The patterns are shifted vertically for clarity. The annealed samples show the presence of NiO peaks. The reflexes of Ni are still observed and arise from the incomplete oxidation of the Ni supporting layer. The stars and tick marks denote the Au-Ni alloy and Au, respectively. From the above, it can be seen that metallic Ni still dominate the XRD spectrum, and it appears necessary to estimate the magnitude of oxidation of the nanostructures. For doing this, we make use of the data published in [33] which shows that Ni oxidation follows

a parabolic law in a wide range of temperature. Through extrapolation and taking into account the surface area of the 1D buy XAV-939 morphology involved (see calculation details in Additional file 1: S1), it can be Repotrectinib shown that sample 2 consists of 60% NiO while sample 3 is completely oxidized. CBL0137 mw Using the same procedure, only a small fraction of oxide (0.37%) is calculated for the underlying Ni layer, which explains the dominance of the Ni peaks in the XRD patterns. The morphology

of the nanostructures obtained is shown in Figure 2. The non-annealed sample 1 (Figure 2a, b) shows solely Ni NTs that form via nucleation and growth at the pore walls because of the presence of an extremely thin Au layer (see the experimental section and our previous paper [32]). The judicious deposition time for Ni to obtain NT is 50 s. Figure 2 SEM images of non-annealed (sample 1) and annealed samples (samples 2 and 3). (a) Cross-sectional and (b) top Carnitine dehydrogenase views of the as-prepared Ni NT (non-annealed sample 1 inside AAO template). (c) Wall thickening after 25-min annealing (sample 2). (d) The complete closure of walls yielding NR morphology after 300-min annealing (sample 3). During annealing, the oxide layer nucleates and grows from the exposed inside walls and thickens in the direction of the inner-tube diameter. This suggests an outward diffusion

of the Ni species toward oxygen ions. On the non-exposed outside walls that are confined by the AAO template, no oxide growth is expected. A short annealing time leads to incomplete oxidation of the Ni NTs, resulting in the formation of an oxide scale supported on a remaining Ni layer (see also the XRD results above and Additional file 1: S1). This is the case of sample 2 (Figure 2c; 25-min annealing). For longer annealing time, complete closure of the NT, to finally give the NR morphology as shown in Figure 2d, is achieved because of the volume increase associated with NiO oxide formation. This is the case of sample 3 (300-min annealing). Figure 3 shows the CV curves of the NiO NTs and NiO NRs recorded using a potential window of 0.5 V (between 0.35 and 0.85 V) at various scan rates (5, 10, 25, 50, and 100 mV/s).