In recent years, more and more metal oxides have been finding critical tribo-catalytic applications. Presently, we have explored the tribo-catalytic conversion of H₂O and CO₂ using Co₃O₄ nanoparticles and obtained some surprising results. In an as-received 150 mL glass reactor enclosed with 10 mL of H₂O, 0.10 g of Co₃O₄ nanoparticles, 1 atm of CO₂, and a Teflon magnetic rotary disk, we observed the production of as much as 57.41 µmol/L of H₂, 0.15 µmol/L of CH₄, and 0.21 µmol/L of CO after 5 h of magnetic stirring. Metallic coatings of Cu, Ni, SUS316, Ti, Nb, Mo, and W were further introduced on reactor bottoms separately. For those coatings of Ni, SUS316, Ti, and Nb, the reduction of CO₂ was dramatically enhanced, and C₂₊ products of C₂H₆ and C₂H₄ were observed. Especially for the Ti coating, the amounts of H₂ and CH₄ were increased by 2 and 26 times from those for the glass bottom, respectively, and the amounts of C₂H₆ and C₂H₄ were very impressive. The Co₃O₄ nanoparticles were proven chemically stable under magnetic stirring in water, and hydroxyl radicals and superoxide radicals have been detected for the Co₃O₄ nanoparticles under magnetic stirring through fluorescence spectroscopy and electron paramagnetic resonance spectroscopy analyses. These findings not only reveal outstanding capability of Co₃O₄ to generate multicarbon products from H₂O and CO₂ through tribo-catalysis but also highlight a promising potential of tribo-catalysis as a whole to harness mechanical energy for addressing energy shortages and environmental pollution.

The friction between nanomaterials and Teflon magnetic stirring rods has recently drawn much attention for its role in dye degradation by magnetic stirring in dark. Presently the friction between TiO₂ nanoparticles and magnetic stirring rods in water has been deliberately enhanced and explored. As much as 1.00 g TiO₂ nanoparticles were dispersed in 50 mL water in 100 mL quartz glass reactor, which got gas-closed with about 50 mL air and a Teflon magnetic stirring rod in it. The suspension in the reactor was magnetically stirred in dark. Flammable gases of 22.00 ppm CO, 2.45 ppm CH₄, and 0.75 ppm H₂ were surprisingly observed after 50 h of magnetic stirring. For reference, only 1.78 ppm CO, 2.17 ppm CH₄, and 0.33 ppm H₂ were obtained after the same time of magnetic stirring without TiO₂ nanoparticles. Four magnetic stirring rods were simultaneously employed to further enhance the stirring, and as much as 30.04 ppm CO, 2.61 ppm CH₄, and 8.98 ppm H₂ were produced after 50 h of magnetic stirring. A mechanism for the catalytic role of TiO₂ nanoparticles in producing the flammable gases is established, in which mechanical energy is absorbed through friction by TiO₂ nanoparticles and converted into chemical energy for the reduction of CO₂ and H₂O. This finding clearly demonstrates a great potential for nanostructured semiconductors to utilize mechanical energy through friction for the production of flammable gases.

Contrasting room-temperature hydrogen sensing behaviors have been revealed for Pt-TiO₂ and Pt-SnO₂ composite nanoceramics. In the case of the Pt-TiO₂ nanoceramics, the ultrahigh hydrogen sensitivities are lost abruptly when the oxygen/hydrogen concentration ratio in ambient atmosphere reaches a critical value. However, in the case of the Pt-SnO₂ nanoceramics, such a phenomenon does not occur, and the extraordinary room-temperature hydrogen sensing capabilities are observed in the presence of oxygen in air. Our combined experimental and theoretical investigations establish a unified mechanism for both the systems, which is rooted in hydrogen chemisorption on the surface and interstitial lattice sites of SnO₂ and TiO₂; the difference in stability of the chemisorbed hydrogen on SnO₂ and TiO₂ is considered responsible for the contrasting hydrogen sensing capabilities. The central findings are helpful in enriching our microscopic understanding of hydrogen interaction with various metal oxide semiconductors (MOSs) at room temperature in varying mixed gaseous concentrations, and they could be instrumental in developing reliable room-temperature hydrogen sensors based on bulk MOSs.

Ceramics of Bi_0.9Ba_0.1Fe_0.925Ti_xO₃ (x = 0.0625, 0.08125, 0.0875, and 0.11) were prepared according to two doping strategies: one is called single-step doping in which Ba and Ti were doped together in calcination, while the other one is called two-step doping in which Ba and Ti were doped in calcination and sintering, respectively. Compared with samples prepared with single-step doping, those prepared with two-step doping have obviously different XRD patterns and small grains, and are dramatically improved in dielectric loss, resistivity, and remnant magnetization. A low dielectric loss of 0.05 at 10³ Hz, a high resistivity of 4×10¹² Ω·cm, and a large remnant magnetization of 1.5 emu/g, have been obtained simultaneously for Bi_0.9Ba_0.1Fe_0.925Ti_0.11O₃ prepared with two-step doping. The contrast between these two doping strategies clearly reveals the importance of establishing a proper doping strategy when two or more elements are co-doped to BiFeO₃.

Pure and noble metal (Pt, Pd, and Au) doped TiO₂ nanoceramics have been prepared from TiO₂ nanoparticles through traditional pressing and sintering. For those samples sintered at 550 ℃, a typical premature sintering occurred, which led to the formation of a highly porous microstructure with a Brunauer–Emmett–Teller (BET) specific surface area of 23 m²/g. At room temperature, only Pt-doped samples showed obvious response to hydrogen, with sensitivities as high as ~500 for 1000 ppm H₂ in N₂; at 300 ℃, all samples showed obvious responses to CO, while the responses of noble metal doped samples were much higher than that of the undoped ones. The mechanism for the observed sensing capabilities has been discussed, in which the catalytic effect of Pt for hydrogen is believed responsible for the room-temperature hydrogen sensing capabilities, and the absence of glass frit as commonly used in commercial thick-film metal oxide gas sensors is related to the high sensitivities. It is proposed that much attention should be paid to metal oxide porous nanoceramics in developing gas sensors with high sensitivities and low working temperatures.