Highly Ordered and Multiple-Responsive Graphene Oxide

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Highly Ordered And Multiple Responsive Graphene Oxide / Azoimidazolium Surfactant Intercalation Hybrids: A Versatile Control Platform Changxu Lin, Mengchun Xu, Wei Zhang, Long Yang, Zheng Xiang, and Xiang-Yang Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00061 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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Highly Ordered And Multiple Responsive Graphene Oxide / Azoimidazolium Surfactant Intercalation Hybrids: A Versatile Control Platform Changxu Lin,a* Mengchun Xu,a Wei Zhang,b Long Yang,a Zheng Xiang,a and Xiang-yang Liua*

a

Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft

Functional Materials Research, College of Physical Science and Technology, Xiamen University, 361005 Xiamen, China. b

C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV 26505

USA

KEYWORDS: Ionic liquids, azobenzenes, graphene, responsive materials

Abstract: To produce graphene materials with better controllability, a new graphene oxide (GO) intercalation hybrid is fabricated with the incorporation and the functionization with the azoimidazolium surfactant (AzoIm+). The hybrid exhibits a highly uniform lamellar structure in which few-layer GO stacks with the AzoIm+ alternatively. Simultaneous control of the mesoscopic structures, aggregation properties and electrochemical behaviour of the hybrid are achieved by inheriting from the photo-, thermal and mechanical responsiveness of the azoimidazolium. UV treatment produces a well-dispersed GO/AzoIm+ suspension aggregate and precipitate, while the

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specific capacitance of the final hybrid decreases. The lamellar stacking becomes anisotropic by uniaxial stretching on a soft polymer. With a liquid crystal unit inserted between the layers, the d spacing of the lamella passes through transformation, disordering and finally recovery stages, associated with the increasing and decreasing temperature. The explosive release of heat generated by the thermal reduction of GO is reduced in the GAI hybrid. The release of heat is tunable by varying the relative amount and UV treatment of AzoIm+. The physical properties of the hybrid allow the controlled preparation of ultra-small Au nanodots between the GO layers. This represents a major step towards multiple responsive integrated graphene applications.

1 Introduction Since the discovery of single-layer graphene, a new branch of materials science devoted to two-dimensional (2D) materials has opened up. Pristine graphene, GO and reduced graphene oxide (RGO), which share the structural characteristics of an sp2-bonded planar carbon network, compose the class of graphenes. Their unique physicochemical properties offer great potential and even real state-of-the-art applications in microelectronics,1-2 energy,3-5 catalysis,6-7 optics8-10, etc. In most of these applications, it is not only the composition of the species interacting with graphene but also the organisational form of the graphene basal planes that is important.11 Hierarchical structures were made by combining graphene and other functional species such as conductive polymers12-13 or other low dimension carbon materials.14-15 Porous graphene was fabricated to construct electrodes for supercapacitors.16 Layer-by-layer assembly of graphene and SnO2 nanoparticles produces electrode materials with high specific energy density that resists degradation after multiple charge and discharge cycles.17 On a larger scale, patterns of graphene were built by in situ growth of graphene


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via chemical vapour deposition18 or plasma etching under a patterned mask.19 The construction of such structures requires highly fine control. Meanwhile, the energetic nature of GO itself is another reason why good controllability is essential. The exothermic process of GO reduction represents a potential energy source for the synthesis of nanoparticle-embedded composites and micro-patterned electronic devices based on RGO. However, the utilisation of the energy is restricted by the self-propagating and even explosive nature of GO reduction.20 Efforts have been made to give this process more controllability by adjusting the pH during reduction21 and introducing oxo-functionalised graphene instead of GO.22 The diversity of application scenarios will necessitate the development of a more convenient fabrication process, with more choice and finer control of energy. To design and construct responsive or “smart” applications of graphene, various forces and interactions have been introduced.11, 23 A photo-responsive monolayer unit was anchored to graphene to form photo-switchable transparent electrodes.24 Numerous other applications have been based on azobenzene units, e.g. graphene structural modulators,25-28 energy storage devices29 and optoelectronic devices.30-32 For example, the perpendicular layer distance, a key parameter in the field effect transistors, was dynamically adjusted via azobenzene surfactants intercalated between the GO basal planes.25 In the direction along the planes, the graphene geometry and graphene-based heterostructure were patterned by uniaxial stretching. As a result, the charge carrier dynamics of graphene was tuned to achieve a programmable extreme pseudomagnetic field.33

The application of

graphene has been extended to many other responsive systems based on the thermal, pH, electric and humidity stimuli.34 All of these instances of control were isolated and far from providing comprehensive controllability. Thus, an integrated means for controlling graphene materials is


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urgently needed. Rather than piling up responsive units, a platform offering multiple control via a single responsive unit would be preferable.

Figure 1. Illustrative scheme of formation and structure of GAIs.

Imidazolium is a good candidate for the above objective due to its good compatibility with graphene in building either functionally orientated structures or complete systems. The imidazolium surfactant was used to exfoliate flakes of graphene from graphite35-36 or to stabilise graphene.37-38 The anion- and solvent-responsive phase transfer of graphene oxide was achieved with the imidazolium-containing copolymers.39 Additional affinity of imidazolium for graphene (or GO) was also manifested in cation–anion Coulombic interactions and π-π interactions.40 In addition, the anion exchange chemistry of imidazolium facilitated the expansion of the components and functionality of


