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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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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
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 Department of Physics, Indian Institute of Science, Bangalore 560012, India 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 b c
a r t i c l e
i n f o
Article history: Received 4 June 2015 Received in revised form 24 September 2015 Accepted 25 September 2015 Available online 28 September 2015 Keywords: Reduced graphene oxide Fiber Bragg gratings Gas sensor Etching
a b s t r a c t We report a simple and highly sensitive methodology for the room temperature NO2 gas sensing using reduced graphene oxide (RGO) coated clad etched fiber Bragg grating (eFBG). A significant shift (>10 pm) is observed in the reflected Bragg wavelength (B ) upon exposing RGO coated on the surface of eFBG to the NO2 gas molecules of concentration 0.5 ppm. The shift in Bragg wavelength is due to the change in the refractive index of RGO by charge transfer from the adsorbing NO2 molecules. The range of NO2 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). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Highly sensitive, selective, cost effective and portable sensors for the detection of toxic and inflammable gases have attracted immense attention in recent years, due to their applications in diverse fields including environmental pollution monitoring. Hazardous gases such as nitrogen dioxide (NO2 ) have very adverse effects if the concentration in atmosphere exceeds the limit of 30 ppb (according to European Commission Air Quality Standards) over an average period of one year. The major amount of NO2 is produced from the fuel consumption and it causes the acid rain, degradation of ozone layer as well as the atmospheric pollution. Metal oxide nanowires, nanorods, fibers, nanoparticles and thin films [1–4], conducting polymers [5], organic semiconductors [6,7], single walled carbon nanotubes (SWNTs) [8], graphene [9] and reduced graphene oxide (RGO) [10] and their composites have been used in fabricating semiconductor gas sensors (chemiresistive sensors), pellistors and electrochemical devices for various gas sensing applications. In particular, graphene has been extensively used to fabricate different types of chemical and gas sensors. Graphene based sensors have the ability to detect even a single molecular
∗ Corresponding author. E-mail address: [email protected] (A.K. Sood). http://dx.doi.org/10.1016/j.snb.2015.09.128 0925-4005/© 2015 Elsevier B.V. All rights reserved.
species due to the tunable electrical conductivity caused by the chemical doping from the adsorbed molecules [9,11,12]. Though these types of devices are highly sensitive to low ppm levels, they have major limitations such as cross response issues to other gases and humidity levels and limited lifetimes along with the complications in the fabrication [13]. Alternatively, optical gas sensing devices [13] have been developed using optical fibers [14,15] and fiber Bragg gratings (FBGs) [16,17] based on probing the changes in the optical absorption or wavelength of the reflected light due to adsorbed gas molecules. Recently, FBGs have been emerged as an efficient biochemical [18,19], gas [17,20], thermal and mechanical [21,22] sensors due to the strong dependence of Bragg wavelength (B ) and optical transmission intensity on the refractive index of surrounding medium and the other external perturbations. Further, the carbon nanomaterials (SWNTs, graphene and its derivatives) which are known for their widely usage in the fabrication of biochemical sensors [8,23], enhance the sensitivity of FBG sensors when they are coated around the clad etched FBG sensor [18,19]. In addition, micro fiber and micro FBGs deposited with graphene grown by chemical vapor 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 fabricating the RGO coated etched FBG (eFBG) sensors for NO2
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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 graphene oxide (GO) flakes coated on the surface of eFBG. The gas sensing experiments have been carried out by probing the changes in Bragg wavelength which occur due to the significant changes in effective refractive index caused by the adsorption of NO2 molecules on RGO film present on the surface of eFBG sensor. Fig. 1a shows the schematic of NO2 gas sensing mechanism on RGO coated eFBG. 2. Materials and experimental methods The graphite powder used in the graphite oxide preparation is purchased from Superior Graphite Co. (Riverside, Chicago) and the analytical grade Hydrazine is purchased from sigma Aldrich. The preparation method for graphite oxide is followed as given in the reference [19]. Further, the GO aqueous dispersion is obtained after sonication of 500 g of graphite oxide in 10 mL of deionized (DI) water. The solution is centrifuged for 3 times at 2000 rpm for 15 min and the supernatant is collected to obtain the most of the single layer GO flakes by avoiding the multilayers. The inscription of FBG in a single mode photosensitive optical fiber (125 m diameter) with germania doped core (purchased from M/s Nufern) is carried out using the phase mask technique; a KrF excimer UV laser of wavelength 248 nm, pulse energy 6 mJ,
