Hydrogen Sulfide Gas Sensor Based on Copper/Graphene Oxide Composite Film-Coated Tapered Single-Mode Fibre Interferometer

1 Introduction

Hydrogen sulfide (H₂S) is a colourless, acidic and highly toxic gas. H₂S gas not only seriously threatens human health, but also causes serious corrosion to metal equipment. The explosion limit of H₂S is 4.3 %∼46 % [1]. It burns with a blue flame and produces sulphurdioxide gas, which is harmful to human eyes and lungs [2], [3]. In a low concentration H₂S environment, local irritation of the eyes and respiratory tract is obvious. While in the high concentration environment, it manifests as symptoms of the central nervous system and asphyxia [4]. In order to avoid H₂S poisoning, effective protective measures must be adopted, so the monitoring of trace H₂S is very important. At present, the common methods for monitoring H₂S gas are as follows: electrochemical analysis [5], gas chromatography [6], heater-type sensor [7]. However, some of them are expensive, and others are liable to cause secondary hazards because they are electrified. Therefore, a novel gas-detection method based on sensitive materials and optical fiber sensing is proposed in recent years [8], [9]. Among the sensitive materials, graphene oxide (GO) has excellent adsorption properties because of its large specific surface area and abundant oxygen-containing functional groups [10], [11], [12], [13], [14]. In optical fibre sensors, Mach–Zehnder interferometric (MZI) sensors have many applications in liquid refractive index, curvature, strain, temperature and gas sensing. Interestingly, by adjusting the parameters of the fusing machine, the fibre taper can be obtained by pushing or stretching the coupling point of the fibres, and then obtain the inter-mode interference. The single-mode fibres (SMFs) were fused at both ends of photonic crystal fibres (PCFs) and the formed fibre tapers between PCF and SMF, constructed MZI structure [15]. An all-fibre Mach–Zehnder high temperature and high sensitivity temperature sensor were produced based on two SMFs series connection with thick cones [16]. The SMF with tapered tapers was fused at both ends of the PCF, and H₂S sensors were fabricated based on the graphene sensitive film on the PCF surface, and the sensitivity of the sensors was improved by depositing copper (Cu) nanoparticles on the graphene film [17]. Considering that Cu can enhance the adsorption of H₂S [17], a novel optical fibre bitaper H₂S sensor based on the Cu deposited GO was constructed in this work. Two enlarged bitapers of optical fibres were fabricated by three-segment SMF. The copper/graphene oxide (Cu/GO) composite sensitive film was coated on the SMF surface to make the interferometer sensitive to H₂S. The concentration of H₂S was measured by detecting the regular movement of interference trough. The sensitivity, selectivity and response-recovery characteristics of the sensor are also studied, and the results are discussed.

Figure 1:

Schematic diagram of the experimental setup.

2 Theoretical Method

The experimental gas sensing setup is shown in Figure 1. The fibre optic sensor was fixed inside the gas chamber with the inlet and outlet of the gas. The rotary pump was used to evacuate the gas chamber, the nitrogen gas was used for purging the chamber to get the reference signal and NaOH was used to treat the exhaust gas of the chamber. The proposed structure of MZI gas sensor is shown in the inset of Figure 1, which consists of three SMFs and two enlarged fibre bitapers. Two segments of SMF are directly spliced to form enlarged bitaper, and the SMF in the middle of the bitapers is the sensing region. After the light in the core passes through the first fibre taper, one part enters the core and the other part enters the cladding. The core mode and different order cladding modes have different effective refractive indexes. Through the transmission of the light in the interference arm, the difference of the optical paths between the modes is emerged. When light propagates to the second fibre taper, the light of the fundamental mode and that of the cladding mode are coupled and the interference comes into being.

The intensity of output light of the MZI gas sensor can be expressed as [18],

where I_cor and I_cla are the intensities of the core and cladding modes, respectively. Δφ is the phase difference between core and cladding modes.

where λ_m is the monitoring wavelength, L is the length of the SMF sensing section, and Δneff is the difference between the effective refractive indices of the core (neffcor) and the cladding (neffcla). When the difference between the cladding mode and the core mode is equal to (2m + 1)π, the light in the core and cladding will be destructive interference, that is to say, the interference dip will be formed. The mth-order wavelength of the interference dip can be expressed as [19],

The length (L) of the sensing region is 50 mm. When the sensing film coated on the cladding surface absorbs the target gas, the refractive index of the film will change and then the effective refractive index of the cladding will also vary, while the refractive index of the core remains unchanged. Therefore, with the change of the effective refractive index of the cladding, the mth-order wavelength of the interference dip will shift [20]. The amount of variation of the mth-order wavelength of the interference dip can be expressed as,

From (4), when the effective refractive index of the cladding increases, the (Δneff+Δn) decreases, i.e. Δn is negative, and the interference dip will be blue-shifted, and conversely, that is red-shifted. The movement of interference dip can be observed by the spectrometer, and the relationship between the concentration of H₂S gas and the movement of interference dip can be established.

