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

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

Solid-state electrolytes are attracting increasing interest for electrochemical energy storage technologies. In this Review, we provide a background overview and discuss the state of the art, ion-transport mechanisms and fundamental properties of solid-state electrolyte materials of interest for energy storage applications. We focus on recent advances in various classes of battery chemistries and systems that are enabled by solid electrolytes, including all-solid-state lithium-ion batteries and emerging solid-electrolyte lithium batteries that feature cathodes with liquid or gaseous active materials (for example, lithium–air, lithium–sulfur and lithium–bromine systems). A low-cost, safe, aqueous electrochemical energy storage concept with a ‘mediator-ion’ solid electrolyte is also discussed. Advanced battery systems based on solid electrolytes would revitalize the rechargeable battery field because of their safety, excellent stability, long cycle lives and low cost. However, great effort will be needed to implement solid-electrolyte batteries as viable energy storage systems. In this context, we discuss the main issues that must be addressed, such as achieving acceptable ionic conductivity, electrochemical stability and mechanical properties of the solid electrolytes, as well as a compatible electrolyte/electrode interface. This Review details recent advances in battery chemistries and systems enabled by solid electrolytes, including all-solid-state lithium-ion, lithium–air, lithium–sulfur and lithium–bromine batteries, as well as an aqueous battery concept with a mediator-ion solid electrolyte.

Lithium battery chemistries enabled by solid-state electrolytes

The chemistry of copper is extremely rich because it can easily access Cu0, CuI, CuII, and CuIII oxidation states allowing it to act through one-electron or two-electron processes. As a result, both radical pathways and powerful two-electron bond forming pathways via organmetallic intermediates, similar to those of palladium, can occur. In addition, the different oxidation states of copper associate well with a large number of different functional groups via Lewis acid interactions or π-coordination. In total, these feature confer a remarkably broad range of activities allowing copper to catalyze the oxidation and oxidative union of many substrates.

Oxygen is a highly atom economical, environmentally benign, and abundant oxidant, which makes it ideal in many ways.1 The high activation energies in the reactions of oxygen require that catalysts be employed.2 In combination with molecular oxygen, the chemistry of copper catalysis increases exponentially since oxygen can act as either a sink for electrons (oxidase activity) and/or as a source of oxygen atoms that are incorporated into the product (oxygenase activity). The oxidation of copper with oxygen is a facile process allowing catalytic turnover in net oxidative processes and ready access to the higher CuIII oxidation state, which enables a range of powerful transformations including two-electron reductive elimination to CuI. Molecular oxygen is also not hampered by toxic byproducts, being either reduced to water, occasionally via H2O2 (oxidase activity) or incorporated into the target structure with high atom economy (oxygenase activity). Such oxidations using oxygen or air (21% oxygen) have been employed safely in numerous commodity chemical continuous and batch processes.3

However, batch reactors employing volatile hydrocarbon solvents require that oxygen concentrations be kept low in the head space (typically <5–11%) to avoid flammable mixtures, which can limit the oxygen concentration in the reaction mixture.4,5,6 A number of alternate approaches have been developed allowing oxidation chemistry to be used safely across a broader array of conditions. For example, use of carbon dioxide instead of nitrogen as a diluent leads to reduced flammability.5 Alternately, water can be added to moderate the flammability allowing even pure oxygen to be employed.6 New reactor designs also allow pure oxygen to be used instead of diluted oxygen by maintaining gas bubbles in the solvent, which greatly improves reaction rates and prevents the build up of higher concentrations of oxygen in the head space.4a,7 Supercritical carbon dioxide has been found to be advantageous as a solvent due its chemical inertness towards oxidizing agents and its complete miscibility with oxygen or air over a wide range of temperatures.8 An number of flow technologies9 including flow reactors,10 capillary flow reactors,11 microchannel/microstructure structure reactors,12 and membrane reactors13 limit the amount of or afford separation of hydrocarbon/oxygen vapor phase thereby reducing the potential for explosions.

Enzymatic oxidizing systems based upon copper that exploit the many advantages and unique aspects of copper as a catalyst and oxygen as an oxidant as described in the preceding paragraphs are well known. They represent a powerful set of catalysts able to direct beautiful redox chemistry in a highly site-selective and stereoselective manner on simple as well as highly functionalized molecules. This ability has inspired organic chemists to discover small molecule catalysts that can emulate such processes. In addition, copper has been recognized as a powerful catalyst in several industrial processes (e.g. phenol polymerization, Glaser-Hay alkyne coupling) stimulating the study of the fundamental reaction steps and the organometallic copper intermediates. These studies have inspiried the development of nonenzymatic copper catalysts. For these reasons, the study of copper catalysis using molecular oxygen has undergone explosive growth, from 30 citations per year in the 1980s to over 300 citations per year in the 2000s.

