3D Graphene Films Enable Simultaneously High Sensitivity...

FULL PAPERwww.afm-journal.de

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1803221 (1 of 10)

Y. Li, Prof. F. QinInstitute for Composites Science Innovation (InCSI)School of Material Science and EngineeringZhejiang UniversityHangzhou 310027, ChinaDr. Y. RenInstitute of Metal ResearchChinese Academy of SciencesShenyang 110016, ChinaProf. Y. ZhuiChEM (Collaborative Innovation Center of Chemistry for Energy Materials)University of Science and Technology of ChinaHefei, Anhui 230026, China

3D Graphene Films Enable Simultaneously High Sensitivity and Large Stretchability for Strain Sensors

Fei Pan, Si-Ming Chen, Yuhan Li, Zhuchen Tao, Jianglin Ye, Kun Ni, Han Yu, Bin Xiang, Yibin Ren, Faxiang Qin, Shu-Hong Yu, and Yanwu Zhu*

Integration of 2D membranes into 3D macroscopic structures is essen-tial to overcome the intrinsically low stretchability of graphene for the applications in flexible and wearable electronics. Herein, the synthesis of 3D graphene films (3D-GFs) using chemical vapor deposition (CVD) is reported, in which a porous copper foil (PCF) is chosen as a template in the atmospheric-pressure CVD preparation. When the 3D-GF prepared at 1000 °C (noted as 3D-GF-1000) is transferred onto a polydimethylsiloxane (PDMS) membrane, the obtained 3D-GF-1000/PDMS hybrid film shows an electrical conductivity of 11.6 S cm−1 with good flexibility, indicated by small relative resistance changes (ΔR/R0) of 2.67 and 0.36 under a tensile strain of 50% and a bending radius of 1.6 mm, respectively. When the CVD temperature is reduced to 900 °C (generating a sample noted as 3D-GF-900), the 3D-GF-900/PDMS hybrid film exhibits an excellent strain-sensing performance with a workable strain range of up to 187% and simultaneously a gauge factor of up to ≈1500. The 3D-GF-900/PDMS also shows a remark-able durability in resistance in repeated 5000 stretching-releasing cycles. Kinetics studies show that the response of ΔR/R0 upon strain is related to the graphitization and conductivity of 3D-GF which are sensitive to the CVD preparation temperature.

DOI: 10.1002/adfm.201803221

intrinsically high strength (≈130 GPa), graphene has a brittle attribute,[4,5] indi-cated by an immediate fracture when the applied tensile strain exceeds 1% in the mechanical testing of suspended gra-phene,[6,7] although the strain could be larger than 20% in the zigzag direction of graphene nanoribbons as obtained from simulations.[8–10] To achieve better mechanical robustness, various attempts have been made by coupling graphene with polymers such as polymethylmeth-acrylate[11] and polydimethylsiloxane (PDMS),[12] e.g., by transferring graphene prepared from chemical vapor deposition (CVD). Specifically, the adhesion between PDMS elastomer and graphene allows the efficient transfer of tensile stress to graphene films;[12] the even mechanical loading across the PDMS/graphene inter-face prevents the development of local fractures in graphene when the elas-tomer is stretched in certain range. By relaxing pre-existing winkles, for instance, a film of CVD graphene transferred onto

PDMS was stretched to a strain of 2.4% without much change of electrical resistance.[12] However, an increase in resistance was observed when it was further stretched to 4.5% while the resistance could be entirely recovered when the tensile strain was released.[12] The reversible change of resistance in the strain range of 2.4–4.5% has been explained by the reversible

Strain Sensors

F. Pan, Z. Tao, J. Ye, K. Ni, H. Yu, Prof. B. Xiang, Prof. Y. ZhuKey Laboratory of Materials for Energy ConversionChinese Academy of Sciences & Department of Materials Science and EngineeringUniversity of Science and Technology of ChinaHefei, Anhui 230026, China E-mail: [email protected]. Chen, Prof. S.-H. YuDivision of Nanomaterials and ChemistryHefei National Research Center for Physical Sciences at the MicroscaleCAS Center for Excellence in NanoscienceHefei Science Center of CASCollaborative Innovation Center of Suzhou Nano Science and TechnologyDepartment of ChemistryUniversity of Science and Technology of ChinaHefei, Anhui 230026, China

1. Introduction

As the 2D allotrope of carbon family, graphene exhibits excel-lent electronic and mechanical properties, such as high room-temperature mobility (≈30 000 cm2 V−1 s−1) and high Young’s modulus (≈1 TPa), among others.[1–3] While with

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201803221.

Adv. Funct. Mater. 2018, 1803221

http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadfm.201803221&domain=pdf&date_stamp=2018-08-13

www.afm-journal.dewww.advancedsciencenews.com

1803221 (2 of 10) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

deformation of hexagonal structure in graphene under tensile strain.[13] An irreversible change of resistance was observed when the tensile strain reached 5%, which was caused by the cracking and fragmenting of graphene sheets.[14]

Gauge factor, defined as the ratio of relative resistance change (ΔR/R0) to applied strain (ε), is often used to indi-cate the electric response when the graphene films are used as strain sensors. For the CVD graphene mentioned above,[12] a gauge factor of 151 was obtained in the strain range of 3.2–4.5%. By using woven copper meshes as template, a free-standing graphene woven fabric (GWF) was prepared by CVD and subsequently transferred to PDMS; the GWF/PDMS hybrid film could be stretched by a strain of up to 8% with a reversible resistance change, resulting in a gauge factor of ≈1000 (in the strain range of 2–6%).[15] The reversible resist-ance change observed in the stretching range, which is much larger than that of continuous graphene films,[12] has been contributed to the formation of high-density cracks, which are irreversible in fracturing of structure but reversible in resist-ance due to the re-bridging of cracks.[15] When the strain is larger than 8%, the propagation of cracks leads to the complete fracture of GWF and thus an irreversible resistance change. To further enhance the stretchability, prewrinkled CVD gra-phene was transferred to PDMS for a maximum stretching of 40%,[16–18] in which an increase of 45% in resistance was observed under tensile strain of 30%.[18] While with the pro-ceedings mentioned above, the stretchability of graphene-based strain sensors remains a challenge due to the intrinsically low tensile strain of graphene in two dimensions. Flexible sensors with high stretchability are desired for unprecedented applica-tions, such as wearable health-monitoring patch, robotic sen-sory skin, and tele-surgery electronic glove.[19] For instance, the movements of human joints would generate strains of as high as 55% upon stretching.[20]

