Int. J. Appl. Ceram. Technol., 8 [6] 1408–1413 (2011)
DOI:10.1111/j.1744-7402.2011.02615.x

Ceramic Product Development and Commercialization

Gas Sensor for Volatile Organochlorine Compounds
Using Percolation Conduction of Organic
Montmorillonite-Carbon Composites
Isao Tsuyumoto,* Yuuki Iida, and Haruna Hori
Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute ofTechnology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan

Gas sensor material, which shows a resistivity change in response to change in concentration of volatile organochlorinecompounds, is first developed based on a percolation principle. A composite of organic montmorillonite and carbon showsa reversible resistivity change in response to the concentration change of chloroform, dichloromethane, trichloroethylene,
tetrachloroethylene, and tetrachloromethane, because the resistivity of the composite is closely dependent on the volume
fraction of the insulating phase and the volume of the insulating phase is increased by incorporation of the organochlorine
molecules. The response time is substantially improved by fabricating thick-film composites using classified organic
montmorillonite.

Introduction
Volatile organochlorine compounds such asdichloromethane, chloroform, trichloroethylene, andtetrachloroethylene are hazardous substances having
central nervous system toxicity and carcinogenic risk,
although widely used for solvents and detergents in var*tsuyu@neptune.kanazawa-it.ac.jp
r 2011 The American Ceramic Society

ious industrial processes. Their allowable concentrations
in working environment are regulated by the lawsof most countries to prevent health hazards. The gas
concentrations of volatile organochlorine compounds
are usually determined by gas chromatography (GC)equipped with an electron capture detector or a flame
ionization detector, or a gas chromatography–massspectrometer (GC-MS).1–3 Such analyses are not performed in situ because of the need for sample pretreatment and the unportabilty of the equipment. Gas

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Gas Sensor for Volatile Organochlorine Compounds

sensors for organochlorine compounds have notbeen developed so far, whereas the concentrations offlammable gases such as hydrogen, methane, propane,and ethanol are easily monitored by resistivity measurements of semiconductor gas sensor materials.4 This isbecause the flammability affects the amount of oxygen
adsorbed on the semiconductor surface, whereas the organochlorine compounds are inflammable and generally
unreactive. Simple detection methods of volatile organochlorine compounds are highly desirable to monitor the working environment.
We have been investigating the percolation behavior of electrical conductivity and its application to sensors.5–9 The electrical resistivity of a composite preparedby mixing conducting and insulating particles is closely
dependent on the volume fraction of each phase, andthus an extremely small volume change of the phase due
to an extrinsic perturbation brings about the resistivity
change of the composite. Pressure can be electricallydetected by monitoring the resistivity of the composite
because the volume fraction of each phase changes withpressure. Carbon-dispersed rubber has been put to practical use as pressure sensors, because the stress to therubber brings about the decrease in the volume fractionof the rubber matrix, and consequently, the decrease in
the resistivity. Our idea is that an objective substancecan also be electrically detected on the condition thatthe volume of one phase increases by incorporation of
the objective substance. Carbon-loaded polymers havebeen found to show resistivity increase when exposed tosolvents and hydrocarbons due to the volume change ofthe polymer matrix.10–13 One of the authors has firstobserved an anomalous resistivity increase of hexagonaltungsten bronzes in a humid atmosphere at room
temperature.7–9 The resistivity was increased because
the conduction path along the c-axis was separated bywater incorporation into oxygen vacancies existing every
six layers perpendicular to the c-axis. In our previous
paper, we have prepared a humidity sensor consisting of
the mixture of montmorillonite and carbon and havedemonstrated the operating principle based on percolation conduction.5 The resistivity was increased in thehumid atmosphere because the volume of the insulating
montmorillonite was increased by water incorporation.
In the present study, we used organic montmorillonite for an insulating phase and first developed asensor material whose resistivity changes with thegas-phase concentration of the volatile organochlorinecompounds.

