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Sensitivity enhanced microfiber interferometer ammonia gas sensor by using WO3 nanorods coatings
Optics and Laser Technology 125 (2020) 106036
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Sensitivity enhanced microfiber interferometer ammonia gas sensor by using WO3 nanorods coatings
T
⁎
Qiqi Wang, Haiwei Fu , Jijun Ding, Chong Yang, Shuai Wang School of Science, Xi’an Shiyou University, Xi’an 710065, China Shaanxi Engineering Research Center of Oil and Gas Resource Optical Fiber Detection, China Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas Wells, China
H I GH L IG H T S
effective method for the determination of ammonia concentration is proposed. • An microfiber interferometer coated by WO nanorods is fabricated. • AThetapered structure has good repeatability and high sensitivity. • Compared with other gases, the sensor has apparent selectivity for ammonia. • 3
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanomaterials Optical microfiber Ammonia sensing Gas selectivity
Rapid and accurate detection of toxic and harmful gases in the environment is an important means to prevent poisoning and leakage accidents. A tapered microfiber interferometer coated by WO3 nanorods for gas sensing is fabricated by using flame melting biconical taper method. As a coated material for the microfiber interferometer (MFI) ammonia gas sensor, the WO3 nanorods are made by hydrothermal method, and are dripped onto the surface of the sensor. The WO3 nanorods coating on the sensor surface can absorb ammonia molecules and generate charge transfer, which results in the shift of transmission spectrum. The experimental results show that the prepared sensor has high sensitivity and selectivity to ammonia gas. When the ammonia gas concentration is from 0 to 11640 ppm, the spectrum shift of the WO3 nanorods coated sensor is 16.23 nm. The sensor has good repeatability, selectivity and wide application prospects in the monitor of harmful gases.
1. Introduction Toxic, harmful, inflammable and explosive gases exist in the daily life of human beings, and sometimes there are sudden major safety accidents. Among them, ammonia is a harmful polluting gas. Ammonia has a wide range of applications in human daily life, such as nitrogen fertilizer used in crop production, additives used in building processing, ammonia monitoring in the laboratory, will produce ammonia [1]. However, ammonia can cause corrosion of skin tissue. A small amount of ammonia inhalation can stimulate the respiratory tract, leading to dizziness and poor resistance. Excessive inhalation can lead to symptoms such as edema and sudden respiratory arrest [2–5]. As an important branch of sensing field, gas sensor has been widely used in agriculture, industry, electronics and other fields [6–8]. For example, monitoring and controlling the leakage of harmful and toxic gases, monitoring the safety of gas in farmhouses, and emission of waste gases
⁎
from chemical plants [9,10]. Therefore, the development of high sensitivity ammonia concentration detection has a very broad application value and research background. As an important sensing material, metal oxides are widely used in the field of gas sensing. Metal oxide gas sensor has the advantages of fast response recovery, simple manufacturing method, high sensitivity and low production cost. It has been widely used in the monitoring, detection, control and prevention of harmful gases [11–15]. However, metal oxide materials usually react with a variety of gases. Therefore, how to improve the sensitivity of materials to specific gases, reduce the cross-response of sensors to various gases, and improve the selectivity of sensors are of great significance to research [16–18]. By controlling the morphology of materials and composites, the surface area and purity of nanomaterials can be effectively improved, and the semiconductor WO3 materials with large transmittance and specific surface area can be prepared. WO3 materials not only can be widely used in
Corresponding author. E-mail address: [email protected] (H. Fu).
https://doi.org/10.1016/j.optlastec.2019.106036 Received 4 August 2019; Received in revised form 30 October 2019; Accepted 23 December 2019 0030-3992/ © 2020 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 125 (2020) 106036
Q. Wang, et al.
industry, agriculture, national defense and other fields, but also have the advantages of small band gap, non-toxicity, good stability [19–21]. WO3 has the advantages of small band gap, high void oxidation ability of valence band, non-toxicity and good stability. It is an ideal choice for many visible-light active photocatalysts. WO3 is selected as a reliable gas-sensitive material for gas detection. Therefore, WO3 material is an ideal gas sensing material and has broad application prospects. How to develop stable, highly selective and highly sensitive sensors has become a hotspot of modern research. WO3 is a common metal oxide n-type semiconductor material. In recent years, metal oxide semiconductor materials WO3 have good application prospects in air purification, gas concentration measurement and other environmental applications [22,23]. In this work, we have adopted a reliable method to prepare WO3 coated tapered microfiber ammonia gas sensor. Micro-structured optical fibers are fabricated by stretching the flame of multi-core fiber (MCF) to micron level. The concentration of ammonia in the environment is measured by coating WO3 material on the surface of fiber taper.
