Gas adsorption and gas-sensing properties of europium bisphthalocyanine derivative Langmuir-Blodgett thin films

Article abstract of DOI:10.1016/S1386-1425(97)00149-2

In this paper, europium bisphthalocyanine derivative (EuPc′₂) LB films transferred to an interdigital electrode have been prepared at room temperature. The dynamic response of the electrical conductance to Cl₂ gas is presented. At room temperature, the response time of 10-layer LB films of EuPc′₂ is 45 s and the recovery time is 1 min after exposure to a concentration of 50 ppm Cl₂. The reproducibility of the conductivity changes can be improved by using pulsed operation to reduce the exposure time to Cl₂. The change in film conductivity upon exposure of different concentrations of Cl₂ gas has been measured. The lowest detectable concentration of Cl₂ gas is 2 ppm. The maximum conductance change varies linearly with film thickness. The response time constants also increase with film thickness. The interaction between EuPc′₂ and Cl₂ in the LB films was monitored by UV-vis spectroscopy. LB films exposed to Cl₂ gave a red-shifted Q-band which can be predicted from the conductance data.

Full text of DOI:10.1016/S1386-1425(97)00149-2

Spectrochimica Acta Part A 54 (1998) 77–83
Gas adsorption and gas-sensing properties of europium
bisphthalocyanine derivative Langmuir–Blodgett thin films
Bingjie Liang *, Lin Gan, Chunwei Yuan, Yu Wei
National Laboratory of Molecular and Bimolecular Electronics, Southeast Uni6ersity, Nanjing 210096, People’s Republic of China
Received 14 April 1997; accepted 11 June 1997
Abstract
In this paper, europium bisphthalocyanine derivative (EuPc%2) LB films transferred to an interdigital electrode have
been prepared at room temperature. The dynamic response of the electrical conductance to Cl2 gas is presented. At
room temperature, the response time of 10-layer LB films of EuPc%2 is 45 s and the recovery time is 1 min after
exposure to a concentration of 50 ppm Cl2. The reproducibility of the conductivity changes can be improved by using
pulsed operation to reduce the exposure time to Cl2. The change in film conductivity upon exposure of different
concentrations of Cl2 gas has been measured. The lowest detectable concentration of Cl2 gas is 2 ppm. The maximum
conductance change varies linearly with film thickness. The response time constants also increase with film thickness.
The interaction between EuPc%2 and Cl2 in the LB films was monitored by UV-vis spectroscopy. LB films exposed to
Cl2 gave a red-shifted Q-band which can be predicted from the conductance data. © 1998 Elsevier Science B.V.
Keywords: Europium bisphthalocyanine; Langmuir–Blodgett thin films; Gas adsorption; Gas-sensing property;
UV-vis spectrum
1. Introduction
Deposition of thin films by the LB technique is
a highly attractive method for producing well-or-
dered thin films of known thickness. Faster re-
sponse was observed with LB films substituted
phthalocyanine molecules when exposed to NO2
than those reported for vacuum evaporated film
[4]. Surface damage is inherent with high energy
techniques such as sputtering or evaporation.
Therefore, fabrication of molecular layers by the
Langmuir–Blodgett technique at room tempera-
ture is expected to produce better gas-sensitivity.
Metal phthalocyanines have two possible sites
for gas adsorption: one is the central metal itself
and the other is the y-electron system. The chem-
Lanthanide bisphthalocyanine complexes are
well known for their semiconductive properties
[1]. The adsorption of certain gaseous molecules
on the surface of phthalocyanine crystals and thin
solid films produces significant changes in the
electrical properties of these materials. Gas ad-
sorption on LB films of conjugated macrocycles
seems to promote rapid changes in electrical con-
ductivity [2,3].
* Corresponding author. Tel.: +86 025 7712719; fax: +86
025 7712719; e-mail: lbj@seu.edu.cn
1386-1425/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved.
PII S1386-1425(97)00149-2

