, Q.; Peng, W.; Yu, H. Sustainable Graphene Suspensions: A Reactive Diluent for Epoxy Composite Valorization. ACS Sustainable Chem. Eng. 2017, 5, 7792-7799. 19. Teng, C.-C.; Ma, C.-C. M.; Lu, C.-H.; Yang, S.-Y.; Lee, S.-H.; Hsiao, M.-C.; Yen, M.-Y.; Chiou, K.-C.; Lee, T.-M. Thermal conductivity and structure of non-covalent functionalized graphene/epoxy composites. Carbon 2011, 49, 5107-5116. 20. Otieno, G.; Kim, J.-Y. Conductive graphite/polyurethane composite films using amphiphilic reactive dispersant: Synthesis and characterization. J. Ind. Eng. Chem. (Amsterdam, Neth.) 2008, 14, 187-193. 21. Al Shboul, A.; Trudeau, C.; Cloutier, S.; Siaj, M.; Claverie, J. P. Graphene dispersions in alkanes: toward fast drying conducting inks. Nanoscale 2017, 9, 9893-9901. 22. Huang, X.; Leng, T.; Zhu, M.; Zhang, X.; Chen, J.; Chang, K.; Aqeeli, M.; Geim, A. K.; Novoselov, K. S.; Hu, Z. Highly Flexible and Conductive Printed Graphene for Wireless Wearable Communications Applications. Sci. Rep. 2015, 5, 18298. 23. Yu, G.; Wu, P. Effect of chemically modified graphene oxide on the phase separation behaviour and properties of an epoxy/polyetherimide binary system. Polym. Chem. 2014, 5, 96104. 24. Wei, X.; Li, D.; Jiang, W.; Gu, Z.; Wang, X.; Zhang, Z.; Sun, Z. 3D Printable Graphene Composite. Sci. Rep. 2015, 5, 11181. 25. Cui, J.; Zhou, S. Polyamine-functionalized perylene bisimide for dispersion of graphene in water with high effectiveness and little impact on electrical conductivity. J. Nanopart. Res. 2017, 19, 357. 26. Sampath, S.; Basuray, A. N.; Hartlieb, K. J.; Aytun, T.; Stupp, S. I.; Stoddart, J. F. Direct Exfoliation of Graphite to Graphene in Aqueous Media with Diazaperopyrenium Dications. Adv. Mater. 2013, 25, 2740-2745. 27. Ayán-Varela, M.; Paredes, J. I.; Guardia, L.; Villar-Rodil, S.; Munuera, J. M.; Díaz-González, M.; Fernández-Sánchez, C.; Martínez-Alonso, A.; Tascón, J. M. D. Achieving Extremely Concentrated Aqueous Dispersions of Graphene Flakes and Catalytically Efficient Graphene-Metal Nanoparticle Hybrids with Flavin Mononucleotide as a High-Performance Stabilizer. ACS Appl. Mater. Interfaces 2015, 7, 10293-10307. 28. Yoon, W.; Lee, Y.; Jang, H.; Jang, M.; Kim, J. S.; Lee, H. S.; Im, S.; Boo, D. W.; Park, J.; Ju, S.-Y. Graphene nanoribbons formed by a sonochemical graphene unzipping using flavin mononucleotide as a template. Carbon 2015, 81, 629-638. 29. Hao, R.; Qian, W.; Zhang, L.; Hou, Y. Aqueous dispersions of TCNQ-anion-stabilized graphene sheets. Chem. Commun. 2008, 0, 6576-6578. 30. Khanra, P.; Lee, C.-N.; Kuila, T.; Kim, N. H.; Park, M. J.; Lee, J. H. 7,7,8,8-Tetracyanoquinodimethane-assisted one-step electrochemical exfoliation of graphite and its performance as an electrode material. Nanoscale 2014, 6, 4864-4873. 31. Yang, S.; Jiang, Y.; Li, S.; Liu, W. Many-body dispersion effects on the binding of TCNQ and F4-TCNQ with graphene. Carbon 2017, 111, 513-518. 32. Michaelis, L.; Schubert, M. P.; Granick, S. The Free Radicals of the Type of Wurster's Salts. J. Am. Chem. Soc. 1939, 61, 1981-1992.

33. Gao, Y.; Chen, H.; Ge, J.; Zhao, J.; Li, Q.; Tang, J.; Cui, Y.; Chen, L. Direct Intertube Cross-Linking of Carbon Nanotubes at Room Temperature. Nano Lett. 2016, 16, 6541-6547. 34. Brinker, C. J. Hydrolysis and condensation of silicates: Effects on structure. J. Non-Cryst. Solids 1988, 100, 31-50. 35. Onitsch, C.; Rosspeintner, A.; Angulo, G.; Griesser, M.; Kivala, M.; Frank, B.; Diederich, F.; Gescheidt, G. Donor-Substituted Diphenylacetylene Derivatives Act as Electron Donors and Acceptors. J. Org. Chem. 2011, 76, 5628-5635. 36. Havenith, R. W. A.; de Wijs, G. A.; Attema, J. J.; Niermann, N.; Speller, S.; de Groot, R. A. Theoretical Study of the Stable Radicals Galvinoxyl, Azagalvinoxyl and Wurster’s Blue Perchlorate in the Solid State. J. Phys. Chem. A 2008, 112, 77347738. 37. Guardia, L.; Fernández-Merino, M. J.; Paredes, J. I.; SolísFernández, P.; Villar-Rodil, S.; Martínez-Alonso, A.; Tascón, J. M. D. High-throughput production of pristine graphene in an aqueous dispersion assisted by non-ionic surfactants. Carbon 2011, 49, 1653-1662. 38. Mustafa, L.; Aliaksandra, R.; John, F. D.; Jonathan, N. C. Measuring the lateral size of liquid-exfoliated nanosheets with dynamic light scattering. Nanotechnology 2013, 24, 265703. 39. Kim, S. G.; Wang, S. H.; Ok, C. M.; Jeong, S. Y.; Lee, H. S. Lateral diffusion of graphene oxides in water and the size effect on the orientation of dispersions and electrical conductivity. Carbon 2017, 125, 280-288. 40. Carrasco, P. M.; Montes, S.; García, I.; Borghei, M.; Jiang, H.; Odriozola, I.; Cabañero, G.; Ruiz, V. High-concentration aqueous dispersions of graphene produced by exfoliation of graphite using cellulose nanocrystals. Carbon 2014, 70, 157-163. 41. Ronan, J. S.; Mustafa, L.; Jonathan, N. C. The importance of repulsive potential barriers for the dispersion of graphene using surfactants. New J. Phys. 2010, 12, 125008. 42. Kyrylyuk, A. V.; Hermant, M. C.; Schilling, T.; Klumperman, B.; Koning, C. E.; van der Schoot, P. Controlling electrical percolation in multicomponent carbon nanotube dispersions. Nat. Nanotechnol. 2011, 6, 364. 43. Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442, 282.


Page 8 of 9

Page 9 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents artwork


High Concentration Self-crosslinkable Graphene Dispersion

Recommend Documents