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Publicly Available Published by De Gruyter November 16, 2018

Dual Fabry–Pérot Interferometric Carbon Monoxide Sensor Based on the PANI/Co3O4 Sensitive Membrane-Coated Fibre Tip

  • Jin Peng , Wenlin Feng EMAIL logo , Xiaozhan Yang , Guojia Huang and Shaodian Liu

Abstract

A novel dual Fabry–Pérot (F-P) interferometric carbon monoxide gas sensor based on polyaniline/Co3O4 (PANI/Co3O4) sensing film coated on the optical fibre end face is proposed and fabricated. Its structure is composed of standard single-mode-fibre (SMF), endlessly photonic crystal fibre (EPCF), and PANI/Co3O4 sensing membrane (PCSM). Therefore, they form three F-P reflectors, the reflector between SMF and EPCF, that between EPCF and PCSM, and interface between PCSM and air. So, the dual F-P interferometer is achieved. The results show that in the range of 0–70 ppm, the interference spectra appear red shift with the increasing carbon monoxide concentration. In addition, the high sensitivity of 21.61 pm/ppm, the excellent linear relationship (R2 = 0.98476), and high selectivity for carbon monoxide are achieved. The response and recovery time are 35 and 84 s, respectively. The sensor has the advantages of high sensitivity, strong selectivity, low cost, and simple structure and is suitable for sensitive detection of trace carbon monoxide gas.

1 Introduction

Carbon monoxide (CO) is a widely inflammable and explosive toxic gas in the atmosphere. It often occurred in the event of CO poisoning and explosion. At present, some sensor types, such as metal-oxide semiconductor [1], [2], electrochemical solid electrolytes [3], [4], electrochemical polymer electrolytes [5], and thermal resistance [6], [7], [8], [9], are commonly used to detect target gases, but there are still some problems in selectivity, stability, and reliability. Fibre optic Fabry–Pérot (F-P) interferometric sensor has small volume, low cost, strong selectivity, high sensitivity, good stability, and simple structure and hence is widely used in temperature, pressure, strain, and filtering modulation measurements [10], [11], [12], [13], [14], [15]. In the gas detection, the intrinsic properties of the sensitive films are important. Among these sensitive materials, polyaniline (PANI), a polymer, has attracted much attention because of its unique optical and excellent charge-transfer features [16], [17]. After PANI is entrapped with Co3O4, the doping can reduce the effective height and width of the tunnel barrier and then improve the conductivity and gas sensitivity [18], [19].

In the present work, we report a novel dual F-P interferometer that is set up by the standard single-mode-fibre (SMF), endlessly photonic crystal fibre (EPCF), and PANI/Co3O4 sensing membrane (PCSM). As an application, CO gas was detected by using the SMF-EPCF-PCSM interferometer, and some useful results were obtained.

2 Theory

The operation principle of the sensor based on interferometer, which uses a sensing structure of dual F-P interferometer, is shown in Figure 1. A section of the EPCF is fused with the SMF, and the first F-P cavity is obtained, and then the EPCF end is coated with a sensitive film to form second F-P cavity. As shown in Figure 1, the dual F-P interferometric gas sensor based on the PANI/Co3O4 sensitive film-coated fibre end surface is fabricated, which can be considered as the interference of three beams by three reflectors (mirrors 1, 2, and 3). As shown in Figure 1, the length of the EPCF and the thickness of the PCSM are d1 and d2, respectively. The reflected lights from three mirrors will be coupled back into SMF and interfere with one another. According to the theory of multi-beam light interferences, the total interference intensity I′0 is given as follows [20], [21], [22], [23]:

(1)I0=I10+I20+I302I10I20cos(φ1)2I30I20cos(φ2)+2I30I10cos(φ3)

where I10, I20, and I30 are the reflected light intensities at three mirrors; φ1, φ2, and φ3 are, respectively, the phase differences of the corresponding phase superposition of every two beam lights.

Figure 1: The sensing structure of dual Fabry–Pérot interferometer.
Figure 1:

The sensing structure of dual Fabry–Pérot interferometer.