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ionic liquid/graphene composites.41-42 Imidazolium is also an efficient carrier for integrating moieties paired to external stimuli and other functional groups. Recently, we have synthesised a new class of surfactants with azoimidazolium as the cation. Azoimidazolium consists of an imidazolium and a phenyl connected by an azo bond, making it an analogue of azobenzene. It exhibits similar photo-responsive, thermal responsive and liquid crystal anisotropic properties to those of azobenzene.43 In this work, azoimidazolium surfactants were incorporated into GO to form GO/AzoIm+ intercalation hybrids (GAIs). (Figure.1) By powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterisation, we found that the GAIs had a highly uniform mesoscopic structure, with almost perfectly one-to-one alternate stacking of the two components, resulting from Coulombic attraction and π-π interactions between basal planes and azoimidazolium units. The orderliness had reached the level of the Graphite Intercalation Compounds (GICs).44 These two classes shared some interlayer molecule species of quaternary ammoniums or surfactants in some examples.45-48 However, unlike the top down mechanism of the GICs formation, the main idea in forming GAIs was the bottom up self-assembly. Only based on the uniform structure of the GAI and the bottom up formation mechanism, good controllability of various properties of the graphene hybrids could be achieved by effectively inheriting the multiple responsive nature of azoimidazolium. Light, temperature and stretched substrate was examined as the stimulus source. Their responsiveness to stimuli could be adjusted by varying the relative content of AzoIm+ and switching the UV treatment on / off during preparation. The adjustable nature of the GAI structure was conveyed directly to energy conversion applications, including controllable energy release during the thermal reduction of GO and the manipulation of electrochemical


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capacitance using light. A new platform for control, which encompasses multiple sources, multiple tasks and a variable scale, was advanced based on the combined results. The study of the GAI also opens a new frontier of graphene application. The multiple responsive property makes it easier to build smart graphene materials. Based on the exchange chemistry of the imidazolium and the control energy release of GO reduction, the GAI is a suitable choice for fabricating smart graphene / functional species composite in situ. Furthermore, the responsiveness on the soft substrate can also serve in the soft electronics.

2 Experimental Section 2.1 Preparation of GAI hybrids All reagents were purchased from commercial supplier and used without further purification. GO was produced using a modified Hummers’ method from natural graphite. The surfactant AzoIm+ ([C14PhAzoImEt]Br) was synthesized according to our previous work.43 The intercalated hybrid was prepared as follows. The freeze dried GO solid was dispersed in mixed ethanol / n-hexane (1:1, w/w) solvent in 1-hour sonication. The AzoIm+ was dissolved in mixed ethanol / n-hexane (1:1,w/w). The GO suspension and AzoIm+ solution was mixed to get the precursor suspension with agitation. The GAI was fabricated by the drop-casting of the GO and AzoIm+ precursor suspension on designated substrates and finally dried in air in room temperature. For GAI-Rtotal, the GO/AzoIm+ mass ratio

Rtotal was 0.69, 3.43 and 6.86, and the overall concentration of hybrids was fixed to 1 mg/ml. For comparison, we prepared an equivalent GO/AzoIm+ hybrid, denoted (GO/AzoIm+)eq, in which the excess surfactant was removed by repeated centrifugation and ultrasonic dispersion. For GAI-Rtotal-UV, the AzoIm+ solution was exposed to 365 nm radiation of UV lamp (Spectronics Co.,


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SB-100PIFA, 5.57 mW·cm-2 at 20cm) for 45 min after mixed with GO dispersion. For Rtotal -S, the mixed dispersion was drop-casted on stretched PDMS substrate (about 20% strain) and dried in air with PDMS release. For GAI- Rtotal-UV-S, the mixed dispersion was UV-treated the same way as GAI- Rtotal -UV before drop-casted on stretched PDMS substrate as GAI- Rtotal-S. For rGAI-3.43-Au (reduced GAI), 140µL HAuCl4 solution (4 wt%) was added into 62g the GO/AzoIm+ precursor suspension for GAI-3.43 (which contained 14mg GO and 48mg AzoIm+ in ethanol:n-hexane solvent). The sample was reduced in the tube furnace under N2 atmosphere, heated to 250°C at a speed of 5°C·min-1 and finally yielded rGAI-3.43-Au sample. The very same procedure was applied for GO/Au composite as the control only without AzoIm+.

2.2 Sample characterization: 1

H NMR spectra were collected using a Bruker 400 MHz spectrometer (Bruker, German). The

Fourier transform infrared spectrum (FT-IR) were recorded using a Nicolet IN10 spectrometer (Thermo Fisher Scientific, US) in the form of KBr tablets. The zeta potential and particle size in GO/AzoIm+ suspension were measured using a Mastersizer 2000 instrument (Malvern Instruments Ltd, UK). Photochemical behaviour of the hybrids was characterized by UV-Vis absorption spectroscopy (Lambda750, PerkinElmer, US). For UV-Vis, zeta potential, and the light diffraction characterization under consecutive light treatment, the precursor suspension was diluted to 0.05 mg/ml (denoted as AzoIm+/GO(d)). The structures of the hybrids were examined by SEM (SU-70, Hitachi Ltd, JP) on the glass substrate and atomic force microscopy (AFM) (DI Multimode V, Bruker, German) in the fast scanning mode on the fresh mica. The order mesostructure of the hybrids was measured by XRD (AXS D8 Bruker, German) using Cu Kα radiation at a scan rate of 5°/min.


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The variable-temperature XRD test was performed on the basis of 12 min of thermal equilibrium time with hot plate. The Raman spectra were obtained using a Raman spectrometer (LabRam-010, JY Ltd, FR). The differential scanning calorimetry (DSC) diagram was performed was performed using a Toledo 1/400 system, 10°C/min heating and cooling in the N2 atmosphere. TEM images were taken using a JEOL 2100 system, and sample was made by microwave assisted dispersion of corresponding GAIs in DI water. X-ray photoelectron spectroscopy (XPS) was conducted on Thermo SCIENTIFIC ESCALAB 250Xi with A1K alpha source, 15kV tube voltage and 10 mA current.

2.3 Electrochemical measurements: Cyclic voltammetry (CV) was performed on a CHI660E (Shanghai Hua Chen, China) workstation using a standard three-electrode cell setup. A glassy carbon electrode (GCE), platinum wire and saturated calomel electrode (SCE) were used as the working electrode, the counter electrode and the reference electrode, respectively. A 1 M PBS (pH = 7) aqueous solution was used as the electrolyte. The GCE was first polished with aluminium powder carefully, rinsed with deionized water, and ultrasonicated in water and ethanol for 5 minutes. The electrode was then dried under nitrogen. The potential scan rate of CV is 20 mV/s, 30 mV/s, 40 mV/s, and 50 mV/s. For sample tests, 50 µL of the sample was dropped on the surface of the GCE and allowed to dry in air at ambient temperature. The mass of the sample can be calculated quantitatively.

