Hydrogen sensing using palladium coated long period
gratings
R R J Maier1, T D P Allsop2, R Neal3, J S Barton1, D J Webb2 and I
Bennion2
1
Heriot Watt University, School of EPS, Edinburgh EH14 4AS UK
2
Electronic Engineering, Aston University, Birmingham B4 7ET UK
3
University of Plymouth, Faulty of Technology, School of Electronic
Communications and Electrical Engineering, Plymouth, PL4 8AA, UK
r.r.j.maier@hw.ac.uk
Abstract. There is an urgent need for all-optical, safe hydrogen sensors to support
future hydrogen based technologies. Palladium, a catalytic material widely used
material in the hydrogen technology, readily, and selectively, reacts with hydrogen by
the formation of Pd hydrides which results in a reduced conductivity. This is a
chemochromic reaction i.e. it exhibits a small change in refractive index via the
Kramers-Kronig relationship. Long period gratings [LPGs] are sensitive to the
refractive index contrast at the cladding / free space interface, making the fibre
sensitive to the surrounding environment. Coating a fibre in the region of the LPG
with a palladium layer 10 nm to several 100 nm thick enables the fast detection of low
concentration of hydrogen via the change in refractive index of the layer.
Experimental data with hydrogen concentrations in the 100 to 8000ppm region will be
presented for a range of differently coated fibres. A shift of 0.18pm ppm-1 of H2 at a
concentration 500ppm (in dry nitrogen) is observed for a 250 nm thick coating. The
sensor response saturates at higher concentrations with a reduced response of
0.05 pm ppm-1 of H2 at 8000ppm. The dependency of sensitivity and temporal
characteristics on palladium layer thickness, temperature and hydrogen concentration
will be discussed. At a concentration of 8000 ppm of H2 the sensor follows an
exponential growth function with a time constant of ~1000 seconds reaching 95% of
the asymptotic limit after 20 minutes.
1. Introduction
Palladium and its alloys with Cu, Ag and Au are widely used materials in hydrogen
technology because of their ability to safely store large amounts of atomic hydrogen..
Palladium and alloys are highly selective to hydrogen and are for example also used in
membrane form for hydrogen separation. Hydrogen molecules are efficiently dissociated by
a catalytic process on a Pd surface after which the atomic hydrogen can readily penetrate
the lattice of the bulk Pd based material where it resides at interstitial spaces. This uptake of
hydrogen into the bulk subtly changes the mechanical and physical properties of the material
and these effects are widely used as a means for quantitative hydrogen detection. Small
hydrogen dependent changes in conductivity have been detected using for example Pd
structures in FET [1] and Schottky diode [2] based electronic devices. This effect is, in part,
based on the reduction of electron mobility through a lattice expansion on hydrogen uptake.
This expansion although, small can be measured using strain sensing devices[3,4].
Changes in material structure affecting its conductivity are intrinsically linked with changes
in optical properties of any material through the Kramers-Kronig relationship and hence it is
not surprising that small changes in reflectivity can been observed on exposure of a Pd
sensing layer to hydrogen[5,6]. Changes in reflectivity of a few % have been observed on
exposure of Pd surfaces to concentrations of several % of hydrogen. Remote monitoring
systems using fibre optic interfaces are susceptible to intensity noise and systems have been
demonstrated using reference sources or dual wavelength interrogation to address this
issue. Evanescent wave sensing of etched Pd coated fibres has been reported[7].
Palladium has a very broad reflection spectrum typical of metals and as such it is non trivial
to implement such schemes. Surface Plasmon resonances [SPR] have been observed in a
number of fibre based systems based on Pd and WO3 sensor layers where the strength and
wavelength is sensitive to hydrogen [8].
Hydrogen reaction dynamics can be expressed by rate constants dx (adsorption) and cx
(desoprption) where x=1, e and i, stand for front surface, bulk and back surface processes
respectively.