On the other hand, the stretching applied on a 3D structure can be decomposed to smaller strains within the local connec-tions and the uniaxial tension may be converted to bending at joints instead of direct stretching.[21,22] Therefore, a 3D network or porous structure potentially provides larger stretchability than a 2D film via bending of network skeletons or rotating of pore walls toward the stretching direction,[23] even both con-sisting of similar structural units. Toward this route, platelets prepared by exfoliating graphite in N-methyl-pyrrolidone were dispersed in lightly cross-linked polysilicone and the obtained hybrid (called “G-Putty”) displayed a gauge factor of 535 under a tensile strain of less than 0.8%.[24] The electromechanical behavior of “G-putty” has been interpreted as breaking of interplatelet connections under strain and the reformation of connections under relaxation. By combining abovementioned graphene platelets with rubber, an ultrahigh strain exceeding 800% was achieved,[25] showing a maximum gauge factor of 35. With better electric conductivity and higher structural integrity, graphene foams prepared by nickel (Ni)-foam-templated CVD have attracted intense attention for stretchable conductors[26,27] and strain sensors.[28] The graphene foam/PDMS hybrids could be stretched to a strain of 95% with a resistance change of 210%, corresponding to a gauge factor of 2.2.[26] With relatively small electrical resistance changes (e.g., 35% under 80% tensile strain for graphene foams-PEDOT:PSS-PDMS hybrid[27]), gra-

phene foam-based hybrids have been mostly used as stretch-able conductors,[26,27] as the gauge factors are not large enough to ensure the high sensitivity as strain sensors. In another work, graphene foams made from Ni foams were fragmen-tized and then integrated with PDMS, resulting in a strain of over 70% and a gauge factor of 15–29.[28] The improved gauge factor is essentially attributed to the effects related to both the deformation of 3D network and the connection change between fragments, as discussed above. Nevertheless, simultaneously achieving high sensitivity and large stretchability remains a challenge for graphene-based strain sensors.

In this work, 3D graphene films (noted as 3D-GFs) are prepared by CVD using a porous copper foil (PCF) as tem-plate. Compared with graphene foams obtained from Ni foams (topically with pore size of 300–500 µm[26]), the 3D-GFs pre-pared in this work contain interconnected pores with much smaller size (typically 1–4 µm). After being transferred to PDMS, the hybrid film made from 3D-GF prepared at 1000 °C (noted as 3D-GF-1000/PDMS) exhibits an electrical conduc-tivity of 11.6 S cm−1 and excellent flexibility, indicated by ΔR/R0 of 2.67 under a tensile strain of 50% and ΔR/R0 of 0.36 for a bending radius of 1.6 mm. In contrast, the hybrid film made from 900 °C preparation (noted as 3D-GF-900/PDMS) shows a high working strain range of up to 187% and a high sensi-tivity with a gauge factor of up to ≈1500 at 100% tensile strain with a stretching rate of 100% s−1. The strain sensor based on 3D-GF-900/PDMS lasts for over 5000 stretching-releasing cycles (under 50% tensile strain), with a small variety of ΔR/R0.

2. Results and Discussion

2.1. Preparation of 3D-GFs and 3D-GFs/PDMS

The fabrication of 3D-GFs/PDMS hybrid is briefly illustrated in Figure 1. The PCF with numerous interconnected pores was prepared by partially removing zinc (Zn) from commer-cial brass foils using vacuum-dealloying method based on sublimation and Kirkendall effect.[29,30] For the preparation of 3D-GFs, a PCF with typical pore size of 1–4 µm (Figure 1a) was treated in an atmospheric-pressure CVD at 900 °C for 10 min or at 1000 °C for 7.5 min in a flow mixture of H2, Ar, and CH4, to make sure a reasonable quality of 3D-GFs and to minimize the melting of pores in the PCF simultaneously. The graphene film prepared shows a similar morphology con-forming the PCF, as shown in the scanning electron micros-copy (SEM) image in Figure 1b. More SEM images in Figure S1 in the Supporting Information show that the morphology of PCFs after CVD at 900 or 1000 °C generally copies that of PCF. The pores are slightly deformed after CVD at 900 °C for 10 min (Figure S1d–f, Supporting Information). The number of pores on the surface shows a significant reduction after CVD at 1000 °C for 7.5 min (Figure S1g–i, Supporting Informa-tion), which may be explained by the partical melting of Cu at 1000 °C.[31] The porosity of PCF is estimated as 18.9% by comparing the density of PCF (≈7.26 g cm−3) to that of com-mercial copper foil (≈8.96 g cm−3).[32] The density is further decreased to ≈6.83 g cm−3 (after treatment at 900 °C for 10 min, corresponding to a porosity of 23.8%) and ≈6.90 g cm−3 (after




treatment at 1000 °C for 7.5 min, corresponding to a porosity of 23.0%), respectively. At the same time, the thickness is reduced to ≈17 µm for both treatment conditions from ≈18 µm of the original PCF. The metallic impurity analysis (Table S1, Supporting Information) performed with inductively coupled plasma atomic emission spectrometry (ICP-AES) shows that the Zn content after treatment is reduced to 20.41 wt% (after treatment at 900 °C for 10 min) or to 11.27 wt% (after treat-ment at 1000 °C for 7.5 min) from 25.03 wt% in the original PCF. Zn deposition was also observed on the wall of quartz tube during the cooling in the CVD. PDMS was infiltrated on one side of CVD-treated PCFs, then thermally solidified in ambience. As shown in the cross-sectional SEM image (Figure 1c), the interface between PDMS and PCF is clearly identified. After the PCF is removed sequentially by 0.5 m FeCl3/HCl aqueous solution mixture and 20% HNO3 aqueous solution, a 3D-GFs/PDMS hybrid is eventually obtained. The SEM image (Figure 1d) taken from the other side without PDMS costing demonstrates the protrusions inversely copying the surface morphology of PCF.

