## 3 Results and Discussion  
### Structural and Morphological Characteristics  
Figure 2 shows the crystalline structure by XRD of the Ti-TiO\({}_{2}\) layers and the as-synthesized Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) heterostructures at different etching times. The presented results show that the thermal oxidation of the Ti foil at 500 \({}^{\circ}\)C conducted to the rutile phase transition (JCPDS 01-075-1010), which was confirmed by the main peaks at 20: 27.65\({}^{\circ}\), 32.47\({}^{\circ}\), 54.64\({}^{\circ}\) (Figure 2a). In addition, the average crystallite sizes of the as-prepared Ti-TiO\({}_{2}\) layers were estimated, using the Debye-Scherrer formula, to be about 62 nm (S1), 40 nm (S2) and 30 nm (S3). It is likely that the subsequent restriction in particle growth can be attributed to the decrease in the diffusion rate of titanium, which is responsible for the reaction speed. The XRD pattern of pristine CuMnO\({}_{2}\) that was previously reported in our paper [26] indicates that the sample crystalized in the crednerite form (JCPDS 00-014-0623) is also confirmed by peaks at 20: 31.32\({}^{\circ}\), 33.45\({}^{\circ}\), 37.0\({}^{\circ}\), 52.42\({}^{\circ}\), 56.85\({}^{\circ}\). The XRD patterns of the as-synthesized heterostructures S1a, S2a and S3a presented in Figure 2b show that the diffraction peaks specific for CuMnO\({}_{2}\) (crednerite form) and TiO\({}_{2}\) (rutile form) are preserved, which demonstrates the good deposition of the CuMnO\({}_{2}\) films onto the Ti-TiO\({}_{2}\) layers. Figure 3 shows the surface morphology of the oxidized titanium samples S1 (Figure 3a), S2 (Figure 3b), and S3 (Figure 3c) for the Ti-TiO\({}_{2}\) layers. The SEM images show that the passivation layer from the surface of Ti foil was completely removed for all three of the samples. TiO\({}_{2}\) layers were grown in the polyhedron form with curved surfaces (broken stone appearance). The growing of the TiO\({}_{2}\) layer was oriented in clearly defined and delimited directions by the curved surfaces of the polyhedral structure. The TiO\({}_{2}\) layers were very well defined, and the sample that was etched for 3 h (S3a) showed a compact growth of elongated grains. The distance between the elongated rows of grains decreased with the increase of corrosion time, and at 3 h the rows of grains were joined over the entire surface of the layer (Figure 3c). The CuMnO\({}_{2}\) films were uniformly deposited, without cracks, and were covering the entire TiO\({}_{2}\) layer surface following its polyhedral shape, as can be seen in Figure 3d-f. Figure 4 shows the 2D (Figure 4a-c) and 3D (Figure 4d-f) surface morphologies by AFM analysis for the Ti-TiO\({}_{2}\) layers, which confirms the results that were obtained by the related SEM images, which indicated that TiO\({}_{2}\) particles have an oriented growth. The Nanosurf EasyScan 2 software was used for the determination of the particle sizes from the surface (Figure 5). In addition, the surface roughness (measured surface area of about 1.326 pm\({}^{2}\)), which is represented by the average roughness \(Sa\) and the mean square root roughness \(Sq\), and the layer thickness were determined from the AFM analysis data. The topographical parameters \(Sp\), which refers to the maximum peak height deviation of the roughness, and \(Sv\), which refers to the maximum valley depth deviation of the roughness that was evaluated over the calculated 3D surface (Figure 4d-f), were used for the calculation of the layers' thicknesses [37]. All of the obtained values are shown in Table 2.  
Figure 2: X-ray patterns for (**a**) Ti-TiO\({}_{2}\) layers and (**b**) as-synthesized Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) heterostructures. Figure 3: SEM images for the Ti-TiO\({}_{2}\) layers as S1 (**a**); S2 (**b**); S3 (**c**) and sensors Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) as S1a (**d**); S2a (**e**); S3a (**f**). Figure 4: 2D AFM surface images of S1 (**a**); S2 (**b**) and S3 (**c**); 3D AFM images for S1 (**d**); S2 (**e**) and S3 (**f**).