JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY
REVIEW
Metal Oxide Nanoarchitectures
for Environmental Sensing
Oomman K. Varghese and Craig A. Grimes¤
Department of Electrical Engineering and Materials Research Institute,
217 Materials Research Laboratory, Pennsylvania State University,
University Park, Pennsylvania, USA
Metal oxide materials are widely used for gas sensing. Capable of operating at elevated temperatures and in harsh environments, they are mechanically robust and relatively inexpensive and
offer exquisite sensing capabilities, the performance of which is dependent upon the nanoscale
morphology. In this paper we Ž rst review different routes for the fabrication of metal oxide nanoarchitectures useful to sensing applications, including mesoporous thin Ž lms, nanowires, and nanotubes.
Two sensor test cases are then presented. The Ž rst case examines the use of highly uniform
nanoporous Al2 O3 for humidity sensing; we Ž nd that such materials can be successfully used as
a wide-range humidity sensor. The second test case examines the use of TiO2 nanotubes for
hydrogen sensing. Going from a nitrogen atmosphere to one containing 1000 ppm of hydrogen, at
290 ž C, 22-nm-diameter titania nanotubes demonstrate a 104 change in measured resistance with
no measurement hysteresis.
Keywords: Nanotubes, Nanowires, Metal Oxide, Gas Sensing, Hydrogen, Humidity, Sensor.
1. INTRODUCTION
As illustrated by Figure 1, it is necessary to simultaneously solve several design variables to achieve a useful
sensing device. Often these design variables, which certainly include but are not limited to cost, size, and durability, are contradictory in nature. Perhaps a starting point
in considering a sensor platform is the transduction mechanism; do we seek, for example, to detect changes in
electrical impedance, electrical phase, magnetic properties, frequency, elasticity, or mass? Is the material being
used as a sensor selective for the target species or does it
require an e-nose approach? Once a target sensor material
is identiŽ ed, operational issues that must be determined
include sensitivity, dynamic range, resolution, hysteresis,
fatigue, and drift. Materials investigated for sensing utility
may possess one or more favorable attributes but be unusable because of a severe limitation in another. Furthermore, since there appears to be no universally applicable
perfect sensor, each sensing application generally requires
the bottom-up design of a suitable sensor. Hence considerable effort has historically been spent in both broad and
speciŽ c development of sensor technologies, and it is still
¤
Author to whom correspondence should be addressed.
J. Nanosci. Nanotech. 2003, Vol. 3, No. 4
an area of vital importance and interest to the medical,
manufacturing, environmental, and defense/security communities.
Chemical or biological sensors outside of a controlled,
clean environment immediately discover that it is a dirty
world out there, with dust, dirt, diesel fumes, smoke, soot,
biologicals, and what have you ready to foul and degrade
operational capabilities. Not only do sensors get dirty,
the more sensitive they are the more susceptible they are
to insult and fouling. Although we seek to develop selfcleaning sensors, our immediate interest lies in developing exquisitely sensitive gas-sensing materials inexpensive
enough to be used on a disposable basis. Metal oxide
materials, such as SnO2 , Al2 O3 , and TiO2 , can be had
at relatively low cost and have long been recognized for
their outstanding gas-sensing properties. Furthermore, it is
generally now recognized that nanoscale control of metal
oxide surface morphologies permits signiŽ cant enhancement of gas-sensing properties. Hence we have focused
our efforts, as described here, at investigating and controlling the gas-sensing properties of metal oxide nanoarchitectures to successfully create an exquisitely sensitive
gas sensor that, when needed, can readily and practically
be used on a disposable basis.
© 2003 by American ScientiŽ c Publishers 1533-4880/2003/04/277/293/$17.00+.25
doi:10.1166/jnn.2003.158
277
Varghese and Grimes/Metal Oxide for Sensing
REVIEW
Stimulus
(s)
Electrical Signal
S(s)
• Materials Design
-useful transduction mechanism
-chemistry
-nano to microstructure
• Device Design
• Circuit Design
• Signal Processing
Fig. 1. Schematic drawing of design variables inherent in the fabrication of a useful sensor.
2. FABRICATION OF METAL OXIDE
NANOARCHITECTURES
2.1. Mesoporous Thin Films
Nanoarchitectured thin Ž lms are of considerable interest
for applications including photocatalysis and sensing and
as templates for cell growth.1–6 Apart from the conventional top-down engineering approach of translating a constructed pattern onto a substrate by lithography7 or soft
lithography,8 self-assembly and self-organization of materials offer a rapid fabrication route at low cost. Considerable effort has focused on the use of a template around
which the material of interest is assembled. Depending on the required pore size, block copolymers,91 10
latex spheres,11–13 water-in-oil emulsions,14 polystyrene
particles,151 16 colloidal crystals,17–22 and bioskeletons23–25
have been used as templates. Problems associated with
template-assisted fabrication of porous structures include
preparation of a high-quality template, complete Ž lling of
the voids in the template, and minimization of shrinkage
upon template removal. Since any of these factors can
J. Nanosci. Nanotech. 2003, 3, 277–293
in uence the Ž nal quality of the porous structure, all three
requirements must be fulŽ lled at the same time.
A nontemplate method for the synthesis of ordered
micrometer-sized honeycomb structures by self-assembly
of block copolymers was reported by Francois et al.26–28
and Jenekhe and co-workers.29 Shimomura et al.301 31
helped pioneer efforts to fabricate patterned thin Ž lms
by self-assembly. An organic chloroform solution with
amphiphiles containing metal acetylacetonates or alkoxides, cast at high atmospheric humidity, was found to
form a closely packed layer of water droplets on top
of the organic solvent, with the water droplets acting as
a template;301 31 after evaporation of the chloroform and
water, a honeycomb structure remains. Finally the pyrolysis of the metal alkoxide Ž lm leads to the formation
of microporous metal oxide such as anatase, with pore
size and wall thickness of the resulting Ž lm controlled
by solution concentration and ambient humidity level. In
a similar vein Nishikawa and co-workers32 reported the
fabrication of porous Ž lms by casting of a dilute solution of amphiphillic polymers onto solid substrates at high
humidity levels.
Several sol-gel methods have been employed to fabricate patterned TiO2 Ž lms, typically by the incorporation of organic polymers with the precursor solution to
obtain precisely controlled macroscopic structures. Tatsuma et al.,15 for example, reported the fabrication of
microporous TiO2 Ž lms prepared with the use of a twodimensional array of polystyrene microspheres as a template. A TiO2 aqueous sol was introduced into the gap
between the polystyrene spheres; upon the evaporation
of water the remaining Ž lms were calcined to incinerate
the polystyrene particles, leaving a porous TiO2 Ž lm.
In an analogous approach, Kajihara et al.33 reported the
Oomman K. Varghese received his Ph.D. degree in physics from the Indian Institute of Technology, Delhi, India, in
2001. Since graduation he has worked as a postdoctoral fellow in the research group of Professor Grimes, Department
of Electrical Engineering and the Materials Science and Engineering Department, the Pennsylvania State University. His
Ž elds of interest include gas sensors based on metal oxide nanodimensional structures and carbon nanotubes, impedance
spectroscopy and transmission electron microscopy for materials characterization, grain boundary space charge segregation
effects in binary oxides, and sol-gel synthesis of metal oxide thin Ž lms.
Craig A. Grimes received B.S. degrees in electrical engineering and physics from the Pennsylvania State University in
1984, and the M.S. and Ph.D. degrees in electrical engineering from the University of Texas at Austin in 1986 and 1990,
respectively. He was employed by Lockheed Research Laboratories from 1990 to 1994. From 1994 to 2001 Dr. Grimes
was a member of the Electrical and Computer Engineering Department at the University of Kentucky, where he was
the Frank J. Derbyshire Professor. He is currently an associate professor at the Pennsylvania State University in the
Department of Electrical Engineering, and Materials Science and Engineering. His research interests include remote query
chemical and environmental sensors, nanodimensional metal-oxide thin-Ž lm architectures for sensing and biotemplating,
propagation and control of electromagnetic energy, and carbon nanotube-based electronic devices. He has contributed over
120 archival journal publications and seven book chapters and is co-author of the book The Electromagnetic Origin of
Quantum Theory and Light, published by the World ScientiŽ c Publishing Company (2002). He is North American editor
of Sensors and an editorial board member of IEEE Transactions on Magnetics.
278
J. Nanosci. Nanotech. 2003, 3, 277–293
the original 33.85 ml sol; the pH of the resulting solution
was approximately 0.5. Topology formation is found to be
a function of (1) ambient humidity, (2) atmosphere  ow
velocity during drying, (3) sol concentration, and (4) sol
pH. Figure 2a–c illustrates the effect of drying rate on the
structure; Figure 2a shows the fastest drying rate and Figure 2c the slowest (the dark areas are pores; i.e., absence
of Ž lm). In the initial stages of the Ž lm growth process the
condensed droplets grow as isolated objects without interaction between neighbors, as seen in Figure 2a. Longer
drying times permit the drops to begin to coalesce, as seen
in Figure 2b, with further coalescence seen in Figure 2c.
Breath Ž gures36–48 are formed when a liquid surface is
REVIEW
fabrication of porous TiO2 Ž lms with the use of an
alkoxide-based sol-gel containing polyethylene glycol,
which served as the template, into which the substrates
were dip coated.
