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Size-dependent hydrogen uptake behavior of Pd nanoparticles revealed by photonic crystal surface waves

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Abstract

We present an optical method of study of nanoparticle properties using photonic crystal surface waves. Palladium nanoparticles were deposited on a surface of a one-dimensional photonic crystal, which supports the propagation of p-polarized optical surface waves. The changes in the nanoparticle properties, such as its dimension and refractive index, were monitored through angle interrogation of the photonic crystal surface waves. The interaction of palladium nanoparticles with hydrogen was detected with this method. The size-different hydrogen uptake behavior by 2 and 6 nm diameter Pd nanoparticles results in qualitatively different response of the optical signal, viz., in the different signs of such a response. This not only confirms the absence of the a- to b-phase transformation for the smallest palladium nanoparticles, but is a plausible indication that hydrogen donates its electrons to a collective electron band of the metal.
Size-dependent hydrogen uptake behavior of Pd nanoparticles revealed by
photonic crystal surface waves
Valery N. Konopsky, Dmitry V. Basmanov, Elena V. Alieva, Sergey K. Sekatskii, and Giovanni Dietler
Citation: Appl. Phys. Lett. 100, 083108 (2012); doi: 10.1063/1.3690085
View online: http://dx.doi.org/10.1063/1.3690085
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i8
Published by the American Institute of Physics.
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Size-dependent hydrogen uptake behavior of Pd nanoparticles revealed
by photonic crystal surface waves
Valery N. Konopsky,
1,a)
Dmitry V. Basmanov,
1
Elena V. Alieva,
1
Sergey K. Sekatskii,
2
and Giovanni Dietler
2
1
Institute of Spectroscopy, Russian Academy of Sciences, Fizicheskaya, 5, Troitsk, Moscow region 142190,
Russia
2
Laboratoire de Physique de la Matie`re Vivante, Institut de Physique des Syste`mes Biologiques,
Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
(Received 23 November 2011; accepted 8 February 2012; published online 23 February 2012)
We present an optical method of study of nanoparticle properties using photonic crystal surface
waves. Palladium nanoparticles were deposited on a surface of a one-dimensional photonic crystal,
which supports the propagation of p-polarized optical surface waves. The changes in the
nanoparticle properties, such as its dimension and refractive index, were monitored through angle
interrogation of the photonic crystal surface waves. The interaction of palladium nanoparticles with
hydrogen was detected with this method. The size-different hydrogen uptake behavior by 2 and
6 nm diameter Pd nanoparticles results in qualitatively different response of the optical signal, viz.,
in the different signs of such a response. This not only confirms the absence of the a-tob-phase
transformation for the smallest palladium nanoparticles, but is a plausible indication that hydrogen
donates its electrons to a collective electron band of the metal. V
C2012 American Institute of
Physics.[http://dx.doi.org/10.1063/1.3690085]
Nanometer sized materials are of great scientific and
technological interest because their physical and chemical
properties are often size-dependent and different from their
bulk counterpart. The palladium-hydrogen system can be
considered as a model system for such studies for several
reasons. First, due to the noble character of Pd, its nanopar-
ticles have only a very thin oxide surface film (about one
atomic monolayer
1
) that can be removed in the initial expo-
sure to H2.
2
Second, the behavior of bulk palladium under
hydrogen exposure has been thoroughly investigated both
experimentally and theoretically.
3,4
Palladium hydride, PdHx, exhibits two distinct phases,
denoted as aand bphases (the latter is sometimes referred to
in literature as a0phase). In the aphase, at low hydrogen
concentration, the hydrogen atoms are incorporated into the
Pd crystal structure and occupy interstitial sites in the lattice
and at the grain boundaries. This leads to internal strain and
a slight expansion of the face-centered cubic (fcc) Pd lattice.
This expansion is approximately linear in a [Pd lattice con-
stant]-[H2pressure] dependence. When the hydrogen con-
centration increases further, a first-order phase transition
occurs. For some metals there is a structural phase transfor-
mation of the metal lattice in passing from the ato bphase;
however, in palladium there is only a change in the lattice
constant of the fcc lattice. In this b(a0) phase the palladium
hydride may be considered as an interstitial alloy, where the
hydrogen atoms occupy the octahedral lattice sites of the fcc
Pd lattice, forming a defective rock-salt (NaCl) structure.
