## 3 Results and Discussion  
### Morphology and Structure of the Films  
From the cross-section of the pure tungsten oxide WO\({}_{31}\)comp thin films prepared from the precursor solution without additives (Figure S1c), a full coverage of the substrate by WO\({}_{3}\) with a film thickness of around 100 nm can be observed, thinner than the reported 250 nm in [14]. This could be caused by differences in the details of experimental conditions during spin-coating. The samples reveal a smooth and compact film structure with less than 2 nm RMS surface roughness (WO\({}_{31}\)comp) on the crystalline FTO substrate as also observed in [14]. While the addition of PEG 400 or PEG 600 in the volume ratio of 3:10 or 5:10 into the precursor solution led to inhomogeneous films, the addition of PEG in the volume ratio of up to 1.5:10 provided films of WO\({}_{31}\)up400 and WO\({}_{31}\)up600 with a smooth coverage of the substrate (Figures 1a and S1b) with RMS surface roughness similar to the WO\({}_{31}\)comp films (Figure S1a). The cross-section of these films indicates a grainy film structure and film thicknesses of about 130-150 nm (Figures 1d and S1d). Such a grainy film structure has been also observed for WO\({}_{3}\) thin films prepared via spin-coating from a peroxotungstic acid precursor solution with PEG 20,000 as structure-directing agent and annealed at 300 \({}^{\circ}\)C [30]. Using PI\({}_{50}\)-\(b\)-PEO\({}_{45}\) as structure-directing agent results in around 200-230 nm thick WO\({}_{31}\)mp films (Figure 1e) with isolated spherical mesopores of around 10-20 nm diameter (Figure 1b,e). These pore sizes are comparable to the ones obtained for mesoporous titanium oxide using a similar block copolymer [35; 38]. Films prepared with PIB\({}_{50}\)-\(b\)-PEO\({}_{45}\) as additive were light-brownish in color, indicating some remaining carbon within the films. The combination of PEG 600 and PIB\({}_{50}\)-\(b\)-PEO\({}_{45}\) as structure-directing agents led to 230-250 nm thick WO\({}_{31}\)up films with interconnected mesopores (Figure 1c,f). Figure 1: Top view (**a**–**c**) and cross-section (**d**–**f**) of the WO\({}_{31}\)up600 (**a**,**d**), WO\({}_{31}\)mp (**b**,**e**), and WO\({}_{31}\)up (**c**,**f**) thin films on FTO. The left part of the film in (**a**) was scratched off to provide a comparison to the bare substrate. The amorphous character of the films which were annealed at temperatures \(T\leq 300\)\({}^{\circ}\)C is revealed by GIXRD at room temperature (30 \({}^{\circ}\)C) as depicted in Figure 2 for a WO\({}_{31}\)up600 film. No reflexes between 20-60\({}^{\circ}\) are observable which would be characteristic of crystalline WO\({}_{3}\). All other reflexes correspond to FTO as confirmed by a GIXRD measurement of a bare FTO glass substrate. Annealing the film during the GIXRD analysis showed the transition of an amorphous film into a crystalline film. At 400 \({}^{\circ}\)C, reflexes assigned to triclinic WO\({}_{3}\) (JCPDS card 83-0947) start to appear and become more obvious from 450 \({}^{\circ}\)C on. For WO\({}_{31}\)comp as well as for the WO\({}_{31}\)up400, WO\({}_{31}\)mp and WO\({}_{31}\)up films, similar series were obtained, indicating the presence of triclinic WO\({}_{3}\) only after annealing the samples at 450 \({}^{\circ}\)C (Figure S2). The crystallite size of the films annealed at 450 \({}^{\circ}\)C was estimated from the reflexes at 2\(\Theta\) around 23\({}^{\circ}\) and 42\({}^{\circ}\) using the Scherrer equation [41; 42; 43]. An average crystallite size of around 21 nm was obtained which is well in line with the crystallite size reported in [44] for WO\({}_{3}\) films prepared by dip-coating and annealed at 500 \({}^{\circ}\)C. [PERSON] et al. [14] and [PERSON] et al. [30] have also obtained triclinic WO\({}_{3}\) from peroxotungstic acid solutions. At 600 \({}^{\circ}\)C and 650 \({}^{\circ}\)C, the relative intensity of the substrate signals increased, presumably caused by thermal expansion of the sample leading to a higher contribution from the substrate.