Investigations of methane gas sensor based on biasing operation of n-ZnO nanorods/p-Si assembled diode and Pd functionalized Schottky junctions

In this work, we developed methane gas sensors based on zinc oxide nanorod (n-ZnO-NR) arrayed assembly, synthesized using aqueous phase deposition on p-Si(100) substrates, fabricated as pn junction besides chemical sensitization by Pd (Pdcat/n-ZnO-NR) and Pd-30wt%Ag (Pd-Agcat/n-ZnO-NR) forming Schottky junctions. These sensors are evaluated under reducing methane [CH4] gas with varying concentration (100-10,000 ppm or 0.01-1.0%) mixed with synthetic air under optimum temperature (<= 200-230 degrees C) for thermal activation and UV A (365 nm) light activation supplemented with 50-100 degrees C, following increased electrical conductivity, bandgap narrowing, decreased barrier height, under biasing operation. The performance metrics include a high response (R; 45-80%), low limit of detection (LOD; 80-270 ppm), fast response-recovery times (<1-67 s), and strong binding constants (0.012-0.021), quantified from saturation current, transient response, and Langmuir adsorption isotherm, respectively. Every factor inducing a change in oxygen content of analyte gas atmosphere above ZnO include: (1) surface chemical reaction with chemisorbed oxygen ions on pristine and Pdcat or Pd-Agcat modified n-ZnO-NR yields electron donors increasing sensor conductance; (2) effective free carrier concentration and amplified interface-dipole allowing reduced barrier height and electron tunneling at high reverse bias prevailing depletion-driven mechanism; (3) faster electron transport shortening the response time due to weakened oxygen ion adsorption removed by analyte gas molecules; and finally, (4) in-depth study of thermal and UV activated processes is supported with space charge model and band-bending theory perspectives.

Automated summary

In this study, methane gas sensors based on zinc oxide nanorod (n-ZnO-NR) arrayed assembly were developed. The sensors were synthesized using aqueous phase deposition and fabricated as pn junctions with chemical sensitization by Pd and Pd-30wt%Ag forming Schottky junctions. The sensors showed high response, low limit of detection, fast response-recovery times, and strong binding constants when evaluated under reducing methane gas with varying concentration, thermal activation, and UV A light activation.