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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

Journal of Nanophotonics 012522-1 Jan–Mar 2016 • Vol. 10(1)


http://dx.doi.org/10.1117/1.JNP.10.012522http://dx.doi.org/10.1117/1.JNP.10.012522http://dx.doi.org/10.1117/1.JNP.10.012522http://dx.doi.org/10.1117/1.JNP.10.012522http://dx.doi.org/10.1117/1.JNP.10.012522mailto:[email protected]:[email protected]:[email protected]

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.

Kondrashov et al.: Electrical properties of gas sensors based on graphene and single-wall carbon nanotubes



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.




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.




[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.



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

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