Liquid crystal graphene oxide with different layers: fabrication, characterization and applications

Abstract

A series of graphene oxide (GO) nanosheets with monolayer, bilayer and multilayer structures were prepared merely via a simple centrifugation method. It is noticeable that the as-prepared bilayer and multilayer GO nanosheet aqueous dispersions exhibit liquid crystal (LC) behavior. Not only the liquid- but also the solid-state of the obtained GO presented prominent functional LC behavior. Then, GO films were fabricated via simple drop-casting of concentrated GO (cGO) aqueous dispersions that retain the radial schlieren texture and show excellent electrical conductivities of 12 S cm⁻¹ after chemical reduction. Thus it can be expected that utilizing the coating property of GO aqueous dispersions as well as the prominent birefringent linear features of the cGO films will have great potential for preparation of functional anti-counterfeiting components and energy materials.

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Introduction

Graphene oxide liquid crystals (GO LCs) are of potential technological interest because self-assembled GO sheets, produced by the liquid-crystalline process, can exhibit multifunctional attributes such as high mechanical strength and excellent flexibility, and can be an important precursor of ordered graphene building blocks.^1,2 Compared to other approaches developed to align GO sheets, the liquid-crystal route is particularly attractive for achieving self-assembly with graphene-based materials in the form of fibers and films.^3,4,7,8 The liquid-crystalline behavior of monolayer GO sheets has been reported.^1–8 Recently, even the liquid-crystalline behavior of graphite oxide sheets has also been confirmed.⁹ Furthermore, it is interesting to know about the pure bilayer or multilayer of GO sheets, and if there is liquid-crystalline behavior for them. In earlier work, size-selective fractionation of GO sheets was reported. Sun et al. developed a density gradient ultracentrifugation method for the fractionation of GO sheets.^10,11 Shi et al. invented a rapid and simple size separation method based on the pH-dependent amphiphilicity of GO sheets.¹² However, thickness-selective fractionation of GO sheets has never been reported.

Here, we report the separation of GO sheets with different layers by simple centrifugation, after which multilayer, bilayer and monolayer GO sheets are obtained. Our experimental results indicate that not only the monolayer, but also the bilayer or multilayer GO sheets are able to form a LC in their aqueous dispersions. More importantly, the GO films that were prepared with GO LCs can retain the radial schlieren texture. Furthermore, graphene films can be obtained by simple drop-casting of GO dispersions, combined with chemical reductions, and possess fine flexibility and high electrical conductivity. Moreover it can be shown that the macroscopic solid materials can be used well in optical fields and as anti-counterfeiting material.

Results and discussion

Scheme 1 shows the fabrication process of GO sheets with varying layers via a series of centrifugations. GO was synthesized from graphite powder by a modified Hummers method.^13,14 GO sheets with different layers were obtained by simple centrifugation (see ESI†), which could be confirmed by atomic force microscopy (AFM) measurements. Fig. 1a–c show the representative AFM images of the GO sheets obtained from three different cycles. It can be clearly observed that the thickness of the GO sheets decreases with the increasing cycle times. According to the AFM measurements (Fig. S3–S5, ESI†), the thickness and width distribution of each sample can be obtained. Typically, for GO-1, GO-6 and GO-12, the thickness is about 2 nm, 1.6 nm and 1 nm, respectively (Fig. 1d–f). According to the uniform thickness of each sample and the linear decrease in the thickness from GO-1 to GO-12, we infer that the three samples have different sheet layers and that they are multilayer, bilayer and monolayer for GO-1, GO-6 and GO-12, respectively.¹⁵ The thickness values of the multilayer GO sheets are much less than three times that of the values of monolayer GO sheets; we think that the multilayer GO sheets, compared with the monolayer sheets, have less oxygen-containing functional groups, resulting in the experimental thickness being lower than the theoretical thickness.¹⁶ Moreover, their lateral widths are also reductive and the average lateral widths are 1148 nm, 933 nm and 746 nm (Fig. 1g–i). For GO-1, GO-6 and GO-12, average values for W/T of individual sheets are 5.13 × 10², 5.5 × 10² and 7.34 × 10², respectively, indicating that the average W/T values are decreasing with the increasing cycle times.

