Crosslinking Stabilizes Electrical Resistance of Reduced Graphene

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Crosslinking Stabilizes Electrical Resistance of Reduced Graphene Oxide in Humid Environment Yiqian Jin, and Woo Lee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03416 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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Crosslinking Stabilizes Electrical Resistance of Reduced Graphene Oxide in Humid Environment Yiqian Jin and Woo Lee Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey, 07030, the United States of America KEYWORDS: reduced graphene oxide, sensors, crosslinking, ethylenediamine, interactions with water ABSTRACT: Reduced graphene oxide (rGO) is an excellent candidate for many sensor applications since its electrical properties can be tailored to become sensitive to temperature, humidity, strain, and chemicals. However, the wide use of rGO may be limited by its susceptibility to humidity changes. Here we report experimental evidence for the first time that: (1) the interlayer between rGO sheets can swell upon exposure to humid environments due to the intercalation of water and (2) the expanded interlayer spacing increases electrical resistance. As a novel means of mitigating this instability, ethylenediamine (EDA) was used as a covalent crosslinker to anchor rGO sheets to limit interlayer expansion and stabilize electrical resistance under humid environments. INTRODUCTION Graphene is a monolayer of carbon atoms which are tightly packed into a two-dimensional honeycomb lattice, and has received significant attraction since its first fabrication from

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graphite by mechanical exfoliation.1 It is also a basic building block for graphitic materials of all other dimensionalities,2–5 which have been investigated in various fields. For example, graphene can be functionalized to provide a new class of 2D materials with exceptional promise for physical, chemical, and biological sensors.6–10 Graphene oxide (GO) has a similar layered structure, but the carbon atoms in the basal GO plane are heavily decorated by oxygen-containing groups. The functional groups enlarge the interlayer distance between GO sheets, and make GO water-soluble and electrically non-conductive.11–13 By thermal, chemical, or photo-catalyst reduction, the 2D interlayer spacing and the electrical conductivity of GO can be restored to a certain extent with most oxygen-containing groups removed.14–17 It has also been found that reduced graphene oxide (rGO) is an excellent candidate for many sensor applications, since it is sensitive to temperature,18–20 humidity,21–24 strain,25–28 gases,29–33 and other stimuli. We previously found34 that rGO behaves as a negative temperature coefficient (NTC) material with its electrical resistance decreasing exponentially with temperature. The NTC behavior could be characterized using the following equation:

R  R0 exp( 

T0  T ) T0  T

(1)

where R is the resistance as a function of temperature (T), R0 is the resistance at the reference temperature (T0=298 K), and β is the value that represents the temperature sensitivity of rGO. The higher the β value is, the more sensitive the rGO sensors are. We determined35 that the structural defects, such as etched areas within rGO planes, can function as thermally activated electron traps to significantly influence the NTC behavior of rGO. We also exploited34 the NTC behavior of rGO to demonstrate very conformal (>1,000 mechanical bending cycles), precise (°C), responsive (~0.1 s), and miniaturizable (10 nm thick and 50 m wide) temperature sensors. Furthermore, we recently integrated36 rGO into textile filaments to 2


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rGO sheet (a) e

rGO

H2O molecule (b)

Hopping effect

EDA molecule rGO Crosslinked with EDA

e

Electron Water absorption

Water absorption Interlayer Expansion

Mitigated Expansion

Figure 1. Illustrations of hypotheses: (a) H2O absorption into the interlayer space between rGO sheets increases d-spacing and decreases electrical conductivity and (b) covalent crosslinking of rGO sheet can reduce water absorption and mitigate expansion.

generate a sensor array that can be used for spatiotemporal mapping of skin temperatures in a non-invasive, wearable, and cost effective manner. Despite these distinctive advantages, we observed that consistent measurements with rGO sensors were susceptible to daily humidity changes. Typically, the resistance and β value of rGO sensors increased about 50% after they were exposed in an ambient environment for 2 weeks. These changes could be significantly accelerated with increased relative humidity (RH). As illustrated in Figure 1a, we hypothesize that: (1) the interlayer space between rGO sheets can be intercalated by water and expanded, (2) the expanded interlayer space requires more energy for electrical carriers to jump over, and (3) this swelling effect is the main cause of the apparent instability of rGO sensors associated with RH changes. Several recent theoretical studies37,38 support our hypothesis since their simulation results suggest that water molecules can interact with GO, in the presence of oxygen, to form new functional groups and make the 2D structure of GO more disordered. Also, it has been suggested that, if a GO structure absorbs 26 wt% water, its d-spacing is expected to increase 3