repetition rate of 200 Hz and a phase mask of 1069 nm pitch are used in the FBG fabrication. The FBGs fabricated as above are dipped in 40% of hydrofluoric (HF) acid to etch the cladding layer around the grating region. The reflected Bragg wavelength is continuously monitored during the etching process and the eFBG sensor is taken out of the HF acid, after 1 nm downshift in Bragg wavelength. The eFBG is subsequently washed with DI water to remove the residual HF molecules. This process reduces the thickness of the clad material from ∼58 m to ∼0.5 m [18]. Further, the surface of eFBG is made hydrophilic by treating it with NH4 OH:H2 O2 :H2 O (1:1:5) solution about 1 hour [19] followed by DI water washing. The GO coating is carried out by immersing the surface modified eFBG sensor in 200 L GO aqueous solution and drying the solution at 35 ◦ C. After completion of GO coating, the hydrazine treatment is undertaken for ∼8 h at 100 ◦ C to reduce the GO film deposited on the eFBG surface. Lastly, the eFBG sensor is baked at 120 ◦ C for 1 h to remove the adsorbed hydrazine molecules. The gas sensing set up used consists of a gas chamber where the FBG sensor is placed under the gas outlet purging nozzle. A mass flow controller with a maintained flow rate of 1000 sccm (standard cubic centimeters per minute), provides the synthetic air (80% of N2 + 20% of O2 ) controlling channel and NO2 channel. The concentration of NO2 gas is set at 0.5 ppm, 1 ppm, 2 ppm and 3 ppm by varying the synthetic air channel flow rate; the same flow rate
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(a)
(b)
(c)
(d)
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 constant pressure on fiber even under different concentrations of NO2 . All the measurements have been carried out at room temperature and the relative humidity of 45 (±1) %. The reflected Bragg wavelength of the eFBGs (B ) is monitored throughout the experiment using an optical interrogator (Micron Optics, SM130) with a wavelength repeatability of 1 pm (picometer).
combinational modes and overtones; 2D band at 2712 cm−1 , D+G band at 2941 cm−1 and 2G band at 3201 cm−1 . After reduction of GO flakes coated on the eFBG sensor, the Raman spectrum shows G-band at 1593 cm−1 , D-band at 1346 cm−1 , 2D band at 2686 cm−1 , D+G band at 2932 cm−1 and 2G band at 3193 cm−1 , which confirm the reduction of the flakes [24]. The peak position of the 2D band is similar to that of a monolayer graphene prepared using mechanical exfoliation.
3. Results and discussion 3.1. Scanning electron microscopy (SEM) characteristics of RGO on eFBG Fig. 1b shows the SEM image of eFBG sensor coated with RGO prepared by the reduction of GO coated on the eFBG surface using hydrazine. Fig. 1c shows the EDAX spectrum of RGO on the eFBG sensor. The area from which the spectrum has been taken is marked in a black color open square in Fig. 1b. From the EDAX data, the atomic ratio of carbon to oxygen is found to be ∼3.5 after reduction, which corroborates the reduction of GO to RGO. Since there is some oxygen atomic contribution from silica fiber, the actual atomic ratio of carbon to oxygen in RGO would be more than 3.5. 3.2. Raman spectrum of GO and RGO on eFBG Fig. 1d shows the Raman spectrum before and after the reduction of GO flakes coated on the eFBG sensor. Raman spectrum of GO typically consists of two prominent bands: defect induced Dband at 1356 cm−1 and G-band at 1612 cm−1 arising due to the in-plane bond stretching of sp2 hybridized carbons along with the
3.3. FBG based NO2 sensing Fig. 2a shows the Bragg wavelength as a function of time for bare FBG and eFBG (without RGO coating) sensors upon cyclic exposure to 3 ppm NO2 gas. The data shown in Fig. 2a, 2b are averaged over 5 points. The bare FBG sensor has shown 1–2 pm change in Bragg wavelength (less than the measurable limit of the Bragg wavelength) and the eFBG (without RGO coating) sensor exhibited ∼4 pm up-shift in the Bragg wavelength. This 4 pm shift can be expected from the strain applied on the eFBG (without RGO coating) sensor due to the 1000 sccm flowrate of the NO2 gas. Fig. 2b shows the Bragg wavelength as a function of time for the cyclic exposure of 3 ppm NO2 gas on the RGO coated eFBG sensor. In contrast to bare FBG and eFBG (without RGO coating) sensors, the RGO coated eFBG sensor shows ∼28 pm up-shift in the Bragg wavelength. The shift occurs due to the change in refractive index of RGO caused by the charge transfer between the RGO and NO2 molecules. The RGO is unintentionally p-doped [24] and adsorption of the electron withdrawing NO2 gas molecules increases the local hole concentration which changes the refractive index of RGO [16]. Thus, the effective refractive index of RGO coated eFBG sensor is
