*2.3. XRD Analysis of the WO3 Xerogles and Powders*

The XRD measurements of the xerogels and powders were performed using a PW 1710 Philips X-ray diffractometer (Philips, Almelo, Netherlands). The XRD spectra of the WO3 xerogels prepared by either a mixture of 2-propoxy ethanol and 2-propanol or solely with 2-propanol are presented in Figure 3a, while the corresponding powders obtained after annealing of the xerogel at 450 ◦C for 1 h are shown in Figure 3b. The results confirmed that both WO3 xerogels, regardless on the solvent used for the sol preparation are amorphous while annealing of the xerogels leads to the crystallization. The analysis of the XRD spectra of the WO3 powders reveals the presence of the monoclinic phase, which is well in agreement with our previous results for the sample C [12].

**Figure 3.** XRD patterns of the WO3 xerogel (**a**) and corresponding powders obtained after annealing of the xerogels for 1 h at 450 ◦C (**b**). Xerogel A and powder A denotes WO3 prepared with 2-propoxy ethanol and 2-propanol. Xerogel C and powder C denotes WO3 prepared with 2-propanol.

#### *2.4. Rheological Characterization*

Rheological measurements were performed with a rotational controlled rate rheometer (Physica MCR302, Anton Paar, Graz, Austria), equipped with a cone and plate sensor system (CP 50/2◦). Temperature of the measurements was controlled with a Peltier HOOD (Anton Paar, Graz, Austria).

All samples were tested under rotational and oscillatory shear conditions. Rotational flow tests were performed with a triangular method by changing the shear rate from 0–1000–0 s<sup>−</sup>1. Oscillatory stress sweep tests at constant frequency of oscillation (1 Hz) were used to determine the linear viscoelastic range (LVR). Frequency tests were performed at constant small deformation in LVR by decreasing the frequency from 20–0.01 Hz.

#### *2.5. IR Spectroscopic Measurements*

The IR spectra of the sols, xerogels and gels have been taken by FT-IR Perkin Elmer System 2000 spectrometer (PerkinElmer, Waltham, MA, USA). The samples have been deposited as thin layers on the double side polished Si-resin.

#### **3. Results and Discussion**

#### *3.1. Structural Analysis of the Sols and Gels*

A characteristic of a WO3 sol prepared by the peroxo sol-gel route is the presence of the peroxopolytungstic acid (P-PTA) clusters. The structure of the P-PTA was assessed in 1991 by Nanba et al. [29]. It is complex and consists of two edge-sharing 3-membered (W3O13) rings, located above and below the corner-shared 6-membered (WO7 pentagonal bipyramids) ring. The contact between these species is established via the H-bonds of water placed between them. The P-PTA structures transform during the sol preparation, drying and gelation process to the network of the tungsten polyhedra (WO6) connected via corners and edges. There are numerous factors influencing this transformation, among them are also the temperature and the alcohol used for the sol preparation.

For the IR spectrum of the WO3 sols a variety of the W-O bond oscillations are characteristic and the bands could be assigned to: the terminal bond, i.e., double bond between tungsten and oxygen, ν(W = O) at 980 cm<sup>−</sup>1, single W-O bond of tungsten polyhedra connected via corners, ν(W-O-W) at 630–650 cm−<sup>1</sup> or via the edges, ν(W-O-W) at 700–720 cm<sup>−</sup>1. In addition, the peroxo bonds are characterized by the absorption peaks of the W-O-O-W and W-O-O oscillations at 800–830 cm–1 and 560 cm–1, respectively. The presence of water in the structure is evident from the broad band in the range 3000–3500 cm−<sup>1</sup> (ν(O-H)) and a band at 1630 cm−<sup>1</sup> (δ(H2O)) [30–33].

Figure 4 shows the IR spectra of the freshly deposited WO3 sols based on different alcohols, the xerogel of the sol that was dried in a recording chamber of the FTIR spectrophotometer and the WO3 gel. For the comparison of the influence of different solvents on the structure of the sols and gel formation we have analyzed beside the printable WO3 ink prepared by 2-propoxy ethanol (Figure 4a) also the IR spectra of the WO3 sols based on ethanol (Figure 4b) or 2-propanol (Figure 4c). It should be mentioned, that the WO3—ethanol and WO3—2-propanol sols have been found inappropriate for the inkjet printing [15] but are very suitable for dip-coating deposition of the active transparent WO3 layers used in chromogenic devices [7,9–12,14].

To compare how the temperature of the WO3 ink influences the cross-linking of the tungsten polyhedra and gel formation we have taken the IR spectra after the gelation of the WO3 sol in a rheometer at the temperatures (20 ◦C, 30 ◦C, 40 ◦C, 50 ◦C and 60 ◦C) at which the rheological properties of the sols were examined (Figure 5).

