Tapered Optical Fiber for Hydrogen Sensing Application Based on Molybdenum Trioxide (MoO₃)  ^†

Abstract

In this work, molybdenum trioxide (MoO₃) was synthesized and deposited on tapered optical fiber using the drop-casting technique for hydrogen (H₂) detection at room temperature. A transducing platform in a transmission mode was constructed using multimode optical fiber (MMF) with a 125 µm cladding and a 62.5 µm core diameter. To enhance the evanescent light field surrounding the fiber, the fibers were tapered from 125 µm in diameter to 20 µm in diameter with a 10 mm waist. The microstructures and chemical compositions of the fabricated sensor were analyzed by field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), differential X-ray (XRD), and atomic force microscopy (AFM). In addition, the gas detection properties of the fabricated sensor were studied by exposing it to various concentrations of hydrogen gas from 0.125% to 2.00%. As a result, the sensitivity, response, and recovery time were 11.96 vol%, 220 s, and 200 s, respectively. Overall, the fabricated sensor exhibits good sensitivity as well as repeatability and stability for hydrogen gas detection.

1. Introduction

Hydrogen gas has a wide explosion range (4–75%), small ignition energy (0.02 mJ), and a high flame propagation speed [1]. It can leak out of storage as H₂ due to the fact that its molecule size is too small [2]. These reasons underscore the importance of H₂ detection to help reduce the risk of human exposure to the gas [3]. The fact that the optical signal is immune to electromagnetic interference, non-inductive with less attenuation than the electrical signal, is ideal for gas detection applications [4]. Therefore, the optical sensors rely on optical fibers, which have unique characteristics, such as lightness, small size, electromagnetic interference resistance, instability, and stiffness in harsh environments [5]. Due to their peculiar properties, optical fibers are perfect candidates for detection in harsh environments. In parallel, depositing the nanostructured metal oxide detection layer can provide new optical detection properties for gas [6]. The basic sensing concept of optical fiber sensors is based on the observation of any changes in the basic parameters of the modified optical signal resulting from the interaction of light through the optical fiber with external physical, chemical, or biological measurement. The optical parameters that can be modulated are the intensity, phase, and wavelength. Among the above-mentioned parameters, intensity-based measurement is the simplest and the most low-cost technique to detect different measurands. Intensity based sensors usually require additional light to quantify the response. As a result, they use large core multimode optical fibers. This type has many advantages including its simplicity, the possibility of it being multiplexed, and its distributed sensing performance. Optimizing the analytical surface interaction contributed by the higher surface-to-volume ratio of nanostructures improves the performance of optical gas detection. So, the main motivation to improve H₂ sensing is due to the health and safety concerns that have arisen with the vast popular use of H₂ and its associated technologies.

In recent years, improved nanotechnology promises an astonishing evolution in gas sensor design and capabilities. Various hydrogen oxide sensors, including SnO₂ [7,8], ZnO [9,10], and Wo₃ [11,12], as well as their mixtures, were examined. However, the response speed and sensitivity are still not satisfactory. Recent work aims to explore another metal oxide that can provide better cross selectivity and response. Molybdenum trioxide (MoO₃) shows promise in energy storage, catalysis, and gas detection applications [13]. Recently, the experiment showed that the interaction of optical fiber-based MoO₃ with hydrogen is highly sensitive, has a fast response to ant time, and an efficient eye-readable gas detection technology has been developed as a bio complement of the conventional H₂ sensors [14]. Yang et al. [15] has prepared a hydrogen sensor based on [001]-oriented α-MoO₃ nanoribbons that has a fast response time of 14.1 s toward 1000 ppm of H₂ at room temperature; Alsaifa et al. [16] has fabricated a hydrogen sensor based on two-dimensional MoO₃ nanoflakes with rapid response time of 7 s and recovery time of 24 s. However, many electrochemical hydrogen sensors based on MoO₃ carry the risk of electric sparks and have to work under elevated temperatures [17].

In this work, a tapered optical fiber coated with MoO₃ is used to detect hydrogen gas. It also explains the possible interaction mechanisms between the MoO₃ sensing layer and hydrogen gas.

