Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties Neeru Sharma a, Vikas Sharma a, d, Rishi Vyas a, e, Mitlesh Kumari c, Akshey Kaushal d, R Gupta b, c, S.K Sharma a, K Sachdev a, b, * a Department of Physics, Malaviya National Institute of Technology, Jaipur 302017, India Materials Research Centre, Malaviya National Institute of Technology, Jaipur 302017, India Department of Chemistry, Malaviya National Institute of Technology, Jaipur 302017, India d Department of Physics, Indian Institute of Technology Delhi, Hauz Khas 110016, India e Department of Physics, Swami Keshvanand Institute of Technology Management and Gramothan, Jaipur 302017, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 16 March 2019 Received in revised form July 2019 Accepted July 2019 Available online xxx In this report, we report a green, rapid and scalable synthetic route for the production of reduced graphene oxide (rGO) using an environment-friendly reducing agent (L-glutathione/L-Glu) to test its feasibility for CO & NO2 gas sensing The structure, morphology, and thermal stability of as-synthesized rGO are investigated via Raman spectroscopy, Fourier infrared spectroscopy, X-ray diffraction, Field emission scanning electron microscope, and thermal gravimetric analysis The L-Glu-rGO shows higher sp2 carbon hybridization (42at.%) than graphene oxide (GO) (29 at.%) The results indicate that L-Glu-rGO exhibits good relative response at 150 C to both gases (10 ppm of NO2 and CO) Further, L-Glu-rGO shows a smaller response time (~10.61 s for NO2 and ~5.05 s for CO) than GO (~16.64 s, ~11.92 s to NO2 and CO respectively) at 150 C, indicating the potential application of L-Glu-rGO for gas sensing © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Green method L-Glutathione Graphene oxide Reduced graphene oxide Gas sensor Introduction Exposure to toxic gases puts our everyday life at risk in a commercial and domestic ambiance This has led to the development of low cost and high performing gas sensors exhibiting a low level of detection for toxic gases to address health issues Gas sensors perform an important role in various areas viz agriculture, medical field, electronics, aerospace, etc Metal oxide gas sensor like Fe2O3, SnO2, In2O3, WO3, ZnO, TiO2, and MoO3 [1e9] are the most investigated ones due to their exclusive benefits such as small response time, large range of target gases, long lifetime, high sensitivity, cost efficiency, but suffer from issues such as long-term stability, and high operating temperature [10] Nanotechnology gives liberty to cultivate the next generation gas sensing layers with improved sensitivity, selectivity, fast recovery, and smaller response time for a small concentration of gas [11] Surface area is one of the favorable * Corresponding author Department of Physics, Malaviya National Institute of Technology, Jaipur 302017, India E-mail address: ksachdev.phy@mnit.ac.in (K Sachdev) Peer review under responsibility of Vietnam National University, Hanoi parameters which decides the sensitivity of any material Graphene is a material contains one atom thick layer of sp2 hybridized carbon atom, which is reported to give promising results in sensing applications due to its intrinsic electrical properties and having large surface area resulting from its nanostructure Graphene has been widely used for gas sensing, in energy storage devices [12,13], as transparent conducting electrode [14], in electrochemical sensors [15], ultrafiltration application on account of its unique properties viz very high mobility-200,000 cm2 vÀ1 sÀ1, mechanical stiffness -1060 GPa, excellent light transmittance -97.7%, large surface area-2630 m2 gÀ1, and thermal conductivity -5000 W mÀ1 kÀ1 [16e18] The graphene derivative, graphene oxide (GO), containing carbon, hydrogen, and oxygen in a varying ratio is hydrophilic and biocompatible in nature and is used in energy storage, as a biosensor, for disease detection, etc GO is a starting point for the synthesis of high quality, cost-efficient, and large yield graphene Reduced graphene oxide (rGO) is the best-known material as graphene derivative, having the same configuration and properties like pristine graphene, hence is suitable for electronic devices https://doi.org/10.1016/j.jsamd.2019.07.005 2468-2179/© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Various methods have been used for the synthesis of graphene viz; sublimation of Si wafer [19], annealing of SiC [20], CVD growth on nickel and copper [21,22], chemical