Materials Chemistry and Physics 88 (2004) 106–109

Palladium capped samarium thin films as potential hydrogen sensors

Pushpendra Kumar, L.K. Malhotra∗

Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

Received 15 July 2003; received in revised form 24 December 2003; accepted 21 June 2004

Abstract

We show here that a palladium capped samarium film at room temperature can be used as an efficient sensor material for hydrogen detection.As low as 194 ppm of hydrogen gas can be detected without any cross sensitivity problem. Depending upon the partial pressure of hydrogen,response times varying from 6 to 30 s have been obtained. An interesting observation is very fast hydrogen desorption (2 s) at room temperature,thus making the sensing element ready for reuse quickly. No external source is needed with this sensor element.© 2004 Elsevier B.V. All rights reserved.

Keywords:Palladium; Samarium; Hydrogen sensor; Electrical switching

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. Introduction

Associated with the increasing use of hydrogen in manyndustries such as chemical, food, metallurgical, electronicnd others and its anticipated greater utilization as a sourcef clean abundant energy, is the issue of its safe storage,

ransportation and handling. A hydrogen leak should bevoided because hydrogen when mixed with air in the ratio of.65–93.9 vol.% is explosive[1]. Highly sensitive hydrogenensors are therefore needed to detect H2 gas leakage. Crit-cal sensor performance requirements include high selectiv-ty, fast response, insensitivity to other gases, durability andotential for low cost production. Different types of hydro-en sensors have been reported[1–9] and many are commer-ially available. Detection of hydrogen using bulk rare earthetal hydrides was proposed as far back as 1973, however,

he proposed approach was specific to steel industry but notdaptable for other applications[10]. The recent discovery

11] of spectacular changes in optical and electrical proper-ies of thin film rare earth metals upon hydrogenation hasriggered large-scale investigations to exploit these materials

samarium films covered with palladium overlayer for thelectrical and optical switching behavior, we observedexposure of these films to hydrogen, leads to a chanelectrical resistance that scales with hydrogen concentrin the gas phase. The reaction is reversible suggesting thcapped samarium films could be used as hydrogen seelements. We show here that it is indeed so and thesedemonstrate highly desirable features, such as fast resand recovery speeds, sensitivity to hydrogen in ppm rangabsence of cross sensitivity to possible contaminant ga

2. Experimental

Samarium films of thickness 55 nm were depositedvacuum evaporation on a 15 mm× 15 mm× 2 mm ultra-sonically cleaned glass substrate, two edges each 2 m×2 mm of which were precoated with aluminum and bonwith copper wire for taking electrical contacts. The chber was repeatedly purged with high purity Argon gasabout 30 min and then evacuated. The pressure in the c

or various technological applications. While investigating

∗ Corresponding author. Tel.: +91-11-26591325/6861977;ax: +91-11-26581114/6862037.

ber was 1× 10−4 Pa prior to deposition. A continuous Pdoverlayer of thickness 12 nm was then deposited over theSm film (except in the region above contact B where thethickness was restricted to 3.5 nm) without breaking the vac-u

d.

E-mail address:lalit@physics.iitd.ernet.in (L.K. Malhotra).

254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.matchemphys.2004.06.038

um. The configuration is shown inFig. 1. The palladium

P. Kumar, L.K. Malhotra / Materials Chemistry and Physics 88 (2004) 106–109 107

Fig. 1. Sample Configuration.

overlayer provides protection to the underlying film againstoxidation, increases the hydrogen sticking coefficient andacts as a catalytic layer, which assists in dissociative absorp-tion and associative desorption of hydrogen molecules at rea-sonably high rates. The diffusion coefficient of hydrogen inpalladium is 3× 10−9 cm2 s−1 [12] at room temperature.Hydrogen in monoatomic forms only reacts with Sm at roomtemperature. Since the Pd overlayer remains metallic even af-ter hydrogenation, 12 nm electrically continuous Pd layer, ifused over the entire surface area of samarium film, masks theobservation of a metal to semiconductor transition becausethe Pd film provides a low resistance path to the applied cur-rent. (It may be noted that the resistance of a 12 nm Pd filmon 15 mm× 15 mm× 2 mm glass substrate is 615� whereasthat of a 3.5 nm Pd film is 700 k�). The Pd capped Sm film(Pd–Sm) was used as a sensing element for detection of hy-drogen. The sensing element was housed in a sample holdercontaining air at normal pressure and at room temperature.Controlled quantity of hydrogen or other gases could be intro-duced into the specimen holder using mass flow controllers.The specimen holder also had a provision for monitoring re-sistance of the sensor element. The resistance was measuredby two probe method using electrometer (Keithley, Model-6517A). By opening the upper lid of the sample holder, thesensing element could be fully exposed to atmosphere.

