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.