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from 0.51 to 0.90 nm.39 Similarly, we expect that the small size of water molecules (0.23 nm) enables them to infiltrate into the d-spacing of rGO (~0.35 nm) and cause the swelling. As the electrical carrier transmission in rGO requires interlayer hopping, the transmission is expected to become more difficult with larger d-spacing of rGO caused by water absorption and swelling. We also postulate that the instability can be controlled by covalently crosslinking rGO with organic molecules to mitigate the expansion of the interlayer space and therefore limit water absorption (Figure 1b), as a novel mechanism of stabilizing the electrical properties of rGO sensors in ambient environments. Here we studied EDA as a crosslinker to stabilize the rGO’s interlayer spacing and electrical resistance. EDA is a short and strongly alkaline alkyldiamine, which can react with carboxylic groups and form amide functional groups in a neutral solution. It has been widely used to build blocks in chemical synthesis40–42 and to modify the interface of GO.43,44 In order to ascertain these hypotheses, this study was aimed at investigating: (1) the effects of RH and oxygen concentrations on the NTC behavior and the average d-spacing of rGO, (2) the effects of adding varying amounts of EDA on the development of crosslinked rGO/EDA structures, and (3) the effectiveness of the crosslinking strategy in stabilizing the interlayer structure and NTC behavior of rGO/EDA upon exposure to humid environments. EXPERIMENTAL Materials: The commercial available high-purity aqueous solution of GO (2 mg/ml, purity > 99%) was supplied by CheapTubes. The thickness of GO was measured to be 0.7-1.2 nm with the diameter of 300-800 nm. The polyimide film (Kapton HN 200) was purchased from DuPont and used as the substrate of rGO sensors. EDA (≥99.5%), NaCl (anhydrous, ACS Reagent, ≥99.0%), Mg(NO3)2 (BioUltra, ≥99.0%), and P2O5 (powder, ACS Reagent, ≥98.0) 4


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(a) GO

EDA/GO Inkjet printhead

(c)

(b)

Mix Polyimide Inkjet printing & drying

(EDA/GO)0

r(EDA/GO)0

(EDA/GO)0.0002

r(EDA/GO)0.0002

(EDA/GO)0.002

r(EDA/GO)0.002

(EDA/GO)0.02

r(EDA/GO)0.02

(EDA/GO)2

r(EDA/GO)2

EDA/GO Thermal reduction r(EDA/GO) Inkjet printing Ag Ag r(EDA/GO) sensor

Figure 2. (a) Illustration of procedures to fabricate an r(EDA/GO) sensor. Optical images of samples produced at various EDA/GO ratios: (b) EDA/GO and (c) r(EDA/GO).

were purchased from Sigma-Aldrich. The silver ink, Metalon JS-B40G, was provided by NovaCentrix. Preparation of rGO and r(EDA/GO) sensors: The rGO thin-film sensors were inkjet-printed onto polyimide substrates containing silver interdigitated footprints (Novacentrix JS-B40G) using the previously established procedures.45 In brief, polyimide substrates were rinsed, prior to printing, with deionized water and isopropyl alcohol and treated with oxygen plasma for 10 minutes using a plasma cleaner (Harrick Plasma) to make the polyimide films hydrophilic. The GO solution was printed on polyimide films using a Dimatrix FujiFilm inkjet printer (DMP-2831). This printer was equipped with a printhead/cartridge configuration consisting of 16 micro-fabricated piezoelectric nozzles with each nozzle programmable and addressable with 20 μm positioning resolution. Cartridge height and substrate temperature were maintained at 0.5 mm and 25°C, respectively. After inkjet printing, the GO samples were reduced at 220°C for 6 hours to produce rGO sensors. The electrodes for resistance measurements on rGO were formed by inkjet printing silver on rGO patterns and then treated at 220°C for 20 minutes. 5


