## 3 Results
### Set Up and Optimization of the Catalyst Deposition Process
This is an interesting procedure for coating only the catalyst where needed (e.g., over one side of the tile only). Unfortunately, despite a mean deposition weight of 6.3 mg after just one cycle, this technique was not reliable, since the layer of titania was less uniform even with respect to the untreated tiles processed via dip coating. In light of these results, more tests were developed to assess the robustness of the double washing (piranha solution and rinsing with carbonate) procedure. As reported in Table S2 (see Supporting Information File), an average of 0.7 mg was added after each cycle, and this was consistent among the series of four glasses, even with a sparse standard deviation. The standard deviation was further optimized, increasing the amount of coating. Six cycles allowed for the collection of 4.2 mg of P25 deposited over the surface of the tiles, which is in line with our requirements as the photocatalytic reduction of CO\({}_{2}\) was previously optimized using 37 mg of photocatalyst powder in 1.2 L of solution. By using 12 tiles with such coatings, a consistent comparison could be ensured. Not only did the deposited layer result in being very homogeneous, it was also sufficiently resistant from an adhesion point of view. This point was first demonstrated via the immersion of a glass tile in a beaker containing water and maintaining stirring for 3 h. After that amount of time, the glass tile was dried at 105 \({}^{\circ}\)C and weighed again, which confirmed that no appreciable amount of coating was lost during the tests (reproducibility was ensured by many repetitions), at least within the limits of the balance used, i.e., 0.1 mg precision. In addition, an abrasion resistance test was carried out. Scratch and abrasion tests are codified differently according to the purpose of the abrasion resistance measurement. Evaluations of the resistance to abrasion and the hardness of the coating are the most important features in this case, which are measured by applying a constant force and measuring the loss of powder, since the coating should not be purposefully used in contact with other surfaces. On the contrary, for surfaces that should be used in continuous contact with other solid objects, an increasing force is applied until the coating is lost completely or up to a predetermined % of weight. In our measurement, the coating was subjected to a constant small force, and the loss of powder was measured. For this, we used a homemade apparatus comprising a 500 mm-long plane inclined at 45\({}^{\circ}\) and covered with sandpaper (P200). With this setup, each tile was pulled down the plane by its own weight (ca. 5 g mass, ca. 0.06 N scratch force on the inclined plane), and as a result, roughly the same force was applied during the tests. Then, the tiles were weighed and observed with a microscope. The surface of the glass tiles was unaffected based on the observation with a microscope. Even after multiple abrasion cycles, none of the glasses lost more than 0.1 mg of coating, and the surface was substantially unaffected. Given the robustness of the deposition technique, we decided to further optimize the parameters to obtain a higher loading of TiO\({}_{2}\) per cycle. All the following tests were performed using two deposition cycles. Table S3 (see Supporting Information File) summarizes all the results, which show that there is an effect of the solution temperature on the deposition of P25. For instance, the weight of the deposited layer increased by 20% when the temperature rose to 30 \({}^{\circ}\)C and by 87% with a further increase to 50 \({}^{\circ}\)C. Both the minimum and the maximum weights of the series (usually composed of twelve samples) increased as well. Another important parameter was pH, which was lowered via the addition of either HCl or HNO\({}_{3}\), which are both strong acids and widely used for activating glass, with the former being, in general, more volatile [32].