Wearable, Flexible, Disposable Plasma-Reduced Graphene Oxide

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Surfaces, Interfaces, and Applications

Wearable, flexible, disposable plasma-reduced graphene oxide stress sensors for monitoring activities in austere environments Haiping Zhou, X. Ye, Wen Huang, Mengqiang Wu, L. N. Mao, Bin Yu, Shuyan Xu, Igor Levchenko, and Kateryna Bazaka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22673 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Materials & Interfaces Revised version AM-2018-22673d

Wearable, flexible, disposable plasma-reduced graphene oxide stress sensors for monitoring activities in austere environments H. P. Zhou,a X. Ye,a W. Huang,b M. Q. Wu,a,* L. N. Mao,b B. Yuc, S. Xu,d I. Levchenko,d,e,* K. Bazakad,e,f School of Materials and Energy, University of Electronic Science and Technology of China, 2006 Xiyuan Ave, West High-Tech Zone, Chengdu, Sichuan 611731, China. [email protected] (M. Q. Wu) b State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronics Science and Technology of China, 2006 Xiyuan Ave, West High-Tech Zone, Chengdu, Sichuan 611731, China c College of Nanoscale Science and Engineering (CNSE), State University of New York, Albany, New York 12203, United States d Plasma Sources and Application Center/Space Propulsion Centre Singapore, NIE, and Institute of Advanced Studies, Nanyang Technological University, 637616, Singapore. [email protected] (I. Levchenko) e School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Brisbane, Australia f Institute for Future Environments, Queensland University of Technology, Brisbane, Australia. [email protected] (K. Bazaka)

a

Supporting Information

ABSTRACT: In austere environments, e.g. in outer space, on surfaces of extra-terrestrial bodies (Moon, Mars) or under water, technologies that can enable continuous, reliable, authentic monitoring of movement of human operators and devices can be critical. We report here the production and human body test of wearable, flexible graphene oxide stress sensors suitable for real-time monitoring of body parameters, state and position of humans and automatic equipment. These sensors have excellent sensitivity and signal strength across a wide strain range, alleviating the need for additional instrumentation for signal processing and amplification. Their low cost makes them virtually disposable, which may benefit such applications as smart clothing. The sensors were fabricated by a concomitant reduction and N-doping of graphene oxide on polydimethylsiloxane in N2-H2 plasma. The direct bias and other plasma parameters have a significant effect on the reduction and properties of graphene oxide sensors, as shown by optical emission, Raman and X-ray photoelectron spectroscopies and X-ray diffraction. Optical emission showed different excitation and ionization processes involving atomic and molecular species in the N2-H2 discharge. The photoelectron spectroscopy has confirmed the graphene reduction and introduction of nitrogen doping into the reduced graphene oxide. The bias efficiently controls plasma-induced electric fields, and plasma-related effects determine the N-doping levels. The reduced graphene oxides demonstrate excellent tensile properties, which make them suitable for efficient but cheap stress sensors. This eco-friendly, fast, room-temperature method shows a great potential for fabrication of efficient, flexible sensors. KEYWORDS: Graphene, Flexible Sensors, Wearable Sensors

1. INTRODUCTION Human ambition to explore remote and often inhospitable areas on Earth, and as far away as Moon and Mars,1 require humans to operate in austere environments. These can range from deserts, highlands, underwater, deep mines and caves, to outer space and eventually surfaces of extra-terrestrial bodies.2 What these hostile environments often have in common is the need for continuous, real-time, reliable, authentic monitoring of human body parameters and the state of automatic instruments and devices to ensure efficient and safe operation. Furthermore, due to specifics of these environments, there are limited options as to the nature, dimensions and power requirements of equipment that could be used for this purpose. Ideally, these should be wearable sensors,3, 4 capable of producing good quality signals over a broad range of signal magnitudes without any auxiliary processing equipment.5-8 Low-cost disposable sensors are particularly attractive for they can be easily replaced to ensure high performance. Novel technologies,9 such as simple, cheap yet reliable wearable sensors based on graphene-type materials can provide an efficient monitoring solution.10 Here we report a novel approach for the synthesis of a graphene oxide platform on which simple, disposable, strong-signal stress sensors can be built. These sensors are robust and versatile, and could be used under various conditions and for different aims, e.g. for continuous monitoring of the state of human health and movement, as well as for the operational control of various automated systems.11 Due to simplicity and low cost, these sensors could be easily replaced and thus used in large quantities, including for applications that require single-use, disposable devices, and under harsh conditions where the damage and extra short service life is expected (Figure 1). ACS Paragon Plus1 Environment

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Page 2 of 18 Revised version AM-2018-22673d

Figure 1. Continuous monitoring of body parameters, health and other characteristics is vitally needed when humans are working under harsh conditions of e.g. a deep mine, or in outer space and on the surface of another planet. Automatic equipment and instruments also require continuous monitoring of their state and activity. Low-cost wearable sensors deployed en masse could provide a continuous flow of vital information to the base, significantly contributing to the safety and efficiency of the worker. Low cost will ensure a single-use mode of the sensors for the humans operating under austere environments, and fast, short-term scheduled replacement and multiple-line redundancy for the sensors installed on automatic and remotely operated instruments.

