Pt nanoparticle-decorated reduced graphene oxide hydrogel for high

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Pt nanoparticle-decorated reduced graphene oxide hydrogel for high-performance strain sensor: Tailoring piezoresistive property by controlled micro-structure of hydrogel Sang-Ha Hwang, Young-Bin Park, Seung Hyun Hur, and Han Gi Chae ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00483 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Nano Materials

Pt nanoparticle-decorated reduced graphene oxide hydrogel for high-performance strain sensor: Tailoring piezo-resistive property by controlled micro-structure of hydrogel Sang-Ha Hwang †, ‡, Young-Bin Park ‡,*, Seung-Hyun Hur §,*, and Han Gi Chae †,* †

School of Materials Science and Engineering, Ulsan National Institute of Science and

Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea ‡

School of Mechanical, Aerospace and Nuclear Engineering, Ulsan National Institute of

Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea §

School of Chemical Engineering, University of Ulsan, Daehakro 93, Nam-gu, Ulsan44610,

Republic of Korea

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 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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TABLE OF CONTENTS

KEYWORDS Graphene, Graphene oxide, Hydrogel, Piezo-resistivity, Platinum nanoparticle, Chemical reduction

ABSTRACT A class of conducting reduced graphene oxide hydrogels is prepared with various porosity, surface area, and electrical conductivity using ethylenediamine (EDA)/functionalized graphene oxide (GO) and ascorbic acid (VC)/GO by hydrothermal method. The 2


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microstructure of hydrogels is tailored by changing the composition of materials. It is observed that the conducing network is formed by crosslinked graphene platelets, and the piezo-resistive behavior of hydrogels under cyclic compressive strain shows a linear trend up to 6.8% strain when the hydrogel is prepared by adding 5 wt% of EDA in aqueous GO solution. In addition, platinum nanoparticle (Pt NP) decorated EDA-GO hydrogel was also prepared, showing greatly improved linear strain range as high as 52.8% with an increase in compressive modulus by 873% because of the multi-scale reinforcing mechanism, which is attributed to the strong interaction between Pt NPs and graphene platelets. The piezoresistivity of the hydrogels can be of great interest in the field of high-performance strain sensors.

INTRODUCTION A unique combination of two-dimensional (2D) crystal structure and sp2 hybridized carbon bonding of graphene, it exhibits an excellent mechanical, electrical, thermal, and optical properties, making it suitable in various applications. The in-plane elastic modulus of an individual graphene is predicted to be ~ 1 TPa 1-3, which was confirmed by nano-indentation experiments using atomic force microscopy (AFM) 4. In addition, graphene exhibits a fracture strength of ~ 125 GPa 5, which is well agreed with the theoretical strength of carboncarbon bond 6 . Considering its low density (1–2 g·cm-3), graphene exhibits excellent specific mechanical properties, higher than that of most other structural materials, including aluminum, titanium, and steel. Graphene also has an zero electronic band gap structure, which makes it particularly important in many electronic and energy applications 7. In recent studies, engineering band gap structure of graphene by breaking its symmetry with defect 3



generation 8, water adsorption 9, applied bias

10

, and interaction with various gases

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11

, has

drawn significant attention to utilize it as a semi-conductive material. It should be also noted that graphene exhibits exceptionally high values of in-plane thermal conductivity (~ 5,000 W m-1·K-1) 12, charge carrier mobility (200,000 cm2·V-1·S-1) 13 and specific surface area (2,630 m2·g-1) 14. Such exceptional electrical properties of graphene have drawn significant attention in the field of sensors 15, electrocatalysts 16, supercapacitors 17, light-emmiting diodes (LEDs) and solar cells

