Carbon nanotubes Sensor properties A Review Author’s Accepted Manuscript Carbon nanotubes Sensor properties A Review Irina V Zaporotskova, Natalia P Boroznina, Yuri N Parkhomenko, Lev V Kozhitov PII S[.]
Author’s Accepted Manuscript Carbon nanotubes: Sensor properties A Review Irina V Zaporotskova, Natalia P Boroznina, Yuri N Parkhomenko, Lev V Kozhitov www.elsevier.com/locate/moem PII: DOI: Reference: S2452-1779(17)30017-8 http://dx.doi.org/10.1016/j.moem.2017.02.002 MOEM50 To appear in: Modern Electronic Materials Cite this article as: Irina V Zaporotskova, Natalia P Boroznina, Yuri N Parkhomenko and Lev V Kozhitov, Carbon nanotubes: Sensor properties A R e v i e w , Modern Electronic Materials, http://dx.doi.org/10.1016/j.moem.2017.02.002 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Carbon nanotubes: Sensor properties A Review Irina V Zaporotskova1,*, Natalia P Boroznina1, Yuri N Parkhomenko2, Lev V Kozhitov2 Volgograd State University, 100 Universitetskii Prospekt, Volgograd 400062, Russia National University of Science and Technology MISiS, Leninskiy Prospekt, Moscow 119049, Russia irinazaporotskova@gmail.com n.z.1103@mail.ru parkh@rambler.ru kozitov@misis.ru * Corresponding author Abstract Recent publications dealing with dealing with the fabrication of gas and electrochemical biosensors based on carbon nanotubes have been reviewed Experimental and theoretical data on the working principles of nanotubes have been presented The main regularities of the structure, energy parameters and sensor properties of modified semiconducting systems on the basis of cabon nanotubes have been studied by analyzing the mechanisms of nanotubule interaction with functional groups (including carboxyl and amino groups), metallic nanoparticles and polymers leading to the formation of chemically active sensors The possibility of using boundary modified nanotubes for the identification of metals has been discussed Simulation results have been reported for the interaction of nanotubes boundary modified by —СООН and —NH2 groups with atoms and ions of potassium, sodium and lithium The simulation has been carried out using the molecular cluster model and the MNDO and DFT calculation methods Sensors fabricated using this technology will find wide application for the detection of metallic atoms and their ions included in salts and alkali Keywords: carbon nanotubes, sensor properties, sensors on the basis of carbon nanotubes, boundary modified nanotubes, carboxyl group, amino group Introduction The current stage of research into the nanotubular forms of materials is characterized by a great interest to their synthesis methods, improvement of these synthesis methods, study of the properties and attempts of industrial applications of these nanomaterials Systems of this type attract interest thanks to a combination of multiple properties that cannot be achieved in conventional single crystal and polycrystalline structures Nanomaterials are defined as materials the sizes of which in at least one dimension are in the 1—100 nm range [1—3] Their shapes may be zero-dimensional (0D) and one-dimensional (1D) nanostructures 0D nanostructures include, for example, quantum dots [1] Quantum dots were used as a structural material for multiple applications including memory modules, quantum lasers and optical sensors The discovery of carbon nanotubes (1D nanostructures) is one of the most important achievements of the advanced science This existence form of carbon is intermediate between graphite and fullerenes However, many of nanotube properties are drastically different from those of the abovementioned forms of carbon Therefore nanotubes (or nanotubulenes) should be considered as a new material with unique physicochemical properties showing good promise for a wide range of applications [4—8] Carbon nanotubes (CNT) can find applications in a great number of areas such as additives to polymers and catalysts, in autoelectron emission for cathode rays of lighting components, flat displays, gas discharge tubes in telecommunication networks, absorption and screening of electromagnetic waves, energy conversion, lithium battery anodes, hydrogen storage, composite materials (fillers or coatings), nanoprobes, sensors, supercapacitors etc [9, 10] The great variety of the new unconventional mechanical, electrical and magnetic properties of nanotubes can become the starting point for a breakthrough in nanoelectronics As a nanotube is a surface structure, its whole weight is concentrated in its surface layers This feature is the origin of the uniquely large unit surface of tubulenes which in turn predetermines their electrochemical and adsorption properties [11] The high sensitivity of the electronic