Role of the paramagnetic donor-like defects in the high n-type conductivity of the hydrogenated ZnO microparticles

Abstract

The magnetic and electronic properties of the hydrogenated highly conductive zinc oxide (ZnO) microparticles were investigated by electron paramagnetic resonance (EPR) and contactless microwave (MW) conductivity techniques in the wide temperature range. The EPR spectra simulation allowed us to resolve four overlapping EPR signals in ZnO microparticles. The Lorentzian EPR line with isotropic g-factor 1.9623(5) was related to the singly ionized oxygen vacancy. Another Lorentzian line with g|| = 1.9581(5), g⊥ = 1.9562(5) was attributed to the zinc interstitial shallow donor center, while EPR signal with g|| = 1.9567(5), g⊥ = 1.9556(5) and Gaussian lineshape was assigned to the hydrogen interstitial shallow effective-mass-like donor. The EPR signal with g|| = 1.9538(5), g⊥ = 1.9556(5) and Lorentzian lineshape was tentatively attributed to the shallow donor center. The charge transport properties in ZnO microparticles have been investigated by the contactless MW conductivity technique at T = 5–296 K. Two conduction mechanisms, including ionization of electrons from the shallow donors to the conduction band and hopping conduction process, have been distinguished. The hopping conduction process follows Mott’s variable-range hopping T−1/4 law at T = 10–100 K. The evaluated values of the average hopping distance (15.86 Å), and hopping energy (1.822 meV at 40 K) enable us to estimate the donor concentration in the investigated ZnO microparticles as ~ 1018 cm−3.

Efficient Cocatalyst-Free Piezo-Photocatalytic Hydrogen Evolution of Defective BaTiO3–x Nanoparticles from Seawater

“…In order to confirm the assignments of these peaks, the EPR was done at 70 °C, as shown in Figure S4. While the faster diffusion of the electrons at the higher temperature is consistent with the observed signal attenuation, 29 the maintenance of the g values confirms that they do not indicate anomalous phenomena and hence are as assigned. Finally, while the progressive Ti 4+ → Ti (4−x)+ reductions are feasible for H 2 reduction, no signal for Ti 2+ and Ti 0 is visible because they are EPR-inactive.…”

About defect phenomena in ZnO nanocrystals

“…4c and d Additionally, for bulk ZnO crystals, the onset of the increase in EPR intensity starts at 50 K and ends at 150 K. However, for nano-ZnO, the onset of the increase in EPR intensity starts at 50 K (similar to bulk ZnO) but ends at 80 K. This is an indication of a different paramagnetic defect type which exists in larger concentration in bulk ZnO with a significant reduction of its spin concentration for nano-ZnO. This additional para-magnetic defect which is observed particularly in the 50 K to 150 K temperature bracket contributes to the EPR signal at g = 1.96 by overlapping an existing V Zn − defect signal 58) and Erdem in the Q-band 30 EPR spectra.…”

“…It must be noted that the core-shell model is specifically and exclusively for nanosized ZnO particles and cannot be applied for microcrystalline (>500 nm) or bulk ZnO samples as reported by Savchenko et al 58 where multiple defect centres exist to compensate macroscopic charge inhomogeneities.…”

Understanding the Sensing Mechanism of ZnO Nanoparticles through Identifying Intrinsic Defects

“…These signals could also form the triplet state of superoxide anions, but the shoulders were not resolved in our experimental spectra. For many investigations of paramagnetic centers in ZnO, there are two important problems that concern identification of the EPR spectra: (i) different spectra are attributed to the same defect; (ii) the same spectrum to different paramagnetic centers. − Therefore, our assignment still needs further experiments to confirm. The observed g value ( g = 2.009) is larger than the value expected for a free-electron spin …”