# Enhancing the Photocatalytic Activity of Immobilized TiO2 Using Laser-Micropatterned Surfaces
## 3 Results and Discussion
### Photocatalytic Activity of Different Surfaces  
Illumination of an aqueous solution of atrazine in the presence of all tested surfaces resulted in the decomposition of atrazine, whereas in their absence, the degradation was remarkably slow (Figure 7). Experiments in the absence of light showed adsorption of atrazine on the photocatalytic surfaces below 3%. Each data point in Figure 7 represents the average value of five independent experiments, carried out using five identical surfaces coated with the same photocatalytic material. Error bars correspond to the standard deviation of the five independent experiments and range from 0.007 to 0.018 \(\upmu\)mol/L, 0.007 to 0.025 \(\upmu\)mol/L, and 0.002 to 0.015 \(\upmu\)mol/L for F-Si-solgel, F-Si-P25-solgel, and M-Si-solgel, respectively. The observed rate constants, \(k_{\mathrm{obs}}\), were determined for each photocatalytic surface assuming first-order kinetics and the results are summarized in Table 2. Only two experiments were carried out for the rippled R-Si-solgel surface, because the morphology of this sample was outside the main scope of this work and it was used only for comparison purposes. The best photocatalytic activity was observed for the reference TiO\({}_{2}\) + Degussa F-Si-P25-solgel surface with a \(k_{\mathrm{obs}}\) mean value of 32.7 \(\times\) 10\({}^{-3}\) min\({}^{-1}\). This value is one order of magnitude higher than the \(k_{\mathrm{obs}}\) of the F-Si-solgel surface (4.9 \(\times\) 10\({}^{-3}\) min\({}^{-1}\)), which consists of the same substrate (flat silicon) with the pure TiO\({}_{2}\) coating. This difference demonstrates the photocatalytic efficiency of the coating alone. However, when the pure TiO\({}_{2}\) film is deposited on the microconical silicon substrate (M-Si-solgel), \(k_{\mathrm{obs}}\) increases by one order of magnitude (24.1 \(\times\) 10\({}^{-3}\) min\({}^{-1}\)) and approaches the value of the reference F-Si-P25-solgel surface. This indicates the improvement in the photocatalytic efficiency of the surface due to the 3D micro-patterned morphology of the substrate. Figure 6: Raman spectra of (**a**) F-Si-P25-solgel, (**b**) F-Si-solgel, and (**c**) M-Si-solgel. The dependence of the photocatalytic improvement on the 3D geometry of the substrate is indicated by the results obtained with the rippled R-Si-solgel surface. Here, the same pure TiO\({}_{2}\) coating is used, albeit with a smoother substrate morphology, consisting of microripples instead of microcones. The mean value of \(k_{\text{obs}}\) for this surface (7.3 \(\times\) 10\({}^{-3}\) min\({}^{-1}\)) is between those of F-Si-solgel and M-Si-solgel, all with the same coating. This remarkable result indicates that the substrate morphology is a leading parameter of the photocatalytic process. Furthermore, the sensitivity of the measurements employed in this work is high enough to distinguish between different morphologies of the same length scale. The approximately threefold improvement of \(k_{\text{obs}}\) for the M-Si-solgel compared with the R-Si-solgel is in agreement with the approximately threefold increase in the height of the microcones (29 \(\  
\(k_{\rm obs}\) on the size of the microstructure. Therefore, the change in the specific surface area of the substrates is reflected in the change in the photocatalytic efficiency of the samples. In order to investigate the effect of the substrate morphology on the TiO\({}_{2}\) + Degussa P25 film photocatalytic activity, we carried out photocatalytic measurements on microconical silicon substrates coated with the composite TiO\({}_{2}\) + Degussa P25 film. These surfaces exhibit a mean \(k_{\rm obs}\) equal to (38.5 \(\pm\) 5.7) \(\times\) 10\({}^{-3}\) min\({}^{-1}\), which is similar to the \(k_{\rm obs}\) value for TiO\({}_{2}\) + Degussa P25 deposited on flat silicon (F-Si-P25-solgel), indicating that surface modification has no significant effect on their performance, unlike the results for the pure TiO\({}_{2}\) films.