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Material and Methods
Organic montmorillonite (average particle size:
40 mm, Esben NX80, Hojun, Gumma, Japan) containing 49.9 wt% of dimethyl dioctadecyl ammonium ion(DMDO) in the interlayer was used to prepare pellets
and thick films.14–21 The chemical composition of thehost montmorillonite is Na2/3Si8(Al10/3Mg2/3)O20(OH)4, and all of the Na1 ion in the host interlayer
are exchanged with the DMDO ion in the organic
montmorillonite. The organic montmorillonite and carbon powders (average particle size: 5 mm, Kojundo
Chemical Laboratory, Saitama, Japan) were mixed in
the weight ratio of 90:10 in an agate mortar, andmolded into pellets of 0.116 g, 13 mm diameter and0.63 mm thickness with relative density 76% by monoaxial pressing at 450 MPa. The pellets were dried in anoven at 1151C for 24 h after gold electrodes were coatedon both sides of the pellets using gold paste (No. 8563,
Tokuriki Chemical Research, Tokyo, Japan). As for the
thick-film samples, the organic montmorillonite powder
was suspended in acetone and the turbid supernatant
was gathered by decantation in order to classify the
powder by particle size and to fabricate smooth thickfilms. The average particle size in the supernatant wasestimated at 26 mm by the laser scattering method, andcoarse particles above around 90 mm were completely
removed by the classification. The carbon powder was
mixed with the supernatant. A viscous suspension was
obtained by evaporating most of acetone at room temperature, and the thick films of carbon-organic montmorillonite composites were prepared by applying thesuspension to a slide glass without adhesives and drying
it at room temperature. The two gold electrodes were
coated at a distance of 10 mm on the same surface, andthe thickness of the thick film was 0.03 mm. The content of the carbon in the thick films was 17.0 wt%(18.5 vol%).
The resistivity was measured by the dc two-probe
method using a digital multimeter (34401A, Agilent
Technologies, Palo Alto, CA) after drying at 1151C for
24 h and subsequently cooling down to room temperature in dry air. The resistivity changes were investigatedin response to stepwise changes of the gas concentrationof volatile organochlorine compounds such as chloroform (trichloromethane), dichloromethane (methylene
dichloride), trichloroethylene, tetrachloroethylene (perchloroethylene), and tetrachloromethane (carbon tetrachloride). Flammable gases such as acetone, ethanol, and1410

International Journal of Applied Ceramic Technology—Tsuyumoto, Iida and Hori

2-propanol were also used for comparison. The gas concentrations over 4 vol% were controlled by passing dry air
through a gas bubbler filled with a liquid of the compound and mixing the gas with the other dry air usingmass flow controllers. The gas concentrations as low as
6200 ppm or below were controlled by injecting the liquid compound into a sample chamber using a microsyringe. The morphology of the sample composites wasobserved using an SEM (S-3000N, Hitachi, Tokyo,
Japan) after carbon coating. The structure change ofthe organic montmorillonite by incorporation of thechloroform was also investigated by powder X-ray diffraction (Miniflex, Rigaku, Tokyo, Japan).

Vol. 8, No. 6, 2011

Fig. 2. Resistivity change of a composite pellet containing
10.0 wt% of carbon when the air containing chloroform wasintroduced and its concentration was changed stepwise at 201C.

The resistivity of the carbon–organic montmorillonite composite pellets is shown in Fig. 1 as a function of
the volume fraction of the organic montmorillonite.
Similar to the case with the inorganic montmorillonite,5the resistivity began to increase above around 80 vol%and showed a steep jump by four orders of magnitude at
93–94 vol%. The critical threshold of percolation (pc),the volume fraction of the conducting phase, is estimated as 7 vol%, and thus the composites containing
7 vol% or a little more of carbon is expected to show asensitive resistivity increase by the volume increase of
the insulating phase. The earlier studies theoretically
predicted that the pc is 7.6 vol% when the particle size
ratio of insulating to conducting particles is 8 to 1.22
This theoretical prediction is coincident with thepresent result.

