Films of substituted zinc phthalocyanines as active layers of chemiresistive sensors for ammonia detection

Abstract

Metal phthalocyanines, which are examples of one-dimensional semiconductors, are widely used as active layers of chemiresistive sensors for detecting various gases, in particular ammonia. Despite the high interest in such sensors, there have been few systematic studies on the effect of the type of substituent and structural features of films on their chemiresistive response to gases. In this work, films of zinc phthalocyanine (ZnPc-1, 2, 3 and 4) bearing alkylthio (–S(CH₂)₅CH₃) and polyoxo (–O(CH₂CH₂O)₃CH₃ and –S(CH₂CH₂O)₃CH₃) substituents, which exhibit liquid crystalline properties and, as a consequence, are capable of forming ordered films, were tested as active layers of chemiresistive sensors for the detection of low concentrations of ammonia (0.1–50 ppm). The effect of the type of substituent on the films’ alignment and sensor response to ammonia was studied for the first time. It was shown that the films of all investigated zinc phthalocyanines demonstrated a reversible chemiresistive sensor response to gaseous ammonia. The sensor response of the as-deposited films of ZnPc-1 and ZnPc-2 with polyoxo-substituents, which are liquid crystalline at room temperature, was noticeably higher than that of ZnPc-3 and ZnPc-4, which form polycrystalline disordered films. The limit of ammonia detection was 17 ppb for ZnPc-1 and ZnPc-2 films. Heat treatment of ZnPc-1 and ZnPc-2 films did not cause any changes in their structure and sensor properties, while ZnPc-3 and ZnPc-4 films became more ordered after heating, and their sensor response increased several times. This led to a reduction in the detection limit of ammonia to 10 ppb in the case of the ZnPc-3 film. All investigated films have high selectivity to ammonia in the presence of carbon dioxide and vapors of some volatile organic compounds.