The a-bphase transformation results in the large increase of
the Pd lattice constant by 3.54% that corresponds to a 11%
volume increase. This sharp, nonlinear increase on the [Pd
lattice constant]-[H2pressure] curve corresponds to the well-
known plateau (miscibility gap) on a pressure-composition
isotherm, i.e., on a [H2pressure]-[x] (from PdHx) curve.
The modification of this bulk Pd behavior at the nano-
scale has been extensively studied over the past decade.
59
It was found that in palladium nanoclusters the miscibility
gap is narrowed and practically disappears for clusters
smaller than 3 nm.
10
Theoretical simulations of hydrogen
uptake in small Pd nanoparticles also confirm the disappear-
ance of the miscibility gap and a-bphase transformation.
11
For Pd nanoparticles less than 3 nm in size, the discontinuity
in the [Pd lattice constant]-[H2pressure] curve, which is spe-
cific for the a-btransition, disappears and this curve becomes
approximately linear in this region.
12
Nevertheless, some re-
sidual features of the a-bphase transition, such as a small
hysteresis on the pressure-composition isotherm, still occur
even for the smallest Pd nanoparticles.
12,13
There is a wide range of tools available to detect the pres-
ence of the hydrogen in an environment under study. In such
hydrogen sensors, a palladium film is commonly used as the
selective layer, in conjunction with a range of transducers,
such as thin film resistors
14
or nanoparticle resistors.
15
How-
ever, the number of instruments able to distinguish between
the aand bphase response of Pd nanoparticles upon hydrogen
injection is limited. As a rule, detailed study of Pd-hydrogen
interaction involves either a gas-loading gravimetric Sartorius
micro-balance measurements or measurements of pressure
change due to hydrogen absorption/release in Sievert’s reactor
(a closed system with constant volume). Then, the aand b
phase response are determined from correspondent parts of
the pressure-composition isotherm.
In this letter we show that photonic crystal surface
waves (PC SWs) can be used as a sensitive measurement
tool of hydrogen uptake by Pd nanoparticles. Moreover, a
peculiarity of the sensing system “Pd nanoparticles on the
PC surface” results in different signs of the optical response
a)
Electronic mail: konopsky@gmail.com.
0003-6951/2012/100(8)/083108/4/$30.00 V
C2012 American Institute of Physics100, 083108-1
APPLIED PHYSICS LETTERS 100, 083108 (2012)
of this system, depending on the presence or absence of
phase transition in palladium during measurement.
PC SWs are excitation of optical modes, which can exist
on the external surface of a photonic crystal in its band gap
region. Sometimes these PC SWs are also called Bloch sur-
face waves
16
or optical Tamm states.
17
In recent years, PC
SWs have been used in ever-widening applications in the
field of optical sensors.
1824
In the present study we used
p-polarized PC SWs, propagated along the external surface
of 29-layer structure on which Pd-nanoparticles were depos-
ited. A sketch of the experimental setup is shown in Fig. 1:
the PC SWs were excited by focusing a laser beam from a
diode laser through a glass prism onto the one-dimensional
(1D) PC structure (a Kretschmann-like excitation scheme).
A cylindrical lens with the focal length f¼70 mm was used
to focus the laser beam on the structure. The excitation
source was a fiber-coupled diode laser with wavelength
k¼737:7 nm and power W¼0.26 mW.
The change of the PC SW propagation constant qSW ¼
n0sinðh0Þwas measured by detecting the shift of the reflec-
tion profile on a CMOS matrix placed 225 mm away from
the prism. A typical reflection profile is also shown in Fig. 1.
The origin of such form of the reflection profile is discussed
in details in the Refs. 19 and 21.
The 1D PC structure used in the experiments is as fol-
lows: substrate/ðHLÞ14H0M/air,whereHis a Ta2O5layer with
a thickness d2¼112:8nm,Lis a SiO2layer with d1¼155:0
nm, H0is a Ta2O5layer with d20¼103:4 nm, and Mis the
final layer of Pd nanoparticles. The prism and the substrate
were made from BK-7 glass. The Ta2O5=SiO2multilayer was
deposited by magnetron sputtering. The initial refractive
indexes (RIs) of optical materials were taken from Ref. 25,
and then they were fitted for the particular deposition through
spectral transition measurements of this multilayer structure.
The RIs of the substrate, Ta2O5,SiO2,andPd were, respec-
tively, at k¼737:7nm: n0¼1:513, n2¼2:076, n1¼1:455,
and n3¼nPd ¼1:9þi4:8.
