More and more metal oxide nanomaterials are being synthesized and investigated for degradation of organic pollutants through harvesting friction energy, yet the strategy to optimize their performance for this application has not been carefully explored up to date. In this work, three commercially available ZnO powders are selected and compared for tribocatalytic degradation of organic dyes, among which ZnO-1 and ZnO-2 are agglomerates of spherical nanoparticles around 20 nm, and ZnO-3 are particles of high crystallinity with a regular prismatic shape and smooth surfaces, ranging from 50 to 150 nm. Compared with ZnO-1 and ZnO-2, ZnO-3 exhibits a much higher tribocatalytic degradation performance, and a high degradation rate constant of 6.566 × 10⁻² min⁻¹ is achieved for RhB, which is superior compared with previous tribocatalytic reports. The stability and universality of ZnO-3 were demonstrated through cycling tests and degradation of different types of dyes. Furthermore, the mechanism of tribocatalysis revealed that h+ was the main active species in the degradation process by ZnO. This work highlights the great significance of high crystallinity rather than a large specific surface area for the development of high-performance tribocatalysts and demonstrates the great potential of tribocatalysis for water remediation.

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.

Hydrogen in metal oxides usually strongly associates with a neighboring oxygen ion through an O-H bond and thus displays a high stability. Here we report a novel state of hydrogen with unusually high mobility and reactivity in metal oxides at room temperature. We show that freshly doped hydrogen in Nb₂O₅ and WO₃ polycrystals via electrochemical hydrogenation can reduce Cu²⁺ ions into Cu⁰ if the polycrystals are immersed in a CuSO₄ solution, while this would not happen if the hydrogenated polycrystals have been placed in air for several hours before the immersion. Time-dependent studies of electrochemically hydrogenated rutile single crystals reveal two distinct states of hydrogen: one as protons covalently bonded to oxygen ions, while the other one is highly unstable with a lifetime of just a few hours. Observation of this mobile and reactive state of hydrogen will provide new insight into numerous moderate and low temperature interactions between metal oxides and hydrogen.