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

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

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Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

The field of self-assembled monolayers (SAMs) has witnessed tremendous growth in synthetic sophistication and depth of characterization over the past 15 years.1 However, it is interesting to comment on the modest beginning and on important milestones. The field really began much earlier than is now recognized. In 1946 Zisman published the preparation of a monomolecular layer by adsorption (self-assembly) of a surfactant onto a clean metal surface.2 At that time, the potential of self-assembly was not recognized, and this publication initiated only a limited level of interest. Early work initiated in Kuhn’s laboratory at Gottingen, applying many years of experience in using chlorosilane derivative to hydrophobize glass, was followed by the more recent discovery, when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions.3 Getting away from the moisture-sensitive alkyl trichlorosilanes, as well as working with crystalline gold surfaces, were two important reasons for the success of these SAMs. Many self-assembly systems have since been investigated, but monolayers of alkanethiolates on gold are probably the most studied SAMs to date. The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. In nature, self-assembly results in supermolecular hierarchical organizations of interlocking components that provides very complex systems.4 SAMs offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. The ability to tailor both head and tail groups of the constituent molecules makes SAMs excellent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. That SAMs are well-defined and accessible makes them good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures.5 However, this will require studies of more complex systems and the combination of what has been learned from SAMs with macromolecular science. The exponential growth in SAM research is a demonstration of the changes chemistry as a disciAbraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in 1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan University in 1971. After a brief period in industry, he moved to the Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for work on heterosubstituted porphyrins. He then spent two years at Northwestern University in Evanston, IL, where his main interest was onedimensional organic conductors. In 1985 he joined the Corporate Research Laboratories of Eastman Kodak Company, in Rochester, NY, where his research interests were molecular design of materials for nonlinear optics and self-assembled monolayers. In 1994 he moved to Polytechnic University where he is the Alstadt-Lord-Mark Professor of Chemistry. His interests encompass self-assembled monolayers, surface engineering, polymers at interface, and surfaces phenomena. 1533 Chem. Rev. 1996, 96, 1533−1554

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Formation and Structure of Self-Assembled Monolayers.

Effects of the use of inert supports for TiO2 loading on photocatalyzed decomposition of propionaldehyde in the gas phase was investigated for mordenite support with various amounts of TiO2 loading and for several kinds of supports such as other zeolites, alumina, silica, and activated carbon. The adsorption constant and the amount of adsorption of propionaldehyde were evaluated for TiO2-loaded supports by obtaining Langmuir adsorption isotherms. By correlating these parameters to the photodecomposition rates of propionaldehyde, the involvement of the support in the photodecomposition reaction is clarified. The photocatalytic activity of TiO2 on mordenite having various amounts of TiO2 loading increases with increase in the amount of loaded TiO2 up to an optimum value (ca. 50 wt %), beyond which a decreasing tendency of the activity appeared. In the region of ascending activity, plenty of adsorbed substrate is available and the activity is controlled by the content of TiO2, while in the region of descending activity, the decrease in the amount of adsorbed substrate due to a decrease in the occupancy of the support by the TiO2 loading is responsible for the activity decrease. The photocatalytic activities are greatly influenced by the kind of inert supports used and show a volcano type dependence on the adsorption constant of the TiO2-loaded supports. In cases where the adsorption constant is low, the decomposition rate is determined by the amount of adsorbed substrate, while if the adsorption constant is very high, plenty of adsorbed substrate is available on the support, but it is not mobile to the loaded TiO2. We conclude that the use of an inert support having a medium adsorption constant is necessary to obtain the highest activity, where a high amount of adsorbed substrate that can be supplied to TiO2 particles is available.

Effect of Inert Supports for Titanium Dioxide Loading on Enhancement of Photodecomposition Rate of Gaseous Propionaldehyde

Photochemical and photocatalytic properties of microcrystalline TiO{sub 2} incorporated in the interlayer space of montmorillonite were investigated in reference to those of TiO{sub 2} powder particles which were prepared by agglomeration of a titania sol that was used in the preparation of the titania-pillared clay. The pillared TiO{sub 2} microcrystallites having ca. 15-{angstrom} pillar height showed a ca. 0.58-eV blue shift in its absorption and fluorescence spectra compared with those of the TiO{sub 2} particles, exhibiting the quantum size effect. The excited electronic states of the pillared TiO{sub 2} were determined to be 0.36 V more negative than that of the TiO{sub 2} powder particles. Photocatalytic activities of the pillared TiO{sub 2} were greater than those of the TiO{sub 2} powder particles for decomposition of 2-propanol to give acetone and hydrogen and of n-carboxylic acids with up to eight carbons (from acetic acid to caprylic acid) to give the corresponding alkanes and carbon dioxide, though the pillared TiO{sub 2} exhibited lower activities for decomposition of capric acid, having 10 carbons in a molecule.

Photocatalytic activities of microcrystalline titania incorporated in sheet silicates of clay

Titanium dioxide/adsorbent hybrid photocatalysts for photodestruction of organic substances of dilute concentrations

Tubules of 200 nm outer diameter spinel LiMn{sub 2}O{sub 4} were prepared by thermal decomposition of an aqueous solution containing lithium nitrate and manganese nitrate at 1:2 molar ratio using a nanoporous alumina membrane as a template. After dissolving the template membrane, the resulting nanotubule array of LiMn{sub 2}O{sub 4} was coated with polypyrrole to investigate the galvanostatic charge-discharge characteristics. The polypyrrole-coated LiMn{sub 2}O{sub 4} tubule electrodes exhibited higher capacities than the polypyrrole-coated LiMn{sub 2}O{sub 4} thin-film electrode prepared under the same conditions, except for the use of the alumina membrane; it was ca. 2.5 times greater at 0.1 mA/cm{sup 2} and became ca. 12 times greater with an increase in current density to 1.0 mA/cm{sup 2}. The observed high capacity of the tubule electrode seems to have resulted from two kinds of effects: a decrease in real current density of high specific surface area at the tubule electrode and a decrease in thickness of LiMn{sub 2}O{sub 4} solid phase for Li{sup +} ions to diffuse through during the charging and discharging reactions.

Template Synthesis of Polypyrrole‐Coated Spinel LiMn2 O 4 Nanotubules and Their Properties as Cathode Active Materials for Lithium Batteries

The effect of the amount of TiO2 loading on activated carbon support on the photodecomposition of dichloromethane was studied The apparent adsorption constant of the TiO2-loaded activated carbon for dichloromethane was found to decrease with increase of the fraction of the loaded TiO2 It was found that the apparent rate constant of CO2 evolution became small with increase of the activated carbon content, suggesting that the absorbed substrate on the activated carbon support was not easily involved in the photodecomposition reaction However, the use of mixed suspensions of TiO2 and activated carbon showed lower activities than the TiO2-loaded activated carbon for photodegradation of dichloromethane The role of the activated carbon support in the photodecomposition of dichloromethane is discussed

Hiroshi Yoneyama

Papers

Effect of Inert Supports for Titanium Dioxide Loading on Enhancement of Photodecomposition Rate of Gaseous Propionaldehyde

Photocatalytic activities of microcrystalline titania incorporated in sheet silicates of clay

Titanium dioxide/adsorbent hybrid photocatalysts for photodestruction of organic substances of dilute concentrations

Template Synthesis of Polypyrrole‐Coated Spinel LiMn2 O 4 Nanotubules and Their Properties as Cathode Active Materials for Lithium Batteries

Effect of activated carbon content in TiO2-loaded activated carbon on photodegradation behaviors of dichloromethane