properties of nanotubes to molecules adsorbed on their surface and the unparalleled unit surface providing for this high sensitivity make CNT a promising starting material for the development of superminiaturized chemical and biological sensors [12, 13] The operation principle of these sensors is based on changes in the V—I curves of nanotubes as a result of adsorption of specific molecules on their surface The use of CNT in sensor devices is one of their most promising applications in electronics These sensors should have high sensitivity and selectivity, as well as fast response and recovery Below we provide a review of recent works that dealt with the development of CNT based sensors and study of their working mechanisms, and generalize available theoretical and experimental literary data on the alkaline metal sensitivity of carbon tubulenes boundary modified by functional groups Structural features of carbon nanotubes Carbon nanotubes were discovered and described by S Injima, Japan, in 1991 One of the amazing phenomena associated with the nanotubes is the dependence of their properties on their shape Nanotubes are elongated cylindrical structures with diameters of to several dozens of nanometers and lengths of up to several microns consisting of one or several hexagonal graphite planes rolled in tubes Their surface consists of regular hexagonal carbon cycles (hexagons) [4— 10] Depending on nanotube synthesis conditions, one- or multilayered tubulenes with open or closed terminations may form The structure of tubulenes is typically described in terms of infinite cylindrical surfaces accommodating carbon atoms interconnected into a single network with hexagonal cells, i.e the sp2-network The mutual orientation of the hexagonal network and the longitudinal axis of a nanotube determines an important structural property of the tubulene, i.e its chirality The chirality of a nanotube is described by two integers (n and m) that locate the hexagon of the network which will match after nanotube rolling with the hexagon that is in the origin of coordinates The chirality of a nanotube can also be uniquely specified by the angle Θ (Θ is the orientation angle or the chiral angle) formed by the nanotube rolling direction and the direction of the common edge of two adjacent hexagons There are multiple nanotube rolling options, but of special interest are those which not distort the structure of the hexagonal network These optional rolling directions are those at the angles Θ = and 30 arc deg corresponding to the (n, 0) and (n, n) chiralities, respectively The orientation (or rolling) angle determines the electrical properties of CNT They can exhibit either metallic or semiconductor conductivity types However, most nanotubes are semiconductors with a 0.1 to 0.2 eV band gap Controlling their band structure one can obtain a variety of electronic devices [10] It is a common practice to subdivide the CNT in two types, i.e the achiral and chiral ones The chiral tubulenes have a screw symmetry, while the achiral ones have a cylindrical symmetry and are further divided in two types In one of the achiral CNT types, two edges of each hexagon are parallel to the cylinder axis These are the so-called zig-zag nanotubes (Fig a) In the other type of the achiral CNT two edges of each hexagon are perpendicular to the cylinder axis, these being the so-called arm-chair nanotubes (Fig b) Generally, the CNT can be described by specifying the chiral vector Ch: Ch = na1 + ma2, (1) as well as the tube diameter dt, the chiral angle Θ and the basic translation vector T (Fig 2) The vector Ch connects the two crystallographically equivalent states O and A on a twodimensional (2D) graphene plane in which carbon atoms are located Figure shows the chiral angle Θ of a zig-zag type nanotube (Θ = 0) and the unit vectors a1 of а2 of the hexagonal lattice The angle Θ = 30 arc deg corresponds to an arm-chair tubulene The pairs of the symbols (n, m) specify different methods of graphene surface rolling to a nanotube The differences in the chiral angle Θ and the tube diameter dt cause differences in the properties of the CNT In the (n, m) notation system used for exactly specifying the chiral vector Ch, the notation (n, m) in Eq (1) refers to chiral symmetry tubulenes, (n, 0) refers to zig-zag tubulenes and (n, n) refers to armchair tubulenes The higher the value of n the greater the diameter of the tube In the terms of the (n, m) indexes, the diameter of a tubulene can be written as follows: dt = Ch/π = 3ac – c m2 mn n2 1/2 , where ac–c is the difference between the nearest carbon atoms (0.1421 nm for graphite) and Сh is the length of the chiral vector Ch The chiral angle Θ is specified by the