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Sensitization of an optical fiber methane sensor with graphene
Optical Fiber Technology 37 (2017) 26–29
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Sensitization of an optical fiber methane sensor with graphene J.Y. Zhang a,b,c,d, E.J. Ding a,b,⇑, S.C. Xu c,d, Z.H. Li c,d, X.X. Wang c,d, F. Song c,d a
IOT Perception Mine Research Center, China University of Mining and Technology, Xuzhou 221008, China School of Information and Control Engineering, China University of Mining and Technology, Xuzhou 221008, China c College of Physics and Electronic Information, Dezhou University, Dezhou 253023, China d Shandong Provincial Key Laboratory of Biophysics, College of Physics and Electronic Information, Dezhou University, Dezhou 253023, China b
a r t i c l e
i n f o
Article history: Received 24 April 2017 Revised 16 June 2017 Accepted 25 June 2017
Keywords: Graphene Optical fiber sensing Tin oxide Side-polished optical fiber
a b s t r a c t We analyze the mechanism by which tin oxide can be utilized for the optical sensing of methane gas via surface adsorption and electromagnetic theory. Single-mode optical fibers with core diameters of 9 lm and cladding diameters of 12 lm were used. A 15 mm-long segment of each optical fiber was polished to the core via wheel side-polishing; the exposed fiber core areas were coated with graphene-doped tin oxide such that a novel graphene-based optical fiber methane sensor was fabricated. The experimental results show that the sensor exhibits excellent linear fitting and reproducibility, making it useful for the detection of low concentrations of methane. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Methane is the main component of mine gas, and it can explode in air very easily; thus, it has been the focus of mining safety concerns. Currently, the main methods for detecting methane include catalytic combustible [1], semiconductor-based [2–4], electrochemical [4–5], and optical sensing [6] methods. Optical fiberbased methane sensors are suitable for use in harsh and dangerous environments due to their excellent electric insulation, strong anti-electromagnetic interference capabilities, and long-distance on-line monitoring. They also consist of elements with simple structures, and they are explosion-proof. Many researchers are investigating these types of sensors. Tin (IV) oxide (SnO2) is an n-type semiconductor, and it contains free charges. When light waves propagate inside SnO2, absorption-induced loss can occur [7]. When methane molecules are in contact with the surface of a SnO2 thin film, methane acts as a reducing gas because the work function of SnO2 is larger than the dissociation energy of the adsorbed molecules. As a result, methane molecules eject electrons into SnO2 and become positively charged [8]. Therefore, the number of charge carriers within the SnO2 increases, resulting in enhanced conductivity and a higher refractive index, n. Graphene is a single-layer, two-dimensional crystal in which carbon atoms are sp2-hybridized to form hexagons that extend in ⇑ Corresponding author at: IOT Perception Mine Research Center, China University of Mining and Technology, Xuzhou 221008, China. http://dx.doi.org/10.1016/j.yofte.2017.06.011 1068-5200/Ó 2017 Elsevier Inc. All rights reserved.
a honeycomb structure. More broadly, carbon films with thicknesses of several atoms are also considered to be graphene. Graphene has many unique properties, such as a remarkable charge mobility [10,000 cm2/(Vs)] [9], a large specific surface area 2
[theoretical value 2630 mg ] [10], superior thermal conductivity [5000 W/(mK)] [11], and low Johnson and 1/f noise values [12]. Scientists have discovered that pristine graphene can easily adsorb polar molecules; F. Schedin of the University of Manchester found that micrometer-sized sensors made from graphene can detect individual events when a gas molecule is adsorbed on or desorbed from the graphene surface. The adsorbed/desorbed molecules change the local carrier concentration in the graphene (as donors/acceptors), leading to changes in its physical parameters such as its conductivity. By detecting the change in output current as a function of the presence of foreign molecules under an applied external voltage, ultra-sensitive gas sensors could be prepared [13]. Researchers in South Korea, India, and Pakistan successfully grew graphene on the cross-section of a plastic, D-type optical waveguide with a large core diameter. The sensitivity of this sensing structure was found to be almost two orders of magnitude higher than that of traditional electronic thin-film sensors and traditional microphotonic sensors based on evanescent waves [14–17]. In 2012, a chemical gas sensor based on a graphenemicro/nanofiber composite structure was proposed. Through the attachment of micro/nanofibers to graphene, evanescent waves are coupled to the graphene waveguide plane. When gas molecules made contact with the graphene, the effective refractive index of
27
J.Y. Zhang et al. / Optical Fiber Technology 37 (2017) 26–29
each mode of the composite waveguide was altered, causing polarization attenuation [18–21]. Variations in chemical gas concentration could be detected by measuring the intensity changes in the output light signal. These results open up the possibility of utilizing graphene-based optical fiber waveguides as gas sensors and provide strong support for the effective combination of graphene and micro/nanofibers. Herein, thin graphene-doped tin oxide films were prepared and coated on side-polished optical fibers to fabricate methane sensors. The sensing characteristics and sensitivity of the as-synthesized methane sensors were investigated experimentally.
