Here we combined experimental and theoretical results to correlate the morphological, optical, and electronic properties of cerium oxide (CeO2) prepared by a microwave-assisted hydrothermal method with varying synthesis times. X-ray diffraction confirmed a cubic structure without deleterious phases. Density functional theory simulations confirmed an indirect (K-L) bandgap energy of 2.80 eV, with an electron transition between O-2p and Ce-4f orbitals, which agrees with the value obtained using diffuse reflectance. Raman spectroscopy shows that changing the synthesis times results in samples with different defect densities at a short range. Theoretical calculations confirmed that the deformations and changes in the experimental Raman spectra area result in oxygen displacement; as the displacement decreases, the crystallinity increases, and only one peak was observed. Scanning electron microscopy and high-resolution transmission electron microscopy show changes in the morphologies as the synthesis time varies. For shorter times, sheet and polyhedral morphologies were noted. With time increases, the sheets turn into nanorods and nanowires until the nanowires decrease and cubes are observed. In addition, an initial study regarding the influence of the surface on the electric response of CeO2 was completed. It was observed that the presence of different surface defects ([CeO6·2Vo x ] or [CeO7·Vo x ]) can alter the material resistance.

Impedance spectroscopy is essential in understanding the physical and chemical processes that affect the electronic behavior of semiconducting metal oxide-based gas sensor materials, but is often overlooked or avoided by researchers because of the complexity of proper data interpretation. These metal oxide nanomaterials are promising as resistive-type gas sensors because they have the potential to meet the critical need that has developed for low-power, low-cost, portable gas sensors. However, the mechanisms that determine these materials' gas-sensing performance is still not understood well enough to advance the field toward bottom-up design for specific applications. Direct current (DC) measurements give information on the device performance such as sensitivity, selectivity and response time. Alternating current (AC) measurements are a lesser-used approach that can give the same information, but also allow the varying contributions from the bulk, surfaces and interfaces, grain boundaries, electrode contacts and even substrate to be quantified, whether we work with polycrystalline materials or single-crystal structures such as nanowires and nanorods. With this technique it is possible to extract fundamental information about intergranular potential energy barrier height, width and donor concentration changes in different atmospheres and temperatures. The equivalent circuit method is the most common method of relating the impedance data to the physical system. However, the main weakness of this method is that the ideal circuit elements used in the fitting can be connected in many different ways to fit the same signal. To properly apply this technique, the model development should consider both the impedance measurement and the physical and chemical characteristics of the system being studied. This review presents a detailed description of impedance spectroscopy techniques for fundamental analysis of materials behavior in gas sensing applications, including proper data interpretation and types of fitting models commonly used, to enable researchers to better apply these techniques toward understanding of their materials systems.

No abstract