Trong-On Do

Shape-Controlled Synthesis of Highly Crystalline Titania Nanocrystals

A versatile synthetic method based on solvothermal technique has been developed for the fabrication of TiO(2) nanocrystals with different shapes such as rhombic, truncated rhombic, spherical, dog-bone, truncated and elongated rhombic, and bar. The central features of our approach are the use of water vapor as hydrolysis agent to accelerate the reaction and the use of both oleic acid and oleylamine as two distinct capping surfactants which have different binding strengths to control the growth of the TiO(2) nanoparticles. We also show that the presence of an appropriate amount of water vapor along with the desired oleic acid/oleylamine molar ratio plays a crucial role in controlling size and shape of TiO(2) nanocrystals.

Three-dimensional ordered assembly of thin-shell Au/TiO2 hollow nanospheres for enhanced visible-light-driven photocatalysis.

An Au/TiO(2) nanostructure was constructed to obtain a highly efficient visible-light-driven photocatalyst. The design was based on a three-dimensional ordered assembly of thin-shell Au/TiO(2) hollow nanospheres (Au/TiO(2)-3 DHNSs). The designed photocatalysts exhibit not only a very high surface area but also photonic behavior and multiple light scattering, which significantly enhances visible-light absorption. Thus Au/TiO(2)-3 DHNSs exhibit a visible-light-driven photocatalytic activity that is several times higher than conventional Au/TiO(2) nanopowders.

Novel Route to Size-Controlled Fe–MIL-88B–NH2 Metal–Organic Framework Nanocrystals

A new approach for the synthesis of uniform metal-organic framework (MOF) nanocrystals with controlled sizes and aspect ratios has been developed using simultaneously the non-ionic triblock co-polymer F127 and acetic acid as stabilizing and deprotonating agents, respectively. The alkylene oxide segments of the triblock co-polymer can coordinate with metal ions and stabilize MOF nuclei in the early stage of the formation of MOF nanocrystals. Acetic acid can control the deprotonation of carboxylic linkers during the synthesis and, thus, enables the control of the rate of nucleation, leading to the tailoring of the size and aspect ratio (length/width) of nanocrystals. Fe-MIL-88B-NH(2), as an iron-based MOF crystal, was selected as a typical example to illustrate our approach. The results reveal that this approach is used for not only the synthesis of uniform nanocrystals but also the control of the size and aspect ratio of the materials. The size and aspect ratio of nanocrystals increase with an increase in the concentration of acetic acid in the synthetic mixture. The non-ionic triblock co-polymer F127 and acetic acid can be easily removed from the Fe-MIL-88B-NH(2) nanocrystal products by washing with ethanol, and thus, their amine groups are available for practical applications. The approach is expected to synthesize various nanosized carboxylate-based MOF members, such as MIL-53, MIL-89, MIL-100, and MIL-101.

In situ X-ray diffraction monitoring of a mechanochemical reaction reveals a unique topology metal-organic framework.

Chemical and physical transformations by milling are attracting enormous interest for their ability to access new materials and clean reactivity, and are central to a number of core industries, from mineral processing to pharmaceutical manufacturing. While continuous mechanical stress during milling is thought to create an environment supporting nonconventional reactivity and exotic intermediates, such speculations have remained without proof. Here we use in situ, real-time powder X-ray diffraction monitoring to discover and capture a metastable, novel-topology intermediate of a mechanochemical transformation. Monitoring the mechanochemical synthesis of an archetypal metal-organic framework ZIF-8 by in situ powder X-ray diffraction reveals unexpected amorphization, and on further milling recrystallization into a non-porous material via a metastable intermediate based on a previously unreported topology, herein named katsenite (kat). The discovery of this phase and topology provides direct evidence that milling transformations can involve short-lived, structurally unusual phases not yet accessed by conventional chemistry.

