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A high mechanical property of laminated electromechanical sensors by carbonized nanolignocellulose/graphene composites Yipeng Chen, Chengmin Sheng, Baokang Dang, Temeng Qian, Chunde Jin, and Qingfeng Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19353 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Applied Materials & Interfaces

A high mechanical property of laminated electromechanical sensors by carbonized nanolignocellulose/graphene composites Yipeng Chen†, Chengmin Sheng†, Baokang Dang†, Temeng Qian†, Chunde Jin†, Qingfeng Sun†,* †School of Engineering, Zhejiang A&F University, Hangzhou, Zhejiang Province, 311300, PR China *Corresponding author: E-mail: [email protected] (Qingfeng Sun)

ABSTRACT Although widely used in nanocomposites, the effect of embedding graphene in carbonized nanolignocellulose substrates is less clear. We added graphene to a carbonized nanolignocellulose to change its mechanical and electromechanical

properties.

Here,

the

laminated

carbonized

nanolignocellulose/graphene composites were fabricated by carbonizing the nanolignocellulose/graphene composites prepared through mechanochemistry and flow-directed assembly process. The resulting composites exhibit excellent mechanical property with the ultimate bending strength of 25.6 ± 4.2 MPa. It is observed reversible electrical resistance change in these composites with strain, which is associated with the tunneling conduction model. This type of high-strength conductive

composite

has

great

potential

applications

in

load-bearing

electromechanical sensors. KEYWORDS: electromechanical sensors; graphene; carbonized nanolignocellulose 1

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composite; laminated structure; mechanical property There is a renewed interest in graphene embedded materials that in a multifunctional capacity in both architectural and commercial systems1-4. One such application is electromechanical sensors. These sensors function as monitoring deformation, pressure, and impact during use. Conor S. Boland5 have added graphene to a lightly crosslinked

polysilicone,

often

encountered

as

Silly

Putty,

changing

its

electromechanical properties substantially. The resulting nanocomposites display unusual electromechanical behavior, such as postdeformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain. Prior research has suggested that an important application area involves the addition of graphene to polymers, usually to enhance electrical, mechanical, or barrier properties6, 7

.

Lignocellulose composite can be used as a precursor to produce the crosslinked polymer, ceramic and carbon composites8 so that the carbonized graphene embedded nanolignocellulose (c-GNLC) composite has been chosen as candidate high-specific strength materials for these electrical applications. The graphene-embedded laminated nanolignocellulose (GNLC) composite is prepared by mechanochemistry combination with flow-directed assembly and hot-pressing process that assemble the laminated nanolignocellulose (NLC)/graphene composite to nanometer scale and make it interlinked, respectively. In the process of carbonization, a stable, low-cost hard carbon is formed due to the uniformity and consistency of nanolignocelluloseboard, which can be easily processed into a complex shape by a variety of conventional 2


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methods9. In addition, laminated structures supply enough movement space for the NLC/graphene sheets when loading. We studied the effect of adding graphene to a lightweight carbonized nanolignocelluloseboard (c-NLC composite) with high specific strength that possesses satisfactory mechanical, thermal, electromagnetics and tribological properties. Addition of graphene to the c-NLC composite increases its stiffness and conductivity. However, it retains its characteristic that is highly specific strength, and because of the deformability, the nanosheets are mobile and respond to deformation in a time-dependent manner. In addition, the load frequency of itself is well received during mechanical deformation. This has led to the development of high-specific strength of mechanical sensing material that reflects the force state of the carbonized nanolignocelluloseboard itself. It is proved that the electromechanical sensor is feasible in the field of civil engineering monitoring. Results The model of the c-GNLC composite was shown in Figure 1. Firstly, the lignocellulose was broken, refined to nanoscale and its specific surface area was increased by mechanical force in the colloid grinder. Correspondingly, the crystallinity of cellulose in the lignocellulose declined, the lattice defects in the crystal structure caused the lattice displacement, and the system temperature increased at the same time10. The free energy of lignocellulose surface increased at this stage. Then, the intrant graphene was adhered to the NLC surface by hydrogen bonding and electrostatic adsorption force. AFM was often used to analyze the micro-topography 3