2.2. Characterizations of 3D-GFs

After the PCF is removed, the integrity of 3D-GFs is pre-served and the film without PDMS protection can float on deionized water (Figure 2a). The film transferred onto a quartz plate is transparent (top inset of Figure 2a) and the transmittance (bottom inset of Figure 2a) is about 60.6% at the wavelength of 550 nm for 3D-GF-900. At the same wavelength, 3D-GF-1000 shows a transmittance of 65.5% (Figure S2a, Supporting Information). Based on the fact that the transmittance of an individual graphene layer was measured as ≈2.3%,[33] the average number of graphitic layers can be roughly estimated as 8.6 or 7.5 for 3D-GF-900 or 3D-GF-1000, respectively, considering that both sides of the PCF are covered by 3D-GFs. Like the morphology of

3D-GF-900 on PCF, the SEM image (Figure 2b) of 3D-GF-900 without PCF shows abundant bubbles; the inset in Figure 2b further highlights the protrusions. The SEM image (Figure S2b, Supporting Information) of 3D-GF-1000 shows the similar features. One shall note that the 3D structure of 3D-GFs either transferred on quartz or copper meshes would collapse, leading to much smaller thickness (e.g., ≈ 90 nm for 3D-GF-1000) and overlapped morphology as observed in SEM (Figure S2b, Supporting Information). The transmission electron microscopy (TEM) image of 3D-GF-900 (Figure 2c) further shows the crumpled and interconnected platelets. The high-resolution TEM (HR-TEM) image (Figure 2d) shows that the pores consist of graphitic layers, which are further verified by the selected area electron diffraction (SAED) pattern shown in the upper inset. The number of graphitic layers for 3D-GF-900 mainly falls in the range of 7–11, as shown in the bottom inset of Figure 2d; the layer number is between 6 and 12 for 3D-GF-1000 (bottom inset of Figure S2d, Supporting Information). The Raman G peak at around 1583 cm−1 (Figure 2e) confirms the sp2 carbon hybridization in 3D-GFs prepared at 900 and 1000 °C.[34] The intensity ratio of D peak to G peak (ID/IG) is ≈0.89 for 3D-GF-900 or ≈0.58 for 3D-GF-1000, indicating that more defects are induced at the lower growth temperature, as the D peak (≈1349 cm−1) is typically considered to originate from the defects.[35] The XPS C 1s spectra in Figure 2f show that C 1s peak mainly consists of two components, corresponding to sp2 carbon bonding (284.7 eV) and sp3 carbon bonding (285.5 eV).[36] The sp3/sp2 ratio, which was estimated by inte-grating the area under sp3 or sp2 peaks, is 0.32 or 0.21 for 3D-GF-900 or 3D-GF-1000, respectively. The higher sp3/sp2 ratio of 3D-GF-900 again indicates the lower graphitization degree from CVD performed at the lower temperature.[37,38] Please note that the overall layer number[39] and the content of defects may be also related to the significant amount of Zn in PCFs and the continuous evaporation of Zn during CVD.


Figure 1. Schematic and SEM images of a) PCF, b) 3D-GFs on PCF after CVD, c) 3D-GFs covered with PDMS before PCF is removed, and d) 3D-GFs on PDMS after PCF is removed.



2.3. Electromechanical Performance of 3D-GF-1000/PDMS

The electromechanical property of 3D-GF-1000/PDMS has been investigated in detail by measuring the electrical resist-ance with increasing the uniaxial tensile strain. When the ten-sile strain is increased from 0% to 200% at a stretching rate of 1% s−1, the relative resistance change (ΔR/R0) of 3D-GF-1000/PDMS experiences a monotonic increase (Figure 3a). ΔR/R0 reaches 26.7 for the tensile strain of 200%. During stretching the sample exhibits gray appearance without any apparent cracks observed. As the strain is applied synchroni-cally with the time by a rate of 1% s−1, the curve of ΔR/R0 versus tensile strain ε in Figure 3a has been fitted directly with the Equation (1)

/ ( 1)0R R A eB∆ = × −ε× (1)

where A is 4.662 and B is 0.958.[25,40] The coefficient A is pro-portional to the number of conducting units within a con-ducting path but inversely proportional to the number of conducting paths, and the coefficient B is related to the average tunneling distance and the height of potential barrier between adjacent conducting units.[40] The fitting indicates that the tunneling probability in the conductive network of 3D-GF-1000/PDMS exponentially decreases with the tensile strain applied.[25,40] The ΔR/R0 response of 3D-GF-1000/PDMS was systematically measured by applying cyclic tensile strains from 0 to maximal values of 1%, 5%, 10%, 15%, 20%, 50%, or 100% at different stretching rates; the detailed results are shown in


Figure 2. a) Optical photograph of 3D-GF-900 floating on deionized water after PCF is removed. Insets show the transparent film and UV–vis tramimi-sion spectrum of 3D-GF-900 on a quartz plate. b) Typical SEM image of 3D-GF-900 transferred on a copper mesh. Inset shows a magnified image, highlighting the protrutions. c) TEM image of 3D-GF-900. d) HR-TEM image of 3D-GF-900, showing nine graphitic layers in the region of focus. Insets show the SAED pattern and the distribution of layer number. e) Raman spectra of 3D-GFs on quartz substrates. f) XPS C 1s spectra of 3D-GF-900 and 3D-GF-1000.