Recent advances in the development of sunlight-driven hollow structure photocatalysts and their applications

The over-exploitation of fossil fuels means that research into alternative sustainable energy sources is crucial for the scientific community. The harvesting of solar energy via photocatalysis is a key approach to developing these alternatives. Furthermore, photocatalytic materials show great promise for degradation of pollutants. However, limitations in incident light utilization and charge separation are major drawbacks that restrict the activity of current artificial photosystems. Construction of hollow nano-sized photocatalysts is emerging as a promising approach to fabricating novel and effective materials, as hollow photocatalysts possess unique properties that may be exploited to overcome these challenges. This review gives a concise overview of the advantages of hollow structures for this purpose, the methodology used to prepare hollow photocatalysts, and the current state-of-the-art in the development of hollow structure photocatalysts for energy production and environmental applications.

Shape- and size-controlled synthesis of monoclinic ErOOH and cubic Er2O3 from micro- to nanostructures and their upconversion luminescence.

A general approach has been developed for the synthesis of monoclinic ErOOH and cubic Er2O3 structures with high yield and controlled size and shape via the solvo-hydrothermal reaction of erbium nitrate in water/ethanol/decanoic acid media. The monoclinic ErOOH phase was formed at relatively low temperature (120-140 degrees C); however, the cubic Er2O3 phase was obtained at higher temperature (160-180 degrees C). By simply tuning different experimental parameters, such as the reaction temperature, the concentration of decanoic acid and erbium precursor etc., different sizes from 3 nm to 3 microm, and a variety of shapes including cores/dots to spheres, wrinkle-surfaced spheres, flowers, dog bonds, wires, rods, bundles, straw sheaves, and brooms can be achieved. The particle size of products decreased from micro- to nanometer as the decanoic acid concentration increased from 0.038 to 0.190 M. Furthermore, by using anhydrous ethanol instead of water-ethanol solvents, the particle size significantly decreased from 18 nm spheres to 3 nm cores. At high precursor monomer concentrations (76.25-152.50 mM), the nanorods were also obtained due to the anisotropic growth. On the basis of this study, a correlation between the experimental parameters and the phase, shape, and size of the products was proposed. The upconversion luminescence properties depend not only on crystalline phase but also on particle size of the products. The luminescence intensity increases with the decrease of particle size from micro- to nanometers.

Solvo-hydrothermal approach for the shape-selective synthesis of vanadium oxide nanocrystals and their characterization.

A new solvo-hydrothermal method has been developed for the synthesis of uniform vanadium oxide nanocrystals (NCs) with various sizes and shapes in aliphatic amine/toluene/water using V(V) diperoxo alkylammonium complexes. The vanadium complex precursors were prepared from an ion exchange reaction of V(V) diperoxo gels and tetraalkylammonium bromide in the water-toluene mixture using H(2)O(2) solution and commercial bulk V(2)O(5) powders as starting vanadium gel source. The obtained VO(2) NC products were characterized by means of transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Fourier transform infrared absorption spectroscopy (FTIR), thermogravimetric differential thermal analysis (TGA-DTA), and nitrogen adsorption/desorption (BET). The size and shape of NCs can be controlled by different synthesis parameters such as water content, steric ligands of complexes, alkyl chain lengths of capping aliphatic amines, as well as nature of solvent. Monodisperse vanadium oxide NCs with various sizes and shapes, nanospheres, nanocubes, nanorices, and nanorods, can be easily achieved. The possible mechanisms for the formation of vanadium complex precursors and vanadium oxide NCs as well as the shape evolution of NCs were also discussed. The as-made vanadium oxide products exhibited the monoclinic rutile VO(2) structure, which was however converted to the orthorhombic V(2)O(4.6) structure after calcination in air. The XPS results also revealed only one V(4+) state for the as-made sample; however, the coexistence of V(5+) and V(4+) states and two components of oxygen associated with OV and O-V for the calcined samples on the vanadium oxide NC surface were observed. The surface chemical composition of both as-made and calcined samples were found to be VO(2) and V(2)O(5-x) (x = 0.4), respectively. Our approach may provide a novel route for the extended synthesis to other inorganic NCs.