therefore difficult to control. Doping can improve the conductivity of SnO2 thin films and help them maintain high transmittance in the visible light region. As a single-layer carbon system, graphene has a large specific area (2600 m2/g) and a high carrier mobility at room temperature. Graphene also displays a perfect quantum tunneling effect, a half-integer quantum hall effect, and permanent conductivity [22]. The doping of SnO2 with graphene will result in a higher carrier concentration for SnO2, thus giving rise to gas sensing thin films with enhanced performance.
2. Sensing mechanism
3.1. Preparation of the optical fibers
Tin (IV) oxide is an n-type semiconductor, and electrons are its majority carriers. When light waves propagate inside SnO2, absorption loss occurs. The electric vector satisfies the damped wave equation,
Single-mode optical fibers with core diameters of 9 lm and cladding diameters of 12 lm were used. A 15 mm-long segment of each optical fiber was polished to the core by wheel sidepolishing. The polishing depth was monitored using a threadlet instrument. The optical fibers were immersed in an ethanolcontaining tube and cleaned with an ultrasonicator to ensure that their surfaces were clean before they were coated with thin films.
@2E @E ¼0 rl0 @t @t2
ð1Þ
where r is the conductivity, e0 is the vacuum permittivity, er is the relative permittivity, and l0 is the vacuum permeability. After simplifying the light waves to plane electromagnetic waves, the optical admittance of the conducting medium can be derived from the wave Eq. (1),
N ¼ n jk
ð2Þ
where n is the refractive index, k is the extinction coefficient, and
n2 k ¼ er 2
ð3Þ
2nk ¼ r=e0 x
ð4Þ
From Eqs. (2) and (4), n follows from the following equation:
n2 ¼
1=2 1 r 2 er 1 þ þ1 2 xe0 er
ð5Þ
Light Intensity (a.u.)
According to Eq. (5), the refractive index, n, increases with increasing conductivity, r. When methane molecules are in contact with the surface of SnO2 thin films, the adsorbed molecules diffuse freely on the surface and lose their motion energy. These molecules eject electrons into SnO2 and become positive ions. As a result, the number of charge carriers in the SnO2 increases, leading to an increase in the conductivity and the refractive index, n. When the methane concentration increases, its transmittance increases. However, the resistivity of the pristine SnO2 thin film is high; its carrier concentration is determined by the number of oxygen vacancies and is
70000
0% 5% 15 % 25 % 35%
(a)
60000 50000 40000 30000 20000 10000 0
1530 1540 1550 1560 1570 1580 1590 1600
Wavelength(nm)
3.2. Reagents and preparation of film coatings Reagents: Tin (IV) chloride pentahydrate (SnCl45H2O, AR), isopropanol (AR), and graphene (AR). Preparation process: Two aliquots of SnCl45H2O were weighed, and each samples was dissolved in 50 mL of isopropanol to prepare solutions with concentrations of 0.05 M; 0.05 g of graphene was added to one of the solutions. After the solutions were stirred at room temperature for 4 h with magnetic stirrers, the solutions were aged for 24 h. Then, the two solutions were drop-coated on the exposed areas of the abovementioned polished fibers. Each layer was allowed to air-dry before the next layer was applied; this procedure was repeated five times. 4. Results and discussion The components of the optical fiber methane sensor setup are as follows: a graphene-doped, SnO2-coated optical sensor, methane, nitrogen, a gas flow control device, a test gas chamber, and a bench top optical spectrum analyzer (MS9740A from Anritsu). In a typical experiment, gas samples with different concentrations (volume ratios) of methane were prepared by modulating the flow rates of methane and nitrogen. The built-in light source of the optical spectrum analyzer was set to a wavelength of 1550 nm, and the output intensities of the coated optical fibers (with SnO2 or graphene-doped SnO2) were measured at different methane concentrations.
Light Intensity (a.u.)
r2 E l0 e0 er
3. Experimental
30000
0% 5% 15% 25% 35% 45% 55%
(b)
25000 20000 15000 10000 5000 0
1530 1540 1550 1560 1570 1580 1590
Wavelength(nm)
Fig. 1. a, Variations in the output light intensities of the SnO2-coated fibers monitored using an optical spectrum analyzer at different methane concentrations. b, Variations in the output light intensities of the graphene-doped, SnO2-coated fibers monitored using an optical spectrum analyzer at different methane concentrations.
