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Enhanced electrical conductivity of cellulose nanofiber/ graphene composite paper with a sandwich structure Minjie Hou, Miao-jun Xu, and Bin Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02683 • Publication Date (Web): 21 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced electrical conductivity of cellulose nanofiber/graphene composite paper with a sandwich structure Minjie Hou†,‡, Miaojun Xu†, Bin Li†,‡* † Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials, College of Science, Northeast Forestry University, Harbin 150040, P. R. China ‡ Key Lab of Bio-based Material Science and Technology (Northeast Forestry University), Ministry of Education, Harbin 150040, P. R. China *Corresponding authors e-mail address: [email protected], Fax: +86 0451 82192699 ABSTRACT: A novel conductive paper based on cellulose nanofiber (CNF) and reduced graphene oxide (RGO) with a sandwich structure was successfully prepared through a step-by-step vacuum filtration followed by chemical reduction process, in which a CNF layer is sandwiched between two thin RGO layers. This unique design strategy not only provides a highly conductive network for its surface, but also maintains the structural integrity of CNF. The sandwich structured paper exhibits a significantly conductive anisotropy, and the in-plane electrical conductivity is drastically enhanced as 4382 S m-1 with only 4 wt% RGO, while it is insulating along the cross-plane direction. This can be attributed to the RGO layers at the top and bottom surface connected in parallel. This high electrical conductivity is greatly superior to most

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of the cellulose/graphene composite papers obtained by conventional blending process. Compared with the similar layer-by-layer assembly technique, the present method is more feasible and time-saving. Moreover, the sandwich structured paper shows excellent mechanical strength and good flexibility which may facilitate its applications in future flexible electronics. KEYWORDS: cellulose nanofibers, reduced graphene oxide, conductive paper, sandwich structure, anisotropy Introduction Flexible conductive papers have attracted great attention in a range of new applications such as thin film transistors, organic solar cells, and energy storage devices.1-4 They are typically built on a flexible substrates including polyethylene terephthalate (PET), polycarbonate (PC) and polyimide (PI). Due to the high transmittance and flexibility, plastic substrates have certainly achieved success in a variety of devices, but their disadvantages are obvious, such as a low coefficient of thermal expansion (CTE) and high processing temperature,5, 6 and in some cases it is not renewable. Recently, cellulose nanofibers (CNF) paper is emerging as a strong competitive substrate for electronics due to its attractive properties, including low CTE,6 high optical transmittance7, 8 and mechanical strength.9 It is consist of nanofibers with 3-4 nm width and a few micrometers in length.10 The foldable, freestanding CNF paper can be obtained by vacuum filtration of the CNF dispersion through a microporous membrane. Its dense nano-sacle network leads to a smooth surface and it is more flexible and printable than plastic.8, 11 These excellent characteristics induces CNF paper possess an excellent performance, and there are no significant loss of optical transparency in high temperature. Meanwhile, it is environmental friendliness and biocompatible with human skin, which can be applied in wearable electronic devices.12


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Graphene is a two-dimensional carbon material with one atom thick and the carbon atoms are bonded together through sp2 hybridization arranged in a honeycomb network.13-16 It exhibits a large specific surface area, high charge mobility of around 10000 cm−2 s−1, thermal conductivity with a value of ∼5000 W mK−1 and high Young's modulus of 1.0 TPa. These characteristics make it an attractive material for the development of the novel electronics.17 Recently, many researchers have been attempting to incorporate graphene into CNF matrix for application in the electronics field.18-22 Blending is the traditional approach to fabricate the CNF/graphene paper, and the electric property is completely depends on the loading amount of graphene.23 Kang et al.17 dispersed the graphene nanosheets (GNS) into the cellulose pulp and fabricated conductive composites by ﬁltration. The electrical conductivity was reached to 11.6 S m-1 by adding 3.2 wt% of GNS. Luong et al.19 prepared the CNF/RGO paper by combining RGO and amine-modified CNF, and the conductivity was improved from 4.79×10-4 to 71.8 S m-1 when the RGO contents increased from 0.3 to 10 wt%. Yang et al. reported another CNF/RGO composite, which was prepared by blending graphene oxide (GO) into CNF matrix followed by chemical reduction.31 The optimized conductivity reached to 4057.3 S m-1 with an RGO loading of 50 wt%. The higher content of RGO in composite paper would severely damage the mechanical properties of paper, which would conceivably limit its application in many fields. Therefore, the preparation of cellulose/graphene composite papers with superior electrical conductivity and mechanical performance simultaneously need to be further investigated. Herein, we report a sandwich structured conductive paper based on CNF and RGO, which is prepared by sequential filtering the desired amount of GO, CNF and GO dispersions through a microporous membrane, and following the hydroiodic acid reduction. The three step vacuum filtration process is more feasible and time-saving, which can avoid the long-time homogenizing