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As illustrated in Figure 2a, r(EDA/GO) sensors were produced by inkjet-printing of EDA/GO suspended in water and subsequent thermal reduction at 220°C in a furnace for 6 hours. The GO solution (2 mg/ml) was mixed with 0, 0.4, 4, 40, or 4000 μg/ml EDA to create EDA/GO solutions that are denoted as: (EDA/GO)0, (EDA/GO)0.0002, (EDA/GO)0.002, (EDA/GO)0.02, and (EDA/GO)2, respectively. The GO/EDA inks were printed on polyimide substrates as a 2 mm×10 mm pattern using a cartridge that generated 10 pL ink droplets. After the EDA/GO inks were dried, the printed sensors were thermally reduced at 220°C for 6 hours. The optical images of the EDA/GO and r(EDA/GO) samples are shown in Figures 2b and 2c, respectively. Fourier Transform Infra-Red (FTIR) spectroscopy: The GO and GO/EDA solutions were dried on clean glass slides to form thin films. The films were peeled off from the glass slides by razor blades, mixed with KBr powders (dried at 120°C) in a mortar, and then compressed into transparent slices for FTIR characterization. FTIR measurements were performed on a Bruker Tensor 27 FTIR spectrometer with a resolution of 4 cm-1 and 128 scans per sample. Ultraviolet-visible spectroscopy (UV-vis): Optical properties of the rGO and r(GO/EDA) samples were characterized by UV-vis spectroscopy. The scan range and the rate of transmission were 250-800 nm and 100 nm/min, respectively. To avoid measurement errors at high absorbance, the concentration of the characterized solutions was diluted to 1/10 of the solutions used for inkjet printing. The samples were prepared on quartz slides to avoid the noise of absorbance from visible light to infrared light. The direct optical band gap (Eg) of the r(EDA/GO) samples was obtained using Tauc’s relation:46

ahv  B(hv  E g ) m

6


(2)

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where α is the absorption coefficient, B is a constant, hv is the photon energy, and m=1/2 for direct allowed transitions. By plotting (ahv)2 vs. hv and extending the linear part and getting an intersection pointer on X-axis, the direct band gap was acquired. Thermal gravimetric analysis (TGA): The EDA/GO samples were prepared using the same procedures described for the FTIR samples. Weight loss of GO/EDA during thermal reduction was determined using a TGA instrument (Q50 TGA, TA Instruments) operated under a constant flow of nitrogen at 20 mL/min at a heating rate of 20°C/min from 70°C to 300°C. The heating process stayed at: (1) 120°C for 10 minutes to remove excess water and EDA and (2) 220°C for 10 minutes to complete thermal reduction. Scanning electron microscopy (SEM): The surface morphology of r(EDA/GO) films was examined by SEM (Zeiss Auriga Small Dual-Beam FIB-SEM microscope). Captured micrographs were processed using the image processing software (ImageJ). Environmental Exposures: A commercial incubator (Heracell vios 160i Tri-gas incubator, Thermo Fisher) was used to simulate physiologically relevant environments at the skin-air interface. The incubator was equipped with oxygen sensors and gas controllers to control the concentration of oxygen in the range of 1 to 21% and temperature in the range of 21 to 55°C. Also, RH could be adjusted by placing saturated inorganic salt solutions in the incubator (e.g., CH3COOK for 23% RH, Mg2(NO3)2 for 52% RH, NaCl for 75%RH, and K2SO4 for 97% RH).23 The r(EDA/GO) sensors were placed at 37°C in the incubator and taken out periodically for resistance measurements. Sensor Measurements: For temperature sensitivity testing, the r(EDA/GO) sensors were placed on a Sawatec HP-150 hot plate for temperature control. Each r(EDA/GO) group contained 5 sensors, and their resistance was measured using a standard 2-probe method with a Keithley 2000 multimeter at 30.0, 40.9, and 52.5°C. 7


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X-ray Diffraction (XRD): The GO/EDA aqueous solution was dropped on (001) silicon wafers (1 cm× 1 cm), dried at room temperature, and then thermally reduced at 220°C for 6 hours. In order to test the moisture swelling effect on r(EDA/GO), after thermal reduction, three groups of samples were treated in a dry environment (20°C, 0% RH, 2 days) and a very humid environment (95°C, >95% RH, 2 hours). The X-ray generated from a sealed Cu tube is monochromated by a graphite crystal and collimated by 0.5mm MONOCAP (λCu-Kα=1.54178 Å). The sample-detector distance is 150 mm, and the exposure time is 600 seconds per run. The data were collected on a Bruke D8 DISCOVER GADDS microdiffractometer equipped with a VANTEC-2000 area detcore in a rotation method. The data were analyzed by XRD2EVAL program in the Bruker PILOT software. The average d-spacing values within the r(EDA/GO) layers were calculated by using the Bragg equation:

  2d sin 

(3)

where λ is the wavelength of the Cu X-ray beam (1.54178 Å), d is the average d-spacing between the adjacent r(EDA/GO) layers, and θ is the diffraction angle. RESULTS Effects of Water Vapor and Oxygen on rGO’s Resistance and D-spacing Figure 3a shows that, at 0% RH, relative resistance changes of rGO stayed within 5% for 192 hours. At 23% RH at 20°C, relative resistance increased by 16% after the first day and 20% after a week. Further increases in RH to 52 and 100% resulted in significantly higher resistance increases. As can be seen by comparing the results in Figures 3a and 3b, decreasing oxygen concentration from 21 to 1% did not cause significant resistance changes in the RH range of 0 to 23%. However, at 52 and 100% RH, the lower oxygen concentration somewhat decelerated resistance increases. Figure 3c shows that the XRD peak of rGO 8


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(a )

(b )

Room temperature 21% oxygen

Room temperature 1% oxygen

(c)

23.43° 24.16°

Figure 3. Effects of controlled environmental exposures on the resistance and interlayer spacing of rGO. Relative resistance changes of rGO as a function of RH at room temperature with: (a) 21% oxygen and (b) 1% oxygen. (c) XRD patterns of rGO before and after exposure to 95%RH at 95°C and 21% oxygen for 2 h.

shifted from 24.16º to 23.43º after exposure to 95% RH at 95°C and 21% O2 for 2 hours, indicating the expansion of average d-spacing in the humid environment from 0.368 to 0.380 nm. Effects of EDA/GO Ratio on Crosslinking The FTIR spectra of the EDA/GO samples are shown in Figure 4. For the GO control sample, i.e., (EDA/GO)0, the characteristic peaks of GO were detected, including the -OH stretching at 3388 cm-1, the carboxylic C=O stretching at 1726 cm-1, the skeletal vibration of unoxidized graphitic domains at 1611 cm-1, and the alkoxy C-O stretching at 1065 cm-1. After EDA was added into GO, it was observed from the (EDA/GO)0.0002, (EDA/GO)0.002, and (EDA/GO)0.02 samples that the carboxyl peak intensity decreased gradually with increasing the EDA/GO ratio. For (EDA/GO)2, the carboxylic C=O peak at 1726 cm-1 disappeared while two new peaks at 1340 cm-1 and 1555 cm-1 were detected. The new peaks were assigned to the C-N

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-CONH1555 -NH2 1340

C=C 1611

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(EDA/GO)2

-COOH 1726

(EDA/GO)0.02 (EDA/GO)0.002 (EDA/GO)0.0002 (EDA/GO)0

Figure 4. FTIR spectra of GO-EDA samples.

stretching and the N-H stretching, respectively, which indicated the existence of amide functional groups in (EDA/GO)2. Effects of Crosslinking on the Structure and Water-Induced Resistance Changes of rGO Figure 2b shows that the transparency of the GO/EDA samples was reduced by increasing the EDA concentration. Due to the high optical absorbance, the difference between rGO and r(EDA/GO) samples could not be visually distinguished by naked eyes (Figure 2c). Figure 5 shows the UV-vis spectra of r(EDA/GO). With the increased ratio of EDA/GO, the absorbance of r(EDA/GO) increased rapidly in the visible region from 380 to 780 nm, and the peak of absorbance red-shifted from 235 nm to 267 nm (Figure 5a). As a semi-conductive material with a bandgap,47 the optical absorbance of r(EDA/GO) can be related to the bandgap energy, Eg. As shown in Figure 5b and summarized in Table 1, the Eg values, obtained using Tauc’s relation using the method described in Experimental section, decreased with increasing the r(EDA/GO) ratio, for example, 3.70 eV for (EDA/GO)0 to 1.97 eV for EDA/GO)2. The thermal reduction process of EDA/GO samples was investigated by TGA as shown in Figure 6. For (EDA/GO)0, absorbed water was evaporated at 120°C, which occupied 5.7% of 10


