Nanosheet-assembled tungsten oxide microspheres have been synthesized using rapid sonochemistry followed by thermal treatment. Transient observation of controllable synthesis reveals that the morphological evolution of the product is highly dependent on the ultrasonication time. An assembly mechanism based on oriented attachment and reconstruction is proposed for the sonochemical formation of the nanosheet-assembled microspheres. The obtained samples possess intrinsic non-stoichiometry and a hierarchically porous nano/microstructure, which is beneficial for their utilization in sensing materials and for fast diffusion of gas molecules. The maximum response of the tungsten oxide hierarchical microspheres is 3 times higher than that of commercial nanoparticles for NO₂ gas. The gas adsorption–desorption kinetics during the sensing process were mathematically simulated by a derivative method. The first-principles calculation reveals that the NO₂ molecule is most likely adsorbed at the terminal O_1c site of tungsten oxide, leading to the introduction of new surface states, which are responsible for the intrinsic NO₂-sensing properties.

Ion-adsorption rare earth element (REE) ores are strategic mineral resources, particularly for heavy REEs. To reduce the amount of nitrogen in the form of ammonia in waste waters and to increase the low recovery efficiency of the current extraction process for REEs, we developed a novel environmentally friendly stepwise solvent extraction method for the separation and enrichment of REEs. Based on their different extraction abilities and a cation-exchange mechanism, two acidic phosphorus extractants (HEH(EHP) and HDEHP) were selected to sequentially separate and enrich REEs. Using this novel technique, ammonia nitrogen emissions were completely avoided and the recovery efficiency of REEs was significantly improved. From the McCabe–Thiele plots, it was found that a three-stage HEH(EHP) extraction at V_A/V_O = 10/1 and two-stage HCl stripping at V_A/V_O = 1/16 were required to enrich heavy REEs, whereas a three-stage HDEHP extraction at V_A/V_O = 25/1 and three-stage HCl stripping at V_A/V_O = 1/20 were required to enrich light REEs. The total recovery efficiency realized for REEs was >99% and the heavy and light REE chloride concentrations in solution were up to 240 and 200 g L⁻¹, respectively. These results suggest an efficient way of recovering REEs from a leaching solution with low concentrations.