Fibre optic hydrogen gas detection based on thin Palladium
Films
Kevin Gleeson, Elfed Lewis
Optical Fibre Sensors Research Centre, Electronic and Computer Engineering Department,
University of Limerick, Limerick, Ireland.
Kevin.Gleeson@ul.ie, Elfed.Lewis@ul.ie
Abstract
A transmission based fibre optic hydrogen sensor using a thin palladium film of 12nm is
presented. The palladium film was deposited onto a glass substrate by thermal evaporation.
The film was tested using 5% hydrogen in a nitrogen atmosphere. The absorption and
desorption of hydrogen causes the optical properties of palladium to change depending on the
concentration of hydrogen present in the atmosphere. Using a deuterium/halogen light source
in conjunction with a UV/VIS spectrometer the changes in the optical transmittance in the VIS
spectra of the Pd film was monitored for 5% hydrogen in a nitrogen atmosphere. The response
and recovery time for the sensor is shown to be 60 and 300 seconds respectively. A strong
change in transmission in the order of 9% was recorded and the sensor shows good
repeatability to continuous exposure cycles to 5% hydrogen using nitrogen as the carrier and
recovery gas.
1. Introduction
Hydrogen (H2) has been developed and utilised in numerous fields such as hydrogenation processes,
the aerospace industry, hydro cracking of oil, production of chemicals etc. With increasing global
warming and climate change from the continuous burning of fossil fuels for our energy need, an
increasing amount of research and development is being focussed on the use of hydrogen as the next
generation energy source. Unlike fossil fuels, which emit harmful pollutants into the atmosphere and
are a finite resource, hydrogen is a clean, renewable energy source emitting only water as it is burned.
There are however certain technological barriers to be overcome before hydrogen can be used
to replace fossil fuels as an energy source. Hydrogen is a combustible substance with a Lower
Explosive Limit (LEL) of 4.65% at room temperature and pressure and since it is the smallest element
on earth one of the main barriers to overcome is hydrogen leak detection. Numerous semiconductor
based hydrogen sensors have been developed and commercialised[1-4].These sensors use palladium
(Pd) as the active sensing layer and work by detecting changes in the electrical properties of palladium
when in contact with hydrogen. Due to the combustible nature of hydrogen, optical monitoring of
hydrogen is considered the most appropriate method owing to its inherent safe nature when compared
with techniques requiring electrical measurements. For this reason there has been considerable interest
in optical detection methods for determining hydrogen concentrations in the atmosphere [5].The
optical properties of palladium change when in contact with hydrogen as a function of hydrogen
concentration[6]. This property is used to detect different levels of hydrogen in the atmosphere.
In this paper, a transmission based optical fibre sensor utilizing a thin Pd film as the sensing
layer for the detection of 5% hydrogen (H2) in a nitrogen (N2) atmosphere is discussed. The response
time for the Pd’s reaction to 0% H2 and 5% H2 in N2 is presented along with evidence of the
repeatability and complete reversibility of the sensors operation.
2. Principle operation
Palladium has the unique property of selectively and reversibly reacting to hydrogen by absorbing it to
form a binary hydride. When palladium is exposed to hydrogen the palladium absorbs the hydrogen
and forms a reversible palladium hydride. Hydrogen molecules are converted into hydrogen atoms (H2
←→ 2H) at the palladium surface with an efficient dissociation rate. The hydrogen atoms diffuse
rapidly through the palladium film leading to the reversible hydrides of the form PdHx where x is the
atomic ratio H/Pd. The palladium hydride film (PdHx) has different mechanical, electrical and optical
properties than those of a hydrogen free palladium film. PdHx has two different phases, the α and β
phase depending on the Pd/H composition. The reversible α-phase is found at low hydrogen
concentration. Without hydrogen Pd is in the α phase. As the hydrogen concentration increases Pd is
transformed to the β-phase. The hydrogen concentration at which the phase transition takes place
depends on the film thickness and temperature and it introduces a hysteresis in the optical and
mechanical parameters of the palladium film. Pd in the β-phase has the same face centred cubic
symmetry of pure palladium but with the lattice parameter expanded up to 3.5%. Transition from the
α-phase to the β-phase for a thin Pd film (10nm-100nm) occurs at a wider hydrogen concentration
range than the transition in the thicker Pd films (>10um) which occurs around 4%.[7]
3. Experimental set-up
Thin films of palladium were deposited onto highly polished glass slides using a BOC/Edwards
E305A vacuum thermal coating system. This system contains an Edwards FTM5 quartz crystal to
monitor the rate of film deposition and to measure the film thickness. The quartz crystal was
positioned directly above the evaporation source. The mass deposited on the quartz crystal during the
evaporation alters its natural frequency of vibration. This frequency change was recorded on the meter
of the film thickness monitor connected to the quartz crystal. Using this system the thickness of the
deposited palladium films was approximated to be 9nm. However the problem of using a vacuum
thermal coating system is that the deposition process is not direct and evenly dispersed over the
substrate. It is worth noting that the quartz crystal monitor system is not very accurate when
monitoring thin film deposition of such a small scale. Thus the recorded thickness using the quartz
crystal is an approximation of the film thickness. Because of this it was decided to use Spectroscopic
ellipsometry (SE) to determine the thickness of the thin films. The SE measurements were taken using
the J.A Woollam M2000U ellipsometer and the data was modelled using the accompanying Wvase32
software. The measured thickness using SE of the Pd film was recorded to be 12nm.