Two types of palladium nanoparticles were used: 2 and
6 nm. It was expected that 2 nm nanoparticles will uptake the
hydrogen without the a-bphase transformation, while for
6 nm nanoparticles the phase transition would take place.
Both palladium nanoparticles sizes were purchased from
PlasmaChem GmbH, Germany (PL-Pd-HPB2—10 mg and
PL-Pd-HPB6—10 mg) in 10 mg hydrophobic nanopowder
dozes and were dissolved in 99.5% cyclohexane (non-polar
solvent) to a concentration of 0.5 mg/mL.
Samples were prepared identically for both palladium
nanoparticles sizes: first, the PC surface with the Ta2O5layer
on the top was sonicated in ethanol and then in acetone for
10 min each. Next, these precleaned and dried samples were
exposed to UV-ozone (185 and 254 nm) for 30 min. Fresh
nanoparticles solution were spin-coated onto this clean
hydrophilic PC surface. A 100 lLdroplet was dropped onto
the PC surface and allowed to spin at 3000 rpm for 30 s.
Then, the coated slides were attached to the glass prism and
placed into a gas chamber under the flow of dry nitrogen
overnight to remove any residual cyclohexane from the PC
surface. The experimental results are shown in Fig. 2. The
response of the uncoated, bare 1D PC on the injection of
0.5% H2was just a result of the change of the RI of the
external gas medium. At normal conditions, RI of the nitro-
gen is nN2¼1:000297, while RI of the hydrogen is nH2
¼1:000139. Therefore, the change of RI due to injection of
0.5% H2is about Dn’0:8106. From Fig. 2(a) one can
see that the propagation constant qSW changed to approxi-
mately this value, as expected.
In Figs. 2(b) and 2(c), it is seen that the signs of Dqin
response to injection of 0.5% H2differ for the 2 and 6 nm
nanoparticle layers. The response of the continuous 8 nm
thick Pd film at the a-bphase transition is also shown in Fig.
2(d) for comparison. A small hysteresis is present at experi-
ments with Pd of all sizes. From these data one can see that
for 2 nm Pd NPs, where the a-bphase transformation does
not occur, the sign of Dqis negative. While for 6 nm Pd
NPs, where the a-bphase transition takes place, the sign of
Dqis positive. Below we give our interpretation of these
experimental results.
The propagation constant qSW and, therefore, the excita-
tion angle h0of the PC SW may undergo a change due to
two reasons: a change of an “effective thickness” of the layer
of Pd nanoparticles and a change of “effective RI” of the Pd
nanolayer. The general rules of PC SW response on the
changes of a metal film that can be concluded from the dis-
persion relation
26
are (1) if the thickness of the metal nano-
layer increases, the qSW increases and (2) if the imaginary
part of RI of the metal increases (i.e., the nanolayer becomes
“more metallic”), qSW decreases (i.e., qSW shifts to the “light
line”). The signs of these contributions in Dqare opposite
due to the negative sign of the real part of permittivity of a
metal (ReðeM<0)).
The permittivity of a metal eMin the red and infrared
range may be characterized by the Drude model
27
eM¼e1x2
p
x2þicx þeintðxÞ;(1)
ReðeMeintðxÞÞ¼e1x2
p
x2þc2;(2)
ImðeMeintðxÞÞ ¼ cx2
p
xðx2þc2Þ;(3)
FIG. 1. (Color online) Sketch of the experimental setup where changes of
Pd nanoparticle’s thickness and RI are detected through angle interrogation
of a PC SW.
083108-2 Konopsky et al. Appl. Phys. Lett. 100, 083108 (2012)
where xpis a plasma frequency, cis collision frequency of
electrons, e1is the optical constant, and eintðxÞis a fitting
permittivity which reflects the contribution of bounded elec-
tron transitions located in the nearest spectral range. The
plasma frequency, in turn, depends on the density of the free
electrons in the metal Ne
x2
p¼4pNee2
m;(4)
where eand mare the charge and the effective mass of the
electrons, respectively.