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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 1 2 3
25.7 28.1 32.2 34.3
26.7 20.5 18.1 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 negligible 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, specifically Ag particles decorated RGO & aniline functionalized RGO for ammonia sensing [25,26], hydrogen plasma treated RGO for CO2 sensing [27] and RGO polymer & SnO2 –RGO composites for NO2 sensing [28,29]. Since the change in local refractive index of RGO depends on the 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 function of time for different concentrations of NO2 varying from 0.5 ppm to 3 ppm. 0.5 ppm is the lower limit of concentration of NO2 which we could achieve with our set up. The data has been averaged over 5 s. It is observed that the shift in Bragg wavelength reaches the saturation limit for NO2 gas exposure of concentration 3 ppm after ∼ 40 min. Further, the sensing experiments have been performed with 2 ppm, 1 ppm and 0.5 ppm of NO2 gas to obtain the saturation limit in the shift in Bragg wavelength followed by the purging process with synthetic air of 1000 sccm flow rate after each exposure of different concentration of NO2 gas. The inset in Fig. 3a shows one complete cycle of adsorption and desorption for 3 ppm NO2 on the RGO coated eFBG surface which causes saturation in the shift in Bragg wavelength as a function of time. The adsorption data was fitted with the equation B (t) = (B )∞ (1 − exp(−t/ a )), where (B )∞ represents the maximum value of B at t = ∞. Similarly, the desorption data is fitted with the equation B (t) = (B )∞ exp(−t/ d ). The values of (B )∞ and a after fitting the data with adsorption equation, are given in Table 1. As the desorption process was carried out using the purging process with synthetic air of 1000 sccm flow rate to remove the NO2 gas molecules, the value of d is 3.8 min for all the NO2 concentration.
Fig. 3. (a) Shift in Bragg wavelength of a RGO coated eFBG sensor as a function of time for different concentrations of NO2 gas varying from 0.5 ppm to 3 ppm. The 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 wavelength of a RGO coated eFBG sensor as a function of concentration of NO2 after 20 min and 40 min exposure. Inset: Time taken for 50% increase in shift in Bragg wavelength of RGO coated eFBG sensor as a function of concentrations of NO2 .
Fig. 3b shows the Bragg wavelength shift as a function of NO2 concentration after exposure of RGO coated eFBG sensor for 20 min and 40 min. It is worth here to mention that after 20 min of exposure even with 0.5 ppm, the observed shift is ∼12 pm (very high in comparison of the measuring limit of the interrogator) and we therefore feel that the detection of NO2 of concentration less than 0.5 ppm can also be achieved using RGO coated eFBG sensor (0.5 ppm was the lower limit we could achieve with our experimental setup). Inset of Fig. 3b shows the time for 50% increase in the shift in Bragg wavelength for different NO2 gas concentrations. These time values have been estimated from the shift in Bragg wavelength as a function of time data shown in Fig. 3a. It is apparent that the time taken for obtaining 50% increase in the shift in Bragg wavelength is decreased from ∼ 20 min to ∼6 min with the increase in concentration from 0.5 ppm to 3 ppm. The results obtained in our experiments clearly shows that the RGO coated eFBG sensors can be used as ultra-sensitive gas sensors that can detect even 0.5 ppm of NO2 gas with great accuracy. The RGO coated eFBG sensors are cost effective, portable with low level of complexity in sensor fabrication, maintenance, operation and these sensors can also be used for the detection of other toxic gases. We have observed no degradation in the performance of the RGO coated eFBG sensor for at least
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Table 2 Comparison of different materials used for NO2 gas sensing with limit of detection with the present work. Material
Device readout
Operating temperature
RGO coated eFBG
Shift in Bragg wavelength of eFBG
RT
ZnO nanostructures SnO2 nanoribbons SnO2-core/ZnO-shell nanofibers TeO2 nanorods Polypyrrole-PET Polythiophene-CuPc Polyaniline-In2 O3
Electrical Resistance
Electrical Resistance
Reference Present work
0.1 ppm 3 ppm 70 ppm 0.5 ppm
[4]
20 ppm 4.3 ppm 0.5 ppm
[5]
RT
[8]
RT–250 ◦ C
0.01 ppm 0.025 ppm 0.1 ppm 0.5 ppm 0.1 ppb
RT
1 molecule
[9] [11]
Electrical Resistance
RT
0.5 ppm 3.6 ppm 0.015 ppm
Change in the emission intensity Differential absorbance
RT
3 ppm
[13]
RT
14 ppm
