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Electrical properties of gas sensorsbased on graphene and
single-wallcarbon nanotubes
Ivan I. KondrashovIgor V. SokolovPavel S. RusakovMaxim G.
RybinAlexander A. BarminRazhudin N. RizakhanovElena D.
Obraztsova
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Electrical properties of gas sensors based on grapheneand
single-wall carbon nanotubes
Ivan I. Kondrashov,a,* Igor V. Sokolov,a,b Pavel S.
Rusakov,a
Maxim G. Rybin,a,c Alexander A. Barmin,b Razhudin N.
Rizakhanov,b andElena D. Obraztsovaa,c
aA. M. Prokhorov General Physics Institute, 38 Vavilov Street,
Moscow 119991, RussiabKeldysh Research Center, 8 Onejskaya Street,
Moscow 125438, Russia
cNational Research Nuclear University MEPhI (Moscow Engineering
Physics Institute), 31Kashirskoye shosse, Moscow 115409, Russia
Abstract. Here, we present investigation of the influence of
different gases (carbon dioxide,ammonia, and iodine vapor) on the
sensory properties of graphene and single-wall carbon nano-tube
films. The gas molecules are adsorbed by carbon films (graphene or
nanotubes) and changethe film’s electrical resistance. In the
course of this work, the setup for studying the electro-physical
properties of carbon nanomaterials has been designed and
constructed in the lab. Withthis home-made equipment, we have
demonstrated a high efficiency of graphene and nanotubesas
adsorbents of different gases and a possibility to use these
materials as gas sensors. We havealso performed a chemical
modification of graphene and carbon nanotubes by attaching
thenanoparticles of calcium carbonate (CaCO3) to improve the
sensitivity and selectivity of sensors.© 2016 Society of
Photo-Optical Instrumentation Engineers (SPIE) [DOI:
10.1117/1.JNP.10.012522]
Keywords: graphene; carbon nanotubes; gas sensors.
Paper 15132SS received Oct. 13, 2015; accepted for publication
Jan. 7, 2016; published onlineJan. 27, 2016.
1 Introduction
Today, the identification of environmental gases is more and
more important for solving variousproblems, such as global warming,
exhaust emissions, acid rain, destruction of the ozone layer,etc.
One of the promising types of gas sensors is the adsorption sensor
based on changing theelectrical conductivity of an active material
(adsorbent) in the case of adsorption of gas mol-ecules (adsorbate)
on the sensor material’s surface. The adsorption sensors are most
promisingbecause of their high compactness, sensitivity, and energy
efficiency.1,2 The adsorption of gaseson the active element surface
changes its electrical resistance due to the donor or acceptor
mecha-nism of redistributing electrons in the surface layer. This
principle is used in sensors with nano-carbon active elements,
namely, nanocrystalline and microcrystalline graphite,
single-walled andmultiwalled carbon nanotubes, fullerenes, and
graphene.3–4
Thus in this work, we present a detailed investigation of the
influence of different gases onthe sensory properties of graphene
and single-wall carbon nanotube (SWCNT) films. The char-acteristics
of gas sensors were measured using special home-made equipment to
obtain the con-tinuous sensor electric resistance change under
exposure to different gases in an air atmosphereat room
temperature. Nanomaterials for various engineering applications
(nanowires, nanotubes,and nanoribbons) are promising for use in
miniaturized chemical and biological sensors. This isthe reason to
try to substantially modulate their electrical properties
(electrical conductivity andcapacitance) in the case of contact
with the sample gas, the possibility to vary their
electricalproperties by changing the chemical composition and
geometry of the nanostructures, as well asan attempt to easily
embed them in nanoelectronic devices.5 The main advantages of
SWCNTsand graphene as the sensor elements are the unique high
absorbent capacity (a high ratio of
*Address all correspondence to: Ivan I. Kondrashov, E-mail:
[email protected]
1934-2608/2016/$25.00 © 2016 SPIE
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surface area to volume),6 the radiation stability (aerospace
industry),7 the high sensitivity, and theconvenience of their
possible functionalization. It is effective to use thin films of