There is general agreement that the electrons of hydro-
gen atoms become the shared free electrons in the metal
28
although other interpretations are still discussed. Hereafter,
we accept this “shared free electrons” interpretation (possi-
ble alternatives will be outlined below). In this case, the
plasma frequency of free electrons is changed, while hydro-
gen atoms donate their electrons to a collective metallic elec-
tron band. Therefore, hydrogen uptake by a Pd nanoparticle
in the aphase leads to increase in the electron density of the
Pd nanoparticle, on one hand, and to an increase in scattering
of the electrons in metal on the other hand (i.e., protons of
hydrogen become additional scattering centers for free
electrons in the metal). The first effect increases the plasma
frequency (see Eq. (4)) and resulting in a more negative real
part value of the Pd permittivity (see Eq. (2):ReðePdþHÞ
>ReðePdÞ), while the second effect increases the cand the
value of the imaginary part of Pd permittivity: (see Eq. (3):
ImðePdþHÞ>ImðePdÞ). Both effects lead to an increase in the
value of the imaginary part of palladium RI (i.e., makes the
Pd “more metallic” in the aphase)
ImðnÞ¼Imðffiffie
pÞ¼ ffiffi
2
p
2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ReðeÞ2þImðeÞ2
qReðeÞ
r:(5)
Here we will consider the nanoparticle layer as a planar
metal film with some equivalent “effective” thickness and
“effective” RI. These equivalent layer parameters may be
determinated experimentally (in principle) from optical and
electrical properties of the nanofilm.
29
The non-continuous
metal film may also support modes, depending on its
“effective” thickness and RIs (Refs. 30 and 31). Hereafter
we will interested only in relative changes of this equivalent
layer parameters at hydrogen injection.
As we have mentioned above, it may be shown, from
the dispersion relation of PC SW, that the increase of imagi-
nary part of RI of a metal nanolayer in 1D PC leads to
decrease of PC SW propagation constant q, while the
increase of the “effective thickness” of the metal nanolayer
leads to increase of q. Therefore, if a Pd nanoparticle is in
the aphase, where the thickness increase under hydrogen
uptake is small, the “effective RI” effect dominates and PC
SW propagation constant qdecreases. This is the explanation
of the negative sign of Dqfor 2 nm Pd nanoparticles at H2
injections.
For 6 nm Pd nanoparticles, the effect of the 3.54%
increase on the “effective thickness” (during the a-bphase
transformation) is predominant, and the PC SW propagation
constant qincreases during this transformation. Moreover,
FIG. 2. (Color online) Changes in the
propagation constant DqSW of the PC
SW in response to hydrogen injection
for different experimental arrangements.
083108-3 Konopsky et al. Appl. Phys. Lett. 100, 083108 (2012)
the 11% volume increase results in a decrease in the electron
density in the Pd nanoparticles during the a-bphase transi-
tion, which also leads to an increase in q. This explains the
positive sign for Dqfor 6 nm Pd NPs. Additionally, one may
speculate that a more regular arrangement of hydrogen pro-
tons in the octahedral lattice sites in bphase leads to less
scattering than with a nonregular distribution in the aphase.
So, the presented results may be considered as an addi-
tional plausible argument that hydrogen donates its electrons
to palladium and becomes (at least partially) ionized inside
the metal. The negative sign of Dqfor 2 nm Pd nanopar-
ticles, in this case, is the result of an increase in the density
of the free electrons Ne(and, therefore, more negative
ReðePdÞ) in a nanoparticle. Two possible alternative interpre-
tations seem less probable for the following reasons.
The 1st alternative is that the imaginary part of RI (see
Eq. (5)) increases due to increase of ImðePd Þ, as a result of
the increase in scattering. But the scattering in a 2 nm nano-
particle is already strongly increased due to collision-
induced scattering of conducting electrons at the walls of the
nanoparticle. At optical frequencies, the dampening in nano-
structures with the characteristic dimension Lis
c¼cbulk þtF
L;(6)
where cbulk is the damping constant for the bulk sample and
tFis the electron velocity on the Fermi surface. For a spheri-
cal nanoparticle (NP) (Ref. 32)
Im eNP
MeintðxÞ

x2
p
x3c¼x2
p
x3cbulk þ3tF
4r

;
Im eNP
M

Im ebulk
M

þ3
4
x2
p
x3
tF
r;(7)
where ris the radius of the sphere. Numerical estimation
shows that this addition to the bulk imaginary part is about
60 for 2 nm NPs, while bulk value of ImðePdÞitself is about
18 in this spectral range. So, it is a reasonable assumption
that the influence of the additional scattering on the hydrogen
in 2 nm Pd NPs is relatively small.