of graphene composite. Figure S1 showed the micro-topography of graphene detected on the NLC surface. The average thickness of NLC/graphene composite was increased from 7.5 nm to 33 nm with the increase in the amount of graphene in the load, which demonstrated the content of graphene has obvious influence on thickness of NLC/graphene composite. In the initial stages of filtration, the GNLC sheets were forced rapidly onto the surface of the filter paper by the water flow and randomly assembled (folded, crumpled, and wrinkled). Layers of the GNLC platelets assembled at this stage were not particularly well-packed and arranged. After a short time, the filter became clogged due to the deposition of the sheets, and the water flow slows down considerably. During the subsequent period of slow filtering (evaporation of water was also occurring), the concentration of the GNLC sheets in the suspension rises, resulting in a significant increased in the sheet-to-sheet interactions. During this stage, the sheets were more likely to be aligned on top of each other in the ever-growing deposit and are probably also “smoothed out” by the water flow11. After the hot-pressed process, the GNLC layers togethered to board with the hydrogen bond, electrostatic and van der Waals attractive forces. The eventual c-GNLC composite was obtained by the GNLC composite carbonized in an argon atmosphere at 1000 °C.

4


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Figure 1. Schematic illustration of the formation of the laminated c-GNLC composite. (a) The mechanochemical process of lignocellulose and graphene. (b) The suspension of NLC/graphene platelets. (c) The laminated NLC/graphene platelets assembling through the vacuum filtration. (d) Laminated GNLC composite was obtained after hot-pressing. (e) GNLC composite was carbonized with 1000 °C. (f) The final c-GNLC composite.

Figure 2 showed SEM images of the NLC/graphene composite and the multi-layered structure of the GNLC composite and c-GNLC composite. As shown in Figure 2b, 5



graphene sheets were supported by NLC to form a 2D sheet. The inset TEM image in Figure 2a further demonstrated that NLC covered by GO nanosheets interconnected into flat ultra-thin mesh. For GNLC composite, there was a 3D laminated materials assembled by 2D NLC/graphene sheets. From the cross section of a fractured surface of the c-GNLC composite (Figure 2c), it was observed that the NLC translated to hollow structure, the wall thickness of the c-NLC/graphene sheets was about 2 µm, which was due to the NLC cells collapsed causing shrinkage in the carbonization process12, 13. The hollow structure of the c-NLC/graphene sheets (Figure 2d) supplied enough movement space for itself when loading. It was superior to the GNLC composite when used as the electromechanical sensors. In addition, the inner surface of the c-NLC/graphene sheets may facilitate the charge storage14.

Figure 2. (a) SEM image GNLC platelets, inset: TEM image of GNLC platelets. SEM image of cross-section morphology of (b) the laminated GNLC composite and (c) the 6


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laminated c-GNLC composite. (d) Enlarged image of (c).

XRD patterns (a) of the NLC composite, GNLC composite and c-GNLC composite were shown in Figure 3. In the Figure 3a, as for the GNLC composite, the diffraction peaks at about 22° and 16° were attributed to the typical crystalline region of cellulose15, an intense peak at 2θ = 25.6° corresponded to the (002) planes of graphene16, which was not detected for NLC composite. However, the peak for (101) plane qualitatively demonstrated graphite formation was available after carbonizing at 1000 °C17. The intensity of the (002) diffraction peaks enhanced with increasing graphene loading, which showed that increased content of graphene was beneficial to improve the microcrystalline structure of carbon in the c-GNLC composite. The Raman spectrum of the c-NLC composite was shown in Figure 3b, the maximum varies by G band at approximately 1580 cm−1 and D band at 1360 cm−1 was observed showing the presence of disordered and graphitic carbon18, which was noticed that they were successfully converted into carbon c-GNLC composite over 1000 °C thermal treatment. Compared with the c-NLC composite, the increased intensity ratio of the G band (IG) to D band (ID) of c-GNLC composite revealed the coat of graphene on c-GNLC composite surface, which represented the sp2-bonded carbon atoms19. What’s more their intensity and the intensity ratio of the G band (IG) to D band (ID) increased with increasing graphene loading, which demonstrated they combined well with each other. The quantification of the amount of sp2-bonding can be conducted by measuring the ratio of π* bonding to π*+δ* bonding using electron energy loss 7