3. Experiment 3.1. Preparation of WO3 nanoroads material Firstly, 2.489 g Na2WO3·2H2O and 3.057 g Na2SO4 are added into 80 ml deionized water. Stir well at room temperature of 20.9 °C. 3 mol/ L HCL is added to the mixture, and the pH value of the solution is adjusted to 1. Then, the mixture is transferred to 100 ml Teflon-lined stainless-steel autoclave and heated for 24 h. Finally, centrifuge with 8000 rpm and wash 3 times. Dry to powder in vacuum drying chamber. 3.2. Sensor manufacturing and experimental device The fabricated optical fiber sensor is fixed on the slide and the impurities on the surface of the optical fiber are cleaned with deionized water. Then, WO3 nanorods are coated on the surface of the sensor. The sensor coated with WO3 nanorods is placed in a vacuum drying chamber for drying. As shown in Fig. 2, the sensors are connected to a broadband light source (BBS, RS-232) and an optical spectrum analyzer (OSA, Anritsu, MS9740A), respectively. The sensor is fixed in a sealed gas chamber with a volume of 25 dm3. Two small fans in the gas chamber rotate continuously to make the gas distribution uniform. During the whole process, the experimental environment has been at room temperature, ignoring other potential external environment. When the measurement is completed, the chamber is opened and the gas in the chamber is released and volatilized rapidly.
2. Theoretical analysis 2.1. Sensor structure The multi-core optical fiber is gradually thinned by using flame fused cone technology. The resulting fused conical MCF sensor is shown in Fig. 1. The MCF used in the experiment is a seven-core optical fiber, which is produced by Futong Group. The center of MCF has one core, and the other six cores are arranged in a regular hexagon. In the experiment, the welding machine selected is Fitel type optical fiber welding machine (S178A) of Guhe Company, the model of flame melting taper system used in the experiment is SCS-4000.
3.3. Characterization of materials
When the beam enters the fiber, one part of the beam is coupled to the core of the MCF fiber, and the other part is coupled to the cone of the MCF fiber. When the beam enters the fiber cone, the core mode is gradually coupled to the cladding mode. In the sensitive region of the optical fiber gas sensor, the evanescent wave interaction between the detected gas and the surface of the tapered microfiber is studied. According to Lambert-Beer law, the input light intensity and output light intensity of the optical fiber gas sensor satisfy the following equation:
The morphology and composition of WO3 materials are analyzed by a scanning electron microscopy (SEM, JAM-6390A). Meanwhile, the phase composition of WO3 is analyzed by X-ray diffractometer (XRD, XRD-6000). The scanning speed is 8.00 deg/min and the sampling interval is 0.02 deg. During the X-ray diffraction detection, no characteristic peaks of other crystalline materials are detected, indicating that pure WO3 products are prepared. Fig. 3a is a scanning electron microscopic image of WO3 nanorods. Fig. 3b is an X-ray diffraction pattern of the WO3 nanorods. Compared with standard cards, no impurity peaks are found, indicating the only crystal phase detected is WO3. Fig. 3c is topography of WO3 material coated on the sensor surface. The enlarged image can clearly show the sensor wrapped in WO3 material. It can be seen that WO3 material is uniformly coated on the surface of the sensor, absorbing ammonia molecules, resulting in wavelength shift of the transmission spectrum.
I (λ ) = I0 (λ ) exp [−2r (λ ) α (λ ) lc ]
4. Results and analysis
2.2. Principle
(1)
where λ is the wavelength of incident light, I0 (λ ) is incident light intensity, α (λ ) is the gas absorption coefficient, l is the sensing length, c is the gas concentration, r (λ ) is the relative action sensitivity. Therefore,
r (λ ) = [nr (λ ) ne (λ )] f (λ )
The concentration of ammonia is measured by WO3 coated sensor. The experimental results are shown in Fig. 4. After coating WO3 nanorods on the surface of the sensor, ammonia concentration in the environment is measured. The measurement results show that the wavelength shift of the transmission spectrum is greatly increased, and the maximum displacement of the wavelength is 16.23 nm. WO3 nanorods have strong adsorption capacity for ammonia molecules. Fig. 5 is a linear fitting curve for detecting the ammonia concentration with WO3 nanorods sensor and uncoated sensor. It is the relationship between the wavelength shift and the ammonia
(2)
where nr (λ ) is the effective refractive index of the measured gas, ne (λ ) is the effective refractive index of the optical fiber guided mode, f (λ ) is the ratio of evanescent field energy to total transmitted field energy.