The 2nd alternative is that the real part of ReðePdÞ
becomes more negative not as a result of x2
pincreasing, but
rather as a result of decreasing of Reðeint ðxÞÞ in Eq. (2).In
other words, the ReðePdÞis changed due to interaction of
hydrogen with bounded electrons in palladium. This alterna-
tive cannot be simply excluded since the interband transi-
tions in infrared range are rather common in transition
metals and palladium does have an interband transition in
the range of 800-900 nm. Some additional investigations
with different laser wavelengths are needed to clarify this
point. Here we assume that this contribution of bounded
electrons to ReðePdÞat k737 nm is small in comparison
with the contribution of free electrons.
To summarize: we presented an experimental technique
where changes in size and RI of a nanoparticle’s layer are
monitored by PC SWs.
Deposition of nanoparticles on 1D PC, which support
p-polarized SWs on its external surface, permits optically
investigate even such lossy objects as 2 nm Pd NLs. This
technique is able to detect hydrogen uptake in small Pd
nanoparticles and to distinguish between the aphase
response and a-bphase transition upon injection. The nega-
tive sign of the response of aphase presumably points out
that the electrons of hydrogen become shared free electrons
in the palladium after injection.
This work was financially supported by the Science and
Technology Cooperation Programme Switzerland–Russia
and by the Russian Foundation for Fundamental Research.
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083108-4 Konopsky et al. Appl. Phys. Lett. 100, 083108 (2012)

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... Twenty years later, excitation of optical surface waves (SW) in photonic crystals in a Kretschmann-like configuration was first demonstrated [5]. These studies rapidly led to broad use of surface waves based on photonic crystals in ever-widening applications in the field of optical sensors [6][7][8][9][10][11][12][13][14]. This technique benefits much from its unique peculiarities, namely, the existence of both p-and s-polarized surface waves enables to discriminate surface and volume effects. ...
... For this case the photonic crystal can be characterized, for a certain wavelength, by the same refractive index as that of an infinite dielectric, thus forming a symmetric sandwich structure. Exploitation of such structures enabled to excite and use in practice surface plasmons in thin Pd [12,20] and Co (to be submitted) layers as well as blue (at 405 nm) [21] and UV (at 375 nm; to be submitted) plasmons in thin gold layers. Note that if a photonic crystal is not exploited, for all these cases there are no reasons to speak about plasmons at all, because their propagation length is just in the order of a light wavelength. ...
... We assume that n S =1.455, n L =2.076 (Ref. 22, such a photonic crystal has been quite successfully used to support long-range surface plasmons in Pd films [12,20] (15). For the parameter a slightly larger than 1, this ratio defines = −0.333 ...
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Nowadays, unique characteristics of surface electromagnetic waves, particularly, surface plasmons supported by a specially designed photonic crystal find numerous applications. We propose to exploit an evident analogy between such a photonic crystal and a structure with a sine-modulated refractive index. The light propagation inside the latter is described by the famous Mathieu’s differential equation. This application of the Mathieu’s equation can be useful for a design of multilayer structures, and also for fundamental understanding of electromagnetic phenomena in inhomogeneous media.
... A scheme of the Kretschmann-like excitation of PC SMs is presented in Figure 1. In recent years, the PC SMs have been used in ever-widening applications in the fields of optical sensors [8][9][10][11][12][13][14], optical biosensors [15][16][17][18][19][20][21][22][23] and in other fields [24][25][26][27][28][29][30]. To excite the PC SM at any predetermined wavelength and at any predetermined wavevector (i.e., at any predetermined angle in the Kretschmann scheme), it is necessary to calculate the thickness of the last dielectric layer, which depends on the thicknesses of the double layers and their refractive indices (RIs), as well as RI of external environment. ...
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An impedance approach has been implemented to design truncated 1D photonic crystals, sustaining optical surface modes, with any predetermined wavelength and wavevector. The implementation is realized as a free Windows program that calculates both the thicknesses of the double layers and the thickness of the final truncated layer at given refractive indices of the layers. The dispersion of the refractive indices can be given in the form of the Sellmeier/Drude formulas or in the form of a wavelength-n-k table. For mixed layers, the Maxwell Garnett theory can be used. This approach is suitable for studying and visualizing the field distribution inside photonic crystals, dispersion, and other aspects of the designed structures that sustain optical surface modes. Therefore, this program should promote scientific development and implementation of practical applications in this area.
... A multilayer structure in the form [PC/M/air] has been proposed to simplify the implementation of LRSP in practical applications, where the external dielectric is air [14]. This approach was tested with diverse systems, including thin palladium layers (for ultrasensitive hydrogen detection [15][16][17]), thin gold layers in blue spectral range (for nitrogen dioxide detection [18]), and thin ferromagnetic cobalt layers (for magnetoplasmonics [19]), among others (see also [20][21][22][23] for examples of other applications). ...
