3.3. Abatement of Rhodamine B

The Rhodamine B degradation mechanism passes through two dierent pathways; the first being the deethylation process and the second being the destruction of the chromophore structure [21]. Such a dual mechanism seems to depend on the nature of the light in the photoinduction. Under UV-Vis conditions, there is no selectivity and the destruction of the chromophore is also involved during the degradation. [22]. Additionally, the results obtained during discoloration of the samples are observed in Figure 3, where ǻ*a*\* is defined as the change in coordinate *a*\* during the exposure time of the samples to UV light; being the initial hue at *t* = 0 and the final coordinate of the same sample at time t as shown below:

$$
\Delta a^\* = a\_0^\* - a\_t^\* \tag{4}
$$

In this case, the coordinate *a*\* corresponds to a reddish hue for positive values, according the color-order system specified by the Commission Internationale de l'Eclairage [20]. The highest photoactivity was observed for samples with a 3% addition of TiO2í*x*N*y* and TiO2, as similarly reported by other works [10,11]. The control samples were also evaluated, to determine the catalytic reactions by photolysis or by temperature.

**Figure 3.** Change in ǻ*a*\* during five hour exposure to UV light with different addition percentages of TiO2í*x*N*y* and TiO2 for samples at 65 h of curing age.

There is a difference between the specimens with 3% of TiO2 facing the other samples was observed, and the values of 0.5% of TiO2í*x*N*y* were similar to the samples with 1% of TiO2. It is noted that 0.5% of TiO2í*x*N*y* showed a more significant change than 1% of TiO2í*x*N*y*; this behavior can be explained in two ways. The first explanation is that the action of the hydrated products forms a layer around nano- TiO2í*x*N*y*, which protects nanoparticles from abrasion and may decrease the photocatalytic activity [23]. Here, the nanoparticles act as nucleation sites in cement hydration increasing the accumulation of hydration products [24]. The second way is that the decrease in photocatalytic activity is explained by re-agglomeration of nanoparticles in the matrix of the cement as a consequence of low dispersion. This occurs also in the samples with 3% of TiO2í*x*N*y*, but the generation of active sites seems to be a more important factor in this case; therefore, the abatement of Rhodamine B is improved. Even with this information, it is necessary to find other evidence, such as the generation of hydration products and the relationship of them with the content of nanoparticles in the cement and the photoactivity, in future works in order to ratify this hypothesis.

In order to determine a percentage of decrease of color, the photocatalytic efficiency coefficient (ഌ) was calculated and is defined as:

$$\mathfrak{E} = \frac{A(a\_0^\*) - A(a^\*)}{A(a\_0^\*)} \times 100\tag{5}$$

where *A*(*a*0\*) corresponds to an ideal state in which the initial coordinate remains constant during *t*f time, as established by Equation (6), and *A*(*a*\*) the area under the real curve of *a*\*, as observed in Equation (7) [10].

$$A(a\_0^\*) = t\_f \times a\_0^\* \tag{6}$$

$$A(a^\*) = \int\_{t\_0}^{t\_f} a^\* \mathrm{d}t \tag{7}$$

Figure 4 shows the photocatalytic efficiency coefficients of the samples (cured 65 h) after 5 h of UV irradiation. Coefficients greater than those obtained for control samples (0%), are considered photocatalytic reactions and are products of the interaction between photons and the nanoparticles of photocatalyst. Otherwise, these coefficients are considered as other kinds of interactions, such as photolysis or thermolysis. It is noted, that all the additions showed photocatalytic activity and efficiency above the standard sample. In addition, a similar coefficient between the samples with 0.5% of TiO2í*x*N*y* and 1% TiO2í*x*N*y* was observed, different to results obtained in another research [11]. As explained above, this could be due to the re-aglomeration effect of the nanoparticles in the matrix cementitious. Similarly, a uniform performance of TiO2 3% was observed for TiO2í*x*N*y* 3%.

**Figure 4.** Photocatalytic efficiency Coefficient (İ) for samples in UV light with different addition percentages of TiO2í*x*N*y* and TiO2 for samples at 65 h of curing age, control sample, TiO2í*x*N*y*, TiO2. Error bars are standard deviation.

Some samples were evaluated as well in visible light. In Figure 5, the comparison between samples of TiO2í*x*N*y* with 0.5%, 1%, 3% and TiO2 with 1%, 3% for the change in ǻ*a*\* *versus*  exposure time is shown. It is noted that TiO2 did not show photoactivity in visible light, mainly because of its wide band gap that only lets the electrons jump to the conduction band by stimuli of wavelengths below 400 nm.

Additionally, the photocatalytic efficiency coefficient for these samples was calculated. In Figure 6, the comparison between coefficients for samples with 0.5%, 1%, 3% of TiO2í*x*N*y*, and 1%, 3% of TiO2, including control samples are reported. TiO2 samples obtained values below the samples with 0%. The minimal change in *a*\* is attributed to other mechanisms, not to photocatalytic reactions. When percentages equal or below are obtained in comparison with the control sample (0%), there is no evidence of photocatalytic interactions; therefore, the change in hue is attributed to photolysis and thermolysis reactions. It is possible that TiO2 nanoparticles create a shielding effect in the samples, blocking photonic absorption in the pigment and preventing photolysis reactions.

**Figure 5.** Change in ǻ*a*\* during 5 h exposure to visible light with different addition percentages of TiO2í*x*N*y* and TiO2 for samples at 65 h of curing age.


**Table 2.** Standard deviation of samples with TiO2í*x*N*y* in visible light.


**Table 3.** Standard deviation of samples with TiO2 in visible light.

The tables above show some data of the samples evaluated in visible light. Standard deviation was calculated for *a*\* coordinate in all the samples. At the same time, the ǻ*E* was calculated. This represents the change in color that is defined as the difference between two points of values, in the Euclidean sense [20]. The higher the ǻ*E*, the higher the change in color. ǻ*E* is defined as:

$$
\Delta E = \sqrt{(L\_0^\*-L\_t^\*)^2 + (a\_0^\*-a\_t^\*)^2 + (b\_0^\*-b\_t^\*)^2} \tag{8}
$$

It is noteworthy that the band gap of TiO2 corresponds to 3.2 eV per atom; therefore, to do the electronic jump from the valence band to the conduction band and generate redox species, it is necessary to induce the particles and a suitable stimulus in terms of light or energy photons located in the range of near ultraviolet (UVA). On the other side, the inclusion of nitrogen atoms in the structure of TiO2, as mentioned above, causes a decrease in the band gap, achieving the previously stated electronic jump more easily. In this form, it is possible to have photocatalytic activity in wavelengths between 400 nm and 700 nm. This can be observed in specimens with 3% of TiO2í*x*N*<sup>y</sup>* that showed a percentage above the others. Samples with a low percentage of TiO2í*x*N*y* showed low photoactivity, possibly due to the characteristics of the nanoparticles used. It is possible that commercial nano-TiO2í*x*N*y* has a low doping degree of nitrogen and, for this reason, it requires high percentages in order to improve the efficiency. These results show that it is possible to obtain cements photoactivated with a self-cleaning property in visible light, but it is necessary to evaluate them in other conditions.

**Figure 6.** Photocatalytic efficiency Coefficient (İ) for samples in Visible light with 1%, 3% TiO2í*x*N*y* and 1%, 3% TiO2 for samples at 65 h of curing age, control sample, TiO2í*x*N*y*, TiO2. Errors bars are standard deviation.
