# TiO2 Modified Geopolymers for the Photocatalytic Dye Decomposition
## 3 Results and Discussion
### Photocatalytic Activity
Figure 8B shows the time dependence of photocatalytic activity (X) for photocatalysts incorporated into the parent geopolymer matrix with a comparable content of TiO\({}_{2}\) powder (\(\approx\)3.2 wt.%, see Table 3), labelled as GP-P25, GP-CG100 and GP-CG300. For the possible schematic mechanism of RhB dye decomposition on TiO\({}_{2}\) modified metakaolin-based geopolymer see, e.g., [59; 62; 63]. All materials were milled to a comparable particle size (d\({}_{50}\)) before testing (see Table A5). Furthermore, the photocatalysts with the higher (\(\approx\)6.0 wt.%) and lower (\(\approx\)1.5 wt.%) content of TiO\({}_{2}\) incorporated powder were also tested (not shown in Tables and Figures). However, as it did not have a positive effect, with the respect to the TiO\({}_{2}\) cost, on the photocatalytic activity (e.g., the X value is equal to 10.5, 18.0 and 20.0% for GP-P25 photocatalyst with \(\approx\)1.7, 3.3 and 6.0 wt.% of TiO\({}_{2}\), respectively), the photocatalysts containing \(\approx\)3.2 wt.% of different TiO\({}_{2}\)sources were compared. The time dependence of X for GP-P25 and GP-CG100 photocatalysts is almost comparable (Figure 8B). After 120 min of illumination, the photocatalytic activity reaches 19.5 and 18.0% for the GP-CG100 and GP-P25 photocatalysts, respectively. Evidently, the difference in X between the pure, industrially produced TiO\({}_{2}\) P25 and CG100 photocatalysts (Figure 8A) were eliminated. Furthermore, the X dropped to 11.0% for the GP-CG300 photocatalyst after 120 min of illumination (Figure 8B). The different photocatalytic activities of the photocatalysts incorporated into the parent geopolymer matrix may be explained as follows: Firstly, the lowest X for the GP-CG300 photocatalyst could be the consequence of the above-mentioned fact that (see Figure 5 and also Table 4) this was the only incorporated photocatalyst not to exhibit an increase in the anatase diffraction lines in comparison with the parent geopolymer matrix. In the GP-CG300 photocatalyst, titanium is thus present in a form other than anatase. To support this idea, we performed FTIR measurements. Figure 9 shows a comparison of FTIR spectra for pure CG300 and CG100 photocatalysts and those incorporated into the parent geopolymer matrix. We compared the results for all the samples in the spectral region of 1300-800 cm\({}^{-1}\), where the pure TiO\({}_{2}\) samples provided the absorption and the absorption of the parent geopolymer matrix in the incorporated photocatalysts was subtracted. The spectra of the pure CG100 photocatalyst and the incorporated photocatalyst GP-CG100 are practically identical. On the other hand, the spectra of the pure CG300 photocatalyst and the incorporated photocatalyst GP-CG300 are slightly different. It is possible to see the broadening for GP-CG300 in comparison with the spectra of the CG300 photocatalyst, which indicates the loss of ordering in the  
Figure 8: The time dependence of corrected photocatalytic activity (X) for (**A**) the parent geopolymer matrix (GP), the mechanical mixture (MM-GP-P25)) and the pure TiO\({}_{2}\) photocatalysts (P25, CG100 and CG300) and (**B**) those incorporated into the geopolymer matrix. The lines are included as a mere eye guide. first case. This corroborates the results obtained by the XRD analysis (see Figure 5 and Table 4). Secondly, the different photocatalytic activities of the incorporated photocatalysts could also be caused by the fact that the average particle size (see Table A4) of the pure CG300 was the lowest of the pure TiO\({}_{2}\) photocatalysts. As stated above, this pure TiO\({}_{2}\) photocatalyst could thus be incorporated into the porous geopolymer structure much more easily than the other pure TiO\({}_{2}\) photocatalysts, and thus the support covers the TiO\({}_{2}\) particles, see, e.g., [16; 17]. We assume that covered TiO\({}_{2}\) particles are unavailable for the photocatalytic reaction.