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Abstract
As a carbon-free energy carrier and an attractive alternative energy source, hydrogen energy has great development potential for future considerations, and it may be the ultimate answer to the global energy crisis. Due to the high combustibility of hydrogen, hydrogen sensors will be a vital component of safe use of hydrogen. Among the various sensors, the optical hydrogen sensor can meet the requirements of intrinsic safety, online detection, surrounding immunity, and lack of spark. Hence, we demonstrate a miniature optics-mechanics synergistic fiber optic hydrogen sensor by using Pd nanofilm, it has a large response range (0.5%-3.5%), high sensitivity of -0.334 nm/1% concentration and a short response time (10s)/recovery time (25s). Experimental results reveal that the proposed optics-mechanics synergistic fiber optic hydrogen sensor is reusable, durable, and low temperature sensitive. In this optics-mechanics synergistic fiber optic hydrogen sensor, nano Pd film with a large surface-to-volume ratio allows for rapid hydrogen dissociation, and Pd lattice expansion caused by Pd-hydrogen reaction is effectively transduced into optical change. This proposed sensor integrated Pd nanofilm with optical fiber by using an optics-mechanics synergistic strategy, resulting in a compact and all-optical solution for the safe measurement of hydrogen concentration, which can be used in hazardous or space-limited environments.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
To meet the world environmental degradation and resource shortages, hydrogen, as a clean and maintainable energy carrier, can play an important part. However, since the high combustibility of hydrogen, hydrogen detecting device will play a crucial role in a hydrogen economy [1]. For safety, any leaks in hydrogen energy storage systems, cars, and installations, as well as the hydrogen distribution infrastructure, must be detected immediately. As a result, hydrogen sensor behavior goals call for a response time of tens of seconds and a measuring scope of 0.5% to 4% at room temperature [2]. To achieve the above detection objectives, hydrogen sensors with electrical readout have been extensively studied [3,4], but they still pose a risk of explosions caused by electric sparks. According to the safe monitoring requirement of hydrogen detection and the intrinsic safety of optical hydrogen sensors, optical hydrogen measuring has been noticed as a new technique, and an assortment of strategies have been utilized to improve the capabilities of optical hydrogen sensors. In optical detection, surface plasmon resonance (SPR) has received a lot of attention [5,6]. Furthermore, SPR sensors that use perfect absorption have been proposed [7–10]. These sensors, in any case, require huge optical setups to guarantee that the input and readout signals are appropriately coupled to optical detectors. Optical fiber hydrogen sensors based on hydrogen sensitive materials can achieve pretty much the same hydrogen detection, but do not need complex optical collection setup [11–19]. Withal the optical response originates from the interstitial sites where the hydrogen molecules are absorbed into the metal/metal oxide bulk, making such sensors highly hydrogen selective by nature. Besides, Fabry-Pérot (FP) interferometer has been widely studied as an important optical device, especially as an optical fiber sensing element [20–24]. These are appealing because the optical signal is non-sparking, therefore suitable for hydrogen concentration safety detection needs.
Among the many hydrogen sensitive materials, palladium (Pd) is the preferred functional material. Pd films can be used in both hydrogen detection and separation devices, which is owing to its capacity to dissolve hydrogen effectively in ambient circumstances, as well as its reversible phase transition at room temperature from metal-to-metal hydride (PdHx), which can result in considerable optical and mechanical property transformation. Normally, the hybrid Pd/PdHx film achieves absorption equilibrium and no longer expands after a period of exposure to a specific concentration of hydrogen. Due to the existence of the absorption equilibrium state, Pd is suitable for optics-mechanics hydrogen sensing and the extent of lattice expansion is highly dependent on hydrogen concentration [25,26]. Commercially, since nanometric films have large surface-to-volume ratio, they are being investigated to enhance hydrogen absorption-desorption capabilities [27]. However, to ensure reusability, high strength and ductility are generally required to resist many percentages of deformation during operation, thus, the mechanical behavior of a Pd thin film with a plane grain size of 30 nm has been studied under uniaxial tension [28], which demonstrated that the nanofilm has strong strength, high strain hardening capability, and moderate ductility.
