In this paper, a response time of the surface plasmon resonance fiber optic hydrogen sensor has successfully improved with keeping sensor sensitivity high by means of hydrogen curing (immersing) process of annealed Au / Ta2O5 / Pd multi-layers film. The hydrogen curing effect on the response time and sensitivity has been experimentally revealed by changing the annealing temperatures of 400, 600, 800°C and through observing the optical loss change in the H2 curing process. When the 25-nm Au / 60-nm Ta2O5 / 10-nm Pd multi-layers film annealed at 600°C is cured with 4% H2 / N2 mixture, it is found that a lot of nano-sized cracks were produced on the Pd surface. After H2 curing process, the response time is improved to be 8 s, which is two times faster than previous reported one in the case of the 25-nm Au / 60-nm Ta2O5 / 3-nm Pd multi-layers film with keeping the sensor sensitivity of 0.27 dB for 4% hydrogen adding. Discussions most likely responsible for this effect are given by introducing the α-β transition Pd structure in the H2 curing process.
© 2014 Optical Society of America
In recent years, hydrogen has attracted much attention as a clean, sustainable, and abundant energy source and would be widely used in the automotive industry and in rocket engines. On the other hand, hydrogen has flammable and highly explosive properties when mixed with oxygen. A leakage of hydrogen in air would lead to an explosive atmosphere of easy ignition if hydrogen concentration exceeds more than 4% (the lower explosive limit: LEL). Therefore, the development of rapid and accurate hydrogen sensors is essential to monitor the hydrogen leakages. In potentially explosive atmospheres optical fiber sensors are preferable to be used because of no electrical contacts in the sensing point and transmission line. Moreover, they are compact, light in weight, immune to electromagnetic interference and resistant to corrosion. Therefore, a number of optical fiber hydrogen sensors have been attractively proposed [1–7]. In the most of optical hydrogen sensors a thin palladium layer is usually employed as the sensitive material because palladium allows the selective absorbability for hydrogen. At present, there are mainly several kinds of optical fiber hydrogen sensors, such as tapered fibers [1,2], unclad fibers [3–5], Fiber Bragg grating [6,7]. These tapered or unclad fibers has been required to eliminate thick cladding layers to access transmitted light in core through evanescent wave, resulting in a loss of mechanical strength, while FBG sensors need to be compensated because of their temperature dependency.
Comparing to the above techniques, our previous work  has reported a surface plasmon resonance (SPR) hydrogen sensor by means of hetero-core structured fiber optics with Au / Ta2O5 / Pd multi-layers for the first time, which can detect Pd hydrogenation at the near infrared wavelength of 850 nm. Although surface plasmon resonance (SPR) has long been developed due to its high sensitivity to small changes in refractive index [9,10], no SPR hydrogen sensors have been reported so far which operate at the near-infrared wavelength of 850 nm because the light incident angles for the guided-modes of optical fibers at this wavelength do not meet the resonant angular condition to excite the SPR phenomenon for a Pd or Pd hydride monolayer. Although some other works using SPR were reported using multi-layers on unclad fibers, their second layers of SiO2  and Si  have to be thick enough to operate at operating at visible wavelength  and at 350nm , respectively, because the refractive indices of SiO2 and Si were insufficient to excite SPR at the near-infrared wavelength of 850 nm, hence they have no choice but to be very insensitive. In contrast to those above, our precious experiment  successfully showed sufficient sensitivities and response time (15 s) with showing a trade-off between sensitivity and response time, depending on the layer thicknesses. One of the advantages of our experiment is brought from the success of realizing a cylindrically uniform coating which was not introduced in other works reported so far.
Since the first demonstration of the hetero-core technique  by the present authors, the hetero-core fiber sensor [12–14] has become well known SPR devices because of the practical advantages in terms of the simplicity of structure and the interesting sensing principles. The structure consists of a peace of single-mode optical fiber between two multi-mode fibers. Due to the core diameter mismatch the transmitted light is guided to the cladding of the single-mode fiber, which works as a sensing region.