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J.Y. Zhang et al. / Optical Fiber Technology 37 (2017) 26–29
22000
1530nm Linear Fit
(a)
Light Intensity(a.u.)
Light Intensity(a.u.)
21000 20000 19000 18000 17000 16000 15000 14000
25000
5%
(b)
24000 23000 22000 21000 20000 19000 18000 17000
15% 25% 35% 45% 55%
5%
15% 25% 35% 45% 55%
Concentration(%)
Concentration(%) 1575nm Linear Fit
28000
(c)
Light intensity(a.u.)
Light Intensity(a.u.)
30000
26000 24000 22000 20000
5%
1550nm Linear Fit
1590nm Linear Fit
(d)
24000 22000 20000 18000 16000
15% 25% 35% 45% 55%
5%
15% 25% 35% 45% 55%
Concentration(%)
Concentration(%)
Fig. 2. Variations in the output light intensities of graphene-doped SnO2-coated optical fibers at different wavelengths.
Fig. 1 shows the output light intensities of the SnO2 films and the graphene-doped SnO2 films measured using the optical spectrum analyzer at different input wavelengths; the ordinate is the output light intensity, and the abscissa is the methane concentration. We adopt center wavelength of 1550 nm with fiber optic light source input. The data show that the output light intensity of the side-polished fiber coated with the graphene-doped SnO2 thin film increased with increasing methane concentration. When the methane concentration increased from 0% to 55%, the output light intensity increased from 14,000 to 25,000. The output light intensity increases with increasing methane concentration because the increasing methane concentration causes an increase in the electric conductivity of the SnO2 film. The increasing conductivity
causes the refractive index, n, to increase in turn, and the increase in refractive index leads to a decrease in the absorption coefficient, a, and an increase in the output light intensity. Because of the presence of graphene, the methane molecules were evenly and tightly immobilized in the thin film; thus, the addition of graphene greatly improves the intensity and stability of the output signal. A comparison of the two experimental results clearly shows that the SnO2 thin film-coated optical fibers did not discriminate between the various methane concentrations as well as the graphene-doped SnO2 thin film-coated optical fibers did. We then studied the variations in the output light intensity at certain input wavelengths. Fig. 2 shows the relationship between the output light intensity and the methane concentration under
36200 36000 35800 35600 35400 35200 35000 34800 34600
(a)
5%
Light Intensity(a.u.)
Light Intensity(a.u.)
1530nm
15%
25%
40000 39800 39600 39400 39200 39000 38800 38600 38400 38200
1550nm
(b)
5%
35%
Concentration(%)
(c)
5%
Light Intensity(a.u.)
Light Intensity(a.u.)
1575nm 59400 59200 59000 58800 58600 58400 58200 58000 57800 57600
15%
25%
Concentration(%)
35%
15%
25%
35%
Concentration(%)
54600 54400 54200 54000 53800 53600 53400 53200 53000 52800 52600 52400
1590nm
(d)
5%
15%
25%
35%
Concentration(%)
Fig. 3. Variations in the output light intensities of the SnO2-coated optical fibers at different input wavelengths.
J.Y. Zhang et al. / Optical Fiber Technology 37 (2017) 26–29
various input wavelengths. Here, 1530 nm, 1550 nm, 1575 nm, and 1590 nm input wavelengths were used. At all of these wavelengths, the output light intensity increased linearly with increasing methane concentration; the degrees of fitting were 0.97226, 0.97075, 0.96202, and 0.99438, respectively. These results indicate that the addition of graphene leads to stronger adsorption of the methane molecules, resulting in stable output signals. After comparing the above experiments, we then coated the same side of the polished optical fibers with a solution containing only SnO2. Fig. 2(a) and (b) show a similar trend relating output light intensity and methane concentration; however, this sensor did not discriminate methane as well as the one made with a graphene-doped SnO2-coated optical fiber, and the output signal was not as stable. Fig. 3 shows the relationship between the output light intensity and the methane concentration under various input wavelengths. The same wavelengths were used here (1530, 1550, 1575, and 1590 nm); the degrees of fitting were 0.98438, 0.64, 0.29111, and 0.76496, respectively. The output light intensity did not strictly increase linearly with increasing methane concentration; the degrees of fitting were very low. These results indicate that graphene doping increases the reliability and sensitivity of this sensor system. For the side-polished optical fibers coated with graphene-doped SnO2 thin films, using the same fiber, five minutes at each interval multiple repeated experiments yielded similar results, exhibiting that the setup displayed excellent reproducibility. 5. Conclusions We designed an optical fiber-based methane sensor based on graphene-doped SnO2 thin films. By exploiting the gas sensing properties of SnO2 and the excellent adsorption characteristics and atomic thickness of graphene (which result in the even and tight adsorption of methane inside the thin film), we have shown that a linear relationship exists between the methane concentration from 0% to 55% and the output light intensity; the sensing system exhibited excellent reproducibility. This work could provide a route to high-precision detection of methane and further investigation of side-polished optical fiber sensors. Acknowledgments The authors are grateful for financial support from National Natural Science Foundation of China (11604040), Shandong Province Natural Science Foundation (ZR2014FQ032), Experimental