By combining the synergistic effects and the inherent features of Pd film, the optical signal transducers can be linked to selective hydrogen absorption, thus, a metal-fiber optical hydrogen sensor platform is proposed. As we demonstrate, we can meet the stringent 10 s room-temperature response time target by using the Pd nanofilm with high volume-to-surface ratio in tandem with FP interferometer via metal–fiber interface engineering. Simultaneously, hysteresis is suppressed, sensor’s response and recovery times are significantly reduced.
2. Methods
2.1 Sensor fabrication
Figure 1 illustrates the schematic of the proposed optics-mechanics synergistic fiber optic hydrogen sensor. Figure 1(a) shows the FP interferometer-based fiber hydrogen sensor. The production procedure is shown in Fig. 2: First, as illustrated in Fig. 2(a), we splice a single mode fiber with a fused silica hollow core optical fiber (Polymicro Technologies [29]) by using a commercial fusion splicer (Fujikura, FSM-50s). The outer diameter of hollow core fiber is 125µm, which is equal to single mode fiber diameter. The inner diameter of the hollow core fiber is 40µm; Then, as shown in Fig. 2(b), we cleave the hollow core fiber to a desired length of 75µm utilizing fiber cleaver under a microscope; Finally, we use the UV curing transfer method to transfer the nano-thickness Pd film to the end face of the hollow core fiber. The UV curing transfer method is as follows: First, we fabricate two ultraviolet curing epoxy gels on a silica substrate and transfer the gels to the end face of the hollow fiber without entering the interior of the hollow under the microscope, as shown in Fig. 2(c). Second, the hollow fiber with the UV-curable epoxy is attached vertically to the substrate coated with Pd nanofilm, as shown in Fig. 2(d). Third, the epoxy is fully cured by continuous exposure to UV light for one hour, as shown in Fig. 2(e). Fourth, rapidly pulling up the hollow fiber vertically, our proposed sensor is successfully prepared. When compared to bulk Pd, the Young's modulus and yield strength of Pd nanofilm are lower. This means that constant stress caused by Pd lattice expansion will cause greater deformation [30]. The Pd film is used as a transducer and hydrogen-sensitive material, which is beneficial to realize high-sensitivity hydrogen detection.
Fig. 1. Schematic diagrams of optics-mechanics synergistic fiber optic hydrogen sensor. (a) Structural representation of highly hydrogen sensitive fiber-based FP interferometer, (b) Action principle of Pd hydrogen sensitive film, (c) Reflection spectrum of proposed interferometer.
Fig. 2. Fabrication process schematic of proposed optics-mechanics synergistic fiber optic hydrogen sensor.
2.2 Principle of operation
At room temperature, Pd can dissolve and chemisorb hydrogen molecule (H2) to hydrogen atoms (H) on its surface. These atoms quickly saturate the surface and diffuse into interstitial lattice spaces in the subsurface area (Fig. 1(b)). In nanoscale systems like nanofilms, the presence of hydrogen in the subsurface layer generates lattice strain. Furthermore, lattice strain leads to deformation of PdHx-Pd composite nanofilms, forming the mechanism of hydrogen detection based on Pd and explaining Pd's innately high hydrogen selectivity. To illustrate the effect of the Pd-hydrogen reaction on Pd film, Fig. 1(b) shows the schematic diagram of the composition of Pd film after Pd-hydrogen reaction. While the Pd film is exposed to hydrogen, H atoms are located mostly at surface of Pd film during Pd-hydrogen reactions. Thus, as shown in Fig. 1(b), after Pd-hydrogen reaction, Pd film can be considered as a two-layer structure, corresponding to PdHx/Pd. PdHx/Pd film can be considered as a thin and elastic diaphragm, and we can calculate the strain of the film (Sc) by using the following equation,
Figure 1(c) shows the reflection spectrum of proposed hydrogen sensor. The interferometric spectrum of the FP interferometer can be expressed with [31],
2.3 Morphology characterization of hydrogen sensitive film
We prepare two Pd films with different thicknesses for sensor preparation by controlling the sputtering time. As shown in Fig. 3, we further characterize the thickness and morphology characterization of the Pd films by using atomic force microscopy (AFM), the thickness estimated from Fig. 3 is 27.42 nm and 31.83 nm, respectively.
