Rhodamine B Photodegradation in Aqueous Solutions Containing Nitrogen Doped TiO2 and Carbon Nanotubes Composites

In this work, new results concerning the potential of mixtures based on nitrogen doped titanium dioxide (TiO2:N) and carbon nanotubes (CNTs) as possible catalyst candidates for the rhodamine B (RhB) UV photodegradation are reported. The RhB photodegradation was evaluated by UV–VIS absorption spectroscopy using samples of TiO2:N and CNTs of the type of single-walled carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWNTs), multi-wall carbon nanotubes (MWNTs), and single-walled carbon nanotubes functionalized with carboxyl groups (SWNT-COOH) having various concentrations of CNTs. The best photocatalytic performance was obtained for sample containing TiO2:N and 2.5 wt.% SWNTs-COOH, when approx. 85% of dye removal was achieved after 300 min. of UV irradiation. The reaction kinetics of RhB aqueous solutions containing TiO2:N/CNT mixtures followed a complex first-order kinetic model. The TiO2:N/CNTs catalyst induced higher photodegradation efficiency of RhB than TiO2:N due to the presence of CNTs, which act as adsorbent and dispersing agent and capture the photogenerated electrons of TiO2:N hindering the electron–hole recombination.


Introduction
Titanium dioxide (TiO 2 ) is a wide bandgap semiconducting material (3.0, 3.2, and 3.5 eV for rutile, anatase, and brookite phases, respectively) that has gained special attention due to its nontoxicity, and chemical and physical stability [1,2]. TiO 2 is the most used n-type semiconductor in different environmental applications and energy fields, such as photocatalysis and dye-sensitized solar cells [3]. Great efforts have been made to solve the water pollution issues; therefore, the main objective is focused on the removal of dyes from wastewater [4]. In order to remove the pollutants, the methods used involve biodegradation, adsorption, and membrane filtering [5]. A practical method for the removal of pollutants considers the use of sunlight for photodegradation of dyes, and the main process involves advanced oxidation. The principal mechanism of advanced oxidation implies the generation of hydroxyl radicals, which are responsible for the degradation of organic pollutants. The ability of materials to be photoactivated by sunlight implies a good knowledge of the charge transport processes that take place. Thus, the charge transfer induced by UV irradiation has been studied using various experimental techniques such as photoluminescence, theoretical modeling, etc.  Table 1.  Table 1. The SEM analysis highlights in Figure 2 the TiO 2 :N irregular platelets, while in the case of the S2, D, M, and SC samples, both TiO 2 :N platelets and one-dimensional structures corresponding to the bundles of SWNTs, DWNTs, MWNTs, and SWNTs-COOH were observed. Figure 3 shows the TEM images of the S2, D, M, and SC samples as well as the size distribution of TiO 2 :N nanoparticles in the S2, D, M, and SC samples. For statistical purposes, 100 particles were measured for each sample. Mean sizes of TiO 2 :N nanoparticles in the S2, D, M, and SC samples were equal to 28.4 nm, 28.81 nm, 26.11 nm, and 26.43 nm, respectively. The minimum and maximum sizes, respectively, of the TiO 2 :N nanoparticles in the case of the samples labeled were as follows: (i) S2, 10 The SEM analysis highlights in Figure 2 the TiO2:N irregular platelets, while in the case of the S2, D, M, and SC samples, both TiO2:N platelets and one-dimensional structures corresponding to the bundles of SWNTs, DWNTs, MWNTs, and SWNTs-COOH were observed. Figure 3 shows the TEM images of the S2, D, M, and SC samples as well as the size distribution of TiO2:N nanoparticles in the S2, D, M, and SC samples. For statistical purposes, 100 particles were measured for each sample. Mean sizes of TiO2:N nanoparticles in the S2, D, M, and SC samples were equal to 28.4 nm, 28.81 nm, 26.11 nm, and 26.43 nm, respectively. The minimum and maximum sizes, respectively, of the TiO2:N nanoparticles in the case of the samples labeled were as follows: (i) S2, 10      In order to confirm if a