A disadvantage of using a template is that the dominant length scale of the resulting porous structure is
Ž xed by the template size; therefore dynamic control of
the length scale becomes almost impossible. Our interest lies in the fabrication of nanoporous Ž lms without the use of a template; for example, we recently
reported the fabrication of metal oxide Ž lms with magnetically modulated nanodimensional porosity.34 The absence
of a template makes the ambient humidity level and
the sol pH key process variables for the fabrication of
controlled structures.35 We describe here the fabrication
of TiO2 mesoporous structures via sol-gel breath Ž gure
formation,36–48 as well as thermocapillary and surface
tension-driven Benard-Marangoni convection.49–56
The overall hydrolysis and condensation reaction for
titanium isopropoxide, the sol-gel precursor for TiO2 , can
be represented as Ti(OC3 H7 )4 C 2H2 O ! TiO2 C
4HOC3 H7 . The condensation reaction leads to the formation of colloidal particles, which can be polymeric or
particulate, depending on the type of precursors and pH
of the sol. Colloidal particle aggregation may be inhibited by the formation of surface charge developed by
either preferential dissociation of one of the lattice ions
of the sol particle or preferential adsorption of charged
species from solution. The surface charge is formed by
either protonation (Ti-OH C HC ! Ti-OHC2 5 or deprotonation (Ti-OH C OHƒ ! Ti-Oƒ C H2 O) of the Ti-OH
bonds. The pH at which the surface is electrically neutral
is called the point of zero charge (PZC). The surface is
negatively charged at a pH > PZC and positively charged
at pH < PZC. For TiO2 the PZC varies at values at least
between 5.2 (Ref. 57) and 5.5 (Ref. 58).
In a nitrogen environment the precursor titanium
tetraisopropoxide (TTIP) was dissolved in isopropanol,
to which deionized water and then nitric acid were
added. The reagents used in the experiment, namely TTIP
(99.999%), isopropanol (99.5%), and nitric acid (70%
redistilled), were procured from Aldrich. The solution was
continuously stirred for 2 h and then stored in a nitrogen
environment. In a typical preparation of 0.1 M TiO2 sol,
1 ml of TTIP, 0.05 ml of HNO3 (70% distilled), 0.1 ml of
deionized water, and 32.7 ml of isopropanol were used.
The Ž lms were deposited by either dip coating of the
glass or silicon substrate, or simply by placing a droplet
of solution on a clean substrate. The ambient humidity
in which the TiO2 Ž lms dried was controlled by passing
nitrogen through a room-temperature bubbler. The humidity and temperature of the chamber were monitored with a
digital hydrometer. Upon drying, all Ž lms were annealed
at 100 ž C for 1 h in a nitrogen environment.
A 0.01 M sol was modiŽ ed by a 1:2 nitric acid/deionized
water solution, with 15 Œl of acid/water solution added to
Varghese and Grimes/Metal Oxide for Sensing
Fig. 2. Drying-time-dependent FE-SEM images of a thin Ž lm
deposited from a 0.1 M TiO2 sol, 1 ml of TTIP, 0.05 ml of HNO3 (70%
distilled), 0.1 ml of deionized water, and 32.7 ml of isopropanol, to
which 15 Œl of a 1:2 nitric acid/deionized water solution was added.
(a) the fastest drying region, and (c) the slowest drying region where
the liquid has coalesced into drops.
279
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
REVIEW
brought into contact with moist air; the solvent vapor pressure and air velocity across the surface drive solvent evaporation, rapidly cooling the surface. This in turn facilitates the nucleation and growth of water droplets from the
atmosphere.36–41 The temperature difference between the
liquid surface and air results in thermocapillary convection within the liquid that acts to stabilize the condensing
water droplets on or at the solution surface.43–48 Air  ow
across the surface, coupled with surface convection currents, drives the ordering of water droplets into hexagonal
arrays.44–46 Once the surface is completely covered with
water droplets the temperature difference between the surface and the droplets diminishes, and the droplets, being
denser than the solvent, sink into the solution deŽ ning the
residual structure. Figure 3 shows the effect of sol pH,
showing the pH-dependent variation in the resulting structure achieved.
Benard-Marangoni convection typically results from a
vertical temperature gradient due to solvent evaporation
from a thin liquid Ž lm.46–54 Preferential solvent evaporation removes heat from the Ž lm surface, resulting in
Benard-Marangoni convection50–56 across the  uid layer,
with the suspension welling up in the center of a convection cell and then  owing back down the cell boundary. As
evaporation proceeds, the colloidal suspension becomes
more concentrated, changing the convection characteristics and, in turn, deŽ ning the ultimate Ž lm structure.
Well-deŽ ned hexagonal, pentagonal, or square patterns
are expected from an ideal time-independent BenardMarangoni convection50 in a homogeneous liquid, whereas
the cell patterns of our Ž lms seen in Figure 2 are typically
irregular. This deviation of structure from the theoretical
ideal may be the result of time-dependent convection  ow
through changes in sol viscosity with preferential evaporation of the propanol solvent, leading to coupled thermosolutal Benard-Marangoni convection51 in the nonhomogeneous  uid.
2.2. Nanotube, Nanopore, and Nanowire
Fabrication via Anodization of Al and Ti
Highly ordered nanoporous alumina Ž lms (see Fig. 4a
and b) are made througha two-step anodizationprocess.59–61
The aluminum substrate is Ž rst anodized in an oxalic or
sulfuric acid solution. The anodization is stopped after a
few microns of aluminum are consumed, and the porous
alumina Ž lm is removed through etching. The etchant, a
mixture of chromic acid and phosphoric acid, is highly
selective, attacking alumina much faster than aluminum.
The remaining aluminum is dimpled, with the dimples
serving as a uniform seed layer upon which a highly uniform porous layer can then be achieved through a second anodization step at the same voltage. Alumina templates are widely used as templates for the fabrication of
nanowires. Figure 4c shows a TiO2 nanowire mat, fabricated by Ž lling pores of an alumina membrane via sol
280
Fig. 3. Images taken of a dried 0.1 M TiO2 sol, 1 ml of TTIP, 0.05 ml
of HNO3 (70% distilled), 0.1 ml of deionized water, and 32.7 ml of
isopropanol, to which varying amounts of a 1:2 nitric acid/deionized
water solution were added to vary pH values. (a) pH 0.17. (b) pH 0.29.
(c) pH 0.37.
gel, letting the Ž lm dry, and then subsequently removing
the alumina template by a sodium hydroxide etch.
Anodization has also been used to fabricate TiO2 nanotube arrays,62 with pore size linearly proportional to
anodization voltage (see Fig. 5). The nanotube array in
Figure 5 was fabricated by anodization of titanium at 20 V
in 0.5% wt HF solution for 20 min, resulting in a wellaligned titanium oxide nanotube array with an approximate average tube diameter of 60 nm and a tube length
of 400 nm. Diameters of fabricated tubes have ranged in
size from 25 nm to 65 nm.
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
REVIEW
Fig. 5.
TiO2 nanotubes made by anodization of titanium.
thermal stability, mechanical strength, and quick response.
However, ceramic humidity sensors still suffer from insufŽ cient sensitivity over wide humidity ranges, as well as
lack of reversibility and drift in base resistance with time
because of water molecule chemisorption.
The humidity-sensing properties of alumina, discovered
almost 50 years ago,83–94 are based upon ionic conduction; the presence of an adsorbed layer of water at the
surface reduces the total sensor impedance because of the
increase in the ionic conductivity, as well as capacitance
due to the high dielectric constant of water. The humiditysensing characteristics of a given sensing element depend
upon the material used, the method of preparation, and
the resulting surface topology. Porous alumina is usually
preferred for sensing applications because of the large
surface area available for water adsorption.
An illustration of water adsorption by a surface,
redrawn from Ref. 95, is shown in Figure 6. Figure
6a shows the adsorption of a water molecule on a
clean alumina surface. Physisorption of water initially
occurs on an active surface site and forms an adsorption
Fig. 4. (a) The surface of a two-step anodized alumina membrane (the
 akes seen in the image are the “dust” associated with tissue paper).
(b) An image of a cleaved sample showing the pore channels. (c) TiO2
nanowires made by Ž lling an alumina template via sol-gel, then subsequently removing the template by a sodium hydroxide etch, resulting in
a TiO2 nanowire mat.
3. EXPERIMENTAL RESULTS:
APPLICATION TO SENSING
3.1. The Water Vapor Sensing Performance
of Highly Ordered Nanoporous
Alumina Films
Humidity sensors have attracted considerable attention
over many years because of their great importance
in applications ranging from monitoring food quality to meteorological studies.63–65 Ceramic humidity
sensors651 71–82 are commercially available and offer major
advantages, with high resistance to chemical attack,
Fig. 6. Schematic illustration of interaction of a water molecule with
an alumina surface, redrawn from Ref. 95.
281
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
Adsorbed Water Layer
O
O
H
O
H
O
H+
H
H
H
O
H
H+ H
O
O
H
O
H
O
H+
H
H
H
O
H+
O
H
O
H
O
H
O
H+
H
H
H
O
H+ H
O
O
H
O
H
O
H+
H
H
H
O
H+
H
O
O
H
O
H
O
H+
H
H
H
O
H+ H
O
H
O
H
H
H
H
H
H+
O
O
H
O
H
O
H
O
H
H
O
H
O
H
H+
O
O
H+
H
O
H
O
H+
H
O
O
H
O
H
O
H
O
H+ H
H
O
H
H +
H+
O
H
O
REVIEW
H+ H
O
H +
H
O
H
O
H+
H
O
H
H
Capillary
Condensation
Capillary
Condensation
H+
H
H+
H
O
O
H
Alumina
H+
H
H+
H
O
H
O
H
O
H
Adsorption
O
H
H
H
H
O
O
O
O
Aluminium
Fig. 7. Schematic illustration of buildup of adsorbed water layers,
redrawn from Ref. 96: building of a water layer upon an alumina surface.
complex (Fig. 6a and b). The negatively charged oxygen
of the water molecule becomes attached to the positively
charged cation site, and one of the positively charged
hydrogen atoms becomes attached to the anionic site by
electrostatic attraction. Eventually, they are converted to
two chemisorbed hydroxyl (OH) groups (Fig. 6c). An
increase in humidity makes the water molecules physisorb
to this chemisorbed hydroxyl layer. The effectiveness of
physisorption depends upon the cation charge complexes
(from alumina or the surface impurities) and the hydroxyl
ions on the surface of the alumina. The physisorption
process is facilitated by higher surface charge densities.