3 Results and discussion 3.1 Formation of highly ordered GAI hybrids GO was prepared by oxidation using Hummer’s method,49 dialysis in H2O for 2 days, and lyophilisation thereafter. GAI hybrids were prepared by dissolving specific amounts of AzoIm+ and


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re-dispersing GO in EtOH/n-hexane (1:1, v/v), into which the AzoIm+ cations and GO were successfully incorporated via Coulombic and π-π interactions. Three samples of GAI-Rtotal (where

Rtotal is the mass ratio of AzoIm+ surfactants / GO, Rtotal = 0.69, 3.43, 6.86) were fabricated to evaluate the effect of increasing AzoIm+ content on the formation of GAI hybrids, as monitored by FT-IR, XRD and SEM characterisation. The chemical composition of the GAI hybrids was verified by the FT-IR spectra, as the absorbance peaks for both GO and AzoIm+ were identified (Figure 2b). The spectrum for AzoIm+ contained bands at 3033 cm−1 (C-H stretching in benzene and the imidazolium ring), 2920 cm−1 (C-H stretching in the tetradecanyl chain), 1463 cm−1 (-C-H bending in the benzene ring), 1385 cm−1 (stretching of -N=N-) and 1149 cm−1 (stretching and asymmetric stretching of the C-N in the imidazolium ring). The spectrum for GO contained vibrational bands at some, but not all, of the positions in the Azolm+ spectrum. In the hybrids, more peaks appeared with an increasing amount of the surfactant. To understand the interactions between the azoimidazolium cations and GO, XPS was performed and discussed. Detailed information on the chemical environment was revealed by fitting the C1s and N1s peaks (Figure S1). The C1s peak was fitted to six groups: C-C sp2 (284.5 eV), C-O in epoxy (285.1), C-C sp3 (285.9 eV), C-N (286.6 eV), C=O (287.2 eV) and -O-C=O (289.3 eV)50. The N1s peak was fitted to four groups: N=N (400.2 eV), C- N, C-N+ in imidazolium (401.4 eV) and N+-O− (404.0 eV). The binding energies of the N1s-fitted groups were all shifted positively by about 3 eV compared with the imidazolium-GO composite, and approached those of N in N-doped graphene.50 This implies the building of a massive π-π interaction network with the excess surfactant molecules in a close and orderly arrangement (Figure S1). The amount of AzoIm+ directly stacked on the GO was determined by XPS spectra (Figure 2c-d).

In the case of (GO/AzoIm+)eq, with no excess


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surfactant, the abundance of N atoms was found to be 3.11%, similar to the ratio of quaternary-N to GO hybrids, which is about 1 surfactant molecule to 75 carbon atoms of GO.25

The results were

calculated and presented as the mass ratio of stacked AzoIm+ / GO calculated as Rstack: Rstack (GAI-0.69) = 0.55, Rstack (GAI-3.43) = 2.10, Rstack (GAI-6.86) = 3.01. The amount of stacked AzoIm+ increased with the increasing Rtotal, while the percentage decreased.


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Figure 2. Formation of GAIs. (a) zeta-potential vs. Rtotal; (b) FT-IR spectra of GAIs and their building blocks (AzoIm+ and GO); (c-d) XPS survey scan of (c) GAI-Rtotal (d) equivalent GO/AzoIm+ composite and GO. .

Figure 3. Formation of GAIs. (a) XRD pattern of GAI-Rtotal (Rtotal = 0.69, 3.43, 6.86); (b) XRD pattern of building blocks and the equivalent GO/AzoIm+ composite. SEM images of (c) GAI-0.69 (d) GAI-3.43 (e) GAI-6.86.

Compared with other surfactant/GO hybrids, a much more ordered structure appeared in our GAI hybrids. The gradual formation of this structure, as shown by XRD, demonstrates the expected


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orderliness (Figure 3a). First, for GAI-0.69, the characteristic broad peak of stacked GO layers around 11.0° disappeared. The ordered stacking of the GO layers was weakened as the azoimidazolium surfactant began to insert into the interlayer space. When the mass ratio of surfactant was increased by five times to form GAI-3.43, a set of peaks containing one primary and two secondary peaks (collectively labelled Peak A) appeared, which was attributable to the (001) reflection of a well-resolved lamellar structure. From the primary peak at 2θ = 3.98°, the d spacing of the lamella was calculated as 2.47 nm. A weaker and broader peak (Peak B), representing a less ordered structure, appeared close to the (001) peak position of the pure AzoIm+ surfactant. For GAI-6.86, which had double the AzoIm+ content, the ordering of the lamella was enhanced and their d spacing was slightly reduced. The broad Peak B shifted towards Peak A and partly merged into it. The changing morphology was also observed in the SEM images, showing the evolution from sheets to sheet-warping crystals as more surfactant molecules were incorporated (Figure 3c-e). In the XRD patterns of the GAIs, the absence of peaks that had appeared for the pure GO or surfactant confirmed that the GAI structure was a new hybrid formed by the intercalation of the surfactant into GO. (Figure 3b) If the GO was considered as the flat and 2D extending part of the hybrids resemble floors in a building, the AzoIm+ surfactant molecules are equivalent to columns between storeys, supporting the GO floors. In GAI-0.69, the small quantity of surfactant molecules was just enough to neutralise the negatively charged GO (Figure 3a), forming a tent-like structure that disrupted the close stacking of GO itself, but insufficient to form new ordered lamella. In GAI-3.43, the additional surfactant molecules acted to physically support the GO sheets as well as surpass the charge saturation point.


