Highly Sensitive Hydrogen Sensor Based on Palladium-Coated Tapered Optical Fiber at Room Temperature †

This paper describes the application of a palladium (Pd)-coated tapered optical fiber in order to develop hydrogen (H2) sensor. A transducing channel was fabricated with multimode optical fiber (MMF) with cladding and core diameters of 125 μm and 62.5 μm, respectively, to enhance the evanescent field of light propagation through the fiber. The multimode optical fiber was tapered from a cladding diameter of 125 μm to a waist diameter of 20 μm, waist-length of 10 mm, and down taper and up of 5 mm and coated with Pd using the drop-casting technique. In order to establish the palladium’s properties, various characterization techniques were applied, such as FESEM, EDX, and XRD. The developed palladium sensor functioned reproducibly at a gas concentration of 0.125% to 1.00% H2 at room temperature in the synthetic air. In this case, the response and recovery times were 50 and 200 s, respectively. Furthermore, this study demonstrated that the production of a dependable, effective, and reproducible H2 sensor by applying a basic, costeffective method is possible.


INTRODUCTION
Hydrogen (H 2 ) has high energy content, making it an ideal clean fuel with several application potentials in different industries.
Optical offers interesting properties, such as light weight, resistance to EI, instability, and rigidity in harsh environments.
Palladium (Pd) is currently receiving interest in many applications, such as H 2 , the hydrogenation process, and the detection of H 2 .
Semiconducting metal oxides can provide good sensitivity, selectivity and stability sensors. To evaluate the optical fiber sensor performance (sensitivity, response and recovery time. repeatability, and selectivity) based on absorbance measurement.
To discuss the sensing mechanism of gas moleculessensing layer interaction of tapered optical fiber sensor.

METHODOLOGY
➢ Multimode Optical fiber (MMF) was fabricated with cladding and core diameters of 125 µm and 62.5 µm respectively, as a transducing platform.
➢ The MMF was tapered from cladding diameter of 125 µm to waist diameter of 20 µm, waist-length of 10 mm, and down taper and up of 5 mm.
➢ The machine works based on a heating and pulling process, using a graphite filament as a heater to achieve the desired geometry of the tapered profile.

Palladium Functionalization of the Tapered Optical Fiber
➢ The Pd sensor was fabricated following a simple one-step process by mixed : • 0.1 mL of hydrochloride acid • 0.9 mL of palladium chloride (PdCl 2 ) • 10 mL of deionized water The solution was placed in an ultrasonic bath and left for 15 minutes to homogenize.
➢ The coating of the tapered optical fiber was done using the drop-casting technique.
• A drop of the mixture (approx. 10 µL) was dropped into the base of the tapered optical fiber.
• Heating the sample at 80 o C for 15 minutes in the oven to ensure complete evaporation of the aqueous medium. • The EDX pattern of Pd revealed that the important elements in Pd films are Pd and O, as evidenced by their respective peaks.

METHODOLOGY
• XRD patterns of the Pd-coated sensor recorded in range 2θ, from 30°to 90°. ➢ The absorption spectra of the sensor coated with Pd to synthetic air at room temperature with different concentration 0.125% to 1.00% H 2 .
• The Pd sensor demonstrated notable changes in absorbance, especially in the wavelength range of 550-850 nm as shown in Figure (a).
➢ The response time and recovery time of the Pd costed sensor was 50 sec and 200 sec respectively. Changes in absorption at 0.125% H 2 are about 24% and 52% higher at 1.00% H 2 as shown in Figure (b).
• The Pd coated sensor showed stronger absorbance and recovery of H 2 at higher absorption changes.
➢ Sensor repeatability was confirmed by exposure of the sensor to 3 cycles of 1.00% H 2 , as shown in Figure (c). Overall, the Pd coated sensor showed a high level of good repeatability of H 2 .
➢ A test for selectivity was done for Pd coated sensor toward NH 3 and CH 4 gas at 1.00% concentration as shown in Figure (b).
• The Pd coated sensor showed a remarkably high H 2 absorbance response with a weak response for other gases.
• CH 4 gas is a stable gas that requires very high energy to dissociate H from C.
• The sensor is less sensitive toward NH 3 , probably because of Pd since it is more suitable for dissociating the H 2 gas.

The Sensing Mechanism for Tapered Pd NPs Coated Optical Fibers
➢ The Pd-coated fiber sensor's optical response occurs because of the reaction of palladium to hydrogen gas.
➢ Pd absorbs H 2 gas molecules, resulting in it changing into PdHx (where a small percentage expands the Pd particle size).
➢ The Pd layer increases in thickness and size while absorbing hydrogen, thereby also changing the layer's optical properties.
➢ The real and imaginary parts alter the permittivity of the Pd layer to result in a corresponding change of boundary conditions on the sensor surface.

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The problem or the proposal

Background information
Main conclusions

Concise analysis
This study demonstrated that optical fiber sensors could be developed from Pd NPs by employing a dropcasting technique.
The performance of the developed sensor was evaluated in terms of its response at room temperature using different concentrations of H 2 gas.
These evaluations indicated that the Pd-coated sensor exhibited a 52% change in the absorbance response when exposed to 1.00% H 2 in synthetic air.
It is possible to develop an efficient, reliable and reproducible H 2 sensor by using a simple and cost-effective approach under real atmospheric conditions.