Ultra Sensitive NO2 Gas Detection Using the Reduced Graphene Oxide Coated Etched ﬁber Bragg Gratings

Sensors and Actuators B 223 (2016) 481–486

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Ultra sensitive NO2 gas detection using the reduced graphene oxide coated etched ﬁber Bragg gratings

Sridevi. S a, K.S. Vasu b, Navakanta Bhat c, S. Asokan a,d, A.K. Sood b,∗ a Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India b Department of Physics, Indian Institute of Science, Bangalore 560012, India c Center of Excellence in Nanoelectronics, Indian Institute of Science, Bangalore 560012, India d Robert Bosch Centre for Cyber physical Systems, Indian Institute of Science, Bangalore 560012, India article info abstract

Article history: We report a simple and highly sensitive methodology for the room temperature NO2 gas sensing using Received 4 June 2015 reduced graphene oxide (RGO) coated clad etched fiber Bragg grating (eFBG). A significant shift (>10 pm) Received in revised form is observed in the reflected Bragg wavelength (B) upon exposing RGO coated on the surface of eFBG 24 September 2015 to the NO gas molecules of concentration 0.5 ppm. The shift in Bragg wavelength is due to the change Accepted 25 September 2015 2 in the refractive index of RGO by charge transfer from the adsorbing NO molecules. The range of NO Available online 28 September 2015 2 2 concentration is tested from 0.5 ppm to 3 ppm and the estimated time taken for 50% increase in B ranges from 20 min (for 0.5 ppm) to 6 min (for 3 ppm). Keywords: Reduced graphene oxide © 2015 Elsevier B.V. All rights reserved. Fiber Bragg gratings Gas sensor Etching

1. Introduction species due to the tunable electrical conductivity caused by the chemical doping from the adsorbed molecules [9,11,12]. Though Highly sensitive, selective, cost effective and portable sensors these types of devices are highly sensitive to low ppm levels, they for the detection of toxic and inflammable gases have attracted have major limitations such as cross response issues to other gases immense attention in recent years, due to their applications in and humidity levels and limited lifetimes along with the compli- diverse fields including environmental pollution monitoring. Haz- cations in the fabrication [13]. Alternatively, optical gas sensing ardous gases such as nitrogen dioxide (NO2) have very adverse devices [13] have been developed using optical fibers [14,15] and effects if the concentration in atmosphere exceeds the limit of fiber Bragg gratings (FBGs) [16,17] based on probing the changes in 30 ppb (according to European Commission Air Quality Standards) the optical absorption or wavelength of the reflected light due to over an average period of one year. The major amount of NO2 is adsorbed gas molecules. produced from the fuel consumption and it causes the acid rain, Recently, FBGs have been emerged as an efficient biochemical degradation of ozone layer as well as the atmospheric pollution. [18,19], gas [17,20], thermal and mechanical [21,22] sensors due to Metal oxide nanowires, nanorods, fibers, nanoparticles and thin the strong dependence of Bragg wavelength (B) and optical trans- films [1–4], conducting polymers [5], organic semiconductors [6,7], mission intensity on the refractive index of surrounding medium single walled carbon nanotubes (SWNTs) [8], graphene [9] and and the other external perturbations. Further, the carbon nanoma- reduced graphene oxide (RGO) [10] and their composites have been terials (SWNTs, graphene and its derivatives) which are known used in fabricating semiconductor gas sensors (chemiresistive sen- for their widely usage in the fabrication of biochemical sensors sors), pellistors and electrochemical devices for various gas sensing [8,23], enhance the sensitivity of FBG sensors when they are coated applications. In particular, graphene has been extensively used to around the clad etched FBG sensor [18,19]. In addition, micro fiber fabricate different types of chemical and gas sensors. Graphene and micro FBGs deposited with graphene grown by chemical vapor based sensors have the ability to detect even a single molecular deposition have also been used for gas sensing based on measuring the changes in transmission intensity and Bragg wavelength [16,17]. ∗ The present study demonstrates an easy and effective way of Corresponding author. E-mail address: [email protected] (A.K. Sood). fabricating the RGO coated etched FBG (eFBG) sensors for NO2 http://dx.doi.org/10.1016/j.snb.2015.09.128 0925-4005/© 2015 Elsevier B.V. All rights reserved. 482 S. Sridevi. et al. / Sensors and Actuators B 223 (2016) 481–486