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B. Liang et al. / Spectrochimica Acta Part A 54 (1998) 77–83
ical properties of the two sites may change consid-
erably and may in turn affect the adsorption
properties of the Pc towards oxidative gases [5].
Cerium bisphthalocyanine [6] and ytterbium bis-
phthalocyanine [7] LB thin films were found to
produce significant change in the electrical con-
ductivity for the vapor of NOx and chlorine,
respectively. H.Y. Wang [8] reported that
phthalocyanine LB films [(C6H13)3SiOSiPcOG-
ePcOH] which had been fabricated into microsen-
sors for oxidizing gases, particularly for halogen
gases had good selectivity when the operating
temperature was controlled at the optimum value
for the gas to be measured.
Fig. 1. The synthesis process of europium bisphthalocyanine
derivative.
In this paper, the electrical response of 10-layer
LB films of Europium bisphthalocyanine deriva-
tive to the presence of Cl2 is reported. The varia-
tion of optical absorbance in the visible region
was also monitored using UV-vis spectroscopy.
spaced 50 mm from the adjacent electrode. The
finger overlap distance was 10 mm. The structure
of gold interdigital electrode is shown in Fig. 2.
These substrates were well washed at first with
detergent, followed with freshly distilled water,
methanol and chloroform using ultrasonic wave
for 15 min each, dried and immediately used.
2. Experimental section
2.1. Synthesis
Due to the intermolecular interaction between
the macrocycles, peripherally unsubstituted metal-
lophthalocyanines are practically insoluble in
common organic solvents. The solubility of ph-
thalocyanines can be increased by introducing
substituents into the periphery of the macrocycle.
These substituents lead to a larger distance be-
tween the inclined stacked phthalocyanine and
enable their solution. In this paper, Eu(III)-octa-
4-(tetrahydrofurfuroxy) phthalocyanine was syn-
thesized. It has good solution in common organic
solvent such as chloroform. The synthesis of solu-
ble EuPc2% is described in Fig. 1.
2.3. Film preparation
Eu bisphthalocyanine derivative was dissolved
in chloroform under ultrasonic waves. 250 ml
chloroform solution (5×10−4 M) was spread
onto the freshly distilled water (ca. pH 6, 20°C).
After complete evaporation of the solvent, the
surface pressure-area of isotherm was measured
by compressing the mobile barrier at a speed of
2×10−3 nm2 mol−1 s−1
.
2.2. Description of the interdigital comb electrode
Since the conductivity of Pc LB films is low, an
interdigital microelectrode is required for reliable
electrical measurements. The substrates used for
conductance measurement of Pc LB films were
glass substrates precovered with gold interdigital
electrodes. The gold interdigital electrode consists
of 50 pairs of finger electrodes of width 50 mm,
Fig. 2. Structure of gold interdigital electrode.

B. Liang et al. / Spectrochimica Acta Part A 54 (1998) 77–83
79
Deposition was Z-type with the substrate being
passed through a clean water surface in one com-
partment and withdrawn through the monolayer
in the other compartment. The LB films were
transferred to the substrate by the vertical dipping
method at a surface pressure 25 mN m−1, up–
down speed is 5 mm min−1
.
For UV-vis measurements, monolayers were
transferred to a Quartz disc that has been cleaned
in chromosulfuric acid maintained at 110°C. After
2 h in the cleaning solution, the slides were copi-
ously rinsed with distilled water and placed in
NaOH solution (pH 11) for 12 h. The resulting
slides were hydrophilic.
2.4. Gas-sensing experiments
The Cl2 used was custom blended with N2 of
ultrahigh purity. Dynamic gas-sensing experi-
ments were conducted using the apparatus.
Before each chlorine test, the entire gas-sensing
system was cleaned with N2. In a typical experi-
ment, the system was purged with the Cl2–N2 gas
mixture for 15 min then the Cl2 gas was switched
off and N2 was turned on for 15 min. The conduc-
tance of the phthalocyanine film was monitored
continuously with an electrometer.
Fig. 3. The y–A isotherm of Europium bisphthalocyanine
derivative at room temperature.
and edge on to the water surface. Since the calcu-
lated area per molecule is larger than the limiting
molecular areas obtained by the experiment, we
suggest that the tilted edge-on phthalocyanine
molecular was organized on the water surface.
3.2. Spectroscopic e6idence of gas adsorption
2.5. Spectroscopic instrumentation
The phthalocyanine compounds exhibit strong
electronic transitions in the visible region (Q-band
at 600–800 nm) and in the near ultraviolet one (B
or Soret band at 300–400 nm) due to y–y*
transition states of Eu symmetry. The absorption
spectrum for a 10-layer Eu bisphthalocyanine LB
film on Quartz disc is illustrated in Fig. 4. The
Q-band splits into two maximum peaks at 690 nm
and 718 nm. Also shoulder peak splits into 620
and 660 nm, respectively. The spectrum is typical
of the spectrum of solid (bulk and thin solid films)
lanthanide bisphthalocyanine derivatives [10].
The lanthanide bisphthalocyanine are sandwich
compounds in which the rare-earth ion occupies a
central position between two parallel but stag-
gered (45°) phthalocyanine ligands. While one of
the phthalocyanine macrocycles is slightly saucer
shaped toward the rare-earth ion, the other is
planar. The 718 nm peak corresponds to the
UV-vis absorption spectra were recorded on a
Shimadzu 2100 UV-vis spectrophotometer.
3. Results and discussion
3.1. Monolayer beha6ior
The isotherm given in Fig. 3 shows that the
monolayer can be compressed to a fully con-
densed close-packed film. The collapsing pressure
of the monolayer is 40 mN m−1. Value for the
average area per molecule obtained by extrapolat-
ing the steeply rising part of the curve to zero
pressure is approximately 0.6 nm2. The area per
molecule calculated by Yuqi Liu et al [9] is ap-
proximately (164.8 A2) if the molecule of LnPc2%
are densely stacked in a face-to-face orientation