Fig. 1. The schematic diagram of the tapered microfiber.
wo3
2
Optics and Laser Technology 125 (2020) 106036
Q. Wang, et al.
Light source
sensing area
Spectrometer
Gas chamber
Fig. 2. Experimental setup for detecting concentration of ammonia gas.
concentration. The red curve is the fitting curve of WO3 nanorods sensor. The fitting results are as follows:
will occur between the gas sensing materials and the adsorbed ammonia molecules. WO3 has photoelectrochemical activity due to the effective carrier separation in its structure. Metal oxide nanorods have larger specific surface area and shorter distances. A small amount of carrier diffusion increases the effective optical conversion efficiency. The refractive index of the microfiber surface changes due to the electron transfer and the spectra red shift when the NH3 is adsorbed on the surface on the surface of WO3 nanorods. The adsorption of NH3 molecule by WO3 leads to the increase of electron density on the sensor surface. Ultimately, the transmission spectrum of the sensor shifts. The repeatability of the sensor is an important index in daily application. After injecting ammonia gas with the concentration of 3492 ppm into the chamber, the wavelength shifts to a certain position
002
20
30
40
50
2theta (degree)
60
70
404
224
200
222 400
110
100
Intensity (a.u.)
WO3 PDF#85-2460
10
×5,000
200
(b)
(a)
220
where Δλ is the wavelength shift and c is the concentration of ammonia. In the sensitive region of the optical fiber gas sensor, the evanescent field interaction between the measured gas NH3 and the surface of the tapered microfiber. Because of there is energy difference between WO3 material and NH3 molecule. When ammonia molecules are adsorbed on the surface of gas sensing materials, charge migration or rearrangement
004
(4)
Δλ = −0.02242 + 3.6089 × 10−5c − 6.3796 × 10−10c 2
202
The blue curve is the fitting curve of without WO3 nanometers, and the fitting result is as follows:
112
(3)
Δλ = 0.3894 − 3.6081 × 10−4c + 1.4415 × 10−7c 2
80
(c)
5 ȝm
×10,000
1 ȝm
Fig. 3. (a) The SEM of the WO3 nanorods, (b) the XRD of the WO3 nanorods, (c) SEM image of WO3 coated sensor. 3
Optics and Laser Technology 125 (2020) 106036
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Wavelength shift (nm)
Transmission (dBm)
-22 -24 -26 -28 -30 0ppm 3492ppm 6984ppm 10476ppm
-32 -34
1164ppm 4656ppm 8148ppm 11640ppm
2328ppm 5820ppm 9312ppm
16
16.23 nm
8 4 0
1520 1530 1540 1550 1560 1570 1580
Wavelength (nm) Fig. 4. The transmission spectra of different ammonia concentration.
a
0.65 nm
0.77 nm
0.54 nm
b
c
d
Analyte
Fig. 7. Selectivity of the response of the sensor to ammonia and other analytes.
16
Wavelength shift (nm)
ammonia methanol alcohol acetone
12
5. Conclusions Experimental data
12
Fitting data Experimental data
8
In conclusion, the selective detection of sensor is realized by coating WO3 nanorods material on the surface of tapered fiber. Owing to the large surface area of WO3 nanorods, it has strong adsorption capacity and good selectivity for ammonia. The wavelength shift of the MFI coated with WO3 nanorods is 54.1 times that of the MFI without WO3 nanorods when the ammonia concentration is between 0 and 11640 ppm. Therefore, the sensor has good recyclability, selectivity and repeatability. Moreover, it has good prospects in the biological fields such as environmental gas monitoring and gas leakage monitoring in chemical plants.
with WO3
Fitting data
4 none
0 0
2000
4000
6000
8000 10000 12000
Concentration (ppm)
Declaration of Competing Interest
Wavelength Shift (nm)
Fig. 5. Fitting curve of wavelength shift and ammonia concentration.
