## 3 Results
### Set Up and Optimization of the Catalyst Deposition Process
A closer analysis with an optical microscope (up to \(\times\)400 magnification power) unveiled that the surface of the glasses was unfinished, and many microscopic debris prevented the homogeneous deposition of the P25 nanoparticles (Figure 3b). To overcome this problem of non-homogeneity, all the glass slides were pretreated and washed with a piranha solution (3:1 H\({}_{2}\)SO\({}_{4}\)/H\({}_{2}\)O\({}_{2}\) 30% \(w/w\)), which proved to be effective  
Figure 3: P25 photocatalyst deposited over glass surface after two cycles, (**a**) washed with piranha solution, and (**b**) untreated surface. Observed through an optic microscope at 100 magnification power. in cleaning and activating the surface to adhere the catalyst. Interestingly, a significant improvement in the coating over the glass surface was observed with an average of 2.3 mg of P25 deposition in two consecutive cycles. Interestingly, the first deposition did not deposit an appreciable amount of titania, barely 0.1 mg (Table S1, Supporting Information File), which is probably due to the left-over traces of piranha solution on the surface. After three deposition cycles, the average photocatalyst layer weight (2.2 mg) was lower than that of the same layer after two cycles, which was 4.8 mg, and we interpreted this as being a result of the fact that some titania was detached again during the subsequent dipping as a result of poor adhesion of the first layer. In any case, at the end of the 6 th cycle, the average layers weighed 5.5 mg.  
To confirm the hypothesis that traces of piranha solution were interfering with the deposition process, the tiles were rinsed with sodium carbonate solution (1 M) and then dipped into the P25/water suspension. This was selected as a weak and inexpensive base, able to tune the surface charges. After three cycles of coating, the average deposition was 3.4 mg, almost 50% higher than the amount that was previously obtained (2.2 mg). The positive effect of rinsing can possibly be explained by the effect of acids or bases on the surface charges present on the glass surface [29]. The point of zero charge of borosilicate is ca. 7.5-8, while that of P25 Titania is ca 6.5 [30]. When treating the glass surface with piranha solution, besides removing impurities, we left the surface enriched with positive charges and enriched it with oxydrils after rinsing with carbonate. This may have affected the surface interaction with suspended particles that, depending on the pH of the slurry, may have been neutral or positively or negatively charged. Since the pH of the slurry was ca. 5.5 when native pH was kept in the deionized water, the P25 particles were positively charged and may have interacted more favorably with a surface where an excess of positive charges, attained after treatment with a strongly acidic solution, had been neutralized with carbonate. One doubt may regard the chemical effects of residual amounts of piranha solution, particularly sulfur. This is in general not considered a problem, since the reaction of CO\({}_{2}\) photoreduction is carried out using a significant amount of sulfite as the hole scavenger, which oxidizes to sulfate. Therefore, it is very unlikely that possible residuals at the interface between the glass and the coated powder exerted a significantly different effect in such a surrounding. This notwithstanding, we checked the effects of glass cleaning with the piranha solution and subsequent washing by performing EDX analyses (Figure S1), reported in the Supporting Information File. A significant concentration of sulfur was noticed only for the sample immediately after treatment with the piranha solution. Subsequent rinsing and washing progressively decreased the S content down to 2 wt% (only one washing with water) or to negligible concentrations for further washing with bicarbonate and then with HCI (See Figure S1, Supporting Information File). Other researchers reported a simple coating technique with a brush rather than a dip coating apparatus, so that route was explored as well [31].