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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.
5–9
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.
18–24
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 ffiffiffi
2
p
2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
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.
1
J. Klikovits, E. Napetschnig, M. Schmid, N. Seriani, O. Dubay, G. Kresse,
and P. Varga, Phys. Rev. B 76, 045405 (2007).
2
C. Sachs, A. Pundt, R. Kirchheim, M. Winter, M. Reetz, and D. Fritsch,
Phys. Rev. B 64, 075 408 (2001).
3
F. A. Lewis, The Palladium Hydrogen System (Academic, London, 1967).
4
R. J. Wolf, M. W. Lee, R. C. Davis, P. J. Fay, and J. R. Ray, Phys. Rev. B
48, 12415 (1993).
5
C. Nu¨tzenadel, A. Zu¨ttel, D. Chartouni, G. Schmid, and L. Schlapbach,
Eur. Phys. J. D 8, 245 (2000).
6
M. Suleiman, J. Faupel, C. Borchers, H. Krebs, R. Kirchheim, and A.
Pundt, J. Alloy. Compd. 404, 523 (2005).
7
C. Langhammer, Z. Yuan, I. Zoric, and B. Kasemo, Nano Lett. 6, 833
(2006).
8
M. Khanuja, S. Kala, B. R. Mehta, and F. E. Kruis, Nanotechnology 20,
015502 (2009).
9
C. Langhammer, V. P. Zhdanov, I. Zoric´, and B. Kasemo, Phys. Rev. Lett.
104, 135502 (2010).
10
M. Suleiman, N. M. Jisrawi, O. Dankert, M. T. Reetz, C. Ba¨ htz, R. Kirch-
heim, and A. Pundt, J. Alloys Compd. 356–357, 644 (2003).
11
M. W. Lee, R. J. Wolf, and J. R. Ray, J. Alloys Compd. 231, 343 (1995).
12
B. Ingham, M. F. Toney, S. C. Hendy, T. Cox, D. D. Fong, J. A. Eastman,
P. H. Fuoss, K. J. Stevens, A. Lassesson, S. A. Brown, and M. P. Ryan,
Phys. Rev. B 78, 245408 (2008).
13
A. Pundt, M. Suleiman, C. Bhtz, M. T. Reetz, R. Kirchheim, and N. M.
Jisrawi, Mater. Sci. Eng. B 108, 19 (2004).
14
R. C. Hughes and W. K. Schubert, J. Appl. Phys. 71, 542 (1992).
15
T. Xu, M. Zach, Z. Xiao, D. Rosenmann, U. Welp, W. Kwok, and G. Crab-
tree, Appl. Phys. Lett. 86, 203104 (2005).
16
E. Descrovi, T. Sfez, M. Quaglio, D. Brunazzo, L. Dominici, F. Michel-
otti, H. P. Herzig, O. J. F. Martin, and F. Giorgis, Nano Lett. 10, 2087
(2010).
17
T. Goto, A. V. Baryshev, M. Inoue, A. V. Dorofeenko, A. M. Merzlikin,
A. P. Vinogradov, A. A. Lisyansky, and A. B. Granovsky, Phys. Rev. B
79, 125103 (2009).
18
A. Shinn and W. Robertson, Sens. Actuator B-Chem. 105, 360 (2005).
19
V. N. Konopsky and E. V. Alieva, Phys. Rev. Lett. 97, 253904 (2006).
20
V. N. Konopsky and E. V. Alieva, Anal. Chem. 79, 4729 (2007).
21
V. N. Konopsky and E. V. Alieva, Opt. Lett. 34, 479 (2009).
22
V. N. Konopsky, D. V. Basmanov, E. V. Alieva, D. I. Dolgy, E. D.
Olshansky, S. K. Sekatskii, and G. Dietler, New J. Phys. 11, 063049
(2009).
23
Y. Guo, J. Y. Ye, C. Divin, B. Huang, T. P. Thomas, J. R. Baker, Jr., and
T. B. Norris, Anal. Chem. 82, 5211 (2010).
24
V. N. Konopsky and E. V. Alieva, Biosens. Bioelectron. 25, 1212 (2010).
25
E. D. Palik, Handbook of Optical Constants of Solids (Academic, London,
1985).
26
V. N. Konopsky, New J. Phys. 12, 093006 (2010).
27
W. Vargas, I. Rojas, D. Azofeifa, and N. Clark, Thin Solid Films 496, 189
(2006).
28
F. A. Lewis, Platinum Met. Rev. 15, 21 (1971).
29
P. Gadenne and G. Vuye, J. Phys. E: Sci. Instrum. 10, 733 (1977).
30
F. Yang, G. W. Bradberry, and J. R. Sambles, Phys. Rev. Lett. 66, 2030
(1991).
31
F. Yang, J. R. Sambles, and G. W. Bradberry, Phys. Rev. Lett. 64, 559
(1990).
32
C. Bohren and D. Huffman, Absorption and Scattering of Light by Small
Particles (Wiley & Sons, New-York, 1983).
083108-4 Konopsky et al. Appl. Phys. Lett. 100, 083108 (2012)