spectroscopy (EELS). A comparison of the carbon K near-edge structure for the c-NLC composite and the c-GNLC composites of similar thickness is shown in Figure 3c. With the assumption that the sp2-bonding in the c-GNLC composites was 1, the c-NLC composite were determined to have 0.84 sp2-bonding. a

c

b

c-GNLC c-NLC

1.5 wt% graphene

0.5 wt% graphene c-NLCB

1.5 wt% graphene

IG/ID=1.02

1 wt% graphene

IG/ID=1.01

0.5 wt% graphene

IG/ID=0.99

GNLCB

20

30

2θ (degree)

40

50 500

1000

π*

IG/ID=0.98

c-NLCB 10

Intensity (a.u.)

Intensity (a.u.)

1 wt% graphene

Intensity (a.u.)

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

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1500

2000

2500

3000 260

-1

Raman Shift (cm )

280

δ*

300

320

340

Energy Loss (eV)

Figure 3. (a) XRD spectra of the GNLC composite, the c-NLC composite and the c-GNLC composites with different content of graphene from 0.5–1.5 wt.%. (b) Raman spectra of the c-NLC composite and the c-GNLC composites with different content of graphene. (c) EELS spectra of c-NLC composite and the c-GNLC composites.

In the X-ray photoelectron spectroscopy (XPS) survey scan (Figure 4a), peaks at 284.17 and 531.5 eV indicated that the NLC composite, GNLC composite and c-GNLC composite was predominantly composed of C and O elements. Compared with the C/O ratio of 2.2 in the NLC, the C/O ratio for the GNLC composite was 3.6, indicated coexist of NLC and graphene in GNLC, and the C/O ratio of the c-GNLC composite were determined to be 17.3 further suggested the high carbonization level of c-GNLC composite. As shown in Figure 4b, the C1s core-level spectrum divided 8


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into three different signals. The peaks located at 284.8, 285.7, and 288.9 were related to the sp2 carbon of graphitic carbon and graphene20; the epoxide, phenol or ether groups (C–O); the carboxyl functional groups, esters or lactones (–C=O or –COOR), respectively. Increased sp2 C peak intensity indicated the presence of graphene and the success of the carbonization process21. There were a large number of oxygen functional groups were attributed to the incomplete carbonization of the carbohydrates in the lignocellulose such as glucose and sucrose. a

C1s

c-GNLC GNLC NLC

b

2

c-GNLC

C 1s sp C

C-O

C=O or O-C=O

Intensity (a.u.)

O1s Intensity (a.u.)

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


C1s O1s

C 1s

GNLC 2

sp C C-O C=O or O-C=O NLC

C 1s

O1s C1s

2

sp C 0

200

400

600

800

1000

1200

280

Binding energy (eV)

C-O 285

C=O or O-C=O 290

295

Binding Energy (eV)

Figure 4. (a) Survey scan, and (b) C1s XPS spectra of the NLC composite, GNLC composite and c-GNLC composite.

