Temperature-dependent surface porosity of Nb2O5 under high-flux, low-energy He+ ion irradiation

The present study reports on high-flux, low-energy He' ion irradiation as a novel method of enhancing the surface porosity and surface area of naturally oxidized niobium (Nb). Our study shows that ion-irradiation-induced Nb surface micro- and nano-structures are highly tunable by varying the target temperature during ion bombardment. Mirror-polished Nb samples were irradiated with 100 eV He+ ions at a flux of 1.2 x 10(21) ions m(-2) s(-1) to a total fluence of 4.3 x 10(24) ions m(-2) with simultaneous sample annealing in the temperature range of 773-1223 K to demonstrate the influence of sample temperature on the resulting Nb surface morphology. This surface morphology was primarily characterized using field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Below 923 K, Nb surfaces form nano-scale tendrils and exhibit significant increases in surface porosity. Above 923 K, homogeneously populated nano-pores with an average diameter of 60 nm are observed in addition to a smaller population of sub-micron sized pores (up to 230 nm in diameter). Our analysis shows a significant reduction in surface pore number density and surface porosity with increasing sample temperature. High-resolution ex situ X-ray photoelectron spectroscopy (XPS) shows Nb2O5 phase in all of the ion-irradiated samples. To further demonstrate the length scales in which radiation-induced surface roughening occurs, optical reflectivity was performed over a spectrum of light between 200 and 1100 nm, showing a recovery of nano-scale surface damage at high sample temperatures. (C) 2015 Elsevier B.V. All rights reserved.