1.4
Ammonia at concentration of 3492 ppm
1.2 1.0
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
4th
3rd
2nd
1th
Acknowledgements
NH3
This work is supported by the Research Foundation of Education Bureau of Shaanxi Province, China, under Grant 14JS073, the Graduate Student Innovation Fund of Xi’an Shiyou University (YCS19211030). Science and Technology Plan Program in Shaanxi Province of China (Grant No. 2019GY-170, Grant No. 2019GY-176).
0.8 0.6 0.4 Air
0.2 0.0
References
0
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[1] S. Vadivel, G. Balaji, S. Rathinavel, High performance ethanol and acetone gas sensor based nanocrystalline MnCo2O4 using clad-modified fiber optic gas sensor, Opt. Mater. 85 (2018) 267–274. [2] S. Sridevi, K. Vasu, N. Bhat, S. Asokan, A. Sood, Ultra sensitive NO2 gas detection using the reduced graphene oxide coated etched fiber Bragg gratings, Sensor. Actuat. B-Chem. 223 (2016) 481–486. [3] R. Kitture, D. Pawar, Ch.N. Rao, R.K. Choubey, S.N. Kale, Nanocomposite modified optical fiber: a room temperature, selective H2S gas sensor: studies using ZnOPMMA, J. Alloy. Compd. 695 (2016) 2091–2096. [4] T. Wang, W. Yasukochi, 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. Actuat. B-Chem. 228 (2016) 573–580. [5] S. Narasimman, L. Balakrishnan, Z.C. Alex, Fiber-optic ammonia sensor based on amine functionalized ZnO nanoflakes, IEEE Sens. J. 18 (2018) 201–208. [6] F. Yavari, E. Castillo, H. Gullapalli, P. Ajayan, N. Koratkar, High sensitivity detection of NO2 and NH3 in air using chemical vapor deposition grown grapheme, Appl. Phys. Lett. 100 (2012) 203120. [7] H. Zeng, S. Terazono, T. Tanuma, A novel catalyst for ammonia synthesis at ambient temperature and pressure: visible light responsive photocatalyst using localized surface plasmon resonance, Catal. Commun. 59 (2015) 40–44. [8] S. Xiao, Y. Jiang, J. Tang, Fiber optic methane sensing system based on temperature-tunable distributed feedback laser, Sensor. Mater. 30 (2018) 2039–2048. [9] S. Devendiran, D. Sastikumar, Fiber optic gas sensor based on light detection from the samarium oxide clad modified region, Opt. Fiber Technol. 46 (2018) 215–220. [10] C. Wu, C. Chiang, Application of notched long-period fiber grating based sensor for CO2 gas sensing, Fiber Integrated Opt. 35 (2016) 21–28.
Time (min. ) Fig. 6. The repeatability of WO3 coated MFI sensor response to the ammonia at concentration of 3492 ppm.
and does not change. Open the gas chamber, the gas quickly releases and volatilizes. The transmission spectrum quickly returns to the initial position. Four measurements are repeated. As shown in Fig. 6, WO3 nanorods coated with MFI have good repeatability and time recovery. Selectivity is an important indicator of gas sensor measurement. It is necessary to detect specific gas in the mixed gases. In Fig. 7, WO3 nanorods coated MFI sensors are used to detect different gases. It is shocking that sensor has obvious responsiveness for ammonia, and the wavelength shift is as high as 16.23 nm. For other gases, methanol, alcohol and acetone, the wavelength shift is very small, far less than 1 nm. Therefore, the sensor has obvious selective response to ammonia.
