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Fe2O3 nanotube coating micro-fiber interferometer for ammonia detection
Sensors & Actuators: B. Chemical 303 (2020) 127186
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fe2O3 nanotube coating micro-fiber interferometer for ammonia detection ⁎
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Haiwei Fu , Qiqi Wang, Jijun Ding, Yi Zhu, Min Zhang, Chong Yang, Shuai Wang Shaanxi Engineering Research Center of Oil and Gas Resource Optical Fiber Detection, Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, 710065, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical fiber interference Gas sensing Optical fiber taper Nano-materials
This paper presents a convenient and effective method for the determination of ammonia concentration. The structure is manufactured of Fe2O3 coated tapered microfiber interferometer (MFI). The Fe2O3 nanotube coated on the MFI acts as sensing layer, which enhance the gas selectivity, adsorptivity for ammonia molecule and also enhances the evanescent field strength distribution of the sensor and changes the mode effective refractive index of guided mode in MFI. When the quantity of ammonia molecule adsorbed on the sensing layer changes, the transmission spectrum of the sensor will shift. By determining the wavelength shift of the dips of the transmission spectrum, the concentration of ammonia will be detected. The experimental results show that the tapered microfiber interferometer (MFI) sensor coated with Fe2O3 nanotube has better time response repeatability and high sensitivity in detecting ammonia gas at room temperature. The sensor has a sensitivity of 1.30 pm/ppm and can be used for the detection of harmful gas ammonia in the environment.
1. Introduction With the rapid development of the industrialization process, serious environmental pollution problems have produced, and the monitoring and control of harmful gases has been paid special attention [1–3]. Ammonia is a harmful gas that excessive intake can cause lung swelling, liver failure and even death [4]. Ammonia has corrosive and irritating effects on our skin. It can absorb water from skin tissue and destroy cell membrane structure. Short-term inhalation of large quantities of ammonia can cause eye pain, sore throat, dizziness, limb weakness and other symptoms. For some people, prolonged exposure to ammonia can lead to skin pigmentation and finger ulcers. In severe cases, symptoms such as pulmonary edema and cardiac arrest may occur [5]. According to different concentrations of ammonia, the human body will produce corresponding physiological reactions. When the ammonia concentration is 5 ppm to10 ppm, it can be smelled. At 400 ppm, it can cause serious irritation to the nasal cavity and upper respiratory tract. When the concentration reaches 5000 ppm, it can cause severe edema, asphyxia, and may be fatal in an instant [6]. In general, ammonia is widely used in our daily lives. About 80% of ammonia is used as agricultural fertilizer, synthetic fiber, plastic and explosives [7]. Ammonia is also found in air, soil and water samples in hazardous waste areas. Therefore, timely and accurate monitoring of harmful gases has become one of the urgent problems in coal, electric power, chemical industry and other industries [8–10]. Because the gas
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to be monitored is usually in a dangerous environment such as flammable, explosive, and corrosive, it limits the use of electrical sensors [11–13]. The fiber optic sensor has been widely used in the detection of gas sensing due to its small size, anti-electromagnetic interference, high sensitivity [14,15]. Therefore, in the perspective of environmental pollution detection and personal safety, ammonia gas sensitivity detection of fiber optic gas sensors has broad application value and prospects. In recent years, gas sensors have been widely used as gas detection tools in gas monitoring and alarm devices in petrochemical industry, natural gas pipelines, closed environment deep wells and pharmaceutical fields [16–18]. High performance gas sensor has become the focus and hot spot of research at domestic and foreign research [19]. In practical industrial production, metal oxide semiconductor gas sensors have been widely used in the detection and alarm of harmful gases and control systems because of their simple structure, fast response speed, low production cost and high sensitivity [20,21]. Fiber optic gas sensors with metal oxide as coated materials have been reported, and high sensitivity and absorption selectivity characteristics in gas detection have also been confirmed [22]. Therefore, to develop new high-performance gas-sensitive materials is of great significance for further improving gas sensing performance to meet actual needs. Based on the above research background, we prepare a Fe2O3 coated tapered microfiber ammonia sensor by a simple and effective method. Microstructure fibers are achieved by a method in which a
Corresponding author. E-mail address: [email protected] (H. Fu).
https://doi.org/10.1016/j.snb.2019.127186 Received 30 April 2019; Received in revised form 12 September 2019; Accepted 22 September 2019 Available online 24 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 303 (2020) 127186
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Fig. 1. (a) The diagrammatic sketch of gas sensing principle for the Fe2O3 coated MFI, (b) sectional view for the multi-core fiber.
length of multi-core fiber (MCF) is stretched by flame and reduced in diameter to the order of microns. Fe2O3 nanotube is fabricated via a facile hydro thermal method. The adsorption of ammonia molecules by the Fe2O3 coated causes a change in the effective refractive index, which in turn leads to a shift in the wavelength of the transmission spectrum. At room temperature, the sensor has good gas selectivity in the range of 0 ppm–11640 ppm ammonia concentration, which provides a new idea for further research on the performance of fiber optic gas sensor.