and thermal reduction of GO [23], etc Large-yield production of graphene is difficult and very expensive; therefore, chemical reduction of GO to rGO is a versatile route and also cost-effective among of all above methods In this report, Hummer's method is used for the production of graphene oxide taking graphite as starting material Hydrazine hydrate [24], sodium borohydride [23], citric acid [25], amino acids [26,27], glucose [28], etc., which are frequently used for the reduction of GO not give complete removal of oxygen functional moieties from the GO Some of the reducing agents are quite toxic and harmful to human beings, therefore, an environment-friendly and costeffective synthesis route for rGO is required Ascorbic acid [28], glycine [27], and many other amino acids [26,29] have also been reported to be used as reducing agents for the production of rGO In a recent report, alanine was used for the reduction of GO leading to the removal of a large amount of oxygen moieties [26] Here, we have used L-Glutathione as a new eco-friendly reducing agent to produce rGO It is a combination of cysteine, glutamate, and glycine, and exists in oxidized (GSSG) as well as reduced (GSH) state Fig shows the reaction pathway for the formation of GO and its reduction to L-Glu-rGO There are no previous reports mentioned the usage of L-glutathione (L-Glu) to synthesize reduced GO In this paper, the motive is to examine a rapid, scalable and, an eco-friendly method for the synthesis of rGO using a small amount of L-glutathione and then test its gas sensing properties towards carbon mono oxide (CO) and nitrogen dioxide (NO2) gases In our daily life, we are exposed to these gases as pollutants in the ambient atmosphere, thus affecting human health [30] In the current work, GO was prepared by Hummer's method using oxidizing agent KMnO4 and then reduced through an environment-friendly route using L-glutathione Resulting GO and L-Glu-rGO were characterized using standard characterization techniques and then tested for detection of CO and NO2 gases for 10 ppm concentration at 100, 150 and 200 C temperature Experimental 2.1 Materials Graphite fine powder (150 mesh, 99.5%), sodium hydroxide pellets (NaOH), potassium permanganate (KMnO4, 99%), and Lglutathione (99%) were purchased from CDH Hydrogen peroxide (H2O2, 30%), and sulfuric acid (H2SO4, 98%) were purchased from RANKEM Hydrogen chloride (HCl, 12 N) was purchased from Merck All chemicals and solvents purchased were of analytical grade and used without further purification 2.2 Preparation of GO and its reduction to L-Glu-rGO Hummer's method was used for the synthesis of GO [24] Graphite fine powder (1 g) was mixed with 50 mL concentrated sulphuric acid (H2SO4) in a 500 mL conical flask at room temperature The mixture was stirred for one hour in ice bath maintained at the temperatures between 10 and 15 C KMnO4 (3 g) was added slowly to the solution to avoid a sudden change in temperature This mixture was again continuously stirred at room temperature (RT) for 30e40 A dark green color solution was obtained which was then alternately stirred and sonicated for 5-min each, ten times 200 mL deionized water (DI) was added to this green color solution, sonicated for h and then left at RT for 48 h Hydrogen peroxide was then added dropwise into the solution until the reaction quenched (to destroy the excess of potassium permanganate) This changed the color of the solution to bright yellow from dark brown which indicates partial reduction of (MnO4)À1 residual permanganate ions [31] We used hydrogen chloride (1 M) for the first wash, which removes sulfate ions from solution followed by centrifugation at the rate of 5000 rpm for 30 Subsequently, DI water was used several times to get the pH of solution ~7 The resultant precipitate was dried in an oven at 40 C for days to obtain GO To prepare rGO, 500 mg of GO was sonicated with L-glutathione solution (200 mL) at a concentration of g/L for 12 h at RT and Fig Proposed reaction pathway for the formation of GO and L-Glu-rGO Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx further h at 50 C The resultant solution was washed sequentially with NaOH solution (to convert epoxy group into hydroxyl and carboxylic group) and then DI water to remove the excess glutathione and make the solution neutral Finally, the filtrate was dried at 80 C for one day to obtain rGO Resultant powder (labeled as LGlu-rGO) was of black color indicating re-graphitization of GO [32] A pictorial