The experimental set up is shown in figure 1. A gas cell was specially constructed to hold the
Pd coated glass slide. It is made from aluminium and consists of two collimating lenses, which are
aligned at an orientation of 180 degrees, and gas inlet and outlet connectors. Light is transmitted into
the gas cell from a DH-2000 UV-VIS-NIR deuterium-halogen light source from ocean optics using the
halogen lamp only. The received light is detected by an S2000-TR spectrometer from ocean optics and
the data from the spectrometer is acquired and stored using a personal computer running Labview. The
light source and detector are connected to the gas cell using multimode polymer optical fibres (Fibre
data A19A15A0) via the collimating lenses.
The gas supply component consists of a 5% Hydrogen-95%Nitrogen (H2) mix and Nitrogen
zero grade (N2) from BOC gases. They are connected to the gas cell using stainless steal piping and
can be mixed together or used separately using shut-off valves.
Figure 1 Experimental Set-up
4. Results
Figure 2 shows the change in transmission spectrum in the visible region induced by the 12nm film
when exposed to 5% Hydrogen in a Nitrogen atmosphere. From the data in figure 2 the main
wavelengths of interest, where the maximum change in transmission occurs, are at 564nm, 622nm,
658nm, 680nm and 767nm. Several experiments were undertaken to determine the signal change and
the response time of the sensor at each of the specified wavelengths.
4500
4000
3500
Intensity
3000
2500
5%H2
0%H2
2000
1500
1000
500
0
300
400
500
600
700
800
Wavelength (nm)
Figure 2 Increase in transmission spectrum from exposure of 0-5% Hydrogen
Figure 3 shows the normalized signal change at 564nm for repeated exposures of 5% H2 gas and the
response time for the palladium to fully absorb and desorb the hydrogen gas. There is a 9% increase in
the transmission spectrum which means that the 12nm Pd becomes more transparent upon exposure to
hydrogen which is expected from literature[5]. The response time for the sensor to 5% H2 is 60
seconds with a recovery time of 300 seconds when exposed to 100% N2. The response and recovery
times are defined as the time required to achieve 100% of the corresponding signal change for the
adsorption and desorption of hydrogen. While the response times are not in the order of a 1-5 seconds
they are considered to be good. It is reasonable to assume that the time taken for hydrogen to fully
adsorb and desorb from the 12nm Pd film is due to the phase transition of the Pd from the α-phase to
the β-phase. The following results also reflect the complete reversibility of the sensors response to
cycles of hydrogen/nitrogen exposures at a concentration of 5% hydrogen. The sensor is also shown to
be very repeatable, as the response does not differ over several cycles of hydrogen exposure.
Wavelength 564nm
Normalized intensity
0.12
0.1
0.08
0.06
0-5% H2
0.04
0.02
0
-0.02
0
1000
2000
3000
4000
Time (seconds)
Figure 3 Response of sensor to 5% H2 at 564nm
Figure 3 The change in the peak response to 5% H2 at 564nm, 622nm, 656nm, 680nm,
and 767nm.
Each of the other wavelengths monitored show similar changes to the transmission spectrum with the
same response time with one small exception of the peak at 767nm. Figure 4 shows the normalized
average peak response for each wavelength of interest. From the data shown there is little difference in
the change in the transmission spectrum in the VIS region but as it nears the infrared there is a small
decrease in the transmission spectre at 767nm. This decrease could be due to a smaller signal response
change as the wavelengths nears the infrared but will be investigated at a later date.
5. Conclusions and future work
An experimental transmission based fibre optic sensor using thin palladium film has been developed to
measure the presence of hydrogen to a concentration of 5%. The preparation of the palladium films via
thermal evaporation is discussed and the thickness of the film is measured to be 12nm using
spectroscopic ellipsometry techniques. A strong signal response change of 9% has been demonstrated
for a change from 0-5% hydrogen gas in a nitrogen atmosphere. Response time for the sensor, to 5%
H2, is shown to be 60 seconds with a recovery time of 300 seconds. The response of the sensor is
shown to be completely reversible and to have good repeatability.
Future work will investigate the response of the sensor to different concentrations of
Hydrogen. Different film thicknesses will also be investigated along with the sensors response to
reflectivity changes. From the results section there was a decrease in the sensors transmission response
towards the infrared. This will be further investigated in the future.
6. Acknowledgements
The authors would like to thank the Material Surface Science Institute (MSSI) of the University of Limerick
for the use of their facilities and equipment. Also the authors would like to thank the staff of Electronic and
Computer Engineering Department of the University of Limerick for their continued assistance during testing.
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