In the fibre F-P interference structure, the relationship between the phase difference and optical path difference can be expressed as follows [23]:

(2)Δφ=2πλδ,δ=2ndcosβ

where λ is the light wavelength, n is the refractive index of F-P cavity material, and β is the angle between reflection light and normal line, and d is length of F-P cavity. The phase difference can be further expressed as follows [23]:

Δφ=4πnλdcosβ,φ1=Δφ1+φ10=4πn2d1cosβλ+φ10
(3)φ2=Δφ2+φ20=4πn3d2cosβλ+φ20φ3=Δφ3+φ10+φ20=4π(n2d1+n3d2)cosβλ+φ10+φ20

where n2 and n3 are the refractive indexes of EPCF and PCSM. φ10, φ20 and φ30 are the initial phases, respectively. The valley of deconstructive interference is used as a reference. The total optical path difference and phase difference can be expressed as follows:

δ=2(n3d2+nd12)
(4)Δφ=2πλmδ=4π(n3d2+nd12)λm=(2m+1)π

where m is the order of interference valley, and λm is the initial valley wavelength. When the sensitive membrane adsorbs gases, the λm becomes λm + Δλm, the n3 changes into n3 + Δn3, and the d3 turns into d2 + Δd2. But n2 and d1 remain unchanged because they are not affected by external environment. The phase value of the cosine term in (3) remains identical to that of the valley wavelength. That is,

(5)4π(n3d2+nd12)λm=4π[(n3+Δn3)(d2+Δd2)+nd12]λm+Δλm=(2m+1)π
(6)Δλm=4[Δn3d2+(n3+Δn3)Δd2]2m+1

From (2) to (6), it can be seen that the phase difference of reflection lights is proportional to the length d and refractive index n of the F-P cavity. When the length d or refractive index n of the optical fibre F-P cavity changes, the phase difference will also transform, and the light intensity of the reflection light will also vary accordingly. Therefore, the design principle of the fibre F-P sensor is that the change of the measured physical quantity is converted to the changes of the cavity length (d + Δd) and refractive index (n + Δn) of F-P cavity, and then the wave valley is moved (λm+Δλm). Therefore, the change of the measured physical quantity can be deduced by measuring the shift of the wave valley.

3 Experimental

3.1 Preparation of the Dual F-P Cavities

The endlessly single mode photonic crystal fibre (EPCF, SM-10 PCF) bought from Yangtze Optical Fibre and Cable Joint Stock Limited Company (YOFC), and its diameter is 125 μm, and the multilayer air hole of 9.5 μm has a hexagonal array in EPCF. A 6-cm-long EPCF and SMF (SMF-28) were employed for a lower fusion loss because they have the same 125-μm diameter. The EPCF cleaved two flat end facets and one side of EPCF spliced with a standard SMF by Furukawa S178C fusion splicer. The automatic operation mode of the splicer is used for splicing the standard SMF and EPCF. In the operation program of the splicer, the default parameters for the splicing of the SMF and EPCF are as follows: the first and first end arc-power setting at +60, the second and second end arc-power setting at +100, the prefusion time of 160 ms, the initial and second arc-duration time of +150 ms, the automatic discharge time of +1300 ms, the first discharge end time of +800 ms, the second discharge end time of +1000 ms, and Z-pull distance of 15 μm. Thus, the EPCF and SMF end faces are completely fused to form the first and the second reflection mirror surfaces (mirrors 1 and 2 in Fig. 1).

The preparation method of the third reflection mirror surface is as follows: 1 g nano-Co3O4 was put into the hydrochloric acid of 0.2 M for 2 h of ultrasonic dispersion and mixed the 1.78 mL aniline solution by the magnetic force for 1 h, and then added slowly ammonium persulfate into the mixture by stirring the magnetic force at the temperature of 30 °C for 10 h; finally, the precipitation was collected by alcohol. The sample was drying 2 h at 60 °C. The sensitive film was coated on the end surface of EPCF by dipping method and dried in a vacuum drying box at 80 °C for 6 h to make it adsorbed stably on the end surface of the EPCF fibre. Therefore, the third reflection mirror surface (mirror 3 in Fig. 1) is obtained. To further determine the structure and components of sensitive membrane, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were measured and analysed.

3.2 Gas Sensing System

Schematic diagram of the experimental setup is shown in Figure 2. Amplified spontaneous emission (ASE) broadband light source with the wavelength range is 1530–1625 nm, which was launched into the circulator of gas test system. Circulator (PIOC315P210-type) is prepared by Jilian Technology Co. Ltd., China. Dual F-P interferometer was placed in the glass gas chamber, which can introduce the CO gas effectively. Transmission spectra were measured by using an optical spectrum analyzer (AQ6370D, OSA; Yokogawa, Tokyo, Japan). The concentration of the target gas was adjusted by the air valve and mass-flow controller, which was designed to export the gas to the chamber.