changes can cause the sensor transmission spectrum to shift. 3. Experimental details 3.1. Preparation and characterization of Fe2O3 structures There is using the simple and reliable hydrothermal method to prepare Fe2O3 nanotube. First of all, 8.0 ml of an aqueous solution of FeCl3 and 7.2 ml of an aqueous NH4H2PO4 solution are added to 200 ml of deionized water, and then the solution is stirred 10 min at room temperature. Meanwhile, the concentration of the aqueous solution of FeCl3 is 0.5 M, and the concentration of the aqueous solution of NH4H2PO4 is 0.02 M. Second, the obtained product is added to 100 ml Teflon-lined stainless-steel autoclave for heating in a dry box at 220 °C for 60 h. Finally, the red product is centrifuged with ethanol three times with, and dried to 80 °C to powder. The morphology and composition of Fe2O3 materials are analyzed by a scanning electron microscopy (SEM, JAM-6390A) and an energy dispersive spectroscopy (EDS, JAM-6390A). At the same time, the phase composition of Fe2O3 is analyzed by X-ray diffractometer (XRD, XRD6000) with a scanning speed of 8.00 deg/min and a sampling pitch of 0.02 deg.
2. The theoretical analysis 2.1. Evanescent wave of the MFI In this experiment, we prepare the MFI by flame-melt stretching method, and the fiber used is a MCF. This is the cross section of MCF, as shown in Fig. 1(b). Since MCF introduce multiple cores into a common single-core fiber, we use flame stretching to bring diameters up to the order of micrometers, making it easier to generate and enhance crosstalk between multiple cores. When use for the detection of the environment to be tested, the transmission spectrum exhibits a high extinction ratio and low loss. When light is transmitted through the fiber, total reflection occurs when incident light reaches the fiber core and cladding interface, and the wave transmitted along the cladding coating layer is called evanescent wave, as shown in Fig. 1(a). When the ammonia concentration in the external environment changes, the gas sensitive material will absorb more gas molecules, which will eventually causing the transmission wavelength shift.
3.2. Fabrication of the sensor In the experiment, the MCF fiber is stretched by flame melt drawing. The MCF is produced by Futong Group, which has a core in the center and six other cores arranged in a hexagon. These cores of the optical fiber have the identical radius (r) of 5.155 μm, which the optical fiber include seven cores. The distance between each core (Λ) is 38.78 μm. The length of the tapered microfiber is 22 mm. The sensor is made by flame heating and stretching, and has a simple structure and low cost. Firstly, the sensor is fixed on a glass slide and cleaned with ethanol to remove foreign substances, which ensured the optical fiber surface clean. Then, the Fe2O3 material is coated on fiber microstructure surfaces by a droplet coated method. Finally, MFIs coated with Fe2O3 material is put into a vacuum drying oven for 60 °C for 240 min.
2.2. Principle Fig. 1(a) shows the structure of the MFI. When the input light enters the first region of the fiber, the MCF simultaneously excites the fundamental mode and the higher-order modes. In the second transition region, the fundamental mode and the higher-order modes are recombined. Meanwhile, there are two modes of existence, the central core mode and the side core mode. Between these two modes, the effective refractive index difference is expressed as Δn. Therefore, the phase difference between the two different modes can be expressed as
δϕ = 2πΔnL λ
3.3. Experimental
(1) As shown in Fig. 2, the sensor is attached to an airtight gas chamber which by the two fiber brackets. One interface of the sensor is attaching to an optical spectrum analyzer (OSA, Anritsu, MS9740A) and the other interface of the sensor attaching to broadband light source (BBS, RS232). Moreover, the BBS has a light-emitting range of 1510 nm–1590 nm, and the OSA has the resolution of 5 pm. The bulk of the gas chamber is 25 dm3, and a small fan is placed on both sides of the gas chamber. In the experiment, the ammonia water is a 25% ammonium hydroxide aqueous solution (Traditional Chinese Medicine, Analytically Pure). There is a heated evaporating dish structure in the gas chamber of measuring ammonia gas with a tapered optical fiber sensor coated with Fe2O3. By injecting ammonia water ammonium hydroxide
Where, λ is the central wavelength of light-wave. L is the length of the effective area in the MFI. Formula (2) describes the relationship between the external RI (next) and the wavelength shift, which is obtained by using a minor variation of next and δϕ. Meanwhile, it is also consider the modal dispersion,
dλ 1 1 dΔn ⎞ =λ ⎛ dnext Γ ⎝ Δn dnext ⎠ ⎜
⎟
(2)
λ dΔn ⋅ Δn dλ
Where, Γ = 1 − is a dispersion factor characterized by a change in refractive index with a change of wavelength. Based on these analyses, the sensor of the effective mode refractive index will change in different external environments. However, these 2
Sensors & Actuators: B. Chemical 303 (2020) 127186
H. Fu, et al.
Fig. 2. The diagram of the experimental configuration.
Fig. 3. (a) SEM image of Fe2O3 nanotube, (b) EDS spectrum of the Fe2O3 sample, (c) XRD patterns of the Fe2O3 nanotube.
volatilized.