Figure 3b and Figure S4 in the Supporting Information. All the measurements reach nearly stable state within the first 20 cycles. For the cyclic testing till a maximum strain of 50%, the ΔR/R0 curve of the 20th cycle coincides well with that of the 11th cycle, which shows a little difference from that of first cycle (inset of Figure 3b). The peak ΔR/R0 value of 2.67 (under the strain of 50%) at the 20th cycle is in the range of previ-ously reported values (e.g., 24 under strain of 40% for wrinkled graphene/PDMS film,[16] 3.1 under strain of 40% for four-layer buckled graphene/PDMS film,[17] and 0.45 under strain of 30% for textured graphene/PDMS film[18]). When the maximum strain is less than 50% (e.g., for 1%, 5%, 10%, 15%, and 20%, shown in Figure S4, Supporting Information), the peak value

of ΔR/R0 gradually decreases with cycling after the first cycle; while for the cycling with a maximum strain of 100%, the peak value of ΔR/R0 slowly increases with cycling. Upon releasing from the maximal tensile strain of 50% or less, the ΔR/R0 could recover to zero or negative, indicating the completely recovered or even reduced resistance. For example, when the 3D-GF-1000/PDMS was subject to the cyclic tensile strain of from 0% to 10%, the ΔR/R0 value of –0.33 was achieved when the strain was totally released (Figure S4c, Supporting Informa-tion). In contrast, after the first 20 cycles of tensile strain from 0% to 100%, the peak value of ΔR/R0 slowly increases from 6.57 to 9.71 while the value under release is 0.56 (Figure S4f, Supporting Information).


Figure 3. a) Response of ΔR/R0 upon stretching of 3D-GF-1000/PDMS at a stretching rate of 1% s−1. Insets show the optical images of the sample under the tensile strain of 0% and 200%, respectively. b) Cyclic ΔR/R0 response of 3D-GF-1000/PDMS in stretching-release cycles under tensile strain varying from 0% to 50% at a stretching rate of 5% s−1. The inset shows the detailed ΔR/R0-strain curves of the 1st, 11th, and 20th cycles, which are marked by red-dotted rectangles. c) Real and d) imaginary parts of the complex permittivity depending on frequency for 3D-GF-1000/PDMS, measured immediately after cyclic strains were applied. e) ΔR/R0 curves of 3D-GF-1000/PDMS under cyclic bending release for the smallest bending radius of 1.6 mm. The left inset shows the optical image of the sample under bending. The right inset shows the detailed ΔR/R0 response of the 1st, 11th, and 20th bending cycles, which are marked by red-dotted rectangles. f) ΔR/R0 curves of the first 100 (of 5000) cycles for 3D-GF-1000/PDMS under tensile strain varying from 0% to 50% and with bending till a radius of 2.2 mm, respectively. The right half shows the results of another 100 cycles after 5000 cycles of stretching or bending.



The cyclic changes of resistance with the strain release could be generally explained by the formation of distortions/cracks under strain and the re-bridging of such cracks or relaxa-tion of distortions when the strain is released, as reported previously.[12,15,41,42] To further understand the evolution of conductive network in 3D-GF-1000/PDMS, high-frequency dielectric spectroscopy (DS) was performed immediately after the certain numbers of cyclic stretching, as DS is sensitive to detect intermolecular interactions and dynamic process.[43] As shown in Figure 3c,d, the real part of permittivity of 3D-GF-1000/PDMS decreases with the cycling number for the strain from 0% to 50%, but the imaginary part increases at the same time. Such results indicate the slightly decreased conductivity in the 3D conductive network with the cyclic stretch release, which may be caused by the accumulated cracks. It has been reported that the transfer of graphene onto PDMS may intro-duce strains such as distortion or buckling.[44,45] On the other hand, the accumulated relaxation of 3D conductive network in such strains and accumulated relaxation of distorted graphene, though tiny for each cycle, may explain the gradual decrease of ΔR/R0 for the maximum strain of 20% or less.[46] For high maximal strains, e.g. 100%, however, the accumulation of cracks in the 3D-GF-1000 may dominate, resulting in the gradually increased ΔR/R0 value.

The ΔR/R0 response of 3D-GF-1000/PDMS upon bending was measured with a home-made two-point bending instru-ment coupled with a high-precision mechanical testing system. A stable ΔR/R0 peak value is also achieved in the first 20 cycles of bending release, for all the bend radii of 10.9, 3.6, 2.2, and 1.6 mm, as shown in Figure 3e and Figure S5 in the Supporting Information. The inset of Figure 3e shows that the ΔR/R0 peak value is ≈0.4 for the bending radius of 1.6 mm in the first cycle and remains 0.36 from the 11th cycle to 20th cycle. The slow decrease in the peak ΔR/R0 value of 3D-GF-1000/PDMS under bending radius of 1.6 mm can also be explained by the accumulative relaxation of 3D conductive network which was induced by cyclic bending. A stable ΔR/R0 of 3.37 is remained after the 3D-GF-1000/PDMS is subject to 5000 cycles of tensile strain from 0% to 50% (Figure 3f). After 5000 cycles of bending release with a bending radius of 2.2 mm, 3D-GF-1000/PDMS shows a stable ΔR/R0 of 0.19, which is close to 0.18 in the first 100 cycles, demonstrating the remarkable stability and dura-bility of 3D-GF-1000/PDMS flexible conductor.

