*3.1. Characterisation of the NW*−*P2W17V and FTO*−*P2W17V Materials*

The multilayer growth process of composite film on the precursor-coated quartz substrate (on both sides) was monitored by UV-Vis spectroscopy (Figure 1b). It exhibited strong absorption of P2W17V with two characteristic absorption peaks at 201 and 289 nm. The peak at 201 nm originates from the terminal oxygen to tungsten charge-transfer transition (Od→W), whereas the peak at 289 nm corresponds with the charge-transfer transition from the bridging-oxygen to tungsten (Ob/Oc→W). The inset of Figure 1b shows the plots of the absorbance values at 201 and 289 nm as a function of the layer number and suggests that growth is uniform during each cycle.

SEM-EDS and TEM were then performed to obtain the detailed information about the surface morphologies and homogeneities of the composite materials. The SEM images of the FTO−P2W17V film are shown in Figure S4, wherein it can be visualised that the FTO substrate was covered by aggregated P2W17V anions. In addition, the cross-sectional view of the FTO−P2W17V film gave a thickness of ~150 nm. As shown in Figure S5, the FTO substrate was covered with densely grown TiO2 NWs, and the cross-sectional image confirmed that the height of the nanowires was approximately 600 nm. After the LbL process, it was apparent that the interspaces of the NWs were filled, and the NWs became wider and more compact owing to the deposition of P2W17V and PEI (Figure 2a). Moreover, the EDS mapping of P, W, Ti, and V confirmed the feasibility of the hydrothermal treatment and LbL process (Figure 2b), since the POMs and the TiO2 NWs were evenly distributed on the surface of the FTO substrate.

Subsequently, AFM was employed to study the surface morphologies and roughness properties of the FTO−P2W17V and NW−P2W17V films (Figure 2c,d and Figure S6). Twodimensional (2D) and three-dimensional (3D) images of the two films confirmed that their surface microstructures were quite different. More specifically, the AFM images of the FTO−P2W17V film displayed some uniformly sized spherical particles, which resulted from the FTO substrate being covered with cross-linked POM anions with a thickness of 100 nm (Figure S6b). From Figure 2d, it was apparent that the surface of the NW−P2W17V film shows a regular cylindrical microstructure, suggesting the presence of TiO2 NWs substrate. The height of the NWs anchored with the POMs was ~500 nm, which corresponded well with the SEM observations. In addition, the root mean square (RMS) roughness for each film was calculated from an area of 5 × 5 μm2 in the AFM image, wherein the surface roughness (i.e., RMS) values of the NW−P2W17V and FTO−P2W17V films were found to be 73.6 and 20.5 nm, respectively. A higher roughness could lead to a larger reactive surface area, thereby improving the electrochemical performance of the material.

**Figure 2.** (**a**) SEM images of NW−P2W17V (inset: the cross-sectional images of prepared films); (**b**) EDS mapping of NW−P2W17V for P, W, Ti and V respectively; (**c**) 2D AFM images; and (**d**) 3D AFM images of NW−P2W17V films; High-resolution XPS spectra for Ti 2p (**e**) and W 4f (**f**).

The surface chemical compositions of the as-prepared films were further determined and quantified by XPS analysis. The high-resolution XPS spectra of the prepared composite film shown in Figure 2 indicates that the composite material mainly contains C, P, Ti, and W [28,29], wherein the Ti should originate from the TiO2 NWs on the FTO substrate. This result further confirms that the POMs and the TiO2 NWs are distributed on the surface of the FTO substrate. As shown in Figure 2e, the most intense doublet peaks are observed at 35.6 and 37.7 eV, which correspond to the binding energies of the electrons in the W4f7/2 and W4f5/2 levels of W in the W(VI) valence state. These results indicate that the majority of W atoms were in a highly oxidised state and could be reduced to W(V), which is the key reaction in the EC process of polyoxotungstate-based materials. With respect to the high-resolution Ti2p peaks, they could be split into peaks at 458.9 and 464.6 eV, which were both attributed to TiO2 (Figure 2f), thereby indicating that the main matrix component was TiO2. Furthermore, the prepared film exhibited a peak corresponding to the C1s level (284.8 eV) of the carbon present in the PEI polycation, whereas the P2p signal (at 133.0 eV) and the V2p signal (at 532.4 eV) [30] were ascribed to P2W17V (Figure S7). Thus, the XPS data suggest that PEI cations and P2W17V anions were incorporated into the TiO2 NW substrate, which is consistent with the UV-vis results.

TEM is indispensable for the characterisation of nanostructured materials, particularly when the particle shape is important in determining its function, and so TEM was employed herein to evaluate the microstructure of the composite and the spatial relationship between TiO2 NWs and P2W17V. Figure 3a–b show the typical TEM images of the TiO2 NWs with a diameter of ~50 nm. The EDS elemental mapping patterns of the TiO2 NW−P2W17V film were also recorded, as shown in Figure 3c–f and Figure S8. Combined with the TEM morphological observations, the distributions of W, P, Ti, and V suggest a uniform distribution of P2W17V on the TiO2 NWs. As shown in the TEM image (Figure 3b), following the LbL assembly process, the P2W17V coating layer covered the surface of the NWs, forming a core-shell structure. As indicated by the arrows, the darker columnar area is a TiO2 NW and the lighter part surrounding it are P2W17V particles. The selected area electron diffraction pattern showed the specific diffraction spots of TiO2 nanowires, and it can be attributed to the rutile phase [23].

**Figure 3.** TEM images of TiO2 nanowires (**a**) and NW−P2W17V (**b**); (**c**–**f**) EDS elemental mapping patterns of Ti, W, and V in the NW−P2W17V films.