After hydroxyl formation, the next water molecule will
be physisorbed through hydrogen double bonds on the
two neighbouring hydroxyl groups (Fig. 6d), and a proton
may be transferred from a hydroxyl group to the water
molecule to form a H3 OC ion. At higher humidity levels,
each water molecule will be singly bonded to a hydroxyl
group, as illustrated in Figure 7 (redrawn from [96]), and
proton hopping between adjacent water molecules in the
continuous water layer takes place. The conduction process in this case occurs by attachment of a proton to a
water molecule, thereby forming a hydronium ion; the
hydronium ion releases another proton to a second water
molecule that accepts this proton while releasing a third
proton, and so on through the liquid. Hence the dominant
charge carrier in a high-moisture atmosphere is the HC
(proton). The concentration of HC increases with increasing humidity, and it moves freely through the water-like
layer.
In the case of porous alumina, capillary condensation
can take place in pores with a radius up to rK at a particular relative humidity and temperature, which is given by
Kelvin’s relation,
rK D
2ƒM
RT ln4Ps =P5
Here P is the water vapor pressure, Ps is the water vapor
pressure at saturation, ƒ is the surface tension, R is the
universal gas constant, T is the temperature in Kelvin,
282
Fig. 8. Schematic representation of water vapor interaction with
nanoporous alumina illustrating some, but not all,97 of the multiple factors inherent in determining the response.
and  and M are, respectively, the density and molecular
weight of water.
According this relation, the condensation occurs inside
small pores at lower humidity levels as illustrated in
Figure 8, which is an illustrative drawing presenting
the different water-surface interaction mechanisms. (Note:
Figure 8 is not meant to be an exhaustive consideration of all possible effects; for example, the drawing
does not address effects associated with capillary surface
curvature.97 )
This condensed water reduces the impedance of alumina considerably. No condensation occurs inside pores
of radius greater than rK at a particular humidity level (see
the largest pore shown in Fig. 8). In such pores, as in the
case of a plane surface, it is the number of physisorbed
layers that decides the impedance.
Since capillary condensation enhances the sensing
capabilities of a material, pore size distribution has been
widely considered to be an important parameter in determining the sensitivity in a particular humidity range.941 98
However, the results of our study on uniform nanoporous
alumina Ž lms, presented below, show that an easily built
and highly reproducible wide-range humidity sensor can
be achieved with nanodimensional pores of a narrow size
distribution made by anodization of aluminum.59–611 99
Starting from adhesive-backed aluminum tape (99%
pure) purchased from Tesa Tape Inc.100 and subsequently
anodized, sensors SO50, SO30, and DO15 were prepared
under the conditions given in Table I. Per sample notation, the Ž rst letter denotes a single S or double D step
anodization process; the second letter denotes the use of
Table I. Anodization conditions used for making nanoporous alumina
Ž lms.
Sample
Anodizing
voltage
(v)
Duration of
anodization
(min)
No. of
anodization
stages
SO50
SO30
DO15
50
30
15
120
120
120
1
1
2
The electrolyte used in all cases was 2 wt% oxalic acid.
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
% of pores
30
20
10
0
37.5
40
42.5
45
47.5
50
Pore diameter (nm)
(a)
% of pores
30
20
10
0
25
32.5
37.5
42.5
47.5
52.5
Pore diameter (nm)
(b)
DO15
25
% of pores
20
15
10
5
0
17.5
20
22.5
25
27.5
30
32.5
37.5
Pore diameter (nm)
(c)
Fig. 9. Pore size distribution of the tested sensors. (a) SO50. (b) SO30.
(c) DO15.
REVIEW
oxalic acid (O) in the anodization bath; the two-digit number denotes the anodization voltage. It was observed that
single-step anodization results in disordered pore structures at low voltages. Hence a double-step process was
used to make sample DO15. These samples were initially
anodized under the conditions given in Table I. The samples were then immersed in an aqueous solution containing 1.8 wt% chromic acid and 4 wt% phosphoric acid for
12 h to remove the initial nanoporous layer while keeping the nonporous bottom layer of alumina intact. The
samples were rinsed in distilled water and again anodized
under the same conditions used for the Ž rst anodization
step. Estimates of pore size distribution were determined
from FE-SEM images and are shown in Figure 9a–c
as a size distribution histograph. Samples SO50, SO30,
and DO15 have, respectively, average pore diameters of
Fig. 10. Digital image of an impedance-based interdigital nanoporous
alumina humidity sensor.
45.2 nm, 38.4 nm, and 13.6 nm, with a corresponding
standard deviation of 3.4 nm, 7.8 nm, and 4.9 nm. All
samples have a predominantly ordered pore structure.
Immediately after anodization all samples were thoroughly washed in distilled water and then dried in a nitrogen atmosphere at 100 ž C. An interdigital capacitor pattern with dimensions of 1.2 cm £ 1.2 cm was formed on
the surface of the alumina Ž lms by the evaporation of gold
(thickness º 250 nm) through a mask. A digital camera
image of a typical sensor is shown in Figure 10. To create
the necessary ambient humidity, argon passed through a
mass  ow controller (MFC) was bubbled through a bottle containing deionized water and then mixed with dry
argon coming from another MFC in appropriate ratios
before being passed to the test chamber. A constant total
 ow was maintained throughout the experiment. Chamber humidity was monitored with the humidity probe
of a Keithley 6517A electrometer, and all measurements
were carried out at room temperature (23 ž C). Sensor
impedance was measured over the frequency range 5 Hz
to 13 MHz with a computer-controlled Hewlett Packard
impedance analyzer (4192A) Ž tted with an impedance test
Ž xture (Agilent 16034E). A signal amplitude of 90 mV
was used for all measurements. Electrical contact was
made between the sensor under test and the impedance
analyzer test Ž xture with 42-gauge 5-cm-long jumpers,
attached to the interdigital electrodes with silver paste.
The contacts were subsequently annealed at 100 ž C for
2 h to cure the silver paste.
Figure 11 shows the measured 5-kHz sensor impedance
of the different sensors as a function of humidity. The
humidity-sensitive region of DO15 is ¹45% to 95% RH.
Sensors SO30 and SO50 become sensitive to humidity at
approximately 65% and 75% RH, respectively. A smaller
pore size increases the range of humidity values over
which the sensor is responsive and increases the sensitivity at lower humidity levels. Therefore an optimal pore
size can be selected for the operating region of interest.
283
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
SO50
6
10
| Z| (W)
| Z| (W)
REVIEW
5
10
Z (SO30)
4
10
Z(SO50)
Z(DO15)
106
105
104
3
10
5kHz
10kHz
100kHz
1MHz
107
0
20
40
60
80
100
3
10
0
RH (%)
20
40
60
80
100
RH (%)
Fig. 11. Measured 5-kHz impedance of sensors over different humidity
levels.
(a)
SO30
284
5kHz
10kHz
100kHz
1MHz
7
10
6
| Z| (W)
10
5
10
4
10
3
10
0
20
40
60
80
100
RH (%)
(b)
DO15
6
10
5kHz
10kHz
100kHz
1MHz
5
| Z| (W)
10
4
10
3
10
2
10
0
20
40
60
80
100
RH (%)
(c)
Fig. 12. Variation of sensor impedance with humidity and measurement frequency. (a) SO50. (b) SO30. (c) DO15.
7.8
45% RH
45% RH
45% RH
7.6
| Z| (x105 W)
Figure 12 shows the response of the different sensors
to humidity at different measurement frequencies. It can
be seen from the Ž gure that as the frequency decreases
the width of the sensitive region increases and the point at
which the sensor becomes responsive shifts to lower relative humidity values. For a given pore size, the humiditysensitive region of operation can be selected by frequency
tuning.
The response time, deŽ ned as the time needed to reach
90% of the Ž nal signal for a given relative humidity,
and the recovery time, deŽ ned as the time taken for
the signal to come to within 10% of the initial value,
were determined by alternately exposing the sensors to
a 2% and a 45% RH ambient with impedance measured
at 5 kHz. A typical response/recovery graph of sensor
SO50 is shown in Figure 13. All sensors were completely reversible, regaining their original impedance values even after repeated exposure to high humidity levels.
The response time and recovery times corresponding to
different pore diameters are shown in Figure 14. The minimum response/recovery times were observed for SO50,
the sample with the largest pore size.
The impedance spectra of the sensors were taken over
a range of humidity levels. The total impedance was
resolved into real (Z0 ) and imaginary (Z00 ) parts, and ColeCole impedance plots were constructed. Figure 15a–c
shows, respectively, the Cole-Cole plot of sensors SO50,
SO30, and DO15. The equivalent circuit models shown in
Figure 16 were used to Ž t the experimental data. The Ž tting was accomplished with the complex nonlinear leastsquare-Ž tting program.101 The dots in the plots represent experimental data, and lines represent the equivalent circuit model Ž t. The equivalent circuit, denoted (a)
or (b) with reference to Figure 16, used for Ž tting is
denoted near the corresponding curves in Figure 15. The
equivalent circuit models are different from the empirical model suggested by Falk et al.102 for porous alumina
sensors fabricated by anodization of aluminum thin Ž lms.