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treatment and a large number of deposition steps in layer-by-layer assembly. 19, 25, 26 Moreover, the thickness of each individual layer can be freely adjusted. In the layered structure, the CNF middle layer sandwiched between the two RGO coating layers not only act as the supporting substrate for the resultant paper but also hinder the electron transport along the cross-plane direction. Therefore, RGO/CNF/RGO paper possesses an anisotropic electrical conductivity along the in-plane and cross-plane directions. This strategy may provide a new idea to fabricate high performance cellulose-based electronic devices. The electrical and mechanical properties of RGO/CNF/RGO papers with different RGO contents were characterized and discussed. Material and Methods Materials. The cellulose material was provided by Hengfeng Paper Co., Ltd. (Heilongjiang, China) and its viscosity-average molecular weight (Mη) was determined to be 9.3×104. Graphite oxide powder was obtained from Nanjing XFNANO Materials Tech Co., Ltd., China. 2, 2, 6, 6-tetramethylpiperidine (TEMPO, 98 wt%) and hydroiodic acid (55 wt% in H2O) was purchased from Shanghai Aladdin Chemical Regent Inc., other reagents were used without further purification. All water used was purified using a Milli-Q century system (Millipore, American). Preparation of the RGO/CNF/RGO papers. The cellulose nanofibers (CNF) were prepared according to the literature27 and graphene oxide (GO) was prepared via ultrasonication for 1h followed by centrifugation. The sandwich structured papers were fabricated by a step-by-step vacuum filtration process. Firstly, a desired amount of GO dispersion (0.01 wt%) was filtered through a polytetrafluoroethylene microporous membrane (0.22 um pore size) to form a filter cake. Then the aqueous CNF dispersion (0.4 wt%) was poured slowly onto the formed cake and ensured the CNF covered uniformly on its surface. Finally, the GO dispersion with equal amount


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was added. After the water was filtering out, the filter cake with three layers was dried and reduced by HI acid at room temperature. The sandwich structured RGO/CNF/RGO papers were prepared with different RGO contents (0.25, 1, 2, and 4 wt%). Pure CNF paper treated with HI was also prepared and used to comparative investigation. Characterization. Transmission electron microscopy (TEM) images were recorded on a JEM-2100 microscope operated at 200 kV and Atomic force microscopy (AFM) was performed by a Dimension Icon (Bruker Germany).The SEM micromorphology of the sample was characterized using a JSM-7500F microscope (JEOL, Japan) at an operating voltage of 5 kV. XRD spectra were collected on an X-ray diffractometer (D/max 2200, Rigaku, Japan) using a Ni-filtered Cu K radiation (1.5406 Å) at 40 kV and 40 mA. Scattered radiation was detected in the range of 2θ= 5-40° at a scan rate of 4°/min. XPS analysis was carried out on a Thermo Escalab 250 Xi system using a spectrometer with a dual Al Kα source. The Raman spectroscopy measurements were performed using a Renishaw Raman Spectrum equipment with a 532 nm wavelength incident laser. A Linkam TST350 tensile stage was used to perform mechanical tests with a 200 N load cell at a load speed of 20 µm s-1. At least 5 specimens were measured from each sample. All of the samples were cut into strips with 40 mm × 5 mm × 23-26 µm. The thermal stabilities were measured by using a thermogravimetric analyzer (Pyris 6, PerkinElmer, USA) from 50 to 700 ºC at a heating rate of 10 ºC/min in a nitrogen environment. The electrical conductivity was measured using a Four-Probe Instrument RTS-8 and the corresponding conductivity was calculated using the following equation:

σ (S m −1 ) =

1

ρ

=

1 dR

(1)

where d is the thickness of the paper and R is the surface resistance.


