Nb₂O₅ Nanostructure Evolution on Nb Surfaces via Low-Energy He⁺ Ion Irradiation

Abstract

We propose low-energy, broad-beam He ion irradiation as a novel processing technique for the generation of NbO surface nanostructures due to its relative simplicity and scalability in a commercial setting. Since there have been relatively few studies involving the interaction of high-fluence, low-energy He ion irradiation and Nb (or its oxidized states), this systematic study explores both effects of fluence and sample temperature during irradiation on resulting surface morphology. Detailed normal and cross-sectional scanning electron microscopy (SEM) studies reveal subsurface He bubble formation and elucidate potential driving mechanisms for nanostructure evolution. A combination of specular optical reflectivity and X-ray photoelectron spectroscopy (XPS) is also used to gain additional information on roughness and stoichiometry of irradiated surfaces. Our investigations show significant surface modification for all tested irradiation conditions; the resulting surface structure size and geometry have a strong dependence on both sample temperature during irradiation and total ion fluence. Optical reflectivity measurements on irradiated surfaces demonstrate increased surface roughening with increasing ion fluence, and XPS shows higher oxidation levels for samples irradiated at lower temperatures, suggesting larger surface roughness and porosity. Overall, it was found that low-energy He ion irradiation is an efficient processing technique for nanostructure formation, and surface structures are highly tunable by adjusting ion fluence and NbO sample temperature during irradiation. These findings may have excellent potential applications for solar energy conversion through improved efficiency due to effective light absorption.

One‐Step Plasma Synthesis of Nb2O5 Nanofibers and their Enhanced Photocatalytic activity

“…Apparently, the optical property was altered by the morphology change. Because the FN increases the optical absorbance significantly, the FN surface could be appropriate for solar-thermal application, [3] similar to W cases. [54] Here, the reflectance includes both the specular and diffuse reflectance.…”

“…[42] Pinholes were formed even at 1000 K; the size of pinholes was much smaller and protrusions were more prominent in Figure 1(a). [38] In previous studies, though protrusions were formed on Nb, no FNs were identified on the surface; [3,24] this would be because of the difference in the He flux and fluence. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Different from W case, where grain orientation dependence with grain boundary was identified, [43] there were no clear grain boundary and grain orientation dependence.…”

“…Niobium (Nb) pentaoxide (Nb 2 O 5 ) is popular semiconductor because of its stability and various applications: gas sensor, [1] solar energy conversion, [2,3] lithium ion battery, [4] catalysts, [5] and photocatalysts. [6,7] Concerning photocatalysis application, after the discovery of hydrogen (H 2 ) production from water splitting using titanium oxide (TiO 2 ) by Honda and Fujishima, [8] development of highly effective photocatalysts has been extensively explored.…”

“…[17] Because highly porous materials are thought to enhance the activity of photocatalysis, [2,18] many works have been reported about fabrication of mesoporous Nb 2 O 5 nanostructures via such as sol-gel process, [19] electrospinning, [15] evaporation induced self-assembly method, [11,13] and solvothermal reaction. [6] Recently, Novakowski et al reported that helium (He) ion beam can be used for one-step fabrication of Nb 2 O 5 nanostructures, [3] nanometer sized protrusion were formed on the surface by He ion irradiation. Effects of He plasma irradiation on metals have been investigated widely during the last decade.…”

“…[20] Later on, formation of FNs was identified on various metals. [3,24] Photocatalytic/ photoelectrochemical application using He plasma treated surfaces have been explored using W oxides, [29][30][31] iron oxides, [32,33] titanium oxides, [34] and vanadium oxides. [26,27] Since Nb has a rather low shear modulus (~32 GPa [28] ), it is likely that FNs is not easily formed.…”