18

. However, thermal and electronic properties close to the ideal values can

only be achieved with free-standing or suspended graphene samples and the properties markedly degrade with graphene samples supported on substrates 19-22. In recent years, 3D free-standing architecture made of 2D graphene sheets has played a critical role in several applications. Assembling graphene hydrogel is one of the routes and the resulting 3D architectures clearly exhibited unique characteristics such as electrical conductivity, mechanical integrity, and chemical or catalytic activity23-26. 3D network structures of reduced graphene oxide (rGO) were prepared by hydrothermal process as reported by Shi et al. 27, based on a physical cross-linking mechanism. Other strategies used so far to manufacture hydrogel employ polymers and small organic molecules as crosslinking agents

28-31

. Although several methods for manufacturing graphene hydrogels have

been reported, few studies have focused on their structural properties and methodological approaches for controlling the microstructure, particularly the pore structure. Therefore, it is still a challenge to obtain graphene hydrogel with controlled pore size thereby controlled mechanical and electrical responses. To address the effect of synthesis method and corresponding pore-structure parameters on the electro-mechanical properties of graphene hydrogel, cylindrical-shaped reduced graphene 4


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oxide hydrogels (rGOHs) have been prepared in the current study using ethylenediamine (EDA)–functionalized graphene oxide and ascorbic acid (Vitamin C, VC)–GO by a hydrothermal method. The composition of EDA or VC with respect to GO was adjusted to tailor the pore structure of hydrogels. Thus, prepared hydrogels exhibited sponge-like structure with a variety of porosity, surface area, and electrical conductivity. The hydrogels were also subjected to a compressive strain to assess the structural integrity as well as piezoresistivity of the hydrogels as high as 16.2% strain. In addition, an attempt to integrate Pt nanoparticles (Pt NPs) in rGOH by using chloroplatinic acid and ethylene glycol was made to prepare Pt-rGOH. Microstructure analysis by high resolution electron microscopy showed Pt NPs with a size distribution of about 6.8 nm are uniformly distributed between rGO sheets. The clusters of Pt NPs as large as few micrometers are also observed and such a multi-scale NP distribution in hydrogel played as an effective reinforcement (strong physical network point), showing the improved linear elastic response as high as 52.8% strain as compared to that of rGOH (6.8%). In addition, the excellent piezo-resistivity behavior of Pt-rGOH was also observed. Figure 1 shows the schematic description of various chemical reaction to prepare different types hydrogels and a photograph of EDA-rGOH after crosslinking reaction.

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Figure 1 Schematic description of preparation methods of various graphene-based hydrogels and a photograph of reduced graphene oxide hydrogel by hydrothermal self-assembly.

RESULTS & DISCUSSION The chemical composition and the extent of oxidation of the EDA-rGOHs and the VCrGOH5 were analyzed using XPS. As shown in Figure 2 (a), the N 1s peak is observed at 399.1 eV for EDA-rGOH5 formed by the reaction between EDA and GO, which cannot be 6


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observed with VC-rGOH5. The presence of a single N 1s peak in the sample suggests that nearly all the nitrogen atoms exist in the form of C-NH-C, confirming the reaction between the amine group of EDA and the epoxy or carboxyl acid group of GO. In the case of PtrGOH, a peak at 70.24 eV corresponding to Pt 4f and a peak at 73.89 eV that can be assigned to 4f7/2 and 4f5/2 states of Pt are observed. As shown in Figure 2 (b), VC-rGOH5 exhibits five characteristic C 1s peaks at 284.5, 285.5, 286.7, 287.9, and 289.1 eV, which correspond to sp2 C, sp3 C, C-O (epoxy and alkoxy), C=O (carbonyl), and –O–C=O (carboxylic acid), respectively

32-34

. Both the EDA-rGOH5 and VC-rGOH5 samples exhibit significant

reduction in the number of oxygen related functional groups. However, EDA-rGOH5 possesses more sp2 C than VC-rGOH5, indicating an improved reduction of the functional groups and a higher restoration of C=C in GO by EDA (Figure 2 (c))