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Results and discussion

Figure 1. Schematic of the fabrication process of the RGO/CNF/RGO papers The cellulose nanofiber (CNF) was prepared from the wood cellulose by TEMPO-mediated oxidation.8,27,39 The morphology of the CNF was characterized by transmission electron microscopy (TEM), and the measured diameter was around 7.5 nm, as shown in Figure S1a. The GO sheets presented irregular shapes and the thickness of around1.3 nm, which indicated the formation of the single-layered GO (Figure S1b).17,18 The CNF and GO sheets can be stably dispersed in water due to their hydrophilic nature16,27,40. Furthermore, the CNF have strong attractive interaction with GO via hydrogen bonding and van der Waals interaction.24 Fabrication process of RGO/CNF/RGO papers is described in Figure 1. Firstly, a certain amount of GO dispersion was filtered, and GO sheets could be assembled into a paper-like material under a directional flow.16 Then the CNF dispersion was dropped to form a bilayer cake. Finally, the same amount of the original GO dispersion was incorporated and covered on CNF surface. After HI acid reduction, the RGO/CNF/RGO papers were obtained, varied visually from transparent to


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fully opaque (Figure S2). The RGO/CNF/RGO papers with 4 wt% RGO displays a shiny metallic luster when it was exposed to light, meanwhile, it presented excellent flexibility when bent to the angles of 180º.

Figure 2. SEM images of (a) CNF surface and (b, c) the top and bottom surface of RGO/CNF/RGO papers with 2 wt% RGO; (d) Cross-sectional images of RGO/CNF/RGO papers with 2 wt% RGO and the inset is CNF substrate; (e, f) the RGO layers adhered on the top and bottom surface, respectively. Compared to the surface of pure CNF paper depicted in Figure 2a, the surface of GO/CNF/GO papers exhibits a relatively rough and wrinkle-like structure (Figure S3), which indicating that GO sheets were successfully coated onto both sides of the CNF substrate. It can be observed that the surface microstructures between the top and bottom surface are different. The top surface


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shows the stripe shaped patterns with a well-aligned structure, caused by the directional flow during vacuum-assisted filtration.28,29 For the bottom surface of GO/CNF/GO papers, many little pits appeared, the fact can be corresponded that a few of GO sheets were left on the micropore membrane. The surface of RGO/CNF/RGO papers with 2 wt% RGO exhibits more rough and uneven after reduction with HI acid, corresponding to the agglomeration of RGO sheets (Figure 2b and 2c). The SEM images for the cross-section of RGO/CNF/RGO papers present a sandwich structure, and the middle layer of CNF is sandwiched between two RGO layers (Figure 2e and 2f). The CNF are compressed more compact, and the structural integrity benefits to an excellent mechanical strength of RGO/CNF/RGO papers (Figure 2d). It can also be seen from Figure 2e and 2f that the RGO sheets existed on the top and bottom surface interacted strongly with the CNF substrate, exhibit a wrinkled and layered structure, this morphology is consist with the characteristics of single RGO film.30


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Figure 3. C1s XPS spectra of (a) GO, RGO reduced by HI acid at (b) 25 ºC and (c) 100 ºC. (d) XRD patterns and (e) Raman spectra of GO and RGO. (f) The surface resistance of RGO with various immersion times in HI acid at 25 ºC. HI acid is selected as the chemical reducing agent for the reduction of GO. The complete reduction of a 5 µm thick GO film needs 45 min at 100 ºC, according to the reported previously work.30 In order to evaluate the reduction of GO by HI acid, the products were measured by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectrum. The C1s spectrum of GO can be divided into three components with binding energies at about 284.3, 286.3 and 286.8 eV, corresponding to carbon atoms in C–C, C–O and C=O species, respectively (Figure 3a).31 When the reduction of GO by HI acid are carried out at 25 ºC, the relative peak intensity of C–O and C=O are dramatically decreased and the peak of C-C appeared dominant at 284.3 eV (Figure 3b), the C/O atomic ratio is also increased from 1.8 to 7.5, close to that of RGO reduced at 100 ºC (Table S1). The result indicates a high efficiency of HI acid reduction at room temperature. Figure 3d present the XRD patterns of GO and RGO. Comparing with the GO, the diffraction peak of RGO reduced at 25 ºC shifts to the higher 24.1º, the corresponding layer-to-layer distance (d-spacing) are decreased from 0.81 to 0.37 nm (Table S1), which further confirm the elimination of oxygen functional groups on the RGO sheets and partial recovery of the graphitic conjugated structure.32 The RGO reduced at 100 ºC exhibits one broad peak which is due to an increase in the disordered carbons.33 The Raman spectroscopy have been widely used to study the structural changes of carbon materials.13,14 Figure 3e displays two prominent peaks at 1360 and 1560 cm-1, corresponding to the characteristic D and G bands, respectively. For the RGO reduced at 25 ºC, the integrated intensity ratio of the D peak to the G peak (ID/IG) raises from 0.95 to 1.13, which reveals an increase in the sp2 cluster size and more structural