nanomaterialsto provide access for the gaseous medium to the
surface of a conductive material for increasingthe adsorption of
gas molecules. We observed different reactions of sensors,
depending on thedonor or acceptor mechanism of the redistribution
of electrons between the gas and the sensorsurface. Due to the
small sensor size, good selectivity can also be achieved in the
case of usingseveral different sensors at the same time. This
allows determining the composition changes ofthe gaseous atmosphere
with good accuracy.8
2 Experimental Details
Graphene films were formed by a chemical vapor deposition (CVD)
method on metal foil.9–11
The foil was attached to a polymer, then placed in the solution
of the etchant. After metal foiletching, graphene can be
transferred onto any substrate from the polymer. Graphene films
usedin the work contained 3 to 4 layers. SWCNT films were
synthesized by the aerosol-CVDmethod.12 They consist of a network
of SWCNTs having an average diameter of about1.9 nm (Fig. 1). The
film consists of 1∕3 metallic CNTs and 2∕3 semiconducting CNTs.The
film thickness is about 100 nm. The SWCNT films were originally
deposited on the filterwith a weak adhesion. Then they simply were
reprinted on the desired substrate.
Later, these materials were used as the sensing elements.
Several types of substrates, namelya coverslip, quartz, silicon,
and sapphire, were used for gas sensors. We transferred carbon
filmson the substrate via two main approaches, depending on the
method of contact connecting: thenanotubes and graphene thin films
were deposited on ceramic substrates with interdigitated
goldelectrodes [Fig. 2(a)] and on a substrate with silver
electrodes [Fig. 2(b)]. In the first case, theelectrodes were
fabricated by photolithography, and Ti or Au sputtering (total
thickness of about100 nm) on silicon oxide.
The interdigitated electrodes were used in experiments with CO2,
the silver electrodes—inexperiments with NH3 and I2. The fabricated
sensors have quite a low electrical resistance, in therange of 20
Ω.
Several types of gases (Ar, CO2, NH3, and iodine vapor) were
employed for gas sensingapplications. The sensors were placed in a
chamber with an electrical feedthrough. Argongas was continuously
used as the carrier gas throughout the work. After purging the
chamberwith pure argon and waiting for stabilization of the
electrical resistance of the carbon film, thegas to be tested was
injected into the chamber (Fig. 3). The electrodes from the films
were con-nected to an analog-to-digital converter which was used to
monitor the values of conductivity
Fig. 1 SEM image of as-prepared SWCNT film.
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every second on the PC. All the measurements were taken at room
temperature and under normalpressure.
In order to improve the gas sensor characteristics, we used
SWCNTand graphene films func-tionalized with nanoclusters of CaCO3.
To functionalize the films with CaCO3, we added asolution of sodium
carbonate in water to a solution of calcium chloride in water, then
the sub-strates with films were dipped into the mixture. CaCO3
particles start to precipitate on the films.After a few minutes, we
removed and dried them. A similar method was developed earlier
andwas used to obtain a better selectivity (compared to that of
as-prepared films) to certain gases.13–15 Relying on similar
methods for CaCO3 particle preparation in other works, we estimate
theaverage cluster size as 50 nm to 30 μm.
3 Results and Discussion
A series of experiments on the sensory properties of graphene
and SWCNTs were conductedwith different gases. It was found that
the sensitivity, selectivity, and response time of the
sensorsstrongly depend on the active material and the tested gas.
The measurements of the sensoryproperties based on the electrical
conductivity of the adsorbent film are presented later. A
typicalform of conductivity graph in normal conditions is a
horizontal line. It looks the same uponexposure to argon, because
argon provides practically no transfer of electrical
charge.Graphene and SWCNT films showed a good temporal stability
and a zero response in theabsence of changes in the gas
environment.
