There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

The surface science of titanium dioxide

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

We report a novel wet-chemical approach at 180 °C for the synthesis of monodispersed ZnO nanorods with high single-crystallinity. The method has successfully brought the ZnO nanorod diameter from a reported 150 nm down to the 50 nm regime in this work. The aspect ratio of the synthesized nanorods achieved is exceptionally high (in the range of 30−40). This simple low-cost approach should promise us a future large-scale synthesis of ZnO nanostructures for many important applications in nanotechnology in a controlled manner.

Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm.

In this work, we report a simple “one pot” approach to prepare hollow anatase TiO2 nanospheres via Ostwald ripening under hydrothermal conditions. Inner nanospace and highly organized crystallites in the shell structure and surface regions can be created with a wide range of controlling parameters. The formation mechanism has been investigated with TEM/ED/SEM/EDX/XRD/XPS methods. The approach shows a high versatility for structural engineering of various targeted morphological products, including inner material refills.

Preparation of Hollow Anatase TiO2 Nanospheres via Ostwald Ripening

A two-tiered organizing scheme with multiple-length scales for construction of dandelion-like hollow CuO microspheres has been elucidated:  (1) mesoscale formation of rhombic building units from smaller nanoribbons via oriented aggregation and (2) macroscopic organization of these units into the CuO microspheres. This self-assembly concept may also be applicable to other metal oxides by creating geometric constraints for constructional units.

Mesoscale Organization of CuO Nanoribbons: Formation of “Dandelions”

Two methodic concepts, symmetric and asymmetric Ostwald ripening, are elucidated by solution-route syntheses of oxide and sulfide semiconductors. While the original shape of a crystallite aggregate forms the exterior appearance, the preorganization of the crystallites determines the ultimate interior space structure of the aggregate upon Ostwald ripening. Further investigations on the design of crystallite preorganization and control of the solution process will allow the construction of complex architectures, including nonspherical configurations.

Symmetric and Asymmetric Ostwald Ripening in the Fabrication of Homogeneous Core–Shell Semiconductors

In this work, a novel hydrothermal route is developed to synthesize cobalt carbonate hydroxide, Co(CO{sub 3}){sub 0.5}(OH).0.11H{sub 2}O. In this method, sodium chloride salt is utilized to organize single-crystalline nanowires into a chrysanthemum-like hierarchical assembly. The morphological evolution process of this organized product is investigated by examining different reaction intermediates during the synthesis. The growth and thus the final assembly of the Co(CO{sub 3}){sub 0.5}(OH).0.11H{sub 2}O can be finely tuned by selecting preparative parameters, such as the molar ratio of the starting chemicals, the additives, the reaction time and the temperature. Using the flower-like Co(CO{sub 3}){sub 0.5}(OH).0.11H{sub 2}O as a solid precursor, quasi-single-crystalline mesoporous Co{sub 3}O{sub 4} nanowire arrays are prepared via thermal decomposition in air. Furthermore, carbon can be added onto the spinel oxide by a chemical-vapor-deposition method using acetylene, which leads to the generation of carbon-sheathed CoO nanowire arrays (CoO rate at C). Through comparing and analyzing the crystal structures, the resultant products and their high crystallinity can be explained by a sequential topotactic transformation of the respective precursors. The electrochemical performances of the typical cobalt oxide products are also evaluated. It is demonstrated that tuning of the surface texture and the pore size of the Co{sub 3}O{sub 4} products is very important in lithium-ion-battery applications. The carbon-decorated CoO nanowire arrays exhibit an excellent cyclic performance with nearly 100% capacity retention in a testing range of 70 cycles. Therefore, this CoO rate at C nanocomposite can be considered to be an attractive candidate as an anode material for further investigation. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Hua Chun Zeng

Papers

Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm.

Preparation of Hollow Anatase TiO2 Nanospheres via Ostwald Ripening

Mesoscale Organization of CuO Nanoribbons: Formation of “Dandelions”

Symmetric and Asymmetric Ostwald Ripening in the Fabrication of Homogeneous Core–Shell Semiconductors

Mesoporous Co3O4 and CoO@C Topotactically Transformed from Chrysanthemum-like Co(CO3)0.5(OH)·0.11H2O and Their Lithium-Storage Properties