Figure 5a showed the variation of the bending strength of the GNLC composite and c-GNLC composite with the content of graphene. It was obvious that the addition of graphene has a significant effect on mechanical behavior of the GNLC composite and c-GNLC composite. The average bending strength for NLC composite and c-NLC composite was 18.3 ± 2.6and 20.2 ± 3.1 MPa, respectively. The strength gradually 9



increased to 20.6 ± 2.7 and 23.5 ± 3.3 MPa for 1 wt.% and further to 22.7 ± 3.3 and 25.6 ± 4.2 MPa for 1.5 wt.% of graphene loading. On the other hand, the average value of elasticity modulus increased to 2.9 ± 0.48 and 3.2 ± 0.68 GPa for 1.5 wt.% graphene was loading from 1.9 ± 0.5 and 2.3 ± 0.6 GPa for the NLC composite and cNLC composite. In addition, compared with the GNLC composite, the c-GNLC composite exhibited the higher mechanical behavior in the same graphene content, which was due to the increase number of graphite microcrystals in the c-GNLC composite, and the decrease margin of micro-crystal layer happened in the carbonization process9, the density of the c-GNLC composite also increased at the same time. When added the content of graphene up to 5%, 10% or 15%, the extent of the increase of the mechanical and electromechanical properties was small or even diminished (Figure S2), which was due to a decrease in the interaction result from the aggregate of grapheme. The toughening mechanism of layered structure was different from the traditional method of removing defects and improving mechanical properties22, 23. The defect layer in the material will block the expansion crack and increase the fracture surface in the vertical direction of the main stress, so as to improve the fracture toughness of the material. Figure 5b showed the fracture morphology of the c-GNLC composite, the fracture is uneven, and a thin sheet of carbon was pulled out to present a bridge state. This was because the c-GNLC composite have broken several times during the whole process of destruction, the crack was extended and deflected along the load direction, and the crack expansion can consume and absorb energy, and form a bridge at the end of the crack to prevent 10


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the crack expansion24. When a layer or a set of fracture, the other layer without damage can continue to bear the load, which was reflected by the >1% inelastic deformation prior to failure in the stress-strain curve (Figure S3), so that the strength and fracture toughness was improved to prevent the sudden catastrophic rupture. In addition, comparison of mechanical properties of the c-GNLC composite with reported lignin-based materials was shown in Table S1, which exhibited the high mechanical properties of our composite.

Figure 5. (a) Bending strength and elasticity modulus of the GNLC composite and the c-GNLC composite with different content of graphene from 0.5–1.5 wt.%. (b) The macrograph (left) and SEM image of the fracture surface of the GNLC composite.

Finally, it was demonstrated that the conductive c-GNLC composite could be used as electromechanical sensors under uniaxial compression. In a moderate range of strain (< 3%), c-GNLC composite could be compressed and revert to the original volume when unloaded, which exhibited definite manoeuvrability. The measurement (Figure 11



6a) of the compressive stress-strain curve and electric property showed that the resistivity was decreased significantly during the compression cycle, and the resistivity of the c-GNLC composite returned from 21.9 to 18.4 Ω·cm at the end of the loop when its compression strain reached 3%. It was attributed to the increased tunneling points between two layers of the c-GNLC. It has been also noticed that the resistance of other graphene composites changed with the compression or stretching25. When the c-GNLC composite was kept the static for 40 s at the 1% to 3% strain, Figure 6b showed the resistivity of graphene content from 0.5 to 1.5% of c-GNLC composite changed from 0.1 to 0.22, 0.06 to 0.18 and 0.04 to 0.15 along stress. As shown in Figure 2a, graphene was covered c-NLC, which changed with the variation of compressive load, and the layered structure was formed (Figure 2c and d). This microstructure resulted to the contact points and distances among the c-GNLC platelets changed obviously with the changement of compression force. Figure S4 also showed stress-strain curves of the c-GNLC composites after 10 cycles, which demonstrated the reversible mechanical properties of c-GNLC composites. In addition, comparison of electromechanical properties of the c-GNLC composite with reported carbon filled materials was shown in Table S2, which exhibited the comparative electromechanical properties of our composite.

12


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Figure 6. Electromechanical properties of the c-GNLC composite. (a) Stress-strain curves and simultaneously recorded resistivity values by compressing a composite with 1.5 wt.% graphene at a maximum strain of 3%. (b) Resistance change associated with compression cycles in which the composites with different graphene loadings was kept stable for 40 s atmaximum (3%) or minimum(1%) strains.

