Manipulating MWCNT/TiO2 heterostructure morphology at nanoscale and its implications to NO2 sensing properties

Abstract Multi-walled carbon nanotubes (MWCNTs) based heterostructures were prepared by oxygen plasma treatment and atomic layer deposition (ALD) of TiO2 carried on MWCNTs. Different TiO2 structures (island, granular and continuous forms) on the nanotubes were obtained by varying the processing parameters. Chemical inertness of MWCNTs led to the ALD of TiO2 in an island form. Oxygen plasma treatment ensured deposition of a continuous TiO2 film, 5 nm in thickness. A granular form of TiO2 was obtained on oxygen plasma treated MWCNTs using shorter duration of Ti-precursor pulse. Different imaging modes in high resolution transmission electron microscopy visualized TiO2 nanostructures and defects in MWCNT walls. The latter were also confirmed by Raman spectroscopy. The morphology of TiO2 and oxygen plasma treatment of the MWCNTs highly affected the NO2 sensing behavior of chemiresistive heterostructure sensors at room and elevated temperatures. All investigated sensors exhibited a decrease in the resistance upon exposure to NO2 gas which implies a p-type sensor response. The granular TiO2/MWCNTs heterostructure showed the highest response towards NO2 gas as compared to the island and continuous forms.