Scheme 1 Schematic protocol to separate GO sheets with different layers.

Fig. 1 Representative AFM images of the GO sheets deposited on silicon wafer from GO-1, GO-6 and GO-12 aqueous solution (a–c) and their corresponding thickness distributions (d–f) and width distributions (g–i).

The decreasing trend of thickness is also confirmed by transmission electron microscopy (TEM), which is shown in Fig. 2. The results illustrate that the thickness of GO-6 and GO-12 is thinner than that of GO-1, which is in good agreement with the results of AFM.

Fig. 2 TEM images of GO-1 (a), GO-6 (b), and GO-12 (c). The samples were deposited on copper screens (325 mesh).

In a previous study, graphene oxide was proven to be a highly heterogeneous material that consists of poorly oxidized graphene sheets and highly oxidized graphene debris.¹⁷ The poorly oxidized graphene sheets were thicker and bigger than the highly oxidized graphene debris; this means that they may be preferentially settled to the bottom of a centrifugal tube by centrifugal forces. To test this hypothesis and further explore the influencing factors of exfoliation, Raman spectroscopy and X-ray-photoelectron-spectroscopy (XPS) were used to characterize the obtained GO sheets. Fig. S6 (ESI†) shows the high frequency Raman spectra of GO sheets with different layers. The Raman spectra of the samples display two characteristic peaks at 1356 and 1595 cm⁻¹, which represent the D-band (C–C, the vibrations of sp³ carbon atoms of defects and disorder) and G-band (C=C, the vibration of sp² carbon atoms in a graphitic 2D hexagonal lattice), respectively.¹⁸ In general, the degree of graphitization is an indicator of the graphene sheets’ disorder level, and is characterized by the intensity ratio of the D and G bands (R = I_D/I_G). The I_D/I_G ratio of GO-1, GO-6 and GO-12 is 0.84, 0.89 and 0.94, respectively. The increasing I_D/I_G ratio indicates that the graphitic 2D hexagonal lattice has been severely disturbed, which weakens the van-der-Waals forces between the GO layers.

As is known, electrostatic repulsion, due to the presence of electronegative oxygen-containing functional groups, also has an influence on the van-der-Waals forces between GO layers.¹⁹ Hence, the oxygen content may be a major factor to affect this interaction. XPS measurement was always carried out to identify the chemical species, which can be used as direct evidence to measure the oxygen content.^20,21 The results from the XPS analysis are shown in Fig. S7 (ESI†). The C1s spectra are compared for the different cycles of the GO samples. As can be seen from the C1s XPS spectrum of GO, a considerable degree of oxidation is clearly indicated with four different components corresponding to the following functional groups: sp²-hybridized C–C/C=C in the aromatic ring (284.6 eV), C–O (286.7 eV), C=O (287.7 eV), and C(O)OH (290 eV). The atomic ratios of C1s/O1s decrease from 2.793 to 2.660 for GO-1 and GO-2. So the oxygen amount increases with the increasing cycle times, implying that it is closely related to the exfoliation of GO. The higher oxygen content means more negative charges, which results in stronger repulsive forces between the GO sheets, making the GO sheets more easily exfoliated.^4,6 From the results of Raman and XPS, it can be concluded that the van-der-Waals forces between GO sheets are affected by the defects of the 2D hexagonal lattice and electronegative oxygen-containing functional groups. The decrease of the van-der-Waals forces between GO sheets, owing to the enhancive defects and/or the increased oxygen content, can contribute to the exfoliation of the GO sheets.

