Low-Temperature Ethanol Sensor via Defective Multiwalled Carbon Nanotubes

Abstract

This paper focuses on the fabrication of defective-induced nanotubes via the catalytic chemical vapor deposition method and the investigation of their properties toward gas sensing. We have developed defective multi-walled carbon nanotubes with porous and crystalline structures. The catalyst layer used in CNTs' growth here was based on 18 and 24 nm of Ni, and 5 nm of Cr deposited by the dc-sputtering technique. The CNTs' defects were characterized by observing the low graphite peak (G-band) and higher defect peaks (D-band) in the Raman spectrum. The defectives sites are the main source of the sensitivity of materials toward different gases. Thus, the current product was used for sensing devices. The device was subjected to various gases such as NO, NO₂, CO, acetone, and ethanol at a low operating temperature of 30 °C and a concentration of 50 ppm. The sensor was observed to be less sensitive to most gas while showing the highest response towards ethanol gas. The sensor showed the highest response of 8.8% toward ethanol at 30 °C of 50 ppm, and a low response of 2.8% at 5 ppm, which was investigated here. The signal repeatability of the present sensor showed its capability to detect ethanol at much lower concentrations and at very low operating temperatures, resulting in reliability and saving power consumption. The gas sensing mechanism of direct interaction between the gas molecules and nanotube surface was considered the main. We have also proposed a sensing mechanism based on Coulomb dipole interaction for the physical adsorption of gas molecules on the surface.

Keywords: 1D nanostructures; defective carbon nanotubes; ethanol sensor; sensing properties.

Figure 1

Schematic diagram for; (a) CNTs’ synthesis, and (b) sensing device fabrication.

Figure 2

Schematic diagram of the gas sensing system.

Figure 3

(a) Raman spectrum of the CNTs prepared with 18 and 24 nm of Ni catalyst layers, (b) FESEM image, and (c) HRTEM image CNTs prepared with 24 nm of Ni catalyst layers.

Figure 4

Single sensor signal at various operating temperatures ranging from 30–60 °C for CNTs prepared with (a) 8 nm and (b) 24 nm of Ni layer.

Figure 5

Response and recovery time constants at various operating temperatures for CNTs prepared with 24 nm of Ni layer.

Figure 6

Sensor signal versus time at various gas concentrations for the sensor prepared with 18 nm and 24 nm of Ni catalyst layer.

Figure 7

The sensor response of the prepared device is a function of; (a) operating temperatures at 50 ppm and (b) gas concentration at an operating temperature of 40 °C.

Figure 8

Proposed gas sensing mechanism based on Coulomb’s interaction.

Figure 9

Cyclic curve of sensor signals at concentrations of 50 ppm, and various temperatures for the sensor fabricated with CNTs/24 nm-Ni layer.

Figure 10

(a) Sensor signal toward various gases: NO, NO₂, acetone, and CO, and (b) the sensor’s selectivity toward these gases compared to ethanol measured at 30 °C, for the sensor fabricated with CNTs/24 nm-Ni layer.

Figure 11

The sensor response at an operating temperature of 30 °C for various humidity conditions for the sensor fabricated with CNTs/24 nm-Ni layer.