In the equivalent circuit models R1 and R2 represent two
frequency-independent resistors in parallel with dispersive frequency-dependent capacitors, Cn1 4—5 and Cn2 4—5,
respectively.103 These non-Debye capacitances can be
7.4
7.2
7
2% RH
2% RH
6.8
0
200
400
600
800
Time (Sec.)
Fig. 13. Response of sensor SO50 upon repeated cycling between relative humidity environments of 2% and 45%.
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
35% (a) 55% (a)
93% (b)
1.2
45
SO50
65% (a)
40
1
SO30
35
Z" (MW)
DO15
30
SO50
25
20
15
REVIEW
Response/Recovery
Time (Sec)
50
81% (b)
0.8
0.6
75% (b)
0.4
10
20
25
30
35
40
45
0.2
50
Pore diameter (nm)
0
0
Fig. 14. Dependence of response time and recovery time on pore size.
0.4
0.6
0.8
1
1.2
Z’ (MW)
(a)
1.4
SO30
35% (a)
1.2
55% (a)
81% (b)
1
Z" (MW)
93% (b)
75% (b)
0.8
0.6
0.4
65% (b)
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Z’ (MW)
(b)
1.2 10 6
17% (a)
27% (a)
DO15
6
1 10
8 105
Z" (W)
expressed in a general form as Cn 4—5 D Bn 4i—5nƒ1 , where
Bn is a constant for a given set of experimental conditions
and n takes a value between 0 and 1 according to the local
microscopic environment through which charge transport
takes place.751 1021 103 Bn behaves as an ideal frequencyindependent capacitor as n approaches 1, and as an ideal
conductor when the value of n approaches 0.101
By examination of Figure 15a, SO50, it can be seen
that at low humidity levels the Cole-Cole plot describes
an arc with a very large radius of curvature. As the humidity increases, the radius decreases, and at an RH of about
80% the complete semicircle comes inside the measurable impedance range. However, at the same humidity
level a spur appears in the low-frequency region that distorts the semicircle. The spur, which can be viewed as
part of a second semicircle, dominates at higher humidity levels, while the semicircle described by the highfrequency impedance values diminishes in size. This
behavior is similar for all sensors. For sensors SO50,
SO30, and DO15, respectively, the RH at which the
high-frequency semicircle comes completely within the
measurable impedance range is º75%, 65%, and 35%;
these values roughly match the in ection points seen in
Figure 11.
The fundamental mechanism that enables ceramic sensors to sense humidity is the physisorption of water
molecules on an initially chemisorbed layer of hydroxyl
ions.651 96 The chemisorbed hydroxyl ions enhance the
electrical conductivity of the sensor either by donating
electrons to the conduction band of the base material
or through proton hopping between adjacent hydroxyl
groups upon the application of an electric Ž eld. The
process of chemisorption occurs at very low humidity
levels and is unaffected by further changes in humidity. However, an increase in humidity makes the water
molecules physisorb to this hydroxyl layer. The effectiveness of physisorption depends upon the cation charge
complexes (from the material or the impurities) and the
hydroxyl ions (water molecules present at the surface of
the base material).96 The physisorption process is facilitated by higher surface charge densities. During formation
of the Ž rst physisorbed layer, a water molecule becomes
attached to two neighboring hydroxyl groups through
0.2
6 105
55% (b) 45% (b)
35% (b)
93% (b)
4 105
2 105
0
0
2 105
4 10 5
6 10 5
8 10 5
1 10 6 1.2 10 6
Z’ (W)
(c)
Fig. 15. Cole-Cole plot of the sensors at different humidity levels.
(a) SO50. (b) SO30. (c) DO15. The points represent the experimental
data, and lines represent the Ž t with the equivalent circuit model given
in Figure 9. The RH values at which the data were collected and the
equivalent circuit used for Ž tting each curve (given as (a) or (b), denoting the circuit of Figure 17a and b, respectively) are shown adjacent to
each curve.
hydrogen double bonds, and a proton may be transferred
from a hydroxyl group to the water molecule to form a
H3 OC ion.
When the physisorption occurs within less than a mono
layer, that is, when clusters of physisorbed molecules
285
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
REVIEW
Cn1 (w)
8
10
R1 (W)
6
R1
(a)
Cn1 (w)
Cn2 (w)
10
SO50
SO30
DO15
4
10
2
10
0
20
40
60
80
100
RH (%)
R1
R2
Fig. 17. Humidity dependence of resistance R1 of different sensors
obtained by Ž tting the equivalent circuit model with experimental data.
(b)
are present at the surface, it is thought that H3 OC diffusion within hydroxyl groups and proton transfer between
adjacent water molecules clusters take place.96 However,
Khanna and Nahar104–106 refute this theory. They argue
that the proton diffusion is less probable in a surface
that contains hydroxyl ions and clusters of physisorbed
molecules and hence cannot account for the conductivity of porous alumina at an RH less than 40%, that is,
before the Ž rst monolayer is formed. Instead they suggested a phonon-assisted electron tunneling mechanism at
low humidity levels in which electron tunneling occurs
from one water molecule to another as enhanced by the
surface anions of the alumina sensors.
At higher humidity levels, the number of physisorbed
layers increases, allowing each water molecule to be
singly bonded to a hydroxyl group, and proton hopping between adjacent water molecules in the continuous water layer takes place. The conduction process is
the same as that of pure water and is called the Grotthuss chain reaction.96 The dominant charge carrier in
a high-humidity environment is therefore HC ions (protons). The concentration of HC increases with increasing humidity, and HC move freely through the water-like
layer. According to the theory of Khanna and Nahar,104
in porous alumina the protons for conduction are donated
not only by H2 O, but also by the Al(OH) 3 formed as
a result of water adsorption and the impurity anions
incorporated into the Ž lm from the electrolyte during
anodization.107–109 The water-like network consists of
singly bonded water molecules, which possess a high
dielectric constant as they form dipoles and reorient freely
under an externally applied electric Ž eld. The impedance
of the nanoporous alumina sensors includes this capacitive contribution as well.
From the analysis of Figure 15, the high-frequency
semicircle that diminishes in size with increasing RH
arises because of the effect of a physisorbed water layer
at the surface. Therefore R1 in the equivalent circuit
of Figure 16 represents the charge transfer resistance
through the adsorbed water layer. The effect of electronic
286
conduction through the alumina bulk is parallel to that
of the surface conduction affected by the adsorbed water
layer. This bulk conduction process is dominant only at
extremely low humidity values when there are only a
few adsorbed molecules. Hence the incomplete semicircle, or arc, in the low-humidity region is partially determined from the base alumina. Bn1 represents the capacitance contribution from the adsorbed water layer. The
large spurs seen in the low-frequency region are a consequence of ion migration in the adsorbed layer toward
electrodes.1011 103 This migration leads to accumulation of
ions at the electrodes. Since the relaxation time of ion
migration is larger than the charge transfer process it
is seen only at low frequencies. The accumulated ions
in the adsorbed water layer give rise to R2 and Bn2 .
From the equivalent circuits given in Figure 16, it can
be seen that the presence of ion accumulation represented by the combination of R2 and Cn2 4—5 increases
the overall impedance of the sensor. Hence the reduction
in impedance caused at high humidity due to the charge
transfer process is countered by ion accumulation.
To better understand the fundamental sensing mechanisms, R1 and Bn1 , obtained through the numerical Ž tting
program for the different sensors, are plotted in Figures 17
and 18, respectively. It can be seen from Figure 17 that
R1 , which is the resistance to charge transport in a static
electric Ž eld by the material containing an adsorbed water
layer in the absence of ion migration, is linear with RH
-7
10
50V
30V
DO15
-8
10
Bn1
Fig. 16. Equivalent circuit used for Ž tting the experimental data.
-9
10
-10
10
-11
0
20
40
60
80
100
RH (%)
Fig. 18. Humidity dependence of Bn1 of different sensors obtained by
Ž tting the equivalent circuit model (Fig. 17) with experimental data.
J. Nanosci. Nanotech. 2003, 3, 277–293
Hence in the region close to the electrode-alumina interface, the adsorbed water layer exhibits a net dipolar orientation, and, therefore, it does not display its normal
dielectric constant.1011 103 Depending on ion size, up to
a monolayer of charge can exist at the electrode;101 the
diffuse (Gouy) layer of counter-ions spreads away from
this inner (Stern) layer. The number of ions available to
form the Stern and Gouy layers at a particular frequency
increases with increasing number of physisorbed layers.
Hence as the number of physisorbed layers increases, the
Stern and Gouy layers start forming at higher frequencies.
With reference to Figure 12, the relative humidity
level at which the sharp decrease in impedance occurs
shifts higher with increasing frequency. This result can be
explained with the equivalent circuit shown in Figure 16a,
that is, the circuit in the absence of ion accumulation. At
high frequencies the reactive impedance Xc of Cn1 4—5,
equal to Xc D 1=j—Cn1 4—5, is low and hence impedance
is largely determined by R1 . As the humidity increases
R 1 decreases; however, this translates into a reduction in
impedance only at relatively low frequencies, where the
value of R1 is comparable to or less than the capacitive
reactance of Cn1 (—). As the measurement frequency is
reduced the reactance of Cn1 (—) decreases; hence the effect
of R1 becomes more signiŽ cant at lower humidity levels.