Electrical Resistance
Mechanically exfoliated graphene
Hall Geometry
Absorption spectroscopy using blue LED Acousto-optic differential optical absorption spectroscopy
0.5 ppm
100–350 ◦ C RT 350 ◦ C 150 ◦ C
Surface acoustic wave operational frequency
Bare CNTs Vertically aligned CNTs Metal decorated CNTs Metal oxide decorated CNTs Polymer coated CNTs
Epitaxial grown graphene Chemically modified RGO Porous RGO
Limit of detection
6 months. We feel that even longer aging times will not affect the sensor performance and RGO coated on the eFBG surface is stable for a much longer time at room temperature. 4. Conclusions In the present work, an optical platform has been demonstrated for the room temperature sensing of NO2 gas, with a lower detection limit of 0.5 ppm with 0.8 pm/min sensitivity. The adsorption and desorption of the NO2 gas molecules on RGO changes the refractive index by increasing and decreasing the local carrier density. The optical read out of the RGO coated eFBG sensor, namely the shift in Bragg wavelength, is highly sensitive to the changes in refractive index of RGO flakes coated on the surface of eFBG sensor. RGO coated eFBG sensor detects the NO2 gas in sub ppm level and the sensor performance can be improved to sub ppb level as well as the response time can be decreased by having multiple gratings in the same fiber. Table 2 shows the comparison of different materials used for NO2 gas sensing with limit of detection with our present work. Acknowledgements Prof. A.K. Sood thanks the Nanomission project of Department of Science and Technology for financial assistance. Prof. S. Asokan, acknowledges the Centre for Strategic Initiates and Robert Bosch Centre for Cyber Physical Systems, Indian Institute of Science, for support. References [1] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors 10 (2010) 2088–2106. [2] M. Tiemann, Porous metal oxides as gas sensors, Chem. Eur. J. 13 (2007) 8376–8388. [3] A.A. Tomchenko, G.P. Harmer, B.T. Marquis, J.W. Allen, Semiconducting metal oxide sensor array for the selective detection of combustion gases, Sens. Actuators B 93 (2003) 126–134. [4] S. Park, S. Kim, G. Sun, W.I. Lee, K.K. Kim, C. Lee, Fabrication and NO2 gas sensing performance of TeO2 -core/CuO-shell heterostructure nanorod sensors, Nanoscale Res. Lett. 9 (2014) 638–645.
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Biographies Sridevi. S received her B.E. degree in Instrumentation Technology and M.Tech degree in Biomedical Signal Processing and Instrumentation from Visvesvaraya Technological University, India. She is currently pursuing Ph.D at Indian Institute of Science, under the supervision of Prof. S. Asokan and Prof. A.K Sood. Her current research interest includes bio-chemical sensors, gas sensors, strain, and temperature sensors. The area of work is predominantly based on nanomaterials coated on etched fiber Bragg gratings.
K.S. Vasu received his M.Sc degree in Physics from Sri Venkateswara University, India. He has finished Ph.D from Department of Physics, Indian Institute of Science and currently working as a research associate in the Department of Physics, The University of Manchester. He has worked on electrical, rheological properties and biosensing based on graphene oxide and SWNTs. He is currently working in desalination project using the graphene oxide membranes. Navakanta Bhat received the B.E. degree (1989) in Electronics and Communication from the University of Mysore, the M.Tech. degree (1992) in microelectronics from IIT Bombay, and the Ph.D. degree (1996) in Electrical Engineering from Stanford University. He worked at Advanced Products R&D Lab, Motorola till 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 Engineering and ECE department. His current research interest includes NanoCMOS technology, Gas sensors and Bio sensors for Diabetes management. He has more than 200 publications and 7 issued US patents. He is the fellow of Indian National Academy of Engineering. He is the editor of IEEE Transactions on Electron Devices. S. Asokan received the M.Sc. degree in Materials Science from the College of Engineering, Guindy, Anna University, Madras, India, and the Ph.D. degree in Physics from the Indian Institute of Science, Bangalore, India. He is currently a Professor at the Department of Instrumentation and Applied Physics and Chairman of the Robert Bosch Center for Cyber Physical Systems, Indian Institute of Science. He has edited two books and published more than 180 papers in international journals/Books. 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 spectroscopy of nanodevices to understand basic science issues in phonon renormalization and mobility of carriers and to use them as nano-sensors. He has published more than 340 papers in refereed international journals and holds several patents. His work has been recognized by many honors, awards and fellowships of the Academies in India, The World Academy of Sciences (TWAS), and The Royal Society, London.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