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Optics and Laser Technology 125 (2020) 106036
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5
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Sensitivity enhanced microfiber interferometer ammonia gas sensor by using WO3 nanorods coatings
T
⁎
Qiqi Wang, Haiwei Fu , Jijun Ding, Chong Yang, Shuai Wang School of Science, Xi’an Shiyou University, Xi’an 710065, China Shaanxi Engineering Research Center of Oil and Gas Resource Optical Fiber Detection, China Shaanxi Key Laboratory of Measurement and Control Technology for Oil and Gas Wells, China
H I GH L IG H T S
effective method for the determination of ammonia concentration is proposed. • An microfiber interferometer coated by WO nanorods is fabricated. • AThetapered structure has good repeatability and high sensitivity. • Compared with other gases, the sensor has apparent selectivity for ammonia. • 3
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanomaterials Optical microfiber Ammonia sensing Gas selectivity
Rapid and accurate detection of toxic and harmful gases in the environment is an important means to prevent poisoning and leakage accidents. A tapered microfiber interferometer coated by WO3 nanorods for gas sensing is fabricated by using flame melting biconical taper method. As a coated material for the microfiber interferometer (MFI) ammonia gas sensor, the WO3 nanorods are made by hydrothermal method, and are dripped onto the surface of the sensor. The WO3 nanorods coating on the sensor surface can absorb ammonia molecules and generate charge transfer, which results in the shift of transmission spectrum. The experimental results show that the prepared sensor has high sensitivity and selectivity to ammonia gas. When the ammonia gas concentration is from 0 to 11640 ppm, the spectrum shift of the WO3 nanorods coated sensor is 16.23 nm. The sensor has good repeatability, selectivity and wide application prospects in the monitor of harmful gases.
1. Introduction Toxic, harmful, inflammable and explosive gases exist in the daily life of human beings, and sometimes there are sudden major safety accidents. Among them, ammonia is a harmful polluting gas. Ammonia has a wide range of applications in human daily life, such as nitrogen fertilizer used in crop production, additives used in building processing, ammonia monitoring in the laboratory, will produce ammonia [1]. However, ammonia can cause corrosion of skin tissue. A small amount of ammonia inhalation can stimulate the respiratory tract, leading to dizziness and poor resistance. Excessive inhalation can lead to symptoms such as edema and sudden respiratory arrest [2–5]. As an important branch of sensing field, gas sensor has been widely used in agriculture, industry, electronics and other fields [6–8]. For example, monitoring and controlling the leakage of harmful and toxic gases, monitoring the safety of gas in farmhouses, and emission of waste gases
⁎
from chemical plants [9,10]. Therefore, the development of high sensitivity ammonia concentration detection has a very broad application value and research background. As an important sensing material, metal oxides are widely used in the field of gas sensing. Metal oxide gas sensor has the advantages of fast response recovery, simple manufacturing method, high sensitivity and low production cost. It has been widely used in the monitoring, detection, control and prevention of harmful gases [11–15]. However, metal oxide materials usually react with a variety of gases. Therefore, how to improve the sensitivity of materials to specific gases, reduce the cross-response of sensors to various gases, and improve the selectivity of sensors are of great significance to research [16–18]. By controlling the morphology of materials and composites, the surface area and purity of nanomaterials can be effectively improved, and the semiconductor WO3 materials with large transmittance and specific surface area can be prepared. WO3 materials not only can be widely used in
Corresponding author. E-mail address: [email protected] (H. Fu).
https://doi.org/10.1016/j.optlastec.2019.106036 Received 4 August 2019; Received in revised form 30 October 2019; Accepted 23 December 2019 0030-3992/ © 2020 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 125 (2020) 106036
Q. Wang, et al.
industry, agriculture, national defense and other fields, but also have the advantages of small band gap, non-toxicity, good stability [19–21]. WO3 has the advantages of small band gap, high void oxidation ability of valence band, non-toxicity and good stability. It is an ideal choice for many visible-light active photocatalysts. WO3 is selected as a reliable gas-sensitive material for gas detection. Therefore, WO3 material is an ideal gas sensing material and has broad application prospects. How to develop stable, highly selective and highly sensitive sensors has become a hotspot of modern research. WO3 is a common metal oxide n-type semiconductor material. In recent years, metal oxide semiconductor materials WO3 have good application prospects in air purification, gas concentration measurement and other environmental applications [22,23]. In this work, we have adopted a reliable method to prepare WO3 coated tapered microfiber ammonia gas sensor. Micro-structured optical fibers are fabricated by stretching the flame of multi-core fiber (MCF) to micron level. The concentration of ammonia in the environment is measured by coating WO3 material on the surface of fiber taper.
