Inhomogeneous Oxygen Vacancy Distribution in Semiconductor Gas Sensors: Formation, Migration and Determination on Gas Sensing Characteristics

Abstract

The density of oxygen vacancies in semiconductor gas sensors was often assumed to be identical throughout the grain in the numerical discussion of the gas-sensing mechanism of the devices. In contrast, the actual devices had grains with inhomogeneous distribution of oxygen vacancy under non-ideal conditions. This conflict between reality and discussion drove us to study the formation and migration of the oxygen defects in semiconductor grains. A model of the gradient-distributed oxygen vacancy was proposed based on the effects of cooling rate and re-annealing on semiconductive thin films. The model established the diffusion equations of oxygen vacancy according to the defect kinetics of diffusion and exclusion. We described that the steady-state and transient-state oxygen vacancy distributions, which were used to calculate the gas-sensing characteristics of the sensor resistance and response to reducing gases under two different conditions. The gradient-distributed oxygen vacancy model had the applications in simulating the sensor performances, such as the power law, the grain size effect and the effect of depletion layer width.

Improving gas sensing performance by oxygen vacancies in sub-stoichiometric WO3−x

“…These results indicate that oxygen deciency enhances chemisorption of oxygen ions on WO 3Àx surface at low temperature by offering a large number of chemically active absorption sites, which in turn facilitates the ionosorption process and improves the lowtemperature gas sensing sensitivity. [34][35][36] Thus, both the temperature-dependent response curves and the O 2 -TPD proles measured on the sub-stoichiometric WO 3Àx sensors evidence that the working temperature of the gas sensors can be lowered down by using sub-stoichiometric nano WO 3Àx. The W 5 O 14 sensor presents the best performance over the narrow temperature range around 280 C. However, W 18 O 49 possesses the highest concentration of oxygen vacancies among all the sub-stoichiometric WO 3Àx ones studied here, and the W 18 O 49 sensor presents considerable response at low temperature.…”

Influence of Oxygen Vacancy Behaviors in Cooling Process on Semiconductor Gas Sensors: A Numerical Analysis

“…Furthermore, the defects would migrate in the grain if the temperature is above the absolute zero K especially when the operating temperature of a semiconductor gas sensor is usually above 200 °C. The importance of V O was recognized and there were some specific studies on it [ 30 , 31 , 32 ]. However, in the theoretical investigations, the roles of V O were simplified since its density had to be assumed as a uniform distribution throughout the grain [ 17 , 18 ].…”

“…The analytical solution of the diffusion equation is not available. Thus, several presumptions had to be made in the previous works [ 31 , 33 , 34 ] to simplify the calculation, which simulated the V O distribution and gas-sensing characteristics of the semiconductor grain. However, some of the presumptions were far from the practical situations, which leads to an inaccuracy in the solutions that describe the distributions of oxygen vacancies.…”

Mesoporous Tungsten Trioxide Photoanodes Modified with Nitrogen-Doped Carbon Quantum Dots for Enhanced Oxygen Evolution Photo-Reaction

“…The syntheses of ordered meso porous materials are classified as template-free or templating methods. Methods involving sol-gel [10], chemical vapor deposition (CVD) [11], and spray pyrolysis deposition [12] mostly produce materials with irregular meso porous structures and disordered pore morphology. Besides, the hard-templating method produces organized meso porous materials with highly crystalline framework that replicates the structure of the meso porous SBA−15 silica, carbon, or cubic KIT-6 silica template [13,14].…”