Tungsten oxide tree-like structures

An interesting micrometer scale tree-like structure has been generated by heating a W foil, partly covered by a SiO2 plate, in an Ar atmosphere at ca. 1600°C. Upon sonication, the trees are broken into nanoneedles (ca. 5–50 nm wide and 20–200 nm long) and planar polyhedral nanoparticles (ca. 10–50 nm cross-section). Structural analysis, using ED, EDX, XRD, and HRTEM, showed that: (1) the trees consist of well-crystallised WOx phases (x=0–3); (2) the nanoneedles are composed mainly of monoclinic W18O49 phases; and (3) the nanoparticles consist primarily of WO3. The tree growth is thought to arise from the intrinsic crystalline feature of WOx, the planar defect or the shear structure of which is responsible for the breakdown of the trees.