High Aspect-Ratio WO3 Nanostructures Grown By Self-Organizing Anodization in Hot Pure O-H3PO4
Authors: Altomare Marco , Nguyen Nhat Truong , Schmuki P. 
Published: February 2020

In the last decades, one-dimensional (1D) nanostructured semiconductor have attracted large attention in the field of materials science owing to their enhanced physico-chemical properties such as large porosity and high specific surface area,1 orthogonal charge carrier separation and facilitated electron transport.2-5 With this contribution we focus particularly on WO3, as the synthesis of such defined architectures could not be achieved using common techniques, such as hydrothermal processes,6 spray pyrolysis,7 sputtering8 and thermal/e-beam evaporation.9,10

We introduce an anodization approach of W metal, based on the use of hot (nominally) pure ortho-phosphoric acid (i.e., o-H3PO4) as anodizing electrolytes, to form self-organized high aspect-ratio structures of WO3.11 Noteworthy, various other anodic growth pathways of porous WO3 are reported that however lead only to oxide architectures lacking of long range order,12-14 mainly because the anodization of W metal in common electrolytes produces highly soluble species (i.e., fast dissolution oxide rate) and this typically leads to a poor control over the anodic growth.15,16

The key of our approach is that the water content in the hot o-H3PO4 electrolyte (typically kept at ca. 100°C during anodization) is limited to traces, that is essential to reach a controlled growth of thick and ordered porous films.11,17 Under optimized electrochemical conditions, the formed anodic layers consist of highly-aligned WO3 nanopores with adjustable diameter (5-20 nm) and length (up to ~ 4 µm) that can reach an aspect-ratio of ~ 400.
These 1D WO3 nanostructures, owing to their electronic and optical properties, advantageous directional charge transfer and enhanced gas diffusion and ion intercalation geometry (in comparison to devices fabricated from classical powder assemblies), can be suitably used in a large palette of applications as photocatalysts, electrodes for electrochromic devices and photo-electrochemical cells (i.e., H2 generation), and gas-sensors.
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