3. Experiment 3.1. Preparation of WO3 nanoroads material Firstly, 2.489 g Na2WO3·2H2O and 3.057 g Na2SO4 are added into 80 ml deionized water. Stir well at room temperature of 20.9 °C. 3 mol/ L HCL is added to the mixture, and the pH value of the solution is adjusted to 1. Then, the mixture is transferred to 100 ml Teflon-lined stainless-steel autoclave and heated for 24 h. Finally, centrifuge with 8000 rpm and wash 3 times. Dry to powder in vacuum drying chamber. 3.2. Sensor manufacturing and experimental device The fabricated optical fiber sensor is fixed on the slide and the impurities on the surface of the optical fiber are cleaned with deionized water. Then, WO3 nanorods are coated on the surface of the sensor. The sensor coated with WO3 nanorods is placed in a vacuum drying chamber for drying. As shown in Fig. 2, the sensors are connected to a broadband light source (BBS, RS-232) and an optical spectrum analyzer (OSA, Anritsu, MS9740A), respectively. The sensor is fixed in a sealed gas chamber with a volume of 25 dm3. Two small fans in the gas chamber rotate continuously to make the gas distribution uniform. During the whole process, the experimental environment has been at room temperature, ignoring other potential external environment. When the measurement is completed, the chamber is opened and the gas in the chamber is released and volatilized rapidly.
2. Theoretical analysis 2.1. Sensor structure The multi-core optical fiber is gradually thinned by using flame fused cone technology. The resulting fused conical MCF sensor is shown in Fig. 1. The MCF used in the experiment is a seven-core optical fiber, which is produced by Futong Group. The center of MCF has one core, and the other six cores are arranged in a regular hexagon. In the experiment, the welding machine selected is Fitel type optical fiber welding machine (S178A) of Guhe Company, the model of flame melting taper system used in the experiment is SCS-4000.
3.3. Characterization of materials
When the beam enters the fiber, one part of the beam is coupled to the core of the MCF fiber, and the other part is coupled to the cone of the MCF fiber. When the beam enters the fiber cone, the core mode is gradually coupled to the cladding mode. In the sensitive region of the optical fiber gas sensor, the evanescent wave interaction between the detected gas and the surface of the tapered microfiber is studied. According to Lambert-Beer law, the input light intensity and output light intensity of the optical fiber gas sensor satisfy the following equation:
The morphology and composition of WO3 materials are analyzed by a scanning electron microscopy (SEM, JAM-6390A). Meanwhile, the phase composition of WO3 is analyzed by X-ray diffractometer (XRD, XRD-6000). The scanning speed is 8.00 deg/min and the sampling interval is 0.02 deg. During the X-ray diffraction detection, no characteristic peaks of other crystalline materials are detected, indicating that pure WO3 products are prepared. Fig. 3a is a scanning electron microscopic image of WO3 nanorods. Fig. 3b is an X-ray diffraction pattern of the WO3 nanorods. Compared with standard cards, no impurity peaks are found, indicating the only crystal phase detected is WO3. Fig. 3c is topography of WO3 material coated on the sensor surface. The enlarged image can clearly show the sensor wrapped in WO3 material. It can be seen that WO3 material is uniformly coated on the surface of the sensor, absorbing ammonia molecules, resulting in wavelength shift of the transmission spectrum.
I (λ ) = I0 (λ ) exp [−2r (λ ) α (λ ) lc ]
4. Results and analysis
2.2. Principle
(1)
where λ is the wavelength of incident light, I0 (λ ) is incident light intensity, α (λ ) is the gas absorption coefficient, l is the sensing length, c is the gas concentration, r (λ ) is the relative action sensitivity. Therefore,
r (λ ) = [nr (λ ) ne (λ )] f (λ )
The concentration of ammonia is measured by WO3 coated sensor. The experimental results are shown in Fig. 4. After coating WO3 nanorods on the surface of the sensor, ammonia concentration in the environment is measured. The measurement results show that the wavelength shift of the transmission spectrum is greatly increased, and the maximum displacement of the wavelength is 16.23 nm. WO3 nanorods have strong adsorption capacity for ammonia molecules. Fig. 5 is a linear fitting curve for detecting the ammonia concentration with WO3 nanorods sensor and uncoated sensor. It is the relationship between the wavelength shift and the ammonia
(2)
where nr (λ ) is the effective refractive index of the measured gas, ne (λ ) is the effective refractive index of the optical fiber guided mode, f (λ ) is the ratio of evanescent field energy to total transmitted field energy.