Fig. 1. (a) Schematic of NO2 gas sensing mechanism on RGO coated eFBG. (b) SEM image of eFBG sensor showing the uniformly coated RGO in the region of FBG. (c) EDAX spectrum recorded from the RGO flake (specified with the black color rectangular box in Fig. 1a) coated on the surface of eFBG sensor. (d) Raman spectrum of the GO flakes (coated on the eFBG sensors) before and after the reduction.

gas sensing. The sensor has been developed by the reduction of repetition rate of 200 Hz and a phase mask of 1069 nm pitch are graphene oxide (GO) flakes coated on the surface of eFBG. The used in the FBG fabrication. The FBGs fabricated as above are dipped gas sensing experiments have been carried out by probing the in 40% of hydrofluoric (HF) acid to etch the cladding layer around changes in Bragg wavelength which occur due to the significant the grating region. The reflected Bragg wavelength is continuously changes in effective refractive index caused by the adsorption of monitored during the etching process and the eFBG sensor is taken NO2 molecules on RGO film present on the surface of eFBG sensor. out of the HF acid, after 1 nm downshift in Bragg wavelength. The Fig. 1a shows the schematic of NO2 gas sensing mechanism on RGO eFBG is subsequently washed with DI water to remove the resid- coated eFBG. ual HF molecules. This process reduces the thickness of the clad material from ∼58 ␮mto∼0.5 ␮m [18]. Further, the surface of eFBG 2. Materials and experimental methods is made hydrophilic by treating it with NH4OH:H2O2:H2O (1:1:5) solution about 1 hour [19] followed by DI water washing. The graphite powder used in the graphite oxide preparation is The GO coating is carried out by immersing the surface modified ␮ purchased from Superior Graphite Co. (Riverside, Chicago) and the eFBG sensor in 200 L GO aqueous solution and drying the solution ◦ analytical grade Hydrazine is purchased from sigma Aldrich. The at 35 C. After completion of GO coating, the hydrazine treatment ∼ ◦ preparation method for graphite oxide is followed as given in the is undertaken for 8 h at 100 C to reduce the GO film deposited on ◦ reference [19]. Further, the GO aqueous dispersion is obtained after the eFBG surface. Lastly, the eFBG sensor is baked at 120 C for 1 h sonication of 500 ␮g of graphite oxide in 10 mL of deionized (DI) to remove the adsorbed hydrazine molecules. water. The solution is centrifuged for 3 times at 2000 rpm for 15 min The gas sensing set up used consists of a gas chamber where the and the supernatant is collected to obtain the most of the single FBG sensor is placed under the gas outlet purging nozzle. A mass layer GO flakes by avoiding the multilayers. flow controller with a maintained flow rate of 1000 sccm (standard The inscription of FBG in a single mode photosensitive opti- cubic centimeters per minute), provides the synthetic air (80% of cal fiber (125 ␮m diameter) with germania doped core (purchased N2 + 20% of O2) controlling channel and NO2 channel. The concen- from M/s Nufern) is carried out using the phase mask technique; tration of NO2 gas is set at 0.5 ppm, 1 ppm, 2 ppm and 3 ppm by a KrF excimer UV laser of wavelength 248 nm, pulse energy 6 mJ, varying the synthetic air channel flow rate; the same flow rate S. Sridevi. et al. / Sensors and Actuators B 223 (2016) 481–486 483

(a) (b)