A number of elegant reviews on the subject of catalytic copper oxidation chemistry have appeared. Most recently, reviews provide selected coverage of copper catalysts14 or a discussion of their use in the aerobic functionalization of C–H bonds.15 Other recent reviews cover copper and other metal catalysts with a range of oxidants, including oxygen, but several reaction types are not covered.16 Several other works provide a valuable overview of earlier efforts in the field.17 This review comprehensively covers copper catalyzed oxidation chemistry using oxygen as the oxidant up through 2011. Stoichiometric reactions with copper are discussed, as necessary, to put the development of the catalytic processes in context. Mixed metal systems utilizing copper, such as palladium catalyzed Wacker processes, are not included here. Decomposition reactions involving copper/oxygen and model systems of copper enzymes are not discussed exhaustively. To facilitate analysis of the reactions under discussion, the current mechanistic hypothesis is provided for each reaction. As our understanding of the basic chemical steps involving copper improve, it is expected that many of these mechanisms will evolve accordingly.

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Aerobic Copper-Catalyzed Organic Reactions

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Titanium dioxide (TiO2) is recognized as the most efficient photocatalytic material, but due to its large band gap energy it can only be excited by UV irradiation. Doping TiO2 with nitrogen is a promising modification method for the utilization of visible light in photocatalysis. In this work, nitrogen-doped TiO2 films were grown by atomic layer deposition (ALD) using TiCl 4 ,N H 3 and water as precursors. All growth experiments were done at 500 ◦ C. The films were characterized by XRD, XPS, SEM and UV–vis spectrometry. The influence of nitrogen doping on the photocatalytic activity of the films in the UV and visible light was evaluated by the degradation of a thin layer of stearic acid and by linear sweep voltammetry. Light-induced superhydrophilicity of the films was also studied. It was found that the films could be excited by visible light, but they also suffered from increased recombination. © 2005 Elsevier B.V. All rights reserved.

Atomic layer deposition of TiO2−xNx thin films for photocatalytic applications

This paper reviews several high-k ALD processes potentially applicable to the production of capacitors, concentrating on very recent developments. A list of the dielectric materials under investigation consists of the oxides of several metals, including the Group 4 (Ti, Zr, Hf) elements. The binary oxides of Group 4 metals, as well as their mixtures with other oxides, doped hosts, or multi-layers in the form of nano-laminates are of interest.Several examples of our recent results are shown, including possible ALD routes to materials not previously grown, as well as advances in process development.

Atomic Layer Deposition of High-k Oxides of the Group 4 Metals for Memory Applications**

This article demonstrates how important it is to find the optimal heating conditions when electrospun organic/inorganic composite fibers are annealed to get ceramic nanofibers in appropriate quality (crystal structure, composition, and morphology) and to avoid their disintegration. Polyvinylpyrrolidone [PVP, (C6H9NO)
 n
 ] and ammonium metatungstate [AMT, (NH4)6[H2W12O40]·nH2O] nanofibers were prepared by electrospinning aqueous solutions of PVP and AMT. The as-spun fibers and their annealing were characterized by TG/DTA-MS, XRD, SEM, Raman, and FTIR measurements. The 400–600 nm thick and tens of micrometer long PVP/AMT fibers decomposed thermally in air in four steps, and pure monoclinic WO3 nanofibers formed between 500 and 600 °C. When a too high heating rate and heating temperature (10 °C min−1, 600 °C) were used, the WO3 nanofibers completely disintegrated. At lower heating rate but too high temperature (1 °C min−1, 600 °C), the fibers broke into rods. If the heating rate was adequate, but the annealing temperature was too low (1 °C min−1, 500 °C), the nanofiber morphology was excellent, but the sample was less crystalline. When the optimal heating rate and temperature (1 °C min−1, 550 °C) were applied, WO3 nanofibers with excellent morphology (250 nm thick and tens of micrometer long nanofibers, which consisted of 20–80 nm particles) and crystallinity (monoclinic WO3) were obtained. The FTIR and Raman measurements confirmed that with these heating parameters the organic matter was effectively removed from the nanofibers and monoclinic WO3 was present in a highly crystalline and ordered form.

Thermal study on electrospun polyvinylpyrrolidone/ammonium metatungstate nanofibers: optimising the annealing conditions for obtaining WO3 nanofibers

Lithium Phosphate Thin Films Grown by Atomic Layer Deposition

In this study, VOx films were grown by atomic layer deposition (ALD) using V(NEtMe)4 as the vanadium precursor and either ozone or water as the oxygen source. V(NEtMe)4 is liquid at room temperature and shows good evaporation properties. The growth was investigated at deposition temperatures from as low as 75 °C, up to 250 °C. When using water as the oxygen source, a region of constant growth rate (ca. 0.8 A/cycle) was observed between 125 and 200 °C, with the ozone process the growth rate was significantly lower (0.31–0.34 A/cycle). The effect of the process conditions and post-deposition annealing on the film structure was investigated. By varying the atmosphere under which the films were annealed, it was possible to preferably form either VO2 or V2O5. Atomic force microscopy revealed that the films were smooth (rms < 0.5 nm) and uniform. The composition and stoichiometry of the films were determined by X-ray photoelectron spectroscopy. Conformal deposition was achieved in demanding high aspect ratio structure.

Mikko Heikkilä

Papers

Atomic layer deposition of TiO2−xNx thin films for photocatalytic applications

Atomic Layer Deposition of High-k Oxides of the Group 4 Metals for Memory Applications**

Thermal study on electrospun polyvinylpyrrolidone/ammonium metatungstate nanofibers: optimising the annealing conditions for obtaining WO3 nanofibers

Lithium Phosphate Thin Films Grown by Atomic Layer Deposition

Atomic layer deposition and characterization of vanadium oxide thin films