29
Technology Program of Dezhou University (SYJS16001, ZZYQ16003 and 320061) and Taishan Scholars Program of Shandong Province. References [1] M. Tong, Catalytic Sensor and Application Technology, China University of Mining and Technology Press, Xuzhou, 2002, pp. 1–6. [2] M. Saha, A. Banerjee, A.K. Halder, Effect of alumina addition on methane sensitivity of tin dioxide thick films, Sensors Actuators B: Chem. 79 (B) (2001) 192–195. [3] R. Rella, P. Siciliano, L. Vasanelli, Physical properties of osmium doped tin oxide thin films, J. Appl. Phys. 83 (4) (1998) 2 369–2 371. [4] J. Qiao, H.F. Li, Y. Chang, et al., Voltammetric study and detection of methane on nickel hydroxide modified nickel electrode, Anal. Lett. 41 (4) (2008) 593– 598. [5] J. Patrick, M. Beat, W. Bernhard, et al., Determination of methane and other small hydrocarbons with a platinum-Nafion electrode by stripping voltammetry, Anal. Chim. Acta 432 (2001) 1–10. [6] J.L. Zhang, Z.B. Wang, X.L. Xiao, et al., Measurement technology of methane concentration based on harmonic detection and characteristic absorption of gases, J. Chin. Coal Soc. 34 (1) (2009) 24–27. [7] Q. Zhao, D. Ju, X. Deng, J. Huang, B. Cao, X. Xu, Morphology-modulation of SnO2 hierarchical architectures by Zn doping for glycol gas sensing and photocatalytic applications, Sci. Rep. 5 (2015) 7874. [8] V.K. Tomer, S. Duhan, In-situ synthesis of Sn O2/SBA-15 hybrid nanocomposite as highly efficient humidity sensor, Sensors Actuators B: Chem. 212 (2015) 517–525. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669. [10] M.D. StollerS, J. Park, Y.W. Zhu, et al., Graphene-based ultracapacitors, Nano Lett. 8 (10) (2008) 3498–3502. [11] A.A. Balandin, S. Ghosh, W.Z. Bao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008) 902–907. [12] F. Schedin, A.K. Geim, S.V. Morozov, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (9) (2007) 652–655. [13] F. Schedin, A.K. Geim, S.V. Morozov, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [14] I.F. Rivera, R.K. Joshi, J. Wang. Graphene based ultra-sensitive gas sensors. IEEE Sensors 2010 Conference, 2010: 1534–1537. [15] S. Rumyantsev, G.G.X. Xiong Liu, M.S. Shur, et al., Selective gas sensing with a single pristine graphene transistor, Nano Lett. 12 (2012) 2294–2298. [16] N. Reckinger, A. Vlad, S. Melinte, et al., Graphene-coated holey metal films: tunable molecular sensing by surface plasmon resonance, Appl. Phys. Lett. 102 (2013) 2111081–20111084. [17] S. Some, Y. Xu, Y. Kim, et al., Highly sensitive and selective gas sensor using hydrophilic and hydrophobic graphenes, Sci. Rep. 2013 (1868) 3. [18] B.C. Yao, Y. Wu, Y. Cheng, et al. Graphene based microfiber gas sensor. OFS 2012 22nd International Conference on Optical Fibre Sensors, 2012: 8421. [19] H. Li, Y. Anugrah, S.J. Koester, Optical absorption in graphene integrated on silicon waveguides, Appl. Phys. Lett. 101 (11) (2012) 1111101–1111103. [20] J.A. Kim, T. Hwang, S.R. Dugasani, et al., Graphene based fiber optic surface plasmon resonance for bio-chemical sensor applications, Sensors Actuators B: Chem. 187 (2013) 426–433. [21] B.C. Yao, Y. Wu, Y. Cheng, et al. Determination of complex refractive index of graphene waveguide by microfiber knot sensor. Asia Pacific Optical Sensors Conference Proceedings, Wuhan, 2013, 89241U–89241U-4. [22] R.T. Weitz, A. Yacoby, Graphene rests easy, Nat. Nanotechnol. 5 (10) (2010) 699–700.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Sensitization of an optical fiber methane sensor with graphene J.Y. Zhang a,b,c,d, E.J. Ding a,b,⇑, S.C. Xu c,d, Z.H. Li c,d, X.X. Wang c,d, F. Song c,d a
IOT Perception Mine Research Center, China University of Mining and Technology, Xuzhou 221008, China School of Information and Control Engineering, China University of Mining and Technology, Xuzhou 221008, China c College of Physics and Electronic Information, Dezhou University, Dezhou 253023, China d Shandong Provincial Key Laboratory of Biophysics, College of Physics and Electronic Information, Dezhou University, Dezhou 253023, China b
a r t i c l e
i n f o
Article history: Received 24 April 2017 Revised 16 June 2017 Accepted 25 June 2017