H2
d1



c1
2H a
de




ce
2H b
di



ci
2H i
(1)
2. Wavelength encoding of changes in refractive index
Signals encoded in the wavelength or frequency domain can be transmitted with very little
or no deterioration over long distances using fibre optic systems. Long period gratings
[LPG], a special case, of in-fibre grating structures can be made into highly sensitive
refractive index sensors.
LPGs are formed in a similar way to the better known fibre Bragg gratings [FBG] by
inscribing a periodic refractive index perturbations into the core of an optical fibre where the
periodicity of the refractive index modulation is typically 100’s of times larger than the
wavelength of light. In a LPG, certain frequencies of forward propagating core modes will be
coupled out of the core and continue to propagate as cladding modes as shown
schematically in Figure 1.
n0
sensor layer
n of cladding to free space
(sensor layer) interface
nco
ncl
Figure 1. Schematic of mode coupling in a LPG.
These modes can remain bound in the cladding or become radiation modes, depending on
boundary conditions i.e. the refractive index difference at the cladding to free-space
interface. In a typical LPG with e.g. 450 μm periodicity, there are several wavelengths for
which the conditions of cladding mode coupling are fulfilled as shown in Figure 2.
The wavelengths at which cladding mode coupling occurs are highly sensitive to changes
in refractive index contrast at the cladding / environment boundary with sensitivities varying
for the different LPxx loss bands. Depending on specific grating details, the loss band
position can change by, for example 5 nm, for a change in n of the environment of only
0.006. LPGs have been used as highly sensitive sensor elements for a variety of liquid
based environmental and chemical sensing tasks. Very small changes in refractive index in
the order of 1×10-6 at the cladding interface have been detected using LPGs.
FWHM
LP03
LP02
LP04
LP05
1100nm
60nm/div
LP06
1580nm
1400nm
10nm/div
1500nm
Figure 2. Left: LPG transmission spectrum of 450 μm periodicity grating (5 cm long) with
400nm thick Pd coating in region of grating. Right: Strongest loss band (LP06) at 1450nm.
The reflectivity of a metallic surface is characterised by the damping, , of an incident
electromagnetic wave,
  0.5  (0 ) 1
(2)
where: 
the frequency of light and o, the
permeability of free space. The complex refractive index n  of a metal is given by
n 
c 1 i 
  
   
(3)
where the imaginary part is very large due to a small 
The reflectivity of a metal surface is then given by
( n R  1) 2  n I
R
2
( n R  1) 2  n I
2
(4)
where nR and nI are the real and imaginary parts of the refractive index. Hence, reflectivity
and conductivity are directly related to the refractive index and it is expected that a change in
refractive index of a metallic Pd based sensing layer on the outside of a LPG containing fibre
should affect the wavelengths of the loss bands, thus converting the change in refractive
index to a wavelength encoded measurand which can be transmitted over long lengths of
fibre without deterioration and free of external interference.
3. Experimental
3.1. Palladium coating:
LPGs used in this study were manufactured at Aston University by point by point inscription
using a doubled Ar+ laser into a single mode fibre (Corning SMF-28) with a periodicity of
450 μm. Subsequent palladium coating of these LPGs was carried out at Plymouth University
by RF sputtering (in argon atmosphere) in a vacuum coating chamber. The fibre was held in
a rotation horizontal mount at a distance of ~10 cm above the sputtering source. Rotation of
the fibre ensured an even coating thickness over the full circumference of the fibre. Nominal
coating thicknesses applied were between 50 and 500nm although only results from a
250nm thick coating are presented here.
3.2. Hydrogen exposure
The spectral characteristics of a LPGs are cross sensitivities to strain, temperature and
bending. It is therefore essential to ensure that a fibre is ideally insulated from these effects
which has been achieved by placing the fibres inside a temperature stabilized tube with 1mm
inner diameter. A schematic of the experimental setup is shown in Figure 3.
H2
source
MFC
OSA
500sccm
exhaust
Vacuum
grease
N2
PT100
MFC
RTV
silicone
heater
sample tube
LPG
Figure 3. Schematic of experimental setup.