80
B. Liang et al. / Spectrochimica Acta Part A 54 (1998) 77–83
planar phthalocyanine y–y* transition, while the
690 nm peak corresponds to saucer shaped ph-
thalocyanine macrocycle. The weak peaks at 620
and 660 nm are due to the existence of a small
amount of associated species in LB films.
The presence of chlorine, Cl2 leads to an irre-
versible oxidation of the EuPc%2 LB films. The
response and recovery curve of the sensor to Cl2
(50 ppm in N2) is shown in Fig. 5. First, Cl2
causes a strong increase of the conductivity. After
the LB films have been exposed in chlorine gas for
15 min and the chlorine is removed, an important
conductivity decrease is observed with a concomi-
š
š
tant transformation into Pc2Eu Cl , as shown
by the optical absorption spectrum. The peak
height and the peak width decreased. The peak
wavelength was a red-shifted by 8 nm which is
characteristic of the ‘oxidized form’. Electrochem-
ical studies of lanthanide bisphthalocyanine films
have also shown that red-shifted Q-band is pro-
duced upon oxidation [11].
Fig. 5. The response and recovery curves to 50 ppm Cl2 in N2
at room temperature.
of the LB film appears to be necessary for maxi-
mum response. The conductivities of the films
increased sharply with time at first, which may be
due to the surface absorption effect, and then
increased slowly, probably due to diffusion of gas
into the bulk. The adsorption and the desorption
processes occurred simultaneously, producing a
dynamic equilibrium and hence saturation in the
conductance values. When the Cl2 gas was re-
moved, the recovery also showed a rapid decrease
followed by a slower decrease of conductance.
After Cl2 was removed, the conductance does not
return to the baseline by 15 min N2 cleaning. This
may be attributed to the strong interaction be-
tween chlorine and the phthalocyanine y-system
and the chemisorption of chlorine in the bulk of
the LB films.
3.3. Gas-sensiti6ity
Since the sensor measurement electrodes are
only in contact with the bottom face of the sens-
ing film, the permeation of the gas into the bulk
Fig. 5 shows the conductivity increased 80%
when the EuPc%2 LB films had been exposed to 50
ppm Cl2 for 45 s and the conductivity decreased
80% when it had been cleaned by N2 for 1 min,
the response and recovery times are 45 s and 1
min.
The reproducibility of the conductivity changes
in an atmosphere containing 50 ppm Cl2 at room
temperature has been studied. Fig. 6(a) shows the
degradation of the sensor response when cycled
Fig. 4. The absorption spectrum of a 10-layer LB film on
Quartz disc (-(- - -)in air; (-) in 50 ppm Cl2.