3. Results and discussion
3.1 Static hydrogen concentration measurement
We use Pd film-based optics-mechanics synergistic fiber optic hydrogen sensor to implement hydrogen concentration detection. The proposed sensors are placed inside a custom-built gas chamber to investigate the H2 response. The flow rates of pure H2 gas and pure N2 gas are adjusted by using two mass flow controllers to control the H2 gas concentration in the chamber. During hydrogen concentration detection, the total flow rate of the gas mixture is set to 2L/min, and we continuously record the sensor reflection spectrum with a time interval of 5 seconds. Figure 4 illustrates the reflection spectrum changes of optics-mechanics synergistic fiber optic hydrogen sensor caused by the Pd-hydrogen reaction. Figure 4(a) shows the reflection spectra of the Pd film optics-mechanics synergistic fiber optic cavity under different hydrogen concentrations. We use a broadband source (1525-1610 nm) and an optical spectrum analyzer to analyze the reflected light from the sensor. According to the equation L = λ1λ2/nair(λ2- λ1) (λ1and λ2 are the adjacent valley wavelengths of the reflection spectrum, and nair is the air refractive index), the length of the FP cavity L can be calculated. The calculated cavity length is ∼74.3µm, which is close to the measured cavity length. The spectrum blue shifts as the hydrogen concentration increases from 0.5% to 3.5%. Figure 4(b) shows the wavelength shift of the sensors with different film thicknesses. The wavelength shift has a polynomial fitting relationship with the hydrogen concentration, and while the thickness of the Pd film is 27.42 nm and 31.83 nm, the average sensitivity is -0.334 nm/1% and -0.248 nm/1%, respectively. Besides, the Pearson correlation coefficient of polynomial fitting curves is 0.998 and 0.999 corresponding to the Pd film thickness 27.42 nm and 31.83 nm. From Fig. 4, the thinner Pd layer has higher sensitivity, and the experimental results are in good agreement with the Eq. (2).
Fig. 4. The reflection spectrum changes of optics-mechanics synergistic fiber optic hydrogen sensor caused by the Pd-hydrogen reaction. (a) The reflection spectrum corresponding to different hydrogen concentrations of the 27.42 nm Pd film optics-mechanics synergistic fiber optic hydrogen sensor, (b) The wavelength shift corresponding to different hydrogen concentrations.
3.2 Dynamic hydrogen concentration measurement
As shown in Fig. 5, feed-in gas is cyclic switched from 100% N2 and diluted H2 with certain concentration, the hydrogen concentration gradually increases from 0% to 3.5%, and then decreases from 3.5% to 0%. For each concentration test, the sensor is subjected to diluted H2 for 300 seconds and pure N2 for 300 seconds. Figure 5 (a1) and (b1) illustrate the wavelength shift corresponding to different hydrogen concentrations. As the hydrogen concentration gradually increases, the wavelength shift increases gradually. Specifically, the total wavelength shift of the sensor with 27.42 nm Pd film is -1.191 nm, however, that of the sensor with 31.83 nm Pd film is -0.887 nm. The average sensitivity of the sensor with 27.4 nm Pd film is -0.334 nm/1%, which is higher than that of the sensor with 31.83 nm Pd film, which is -0.248 nm/1%. Subsequently, the hydrogen concentration gradually decreases to 0%, and we compare the wavelength shift at the rising/falling process. As shown in Fig. 5 (a2) and (b2), the wavelength shift is in good agreement at the same hydrogen concentration in both processes. By comparing the wavelength shift, the maximum wavelength shift difference is 0.061 nm (@2.5%) for the sensor with 27.42 nm Pd film, and it is 0.055 nm (@1.5%) for the sensor with 31.8 nm Pd film. Furthermore, as illustrated in Fig. 5 (a2) and (b2), there is barely any optical response hysteresis.