The electrical properties of the conducting c-GNLC composite were controlled by a combination of layered morphology and electrical properties of the c-GNLC platelets (supplementary text S1). The microstructure was observed with SEM to study the characteristics of the conductive network of c-GNLC composite. Figure S5 showed the microstructure of the c-GNLC composite, in which Figure S5a denoted the c-GNLC network and Figure S5b denoted the graphene platelets on the c-NLC surface. Figure 7a showed a portion of an electrons journey through a c-GNLC platelets network. The resistance of p2 (Rp2) was connected in series to form a resistor element the resistance of p1 (Rp1). Then, Rp1 forms the conductive network by connections in parallel and then series. It can be easily derived that the fractional change of resistance of the c-GNLC composite was equal to that of Rp1 + Rp2, which 13



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was similar to the system of carbon black filled cement-based composites26. Therefore, the total resistance of the electron in this part of the journey was the combined resistance of the platelets and the resistance of the moving resistance of the platelets. The conductivity for the c-GNLC composite changed underlying the movement of c-GNLC platelets was due to three possible factors: (i) The compressive applying strain predominately effects the structure of the networks (i.e. χ=f(ε)) (ii) The compressive applying strain predominately effects the electrical properties of the nanosheets themselves (i.e. Ri,i=f(ε)) (iii) The compressive applying strain predominately effects the electrical properties of the internanosheets junctions (i.e. Ri,i+1=f(ε)) The electrical percolation of the c-GNLC composite was simplified as a series of paths

of

tunnel transduction

connections

between

the

c-GNLC

platelets

interconnections and adjacent graphene embedded in the carbonizing NLC matrix. In the path of contact with no or small distance, the tunneling of c-GNLC platelets becomes a bottleneck, and the path can be more conductive under the compressive applying strain. When the nanoscale graphene distributed in c-GNLC composite host, the current density (J) in the composite formed by two c-GNLC platelets with a small distance (d) at low voltage (U) can be determined by the following equality26-28:

= 32⁄2 ⁄ℎ −4 2 ⁄ℎ

(1)

where m is the electron mass, e is the charge, h and j are the Planck constant and the 14


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barrier height between two adjoining c-GNLC platelets, respectively. Rm represents the resistance of a composite, n and a2 represent the effective cross-section area and a total number of such junctions, respectively, the resistance of a composite is obtained as

= 2 3 ⁄ℎ 4 2⁄ℎ

(2)

When the distance between the adjacent c-GNLC platelets is reduced from the initial d0 to the intermediate level (d) in the load period, the resistance of c-GNLC composite (Rm) changes from R0 to R at the same time, then the relative resistance (R/R0) is obtained as follows

⁄ = ⁄ −42 × − ⁄ℎ

(3)

Where d0 is determined by the fabrication process, under uniaxial compression, the distance of adjacent c-GNLC platelets depending on external strain. Only c-GNLC platelets with a lower distance can form new tunnel points and help increase conductivity. Assuming the strain (ε) through the entire c-GNLC composite is homogeneous, the relative resistance changes ∆/ can be quantified with

∆⁄ = 1 − 1 − # −42 × # ⁄ℎ

(4)

As shown in Figure 7b, the fitting results by eq 4 accords well with the experimental results obtained from the c-GNLC composite with 1.5 wt.% to corresponding strain regions. Resistance decreases by about 60% with increase in an ultimate compressive strain of 12%.

15



Figure 7. (a) Analysis of conductivity change in the c-GNLC composite during compression. (b) Experimental data and theoretical results by eq 4 of compressing c-GNLC composite (with 1.5 wt.% graphene).