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 Department of Physics, Indian Institute of Science, Bangalore 560012, India 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 b c
a r t i c l e
i n f o
Article history: Received 4 June 2015 Received in revised form 24 September 2015 Accepted 25 September 2015 Available online 28 September 2015 Keywords: Reduced graphene oxide Fiber Bragg gratings Gas sensor Etching
a b s t r a c t We report a simple and highly sensitive methodology for the room temperature NO2 gas sensing using reduced graphene oxide (RGO) coated clad etched fiber Bragg grating (eFBG). A significant shift (>10 pm) is observed in the reflected Bragg wavelength (B ) upon exposing RGO coated on the surface of eFBG to the NO2 gas molecules of concentration 0.5 ppm. The shift in Bragg wavelength is due to the change in the refractive index of RGO by charge transfer from the adsorbing NO2 molecules. The range of NO2 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). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Highly sensitive, selective, cost effective and portable sensors for the detection of toxic and inflammable gases have attracted immense attention in recent years, due to their applications in diverse fields including environmental pollution monitoring. Hazardous gases such as nitrogen dioxide (NO2 ) have very adverse effects if the concentration in atmosphere exceeds the limit of 30 ppb (according to European Commission Air Quality Standards) over an average period of one year. The major amount of NO2 is produced from the fuel consumption and it causes the acid rain, degradation of ozone layer as well as the atmospheric pollution. Metal oxide nanowires, nanorods, fibers, nanoparticles and thin films [1–4], conducting polymers [5], organic semiconductors [6,7], single walled carbon nanotubes (SWNTs) [8], graphene [9] and reduced graphene oxide (RGO) [10] and their composites have been used in fabricating semiconductor gas sensors (chemiresistive sensors), pellistors and electrochemical devices for various gas sensing applications. In particular, graphene has been extensively used to fabricate different types of chemical and gas sensors. Graphene based sensors have the ability to detect even a single molecular
∗ Corresponding author. E-mail address: [email protected] (A.K. Sood). http://dx.doi.org/10.1016/j.snb.2015.09.128 0925-4005/© 2015 Elsevier B.V. All rights reserved.
species due to the tunable electrical conductivity caused by the chemical doping from the adsorbed molecules [9,11,12]. Though these types of devices are highly sensitive to low ppm levels, they have major limitations such as cross response issues to other gases and humidity levels and limited lifetimes along with the complications in the fabrication [13]. Alternatively, optical gas sensing devices [13] have been developed using optical fibers [14,15] and fiber Bragg gratings (FBGs) [16,17] based on probing the changes in the optical absorption or wavelength of the reflected light due to adsorbed gas molecules. Recently, FBGs have been emerged as an efficient biochemical [18,19], gas [17,20], thermal and mechanical [21,22] sensors due to the strong dependence of Bragg wavelength (B ) and optical transmission intensity on the refractive index of surrounding medium and the other external perturbations. Further, the carbon nanomaterials (SWNTs, graphene and its derivatives) which are known for their widely usage in the fabrication of biochemical sensors [8,23], enhance the sensitivity of FBG sensors when they are coated around the clad etched FBG sensor [18,19]. In addition, micro fiber and micro FBGs deposited with graphene grown by chemical vapor 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 fabricating the RGO coated etched FBG (eFBG) sensors for NO2
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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 graphene oxide (GO) flakes coated on the surface of eFBG. The gas sensing experiments have been carried out by probing the changes in Bragg wavelength which occur due to the significant changes in effective refractive index caused by the adsorption of NO2 molecules on RGO film present on the surface of eFBG sensor. Fig. 1a shows the schematic of NO2 gas sensing mechanism on RGO coated eFBG. 2. Materials and experimental methods The graphite powder used in the graphite oxide preparation is purchased from Superior Graphite Co. (Riverside, Chicago) and the analytical grade Hydrazine is purchased from sigma Aldrich. The preparation method for graphite oxide is followed as given in the reference [19]. Further, the GO aqueous dispersion is obtained after sonication of 500 g of graphite oxide in 10 mL of deionized (DI) water. The solution is centrifuged for 3 times at 2000 rpm for 15 min and the supernatant is collected to obtain the most of the single layer GO flakes by avoiding the multilayers. The inscription of FBG in a single mode photosensitive optical fiber (125 m diameter) with germania doped core (purchased from M/s Nufern) is carried out using the phase mask technique; a KrF excimer UV laser of wavelength 248 nm, pulse energy 6 mJ,
