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Platinum Metals Rev., 1999, 43, (4), 155

Optical Response of Thin Supported Palladium Films to Hydrogen

Non-Destructive Testing for Hydrogen Detection

  • By C. Christofides
  • K. Kalli
  • A. Othonos
  • Department of Natural Sciences (Physics), University of Cyprus, Nicosia
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Many variants of hydrogen gas sensors, based on the interaction between palladium metal and hydrogen gas, have been demonstrated in recent years, and the growing use of hydrogen as a raw material has promoted further activity in this area. This general interest led to our original motivation. Prior research, described in the literature, has been directed towards fabricating “sensors-on-a-chip”, by measuring changes in the electrical properties of thick palladium layers upon exposure to hydrogen: for example, values of their resistance or capacitance; the advantage here being that a signal is derived in an electrical form (1).

Signal recovery from sensors, based on direct electrical measurements, demands a continuous and hence a thick (> 100 nm) palladium film, which can be characterised in the range where bulk palladium properties dominate. These bulk properties are well understood (2).

However, our research took a different direction – by examining the response of extremely thin (∼ 10 nm) palladium layers, in which deviations from bulk behaviour are known to occur, and we investigated properties of thin palladium film on silicon dioxide/silicon substrates, Pd-SiO2/Si(3). As the electrical properties of very thin palladium films cannot be measured, due to the highly discontinuous nature of the films resulting from the low level of surface coverage, we turned to the use of simple laser diagnostic techniques (4). Our goal was to examine the performance of devices that might be compatible with the present day silicon-driven revolution in technology and with the use of lasers, thereby achieving a simple “sensor-on-a-chip” device, which would directly incorporate the sensor, source and detector in one component. In our experiments to date we have measured changes in the absolute reflectivity of Pd-SiO2 films upon exposure to various concentrations of hydrogen gas as a function of temperature, see Figure 1 (5).

Palladium-Silica/Silicon Structures


Silicon wafers (p -type 5-13 ohm cm) were cleaned, prior to palladium evaporation, by a standard technique. A layer of silicon dioxide was formed, by oxidation in dry oxygen at 1100°C, to a thickness of 100 nm. This process was followed by a 15 minute anneal in argon at the same temperature. Palladium was then evaporated onto the silica samples, which were kept at room temperature during the process. The Pd/SiO2 film was not annealed before measurements were taken. The background pressure during the evaporation was ∼ 107 Torr, for a deposition rate of 2 to 3 Å per second.

Fig. 1

Schematic diagram of the optical measurement technique. The test cell is equipped with inlet and exhaust ports to allow for the cycling of gas mixtures. The sample held in the test cell is a palladium/silica/silicon thin film of area 1 cm x 1 cm held at room temperature and ambient pressure. The laser emits light of wavelength 632.8 nm. L are lenses and M are mirrors

Schematic diagram of the optical measurement technique. The test cell is equipped with inlet and exhaust ports to allow for the cycling of gas mixtures. The sample held in the test cell is a palladium/silica/silicon thin film of area 1 cm x 1 cm held at room temperature and ambient pressure. The laser emits light of wavelength 632.8 nm. L are lenses and M are mirrors

Fig. 2

The fractional change in reflectivity as a function of hydrogen concentration in air for an 8 nm thick Pd-SiO2 film. The α ← β phase transition occurs at a hydrogen concentration of ∼ 2 per cent. The inset shows a typical example of the decrease and increase in reflectivity upon exposure to hydrogen and air, respectively. The change in slope occurs 4 minutes after exposure to hydrogen,for a gas flowing into the cell at a rate of 160 ml min−1. Values 4, 3, 2 per cent, etc., are the amounts of hydrogen in air

The fractional change in reflectivity as a function of hydrogen concentration in air for an 8 nm thick Pd-SiO2 film. The α ← β phase transition occurs at a hydrogen concentration of ∼ 2 per cent. The inset shows a typical example of the decrease and increase in reflectivity upon exposure to hydrogen and air, respectively. The change in slope occurs 4 minutes after exposure to hydrogen,for a gas flowing into the cell at a rate of 160 ml min−1. Values 4, 3, 2 per cent, etc., are the amounts of hydrogen in air