chemical adsorption took place between TiO 2 :N nanoparticles and carbon nanotubes of the types SWNTs, DWNTs, MWNTs, and SWNTs-COOH, Figures 4 and 5 show the Raman spectra of SWNTs, DWNTs, MWNTs, and SWNTs-COOH and the S2, D, M, and SC samples, recorded at the excitation wavelengths of 1064 and 633 nm, resonant for semiconductor and metallic tubes, respectively. In the case of a physical adsorption of carbon nanotubes on the surface of TiO 2 :N nanoparticles, in the Raman spectra of the TiO 2 :N/carbon nanotube composites, a sum effect of the Raman lines of the two constituents should be observed. According to the changes reported below, the changes in position and intensity of the Raman lines of the two constituents, i.e., TiO 2 :N and carbon nanotubes, indicate a chemical adsorption of carbon nanotubes on the surface of TiO 2 :N nanoparticles. According to Figure 4, the main Raman lines of the carbon nanotubes are localized in the following spectral ranges: (i) 100-350 cm −1 , which are assigned to the radial breathing mode [25]; the maxima of Raman lines are peaked in the cases of SWNTs, DWNTs, and SWNTs-COOH at 164 cm −1 , 179 cm −1 -264 cm −1 -328 cm −1 , and 166 cm −1 , respectively; these Raman lines were often used to calculate the diameter of carbon nanotubes [25]. In this context, using (a) the equation ν (cm −1 ) = 248/d (nm) [25], the diameters of the SWNTs and SWNTs-COOH samples are calculated to be 1.  In order to confirm if a chemical adsorption took place between TiO2:N nanoparticles  and carbon nanotubes of the types SWNTs, DWNTs, MWNTs, and SWNTs-COOH, Figures 4 and 5 show the Raman spectra of SWNTs, DWNTs, MWNTs, and SWNTs-COOH and the S2, D, M, and SC samples, recorded at the excitation wavelengths of 1064 and 633 nm, resonant for semiconductor and metallic tubes, respectively. In the case of a physical adsorption of carbon nanotubes on the surface of TiO2:N nanoparticles, in the Raman spectra of the TiO2:N/carbon nanotube composites, a sum effect of the Raman lines of the two constituents should be observed. According to the changes reported below, the changes in position and intensity of the Raman lines of the two constituents, i.e., TiO2:N and carbon nanotubes, indicate a chemical adsorption of carbon nanotubes on the surface of TiO2:N nanoparticles. According to Figure 4, the main Raman lines of the carbon nanotubes are localized in the following spectral ranges: (i) 100-350 cm −1 , which are assigned to the radial breathing mode [25]; the maxima of Raman lines are peaked in the cases of SWNTs, DWNTs, and SWNTs-COOH at 164 cm −1 , 179 cm −1 -264 cm −1 -328 cm −1 , and 166 cm −1 , respectively; these Raman lines were often used to calculate the diameter of carbon nanotubes [25]. In this context, using (a) the equation ν (cm −1 ) = 248/d (nm) [25], the diameters of the SWNTs and SWNTs-COOH samples are calculated to be 1. band, these being assigned to the disorder state or the defects in the graphitic lattice of carbon nanotubes and the tangential mode, respectively [25]. In the case of SWNTs, DWNTs, MWNTs, and SWNTs-COOH, the maximum of the D band is peaked at 1269, 1279, 1288, and 1281 cm −1 , respectively, while the maximum of the TM band is situated at 1593, 1587, 1600, and 1593 cm −1 , respectively.   Raman lines of the TiO2:N nanoparticles peaked at 144, 395, 515, and 636 cm −1 , being assigned to the Eg(1), B1g(1), A1g, and Eg(3) vibrational modes [27,28,29]. The following changes are remarked in the following cases: (i) S2 sample, a down-shift of the Raman lines from 164 cm −1 and 1269 cm −1 (Figure 4a (Figure 4h); the Raman line at 149 cm −1 belongs to the TiO2:N nanoparticles. These variations indicate in the case of (i) the D and M samples, a higher disorder state in the graphitic lattice of semiconducting tubes highlighted by the increase in the intensity of the D band, as a consequence of adsorption of carbon nanotubes onto the TiO2:N nanoparticles surface; and (ii) the S2 and SC samples, a compression induced to semiconducting carbon nanotubes [30] in the presence of the TiO2:N nanoparticles evidenced by the down-shift of the D band.
