Metal oxides are the key ingredients for the development of many advanced functional materials and smart devices. Nanostructuring has emerged as one of the best tools to unlock their full potential. Tungsten oxides (WOx) are unique materials that have been rigorously studied for their chromism, photocatalysis, and sensing capabilities. However, they exhibit further important properties and functionalities that have received relatively little attention in the past. This Feature Article presents a general review of nanostructured WOx, their properties, methods of synthesis, and a description of how they can be used in unique ways for different applications. Tungsten oxides (WOx) are unique functional materials that can be obtained in a vast variety of nanostructured forms. This Feature Article presents a comprehensive review on the properties of WOx that goes beyond chromism and photocatalysis, for which they are usually investigated for. This is followed by a survey of their synthesis methods and implementations for different applications.

Nanostructured Tungsten Oxide – Properties, Synthesis, and Applications

Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni2O3. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

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Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these

ZnS nanostructures: From synthesis to applications

We report on the development of highly conductive NiCo2S4 single crystalline nanotube arrays grown on a flexible carbon fiber paper (CFP), which can serve not only as a good pseudocapacitive material but also as a three-dimensional (3D) conductive scaffold for loading additional electroactive materials. The resulting pseudocapacitive electrode is found to be superior to that based on the sibling NiCo2O4 nanorod arrays, which are currently used in supercapacitor research due to the much higher electrical conductivity of NiCo2S4. A series of electroactive metal oxide materials, including CoxNi1–x(OH)2, MnO2, and FeOOH, were deposited on the NiCo2S4 nanotube arrays by facile electrodeposition and their pseudocapacitive properties were explored. Remarkably, the as-formed CoxNi1–x(OH)2/NiCo2S4 nanotube array electrodes showed the highest discharge areal capacitance (2.86 F cm–2 at 4 mA cm–2), good rate capability (still 2.41 F cm–2 at 20 mA cm–2), and excellent cycling stability (∼4% loss after the repetitive ...

Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors

Energy will be one of the most important factors to influence human society in the 21st century.1,2 Cost, availability, and sustainability of energy have a significant impact on the quality of our lives, development of global economies, relationships between nations, and the stability of our environment. Scientists are now focusing on the development of renewable energies3 generated from natural resources such as sunlight, wind, rain, tides, and geothermal heat. Among these, the sun has the potential to make the largest energy contribution: only one hour of sunshine (3.8 × 1023 kW) is more than enough to satisfy the highest human demand for energy for an entire year (1.6 × 1010 kW in 2005).4-7 Solar cells, also called photovoltaics,8 are devices based on solar technology which convert sunlight directly into electricity under the photovoltaic effect. Becquerel was the first to recognize this effect in 1839, when he shined light * To whom correspondence should be addressed. E-mail: muellen@ mpip-mainz.mpg.de. † Max Planck Institute for Polymer Research. ‡ BASF SE. Chen Li studied chemistry and completed his Master’s thesis under the supervision of Professor He Tian at the East China University of Science and Technology in 2003. In 2004, he joined the group of Professor Klaus Müllen at the Max Planck Institute for Polymer Research to work on functional rylene dyes for dye-sensitized solar cells. He received his Ph.D. in 2008 from the Johannes Gutenberg University of Mainz. Presently he works in the same group as a project leader, and his research interests are functional dyes and pigments as well as their applications.

M. Regragui

Papers

Structure, composition and optical properties of ZnS thin films prepared by spray pyrolysis

Structural and optical properties of CeO2 thin films prepared by spray pyrolysis

Preparation and characterization of pyrolytic spray deposited electrochromic tungsten trioxide films

Physico-chemical, optical and electrochemical properties of iron oxide thin films prepared by spray pyrolysis

Electrical and optical properties of WO3 thin films