In 1 Hz- to 100-kHz range, the space charge region rules the conductivity process. There is a sharp decrement in the sensitivity with the increment of frequency and little variation in the gain values at frequency higher than 100 kHz, where the conductivity is mainly dependent on the surface charge of the grains. This revealed that a suitable selection of frequency could achieve maximum gain in sensitivity. The sensing mechanism can be described from the following aspects: The oxygen molecules from the ambient atmosphere were initially adsorbed onto the ZnO surface. The electrons were extracted from the conduction band of the ZnO material and were converted to a single or a double oxygen ion
and became ionosorbed on the surface . This led to a decrease in electron concentration and consequently an increase in resistance. This mechanism can be find more described as follows [2, 37]: (5) The reaction of the hydrogen or any reduction gases with the ionosorbed results in the release of the captured electrons back to
the conduction band. This results in an increase in electron concentration, decreasing the resistance which could be explained by the following reaction : (6) When the hydrogen is introduced, PdO is reduced to metallic palladium, returning electrons to ZnO. Hydrogen molecules adsorbed on palladium simultaneously click here spill over the surface of ZnO, activating the reaction between hydrogen and the adsorbed
oxygen: (7) At elevated temperature, Pd is oxidized by the chemisorbed oxygen: (8) The weak bonding of Pd atoms with the oxygen gas results in the dissociation of the complex at relatively low temperature releasing atomic oxygen. The oxygen atoms migrate along the surface of the grains. This migration is induced by the Pd catalyst and is known as spillover of the gaseous ions . Thus, the oxygen atoms capture electrons from the surface layer selleck products forming an acceptor surface at the grain boundary. The presence of catalyst atoms activates the reaction between reducing gases and the adsorbed oxygen [39–41]. Thus, the Pd sensitization on the ZnO nanorod surface enabled the hydrogen sensing at relatively low operating temperature. Conclusions A hydrogen Phosphoprotein phosphatase sensor was successfully developed using Pd-sensitized ZnO nanorods synthesized on oxidized silicon substrate using a sol-gel spin coating technique. The sensor detected ppm level hydrogen at room temperature with more sensitivity over the literature-reported values for the ZnO-based sensors. The variation in the resistance value of the grain boundary which was the basis of analyte detection mechanism was due to the sole variation in hydrogen concentration. Nyquist plot strongly supported the impedance findings. Acknowledgments MK acknowledges the financial support of the Malaysian Ministry of Higher Education (MOHE) through FRGS grant number 9003-00276 to Professor UH.