35, 36

. A peak

corresponding to Pt 4f is observed in Pt-rGOH (Figure 2 (d)). XPS signals were deconvoluted to obtain the corresponding peaks for Pt metal, Pt (II), and Pt (IV) species, which indicates the presence of Pt0 along with small amounts of Pt2+ and Pt4+ species. Pt 4f7/2 and Pt 4f5/2 peaks are also observed at 70.24 eV and 73.89 eV, respectively, which are slightly shifted to lower binding energy values as compared with the standard binding energy values of Pt 4f7/2 and Pt 4f5/2 for Pt0 state (70.83 eV and 74.23 eV). This could be attributed to the electron transfer from the graphene sheet to the Pt NPs. Because the work function of graphene (4.48 eV) is smaller than that of Pt (5.65 eV), the electron transfer from the graphene sheets to Pt NPs could occur during the formation of Pt-graphene hybrid structures 37.

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Figure 2 (a) XPS survey data of VC-rGOH5, EDA-rGOH5, and Pt-rGOH. C 1s XPS spectra of (b) VC-rGOH5, (c) EDA-rGOH5, and (d) Pt 4f XPS spectrum was observed from PtrGOH.

Figure S1 (a) is Raman spectra of various samples, showing that two peaks are observed around 1358 cm-1 (D band) and 1595 cm-1 (G band) which are attributed to the defective and graphitic structures of graphene, respectively. Generally, The intensity ratio of the D and G bands (ID/IG) is inversely proportional to the average crystallite size in graphite materials, so the increase in ID/IG intensity ratio after reduction indicated a decrease in the average size of the graphitic domains, which is caused by the re-graphitized small sp2 domains 38. Due to the selective reduction of GO with low temperature (80°C), the small graphitic domains in rGOHs are created merely by reduction of the epoxy groups. As a result, the ID/IG intensity 8


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ratio of rGOHs (1.36-1.38) is lower than that of GO (0.9). The similar phenomena are also found in Raman spectra of reduced graphene oxides reported by Song et al.39 and Xu et al.40 . The interlayer spacing in each sample was investigated using XRD as shown in Figure S1 (b). The characteristic diffraction peaks are observed at 2θ = 26.3° for graphite, 9.78° for GO, 25.0° for EDA-rGOH5, and 25.9° for VC-rGOH5, corresponding to the interlayer spacing of 3.34 Å, 9.04 Å, 3.56 Å, and 3.44 Å, respectively. The increase in the interlayer spacing of GO after the oxidation can be attributed to the presence of enriched functional groups and captured water molecules

41

. EDA-rGOH5 exhibits a slightly larger interlayer spacing than

that of VC-rGOH5, which could be attributed to the EDA linkages present between the GO sheets after performing the crosslinking reaction between GO and EDA

34, 36

. Pt-rGOH

exhibits characteristic peaks of a face-centered cubic (fcc) Pt lattice. Diffraction peaks are observed at 39.9°, 46.3°, 67.7°, and 81.4° corresponding to Pt(111), Pt(200), and Pt(311), respectively, confirming the reduction of the Pt precursor, H2PtCl6 to Pt. Additionally, the Scherrer equation yields an average crystallite size of 6.8 nm for Pt (normal to Pt(220)) on graphene, which is in agreement with the results obtained using TEMs (Figure 3 (e), (f)). With the addition of Pt NPs, the C(002) peak, which indicates the interlayer spacing between the graphene sheets in rGOH, widens and varies owing to the Pt hybridization. The pore structure of rGOHs was analyzed using the BET apparatus. As shown in Figure S2 (a), all the samples exhibit distinct H2 type adsorption and desorption isotherms, indicating the formation of a mesoporous structure as a result of the crosslinking reaction in EDArGOHs and the restored π-π interactions between the rGO sheets in VC-rGOH5 42. The BET surface area and the pore volume increase as the GO concentration increases when EDA is used. EDA-rGOH15 exhibits the highest surface area of 745 m2·g-1 among all the GO hydrogel samples prepared in this study. The bulk density of VC-rGOH5 is higher than that 9