pathway for the preparation of GO and L-Glu-rGO is shown in Fig As prepared GO and L-Glu-rGO samples were then characterized for their physical and chemical characteristics Thin films (of thickness 700e740 nm) of prepared samples were deposited on a glass substrate using spin coating method (shown in Fig 3) Before the deposition, glass substrates were cleaned by standard cleaning method (using acetone, de-ionized water, aqua regia, and isopropanol) The substrate was dipped in Aqua regia solution to obtain good adhesion of solution A solution of (GO/LGlu-rGO) in ethylene glycol was sprayed on glass substrate using microsyringe and spinned at 600 rpm spin speed for This process was repeated three times and after every step, the samples were dried at 60 C The film thickness was measured using AFM, the detail is given in supplementary file (Fig S3(a) and (b)) After film deposition of sensing material, the silver paste was used to form contact and then again the films were dried at 60 C 3 Result and discussion 3.1 Structural investigation X-ray diffraction, Raman spectroscopy, and Fourier infrared spectroscopy techniques affirm the production of GO and L-GlurGO 3.1.1 X-ray diffraction Fig 4(a) and (b) shows XRD pattern of GO and L-Glu-rGO respectively, showing crystalline nature for both samples Bragg's relation was used to calculate the d-spacing of GO and L-Glu-rGO [24] GO displays a strong diffraction peak at 11.08 corresponding to 0.797 nm d-spacing with an index plane of (001) [24] and large d-spacing shows a high degree of oxidation The diffraction pattern of L-Glu-rGO does not show characteristic peak of GO, and new diffraction peaks appear at 12.51, 26.79 and 35.84 corresponding to index planes (001), (002) and (111) [33] The d-spacing for L-GlurGO is found to be 0.332 nm corresponding to (002) plane, and the decrement in d-spacing indicates the removal of oxygen moieties during the reduction process Another reason for variation in dspacing of L-Glu-rGO is the reestablishment of the sp2 network [34] 2.3 Characterization Synthesized samples of GO and L-Glu-rGO were characterized by X-ray diffraction (XRD); X-pert powder diffractometer using CuKa radiation of wavelength 1.54 Å), Fourier transform infrared spectroscopy (FTIR; PerkinElmer system under transmission mode), Raman Spectroscopy (STR 500 Confocal Micro Raman Spectrometer), thermo-gravimetric analysis (TGA; PerkinElmer STA 6000), field emission scanning electron microscope (FESEM; 450 FEI, NOVA nano SEM), Atomic force microscope (Multimode probe microscope AFM), X-ray photoelectron spectroscopy (XPS, Omicron nanotechnology, Oxford instrument Germany) XPS wide scans and C1s and O1s spectra were recorded for synthesized samples by using Al-Ka radiation (hn ¼ 1486.6 eV) The pass energy for survey and wide spectra of synthesized samples was 50 eV and 20 eV respectively IeV Characteristics and gas sensing measurements of samples were recorded by digital multimeter Keithley-2400 source meter controlled by LabVIEW™ 2010 software 3.1.2 Raman spectroscopy Fig shows the Raman spectra of GO and L-Glu-rGO, giving information about the structural disorders, crystallization, defects and quality of carbon materials during oxidation and then reduction of the samples Usually, two major peaks are observed in Raman spectra of carbon materials; due to (1) vibration mode of the disorder (D band) and (2) vibration mode for sp2 carbon atoms (G band) [23] The D band of GO is found at 1344 cmÀ1 which is associated with the disorder due to oxygen moieties and G band is found at 1595 cmÀ1 due to CeC stretching [26] After the reduction of GO, D and G bands shift to 1351 cmÀ1 and 1599 cmÀ1 respectively A peak at 2697 cmÀ1 in L-Glu-rGO corresponding to 2D band is an affirmation of graphene structure [35] The observed intensity ratio (ID/IG) was 0.93 for GO It is noticed that the ID/IG ratio increases to 1.06 for L-Glu-rGO indicating reduction of graphene oxide [36] Another reason for the increment in peak intensity ratio could be due to more sp2 character in L-Glu-rGO which leads to Fig Pictorial depiction for step by step, synthesis of GO and its reduction using L-Glutathione to form L-Glu-rGO Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig Flow chart for the fabrication of device based on GO and L-Glu-rGO using spin coating method Fig The X-ray diffraction pattern of GO with major peak along with the (001) direction (a) and after reduction, L-Glu-rGO showing peaks with (001), (002) and (111) directions (b) larger