Graphenes have attracted a great deal of interest due to their extremely high charge-carrier mobilities, extraordinary thermodynamic stability, excellent mechanical stiffness,12 exceptional electron emission properties, applicability in photonic propulsion13 and green chemistry processes,14 suitability for space applications,15 and many others. Graphene-based technology have the potential to deliver a favourable combination of desirable properties, excellent performance and cost effectiveness, yet large-scale economic production of high-quality graphenes remains a challenge16, 17. Graphene oxide (GO) is a promising precursor for mass-production of graphene for broad range of applications, including composite materials, electrodes for batteries, medical devices, supercapacitors and sensing devices.18, 19 GO is always decorated with abundant oxygen-containing functional groups, including hydroxyl, epoxy, ketone and carboxyl groups either on the basal plane or at the edges with a mixture of sp2-and sp3-hybridized carbon atoms20 that make GO an electrically insulating and mechanically weak material so that the reduction of GO is often needed to enable real life applications. Nitrogen doping is an effective approach for tuning the band gap and charge-carrier concentration in graphene.21 In addition, as GO is rarely completely reduced and as such still contains a small amount of carboxyl groups that reduce its conductivity, nitrogen doping brings about positive effects including higher conductivity, good wettability and excellent electrochemical properties.22 Nitrogen-doped reduced graphene oxides (N-RGO) have broad application prospects in the field of lithium batteries,23 supercapacitors,24 sensor25 and fuel cells.26 A number of approaches have been explored for the preparation of N-RGO, including hydrothermal reaction,27 chemical reduction,23, 28 thermal annealing graphene oxide in NH3 atmosphere29, 30 and so on. Tao et al. synthesize N-RGO by ultrasonic treatment of GO in aqueous solution of ammonia for 3 h.27 However, this method is time consuming with respect to the treatment as well as preparation processes. In another example reported by Stankovich et al., N-RGO was obtained via hydrazine monohydrate vapor reduction method at 100 ºC under a water-cooled condenser for 24 h.28 Thus-produced N-RGO need to be washed with copious amounts of water and methanol, with many other potentially harmful chemicals involved in approaches based on chemical reduction. Indeed, hydrazine hydrate is highly toxic and dangerously unstable, and as such is considered to be potentially harmful to humans. Another popular route is based on thermal annealing of GO in NH3 atmosphere at ∼900 °C.29 It has been widely used to produce N-RGO or N-doped carbon composite materials for energy applications.31 Thermal annealing route is a simple approach to obtain N-RGO, ACS Paragon Plus2 Environment

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however, high temperature brought in from thermal reduction method can bring about potential risks. More importantly, a great many materials and substrate are not sufficiently stable to withstand such high temperatures. Overall, the aforementioned methods for N-RGO fabrication are potentially environmentally hazardous, associated with the use or production of toxic substances, treatment under high temperature, and long processing times. Compared with chemical and reduction methods, plasma-assisted simultaneous reduction and nitrogen doping is a simple, eco-friendly and safe approach. Low-energy N2+ implantation has been used for the reduction and nitrogen doping of GO,21, 32 but it is limited by the sputtering area when using ion sputtering gun. In addition, NH3 plasma is often applied for the reduction and nitridation of GO. However, it would be more beneficial if the proportion of N and H could be controlled to enable greater degree of precision, and to facilitate better control over the etching by active hydrogen atoms. Herein, we present a room-temperature simultaneous reduction and N-doping of graphene oxide in a N2 and H2 plasma (note that nitrogen has relative high ionization potential). Optical emission spectroscopy (OES) technique was applied for N2 and H2 gas plasma diagnostic. In this treatment process, the presence of a large number of reactive H radicals and N2+ ions promote the reduction and doping of GO in a high density chemically active environment. The reactive H radicals act as reactant to remove oxygen containing functional groups and high energy ion collisions enable GO doping. Besides, we use the negative DC bias creatively to control the plasma induced in-built electric field. According to the OES analysis, the effect of negative DC bias significantly enhances electron temperature. It has been widely established that N atoms can be substitutionally incorporated into RGO either in a form of graphitic N atom, pyrrolic N atom or pyridinic N atom, and those three N species can be effectively distinguished by XPS33. The DC bias-related effects also determined the N-doping levels of three N species. The N-RGO film fabricated on PDMS displays excellent tensile property for stress sensor and the plasma based eco-friendly, room-temperature and fast method for simultaneous reduction and N-doping of graphene oxide shows great potential for fabrication of flexible sensors.