following expression: 3m tan 1 m 2n To study the properties of the CNT as one-dimensional (1D) systems one should specify the lattice vector Т oriented along the tubulene axis orthogonally to the chiral vector Ch (Fig 2) The vector Т of a chiral tubulene can be written as follows: T 2m n a1 2n m a2 dk whereas the following statement is true for dk: d if n – m is not a multiple of 3d dk = 3d if n = m is a multiple of 3d where d is the greatest common divisor of (n, m) Gas sensors based on carbon nanotubes As a nanotube is a surface structure, its whole weight is concentrated in the surface of its layers This feature is the origin of the uniquely large unit surface of tubulenes which in turn predetermines their electrochemical and adsorption properties The extremely high adsorption capacity of the CNT and the excellent sensitivity of the CNT properties to atoms and molecules adsorbed on their surface [8] provide the possibility of designing sensors on the basis of nanotubes [12—14] Currently, several types of gas sensors (detectors) on the basis of the CNT are discussed in luterature: – sorption gas sensors; – ionization gas sensors; – capacitance gas sensors; – resonance frequency shift gas sensors We will consider these types of sensors in detail Sorption gas sensors Sorption gas sensors are the largest group of gas sensors [13] Their main operation principle is adsorption during which an adsorbed gas molecule transfers an electron to or takes it from a nanotube This changes the electrical properties of the CNT, and this change can be detected There are gas sensors based on pure CNT including mono- and multilayered ones, as well as those based on CNT modified by functional groups, metals, polymers or metal oxides It is well known that monolayer CNT are sensitive to gases, e.g NO2, NH3 and some volatile organic compounds due to a change in the conductivity of the nanotubes as a result of gas molecule adsorption on their surface [15, 16] A sensor was designed [16] for detecting gases and organic vapors at room temperature the detection limit of which was as low as 44 ppb for NO2 and 262 ppb for nitrotoluene The recovery time of that sensor was ~10 h due to the high bond energy between the CNT and some gases Then [17] this recovery time was reduced to 10 by exposing to UV radiation which facilitated the desorption of gas molecules The same gases were detected with another sensor [18] based on a field-effect transistor in which the conducting channel was one semiconducting monolayer CNT The response time of the device was within a few seconds, and the response defined as the ratio between the resistivity before and after gas exposure was approx 100—1000 ppm for NO2 Three models were proposed for explaining the action of that sensor: – charge transfer between a nanotube and a molecule adsorbed on its surface; – molecular strobing of nonpolar molecules e.g NO2 which shift the conduction threshold of the CNT; – change of the Schottky barrier between the nanotube and the electrodes [19, 20] In transistor based sensors the energy barrier of CNT adsorption for dimethylmethylphosphonate [21], NH3 [22] or NOx [23] can be reduced by applying positive bias to the gate This causes electron tunneling through a narrow barrier To reduce the sensor recovery time after gas detection by a sorption mechanism, attempts were made to accelerate gas desorption by heating sensor detectors The operation of a monolayer CNT based sensor for NH3 detection was analyzed [24] Gas exposure leads to electron transfer from NH3 to the tube resulting in the formation of a spatial charge region on the surface of the semiconductor CNT and hence an increase in its electrical resistivity The device reached saturation at a concentration of ~40 ppm The sensor recovered completely to the initial state at 80 °C Fabrication of sensor detectors by template printing followed by annealing in air at different temperatures for h was reported [25] Those sensors were used for NH3 detection After 10 gas exposure at room temperature the sensor resistivity increased by 8% compared to the initial level The conduction type of the CNT changed from semiconducting at moderate temperatures (< 350 °С) to metallic at high temperatures (> 350 °C) The possibility of fabricating multilayered CNT (MCNT) based sensors was discussed [26, 27] The resistivity of the sensor proved to change due to the p conductivity type in semiconducting MCNT and the formation of Schottky barriers between nanotubes having metallic conductivity type and those having semiconductor conductivity type during gas adsorption An electrochemical gas sensor was designed on the basis of modified multilayered CNT films for Cl2 detection [28] The sensor’s