Morphology-conserved transformation: synthesis of hierarchical mesoporous nanostructures of Mn(2)O(3) and the nanostructural effects on Li-ion insertion/deinsertion properties

By means of morphology-conserved transformation, we have synthesized hierarchically structured Mn(2)O(3) nanomaterials with different morphologies and pore structures. The key step of this method consists of the formation of a precursor containing the target materials interlaced with the judiciously chosen polyol-based organic molecules, which are subsequently knocked out to generate the final nanomaterials. In the present work, two kinds of precursor morphologies, oval-shaped and straw-sheaf-shaped, have been selectively prepared by hydrothermal treatment of different functional polyol molecules (oval-shape with fructose and straw-sheaf-shape with beta-cyclodextrin) and potassium permanganate. Thermal decomposition of the precursors resulted in the formation of mesoporous Mn(2)O(3) maintaining the original morphologies, as revealed by extensive characterization. These novel hierarchical nanostructures with different pore sizes/structures prompted us to examine their potential as anode materials for lithium ion batteries (LIBs). The electrochemical results with reference to LIBs show that both of our mesoporous Mn(2)O(3) nanomaterials deliver high reversible capacities and excellent cycling stabilities at a current density of 200 mA g(-1) compared to the commercial Mn(2)O(3) nanoparticles. Moreover, the straw-sheaf-shaped Mn(2)O(3) exhibits a higher specific capacity and a better cycling performance than the oval-shaped one, due to the relatively higher surface area and the peculiar nanostrip structure resulting in the reduced length for lithium ion diffusion. Morphology-conserved transformation yields two kinds of hierarchical mesoporous Mn(2)O(3) nanomaterials with high capacities and cycling stabilities for lithium ion batteries.NSFC/HK-RGC[NSFC 20931160426, N_HKUST609/09]; HK-RGC[HKUST 604809, 605710

Branched ZnO nanostructures as building blocks of photoelectrodes for efficient solar energy conversion

ZnO nanotetrapods are distinguished by their unique nanocrystalline geometric form with four tetrahedrally directed arms, which endows them the ability to handily assemble three-dimensional network structures. Such network structures, coupled with the intrinsically excellent electronic properties of the semiconducting ZnO, have proved advantageous for building photoelectrodes in energy conversion devices since they allow fast vectorial electron transport. In this review article, we summarize recent efforts, with partial emphasis on our own, in the development of ZnO nanotetrapod-based devices for solar energy conversion, including dye-sensitized solar cells and photoelectrochemical cells for water splitting. A pure ZnO nanotetrapod network was firstly demonstrated to have excellent charge collection properties even with just physical contacts. Composition design of ZnO nanotetrapods/SnO2 nanoparticles yielded a high efficiency of 4.91% in flexible DSSCs. More significantly, by secondary branching and nitrogen doping, a record performance for water splitting has been achieved. A perspective on future research directions in ZnO nanotetrapod-based solar energy conversion devices is also discussed together with possible strategies of pursuit. It is hoped that the results obtained so far with the ZnO nanotetrapods could inspire and catalyze future developments of solar energy conversion systems based on branched nanostructural materials, contributing to solving global energy and environmental issues

High performance supercapacitors based on highly conductive nitrogen-doped graphene sheets

Thermal nitridation of reduced graphene oxide sheets yields highly conductive (similar to 1000-3000 S m(-1)) N-doped graphene sheets, as a result of the restoration of the graphene network by the formation of C-N bonded groups and N-doping. Even without carbon additives, supercapacitors made of the N-doped graphene electrodes can deliver remarkable energy and power when operated at higher voltages, in the range of 0-4 V

General surfactant-free synthesis of MTiO3 (M = Ba, Sr, Pb) perovskite nanostrips

We report on the first general preparation of one dimensional (1D) MTiO3 (M = Ba, Sr, Pb) nanostrips by a surfactant-free approach in nonaqueous molten salt media. The as-synthesized MTiO3 nanostrips were characterized by electron microscopic techniques, X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The MTiO3 nanostrips are characterised by a width of 50-200 nm, a thickness of 20-50 nm, and a length of several to tens of micrometers. The growth processes of the MTiO3 nanostrips have been studied and discussed

Double-Layered Photoanodes from Variable-Size Anatase TiO2 Nanospindles: A Candidate for High-Efficiency Dye-Sensitized Solar Cells

Chemical Equation Reprentation Little and large: A double-layered photoanode constructed from small and large single-crystal anataseTiO ₂ nanospindles achieves 8.3% energy-conversion efficiency in a dye-sensitized solar cell (see picture). One layer of the double-layer structure serves mainly to harbor numerous dye molecules, and the other primarily enhances light harvesting by multiple scattering. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA

Facile hydrothermal preparation of hierarchically assembled, porous single-crystalline ZnO nanoplates and their application in dye-sensitized solar cells

Bottom-up engineering of semiconductor nanostructures holds high potential for designing the next generation of solar cells. A superstructure of meso/micro-porous single-crystalline ZnO nanoplates is created by controlled thermal decomposition of a nanoplate precursor prepared from a hydrothermal process. This unique porous nanoplate structure has proved to be an excellent candidate for constructing photoanodes of low-cost and high-performance dye-sensitized solar cells (DSSC)