As far as we know, a high concentration of the GO aqueous dispersion is a prerequisite to form a lyotropic LC. In this work, twelve groups of concentrated GO (cGO) aqueous dispersions were obtained via centrifuging. The f_m (mass fraction) values of the cGO aqueous dispersions are 2.5 × 10⁻², 1.7 × 10⁻², 1.2 × 10⁻², 1.0 × 10⁻², 0.8 × 10⁻², 0.8 × 10⁻² and 0.9 × 10⁻² for cGO-1, cGO-2, cGO-4, cGO-6, cGO-8, cGO-11 and cGO-12, respectively. The f_m values fall with the increase of the cycle times, except for cGO-12 (see ESI†). Fig. 3 shows the prominent optical birefringence of the twelve concentrated GO (cGO) dispersions in capillaries between two crossed polarizers, viewed by polarized-light optical microscopy (POM), which confirms the formation of the lyotropic LC. Comparing Fig. 1 and 3, we can see that cGO-12, with a thickness of 1.0 nm, presents a typical optical birefringence, which is in agreement with the previous reports. Moreover, we find that both GO-6 and GO-1, with a bilayer and multilayer, also have LC behavior. However, the optical birefringence of the twelve capillaries shows no distinct difference with the decreasing cGO concentration, except for cGO-12, which is due to the increased concentration of cGO-12 resulting in a more colorful optical birefringence. Using Onsager’s theory, the concentration for phase transition from the isotropic phase to the nematic phase is approximately linearly dependent on the thickness/width (f_m ≈ kT/W, k is a constant).^2,22 This means that a higher value of W/T indicates a lower concentration for transition to an ordered mesophase, and the increasing values of W/T for cGO-1, cGO-6 and cGO-12 are 5.13 × 10², 5.5 × 10² and 7.34 × 10² from the data of Fig. 1, suggesting a reductive trend of concentration; this is in accordance with our actual measured concentration changes. Moreover, the optical birefringence became more and more colourful from cGO-1 to cGO-12 when cGO-1 and cGO-6 were diluted to the same concentration as cGO-12 (Fig. S8†). This also conforms to Onsager’s theory.

Fig. 3 POM images of GO aqueous dispersions from cGO-1 to cGO-12 in capillaries, observed between crossed polarizers.

To ascertain the liquid-crystalline phases of the cGO samples, they were observed by POM on a glass slide. The liquid-crystalline schlieren texture in Fig. 4a and b is very similar to typical nematic samples. Meanwhile, the corresponding solid-states of the cGO dispersions also demonstrate radial schlieren textures (Fig. 4c and d). To investigate the radial schlieren textures of organized solids, scanning electron microscopy (SEM) images were collected. As shown in Fig. 5, the fracture morphology of the freeze-dried solid derived from the cGO dispersions displays an orientational arrangement. In the solid films, ordered alignments of wrinkles perpendicular to the sheet planes are clearly observed, with the orientational directions marked by arrows. These radial schlieren textures appearing in the solid-state of the cGO dispersions may be attributed to the arrangement of the GO wrinkles.²³

Fig. 4 POM images of the liquid- and solid-state of cGO aqueous dispersions for cGO-1 (a and c) and cGO-2 (b and d) on microslides observed between crossed polarizers. The scale bar represents 200 μm. Optical photographs of cGO-1 films (e) before and (f) after chemical reduction and the flexible rGO-1 film (g).

Fig. 5 SEM images of the fracture morphology of the freeze-dried solid of cGO-1 (a), cGO-4 (b), cGO-8 (c) and cGO-12 (d); the arrows indicate the alignment directions of the wrinkles.

Fig. 6a shows the SAXS results of the corresponding solid-state of cGO dispersions for cGO-1, cGO-2 and cGO-3. The scattering peak at q = 0.09 nm⁻¹ arising from the diffraction of the ordered flakes, reflects the uniformity of the spacing between the flakes, indicating that the three different cycle samples have the same oriented alignment of sheets, which is also in agreement with the results of the optical birefringence of the twelve concentrated GO (cGO) dispersions in capillaries in Fig. 3.²⁴

Fig. 6 SAXS spectra of films for cGO-1, cGO-2 and cGO-3; the spectra depict the scattering intensity as a function of the scattering vector q (q = (4π sin θ)/λ, where 2θ is the scattering angle) (a). The XRD patterns of cGO-1 and rGO-1 films (b).