As shown in Fig. 2, the resistivity of the composite
pellet containing 10.0 wt% (10.9 vol%) of carbon
showed a stepwise increase when the gas concentrationof chloroform was increased stepwise, and it returned to
its initial level when chloroform-free air was introduced.
It is well known that organic montmorillonite incorporates organic compounds in the interlayer.14–16,18,20,21
The volume of the organic montmorillonite phase isconsidered to be increased due to the extension of theinterlayer spacing. The 90% response time was in therange of 180–230 min and not fast enough for practicaluse. Figure 3 shows the result for the thick-film sample
containing 17.0 wt% (18.5 vol%) of carbon in responseto the change of dichloromethane concentration. The
90% response time was drastically improved to 2–5 min
by fabricating thick films using the collected finer particles. We can say that the response time reached thelevel of practical use. The contact area with the gas was
substantially increased compared with the pellet sample,

Fig. 1. Resistivitiy of an organic montmorillonite–carbon
composite as a function of the volume fraction of the organicmontmorillonite.

Fig. 3. Resistivity change of a composite thick film containing
17.0 wt% of carbon when the air containing dichloromethane wasintroduced and its concentration was changed stepwise at 201C.

Results and Discussion

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Gas Sensor for Volatile Organochlorine Compounds

and this accelerated the diffusions into both the composite and the interlayer. From our adsorption rate
study using pellets, thick films, and powders, we found
the form of the thick films contributes greater to theimprovement of the response time than using smaller
particles. We must mention that the resistance values
were slightly different at the same dichloromethane concentration in increasing and decreasing processes. We
consider this is because smaller particles that surroundlarger particles in the thick film are difficult to return totheir original position after expansion and contraction.
This can be solved by using homogeneous nanoparticles. Figure 4 shows the resistance change in response toincrease of trichloroethylene in low concentration range
476–6188 ppm. The resistance reached equilibrium values in 10 min after going through sharp maximum values. Such overshooting behaviors are considered to
become obvious in the low concentration range due tocompetitive adsorption by the composite and the chamber walls. The detection limit for trichloroethylene wasestimated at 116 ppm from the S/N ratio of the resistance change.
The resistance changes similar to Fig. 3 were also
observed for chloroform, trichloroethylene, tetrachloroethylene, tetrachloromethane, acetone, ethanol, and2-propanol so far. The composites of organic montmorillonite and carbon detect the flammable volatileorganic compounds as well as the inflammable volatileorganochlorine compounds, because the organic molecules are incorporated due to the organic affinity of theinterlayer compound (DMDO) in organic montmorillonite. The inflammable organochlorine compounds
can be distinguished from the flammable organic com-

Fig. 4. Resistivity change of a composite thick film containing
17.0 wt% of carbon when the concentration of trichloroethylene
was increased stepwise from 476 to 6190 ppm at 201C.

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Fig. 5. SEM image of the composite thick film. The flake-likeorganic montmorillonite particles are observed in the left twothirds. The conduction path is provided by the carbon powders
segregated and aggregated in the grain boundaries between themontmorillonite particles as observed in the crack in the right
one-third.

pounds by using concurrently with the flammable gas
sensor. Our future challenge is to give the composites
the selectivity among the organochlorine compounds.
Figure 5 shows the SEM image of the thick-film
surface. The flake-like organic montmorillonite particles
are accumulated and oriented along the [001] plane in
the left two-thirds of the image. The carbon powders aresegregated and aggregated providing a conduction pathin the grain boundaries between the montmorillonite
particles, which is observed in the crack in the right onethird of the image. Figure 6 shows powder XRD patterns of the organic montmorillonite powders before
and after exposition to chloroform-saturated atmosphere for 24 h. In general, powder XRD patterns of
two dimensional crystals show asymmetric (hk0) reflections tailing to higher angles, (00l ) reflections like thoseof the three-dimensional crystals, and no (hkl ) reflections (h, k, la0).23,24 The organic montmorillonite
showed the XRD pattern characteristic of the two dimensional crystals. The d value 3.633 nm for (001) isthree times larger than that of ordinary inorganic montmorillonite 1.241 nm, and this corresponds with thefact that the organic montmorillonite contains 49.9% of
DMDO in the interlayer and the density is about twothirds of the ordinary one. The d values of (001), (002),
and (003) calculated from peak positions after Lorentzpolarization correction were increased by 6.9–7.8%
from 3.344, 1.847, and 1.230 to 3.603, 1.975,
and 1.322 nm, respectively, by the incorporation of