Keywords: Graphene Optical fiber sensing Tin oxide Side-polished optical fiber
a b s t r a c t We analyze the mechanism by which tin oxide can be utilized for the optical sensing of methane gas via surface adsorption and electromagnetic theory. Single-mode optical fibers with core diameters of 9 lm and cladding diameters of 12 lm were used. A 15 mm-long segment of each optical fiber was polished to the core via wheel side-polishing; the exposed fiber core areas were coated with graphene-doped tin oxide such that a novel graphene-based optical fiber methane sensor was fabricated. The experimental results show that the sensor exhibits excellent linear fitting and reproducibility, making it useful for the detection of low concentrations of methane. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Methane is the main component of mine gas, and it can explode in air very easily; thus, it has been the focus of mining safety concerns. Currently, the main methods for detecting methane include catalytic combustible [1], semiconductor-based [2–4], electrochemical [4–5], and optical sensing [6] methods. Optical fiberbased methane sensors are suitable for use in harsh and dangerous environments due to their excellent electric insulation, strong anti-electromagnetic interference capabilities, and long-distance on-line monitoring. They also consist of elements with simple structures, and they are explosion-proof. Many researchers are investigating these types of sensors. Tin (IV) oxide (SnO2) is an n-type semiconductor, and it contains free charges. When light waves propagate inside SnO2, absorption-induced loss can occur [7]. When methane molecules are in contact with the surface of a SnO2 thin film, methane acts as a reducing gas because the work function of SnO2 is larger than the dissociation energy of the adsorbed molecules. As a result, methane molecules eject electrons into SnO2 and become positively charged [8]. Therefore, the number of charge carriers within the SnO2 increases, resulting in enhanced conductivity and a higher refractive index, n. Graphene is a single-layer, two-dimensional crystal in which carbon atoms are sp2-hybridized to form hexagons that extend in ⇑ Corresponding author at: IOT Perception Mine Research Center, China University of Mining and Technology, Xuzhou 221008, China. http://dx.doi.org/10.1016/j.yofte.2017.06.011 1068-5200/Ó 2017 Elsevier Inc. All rights reserved.
a honeycomb structure. More broadly, carbon films with thicknesses of several atoms are also considered to be graphene. Graphene has many unique properties, such as a remarkable charge mobility [10,000 cm2/(Vs)] [9], a large specific surface area 2
[theoretical value 2630 mg ] [10], superior thermal conductivity [5000 W/(mK)] [11], and low Johnson and 1/f noise values [12]. Scientists have discovered that pristine graphene can easily adsorb polar molecules; F. Schedin of the University of Manchester found that micrometer-sized sensors made from graphene can detect individual events when a gas molecule is adsorbed on or desorbed from the graphene surface. The adsorbed/desorbed molecules change the local carrier concentration in the graphene (as donors/acceptors), leading to changes in its physical parameters such as its conductivity. By detecting the change in output current as a function of the presence of foreign molecules under an applied external voltage, ultra-sensitive gas sensors could be prepared [13]. Researchers in South Korea, India, and Pakistan successfully grew graphene on the cross-section of a plastic, D-type optical waveguide with a large core diameter. The sensitivity of this sensing structure was found to be almost two orders of magnitude higher than that of traditional electronic thin-film sensors and traditional microphotonic sensors based on evanescent waves [14–17]. In 2012, a chemical gas sensor based on a graphenemicro/nanofiber composite structure was proposed. Through the attachment of micro/nanofibers to graphene, evanescent waves are coupled to the graphene waveguide plane. When gas molecules made contact with the graphene, the effective refractive index of
27
J.Y. Zhang et al. / Optical Fiber Technology 37 (2017) 26–29
each mode of the composite waveguide was altered, causing polarization attenuation [18–21]. Variations in chemical gas concentration could be detected by measuring the intensity changes in the output light signal. These results open up the possibility of utilizing graphene-based optical fiber waveguides as gas sensors and provide strong support for the effective combination of graphene and micro/nanofibers. Herein, thin graphene-doped tin oxide films were prepared and coated on side-polished optical fibers to fabricate methane sensors. The sensing characteristics and sensitivity of the as-synthesized methane sensors were investigated experimentally.
