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• Monodisperse ZrO 2 nanocrystals were controllably prepared by a phase-transfer route. • The as-prepared ZrO 2 nanocrystals have tunable shapes, crystal forms and sizes. • DFT calculation was used to study the root causes of underpinning growth mechanisms. • Fluorescent monodispersed ZrO 2 :Yb3+/Er3+ nanocrystals could be similarly prepared. Colloidal nanocrystals can offer exquisite control of physical and chemical properties for technological applications by tuning their sizes, shapes and crystal forms. Herein, a convenient phase-transfer route was presented to controllably prepare transparent dispersions of monodispersed ZrO 2 nanocrystals in an aqueous-organic two-phase medium. By adjusting the surfactants, solvents and basicity, the controllability of crystal forms from monoclinic to tetragonal phases, the particle length from 5 to 20 nm, and the different shapes including spindle, cobblestone, sphere, branch, rod and cube can be achieved. The as-prepared ZrO 2 nanocrystals can be readily dispersed in nonpolar solvents, thereby forming the ultra-highly concentrated (60 wt%), highly stable (>18 months) and highly transparent nanodispersions. Density functional theory (DFT) calculations were further used to elucidate the root causes of underpinning growth mechanisms. This work not merely enriches the methodology of controllable fabrication of monodispersed sub-10 nm ZrO 2 nanocrystals, but also enables a platform to synthesize rare-earth-doped nanophosphors using ZrO 2 as the host matrix. The fluorescent ZrO 2 :Yb3+/Er3+ nanocrystals, which can be easily monodispersed in organic solvents, were also prepared with visible luminescene properties under near-infrared irradiation. [ABSTRACT FROM AUTHOR]

• Size-tunable monodisperse Pd nanocrystals were prepared in a rotating packed bed. • They were adopted as pseudohomogeneous catalyst for nitrobenzene (NB) reduction. • 2.6 nm Pd nanocrystals displayed excellent catalytic property and good stability. • The obtained catalytic kinetics data confirm to the Langmuir-Hinshelwood model well. • DFT calculation was used to study the adsorption of NB on the surface of catalyst. Nitrobenzene (NB) reduction is an important catalytic reaction, which is commonly performed in aqueous medium mainly with heterogeneous catalysts. In this work, highly stable aqueous nanodispersions of Pd nanocrystals with a tunable average size between 2 and 11 nm are conveniently prepared in a high-gravity rotating packed bed (RPB) reactor, and firstly serve as a pseudohomogeneous catalyst for NB reduction. As compared to a conventional stirred tank reactor (STR), the product prepared in the RPB reactor has a more uniform particle size. More importantly, the reaction time can be greatly shortened from 3 min (STR) to 1 s (RPB), thereby realizing a continuous preparation. This Pd pseudohomogeneous catalyst displays a significantly size-dependent catalytic activity and a good stability performance. Ultra-small 2.6 nm Pd nanocrystals have a reaction rate five times as fast as 10.1 nm counterpart. The obtained kinetics data confirm to the Langmuir-Hinshelwood model well, having a small error of <10%. Furthermore, the adsorption behavior of NB molecules on the surface of Pd nanocrystals was also studied with density functional theory (DFT) calculation. Since the processing capacity of the used lab-scale RPB reached 44.2 g/h, this route may offer a general platform for mass production of monodisperse nanocrystals as pseudohomogeneous catalyst for highly efficient catalysis. [ABSTRACT FROM AUTHOR]