In this work, lepidocrocite (γ-FeOOH) nanobundles were prepared by a facile NH4F assisted epoxide precipitation route. The reactions between epoxide and [Fe(H2O)6]2+ promoted the hydrolysis and condensation of [Fe(H2O)6]2+, resulting in the formation of iron oxyhydroxide. After calcination of γ-FeOOH nanobundles at 400 °C, the produced α-Fe2O3 still kept the bundle morphology. Due to the unique chemistry of epoxide, the morphology and phase of iron oxide polymorph nanoparticles (goethite, akaganeite, lepidocrocite, magnetite) were well-controlled through controlling reaction conditions such as Fe2+ concentration, NH4F additive and reaction temperature. It is particularly interesting that NH4F working as phase controlling agent is able to control the phase development of iron oxyhydroxides. This phase control effect of NH4F is attributed to the promoted reaction rate of epoxide originating from the higher electronegativity of fluoride ions than chloride ions. Based on the results in this work and our other preliminary works, it is considered that this route can be used as a general strategy for controlling the morphology and phase of transition element compounds.
Noble metal chalcogenides (NMCs) are an important class of materials with unique properties and widespread applications; however, the synthesis of these materials in a well-controlled manner remains challenging. Cation exchange (CE) transformation is a versatile, topotactic strategy which enables the synthesis of a range of materials, but its use for preparing NMCs has not been studied in detail, impeding the application of these materials. Here we demonstrate a robust and general strategy for the fabrication of a wide variety of NMCs via CE transformations, in which CuTe metal chalcogenides are used as sacrificial templates for CE with noble metal cations. The generality of the CE strategy is validated by the fabrication of NMCs with tailored morphologies (zero-dimension (0D), 1D, 2D and 3D), compositions (Pd/Pt/Rh/Ru/Ag/Au-based NMCs) and phases. In-depth mechanistic study shows that the well-controlled synthesis of NMCs is realized by kinetic control and solvent-dependent thermodynamic control.
Jili Wei,Yihua Ren,Yiyang Zhang,Baolu Shi,Shuiqing Li
This work examines the effect of temperature-time history on the flame synthesis of titania nanoparticles using a specially designed swirl-stabilized tubular burner with two kinds of precursor feeding modes, the tangential inlet and the central jet inlet. When the Damköhler number is less than unity, a rapidly mixed type tubular flame is well-formed, which is chosen to be the condition for flame synthesis. The ex situ characterization of the synthesized TiO 2 nanoparticles shows that the particle samples from the tangential inlet feeding mode are mainly anatase (up to 84.3%) with a mean primary particle size of 13.0 ± 3.2 nm, whereas those from the central jet inlet feeding mode are large polydisperse rutile (up to 74%) nanoparticles with a mean size of 16.9 ± 8.8 nm. The significant differences in particle size and crystal phase are attributed to the varied temperature-time histories of two feeding modes, as the residence time of nanoparticles in high-temperature zone of the central jet inlet mode is much longer with larger variation than that of the tangential inlet feeding mode. Finally, the well-dispersed Pd/TiO 2 nanocatalysts are generated by elaborately feeding palladium acetate through the central jet inlet and titanium tetraisopropoxide through the tangential inlets, respectively, to better match the temperature-time histories with the decomposition properties of two different precursors. The study demonstrates the potential of this swirl-stabilized burner for producing tailored single- and multi-component nanomaterials for a variety of applications.
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