Fig. 2. (a) Bragg wavelength as a function of time for bare FBG and eFBG sensors without coating with RGO ﬂakes. The reproducible shift in Bragg wavelength of a RGO coated eFBG sensor as a function of time for the cyclic exposure of (b) 3 ppm of only NO2 (c) 3 ppm of NO2 + 3 ppm of CO2 and (d) 3 ppm of NO2 + 3 ppm of CO. All the measurements were performed at room temperature.

of 1000 sccm is maintained throughout the experiment to have a combinational modes and overtones; 2D band at 2712 cm−1, D+G constant pressure on fiber even under different concentrations of band at 2941 cm−1 and 2G band at 3201 cm−1. After reduction of NO2. All the measurements have been carried out at room temper- GO flakes coated on the eFBG sensor, the Raman spectrum shows ature and the relative humidity of 45 (±1) %. The reflected Bragg G-band at 1593 cm−1, D-band at 1346 cm−1, 2D band at 2686 cm−1, −1 −1 wavelength of the eFBGs (B) is monitored throughout the exper- D+G band at 2932 cm and 2G band at 3193 cm , which confirm iment using an optical interrogator (Micron Optics, SM130) with a the reduction of the flakes [24]. The peak position of the 2D band is wavelength repeatability of 1 pm (picometer). similar to that of a monolayer graphene prepared using mechanical exfoliation. 3. Results and discussion

3.3. FBG based NO2 sensing 3.1. Scanning electron microscopy (SEM) characteristics of RGO on eFBG Fig. 2a shows the Bragg wavelength as a function of time for bare FBG and eFBG (without RGO coating) sensors upon cyclic Fig. 1b shows the SEM image of eFBG sensor coated with RGO exposure to 3 ppm NO2 gas. The data shown in Fig. 2a, 2b are prepared by the reduction of GO coated on the eFBG surface using averaged over 5 points. The bare FBG sensor has shown 1–2 pm hydrazine. Fig. 1c shows the EDAX spectrum of RGO on the eFBG change in Bragg wavelength (less than the measurable limit of sensor. The area from which the spectrum has been taken is marked the Bragg wavelength) and the eFBG (without RGO coating) sen- in a black color open square in Fig. 1b. From the EDAX data, the sor exhibited ∼4 pm up-shift in the Bragg wavelength. This 4 pm atomic ratio of carbon to oxygen is found to be ∼3.5 after reduction, shift can be expected from the strain applied on the eFBG (without which corroborates the reduction of GO to RGO. Since there is some RGO coating) sensor due to the 1000 sccm flowrate of the NO2 gas. oxygen atomic contribution from silica fiber, the actual atomic ratio Fig. 2b shows the Bragg wavelength as a function of time for the of carbon to oxygen in RGO would be more than 3.5. cyclic exposure of 3 ppm NO2 gas on the RGO coated eFBG sensor. In contrast to bare FBG and eFBG (without RGO coating) sensors, 3.2. Raman spectrum of GO and RGO on eFBG the RGO coated eFBG sensor shows ∼28 pm up-shift in the Bragg wavelength. The shift occurs due to the change in refractive index Fig. 1d shows the Raman spectrum before and after the reduc- of RGO caused by the charge transfer between the RGO and NO2 tion of GO flakes coated on the eFBG sensor. Raman spectrum of molecules. The RGO is unintentionally p-doped [24] and adsorption GO typically consists of two prominent bands: defect induced D- of the electron withdrawing NO2 gas molecules increases the local band at 1356 cm−1 and G-band at 1612 cm−1 arising due to the hole concentration which changes the refractive index of RGO [16]. in-plane bond stretching of sp2 hybridized carbons along with the Thus, the effective refractive index of RGO coated eFBG sensor is 484 S. Sridevi. et al. / Sensors and Actuators B 223 (2016) 481–486

Table 1

The extracted values of (B)∞ and a from the ﬁtting of adsorption data shown in Fig. 3a.