Figure 2:

Diagram of spliced region.

Figure 3:

Scanning electron miscroscopy images of (a) the cross section and (b) the side surface of single-mode fibres after coating.

3 Experiment

4 Results and Discussion

4.2 Raman Spectrum of Cu/GO

Raman spectroscopy is one of the effective ways to characterise carbon (C) nanomaterials. The Raman D and G peaks of GO are at 1338 and 1583 cm⁻¹, respectively (Fig. 4). The D peak represents the defects and amorphous structure at the edge of GO, and the G peak represents the ordered sp² bond. The calculated intensity ratio I_D/I_G = 1.01, which indicates that the sensing film is multilayer GO [22].

Figure 4:

Raman spectra of graphene oxide (GO).

Figure 5:

(a) X-ray photoelectronspectroscopy (XPS) image of copper/graphene oxide (Cu/GO) membrane, (b) high-resolution XPS scan of the C 1s, (c) high-resolution XPS scan of the O1s.

4.4 Gas Sensitivity

The H₂S gases of 5, 10, 20, 30, 40, 50 and 60 ppm are prepared respectively. The prepared MZI gas sensor is used to measure different concentrations of H₂S. The transmission spectra without target gas are show in Figure 6a.

Figure 6:

(a) Transmission spectra of Mach–Zehnder interferometer coated with Cu/GO film. (b) Wavelength shift upon the concentration of hydrogen sulfide (H₂S) and spectral response of the sensor in various concentrations of H₂S. (c) The values of selectivity for nitrogen, carbon dioxide, argon, oxygen and H₂S. (d) Dynamic responses of H₂S gas sensor.

Figure 7:

Schematic diagram of sensing mechanism.

The dip wavelength at 1547 nm was selected as the monitoring site, and the deviation of the dip was observed in the range of 0–60 ppm H₂S gas concentration. As shown in Figure 6b, with the increase of H₂S concentration, the transmission spectra show obvious red shift. The film in the sensing region adsorbs H₂S gas molecules, reduces the refractive index of the cladding and increases the difference between the effective refractive indices of the core and the cladding, and makes the central wavelength red shift, which is consistent with the analysis of (4). In the range of 0–60 ppm H₂S gas concentration, the offsets around 1547 nm at the different gas concentrations are linearly fitted. The linearity is 0.99474, and the sensitivity of the sensor is 4.42 pm/ppm. The detection limit (LOD) of the sensor can be calculated according to the method in literature [23], [24], that is LOD = 3σ/K, in which σ (0.00411 nm) is the standard deviation for the wavelength of sensing system in the absence of H₂S and K (0.00442 nm/ppm) represents the slope of the fitting curve, and the LOD of this sensor (2.79 ppm) is obtained. Figure 6c is the gas selectivity of the sensor. It has high selectivity for H₂S by immersing N₂, CO₂, Ar, O₂ and H₂S with the same concentration. Figure 6d is the recovery-response time curve of the sensor. Its response and recovery time are about 31 and about 48 s, respectively.

The schematic diagram of H₂S sensing mechanism is given in Figure 7. As seen in Figure 7, Cu nanoparticles are entrapped into the GO matrix. The first interaction between sensitive film and the analyte (here, H₂S gas) occurs on the GO matrix. The oxygen-containing groups, such as carboxyl and hydroxyl groups, and the defective sites of GO, can be the adsorption sites for H₂S molecules [25], which will increase the refractive index of Cu/GO and then the wavelength will be red shift in the spectra. Moreover, Cu nanoparticles can enhance the adsorption properties of sensitive membranes to H₂S molecules. The reason is that Cu nanoparticles readily donate electron to GO and form Cu⁺ or Cu²⁺, and the Cu ions bind H₂S gas easily and produce Cu₂S or CuS [see (5)], this reaction also increases the refractive index and enhances the wavelength red shift [7].

(5)Cu(Cu2+)++H2S+O2−→←−O2−Cu2S(CuS)+H2O

5 Conclusion

A H₂S Mach–Zehnder gas sensor with two waist-enlarged tapers based on Cu/GO composite membrane was proposed. The experimental results indicate that the interference dips show a red shift with the increase of H₂S concentration in the range of 0–60 ppm. The sensitivity of the sensor is 4.42 pm/ppm, and the LOD of this sensor is 2.79 ppm. The response time and recovery time are about 31 and 48 s, respectively. There is a good linear relationship between H₂S concentration and wavelength shift, and the high selectivity for H₂S is achieved. The sensitive mechanism is proposed and analysed in detail. The sensor has the advantages of low cost, small size, simple structure and easy preparation. It has a potential application for H₂S low concentration measurement in production environments.

6 Highlights

1. Cu/GO composite film-coated tapered SMF interferometer was proposed.

2. The interference spectra appear red shift with the increasing H₂S concentration.

3. A high linearity of 0.99474 and a good selectivity are obtained.

4. Calculated the LOD of this sensor is as low as 2.79 ppm.

These highlights are new and were not reported in previous papers.