2.4. Electromechanical Performance of 3D-GF-900/PDMS

The ΔR/R0 response of 3D-GF-900/PDMS was measured by applying a uniaxial tensile strain from 0% to 200% at a stretching rate of 1% s−1. As shown in Figure 4a, a mono-tonic increase in ΔR/R0 was observed when the tensile strain increases from 0% to 187%. The ΔR/R0 value reaches 295 for the tensile strain of 187%, much higher than 23.4 from 3D-GF-1000/PDMS under the same strain. For the strains higher than 187% the ΔR/R0 dramatically increases, till ≈640 under the strain of 200% (inset of Figure 4a), suggesting that the conduc-tive network of 3D-GF-900/PDMS is nearly broken in this strain range. In the strain range of 0–160%, fitting the curve with the Equation (1) generates A = 8.863 and B = 1.769. Based on the

previous discussion, the higher A and B values of 3D-GF-900/PDMS, compared with those obtained from 3D-GF-1000/PDMS in the similar stretching range, can be attributed to the fewer conducting paths, the larger tunneling distance and/or the higher potential barrier between conducting units. Figure 4b and Figure S6 in the Supporting Information show the cyclic behavior of 3D-GF-900/PDMS by applying strains from 0 to various maximal of 1%, 5%, 10%, 20%, 50%, and 100% at the same stretching rate of 10% s−1. All cycling reaches nearly stable state after the gradual decrease of ΔR/R peak values in the first 20 cycles. For the maximal strain of 100%, Figure 4b shows that the ΔR/R peak value reaches 204 for the first cycle and stabilizes at ≈123 after 20 cycles. The ΔR/R0 curves of the 1st, 11th, and 20th cycles (inset of Figure 4b) indicate that the ΔR/R0 response has a large hysteresis loop for the first cycle but remains similar from 11th to 20th cycles. The DS study performed immediately after various cycles under the maximal strain of 50% (Figure S7, Supporting Information) shows that both real and imaginary parts of the permittivity of 3D-GF-900/PDMS are lower than those obtained from 3D-GF-1000/PDMS, suggesting that the 3D-GF-900/PDMS is less conductive than 3D-GF-1000/PDMS. The results are consistent to the above-mentioned Raman and XPS measurements, showing that 3D-GF-900 has more defects and lower graphitization degree. These explain the more remarkable ΔR/R0 of 3D-GF-900/PDMS under the same tensile strains. To evaluate the response time of 3D-GF-900/PDMS, rapid step tensile strains of 1% and 5% were applied on the strain sensors under stretching rate of 100% s−1 (≈720 mm min−1). As shown in Figure S8 in the Sup-porting Information, the 3D-GF-900/PDMS strain sensor could rapidly respond to step strains within 50 or 81 ms, for the max-imal strain of 1% or 5%, respectively, indicating an excellent response performance. Figure 4c shows the ΔR/R0 evolution of 3D-GF-900/PDMS under cyclic tensile strains from 0 to 50% at 10% s−1 stretching rate for 5000 cycles. In the consequent 5000 times of stretching-releasing cycles, the ΔR/R0 peak value gradually decreases and stabilizes to 4.95 (5000th cycle; inset of Figure 4c). The detailed ΔR/R0 curves of the 90th–95th cycles coincide well with those of 5065th–5070th cycles. The high sensitivity and large workable strain range enable the 3D-GF-900/PDMS strain sensors to monitor the bending of finger. As shown in Figure 4d, when the extended index finger was repeatedly bent and straightened with a frequency of ≈4 Hz, the ΔR/R0 responds to the motion quickly with strong signals, jus-tifying the ability of 3D-GF-900/PDMS strain sensors to mon-itor the large deformation of human body.

The gauge factor of 3D-GF-900/PDMS has been calculated from the ΔR/R0 versus strain curves (Figure S9, Supporting Information) in the tensile strain range from 0% to 100% at stretching rates of 1, 5, 10, 20, 50, and 100% s−1, respectively; the values are shown in Figure 4e. As can be seen, the gauge factor generally increases with the strain and the stretching rate. For example, at the stretching rate of 10% s−1 the gauge factor increases from ≈30 for the strain of 10% to ≈125 for the strain of 100%. The highest gauge factor reaches ≈1500 for 100% tensile strain at the stretching rate of 100% s−1. Figure 4f compares the gauge factors of 3D-GF-900/PDMS strain sen-sors with those from other graphene-based strain sensors reported recently.[12,15,24,25,28,40,47,48] Among them, the gauge




factor calculated by result of monolayer CVD graphene film on PDMS substrate was 151 (under the tensile strain only from 3.2% to 4.5%).[12] The GWF/PDMS hybrid film based on the CVD growth on woven copper mesh exhibited ultrahigh gauge factors of up to ≈1000 but the workable strain range was lim-ited to less than 8%.[15] The UGF/PDMS hybrid film based on electrochemical exfoliated graphene interface-assembly dem-onstrated a gauge factor of 1037 (under the tensile strain of 2%).[48] On the other hand, the graphene–rubber-composite-based strain sensors showed a large workable strain exceeding 800% while a limited gauge factor of 35.[25] Clearly, the strain

sensor based on 3D-GF-900/PDMS made in this work has shown simultaneously high gauge factors and large workable strains, providing opportunities for detecting strain when the device must be stretched.

