Abstract: This paper reviews the preparation, structural characteristics, gas sensing performance and future development direction of novel single-walled carbon nanotubes, multi-walled carbon nanotubes and multi-walled carbon nanotube array gas sensors. Keywords: carbon nanotubes; gas sensors; current status and development I. Introduction In 1991, Professor S. Iijima[1] of NEC Corporation discovered carbon nanotubes, which are the fifth allotrope in the carbon family discovered in the 1990s. They are formed by the strongest C-C covalent bonds in nature. The structure of carbon nanotubes can be regarded as a cylinder rolled up from graphene. Carbon atoms are arranged in a spiral on its surface, and in special cases, they can be arranged in an armchair or serrated shape. According to the number of wall layers, it can be divided into single-walled and multi-walled types; at the same time, according to the chirality vector (n, m), it is divided into metallic and semiconductor types: when nm is an integer multiple of 3, it is metallic, and in other cases it is semiconductor[2]. Because of its unique mechanical, electronic, and chemical properties, quasi-one-dimensional tubular molecular structure, and potential application value, carbon nanotubes have become a rising star in the chemical community, attracting great interest from physicists, chemists, and materials scientists. Various countries have invested a lot of manpower and resources in a series of studies on its properties, preparation, and applications, and have achieved gratifying results. Carbon nanotubes have a hollow structure and a large wall surface area, and have a great adsorption capacity for gases. Since the adsorbed gas molecules interact with the carbon nanotubes and change its Fermi level, the macroscopic resistance changes significantly. The composition of the gas can be detected by measuring the change in resistance. Therefore, carbon nanotubes can be used to make gas molecule sensors. Currently, J. Kong et al. [3] have successfully studied the gas-sensing characteristics of single-walled semiconductor carbon nanotubes, opening the door to the study of one-dimensional carbon nanotubes as sensitive materials to form gas-sensing sensors. II. Current Status of Research on Carbon Nanotube Gas Sensors 1. Fabrication of Gas Sensors Using Single-Wall Carbon Nanotubes J. Kong et al. [3] fabricated a single-wall semiconductor carbon nanotube on a SiO2/Si substrate with dispersed catalyst by chemical vapor deposition. Among them, two metal electrodes are connected to a single semiconductor single-wall carbon nanotube (S-SWNT) to form a metal/S-SWNT/metal structure (Figure 1(a)) and exhibit the properties of a P-type semiconductor. The SWNT has a diameter of 1.8 nm, and the metal electrode is composed of a 20 nm nickel layer covered with a 60 nm gold layer. Now, the gas detection test is used to detect the change in resistance of a single SWNT in different gases. A SWNT sample is placed in a sealed 500 ml glass bottle, and NO2 ((2~200)×10-6) or H3 (0.1%~1%) diluted in air or argon is introduced to obtain the I/V relationship curve (as shown in Figure 1(b) and (c)). As shown by the curves, the conductivity decreases by two orders of magnitude in an NH3 atmosphere, while it increases by three orders of magnitude in a NO2 atmosphere. This is because when the semiconductor single-walled carbon nanotube is placed in an NH3 atmosphere, the valence band deviates from the Fermi level, resulting in hole losses and a decrease in conductivity; while in a NO2 atmosphere, the valence band moves closer to the Fermi level, resulting in an increase in hole carriers and thus an increase in conductivity. Since the metal/S-SWNT/metal structure is similar to a field-effect transistor with holes as the main charge carriers, when the voltage between the source and drain is constant, the current decreases as the gate voltage increases (as shown in Figure 2). Curve b in Figure 2 is the gate voltage-current relationship curve without any gas introduced, while curves a and c are the gate voltage-current relationship curves measured in NH3 and NO2 atmospheres, respectively. Without any gas introduced, the current is 15 μA at a gate voltage of 0V, while the current becomes almost 0A when an NH3 atmosphere is introduced. So, if we measure NH3 gas, we set the initial grid voltage to 0V, and as shown in the figure above, the conductivity of the sample will decrease by two orders of magnitude. If we measure NO2 gas, we first set the grid voltage to +4V. Before NO2 gas is introduced, the current is almost zero. After NO2 is introduced, the current increases greatly, and its