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The position of Peak B was attributable to the weaker crystalline structure of the AzoIm+ surfactant in the hybrid than in the pure compound. Moreover, Peak B shifted towards Peak A when the amount of surfactant in the hybrids increased from GAI-3.43 to GAI-6.86, evidence of closer interactions between surfactant and GO. The additional π-π interactions between surfactant cations and GO acted to compress the spacing between GO layers by making the supporting surfactant more compact and ordered. It was also an uncommon example for the defective GO to reach the GIC-level orderliness. The (GO/AzoIm+)eq structure, as revealed by the XRD pattern, was similar to that of GO assemblies in the previous work of Xu et al.,

25

which contained regions of both mutually repelling

and re-aggregated GO (Figure 3b). Compared to this report, a new re-assembled GO hybrid with an unprecedentedly high degree of ordering was constructed, and the ordering was further enhanced upon doubling the surfactant ratio. The organization of GAI on a larger scale was revealed by taking both Rtotal and Rstack into consideration. The key fabrication step of GAIs was drop-casting which led to the preservation of almost all AzoIm+ from the precursor suspension. Thus, the Rtotal was still the overall ratio of AzoIm+ in the GAIs. We proposed a dual-domain model for the explanation of the difference between Rtotal and Rstack. An ordered domain was composed of alternatively stacking of GO and AzoIm+, which was related to the Peak A in XRD. A disordered domain was formed by the GO and excess AzoIm+ which roughly equaled to Rtotal - Rstack. The free GO platelets disrupted the crystallization of AzoIm+, which led to the Peak B in XRD close to the original 2θ position of AzoIm+. The GAI hybrids was composed by these two types of domains with a probable random distribution.


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Figure 4. (a-c) UV-Vis spectra of suspensions of the AzoIm+/GO(d) with different Rtotal under continual UV exposure (a) Rtotal = 6.86, (b) Rtotal = 3.43 (c) Rtotal = 0.69. (d-e) AFM images and measurements of Rtotal = 6.86 (d) before

and (e) after UV exposure. (f) optical images before and

after UV exposure. (g-h) SEM images of AzoIm+/GO(d) (g) without / (h) with UV treatment after drop-casting and air drying. The dashed line in (g) marked the bounder of flat and wrinkle GO.


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3.2 Photo-responsive assembly behaviour Azoimidazolium, like other azo compounds, has been shown to be photo-responsive, more strongly so in non-polar solutions than in aqueous solution.43 Precursor suspensions of its hybrids also inherit this responsiveness. To fit the UV-Vis characterization range and to check the status of the isolated GO platelet, the concentration was diluted to 0.05 mg/ml (1/50 concentration of precursor suspension for GAIs). The diluted suspension used in this section was denoted as AzoIm+/GO(d). After 30 min of exposure to UV light, the time-dependent UV-Vis spectra of AzoIm+/GO(d) with different AzoIm+/GO ratios exhibited a decrease in the absorption intensity at 358 nm over time, caused by the gradual conformational change in the azoimidazolium (Figure 4a-c). With increasing ratios of surfactants in the suspensions, the percentage change and the speed of this reduction in intensity both decreased. Meanwhile, the baseline intensity, representing the turbidity of the solution, increased with surfactant ratio. This is evidence of heterogeneity, even at the dilute concentration of 0.05 mg/ml necessary for the UV-Vis experiment. Another sign of aggregation in the spectra under continual UV exposure was the emergence of scattered fluctuations in the precursor suspensions with AzoIm+/GO ratios of 3.43 and 0.69. The higher the content of AzoIm+ in suspension, the higher the baseline rose, and the smoother the spectral data. All of these phenomena were attributable to the photo-induced aggregation of the AzoIm+/GO mixture on the mesoscopic scale via the E-Z conformational change in the AzoIm+ surfactant. To further verify this assumption, we scanned the dilute precursor before and after UV exposure by AFM (Figure 4d & e). The presence of E-rich surfactants caused the GO to shrink and the thickness of the hybrid to rapidly increase to several dozen nanometres from less than 1 nm – the typical thickness of a few layers of GO. The morphological change and the aggregation were so intense as to


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be visible with the naked eye (Figure 4f). The particle sizes of the AzoIm+/GO(d) determined by the laser diffractometer, under optional 45min UV exposure, were all listed in Table 1. In the series of samples without UV exposure, the pure GO in the suspension was ~ 15 µm. With small amount of AzoIm+ stacked on the GO (Rtotal = 0.69), the Z-average size of AzoIm+/GO(d) dramatically increased to 57.9 µm. However, when more AzoIm+ involved, the size of AzoIm+/GO(d) decreased to 4.83 (Rtotal = 3.43) and 4.75 µm (Rtotal = 3.43). More excess AzoIm+ resulted in a slight decrease of size. After UV exposure only the AzoIm+/GO(d) with Rtotal = 0.69 showed exceptional increase in size, unlike the other three. A SEM observation was performed on this sample. A major shrink and aggregation of GO was observed in the sample of Rtotal = 0.69 after UV exposure. A slight shrink was also observed on the GO surface without UV exposure. Based on these results, the photo-induced aggregation of GO was the joint effect of the neutralization and photoisomerization by the AzoIm+. Via their azoimidazolium heads, the surfactant molecules interacted with the GO sheets, both Coulombically and by π–π interaction. Due to the greater quantity of AzoIm+ than in previously reported hybrids (as determined from the XPS data),25 the two aromatic rings of the azoimidazolium heads were coplanar with the GO planes, benefitting both kinds of interaction. However, once the azoimidazolium heads converted to the E-isomer, the interactions became much weaker due to the disruption of planarity by the resulting structural distortion. Thus, with the bending of the N=N bond and reorientation of the two rings, it was impossible for them to maintain their original position. The GO, being less strongly stabilised by the surfactants, eventually became aggregated. The full association with AzoIm+ could cause the shrink of GO but also the inhibition of aggregation from charge repellence. Only the partial-association with AzoIm+ could reach the critical


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point for the Z – E photo isomerization to decrease the zeta potential by about 9 mV. The fluctuation of the spectral data in the surfactant-less dispersion was also attributable to the above mechanism. Table 1. Z-average particle size (µm) of AzoIm+ binded GO at different Rtotal and UV treatment on / off in the diluted precursor suspension pure GO (Rtotal = 0)

Rtotal = 0.69

Rtotal = 3.43

Rtotal = 6.86 b

No UV

15.4

57.9

4.83

4.75

UV treated

11.1

61.0

4.37

3.93

Figure 5. (a-c) XRD pattern, (d-f) Raman spectra of (a & d) GAI-6.86 vs. GAI-6.86-UV (b & e) GAI-3.43 vs. GAI-3.43-UV (c & f) GAI-0.69 vs. GAI-0.69-UV; (g-i) SEM images of (g) GAI-6.86-UV (h) GAI-3.43-UV (i) GAI-0.69-UV.