2. EXPERIMENT AND DISCUSSIONS 2.1. Synthesis of Graphene Oxide. We have used in this work a convenient and powerful radio frequency (RF) plasma technology which has already demonstrated promising potential for the synthesis of polymers,34, 35 complex metamaterials,36, 37 treatment of novel materials for biomedical38, 39 and space applications,40, 41 various plasma-based material processing techniques,42, 43 and many other techniques.44 In the described experiments, the glass substrates were ultrasonically cleaned using acetone for 10 min followed by an ultrasonic clean in ethyl alcohol for 10 min and then cleaned in DI water for 10 min. GO was fabricated using a modified Hummer's method (Tanfeng Tech) and the layer number of GO is few layers (3~5 layers). The GO was dispersed into ethanol solution at 1 mg/mL and then spray coated on the glass substrate by a spray device (Holder) with a nozzle with a diameter of 0.2 mm. The thickness of GO film on glass is about 100 µm. In addition, GO films on flexible substrates were also prepared by spraying the GO dispersion onto a polydimethylsiloxane (PDMS) substrate, followed by low-temperature drying on a heat plate at 45 °C for 3 hours. The PDMS monomer and curing agent (Sylgard 184) were mixed with a mass ratio of 10:1.45 After that, the GO samples were transferred to a home-made plasma facility (Figure 2a) with an ultrahigh vacuum chamber. The vacuum chamber was pre-evacuated to a base pressure of 10-4 Pa using a molecular pump and mechanical pump. A mixture of high purity N2 (40 sccm (standard cubic centimetres per minute)) and H2 (10 sccm) was used as a working gas for a synchronous inductively coupled plasma (ICP) discharge. The flows of hydrogen and nitrogen were monitored with gas flow meters, and the pressure in the vacuum chamber was recorded using a resistance vacuum gauge and ionization gauge. The applied radio frequency (RF) power was 1400W, and low frequency (~370 kHz) inductively coupled plasma source

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Figure 2. (a) Schematic of the plasma-based experimental setup, (b) schematic drawing of nitrogen doped and reduced graphene oxide (N-RGO) synthesis by plasma and (c) the OES measurement system. (d, e) Three-dimensional reliefs of the graphene patterns of the graphene oxide (d) and nitrogen doped and reduced graphene oxide (e).

produced a high-density plasma at low (mTorr) pressure range to facilitate independent control of electron density and energy of ions impinging onto the growing surface, and low plasma sheath potential. The OES measurements were carried out using spectrometer (SeemanTech, S3000-UV-NIR) and computer, with the design of the experimental system shown in Figure 2c. The substrate holder is connected with a negative DC potential (Figure 2a). Figure 2b shows the concept of conversion of GO to N-RGO films after N2 and H2 plasma treatment. Thus prepared N-RGO were termed N-RGO-0V and N-RGO-35V to refer to N-RGO films reduced by N2 and H2 plasma under 0 V and 35 V negative bias, respectively. HRGO-35V was used to refer to RGO films reduced under 35 V negative bias by just H2 plasma. 2.2. Characterization of Graphene Oxide. We have examined the Raman spectra for GO,

N-RGO-0V, N-RGO-35V and H-RGO-35V samples. Raman spectra were obtained using a Raman spectroscope with a 532 nm laser excitation (HORIBA iHR 550, 50mW). The typical measuring time for one Raman spectrum was ∼5 s. X-ray photoelectron spectroscope (XPS, Thermo SCIENTIFIC ESCALAB 250Xi, ThermoFisher Scientific) was used to detect the surface composition of GO and N-RGO. The crystallographic changes of the treated GO were evaluated using X-ray diffraction (XRD, LabX XRD-6000). ACS Paragon Plus4 Environment