repetition rate of 200 Hz and a phase mask of 1069 nm pitch are used in the FBG fabrication. The FBGs fabricated as above are dipped in 40% of hydrofluoric (HF) acid to etch the cladding layer around the grating region. The reflected Bragg wavelength is continuously monitored during the etching process and the eFBG sensor is taken out of the HF acid, after 1 nm downshift in Bragg wavelength. The eFBG is subsequently washed with DI water to remove the residual HF molecules. This process reduces the thickness of the clad material from ∼58 m to ∼0.5 m [18]. Further, the surface of eFBG is made hydrophilic by treating it with NH4 OH:H2 O2 :H2 O (1:1:5) solution about 1 hour [19] followed by DI water washing. The GO coating is carried out by immersing the surface modified eFBG sensor in 200 L GO aqueous solution and drying the solution at 35 ◦ C. After completion of GO coating, the hydrazine treatment is undertaken for ∼8 h at 100 ◦ C to reduce the GO film deposited on the eFBG surface. Lastly, the eFBG sensor is baked at 120 ◦ C for 1 h to remove the adsorbed hydrazine molecules. The gas sensing set up used consists of a gas chamber where the FBG sensor is placed under the gas outlet purging nozzle. A mass flow controller with a maintained flow rate of 1000 sccm (standard cubic centimeters per minute), provides the synthetic air (80% of N2 + 20% of O2 ) controlling channel and NO2 channel. The concentration of NO2 gas is set at 0.5 ppm, 1 ppm, 2 ppm and 3 ppm by varying the synthetic air channel flow rate; the same flow rate
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(a)
(b)
(c)
(d)
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 constant pressure on fiber even under different concentrations of NO2 . All the measurements have been carried out at room temperature and the relative humidity of 45 (±1) %. The reflected Bragg wavelength of the eFBGs (B ) is monitored throughout the experiment using an optical interrogator (Micron Optics, SM130) with a wavelength repeatability of 1 pm (picometer).
combinational modes and overtones; 2D band at 2712 cm−1 , D+G band at 2941 cm−1 and 2G band at 3201 cm−1 . After reduction of GO flakes coated on the eFBG sensor, the Raman spectrum shows G-band at 1593 cm−1 , D-band at 1346 cm−1 , 2D band at 2686 cm−1 , D+G band at 2932 cm−1 and 2G band at 3193 cm−1 , which confirm the reduction of the flakes [24]. The peak position of the 2D band is similar to that of a monolayer graphene prepared using mechanical exfoliation.
3. Results and discussion 3.1. Scanning electron microscopy (SEM) characteristics of RGO on eFBG Fig. 1b shows the SEM image of eFBG sensor coated with RGO prepared by the reduction of GO coated on the eFBG surface using hydrazine. Fig. 1c shows the EDAX spectrum of RGO on the eFBG sensor. The area from which the spectrum has been taken is marked in a black color open square in Fig. 1b. From the EDAX data, the atomic ratio of carbon to oxygen is found to be ∼3.5 after reduction, which corroborates the reduction of GO to RGO. Since there is some oxygen atomic contribution from silica fiber, the actual atomic ratio of carbon to oxygen in RGO would be more than 3.5. 3.2. Raman spectrum of GO and RGO on eFBG Fig. 1d shows the Raman spectrum before and after the reduction of GO flakes coated on the eFBG sensor. Raman spectrum of GO typically consists of two prominent bands: defect induced Dband at 1356 cm−1 and G-band at 1612 cm−1 arising due to the in-plane bond stretching of sp2 hybridized carbons along with the
3.3. FBG based NO2 sensing Fig. 2a shows the Bragg wavelength as a function of time for bare FBG and eFBG (without RGO coating) sensors upon cyclic exposure to 3 ppm NO2 gas. The data shown in Fig. 2a, 2b are averaged over 5 points. The bare FBG sensor has shown 1–2 pm change in Bragg wavelength (less than the measurable limit of the Bragg wavelength) and the eFBG (without RGO coating) sensor exhibited ∼4 pm up-shift in the Bragg wavelength. This 4 pm shift can be expected from the strain applied on the eFBG (without RGO coating) sensor due to the 1000 sccm flowrate of the NO2 gas. Fig. 2b shows the Bragg wavelength as a function of time for the cyclic exposure of 3 ppm NO2 gas on the RGO coated eFBG sensor. In contrast to bare FBG and eFBG (without RGO coating) sensors, the RGO coated eFBG sensor shows ∼28 pm up-shift in the Bragg wavelength. The shift occurs due to the change in refractive index of RGO caused by the charge transfer between the RGO and NO2 molecules. The RGO is unintentionally p-doped [24] and adsorption of the electron withdrawing NO2 gas molecules increases the local hole concentration which changes the refractive index of RGO [16]. Thus, the effective refractive index of RGO coated eFBG sensor is