Figure 2: Schematic diagram of experimental setup.
Figure 2:

Schematic diagram of experimental setup.

4 Results and Discussion

The XRD patterns of Co3O4 and PANI/Co3O4 composite are shown in Figure 3. All diffraction peaks can be indexed as cubic structure (Fd-3m space group) with lattice constant a ≈ 0.8085 nm for Co3O4 (PDF#76-1802 [24]). When Co3O4 and PANI constitute a composite membrane, the positions of the diffraction peak at 17.957°, 19.027°, 23.628°, 26.472°, 30.584°, 31.317°, 32.932°, 36.902°, 38.606°, 40.852, 44.879°, 55.743°, 59.451°, and 65.342° should be attributed to the PANI/Co3O4.

Figure 3: X-ray diffraction patterns of Co3O4 and PANI/CO3O4 membrane.
Figure 3:

X-ray diffraction patterns of Co3O4 and PANI/CO3O4 membrane.

The composition of PANI/Co3O4 specimen was investigated by XPS in Figure 4a. C, N, O, and Co were measured in the membrane where they were anticipated to be. The Co 2s and 2p1 peaks are, respectively, 925.3 and 793.7 eV. C 1s and Auger peaks are located at 285 and 1221.4 eV, respectively. N 1s is at 398.4 eV. O 1s, 2s. Auger and loss peaks are situated at 531.8, 23.1, 979.7, 999, and 553.2 eV, respectively; S 2s and 2p peaks are 229.0 and 164 eV, respectively. All photopeaks of these elements agree well with the standard spectrum values. It should be noted that the hydrogen elements in the membrane have not been obtained because hydrogen has no inner electron, and its outer electron is used for bonding, and no any photoelectrons can be excited by X-ray [25]. The S elemental peaks in Figure 4a should come from the ammonium persulfate in the reaction process of the sensing membrane. The high-resolution XPS profile of C 1s region in Figure 4b shows four characteristic peaks at around 287, 284.7, 284, and 283 eV, corresponding to C=O, C–O, C–N, and C–C, respectively.

Figure 4:  (a) X-ray photoelectron spectroscopy image of PANI/Co3O4 membrane and the corresponding high-resolution XPS scan of the (b) C 1s, (c) N 1s, (d) O 2s, and (e) Co 2p.
Figure 4:

(a) X-ray photoelectron spectroscopy image of PANI/Co3O4 membrane and the corresponding high-resolution XPS scan of the (b) C 1s, (c) N 1s, (d) O 2s, and (e) Co 2p.

Figure 4c is a narrow-band XPS high-resolution scan of N 1s. It is shown that the binding energies of –N=, –NH–, and N+ are 398.7, 399.2, and 401.2 eV, respectively. The O 2s peaks in Figure 4d indicate that the binding energies corresponding to Co–O, –OH and C=O are 530, 531.3, and 532.5 eV, respectively. Figure 4e is a narrow-band XPS scan of Co 2p after peak splitting. It is shown that the binding energies of Co 2p1/2 and Co 2p3/2 are 779.8 and 775.7 eV, respectively. 780.8 and 779.7 eV are corresponding to Co2+ and Co3+ of Co 2p3/2, which are in good agreement with XPS results of Co3O4 [26], [27], [28]. The weak peak at 786 eV may be caused by the interaction between hydroxyl and Co adsorbed on the surface of Co3O4.

Raman spectra of PANI/Co3O4 composite film are given in Figure 5. The Raman peaks are, respectively, 191, 462, 527, 609, and 678 cm−1, corresponding to the Eg, E2g1, F2g2, and A1g vibration modes of Co3O4 [29], [30], [31], [32], [33]. The peaks at around 415, 583 (two out-of-plane CH shift motions), 1169 (C–H bending of quinoid ring), 1225 (C–N stretching mode of the polaronic units), 1345 (C–N•+ stretching modes of delocalized polaronic charge carriers), 1400 (vibrations of C–N+ fragments), 1497 (C=N stretching of the quinoid rings), 1540 (N–H bending deformation band of protonated amine), and 1592, 1621 cm−1 (C–C stretching of benzene ring) are in agreement with those of Raman modes in high-quality PANI [32], [33], [34], [35], [36], [37], [38].

Figure 5: Raman spectrum of PANI/CO3O4 membrane.
Figure 5:

Raman spectrum of PANI/CO3O4 membrane.