1412International Journal of Applied Ceramic Technology—Tsuyumoto, Iida and Hori

Fig. 6. Powder XRD patterns of the organic montmorillonite
before and after chloroform incorporation. The organic
montmorillonite powder was left in a chloroform-saturated
atmosphere at 201C for 24 h. Inset shows the XRD pattern change
in the desorption process of the organic montmorillonite having
adsorbed chloroform (the bottom black line, the sample having
adsorbed 127 wt% of chloroform; the second line from the bottom,
the sample left for 8 min in air; the third line, the sample left for
16 min in air; the top gray line, the sample left for 24 min in air).
The patterns in the inset are deviated for clarity after Lorentzpolarization correction.

chloroform, whereas the d value of (100) remained thesame at 0.4485 nm. This indicates that the interlayer
spacing was increased by the incorporation of theorganic compounds due to the organic affinity of theinterlayer DMDO and supports the sensing mechanismthat the volume increase of the insulating phase brings
about the resistivity increase. A little discrepancy in lattice parameters c calculated from each (00l ) (l 5 1–3)
reflection is due to profile broadening derived from bothsize and distortion of interferential coherence domain,and such discrepancy has been also reported for ordinary hydrated montmorillonite.25 It is interesting
to note that the intensity of (001) was remarkably
decreased by the incorporation and increased by thedesorption whereas the intensities of (002) and (003)
showed little changes. This suggests that the incorporation of chloroform brought about the distortion of thedomain rather than the change in the size, because thesize effect on the profile is independent of the orders(00l ) and the distortion effect is dependent on theorders.24 Additionally, the XRD pattern of the sample
pellet showed higher relative intensities of (00l ) thanthose of the powder sample, indicating the orientation
along [001] plane in the pellet.
In our present and previous papers,5 the new chemical sensing principle based on the percolation was formulated using the composite of clay and carbon. So far,Vol. 8, No. 6, 2011

we demonstrated the detection of volatile organochlorine compounds and flammable organic compounds
using organic montmorillonite as well as water vapor
using ordinary montmorillonite. The sensing principle
is inherently advantageous in sensitivity as well as cost
reduction, and thus there are various potential applications such as gas alarms in working environment, disposable test strips, water quality monitor, odor sensors,
and taste sensors. Selectivity in detection can beachieved by combining the present principle with antigen–antibody reactions, for example, by modifying theinterlayer with antibody, which specifically reacts withthe objective substance. It is our future challenge to apply specific immune reactions to the present gas sensors
and to establish a new category of biosensors. Furthermore, immunoassay tests using the absorptiometry havebeen used for semiquantitative analysis of environmental pollutants such as pentachlorophenol, polychlorinated biphenyl, p,p0 -dichlorodiphenyltrichloroethane, andthese test methods were approved by the U.S. EPA.26
Electrical detection in immunoassay tests will be possible by using antibody-modified clay composites insteadof test tubes or microtiter plates, and this technique will
be useful in detection on a microchip.

Conclusions
We demonstrated that volatile organochlorinecompounds such as chloroform, dichloromethane,
trichloroethylene, tetrachloroethylene, and tetrachloromethane can be detected by a gas sensor consisting of
the composite of organic montmorillonite and carbon.
The resistivity of the composite was increased in
response to the concentration increase of the volatileorganochlorine compounds, because the volume of theinsulating phase in the composite was increased byincorporation of the organochlorine molecules. The
response time was greatly improved from 180–230 to
2–5 min by changing the sample form from the pellet
composite to the thick film composite, because thediffusions into both the composite and the interlayer
is accelerated due to the increase of the contact area withthe gas. The XRD patterns indicated that the lattice
parameters c was increased by incorporation of the organochlorine compounds, and this supports the mechanism that the volume increase of the insulating phasebrings about the resistivity increase. The detection limit
for trichloroethylene was estimated at 116 ppm. The

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Gas Sensor for Volatile Organochlorine Compounds

resistivity change was also observed for flammable gases
such as acetone, ethanol, 2-propanol, and water vapor.
More sensitive and more selective detection will be possible by modifying the interlayer with peptide or antibody, which specifically reacts with the objective
substance.
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