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Plasmas are commonly referred to as the “fourth state of the matter.” Plasmas represent a fully or partially ionized gas, i.e., ions, electrons, and neutral species.46 Importantly, the densities of positive and negative particles in plasma are equal, i.e., plasma is a quasineutral medium. OES is commonly used to study the anatomy of the RF discharge. Comparative SEM images from the top surface of GO and N-RGO-35V films are presented in Figure 3 and were used to characterize structural changes after plasma treatment. The top view of GO film shows a micro-rough surface comprised of the wrinkled, randomly aggregated and thin graphene oxide sheets (Figure 3a), while the surface of plasma-induced N-RGO-35V films is more smooth (Figure 3b). The smoother surface of N-RGO-35V indicates the reduction in the number of defects in GO sheets, including the reduction of oxygen function groups, the removal organic impurities and the recovery of sp2 carbon structures. This result are consistent with the previous report by Kim et al. which shows the decrease in surface roughness for GO films with the increasing time of exposure to NH3 plasma at room temperature.47 Figures 3c,d illustrate the typical surface topography profiles for the samples, and Figures 3e,f show the typical 2D Fast Fourier Transform (2D FFT) (real) distributions. The 2D FFT distributions do not suggest many differences between the samples, while the GO pattern is apparently denser. Figure 3g shows typical emission spectra of N2 and H2 (40:10 sccm) plasmas, with emission spectra recorded over 200–800 nm. As reported in previous studies, hydrogen Balmer lines (Hα at 656.1 nm and Hβ at 485.4 nm) are obvious, and numerous H2 molecular bands are also visible being centered at 595.7 nm.48 A variety of nitrogen emission peaks are spread across the measured spectral region. The N2+ lines, with peaks at 390.5 nm, 426.4 nm and 469.3 nm, correspond to the first negative system.49 Bandhead at approximately 540.0 nm, 650.0 nm and 750.0 nm are assigned to the first positive system of neutral nitrogen molecule which are supposed to represent the recombination of ground-sate nitrogen atoms through a three-body collision process with a third body.50 The peaks in the spectral region centered at 315.5 nm, 380.0 nm and 399.2 nm correspond to the most intense peaks of the second positive molecular series of neutral nitrogen. The second positive N2 molecules may be generated by radiative recombination of an electron with an ionized N2 molecule in its ground state.48 The first negative band of nitrogen was used to study the higher-energy electrons, while the second positive band was used to give information on the behaviour of middle-energy electrons.51 One can see that the first negative band of N2+ lines with highest intensity dominates the spectrum. This observation indicates a high-density chemically active environment, consisting of abundant radicals, ions and high-energy electrons inside N2 and H2 plasmas. What is more, the NH line is observed at 336.6 nm, which is important in nitriding process and is an indicator of high reactivity. The emission spectra intensity at a particular wavelength is proportional to the concentration of the substance in excited state (Figure 3). In comparison with the Langmuir probe, OSE is a non-intrusive technology for the measurement of plasma parameters. Detection of those emission intensities provides a qualitative indicator of concentrations of reactive species in plasmas, which can then be converted into a quantitative relative or absolute species number density using calculations.52 The intensity of Hα and Hβ can be used to measure electron temperature. The intensity of these peaks increased with increasing plasma power (see the support information). When the negative DC bias connected to the substrate increased to -35 V, the electron temperature Te (0.79 eV) is greater than Te (0.55 eV) when Vs=0 V. It is indicated that the nitrogen is expected to be more highly activated when ACS Paragon Plus5 Environment



biased to -35 V, leading to longer-lived nitrogen atoms.

Figure 3. SEM images of (a) GO and (b) N-RGO-35V; profiles along the centerlines (c, d), shown by red lines on (a, b); and 2D Fast Fourier Transform (2D FFT) (real) distributions (e, f). The 2D FFT distributions do not elucidate many differences, while the GO pattern is apparently denser. (g) Optical emission lines from 200 nm to 800 nm of N2 and H2 plasma with 1400 W input power at the pressure of 1.6 Pa. Large variations in the thickness of the samples at different areas could be noticed, due to a specific orientation of graphenes, as illustrated in Figures 2d and 2e. The thickness of GO was estimated as 3 to 5 layers.

Because of the negative DC bias, the plasma-induced electric field would change,53 resulting in heavy particle collisions involving sufficiently accelerated cations in the near-substrate sheath. Raman spectroscopy is a very reliable, non-destructive characterization technique54 and it has been widely used to characterize the structure and quality of carbon materials, especially graphene and GO. There are two prominent peaks in the Raman spectra of four samples, namely, the D-band at 1340 cm-1 and the G-band at 1581 cm-1 and a weak 2D-band at 2667 cm-1.55 The D peak is considered to be a signature of structural defects present in graphene, while the G peak is associated with the formation of the sp2 hybridized carbon network and originates from the doubly degenerated phonon vibrations at the Brillouin zone centre (Figure 4a).56 The ~2450 ACS Paragon Plus6 Environment

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cm-1 band, always observed in carbon materials’ Raman spectra, has been described in the literature.57

Figure 4. (a) Raman spectra of GO, N-RGO-0, N-RGO-35 and H-RGO-35, and (b) XRD spectra of GO, N-RGO-0V-5min, H-RGO-35V-5min, N-RGO-35V-5min, N-RGO-35V-15min and N-RGO-35V-80min.

