The optical properties of palladium (Pd) / yttrium (Y) double-layered thin films have been investigated to realize an optical hydrogen sensor with high sensitivity at room temperature. A remarkable increase in transmittance of the Pd/Y film to about 17 times was observed when the sensor detected 100% hydrogen gas. Moreover, the response time within 8 s was much shorter than that reported previously and the recovery time of 30 s was shorter than that of the Pd film. The sensor exhibited a considerable increase in transmittance to about 8 times even with a detection of 0.1% hydrogen. Compared with previously reported Pd-based optical sensor, the Pd/Y films had a great advantage of high sensitivity and excellent response and recovery properties.
Keywords

Hydrogen Sensor; Optical Sensing; Pd/Y Thin Films; Sensitivity; Response and Recovery Properties
Introduction

The hydrogen is expected to play an important role as a secondary energy resource because it is clean and inexhaustible. The use of the hydrogen gas involves a great risk because it has a wide range of explosive concentrations and low ignition energy, as well as its colorless and odorless nature. For that reason, a sensing device for monitoring hydrogen is required to use hydrogen safely. Conventional electrical sensors, such as semiconducting ceramics (SnO2 or ZnO) [1, 2] and simple platinum wire, must be heated up to 150-400˚C to enhance the sensitivity. Therefore, this type of sensor has a risk of ignition on account of its high operating temperature and the possibility that an electrical spark could be generated on the sensing circuit.

To avoid these problems, we have developed a new type of optically readable hydrogen sensors. The principle of the optically readable sensors is based on the change in the transmittance and the reflectance of thin films such as Pd [3], Pd/WO3 [4] and Pd/MoO3 [5] on a glass substrate, due to hydride formation. These sensors have a great advantage of high safety and prevention of an explosion.

A major change in the optical properties of rare-earth metals with forming their hydrides has been reported by Huiberts et al. [6]. Yttrium (Y) was selected out of the rare-earth metals as a layer detecting hydrogen. Moreover, Palladium (Pd) with outstanding hydrogen sorption was used as a layer of catalyst on the Y layer. There is the possibility that an optical sensor with high sensitivity can be realized using these materials. However, the characteristics of their materials have not been investigated from the point of view of hydrogen sensing characteristics such as response speed, repeatability and hydrogen concentration dependence of sensitivity. In this paper, the optical properties of the Pd/Y double-layered thin films were investigated.
Experimental

The sensing devices consisting of a Y film covered with a Pd film were prepared on a glass substrate (Corning #7059) using a R. F. magnetron sputtering apparatus (Anelva, SPF-332H), which was equipped with 99.99% Pd and 99.9% Y plates. The sputtering was carried out under 1 Pa Ar atmosphere. After the deposition of the Y film on the substrate, the Pd film was continuously formed on the Y film. The Pd film plays an important role not only in protecting the Y film against its oxidation but also in catalyzing hydrogen dissociation. To compare the Pd/Y sensor with previously [3] reported Pd-based sensor, the Pd monolayered film was also prepared through the similar procedure. A 50 nm Y film covered with a 20 nm Pd film and the Pd monolayered film were prepared so as to have the total thickness of 70 nm. Moreover, the thickness of the Y film covered with the 20 nm Pd film was varied from 20 nm to 150 nm.