therefore difficult to control. Doping can improve the conductivity of SnO2 thin films and help them maintain high transmittance in the visible light region. As a single-layer carbon system, graphene has a large specific area (2600 m2/g) and a high carrier mobility at room temperature. Graphene also displays a perfect quantum tunneling effect, a half-integer quantum hall effect, and permanent conductivity [22]. The doping of SnO2 with graphene will result in a higher carrier concentration for SnO2, thus giving rise to gas sensing thin films with enhanced performance.
2. Sensing mechanism
3.1. Preparation of the optical fibers
Tin (IV) oxide is an n-type semiconductor, and electrons are its majority carriers. When light waves propagate inside SnO2, absorption loss occurs. The electric vector satisfies the damped wave equation,
Single-mode optical fibers with core diameters of 9 lm and cladding diameters of 12 lm were used. A 15 mm-long segment of each optical fiber was polished to the core by wheel sidepolishing. The polishing depth was monitored using a threadlet instrument. The optical fibers were immersed in an ethanolcontaining tube and cleaned with an ultrasonicator to ensure that their surfaces were clean before they were coated with thin films.
@2E @E ¼0 rl0 @t @t2
ð1Þ
where r is the conductivity, e0 is the vacuum permittivity, er is the relative permittivity, and l0 is the vacuum permeability. After simplifying the light waves to plane electromagnetic waves, the optical admittance of the conducting medium can be derived from the wave Eq. (1),
N ¼ n jk
ð2Þ
where n is the refractive index, k is the extinction coefficient, and
n2 k ¼ er 2
ð3Þ
2nk ¼ r=e0 x
ð4Þ
From Eqs. (2) and (4), n follows from the following equation:
n2 ¼
1=2 1 r 2 er 1 þ þ1 2 xe0 er
ð5Þ
Light Intensity (a.u.)
According to Eq. (5), the refractive index, n, increases with increasing conductivity, r. When methane molecules are in contact with the surface of SnO2 thin films, the adsorbed molecules diffuse freely on the surface and lose their motion energy. These molecules eject electrons into SnO2 and become positive ions. As a result, the number of charge carriers in the SnO2 increases, leading to an increase in the conductivity and the refractive index, n. When the methane concentration increases, its transmittance increases. However, the resistivity of the pristine SnO2 thin film is high; its carrier concentration is determined by the number of oxygen vacancies and is
70000
0% 5% 15 % 25 % 35%
(a)
60000 50000 40000 30000 20000 10000 0
1530 1540 1550 1560 1570 1580 1590 1600
Wavelength(nm)
3.2. Reagents and preparation of film coatings Reagents: Tin (IV) chloride pentahydrate (SnCl45H2O, AR), isopropanol (AR), and graphene (AR). Preparation process: Two aliquots of SnCl45H2O were weighed, and each samples was dissolved in 50 mL of isopropanol to prepare solutions with concentrations of 0.05 M; 0.05 g of graphene was added to one of the solutions. After the solutions were stirred at room temperature for 4 h with magnetic stirrers, the solutions were aged for 24 h. Then, the two solutions were drop-coated on the exposed areas of the abovementioned polished fibers. Each layer was allowed to air-dry before the next layer was applied; this procedure was repeated five times. 4. Results and discussion The components of the optical fiber methane sensor setup are as follows: a graphene-doped, SnO2-coated optical sensor, methane, nitrogen, a gas flow control device, a test gas chamber, and a bench top optical spectrum analyzer (MS9740A from Anritsu). In a typical experiment, gas samples with different concentrations (volume ratios) of methane were prepared by modulating the flow rates of methane and nitrogen. The built-in light source of the optical spectrum analyzer was set to a wavelength of 1550 nm, and the output intensities of the coated optical fibers (with SnO2 or graphene-doped SnO2) were measured at different methane concentrations.
Light Intensity (a.u.)
r2 E l0 e0 er
3. Experimental
30000
0% 5% 15% 25% 35% 45% 55%
(b)
25000 20000 15000 10000 5000 0
1530 1540 1550 1560 1570 1580 1590
Wavelength(nm)
Fig. 1. a, Variations in the output light intensities of the SnO2-coated fibers monitored using an optical spectrum analyzer at different methane concentrations. b, Variations in the output light intensities of the graphene-doped, SnO2-coated fibers monitored using an optical spectrum analyzer at different methane concentrations.