REVIEW
in a semilog plot. For sensor DO15 R1 decreases to a
few hundred ohms at high RH levels, indicating that the
observed conductivity is not due merely to the protons
provided by the adsorbed water layer. Since deionized
water and argon gas are used to create the required humidity environment, it is reasonable to assume that the surfaces of the alumina Ž lms contain anions, presumably
from the electrolyte used for anodization, which controls
conduction through the adsorbed layer.108
With reference to Figure 18, Bn1 of the larger pore samples SO50 and SO30 increases signiŽ cantly only at the
higher humidity levels. This result is in contrast to that
of the smaller pore size sensors, which shows that Bn1
increases at lower humidity levels. The relative humidity values where a signiŽ cant increase in Bn1 takes place
generally match the point seen in Figure 11 where the
impedance drops signiŽ cantly, which in turn is a function of pore size. These same relative humidity values
correlate with the point at which the spur appears in
the impedance diagrams given in Figure 15. The results
lead to the following conclusions. Since hydrogen-bonded
water molecules possess a high dielectric constant, the
increase in Bn1 indicates the presence of these bonds
on the surface, which means that a liquid-like network,
more than one monolayer of physisorbed molecules, has
formed at that humidity level. This liquid-like network is
responsible for the sharp reduction in impedance seen in
Figure 11 and the ion migration that creates the spur in
the low-frequency portion of the Cole-Cole plots as seen
in Figure 15. According to Eq. (1), capillary condensation
does not take place in the alumina test sensors.
In light of the above observations, we propose the
following model to explain the behavior of nanoporous
alumina in response to humidity. The anodization process results in a certain amount of electrolyte anions
trapped within the pores.1041 1071 108 Even in a low-humidity
regime the presence of these anions on the sensor surface provides a high charge density for easy physisorption of water molecules. The result is the formation
of a liquid-like network within the pores. The impurity anions also act as proton donors by a dissociative
mechanism,104 joining the protons donated by the water
molecules. The smaller the pore size, the lower the RH
level at which the liquid-like network is formed. Hence
for the small pore sensors there is variation in Bn1 as
well as impedance magnitude in low-humidity regimes;
the exponential dependence of R1 on RH also suggests
the same phenomenon.
In the case of DO15 the Ž rst semicircle is highly distorted by the low-frequency spur originating with ion
migration. A relatively large number of mobile ions might
be present in these sensors, leading to an enhanced
accumulation of protons at the electrode-sample contact.
As mentioned earlier, the dipoles in the singly bonded
adsorbed water layer are free to rotate in an electric Ž eld.
Varghese and Grimes/Metal Oxide for Sensing
3.2. Hydrogen Sensing with Titania Nanotubes
We recently reported62 the fabrication of self-organized
titania nanotube arrays with an anodization technique.
Although the as-prepared nanotubes are amorphous, they
crystallize on annealing at elevated temperatures and are
structurally stable to at least 600 ž C. This stability of
structure, which is an essential criterion of a gas-sensing
material, prompted us to study the gas-sensing behavior of
these nanotubes for technologically important gases such
as oxygen, carbon monoxide, ammonia, carbon dioxide,
and hydrogen.
Titania has earned much attention for its oxygensensing capabilities.110–115 Furthermore, with proper
manipulation of the microstructure or crystalline phase
and/or the addition of the proper impurities or surface
functionalization, titania can also be used as a reducing
gas sensor.116–127 The interaction of a gas with a metal
oxide semiconductor is primarily a surface phenomenon,
and nanoporous metal oxides1221 1231 1281 129 offer the advantage of providing large sensing surface areas.
Room-temperature metal oxide hydrogen sensors are
generally based on Schottky barrier modulation of devices
like Pd/TiO2 or Pt/TiO2 by hydrogen.130–132 Elevatedtemperature hydrogen sensors examine electrical resistance with hydrogen concentration; for example, Birkefeld et al.133 observed that the resistance of the anatase
phase of titania varies in the presence of carbon monoxide
and hydrogen at temperatures above 500 ž C, but on doping with 10% alumina it becomes selective for hydrogen.
287
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
Platinum electrodes
REVIEW
(a)
Nano tube array
Titanium
Barrier layer
Fig. 19. Schematic representation of the hydrogen sensor geometry.
Munuera et al.134 used rhenium doping in the anatase
phase to perform room-temperature hydrogen monitoring. Recently Shimizu et al.122 reported that anodized
nanoporous titania Ž lms with a Pd Schottky barrier are
sensitive to hydrogen at 250 ž C.
Titania nanotubes were grown from titanium foil
(99.5% pure from Alfa Aesar, Ward Hill, MA) with a
thickness of 0.25 mm. The anodization was performed in
an electrolyte medium of 0.5% hydro uoric acid in water,
with the use of a platinum foil cathode. Well-deŽ ned
nanotube arrays were grown with anodizing potentials
ranging from 12 V to 20 V. Nanotube length increases
with anodization time, reaching 400 nm in approximately
20 min, and then remains constant. For the present study
the samples were anodized for 25 min. The samples were
then annealed at 500 ž C in a pure oxygen ambient for 6 h,
with a heating and cooling rate of 1 ž C/min.
The electrode geometry of the titania nanotube sensors is shown in Figure 19. The sensor consists of a base
titanium metal foil with a nanotube array grown on top.
An insulating barrier layer separates the nanotubes from
the conducting titanium foil. Electrical connections were
made by two parallel 10 mm £ 2 mm platinum pads of
100-Œm thickness. The test chamber consists of a 1.3-liter
quartz tube, with stainless steel end caps, placed inside
a furnace (Thermolyne, USA; model 21100 tubular furnace). Gas  ow through the test chamber was controlled
via a computer-controlled mass  ow controller. The electrical resistance of the titania sensors was measured with
a computer-controlled Keithly digital multimeter. Prior to
data collection the test chamber was initially evacuated
with a mechanical pump, whereupon nitrogen (99.999%
pure) was passed while the sensor under test was heated
to the desired temperature. The test gases were mixed in
appropriate ratios with nitrogen to create the necessary
test gas ambient.
The surface morphology of nanotube arrays prepared
with an anodization potential of 20 V and annealed at
500 ž C for 6 h in a pure oxygen ambient is shown in
Figure 20a and b. It can be seen from these images that
the nanotubes are uniform over the surface. The nanotubes are approximately 400 nm in length and have a
barrier layer62 thickness of º50 nm. For the nanotubes
288
100 nm
(b)
1 mm
(c)
100 nm
Fig. 20. The surface morphology of the titania nanotubes after annealing at 500 ž C. (a) High- and (b) low-magniŽ cation images of a 20-V
sample. (c) A high-magniŽ cation image of a 12-V sample.
fabricated with 20-V anodization the average pore diameter, as determined from FESEM images, is 76 nm (the
standard deviation 15 nm), with a wall thickness of 27 nm
(standard deviation 6 nm). The sample anodized at 12 V,
shown in Figure 2c, was found to have an average pore
diameter of 46 nm (standard deviation 8 nm) with a wall
thickness of 17 nm (standard deviation 2 nm). The porosities of the 20-V and 12-V samples were calculated as 45%
and 61%, respectively.
Glancing-angle X-ray diffraction patterns of a 20-V
sample annealed at 500 ž C for 6 h in oxygen ambient
is shown in Figure 21. It can be seen that both anatase
and rutile phases of titania are present in the sample.
A detailed study135 of these structures by high-resolution
transmission electron microscope (HRTEM) showed that
Varghese and Grimes/Metal Oxide for Sensing
R
T
A
TR
20
30
TR R
A
R R
40
50
TR
RT
60
TT
70
80
Fig. 21. Glancing-angle X-ray diffraction pattern of a 20-V sample
(glancing angle D 2ž ) annealed at 500 ž C in oxygen ambient. A, R, and
T represent, respectively, the re ections from anatase crystallites, rutile
crystallites, and the titanium substrate.
the anatase crystallites were concentrated on the walls of
the nanotubes and rutile on the barrier layer.
Nanotubes annealed in a pure oxygen ambient were
found to be stable (intact) to temperatures of approximately 580 ž C. Above this temperature protrusions were
seen to be coming out through the nanotubes, an effect
that spread with increasing temperature. These protrusions, which are due to oxidation of the titanium substrate,
collapse the nanotubes.
Figure 22 shows the response of the 20-V (76 nm) sample as a function of ambient temperature, as it is switched
from a nitrogen environment to one containing 1000 ppm
hydrogen, and then back to nitrogen. The plot was made
using 4Rg =R0 5ƒ 1 versus time, where R0 is the base resistance of the sensor, that is, the sensor resistance before the
test gas was introduced, and Rg is the measured resistance
in the presence of test gas. The sensor shows increasing
hydrogen sensitivity with temperature, with a change by
3 orders of magnitude in resistance at temperatures above
º300 ž C. At all of the temperatures the original resistance
recovered without hysteresis.
The sensitivity S is deŽ ned by the formula
10 3
R0 ƒ R gs
Rgs
375°C
N2
240°C
180°C
5 10 2
1 10 3
1.5 10 3
10
1
100
2 10 3
2.5 10 3
3 10 3
3.5 10 3
4 10 3
Time (Sec.)
Fig. 22. Variation of resistance Rg , normalized with respect to baseline
resistance R0 , of a 20-V sample with time on exposure to 1000 ppm
hydrogen at different temperatures. It may be noted that the inverse of
Rg =R0 was used in the plot to represent data in the positive y direction.
200
250
300
350
400
450
Fig. 23. The sensitivity temperature dependence of a 20-V sample to
1000 ppm hydrogen.
where R0 is the resistance of the sensor before the gas is
passed and Rgs is the resistance after gas is passed and
reaches the saturation value. The temperature-dependent
sensitivity of a 20-V sample to 1000 ppm hydrogen is
shown in Figure 23. Sensitivity is seen to increase with
temperature to approximately 380 ž C, where the increase
in sensitivity with temperature begins to saturate.
