Chemiresistor Gas Sensors Based on Photoconductivity Changes in Phthalocyanine Thin Films: Enhancement of Response toward Ammonia by Photoelectrochemical Deposition with Metal Modifiers

Determination of NH₃ in flowing N₂ or air is demonstrated by monitoring changes in the photoconductivity of chlorolndium phthalocyanine (InPc-CI) and chlorogalllum phthalocyanine (GaPc-CI) thin films vacuum deposited on platinum silicide interdigilated array microcircuits. Photoconductivity changes were measured, as opposed to the more commonly measured dark conductivity changes, yielding reasonable sensitivity to the adsorption of submonolayer amounts of gas analytes such as NH₃. Photoelectrochemical modification of Pc-coated microcircuits was carried out in order to obtain submonolayer coverages of a variety of different metals (Ag, Au, Cu, Pt, and Hg) on the Pc film. This modification apparently provides new chemisorptlon sites for NH₃, thus improving the performance of these microcircuit assemblies as NH₃ sensors in N₂ and In air. The Hg-modified microcircuits demonstrated a superior performance compared to the microcircuits modified with other metals, by showing a linear photocurrent response as a function of NH₃ concentration in N₂ at room temperature over the range from ca. 1.5 to 5.4 ppm, and a logarithmic relationship above that level (up to ca. 84 ppm). Photoconductivity measurements in NH₃-air mixtures were carried out on microcircuit assemblies “as-prepared” and after photoelectrochemical modification. The presence of the metal modifier on the InPc-CI film markedly Improves the assembly performance in air, providing a detection limit for NH₃ in air of ca. 1.0 part per thousand (ppt). The effect on the photoconductivity of 0₂ and other impurities present In GaPc-CI and InPc-CI is discussed, and a mechanism is proposed to explain the variations observed In the photocurrent response to NH₃ in these assemblies before and after modification.