structural defects [37] ID/IG ratio is more in L-Glu-rGO indicating the reduced size of the sp2 domain [38] as per the equation given below ID f IG Sp2 domain Size Also, more graphitic domains are formed [39,40], suggesting the restoration of sp2 network in L-Glu-rGO [41] Raman spectra at low temperatures taken with the same (532 nm) excitation for GO and L-Glu-rGO are given in supplementary information (Fig S1(a) and (b)) and shows similar behavior like that at room temperature 3.1.3 Fourier transform infrared spectroscopy Fig 6(a) and (b) shows FT-IR plots of synthesized GO and L-GlurGO respectively GO shows the peaks at 1058, 1148, 1636, 1728, 2925 and 3408 cmÀ1 in FT-IR spectra FT-IR spectra of GO shows absorption peaks at 3408 cmÀ1 corresponding to stretching of hydroxyl group and at 2925 cmÀ1 attributed to sp3 CeH stretching [42] The peak at 1728 cmÀ1 is assigned to C]O and at 1636 cmÀ1 Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig Raman spectra of GO and green synthesized rGO (L-Glu-rGO) Higher ID/IG ratio (1.06) in L-Glu-rGO suggests a higher reduction of oxygen functionalities and higher amount of defects present in L-Glu-rGO Fig FTIR spectra of GO (a), and L-Glu-rGO (b) GO prepared by Hummer's method showing a larger proportion of oxygen functional groups while the disappearance of oxygen moieties in L-Glu-rGO suggests removal of these oxygen groups corresponds to C]CeO stretching [43] Peaks corresponding to CeO of alkoxy and epoxy groups are situated at 1148 and 1058 cmÀ1 respectively [37] These oxygen-containing groups reveal the formation of GO from graphite powder during the oxidation process Reduction of GO using L-glutathione leads to a drastic decrease in bands related to functional groups containing oxygen The FT-IR spectrum of L-Glu-rGO shows peaks centered at positions 3423, 1572 and 1367 cmÀ1 It is observed that the intensity of peaks associated with oxygen functional groups (eCeH, C]C, and eOH) decreases; peaks are observed to disappear in some cases (C]O, CeH, CeO, and ]CeH) The disappearance of some oxygen functional groups indicates the successful reduction of GO [44] A small peak at 1572 cmÀ1 in L-Glu-rGO is attributed to the presence of C] C stretching showing enhanced sp2 character [23] These results of FTIR are in agreement with XRD and Raman results, establishing oxidation of graphite to GO and then removal of oxygen functional groups from GO in large proportions on reduction by L-Glutathione 3.1.4 X-ray photoelectron spectroscopy The elemental composition of the surface was analyzed using XPS via classification of the binding energies of ejected electrons from the component elements C1s and O1s spectra were deconvoluted using Casa XPS™ software with Shirley background correction The survey spectra of GO and L-Glu-rGO record significant signals of carbon and oxygen in both sample shown in Fig 7(a) Fig 7(b, c) shows C1s and O1s XPS spectra of GO respectively The deconvoluted peaks of C 1s spectrum of GO were obtained as indicative of C]C (sp2) at 284.3 eV, CeO (sp3) at Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig (a) Wide survey spectra for GO and L-Glu-rGO, (b) C 1s spectra, (c) O 1s XPS spectra recorded for GO, (d) C 1s spectra, (e) O 1s XPS spectra recorded for L-Glu-rGO; XPS spectra of green synthesized L-Glu-rGO shows higher sp2 character than GO confirm removal of oxygen moieties from L-Glu-rGO 284.5 eV, CeC at 285.1 eV and C]O at 286.6 eV [45] with FWHM 1.0, 1.85, 1.91, and 3.3 respectively Separation of sp2 and sp3 represents effective XPS observation Fig 7(b) gives 29% of sp2 carbon, 34% of sp3 carbon, 16.7% of (CeC) and 16.7% of the carbonyl group (C]O) for GO C1s spectra of GO shows higher content of sp3 than sp2 hybridized carbons pertaining to oxidation of graphite to GO In Fig 7(c), O 1s spectra of asprepared GO was seen as decomposed into three peaks with binding energy (BE) 531.0 eV corresponding to a carboxyl group (C]O), 532.3 eV corresponding to a carbonyl group (C]O), and 532.6 eV corresponding to a hydroxyl group (CeO) O 1S spectra fitting of GO represents 54% content of carboxyl group, 26% Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx content of carbonyl and 20% content of hydroxyl group Thus, both C 1s and O 1s spectrum of GO are indicative of a significant degree of oxidation Fig 7(d) and (e) shows C1s spectra, and O1s spectra of L-Glu-rGO respectively The BE values for different hydrocarbon peaks in C 1s spectra of L-Glu-rGO are 284.3 eV for (C]C/CeC), 284.7 