Since the SWCNTs and the graphene layers originally had p-type
conductivity, the injectionof electrons led to the decrease of hole
concentration, i.e., the conductivity of the sensory ele-ments was
reduced. The amplitude of the response to the exposure of NH3 has a
large value,which indicates the high sensitivity of the sensor to
this gas. As seen in Fig. 4(a) (bottom), the
Fig. 3 The scheme of installation for investigation of SWCNT-
and graphene-based gas sensorproperties.
Fig. 2 (a) The image of a substrate with interdigitated gold
electrodes; and (b) an image of a sub-strate with silver
electrodes.
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graphene and single-wall carbon nanotubes
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characteristics of the sensor based on graphene layers have the
form of steps. This means that theadsorption on defects and the
intercalation are the main mechanisms of the adsorption ofNH3 bythe
sensory nanomaterial. At the same time, the steps also appear in
the characteristics of thesensor based on SWCNTs. This means that
the adsorption of NH3 on the surface of sensorsbased on SWCNTs
occurs in two ways: by a physical gas adsorption (peak components
andfast relaxation) and by a slow mechanism of adsorption on
defects and intercalation (plateau).
The maximum sensor response was found upon the exposure to
iodine molecules [Fig. 4(b)].This is due to the fact that the
active element of the gaseous element has an extremely
highefficiency of hole injection into the sensory nanomaterial. In
general, the gas sensors haveshown a good performance in terms of
sensitivity and response time. For both gases, the sensorsshowed a
fast response of not more than 30 s. This indicates that the
sensors are highly sensitivetoward NH3 and iodine adsorption at
room temperature.
In Fig. 5, the electrical conductivity variations of pure
graphene and SWCNT films uponinjection of CO2 are presented. The
response is an additional peak of increased conductivitywhich
occurs when CO2 molecules are adsorbed onto the surface of SWCNTs.
The adsorbedmolecules inject holes into the nanotubes and graphene,
and their conductivity (originally, of p-type) increases
dramatically. When the gas injection is completed, the adsorbed
molecules rap-idly desorb from the surface. A fast response of the
sensor is explained by the fact that in thiscase, a physical gas
adsorption takes place. Moreover, the sensor after the injection of
gas wasnot recovered. At the same time, the response of graphene
was much weaker. Such difference inthe responses of the two
materials is associated with the different mechanisms of CO2
adsorptionon the graphene layers: molecules are adsorbed on defects
and penetrate into the spaces betweengraphene layers
(intercalation). On one hand, these processes are slower,
therefore, there is alonger response of the sensor based on
graphene layers compared with that based onSWCNTs. On the other
hand, in this way, the adsorbed CO2 molecules bond with the
surfaceof the sensor nanomaterial more effectively.
The graphene and SWCNT films were covered with CaCO3
nanoclusters to improve theselectivity and sensitivity of the
sensors. A significant improvement of the amplitude
responsecompared with the sensors based on unmodified SWCNT and
graphene layers can be seen
Fig. 4 The relative electrical conductivity changes of graphene
and SWCNT films upon injection ofNH3 gas (a) and iodine molecules
(b).
Fig. 5 The relative electrical conductivity changes of pure (a)
and modified (b) graphene andSWCNT films upon injection of CO2
gas.
Kondrashov et al.: Electrical properties of gas sensors based on
graphene and single-wall carbon nanotubes
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[Fig. 5(b)]. This is obvious for the sensor based on graphene
layers: in the bottom graph ofFig. 5(a), there is no response upon
CO2 exposure, while in the bottom graph in Fig. 5(b),there is a
significant response. However, the response character has changed.
Since a basicmechanism of the interaction of sensory elements with
a gas is a chemical adsorption, the sensorresponse was slower [the
peak intensity decreases slowly in the two graphs in Fig. 5(b)]. In
thecharacteristics of the sensor based on modified graphene layers,
a fast response was noted. Thisprobably happens due to the
appearance of additional channels of physical CO2 gas adsorptionand
the impossibility of gas adsorption on graphene defects and
intercalation between the layers(the defects are occupied by CaCO3
nanoclusters, and that complicates the gas penetration intothe
interlayer space).
