Metal oxide nanostructures hold great potential for photovoltaic (PV), photoelectrochemical (PEC), and photocatalytic applications. Whereas thin films of various materials of both nanoparticle and nanorod morphologies have been widely investigated, there have been few inquiries into nanodisk structures. Here, we report the synthesis of ultrathin WO 3 nanodisks using a wet chemical route with poly(ethylene glycol) (PEG) as a surface modulator. The reported nanodisk structure is based on the interaction of the nonionic 10000 g/mol PEG molecules with tungsten oxoanion precursors. The WO 3 nanostructures formed are dominated by very thin disks with dimensions on the nanometer to micrometer scale. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images reveal the structures to have dimensions on the order of 350-1000 nm in length, 200-750 nm in width, and 7-18 nm in thickness and possessing textured single-crystalline features. A number of analytical techniques were used to characterize the WO 3 nanodisks, including selected-area electron diffraction (SAED), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), Raman scattering spectroscopy, UV-visible spectrophotometry, and cyclic voltammetry (CV). The growth of the WO 3 nanodisks was inhibited in the [010] crystal direction, leading to ultrathin morphologies in the monoclinic crystal phase. The large flat surface area and high aspect ratio of the WO 3 nanodisks are potentially useful in PEC cells for hydrogen production via direct water splitting, as has been demonstrated in a preliminary experiment with external bias.

In-situ electron microscopy provides a means to directly visualize interfacial processes in real time and with high spatial resolution in real and in reciprocal spaces. However, the traditional preparation of electron-transparent samples from thin films often alters the interface(s) of interest. An innovative approach is to deposit a layer of a chosen material onto a miniature substrate that is already electron-transparent and mounted for TEM imaging. Cu/SnO 2 bilayer nanoribbons were made by coating about 10 nm-thick face centered cubic Cu layers on tetragonal single crystal SnO 2 nanobelts so to study as-made, extending hetero-structure interfaces. The SnO 2 ribbons typically have width and thickness ranging from 10 nm to 1 µm and width/thickness ratios as high as ten [1].The thick and thin sides of a given ribbon can be either of the SnO 2 (101) or ( 010 ) surfaces.TEM observations of bilayers made at room temperature reveal the existence of distinct structural Cu types for growth on SnO 2 (101) and ( 010 ) surfaces, respectively. Cu on SnO 2 ( 010 ) always forms flat and epitaxial Cu(111) films, Fig.1a. In contrast, growth on SnO 2 (101) produces dense and continuous films of Cu grains with no preferred orientation relative to the substrate, Fig.1b. The response of the bilayers to in-situ TEM heating is studied using a 300kV JEOL 3010 transmission electron microscope equipped with a double-tilt heating stage. When subjected to repeated heatingcooling temperature cycles between 25 and 200 o C, the epi-bilayers bent reversibly governed by the theory for macroscopic bimetallic strips, Fig.2. In contrast, the untextured bilayers always displayed a degree of plastic deformation.The Cu layers became unstable when heated to above ~225ºC. Thermodynamic considerations indicate that Cu does not wet SnO 2 (010) or (101) at equilibrium. Both the untextured and epitaxial Cu films irreversibly converted to thick, flat, pure Cu islands between 225 and 500ºC, Fig.3. The onset of island formation was followed by a rapid but brief increase in island number and then a sustained period of slow island thickening through surface diffusion of Cu.Above 550ºC the Cu islands underwent a series of solid-state reactions with the SnO 2 substrate, leading to various phases and major changes in morphology. Sn was first diffused into many of the thickening Cu islands. On the SnO 2 (101) surface, these alloy islands began to etch rapidly into the ribbon substrate at about 600ºC, Fig.4, while the SnO 2 (010) surface proved to be more chemically resistant. As etching continued, many of the flat islands lost their faceting, became quasi-spherical in shape. By 725ºC the majority of these particles had transformed into Cu-Sn phases. Further heating fused the nanoribbons.