Hydrogen at variable concentration, adjusted by means of 2 mass flow controllers) was
flowed through the system at a rate of 500 cm3 min-1 [sccm] of hydrogen was tube through.
Fibres were sealed at one end to the system by RTV silicone whereas the other end was
sealed by a non setting vacuum grease to eliminate strain effects on the fibre.
In data presented here the tube was held horizontally although we have since then found
it to be beneficial to convert the set up to a vertically held tube with the fibre held fixed at the
top and to apply a constant stress to the fibre via a small weight of 5 grams and secondly
preheat the gas mixture prior to entry into the sample tube.
3.3. Optical interrogation
LPGs are transmission filters and are ideally interrogated in transmission. A broadband
continuum source based on a non-linear conversion of a ns pulsed Nd:YAG laser operating
at 1064 in a microstructured fibre is used for illumination and an optical spectrum analyser
(Advantest Q8384) is used as detector. The characteristic response of an LPG to a change
in refractive index contrast at the cladding / sensor layer interface is different for each loss
band. Suffice to say here that the strongest and longest wavelength loss band band visible
in the spectrum shown in Figure 2 is also the band with the strongest response top a given
n. Spectra covering the 1400 to 1500 nm region are recorded every 30 seconds and the
peak position (valley) is extracted automatically using a LabView script.
4. Results
LPG resonance [LP06] shift / nm
4.1. Hydrogen loading
On exposure to hydrogen a catalytic reaction takes place on the Pd surface dissociating
molecular hydrogen. Following dissociation atomic hydrogen can readily penetrate and
propagate through the bulk forming a palladium hydride. After a delay hydrogen begins to
appear on the “backside” of the Pd film at the fibre / Pd film interface and modifies the local
(complex) refractive index of the sensor layer which in turn causes the wavelength of the loss
band position to shift to shorter wavelength.
H2 conc. / ppm
0.0
500
-0.1
1000
-0.2
2000
4000
-0.3
-0.4
8000
-0.5
0
10
20
30
40
50
60
70120
240
360
time / minutes
Figure 4. Shift in peak position (valley) of the LP06 loss band in a LPG coated with 250nm Pd on
exposure to hydrogen concentrations from 500 to 8000 ppm at time zero.
Figure 4 shows the co-plotted observed wavelength shifts of the LP06 band in a Pd coated
LPG on exposure to a range of hydrogen concentrations ranging from 500 ppm to 8000 ppm
in nitrogen at ~23 °C.
The Pd film responds rapidly to the presence of hydrogen with a change in refractive
index causing a shift of the LPG loss band towards shorter wavelength. The observed LPG
loss band (@ 1450 nm) reaches an asymptotic limit proportional to the hydrogen
concentration when an equilibrium of concentration inside and outside the Pd sensor layer
has been reached and the loading / out-diffusion rate constants are balanced. At higher
hydrogen concentrations (≥4000 ppm) we observe, after a rapid initial shift, a more gradual
continued shift towards lower wavelength although we have currently no concrete
explanation for this particular observation. Data were recorded with a data point spacing of
100 pm with the OSA set to a nominal resolution of 100pm. The FWHM of the loss band is
~20nm, hence the total observed peak shift for the 500 ppm exposure, in the order of 100
pm, is equivalent to a shift of 1/200 of the peaks FWH, i.e. at the limit of the systems
resolution. Furthermore, LPGs are, in general, characterised by substantial thermo optic
coefficients and this LPG exhibits a coefficient in the order of 60 pm K-1 necessitating a high
quality of temperature control. Subsequent experiments use a preheated gas flow to
eliminate supply induced thermal fluctuations.
