*3.2. Viscosity of WO3 Sols*

At all temperatures examined (20, 30, 40, 50 and 60 ◦C), the prepared WO3 sol exhibited Newtonian viscosity, which exponentially decreased with increasing temperature (Figure 6). However, after gelation process was finished, the viscosity of all gels, formed at different temperatures, was in the same range, regardless of the temperature at which the gelation took place. The formed gels exhibited similar complex viscosity with much higher values (η \*~400 Pa.s) compared to the initial sols (η \*~0.001–0.01 Pa.s) (Figure 6). This is well in agreement with the results of the IR analysis of the gels taken after the rheology study at

different temperature (Figure 5). The dependencies of the viscosities on the temperature were for the initial sols as well as for the formed gels expressed by Arrhenius model:

$$
\eta = \eta\_0 e^{\left(\frac{E}{RT}\right)} \tag{1}
$$

where *T* is temperature, *η*<sup>0</sup> is a material constant, *E* is the activation energy and *R* is the universal gas constant [34].

**Figure 6.** Temperature dependence of the dynamic viscosity of freshly prepared WO3 sol and complex viscosity of gels, formed at different temperatures.