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defects during the reduction process.34-36 The surface resistances of RGO with various immersion times in HI acid solution are plotted in Figure 3f and the resistance of RGO reduced at 100 ºC is recorded for comparative investigation. The sheet resistance of RGO decreases with the immersion time in the first 30min. After that, the smallest sheet resistance is obtained as ~ 620 Ω/sq and remains constant even up to 3 h. It is highly close to 531 Ω/sq of RGO reduced at 100 ºC. Moreover, RGO/CNF/RGO papers remains a good structural integrity and no destruction under the acidic or alkaline environment. It also can be proved by the fact that the tensile strength of pure CNF paper is unaffected by HI acid treatment at 25 ºC for 1h (Figure S5). On the contrary, the structure of RGO/CNF/RGO papers have been destroyed after 45 min reduction by HI acid at 100 ºC (Figure S4). As a result, the HI treatment at room temperature is more suitable for the preparation of RGO/CNF/RGO papers. The surface resistances of the sandwich structured RGO/CNF/RGO papers are a function of the RGO contents, and the result is shown in Figure 4a. For most of the conductive papers or films, the level of electrical conductivity are directly characterized by the value calculated using the formula (1).18,19 The RGO/CNF/RGO papers containing 0.25 wt% RGO has a sheet resistance of ∼3×105 Ω/sq, which reveals that the conductive paths have been formed. The surface resistance of RGO/CNF/RGO papers containing 4 wt% decreases sharply to ∼1×103 Ω/sq, three order of magnitude smaller than the former, which is due to more conductive paths have been formed. It can also be seen that there is a significant difference in the resistance between the top and bottom surface. For the RGO/CNF/RGO papers containing 4 wt% RGO, the resistances of the top and bottom surface were 670 and 880 Ω/sq, respectively. It is noteworthy that the sheet resistance of the top surface is smaller, which mainly attribute that a portion of GO sheets are lost during the fabrication process, and also revealed in Figure S3b. The typical


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stress-strain curves of RGO/CNF/RGO papers are shown in Figure 4b. The pure CNF paper possesses a high tensile strength (109.1 MPa) and Young's modulus (7.2 GPa). For RGO/CNF/RGO papers, the CNF layer acts as an important role in their tensile behavior. The detailed mechanical properties are listed in Table S2. With the increase of RGO contents in RGO/CNF/RGO papers from 0.25 to 4 wt%, the tensile strength and modulus decreased from 109.9 to 72.7 MPa and 7.6 to 4.8 GPa, corresponding to a 33.8 and 50 % decrease, relative to that of pure CNF paper. This is mainly attributed to the reduction of CNF proportion in RGO/CNF/RGO papers. The elongation at break of all RGO/CNF/RGO papers is lower than that of the pure CNF paper, and it can be explained that the RGO layers acted as the moisture barrier for the hydrophilic CNF.37 The RGO/CNF/RGO papers containing 4 wt% RGO can be twisted and folded, and an LED lamp can be lighted well with no significant difference in the light intensity, further verifying the excellent flexibility of the sandwich structured papers. The electrical conductivity of RGO/CNF/RGO papers along the in-plane and cross-plane directions are also studied using a multimeter, and the results are shown in Figure 4c. The resistivity values are 790 and 967 Ω for the top and bottom surface of RGO/CNF/RGO papers containing 4 wt% RGO (Figure 4c I and II), respectively. The smaller resistance value of 370 Ω is obtained along the in-plane direction (Figure 4c III), but there are no electrical current along the cross-plane direction (Figure 4c IV). The different performance in the electrical conductivity along the in-plane and cross-plane directions show that the RGO/CNF/RGO papers possess a high electrical conductivity anisotropy. The corresponding circuit model is illustrated in Figure 4d and the phenomenon can be attributed to the parallel connection of the two RGO layers.38 When RGO/CNF/RGO papers containing 4 wt% RGO was inserted in the circuit and formed a current loop, the total resistance was calculated using the parallel resistance formula to be 345 Ω/sq,38