The superior electromechanical response of 3D-GFs/PDMS under large tensile strains is closely related to the 3D structure and the temperature-sensitive graphitization degree, leading to the dramatically different ΔR/R0 response and thus distinct applications of 3D-GF-900/PDMS and 3D-GF-1000/PDMS. As shown in Figure S10 in the Supporting Informa-tion, more and wider cracks were observed in SEM when the


Figure 4. a) ΔR/R0 response of 3D-GF-900/PDMS upon tensile strain from 0% to 200% at a stretching rate of 1% s−1. The inset shows details in the strain range of 180–200%. The red line indicates the exponential fitting. b) ΔR/R0 of 3D-GF-900/PDMS under cyclic stretching release with tensile strain from 0% to 100% at a stretching rate of 10% s−1. The inset shows the detailed ΔR/R0–strain curves of the 1st, 11th, and 20th cycles, which are marked by red-dotted rectangles. c) ΔR/R0 of 3D-GF-900/PDMS under cyclic tensile strain from 0% to 50% (at stretching rate of 10% s−1) for 5000 cycles. The inset shows the ΔR/R0 evolution as a function of cycles and the detailed curves in cycles of 90–95 and 5065–5070, respectively. d) Detection of rapid finger bending with a frequency of ≈4 Hz. e) Gauge factor versus strain of 3D-GF-900/PDMS for maximal strain of 100% and stretching rate varying from 1 to 100% s−1. f) Gauge factor comparison of 3D-GF-900/PDMS (at stretching rate of 10 and 100% s−1) with recently reported graphene-based strain sensors, such as G-putty,[24] graphene–rubber composites,[25] monolayer CVD graphene,[12] GWFs,[15] ultrathin graphene films,[48] fragmentized graphene foam,[28] and nanographene films.[40]


1803221 (8 of 10) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2018, 1803221

3D-GF-900/PDMS was stretched to strains of ≈10% and 35%. As schematically shown in Figure 5, the 3D-GFs/PDMS may first experience deformation and/or rotation of carbon scaf-fold under stretching, in addition to the relaxation of crumples in graphitic layers. In contrast to 2D graphene/PDMS hybrid films, in which the strain would be directly transferred to the graphene film from PDMS across the interface, leading to breaking under low strains, e.g., 5%,[12] the strain applied on 3D-GFs would be largely relaxed within the 3D structure (e.g., through bending at joints), explaining the reversible electric response of 3D-GFs under large strains of more than 100%. On the other hand, the stretching-rate-dependent sensitivity of 3D-GF-900/PDMS could also be related to the mechanical properties of PDMS, as molecular dynamics simulations have shown that the fracture strength and strain of graphene varies only slightly with the strain rate.[49,50] As PDMS demon-strates both elastic and viscoelastic behaviors[51,52] when being stretched, the tensile strain, which is mostly applied on the PDMS and then transferred to 3D-GF-900 through the inter-face, shall cause both the elastic and viscoelastic response in PDMS. The elastic response typically features a timing of less than 0.5 s but the viscoelastic response needs a few seconds or longer for relaxation.[53] Thus, for the high stretching rates such as 50% s−1 or 100% s−1, the immediate transfer of quick elastic response would cause large local deformations and high-density cracks, leading to high ΔR/R0 response of 3D-GF-900. On the contrary, the low stretching rates allow more relaxation of PDMS and essentially low local strains transferred to 3D-GF-900, leading to relatively lower ΔR/R0 response. To evaluate the effect of viscoelasticity of PDMS on the sensing performance of 3D-GF-900/PDMS, a tensile stretching of 50% was kept for ≈50 s after 3D-GF-900/PDMS was stretched from 0% to 50% under various stretching rates. As shown in Figure S11 in the

Supporting Information, a stable ΔR/R0 with a small change (from 3.08 to 2.76) in ≈50 s was observed for the low stretching rates, e.g., 1% s−1, while the ΔR/R0 drop (from 6.87 to 4.53) was larger for the stretching rate of 50% s−1. The significant decay of ΔR/R0 under 50% s−1 may be caused by the slow relaxation of PDMS and thus the conducting network of 3D-GF-900. The 3D-GF-900/PDMS strain sensors show high versatility as the sensitivity can be designed to fit various stretching applications.

3. Conclusions

In summary, 3D carbon macrostructures containing intercon-nected graphitic pores have been synthesized by template-directed CVD. The obtained 3D-GFs transferred on flexible PDMS substrate have enabled a reversible change of resistance under large stretching or bending. By tuning the CVD tem-perature, 3D-GFs/PDMS can be used as flexible conductors or highly sensitive strain sensors. For the former, the 3D-GF-1000/PDMS can be cyclically stretched (till 50% tensile strain) with ΔR/R0 of 2.67 and bent (till a radius of 1.6 mm) with ΔR/R0 of 0.36. More impressively, the 3D-GF-900/PDMS has exhibited superior strain-sensing performance, indicated by simultane-ously a large workable strain of up to 187% and a gauge factor of up to ≈1500 (for 100% tensile strain at 100% s−1). The sensor has also shown durability over 5000 cycles (for 50% tensile strain at 10% s−1). The kinetics study shows that the 3D con-ducting networks, which are sensitive to CVD temperature, are responsible to the excellent yet designable strain-sensing per-formance; the electromechanical response of 3D-GFs/PDMS sensors is related to the maximal strain, stretching rate, and interface between 3D-GFs and PDMS. Our work has demon-strated a potentially scalable and versatile technique for future flexible graphene-based electromechanical devices.

4. Experimental SectionFabrication of 3D-GFs and 3D-GFs/PDMS: In a typical experiment,

a PCF with numerous interconnected pores (typical size of 1–4 µm) was ultrasonically washed in 0.3 m HCl aqueous solution for 2 min to remove copper oxide. Then the PCF was ultrasonically washed in a mixture of ethanol and acetone (volume of ≈1:1) for another 30 min to remove organic contamination. A clean PCF in a size of 40 × 20 mm2 was placed in a quartz tube and rapidly (within 2 min) heated to 900 °C (or 1000 °C) in a horizontal tube furnace, followed by annealing for 5 min (or 2.5 min) in a mixture of Ar (100 sccm) and H2 (10 sccm) to eliminate the surface oxide layer. The quick annealing was also to minimize the melting of pores in PCF (annealing at 1000 °C would make the melting faster). After that a small amount of CH4 (5 sccm) was introduced into the quartz tube at ambient pressure for 5 min growth. After growth, the samples were rapidly cooled to room temperature by removing the heating source of furnace in the atmosphere of Ar and H2. To etch the copper, one side of CVD-treated PCF was infiltrated with a viscous mixture of PDMS base/curing agent (the weight ratio 10:1, Sylgard 184, Dow Corning), placed for 12 h at room temperature in the ambience and then thermally solidified at 120 °C for 30 min in the ambience. After the PCF was dissolved in 0.5 m FeCl3/HCl aqueous solution mixture and 20% HNO3 aqueous solution, the obtained sample was washed by deionized water, generating 3D-GFs/PDMS hybrid film.