These results prove a need to use the modified films for
improving the sensory properties ofgas sensors.
4 Conclusion
SWCNT and graphene films on different substrates have been used
for gas sensing applications.These sensors possess a high
sensitivity and a fast response to exposure with NH3, iodine,
andCO2 molecules. The recovery process is completed only for SWCNT
films. Our results haveshown that SWCNT and graphene have a great
potential for application as excellent gas sensorsat room
temperature. To improve the sensitivity and selectivity of the
sensor, it is necessary to usefunctionalized graphene and SWCNT
films. Thus, our future work will concentrate on furtherenhancing
the performance of these nanocarbon-based gas sensors.
Acknowledgments
The work was supported by the RSF project 15-12-30041 and a
research project with theKeldysh Research Center. Kondrashov I.I.
thanks the RFBR project 14-02-31639_mol_a forthe partial
support.
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Ivan I. Kondrashov graduated from the photonics and microwave
physics chair of the physicsdepartment of M.V. Lomonosov Moscow
State University (MSU) in 2012. Since 2012, he hasworked at the
A.M. Prokhorov General Physics Institute (GPI) as a PhD student.
His scientificinterests concern synthesis and gas sensory
properties of graphene and SWCNTs. He is a co-author of two papers
and more than 10 theses in reviewed journals.
Igor V. Sokolov received his master’s and PhD degrees from the
Russian State TechnologicalUniversity named after K.E. Tsiolkovskii
in 2002 and 2006, respectively. Currently, he is a lead-ing
engineer at the All-Russia Research Institute of Automatics. The
scope of his scientific inter-ests includes plasmo-chemical
synthesis and etching of nanomaterials.
Pavel S. Rusakov graduated from the photonics and microwave
physics chair of the physicsdepartment of MSU in 2012. Since 2012,
he has worked at the A.M. Prokhorov General PhysicsInstitute (GPI)
as a PhD student. His scientific interests concern synthesis of
graphene and itsapplication in laser physics. He is a coauthor of
four papers in reviewed scientific journals.
Maxim G. Rybin graduated from the faculty of physics at
LomonosovMoscow State Universityin 2009. He received his PhD in
laser physics from GPI in 2012. He received his second PhDfrom
Central Lyon School, Lyon, France, in 2013. Since 2007, he has
worked at GPI as aresearcher. His scientific interests concern
synthesis, characterization, and application of gra-phene
structures. He is a coauthor of 12 papers in reviewed journals.
Alexander A. Barmin graduated from the department of general and
applied physics at theMoscow Institute of Physics and Technology
(MIPT) in 2001. He received his PhD in thermalphysics and
thermology at MIPT in 2012. Since 2000, he has worked at the SSC
FSUE KeldyshResearch Center. His scientific interests concern
physical properties and mechanical character-istics of functional
and engineered nanostructure materials. He is a coauthor of more
than 20papers in reviewed journals.
Razhudin N. Rizakhanov has received his Ms, PhD, and Doctorate
degrees from the MoscowInstitute of Physics and Technology in 2009.
Currently, he is a director of the NanotechnologyDepartment at the
SSC FSUE Keldysh Research Center. His scientific interests concern
synthe-sis, modification, and applications of different (including
carbon) nanomaterials.
Elena D. Obraztsova graduated from the physics department of MSU
in 1981. She received herPhD in optics at MSU in 1990. Since 1992,
she has worked at the A.M. Prokhorov GeneralPhysics Institute, RAS,
heading the Nanomaterials spectroscopy laboratory since 2001. Her
sci-entific interests concern optical spectroscopy of
low-dimensional materials. She is a coauthor ofmore than 230 papers
in reviewed journals. She was a supervisor of 10 PhD defended
theses.
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graphene and single-wall carbon nanotubes
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http://dx.doi.org/10.1021/la7026797http://dx.doi.org/10.1016/S0925-4005(00)00429-9http://dx.doi.org/10.1016/S0925-4005(99)00062-3