aqueous solution into the evaporating dish, the evaporating dish is heated continuously until the ammonia gas in the ammonia water evaporates completely and fills the whole chamber. Waiting for the experiment to stabilize, the sensor spectrum is stable and no longer drift, recording the response of the fiber optic sensor under the concentration of ammonia at this time. The experiment was carried out at a temperature of 22.7 °C in the external environment. When the power is turned on, the ammonia water is injected into the evaporating dish in the air inlet chamber, and the evaporating dish is continuously heated, the ammonia water is quickly evaporated into gas, and the fan starts to rotate, and the gas of the air chamber can be blown to various positions in the air chamber. Therefore, it is assumed that the gas in the gas chamber is uniformly dispersed. In this process of the experiment, at room temperature and ignoring other potential external environment, the gas chamber has the identical pressure with the atmospheric pressure. Fe2O3 coated can enhance the local evanescent field. Ammonia gas contacts the Fe2O3 coated sensor area, causing the transmission spectrum to shift. When the measure is completed, open this gas chamber, the gas is rapidly
4. Results and discussion The metal oxide material is a particularly important gas sensing material, and it is necessary to understand its microstructure. Morphology and composition analysis of the prepared Fe2O3 are carried out by scanning electron microscopy. The large-size morphology of the Fe2O3 material can be clearly seen in the high-magnification electron microscope Fig. 3(a). Fig. 3(b) shows EDS spectrum of Fe2O3 material coated around the fiber, and the area of the blue rectangular box is marked shooting area. Fig. 3(c) shows XRD patterns of the Fe2O3 material. Meanwhile, Fe2O3 material coated around the fiber coincides with the standard Fe2O3 (PDF#24-0072) and standard Fe0.98O (PDF#39-1088). The comparison results show that Fe2O3 material is successfully prepared. In the preparation process, the lack of oxygen leads to an underoxidation state, and the stoichiometric ratio of iron to oxygen are close to 1:1, forming ferrous oxide. However, this oxide is extremely 3
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H. Fu, et al.
Fig. 4. (a) The transmission spectra of the MFI with and without Fe2O3 coated, (b) the transmission spectra of the MFI without Fe2O3 coated for concentration of ammonia gas, (c) the transmission spectra of the MFI with Fe2O3 coated for concentration of ammonia gas, (d) an enlarged view of the wavelength shift in the blue region in Fig. 4(c) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
concentration, the transmission spectrum of MFI without Fe2O3 coated is shown in Fig. 4(b), the red shifted is 1.28 nm. After absorbing ammonia gas, the pore aggregates of the Fe2O3 material are reduced, resulting in increase in the effective refractive index. Therefore, Fe2O3 coated MFI fiber can significantly enhance the disappearing field of the surface, and is particularly sensitive to local refractive index, which is very suitable for the detection of chemical gases. The transmission spectrum at different ammonia concentrations is shown in Fig. 4(c). As the ammonia concentration increases from 0 ppm to 11,640 ppm, the spectrum drifts toward the long wavelength. For the sake of show that the spectral drift, the spectrum is red shifted at 1554.92 nm to 1570.03 nm. The magnification is shown in Fig. 4(d), and the wavelength shift amount is 15.11 nm. The experimental results show that the Fe2O3 material has good adsorption to ammonia molecules, and the sensitivity of the ammonia-based measurement based on the coated iron oxide material is 11.80 times that of the uncoated. The gas sensing property of Fe2O3 nanotube is surface controlled. When the Fe2O3 nanotube on the sensor surface are exposed to the air, oxygen molecules in the air are adsorbed on the surface of the gas sensing material, and electrons are trapped from the conduction band of the material to form oxygen anions form an electron depletion layer at the surface of the material, resulting in decrease in the concentration of carriers in the gas-sensitive material and an increase in electrical resistance. When the sensor is placed in a gas chamber filled with ammonia gas, the ammonia molecules are brought back into the conduction band of the material by the oxygen molecules to achieve detection of the ammonia concentration. Fig. 5 shows the relationship about wavelength shift and ammonia, which is the nonlinearity curve graph.
Fig. 5. Wavelength shift fit curve of Fe2O3 coated microfiber sensor when changing the ammonia concentrations.
unstable, Fe2+ is unstable and will be slowly oxidized into Fe3+ in the air. Maybe, it cannot form a stable iron oxide and present in the material. In the experiment, the change of the external ammonia concentration is measured by a sensor with Fe2O3 coated and a sensor without Fe2O3 coated, and the results are shown in the figure. Fig. 4(a) is the transmission spectrum of the MCF microstructure fiber optic sensor with and without Fe2O3 coated. With the increase of ammonia 4
Sensors & Actuators: B. Chemical 303 (2020) 127186
H. Fu, et al.
Fig. 6. (a) the repeatability of the MCF microfiber sensor with Fe2O3 coated to ammonia of 3492 ppm at room temperature, (b) the steady-state response properties of the sensor.
Fig. 9. Humidity response at different ammonia concentrations.
Δλ = 0.076 + 7.894 × 10−5c + 1.807 × 10−9c 2
Fig. 7. The selectivity of the MCF microfiber sensor with Fe2O3 coated to ammonia and other analytes.
Where, Δλ is the wavelength shift, and c is the ammonia concentration. The Fe2O3 nano-tube coatings on the surface of the MFI can absorb ammonia molecules and generate charge transfer, which change the refractive index of the coatings of tapered microfiber. Therefore, compared with uncoated MFI sensors, it significantly changes the refractive index of the tapered microfiber coatings. So the mode effective refractive index of the high-order guided modes transmitted in the sensor will be changed. Ultimately it causes the shift of transmission spectrum. In the experiment, it is very important to detect the repeatability of the sensor. Therefore, it can be detected via retest whether the transmission spectrum could revert to the initial position. The repeatability and response recovery characteristics of the structure are determined by retesting whether the transmission spectrum could be restored to the initial position. Fig. 6(a) shows the response characteristics for the wavelength over time when ammonia is detected in four cycles. In the first stage, when the chamber is injected with 3492 ppm ammonia gas at room temperature, the MCF microfiber sensor coated with Fe2O3 coated has a relatively stable and stable growth response to ammonia; when the sensor is placed in the air, the transmission spectrum of the sensor returns to its original position. Repeats the experiment process several times and found that the wavelength drift of the sensor is basically constant, and the recovery time in the air is relatively fast. Fig. 6(b) measured the steady-state response properties of the Fe2O3 coated optical fiber structure. When the concentration of ammonia is 1164 ppm, the evaporating dish is heated, the ammonia gas fills the whole chamber rapidly, and the transmission spectrum shift. When the spectral drift is stable, the transmission spectrum shift is basically
Fig. 8. Humidity measurement of the sensor.
