A new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties

Original ArticleA new sustainable green protocol for production of reduced graphene oxide and its gas sensing properties Neeru Sharmaa, Vikas Sharmaa,d, Rishi Vyasa,e, Mitlesh Kumaric, A

Original Article

A new sustainable green protocol for production of reduced graphene

oxide and its gas sensing properties

Neeru Sharmaa, Vikas Sharmaa,d, Rishi Vyasa,e, Mitlesh Kumaric, Akshey Kaushald,

R Guptab,c, S.K Sharmaa, K Sachdeva,b,*

a Department of Physics, Malaviya National Institute of Technology, Jaipur 302017, India

b Materials Research Centre, Malaviya National Institute of Technology, Jaipur 302017, India

c 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

a r t i c l e i n f o

Article history:

Received 16 March 2019

Received in revised form

2 July 2019

Accepted 9 July 2019

Available online xxx

Keywords:

Green method

L -Glutathione

Graphene oxide

Reduced graphene oxide

Gas sensor

a b s t r a c t

In this report, we report a green, rapid and scalable synthetic route for the production of reduced gra-phene oxide (rGO) using an environment-friendly reducing agent (L-glutathione/L-Glu) to test its feasibility for CO& NO2gas 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

sp2carbon 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 NO2and CO) Further, L-Glu-rGO shows a smaller response time (~10.61 s for NO2and ~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/)

1 Introduction

Exposure to toxic gases puts our everyday life at risk in a

com-mercial 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

inves-tigated 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

parameters which decides the sensitivity of any material Graphene

is a material contains one atom thick layer of sp2hybridized carbon atom, which is reported to give promising results in sensing ap-plications 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 cm2v1s1, mechanical stiffness -1060 GPa, excellent light transmittance -97.7%, large surface area-2630 m2g1, and thermal conductivity -5000 W m1k1[16e18]

The graphene derivative, graphene oxide (GO), containing car-bon, 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 gra-phene derivative, having the same configuration and properties like pristine graphene, hence is suitable for electronic devices

* 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.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

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/ ).

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

do 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

cost-effective 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 usedL-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 1

shows the reaction pathway for the formation of GO and its

reduction to L-Glu-rGO

There are no previous reports mentioned the usage ofL

-gluta-thione (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 ofL-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 KMnO4and 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

2 Experimental 2.1 Materials Graphite fine powder (150 mesh, 99.5%), sodium hydroxide pellets (NaOH), potassium permanganate (KMnO4, 99%), and L -glutathione (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] Graphitefine powder (1 g) was mixed with 50 mL concentrated sulphuric acid (H2SO4) in a 500 mL conicalflask at room temper-ature 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 min 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 2 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)1 residual permanganate ions [31] We used hydrogen chloride (1 M) for thefirst wash, which removes sulfate ions from solution followed by centrifugation at the rate of 5000 rpm for

30 min 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 3 days to obtain GO

To prepare rGO, 500 mg of GO was sonicated withL-glutathione solution (200 mL) at a concentration of 1 g/L for 12 h at RT and

Fig 1 Proposed reaction pathway for the formation of GO and L-Glu-rGO.

further 5 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

gluta-thione and make the solution neutral Finally, thefiltrate was dried

at 80
C for one day to obtain rGO Resultant powder (labeled as

L-Glu-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 inFig 2 As prepared GO and L-Glu-rGO samples were then

characterized for their physical and chemical characteristics

Thinfilms (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/L-Glu-rGO) in ethylene glycol was sprayed on glass substrate using

microsyringe and spinned at 600 rpm spin speed for 2 min This

process was repeated three times and after every step, the samples

were dried at 60
C Thefilm thickness was measured using AFM,

the detail is given in supplementaryfile (Fig S3(a) and (b)) After

film deposition of sensing material, the silver paste was used to

form contact and then again thefilms were dried at 60
C.

2.3 Characterization

Synthesized samples of GO and L-Glu-rGO were characterized

by X-ray diffraction (XRD); X-pert powder diffractometer using

CuKaradiation of wavelength 1.54Å), 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-Karadiation (hn¼ 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 Result and discussion 3.1 Structural investigation X-ray diffraction, Raman spectroscopy, and Fourier infrared spectroscopy techniques affirm the production of GO and L-Glu-rGO

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-Glu-rGO 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 d-spacing of L-Glu-rGO is the reestablishment of the sp2 network [34]

3.1.2 Raman spectroscopy Fig 5shows 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 reduc-tion 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 sp2carbon atoms (G band) [23] The D band of GO is found at 1344 cm1 which is associated with the disorder due to oxygen moieties and G band is found at 1595 cm1due to CeC stretching[26] After the reduction

of GO, D and G bands shift to 1351 cm1and 1599 cm1 respec-tively A peak at 2697 cm1in 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/IGratio in-creases 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 sp2character in L-Glu-rGO which leads to

Fig 2 Pictorial depiction for step by step, synthesis of GO and its reduction using L -Glutathione to form L-Glu-rGO.