A new ZnO nanotetrapods/SnO2 nanoparticles composite photoanode for high efficiency flexible dye-sensitized solar cells

We report a new photoanode for flexible dye sensitized solar cells based on a judicious combination of SnO2 nanoparticles and ZnO nanotetrapods and the development of an effective low temperature fabrication technique of ``acetic acid gelation-mechanical press-ammonia activation''. A 4.91\% efficiency has been achieved on an ITO-coated polyethylenenaphthalate (ITO/PEN) substrate under 1 sun equivalent illumination. The peculiar symmetrical branching morphology of ZnO nanotetrapods is beneficial to charge collection of the SnO2/ZnO composite photoanode. The formation of a thin ZnO shell on SnO2 nanoparticles, after ammonia activation, is critical to boosting the open-circuit photovoltage and to improving inter-particle contacts as well as photoelectron injection. The mechanical press, apart from enhancing film durability, has also significantly improved charge collection

Mesoporous TiO2 Single Crystals: Facile Shape-, Size-, and Phase-Controlled Growth and Efficient Photocatalytic Performance

In this work, we have succeeded in preparing rutile and anatase TiO2 mesoporous single crystals with diverse morphologies in a controllable fashion by a simple silica-templated hydrothermal method. A simple in-template crystal growth process was put forward, which involved heterogeneous crystal nucleation and oriented growth within the template, a sheer spectator, and an excluded volume, i.e., crystal growth by faithful negative replication of the silica template. A series of mesoporous single-crystal structures, including rutile mesoporous TiO2 nanorods with tunable sizes and anatase mesoporous TiO2 nanosheets with dominant {001} facets, have been synthesized to demonstrate the versatility of the strategy. The morphology, size, and phase of the TiO2 mesoporous single crystals can be tuned easily by varying the external conditions such as the hydrohalic acid condition, seed density, and temperature rather than by the silica template, which merely serves for faithful negative replication but without interfering in the crystallization process. To demonstrate the application value of such TiO2 mesoporous single crystals, photocatalytic activity was tested. The resultant TiO2 mesoporous single crystals exhibited remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single-crystal nature, and the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. It was found that {1 10} facets of rutile mesoporous single crystals can be considered essentially as reductive sites with a key role in the photoreduction, while {001} facets of anatase mesoporous single crystals provided oxidation sites in the oxidative process. Such shape- and size-controlled rutile and anatase mesoporous TiO2 single crystals hold great promise for building energy conversion devices, and the simple solution-based hydrothermal method is extendable to the synthesis of other mesoporous single crystals beyond TiO2

A double layered photoanode made of highly crystalline TiO(2) nanooctahedra and agglutinated mesoporous TiO(2) microspheres for high efficiency dye sensitized solar cells

We report the development of a novel double layered photoanode for dye sensitized solar cells made of highly crystalline TiO(2) octahedral nanocrystals and agglutinated mesoporous TiO(2) microspheres. The underlayer of nanooctahedra serves as a transparent photoanode for copious and strong dye adsorption on the smooth (101) surfaces and for facilitated electron transport. Although the nanooctahedra are extremely small, our synthetic route has ensured a well-faceted crystalline shape with sharp edges and smooth surfaces, resulting in a 7.61% power conversion efficiency, much higher than that of P25 (5.76%). Separately, the overlayer of hierarchical TiO(2) mesoporous microspheres plays the multiple roles of efficient light scattering, dye absorption and electrolyte permeation. Especially noteworthy is the agglutination of the microspheres through our 3D necking process, which has yielded an electron diffusion coefficient five times that of the P25 network and four times that of the nanooctahedra network. This is a significant breakthrough in DSSCs, which ensures that the photogenerated electrons in the overlayer can be effectively transported through such highway-like paths and ultimately collected at the FTO electrode. Therefore, in this double layered photoanode we have taken into consideration a number of disparate factors aimed at enhancing the overall DSSC performance. Drawing on the judicious combination of materials synthesis and engineering of nanoarchitectures and interfaces, solar cells based on this double layered structure have achieved 8.72% power conversion efficiency even with simple device fabrication procedures, showing promise as a new photoanode design for high efficiency dye sensitized solar cells

Self-assembly of Ni2P nanowires as high-efficiency electrocatalyst for dye-sensitized solar cells

We report an easy way to assemble porous one-dimensional (1D) Ni2P nanowires through phosphatization of a Ni(SO4)(0.3)(OH)(1.4) nanobelt precursor. The peculiar synthetic process endows the Ni2P nanowires with large surface area, hierarchical porous structure and the ability to form closely connected network for transporting both electrons and electrolytes, which in conjunction with the high intrinsic electrocatalytic activity make it an excellent low-cost counter electrode material for dye-sensitized solar cells (DSSCs). Indeed, the first investigation of such novel counter electrode for DSSC presented superb photovoltaic performance rivaling the conventional Pt counter electrode