*3.1. Characterisation of PO4-TiO2 Nanotube and Nanocomposite Membranes*

The FT-IR spectra of prepared nanoparticle TiO2 and PO4-TiO2 were presented separately in Figure 2. For TiO2 nanoparticles, Ti-O bonds are responsible for the bands at 715 cm−<sup>1</sup> and 1025 cm<sup>−</sup>1. the bands at 1622 cm<sup>−</sup><sup>1</sup> refer to the bending vibration of the Ti-OH band. The band at 3387 cm−<sup>1</sup> are assigned to O-H stretching vibration bonds due to moisture adsorption on the material's surface [20,33]. For the chart of PO4-TiO2 particles, the band at 690 cm−<sup>1</sup> corresponds to the stretching of the Ti-O bond. The bands at 890, 1085 and 1270 cm−<sup>1</sup> are referred to as P-O bonds vibration. The band located at 1425 cm−<sup>1</sup> is attributed to the stretching vibration band of the P=O bond. The O-H bonds from H2O molecules adsorption are proofed by the bands at 1630 and 3117 cm<sup>−</sup>1. The band located at 2374 cm−<sup>1</sup> is related to the presence of CO2 [34,35].

**Figure 2.** FTIR spectra of PO4TiO2 (**left chart**), PVA/PEO/PO4TiO2 membranes (**right chart**).

For the membranes, the Figure 2 shows that the bands around 3250 cm−<sup>1</sup> refer to the characteristic stretching vibration band of hydroxyl groups on PVA and PEO. the bands at 1650 cm−<sup>1</sup> are attributed to the bending vibration O-H bonds. The band at 1112 cm−<sup>1</sup> is the characteristic band of PEO [36]. Bands at 2840 cm−<sup>1</sup> can be assigned to the vibration of methylene C-H bonds in the polymer's structure. The characteristic peak for sulfate groups of sulfophithalic acid (SPA) was cited at 900 cm<sup>−</sup>1, while the small bands at 1700 cm−<sup>1</sup> indicate C=O bonds of the sulfophithalic acid (SPA), which confirms the crosslinking process. The band at 1100 cm−<sup>1</sup> is assigned to P-O bonds of phosphate titanium oxide.

In Figure 3, show the XRD pattern of TiO2 and PO4TiO2 (on the left side) and composite membranes on the right side. The constructed membranes' amorphous shape shows good ion conduction [37], while the titanium dioxide rutile characteristic peaks intensity at two angles of 28, 36, 41 and 54 [38]. This is because the phosphate entering the titanium oxide lattice changed its original crystalline phase due to the different synthesis processes for PO4TiO2. Therefore, the intensity of the sharp peak of the original titanium oxide at 28◦ is disappeared in the diffractogram of phosphate titanium oxide. In comparison, the ridge at 54◦ of the TiO2 is absent in the diffractograms of PO4TiO2.

Morphological analysis of membranes was studied using SEM and presented in Figure 4. Figure 4a, b demonstrates SEM images of membranes that show a smooth surface with no defects for the undoped crosslinked membrane. At the same time, particles of phosphate titanium oxide tubes appeared in the doped membrane, which was further confirmed by EDX spectra as shown in Figure 4e. However, the SEM image in Figure 4c illustrate the porous structure of the cross-sectional of the doped membrane. Consequently, these voids lead to an improvement in the ionic conductivity of the films [39]. While TEM image of phosphate titanium oxides shown in Figure 4e proofed the forming of nanotubes shape with nanoscale size as illustrated in Figure 4f.

**Figure 3.** XRD patterns of PO4TiO2 (**left chart**) and PVA/PEO/PO4TiO2 membranes (**right chart**).

(**c**) (**d**)

**Figure 4.** *Cont.*

**Figure 4.** SEM images for (**a**) undoped membrane, (**b**,**c**) doped membrane (PVA/PEO/PO4TiO2-1) surface and crosssection (**d**) EDX analysis for PO4TiO2, (**e**) TEM image for PO4TiO2nanotubes and (**f**) the frequency distribution plot of PO4TiO2nanotubes size from TEM image.