2. Experimental

2.1. Tapering Process of Optical Fiber

The H₂ gas sensor was fabricated utilizing a multimode fiber (MMF) with cladding and core diameters of 125 µm and 62.5 µm, respectively, as a conversion platform. A Vytran glass (Vytran GPX-3400) processing machine was used for the taper. To obtain the desired geometry of the tapered fibers, the machine heating and pulling the fiber uses graphite filament as a heater. The MMF was tapered from the cladding with a 125 µm to a waist diameter of 20 µm, a waist length of 10 mm, and a top and bottom taper of 5 mm. Figure 1 depicts the fabricated tapered optical fiber with the tapered region. According to [18], this tapered shape responds significantly between the gas sensing layer and the evanescent field.

2.2. Fabrication of MoO₃ on Tapered Optical Fiber

Molybdenum trioxide (MoO₃) powder was synthesized by the simple solid decomposition method [19]. A 2.50 g of ammonium heptamolybdate tetrahydrate was taken and ground in suspension for one hour and annealed in an alumina crucible at 500 °C for three hours in the air. The resulting product was washed with distilled water and dried in an air oven. Prepared MoO₃ (0.25 g) was dispersed in 10 mL of deionized water to obtain a milk-white suspension after ultrasonic treatment for 30 min.

The tapered optical fiber was coated with MoO₃ using the drop-casting technique. Using a micropipette, we drooped about 15 μL of the mixture to the tapered base and heated the sample in the oven at 80 °C for 30 min to complete evaporation of the aqueous medium.

The experimental equipment’s of the optical gas sensing system includes a light source with a wavelength of 360 to 2500 nm (tungsten halogen, HL-2000, Ocean Optics, Dunedin, FL, USA) and spectrophotometer (USB 4000, Ocean Optics, Dunedin, FL, USA). The detection range of the optical absorbance spectrum is 200–1100 nm. Use standard (S.M.A. fiber) cables to connect the light source to the fiber optic. The inside diameter of the S.M.A. cable is 600 μm, and the FC/PC type is used as the MMF terminal. The adapter data file recognizes the standard input loss format for MMF terminated with FC/PC-SMA cables determined to be 1.4 dB. Figure 2 shows the experimental installation of sensor H₂. Before the computer-controlled mass flow controller performs the gas purification at a gas flow rate of 200 sccm, the MoO₃ coated-based sensor is placed in a closed gas unit. The H₂ gas is purified by pure synthetic air to achieve a concentration range from 0.125% to 2.00%. The dynamic response and cumulative absorbance were determined while purging synthetic air.

2.3. Structural Characterization of MoO₃

Various characterization techniques have been applied to characterize MoO₃ thin film. The morphology was observed by FESEM (Nova Nanosem 230), while the initial composition was determined by EDX analysis. The identification of the material, crystallization, and the phase transition of the MoO₃, was determined by analysis of XRD (PW 3040-Philips) and AFM (Dimension Edge with ScanAsyst).

The FESEM images demonstrate the successful deposition of MoO₃ on the surfaces of the tapered optical fibers, as shown in Figure 3a. The standard features of the MoO₃ nanoparticles are shown in Figure 3b,c. The particles agglomerated in nature after attempting to form thin plate or flake-like formations. The gas analyte will interact more effectively with this plate-like structure [20].

As illustrated in Figure 4a, EDX was used to identify the elements in the synthesized MoO₃. The MoO₃ film shows the presence of Mo, O, and Si. The silicon peak (Si) was used by the silica fibers used as a substrate. The X-ray diffraction (XRD) patterns of MoO₃ were recorded within the 2θ range of 5° to 70°, as shown in Figure 4b. The sample shows highly crystalline hexagonal and orthogonal structures, respectively, since the intensity is strong and sharp enough with narrow full-width maximum half (FWHM) of diffraction peaks such as 11.25° (100), 34.80° (101), 17.40° (110), 20.10° (200), 27.32° (210), 30.02° (300), 36.15° (310), 44.20° (320), 46.60° (410), 51.15° (002), 56.55° (211), and 58.24° (042). All of the XRD peaks can be distinguished for the sample, and single-phase can be assigned to crystal structures. The prominent diffraction peak corresponding to planes (100) and (210) was placed at the highest density of MoO₃.

Atomic force microscopy (AFM) was conducted to characterize the average surface roughness and thicknesses of MoO₃. Figure 5a depict 3D AFM image of the MoO₃. A 10 × 10 µm section of the boundary area was scanned for the AFM analysis. The average surface roughness values were 44.98 nm. Such low roughness levels indicate that light scattering has no significant effect on sensing performance [21].