conductivity increases by three orders of magnitude. This makes the sensor selective in complex gas environments. The Zettle research group [4] found that the electrical properties of single-walled carbon nanotubes are closely related to the adsorption of oxygen. When single-walled carbon nanotubes are exposed to air or oxygen, the semiconducting carbon nanotubes can be transformed into metallic carbon nanotubes. This not only shows that carbon nanotubes can be used as sensor materials, but also indicates that the performance of carbon nanotubes measured in air is likely related to oxygen. This helps to understand the gas sensing mechanism of carbon nanotubes as sensitive materials for gas sensors more deeply. J. Zhao et al. calculated the changes in the electronic structure of NO2, O2, NH3, H2 and other gases adsorbed on the walls of single-walled carbon nanotubes and between the tube bundles. Theoretically, they explained that the gas adsorption process changed the charge distribution in the carbon nanotubes, causing fluctuations and transfers, which in turn caused changes in the macroscopic resistance of the single-walled carbon nanotubes. J. Kong et al. [5] then made a semiconductor single-walled carbon nanotube modified with Pt. Its surface has a discontinuous Pt metal film, which is more sensitive to H2. Its resistance recovers rapidly after H2 decreases. This semiconductor single-walled carbon nanotube sensor not only has higher sensitivity and selectivity, but also has the advantage of being able to work at room temperature. 2. Gas sensor made with multi-walled carbon nanotubes OK Varghese et al. [6] studied the fabrication of sensors using MWNTs (multi-walled carbon nanotubes). They designed two types of sensors: one is a structure with a layer of MWNTs/SiO2 film on a planar interdigitated capacitor (as shown in Figure 3), which is called a capacitive sensor; the other is a MWNTs bending resistance type, which is to use photolithography to etch a curved groove on the SiO2 film attached to the Si substrate, and then grow MWNTs on SiO2 using chemical vapor deposition, which is called a resistive sensor. (1) Making a capacitive sensor with multi-walled carbon nanotubes First, MWNTs were obtained on the quartz tube wall by high-temperature pyrolysis [7]. Then, the sensor was made on a printed circuit board using a planar interdigitated electrode and MWNTs/SiO2 composite material. The MWNTs/SiO2 composite material, which is the sensitive material, was made by scraping off the available MWNTs on the quartz tube wall, dispersing them in toluene by ultrasonic bath, then washing and drying with isopropanol, and finally dispersing MWNTs into a SiO2 system—this system is formed by dispersing 20% ​​nano SiO2 particles in water. The dry weight ratio of MWNTs to SiO2 in the MWNTs/SiO2 composite material is 2:3. The structure of the capacitive sensor is shown in Figure 3. The sensor is placed in a sealed 60cm3 gas chamber for impedance testing. Argon is used as the carrier gas, the total flow rate is 1000sccm, the pressure of the test gas is controlled by the main flow controller, and the impedance is measured by a Hewlett Packard 4192A impedance analyzer. Before each measurement, in order to remove chemically adsorbed molecules, the sensor must be heated in a vacuum and kept at 100℃ for 1 hour. The finally measured impedance Z is divided into two parts: the real part Z′ and the imaginary part Z″, which constitute the Cole-Cole impedance diagram. As the humidity increases, the diameter of the arc of the Cole-Cole impedance diagram also changes significantly. From this change, it can be seen that the capacitive sensor is sensitive to a certain gas or humidity. In addition, the capacitive sensor is also relatively sensitive to CO2. CAGrimes et al. [8] also successfully used the capacitive MWNTs sensor to monitor CO2. (2) Fabrication of a resistive sensor using multi-walled carbon nanotubes. The resistive sensor is fabricated by growing a thick layer of SiO2 on a Si substrate using thermal oxidation, and then using photolithography to create a curved groove with a total length of about 45 cm, an arm width of about 350 μm, and a gap of 290 μm between the arms. By controlling the reactant dosage, the nanotubes can be grown on the SiO2 layer instead of on the Si substrate. The Cole-Cole diagram of the resistive sensor is similar to that of the capacitive sensor. The equivalent circuit of the resistive sensor is shown in Figure 6. Two capacitors that vary with frequency are connected in parallel with resistors R1 and R2, and then connected in series with R0. In addition, the resistive sensor can also be made into a humidity sensor. Both types of sensors are relatively sensitive to NH3. In the detection of NH3, the resistance R1 and the sensitivity change almost linearly, which can be used as a dosimeter for ammonia. As the concentration of ammonia increases, the response time of the sensor reaches 2-3 min, but the sensor needs to be heated in a vacuum and kept at 100℃, and it takes several days to recover. 