28
J.Y. Zhang et al. / Optical Fiber Technology 37 (2017) 26–29
22000
1530nm Linear Fit
(a)
Light Intensity(a.u.)
Light Intensity(a.u.)
21000 20000 19000 18000 17000 16000 15000 14000
25000
5%
(b)
24000 23000 22000 21000 20000 19000 18000 17000
15% 25% 35% 45% 55%
5%
15% 25% 35% 45% 55%
Concentration(%)
Concentration(%) 1575nm Linear Fit
28000
(c)
Light intensity(a.u.)
Light Intensity(a.u.)
30000
26000 24000 22000 20000
5%
1550nm Linear Fit
1590nm Linear Fit
(d)
24000 22000 20000 18000 16000
15% 25% 35% 45% 55%
5%
15% 25% 35% 45% 55%
Concentration(%)
Concentration(%)
Fig. 2. Variations in the output light intensities of graphene-doped SnO2-coated optical fibers at different wavelengths.
Fig. 1 shows the output light intensities of the SnO2 films and the graphene-doped SnO2 films measured using the optical spectrum analyzer at different input wavelengths; the ordinate is the output light intensity, and the abscissa is the methane concentration. We adopt center wavelength of 1550 nm with fiber optic light source input. The data show that the output light intensity of the side-polished fiber coated with the graphene-doped SnO2 thin film increased with increasing methane concentration. When the methane concentration increased from 0% to 55%, the output light intensity increased from 14,000 to 25,000. The output light intensity increases with increasing methane concentration because the increasing methane concentration causes an increase in the electric conductivity of the SnO2 film. The increasing conductivity
causes the refractive index, n, to increase in turn, and the increase in refractive index leads to a decrease in the absorption coefficient, a, and an increase in the output light intensity. Because of the presence of graphene, the methane molecules were evenly and tightly immobilized in the thin film; thus, the addition of graphene greatly improves the intensity and stability of the output signal. A comparison of the two experimental results clearly shows that the SnO2 thin film-coated optical fibers did not discriminate between the various methane concentrations as well as the graphene-doped SnO2 thin film-coated optical fibers did. We then studied the variations in the output light intensity at certain input wavelengths. Fig. 2 shows the relationship between the output light intensity and the methane concentration under
36200 36000 35800 35600 35400 35200 35000 34800 34600
(a)
5%
Light Intensity(a.u.)
Light Intensity(a.u.)
1530nm
15%
25%
40000 39800 39600 39400 39200 39000 38800 38600 38400 38200
1550nm
(b)
5%
35%
Concentration(%)
(c)
5%
Light Intensity(a.u.)
Light Intensity(a.u.)
1575nm 59400 59200 59000 58800 58600 58400 58200 58000 57800 57600
15%
25%
Concentration(%)
35%
15%
25%
35%
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54600 54400 54200 54000 53800 53600 53400 53200 53000 52800 52600 52400
1590nm
(d)
5%
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Fig. 3. Variations in the output light intensities of the SnO2-coated optical fibers at different input wavelengths.
J.Y. Zhang et al. / Optical Fiber Technology 37 (2017) 26–29
various input wavelengths. Here, 1530 nm, 1550 nm, 1575 nm, and 1590 nm input wavelengths were used. At all of these wavelengths, the output light intensity increased linearly with increasing methane concentration; the degrees of fitting were 0.97226, 0.97075, 0.96202, and 0.99438, respectively. These results indicate that the addition of graphene leads to stronger adsorption of the methane molecules, resulting in stable output signals. After comparing the above experiments, we then coated the same side of the polished optical fibers with a solution containing only SnO2. Fig. 2(a) and (b) show a similar trend relating output light intensity and methane concentration; however, this sensor did not discriminate methane as well as the one made with a graphene-doped SnO2-coated optical fiber, and the output signal was not as stable. Fig. 3 shows the relationship between the output light intensity and the methane concentration under various input wavelengths. The same wavelengths were used here (1530, 1550, 1575, and 1590 nm); the degrees of fitting were 0.98438, 0.64, 0.29111, and 0.76496, respectively. The output light intensity did not strictly increase linearly with increasing methane concentration; the degrees of fitting were very low. These results indicate that graphene doping increases the reliability and sensitivity of this sensor system. For the side-polished optical fibers coated with graphene-doped SnO2 thin films, using the same fiber, five minutes at each interval multiple repeated experiments yielded similar results, exhibiting that the setup displayed excellent reproducibility. 