PdCl2–Polyaniline Composite for CO Detection Applications: Electrical and Optical Response

Different mass ratios of PdCl₂ were incorporated into polyaniline emeraldine base (PdCl₂–PANI) by sonication in acetonitrile. The Pd^II-doped PANI composites readily interact with CO gas at ambient conditions (1 atm, 27 °C) resulting in the reduction of Pd^II ions into metallic Pd⁰ and the release of HCl to afford Pd⁰–PANI·HCl. The dramatic structural changes associated with CO exposure were examined by different spectroscopic (UV–Visible, FT-IR), powder X-ray diffraction (P-XRD) and electrical conductivity (DC) techniques. The P-XRD data shows that the resulting nano-scale metallic Pd⁰ clusters (~10–15 nm) remain tightly bound to the PANI chains. The observed chemical transformation of PdCl₂–PANI into Pd⁰–PANI·HCl as a function of exposure to CO gas was exploited to monitor this poisonous gas in CO/air mixture; the optical and DC electrical response functions were examined in solution and solid state, respectively. The DC electrical response signal is relatively ten-fold more sensitive to CO exposure (% S_electrical = 471–496 %) compared to the corresponding optical response (%S_{optical at 460 nm} = 40–51 %). This responsive action demonstrates that PdCl₂–PANI composite can be employed as a feasible low-cost solid-state CO detecting material in chemical sensor devices.