The defect level was investigated based on the average peak intensity of the D band relative to the G band (ID/IG).58 The ID/IG ratio of GO, N-RGO-0V, N-RGO-35 and H-RGO-35V are 1.04, 0.96, 0.90 and 0.97, respectively. The pristine GO has a ID/IG ratio of 1.04. The ID/IG ratio of N-RGO-0V was lower than that for GO; this observation may indicate the partial recovery of the graphitic structures during plasma reduction of GO. These observations are in agreement with the results reported by Xia et al. where the N-RGO was prepared by thermally annealing GO with melamine: the ID/IG ratio is 1.02 for GO, whereas the ratio decreases to 0.86–0.98 for N-RGO.33 However, incorporation of nitrogen into graphitic structures also breaks the graphitic structure inevitably and causes the structural defects, possibly because more oxygen-containing functional groups were removed relative to the defects caused by nitrogen incorporation. What is more, The ID/IG ratio of N-RGO-35V is lowest, meaning that appropriate bias contributes to the lower level of defects in N-RGO. In addition, The ID/IG ratio of H-RGO-35V is even higher than that in N-RGO-0V, suggesting that the H radical etching effect may be counterproductive. Cumulatively, these results suggest that the use of N2 and H2 plasmas in combination with the application of negative DC bias positively contributes to the defect reduction and restoration of structure of GO. ACS Paragon Plus7 Environment



Figure 5. (a) XPS survey scan, (b) C1s spectra, and (c) N1s spectra of GO, NRGO-0V-5min and NRGO-35V-5min.

The restoration of the graphene structures is confirmed by X-ray diffraction (XRD) patterns (Figure 4b). The XRD pattern for GO showed a major 2-theta peak at 9.8°,59 which corresponds to the interlayer distance of 0.88 nm. A new 002 peak at 20.8° is observed in the samples treated by N2 and H2 plasma biased at 0 V for 5 min (N-RGO-0V-5 min), and the peak became stronger when the bias is -35 V for the same treating time (N-RGO-35V-5 min). The H-RGO-35V-5 min sample shows the characteristics similar to that of N-RGO-35V-5 min. The appearance of this peak after partial reduction in plasma indicates that oxygen functional groups in GO have been partially removed, thereby reducing the expansion and restoring the hexagonal graphite structure. Furthermore, the diffraction peak of GO is located at 10.3° for N-RGO-35V-5 min, with the weak shift attributed to the change in the interlayer space in GO. As for N-RGO-35V-15 min, the diffraction peak of GO is located at 10.3° and the diffraction peak of graphene is located at 21.8°. The intensity of GO peak becomes relatively weaker and the intensity of graphene peak become stronger compared to that of N-RGO-35V-5 min. According to the XRD pattern, the increasing bias contributed to a much higher level of reduction. With effect of DC bias, plasma-induced electric field would change, resulting in heavy particle collisions involving sufficiently accelerated cations in the near-substrate sheath. However, the diffraction peak for GO still remains after 15 min of treatment; this may due to only partial reduction of GO. With longer plasma expose time of 80 min at the same bias of -35 V, a broader peak of N-RGO-35V-80min is observed at 22.8°. Compared with N-RGO-35V-5min, the 2-theta peak shifted to 22.8° with a decreasing interlayer space of 0.42 nm after longer plasma-induced reduction, which means longer treatment duration results in ACS Paragon Plus8 Environment

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virtually complete removal of oxygen functional groups from GO. What is more, the diffraction peak of GO almost disappeared, confirming that it is possible to achieve a full conversion from GO to RGO. Table 1. Nitrogen atomic percentage of various chemical states in NGs prepared with 0 and -35 V biases, respectively

Pyridinic N (%)

Pyrrolic N (%)

Graphitic N (%)

N-RGO-0V

27.5576

55.5534

16.8890

N-RGO-35V

38.7048

40.8890

21.1172

Further corroborative evidence for the reduction of GO during the N2 and H2 plasmas treatment process is provided by XPS spectra. XPS was adopted to detect the surface composition of GO and N-RGO, and the results are shown in Figure 5. In the spectrum for GO, only C and O signals can be detected, which is consistent with the molecular structure of GOError!