Article
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A current-driven source of long-range surface plasmons (LRSPs) on a duplex metal nanolayer is reported. Electrical excitation of LRSPs was experimentally observed in a planar structure, where an organic light-emitting film was sandwiched between two metal nanolayers that served as electrodes. To achieve the LRSP propagation in these metal nanolayers at the interface with air, the light-emitting structure was bordered by a one-dimensional photonic crystal (PC) on the other side. The dispersion of the light emitted by such a hybrid PC/organic-light-emitting-diode structure (PC/OLED) comprising two thin metal electrodes was obtained, with a clearly identified LRSP resonance peak.
... Since then, several works on SPRs in Pd have been presented using various ways for excitation: prism, 69,70 grating, 71 and waveguides. 72À74 It is also worth mentioning the work by Konopsky et al., 75 where they used surface waves on a photonic crystal coupled by a prism to study the hydride formation and decomposition in Pd nanoparticles in the sub-10 nm size range and found that the sign of the signal they observe depends on the studied particle size. From this observation, they conclude that no phase transition between metallic and A very practical type of SPR-based hydrogen sensor for remote sensing is based on optical fibers. ...
Conference Paper
In this review we discuss the evolution of surface plasmon resonance and localized surface plasmon resonance based hydrogen sensors. We put particular focus on how they are used to study metal-hydrogen interactions at the nanoscale, both at the ensemble and the single nanoparticle level. Such efforts are motivated by a fundamental interest in understanding the role of nanosizing on metal hydride formation processes. However, nanoplasmonic hydrogen sensors are not only of academic interest but may also find more practical use as all-optical gas detectors in industrial and medical applications, as well in a future hydrogen economy, where hydrogen is used as a carbon free energy carrier.
... The main three ways to couple light into a metal film to excite a SPR mode are: (i) coupling with a prism, (ii) coupling with a grating, or (iii) using an optical waveguide 23 , as summarized in grating 60 and waveguides [61][62][63] . Worth to mention is also the work by Konopsky et al. 64 where they used surface waves on a photonic crystal coupled by a prism to study the hydride formation and decomposition in Pd nanoparticles in the sub-10 nm size range. ...
Article
In this review we discuss the evolution of localized surface plasmon resonance (LSPR) and surface plasmon resonance (SPR) hydrogen sensors based on nanostructured metal hydrides, which has accelerated significantly during the past five years. We put particular focus on how, conceptually, plasmonic resonances can be used to study metal-hydrogen interactions at the nanoscale, both at the ensemble and the single nanoparticle level. Such efforts are motivated by a fundamental interest in understanding the role of nanosizing on metal hydride formation processes in the quest to develop efficient solid-state hydrogen storage materials with fast response times, reasonable thermodynamics and acceptable long-term stability. Therefore a brief introduction to the thermodynamics of metal hydride formation is also given. However, plasmonic hydrogen sensors are not only of academic interest as research tool in materials science but are predicted to find more practical use as all-optical gas detectors in industrial and medical applications, as well as in a future hydrogen economy, where hydrogen is used as a carbon free energy carrier. Therefore, the wide range of different plasmonic hydrogen sensor designs already available is reviewed together with theoretical efforts to understand their fundamentals and optimize their performance in terms of sensitivity. In this context we also highlight important challenges to be addressed in the future to take plasmonic hydrogen sensors form the laboratory to real applications in devices, including poisoning/deactivation of the active materials, sensor liftetime and cross-sensitivity towards other gas species.
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A low power, reusable optical hydrogen sensor using long-range surface plasmon polariton (LRSPP) cladded membrane waveguides is demonstrated. The sensor incorporates a 5 μm wide, 20 nm thick gold stripe embedded in a 160 nm thick free-standing Cytop membrane with a 5 nm thick Pd over-layer. Input and output coupling is achieved with directly integrated broadside grating couplers. The sensor is tested with hydrogen concentrations up to 3% and demonstrates a strong response with an estimated detection limit of 290 ppm, and a response time of 7 s to a 0.6% H2 step - this level of performance is among the best reported for optical H2 sensors. Cycling of the hydrogen exposure shows a significant hysteresis response, however no film deformation or delamination was observed over many cycles. The high stress that is induced in clamped films during hydrogenation is relaxed in due to the film being deposited on the flexible and elastic Cytop membrane. This could result in improved lifetimes for this sensor and increased uptake ability.