Characterizations and Electromechanical Measurements: The surface morphologies of PCF, 3D-GFs, and 3D-GFs/PDMS were characterized by SEM (JSM-6700F, Japan). HR-TEM (JEOL-2100F, Japan) was employed

Figure 5. Schematic of the structural change for 3D-GFs/PDMS before and after stretching. The blue substrate represents PDMS. The green arrows show the stretching directions.



to characterize the microstructure and layer numbers of 3D-GFs. Raman spectra of 3D-GFs with or without PDMS substrate were measured with a Raman spectroscopy (Renishaw inVia Raman Micro-scope, 532 nm laser with a power of 10 mW, UK). ICP-AES was used to confirm the metal contents in PCF before and after CVD treatment by using an Optima 7300 DV (PerkinElmer, US). X-ray photoelectron spectroscopy of 3D-GFs on PCF was performed on an XPS facility with Al Kα radiation (hν = 1486.6 eV) (Thermo Scientific, ESCALAB 250, US). UV–vis spectra of 3D-GFs on quartz substrate were taken at room temperature with a UV–visible spectrophotometer (Shimadzu Solid 3700 spectrometer, Japan).

To evaluate the electromechanical performance, the 3D-GFs/PDMS (length 35–40 mm, width 7–10 mm, and thickness 0.7–2 mm) were placed on a computer-controlled, universal mechanical testing machine (Instron 5565 A, US) for stretching and bending test. The effective sample length undergone stretching and bending was in the range of 10–16 mm. The resistance of 3D-GFs/PDMS was measured by a two-probe method using a CHI660D electrochemical workstation (Shanghai, China) at a constant bias voltage of 1 V. The electromechanical stability as a function of stretching and bending cycles were evaluated by repeatedly stretching and bending the hybrid films with a two-point bending device. Dielectric measurements were conducted by vector network analyzer (R&S, ZNB20, Germany) in the frequency range of 8.2–12.4 GHz (waveguide method) immediately after 0, 1, 5, 10, 15, and 20 cycles of tensile loading (Instron 5943, US). The stretching and releasing rate was set as 10% s−1 and the maximum tensile strain was 50%. Before testing 3D-GFs/PDMS hybrid films were cropped into the size of waveguide method (≈22.86 mm × 10.16 mm × 2 mm).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsF.P. and S.-M.C. contributed equally to this work. This work was supported by the China Government 1000 Plan Talent Program, China MOE NCET Program, Natural Science Foundation of China (51772282), and funding from Hefei Center for Physical Science and Technology.

Conflict of InterestThe authors declare no conflict of interest.

Keywords3D graphene films, porous copper foils, sensitivity, strain sensors, stretchability

Received: May 10, 2018Revised: July 1, 2018

Published online:

[1] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183.[2] A. K. Geim, Science 2009, 324, 1530.[3] Y. Hao, M. Bharathi, L. Wang, Y. Liu, H. Chen, S. Nie, X. Wang,

H. Chou, C. Tan, B. Fallahazad, Science 2013, 342, 720.[4] N. Liu, A. Chortos, T. Lei, L. Jin, T. R. Kim, W.-G. Bae, C. Zhu,

S. Wang, R. Pfattner, X. Chen, Sci. Adv. 2017, 3, e1700159.

[5] C. Cao, S. Mukherjee, J. Y. Howe, D. D. Perovic, Y. Sun, C. V. Singh, T. Filleter, Sci. Adv. 2018, 4, eaao7202.

[6] C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 2008, 321, 385.[7] P. Zhang, L. Ma, F. Fan, Z. Zeng, C. Peng, P. E. Loya, Z. Liu,

Y. Gong, J. Zhang, X. Zhang, Nat. Commun. 2014, 5, 4782.[8] H. Zhao, K. Min, N. Aluru, Nano Lett. 2009, 9, 3012.[9] Z. Xu, J.Comput. Theor. Nanosci. 2009, 6, 625.

[10] Q. Lu, W. Gao, R. Huang, Modell. Simul. Mater. Sci. Eng. 2011, 19, 054006.

[11] X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, R. S. Ruoff, Nano Lett. 2009, 9, 4359.

[12] X.-W. Fu, Z.-M. Liao, J.-X. Zhou, Y.-B. Zhou, H.-C. Wu, R. Zhang, G. Jing, J. Xu, X. Wu, W. Guo, Appl. Phys. Lett. 2011, 99, 213107.

[13] M. Topsakal, V. Bagci, S. Ciraci, Phys. Rev. B 2010, 81, 205437.[14] C. Jin, H. Lan, L. Peng, K. Suenaga, S. Iijima, Phys. Rev. Lett. 2009,

102, 205501.[15] X. Li, R. Zhang, W. Yu, K. Wang, J. Wei, D. Wu, A. Cao, Z. Li,

Y. Cheng, Q. Zheng, Sci. Rep. 2012, 2, 00870.[16] T. Chen, Y. Xue, A. K. Roy, L. Dai, ACS Nano 2013, 8, 1039.[17] P. Xu, J. Kang, J.-B. Choi, J. Suhr, J. Yu, F. Li, J.-H. Byun, B.-S. Kim,

T.-W. Chou, ACS Nano 2014, 8, 9437.[18] J.-Y. Hong, W. Kim, D. Choi, J. Kong, H. S. Park, ACS Nano 2016, 10,

9446.[19] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida,

A. Izadi-Najafabadi, D. N. Futaba, K. Hata, Nat. Nanotechnol. 2011, 6, 296.

[20] R. Wang, N. Jiang, J. Su, Q. Yin, Y. Zhang, Z. Liu, H. Lin, F. A. Moura, N. Yuan, S. Roth, Adv. Funct. Mater. 2017, 27, 1702134.