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According to the survey spectrum, the atomic composition of GO

revealed 65.6 at. % of carbon and 34.4 at. % of oxygen. The C1s spectra of GO were assigned to four main components, C-C (284.5 eV), C-O (286.5 eV), C=O (287.6 eV) and COOH (288.9 eV).60 The spectrum shows that the C-O component dominates in GO. Furthermore, a new species C-N appeared at 258.8 eV in spectra for N-RGO-0V and N-RGO-35V samples. In comparison to the C1s spectra for GO, the decrease in the magnitude of C–O, COOH and C=O peaks reveals the partial reduction of N-RGO during plasma treatment. In addition, the C-C peaks in N-RGO-0V and N-RGO-35V dominate after plasma treatment. In contrast, C-O component corresponding to hydroxyl and epoxy/ether groups and C=O component corresponding to carbonyl and carboxyl groups considerably diminish. 2.3. Characterization of Graphene Oxide Composition. Analysis of the molecular dynamics

simulations done by Bagri61 revealed that the hydroxyl functional groups required lower temperatures or energy for desorption than the epoxy groups by the release of H2O. So the hydroxyl can be easily removed by the reactive H ions (Hα and Hβ). Based on the C1s spectrum results, there is still oxygen that remains in the RGO. The remaining oxygen atoms are basically in the form of highly thermally stable epoxy or carbonyl groups. The oxygen content that remained in RGO was dependent on the hydroxyl/epoxy ratio, the initial oxygen concentration and temperature or energy, as proved by Bagri.61 Epoxy groups require higher energy for desorption and thus are likely to remain in RGO. What is more, analysis of the Raman spectra shows a lower ID/IG ratio of N-RGO-35V than H-RGO-35V and N-RGO-0V. The higher electron temperature during treatment of N-RGO-35V sample, when compared to N-RGO-0V reveals ACS Paragon Plus9 Environment



that radicals with higher energy contribute to desorption of oxygen groups. Reduction by charged radicals was the primary reason for the differences between the outcomes of reduction in N2-H2 and H2 plasmas. According to the OES, NH radicals were readily observed in N2 and H2 plasma, which can effectively reduce isolated epoxides.62 There are strong dipole–dipole attractions between the polar NH radicals and hydroxyl when they approach each other, which contributes to the reduction process. In addition, the epoxide groups were difficult to reduce using charge-neutral species and they are usually removed by the active radicals in plasma, as the adsorption energy of epoxyl groups on pristine graphene is -4.73 eV , while that of hydroxyl groups is only -0.53 eV, as reported by Xu et al..62 That explains the lower ID/IG ratio in N-RGO-35V compared to that in H-RGO-35V and at the same ICP power, N2-H2 plasma can reduce more oxygen than H2 plasma. The N 1s spectrum for the N-RGO-0V and N-RGO-35V samples (Figure 5c, both spectra were measured after plasma treatment for 5 min) can be assigned to several peaks centred at 399.0 eV, 400.1 eV, 401.3 eV, corresponding to pyridinic, pyrrolic, and graphitic nitrogen, respectively,63 as illustrated in Figure 1. The pyridinic N and pyrrolic N species contribute to the π-conjugated system with a pair of p-electrons in the graphene layers.33 The graphitic nitrogen, appeared as the carbon atoms within the graphitic structure substituted by nitrogen atoms, can impact the electrical conductivity. The specific content of three are list in Table 1. It is worth to note that the content of pyridinic N increased from 27.5% to 38.7% and graphitic N also increased from 16.9% to 21.1% with increasing bias from 0 V to 35 V. After N2 and H2 plasma treatment for 5 min, the pyridinic N and pyrrolic N dominated in the spectra. This may be due to the substitution of the –OH groups at the edges of aromatic domains of GO. It seems that the content of graphitic nitrogen is always lower than that of pyrrolic N and pyridinic N. Indeed, among the possible nitrogen-doping configurations, the incorporation of N atoms into GO using N2 and H2 plasma is energetically favorable and preferential at sites with low coordination numbers (pyridinic N and pyrrolic N)64. The N atoms substitution energetically prefers to occur at the carbon atoms near the defect. In the case of monovacancy, the most stable position for N dopant is the pyridinic N, while for other point defects such as divacancies, N doping tends towards pyrrolic N65. Importantly, it is the pyridinic N that creates Lewis basic sites contributing to high electrocatalytic activity towards oxygen reduction reaction, as proved by the above discussed results of the synthesis of nitrogen-doped carbon materials. It has been reported that 10Environment ACS Paragon Plus

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the structural variety of nitrogen defects in carbon materials provides rich electronic properties such as p-type and n-type conductivity depending on how the N-doped configurations are formed66, 67.