Fig. 5. Time-dependent response corresponding to Pd nanofilm thickness. (a1) The wavelength shift corresponding to different hydrogen concentrations, Pd thickness 27.42 nm, (a2) The wavelength shift corresponding to the specific hydrogen concentrations, Pd thickness 27.42 nm, (b1) The wavelength shift corresponding to different hydrogen concentrations, Pd thickness 31.83 nm, (b2) The wavelength shift corresponding to the specific hydrogen concentrations, Pd thickness 31.83 nm.
The sensor’s response/recovery time is an important parameter indicator. Figure 6 illustrates the temporal response of proposed optics-mechanics synergistic fiber optic hydrogen sensors. Experimental results show that the hydrogen concentration and the Pd thickness impact the response and recovery times. At low hydrogen concentration, the response time of the sensor with 27.42 nm Pd film is ∼50s at 0.5%, as shown in Fig. 6(a). The corresponding recovery time is about 45s, as shown in Fig. 6(b). Furthermore, as shown in Fig. 6(a) and 6(b), the response time and recovery time decrease as the increasing hydrogen concentration. Therefore, the shortest response time of the sensor with 27.42 nm Pd film is 10 seconds while the hydrogen concentration is 3.5%, the corresponding recovery time at this concentration is 25 seconds, and the sensor with 31.83 nm Pd film shows shortest response time (55s) at 3.5%, corresponding to shortest recovery time (55s) at the same concentration. As shown in Fig. 6, at each concentration, the sensor with 27.42 nm Pd film exhibits substantially shorter response times than the sensor with 31.83 nm Pd film. The sensor with 27.42 nm Pd film exhibits superior time response performance, which comes from the short dislocation and diffusion time for the thinner Pd film.
Fig. 6. Temporal response of proposed optics-mechanics synergistic fiber optic hydrogen sensors. (a) Response time, (b) Recovery time.
To investigate the effect of air cavity length on sensor sensitivity and response/recovery time, we fabricate a sensor with a cavity length of 53.2 µm and a Pd film thickness of 31.83 nm. We conduct dynamic hydrogen concentration measurement experiments, and the results are shown in Fig. 7 Fig. 7(d) show the reflection spectrum of the optics-mechanics synergistic fiber optic hydrogen senor with a cavity length of 53.2 µm. In the dynamic hydrogen concentration measurement experiments, feed-in gas is cyclic switched from 100% N2 and diluted H2 with certain concentration, the hydrogen concentration gradually increases from 0% to 3.5%, and then decreases from 3.5% to 0%. For each concentration test, the sensor is subjected to diluted H2 for 300 seconds and pure N2 for 300 seconds. Figure 7 (a) and (b) illustrate the wavelength shift corresponding to different hydrogen concentrations. As the hydrogen concentration gradually increases, the wavelength shift increases gradually. Subsequently, the hydrogen concentration gradually decreases to 0%, and we compare the wavelength shift corresponding to different cavity lengths. As shown in Fig. 7 (c), at the same hydrogen concentration in both processes, the wavelength shifts for different cavity lengths are only slightly different. By comparing the wavelength shift, the maximum wavelength shift difference is 0.053 nm (@3.5%) for the sensors in the concentration increasing phase, and it is 0.089 nm (@0.5%) for the sensor in the concentration decreasing phase. Furthermore, as illustrated in Fig. 7 (c), there are barely any optical response hysteresis in both sensors.
