*2.1. Preparation of the WO3 Sol*

Step one involved the synthesis of a WO3 sol. First, peroxo-tungstic acid (PTA) was synthesized by reacting 5 g of tungsten monocrystalline powder (mean particle size < 1 micron, purity 99.95%, Sigma-Aldrich GmbH, Schneldorf, Germany) with 20 mL of hydrogen peroxide (30%, Belinka). This reaction is strongly exothermic. Sols where then prepared by adding solvent to the PTA solution at 120 ◦C. The addition of alcohol resulted in the formation of the W-ether that polymerizes to peroxopolytungstic acid (P-PTA) [29]. Solvent for inkjet printing ink should have boiling point higher than 100 ◦C, therefore WO3 sol used in this study as ink was prepared with a mixture of two solvents with different boiling points, i.e., 2-propanol (ACS reagent, Sigma-Aldrich GmbH, Schneldorf, Germany) and 2-propoxy ethanol (Puriss, Sigma-Aldrich, Schneldorf, Germany). The ink appeared slightly orange and contained 27.7 mmol of tungsten per 30 mL of sol. Figure 1 presents the WO3 sol and gel state—Figure 1a, the WO3 xerogel dried at RT—Figure 1b and the WO3 powder annealed for 1 h at 450 ◦C—Figure 1c. More details on the preparation of the sols could, also the ones based solely on ethanol and 2-propanol could be found in Ref [15].

**Figure 1.** WO3 sol and gel: (**a**), xerogel (**b**) and powder (**c**).
