This section focuses on the fundamental properties of nanostructured WO x and start with its various crystal structures and the conditions for phase transitions between these structures. The structures of nonstoichiometric WO x and WO 3 hydrates Nanostructured Tungsten Oxide -Properties, Synthesis, and Applications 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 (WO x ) 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 WO x , their properties, methods of synthesis, and a description of how they can be used in unique ways for different applications.

In research on alternative photoanode materials for dye-sensitized solar cells (DSCs), there is rarely any report on WO(3), probably due to its acidic surface and more positive (vs NHE) conduction band edge position compared to TiO(2) and ZnO. For the first time, dye-sensitized solar cells based on porous WO(3) nanoparticle films were successfully fabricated with efficiency of up to 0.75%. The multicrystalline structure of WO(3) was examined by Raman spectroscopy and X-ray diffraction analysis. It was found that significant performance enhancement can be obtained from treating the WO(3) nanoparticle film with TiCl(4); the TiCl(4)-treated WO(3) DSCs were recorded with efficiency reaching 1.46%.

Synthesis of Atomically Thin WO 3 Sheets from Hydrated Tungsten Trioxide. -Nanosheets of atomically thin WO3 layers are prepared by mechanical exfoliation of WO3·2H2O (obtained from pieces of tungsten foil in HNO3 at 80°C for 6 h) using a piece of an adhesive tape, followed by annealing at 300°C. The samples are characterized by XRD, SEM, TEM, and AFM. The sheets have thicknesses that are multiples of the unit cell height (about 1.4 nm). Raman spectroscopy demonstrates that thinning of WO3 significantly affects physical and chemical behavior. Li intercalation causes significant changes in the Raman spectra for thin samples. The prepared sheets may have great potential in developing two-dimensional electronic and photonic devices such as planar photodiodes and transistors. -(KALANTAR-ZADEH*, K.; VIJAYARAGHAVAN, A.; HAM, M.-H.; ZHENG, H.; BREEDON, M.; STRANO, M. S.; Chem.

Here, we demonstrate that niobium pentoxide (Nb(2)O(5)) is an ideal candidate for increasing the efficiencies of dye-sensitized solar cells (DSSCs). The key lies in developing a Nb(2)O(5) crisscross nanoporous network, using our unique elevated temperature anodization process. For the same thicknesses of ∼4 μm, the DSSC based on the Nb(2)O(5) layer has a significantly higher efficiency (∼4.1%) when compared to that which incorporates a titanium dioxide nanotubular layer (∼2.7%). This is the highest efficiency among all of the reported photoanodes for such a thickness when utilizing back-side illumination. We ascribe this to a combination of reduced electron scattering, greater surface area, wider band gap, and higher conduction band edge, as well as longer effective electron lifetimes.

Anodization at elevated temperatures in nitric acid has been used for the production of highly porous and thick tungsten trioxide nanostructured films for photosensitive device applications. The anodization process resulted in platelet crystals with thicknesses of 20-60 nm and lengths of 100-1000 nm. Maximum thicknesses of approximately 2.4 microm were obtained after 4 h of anodization at 20 V. X-ray diffraction analysis revealed that the as-prepared anodized samples contain predominantly hydrated tungstite phases depending on voltage, while films annealed at 400 degrees C for 4 h are predominantly orthorhombic WO3 phase. Photocurrent measurements revealed that the current density of the 2.4 microm nanostructured anodized film was 6 times larger than the nonanodized films. Dye-sensitized solar cells developed using these films produced 0.33 V and 0.65 mA/cm2 in open- and short-circuit conditions.