Different concentrations (i.e. 0, 10, 20, 30, 40, 50, 60 and 70 ppm) of CO gas were detected, and the spectral responses of dual F-P interferometric sensor to different concentrations of CO were recorded in air at room temperature by the OSA (see the inset of Fig. 6). There are obvious red shifts of the dip wavelengths of the resonance bands of the dual F-P interferometric sensor with the increase of CO concentration. When the CO gas contacts with the sensing layer of PANI/Co3O4 membrane, the effective refractive index (n3) of the tip will be increased [25], [39], [40]. At the same time, the film thickness will be expanded [41]. That is to say, the length (d2) of the second F-P cavity becomes longer. Considering that the physical quantities (n2, d1) of the first F-P cavity are invariable values, thus, for the same mth-order interference, the formulas (5) and (6) show that with the increase of CO gas concentration the output transmission spectra of the sensor will be red shift. The experimental results agree well with the theoretical analysis.

Figure 6: Wavelength shift upon the concentration of carbon monoxide. The inset shows the reflectance spectra of the sensor in various concentrations of carbon monoxide.
Figure 6:

Wavelength shift upon the concentration of carbon monoxide. The inset shows the reflectance spectra of the sensor in various concentrations of carbon monoxide.

As shown in Figure 6, the data of dip wavelength versus concentration are fitted by a linear regression model. The result shows that the correlation coefficient R2 of the calibration curve is about 0.98476, which indicates a very good linear response of the dual F-P interferometric sensor in the given concentration range (0–70 ppm) of CO. The sensitivity of the PANI/Co3O4 composite film to CO is 21.61 pm/ppm.

The reflectance spectral data were monitored in order to determine the dynamic behavior of the dual F-P interferometric gas sensor. As shown in Figure 7, the dynamic response rising time (tr) and falling time (tf) of the dual F-P interferometric CO sensor were determined to be, respectively, about 35 and 84 s in the range of 0–70 ppm CO.

Figure 7: Dynamic responses of the carbon monoxide sensor.
Figure 7:

Dynamic responses of the carbon monoxide sensor.

As shown in Figure 8, the relative wavelength shifts of the reflectance spectrum of the dual F-P interferometric sensor for common ingredients in air, such as carbon dioxide, nitrogen, hydrogen sulfide, oxygen, and argon, are examined at 70 ppm. When the sensor is immersed in carbon dioxide, nitrogen, hydrogen sulfide, oxygen, and argon, respectively, no obvious changes are observed. The sensor exhibits excellent sensitivity and selectivity toward CO in comparison with other gases. The relative wavelength shift for CO is at least about eight times more than that of other gas environments. This result indicates that CO can be better absorbed than other five gases by the PANI/Co3O4 composite membrane.

Figure 8: Selectivity for carbon dioxide, nitrogen, hydrogen sulfide, argon, oxygen, and carbon monoxide.
Figure 8:

Selectivity for carbon dioxide, nitrogen, hydrogen sulfide, argon, oxygen, and carbon monoxide.

5 Conclusions

In summary, a method of high-selectivity dual F-P interferometric CO sensor was demonstrated based on the PANI/Co3O4-coated fibre tip of EPCF. The results indicate that with the increase of CO concentration the dip wavelength of the transmission spectrum appears red shift. The sensitivity is 21.61 pm/ppm within the range of 0–70 ppm, and the linear response is very stable. The response time of dual F-P interferometric sensor is about 35 s with excellent selectivity for CO. The gas sensor has the advantages of easy fabrication, high selectivity, and low cost and is suitable for sensitive detection of toxic CO gas monitoring.

Award Identifier / Grant number: 51574054

Funding statement: This work was supported by the National Natural Science Foundation of China (Funder Id: 10.13039/501100001809, 51574054), University Innovation Team Building Program of Chongqing (CXTDX201601030), Livelihood and Natural Science Foundations of Chongqing Science and Technology Commission (cstc2017shmsA20017, cstc2018jcyjAX0294), Science and Technology Research Programs of Guangzhou City (201804010395), Scientific and Technological Project of General Administration of Quality Supervision, Inspection and Quarantine of China (2017QK109), and Postgraduate Research Innovation Project of CQUT (ycx2018104).

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Received: 2018-10-07
Accepted: 2018-10-28
Published Online: 2018-11-16
Published in Print: 2019-01-28

©2018 Walter de Gruyter GmbH, Berlin/Boston

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