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(a)

(b)

Figure 5. (a) UV-vis absorption spectra of r(EDA/GO) samples. (b) Relation between (αhν)2 and photon energy (hν) of r(EDA/GO).

the total mass. From 120°C to 220°C, 32.2% of the mass was further lost during the thermal reduction process. The TGA data of (EDA/GO)0.0002, (EDA/GO)0.002, and (EDA/GO)0.02 showed that small amounts of EDA would not greatly change the mass loss during the thermal reduction process as the EDA molecules were bonded with the carboxylic groups on GO sheets and formed thermally stable amide functional groups. However, for (EDA/GO)2, only 44.1% and 23.0% of weight remained at 120°C and 220°C, respectively. The data indicated that a significant portion of excess EDA was physically absorbed on GO sheets for (EDA/GO)2 and thus evaporated during the heat treatment. Note that the boiling point of EDA is 116°C. The surface morphology of rGO and r(GO/EDA) was investigated using SEM. Figure 7a shows that wrinkles

Table 1. Optical band gap values of r(EDA/GO) samples.

and lumps of about 1 to 10 m were observed on the surface of rGO. The formation of features was because of the uneven dehydration on the GO films. Where the GO sheets contained more H2O, the dehydration process became slower and the lumps could be more 11


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(a)

(c)

(e)

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10μm

(b)

10μm

10μm

(d)

10μm

10μm

Figure 7. SEM images of: (a) r(EDA/GO)0, (b) r(EDA/GO)0.0002, (c) r(EDA/GO)0.002, (d) r(EDA/GO)0.02, and (e) r(EDA/GO)2 .

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(a)

(b)

0

0.0002

0.002

0.02

r(EDA/GO)

(c)

0

2

0.0002 0.002

(d)

0.02

2

r(EDA/GO)

Figure 8. Effects of EDA/GO ratio on the NTC behavior of r(EDA/GO) sensor prior to environmental exposure: (a) Ro and (b) β. Effects of environmental exposure (95% RH, 37°C, 3 weeks) on the NTC behavior of r(EDA/GO) sensor: (c) R/Ro and (d) β. 5 samples were used for each sensor type for statistical analysis.

likely formed. At low EDA/GO ratios, the formation of wrinkles was considerably mitigated (Figures 7b, 7c, and 7d). With EDA crosslinking like r(GO/EDA)0.02, the interlayer displacement of GO sheets was suppressed and the morphological irregularity was mitigated. However, with higher EDA/GO ratios (Figures 7e), new surface features of about 5 μm were observed. With excessive EDA like r(EDA/GO)2, the evaporation of EDA puffed the GO films up and formed new lumps. The SEM pictures showed that with proper concentration, EDA could be a useful crosslinker to anchor the GO sheets against the displacement. Figure 8a shows that the resistance of r(EDA/GO) significantly decreased by increasing the EDA/GO ratio, for example, 231 kΩ for r(EDA/GO)0 vs 42 kΩ for r(EDA/GO)2. β also decreased from 1085K to 583K (Figure 8b). Figure 8c shows relative resistance changes of the r(EDA/GO) samples 13


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(a)

(b)

Figure 9. (a) XRD patterns of r(EDA/GO) samples before and after exposure to 95%RH at 95°C for 2 h. (b) Average d-spacing in rGO and r(EDA/GO) samples before and after environmental exposure to 95%RH at 95°C for 2 h.

upon exposure to 95% RH and 37°C over a period of 3 weeks. The resistance of the r(EDA/GO)0 control sample increased by 2.2 times after 4 days and 3 times after three weeks. But, for all the (EDA/GO)0.0002, (EDA/GO)0.002, (EDA/GO)0.02, and (EDA/GO)2 samples, the resistance increased to about 1.6 times after one week and remained stable for subsequent two weeks. Also, Figure 8d shows that the EDA modified samples exhibited lower β values before and during the humidity exposure. For example, the initial β value of r(EDA/GO)0 was 1085K whereas that of that of r(EDA/GO)2 was 583K. The final β value of r(EDA/GO)0 was 2291K whereas that of that of r(EDA/GO)2 was 1052K. XRD was used to characterize the effects of EDA concentration and water vapor on the average d-spacing of the r(EDA/GO) samples (Figure 9a). The d-spacing change of r(EDA/GO) samples was summarized in Figure 9b. At 0% RH, the (002) peak for rGO was centered at 24.3° with the corresponding average d-spacing of 0.368 nm. After increasing the EDA concentration, the (002) peak shifted gradually higher angles, indicating that the average d-spacing was decreased. For r(EDA/GO)2, the average d-spacing was 0.346 nm, which approached to that of pristine graphite (0.335 nm).

