The NH3, H2O, and C2H5OH cross-sensitivities to the NO2 electrical response of carbon nanotubes (CNTs) thin films for gas sensing applications is reported. CNTs have been deposited by plasma enhanced chemical vapor deposition (PECVD) on Si3N4/Si substrates provided with Pt electrodes. Microstructural features as determined by SEM, TEM (electronic scanning and transmission microscopy) and Raman spectroscopy have highlighted the growth of well developed tubular carbon structures of 30–40 nm diameter and 100–200 nm length. CNTs have shown a p-type response with decrease in resistance upon exposure to NO2 gas (10–100 ppb) and the highest sensitivity at 165 °C working temperature. The NO2 gas sensitivity has resulted to be improved by annealing the as-grown films at temperatures higher than 330°. No response has been found by exposing the films to CO and CH4 in the working temperature range 25–250 °C. An amount of 500 ppm of NH3 and ethanol, as well as 80% relative humidity (RH), have resulted to increase the electrical resistance of the films. Cross-sensitivity test have highlighted strong interference of ethanol and ammonia gases to the NO2 response, while negligible cross-sensitivity effects have been found with humidity at 80 RH. The reproducibility of the electrical response to NO2 is also reported.
The high surface area, size, hollow geometry and chemical inertness of carbon nanotubes (CNTs) makes them attractive for demanding applications in the field of gas sensing. To date studies on possible applications of CNTs have been focused either on individual single-walled carbon nanotubes (SWNTs) as sensitive materials towards O2, NO2 and NH3[1], [2] and [3] or on multi-walled carbon nanotubes (MWNT) mats as NH3, CO, CO2 humidity and O2 gas sensors [4], [5] and [6]. More recently, we have reported on the preparation of thin films CNTs by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) on Si/Si3N4 substrates, provided with interdigital Pt electrodes, for NO2 monitoring at low concentrations (10–100 ppb in air) [7].
Although considerable theoretical efforts have been devoted to the study of the possible interaction of a variety of gas molecules including NO2, O2, NH3, N2, H2O, CO2, CH4, and Ar with CNTs materials [8], [9], [10] and [11], few experimental data are available elucidating the effects of gas adsorption on the electrical properties of CNTs [1], [2], [3], [4], [5], [6], [7], [12], [13] and [14] for gas sensing applications. By first principles calculations on individual SWCNTs, it has been estimated that NO2 and O2 molecules would yield considerable larger adsorption energies than H2O, NH3, CH4, CO2, H2, N2, and Ar [8], [9], [10] and [11]. NO2 and O2, chemisorbed on the CNTs surface, behave as electrons acceptors, while physisorbed molecules like H2O, NH3, CH4, CO2, H2, N2, and Ar act as electron donor, respectively. These theoretical predictions have been partially confirmed in practice as highlighted by previous experimental research. It was indeed demonstrated how NO2 and O2, oxidizing gases decrease the electrical resistance of p-type CNTs [1], [2] and [7], while the opposite effects is demonstrated by ammonia and humidity [1], [4] and [14]. Nevertheless, to date, both theoretical and experimental approaches have not yet fully investigated the effects of simultaneous interactions between competitive oxidizing and reducing gaseous species on the electrical response of CNTs gas sensors.
In this paper we report preliminary investigations on: (i) the effects of a thermal treatment process which enables to improve the sensor sensitivity to sub-ppm concentrations of NO2 gas, (ii) the sensitivity response to oxidizing/reducing gases and vapors like NH3, humidity, C6H6 and ethanol, and (iii) the cross-sensitivity analysis by comparing how the NO2 electrical response is affected by the presence of interfering gases like NH3, humidity and ethanol vapor. The aim of this work is to assess the possibility, if CNTs films may be applied as innovative NO2 sensor for environmental applications.
CNTs thin film have been prepared by PECVD on Si/Si3N4 substrates for gas sensing applications. The temperature-dependence of the electrical resistance highlight a change from metallic to semiconductor response after annealing the as-grown films at temperatures higher than 300 °C. By heating the as-grown films at these temperature improves the NO2 gas sensitivity. CNTs films resistance resulted to decrease when in contact with NO2, whereas it increased with NH3, ethanol, humidity and C6H6. Cross response analysis highlighted interference effects to the NO2 response in the order ethanol>NH3>humidity. CNTs thin films prepared by PECVD have demonstrated their potentiality as a new class of materials for NO2 detection, for environmental applications, with detection limits as low as 10 ppb.
