## 3 Results and Discussion
### Elemental Composition, Density, Surface Area and Porosity  
To verify that WO3 and not any other related compound was obtained when using the precursor solutions containing different polymers as additives, the O:W ratio of the films was determined by EDX (Table 2, (O:W)EDX). Since the tungsten oxide films are thinner than 300 nm and the penetration depth of EDX into the sample is in the micrometer range [47] emission from the substrate has to be accounted for. The spectra revealed the presence of tin in addition to tungsten and oxygen as well as some carbon contamination. Assuming a ratio of O:Sn = 2 for the FTO substrate, meaningful values of (O:W)EDX in the films can still be calculated. At first glance, (O:W)EDX was found to be around 3 for all the films (Table 2) which can be considered a confirmation that WO3 was obtained from the different precursor  
Figure 3: TG curves of the dried peroxotungstic acid precursor (**a**) without PEG (black), with PEG 400 (green), with PEG 600 (blue) and (**b**) with PIB50-\(b\)-PEO45 (orange) and PIB50-\(b\)-PEO45 + PEG 600 (purple). The dashed red lines show the temperature profile. solutions. However, (O:W)\({}_{\text{EDX}}\) decreases for the films prepared with PEG indicating the presence of a higher concentration of oxygen vacancies. It has been reported before that PEG can inhibit the crystallization of the films and might lead to formation of oxygen-deficient WO\({}_{3}\)[48]. For the WO\({}_{31}\)ump films, (O:W)\({}_{\text{EDX}}\) deviates in a larger range compared to the other samples. Thus, the presence of mesopores interconnected by micropores (Figure 1c,f) might lead to locally differing transmission of X-rays emitted from Sn in FTO within a given sample area and, therefore, to larger variations in (O:W)\({}_{\text{EDX}}\). Further, for the WO\({}_{31}\)mp films, the amount of carbon detected in the films was higher directly indicating that polymer fragments were still present in the films. X-ray photoelectron spectroscopy (XPS) was performed to complement the elemental analysis by EDX and, in particular, to obtain direct insight into the W oxidation state in pristine films. The overview spectra, which are quite similar for all the films (Figures 4a and 54a-57a), only show the expected elements such as tungsten and oxygen as well as some carbon impurities, to similar extent in all samples. The W 4f spectra (Figures 4b and 54b-57b) exhibit a doublet at binding energies around 38 eV and 36 eV, which corresponds to W\({}^{6+}\)\(4f_{5/2}\) and W\({}^{6+}\)\(4f_{7/2}\) states [14; 24; 49; 50]. The small shoulders at lower binding energies of about 36.4 eV and 34.2 eV point out the presence of W\({}^{5+}\)\(4f_{5/2}\) and W\({}^{5+}\)\(4f_{7/2}\) states assigned to defects in the film surface [24; 49]. The positions of these peaks are in good accordance with the ones reported for WO\({}_{3}\) thin films prepared via sol-gel processes [14; 24] or sputter deposition [50]. The O 1s spectra (Figures 4c and 54c-57c) show a main peak around 530.8 eV which can be assigned to the W=O bonds [24]. The shoulder with less intensity at higher binding energy of about 532.0 eV can be assigned to W-OH groups [24; 51]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Film Type** & **W/At\%** & **O/At\%** & **Sn/At\%** & **C/At\%** & **(O:W)\({}_{\text{EDX}}\)** \\ \hline WO\({}_{31}\)comp & 10.0 \(\pm\) 0.5 & 66.6 \(\pm\) 0.2 & 17.5 \(\pm\) 0.6 & 5.9 \(\pm\) 0.2 & 3.16 \(\pm\) 0.06 \\ WO\({}_{31}\)up400 & 12.9 \(\pm\) 0.9 & 66.0 \(\pm\) 0.3 & 14.0 \(\pm\) 1.6 & 7.1 \(\pm\) 0.7 & 2.94 \(\pm\) 0.05 \\ WO\({}_{31}\)up600 & 12.7 \(\pm\) 1.0 & 66.1 \(\pm\) 0.2 & 14.7 \(\pm\) 1.3 & 6.6 \(\pm\) 0.2 & 2.90 \(\pm\) 0.04 \\ WO\({}_{31}\)mp & 14.0 \(\pm\) 0.9 & 63.8 \(\pm\) 1.4 & 10.3 \(\pm\) 2.0 & 11.9 \(\pm\) 2.5 & 3.08 \(\pm\) 0.08 \\ WO\({}_{31}\)ump & 16.8 \(\pm\) 1.3 & 65.8 \(\pm\) 1.4 & 9.9 \(\pm\) 1.0 & 7.5 \(\pm\) 1.7 & 2.82 \(\pm\) 0.20 \\ \hline \hline \end{tabular}
\end{table}