Figure 6. (a) Schematics of the nitrogen-doped graphene oxide-based sensor and schematics of the measurements; (b) Photograph of the test bench; (c) Percentage of resistance change of a NGO sensor under varying strain deformation, and (d) the responsive electrical signal collected by the sensor (the Y axis is resistance, and 0.5, 1.0, and 1.5% are different deformations ∆L/L).

The mechanism behind the reduction and N-doping of GO under the inﬂuence of in-built electric field is also proposed. It should be noted that plasma is a complex medium containing a dynamic cocktail of active species that include ions, radicals, electrons and neutrals. In fact, the discharge of N2 and H2 plasmas is very intricate. There is a very strong correlation between ionization, dissociation, electron dynamics, multispecies sheath dynamics and various reactions which together lead to the formation of various active species. There are various species shown in the optical emission spectra (Figure 3g) which correspond to different transition states (see supporting information). Since the chamber area is usually big, the bias voltage can affect the bulk plasma, in particular the plasma potential near the substrate. Due to their lighter mass, electrons are more mobile.68 Therefore, when bias is applied, the probability of electrons arriving at the surface of samples on the substrate is much greater. Moreover, it is still uncontrollable that where ions, electrons and neutrals will go. In this way, 11Environment ACS Paragon Plus



the negative DC bias connected to the substrate plays an important role in controlling an in-built electric field that is induced by the plasma onto the samples surface. The negative DC bias acts as a driving force to attract the impinging ions, that is positive ions, towards the sample surface. In this case, the N2+ and H+ ions are sufficiently accelerated by the DC electric field in the near-substrate sheath.

Figure 7. N-RGO/PDMS stress Figureplaced 7. N-RGO/PDMS sensor on (a) neck,stress (c) sensor placed ontemple (a) neck, opisthnar, and (e) for(c) opisthnar, and (e) temple monitoring. The relativefor monitoring. The ofrelative resistance response the resistance of to the stress sensorsresponse corresponds stress sensors various motion corresponds signals suchto motion signals such asvarious (b) forward bending of the as (b) forward bending of of the neck, (d) bending neck, (d) andbending opisthenar, (f) eyeof opisthenar, and (f) eye blinking. Astronaut photo: blinking.NASA. Astronaut photo: courtesy courtesy NASA.

This accounts for the notable increase in pyridinic N and graphitic N when biased at -35 V. It is also worth emphasizing that the plasma usually interacts with the surface of sample, the DC electric field-induced acceleration of the incoming positive ions results in their deeper penetration into the topography of the film. Hence, the above reported XPS and XRD results clearly evidence that the DC bias plays a key role in tuning the doping and reducing levels. 2.4. Fabrication and test of Stress Sensor. This approach paves the way towards fast,

room-temperature and eco-friendly production of RGO for the applications in flexible electronics and piezoresistive sensors. In our study, the GO films were first deposited onto PDMS substrates pre-treated by Ar and N2 plasma to improve the hydrophilicity by introducing hydrophilic surface functional group containing nitrogen. Comparing with pristine PDMS, plasma-activated PDMS show good wettability and the GO dispersion can coat the plasma-induced PDMS easier and more evenly, with virtually the same thickness across the area of the film. Then, GO films on PDMS were reduced by N2 and H2 plasma biased -35 V for 10 min at room temperature. The electrical and stress properties of a thus-produced flexible sensor were measured by a source measurement unit (SMU) instrument (Keithley 2400) at room temperature. The two ends of the sensor were mounted on a customized micrometer moving stage (Figure 6). The sensing film can be bent by moving the stage closer, and the corresponding resistance change with different 12Environment ACS Paragon Plus

