# Growth of Zr/ZrO\({}_{2}\) Core-Shell Structures by Fast Thermal Oxidation
## 3 Results and Discussion  
X-ray diffraction patterns were recorded under a grazing angle to avoid the signal of the Zr metal core. However, due to the morphology and low thicknesses of most of the oxide shells, the patterns are very noisy. For that reason, in Figure 3, we show the comparison between the main peaks of the pattern and the peak lists of the corresponding phases according to JCDPS cards. The XRD pattern is shown in Figure 3a. Figure 3b (upper part) shows the peak list obtained from a typical X-ray diffraction pattern from our samples. The peaks fit with the presence of two zirconium oxide phases. The peaks shown in the middle graph correspond to a non-stoichiometric Zr\({}_{3}\)O hexagonal phase (JCDPS 01-089-8002) with lattice  
Figure 3: **(a)** XRD diffraction pattern; arrows mark the different phases found (green ones are for the monoclinic ZrO\({}_{2}\) phase, blue ones are for the hexagonal Zr\({}_{3}\)O phase and red ones are for peaks related to both phases). **(b)** Comparison of the peaks obtained (upper) to those listed in the JCDPS cards for the corresponding phases: Zr\({}_{3}\)O (middle) and ZrO\({}_{2}\) (lower). **(c)** Crystal structure of the Zr\({}_{3}\)O phase (O: red; Zr: blue). **(d)** Crystal structure of the ZrO\({}_{2}\) phase (O: red; Zr: green). Figure 2: Current–time profiles used in the different experiments: **(a)** intensity gradually increased from 0 to 2.1 A and held 10 s, then risen slowly to 2.2 A and held 30 s; **(b)** intensity gradually increased from 0 to 2.2 A and afterwards held 60 s; **(c)** intensity directly established at 2.2 A and held 30, 60 and 120 s.  
parameters a = 5.6172 A, b = 5.6172 A and c = 5.1850 A (space group P63 2 2) (Figure 3c). The ZrO\({}_{2}\) monoclinic phase was identified with the JCDPS 00-037-1484 card, with lattice parameters a = 5.1473 A, b = 5.2088 A and c = 5.3166 A (space group P21/c) (Figure 3d). No additional phases related to the presence of impurities were detected. The presence of non-stoichiometric phases might be expected since the oxidation process is extremely fast and might not be completed uniformly. On the other hand, this would lead to low-crystalline-quality areas that would be responsible for the noisy patterns. To further assess the phase content, we performed \(\mu\)-Raman experiments. Then, we recorded Raman spectra from local regions, particularly from the central parts of the wires where the shells are better formed, and the oxidation process was completed. The Raman spectra from all the samples studied are quite similar. An example of these typical Raman spectrum is shown in Figure 4. The peaks observed correspond to the different A\({}_{\rm g}\) (102, 188, 308, 346, 475, 561 and 637 cm\({}^{-1}\)) and B\({}_{\rm g}\) modes (175, 221, 334, 381 and 614 cm\({}^{-1}\)) of the monoclinic phase of ZrO\({}_{2}\). The peaks are labelled according to the data found in the works of [PERSON] et al. [22], [PERSON] et al. [23] and [PERSON] et al. [24]. All the peaks reported previously for the monoclinic phase are observed, and most of them are well-defined and not very broad, so the oxide shell is meant to have a good crystal quality. Red labels correspond to O-O bonds; the green ones correspond to Zr-Zr bonds; the orange ones correspond to Zr-O bonds. The peak at 175 cm\({}^{-1}\) is assigned to a combination of both A\({}_{\rm g}\) and B\({}_{\rm g}\) modes. Some modes (for instance, that at 636 cm\({}^{-1}\)) could be also ascribed to the tetragonal phase. Nevertheless, since only the monoclinic phase was revealed through the X-ray diffraction experiments, we discard this possibility, and we opt for assigning all the modes to the monoclinic structure. The first set of samples described in the Experimental Method Section was used to check the influence of the temperature gradient established along the wire. Figure 5 shows typical topography images obtained from these samples, which upon inspection, do not appear to show any big differences between them.