eV for (CeO), 285.1 eV for CeC and 287.6 eV corresponding to (C]O) [29] The percentage of these different carbon peaks in L-Glu-rGO obtained from the spectrum are 42%, 37%, 7% and 14% attributed to sp2, sp3, CeC and C]O bonding respectively Higher content of sp2 in L-Glu-rGO than that in GO clearly confirms the reduction of GO [46] FWHM in C 1s spectra of L-Glu-rGO was 1.0, 2.6, 1.9, and 2.1 attributed to C]C, C]O, CeC, and CeO respectively Fig 7(e) represents core level spectra of O 1s of L-Glu-rGO which is deconvoluted into three peaks of C]O, CeO, and CeO bonding present at 531.2 eV, 532.2 eV, and 533.5 eV respectively C]O (60.4%), CeO (24%) and CeO (15.6%) are associated with carboxyl, carbonyl, and hydroxyl group respectively [44] Table shows the relative atomic percentage of C 1s and O 1s spectra in the XPS spectra Thus, the results of XPS, Raman, FTIR, and XRD of L-Glu-rGO are a clear indication of the loss of functionalized oxygen groups, and hence a decrease in interlayer spacing on reduction 3.2 Morphological investigation 3.2.1 Field emission scanning electron microscopy Fig 8(a) and (b) shows the morphology of GO and L-Glu-rGO respectively, examined by FESEM FESEM micrograph of reduced graphene oxide indicates stacked layered and corrugated structure The surface of L-Glu-rGO sample becoming corrugated is evidence of the removal of oxygen molecules from GO, yielding L-Glu-rGO that is dominated by defects FESEM micrographs of both samples show wrinkled structure also but the numbers of layers in L-GlurGO are less than that in GO The aggregated and wrinkled L-GlurGO sheets are observed in FESEM image Fig 8(b) which is again an indication of reduction [47e49] Wrinkled structure of GO and LGlu-rGO shows their suitability as gas sensing materials [50] Table The relative atomic percentage of C 1s and O 1s peak for GO and L-Glu-rGO determines by XPS Sample GO L-Glu-rGO C 1s Deconvolution O 1s Deconvolution C]C (sp2) CeO (sp3) CeC C]O C]C CeO C]O 29 42 34 37 16.7 20.1 14 54 60.4 26 24 20 15.6 3.3 Gas sensing measurements The gas sensing behavior of GO and L-Glu-rGO was characterized by measurement of the relative change of resistance on exposure to test gases; nitrogen dioxide (NO2) and carbon monoxide (CO) Both of these are lethal gases since they are colorless as well as odorless and thus the increased concentration in confined space may remain undetected without the use of electronic nose The thermogravimetric analysis of GO and L-Glu-rGO is shown in supplementary information in Fig S 2(a) and (b) Both GO and LGlu-rGO are seen to be thermally stable up to 800 C, which would be helpful for choosing the temperatures for gas sensing measurements The IeV characteristics of synthesized samples GO and L-Glu-rGO shown in supplementary Fig S 4(a) and (b), display ohmic behavior which is in favor of suitability of both samples for gas sensing properties A constant current was sourced and voltage was recorded to calculate the resistance of specimen in a two probe configuration The variation in resistance of specimen on tested gas exposure is preamble of active surface sites available on the surface of GO and L-Glu-rGO and therefore the as-prepared GO and L-Glu-rGO must show comparable sensitivity to the test gases as given in several reports [37,51e53] Multiple operating temperatures (100 C, 150 C, 200 C) were selected for the gas sensing measurement Both the specimens were found to be exhibiting sensitivity towards moderate concentration (10 ppm) of tested gases Multiple exposures to tested gases were carried out (inset of Fig 9(a)) to obtain the average value of sensitivity, response time, and recovery time after obtaining a smooth baseline with no drift A sample calculation of response and recovery time is also shown in Fig 9(a) The response time is the time taken by the signal to obtain 90% of the maximum height of signal from baseline and recovery time is the time taken to reach to the 10% value of the maximum height of the signal from baseline [54] Fig 9(b) summarizes the variation of response for GO and L-GlurGO to 10 ppm NO2/CO with operating temperature It is indicative of higher sensitivity of both the specimen towards NO2 gas as compared to CO which is in accordance with other reports from Guo et al [55] It is further seen that tested gas sensitivity of L-GlurGO sensor (1.06 for NO2 and 0.97 for CO at 100 C; 1.15 for NO2 and 0.99 for CO at 150 C; 1.10 for NO2 at 200 C, L-Glu-rGO does not show sensitivity toward 10 ppm CO at 200 C, due to