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of EDA-rGOHs owing to the influence of the interlayer spacing and the pore size. In the case of Pt-rGOH, the formation of Pt NPs on the graphene surface resulted in a more densely packed microstructure having a decreased pore size as compared to the EDA-rGOHs (pore size decreased from 68 nm to 46 nm). Based on the XRD results, the more intercalated and a wide range of interlayer spacings are observed for Pt-rGOH. Therefore, the decreased pore size and the densely packed microstructure suggest that the Pt NPs are formed effectively on the defect region of graphene and anchored between two or more graphene layers. This causes the graphene layers to become more closely packed rather than synthesized (reduced) between fully restacked sp2 region to widen them. This is confirmed by the Pt 4f XPS spectrum as shown in Figure 2 (d). Furthermore, the oxygen-containing functional groups are known to provide binding sites for anchoring precursor metal ions or metal NPs 43, 44. Overall Pt NP content measured by thermogravimetric analysis (TGA) was 22% by weight (Figure S2 (b)).

Table 1 BET surface area, pore sizes and bulk densities of rGOHs SAMPLE BET surface area (m2·g-1) Pore size (nm) Bulk density (mg·mL-1)

VC-rGOH5

EDA-rGOH5

EDA-rGOH-10

EDA-rGOH-15

Pt-rGOH

285

174

293

745

198

46

68

54

47

48

20.44

17.52

19.02

19.9

29.43.

SEM images of EDA-rGOH5, VC-rGOH5, and Pt-rGOH are shown in Figures 3 (a–d). EDArGOH5 exhibits larger pore and particle sizes as compared with that of VC-rGOH5. This observation is in good agreement with the BET results summarized in Table 1. SEM images 10


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shown in Figure 3 (c) exhibits the presence of well-distributed Pt particles across the fracture surface of rGOH, and the less pore structure as the Pt NPs are formed. However, it should be also noted that the Pt particle size is in the range of 1–1.5 µm as shown in Figure 3 (d), which is inconsistent with the XRD results (6.8 nm). High resolution TEM images are shown in Figures 3 (e, f), exhibiting the individual Pt NPs whose size is well agreed with the calculated crystal size based on the XRD results and the clustering behavior of Pt NPs as well. The HRTEM image given in Figure 3 (f) shows clearly visible lattice fringes having a spacing of 0.23 nm, which matches with the spacing of Pt(111) planes obtained from the XRD results. The spectroscopic and microscopic results obtained for Pt-rGOH suggest that the formation of Pt NPs and clusters mainly occur in the defect rich areas. This may cause the unstacked region in rGOH to reassemble and restack (Figure 3 (g)) owing to the strong interaction between graphene and Pt (known as chelation). .

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Figure 3 SEM images of (a) EDA-rGOH5, (b) VC-rGOH5, and (c, d) Pt-rGOH. Bright field TEM image of (e) Pt NPs and (f) Pt NP cluster. (d) SEM images of Pt cluster on graphene layers. (g) Schematic description of formation of Pt NPs and Pt NP clusters between the graphene layers.

Piezo-resistive performance of the various hydrogel specimens under static and cyclic compressive strains was measured. The resistance change due to the compression-induced densification of porous materials indicates a unique piezo-resistive behavior that depends on the pore size and the bulk density. EDA-rGOH5 and VC-rGOH5 subjected to various cyclic compression frequencies at an amplitude of 2.5% strain exhibit a well-defined piezo-resistive 12