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During the final preparation of the GAI hybrids, UV irradiation also caused changes in both mesoscopic and morphological aspects of the products. The sensitivity of the lamellar structure factor to UV exposure was directly linked to the amount of surfactant in the GAI hybrids (Figure 5a-c). Both sets of Peaks, A and B, in the GAI-3.43 sample were intensified after UV exposure. In the case of GAI-6.86, UV irradiation promoted the intensity of Peak A but suppressed that of Peak B. The greater the excess of surfactant, the more sensitively the GAIs reacted to irradiation, due to the ordering effect induced by the conformational change. The GAI-0.69 sample, which showed no ordering before irradiation, did not undergo improved ordering under UV exposure and still showed no XRD peak. In the SEM pictures, more isolated crystal-like ‘knolls’ appeared in the GAI-6.86 and GAI-3.43 samples on the flat GO ‘plain’. These were presumed to be aggregates of the excess surfactants, and this presumption was supported by the correlation between the quantity of ‘knolls’ and the surfactant ratio (Figure 5g-h). For GAI-0.69, the light-induced morphological evolution of a more wrinkled surface was observed, which resembled that of the low-concentration sample used for the UV-Vis tests (Figure 5i). Raman spectroscopy is an important tool for investigating the electronic structure of GO, and in our case it can be used to detect changes in the number of layers induced by exposure to light. The Raman spectra of the three GAI-Rtotal and the corresponding UV-treated samples are shown in Figure 5d-f. All six display the characteristic Raman peaks of GO at 1353 – 1357 cm−1 (D band), 1587 – 1602 cm−1 (G band) and 2674 – 2689 cm−1 (2D band). In general, the position of the G and 2D bands is affected by the number of GO layers and the internal stress between them.51 A red-shifted G band was shown in the GAI-6.86 due to the more GO layer number resulted from the increased excess surfactant amount. More excess AzoIm+ resulted in an increased blue-shift level in 2D band from GAI-0.69 to GAI-6.86, which was caused by the reduced internal


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stress from the excess surfactant. The intensity ratio of the D and G bands (ID/IG) is related to the structural defects of graphene. The ID/IG of GAI and GAI-UV was marked in Figure 5(g-i). The ID/IG ratio of GAI-0.69 was lower than the other two. More defects were brought in with more excess AzoIm+ involved. The UV treatment to GO and AzoIm+ mixture resulted in decrease of ID/IG ratio by about 0.1 for each case. The twisted cis conformation of the surfactant slightly weakened the interaction between the GO and AzoIm.

3.3 Thermal responsive behaviour of mesoscopic structure The azoimidazolium unit is analogous to azobenzene in its liquid crystal properties, such as thermal and mechanical responsiveness. The XRD patterns of the GAI changed throughout a cycle of heating followed by standing after cooling to room temperature. The variable-temperature XRD characterisation of four orthogonal samples selected for high/low amounts of excess surfactant and with UV treatment on/off was conducted to investigate the dynamic effect of azoimidazolium on the hybrid assembly. To support the analysis of the heat-related behaviour, the DSC was performed to obtain structural change vs. temperature profiles for each sample. GAI-6.86 and GAI-6.86-UV will be discussed first (Figure 6a & b). Generally, as the samples underwent the sequential operations of gradual heating followed by standing at room temperature for a set period, their XRD patterns displayed a series of changes corresponding to transformation, disordering and recovery phases. The original pattern was transformed, as Peak A disappeared in the early stages of heating, while a new lamellar structure appeared with a d spacing of 1.43 nm (2θ = 6.18) and a (002) secondary diffraction, and was denoted as Peak C. Meanwhile, the neighbouring Peak B was further exposed by the disappearance of Peak A. However, in GAI-6.86-UV, Peak A was able to survive at 60°C. A


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common tendency for the orderliness to be weakened during the heating process was shared by both samples. Nevertheless, an exception was observed at 140°C, when both samples suffered a temporary additional loss of intensity at Peak B and Peak C, possibly caused by a change in the surfactant aggregation status. At 200°C, both sets of structures had been almost completely disrupted, leaving only a small residue of Peak B at 2θ around 3.4 – 3.5°. After standing for 48 h at room temperature, a pattern of Peak A returned, which was almost identical to that present before heating, indicating that the structure had achieved recovery. The three phases of the temperature-induced changes of GAI can be summarised by three keywords: transformation from Peak A to Peak C, disordering of both Peak B and Peak C, and recovery of Peak A and Peak C after cooling and standing.


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Figure 6. XRD patterns of temperature sequence. (a) GAI-6.86; (b) GAI-6.86-UV; (c) GAI-3.43; (d) GAI-3.43-UV; (e) mechanism of transformation to disorder to recovery.

The Peak C was related to the ordered structure transformed from the structure corresponded to Peak A. The AzoIm+ intercalated between lamellar structure of Peak C had smaller angle with the GO basal plane than that of Peak A. The more compact structure behind Peak C was caused by －

higher mobility of the Br anion in higher temperature which offered space for enhance interaction between AzoIm+. After cooling down to room temperature, anions moved back to the most probable position and the major peak recovered to Peak A. The transformation phases and the disordering phases were also evident for GAI-3.43 and GAI-3.43-UV, with only a few differences caused by the lower quantity of excess surfactants (Figure 6c & d). Peak C was more sensitive to temperature in both GAI-3.43 and GAI-3.43-UV and became very weak in intensity at 120°C and above. Some of GO diffused into less ordered domain and participated in Peak B. Peak A was also less intense than in the GAI-6.86 series. Recovery behaviour was not observed in the GAI-3.43 series at all. A clearer insight into the dynamic relationship between surfactant and GO emerged from the comprehensive comparison of the four samples. With more excess surfactant in the GAI hybrids, the newly emerged Peak C was more resistant to thermal treatment, and the original Peak A was more likely to recover after the heat source was withdrawn. Following UV treatment of the hybrids, Peak C was less sensitive to disordering and Peak A was more intense at higher temperatures, these changes being attributable to the energy associated with conformational change. Control over the properties of the GAI, with either coarse or fine adjustment, was made possible by varying the surfactant amount and switching the UV treatment on / off, respectively, due to the resulting structural variation.