The response time, deŽ ned as the time needed for the
sensor to reach 90% of the Ž nal signal for a given concentration of gas, is plotted against temperature in Figure 24
(the time includes that required for the gas to equilibrate inside the measurement chamber, estimated to be
º30 s). The response time decreases exponentially with
temperature.
To check the behavior of the sensor on repeated hydrogen exposure, the hydrogen concentration was varied in
discrete steps of 100 ppm from 0 to 500 ppm while the
temperature was kept constant at 290 ž C; the chamber was
 ushed with nitrogen after each exposure to hydrogen.
The response of the 20-V prepared nanotube sensors,
kept at 290 ž C, is shown in Figure 25. The behavior
of the sensor is consistent, recovering its original resistance after repeated exposure to varying hydrogen gas
concentrations. The sensitivities of the 76-nm-, 53-nm-,
and 22-nm-diameter nanotube sensors to hydrogen concentrations ranging from 100 ppm to 1% are shown in
Figure 26. The sensitivity of the 76-nm (20-V) sensor to
low concentrations is shown in Figure 27; there is a linear
increase in sensitivity at low concentrations. The lower
Response Time (Sec.)
(Rg/R0)-1
265°C
10 0
0 10 0
2
10
290°C
10 1
10
Temperature (°C)
345°C
10 2
H 2+N 2
3
10-1
150
2 q (deg.)
SD
10
REVIEW
(101)
T
Sensitivity (R0-Rgs)/Rgs
(110)
J. Nanosci. Nanotech. 2003, 3, 277–293
4
10 3
10 2
10 1
150
200
250
300
350
400
450
Temperature (°C)
Fig. 24. Variation in response time for a 20-V sensor as a function of
temperature. The dots represent measured data.
289
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
10
100
200
300
400
500
400
300
200
Sensitivity (R 0-Rgs)/Rgs
Resistance (W)
REVIEW
100
H2 (ppm)
3
100
10
1
0
100
200
300
400
500
Hydrogen concentration (ppm)
2
10
0 10 0
1 10 4
2 10 4
3 10 4
4 10 4
5 10 4
Time (Sec.)
Fig. 25. Resistance of a 20-V sample when exposed to different concentrations of hydrogen at 290 ž C. The nitrogen-hydrogen mixture was
passed for 1500 s; the chamber was then  ushed with nitrogen for 3000 s
before the nitrogen-hydrogen mixture was passed again.
test limit of 20 ppm is determined by the experimental
apparatus and not the sensor.
Figure 28 shows the change in resistance of the 76-nm-,
46-nm-, and 22-nm-diameter nanotube sensors with exposure to 1000 ppm hydrogen at 290 ž C. Whereas smaller
diameter nanotubes had greater sensitivity to hydrogen,
the samples made at lower anodizing voltages tended to
be more brittle and harder to handle mechanically without
breaking.
The 20-V, 76-nm-diameter sample was exposed to oxygen, carbon monoxide, ammonia, and carbon dioxide at
290 ž C to study the cross-sensitivity. The sensor was
found to have no detectable variation in resistance on
exposure to carbon dioxide. The sensitivities of the titania nanotubes to the other gases are shown in Figure 29.
The sensitivities of the nanotubes to carbon monoxide and
ammonia are negligible compared with that of hydrogen.
The resistance of the nanotubes increased in the presence
of oxygen and did not regain their original electrical conductivity, even after several hours in a 290 ž C nitrogen
environment.
Fig. 27. Sensitivity of a 20-V (76 nm) sample at 290 ž C for low hydrogen concentrations.
Since the sensor measurements were made in atmospheres without oxygen, the increase in conductivity
cannot be due to hydrogen removing oxygen from the
lattice132–134 or the removal of chemisorbed oxygen.135–137
Hydrogen molecules can dissociate at defects on the
titania surface, which can subsequently diffuse into the
titania lattice, acting as electron donors.1281 1381 139 However, a dissociation-driven process would result in a
slow response and recovery times with complete recovery unlikely; since the sensor completely regains its original resistance with hydrogen cycling, it appears that this
is not the dominant mechanism behind high hydrogen
sensitivity. We believe chemisorption of the dissociated
hydrogen on the titania surface is the underlying sensing
mechanism.137 During chemisorption hydrogen acts as a
surface state, and a partial charge transfer from hydrogen
to the titania conduction band takes place. This creates
an electron accumulation layer on the nanotube surface,
enhancing its electrical conductance. Upon removal of the
hydrogen ambient, electron transfer back to the hydrogen molecule takes place, which subsequently desorbs,
restoring the original electrical resistance of the material. Another factor that may play a role in the hydrogen
sensitivity (and selectivity) is the platinum electrodes. At
elevated temperatures hydrogen dissociation can occur on
platinum surfaces. These dissociated hydrogen atoms may
spill1361 140 onto the nanotube surface, where they diffuse
105
105
Pore diameter = 53 nm
1000
100
(R g/R0)-1
Sensitivity (R 0-Rg )/Rg
Pore diameter = 22nm
104
Pore diameter = 76 nm
103
Pore size - 53 nm
101
Pore size - 76 nm
10
10-1
0
1
0
0.2
0.4
0.6
0.8
1
Hydrogen Concentration (%)
Fig. 26. The sensitivity variation of a samples with pore diameters of
22 nm, 53 nm, and 76 nm versus time and hydrogen concentrations.
290
Pore size - 22 nm
0.5
1
1.5
Time (Hour)
Fig. 28. A comparison of the variation in resistance of samples having
pore diameters of 22 nm, 53 nm, and 76 nm versus time, upon exposure
to 1000 ppm of hydrogen at 290 ž C.
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
Oxygen
Ammonia
Carbon monoxide
(Rg/R0)-1
0
-1
10
-2
10
0 10
0
2 10
2
4 10
2
6 10
2
8 10
2
1 10
3
Time (Sec.)
Fig. 29. Variation of resistance of a 20-V sample when exposed to
1000 ppm carbon monoxide, 5% ammonia, and 1% oxygen at 290 ž C.
into the material, affecting its electrical properties. From
the present study it was not clear how signiŽ cant a role
the platinum electrodes play.
Anatase, the polymorph of titania, has been reported to
have high sensitivity for reducing gases like hydrogen and
carbon monoxide.1211 1241 133 Our nanotube samples contain
anatase phase mainly on the walls and rutile in the barrier
layer. As the diffusing hydrogen atoms go to the interstitial sites1331 142 and as the c/a ratio of anatase is almost
four times that of rutile, it appears that the anatase lattice
accommodates hydrogen easily and hence makes a higher
contribution to hydrogen sensitivity.
The effect of chemisorption can be neglected in the
oxygen-sensing experiments. As the recovery requires
several hours, it appears that the nanotubes contain oxygen vacancies or titanium interstitial defects in the presence of nitrogen. On exposure of the sensor to oxygen
ambient, the lattice reoxidizes and hence the conductivity of the sensor decreases. On the removal of oxygen,
the reduction of the lattice will not immediately occur;
hence the sensor requires several hours to regain its original conductivity.
4. CONCLUSIONS
This paper summarizes different routes of fabrication of
metal oxide nanoarchitectures useful for sensing applications. Structures made via anodization of a starting metal
and breath Ž gure formation from a sol-gel are discussed
and presented, as are TiO2 nanowires made by Ž lling a
nanoporous alumina membrane with subsequent removal
of the supporting template. Two test cases illustrating
the utility of these nanoporous metal oxide structures for
sensing are presented: highly ordered nanoporous alumina
is used for humidity sensing, and TiO2 nanotubes are used
for hydrogen sensing. These study test cases reveal several points worth brie y noting.
In the Ž rst sensing test case uniform nanoporous alumina sensors exhibit pore size and frequency-dependent
REVIEW
10
humidity-sensing characteristics. The mechanism responsible for the sensing action of the nanoporous alumina sensors was studied by representing the impedance spectra at
different humidity levels in Cole-Cole plots. This makes it
possible to identify the competing impedance-determining
processes occurring in the sensors, namely charge transfer
and mass transport through the physisorbed water layer,
as dependent upon the humidity level and input signal
frequency. Charge transfer was found to enhance the sensitivity by causing a large variation in impedance with
respect to humidity. Mass transport leads to accumulation
of ions at the electrodes, which tends to decrease the measurement sensitivity by increasing the impedance. Hence
a proper choice of operating frequency, high enough to
avoid ion accumulation, is essential for optimum sensor
performance.
It has been inferred from our results that anions become
incorporated into the alumina Ž lm during the anodization process, supporting the hypothesis of Khanna and
Nahar,104–106 where they act as proton donors and facilitate the adsorption of water molecules. Hence liquid-like
charge transfer networks are formed at relatively lower
humidity levels for the smaller pore-size sensors. This
process, rather than capillary condensation, is primarily
responsible for the observed humidity-sensing behavior of
the sensors. Our studies show that a distribution of pore
sizes, as suggested by Shimizu et al.,94 is not necessary to
obtain sensitive performance over a wide humidity range;
rather such optimal performance can be obtained with uniform nanodimensional pores containing adsorbed anionic
impurities.
In the second sensing test case titania nanotubes, prepared by anodization and annealed in an oxygen atmosphere at a temperature of 500 ž C, were found to be
highly sensitive to hydrogen. The nanotube sensors contained both anatase and rutile phases of titania and showed
appreciable sensitivity toward hydrogen at temperatures
as low as 180 ž C. The sensitivity increased drastically
with temperature, showing a variation of 3 orders in magnitude of resistance to 1000 ppm of hydrogen at 400 ž C.