NO2 concentration (ppm) (B)∞ (pm) ␶a (min) 0.5 25.7 26.7 1 28.1 20.5 2 32.2 18.1 3 34.3 12.3

increased due to the increase in refractive index of RGO and hence the Bragg wavelength is increased after NO2 adsorption. As NO2 desorbs very slowly from RGO surface, the purging process with synthetic air of 1000 sccm flow rate has been performed to remove the NO2 gas molecules after each exposure. Bragg wavelength value of the RGO coated eFBG sensor exposed to the 3 ppm NO2, quickly dropped close to the starting value after purging with the synthetic air of 1000 sccm flow rate. The reproducibility is observed subsequently, with two more cyclic exposures of 3 ppm NO2 on the same sensor. Fig. 2c and d shows the data of Bragg wavelength as a function of time (averaged over 5 points) for the cyclic exposure of NO2 (3 ppm) + CO2 (3 ppm) and NO2 (3 ppm) + CO (3 ppm) gases on the RGO coated eFBG sensor. In the case of cyclic exposure of only NO2 (3 ppm), the RGO coated eFBG sensor has shown ∼28 pm shift in the Bragg wavelength. While for the case of cyclic exposure of NO2 (3 ppm) + CO2 (3 ppm) and NO2 (3 ppm) + CO (3 ppm) gases on the same sensor, a shift of ∼31.7 pm and ∼30.5 pm is observed, respectively. The increased shift of ∼3.7 pm in the case of NO2 (3 ppm) + CO2 (3 ppm) and ∼2.5 pm in the case of NO2 (3 ppm) + CO (3 ppm) can be attributed to the physical adsorption of a few CO2 and CO molecules on the RGO. Thus, the observed negli- gible changes in Bragg wavelength for NO2 (3 ppm) in the presence of CO2 (3 ppm) and CO (3 ppm) confirm the repeatability of RGO coated eFBG sensor and the selectivity of it toward NO2 over the other gases. However, the selectivity of RGO can be improved by specific functionalization or modification with different molecules, Fig. 3. (a) Shift in Bragg wavelength of a RGO coated eFBG sensor as a function of specifically Ag particles decorated RGO & aniline functionalized time for different concentrations of NO2 gas varying from 0.5 ppm to 3 ppm. The RGO for ammonia sensing [25,26], hydrogen plasma treated RGO inset shows one complete adsorption and desorption cycle for 3 ppm NO2 gas exposure. And the smooth lines are the fits as mentioned in the text. (b) Shift in Bragg for CO2 sensing [27] and RGO polymer & SnO2–RGO composites for wavelength of a RGO coated eFBG sensor as a function of concentration of NO after NO sensing [28,29]. 2 2 20 min and 40 min exposure. Inset: Time taken for 50% increase in shift in Bragg Since the change in local refractive index of RGO depends on the wavelength of RGO coated eFBG sensor as a function of concentrations of NO2. amount of adsorbed gas molecules, the rate of increase in the shift in Bragg wavelength with time can be modulated by the concentration of NO2 gas. Fig. 3a shows the shift in Bragg wavelength as a Fig. 3b shows the Bragg wavelength shift as a function of NO2 function of time for different concentrations of NO2 varying from concentration after exposure of RGO coated eFBG sensor for 20 min 0.5 ppm to 3 ppm. 0.5 ppm is the lower limit of concentration of NO2 and 40 min. It is worth here to mention that after 20 min of exposure which we could achieve with our set up. The data has been averaged even with 0.5 ppm, the observed shift is ∼12 pm (very high in com- over 5 s. It is observed that the shift in Bragg wavelength reaches the parison of the measuring limit of the interrogator) and we therefore saturation limit for NO2 gas exposure of concentration 3 ppm after feel that the detection of NO2 of concentration less than 0.5 ppm ∼ 40 min. Further, the sensing experiments have been performed can also be achieved using RGO coated eFBG sensor (0.5 ppm was with 2 ppm, 1 ppm and 0.5 ppm of NO2 gas to obtain the saturation the lower limit we could achieve with our experimental setup). limit in the shift in Bragg wavelength followed by the purging pro- Inset of Fig. 3b shows the time for 50% increase in the shift in Bragg cess with synthetic air of 1000 sccm flow rate after each exposure wavelength for different NO2 gas concentrations. These time val- of different concentration of NO2 gas. The inset in Fig. 3a shows ues have been estimated from the shift in Bragg wavelength as one complete cycle of adsorption and desorption for 3 ppm NO2 on a function of time data shown in Fig. 3a. It is apparent that the the RGO coated eFBG surface which causes saturation in the shift time taken for obtaining 50% increase in the shift in Bragg wave- in Bragg wavelength as a function of time. The adsorption data was length is decreased from ∼ 20 min to ∼6 min with the increase in fitted with the equation B (t)=(B)∞ (1 − exp(−t/a)), where concentration from 0.5 ppm to 3 ppm. The results obtained in our (B)∞ represents the maximum value of B at t = ∞. Similarly, experiments clearly shows that the RGO coated eFBG sensors can be the desorption data is fitted with the equation B(t)=(B)∞ used as ultra-sensitive gas sensors that can detect even 0.5 ppm of − exp( t/ d). The values of ( B)∞ and a after fitting the data with NO2 gas with great accuracy. The RGO coated eFBG sensors are cost adsorption equation, are given in Table 1. As the desorption pro- effective, portable with low level of complexity in sensor fabrica- cess was carried out using the purging process with synthetic air of tion, maintenance, operation and these sensors can also be used for 1000 sccm flow rate to remove the NO2 gas molecules, the value of the detection of other toxic gases. We have observed no degrada- d is 3.8 min for all the NO2 concentration. tion in the performance of the RGO coated eFBG sensor for at least S. Sridevi. et al. / Sensors and Actuators B 223 (2016) 481–486 485