Article
The fabrication process for a long-range surface plasmon polariton hydrogen sensor is presented. The device, referred to as the cladded membrane waveguide, features a 5 μm wide and 20 nm thick gold stripe embedded in a 160 nm free standing Cytop membrane. Broadside excitation and output are achieved with integrated grating couplers. Hydrogen sensitivity is provided by an overlaid 5 nm thick palladium patch, which acts as a transduction medium. The device is fabricated by integrating several process techniques including blind through-wafer alignment, optical photolithography, overlaid electron beam lithography, metal lift-off, and through-substrate silicon wet etching. Fabricated results are presented along with a detailed discussion. The devices are characterized optically via a cutback measurement with the measured waveguide attenuation being consistent with simulated values.
Article
Palladium can readily dissociate and absorb hydrogen from the gas phase, making it applicable in hydrogen storage devices, separation membranes, and hydrogenation catalysts. To investigate hydrogen transport properties in Pd on the atomic scale, we derived a ReaxFF interaction potential for Pd/H from an extensive set of quantum data for both bulk and surface properties. Using this potential, we employed a recently developed hybrid grand canonical-Monte Carlo/molecular dynamics (GC-MC/MD) method to derive theoretical hydrogen absorption isotherms in Pd bulk crystals and nanoclusters for hydrogen pressures ranging from 10–1 atm to 10–14 atm, and at temperatures ranging from 300 to 500 K. Analysis of the equilibrated cluster structures reveals the contributing roles of surface, subsurface, and bulk regions during the size-dependent transition between the solid solution α phase and the hydride β phase. Additionally, MD simulations of the dissociative adsorption of hydrogen from the gas phase were conducted to assess size-dependent kinetics of hydride formation in Pd clusters. Hydrogen diffusion coefficients, apparent diffusion barriers, and pre-exponential factors were derived from MD simulations of hydrogen diffusion in bulk Pd. Both the thermodynamic results of the GC-MC/MD method and the kinetic results of the MD simulations are in agreement with experimental values reported in the literature, thus validating the Pd/H interaction potential, and demonstrating the capability of the GC–MC and MD methods for modeling the complex and dynamic phase behavior of hydrogen in Pd bulk and clusters.
Article
Applications that involve the use of hydrogen gas (H2) have an inherent risk in that hydrogen is combustible in air and hence accurate detection of its concentration is critical for safe operation. Long-Range Surface Plasmon Polaritons (LRSPPs) are optical surface waves that are guided along thin metal films or stripes which are symmetrically cladded by a dielectric and have been demonstrated to be highly sensitive for biological and chemical sensing. The sensor presented here consists of a gold (Au) stripe suspended on an ultrathin Cytop membrane. This architecture is referred to as the membrane waveguide and has previously been demonstrated to support LRSPP propagation. Hydrogen sensing is achieved by overlaying a palladium (Pd) patch on a straight waveguide section, which induces a measureable insertion loss change under the presence of hydrogen. The design and optimization of the sensor through finite element method (FEM) simulation will be discussed. This will include the design of the optimal waveguide geometry along with the design of an integrated grating coupler for broadside light coupling. In addition, details on the fabrication process are presented.
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Full-text available
In situ synchrotron x-ray diffraction experiments on bare palladium nanoclusters prepared by inert-gas aggregation and size selected (1.7–6.0 nm) show significant changes in lattice parameter upon hydrogen loading and a narrowing of the miscibility gap, as the cluster size decreases. The results show that the miscibility gap is open for all cluster sizes studied, in contrast to previous literature results from surfactant-encapsulated palladium clusters. We interpret these results by showing that the nature of the surface is critical in the hydrogenation behavior of the nanoclusters.
Chapter
There is always a requirement for refractive index n and extinction coefficient k values for estimating an optical effect, internal reflection in a prism with multilayers, band-edge transmission of a thin semiconductor film on a substrate, and free-carrier reflection effects. This chapter deals with various aspects of optical constants with emphasis on experimental techniques and sample preparation. It collects references and tabulates n and k for a large number of materials of technological and physics interest, so that when required, the numbers over the widest spectral range, ideally x-ray to microwave, in a grid fine enough to yield the number could be easily found by inspection or by a simple interpolation.
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Hydrogen loading on nanometer-size palladium clusters and defect crystals are simulated by the Monte Carlo method. The pressure-composition isotherms are calculated. In contrast with simulations of bulk samples, the clusters show no evidence of a phase transformation. Simulation shows that hydrogen is loaded on the surface first and then the interior of the cluster. This behavior is related to the observed narrowing of the plateau in the phase boundary of the pressure-composition phase diagram of nanocrystalline palladium.