The response time decreased with increasing temperature;
at 290 ž C full switching of the sensor took approximately
3 min. Results were highly reproducible, with no indication of hysteresis. Our results showed that these sensors
are at least capable of monitoring hydrogen levels from
20 ppm to 4%. At 290 ž C nanotubes with smaller pore
diameters showed higher sensitivity to hydrogen compared with their larger pore counterparts. The sensors
showed high selectivity to hydrogen compared with carbon monoxide, ammonia, and carbon dioxide. Although
the sensor was sensitive to high concentrations of oxygen, the response time was high and the sensor did not
completely regain the original condition. We believe the
hydrogen sensitivity of the nanotubes is due to hydrogen
diffusion into the titania lattice, where they act as electron
donors.
291
Varghese and Grimes/Metal Oxide for Sensing
REVIEW
Acknowledgments: Support of this work by the National
Science Foundation through grant ECS-0210033 is gratefully acknowledged.
References and Notes
1. K. Kajihara, K. Tanaka, K. Horao, and N. Soga, Jpn. J. Appl.
Phys. 35, 6110 (1996).
2. B. O. Regan and M. Gratzel, A low cost, Nature 353, 737 (1991).
3. A. Fujishima and K. Honda, Nature 238, 37 (1972).
4. K. Sato, A. Tsuzuki, H. Taoda, Y. Torii, T. Kato, and Y. Butsugan,
J. Mater. Sci. 29, 5911 (1994).
5. K. Sato, A. Tsuzuki, Y. Torii, H. Taoda, T. Kato, and Y. Butsugan,
J. Mater. Sci. 30, 837 (1995).
6. N. Ozer and C. M. Lampert, Sol. Energy Mater. Sol. Cells 54, 147
(1998).
7. A. Manz and H. Becker, editors, Microsystem Technology in
Chemistry and Life Sciences. Springer-Verlag, Berlin, 1998.
8. E. Kim, Y. Xia, and G. M. Whitesides, Nature 376, 581 (1995).
9. M. Templin, A. Franck, A. Du Chesne, A. Leist, A. Zhang,
R. Ulrich, V. Schadler, and U. Weisner, Science 278, 1795 (1997).
10. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Frederickson,
B. Chmelka, and G. D. Stucky, Science 279, 548 (1998).
11. O. D. Velev, T. A. Jede, R. F. Lobo, and A. M. Lenhoff, Nature
389, 447 (1997).
12. B. T. Holland, C. F. Blanford, and A. Stein, Science 281, 538
(1998).
13. S. Matsushita, T. Miwa, and A. Fujishima, Chem. Lett. 9, 925
(1997).
14. A. Imhof and D. J. Pine, Adv. Mater. 10, 697 (1998).
15. T. Tatsuma, A. Ikezawa, Y. Ohko, T. Miwa, T. Matsue, and
A. Fujishima, Adv. Mater. 12, 643 (2000).
16. Z. Zhong, Y. Yin, B. Gates, and Y. Xia, Adv. Mater. 12, 206 (2000).
17. O. D. Velev and A. M. Lenhoff, Curr. Opin. Colloids Interface
Sci. 5, 56 (2000).
18. O. D. Velev and E. W. Kaler, Adv. Mater. 12, 531 (2000).
19. P. Jiang, J. Cizeron, J. F. Bertone, and V. L. Colvin, J. Am. Chem.
Soc. 121, 7957 (1999).
20. M. E. Turner, T. J. Trentler, and V. L. Colvin, Adv. Mater. 13, 180
(2001).
21. K. M. Kulinowski, P. Jiang, H. Vaswani, and V. L. Colvin, Adv.
Mater. 12, 833 (2000).
22. P. Jiang, J. F. Bertone, and V. L. Colvin, Science 291, 453 (2001).
23. R. Seshadri and F. C. Meldrum, Adv. Mater. 12, 1149 (2000).
24. F. C. Meldrum and R. Seshadri, Chem. Commun. 29 (2000).
25. Q. B. Meng, Z. Z. Gu, O. Sato, and A. Fujishima, Appl. Phys.
Lett. 77, 4313 (2000).
26. G. Widawski, B. Rawiso, and B. Francois, Nature 369, 3897
(1994).
27. B. Francois, O. Pitois, and J. Francois, Adv. Mater. 7, 1041 (1995).
28. O. Pitois and B. Francois, Eur. Phys. J. B 8, 225 (1999).
29. S. A. Jenekhe and X. L. Chen, Science 283, 372 (1999).
30. M. Shimomuara and T. Sawadaishi, Curr. Opin. Colloid Interface
Sci. 6, 11 (2001).
31. O. Karthaus, N. Maruyama, X. Cieren, M. Shimomura,
H. Hasegawa, and T. Hashimoto, Langmuir 16, 6071 (2000).
32. T. Nishikawa, J. Nishida, R. Ookura, S. Nishimura, S. Wada,
T. Karino, and M. Shimomura, Mater. Sci. Eng., C 10, 141 (1999).
33. K. Kajihara, K. Nakanishi, K. Tanaka, K. Hirao, and N. Soga,
J. Am. Ceram. Soc. 81, 2670 (1998).
34. C. A. Grimes, R. S. Singh, E. C. Dickey, and O. K. Varghese,
J. Mater. Res. 16, 1686 (2001).
35. M. Srinivasrao, D. Collings, A. Phillips, and S. Patel, Science 292,
79 (2001).
36. P. G. de Gennes, Rev. Mod. Phys. 57, 827 (1985).
292
J. Nanosci. Nanotech. 2003, 3, 277–293
37. D. Beysens, A. Steyer, P. Guenoun, D. Fritter, and C. M. Knobler,
Phase Transitions 31, 219 (1991).
38. B. J. Briscoe and K. P. Galvin, J. Phys. D 23, 422 (1990).
39. B. J. Briscoe and K. P. Galvin, J. Phys. D 23, 1265 (1990).
40. D. Beysens and C. M. Knobler, Phys. Rev. Lett. 57, 1433 (1986).
41. F. Family and P. Meakin, Phys. Rev. Lett. 61, 428 (1988).
42. A. V. Limaye, R. D. Narhe, A. M. Dhote, and S. B. Ogale, Phys.
Rev. Lett. 76, 3762 (1996).
43. P. Dell’ Aversana and G. P. Neitzel, Phys. Today 51, 38 (1998).
44. P. Dell’ Aversana, J. R. Banavar, and J. Koplik, Phys. Fluids 8, 15
(1986).
45. P. Dell’ Aversana, V. Tontodonato, and L. Carotenuto, Phys. Fluids
9, 2475 (1997).
46. M. Q. Li, S. Q. Xu, and E. Kumacheva, Langmuir 16, 7275 (2000).
47. P. Freymuth, Rev. Sci. Instrum. 64, 1 (1993).
48. P. Ball, The Self-Made Tapestry: Pattern Formation in Nature.
Oxford University Press, New York, 1999.
49. M. F. Schatz, S. J. VanHook, W. D. McCormick, J. B. Swift, and
H. L. Swinney, Phys. Fluids 11, 2577 (1999).
50. S. R. Coriell, M. R. Cordes, and W. J. Boettinger, J. Cryst. Growth
49, 13 (1980).
51. T. S. Sullivan, Y. M. Liu, and R. E. Ecke, Phys. Rev. E 54, 486
(1996).
52. J. L. Rogers, M. F. Schatz, J. L. Bougie, and J. B. Swift, 84, 87
(2000).
53. K. Nitschke, 52, R5772 (1995).
54. E. Bodenschatz, J. R. De Bruyn, G. Ahlers, and D. S. Cannell,
Phys. Rev. Lett. 67, 3078 (1991).
55. C. W. Meyer, D. S. Cannell, G. Ahlers, J. B. Swift, and P. C.
Hohenberg, Phys. Rev. Lett. 61, 947 (1988).
56. R. F. Ismagilov, D. Rosmarin, D. H. Gracias, A. D. Stroock, and
G. M. Whitesides, Appl. Phys. Lett. 79, 439 (2001).
57. E. A. Barringer and H. K. Bowen, Langmuir 1, 420 (1985).
58. M. Tschapek, C. Wasowski, and R. M. T. Sanchez, J. Electroanal.
Chem. 74, 167 (1976).
59. H. Masuda and K. Fukuda, Science 268, 1466 (1995).
60. H. Masuda, F. Hasegwa, and S. Ono, J. Electrochem. Soc. 144,
L 127 (1997).
61. O. Jessensky, F. Muller, and U. Gosele, Appl. Phys. Lett. 72, 1173
(1998).
62. D. Gong, C. A. Grimes, O. K. Varghese, W. Hu, R. S. Singh,
Z. Chen, and E. C. Dickey, J. Mater. Res. 16, 3331 (2001).
63. B. M. Kulwicki, J. Am. Ceram. Soc. 74, 697 (1991).
64. H. Arai and T. Seiyama, in Sensors: A Comprehensive Survey,
edited by W. Gopel, J. Hesse, and J. N. Zemel, VCH, Weinheim
(1992), Vol. 3, pp. 981–1012.
65. E. Traversa, Sens. Actuators, B 23, 135 (1995).
66. N. Yamazoe and Y. Shimizu, Sens. Actuators 10, 379 (1986).
67. A. Bearzotti, I. Fratoddi, L. Palummo, S. Petrocco, A. Furlani,
C. Lo Sterzo, and M. V. Russo, Sens. Actuators B 76, 316 (2001).
68. Y. Sakai, Y. Sadaoka, and M. Matsuguchi, Sens. Actuators, B 35,
85 (1996).
69. C. A. Grimes and D. Kouzoudis, Sens. Actuators, A 84, 205
(2000).
70. K. G. Ong, C. A. Grimes, C. L. Robbins, and R. S. Singh, Sens.
Actuators, A 93, 33 (2001).