For MFI sensors with Fe2O3 coated, the nonlinearity curve of the transmission wavelength versus ammonia concentration is fitted as:
Δλ = 0.139 + 9.724 × 10−5c + 9.936 × 10−8c 2
(4)
(3)
For MFI sensors without Fe2O3 coated, the nonlinearity curve of the transmission wavelength versus ammonia concentration is fitted as: 5
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unchanged after 25 min of repeated recording. Open the chamber, air enters, ammonia is released quickly, and the transmission spectrum returns to the initial position. At the same time, the experiment is repeated four times. Studies have shown that the sensor has good repeatability, reversibility and steady-state response properties. Selectivity is one of the important indicators for evaluating sensor performance. We also test other gases. The results are shown in Fig. 7. Surprisingly, the wavelength shift of the sensor towards methanol, ethanol and acetone is close to and much less than 1 nm. The sensor has a wavelength shift of up to 15.11 nm for ammonia. It can be seen that the sensor has obvious selectivity to ammonia. This shows that, compared with methanol, ethanol and acetone gases, the sensor has apparent response characteristics to ammonia at room temperature. The sensor surface has different adsorption capacities for different gases, and the unique electronics of ammonia make it easier to detect than other analytes. Therefore, the MCF microstructure fiber sensor with Fe2O3 coated does not respond to other gases, providing new ideas and methods for future work. Changes in the external environment have an impact on the practical application of the sensor, especially the humidity outside. As shown in Fig. 8, the transmission spectrum of the sensor is shifted in the response of the externally controlled humidity change of the Fe2O3 coated sensor and the increase in humidity between 30% and 95%, and the maximum shift is 1.36 nm. As shown in Fig. 9, the response of the sensor transmission spectrum to the humidity change at a certain ammonia concentration is measured. In the five groups of comparative experiments, the humidity detection range is 45% to 90%. As can be seen from the figure, when the concentration of ammonia in the environment is constant, the transmission spectrum shift corresponding to the change of humidity is almost the same. Because humidity has little effect on the transmission spectrum of the sensor, it is far less than the response of ammonia to the sensor. When measuring gas concentration, we can ignore the effect of humidity. Even if the sensor is affected by external humidity in the application, the spectrum of the sensor is hardly affected by humidity in the relatively low humidity range of 30% to 90%.
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5. Conclusions In this paper, we study a miniature ammonia gas sensor based on Fe2O3 material coated. The vanishing field of the fiber boundary is enhanced by Fe2O3 coated around the microstructure fiber. An increase in the concentration of ammonia causes an effective refractive index of Fe2O3 to increase, and a wavelength of MFI shifts. The experimental results show that the sensor has good selectivity, repeatability and rapid recovery ability under the action of 0–11640 ppm ammonia gas at room temperature. Due to its low cost, good repeatability and ease of fabrication, this structure puts forward a new direction for gas concentration monitoring and has the huge potential application background in the field of gas sensing.
Haiwei Fu received his Ph. D degree in electronics since and technology from Xi’an Jiaotong University, Xi’an, China, in 2006. He is currently a professor in the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Now, his major research interests are photonics devices and microfiber sensors.
Acknowledgements
Qiqi Wang received her Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2017. She is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Her current research interests are mainly focused on optical fiber sensing.
This work was supported by Science and Technology Plan Program in Shaanxi Province of China (Grant Nos. 2019GY-176 and 2019GY–170), the Research Foundation of Shaanxi Educational Committee (Grant No. 14JS073), the Graduate Student Innovation Fund of Xi’an Shiyou University (Grant No. YCS18112032).
Jijun Ding received his Ph. D. degree in electronics and Information Engineering from Xi’an Jiaotong University in 2014. Now, he is a professor and also is a researcher in the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. His current research is focused on synthesis and application on gas sensor of graphene and its derivatives.
References [1] T. Wang, S. Korposh, S.W. James, R.P. Tatam, S. Lee, A long period grating optical fiber sensor with nano-assembled porphyrin layers for detecting ammonia gas, Sens. Actuators B-Chem. 228 (2016) 573–580. [2] T. Chang, X. Wang, A. Hsiao, Z. Xu, G. Lin, M.R. Gartia, X. Liu, G.L. Liu, Bifunctional nano lycurgus cup array plasmonic sensor for colorimetric sensing and surfaceenhanced Raman spectroscopy, Adv. Opt. Mater. 3 (2015) 1397–1404.
Yi Zhu received her Bachelor’s degree in macro molecular material and engineering at 2016. She is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Her current research interests are mainly focused on synthesis and application of gas sensitive materials.
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optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China.
Min Zhang received her Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2016. She is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Her current research interests are mainly focused on optical fiber sensing.