surface which was the cathode was exposed to chlorine gas, and the resulting galvanic effect was measured The nanotubes acted as the microelectrode The recovery time of that sensor was ~150 s Another sensor on the basis of ultrathin CNT films [29] was used for NO2 and NH3 detection at room temperature The authors proposed a method of synthesizing ~5 nm thick films with a high density of nanotubes ensuring high sensitivity and reproducibility of the sensor, i.e ppm for NO2 and ppm for NH3 Gas desorption was accelerated by UV exposure Gas sensors on the basis of oriented CNT were described [30] The resistivity of the CNT films declined after NO2 exposure and increased after NH3, ethanol and C6H6 exposure A nanotube film can be described as a network of highly efficient resistors consisting of the resistances of every single CNT and the resistivity of the sites and tunnels between adjacent nanotubes A vertical transport type detector was suggested [31] on the basis of regular CNT arrays for an NH3 gas sensing The detector had high sensitivity and response time (less than min) and good recovery at atmospheric pressure and room temperature It provided NH3 detection in the 0.1—6 % range CNT modification by functional groups, metal nanoparticles, oxides and polymers changes the electronic properties of the nanotubes and increases their selectivity and response to specific gases Noteworthy, the interaction of target molecules with different functional groups or additives varies significantly CNT are often modified by adding the carboxyl group — СООН This group creates reactive sections at the terminations and the side walls of the CNT where active interaction with various compounds occurs For example, it was shown [32] that sensors synthesized from carboxylated monolayer CNT were sensitive to CO with a ppm detection limit whereas pure monolayer CNT did not respond to this gas The NO2 gas sensitivity of monolayer CNT functionalized by the amino group —NH2 was studied [33] The amino group acts as a charge transfer agent of the semiconducting CNT that increases the number of electrons transferred from the nanotube to the NO2 molecule There are also gas sensors on the basis of CNT functionalized by polymers that show good performance at room temperature [34, 35] They can be used as conductometric, potentiometric, amperometric and volt-amperometric converters for the detection of a wide range of gases it was shown [36] that field effect transistors based on monolayer CNT modified by polyethyleneimine can be used as gas sensors with improved response and selectivity for NO2, CO, CO2, CH4, H2 and O2 These sensors were able to detect less than ppm NO2 within a response time of 1—2 It was demonstrated [37] that functionalized monolayer CNT with attached poly(sulfonic acid m-aminobenzene) have higher sensitivity to NH3 and NO2 compared with carboxylated nanotubes These systems exhibited sensitivity to ppm NH3 CNT modification by polymers also improves their sensitivity to organic compound vapors A compact wireless gas sensor was designed [38] on the basis of monolayer CNT + polymethylmetacrylate (PMMA) The sensor exhibited a fast response (2—5 s) and an increase in resistivity by 100 orders of magnitude after exposure to dichloromethane, chloroform and acetone vapors The sensor recovered to the initial state immediately after gas removal The sensor’s action mechanism was accounted for by polymer response to the adsorption of organic vapors by PMMA and charge transfer from polar organic molecules adsorbed on the surface of the nanotubes The working principle of an integrated system on the basis of monolayer CNT and polymer cellulose was described [39] A cellulose layer was applied to the surface of the conducting CNT which was used as a gas sensor for the detection of benzene, toluene and xylene vapors There are gas sensors based on CNT modified by metallic nanoparticles [40] The working principle of a sensor on the basis of monolayer CNT with palladium (Pd) nanoparticles for hydrogen detection at room temperature was described [41] The response time of the sensor was 5—10 s, the recovery time being ~400 s Adsorbed H2 molecules are known to dissociate at room temperature into hydrogen atoms that are dissolved in Pd and reduce the metal work function As a result the carrier concentration in the nanotubes decreases and their conductivity drops The process is reversible for dissolved atomic hydrogen can react with atmospheric oxygen to form OH This causes the formation of water which eventually leaves the Pd-CNT system and the initial conductivity of the sensor is restored Two methods of monolayer CNT functionalizing by palladium for the fabrication of hydrogen detectors was