Baoping Jia*ab,
Qiuze Wangb,
Wei Zhangc,
Bencai Linad,
Ningyi Yuanad,
Jianning Ding*ad,
Yurong Rena and
Fuqiang Chua
aSchool of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China. E-mail: baoping.jia@cczu.edu.cn; dingjianning@tsinghua.org.cn; Fax: +86 519 81085951
bJiangnan Graphene Research Institute, Changzhou, Jiangsu 213100, China
cSchool of Natural and Built Environments, University of South Australia, Adelaide, 5001 South Australia
dJiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou, Jiangsu 213164, China
First published on 14th July 2014
A new oil/water interfacial assembling approach has been developed to fabricate ultrathin graphene films based on a combination of two established interfacial assembling techniques. This new approach integrates multiple advantages such as simple equipment requirements in traditional oil/water interfacial assembly and minimum demand on the feeding source in air/water interfacial assembly (Langmuir–Blodgett deposition). By simply injecting a very small amount (1 ml) of ethanol dispersion of sulphonated graphene sheets (GSs) to the hexane/water interface, GSs can be effectively confined at the interface and self-organize into monolayer film with high uniformity and a controllable topographical feature. Multilayered films can also be obtained by layer-by-layer deposition without extra binding agent, demonstrating a cost-efficient, convenient, flexible and controllable approach for high quality ultrathin graphene films.
Much recently, the planar interfaces of liquid/liquid (oil/water) and air/water (Langmuir–Blodgett) systems have been applied as templates for two-dimensional assembly of nanomaterials, including GO and GS, and have shown capability of accurate control over the film thickness in the order of nanometers and in-plane packing density.16–27 Normally, in the oil/water system, nanoparticles in aqueous phases were driven to the interface with external assistance, such as sonication, gas bubble and inducing agent, and then self-organized into film.16–20 In air/water system, nanoparticles in volatile oil phase were spread over the water subphase to form floating monolayer, which can be further compressed into compacted feature.21–27 In the present work, we aim to develop a modified oil/water interfacial assembling approach for ultrathin graphene films based on a comprehensive understanding and combination of the above two systems. In our design, the major advantage of the oil/water system (such as simple requirement in equipment, which is mainly a beaker and syringe) and some merits from air/water system (such as the minimum demand on the feeding source) will be integrated into the new developed method. As a key issue, how to appropriately introduce GSs to the oil/water interface will be decisive to the final quality and performance of the products. One initial idea is GSs dispersed in certain solvent being directly injected into the system and simultaneously entrapped at the interface. Hence, the selected solvent needs to play two roles at the same time, namely, (1) good dispersant for GSs or functionalized ones; (2) help with the interfacial accumulation of GSs. The relationship between the proposed strategy and its parental systems is illustrated in Fig. 1a. To the best of our knowledge, there is no report on this kind of method thus far, and none of the well-known dispersants for graphene, such as N-methylpyrrolidone (NMP), N,N′-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), can be able to fulfill the expected multi-functions.
More recently, it is reported that inducing agents, specifically ethanol and methanol, can effectively accelerate the collection of GO at either oil/water or air/water interface.23–27 Moreover, according to our latest research, sulphonated graphene could show satisfactory dispersiblity in ethanol due to their modified surface polarity.27,28 Therefore, considering these two facts may offer us a possible solution to the above question. As a proof-of-concept, here we demonstrate a novel self-assembly of graphene film by direct introducing GS/ethanol dispersion at the hexane/water interface (Fig. 1b), and their potential application as transparent conducting electrode and thin film supercapacitor.
Optical microscopy image in Fig. 3a confirmed the integrity and smoothness of the graphene films collected on quartz substrate, and it is easy to distinguish the film from the substrate (Fig. 3b). Further zooming into the film, a densely packed structure with occasional vacancy was observed and the total coverage of nanosheets on the substrate reaches up to 96.2% (Fig. 3c). Based on the AFM image and corresponding height profile (Fig. 3f and g), the thickness of the film is 1.2–1.5 nm, consistent with that of single-layered sulphonated graphene.11,28 In some area, irregularities, such as folding and wrinkles, could be noticed, which count up to ∼7.8% of total coverage of the monolayer film. To understand the formation of the film, it is necessary to discuss the surface chemistry of the sulphonated GSs, which could be considered as amphiphilic in some sense with most part of the sulphonic acid groups and residual carboxyl groups gathered around the edges, and hydrophobic graphitic basal region.11 According to Pieranski's theory and Young's equation,30–32 the driving force for the self-assembly of nanoparticles at oil/water interface could be attributed to the minimization of total interfacial free energy, in which the particle size and shape are both important for the energy decrease. Particularly, a particle with high aspect ratio will show superior stability to the spherical one at the oil/water interface.33 Bowden et al. have pointed out that menisci-like structures would form around nanoparticles, and attractive lateral capillary force derived from the overlap of such menisci could draw the adjacent blocks closer to maximize their hydrophobic surface area.34 This kind of lateral capillary force is especially significant for particles with high two-dimensional aspect ratio and is sufficient to drive them closely packed. On the other hand, when the nanosheets were forced together in an edge-to-edge manner, the electrostatic repulsion may result in folding, overlapping and wrinkling at the contacted edges and sometimes in the interior regions.24 The second possible reason for these irregularities could be assigned to the highly flexible nature of graphene which will tend to shrink and crumple during the drying process.
