*2.2. Preparation of the Composite Films*

The TiO2 nanowire arrays were prepared via a hydrothermal synthesis method according to our previous report [22], and this was followed by the preparation of the composite films. More specifically, the surface of the FTO substrate was cleaned in NH3/H2O/H2O/H2O2 (volume ratio 1:1:1) at 80 ◦C for 20 min and then rinsed with deionised water. This step was repeated 3*–*5 times to remove any inorganic and organic impurities from the FTO substrate. The composite film was prepared via the LbL assembly method. Initially, the cleaned FTO substrate was modified with TiO2 NWs. Subsequently, the pure FTO and the modified FTO were immersed in APS overnight. After this time, the samples were placed in HCl (pH 2.0) for 20 min, rinsed with deionised water, and dried under a stream of nitrogen to give the precursor. Finally, the composite film (NW−P2W17V) was constructed by depositing negatively charged P2W17V (5 × 10−<sup>3</sup> mol L−<sup>1</sup> in 0.2 mol L−<sup>1</sup> HOAc-NaAc at pH 3.99) and positively charged PEI (5 × 10−<sup>3</sup> mol L−<sup>1</sup> at pH = 4) onto the TiO2 NWs, according to the LbL method. For comparison, an additional film was prepared on the pure FTO substrate using the same method, and this was designated as FTO−P2W17V. A schematic outline of the fabrication process is shown in Figure 1a.