Fig. 7. Time-dependent response corresponding to cavity length. (a) The wavelength shift corresponding to different hydrogen concentrations, cavity length 74.3μm, (b) The wavelength shift corresponding to different hydrogen concentrations, cavity length 53.2μm, (c) The wavelength shift corresponding to the specific hydrogen concentrations, (d) The reflection spectrum of optics-mechanics synergistic fiber optic hydrogen sensor with 53.2μm cavity length.
Figure 8 illustrates the temporal responses of proposed optics-mechanics synergistic fiber optic hydrogen sensors corresponding to cavity lengths. Figure 8(a) shows the response time and Fig. 8(b) shows the recovery time. For different cavity lengths, experimental results show that the hydrogen concentration correlates with the response and recovery times. As shown in Fig. 8(a) and 8(b), the response time and recovery time decrease as the increasing hydrogen concentration. Furthermore, the response and recovery times corresponding to different cavity lengths are essentially the same for the same hydrogen concentration. The maximum difference in response time is 10s (@0.5%), the percentage of deviation is 6.7%. On the other hand, the maximum difference in recovery time is 20s (@0.5%), the percentage of deviation is 10.5%.
Fig. 8. Temporal response of proposed optics-mechanics synergistic fiber optic hydrogen sensors with different cavity lengths. (a) Response time, (b) Recovery time.
3.3 Energy spectrum analysis
To investigate the morphology of the Pd film before and after hydrogen absorption, we use scanning electron microscope (SEM) to characterize the end surface morphology to clarify the effect of the hydrogen reaction process on the Pd film, as shown in Fig. 9. Figure 9 (a) illustrates cross profile of proposed hydrogen sensor, the Pd film stays flat due to the stable tension of the robust structure after preparation. After hydrogen absorption, the Pd film is deformed, which caused by hydrogen induced lattice expansion, as shown in Fig. 9(b). Energy spectrum analysis are performed in the area marked by the pink rectangle. The Si (SiO2) mapping shows the structure of the microscale cavity. The comparison of Pd mapping before and after hydrogen absorption indicates the operating principle of the sensor.
Fig. 9. Scanning electron microscope (SEM) photographs and energy spectrum mapping of proposed optics-mechanics synergistic fiber optic hydrogen sensor. (a) Before hydrogen absorption, (b) After hydrogen absorption.
3.4 Stability and durability
To verify the stability of the proposed optics-mechanics synergistic fiber optic hydrogen sensor, the sensor with 31.83 nm Pd film is selected to perform stability experiment. As shown in Fig. 10 (a), the injected gas order is N2-1% H2-N2-3% H2-N2, we find that the total wavelength fluctuation is about 0.02 nm in 2 hours. The measurement error of this sensor is ∼0.1%, it demonstrates that this proposed optics-mechanics synergistic fiber optic hydrogen sensor has good stability. Further, to verify the long-term stability of the sensor, we perform stability experiments using a sensor with a Pd film thickness of 27.42 nm after the sensor has been subjected to several hydrogen detection experiments for six months. As shown in Fig. 10 (b), the injected gas order is N2-1% H2-N2-3% H2-N2, we find that the total wavelength fluctuation is about 0.076 nm in 2 hours. The measurement error of this sensor is ∼0.4%.
Fig. 10. Stability of proposed optics- mechanics synergistic fiber optic hydrogen sensor. (a) Pd thickness 31.83 nm, (b) Pd thickness 27.42 nm.
Meanwhile, to verify the long-term durability of proposed sensor with 27.42 nm Pd film, after six months of sensor preparation, we perform dynamic hydrogen concentration detection experiments. As shown in Fig. 11, feed-in gas is cyclic switched from 100% N2 and diluted H2 with certain concentration, the hydrogen concentration gradually increases from 0% to 3.5%, and then decreases from 3.5% to 0%. For each concentration test, the sensor is subjected to diluted H2 for 300 seconds and pure N2 for 300 seconds. Figure 11 (a) illustrate the wavelength shift corresponding to different hydrogen concentrations. As the hydrogen concentration gradually increases, the wavelength shift increases gradually. As shown in Fig. 11(b), the total wavelength shift of the sensor with 27.42 nm Pd film is -0.854 nm. Subsequently, the hydrogen concentration gradually decreases to 0%, and we compare the wavelength shift data at six-month intervals. First, the wavelength shift is in good agreement at the same hydrogen concentration in increasing/falling processes, so, there is barely any optical response hysteresis. Second, by comparing the wavelength shift, due to the aging caused by repeated absorption and desorption of hydrogen, the sensor sensitivity decreases to more than seventy percent of the original.
