Adsorbed Oxygen Ions and Oxygen Vacancies: Their Concentration and Distribution in Metal Oxide Chemical Sensors and Influencing Role in Sensitivity and Sensing Mechanisms

Abstract

Oxidation reactions on semiconducting metal oxide (SMOs) surfaces have been extensively worked on in catalysis, fuel cells, and sensors. SMOs engage powerfully in energy-related applications such as batteries, supercapacitors, solid oxide fuel cells (SOFCs), and sensors. A deep understanding of SMO surface and oxygen interactions and defect engineering has become significant because all of the above-mentioned applications are based on the adsorption/absorption and consumption/transportation of adsorbed (physisorbed-chemisorbed) oxygen. More understanding of adsorbed oxygen and oxygen vacancies (VO•,VO••) is needed, as the former is the vital requirement for sensing chemical reactions, while the latter facilitates the replenishment of adsorbed oxygen ions on the surface. We determined the relation between sensor response (sensitivity) and the amounts of adsorbed oxygen ions (O2ads−, Oads, −O2ads2−, Oads2−), water/hydroxide groups (H2O/OH−), oxygen vacancies (VO•, VO••), and ordinary lattice oxygen ions (Olattice2−) as a function of temperature. During hydrogen (H2) testing, the different oxidation states (W6+, W5+, and W4+) of WO3 were quantified and correlated with oxygen vacancy formation (VO•, VO••). We used a combined application of XPS, UPS, XPEEM-LEEM, and chemical, electrical, and sensory analysis for H2 sensing. The sensor response was extraordinarily high: 424 against H2 at a temperature of 250 °C was recorded and explained on the basis of defect engineering, including oxygen vacancies and chemisorbed oxygen ions and surface stoichiometry of WO3. We established a correlation between the H2 sensing mechanism of WO3, sensor signal magnitude, the amount of adsorbed oxygen ions, and sensor testing temperature. This paper also provides a review of the detection, quantification, and identification of different adsorbed oxygen species. The different surface and bulk-sensitive characterization techniques relevant to analyzing the SMOs-based sensor are tabulated, providing the sensor designer with the chemical, physical, and electronic information extracted from each technique.

Nanostructured CeO2 ultrathin film deposited by the Langmuir Blodgett technique for highly sensitive and specific detection of sub ppm level NO2 gas at room temperature

“…Ciftyurek et al reported exceptionally high gas sensing characteristics due to oxygen gas rapidly adsorbing on single and double-charged oxygen vacancy defect sites created by the presence of lower oxidation state of the metal in the metal oxide. 48 Similarly, we observed 12% Ce 3+ (XPS data) in the ultrathin CeO 2 surface, which forms oxygen defect sites leading to more surface adsorbed oxygen resulting in high NO 2 sensitivity.…”

Delafossite CuGaO2-Based Chemiresistive Sensor for Sensitive and Selective Detection of Dimethyl Disulfide

“…The types of surface ROS closely depend on the temperature. Molecular type O 2 – ions with relatively weak oxidizing capability are dominant below ∼150 °C (reaction 6), and atomic type O – ions with high oxidizing capability generally require high activation energy and become the primary species from ∼150 to 500 °C (via reaction 7). , Those ROS ions enable to oxidize the adsorbed DMDS molecules (via reactions 8–10) and donate electrons to the CuGaO 2 , resulting in an increase of sensor resistance (Figure ): 4 CH 3 SSCH 3 + 7 O 2 − ( ads ) → 4 CH 3 S + 4 CO 2 false( ad false) + 6 H 2 O + 7 e − false( prefix< 150 ° normalC false) 2 CH 3 SSCH 3 + 7 O − ( ads ) → 2 CH 3 S + 2 CO 2 false( ad false) + 3 H 2 O ( ad ) + 7 …”

Functionalized Multiwalled Carbon Nanotubes for Highly Stable Room Temperature and Humidity-Tolerant Triethylamine Sensing

“…The peak area ratio of the adsorbed to hydroxyl species represents the oxygen vacancy states of the materials. 52,53 Their proportion can be calculated using the following eqs 4 and 5:…”

“…Additionally, the intensity of oxygenated functional groups (with peak locations at 285.3, 286.2, and 288.8 eV) corresponds to C–O, CO, and plasmon π–π* transitions, respectively. , In Figure e,f, the O 1s XPS spectra is deconvoluted into two peaks belonging to the adsorbed and hydroxyl oxygen species on the surface of MWCNT and MWCNT-COOH. The peak area ratio of the adsorbed to hydroxyl species represents the oxygen vacancy states of the materials. , Their proportion can be calculated using the following eqs and : where A OH and A ads . are the integrated peak areas of the hydroxyl and adsorbed oxygen states, respectively.…”