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Palladium (Pd) thin films have been deposited by electron beam evaporation, and exposed to increasing hydrogen pressures. Transmittance spectra in the range of visible light have been measured to obtain from them, by means of a spectral projected gradient method, the wavelength dependence of the dielectric function. The decreasing metallic character of Pd with hydrogen absorption is displayed. This effect is more pronounced when Pd is deposited on metallic substrates, and there is a correlation with an increase in the effective polarization of the core electrons determining the optical dielectric constant value. Another optimization approach is devised to separate the contribution of the free carriers and of the interband transitions to the optical conductivity and to the dielectric function. Very good agreement is found between the optimized parameters characterizing the free carrier contribution and the corresponding values reported in the literature and obtained by independent experimental methods.
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Hydrogen concentration-pressure isotherms of surfactant-stabilized palladium clusters and polymer-embedded palladium clusters with diameters of 2, 3, and 5 nm are measured with the gas sorption method at room temperature. The results show that, compared to bulk palladium, the hydrogen solubility in the α phase of the clusters is enhanced fivefold to tenfold, and the miscibility gap is narrowed. Both results can be explained by assuming that hydrogen occupies the subsurface sites of the palladium clusters. The Pd-H isotherms of all clusters show the existence of hysteresis, even though the formation of misfit dislocations is unfavorable in small clusters. Compared to surfactant-stabilized clusters, the polymer-embedded clusters show slow absorption and desorption kinetics. The absorption kinetics can be described by a diffusion model for the composite polymer-cluster system.
Book
While bits and pieces of the index of refraction n and extinction coefficient k for a given material can be found in several handbooks, the Handbook of Optical Constants of Solids gives for the first time a single set of n and k values over the broadest spectral range (ideally from x-ray to mm-wave region). The critiquers have chosen the numbers for you, based on their own broad experience in the study of optical properties. Whether you need one number at one wavelength or many numbers at many wavelengths, what is available in the literature is condensed down into a single set of numbers. Contributors have decided the best values for n and k. References in each critique allow the reader to go back to the original data to examine and understand where the values have come from Allows the reader to determine if any data in a spectral region needs to be filled in Gives a wide and detailed view of experimental techniques for measuring the optical constants n and k Incorporates and describes crystal structure, space-group symmetry, unit-cell dimensions, number of optic and acoustic modes, frequencies of optic modes, the irreducible representation, band gap, plasma frequency, and static dielectric constant.
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Resistive-type palladium structures for hydrogen sensing remains as a research focus for their simplicity in device construction. We demonstrate that a siloxane self-assembled monolayer placed between a substrate and an evaporated ultrathin Pd film promotes the formation of small Pd nanoclusters and reduces the stiction between the palladium and the substrate. The resulting Pd nanocluster film can detect 2% H2 with a rapid response time of ∼ 70 ms and is sensitive to 25 ppm hydrogen, detectable by a 2% increase in conductance due to the hydrogen-induced palladium lattice expansion.
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The reduction in the length scale of materials to the nanometer range brings about fundamental changes that lead to novel and unusual phenomena, very different from their coarse-grained counterparts. These differences are not only due to the different physical properties of the small-size system but it is also affected by the type of the stabiliser used on these materials.In situ X-ray diffraction (XRD) investigations of the hydrogen absorption behaviour in different nanometer sized palladium samples were performed during loading and unloading. Pressure-lattice parameter isotherms were constructed for three different samples: surfactant stabilised clusters, and two types of polymer stabilised samples (clusters and closed clusters layers sample). The pressure-lattice parameter isotherms for the samples show a narrowed lattice parameter miscibility gap. The closed clusters layers sample shows the smallest lattice parameter expansion values. The effect of the samples morphology on the lattice expansion will be discussed. It will be shown that not only the sample sizes affect the expansion but also the cluster surrounding plays an important rule.
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An experimental arrangement is described for measuring in situ in a nearly continuous way the reflectance R and transmittance T near normal incidence, as well as the electrical DC resistance of thin films throughout their deposition and subsequent heat treatment. It is then possible to determine the effective complex dielectric constant epsilon and the electrical DC resistivity rho as functions of the mass thickness of the deposit and to follow their evolution during film growth. The possibilities of the method are demonstrated by an application to thin films of gold.