Fig. 11. Durability and repeatability experiments of proposed optics- mechanics synergistic fiber optic hydrogen sensor with 27.42 nm Pd film for hydrogen concentration detection. (a) Time-dependent response to hydrogen concentration after six-month interval, (b) Comparison on the relationship between wavelength shift and hydrogen concentration after six-month interval.
Due to the intrinsic hysteresis of Pd, Pd film-based hydrogen sensors’ dynamic range and accuracy are reduced dramatically [32–34]. Hysteresis suppression can be achieved by using a thinner Pd film [35]. The suppression is generated by the clamping action of the support, which is amplified in a thin Pd layer. The Pd lattice is constrained by this clamping, making the formation of β-phase energy-intensive. Pd films prepared by magnetron sputtering can be scaled down to the nanoscale level and deliver hysteresis suppression characteristic. The comparison of recovery performance of the proposed optics-mechanics synergistic fiber optic hydrogen sensors, as shown in Fig. 12, it confirms that a thinner Pd film is beneficial to suppresses hysteresis.
Fig. 12. Recovery performance of proposed optics-mechanics synergistic fiber optic hydrogen sensors.
3.5 Temperature response
To investigate the temperature characteristics of our proposed optics-mechanics synergistic fiber optic hydrogen sensor, we use the sensor with Pd film thickness of 27.4 nm to conduct temperature tests in the range of 30-70°C. The reflectance spectrum versus ambient temperature is shown in Fig. 13. As shown in Fig. 13(a), the reflectance spectrum is red shifted as the temperature increases. As shown in Fig. 13(b), the average temperature sensitivity is 0.004 nm/°C. Therefore, the sensor has a low temperature sensitivity, which can be counteracted by adding temperature control or temperature compensation in operation.
Fig. 13. The reflectance spectrum versus ambient temperature of proposed optics-mechanics synergistic fiber optic hydrogen sensor. (a) Temperature-dependent reflectance spectrum, (b) The wavelength shift corresponding to different ambient temperature.
4. Conclusion
To summarize, we propose a Pd film-based miniature optics-mechanics synergistic fiber optic hydrogen sensor with a large operation range (0.5%-3.5%), high sensitivity of -0.334 nm/1%, and short response time (10s)/recovery time (25s). Further, experimental results reveal that the proposed optics-mechanics synergistic fiber optic hydrogen sensor is reusable, durable, and low temperature sensitive. The Pd film with nanoscale thickness and a large surface-to-volume ratio allows for rapid hydrogen dissociation, and the hydrogen induced lattice expansion can be effectively transduced into optics-mechanical behavior. This proposed sensor integrated Pd nanofilm with optical fiber, combining with an optics-mechanics synergistic strategy, delivering a miniature, all-optical solution for safe hydrogen concentration detection. Furthermore, by replacing the sensitive film layer, the proposed sensor structure is a versatile platform for a variety of space limited gas and physical sensing scenarios.
Acknowledgments
The authors would like to thank Erica J. Li for her help in language polishing and editing.
Funding
National Natural Science Foundation of China (61727816, 62171076, 61705189, 61520106013); Fundamental Research Funds for the Central Universities (DUT21RC(3)021); State Key Laboratory of Advanced Optical Communication Systems and Networks (2022GZKF001); Liaoning Revitalization Talents Program (XLYC1802120); Key Research and Development Program of Liaoning Province (2019JH2/10300019).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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