Supporting Information High-Performance Strain Sensors with Fish Scale-Like Graphene Sensing Layers for Full-Range Detection of Human Motions Qiang Liu, Ji Chen, Yingru Li, and Gaoquan Shi* Department of Chemistry, Tsinghua University, Beijing 100084, People s Republic of China * Corresponding authors, E-mail:gshi@tsinghua.edu.cn Table of contents 1. Methods 2. Supplementary Table 3. Supplementary Figures 4. Mechanism Analysis 5. Supplementary References 1
1. Methods GO dispersion. GO was synthesized by the oxidation of 325 mesh graphite flakes according to a modified Hummers method. S1 Typically, natural graphite flakes (1.0 g) were dispersed in concentrated sulfuric acid (23 ml) under stirring in an ice bath, potassium permanganate (3.0 g) was added slowly to keep the temperature of the suspension lower than 20 o C. Then, the reaction system was transferred to a 40 o C oil bath and vigorously stirred for about 30 min. Then, 50 ml deionized water was added, stirred for another 15 min at 95 o C. Successively, additional 150 ml water was added, and hydrogen peroxide (30 wt%) was added dropwise until gas generation was completed, turning the color of the solution from dark brown to yellow. The suspension was filtered and the filter cake was washed 3 times with 1:9 HCl aqueous solution (50 ml) to remove metal ions, and then, washed with deionized water to remove the residual acid. The filter cake was dried in air and dispersed in 300 ml deionized water to form GO aqueous dispersion. Finally, it was purified by dialysis for two weeks using a dialysis membrane with a molecular weight cut off (8,000 14,000 g mol 1 ) to remove the remaining acid and metal species. The resultant GO aqueous dispersion was stirred 24 h and sonicated for 30 min to exfoliate it to GO sheets. After that, the GO dispersion was centrifuged at 3,000 rpm for 40 min twice to remove the unexfoliated particles. Successively, the as-obtained GO dispersion was diluted to 0.5 mg ml 1 for use. rgo films. GO dispersion was reduced to rgo according to a previously reported method. S2 In a typical procedure, the resulting homogeneous GO dispersion (0.5 mg ml 1, 50 ml) was mixed with 50 ml of deionized water, 17.5 µl of hydrazine monohydrate (>99 wt%) and 350 µl of ammonia solution (25 wt%) in a 250-mL reaction flask. After being vigorously stirred 2
for a few minutes, the reaction system was set in a 95 o C oil bath for 1 h. When the reaction was completed, gauze was used to filter out a small amount of residue, resulting a homogeneous rgo suspension. Successively, the rgo film was made by vacuum filtration of an rgo suspension through a cellulose ester membrane with a pore size of 0.22 µm, similar to the method reported for fabricating a GO paper. S3 After the filtration, the rgo film was dried at ambient temperature. The thickness of rgo film was controlled to be 1 µm by the volume of filtered rgo dispersion. 3
2. Supplementary Table Table S1. The performance comparison of graphene-based strain sensors Graphene-based strain sensors Maximum sensing range Limit of detection Average gauge factor Reference Fish scale-like rgo/tape film 82% 0.1% 16.2, 150 This work Graphene monolayer on PDMS 5% [a] 151 S4 Nanographene film on mica 0.4% 0.105% 300 S5 Percolative layered graphene film 1.7% [a] 150 S6 Few-layer graphene on PDMS 7.1% 0.015% 2.4, 4-14 S7 Laser scribed graphene on PET 10% [a] 9.49 S8 Graphene woven fabrics on PDMS 10% 0.2% 35 at 0.2% S9 Graphene-nanocellulose paper 100% 6% 7.1 S10 Fragmentized graphene foam 70% 0.25% 15 S11 Fabricated by pencil drawn 0.6% 0.13% 536.6 S12 Graphene mesh fabric on stretchable tape 7.5% [a] 20 S13 Graphene based on yarns 150% [a] 1.4 S14 Fiber of compression spring architecture 100% 0.2% 3.7 S15 Nanographene films on PDMS 1.6% 0.2% 500 S16 [a] Not shown 4
3. Supplementary Figures Figure S1. a) SEM and b) AFM images of GO sheets. Figure S2. Cross-section SEM image of an rgo film. Figure S3. C1s XPS spectra of a) GO and b) rgo. 5
The C1s X-ray photoelectron spectroscopy (XPS) spectra of GO and rgo indicate the presence of four types of carbon atoms: C C/C=C (284.8 ev), C O (286.8 ev), C=O (287.8 ev) and O C=O (289.0 ev), while the peaks of oxygenated groups in the GO spectrum (Figure S3a) is much stronger than those in the rgo spectrum (Figure S3b), implying the removal of oxygen containing groups on GO sheets by hydrazine hydrate treatment. Figure S4. Raman spectra of GO (black), rgo (red), rgo/tape (blue) films. Raman spectra of freeze-dried GO, rgo film, and rgo/tape composite film are illustrated in Figure S4. Each spectrum displays a D-band at 1350 cm 1 and a G-band at 1590 cm 1. The intensity ratio of D- and G- band (I D /I G ) is increased from 0.95 for GO to 1.30 for rgo film, implying the partial restoration of conjugation structure by hydrazine hydrate treatment. In addition, the I D /I G value of rgo/tape composite film is 1.32, close to that of rgo film, implying that the chemical structure of rgo was unchanged during pre-stretching/releasing process. 6
