Photoinduced reactivity of titanium dioxide

This feature article summarizes recent research on and development of semiconductor-based photocatalyst materials that are applicable to environmental remediation and/or chemical synthesis purposes. A wide variety of TiO2 particles and/or films have been studied during the past 30 years because they are the most stable and powerful photocatalysts leading to the degradation of various organic pollutants. The photocatalytic performance of other semiconductor materials such as ZnO, SnO2, WO3, Fe2O3 and CdS has also been intensively investigated. A general limitation in the efficiency of any photocatalytic process is the recombination of the photogenerated charge carriers, i.e., of electrons and holes, following bandgap illumination. Considerable efforts have been made to suppress this recombination and hence to enhance the charge carrier separation and the overall efficiency by means of coupling of different semiconductors with desirable matching of their electronic band structures, or incorporation of noble metal nanoclusters onto the surface of semiconductor photocatalyst particles. Modification of the physicochemical properties, such as particle size, surface area, porosity and/or crystallinity of the semiconductor materials, and optimization of the experimental conditions, such as pH, illumination conditions and/or catalyst loading, during photocatalytic reactions have also been carefully addressed to achieve high reaction rates or yields. To utilize solar energy more efficiently, i.e., to extend the optical absorption of the mostly UV-sensitive photocatalysts into the visible light range, numerous research groups have contributed to the development of novel visible light active photocatalysts. With the application of semiconductors with narrower bandgaps such as CdS, Fe2O3 and WO3 being straightforward choices, doping of wide bandgap semiconductors like TiO2 has been the most popular technique to enhance the catalysts' optical absorption abilities. Research on mixed-oxide-based semiconductor photocatalysts with deliberately modulated band structures has also attracted tremendous attention in the past decade, concentrating on, for example, the generation of H2 and/or O2 from H2O splitting, and the degradation of organic pollutants under visible light irradiation. Both theoretical calculations and experimental results have convincingly shown that the developed materials can serve as highly efficient photocatalysts that are both environmentally and economically significant.

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Photoelectrocatalytic materials for environmental applications

Ordered magnetic nanostructures: fabrication and properties

TOPICAL REVIEW: Nanomagnetics

The role of electrical potential, charge transport, and recombination in determining the photopotential and photocurrent conversion efficiency (IPCE) of dye-sensitized nanocrystalline solar cells was studied. Electrostatic arguments and electrical impedance spectroscopy (EIS) are used to obtain information on the electrical and electrochemical potential distribution in the cell. It is shown that on the macroscopic level, no significant electrical potential drop exists within the porous TiO2 when it contacts the electrolyte and that the electrical potential drop at the transparent conducting oxide substrate (TCO)/TiO2 interface occurs over a narrow region, one or two layers of TiO2. Analyses of EIS and other data indicate that both the photopotential of the cell and the decrease of the electrical potential drop across the TCO/TiO2 interface are caused by the buildup of photoinjected electrons in the TiO2 film. The time constants for the recombination and collection of the photoinjected electrons are measur...

Influence of Electrical Potential Distribution, Charge Transport, and Recombination on the Photopotential and Photocurrent Conversion Efficiency of Dye-Sensitized Nanocrystalline TiO2 Solar Cells: A Study by Electrical Impedance and Optical Modulation Techniques

Magnetoresistance of single Ni and Co nanowires, of about 60 nm in diameter and 6000 nm in length, was measured at room temperature. The full magnetoresistive hysteresis loops of single Ni nanowires, including the irreversible jump, are understood qualitatively, and major progress has been made towards their quantitative description, on the basis of anisotropic magnetoresistance. In contrast, the magnetoresistive hysteresis loops of single Co nanowires could not be described quantitatively, due to the presence of nucleation processes of domain walls or vortices.

