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

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

Mesoporous organic-inorganic hybrid materials, a new class of materials characterized by large specific surface areas and pore sizes between 2 and 15 nm, have been obtained through the coupling of inorganic and organic components by template synthesis. The incorporation of functionalities can be achieved in three ways: by subsequent attachment of organic components onto a pure silica matrix (grafting), by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (co-condensation, one-pot synthesis), and by the use of bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs). This Review gives an overview of the preparation, properties, and potential applications of these materials in the areas of catalysis, sorption, chromatography, and the construction of systems for controlled release of active compounds, as well as molecular switches, with the main focus being on PMOs.

Silica-based mesoporous organic-inorganic hybrid materials.

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

On the Controllable Soft-Templating Approach to Mesoporous Silicates

In a physically confined environment, interfacial interactions, symmetry breaking, structural frustration and confinement-induced entropy loss can play dominant roles in determining molecular organization. Here we present a systematic study of the confined assembly of silica-surfactant composite mesostructures within cylindrical nanochannels of varying diameters. Using exactly the same precursors and reaction conditions that form the two-dimensional hexagonal SBA-15 mesostructured thin film, unprecedented silica mesostructures with chiral mesopores such as single- and double-helical geometries spontaneously form inside individual alumina nanochannels. On tightening the degree of confinement, a transition is observed in the mesopore morphology from a coiled cylindrical to a spherical cage-like geometry. Self-consistent field calculations carried out to account for the observed mesostructures accord well with experiment. The mesostructures produced by confined syntheses are useful as templates for fabricating highly ordered mesostructured nanowires and nanowire arrays.

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Composite mesostructures by nano-confinement.

There has been an active search for cost-effective photovoltaic devices since the development of the first solar cells in the 1950s (refs 1-3). In conventional solid-state solar cells, electron-hole pairs are created by light absorption in a semiconductor, with charge separation and collection accomplished under the influence of electric fields within the semiconductor. Here we report a multilayer photovoltaic device structure in which photon absorption instead occurs in photoreceptors deposited on the surface of an ultrathin metal-semiconductor junction Schottky diode. Photoexcited electrons are transferred to the metal and travel ballistically to--and over--the Schottky barrier, so providing the photocurrent output. Low-energy (approximately 1 eV) electrons have surprisingly long ballistic path lengths in noble metals, allowing a large fraction of the electrons to be collected. Unlike conventional cells, the semiconductor in this device serves only for majority charge transport and separation. Devices fabricated using a fluorescein photoreceptor on an Au/TiO2/Ti multilayer structure had typical open-circuit photovoltages of 600-800 mV and short-circuit photocurrents of 10-18 micro A cm(-2) under 100 mW cm(-2) visible band illumination: the internal quantum efficiency (electrons measured per photon absorbed) was 10 per cent. This alternative approach to photovoltaic energy conversion might provide the basis for durable low-cost solar cells using a variety of materials.

A photovoltaic device structure based on internal electron emission

A general methodology that utilizes confined mesoporous silica as template for preparing highly ordered mesostructured nanowires and nanowire arrays is developed. The prepared Ag, Ni, and Cu2O nanowires, with unprecedented mesostructures of coaxially multilayered helical, and stacked-donut structures, have the unique features of hierarchical organization, modulated surface morphology, high surface area, and chirality. Surface-enhanced Raman spectra from a silver mesostructured-nanowire bundle are presented.

Templated Synthesis of Highly Ordered Mesostructured Nanowires and Nanowire Arrays

Synthesis and photocatalytic properties of highly crystalline and ordered mesoporous TiO2 thin films.

Since the discovery of mesostructured materials in the early 1990s a great deal of effort has been devoted to developing new structures, compositions, and functionalities. The ability to synthesize semiconducting singleor multicomponent metal-oxide mesostructured materials with controlled morphologies has been a major step towards functional materials with potential applications in sensing, catalysis, and optoelectronics. We describe herein the synthesis and properties of a new generation of ordered mesostructured composites with mesopore walls composed of integrated 3D arrays of different types of wide and narrow band-gap semiconductor nanocrystals including oxides, sulfides, and selenides. Such an integrated 3D coassembly of different functional units is of great importance for optoelectronic applications that rely on the transfer of electrons or holes between two or more active components. Examples of such systems are sensitized titania solar cells and photocatalysts, for which it has been shown that solar conversion efficiency can be strongly increased in the presence of sensitizing dye molecules or narrow band-gap semiconductor nanocrystals. Fabrication techniques previously developed for such functionalized composite titania materials are based on step-bystep methods in which organic molecules or premade semi-

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Jing Tang

Papers

Composite mesostructures by nano-confinement.

A photovoltaic device structure based on internal electron emission

Templated Synthesis of Highly Ordered Mesostructured Nanowires and Nanowire Arrays

Synthesis and photocatalytic properties of highly crystalline and ordered mesoporous TiO2 thin films.

Cubic Mesoporous Frameworks with a Mixed Semiconductor Nanocrystalline Wall Structure and Enhanced Sensitivity to Visible Light