Recently, in order to improve the response time, the hydrogen sensors have been reported based on the Pd/Au alloy  or nano-sized cracks  on the Pd surface. For instance, D. Monzon-Hernandez et al.  made an effort to produce a non-SPR sensor with a faster response by annealing Pd / Au alloy films using the hetero-core technique [11–13] and the finding by Z. Zhao et al. , where annealing and curing effect to response time was interestingly reported. The Pd / Au alloy films annealed at temperatures of 200°C for 1h in an Ar atmosphere. Their experimental results showed a response time of 5 s for 4% hydrogen with the 4-nm Pd / Au alloy films. However, the sensitivity of the sensor for hydrogen is very low because of the usage of thin Pd films and of non-SPR operation. On the other hand, Y. Tack Lee  proposed an electrical hydrogen sensor with nano-sized cracks on the Pd surface onto the Si substrates, which was generated by the reduction process of palladim oxide (PdO) films with hydrogen. The electrical resistance on Si substrates is changed for 4% hydrogen concentration with a response time of about 15 s.
In this paper, we have reported morphological findings due to annealing of Au / Ta2O5 / Pd multi-layers films and the following hydrogen curing process and have obtained the resultant improvement on the response time compared with the previous reported SPR fiber optic hydrogen sensor . The hydrogen curing effect on the response time and sensitivity has been experimentally demonstrated by changing the annealing temperatures of 400, 600, 800°C and measuring the optical loss change in the H2 curing process. After H2 curing process of the 25-nm Au / 60-nm Ta2O5 / 10-nm Pd multi-layers film annealed at 600°C, a lot of nano-sized cracks were appeared on the Pd surface. As a result, it is observed that the response time and sensitivity indicate to be 8 s and 0.27 dB for 4% hydrogen. The response time of our proposed sensor becomes two times faster with keeping the sensor sensitivity high, than previous reported one based on 25-nm Au, 60-nm Ta2O5 and 3-nm Pd multi-layers film. An explanation most likely responsible for this effect is made by introducing the α-β transition in the H2 curing process. The sensor performance has successfully shown a response time as rapid as the most comparable work  reported and more importantly a higher sensitivity suitable to practical use.
2. Experimental set up
A schematic drawing of SPR hydrogen sensor based on a hetero-core fiber is shown in Fig. 1. The hetero-core sensor consists of a short segment of single-mode (SI) fiber with a length of 15 mm and spliced multi-mode (GI) fibers at both ends. Owing to the core diameter difference between single and multi-mode fibers, most of the transmitted lights in the transmission line would largely leak into the cladding region of the SI sensor element. Light in the cladding generates evanescent waves at the surface of the cladding layer of the SI fiber when it is bounced off at the boundary between the cladding region and the surrounding under the condition of total internal reflection. At the other end of the SI sensor element, some lights are re-coupled to the core of the multi-mode fiber. Therefore, SPR waves could be induced in a similar way of the Kretschmann configuration sensor if the surface is coated with a thin metal film. In our previous work , it was reported that novel SPR spectra for hydrogen absorption at the near infrared wavelength of 850 nm were observed when Au / Ta2O5 / Pd films were uniformly coated onto a SI sensor element.
Several samples of hetero-core sensors were fabricated by using a multi-mode GI fiber as the transmission line whose core diameter is 50 µm and the inserted SI fiber with 3 µm of core diameter. The cladding diameters of both fibers were 125 µm. Au, Ta2O5 and Pd were cylindrically coated on the fabricated hetero-core region with thicknesses of 25 nm, 60 nm and 10 nm, respectively, by means of an RF sputtering machine (CFS-4ES-231, Shibaura Mechatronics Co. Ltd.), which was specially designed to introduce a rotating mechanism to realize axial symmetrically uniform deposition on the cladding surface. After the coating process, the sensors were annealed at temperatures of 400, 600, and 800°C in an air atmosphere for 1h. In order to investigate H2 response properties of the annealed films, an experimental setup was used to measure the optical loss change of the sensors, as illustrated in Fig. 1. It consists of a set of flow meters and a 15-ml volume acrylic gas chamber which has an inlet and an outlet to allow a continuous flow of gas. N2 or 4% H2 in N2 was alternatively introduced by controlling the two flow meters. The flow rate of nitrogen was kept at 1000 ml / min and the flow rate of hydrogen was adjusted to given the required concentration. The gas filling time for the chamber is calculated to be approximately 1 s from the chamber volume and the flow rate. The fiber is illuminated by an LED light source whose wavelength is 850 nm and the optical loss is detected using an optical power meter.