Fig. 1. The schematic diagram of the tapered microfiber.
wo3
2
Optics and Laser Technology 125 (2020) 106036
Q. Wang, et al.
Light source
sensing area
Spectrometer
Gas chamber
Fig. 2. Experimental setup for detecting concentration of ammonia gas.
concentration. The red curve is the fitting curve of WO3 nanorods sensor. The fitting results are as follows:
will occur between the gas sensing materials and the adsorbed ammonia molecules. WO3 has photoelectrochemical activity due to the effective carrier separation in its structure. Metal oxide nanorods have larger specific surface area and shorter distances. A small amount of carrier diffusion increases the effective optical conversion efficiency. The refractive index of the microfiber surface changes due to the electron transfer and the spectra red shift when the NH3 is adsorbed on the surface on the surface of WO3 nanorods. The adsorption of NH3 molecule by WO3 leads to the increase of electron density on the sensor surface. Ultimately, the transmission spectrum of the sensor shifts. The repeatability of the sensor is an important index in daily application. After injecting ammonia gas with the concentration of 3492 ppm into the chamber, the wavelength shifts to a certain position
002
20
30
40
50
2theta (degree)
60
70
404
224
200
222 400
110
100
Intensity (a.u.)
WO3 PDF#85-2460
10
×5,000
200
(b)
(a)
220
where Δλ is the wavelength shift and c is the concentration of ammonia. In the sensitive region of the optical fiber gas sensor, the evanescent field interaction between the measured gas NH3 and the surface of the tapered microfiber. Because of there is energy difference between WO3 material and NH3 molecule. When ammonia molecules are adsorbed on the surface of gas sensing materials, charge migration or rearrangement
004
(4)
Δλ = −0.02242 + 3.6089 × 10−5c − 6.3796 × 10−10c 2
202
The blue curve is the fitting curve of without WO3 nanometers, and the fitting result is as follows:
112
(3)
Δλ = 0.3894 − 3.6081 × 10−4c + 1.4415 × 10−7c 2
80
(c)
5 ȝm
×10,000
1 ȝm
Fig. 3. (a) The SEM of the WO3 nanorods, (b) the XRD of the WO3 nanorods, (c) SEM image of WO3 coated sensor. 3
Optics and Laser Technology 125 (2020) 106036
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Wavelength shift (nm)
Transmission (dBm)
-22 -24 -26 -28 -30 0ppm 3492ppm 6984ppm 10476ppm
-32 -34
1164ppm 4656ppm 8148ppm 11640ppm
2328ppm 5820ppm 9312ppm
16
16.23 nm
8 4 0
1520 1530 1540 1550 1560 1570 1580
Wavelength (nm) Fig. 4. The transmission spectra of different ammonia concentration.
a
0.65 nm
0.77 nm
0.54 nm
b
c
d
Analyte
Fig. 7. Selectivity of the response of the sensor to ammonia and other analytes.
16
Wavelength shift (nm)
ammonia methanol alcohol acetone
12
5. Conclusions Experimental data
12
Fitting data Experimental data
8
In conclusion, the selective detection of sensor is realized by coating WO3 nanorods material on the surface of tapered fiber. Owing to the large surface area of WO3 nanorods, it has strong adsorption capacity and good selectivity for ammonia. The wavelength shift of the MFI coated with WO3 nanorods is 54.1 times that of the MFI without WO3 nanorods when the ammonia concentration is between 0 and 11640 ppm. Therefore, the sensor has good recyclability, selectivity and repeatability. Moreover, it has good prospects in the biological fields such as environmental gas monitoring and gas leakage monitoring in chemical plants.
with WO3
Fitting data
4 none
0 0
2000
4000
6000
8000 10000 12000
Concentration (ppm)
Declaration of Competing Interest
Wavelength Shift (nm)
Fig. 5. Fitting curve of wavelength shift and ammonia concentration.
1.4
Ammonia at concentration of 3492 ppm
1.2 1.0
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
4th
3rd
2nd
1th
Acknowledgements
NH3
This work is supported by the Research Foundation of Education Bureau of Shaanxi Province, China, under Grant 14JS073, the Graduate Student Innovation Fund of Xi’an Shiyou University (YCS19211030). Science and Technology Plan Program in Shaanxi Province of China (Grant No. 2019GY-170, Grant No. 2019GY-176).
0.8 0.6 0.4 Air
0.2 0.0
References
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