Biomolecule-assisted route for shape-controlled synthesis of single-crystalline MnWO4 nanoparticles and spontaneous assembly of polypeptide-stabilized mesocrystal microspheres

Single-crystalline mixed metal oxide nanoparticles and 3D hierarchical mesocrystal microspheres of MnWO4 have been synthesized on a large scale by a facile single-step hydrothermal method using Mn(NO3)2 and Na2WO4 precursors, and capping bifunctional amino acid biomolecules with different alkyl chain lengths, and water or water/ethylene glycol medium. The resulting single-crystalline MnWO4 nanoparticles with different uniform shapes including bar, rod, square, quasi-sphere, sphere, hexagonal crystals were obtained by tuning the synthetic conditions such as the concentration and the alkyl chain length of amino acids, pH, and reaction temperature. By decreasing the Mn2+ and WO42− precursor monomer concentration from 0.0150 to 0.0076 M in aqueous media, polypeptide-stabilized MnWO4 mesocrystal hierarchical microspheres were achieved, due to the spontaneous-assembly of primary nanoparticles through the back-bone–back-bone intermolecular hydrogen bonding interactions of polypeptide chains. Nanoplatelet-based microapples with two holes on their poles were also obtained under the same synthetic conditions for the microspheres, except that the use of water/ethylene glycol (10:30 mL) instead of water medium. The photoluminescence (PL) results revealed that the PL emission intensity of the MnWO4 nanobars is higher than that of the self-assembled MnWO4 microspheres. This green chemistry method is simple and highly reproducible, using inexpensive reagents, and water as reaction solvent. The uniform MnWO4 nanorods with the control of aspect ratio (length/width) can be produced in a large quantity as much as 16 g in a single preparation. The current approach is quite general and able to be extended to a variety of other well-defined metal oxide and mixed oxide nanomaterials with controlled shapes.

Mineral neogenesis as an inspiration for mild, solvent-free synthesis of bulk microporous metal–organic frameworks from metal (Zn, Co) oxides

“Accelerated aging” is a simple and conceptually novel methodology for the synthesis of functional metal–organic materials, which seeks to provide a scalable, mild and environmentally-friendly alternative to solution-based or mechanochemical syntheses. Accelerated aging draws inspiration from slow processes of geological biomineralization and mineral neogenesis, and adapts them for the low-energy and solvent-free synthesis of modern metal–organic materials. This systematic study outlines the development of an accelerated aging synthesis of microporous frameworks from metal oxides CoO and ZnO. Whereas metal oxides often require high temperatures or aggressive reagents, accelerated aging allows their spontaneous transformation into porous materials under surprisingly mild conditions, akin to those of molecular self-assembly (humid air, up to 45 °C). Here, we describe how accelerated aging can be optimized for the one-step synthesis of multi-gram amounts of microporous solids. As targets, we selected popular zeolitic imidazolate frameworks (ZIFs): the sodalite-topology ZIF-8, its cobalt analogue ZIF-67, and a related zeolite RHO framework. Unlike conventional solution or mechanochemical syntheses, accelerated aging is diffusion-controlled and does not require continuous agitation, bulk solvent or high temperature. The syntheses of ZIF-8 and ZIF-67 contrast the conventional paradigm of metal–organic framework synthesis, by demonstrating that microporous materials can be spontaneously and efficiently assembled from a close-packed metal oxide without using solvents, high temperature or other activation (e.g. microwave, sonochemical, mechanochemical).

Monodisperse samarium and cerium orthovanadate nanocrystals and metal oxidation states on the nanocrystal surface.

A new solvothermal method has been developed for the synthesis of monodisperse SmVO4 and CeVO4 nanocrystals with controlled size and shape. The obtained materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) techniques. The results reveal that uniform nanocrystals and pure tetragonal phase of SmVO4 and CeVO4 can be achieved. To investigate the oxidation states of the metals on the mixed oxide nanocrystal surface, the XPS technique was employed. The results exhibit that only one oxidation state of samarium, cerium, and vanadium for each metal (e.g., Sm3+, Ce3+, V5+) was surprisingly well stable on the particle surface at the nanoscale, even after calcination, while the existence of two oxidation states of these metals is observed (e.g., Sm3+/Sm2+, Ce4+/Ce3+, V5+/V4+) in the corresponding single metal oxide nanocrystals.