Shuai Wang received his Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2018. He is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China.
Chong Yang received his Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2017. He is currently working toward the M.S. degree for
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fe2O3 nanotube coating micro-fiber interferometer for ammonia detection ⁎
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Haiwei Fu , Qiqi Wang, Jijun Ding, Yi Zhu, Min Zhang, Chong Yang, Shuai Wang Shaanxi Engineering Research Center of Oil and Gas Resource Optical Fiber Detection, Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, 710065, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical fiber interference Gas sensing Optical fiber taper Nano-materials
This paper presents a convenient and effective method for the determination of ammonia concentration. The structure is manufactured of Fe2O3 coated tapered microfiber interferometer (MFI). The Fe2O3 nanotube coated on the MFI acts as sensing layer, which enhance the gas selectivity, adsorptivity for ammonia molecule and also enhances the evanescent field strength distribution of the sensor and changes the mode effective refractive index of guided mode in MFI. When the quantity of ammonia molecule adsorbed on the sensing layer changes, the transmission spectrum of the sensor will shift. By determining the wavelength shift of the dips of the transmission spectrum, the concentration of ammonia will be detected. The experimental results show that the tapered microfiber interferometer (MFI) sensor coated with Fe2O3 nanotube has better time response repeatability and high sensitivity in detecting ammonia gas at room temperature. The sensor has a sensitivity of 1.30 pm/ppm and can be used for the detection of harmful gas ammonia in the environment.
1. Introduction With the rapid development of the industrialization process, serious environmental pollution problems have produced, and the monitoring and control of harmful gases has been paid special attention [1–3]. Ammonia is a harmful gas that excessive intake can cause lung swelling, liver failure and even death [4]. Ammonia has corrosive and irritating effects on our skin. It can absorb water from skin tissue and destroy cell membrane structure. Short-term inhalation of large quantities of ammonia can cause eye pain, sore throat, dizziness, limb weakness and other symptoms. For some people, prolonged exposure to ammonia can lead to skin pigmentation and finger ulcers. In severe cases, symptoms such as pulmonary edema and cardiac arrest may occur [5]. According to different concentrations of ammonia, the human body will produce corresponding physiological reactions. When the ammonia concentration is 5 ppm to10 ppm, it can be smelled. At 400 ppm, it can cause serious irritation to the nasal cavity and upper respiratory tract. When the concentration reaches 5000 ppm, it can cause severe edema, asphyxia, and may be fatal in an instant [6]. In general, ammonia is widely used in our daily lives. About 80% of ammonia is used as agricultural fertilizer, synthetic fiber, plastic and explosives [7]. Ammonia is also found in air, soil and water samples in hazardous waste areas. Therefore, timely and accurate monitoring of harmful gases has become one of the urgent problems in coal, electric power, chemical industry and other industries [8–10]. Because the gas
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to be monitored is usually in a dangerous environment such as flammable, explosive, and corrosive, it limits the use of electrical sensors [11–13]. The fiber optic sensor has been widely used in the detection of gas sensing due to its small size, anti-electromagnetic interference, high sensitivity [14,15]. Therefore, in the perspective of environmental pollution detection and personal safety, ammonia gas sensitivity detection of fiber optic gas sensors has broad application value and prospects. In recent years, gas sensors have been widely used as gas detection tools in gas monitoring and alarm devices in petrochemical industry, natural gas pipelines, closed environment deep wells and pharmaceutical fields [16–18]. High performance gas sensor has become the focus and hot spot of research at domestic and foreign research [19]. In practical industrial production, metal oxide semiconductor gas sensors have been widely used in the detection and alarm of harmful gases and control systems because of their simple structure, fast response speed, low production cost and high sensitivity [20,21]. Fiber optic gas sensors with metal oxide as coated materials have been reported, and high sensitivity and absorption selectivity characteristics in gas detection have also been confirmed [22]. Therefore, to develop new high-performance gas-sensitive materials is of great significance for further improving gas sensing performance to meet actual needs. Based on the above research background, we prepare a Fe2O3 coated tapered microfiber ammonia sensor by a simple and effective method. Microstructure fibers are achieved by a method in which a
Corresponding author. E-mail address: [email protected] (H. Fu).
https://doi.org/10.1016/j.snb.2019.127186 Received 30 April 2019; Received in revised form 12 September 2019; Accepted 22 September 2019 Available online 24 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) The diagrammatic sketch of gas sensing principle for the Fe2O3 coated MFI, (b) sectional view for the multi-core fiber.
length of multi-core fiber (MCF) is stretched by flame and reduced in diameter to the order of microns. Fe2O3 nanotube is fabricated via a facile hydro thermal method. The adsorption of ammonia molecules by the Fe2O3 coated causes a change in the effective refractive index, which in turn leads to a shift in the wavelength of the transmission spectrum. At room temperature, the sensor has good gas selectivity in the range of 0 ppm–11640 ppm ammonia concentration, which provides a new idea for further research on the performance of fiber optic gas sensor.
changes can cause the sensor transmission spectrum to shift. 3. Experimental details 3.1. Preparation and characterization of Fe2O3 structures There is using the simple and reliable hydrothermal method to prepare Fe2O3 nanotube. First of all, 8.0 ml of an aqueous solution of FeCl3 and 7.2 ml of an aqueous NH4H2PO4 solution are added to 200 ml of deionized water, and then the solution is stirred 10 min at room temperature. Meanwhile, the concentration of the aqueous solution of FeCl3 is 0.5 M, and the concentration of the aqueous solution of NH4H2PO4 is 0.02 M. Second, the obtained product is added to 100 ml Teflon-lined stainless-steel autoclave for heating in a dry box at 220 °C for 60 h. Finally, the red product is centrifuged with ethanol three times with, and dried to 80 °C to powder. The morphology and composition of Fe2O3 materials are analyzed by a scanning electron microscopy (SEM, JAM-6390A) and an energy dispersive spectroscopy (EDS, JAM-6390A). At the same time, the phase composition of Fe2O3 is analyzed by X-ray diffractometer (XRD, XRD6000) with a scanning speed of 8.00 deg/min and a sampling pitch of 0.02 deg.