71. T. Seiyama, N. Yamazoe, and H. Arai, Sens. Actuators 4, 85
(1983).
72. L. Ketron, Ceram. Bull. 68, 860 (1989).
73. E. Traversa, G. Gusmano, and A. Montenero, Eur. J. Solid State
Inorg. Chem. 32, 719 (1995).
74. M. K. Jain, M. C. Bhatnagar, and G. L. Sharma, Sens. Actuators,
B 55, 180 (1999).
75. O. K. Varghese and L. K. Malhotra, J. Appl. Phys. 87, 7457 (2000).
76. G. Sberveglieri, R. Murri, and N. Pinto, Sens. Actuators, B 23,
177 (1995).
Varghese and Grimes/Metal Oxide for Sensing
J. Nanosci. Nanotech. 2003, 3, 277–293
113. V. Demarne, S. Balkanova, A. Grisel, D. Rosenfeld, and F. Levy,
Sens. Actuators, B 13–14, 497 (1993).
114. S. Hasegawa, Y. Sasaki, and S. Matsuhara, Sens. Actuators, B
13–14, 509 (1993).
115. A. Rothschild, F. Edelman, Y. Komem, and F. Cosandey, Sens.
Actuators, B 67, 282 (2000).
116. G. Sakai, N. S. Baik, N. Miura, and N. Yamazoe, Sens. Actuators,
B 77, 116 (2001).
117. V. N. Misra and R. P. Agarwal, Sens. Actuators, B 21, 209
(1991).
118. O. K. Varghese, L. K. Malhotra, and G. L. Sharma, Sens. Actuators, B 55, 161 (1999).
119. V. A. Chaudhary, I. S. Mulla, and K. Vijayamohanan, Sens. Actuators, B 50, 45 (1998).
120. H. Tang, K. Prasad, R. Sanjines, and F. Levy, Sens. Actuators, B
26–27, 71 (1995).
121. S. A. Akbar and L. B. Younkman, J. Electrochem. Soc. 144, 1750
(1997).
122. Y. Shimizu, N. Kuwano, T. Hyodo, and M. Egashira, Sens. Actuators, B 83, 195 (2002).
123. M. C. Carotta, M. Ferroni, D. Gnani, V. Guidi, M. Merli, G. Martinelli, M. C. Casale, and M. Notaro, Sens. Actuators, B 58, 310
(1999).
124. N. O. Savage, S. A. Akbar, and P. K. Dutta, Sens. Actuators, B
72, 239 (2001).
125. E. Comini, G. Faglia, G. Sberveglieri, Y. X. Li, W. Wlodarski, and
M. K. Ghantasala, Sens. Actuators, B 64, 169 (2000).
126. I. Hayakawa, Y. Iwamoto, K. Kikuta, and S. Hirano, Sens. Actuators, B 62, 55 (2000).
127. N. Savage, B. Chwieroth, A. Ginwalla, B. R. Patton, S. A. Akbar,
and P. K. Dutta, Sens. Actuators, B 79, 17 (2002).
128. C. C. Koch, editor, Nanostructured Materials: Processing, Properties and Applications, Noyes Publications, New York (2002).
129. H.-M. Lin, C.-H. Keng, and C.-Y. Tung, Nanostruct. Mater. 9, 747
(1997).
130. H. Kobayashi, K. Kishimoto, and Y. Nakato, Surf. Sci. 306, 393
(1994).
131. L. A. Harris, J. Electrochem. Soc. Solid State Sci. Technol. 127,
2657 (1980).
132. K. D. Schierbaum, U. K. Kirner, J. F. Geiger, and W. Gopel, Sens.
Actuators, B 4, 87 (1991).
133. L. D. Birkefeld, A. M. Azad, and S. A. Akbar, J. Am. Ceram. Soc.
75, 2964 (1992).
134. G. Munuera, A. R. G. Elipe, A. Munoz, A. Fernandez, J. Soria,
J. Conesa, and J. Sanz, Sens. Actuators 18, 337 (1989).
135. O. K. Varghese, D. Gong, M. Paulose, C. A. Grimes, and E. C.
Dickey, J. Mater. Res., 18, 156 (2003).
136. R. D. Shannon, J. Appl. Phys. 35, 3414 (1964).
137. K. H. Kim, E. J. Ju, and J. S. Choi, J. Phys. Chem. Solids 45,
1265 (1984).
138. C. A. Grimes, J. Waves Random Media 1, 265 (1991).
139. R. M. Walton, D. J. Dwyer, J. W. Schwank, and J. L. Gland, Appl.
Surf. Sci. 125, 199 (1998).
140. M. J. Madou and S. R. Morrison, Chemical Sensing with Solid
State Devices, Academic Press, New York (1989).
141. G. B. Raupp and J. A. Dumesic, J. Phys. Chem. 89, 5240 (1985).
142. J. B. Bates, J. C. Wang, and R. A. Perkins, Phys. Rev. B 19, 4130
(1979).
143. G. J. Hill, Br. J. Appl. Phys. 1, 1151 (1968).
144. U. Roland, T. Braunschweig, and F. Roessner, J. Mol. Catal., A:
Chem. 127, 61 (1997).
REVIEW
77. E. Traversa, M. Baroncini, E. D. Bartolomeo, G. Gusmano,
P. Innocenzi, A. Martucci, and A. Bearzotti, J. Eur. Ceram. Soc.
19, 753 (1999).
78. K.-S. Chou, T.-K. Lee, and F.-J. Liu, Sens. Actuators, B 56, 106
(1999).
79. E. Traversa, G. Gnappi, A. Montenero, and G. Gusmano, Sens.
Actuators, B 31, 59 (1996).
80. Y.-C. Yeh, T.-Y. Tseng, and D.-A. Chang, J. Am. Ceram. Soc. 72,
1472 (1989).
81. E. Traversa, J. Am. Ceram. Soc. 78, 2625 (1995).
82. T. Yamamoto and K. Murukami, in Chemical Sensor Technology,
edited by T. Seiyama (1989), Vol. 2, pp. 133–149.
83. F. Ansbacher and A. C. Jason, Nature 171, 177 (1953).
84. V. K. Khanna and R. K. Nahar, Sens. Actuators 5, 187 (1984).
85. S. Basu, S, Chatterjee, M. Saha, S. Bhandyopadhay, K. K. Mistry,
and K. Sengupta, Sens. Actuators, B 79, 182 (2001).
86. G. Sberveglieri, R. Anchisini, R. Murri, C. Ercoli, and N. Pinto,
Sens. Actuators, B 32, 1 (1996).
87. L. H. Mai, P. T. M. Hoa, N. T. Binh, N. T. T. Ha, and D. K. An,
Sens. Actuators, B 66, 63 (2000).
88. S. Chatterjee, S. Basu, S. Bandyopadhay, K. K. Mistry, and
K. Sengupta, Rev. Sci. Instrum. 72, 2792 (2001).
89. R. K. Nahar, Sens. Actuators, B 63, 49 (2000).
90. Z. Chen, M.-C. Jin, and C. Zhen, Sens. Actuators 2, 167 (1990).
91. S. Basu, M. Saha, S. Chatterjee, K. K. Mistry, and K. Sengupta,
Mater. Lett. 49, 29 (2001).
92. Y. Sadaoka, Y. Sakai, and S. Matsumoto, J. Mater. Sci. 21, 1269
(1986).
93. T. Seiyama, K. Fueki, J. Shiokawa, and S. Suzuki, editors, Chemical Sensors, Elsevier, New York (1983).
94. Y. Shimisu, H. Arai, and T. Seiyama, Sens. Actuators 7, 11 (1985).
95. T. Morimoto, M. Nagao, and F. Tokuda, J. Phys. Chem. 73, 243
(1969).
96. W. J. Fleming, Soc. Automot. Eng. Trans. Section 2, 90, 1656
(1981).
97. H. Kamiya, M. Mitsui, H. Takano, and S. Miyazawa, J. Am.
Ceram. Soc. 83, 287 (2000).
98. S. H.-Tao, W. M.-Tang, L. Ping, and Y. Xi, Sens. Actuators 19, 61
(1989).
99. D. Gong, V. Yadavalli, M. Paulose, M. Pishko, and C. A. Grimes,
J. Biomed. Microdevices (2002).
100. http://www.tesatape.com.
101. J. R. Macdonald, Impedance Spectroscopy, Wiley, New York (1987).
102. A. E. Falk, B. M. Lacquet, and P. L. Swart, Electron. Lett. 28, 166
(1992).
103. A. K. Jonscher, J. Mater. Sci. 13, 553 (1978).
104. V. K. Khanna and R. K. Nahar, Appl. Surf. Sci. 28, 247 (1987).
105. V. K. Khanna and R. K. Nahar, J. Phys. D: Appl. Phys. 19, L141
(1986).
106. R. K. Nahar, V. K. Khanna, and W. S. Khokle, J. Phys. D 17,
2087 (1984).
107. R. K. Nahar and V. K. Khanna, Sens. Actuators, B 46, 35 (1998).
108. G. C. Wood, P. Skeldon, G. E. Thompson, and K. Shimizu, J. Electrochem. Soc. 143, 74 (1996).
109. G. Patermarakis and K. Moussoutzanis, J. Electrochem. Soc. 142,
737 (1995).
110. T. Takeuchi, Sens. Actuators 14, 109 (1988).
111. U. Kirner, K. D. Schierbaum, and W. Gopel, Sens. Actuators, B 1,
103 (1990).
112. Y. Xu, K. Yao, X. Zhou, and Q. Cao, Sens. Actuators, B 13–14,
492 (1993).
Received: 19 September 2002. Revised/Accepted: 26 November 2002.
293