Sensor response of sol–gel multiwalled carbon nanotubes-TiO2 composites deposited by screen-printing and dip-coating techniques

The comparison of two deposition techniques, screen-printing and dip-coating, to cover non-conductive glass substrates with TiO2:Multiwall Carbon Nanotubes (MWCNT), and its application as chemical sensors are reported. A sol–gel solution containing Ti-isopropoxide and acid treated MWCNT was either precipitated or kept as a sol by adjusting the pH and surfactant concentration. In the first case, screen-printing and annealing techniques were used to coat the substrates, while in the second case the substrates were dip coated and annealed several times. XRD data show the abundance of oriented rutile and anatase planes in the TiO2 dip-coated films, when compared to the screen-printed films. For the TiO2:MWCNT composites, the presence of carbon induces the growth of rutile in both screen-printed and dip-coated films. Additionally, dip-coated composite films are more crystalline and compact than screen-printed films, showing an average carbon content of 5–7 wt%, which is close to the 7 wt% of screen-printed films. Dip-coated composite films show n-type behaviour when sensing ammonia (NH3), similar to the one observed for dip-coated TiO2 but opposite to the p-type behaviour of screen-printed composites. The abundance of Ti3+ in dip-coating films, and/or differences in the coordination environment around the surface Ti sites, is proposed to be responsible for the differences in p/n conductivity of the composite films.