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lower than either of the surface resistances. In this study, the thickness of the individual RGO ‘papers’ were 600 nm measured by the SEM images (Figure S6). Based on the formula (1), the electrical conductivity were calculated using to be 2488 S m-1 and 1894 S m-1 for the top and bottom surface, respectively. Therefore the RGO/CNF/RGO papers containing 4 wt% RGO have a high in-plane electrical conductivity of 4382 S m-1, resulted from the sum of the conductivity values of the two individual RGO ‘papers’. This high in-plane electrical conductivity is superior to that of other previously reported cellulose/graphene composite papers (Table 1). Furthermore, the mechanical properties of RGO/CNF/RGO papers are comparatively investigated with those of CNF/RGO composite papers reported in the literatures.18,19,21,26,31 It is worth noting that the mechanical and electrical properties cannot be simultaneously improved by simple blending. According to the results reported by Yang et al.,31 as shown in Table S3, the decrement of the tensile strength range from 17.2 to 64.2 %, which is mainly due to the high loading of RGO in CNF matrix. However, only a 33.8 % decrease in the tensile strength of RGO/CNF/RGO papers with the RGO content increased to 4 wt%. The fact implies that the RGO/CNF/RGO papers exhibit the best balance in electrical and mechanical performance among the CNF/RGO composites, indicating that the present strategy is beneficial for developing high-performance cellulose-based electronics. A short video is provided to show the excellent conductivity along in-plane direction, in which the sandwich paper can light a LED lamp with either of the top and bottom surface, and its brightness become stronger with both of two RGO layers connected in parallel.


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Figure 4. (a) Sheet resistance of RGO/CNF/RGO papers with different RGO contents. (b) Tensile stress–strain curves of RGO/CNF/RGO papers, the inset is the lighting of an LED using the RGO/CNF/RGO papers containing 4 wt% RGO under different conditions. (c) The resistivity test of the RGO/CNF/RGO papers containing 4 wt% RGO using a multimeter: (I) and (II) represent the top and bottom surface resistivity values, respectively; (III) and (IV) represent the resistivity values along the in-plane and cross-plane directions, respectively. (d) Circuit illustration with RGO/CNF/RGO paper in parallel for lightening a LED lamp.

Table 1. Comparison of the electrical conductivity of RGO/CNF/RGO paper with the results of other related cellulose/graphene composite literatures.


13


Substratea

σd (S m-1) 15.4

Ref.

Blending + VF

Rc (Ω/sq) —

10

Blending+ VF

—

71.8

19

3.2

Blending

1063

11.6

17

CNF

Conductive materialsb RGO

Content (wt%) 10

A-NFC

RGO

Cellulose pulp

GNS

CNF

RGO

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—

Method e

layer-by-layer assembly

2.5×10

3

—

18

26 -4

BC

RGO

10

Blending+ VF

—

1.1×10

BC

xGnP

2

Impregnation

—

75

22

CNF

RGO

50

Blending + VF

—

4057.3

31

CNF

NGO

3

Blending+ casting

—

2.7×10-4

20

Cellulose

GO

5

CNF

RGO

4

Blending+ casting

-2

—

∼10

311

4382

21

23 This work

Step-by-step VF a

In this column: A-NFC = amine-modified nanofibrillated cellulose, BC = Bacterial cellulose. b In this column:

GNS= Graphene nanosheet, xGnP = exfoliated graphite nanoplatelets, NGO = ammonia-functionalized graphene oxide. c R represent the Sheet resistance, d σ represent the electrical conductivity. e VF= Vacuum filtration.

