Porous Tungsten Oxide: Recent Advances in Design, Synthesis, and Applications

Tungsten oxide (WO₃ ) has received ever more attention and has been highly researched over the last decade due to its being a low-cost transition metal semiconductor with tunable, yet widely stable, band gaps. This minireview briefly highlights the challenges in the design and synthesis of porous WO₃ including methods, precursors, solvent effects, crystal phases, and surface activities of the porous WO₃ base material. These topics are explored while also drawing a connection of how the morphology and crystal phase affect the band gap. The shifts in band gap not only impact the optical properties of tungsten but also allow tuning to operate on different energy levels, which makes WO₃ highly desirable in many applications such as supercapacitors, batteries, solar cells, catalysts, sensors, smart windows, and bioapplications.

Electrochromic evaluation of airbrushed water-dispersible W18O49nanorods obtained by microwave-assisted synthesis

Motivated by the technological relevance of tungsten oxide nanostructures as valuable materials for energy saving technology, electrochemical and electrochromic characteristics of greener processed nanostructured W₁₈O₄₉-based electrodes are discussed in this work. For the purpose, microwave-assisted water-dispersible W₁₈O₄₉nanorods have been synthesized and processed into nanostructured electrodes. An airbrushing technique has been adopted as a cost-effective large-area scalable methodology to deposit the W₁₈O₄₉nanorods onto conductive glass. This approach preserves the morphological and crystallographic habit of native nanorods and allows highly homogeneous transparent coating where good electronic coupling between nanowires is ensured by a mild thermal treatment (250 °C, 30 min). Morphological and structural characteristics of active material were investigated from the synthesis to the nanocrystal deposition process by transmission and scanning electron microscopy, x-ray diffraction, atomic force microscopy and Raman spectroscopy. The as-obtained nanostructured film exhibited good reversible electrochemical features through several intercalation-deintercalation cycles. The electrochromic properties were evaluated on the basis of spectro-electrochemical measurements and showed significant optical contrast in the near-infrared region and high coloration efficiency at 550 nm.

ZnO/Si nanowires heterojunction array-based nitric oxide (NO) gas sensor with noise-limited detectivity approaching 10 ppb

We report a ZnO/Silicon nanowire (ZnO/Si NWs) heterojunction array-based NO gas sensor operating at room temperature with an extremely high response (noise limited response ∼10 ppb). The sensor shows very high selectivity towards NO gas sensing and limited perturbation in response due to the presence of moisture. The sensor has been fabricated by using cost-effective chemical processing that is compatible with wafer-level processing. The vertically aligned Si NWs array has been made by an electroless etching method and the ZnO nanostructure was made by chemical solution deposition and spin-coating. Extensive cross-sectional electron microscopy and composition analysis by line EDS allowed us to make a physical model. The electrical characteristic of the model was to fit the I-V data before and after exposure to gas and essential changes in electrical parameters were obtained. This was then explained based on a proposal for the mechanism of gas sensing. We observe that the heterostructure leads to a synergetic effect where the sensing response is more than the sum total of the individual components, namely the ZnO and the Si NWs. The response is much enhanced in the p-n junction when the n-ZnO nanostructure interfaces with p-Si NW compared to that in the n-n junction formed by ZnO on n-Si NW.