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

Sp2domain Size

Also, more graphitic domains are formed[39,40], suggesting the

restoration of sp2network 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-Glu-rGO respectively GO shows the peaks at 1058, 1148, 1636, 1728,

2925 and 3408 cm1in FT-IR spectra FT-IR spectra of GO shows absorption peaks at 3408 cm1corresponding to stretching of hy-droxyl group and at 2925 cm1attributed to sp3CeH stretching [42] The peak at 1728 cm1is assigned to C]O and at 1636 cm1

Fig 3 Flow chart for the fabrication of device based on GO and L-Glu-rGO using spin coating method.

Fig 4 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).

corresponds to C]CeO stretching [43] Peaks corresponding to

CeO of alkoxy and epoxy groups are situated at 1148 and 1058 cm1

respectively[37] These oxygen-containing groups reveal the

for-mation of GO from graphite powder during the oxidation process

Reduction of GO usingL-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 cm1 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

func-tional groups indicates the successful reduction of GO[44] A small

peak at 1572 cm1in L-Glu-rGO is attributed to the presence of C]

C stretching showing enhanced sp2character[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 byL-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

sig-nificant 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

Fig 5 Raman spectra of GO and green synthesized rGO (L-Glu-rGO) Higher I D /I G 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 6 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.

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 as-prepared 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%

Fig 7 (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 sp 2 character than GO confirm removal of oxygen moieties from L-Glu-rGO.

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 sp2in 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 1shows 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

rGO are less than that in GO The aggregated and wrinkled

L-Glu-rGO sheets are observed in FESEM imageFig 8(b) which is again an

indication of reduction[47e49] Wrinkled structure of GO and

L-Glu-rGO shows their suitability as gas sensing materials[50]

3.3 Gas sensing measurements The gas sensing behavior of GO and L-Glu-rGO was character-ized by measurement of the relative change of resistance on exposure to test gases; nitrogen dioxide (NO2) and carbon mon-oxide (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 L-Glu-rGO are seen to be thermally stable up to 800
C, which would

be helpful for choosing the temperatures for gas sensing mea-surements 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 con-centration (10 ppm) of tested gases Multiple exposures to tested gases were carried out (inset ofFig 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 inFig 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-Glu-rGO 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-Glu-rGO sensor (1.06 for NO2and 0.97 for CO at 100
C; 1.15 for NO2and 0.99 for CO at 150
C; 1.10 for NO2at 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 NO2and 1.15 for CO at 150
C; 1.18 for NO2and 1.10 for CO at 200
C) The sensitivity of thesefilms for both test gases is due to the interaction of test gas with sp2-bonded

Table 1

The relative atomic percentage of C 1s and O 1s peak for GO and L-Glu-rGO

de-termines by XPS.

Sample C 1s Deconvolution O 1s Deconvolution

C]C (sp 2 ) CeO (sp 3 ) CeC C]O C]C CeO C]O

Fig 8 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.

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

(pre-sented inTables 2 and 3) These results suggest that green

syn-thesized rGO (L-Glu-rGO) is potential candidate for sensor

applications and the performance of the sensor could be further

improved by tuning the amount ofL-glutathione used for reduction

of GO Higher amount ofL-glutathione could lead to the removal of

more oxygen functional groups which may enhance adsorption site A comparativeTable S2of gas sensing response is given in the supplementaryfile

4 Mechanism Although the stabilization mechanism of reduced graphene oxide by glutathione not so much clear, however, we have tried to

Fig 9 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 NO 2 gases at 100,

150 and 200
C (b).

Table 2

Comparative response and recovery time for GO and L-Glu-rGO towards CO gas.

Temperature (ºC) Response time (s) Recovery time (s) Response time (s) Recovery time (s)

Table 3

Comparative response and recovery time of GO and L-Glu-rGO towards NO 2 gas.

Temperature (ºC) Response time (s) Recovery time (s) Response time (s) Recovery time (s)

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.

explain the reduction mechanism as below (shown inFig 10) GO

haseOH (hydroxyl), eCOOH (carboxylic) and CeO (epoxy)

oxygen-containing 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

5 Conclusions

Reduced graphene oxide was successfully synthesized usingL

-glutathione as a green reducing agent Green synthesized

L-Glu-rGO 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

sub-strate 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

ther-mal 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

L-Glu-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 Thinfilm 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 data

Supplementary data to this article can be found online at

https://doi.org/10.1016/j.jsamd.2019.07.005

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