# TiO2 Modified Geopolymers for the Photocatalytic Dye Decomposition
## 3 Results and Discussion
### Photocatalytic Activity  
Before the photocatalytic testing, we studied (i) the possible decomposition of RhB under the used UV illumination without the presence of photocatalysts and (ii) the possible absorption of RhB dye by the parent geopolymer material (pure or with incorporated TiO\({}_{2}\)) due to its porous structure. Both of these phenomena could significantly influence the as-measured photocatalytic activity. Figure A3A-C in Appendix A shows the change of the RhB dye absorption spectra during the illumination under different UV illumination conditions. It is evident that the RhB dye is decomposed under the illumination by UV photons generated either at \(\lambda\) = 250, 310 and 360 nm (without any filter) and 50% of the total intensity (Figure A3A) or only at \(\lambda\) = 360 nm (using a filter transmitting in the range of 320-410 nm) and 100% of the total light source intensity (Figure A3B). In both cases, RhB dye absorbance at the absorption maxima gradually decreases during illumination. After 120 min of illumination, the absorbance of the RhB dye decreases from 0.883 to 0.325 and to 0.281, respectively (Figure A1A,B, respectively). Under milder illumination conditions, with UV photons generated only at \(\lambda\) = 360 nm (using a filter transmitting in the range of 320-410 nm) and 50% of the total light source intensity, the RhB was stable (Figure A3C). Under these illumination conditions, the absorbance at the absorption maxima at \(\lambda\) = 555 nm of the RhB dye remained the same for 120 min of illumination and the RhB dye did not decompose. Therefore, only these illumination conditions were used in further photocatalytic tests. The representative change of RhB dye absorption spectra during the illumination under the above-mentioned conditions in the presence of the GP-P25 photocatalyst is shown in Figure 6A. Based on the decrease in the RhB dye absorbance at the absorption maxima (marked as \"absorption\" in Figure 6A), the RhB dye is absorbed by the porous geopolymer structure, due to the method of synthesis (polycondensation reactions) [27; 28; 30], before the photocatalytic testing during the photocatalyst dispersion. The absorbance of RhB dye at the absorption maxima (\(\lambda\) = 555 nm) decreased from 0.883 to 0.793 after 5 min of dispersion (Figure 6A). After this, the light source was switched on and the photocatalytic activity induced by UV illumination was monitored. With an increase in the illumination time, RhB dye absorbance gradually decreased. The absorbance decreased from 0.793 to 0.702 and 0.611 after 60 and 120 min of illumination, respectively (see Figure 6A and the difference between the lines \"0 min\" and \"120 min\" marked as \"as-measured photocatalytic activity\"). We continued to monitor the possible absorption of the RhB dye by the porous structure of the photocatalyst by the analysis of the identical dispersion, but without the illumination. The absorption of the RhB dye continued to proceed as the absorbance of the RhB dye decreased from 0.793 to 0.782 and 0.778 after 60 and 120 min, respectively (see Figure 6B and also Figure A3D). The photocatalytic activity X (%) was calculated according to the following equation: X (%) = (A\({}_{0}\) - A\({}_{\mathrm{t}}\))/A\({}_{0}\)\(\times\) 100, where, based on the above-mentioned experiments A\({}_{0}\), is the initial dye absorbance at the absorption maxima after 5 min of dispersion and A\({}_{\mathrm{t}}\) is the dye absorbance at the absorption maxima at the specific time of photocatalysis corrected by the RhB dye absorption by the porous geopolymer structure.