The exceptionally high moisture responsiveness of a new conductive-coordination-polymer based chemiresistive sensor

Abstract

A new conductive coordination polymer (CCP), Ag(SPh–NO₂)·AgNO₃, with a three-dimensional (3D) non-porous structure composed of a unique 2D inorganic layered structure linked by an organic linker, was reported. The 3D CCP Ag(SPh–NO₂)·AgNO₃ exhibits semiconductor behavior, with electronic conductivity as high as 2.7 × 10⁻⁶ S cm⁻¹ at 295 K, which is comparable to the vast majority of highly conductive 3D coordination polymers. The chemiresistive humidity sensing ability of Ag(SPh–NO₂)·AgNO₃ was studied over a wide relative humidity (RH) range (10–90% RH) at room temperature. The sensor showed exceptionally high moisture responsiveness, with a 10⁶-fold increase in response at 90% RH and excellent sensitivity to humidity in the range of 10–80% RH. The sensing mechanism was further studied using alternating current (AC) impedance spectroscopy and direct current (DC) instantaneous reverse polarity experiments. The exceptionally high response of the Ag(SPh–NO₂)·AgNO₃-based sensor at high RH is attributed to the periodic arrangement of the ion species via reversible coordination bonds in the structure. H₃O⁺ and Ag⁺ are easily formed on its hydrophilic surface, through which the free transport of ionic carriers dominates the conductivity change. Using density functional theory (DFT) calculations, it is further demonstrated that the reversibly coordinated AgNO₃ in the structure can be easily occupied by water molecules, which is thermodynamically spontaneous in the presence of humidity. Furthermore, the functional AgNO₃ groups and their reversible coordination bonds enabled Ag(SPh–NO₂)·AgNO₃ to show exceptionally high moisture responsiveness.