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After exposure to the highly humid environment (95% RH, 95°C, 2h), the average d-spacing of r(EDA/GO)0 changed from 0.368 to 0.380 nm. However, the increase of the average d-spacing became significantly less with increasing the EDA concentration. For r(EDA/GO)2, the average d-spacing only increased from 0.346 to 0.347 nm (see Figure 9b), which represented only 8.3% of the average d-spacing change observed for r(EDA/GO)0. DISCUSSION Despite the hydrophobic nature of rGO, we determined from the XRD data that the average d-spacing increased 3.07% on rGO upon exposure to the humid condition (Figure 3). To the best of our knowledge, this is the first direct and robust experimental evidence to show that the d-spacing of rGO can be expanded upon exposure to humid environments. This result supports our first postulation (Fig. 1a) that the small size of water molecules (0.23 nm) enables them to infiltrate into the d-spacing of rGO (~0.35 nm) and cause their swelling. Our result is also consistent with the previous theoretical predictions37–39 that water molecules interact with GO to: (1) form new functional groups, (2) make the 2D structure of GO to become more disordered, and (3) increase its d-spacing. However, we notice that the d-spacing expansion of rGO in our experiments (from 0.368 to 0.380 nm) is much less in the comparison with the reported d-spacing change of GO (from 0.51 to 0.90 nm).39 This phenomenon can be attributed to the reduction of hydrophilic functional groups on rGO. With greatly fewer functional groups, the rGO became less capable of absorbing water molecules and was expected to have a smaller d-spacing change upon the exposure to the moisture effect. The results in Figure 3a support that water vapor is the main factor in increasing the resistance of rGO over the 200-h period. The average d-spacing increase observed in rGO upon exposure to the humid condition (Figure 3c) also supports that the increased resistance

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can be explained by the intercalation of water molecules into the interlayer space between rGO sheets. Since the lateral dimension of rGO sheets (300-800 nm) is much smaller than that of the sensors (6 mm), electrical conduction cannot be possible without electron hopping through the interlayer space between the rGO sheets. The energy required for interlayer hopping is highly controlled by the distance between carbon atoms.48–50 Expanded d-spacing is therefore expected to: (1) make the hopping process more difficult, (2) limit the movement of electrical carriers in the interlayer space, and (3) increases the resistance of rGO. Interestingly, our results in Figures 3b and 3c also suggest that the role of oxygen appears to be relatively minor. Although several studies37–39 reported that both water and oxygen may facilitate the formation of new functional groups in rGO, our results indicate that the resistance increase is predominantly caused by water vapor. Although it is theoretically possible that new functional groups can be generated from oxidation and act as electron traps to decrease the intralayer conductivity of rGO, the rate of oxidation rate was most likely much slower than the rate of water intercalation due to the high activation energy of oxidation (~4.9 eV).37 Our results show that the concentration of EDA, as a crosslinking agent, strongly influences the structural development and properties of r(EDA/GO). In regard to morphological structure, the formation of wrinkles and lumps observed in r(EDA/GO)0 (Figure 7a) may be caused by nonuniform dehydration. First, based on the TGA results (Figure 6), rGO lost about 7.5wt% at 120°C mainly due to the dehydration. Second, we suspect that the hydrophilic GO sheets tended to move and concentrate at the wet regions once the dehydration was not uniform. However, when EDA molecules and GO sheets were bonded, the movement of GO sheets got suppressed, thus resulting in the development of more uniform surface morphology as can be seen from the SEM images of r(EDA/GO)0.0002, r(EDA/GO)0.002, and r(EDA/GO)0.02. With the excessive EDA concentration of r(EDA/GO)2,