Table 2

Comparison of different materials used for NO2 gas sensing with limit of detection with the present work.

Material Device readout Operating temperature Limit of detection Reference

RGO coated eFBG Shift in Bragg RT 0.5 ppm Present work wavelength of eFBG

ZnO nanostructures 100–350 ◦C 0.1 ppm [4] SnO2 nanoribbons RT 3 ppm Electrical Resistance ◦ SnO2-core/ZnO-shell nanoﬁbers 350 C 70 ppm ◦ TeO2 nanorods 150 C 0.5 ppm

Polypyrrole-PET 20 ppm [5] Electrical Resistance Polythiophene-CuPc RT 4.3 ppm Polyaniline-In2O3 Surface acoustic wave 0.5 ppm operational frequency

Bare CNTs 0.01 ppm [8] Vertically aligned CNTs 0.025 ppm Metal decorated CNTs Electrical Resistance RT–250 ◦C 0.1 ppm Metal oxide decorated CNTs 0.5 ppm Polymer coated CNTs 0.1 ppb

Mechanically exfoliated graphene Hall Geometry RT 1 molecule [9]

Epitaxial grown graphene 0.5 ppm [11] Chemically modiﬁed RGO Electrical Resistance RT 3.6 ppm Porous RGO 0.015 ppm

Absorption spectroscopy using blue Change in the emission RT 3 ppm [13] LED intensity Acousto-optic differential optical Differential absorbance RT 14 ppm absorption spectroscopy

