No abstract

Glancing angle co-deposition of well-separated W-Fe nanocolumns was carried out using a W oblique angle sputter source and a Fe confocal incidence source. As-deposited nanocolumns with an overall composition of W64.6Fe35.4 (at.%) exhibited an average column width w nc of 77 ± 15 nm with predominant growth in the β-W phase. With the aim of synthesizing highly porous nanostructures, the as-deposited precursor W-Fe nanocolumnar thin films were immersed in aqueous HNO3 solution for various dealloying durations (t d ). Formation of nanoflake-, nanocactus-, and nanoblade-like structures were observed during the dealloying treatment, as a result of selective dissolution of Fe from the W-Fe precursor films and simultaneous oxidation of W adatoms. By increasing the dealloying duration, the Fe concentration within the film reduced drastically and the film thickness increased by about three times in comparison to the as-deposited film. The dealloyed film exhibited an overall composition of W95.6Fe4.4, where the effective surface area of the film increased substantially. It was found that W adatom diffusion and subsequent rearrangement are crucially important in determining the resultant thin film morphology. The morphological development, corresponding compositions and crystallographic properties of different nanostructures were found to be significantly dependent on the dealloying duration. For optimized processing parameters, the selective dissolution process led to formation of single crystal monoclinic WO3 nanoblades, with growth along [002] and [020] axes.

Porous and photoelectrochemically active Fe-doped WO nanostructures were obtained by a combinatorial dealloying method. Two types of precursor materials libraries, exhibiting dense and nano-columnar morphology were fabricated by using two distinct magnetron sputter deposition geometries. Both libraries were subjected to combinatorial dealloying enabling preparation and screening of a large quantity of compositions having different nanostructures. This approach allows identifying materials with interesting photoelectrochemical characteristics. The dealloying process selectively dissolved Fe from the composition gradient precursor W-Fe materials library, resulting in formation of monoclinic single crystalline nanoblade-like structures over the entire surface. Photoelectrochemical properties of nanostructured Fe:WO films were found to be composition-dependent. The measurement region doped with ∼1.7 at % Fe and a film thickness of ∼ 900-1100 nm displayed highly porous WO nanostructures and exhibited the highest photocurrent density of ∼ 72 μA cm. This enhanced photocurrent density is attributed to the decreased bandgap values, suppressed recombination of electron-hole pairs, improved light absorption as well as efficient charge transport in the highly porous Fe-doped film with single crystalline WO nanoblades.