In order to explore the advantages of the sandwich structured paper, another cellulose/graphene composite paper is also prepared by blending, followed by HI acid reduction, which named RGO-b-CNF as a convenience. The properties of two type of composite papers with 2 wt% of RGO are comparatively studied. As shown in Figure 5a, it is easy to distinguish RGO/CNF/RGO paper owing to its shiny metallic luster on both surfaces. The water contact angle measured on the surface of RGO/CNF/RGO paper is the highest even prolong the time to 180s, which indicates a strong hydrophilic character. XRD diffractograms show that two peaks around 2θ = 15.2º and 22.8º, represent the typical cellulose I crystalline structure of CNF (Figure 5b).39 Therefore, both RGO-b-CNF and RGO/CNF/RGO paper can inherit the excellent mechanical strength of CNF paper (Figure 5c). For RGO/CNF/RGO paper, the tensile strength is 90.9 MPa, approach to the values of 109.1 and 106.2 MPa for pure CNF and RGO-b-CNF paper,


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respectively. But its elongation at break is higher than that of RGO/CNF/RGO paper, which mainly attribute to the middle-layer of CNF. It can be observed that the residue char at 700 ºC of RGO/CNF/RGO paper is higher than that of CNF and RGO-b-CNF paper, as shown in Figure 5d, which benefits to the superior flame retardancy for paper. The electromagnetic interference shielding effectiveness (EMI SE) of these papers are presented in Figure 5e. The EMI SE value are around 0, 1and 2.8 dB, corresponding to the pure CNF, RGO-b-CNF and RGO/CNF/RGO paper, respectively. It indicates that the EMI SE can be effectively improved by altering the construction of paper with the corresponding increase of RGO content in paper. In addition, RGO/CNF/RGO paper shows high electrical conductivity on both top and bottom surface, while CNF and RGO-b-CNF paper are not conductive, as shown in Figure 5f. These improvements of the sandwich structured paper make it to show a good application prospect.

Figure 5. Morphology and properties of CNF, RGO-b-CNF and RGO/CNF/RGO papers were investigated for comparison: (a) Their photographs and changes of the water contact angle with time prolonging; (b) XRD, (c) Stress−strain, (d) TGA curves and (e) EMI shielding effectiveness


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studied in this work (30 MHz to 1.5 GHz). (f) The resistivity test of CNF and RGO-b-CNF paper, the top and bottom surface of RGO/CNF/RGO paper using a multimeter. Conclusions In summary, a sandwich structured RGO/CNF/RGO paper is successfully prepared via a simple and low-cost method through vacuum filtration followed by reduction with HI acid. The RGO sheets can adhere tightly with CNF substrate and form the continuous conductive networks on the top and bottom surface. The as-fabricated paper exhibits an intrinsic electrical conductivity anisotropy, and the in-plane electrical conductivity are significantly enhanced, while it is insulating along the cross-plane direction. The optimized in-plane electrical conductivity can reach up to 4382 S m-1 with 4 wt% RGO, owing to the two surface RGO layers connected in parallel, which is more higher than that of other reported cellulose/graphene composite papers. The sandwich structured RGO/CNF/RGO paper also possesses excellent mechanical strength comparing with the CNF/RGO composite paper obtained by blending, meanwhile, there are obviously enhancement in char-forming and EMI shielding effectiveness. As a result, the presented effective strategy with low cost provides new insights for developing bio-based flexible electronic devices. ASSOCIATED CONTENT Supporting Information. TEM image of cellulose nanofibers, AFM image of GO sheets, Photographs for pure CNF paper and RGO/CNF/RGO papers, SEM images of GO/CNG/GO paper, XRD and Raman data of GO and RGO, cross-section SEM image of RGO/CNG/RGO paper containing 4 wt% RGO.


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The d-spacing and ID/IG values of RGO, the data of the mechanical properties of RGO/CNG/RGO papers and a comparison of the mechanical properties of CNF/RGO composite papers. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86 0451 82192699. Notes The authors declare no competing financial interest. Acknowledgements We are grateful for funding supported by National Natural Science Fund of China (No. 31370709) and Fundamental Research Funds for the Central Universities (No. 2572016AB65). References (1)

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For Table of Contents Use Only

Synopsis A sandwich structure conductive paper was fabricated based on cellulose nanofiber and reduced graphene oxide, which exhibit a high electrical conductivity along the in-plane direction.


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Graphene

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