As a key advantage of interfacial assembly, the ability to fabricate multilayer film with a controllable layer number is highly desired for many applications. Because of the hydrophobic nature and poor wettability of graphene, binding agents (usually polymers) were typically necessary in their layer-by-layer process. However, these external impurities may be detrimental to general performance of the resulted films. In our previous research, it has been testified that hydrophilicity of graphene can be remarkably enhanced by controlled sulphonation.27,28 The sulphonated GSs show much better wetting behavior and a lower contact angle with water, making their direct multilayered deposition feasible. As shown in Fig. 3a and d, the second monolayer with coverage of ∼81.7% was successfully deposited over the first one. Moreover, the film surface became rougher after the second deposition, and coverage of the irregular area increased to 16.8% (as shown in Fig. 3e). Similar results have been reported by Huang et al. in their research on multilayer assembly of GO.23–25 Accordingly, when two layers of nanosheets were overlapped in a face-to-face manner, their electrostatic repulsion will result in heavy degree of defects, especially at high packing density, despite the dominant attraction between them. Moreover, due to the soft nature of graphene, roughness in under layer will reflect on and multiply in the upper layer. Experimental parameters, such as the amount of GS, injecting speed of ethanol and the uplifting rate of substrate, are all influential in the structure of final products. Generally, over dosage of the GSs will result in wave-like inhomogeneity and aggregates (as shown in Fig. 4a), and too fast lifting the substrate will bring rupture (as shown in Fig. 4b) into the film.
Optoelectronic properties of as-prepared films were discussed to seek their possible application as transparent conducing electrode. As shown in Fig. 5a, the monolayer film showed low absorption over the whole region of visible spectrum and the transmittance at 550 nm reached up to 92.3%, slightly lower than the theoretic value of single-layer graphene (97%). This decrease in transmittance can be mainly assigned to the scattering and absorption of incident light at the defective areas. The characteristic absorption at around 270 nm correspond to π–π* transitions, reflecting the recovery of sp2 bonds. Sheet resistivity of the film is 13.5 kΩ/□ which is much higher than the ITO standard. After the second deposition, the sheet resistivity reduced to 9.6 kΩ/□ with a transmittance of 85.3% (Fig. 5b), which is consistent with the calculated value (86%) when considering the coverage of the second layer. There are two possible reasons for the high sheet resistivity of as-prepared films: (1) the GSs studied in this work is hetero-dispersed with a wide size distribution from hundreds of nanometres to tens of micrometers, and the existence of GSs with small size will undoubtedly result in a large number of sheet-to-sheet junctions and increase the overall resistivity; (2) the introduction of functional groups, specifically sulphonic groups in current case, also bring negative effect on the conductivity of the film. To further improve the optoelectronic properties of current films, additional pre- or post-treatments, such as size fractionation of the GSs, control over the sulphonation degree and chemical doping of resulted films, should be preferable.
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Fig. 5 Transmittance of (a) single- and (b) double-layered graphene films and their corresponding sheet resistivities. |
Other than optoelectronic devices, the graphene-based thin films also show high potential in energy storage systems such as supercapacitors and secondary batteries. In current research, multilayered film with a layer number of 20 (1 × 2 cm2) was fabricated in electrochemical tests. As shown in Fig. 6a, the cyclic voltammetry (CV) results displayed a series of symmetrical curves at different scan rates, indicating that the capacitive process was both reversible and stable. Their nearly rectangular shapes indicate that efficient electrochemical double-layer (EDL) capacitance behavior has been established in the electrode.35,36 Calculated specific capacitance reached up to 292 F g−1 at a scan rate of 2 mV s−1, nearly twice the value of its powder counterpart.28 With the scan rate being increased up to 20 mV s−1, the specific capacitances gradually reduced to 254 F g−1, demonstrating a good rate capability (∼87%) (Figure 6b). Galvanostatic cycling of the electrode was performed with a current density of 100 mA g−1. As shown in Fig. 6c, the charge/discharge curves are almost straight lines, confirming the formation of an efficient EDL and good charge propagation. The calculated specific capacitance (287 F g−1) is consistent with that from CV test. A decrease of only 1.9% on the specific capacitance happened after 50 cycles (Fig. 6d), exhibiting excellent cycle stability and a very high degree of reversibility. To understand the high electrocapacitive performance of the current multilayered film, it is necessary to understand its unique micro-geometry. Different from the randomly oriented stacking as in the powder sample with the wide pore distribution, GSs in current thin film electrode are horizontally arranged with very low degree of aggregates, maximally reserving their surface area. Moreover, the increasing in-plane irregularities, although undesirable for transparent conducting film, could effectively expand the interlayer spacing as pillars and build up interconnected channels for the access of electrolytes. In other words, a self-sustained, three-dimensional hierarchical structure could be established within current ultrathin film. Detailed research on the formation and development of this interior structure with the increase of layer number is ongoing and will be reported later.
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