Room temperature gas sensing properties of ultrathin carbon nanotubes by surfactant-free dip coating

Large-scale production of reliable carbon nanotubes (CNTs) based gas sensors involves the development of scalable and reliable processes for the fabrication of films with controlled morphology. Here, we report for the first time on highly scalable, ultrathin CNT films, to be employed as conductometric sensors for NO2 and NH3 detection at room temperature. The sensing films are produced by dip coating using dissolved CNTs in chlorosulfonic acid as a working solution. This surfactant-free approach does not require any post-treatment for the removal of dispersants or any CNTs functionalization, thus promising high quality CNTs for better sensitivity and low production costs. The effect of CNT film thickness and defect density on the gas sensing properties has been investigated. Detection limits of 1 ppm for NO2 and 7 ppm for NH3 have been achieved at room temperature. The experimental results reveal that defect density and film thickness can be controlled to optimize the sensing response. Gas desorption has been accelerated by continuous in-situ UV irradiation.

Fabrication of Fe-doped WO3 films for NO2 sensing at lower operating temperature

Fe-doped tungsten oxide thin films with different concentrations (0 to 2.6 at%) were synthesized on glass and alumina substrates at room temperature using DC reactive sputtering and subsequently annealed at 300oC for 1 hour in air. The alumina substrate has pre-printed interdigitated Pt-electrodes for gas sensing measurements. The effects of Fe-doping on the film structure and morphology, electronic and optical properties for gas sensing were investigated. The grain size of the different films on the alumina and Pt regions of the substrate vary only slightly between 43-57 nm with median size of about 50 nm. Raman spectra showed that the integrated intensity of W=O to O–W–O bands increases with increasing Fe concentrations and this indicated an increase in the number of defects. From XPS the different concentrations of the Fe-doped films were 0.03 at%, 1.33 at% and 2.6 at%. All the films deposited on glass substrate have shown similar visible transmittance (about 70%) but the optical band gap of the pure film decreased form 3.30 eV to 3.15 eV after doping with 2.6 at% Fe. The Fe-doped WO3 film with the highest Fe concentration (2.6 at% Fe) has shown an enhanced gas sensing properties to NO2 at relatively lower operating temperature (150oC) and this can be attributed to the decrease in the optical band gap and an increase in the number of defects compared to the pure WO3 film.