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behavior (Figure 4). Under a slow (6.5% min-1) cyclic compressive strain, both VC-rGOH5 and EDA-rGOH5 exhibit similar changes in resistance and no retardation effect is observed (Figure 4 (a)), while the EDA-rGOH5 gives almost two times higher sensitivity as compared to the VC-rGOH5. However, as the frequency increases (13 % min-1 and 31.25 % min-1), the phase difference of EDA-rGOH5 increases from 0.8 sec for 13% min-1 (Figure 4 (b)) to 1.4 sec for 31.25% min-1 (Figure 4 (c)), whereas the resistance change of VC-rGOH5 is consistent without retardation effect due to the fact that VC-rGOH5 has smaller pore structure and high bulk density, resulting in a more elastic behavior. The structure-related piezo-resistive response is also assessed by varying cross-linking density of the hydrogels. As noted in the experimental section, the concentration of GO with respect to the EDA content, leading to EDA-rGOH5, EDA-rGOH10, and EDA-rGOH15. With increasing GO content, the smaller pore structure and denser hydrogel was formed as listed in Table 1. As a result, one can expect that EDA-rGOH15 may exhibit more elastic response than other EDA-based hydrogels. Figure 5 shows the piezo-resistive responses of EDA-based hydrogels under compressive strain with an amplitude of 2.5% at various cyclic frequencies. At low frequency (6.5 % min-1), all the EDA-based hydrogels showed no noticeable phase difference with a slight difference in sensitivity. At high frequency (31.25 % min-1), unlike EDA-rGOH5 specimen, other EDA-based hydrogels showed dramatically decreased time lag (0.4 and 0.1 sec for EDA-rGOH10 and EDA-rGOH15, respectively), confirming that structural uniformity (higher density and narrow pore size distribution) and strong interparticle interaction lead to more linear and stable piezoresisvie behavior. It can also be concluded that the piezo-resistive sensitivity of hydrogel can be tailored by altering microstructure of hydrogel. The highly elastic and linear piezo-resistive behavior of Pt-rGOH is also observed (Figure 6). Although there is no retardation nor drift effect, less sensitivity (∆R/R0~0.1 at a compressive strain of

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2.5%) is observed due to the densely populated conductive networks with the strong interaction between conductive particles.

Figure 4 Cyclic compressive strain induced resistance change of EDA-rGOH5 and VCrGOH5 at strain frequencies of (a) 6.5 % min-1, (b) 13 % min-1, and (c) 31.25 % min-1. 14


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Figure 5 Cyclic compressive strain induced resistance changes of EDA-rGOH5, EDArGOH-10, and EDA-rGOH-15 at strain frequencies of (a) 6.5 % min-1 and (b) 31.5 % min-1.

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Figure 6 Cyclic compressive strain induced resistance changes of Pt-rGOH at strain frequencies of (a) 6.5 % min-1, (b) 13 % min-1, and (c) 31.25 % min-1.

A static compressive strain with a strain rate of 6.5%-1 min was applied to VC-rGOH5, EDArGOHs, and Pt-rGOH to monitor the compressive stress as well as the resistance changes. The test was stopped when the rGOH samples were physically damaged. As shown in Figure 16


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7 (a), VC-rGOH5 exhibits a higher compressive modulus and larger resistance change as compared to that of EDA-rGOH5 due to the differences in microstructure as discussed earlier. Since the pore structure is collapsing under compressive strain, their stress increment at low strain level is not significant. However, after complete collapse of pore structure at high strain level, stress increment is noticeably higher than the low strain level. Because of this stepwise strain-hardening behavior of porous structure, their piezo-resistive behavior showed similar tendency. The initial resistance changes observed under compressive strain strongly reflect the initial structural change occurred by pore collapsing. Therefore, lowest bulk density (porosity) of rGOH. EDA-rGOH5 exhibits the highest sensitivity (gauge factor) of 7.98. EDA-rGOH10 and EDA-rGOH15 showed similar values (around 4.9) of initial gauge factor (Figure 7 (b)), which is similar to that of VC-rGOH (4.87). Hence, EDA-rGOH10, EDArGOH15, and VC-rGOH showed similar initial piezo-resistive behaviors owing to the similar porosity regardless of the differences in the surface area and the pore size. In the case of PtrGOH as shown in Figure 7 (c), the resistance changes more linearly as compared to that of VC-rGOH5 or EDA-rGOHs. Although Pt-rGOH exhibits the lowest gauge factor (3.99) among all the rGOHs, it exhibits a drastic increase in the measurable strain range of 52.8 % without any physical damage and a unique elastic height and resistivity recovery are achieved when the compression is removed. Pt-rGOH exhibits the highest compressive modulus of 13.5 MPa, which is nearly 873% higher than that of EDA-rGOH5 (Figure 7 (d)). This extraordinary reinforcing efficiency of Pt NPs on rGOH is because of the unique and effective cluster formation as indicated in Figure 3 (g).