2. The theoretical analysis 2.1. Evanescent wave of the MFI In this experiment, we prepare the MFI by flame-melt stretching method, and the fiber used is a MCF. This is the cross section of MCF, as shown in Fig. 1(b). Since MCF introduce multiple cores into a common single-core fiber, we use flame stretching to bring diameters up to the order of micrometers, making it easier to generate and enhance crosstalk between multiple cores. When use for the detection of the environment to be tested, the transmission spectrum exhibits a high extinction ratio and low loss. When light is transmitted through the fiber, total reflection occurs when incident light reaches the fiber core and cladding interface, and the wave transmitted along the cladding coating layer is called evanescent wave, as shown in Fig. 1(a). When the ammonia concentration in the external environment changes, the gas sensitive material will absorb more gas molecules, which will eventually causing the transmission wavelength shift.
3.2. Fabrication of the sensor In the experiment, the MCF fiber is stretched by flame melt drawing. The MCF is produced by Futong Group, which has a core in the center and six other cores arranged in a hexagon. These cores of the optical fiber have the identical radius (r) of 5.155 μm, which the optical fiber include seven cores. The distance between each core (Λ) is 38.78 μm. The length of the tapered microfiber is 22 mm. The sensor is made by flame heating and stretching, and has a simple structure and low cost. Firstly, the sensor is fixed on a glass slide and cleaned with ethanol to remove foreign substances, which ensured the optical fiber surface clean. Then, the Fe2O3 material is coated on fiber microstructure surfaces by a droplet coated method. Finally, MFIs coated with Fe2O3 material is put into a vacuum drying oven for 60 °C for 240 min.
2.2. Principle Fig. 1(a) shows the structure of the MFI. When the input light enters the first region of the fiber, the MCF simultaneously excites the fundamental mode and the higher-order modes. In the second transition region, the fundamental mode and the higher-order modes are recombined. Meanwhile, there are two modes of existence, the central core mode and the side core mode. Between these two modes, the effective refractive index difference is expressed as Δn. Therefore, the phase difference between the two different modes can be expressed as
δϕ = 2πΔnL λ
3.3. Experimental
(1) As shown in Fig. 2, the sensor is attached to an airtight gas chamber which by the two fiber brackets. One interface of the sensor is attaching to an optical spectrum analyzer (OSA, Anritsu, MS9740A) and the other interface of the sensor attaching to broadband light source (BBS, RS232). Moreover, the BBS has a light-emitting range of 1510 nm–1590 nm, and the OSA has the resolution of 5 pm. The bulk of the gas chamber is 25 dm3, and a small fan is placed on both sides of the gas chamber. In the experiment, the ammonia water is a 25% ammonium hydroxide aqueous solution (Traditional Chinese Medicine, Analytically Pure). There is a heated evaporating dish structure in the gas chamber of measuring ammonia gas with a tapered optical fiber sensor coated with Fe2O3. By injecting ammonia water ammonium hydroxide
Where, λ is the central wavelength of light-wave. L is the length of the effective area in the MFI. Formula (2) describes the relationship between the external RI (next) and the wavelength shift, which is obtained by using a minor variation of next and δϕ. Meanwhile, it is also consider the modal dispersion,
dλ 1 1 dΔn ⎞ =λ ⎛ dnext Γ ⎝ Δn dnext ⎠ ⎜
⎟
(2)
λ dΔn ⋅ Δn dλ
Where, Γ = 1 − is a dispersion factor characterized by a change in refractive index with a change of wavelength. Based on these analyses, the sensor of the effective mode refractive index will change in different external environments. However, these 2
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Fig. 2. The diagram of the experimental configuration.
Fig. 3. (a) SEM image of Fe2O3 nanotube, (b) EDS spectrum of the Fe2O3 sample, (c) XRD patterns of the Fe2O3 nanotube.
volatilized.