Magnetoresistance of Ferromagnetic Nanowires

A non-volatile random access memory (NVRAM) of the type with magnetoresistive memory elements ( 1 ) connected by sets of non-intersecting conductor sense lines ( 3, 4 ) which define the address of each memory element ( 1 ) and are connectable to a magnetic write/read recording unit. The memory elements are a plurality of magnetoresistive submicron dots or wires ( 1 ) embedded in a membrane ( 2 ) through which the submicron dots or wires extend. The sets of non-intersecting conductor sense lines ( 3, 4 ) are connected to the opposite ends of the submicron dots or wires ( 1 ) on opposite sides of the membrane. Each magnetoresistive submicron dot or wire ( 1 ) is composed of ferromagnetic material or a combination of ferromagnetic and non-ferromagnetic materials having at least two magnetic states (“0”; “1”), writeable by passing at an appropriate external field a writing current pulse (i w ) in its conductor lines ( 3, 4, 5 ) sufficient to switch its magnetic states and readable by passing a an AC or DC current (i r ) in its conductor lines below the level for switching its magnetic states.

Non-volatile magnetic random access memory

Magnetization reversal of individual Ni nanowires has been studied by anisotropic magnetoresistance (AMR) measurements. The wires are homogeneous cylinders 6/spl mu/ long and averaging 80 nm in diameter. Nanowires were produced by electrodeposition in track-etched membrane templates. Electrical contacts to as-deposited single nanowires were obtained by a novel in-situ method where contact to a single wire is accomplished automatically and reliably during electrodeposition. Steps of the magnetoresistance are detected, corresponding to irreversible switching of the magnetization in a single wire. The dependence of the switching field on the orientation of the applied field as seen by AMR is identical to that recently observed by using micro-SQUID techniques.

Anisotropic magnetoresistance as a probe of magnetization reversal in individual nono-sized nickel wires

Time-resolved photocharge (TRPC) measurements are applied for the first time to demonstrate the presence of a contact potential between lightly sintered colloidal nanocrystalline TiO2 films deposited on transparent highly conductive tin oxide films degenerately doped with fluorine, SnO2(F). By virtue of its contactless nature and absence of an externally applied field, TRPC has previously been demonstrated to possess the capability to directly probe the electric field in semiconductor heterojunction particulate materials without the complications and ambiguities encountered with many other measurement techniques. Using TRPC, vectorial directions of photoelectron currents in glass/SnO2(F)/TiO2 samples, as determined by signal polarity, are found to be dependent on their orientation with respect to the incident photon flux (i.e., negative polarity for exposure through the glass; positive polarity for exposure directly on the TiO2). This implies that photoelectrons always flow toward the SnO2(F)/TiO2 interfa...

Directed Photocurrents in Nanostructured TiO2/SnO2 Heterojunction Diodes

Copper electrodeposition has been followed by in situ electrochemical STM on Au(111) electrodes covered by complete decanethiol monolayers. It has been found that Cu nanoparticles (2−5 nm) were formed at potentials comprising the underpotential deposition (UPD) region on clean gold. The nanoparticle clusters appear to follow a nucleation and sudden growth process as their maximal size is attained instantaneously on the time scale of the STM imaging process. Nanoparticle heights correspond to one atomic layer of Cu. The distribution density of the Cu deposits reaches a maximal value at potentials within the UPD window, as no new formation of clusters nor growth of already existing clusters is seen at potentials well into the bulk deposition potential region. Bulk deposition of isolated Cu nodules is finally seen at potentials 200 mV negative of the Nernstian potential for Cu reduction, probably resulting from thiol film breakdown. Moreover, nanoparticles remain on the Au surface at potentials as high as 10...

Scott E. Gilbert

Papers

Magnetoresistance of Ferromagnetic Nanowires

Non-volatile magnetic random access memory

Anisotropic magnetoresistance as a probe of magnetization reversal in individual nono-sized nickel wires

Directed Photocurrents in Nanostructured TiO2/SnO2 Heterojunction Diodes

Electrodeposition of Cu Nanoparticles on Decanethiol-Covered Au(111) Surfaces: An in Situ STM Investigation