High Aspect-Ratio WO3 Nanostructures Grown By Self-Organizing Anodization in Hot Pure O-H3PO4

Altomare, Marco

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.

(1) Roy, P.; Berger, S.; Schmuki, P. Angew. Chem. Int. Ed. 2011, 50, 2904; (2) Roy, P.; Kim, D.; Lee, K.; Spiecker, E.; Schmuki, P. Nanoscale 2010, 2, 45; (3) Crossland, E. J. W.; Noel, N.; Sivaram, V.; Leijtens, T.; Alexander-Webber, J. A.; Snaith, H. J. Nature 2013, 495, 215; (4) Altomare, M.; Lee, K.; Killian, M. S.; Selli, E.; Schmuki, P. Chem. Eur. J. 2013, 19, 5841; (5) So, S.; Schmuki, P. Angew. Chem. Int. Ed. 2013, 52, 7933; (6) Yang, P.; Zhao, D.; Margolese, D. Nature 1998, 396, 152; (7) Matei Ghimbeu, C.; van Landschoot, R. C.; Schoonman, J.; Lumbreras, M. Thin Solid Films 2007, 515, 5498; (8) Sberveglieri, G.; Depero, L.; Groppelli, S.; Nelli, P. Sensors Actuators B Chem. 1995, 26, 89; (9) Lee, S.; Cheong, H.; Tracy, C. E. Electrochim. Acta 1999, 44, 3111; (10) Antonaia, A.; Addonizio, M.; Minarini, C. Electrochim. Acta 2001, 46, 2221; (11) Altomare, M.; Pfoch, O.; Tighineanu, A.; Kirchgeorg, R.; Lee, K.; Selli, E.; Schmuki, P. J. Am. Chem. Soc. 2015, 137, 5646; (12) Mukherjee, N.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. a. J. Mater. Res. 2011, 18, 2296; (13) Tsuchiya, H.; Macak, J. M.; Sieber, I.; Taveira, L.; Ghicov, A.; Sirotna, K.; Schmuki, P. Electrochem. commun. 2005, 7, 295; (14) Berger, S.; Tsuchiya, H.; Ghicov, A.; Schmuki, P. Appl. Phys. Lett. 2006, 88, 203119; (15) Di Paola, A.; Di Quarto, F.; Sunseri, C. Corros. Sci. 1980, 20, 1067; (16) Di Paola, A.; Di Quarto, F.; Sunseri, C. Corros. Sci. 1980, 20, 1079; (17) Wei, W.; Shaw, S.; Lee, K.; Schmuki, P. Chem. - A Eur. J. 2012, 18, 14622.

2123 High Aspect-Ratio WO3 Nanostructures Grown By Self-Organizing Anodization in Hot Pure O-H3PO4

2123
High Aspect-Ratio WO₃ Nanostructures Grown By Self-Organizing Anodization in Hot Pure O-H₃PO₄