3

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T eas-i ast useo step( fort ei tran-s tiono ctort

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Fig. 2. Response time vs. hydrogen concentration of the sensing element inair.

a function of hydrogen concentration [ts is defined as thetime required for the initial resistance of Pd–Sm film (60�)to reach 90% of the saturation value of Pd:H0.6–SmH3− �

film (15 k�)] (0.6 hydrogen atoms are absorbed per Pd atom[14]). The magnified view for hydrogen concentrations from194 to 10,000 ppm is shown in the inset of the same figure. Itis clear that the switching is very fast, the switching time vary-ing from 30 s for 194 ppm of hydrogen to 6 s for 10,000 ppmof hydrogen. The response time of the sensing element (ex-cept at a very low hydrogen concentration 97 ppm) decreasesexponentially with increase in hydrogen concentration. Oncethe hydrogen supply to the sensor element is cut off and thelid of specimen holder lifted, the hydrogen incorporated inthe sensor element desorbs to the atmosphere within 2 s. Thesensing element is then again ready for detection of hydro-gen and the process can be repeated over a large number ofcycles. This is shown inFigs. 3 and 4for 10,000 and 194 ppmof H2, respectively. This fast desorption is possible becausethe desorption process in all rare earth metal hydrides is en-dothermic[15] and hydrogen has a large diffusion coeffi-cient (0.61 cm2 s−1) in air [16]. Since the sensing elementis in atmosphere, it appears that it is able to absorb requi-site heat from the atmosphere leading to quick desorption of

F pm hy-d

. Results and discussions

The reaction of samarium with hydrogen proceeds asows:

m+ 32H2 → SmH2 + 1

2H2 ⇔ SmH3

he second step is a reversible transition, which canly be induced by changing the H2 gas pressure, wherehe first step is essentially unidirectional. This is becaf the relative small heat of formation for the second−39.6 kJ mol−1 H) compared to the heat of formationhe first step (−202.6 kJ mol−1 H)[13]. The dihydride stats metallic and the trihydride state semiconducting. Theition from the dihydride to trihydride state on absorpf hydrogen is responsible for the metal to semicondu

ransition.Fig. 2 shows the variation of response timets required

o switch from the metallic to the semiconducting stat

ig. 3. Response of the sensing element in the presence of 10,000 progen in air.

108 P. Kumar, L.K. Malhotra / Materials Chemistry and Physics 88 (2004) 106–109

Fig. 4. Response of the sensing element in the presence of 194 ppm of hy-drogen in air.

hydrogen, a fact substantiated by a slower observed desorp-tion in vacuum as discussed later.

One of the prime concerns in sensors is that the sensor el-ement should not respond to gases other than the one, whichit is supposed to detect and monitor. Cross sensitivity to con-taminant gases was checked by introducing 10,000 ppm ofpotential contaminants, like hydrogen sulfide, carbon diox-ide, mixture of argon and methane, argon and ethyl alcoholinto the specimen holder assembly and the resistance mon-itored as a function of time. No change in resistance wasdetected (Fig. 5) suggesting that this sensor element is freefrom cross-sensitivity problems. We have also checked oursensing element (Pd–Sm) inside a vacuum chamber. Whenhydrogen gas was introduced inside the chamber (at 4×10−1 Pa), the response time was found to be almost the sameas that in air (Fig. 6). However, the recovery time was foundto be much larger even though we used a rotary pump forremoving hydrogen absorbed in Pd:H0.6/SmH3− � film.

It may be pointed out that in general, a sensor elementafter detection, needs some external source to restore it to it’soriginal state e.g. temperature[17], power (8) etc. Neitherany temperature control nor any power etc are needed in thepresent case. Further, the problem of water formation doesnot occur unless the sensing element continues to be exposedto hydrogen for a prolonged period (>6 h). However, once

F withPa

Fig. 6. Response of a sensing element in the presence of 10,000 ppm ofhydrogen in vacuum.

water formation takes place, the response time does increase.Though we have tested our element for 100 cycles, moreinvestigations on its stability are needed, especially in veryhumid conditions.

In conclusion, we have shown that a palladium cappedsamarium film exhibits a very high absorption and desorp-tion efficiency for hydrogen at room temperature without anyneed for on external source. It can detect low concentrationsof hydrogen, which makes it a potential base material forhydrogen sensing applications.

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