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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 1 2 3
25.7 28.1 32.2 34.3
26.7 20.5 18.1 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 negligible 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, specifically Ag particles decorated RGO & aniline functionalized RGO for ammonia sensing [25,26], hydrogen plasma treated RGO for CO2 sensing [27] and RGO polymer & SnO2 –RGO composites for NO2 sensing [28,29]. Since the change in local refractive index of RGO depends on the 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 function of time for different concentrations of NO2 varying from 0.5 ppm to 3 ppm. 0.5 ppm is the lower limit of concentration of NO2 which we could achieve with our set up. The data has been averaged over 5 s. It is observed that the shift in Bragg wavelength reaches the saturation limit for NO2 gas exposure of concentration 3 ppm after ∼ 40 min. Further, the sensing experiments have been performed with 2 ppm, 1 ppm and 0.5 ppm of NO2 gas to obtain the saturation limit in the shift in Bragg wavelength followed by the purging process with synthetic air of 1000 sccm flow rate after each exposure of different concentration of NO2 gas. The inset in Fig. 3a shows one complete cycle of adsorption and desorption for 3 ppm NO2 on the RGO coated eFBG surface which causes saturation in the shift in Bragg wavelength as a function of time. The adsorption data was fitted with the equation B (t) = (B )∞ (1 − exp(−t/ a )), where (B )∞ represents the maximum value of B at t = ∞. Similarly, the desorption data is fitted with the equation B (t) = (B )∞ exp(−t/ d ). The values of (B )∞ and a after fitting the data with adsorption equation, are given in Table 1. As the desorption process was carried out using the purging process with synthetic air of 1000 sccm flow rate to remove the NO2 gas molecules, the value of d is 3.8 min for all the NO2 concentration.
Fig. 3. (a) Shift in Bragg wavelength of a RGO coated eFBG sensor as a function of time for different concentrations of NO2 gas varying from 0.5 ppm to 3 ppm. The 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 wavelength of a RGO coated eFBG sensor as a function of concentration of NO2 after 20 min and 40 min exposure. Inset: Time taken for 50% increase in shift in Bragg wavelength of RGO coated eFBG sensor as a function of concentrations of NO2 .
Fig. 3b shows the Bragg wavelength shift as a function of NO2 concentration after exposure of RGO coated eFBG sensor for 20 min and 40 min. It is worth here to mention that after 20 min of exposure even with 0.5 ppm, the observed shift is ∼12 pm (very high in comparison of the measuring limit of the interrogator) and we therefore feel that the detection of NO2 of concentration less than 0.5 ppm can also be achieved using RGO coated eFBG sensor (0.5 ppm was the lower limit we could achieve with our experimental setup). Inset of Fig. 3b shows the time for 50% increase in the shift in Bragg wavelength for different NO2 gas concentrations. These time values have been estimated from the shift in Bragg wavelength as a function of time data shown in Fig. 3a. It is apparent that the time taken for obtaining 50% increase in the shift in Bragg wavelength is decreased from ∼ 20 min to ∼6 min with the increase in concentration from 0.5 ppm to 3 ppm. The results obtained in our experiments clearly shows that the RGO coated eFBG sensors can be used as ultra-sensitive gas sensors that can detect even 0.5 ppm of NO2 gas with great accuracy. The RGO coated eFBG sensors are cost effective, portable with low level of complexity in sensor fabrication, maintenance, operation and these sensors can also be used for the detection of other toxic gases. We have observed no degradation in the performance of the RGO coated eFBG sensor for at least
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Table 2 Comparison of different materials used for NO2 gas sensing with limit of detection with the present work. Material
Device readout
Operating temperature
RGO coated eFBG
Shift in Bragg wavelength of eFBG
RT
ZnO nanostructures SnO2 nanoribbons SnO2-core/ZnO-shell nanofibers TeO2 nanorods Polypyrrole-PET Polythiophene-CuPc Polyaniline-In2 O3
Electrical Resistance
Electrical Resistance
Reference Present work
0.1 ppm 3 ppm 70 ppm 0.5 ppm
[4]
20 ppm 4.3 ppm 0.5 ppm
[5]
RT
[8]
RT–250 ◦ C
0.01 ppm 0.025 ppm 0.1 ppm 0.5 ppm 0.1 ppb
RT
1 molecule
[9] [11]
Electrical Resistance
RT
0.5 ppm 3.6 ppm 0.015 ppm
Change in the emission intensity Differential absorbance
RT
3 ppm
[13]
RT
14 ppm
Electrical Resistance
Mechanically exfoliated graphene
Hall Geometry
