High-performance room-temperature NO2 sensors based on microstructures self-assembled from n-type phthalocyanines: Effect of fluorineehydrogen bonding and metaleligand coordination on morphology and sensing performance

To investigate the effect of fluorineehydrogen bonding and metaleligand coordination on the morphology and gas sensing properties of self-assembled nanostructures, metal free 2,3,9,10,16,17,24,25octakis( heptafluorobutoxy) phthalocyanine H-2[Pc(OCH(2)c(3)F(7)) (8)] (1) and its zinc complex counterpart Zn [Pc(OCH2C3F7) (8)] (2) are synthesized and fabricated into organic microstructures by a phase-transfer method. The self-assembling properties have been comparatively investigated by electronic absorption and Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and currentevoltage (IeV) measurements. Competition and cooperation between the inter-molecular pi-pi interaction and C-H/F-C hydrogen bonding in the direction perpendicular to the pi-pi interaction direction for metal-free phthalocyanine 1 lead to the formation of uniformed two-dimensional helical microribbons with an aspect ratio (length/width) of ca 20. The additional Zn-O coordination interactions between the central zinc ion and the oxygen atom of the adjacent molecule increases the intermolecular interaction, and results in the formation of long onedimensional nanowires for 2 with an aspect ratio of more than 200. Notably, the resulting microstructures were revealed to show good semiconducting properties with the increasing conductivity from 5.99x 10-5 S cm -1 for the nanowires of 2 to 1.68 x 10-4 S cm-1 for the helical ribbons of 1. Moreover, the excellently sensitive, stable and reproducible responses to NO2 in 50-900 ppb range are observed for the helical ribbons of 1 at room temperature, implying the excellent potential as the NO2 sensor for applications in practical environments. Both sensitivity and detection limit of the helical ribbons of 1 for NO2 gas at room temperature was found to be better than those of the nanowires of 2. This suggests that the analyte-phthalocyanine interaction is dominated by not only high conductivity and great surface area but more importantly binding strength of analyte to the central cavity of the phthalocyanine. (C) 2017 Elsevier B. V. All rights reserved.