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Figure 7 Piezo-resistive behavior (straight line) and s-s curve (dashed line) of (a) EDArGOH5 and VC-rGOH5, (b) EDA-rGOH10 and EDA-rGOH15, and (c) Pt-rGOH under static compressive strain with a strain rate of 6.5 cm min-1. (d) Compressive modulus of rGOHs.

CONCLUSIONS A series of hydrogels with various pore structures was prepared using either EDA- or VCfunctionalized graphene oxides and assembled together by the hydrothermal reduction. By further treatment of EDA-rGOH5, Pt NPs were hybridized on graphene sheets in rGOH. XPS and XRD results on Pt-rGOH indicated that the Pt NPs were mainly formed on the defectrich, unstacked region of graphene. BET and SEM results confirmed the microstructure of 18


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rGOHs, which could be tailored by changing the GO concentration, chemical crosslinking agent, or by adding of Pt NPs. The densification mechanism of Pt NPs on rGOH was affected by the presence of Pt NP clusters. Mechanical properties of the samples were examined under cyclic and static compressive strains, exhibiting the piezo-resistive behavior of rGOHs. It is also observed that the porous structure of rGOHs naturally led to the strong piezo-resistive behavior. Because of the large pore size and the high bulk density, EDA-rGOH5 exhibited the highest time lag of 1.4 sec under cyclic compressive strain (non-linear piezo-resistive behavior); however, the highest gauge factor of 7.98 at the initial region under static compressive strain was obtained. With increasing concentration of EDA (EDA-rGOH10 and EDA-rGOH15), the resulting hydrogels showed lower sensitivity but improved linearity in piezo-resistivity. As compared to the EDA-rGOH5, the VC-rGOH5 showed decreased pore size and increased density. As a result, it showed linear piezo-resistivity, which is similar to that of EDA-rGOH15. In addition, upon the successful assembly of Pt NPs in hydrogel, PtrGOH exhibited significantly improved performance in terms of maximum measurable strain range (52.8%) with linear elastic response (improved secant modulus by 873%) that can be explained by the effective hybridization and reinforcing mechanism of Pt NPs and their clusters. These findings provide insights into the potential use of well-defined porous functional nanomaterials for highly stable, piezo-resistive sensor exhibiting controllable sensitivity.

METHODS Preparation of GOs. GO was prepared from expanded graphite using the modified Hummer’s method described elsewhere

45

. Initially, 500 mL of concentrated sulfuric acid 19



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(H2SO4) was added into a beaker under stirring with a Teflon impeller to which 5 g of graphite was slowly added at 0 °C. Then, 30 g of potassium permanganate (KMnO4) was slowly added while maintaining the temperature below 20 °C using an ice bath, and the suspension was stirred for 2 h at 35 °C to oxidize and exfoliate graphite to form GO. In order to complete the reaction, 6 L of deionized water was slowly added for 1 h followed by the slow addition of 50 mL hydrogen peroxide (H2O2, 30 wt%). Vigorous bubbles were formed accompanied by a color change from dark brown to yellow, indicating the quenching of KMnO4. The suspension was treated with 10% HCl solution to remove the metal ions present, followed by washing with deionized water to completely remove the acids, until the pH of the GO suspension reached 7. The concentration of the GO solution was adjusted to 15 mg mL-1 in water for further experiment.