Free-standing GO film was fabricated via simple drop-casting of cGO aqueous dispersions and is shown in Fig. 4e. It is well known that the insulating GO can be easily converted to conductive graphene by either chemical or thermal reduction.^25–27 Herein, the rGO-1 flexible films can be prepared by chemical reduction of cGO-1 films using aqueous hydroiodic acid solution as the reducing reagent.^28,29 After chemical reduction, rGO-1 flexible film has a metallic luster (Fig. 4f and g). In the chemical reduction process, there are two chemical structural evolutions: the elimination of pendant oxygen-containing functional groups, such as hydroxyl and epoxy groups, and the restoration of the conjugated carbon net, which are proven by the detailed characterizations of XPS, Raman and X-ray diffraction (XRD). Fig. S7a and S9 (ESI†) show the XPS survey spectra of cGO-1 films and rGO-1 flexible films, and the atomic ratios of C1s/O1s increase from 2.79 to 8.75. This verifies that the removal of oxygen functional groups may accompany the recovery of the conjugated network. The 2D peak (2680 cm⁻¹) in the Raman spectrum (Fig. S10, ESI†) is obviously increased after reduction, also strongly suggesting the restoration of sp² carbon in rGO films.^5,29,30 The increase in the ratio of I_(D)/I_(G) (1.30) for rGO-1 compared to cGO (0.84) suggests an increase of disorder in the microstructures of the reduced GO, thanks to the presence of unrepaired defects that remained after the removal of oxygen moieties. The XRD spectrum shows that the interlayer distance of the rGO-1 film (Fig. 6b) decreases to 3.68 Å (2θ = 24.16°) from 7.69 Å (2θ = 11.5°) for the cGO-1 film, which is attributed to the elimination of the oxygen-containing groups on the graphene sheets.

Mechanical tensile measurements demonstrate that both GO films and rGO films exhibit a typical plastic deformation under tensile loading at room temperature (Fig. S11, ESI†). The initiating regions in tensile curves are possibly due to the stretching of the wrinkled GO and rGO sheets. The Young’s moduli of GO films and rGO films are 96 MPa and 46 MPa, individually. Upon further straining, GO films and rGO films show typical fracture strengths of 1.9 and 3.9 MPa, and ultimate elongations of 2.6% and 3.9%, respectively. This enhanced strength could be ascribed to the stronger interactions between graphene sheets coming from the more compact stacking of the graphene films.^29,31 Moreover, rGO films show excellent electrical conductivities of 12 S cm⁻¹, which is higher than in previous reports thanks to the fine alignment of graphene sheets resulting from the self-assembly of GO LCs.^32–35 Utilizing the prominent birefringent linear features of cGO films, we tried to employ them for optical anti-counterfeit technology. As shown in Fig. 7, the cGO film (Fig. 7b) shows a prominent emergence of microscopic birefringence between two crossed polarizers (Fig. 7a), while no obvious patterns can be observed from the cGO film in an ordinary optical microscope (OM) (Fig. 7c). The appearance and disappearance of the patterns upon POM and OM are completely reversible due to this simple physical optical transition.

Fig. 7 Optical photographs of cGO film (b); the film acting as an anti-counterfeiting material observed by a polarizing microscope (POM) (a) and optical microscope (OM) (c).

Conclusions

In summary, GO LCs with different sheet layers were successfully prepared, and GO films fabricated via simple drop-casting of cGO aqueous dispersions could show excellent electrical conductivities of 12 S cm⁻¹ after chemical reduction. Also, their extraordinary birefringent linear features allow them to be used as functional anti-counterfeiting materials.^36,37 Moreover, considering GO has a favorable dispersibility in many solvents (water, ethylene glycol, N-methyl pyrrolidone, N,N-dimethylformamide, etc.),³⁸ the realization of wide dispersibility and simplicity in processing means that GO LCs with different layers may have great potential applications in functional energy materials, such as flexible electrodes, electric circuits and chemical sensors, etc.^39–42

Acknowledgements

We are grateful to the National Natural Science Foundation of China (No. 51173170, 21101141), the financial support from the Innovation Talents Award of Henan Province (114200510019), State Key Laboratory of Chemical Engineering (No. SKL-ChE-13A04), and the Key program of science and technology (121PZDGG213) from Zhengzhou Bureau of science and technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16539c

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