[21] W. Warren, A. Kraynik, J. Appl. Mech. 1988, 55, 341.[22] Y. Yu, J. Zeng, C. Chen, Z. Xie, R. Guo, Z. Liu, X. Zhou, Y. Yang,

Z. Zheng, Adv. Mater. 2014, 26, 810.[23] M. Chen, L. Zhang, S. Duan, S. Jing, H. Jiang, C. Li, Adv. Funct.

Mater. 2014, 24, 7548.[24] C. S. Boland, U. Khan, G. Ryan, S. Barwich, R. Charifou, A. Harvey,

C. Backes, Z. Li, M. S. Ferreira, M. E. Möbius, Science 2016, 354, 1257.

[25] C. S. Boland, U. Khan, C. Backes, A. O’Neill, J. McCauley, S. Duane, R. Shanker, Y. Liu, I. Jurewicz, A. B. Dalton, ACS Nano 2014, 8, 8819.

[26] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.-M. Cheng, Nat. Mater. 2011, 10, 424.

[27] M. Chen, S. Duan, L. Zhang, Z. Wang, C. Li, Chem. Commun. 2015, 51, 3169.

[28] Y. R. Jeong, H. Park, S. W. Jin, S. Y. Hong, S. S. Lee, J. S. Ha, Adv. Funct. Mater. 2015, 25, 4228.

[29] Y. Sun, Y. Ren, Vacuum 2015, 122, 215.[30] Y.-B. Ren, Y.-X. Sun, K. Yang, Acta Metall. Sin. (Engl. Lett.) 2016, 29,

1144.[31] K. Qin, E. Liu, J. Li, J. Kang, C. Shi, C. He, F. He, N. Zhao, Adv.

Energy Mater. 2016, 6, 1600755.[32] Y. Hao, L. Wang, Y. Liu, H. Chen, X. Wang, C. Tan, S. Nie, J. W. Suk,

T. Jiang, T. Liang, Nat. Nanotechnol. 2016, 11, 426.[33] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth,

T. Stauber, N. M. Peres, A. K. Geim, Science 2008, 320, 1308.[34] M. S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus, R. Saito,

Nano Lett. 2010, 10, 751.[35] A. C. Ferrari, Solid State Commun. 2007, 143, 47.[36] C. Tang, W. Guo, C. Chen, J. Appl. Phys. 2010, 108, 026108.[37] F. Banhart, J. Kotakoski, A. V. Krasheninnikov, ACS Nano 2011, 5,

26.[38] A. Eckmann, A. Felten, A. Mishchenko, L. Britnell, R. Krupke,

K. S. Novoselov, C. Casiraghi, Nano Lett. 2012, 12, 3925.[39] W. Liu, H. Li, C. Xu, Y. Khatami, K. Banerjee, Carbon 2011, 49,

4122.[40] J. Zhao, G. Wang, R. Yang, X. Lu, M. Cheng, C. He, G. Xie, J. Meng,

D. Shi, G. Zhang, ACS Nano 2015, 9, 1622.



[41] C. Wang, X. Li, E. Gao, M. Jian, K. Xia, Q. Wang, Z. Xu, T. Ren, Y. Zhang, Adv. Mater. 2016, 28, 6640.

[42] M. Zhang, C. Wang, H. Wang, M. Jian, X. Hao, Y. Zhang, Adv. Funct. Mater. 2017, 27, 1604795.

[43] Y. Feldman, A. Puzenko, Y. Ryabov, Adv. Chem. Phys., John Wiley & Sons, New York 2006.

[44] K. Xu, P. Cao, J. R. Heath, Nano Lett. 2009, 9, 4446.[45] O. Frank, G. Tsoukleri, J. Parthenios, K. Papagelis, I. Riaz, R. Jalil,

K. S. Novoselov, C. Galiotis, ACS Nano 2010, 4, 3131.[46] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni,

I. Jung, E. Tutuc, Science 2009, 324, 1312.[47] Y. Wang, L. Wang, T. Yang, X. Li, X. Zang, M. Zhu, K. Wang, D. Wu,

H. Zhu, Adv. Funct. Mater. 2014, 24, 4666.

[48] X. Li, T. Yang, Y. Yang, J. Zhu, L. Li, F. E. Alam, X. Li, K. Wang, H. Cheng, C. T. Lin, Adv. Funct. Mater. 2016, 26, 1322.

[49] H. Zhao, N. Aluru, J. Appl. Phys. 2010, 108, 064321.[50] L. Yi, Z. Yin, Y. Zhang, T. Chang, Carbon 2013, 51, 373.[51] M. R. VanLandingham, N. K. Chang, P. Drzal, C. C. White,

S. H. Chang, J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 1794.

[52] C. C. White, M. R. VanLandingham, P. Drzal, N. K. Chang, S. H. Chang, J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 1812.

[53] M. Fincan, Assessing Viscoelastic Properties of Polydimethylsiloxane (PDMS) Using Loading and Unloading of the Macroscopic Compres-sion Test. University of South Florida 2015.

3D Graphene Films Enable Simultaneously High Sensitivity...

Documents

Transcript of 3D Graphene Films Enable Simultaneously High Sensitivity...