smaller signal to noise ratio) is almost similar to that of GO sensor (1.15 for NO2 and 1.07 for CO at 100 C; 1.41 for NO2 and 1.15 for CO at 150 C; 1.18 for NO2 and 1.10 for CO at 200 C) The sensitivity of these films for both test gases is due to the interaction of test gas with sp2-bonded Fig FESEM images of GO (a) and L-Glu-rGO (b) showing stacked and layered structures Corrugated and wrinkled structure makes them good gas sensing samples Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig Response and recovery curve of GO based gas sensor toward 10 ppm CO at 150 C (a), sensitivity of GO and L-Glu-rGO based sensor toward 10 ppm CO and NO2 gases at 100, 150 and 200 C (b) Table Comparative response and recovery time for GO and L-Glu-rGO towards CO gas Gas CO Samples GO Temperature (ºC) Response time (s) Recovery time (s) Response time (s) L-Glu-rGO Recovery time (s) 100 150 200 12.88 11.92 12.22 20.92 21.72 22.52 10.65 5.05 e 14.94 14.86 e Table Comparative response and recovery time of GO and L-Glu-rGO towards NO2 gas Gas NO2 Samples GO Temperature (ºC) Response time (s) Recovery time (s) Response time (s) Recovery time (s) 100 150 200 14.98 16.64 16.21 14.24 16.57 21.04 10.81 10.61 7.44 14.97 10.29 10.36 carbon, oxygen functional group, vacancies, and structural defects Green synthesized reduced graphene oxide (L-Glu-rGO) shows much smaller response and recovery time than that of GO (presented in Tables and 3) These results suggest that green synthesized rGO (L-Glu-rGO) is potential candidate for sensor applications and the performance of the sensor could be further improved by tuning the amount of L-glutathione used for reduction of GO Higher amount of L-glutathione could lead to the removal of L-Glu-rGO more oxygen functional groups which may enhance adsorption site A comparative Table S2 of gas sensing response is given in the supplementary file Mechanism Although the stabilization mechanism of reduced graphene oxide by glutathione not so much clear, however, we have tried to Fig 10 Mechanism of formation of L-Glu-rGO from GO (a) GSH state of Glutathione releases proton and changes to GSSG state (b) conversion of GO to L-Glu-rGO through GSH Please cite this article as: N Sharma et al., A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.07.005 N Sharma et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx explain the reduction mechanism as below (shown in Fig 10) GO has eOH (hydroxyl), eCOOH (carboxylic) and CeO (epoxy) oxygencontaining groups Reduced graphene oxide via glutathione in reduced state release proton and bind to another molecule of glutathione to make GSSG (glutathione disulfide) [29] The proton can bind to active oxygen species of GO, produce water molecule and black homogenous solution of reduced graphene oxide The black color of rGO indicates re-graphatization Conclusions Reduced graphene oxide was successfully synthesized using Lglutathione as a green reducing agent Green synthesized L-GlurGO showed higher amount of sp2 character and lesser oxygen content than that for GO, examined through XRD, FTIR, Raman and XPS techniques Thicknesses of deposited samples on glass substrate were investigated by AFM and were obtained as 700e740 nm A favorable highly corrugated and layered structure of GO and L-Glu-rGO is seen by FESEM, and hence makes them beneficial for gas sensing properties Both samples show ohmic behavior in IeV characteristics Higher values of current and thermal stability are observed for L-Glu-rGO as compared to that of GO giving an advantage of using L-Glu-rGO based sensors at lower voltages The investigation has proved that green synthesized LGlu-rGO exhibit significant sensitivity for 10 ppm concentration for both gases at 150 C and at the same time exhibit smaller response time and recovery time It is believed that these results provide a pathway to further explore the feasibility of green synthesized rGO (L-Glu-rGO) for gas sensing properties It also provides further motivation to investigate a sustainable green method for synthesis of rGO with minimum harm to the environment Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors Acknowledgment The authors would like to thank Materials Research Center, MNIT J India, and Thin film lab, IIT Delhi, New Delhi India for characterization and gas sensing facilities for present study Thanks to Dr Satyavir Singh and Dr Rajkumar, MNIT Jaipur, for helping me to carry out this work Appendix A Supplementary 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