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[22] X. Shu, K. Chisholm, I. Felmeri, K. Sugden, A. Gillooly, L. Zhang, I. Bennion, K.S. Vasu received his M.Sc degree in Physics from Sri Venkateswara University, Highly sensitive transverse load sensing with reversible sampled fiber Bragg India. He has finished Ph.D from Department of Physics, Indian Institute of Sci- gratings, Appl. Phys. Lett. 83 (2003) 3003–3005. ence and currently working as a research associate in the Department of Physics, [23] K.S. Vasu, K. Naresh, R.S. Bagul, N. Jayaraman, A.K. Sood, Detection of The University of Manchester. He has worked on electrical, rheological properties sugar-lectin interactions by multivalent dendritic sugar functionalized and biosensing based on graphene oxide and SWNTs. He is currently working in single-walled carbon nanotubes, Appl. Phys. Lett. 101 (2012) 053701. desalination project using the graphene oxide membranes. [24] K.S. Vasu, B. Chakraborty, S. Sampath, A.K. Sood, Probing top-gated field effect transistor of reduced graphene oxide monolayer made by dielectrophoresis, Navakanta Bhat received the B.E. degree (1989) in Electronics and Communica- Solid State Commun. 150 (2010) 1295–1298. tion from the University of Mysore, the M.Tech. degree (1992) in microelectronics [25] S. Cui, S. Mao, Z. Wen, J. Chang, Y. Zhang, J. Chen, Controllable synthesis of from IIT Bombay, and the Ph.D. degree (1996) in Electrical Engineering from silver nanoparticle-decorated reduced graphene oxide hybrids for ammonia Stanford University. He worked at Advanced Products R&D Lab, Motorola till detection, Analyst 138 (2013) 2877–2882. 1999. Since then he has been with the Indian Institute of Science, Bangalore, India, where he is currently a Professor with the Center for Nano Science and [26] X. Huang, N. Hu, L. Zhang, L. Wei, H. Wei, Y. Zhang, The NH3 sensing properties of gas sensors based on aniline reduced graphene oxide, Synth. Engineering and ECE department. His current research interest includes Nano- Met. 185–186 (2013) 25–30. CMOS technology, Gas sensors and Bio sensors for Diabetes management. He has [27] S.M. Hafiz, R. Ritikos, T.J. Whitcher, N.Md. Razib, D.C.S. Bien, N. Chanlek, H. more than 200 publications and 7 issued US patents. He is the fellow of Indian Nakajima, T. Saisopa, P. Songsiriritthigul, N.M. Huang, S.A. Rahman, A National Academy of Engineering. He is the editor of IEEE Transactions on Electron practical carbon dioxide gas sensor using room-temperature hydrogen Devices. plasma reduced graphene oxide, Sens. Actuators B 193 (2014) 692–700. S. Asokan received the M.Sc. degree in Materials Science from the College of Engi- [28] Y. Yang, S. Li, W. Yang, W. Yuan, J. Xu, Y. Jiang, In situ polymerization neering, Guindy, Anna University, Madras, India, and the Ph.D. degree in Physics deposition of porous conducting polymer on reduced graphene oxide for gas from the Indian Institute of Science, Bangalore, India. He is currently a Profes- sensor, ACS Appl. Mater. Interfaces 6 (2014) 13807–13814. sor at the Department of Instrumentation and Applied Physics and Chairman of [29] H. Zhang, J. Feng, T. Fei, S. Liu, T. Zhang, SnO nanoparticles-reduced graphene 2 the Robert Bosch Center for Cyber Physical Systems, Indian Institute of Science. oxide nanocomposites for NO sensing at low operating temperature, Sens. 2 He has edited two books and published more than 180 papers in international Actuators B 190 (2014) 472–478. journals/Books.

Biographies A.K. Sood is a Professor in the Department of Physics at Indian Institute of Science, Bangalore. His research interests include physics of nanosystems (e.g. nanotubes and graphene) and soft condensed matter. The former includes transport and Raman Sridevi. S received her B.E. degree in Instrumentation Technology and M.Tech degree spectroscopy of nanodevices to understand basic science issues in phonon renormal- in Biomedical Signal Processing and Instrumentation from Visvesvaraya Technolog- ization and mobility of carriers and to use them as nano-sensors. He has published ical University, India. She is currently pursuing Ph.D at Indian Institute of Science, more than 340 papers in refereed international journals and holds several patents. under the supervision of Prof. S. Asokan and Prof. A.K Sood. Her current research His work has been recognized by many honors, awards and fellowships of the interest includes bio-chemical sensors, gas sensors, strain, and temperature sen- Academies in India, The World Academy of Sciences (TWAS), and The Royal Society, sors. The area of work is predominantly based on nanomaterials coated on etched London. ﬁber Bragg gratings.