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Figure 7. DSC data of (a) GAI-6.86 vs. GAI-6.86-UV (b) GAI-3.43 vs. GAI-3.43-UV (c) GAI-0.69 vs. GAI-0.69-UV. (d) AzoIm+ and GO.

3.4 Controlled energy output in thermal reduction As a reference for all samples, the DSC curve of pure AzoIm+ surfactant had three endothermic peaks, corresponding to phase transition, conformational change and melting, respectively, at 51.6°C, 70.8°C and 108.1°C, plus one exothermic peak at 198.9°C linked to the thermal decomposition of the surfactant itself (Figure 7d). The DSC curves of the GAI samples could be divided into two groups at the end of the surfactant melting process: three negative endothermic peaks and two positive exothermic peaks, which each had unique thermal behaviour (Figure 7a-c). With more GO present in the hybrids, the three endothermic peaks all moved towards lower temperature and diminished. Graphene is well known for its heat-conducting properties.52 In GAIs of lower Rtotal, the presence of more heat-conducting GO in the environment of the AzoIm+ surfactant


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molecules caused them to exhibit their native thermal behavior at a lower apparent temperature. In the variable-temperature XRD, the transition from Peak A to Peak C happened before 60°C, which corresponded to the first two endothermic peaks. The weakening and disappearance of Peak C in XRD between 90°C and 120°C was caused by the melting process (melting point at 108.1°C) , which also occurred within this range. In general, the peaks of UV-irradiated (GAI- Rtotal-UV) samples shifted to higher temperature. The trans to cis conformational change in the AzoIm+ surfactant blocked interlayer heat flow. The endothermic peak at 53.2°C of GAI-6.86-UV indicated a reduced enthalpy change compared with GAI-6.86, which was attributable to the absorption of energy stored in the UV irradiation. The exothermic group was identified as the peak of GO reduction at 178– 188°C and peak of AzoIm+ decomposition at 208–231°C. This identification was made on the basis of the change in the enthalpy (peak area) of each peak with decreasing R. In the higher-temperature range, the molten and more mobile ionic liquid ([AzoIm]Br) served as a better heat conductor in turn. Thus, the presence of more surfactant in the hybrids suppressed the reduction of GO and the surfactant decomposition temperature. The GAI-Rtotal-UV samples also had lower surfactant decomposition enthalpy than the non-UV-treated samples.

Figure 8. TEM images of (a) rGAI-3.43-Au (b) high resolution TEM image of rGAI-3.43-Au (c) TEM images of GO/Au.


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As mentioned in the introduction, the thermal reduction of GO is exothermic and self-propagating, and therefore cannot be controlled or interrupted. This hampers the attempt to exploit the energy stored in GO to prepare GO / nano-material composites, which requires delicate energy manipulation. In our hybrids, the energy output could be adjusted either roughly by changing the relative content or finely by choosing the UV treatment conditions. After completion of the controllable heat output, the key AzoIm+ additive can be decomposed at a similar temperature. By adding HAuCl4 into the AzoIm+ / GO suspension, AuCl4− anions were introduced into the GAI-3.43 hybrid. The sample was heated to 250°C at 5°C·min−1 under N2 atmosphere and the final product was denoted rGAI-3.43-Au. The same procedure was applied to a GO/Au composite as a control without AzoIm+. In the TEM images, monodispersed Au nanodots of less than 5 nm were observed for rGAI-3.43-Au. The most noteworthy result was the approximately uniform distribution of the nanoparticles, indicating that their formation occurred under mild conditions. The affinity between GO and the Au nanodots was not affected by water and ultrasound processing in TEM sample preparation (Figure 8a). The crystal face of Au could be identified in the high resolution TEM image, though not very clearly as it was covered by the GO layer (Figure 8b). The GO/Au composite contained Au nanoparticles of various shapes, polydispersity dimensions and scattering distributions (Figure 8c). The comparison clearly supports the notable potential of GAI as a controllable alternative form of GO for exploiting the energy released during its reduction. In previous studies of the size of metal nanoparticles in catalysis, smaller nanoparticles were generally beneficial but suffered from aggregation and sintering.53-56 A method to fabricate well-dispersed metal nanoparticles would be of great value in catalyst design. Meanwhile, this represents an illustrative


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example of the introduction of functional materials into intercalation layers using the anion exchange chemistry of imidazolium. 3.5 Anisotropic structure on uniaxially stretched soft substrate By investigating the well-formed GAI-Rtotal-S and GAI-Rtotal-UV-S hybrids on the stretched, soft substrate poly(dimethylsiloxane) (PDMS), the anisotropic response of the GAI hybrids to mechanical stimuli was uncovered. We firstly compared them with GAI-Rtotal in the bulk state and found that the previously split XRD peaks resolved into a single, sharp peak for each GAI-Rtotal-S or GAI-Rtotal-UV-S. The 2θ positions of those peaks were also shifted from the original bulk, exhibiting a substrate-responsive behaviour of AzoIm+. The offset of the peak positions was related to the AzoIm+ ratio and UV-on/off switching. GAI-6.86-UV-S and GAI-3.43-UV-S presented lamella with d spacings closest to those of AzoIm+. However, in the UV-off case, a compressed d spacing appeared for GAI-6.86-S, while conversely, an expanded spacing appeared for GAI-3.43-S. Anisotropy was manifested by the intensity of the XRD patterns in different directions, reflecting the structural orderliness of the corresponding lamellar hybrids (Figure 9a). The E-rich form of GAI-6.86-UV-S showed only about two times the difference in the XRD peak intensity, indicating that the ordered structure was distributed preferentially along the substrate stretching direction (0°) (Figure 9c). Meanwhile, the Z-rich form of GAI-6.86-S reacted more strongly to substrate stretching, and the 2.53 nm (2θ = 3.49°) lamella grew almost exclusively in the stretching direction and left nearly no (001) peak in any other direction (Figure 9b). A similar uniaxial phenomenon occurred for E-rich GAI-3.43-UV-S, but the preferential direction of ordering switched to perpendicular (90°). The Z-rich form of GAI-3.43-S showed limited change on substrate stretching except for a slight d spacing compression in the perpendicular direction than the along direction. (Figure 9d & e). Thus,