aqueous solution into the evaporating dish, the evaporating dish is heated continuously until the ammonia gas in the ammonia water evaporates completely and fills the whole chamber. Waiting for the experiment to stabilize, the sensor spectrum is stable and no longer drift, recording the response of the fiber optic sensor under the concentration of ammonia at this time. The experiment was carried out at a temperature of 22.7 °C in the external environment. When the power is turned on, the ammonia water is injected into the evaporating dish in the air inlet chamber, and the evaporating dish is continuously heated, the ammonia water is quickly evaporated into gas, and the fan starts to rotate, and the gas of the air chamber can be blown to various positions in the air chamber. Therefore, it is assumed that the gas in the gas chamber is uniformly dispersed. In this process of the experiment, at room temperature and ignoring other potential external environment, the gas chamber has the identical pressure with the atmospheric pressure. Fe2O3 coated can enhance the local evanescent field. Ammonia gas contacts the Fe2O3 coated sensor area, causing the transmission spectrum to shift. When the measure is completed, open this gas chamber, the gas is rapidly
4. Results and discussion The metal oxide material is a particularly important gas sensing material, and it is necessary to understand its microstructure. Morphology and composition analysis of the prepared Fe2O3 are carried out by scanning electron microscopy. The large-size morphology of the Fe2O3 material can be clearly seen in the high-magnification electron microscope Fig. 3(a). Fig. 3(b) shows EDS spectrum of Fe2O3 material coated around the fiber, and the area of the blue rectangular box is marked shooting area. Fig. 3(c) shows XRD patterns of the Fe2O3 material. Meanwhile, Fe2O3 material coated around the fiber coincides with the standard Fe2O3 (PDF#24-0072) and standard Fe0.98O (PDF#39-1088). The comparison results show that Fe2O3 material is successfully prepared. In the preparation process, the lack of oxygen leads to an underoxidation state, and the stoichiometric ratio of iron to oxygen are close to 1:1, forming ferrous oxide. However, this oxide is extremely 3
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Fig. 4. (a) The transmission spectra of the MFI with and without Fe2O3 coated, (b) the transmission spectra of the MFI without Fe2O3 coated for concentration of ammonia gas, (c) the transmission spectra of the MFI with Fe2O3 coated for concentration of ammonia gas, (d) an enlarged view of the wavelength shift in the blue region in Fig. 4(c) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
concentration, the transmission spectrum of MFI without Fe2O3 coated is shown in Fig. 4(b), the red shifted is 1.28 nm. After absorbing ammonia gas, the pore aggregates of the Fe2O3 material are reduced, resulting in increase in the effective refractive index. Therefore, Fe2O3 coated MFI fiber can significantly enhance the disappearing field of the surface, and is particularly sensitive to local refractive index, which is very suitable for the detection of chemical gases. The transmission spectrum at different ammonia concentrations is shown in Fig. 4(c). As the ammonia concentration increases from 0 ppm to 11,640 ppm, the spectrum drifts toward the long wavelength. For the sake of show that the spectral drift, the spectrum is red shifted at 1554.92 nm to 1570.03 nm. The magnification is shown in Fig. 4(d), and the wavelength shift amount is 15.11 nm. The experimental results show that the Fe2O3 material has good adsorption to ammonia molecules, and the sensitivity of the ammonia-based measurement based on the coated iron oxide material is 11.80 times that of the uncoated. The gas sensing property of Fe2O3 nanotube is surface controlled. When the Fe2O3 nanotube on the sensor surface are exposed to the air, oxygen molecules in the air are adsorbed on the surface of the gas sensing material, and electrons are trapped from the conduction band of the material to form oxygen anions form an electron depletion layer at the surface of the material, resulting in decrease in the concentration of carriers in the gas-sensitive material and an increase in electrical resistance. When the sensor is placed in a gas chamber filled with ammonia gas, the ammonia molecules are brought back into the conduction band of the material by the oxygen molecules to achieve detection of the ammonia concentration. Fig. 5 shows the relationship about wavelength shift and ammonia, which is the nonlinearity curve graph.
Fig. 5. Wavelength shift fit curve of Fe2O3 coated microfiber sensor when changing the ammonia concentrations.
unstable, Fe2+ is unstable and will be slowly oxidized into Fe3+ in the air. Maybe, it cannot form a stable iron oxide and present in the material. In the experiment, the change of the external ammonia concentration is measured by a sensor with Fe2O3 coated and a sensor without Fe2O3 coated, and the results are shown in the figure. Fig. 4(a) is the transmission spectrum of the MCF microstructure fiber optic sensor with and without Fe2O3 coated. With the increase of ammonia 4
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Fig. 6. (a) the repeatability of the MCF microfiber sensor with Fe2O3 coated to ammonia of 3492 ppm at room temperature, (b) the steady-state response properties of the sensor.
Fig. 9. Humidity response at different ammonia concentrations.
Δλ = 0.076 + 7.894 × 10−5c + 1.807 × 10−9c 2
Fig. 7. The selectivity of the MCF microfiber sensor with Fe2O3 coated to ammonia and other analytes.
Where, Δλ is the wavelength shift, and c is the ammonia concentration. The Fe2O3 nano-tube coatings on the surface of the MFI can absorb ammonia molecules and generate charge transfer, which change the refractive index of the coatings of tapered microfiber. Therefore, compared with uncoated MFI sensors, it significantly changes the refractive index of the tapered microfiber coatings. So the mode effective refractive index of the high-order guided modes transmitted in the sensor will be changed. Ultimately it causes the shift of transmission spectrum. In the experiment, it is very important to detect the repeatability of the sensor. Therefore, it can be detected via retest whether the transmission spectrum could revert to the initial position. The repeatability and response recovery characteristics of the structure are determined by retesting whether the transmission spectrum could be restored to the initial position. Fig. 6(a) shows the response characteristics for the wavelength over time when ammonia is detected in four cycles. In the first stage, when the chamber is injected with 3492 ppm ammonia gas at room temperature, the MCF microfiber sensor coated with Fe2O3 coated has a relatively stable and stable growth response to ammonia; when the sensor is placed in the air, the transmission spectrum of the sensor returns to its original position. Repeats the experiment process several times and found that the wavelength drift of the sensor is basically constant, and the recovery time in the air is relatively fast. Fig. 6(b) measured the steady-state response properties of the Fe2O3 coated optical fiber structure. When the concentration of ammonia is 1164 ppm, the evaporating dish is heated, the ammonia gas fills the whole chamber rapidly, and the transmission spectrum shift. When the spectral drift is stable, the transmission spectrum shift is basically
Fig. 8. Humidity measurement of the sensor.