5. Conclusions We designed an optical fiber-based methane sensor based on graphene-doped SnO2 thin films. By exploiting the gas sensing properties of SnO2 and the excellent adsorption characteristics and atomic thickness of graphene (which result in the even and tight adsorption of methane inside the thin film), we have shown that a linear relationship exists between the methane concentration from 0% to 55% and the output light intensity; the sensing system exhibited excellent reproducibility. This work could provide a route to high-precision detection of methane and further investigation of side-polished optical fiber sensors. Acknowledgments The authors are grateful for financial support from National Natural Science Foundation of China (11604040), Shandong Province Natural Science Foundation (ZR2014FQ032), Experimental
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Technology Program of Dezhou University (SYJS16001, ZZYQ16003 and 320061) and Taishan Scholars Program of Shandong Province. References [1] M. Tong, Catalytic Sensor and Application Technology, China University of Mining and Technology Press, Xuzhou, 2002, pp. 1–6. [2] M. Saha, A. Banerjee, A.K. Halder, Effect of alumina addition on methane sensitivity of tin dioxide thick films, Sensors Actuators B: Chem. 79 (B) (2001) 192–195. [3] R. Rella, P. Siciliano, L. Vasanelli, Physical properties of osmium doped tin oxide thin films, J. Appl. Phys. 83 (4) (1998) 2 369–2 371. [4] J. Qiao, H.F. Li, Y. Chang, et al., Voltammetric study and detection of methane on nickel hydroxide modified nickel electrode, Anal. Lett. 41 (4) (2008) 593– 598. [5] J. Patrick, M. Beat, W. Bernhard, et al., Determination of methane and other small hydrocarbons with a platinum-Nafion electrode by stripping voltammetry, Anal. Chim. Acta 432 (2001) 1–10. [6] J.L. Zhang, Z.B. Wang, X.L. Xiao, et al., Measurement technology of methane concentration based on harmonic detection and characteristic absorption of gases, J. Chin. Coal Soc. 34 (1) (2009) 24–27. [7] Q. Zhao, D. Ju, X. Deng, J. Huang, B. Cao, X. Xu, Morphology-modulation of SnO2 hierarchical architectures by Zn doping for glycol gas sensing and photocatalytic applications, Sci. Rep. 5 (2015) 7874. [8] V.K. Tomer, S. Duhan, In-situ synthesis of Sn O2/SBA-15 hybrid nanocomposite as highly efficient humidity sensor, Sensors Actuators B: Chem. 212 (2015) 517–525. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669. [10] M.D. StollerS, J. Park, Y.W. Zhu, et al., Graphene-based ultracapacitors, Nano Lett. 8 (10) (2008) 3498–3502. [11] A.A. Balandin, S. Ghosh, W.Z. Bao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008) 902–907. [12] F. Schedin, A.K. Geim, S.V. Morozov, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (9) (2007) 652–655. [13] F. Schedin, A.K. Geim, S.V. Morozov, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [14] I.F. Rivera, R.K. Joshi, J. Wang. Graphene based ultra-sensitive gas sensors. IEEE Sensors 2010 Conference, 2010: 1534–1537. [15] S. Rumyantsev, G.G.X. Xiong Liu, M.S. Shur, et al., Selective gas sensing with a single pristine graphene transistor, Nano Lett. 12 (2012) 2294–2298. [16] N. Reckinger, A. Vlad, S. Melinte, et al., Graphene-coated holey metal films: tunable molecular sensing by surface plasmon resonance, Appl. Phys. Lett. 102 (2013) 2111081–20111084. [17] S. Some, Y. Xu, Y. Kim, et al., Highly sensitive and selective gas sensor using hydrophilic and hydrophobic graphenes, Sci. Rep. 2013 (1868) 3. [18] B.C. Yao, Y. Wu, Y. Cheng, et al. Graphene based microfiber gas sensor. OFS 2012 22nd International Conference on Optical Fibre Sensors, 2012: 8421. [19] H. Li, Y. Anugrah, S.J. Koester, Optical absorption in graphene integrated on silicon waveguides, Appl. Phys. Lett. 101 (11) (2012) 1111101–1111103. [20] J.A. Kim, T. Hwang, S.R. Dugasani, et al., Graphene based fiber optic surface plasmon resonance for bio-chemical sensor applications, Sensors Actuators B: Chem. 187 (2013) 426–433. [21] B.C. Yao, Y. Wu, Y. Cheng, et al. Determination of complex refractive index of graphene waveguide by microfiber knot sensor. Asia Pacific Optical Sensors Conference Proceedings, Wuhan, 2013, 89241U–89241U-4. [22] R.T. Weitz, A. Yacoby, Graphene rests easy, Nat. Nanotechnol. 5 (10) (2010) 699–700.