Measuring the optical properties of thin metal films is a complex issue, as they are very sensitive to the film deposition conditions and thickness, with the morphology and roughness of the film varying with thickness. Nevertheless, measuring the reflectivity of the film can prove to be a powerful and simple technique for deriving changes in the sample thickness. The absolute reflectivity, as a function of film thickness, is found to have a characteristic form (this is generic to all thin metal films regardless of the metal type) and can be classically modelled as a three-layer system (gas-metal-substrate) which behaves well, even for very thin nanometre-scale films.

The changes in the properties of the metal film depend on the exposure conditions to a particular gas, and can be derived from variations in the effective thickness of the film. Changes in the effective film thickness directly affect the phase of the light passing through the metal. The thickness variations also cause changes in the Fresnel reflection coefficients. However, in our case the film thickness is close to the penetration depth of the laser and changes to the phase of the light is the dominant factor.

The exposure of 8 nm thick Pd-SiO2 films to hydrogen concentrations exceeding 2 per cent in air (at room temperature and pressure) produces a fractional decrease in reflectivity of up to 35 per cent. Purging the system with air to remove the hydrogen gas results in the complete reversibility in the signal change.

The fractional change in reflectivity (FCR) is proportional to the hydrogen concentration both above and below 2 per cent, but with different slopes. Above a hydrogen concentration of 2 per cent, the FCR response is linear but almost flat, whereas, below 2 per cent the palladium exhibits a large increase in FCR response with increasing hydrogen concentration, and remains linear. The key result therefore is the observation of the room temperature changes in the composition of PdHx which produce changes to the FCR.

The palladium-hydrogen system has two phases, the α- and β-phases, with a mixed phase region at room temperature, for which the α-phase extends to values where x = 0.015 and the β-phase to x = 0.58 (2). The (β-phase marks a transition point at which the palladium crystal lattice undergoes a significant transformation with hydrogen concentration, see Figure 2. Therefore the α ← β phase transition occurs at ∼ 2 per cent hydrogen concentration in air, causing the lattice to expand and thus allowing for further clustering of hydrogen at high densities. We have observed this phase transition for palladium film of thickness between 3 and 30 nm.

However, for films of thickness 1 to 2 nm, the graph of FCR with hydrogen partial pressure is linear and of constant slope at all measured hydrogen concentrations, indicating that the sample is constrained to operate well within the α-phase, as there is plainly not enough material to allow for a comprehensive lattice expansion. This results from the island nature of the film.

We have also observed that for palladium films with thickness exceeding 30 nm, there appears to be a catastrophic failure in the response; this can be linked to the dimensional changes associated with the transition to the β-phase hydride.

The primary application of this technique – of using the fractional changes in reflectivity of palladium thin films to observe changes in the hydrogen content in the surrounding gas – will be for sensing various gases and monitoring the rate and degree of surface contamination of the thin sensing film. The laser technique can be applied to any gas-metal system.

We conclude by noting that this very simple technique, namely monitoring the FCR of palladium films exposed to various hydrogen gas concentrations at room temperature and pressure, can recover important information regarding the properties of palladium thin films which would be impossible to make via electrical measurements.

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References

  1. 1
    A. Mandelis and C. Christofides, ‘ Physics, Chemistry and Technology of Solid State Gas Sensor Devices ’, Vol. 125 in “ Chemical Analysis ”, Wiley Interscience, New York, 1993
  2. 2
    F. A. Lewis, in “The Palladium/Hydrogen System”, Academic Press, New York, 1967
  3. 3
    G. A. Frazier and R. Glosser, J. Less-Common Met ., 1980, 74, 89
  4. 4
    M. A. Butler, J. Electrochem. Soc ., 1991, 138,( 9 ), L46
  5. 5
    K. Kalli,, A. Othonos,, C. Christofides,, A. L. Spetz and I. Lundströ;m, Rev. Sci. Instrum ., 1998, 69,( 9 ), 3331

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