Phthalocyanine-Carbon Nanotube Hybrid Materials: Mechanism of Sensor Response to Ammonia from Quantum-Chemical Point of View

Quantum chemical calculations of the geometric and electronic structure of periodic hybrid compounds representing carbon nanotubes (10,0) with zinc phthalocyanine molecules ZnPc-xpy (x = 0, 1, 2, 4) on their surface and their interaction with ammonia were carried out to explain the dependence of the sensor response of the hybrid materials to ammonia on the number of substituents in the ZnPc-xpy macrocycle and to clarify the nature of the interaction between ammonia and phthalocyanine molecules. It was found that the key feature of these materials, which determines their sensor response toward ammonia, is the presence of an impurity band in the band gap of a carbon nanotube, formed by the orbitals of macrocycle atoms. When ammonia adsorbs through the formation of hydrogen bonds with the side atoms of phthalocyanine, the energy of this impurity band decreases. As a consequence, the electron population of the conduction band and, accordingly, the electrical conductivity of the hybrid materials become lower. Moreover, with an increase in the number of oxypyrene substituents in ZnPc-xpy, the interaction energy of ammonia increases and, as a result, the decrease in the energy of the impurity band becomes higher. These facts may explain recent experimental measurements of the parameters of the sensor response of similar hybrid materials to ammonia, where, in particular, it was shown that the sensor response is reversible, and its value increases with an increase in the number of oxypyrene substituents in the phthalocyanine macrocycle.

Automated summary

Quantum chemical calculations were used to investigate the interaction between carbon nanotubes and zinc phthalocyanine molecules with ammonia. The results revealed that the sensor response of the hybrid materials to ammonia depends on the number of substituents in the phthalocyanine macrocycle and the interaction between ammonia and phthalocyanine molecules. The presence of an impurity band in the carbon nanotube's band gap was found to be the key feature determining the sensor response. Ammonia adsorption led to a decrease in the energy of this impurity band, resulting in lower electrical conductivity of the hybrid materials.