Furthermore, this electromechanical sensors can be extended to large and the thick laminated c-GNLC composite (30 × 25 × 5 cm3) (Fig. 8a). An on-board active sensor system as shown in Fig. 8b, which consisted of a miniaturized voltage amplifier (AD620) and the large size c-GNLCB composite. The multimeter was used to monitor the changes of resistance. When people weighing 65 kg stands on this electromechanical sensors, the variation of conductivity was reflected by luminance of bulbs (Figure 8c). 16


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Fig. 8 (a) Macrograph of large and thick laminated c-GNLC composite. (b) Macrograph of on-board active sensor system. (c) The variation of conductivity of this electromechanical sensors when people weighing 65 kg stands on it. In this investigation, we fabricated the laminated c-GNLC composite by carbonizing the

NLC/graphene

composite

synthesized

through

mechanochemistry

and

flow-directed assembly process, with homogeneous graphene dispersion and well-preserved interlayer contact. Laminated c-GNLC composite exhibit superior mechanical property that increase with the content of graphene, combined with the reversible and controllable resistivity change demonstrated by electromechanical tests, we can make high-strength conductive composite be useful in load-bearing 17



electromechanical sensors. The results of our study demonstrate an alternative preparation of composite with high mechanical strength and electromechanical properties. The fabrication strategy paves the way toward electromechanical sensors of the carbonized lignocellulose-based composites. Methods Materials Lignocellulose based on populus tomentosa Carr.; graphene was supplied by XianFeng NANO Co., Ltd. Fabrication of c-GNLC composite. The lignocellulose suspension mixed with adjusting the content of graphene (0.5-1.5 wt.%) were simultaneously added rapidly to a colloid grinder with rotor speed set at around 2880 rpm and mechanico-chemical manufactured for 6h. Lignocellulose/graphene suspension was fed into the colloid grinder continuously. After redundant water of NLC/graphene suspension was vacuum filtered, the composites were hot-pressed at 220 °C, 2.5MPa for 30 min and cured into the GNLC composite. Carbonization was performed in an inconel-lined retort furnace under a gas flow of argon (0.4 l/min). The c-GNLC composite was prepared by calcining these pre-synthesized GNLC composite at 1000 °C for 2 h with a heating rate of 5 °C min−1. Characteristics The distribution and morphology of the composites were characterized by a scanning electron microscope (SEM, Quanta FEG 250, USA) and transmission electron microscope (TEM, Tecnai G2 F20, USA). Crystalline structures were identified by X-ray diffraction technique (XRD, Bruker D8 Advance, Japan) operating with Cu Kα 18


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radiation (λ = 1.5418 Å) at a scan rate (2θ) of 4° min−1. Raman spectroscopy was measured using a multi-wavelength micro-Raman spectroscope (Renishaw RM2000) using 532 nm incident radiation and a 500× aperture. The surface elemental composition analyses were conducted based on the X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific-K-Alpha 1063, UK) with an Al Ka monochromatic X-ray source. EELS was carried out in a JEOL 2100-F TEM. Fracture and Electromechanical Test. Fracture test was carried out on Instron 3382 with a span length of 30 mm. The specimens for the binding test were made into a dimension of 60 mm (length)×10 mm (width)×5 mm (thickness), and the specimens were loaded at a rate of 0.05 mm s−1 up to failure. Electrical properties of the composites were characterized by a standard two-probe method using a source meter (Keithley 2400). Specimens for electromechanical test were made into rectangular blocks. A layer of Cu paste was uniformly pasted on two opposite sides along thickness as electrode pairs. During the compressive tests, a constant compression speed was maintained to give a strain rate of 0.025 mm min−1 for simultaneous stress–strain–resistance measurement. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements. This research was supported by Special Fund for Forest Scientific Research in the Public Welfare (Grant No. 201504501), Key Laboratory of Bio-based Material Science & Technology (Northeast Forestry University), Ministry 19



of Education (Grant No. SWZCL2016-3) and Scientific Research Foundation of Zhejiang A&F University (Grant No. 2014FR077). Supporting Information Available. Possible mechanisms leading to the variation in resistivity at low strain, stress-strain curves. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES 1.

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High Mechanical Property of Laminated ... - ACS Publications

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