3. A joint research group of Professor Pulichel M. Ajayan of the Department of Materials Science and Engineering and Associate Professor Nikhil Koratkar of the Department of Mechanical Engineering at Rensselaer Polytechnic Institute in the United States recently successfully developed a miniature gas sensor sample using a carbon nanotube array. The sample can very sensitively analyze various gases in the atmosphere quantitatively and qualitatively. The sensor has a very simple structure (as shown in Figure 7 [9]). The fabrication method is as follows: First, a MWNT array is generated on a SiO2 (silicon dioxide) substrate using CVD (chemical vapor deposition). Each MWNT has a diameter of about 25-30 nm and a length of about 30 μm. The MWNTs are arranged at intervals of about 50 nm. Then, an insulating glass plate with a thickness of about 180 μm is added to both ends of the MWNTs. Finally, it is covered with an aluminum film to make a gas sensor. The sensor shown in Figure 7 has the following dimensions: about 20 mm wide, about 20 mm long, and about 700 μm thick. When measuring the composition of surrounding gas using a gas sensor, a DC voltage is applied with the MWNTs end as the anode (+) and the aluminum film end as the cathode (-). At the top of the MWNTs, a very low voltage generates a strong electric field, causing dielectric breakdown in the surrounding ionized gas. Experimental results show that the breakdown voltage varies significantly depending on the gas type, allowing for qualitative analysis (Figure 8a). Furthermore, the figure shows a wide range of analyzable gases, even including inert gases such as argon (Ar) and helium (He). Additionally, although the breakdown voltage is independent of gas concentration, it is known that the generated current is proportional to the logarithm of the concentration (Figure 8b), indicating that the gas can be quantitatively analyzed. III. Development Direction of Carbon Nanotube Gas Sensors Gas sensors made of one-dimensional carbon nanotubes as sensitive materials have advantages that conventional sensors cannot replace: First, nano-solid materials have a large interface, providing a large number of gas channels, which greatly improves the sensitivity; second, the sensor operating temperature is greatly reduced; and third, the sensor size is greatly reduced [10]. Therefore, it has a wide range of development prospects in biology, chemistry, mechanics, aviation, and military fields. Using carbon nanotubes to modify electrodes can improve the selectivity for H+, etc., thus making electrochemical sensors. By utilizing the selectivity of carbon nanotubes for gas adsorption and the conductivity of carbon nanotubes, gas sensors can be made. The adsorption of oxygen at different temperatures can change the conductivity of carbon nanotubes. Nano-sensitive materials have small surface areas and large surface energy, and are easy to aggregate, which affects their original characteristics. By assembling nano-scale photosensitive, humidity-sensitive, gas-sensitive, pressure-sensitive and other materials with carbon nanotubes, various nano-scale functional sensors can be made. In nanotechnology, the research level and application level of nanodevices mark the overall level of a country's nanotechnology[11], and carbon nanotube sensors are precisely an extremely important field in the research of nanodevices. Of course, there are still many problems in the field of carbon nanotube sensing, such as the immaturity of carbon nanotube fabrication technology, its unsatisfactory performance, and the long recovery time of gas sensors made of carbon nanotubes. In addition, the synthesis of single-walled carbon nanotubes produces a mixture of metallic and semiconductor tubes. The current preparation method cannot produce nanotubes with completely semiconductor properties. Since metallic tubes have no function, it is very difficult to conduct systematic research. Moreover, no flexible method has been found to modify the surface of nanotubes to make the surface of nanotubes selective in complex gas environments[12]. Although these problems are complex, they will be well solved with the further development of carbon nanotube technology, and carbon nanotube sensors will also achieve great development.