4.2. Unloading of hydrogen
On reduction of the hydrogen concentration around the fibre, hydrogen readily out-diffuses
recombining to H2 on the surface. Figure 5 shows the co plotted recovery process after
shutting off the exposure to hydrogen. All exposures eventually return back to the base line
with time constants depending on the scale of the initial loading. Again the two highest
loadings result in a slightly different characteristic and require longer to recover as shown by
the offset between the two sets of curves. Time constants have been determined using non
linear least squares fitting routines of exponential functions of the forms y=Ae(-t/) and
y=A(1-e(-t/)) for loading and unloading respectively.
0
LPG resonance shift / nm
0.0
10
20
30
40
50
initial hydrogen
concentration / ppm
60
70
80
90 100
120 240 360 480 600 720
A1=0.167 t1=1541
A1=0.095 t1=2533
-0.1
y0=-0.338; A1=0.170 t1=614; A2=0.126 t2=4597
500
y0=-0.45; A1=0.366 t1=941 , A2=0.096 t2=28657
1000
-0.2
2000
-0.3
4000
-0.4
8000
0
10
20
30
40
50
60
70
80
90 100
120 240 360 480 600 720
time / minutes
Figure 5. Recovery of Pd coated LPG after reaching equilibrium.
(note: equilibrium was not reached fully at 4000 and 8000 ppm prior to unloading).
4.3. Summary of results
A summary of the scale of shift versus hydrogen concentration and time constants of the
response is shown in Figure 6 (left) where it can be seen that the sensitivity is substantially
larger at low concentrations reaching >0.18 pm ppm-1H2 for concentrations below 500 ppmH2
dropping off to ~0.03 pm ppm-1H2 of H2 for concentrations in the >4000 ppm regime.
It is well known fact that the reaction dynamics of the Pd / hydrogen system are highly
temperature sensitive and that a substantial increase in speed of response can be achieved
by operating the sensor element at elevated temperature. Studies using the strain response
of Pd on hydrogen uptake show that a reduction in time constants by a factor of 13 can be
observed for a temperature increase from 20 °C to 85 °C although at the same time the
response is decreased by a factor of 1.7 through a reduced solubility of hydrogen in the
palladium lattice [9]. Further increases in reaction speed can be achieved by using a thinner
sensor layer resulting in a faster penetration to the back face although such thin layers can
become structurally unreliable[10].
B
e
400
300
sensitivity
0.03pm/ppm
@ 4000 ppm
200
sensitivity
>0.18pm/ppm
for H2<500ppm
100
0
2000
4000
6000
8000
Hydrogen concentration / ppm
time constant / seconds
observed LPG shift / pm
500
2500
2000
recovery process (out-diffusion)
1500
1000
500
loading process
0
0
2000
4000
6000
8000
Hydrogen concentration / ppm
Figure 6. Left: Shift of LPG loss band as a function of hydrogen concentration.
Right: Time constants of loading and recovery process versus hydrogen concentration.
.
5. Conclusions
A fast response, all optical, wavelength encoded hydrogen sensor based on a palladium
coated LPG has been demonstrated where complex refractive index changes as a function
of hydrogen uptake resulting in a shift of the LPG loss band. A significant benefit of using
palladium is its high hydrogen specificity, although strong cross sensitivities with even low
oxygen concentrations, CO and sulfides exist which strongly modulate the catalytic activities
of the Pd surface. Similar, if not larger responses can be expected by coating LPGs with
alternative chemochromic materials, notably of the WO3 class [11], which are known to
exhibit strong colour changes on reaction with hydrogen although they also show strong
cross sensitivities with other environmental gases especially H2O and are significantly less
specific to H2 than Pd. Currently ongoing studies also point to potential drifts problems
(baseline, speed) in Pd coated LPG based sensors requiring pre ageing of sensor elements
before reaching a stable performance. Improvements in resolution and accuracies are
possible by optimising the LPG parameters to show the strongest n/ for the observed
LPxx loss band. Narrowing the FWHM of the loss band will allow a more accurate
determination of the centroid position of the valley improving resolution and accuracy.
Investigations of thinner coatings, known to have faster responses, are in progress.
Acknowledgement: This study was support by AWE plc.
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