In the IR spectra of the wet sols prepared by different solvents (Figure 4) the bands characteristic for the –CH groups (2840–3000 cm−1) of the alcohol are present. The comparison of the IR spectra of the wet and dried sols-xerogels shows a noticeable difference in the intensity of the –CH bands. The results confirm that the alcohol entirely evaporates during drying of the sols at room temperature when the sols are prepared by ethanol or 2-propanol (Figure 4b,c), while it remains present when the 2-propoxy ethanol (Figure 4a) is used.

The IR spectra of the sols (Figure 4) show the presence of all the peaks characteristic for the P-PTA structure. However, the intensity of the peaks, characteristic for the crosslinking of the tungsten polyhedral, differs among the studied sols. The highest intensity of the band typical for peroxo groups (810 cm–1 and 560 cm−1) is characteristic for the fresh sol and the sol dried at RT (xerogel) prepared by 2-propoxy-ethanol (Figure 4a) which still contains some alcohol. While the intensity of the peroxo groups in the xerogel is much smaller in the case of ethanol and 2-propanol based WO3 xerogel that are solvent (alcohol) free. From these we conclude that the decomposition of the peroxo groups present in the P-PTA structure is associated with drying and evaporation of the solvent from the sol during the xerogel formation. The evaporation process is fast and complete for the sols prepared by ethanol and 2-propanol (Figure 4b,c). A slower decomposition of the peroxo groups has been found in the case of the sol containing 2-propoxy-ethanol (Figure 4a). The slower evaporation could have resulted also in slower gelation process, but the rheological studies showed the opposite. The rheological studies showed that the gelation is even faster in case of the WO3 sol prepared by 2-propoxy-ethanol. The reason for the faster gelation can be attributed to a different way of cross-linking of the WO3 polyhedra when different alcohols are used for the sol preparation which has been confirmed by the IR spectra analysis.

**Figure 4.** The IR spectra of the WO3 sol, xerogel and gel prepared by different alcohols, (**a**) 2-propoxy ethanol, (**b**) ethanol and (**c**) 2-propanol.

In addition to the IR spectra of dried sols, xerogel the spectra of the gels (Figure 4) confirmed that only in the case of the sol containing the 2-propoxy ethanol the alcohol remains trapped in the gel structure (Figure 4a). The results demonstrate a strong influence of the solvent on the cross-linking of the tungsten polyhedra. The analysis of the IR spectra of the gels prepared by ethanol and by 2-propanol reveals that the intensity of the band attributed to the terminal W = O bond (980 cm<sup>−</sup>1) strongly decreases during gelation, while the skeletal W-O bonds typical for the connection of the WO6 polyhedra at 630–650 cm−<sup>1</sup> and 700–720 cm−<sup>1</sup> increase in the intensity (Figure 4b,c). The results confirm also the lowest intensity of the terminalW=O bonds for the gels prepared with ethanol (Figure 4b) which implies that the strongest cross-linking of the tungsten polyhedra took place in the gel formed from the ethanol-based sol.

On the other hand, in the IR spectra of the gel formed from the sol based on 2-propoxy ethanol a peak characteristic for the terminal doubleW=O bond remains intensive as well as the bands characteristic for the peroxo groups (Figure 4a). This leads to the conclusion that in the WO3 gel based on 2-propoxy ethanol the P-PTA structure remains present to some degree, while the cross-linking of the tungsten polyhedra is hindered. In addition, the results confirmed the presence of the –CH bonds (peak at 2840–3000 cm−1) characteristic for the 2-propoxy ethanol meaning that the solvent remained trapped in the cross-linked WO3 gel structure (Figure 4a).

To summarize, the IR spectra analysis of the gels showed that the alcohol used for the WO3 sol preparation strongly influences the cross-linking process taking place in the gel formation. The WO3 gel based on 2-propoxy ethanol has the solvent kept in the gel structure, the structure of the P-PTA is to some extent preserved and the bonding of the tungsten polyhedra is not complete which results in a weaker and softer WO3 gels compared to the gels formed from the ethanol and 2-propanol tungsten sols (Figure 4).

In a further IR analysis, we followed the gelation of the WO3 sol based on 2-propoxy ethanol at different temperatures that the WO3 ink could be exposed during the inkjet processing while adjusting the printing parameters. The IR spectra of the WO3 gels were obtained after the gelation of the WO3 sol in a rheometer at 20, 30, 40, 50 and 60 ◦C (Figure 5). In the IR spectra of the gel a high intensity peaks characteristic for the peroxo bonds (peaks at 810 and 550 cm−1) were observed regardless of the temperature at which gelation took place. In addition, no significant difference of the skeletal W-O modes typical for corner or edge shared tungsten polyhedra has been noticed that might suggest different cross-linking mechanisms taking place at different temperature during the gel formation. Moreover, regardless of the temperature at which the gel has been formed the solvent, 2-propoxy ethanol remains trapped in the WO3 gel structure (Figure 5). The IR analysis shows no significant influence of the temperature, in the range between 20 and 60 ◦C, on the chemical structure of the WO3 gel.