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the surfactant ratio and on/off switching of the UV treatment were both major factors affecting the anisotropic mechanical responsiveness. They were correlated directly with the amount and dominant conformation of the surfactants serving to support the GO layers. Generally, GAI-6.86-S, which was more strongly supported by the surfactant, tended to arrange into ordered layers along the stretching direction. Sufficient excess AzoIm+ surfactant provided enough liquid crystal units to orientate by an interlayer motion through stretching the soft substrate, with their positions becoming fixed after release. In contrast, smaller amounts of excess liquid crystal units were not able to orientate by interlayer motion or to fix the temporary molecular arrangement during solvent evaporation. Thus, after the release of the substrate and solvent removal, the isotropic arrangement dominated. This mechanism also explains the effect of UV treatment. The bending E-rich conformation was unable to undergo the aligning mechanical motion. In the case of GAI-6.86-UV-S, the orientation was fixed. However, in the case of the UV-treated GAI with less supporting surfactant, a compensatory effect was responsible for the perpendicular preference of the order orientation.


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Figure 9. (a) Illustrative diagram of preparation of GAI-Rtotal-S and GAI-Rtotal(-UV)-S. Measuring-angle-dependent XRD patterns of (b) GAI-6.86-S (c) GAI-6.86-UV-S (d) GAI-3.43-S (e) GAI-3.43-UV-S.

3.6 Electrochemical application for energy storage The electrochemical properties of the hybrids were explored via their photo-responsive behaviour and structural stability in an electric field. As before, the GAIs for the electrochemical tests were prepared in EtOH/n-hexane and irradiated for 30 min to prepare GAI-Rtotal-UV samples. They were re-dispersed by sonication before being drop-casted on an Au electrode for modification. The CV


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was performed on the modified electrode (Figure 10). The redox peaks of azoimidazolium were superimposed on the ideal rectangular curve of GO in the CV test. For GAI-6.86, the redox peak pair was located at −0.12 V (ox), −0.24 V (red) vs. SCE. With lower excess surfactant content, i.e. GAI-3.43 to GAI-0.69, the oxidation peaks were up-shifted by −0.05 V to 0.05 V, while the reduction peaks down-shifted by −0.28 V to −0.36 V. At the same time, the redox peak pair deviated increasingly far from a symmetrical shape. The pure surfactant under the same conditions exhibited a symmetrical −0.15 V/−0.26 V oxidation/reduction peak pair (Figure S3). From these results, we concluded that the oxidation and reduction process of the GAI loaded with less surfactant was more difficult and less reversible. From the perspective of the surfactants, this was the direct consequence of their being better enveloped by GO in the less surfactant-loaded GAIs, shielding them from the electrons necessary to trigger the electrochemical reaction. Taking the UV-on/off parameter into consideration, we observed that the three GAI-Rtotal-UV samples behaved differently in terms of the shift of the redox peaks from the GAI-Rtotal reference. The peaks of GAI-6.86-UV shifted in opposite directions, with a higher oxidation voltage and lower reduction voltage. Both peaks of GAI-3.43-UV moved to higher voltage. The redox peaks of GAI-0.69-UV were negligible. A remarkable effect of the integration of GO with the AzoIm+ surfactant was the photoresponsive behaviour of the current and capacitance. In the UV-off state, GAI-3.43 had maximum peak currents of 2.88 A·g−1 and −4.48 A·g−1 in oxidation and reduction, respectively. The specific capacitance was calculated as between 55.7 F·g−1 and 40.9 F·g−1 depending on the scan rate (Table 2). Increasing the surfactant amount to GAI-6.86 or decreasing it to GAI-0.69 both resulted in the capacitance falling to around 2 – 6 F·g−1. In the samples with UV on, all currents and capacitances of


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the GAI decreased, but to different degrees. The remaining percentage of capacitance was 50 – 80%, 5 – 10% and 33

–35% for GAI-6.86-UV, GAI-3.43-UV and GAI-0.69-UV, respectively. The

GAI-3.43 series also achieved the best responsiveness. Interestingly, in the CV curve of the pure surfactant, only the oxidation peak shifted slightly after UV exposure (Figure S3). Therefore, this photo-responsiveness, although it originated from AzoIm+, was a result of both AzoIm+ and GO. In addition to the output, the electrochemical energy input to graphene could also be regulated by light-induced modification of the GAI. The GAIs were therefore characterised by multiple interactions and a close and uniform packing of the AzoIm+ intercalated between the GO layers, which amplified the slight deformation of azoimidazolium into major changes of the structure and energy storage behaviour.

Figure 10. Cyclovoltammetry curves of

(a) GAI-6.86 vs. GAI-6.86-UV (b) GAI-3.43 vs.