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deformations (S1 to S4) were recorded. The resistance change under strain is shown in Figure 6c. ∆R/R0 is the normalized resistance change in RGO and ∆L/L is the displacement variation in the sensing film. The resistance changes under different strain (from 0% to 1.5%) increases from 0% to 36%, which shows a good strain-response behaviour. Figure 6d illustrates the time dependence of the sensor resistance during loading. The conductive channel of the sensor consists of well-connected N-RGO stacking layers. The resistance of the conducting channel is influenced by the intrinsic resistance of N-RGO fragments and the contact resistance of stacked N-RGO fragments. The contact resistance is attributed to the stacking of adjacent N-RGO fragments, and the stacked N-RGO fragments may slide and fracture under strain. During the bending or under strain, the fractured current pathway eventually leads to the resistance change69. The strain sensor with superior stability and wide strain range has then been mounted on different locations of human body to detect different types of motion, see Figure 7. Specifically, several sensors were attached directly to the skin on the neck, hand and temporal fossa to detect response signals corresponding to bending. Forward bending of the neck was used to record periodical signals (Figure 7a, b), and the relative resistance change was almost 10, i.e. the sensor demonstrated a high resolution and excellent signal-to-noise ratio. Thus, this information could be transmitted to the monitoring processor without any amplification, filtering and other data processing operations. These operations could involve additional equipment and hence increase the cost, as well as make the sensor essentially not acceptable for a disposable type of application. Interesting, a signal from the neck bending forward displays a sharp peak followed by a broad shoulder; apparently, this movement can be decomposed into two stages, and the first stage is faster and acuter than the second one, thus producing the characteristic shape of the signal. The quality of the signal after repeated use was also investigated. For opisthenar position, the relative resistance variation between bending repetitions is shown in Figures 7c, d. This recording also demonstrated excellent sensor sensitivity and strong response. Finally, the eye blinking response was also detected with the strain sensor attached onto the temple. The eye blinking and relaxation were repeated, and strong (with the peak ratio reaching 5) response signals were recorded directly from the sensor. Thus, various types of body movements and postures, i.e. from those with large amplitude of motion such as neck and wrist bending, and down to the slight motion of eye blinking can be effectively detected by thus-produced strain sensors. This is due to a combination of compressive and tensile strain effects within the sophisticatedly treated graphene oxide, which makes this sensor platform a promising candidate for intelligent sensing, monitoring human/machine interactions, and controlling body and automatic apparatus parameters in the remote and austere environments. Further studies on the wearable sensors may involve complex materials combining graphene oxide and other surface-grown carbonous nanostructures.70,71

3. CONCLUSIONS In summary, we have demonstrated a convenient, practical technology for the production of cheap single-use graphene oxide sensors for continuous monitoring and controlling various parameters and movements of human body working in the remote and austere environments. This sensor is also suitable for monitoring of motion in the automated equipment under various 13Environment ACS Paragon Plus



conditions, from extra-terrestrial scientific instruments to remote areas on Earth, i.e. in deserts, under water and under harmful environmental conditions. Fast, eco-friendly method for simultaneous reduction and N-doping of graphene oxide via inductively coupled N2 and H2 plasma at room temperature was implemented to produce these cheap, reliable sensors. A significant influence of DC bias on the reduction of GO has been demonstrated by OES, Raman spectra, XPS and XRD. The DC bias contributes to the more efficient reduction and recovery of GO evidenced by the lowest ID/IG ratio in N-RGO-35V-5min samples. Moreover, the glow charge of N2 and H2 mixture plasmas are detected by OES to reveal the complex features of thus-generated plasmas under different biases. And higher electric temperature of biased plasmas indicates an extremely active environment inside these plasmas. Besides, simultaneous reduction and N-doping of GO can be achieve at room-temperature. Moreover, the incorporation of nitrogen into GO leads to the formation of three different N-bonded species. Our findings suggest that thus-biased N2 and H2 plasma treatment can be an effective non-thermal method to prepare not just N-RGO but other materials with controllable N-doping and reduction level, which is significant not only for sensor preparation demonstrated in this study but also other practical application in the future. Direct tests of thus-produced sensors attached to the human skin in various locations, as well as mounted onto specialized test equipment, have demonstrated strong response, high selectivity and excellent signal-to-noise levels, thus guaranteeing efficient applications of this equipment in the critical environments.

 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. 1. Table 1: Most intense spectral lines observed in N2 and H2 inductively coupled plasma discharges 2. Figure S1: The function of ICP power and Te 3. Figure S2: Contact angle of pristine PDMS 104.5°, and plasma-activated PDMS 57.5°. 4. References

 ACKNOWLEDGEMENTS This work was jointly supported by the research project of Sichuan Provincial Department of Science and Technology (2017GZ0134); Open Project of Key Laboratory of Artificial Structures & Quantum Control (Ministry of Education); Shanghai Jiao Tong University; Fundamental Research Funds for Central Universities No. ZYGX2016J055; open fund of State Key Laboratory of Luminescent Materials and Devices (No. 2018-skllmd-06); Australian Research Council; National Research Foundation and AcRF (Rp6/16 Xs), Singapore; I. L. acknowledges support from the School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology.

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Wearable, Flexible, Disposable Plasma-Reduced Graphene Oxide

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