Useful High-Entropy Source on Spinel Oxides for Gas Detection

This study aimed to identify a useful high-entropy source for gas detection by spinel oxides that are composed of five cations in nearly equal molar amounts and free of impurities. The sensor responses of the spinel oxides [1# (CoCrFeMnNi)₃O₄, 2# (CoCrFeMnZn)₃O₄, 3# (CoCrFeNiZn)₃O₄, 4# (CoCrMnNiZn)₃O₄, 5# (CoFeMnNiZn)₃O₄, and 6# (CrFeMnNiZn)₃O₄] were evaluated for the test gases (7 ppm NO₂, 5000 ppm H₂, 3 ppm NH₃, and 3 ppm H₂S). In response to NO₂, 1# and 2# showed p-type behavior while 3-6# showed n-type semiconductor behavior. There are three p-type and one n-type AO structural compositions in AB₂O₄[AO·B₂O₃] type spinel, and 1# showed a stable AO composition because cation migration from site B to site A is unlikely. Therefore, it was assumed that 1# exhibited p-type behavior. The p-type behavior of 2# was influenced by Cr oxide ions that were present at the B site and the stable p-type behavior of zinc oxide at the A site. The spinel oxides 3# to 6# exhibited n-type behavior with the other cationic oxides rather than the dominant p-type behavior exhibited by the Zn oxide ions that are stable at the A site. In contrast, the sensor response to the reducing gases H₂, NH₃, and H₂S showed p-type semiconductor behavior, with a particularly selective response to H₂S. The sensor responses of the five-element spinel oxides in this study tended to be higher than that of the two-element Ni ferrites and three-element Ni-Zn ferrites reported previously. Additionally, the susceptibility to sulfurization was evaluated using the thermodynamic equilibrium theory for the AO and B₂O₃ compositions. The oxides of Cr, Fe, and Mn ions in the B₂O₃ composition did not respond to H₂S because they were not sulfurized. The increase in the sensor response due to sulfurization was attributed to the decrease in the depletion layer owing to electron sensitization, as the top surface of the p-type semiconductors, ZnO and NiO, transformed to n-type semiconductors, ZnS and NiS, respectively. High-entropy oxides prepared using the hydrothermal method with an equimolar combination of five cations from six elements (Cr, Mn, Fe, Co, Ni, and Zn) can be used as a guideline for the design of high-sensitivity spinel-type composite oxide gas sensors.

Large magnetodielectric effect and negative magnetoresistance in NiO nanoparticles at room temperature

Nickel oxide nanoparticles having a mean particle size of 19.5 nm were synthesized by a simple chemical method. Those nanoparticles exhibited a spin glass like behaviour at a temperature around 9 K. The samples showed electronic conduction arising out of small polaron hopping between the Ni2+ and Ni3+ species present in the material. A large magnetodielectric parameter with a maximum value of 52.2% was observed in the sample at room temperature which resulted from the Maxwell-Wagner polarization effect. This was explained as arising due to a large negative magnetoresistance caused by spin polarized electron hopping between Ni2+ and Ni3+ sites with the consequential formation of space charge polarization at the interfaces of the NiO nanoparticles. This was substantiated by direct measurement of magnetoresistance of the samples which gave identical results. It is believed that negative magnetoresistance after direct measurement occurred due to the interaction between ferromagnetic and antiferromagnetic phases and the value was 37%, the highest reported in the literature so far. As a result of the presence of Ni3+ ions, antiferromagnetic phase and ferromagnetic like behaviour of NiO nanoparticles gave higher magnetization than other reported nanoparticles. Such large values of magnetoresistance of the samples will make the material useful as an ideal magnetic sensor.