Absorption spectroscopy using blue LED Acousto-optic differential optical absorption spectroscopy
0.5 ppm
100–350 ◦ C RT 350 ◦ C 150 ◦ C
Surface acoustic wave operational frequency
Bare CNTs Vertically aligned CNTs Metal decorated CNTs Metal oxide decorated CNTs Polymer coated CNTs
Epitaxial grown graphene Chemically modified RGO Porous RGO
Limit of detection
6 months. We feel that even longer aging times will not affect the sensor performance and RGO coated on the eFBG surface is stable for a much longer time at room temperature. 4. Conclusions In the present work, an optical platform has been demonstrated for the room temperature sensing of NO2 gas, with a lower detection limit of 0.5 ppm with 0.8 pm/min sensitivity. The adsorption and desorption of the NO2 gas molecules on RGO changes the refractive index by increasing and decreasing the local carrier density. The optical read out of the RGO coated eFBG sensor, namely the shift in Bragg wavelength, is highly sensitive to the changes in refractive index of RGO flakes coated on the surface of eFBG sensor. RGO coated eFBG sensor detects the NO2 gas in sub ppm level and the sensor performance can be improved to sub ppb level as well as the response time can be decreased by having multiple gratings in the same fiber. Table 2 shows the comparison of different materials used for NO2 gas sensing with limit of detection with our present work. Acknowledgements Prof. A.K. Sood thanks the Nanomission project of Department of Science and Technology for financial assistance. Prof. S. Asokan, acknowledges the Centre for Strategic Initiates and Robert Bosch Centre for Cyber Physical Systems, Indian Institute of Science, for support. References [1] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors 10 (2010) 2088–2106. [2] M. Tiemann, Porous metal oxides as gas sensors, Chem. Eur. J. 13 (2007) 8376–8388. [3] A.A. Tomchenko, G.P. Harmer, B.T. Marquis, J.W. Allen, Semiconducting metal oxide sensor array for the selective detection of combustion gases, Sens. Actuators B 93 (2003) 126–134. [4] S. Park, S. Kim, G. Sun, W.I. Lee, K.K. Kim, C. Lee, Fabrication and NO2 gas sensing performance of TeO2 -core/CuO-shell heterostructure nanorod sensors, Nanoscale Res. Lett. 9 (2014) 638–645.
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Biographies Sridevi. S received her B.E. degree in Instrumentation Technology and M.Tech degree in Biomedical Signal Processing and Instrumentation from Visvesvaraya Technological University, India. She is currently pursuing Ph.D at Indian Institute of Science, under the supervision of Prof. S. Asokan and Prof. A.K Sood. Her current research interest includes bio-chemical sensors, gas sensors, strain, and temperature sensors. The area of work is predominantly based on nanomaterials coated on etched fiber Bragg gratings.
K.S. Vasu received his M.Sc degree in Physics from Sri Venkateswara University, India. He has finished Ph.D from Department of Physics, Indian Institute of Science and currently working as a research associate in the Department of Physics, The University of Manchester. He has worked on electrical, rheological properties and biosensing based on graphene oxide and SWNTs. He is currently working in desalination project using the graphene oxide membranes. Navakanta Bhat received the B.E. degree (1989) in Electronics and Communication from the University of Mysore, the M.Tech. degree (1992) in microelectronics from IIT Bombay, and the Ph.D. degree (1996) in Electrical Engineering from Stanford University. He worked at Advanced Products R&D Lab, Motorola till 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 Engineering and ECE department. His current research interest includes NanoCMOS technology, Gas sensors and Bio sensors for Diabetes management. He has more than 200 publications and 7 issued US patents. He is the fellow of Indian National Academy of Engineering. He is the editor of IEEE Transactions on Electron Devices. S. Asokan received the M.Sc. degree in Materials Science from the College of Engineering, Guindy, Anna University, Madras, India, and the Ph.D. degree in Physics from the Indian Institute of Science, Bangalore, India. He is currently a Professor at the Department of Instrumentation and Applied Physics and Chairman of the Robert Bosch Center for Cyber Physical Systems, Indian Institute of Science. He has edited two books and published more than 180 papers in international journals/Books. 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 spectroscopy of nanodevices to understand basic science issues in phonon renormalization and mobility of carriers and to use them as nano-sensors. He has published more than 340 papers in refereed international journals and holds several patents. His work has been recognized by many honors, awards and fellowships of the Academies in India, The World Academy of Sciences (TWAS), and The Royal Society, London.