3. Results and discussions
Curing  is reported as an initial process to stabilize hydrogen sensor performance by exposing an annealed layer to hydrogen atmosphere to absorb it soon after annealing is completed. Morphological findings are described in this section in more details than other previously reported works [4,5,15–17]. One of the purposes of this work is to confirm whether the cylindrically produced Pd /multi-layers is strong enough to stably operate during the experiment and other usage after high temperature annealing. Comparing with a single layer case , multi-layer was fraught with film deterioration due to the internal stresses in the different material coatings. It has been found that annealing and curing shows no defect on the cylindrical three-layer-structure which was successfully coated using a specially designed rotating RF sputtering machine  in our work. During a series of our repeated experiment, the hydrogen detecting performance is reproducibly made with a reliable lifetime.
Figure 2 shows the light intensity change during the H2 curing detected at the optical power meter for the case of 25-nm Au / 60-nm Ta2O5 / 10-nm Pd multi-layers film annealed at a temperature of 600°C when the sensor is exposed to pure N2 and 4% H2 / N2 mixture. When the annealed multi-layers film was exposed to 4% H2 diluted with N2, the transmitted light intensity largely reduced and then showed a stable state in 12 min soon after hydrogen was initially introduced into the chamber. In addition, it is observed that the light intensity of the annealed film after H2 curing is different from that of the annealed film before H2 curing. It is generally known that when Pd is exposed to hydrogen, hydrogen molecules are first adsorbed onto the Pd surface and then dissociate into atomic hydrogen that diffuses into the interstitial sites in the bulk Pd, leading to formation palladium hydride (PdH). In the α-β phase transition of Pd structure, the drastic lattice expansion is occurred [18,19].
Since the H2 curing process would change the corresponding structure of the Pd films, the surface morphologies of the multi-layers film were observed using the SEM images. Figure 3(a), 3(b) and 3(c) show the SEM images for the case of only sputtering before annealing, after 600°C annealing, and H2-cured multi-layers film, respectively. The surface of the multi-layers film before annealing seems to be uniformly coated, as shown in Fig. 3(a). On the other hand, Fig. 3(b) shows that the numerous grain aggregates were appeared on the annealed multi-layers film. This phenomenon is similarly found in the previous study reported by Z. Zhao et al.  where they found that a large number of blisters were appeared on the Au / Pd alloy films annealed at a temperature of 350 or 400°C for 1h in an argon atmosphere. Therefore, it is indicated that grain aggregates would be likely stemmed from the annealing temperature effect. As can be seen in Fig. 3(c), the number of nano-sized cracks was produced after the H2 curing process. The reason for this would come from the lattice expansion of Pd structure. In the H2 curing process shown in Fig. 2, while the annealed film absorbs the atomic hydrogen, the resonant wavelength shifts to longer wavelength region due to the increase of the dielectric function of the phase shifted Pd layer, hence, the light intensity is reduced. The α-β phase transition of Pd structure with producing PdH leads to the lattice expansion, consequently causing the formation of the nano-sized cracks, then the light intensity is increased again. This is because that the SPR spectra shifted back toward shorter wavelengths due to the change of dielectric function of the deformed Pd layer with PdH and nano-cracks by the H2 curing process, although it is difficult to measure the change in dielectric function of cylindrically coated film with and without hydrogen absorption.
