## 3 Results
### Net Irradiance Measurements and Evaluation  
One of the problems of using dip-coated glass slides in this setup is that both faces of the glass are covered with titania (supposedly equally dispersed). Therefore, the face in front of the bulb receives most of the light, while the rear one is irradiated with the radiation either reflected by the wall of the photoreactor (not determined in our layout) or  
Figure 5: DR–UV spectra and band gap of immobilized titania, as such and decorated with Ag nanoparticles.  
that passes through the glass. To check whether all the immobilized catalyst contributes to the photocatalyzed reaction, an apparatus was built to evaluate the portion of the light absorbed by P25 on the rear face of the glass. The setup comprised a box containing a UV light source (i.e., a UV-LED lamp, 48 W, 365 nm, Suntech Ltd., Salisbury, UK) connected to a cylinder pipe through a hole, while on the other end of the pipe, the probe of a photoradiometer was placed (Figure 6). A 26 mm cut in the middle of the pipe allowed for the insertion of glass slides into its housing, in a way that left no gap and forced the light from the lamp to pass through the coated glasses. First, the background irradiance was collected and an uncoated glass slide was inserted into the apparatus. Then, we analyzed a batch of 15 glasses coated in the same session (recipe #9) with this setup by placing them in the housing, first with one face exposed to the lamp and then with the other one. Five of these glasses were carefully cleaned on one side to estimate the impact of just one layer on the attenuation of the radiation. Table 3 reports the results as functions of the weight of the deposited layer, assuming that it was evenly distributed over the two faces. It was found that the glass itself absorbed ca. 11.6% of the incident radiation, while the one-layer tiles absorbed, on average, 76.8% of the light that passed through, with a minimum of 73.4% and a maximum of 79.5%, upon subtraction of the attenuation due to the glass itself. As expected, the absorption increased when both sides were coated, with an average loss of irradiance of 84.7% (min. 83.0%, max. 86.3%).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Slide \#** & **Side** & **Loading** & **Mean** & **Transmission** \\
**Coated** & **Coated** & **(mg)** & **Irradiance** & **(\%)** \\ \hline none & / & / & 825 & 100 \\ uncoated & 0 & 0 & 729 & 11.6 \\
1.3 & 1 & 3.6 & 85.7 & 10.4 \\
2.3 & 1 & 2.9 & 79.9 & 9.7 \\
3.3 & 1 & 3.3 & 73.8 & 9.0 \\
4.3 & 1 & 2.0 & 124 & 15.0 \\
5.3 & 1 & 2.6 & 116 & 14.0 \\
1.1 & 2 & 14.4 & 24.5 & 3.0 \\
1.2 & 2 & 7.0 & 40.7 & 4.9 \\
2.1 & 2 & 8.4 & 35.9 & 4.3 \\
2.2 & 2 & 6.4 & 17.3 & 2.1 \\
3.1 & 2 & 6.5 & 17.3 & 2.1 \\
3.2 & 2 & 4.3 & 32.8 & 4.0 \\
4.1 & 2 & 5.7 & 18.5 & 2.2 \\
4.2 & 2 & 4.2 & 44.3 & 5.4 \\
5.1 & 2 & 5.0 & 27.4 & 3.3 \\
5.2 & 2 & 3.7 & 43.8 & 5.3 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Attenuation measurements obtained after testing both single-side coated and full-coated glass slides.  
Figure 6: Setup used to measure the amount of light transmitted through the coated glasses.  
Therefore, on average, the rear face of the coated slide absorbed roughly 8% of the incident radiation. On one side, there was a waste of material that contributed only a fraction to the overall photocatalytic activity; on the other hand, this dip coating technique is very simple, and titania P25 is inexpensive. Thus, it is likely that this is still a favorable trade-off between the simplicity of the deposition technique and the inefficient usage of half of the photocatalyst. However, it should be underlined that the photoreactor is supplied with a reflective wall, and the rear face can contribute to light harvesting through direct irradiation.