GAI-3.43-UV (c) GAI-0.69 vs. GAI-0.69-UV. Scan rate: 50 mV/s. Reference: SCE. Table 2. Specific capacitance (F·g−1) of GAIs at different scan rates 20

30

40

50

mV/s

mV/s

mV/s

mV/s

6.11

3.83

3.12

2.72

GAI-6.86-UV 5.05

1.94

1.64

1.44

Samples GAI-6.86


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GAI-3.43

55.7

47.6

43.3

40.9

GAI-3.43-UV 5.96

3.83

3.12

2.71

GAI-0.69

4.52

4.15

3.96

3.83

GAI-0.69-UV 1.60

1.46

1.36

1.27

To better understand the interaction between GO and AzoIm+ surfactant in a ramped electric field, we examined the relationship of voltage scanning speed vs. current for each of the GAI (Figure S4). With the voltage ramped from −0.5 V to 0.5 V and back to −0.5 V, the current of each sample was measured for 20 cycles to observe its dynamic variation. In all six samples including UV-on/off states, the curves from a scanning speed of 20 mV/s were the most pronounced. All of the samples presented a gradual shrinking of the curves through repeated cycles, to a degree that was negatively correlated with the ratio of surfactant. In the samples GAI-6.86, GAI-6.86-UV and GAI-3.43-UV, the 20 mV/s curves were all larger than at the other three speeds. In classical chemical adsorption on the electrode, the peak current is proportional to the voltage scanning speed, and this was consistent with the curves scanned at 30 – 50 mV/s in our work, excluding only the curve of 20 mV/s. In the slowest scan, at 20 mV/s, desorption of the active materials from the electrode surface was accountable for the curve shrinkage, while faster scanning mitigated this process. Too much or too little surfactant, departing from equilibrium, would hinder the optimum structure for capacitance and the resulting photo-responsiveness.

4 Conclusion By modification of GO with azoimidazolium surfactant, a new series of graphene intercalation hybrids was achieved. The strong interaction between GO and the azoimidazolium surfactant components delivered the first reported GIC-level-uniform lamellar structure of GO. The multiple


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responsive nature of azoimidazolium was preserved in the GAI hybrids upon incorporation into GO. As a result, and beyond GICs, behaviours such as aggregation, dominant ordering direction, exothermic effects in the GO thermal reduction process and the electrochemical capacitance of the GAI responded to photo-, thermal and anisotropic mechanical stimuli, respectively. As a further step towards energy conversion applications, the energy input and output were controlled and regulated by UV light and the amount of azoimidazolium. The controlled output of chemical energy from the thermal reduction of GO, by exploiting the anion exchange chemistry of imidazolium, allowed the fabrication of Au nanodots sized below 5 nm. The controlled input of electrochemical energy made possible the manipulation of the capacitance of the GAI. From a broader perspective, the establishment of a good control method for graphene-class materials, which are highly representative of 2D materials, would be a prelude to major developments in controllable 2D materials.

Notes and references 1.

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for Electronic Sensing. Small 2014, 10 (20), 4042-4065. 2.

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Electrical Properties of Graphene Nanodevices. ACS Nano 2015, 9 (4), 3428-3435. 3.

Wu, Z.-S.; Parvez, K.; Li, S.; Yang, S.; Liu, Z.; Liu, S.; Feng, X.; Müllen, K., Alternating Stacked

Graphene-Conducting Polymer Compact Films with Ultrahigh Areal and Volumetric Capacitances for High-Energy Micro-Supercapacitors. Adv. Mater. 2015, 27 (27), 4054-4061. 4.

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. Chemical Reviews 2016, 116 (9), 5464-5519. 5.

Yang, C.; Chen, Z.; Shakir, I.; Xu, Y.; Lu, H., Rational synthesis of carbon shell coated polyaniline/MoS2

monolayer composites for high-performance supercapacitors. Nano Research 2016, 9 (4), 951-962.


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Chang, H.; Wu, H., Graphene-Based Nanomaterials: Synthesis, Properties, and Optical and Optoelectronic

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Devices: A Review. Adv. Func. Mater. 2015, 25 (31), 4929-4947. 10. Sun, Z.; Chang, H., Graphene and Graphene-like Two-Dimensional Materials in Photodetection: Mechanisms and Methodology. ACS Nano 2014, 8 (5), 4133-4156. 11. Wei, D.; Liu, Y., Controllable Synthesis of Graphene and Its Applications. Adv. Mater. 2010, 22 (30), 3225-3241. 12. Damlin, P.; Suominen, M.; Heinonen, M.; Kvarnström, C., Non-covalent modification of graphene sheets in PEDOT composite materials by ionic liquids. Carbon 2015, 93, 533-543. 13. Xu, J.; Wang, K.; Zu, S.-Z.; Han, B.-H.; Wei, Z., Hierarchical Nanocomposites of Polyaniline Nanowire Arrays on Graphene Oxide Sheets with Synergistic Effect for Energy Storage. ACS Nano 2010, 4 (9), 5019-5026. 14. 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. Chemical Reviews 2012, 112 (11), 6156-6214. 15. Han, K.; Liu, Z.; Shen, J.; Lin, Y.; Dai, F.; Ye, H., A Free-Standing and Ultralong-Life Lithium-Selenium Battery Cathode Enabled by 3D Mesoporous Carbon/Graphene Hierarchical Architecture. Adv. Func. Mater. 2015, 25 (3), 455-463. 16. Wang, M.; Duan, X.; Xu, Y.; Duan, X., Functional Three-Dimensional Graphene/Polymer Composites. ACS Nano 2016, 10 (8), 7231-7247. 17. Wang, D.; Kou, R.; Choi, D.; Yang, Z.; Nie, Z.; Li, J.; Saraf, L. V.; Hu, D.; Zhang, J.; Graff, G. L.; Liu, J.; Pope, M. A.; Aksay, I. A., Ternary Self-Assembly of Ordered Metal Oxide−Graphene Nanocomposites for Electrochemical Energy Storage. ACS Nano 2010, 4 (3), 1587-1595. 18. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H., Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457 (7230), 706-710.


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Supporting Information. XPS spectra of GAIs Supplementary cyclovoltammetry curves. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Dr. Changxu Lin. Email: [email protected].

*Prof. Xiang-yang Liu. Email: [email protected].

Acknowledgements This research was supported by China NSFC (Grand No. 21403077), Research Fund for the Doctoral Program of Higher Education of China (Grand No. 20133501120004), China Postdoctoral Science Foundation (Grand No. 2015M582037), Fujian Provincial Department of Education (JA14016). Also supported by “111”Project (B16029), NSFC (No. U1405226), Fujian Provincial Department of Science & Technology (2014H6022) and the 1000 Talents Program from Xiamen University.

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