Crown-ether-substituted asymmetric phthalocyanine derivatives/CdS self-assembled hybrid films with an unprecedented high response toward NO2

Two asymmetrical amphiphilic phthalocyanines simultaneously containing hydrophobic alkoxy and hydrophilic 15-crown-5-ether substituents at the phthalocyanine periphery H-2{Pc(15C5)(3)[(OC8H17)(2)]} (Pc-1) and H-2{Pc(15C5)[(OC8H17)(6)]} (Pc-2) and their symmetrical analogue H-2[Pc(OC8H17)(8)] (Pc-3) have been synthesized and characterized. The Pc-n/CdS (n = 1, 2 and 3, respectively) hybrid films are obtained successfully via a simple quasi-Langmuir-Shafer (QLS) method using H2S-vapor annealing over the Pc-n/Cd2+ self-assembled film formed at the interface of the air/CdCl2 aqueous solution. The film-structure and properties of both the hybrid Pc-n/CdS and pure Pc-n films are comparatively studied by a wide range of methods including UV-vis, polarized UV-vis, XRD, SEM and I-V measurements. Experimental results exhibit a slipped co-facial stacking mode in an "edge-on" conformation (H-type aggregate) formed for the phthalocyanine molecules in both pure Pc-n films and the corresponding Pc-n/CdS hybrid films, with increasing intermolecular pi-pi interactions in the order of Pc-n < Pc-n/CdS and Pc-3/CdS < Pc-2/CdS < Pc-1/CdS, respectively. Accordingly, film-microstructures, crystallinity and conductivity are effectively improved by introducing CdS nanoparticles into the 15-crown-5-substituented phthalocyanines forming Pc-1/CdS and Pc-2/CdS hybrid films. These render excellent sensing performance towards NO2 in the 0.05-2.5 ppm range within a fast dynamic exposure period of 30 s. Strikingly, Pc-1/CdS hybrid film presents an unprecedented high sensitivity of 157.3%.ppm(-1) vs. very low NO2 concentration range of 0.05 similar to 0.25 ppm, achieving one of the best room temperature sensing performances in terms of high sensitivity, rapid responsibility and low detection limit among self-assembled film-based NO2 sensors.