Figure S5. Top-view SEM images of a) the rgo/tape bilayer film was stretched for 50%, b) the second rgo film was adhered to the 50% pre-stretched rgo/tape bilayer (red line is the boundary of the second layer). c) The smaller rgo slices generated in the first layer when the composite film with two rgo layers was further stretched to 100%. Figure S6. Top-view SEM images of forming curled slices upon releasing as the first pre-strain was as large as 80%. 7
Figure S7. Top-view SEM images of a FSG strain sensor prepared by first pre-strain of rgo/elastic bilayer to 50%, then adhered the second rgo layer and further stretched to 100%, and finally slowly released to its original state. Figure S8. Stress-strain curves of an elastic tape (VHB4910, 3M) and the rgo/tape composite film. Figure S9. The method of calculating the strain. 8
Figure S10. The typical current-strain curve of FSG strain sensor (stretching rate =10% min 1 ). Figure S11. Comparison of the sensing range of FSG strain sensors with that of the sensor with single rgo film. 9
Figure S12. Top-view SEM images of the crack's evolution on single graphene film under tensile strain. Figure S13. Cycling test of FSG strain sensor at a stretching/releasing rate of 30% min 1 ; its relative resistance changes synchronously with strain. 10
Figure S14. Variations of relative resistance of FSG strain sensor during the process stretching/releasing for 10% at the rates of 20% min 1, 50% min 1, 100% min 1, 200% min 1, and 500% min 1, respectively. Figure S15. Mechanical stability of the rgo/tape composite film upon cyclic stretching/releasing to 10% for 5000 cycles. 11
Figure S16. The relative resistance variations at the maximum strain of 10% upon cyclic stretching/releasing test. Figure S17. Schematic illustration of bending a FSG strain sensor and the equation for calculating the degree of bending. Figure S18. The relative resistance variations during the process of repeated bending a 15.0 mm FSG strain sensor to 3.0 mm for 5000 cycles. 12
Figure S19. Top-view SEM image of FSG strain sensor stretching for 60% (the white arrows indicate the cracks). Figure S20. Relative resistance responses of FSG strain sensor in detecting finger bending with good stability. 13
4. Mechanism Analysis On the basis of the model shown in Figure 4c and 4d, the sensing mechanism of FSG strain sensor is theoretically analyzed as follows. The initial resistance of this strain sensor, R 0, can be depicted by equation S1: R0 = Rg + Rc (S1) where R g is the inherent resistance of rgo slices, R c is their contact resistance at 0% strain. We reasonably assume R c is much larger than R g, based on the fact that the resistivity perpendicular to the crystal layers of graphite is about 150 times larger than that of graphene basal plane. S17 At a strain of ε, the resistance R is described as: R= R + R (S2) g ' c where ' R c is the contact resistance of graphene slices at the strain of ε. Therefore, the relative resistance variation could be determined as follows: ' ' R R R ( R ) ( ) 0 g Rg + Rc Rc ( Rc Rc ) = = = (S3) R R R R 0 0 0 0 The contact resistance of graphene slices increases with ε because of reducing their overlapping area. According to the law of resistance, the contact resistance of graphene slices could be defined as: Rc = a / S (S4) where a is a constant related to surface condition, contact pressure, and electrical resistivity of graphene slices, S is the overlapping area of FSG slices. After the sensor being stretched to a strain of ε, the overlapping area of FSG slices can be described as: 14
S ' S bε = (1 ) (S5) 0 where b (b < 1) is a constant related to the overlapping mode of FSG slices at the initial state, S 0 is the overlapping area of FSG slices at the initial state. Then the relative resistance variation at strain of ε could be determined as follows: ' ' R ( Rc Rc ) a(1/ S 1/ S0) aε = = = R R R S R (1/ b ε ) 0 0 0 0 0 (S6) When the strain of ε is far smaller than 1/b, the expression of relative resistance variation simplifies as: R abε (S7) R S R 0 0 0 During the stretching process, it is reasonable to regard a, b as constants. The relative resistance variation is nearly proportional to applied strain of ε. 5. Supplementary References S1. Chen, J.; Yao, B.; Li, C.; Shi, G. An Improved Hummers Method for Eco-friendly Synthesis of Graphene Oxide. Carbon 2013, 64, 225 229. S2. Li, D.; Mueller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101 105. S3. Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. Flexible Graphene Films via the Filtration of Water-soluble Noncovalent Functionalized Graphene Sheets. J. Am. Chem. Soc. 2008, 130, 5856 5857. 15
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