Preparation of rGOHs. 1 mL of an aqueous solution of EDA was slowly added to 10 mL of GO suspension having concentrations of 5, 10, or 15 mg mL-1, and the mixture was ultrasonicated for 2 h at 10 °C. The hydrogel formation reaction through the crosslinking between GO and EDA was achieved for 8 h at 80 °C. The obtained GOH was added to 2 L deionized water for 1 h under stirring in order to remove the unreacted EDA, and this process was repeated 5 times. All the samples were freeze-dried at - 37 °C for 2 days. The prepared samples were coded by the corresponding GO concentrations used (EDA-rGOH5, EDArGOH10, and EDA-rGOH15 indicate samples obtained using 5, 10, and 15 mg mL-1 of GO solutions, respectively). To prepare rGOH with VC, 10 mg of VC was dispersed in 10 mL of the GO solution with a GO concentration of 5 mg mL-1 followed by the reaction for 24 h at 80 °C (sample code as VC-rGOH5) while the washing and freeze-drying steps were same as those of EDA-rGOHs.

20


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Preparation of Pt/reduced graphene oxide hydrogels (Pt-rGOH). The Pt-rGOH was prepared by submerging EDA-rGOH5 in a mixed solution of 200 mL deionized (DI) water and 150 mg chloroplatinic acid hydrate, which was maintained at 50 °C for 24 h under mild stirring. Then, 3 mL of EG and 5 mL of 1M NaOH were added (the pH of solution was adjusted to 11). The reduction of chloroplatinic acid to Pt using EG was conducted at 70 °C for 24 h. The color of rGOH was changed from black to gray due to the formation of Pt NPs on the rGOH surface. The final product was obtained after washing with DI water and freeze drying at - 37 °C for 2 days.

Characterization. The crystal structure of the sample was characterized using X-ray diffraction (XRD, Rigaku, D/MAZX 2500V/PC) using Cu-Kα radiation (35 kV, 20 mA, λ = 1.5418 Å) at a scan rate of 2° (2θ) per minute. X-ray photoelectron spectroscopy (XPS, KAlpha, ThermoFisher Scientific ESCALAB 250Xi) measurements were performed using an Al-Kα X-ray source (1486.6 eV), and Raman spectra were collected from 200 to 3,500 cm-1 using a DXR Raman microscope (Thermo Scientific) with 633 nm laser source. The bulk structures of the composites were analyzed using cross-sectional images obtained using a field emission-scanning electron microscope (FE-SEM, JOEL JSM-6500FE). The specific surface area was measured with a surface area and porosity analyzer (Micromeritics, ASAP 2020) and was calculated using the Brunauer-Emmett-Teller (BET) equation. The electrical response of graphene hydrogel samples under compressive strain were measured using an universal materials testing system as shown in Figure 8. Before the test, a pair of electrodes was attached to each side of rGOHs using silver paste and epoxy adhesive (Figure 8 (a)). A simple schematic of the experimental set-up used for performing electro-mechanical measurements of rGOH using a digital multimeter while applying compressive strain is also shown in Figure 8 (b). 21



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Figure 8 (a) Schematic for the sample preparation for conducting electro-mechanical property analysis. (b) Schematic shows the experimental set-up used for the in-situ measurement of electrical conductivity with the application of a compressive load.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected], [email protected], [email protected]

ACKNOWLEDGEMENT This study was supported by the Basic Science Research Program (Mid-career Research Program) through the National Research Foundation of Korea (NRF) funded by the Ministry of

Science

and

ICT,

Korea

(Grant

Nos.

NRF-2016R1A2B4015335,

NRF-

2016R1A2B2006311, NRF-2016R1A2B4007452).

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SUPPORTING INFORMATION AVAILABLE: Raman spectrum, Wide angle X-ray diffraction patterns, Nitrogen adsorption/desorption isotherms, and, Thermogravimetric analysis result are available.

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Pt nanoparticle-decorated reduced graphene oxide hydrogel for high

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