B. Liang et al. / Spectrochimica Acta Part A 54 (1998) 77–83
81
for 15 min exposure to Cl2 followed by 15 min N2
cleaning. The degradation can be improved by
using pulsed operation to reduce the exposure
time to Cl2 from 15 to 1 min with solenoid valve
and time control. The result is shown in Fig. 6(b).
The conductance can recover the original value
after each test since the LB films had been ex-
posed in chlorine for a shorter time.
A plot of the maximum conductance change
versus film thickness is shown in Fig. 7, which
Fig. 7. The reproducibility of the sensor response using pulsed
operation to reduce the exposure time to Cl2 from 15 to 1 min.
shows a linear relationship. Fig. 8 shows the
conductivity change versus time for films 5, 10
and 15 layers. It was observed that the sensor
response increased with the film thickness.
If the film thickness is small, the contact be-
tween the electrode and the film around the cor-
Fig. 6. The reproducibility of the sensor response when cycled
at 15 min exposure to Cl2 flowed by 15 min N2 cleaning.
Fig. 8. The maximum conductance change vs. film thickness.

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B. Liang et al. / Spectrochimica Acta Part A 54 (1998) 77–83
A 200% increase in conductivity has been ob-
served on the addition of 50 ppm Cl2 to a nitro-
gen atmosphere at room temperature. The
conductivity activation energies were calculated
from conventional Arrhenius plots of temperature
dependence of conductivity:
|=|0 exp(−Ea/kT)
where Ea is an activation energy, assumed be an
excitation energy for single donor carriers, T is in
K, k is Boltzmann’s constant and |0 is a constant
of proportionality.
Then DE=kT ln |2/|1, where |1 is the maxi-
mum conductance of the LB film before doping
and |2 is the conductance of LB film after doping.
DE also can be written as:
DE=E2−E1=hc/u2−hc/u1
Thus
Fig. 9. Response to 50 ppm Cl2 at room temperature with film
thickness of 5, 10 and 15 layers.
u2=(DE/hc+1/u1)−1
=[(−kT ln |2/|1)/hc+1/u1]−1
Du=u2−u1=7.5 nm
ners of the electrodes will be poor, which intro-
duces an extra contact resistance. With increase of
the film thickness, the contact will be improved
and the contact resistance decreases. The sensor
with smaller 2 layers did not show conductivity
change.
At room temperature, the conductivity changes
for sensor when ten layers on exposure to various
concentrations of Cl2 are measured. At low con-
centration levels (up to 80 ppm), the relationship
is linear (Fig. 9). The conductivity change trends
to saturation when the concentration is higher
than 80 ppm. When exposed to Cl2 even at the 2
ppm level, LB films of EuPc%2 produced very fast
response.
We observed that the peak wavelength in-
creased by 8 nm. So, it is possible to predict the
changes in optical absorption of the Pc LB film
from the conductance data.
4. Conclusion
The y-A isotherm shows that the soluble EuPc2%
derivative molecules in a monolayer at the air–
water interface are stacked and inclined. LB films
of the compound EuPc2% transferred to an inter-
digital electrode are sensitive to Cl2. At room
temperature, the response time of the 10-layer LB
films to a concentration of 50 ppm Cl2 is 45 s, and
the recovery time is 1 min. The lowest detectable
concentration of Cl2 gas is 2 ppm. The reproduci-
bility of the conductivity changes shows the
degradation which can be improved by using
pulsed operation to reduce the exposure time to
Cl2 from 15 min to 1 min. The sensor response
increased with the film thickness. The maximum
conductance change varies linearly with film
thickness. The change in film conductivity upon
Chemisorption of a gas capable of inducing
donor–acceptor interaction produces an ionized
state and deforms the energy band structure. Cl2
š
first induces the formation of Pc2Eu and conse-
quently the concentration of charge carriers in-
creases. However, when the concentration of
š
Pc2Eu predominates over that of Pc2Eu, the
conductivity of the thin films must decrease, since
charge transport via electron exchange can no
longer occur:
š
š
Pc2Eu , Pc2Eu “Pc2Eu2+, Pc2Eu

B. Liang et al. / Spectrochimica Acta Part A 54 (1998) 77–83
83
exposure to different concentrations of Cl2 shows
a linear relation between the conductive change and
the Cl2 gas concentration.
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region has been compared in the presence of air and
chlorine by UV-vis spectroscopy. Films exposed to
the 50 ppm Cl2 gave a red-shifted Q-band. The
changes in optical absorption of the material can
be predicted from the conductance data. The peak
height and width decreased in the presence of 50
ppm chlorine.
The lanthanide bisphthalocyanine is a promising
gas-sensitive material. The sensitivity can be im-
proved by searching for a suitable central substitu-
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