described [42] Nanotubes can be either chemically functionalized by Pd or coated with sputtered metal films In another work [43] an H2 nanosensor functionalizing method was developed that implied electrodeposition of Pd particles on monolayer CNT The sensor exhibited a good room temperature response The detection limit was 100 ppm, the recovery time being 20 Other metals can also be used for the design of CNT based gas sensors Sensors on the basis of multilayered nanotubes functionalized by Pt or Pd were fabricated [44, 45] They showed good H2 sensitivity and recovery at room temperature The response and recovery times were 10 for CNT functionalized by Pd and 15 for CNT functionalized by Pt Another hydrogen detector was designed on the basis of monolayer CNT decorated by gold particles [46] The effect of point heterocontacts between CNT and gold microwires on the detection of NH3 and NO2 with fast response and recovery was demonstrated [47] The working mechanism of the probe was based on the formation of a thin conducting channel between Au and a nanotube and a change in the resistivity of the tubulene Gas detectors on the basis of monolayer nanotubes modified by Au, Pt, Pd and Rh were reported [48] The difference in the catalytic activity of the metal nanoparticles determines the selectivity of the sensors for Н2, CH4, CO, H2S, NH3 and NO2 The working principle of a high-efficiency gas sensor based on MCNT–Pt composite material sensitive to toluene C7H8 was described [49] The sensor responses at a concentration of ppm and 150 °C were measured The efficiency of the sensor was noticeably higher than that of earlier described sensors [50] There were also reports on the fabrication of gas sensors on the basis of CNT and nanostructured metal oxides [50—56] Sensors modified by SnO2 or TiO2 were sensitive to NO2, CO, NH3 and ethanol vapors at low working temperatures Nanotubes in metal oxide matrices produced the main conducting channels which efficiently changed the conductivity of the composite material during gas adsorption The recovery time of the sensors depended on the energy of the bond between gas molecules and the CNT surface A sensor on the basis of MCNT coated by SnO2 was described [57] that exhibited a good response to oil gas and ethanol vapors and recovered within a few seconds at 335 °C The sensor’s response grew with gas concentration The high sensitivity and low resistivity of that system was accounted for by the specific features of its electron transfer mechanism Electrons move through SnO2 grains in MCNT that have a low resistivity Furthermore, the sensor’s gas response could increase due to the formation of the p—n junction between the nanotubes and the SnO2 nanoparticles [58] Acetone and NH3 can be detected with TiO2 + MCNT composition sensors fabricated using the sol gel method [59] Sensors on the basis of SnO2–TiO2 oxide mixture and MCNT embedded into thin SnO2–TiO2 films were described [60] The response and recovery times of those sensors were less than 10 s at working temperatures of 210—400 °C The improved sensor characteristics and the lower working temperatures can be attributed to an enhancement of the p—n junction influence in addition to the grain boundary effects An interesting working principle of a CNT based sensor device was demonstrated by a scientists team of the Research Center at the Toulouse University, France [61] They found a significant dependence of microwave radiation transmission pattern in a material containing twolayered nanotubes on the concentration of impurities in atmosphere [61] Specimens of twolayered nanotubes ~2 nm in diameter and ~10 m in length that had high purity and high reproducibility of the electric, magnetic and optical parameters were introduced in a powdered form into the cavity of a silicon waveguide mounted on a thin dielectric membrane The membrane material had a dielectric constant close to unity and a high microwave radiation transmission coefficient in the 1—110 GHz range To study the sensor characteristics the authors exposed the device to nitrogen at a atm pressure for 16 h Experimental data on the microwave ... a wide range of applications [4—8] Carbon nanotubes (CNT) can find applications in a great number of areas such as additives to polymers and catalysts, in autoelectron emission for cathode rays... are capacitance gas sensors A capacitance sensor was described [67] the sensitive element of which was an array of misoriented nanotubes grown on a SiO2 layer The first plate of the sensor was... functional groups Structural features of carbon nanotubes Carbon nanotubes were discovered and described by S Injima, Japan, in 1991 One of the amazing phenomena associated with the nanotubes