As part of this study, the thicknesses of the MoO₃ coatings were measured. Measurements were taken by covering portions of fibers with aluminum tape and then determining the differences between the thicknesses of coated and uncoated fibers. The average thickness of the MoO₃ coatings was 181 nm, as shown in Figure 5b.

3. Results and Discussion

The relationship between the sensing layer absorption and operating temperature is shown in Figure 6a. The sample was tested with 1% H₂ on different operating temperatures from room temperature to 200 °C. According to the curve, it indicates that the absorption capacity decreases with increasing temperature, possibly due to the faster reaction rate of the absorbed hydrogen atoms at the active sites at higher temperatures. In this work, the optimum operating temperature of Pd NPs based tapered optical fiber sensors to H₂ is 25 °C.

At room temperature, the absorption spectra of the developed sensor was coated with MoO₃ to synthetic air and 2.00% H₂. The MoO₃ coated sensor exhibits noticeable changes in absorption, as illustrated in Figure 6b (especially in the wavelength range of 550 to 850 nm). The cumulative absorbance of the MoO₃ coated sensor was analyzed to determine its performance. The dynamic response of a MoO₃ coating based sensor toward H₂ concentrations (synthetic) from 0.125% to 2.00% at room temperature is shown in Figure 6c. The response and recovery times of the developed sensor were 220 s and 200 s, respectively. At 0.125% H₂, the absorbance changed by about 8%, while at 2.00% H₂ it changed by about 25%. The MoO₃ coated sensor had higher H₂ absorbance and recovery, as well as advanced compromise differences. Three cycles of 2.00% H₂ were used to test the repeatability of the MoO₃ coated based sensor. Overall, the MoO₃ coated sensor demonstrated a strong and stable absorbance response and excellent repeatability towards H₂.

The absorbance change in the MoO₃ layer can be explained with the following mechanism. During H₂ adsorption onto the MoO₃ sensing layer, the H₂ molecules dissociated onto Pd, resulting in the generation of H⁺ ions and electron. Ions of H⁺ spilled over the MoO₃ nanostructures layer which then react with chemisorbed oxygen (O⁻ and O²⁻), thereby producing H₂O molecules. The generated electrons reduced the transparent Mo⁶⁺ in the center of MoO₃ crystal lattice to Mo⁵⁺ (blue color). The change in sensing layer properties has altered the absorption of light, hence the change in absorbance magnitude. Upon exposure to synthetic air at elevated temperature, the adsorbed oxygen as well as the desorbed H⁺ ions in/out of the sensing layer which returns the absorbance spectrum to its original baseline. The stoichiometry of the MoO₃ is restored from Mo⁵⁺ to Mo⁶⁺ [6].

Figure 7a shows the absorption versus H₂ concentration for MoO₃ coated based sensors. The MoO₃ coated based sensors had a sensitivity of 11.96/vol% and a linearity slope of 98%. When measuring gas sensing properties, selectivity is an important factor to consider. Selectivity is the most critical parameter of the sensor properties. The fabricated sensor with different gases such as ammonia (NH₃) and methane (CH₄) at a concentration of 1.00% was investigated. The MoO₃ coated based sensor had a highly NH₃ absorption response but a substantially lower response for the other gases. Furthermore, the adsorption of MoO₃ based materials was highly selective for polar molecules such as NH₃, whereas sensitivity was low for non-polar molecules such as H₂ and CH₄ [22].

Stability is an important parameter which specifies the durability for all sensors [23]. The fabricated MoO₃ coated based sensors have been tested towards 2.00% H₂ gas concentration in synthetic air at room temperature for 14 days, and the response changed nominally by 2% (as shown in Figure 7c). The finding indicates that among the fabricated gas sensors, MoO₃ coating based sensors present excellent long-term stability.

4. Conclusions

In the present study, molybdenum oxide was deposited on tapered optical fiber using the drop-casting technique for hydrogen sensing decoction. The H₂ gas sensor is fabricated of the as-deposited structure, which is exposed to various concentrations (0.125–2.00%) at room temperature. According to the findings, the MoO₃ coated based sensor enhanced its absorption response by 25% when exposed to 2.00% H₂ in synthetic air. The response and recovery times of the developed sensor were 220 s and 200 s, respectively. The selectivity investigation indicates that the MoO₃ based optical sensor responds strongly towards ammonia, methane, and hydrogen chemicals. The findings suggest an affordable and accessible methodology which may be utilized to enhance an effective, accurate, and repeatable H₂ sensor in real-world atmospheric conditions.