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the SEM image (Figure 7) suggests that cavities were generated underneath the film due to the evaporation of EDA observed during thermal reduction (Figure 6), which damaged the morphological uniformity. The environmental test results in Figure 8 show that the crosslinked rGO samples exhibited more stability in humid environments. The EDA crosslinking improved significantly the environmental stability of rGO by reducing the first 2-day resistance change from 89% to 51%. The crosslinking also improved longer-term stability resistance after 1 week. We noticed that, even after crosslinking, the resistance of r(EDA/GO) still increased 51% after 2 days. Based on prior theoretical research by S. Zhou et al.,37 we postulate that the resistance increase may be caused by H2O reacting slowly with epoxides and hydroxyls remaining on r(EDA/GO). The hydroxyls may function as electron-withdrawing, thus reduce electron density on r(EDA/GO), and increase resistance. The improvements electrical properties correspond with the XRD results (Figure 9b) that, in the extremely humid environment, the average d-spacing of the heavily crosslinked rGO like r(EDA/GO)2 increased from 0.346 to 0.347 nm whereas that of the uncrosslinked r(EDA/GO)0 increased from 0.368 to 0.380 nm. Though the d-spacing changes (0.001-0.012 nm) were small compared with the d-spacing itself (~0.35nm), they were still detectable with the ~0.001nm accuracy of the XRD equipment and the peak shifting decrease of the crosslinked rGO samples can be clearly seen in Figure 9a. In a word, the XRD results make a good agreement with the electrical measurement above. The optical bandgap energy, Eg of rGO decreased from 3.70 to 1.97 eV with increasing EDA concentration. This could be possibly explained by that amine functional groups can act as n-type dopants in rGO.51–53 This optical Eg change is consistent with the significant decreases in the  values observed with increasing the EDA concentration (Figure 8b). The  value 17


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decreases suggest that adding EDA decreases the activation energy barrier associated with generating carriers, makes rGO more conductive and less sensitive to temperature changes. Overall, our results provide us important new insights that; (1) rGO’s environmental instability is mainly due to the interlayer space expansion caused by the intercalation of water molecules and (2) crosslinkers, such as EDA, can be used to mechanically anchor rGO sheets, inhibit the swelling effect, and stabilize the electrical properties especially in humid environments. To the best of our knowledge, this is the first direct and robust experimental evidence to show that the d-spacing of rGO can be expanded by water intercalation upon exposure to humid environments. Also, although crosslinking of GO sheets has been extensively studied for many applications,55–59 this is the first time to our best knowledge that that crosslinking has been used to produce and demonstrate an environmentally stable rGO material. Since rGO has been widely explored as a promising sensor material with its sensing attributes being primarily relied on their electrical properties, the apparent instability of rGO to moisture60–62 has been a problem that undermines its robust use in many sensing applications. Our new insights provide the understanding required to rationally modify rGO as a sensor material to be widely and reliably used in humid environments. Nevertheless, there are several challenges to overcome. First, from a sensor application perspective, EDA crosslinking reduces the temperature sensitivity of rGO. Second, EDA crosslinking may not completely eliminate the moisture effect, especially at the beginning of the exposure. Third, EDA is potentially hazardous and therefore could be replaced with a more benign material. CONCLUSIONS Upon exposure to humidity, the interlayer spacing between rGO sheets increased due to the intercalation of water and the expanded interlayer spacing increased rGO’s electrical 18


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resistance. This intercalation mechanism was used to explain the susceptibility of rGO sensors to humidity changes. As a novel means of mitigating this instability, EDA was used as a covalent crosslinker to anchor rGO sheets, limit interlayer expansion, and thus stabilize electrical resistance under humid environments. ACKNOWLEDGEMENTS Research in this publication was supported by: (1) a grant from the National Science Foundation (SBIR 1648057) which was conducted in collaboration with Linh Le at FlexTraPower and (2) the Stevens Innovation and Entrepreneurship Doctoral Fellowship. We thank Dr. Chunhua Hu at the NYU Department of Chemistry X-ray Diffraction Facility for his help with data collection and the support by the National Science Foundation under Award Number CRIF/CHE-0840277 and by the NSF MRSEC Program under Award Number DMR-0820341. We are grateful for the SEM characterization support from Dr. Alex Chou. We appreciate Kai Zong for his help with UV-vis characterization.

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Crosslinking Stabilizes Electrical Resistance of Reduced Graphene

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