For MFI sensors with Fe2O3 coated, the nonlinearity curve of the transmission wavelength versus ammonia concentration is fitted as:
Δλ = 0.139 + 9.724 × 10−5c + 9.936 × 10−8c 2
(4)
(3)
For MFI sensors without Fe2O3 coated, the nonlinearity curve of the transmission wavelength versus ammonia concentration is fitted as: 5
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unchanged after 25 min of repeated recording. Open the chamber, air enters, ammonia is released quickly, and the transmission spectrum returns to the initial position. At the same time, the experiment is repeated four times. Studies have shown that the sensor has good repeatability, reversibility and steady-state response properties. Selectivity is one of the important indicators for evaluating sensor performance. We also test other gases. The results are shown in Fig. 7. Surprisingly, the wavelength shift of the sensor towards methanol, ethanol and acetone is close to and much less than 1 nm. The sensor has a wavelength shift of up to 15.11 nm for ammonia. It can be seen that the sensor has obvious selectivity to ammonia. This shows that, compared with methanol, ethanol and acetone gases, the sensor has apparent response characteristics to ammonia at room temperature. The sensor surface has different adsorption capacities for different gases, and the unique electronics of ammonia make it easier to detect than other analytes. Therefore, the MCF microstructure fiber sensor with Fe2O3 coated does not respond to other gases, providing new ideas and methods for future work. Changes in the external environment have an impact on the practical application of the sensor, especially the humidity outside. As shown in Fig. 8, the transmission spectrum of the sensor is shifted in the response of the externally controlled humidity change of the Fe2O3 coated sensor and the increase in humidity between 30% and 95%, and the maximum shift is 1.36 nm. As shown in Fig. 9, the response of the sensor transmission spectrum to the humidity change at a certain ammonia concentration is measured. In the five groups of comparative experiments, the humidity detection range is 45% to 90%. As can be seen from the figure, when the concentration of ammonia in the environment is constant, the transmission spectrum shift corresponding to the change of humidity is almost the same. Because humidity has little effect on the transmission spectrum of the sensor, it is far less than the response of ammonia to the sensor. When measuring gas concentration, we can ignore the effect of humidity. Even if the sensor is affected by external humidity in the application, the spectrum of the sensor is hardly affected by humidity in the relatively low humidity range of 30% to 90%.
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5. Conclusions In this paper, we study a miniature ammonia gas sensor based on Fe2O3 material coated. The vanishing field of the fiber boundary is enhanced by Fe2O3 coated around the microstructure fiber. An increase in the concentration of ammonia causes an effective refractive index of Fe2O3 to increase, and a wavelength of MFI shifts. The experimental results show that the sensor has good selectivity, repeatability and rapid recovery ability under the action of 0–11640 ppm ammonia gas at room temperature. Due to its low cost, good repeatability and ease of fabrication, this structure puts forward a new direction for gas concentration monitoring and has the huge potential application background in the field of gas sensing.
Haiwei Fu received his Ph. D degree in electronics since and technology from Xi’an Jiaotong University, Xi’an, China, in 2006. He is currently a professor in the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Now, his major research interests are photonics devices and microfiber sensors.
Acknowledgements
Qiqi Wang received her Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2017. She is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Her current research interests are mainly focused on optical fiber sensing.
This work was supported by Science and Technology Plan Program in Shaanxi Province of China (Grant Nos. 2019GY-176 and 2019GY–170), the Research Foundation of Shaanxi Educational Committee (Grant No. 14JS073), the Graduate Student Innovation Fund of Xi’an Shiyou University (Grant No. YCS18112032).
Jijun Ding received his Ph. D. degree in electronics and Information Engineering from Xi’an Jiaotong University in 2014. Now, he is a professor and also is a researcher in the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. His current research is focused on synthesis and application on gas sensor of graphene and its derivatives.
References [1] T. Wang, S. Korposh, S.W. James, R.P. Tatam, S. Lee, A long period grating optical fiber sensor with nano-assembled porphyrin layers for detecting ammonia gas, Sens. Actuators B-Chem. 228 (2016) 573–580. [2] T. Chang, X. Wang, A. Hsiao, Z. Xu, G. Lin, M.R. Gartia, X. Liu, G.L. Liu, Bifunctional nano lycurgus cup array plasmonic sensor for colorimetric sensing and surfaceenhanced Raman spectroscopy, Adv. Opt. Mater. 3 (2015) 1397–1404.
Yi Zhu received her Bachelor’s degree in macro molecular material and engineering at 2016. She is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Her current research interests are mainly focused on synthesis and application of gas sensitive materials.
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optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China.
Min Zhang received her Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2016. She is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China. Her current research interests are mainly focused on optical fiber sensing.
Shuai Wang received his Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2018. He is currently working toward the M.S. degree for optical engineering at the Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas wells, Xi’an Shiyou University, Xi’an, China.
Chong Yang received his Bachelor’s degree in optical information science and technology from Xi’an Shiyou University at 2017. He is currently working toward the M.S. degree for
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