Flame-made Zn-substituted SnO2 nanoparticulate compound for ultra-sensitive formic acid gas sensing

Highlights: • SnO₂ nanoparticles were made and doped with 0.1–1 wt% Zn by flame spray pyrolysis. • Structural analyses indicated that Zn²⁺ species were substitutionally doped in SnO₂ lattice. • Response to 1000 ppm HCOOH at 350 °C was critically enhanced from 330 to 46,374 with 0.2 wt% Zn. • High HCOOH selectivity was attained against eleven other volatile organic compounds. • The optimal sensor exhibited moderately low negative HCOOH response to humidity. -- Abstract: This research conducts a detailed study on the sensitivity and selectivity of Zn-substituted SnO₂ nanoparticles towards formic acid (HCOOH), an important volatile organic acid (VOA) for various technological applications. SnO₂ nanoparticles containing 0.1–1 wt% Zn were made by flame spray pyrolysis (FSP) and sensing films were manufactured by powder pasting and spin coating processes. The physical and chemical properties of the samples were assessed by XRD, EDS, XPS, nitrogen adsorption and electron microscopy. The results indicated that Zn was substitutionally doped in nanocrystalline SnO₂ particles. Gas sensing properties of the materials were evaluated towards 50–1000 ppm HCOOH and other volatile organic compounds at varying temperatures from 200° to 400 °C in dry and humid air. The test data revealed that the sensing layer with the optimum Zn content of 0.2 wt% provided a remarkably high response of ~ 46,374 toward 1000 ppm HCOOH, which was more than two orders of magnitude higher than undoped ones at 350 °C. Additionally, good HCOOH selectivity was achieved against several volatile organic species including formaldehyde, methanol, ethanol, acetone, benzene, xylene, acetic acid, propionic acid, butyric acid, pentanoic acid and lactic acid. Therefore, the flame-made Zn-substituted SnO₂ sensors were potential for ultra-sensitive and selective detections of HCOOH in practical applications.