In order to investigate the H2 curing effect on the H2 sensor characteristics, the real-time response was obtained as Fig. 4 in the optical loss change of multi-layers film, which was cured with H2 as mentioned in the above. It is found from Fig. 4 that the H2 cured film at the stable state can absorb and desorb the H2 gas rapidly with a high sensitivity for 4% H2 in N2. The rise time and recovery time are defined as the period of the time for a loss change from 10% above ground to 90% of the maximum value and as the time required to fall from 90% to 10%, respectively. The rise time was 8 s at 4% H2 in N2 with a recovery time of 20 s. The experimental result showed that both of the response and recovery times for the proposed sensor are two times faster than those for the 3-nm Pd sensor  which was subjected to no H2 curing after annealing, with keeping the sensor sensitivity as high as 0.27 dB for 4% hydrogen adding, because a lot of nano-sized cracks on the Pd surface helps the rapid absorption of hydrogen. In addition, the sensor with forming the nano-cracks on Pd surface showed excellent reproducibility for the pure N2 and 4% H2 in N2. As a result, it is confirmed that the nano-cracks make it possible to improve the response time. The optical losses as a function of hydrogen concentrations of the H2-cured sensor (square plots) and 3-nm Pd sensor (circle plots) are shown in Fig. 5. It can be seen from Fig. 5 that the optical losses of both sensors increase with the hydrogen concentration and saturate near 4% hydrogen concentration. The maximum optical loss change of the H2-cured film is slightly larger than that of the 3-nm Pd film. This difference could be attributed to the expansion of the Pd surface caused by a lot of nano-sized cracks.
Figure 6 shows the optical loss changes in the real-time response of multi-layers film, which was cured with H2 after 400°C annealing process. From Fig. 6, the response time of this film was 17 s when the sensor was exposed to 4% H2 in N2 and recovery time is 35 s. The optical loss change is indicated to be about 0.45 dB for the H2-cured film after 400°C annealing process, which shows that the sensitivity at 4% H2 of the H2-cured film after 400°C annealing process is higher than that of the H2-cured film after 600°C annealing process. The light intensity change of the multi-layers film annealed at a temperature of 400°C for exposure to 4% H2 in N2 is shown during the H2 curing in Fig. 7. When the annealed multi-layers film was exposed to 4% H2 in N2 in the H2 curing process, the transmitted light intensity slightly reduced and then reached back to a stable state in 3 min after initial exposure of hydrogen with the light intensity change of 0.7 dB, which was observed differently from the case of 600°C annealing temperature as shown in Fig. 2. This is probably because the deformed Pd layer differently shows the dielectric functions, depending on the crack formation condition. Comparing with 600°C annealing, the 800°C annealing films showed a very small response to 4% H2 / N2 mixture during the H2 curing process, more likely because a very small amount of grain aggregates would be produced on the Pd surface.
In this paper, fast response and high sensitive hetero-core structured fiber optic SPR hydrogen sensors have been successfully developed using the hydrogen curing process of annealed Au / Ta2O5 / Pd multi-layers film. We have investigated the H2 response properties of the annealed films, which are annealed at temperatures of 400, 600 and 800°C, with observing the optical loss change in the H2 curing process. In the case of H2-cured multi-layers film for the 25-nm Au / 60-nm Ta2O5 / 10-nm Pd, which is annealed at 600°C, many nano-sized cracks are generated on the Pd surface. In this process, it is estimated that the lattice expansion would be occurred due to the α-β phase transition of Pd structure, consequently would change of dielectric function of the deformed Pd layer with PdH and nano-cracks by the H2 curing process. As a result, it is observed that the rise time and sensitivity are 8 s and 0.27 dB, respectively, for 4% hydrogen in the case of the H2-cured film after 600°C annealing process. The response time of this film shows to be two times faster without any degradation sensor sensitivity, than previous reported one using 25-nm Au, 60-nm Ta2O5 and 3-nm Pd multi-layers film. The nano-cracks are attributed to the improvement of the response time. Reasons in terms of this effect are explained by introducing the α-β transition Pd structure in the H2 curing process. Although further works will be required on cross-sensitivity with gases other than nitrogen and on a practical lifetime and stability in a full-scale experiment, the findings obtained in this work will be useful to explore such next steps of development.
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