The Raman spectra ( Figure 5) highlights in the case of (i) SWNTs, a line with the maximum at 161 cm −1 belonging to the RBM vibrational mode (characteristic of the diameter of 1.54 nm), a D band at 1318 cm −1 assigned to the disorder state or defects induced to the graphitic lattice of carbon nanotubes, a TM band with the maximum at 1590 cm −1 having an asymmetric profile of Breit-Wigner-Fano (BWF) type at ca. 1543 cm −1 , as well as two lines at 2624 and 3159 cm −1 , belonging to the vibrational mode of second order of the D and TM bands (2D and 2TM) [25]. The origin of BWF was reported to have the origin in the electron-phonon interaction [31]; (ii) DWNTs, two Raman lines belonging to the RBM vibrational modes of the inner/external diameter of nanotubes equal to 1.53/2.19 nm and 1.07/1.73 nm, which are peaked at 159 cm −1 and 219 cm −1 , respectively, the D and TM bands peaked at 1320 cm −1 and 1586 cm −1 , respectively, and last but not least the 2D and 2TM bands at 2635 cm −1 and 3166 cm −1 , respectively; (iii) MWNTs, the D, TM, 2D, D + M, and 2TM bands are peaked at 1323 cm −1 , 1586 cm −1 , 2653 cm −1 , 2913 cm −1 , and 3186 cm −1 , respectively; and (iv) SWNTs-COOH, the RBM, D, TM, 2D, and 2TM bands are localized at 163 cm −1 (tube diameter being equal to 1.52 nm), 1337 cm −1 , 1580-1600 cm −1 , 2666 cm −1 , and 3191 cm −1 , respectively. Figure 5i highlights for TiO2:N nanoparticles, four Raman lines peaked at 143 cm −1 , 395 cm −1 , 514 cm −1 , and 637 cm −1 . As seen in Figure 5     Raman lines of the TiO 2 :N nanoparticles peaked at 144, 395, 515, and 636 cm −1 , being assigned to the E g(1) , B 1g(1) , A 1g , and E g (3) vibrational modes [27][28][29]. The following changes are remarked in the following cases: (i) S2 sample, a down-shift of the Raman These variations indicate in the case of (i) the D and M samples, a higher disorder state in the graphitic lattice of semiconducting tubes highlighted by the increase in the intensity of the D band, as a consequence of adsorption of carbon nanotubes onto the TiO 2 :N nanoparticles surface; and (ii) the S2 and SC samples, a compression induced to semiconducting carbon nanotubes [30] in the presence of the TiO 2 :N nanoparticles evidenced by the down-shift of the D band.
The Raman spectra ( Figure 5) highlights in the case of (i) SWNTs, a line with the maximum at 161 cm −1 belonging to the RBM vibrational mode (characteristic of the diameter of 1.54 nm), a D band at 1318 cm −1 assigned to the disorder state or defects induced to the graphitic lattice of carbon nanotubes, a TM band with the maximum at 1590 cm −1 having an asymmetric profile of Breit-Wigner-Fano (BWF) type at ca. 1543 cm −1 , as well as two lines at 2624 and 3159 cm −1 , belonging to the vibrational mode of second order of the D and TM bands (2D and 2TM) [25]. The origin of BWF was reported to have the origin in the electron-phonon interaction [31]; (ii) DWNTs, two Raman lines belonging to the RBM vibrational modes of the inner/external diameter of nanotubes equal to 1.53/2.19 nm and 1.07/1.73 nm, which are peaked at 159 cm −1 and 219 cm −1 , respectively, the D and TM bands peaked at 1320 cm −1 and 1586 cm −1 , respectively, and last but not least the 2D and 2TM bands at 2635 cm −1 and 3166 cm −1 , respectively; (iii) MWNTs, the D, TM, 2D, D + M, and 2TM bands are peaked at 1323 cm −1 , 1586 cm −1 , 2653 cm −1 , 2913 cm −1 , and 3186 cm −1 , respectively; and (iv) SWNTs-COOH, the RBM, D, TM, 2D, and 2TM bands are localized at 163 cm −1 (tube diameter being equal to 1.52 nm), 1337 cm −1 , 1580-1600 cm −1 , 2666 cm −1 , and 3191 cm −1 , respectively. Figure 5i highlights for TiO 2 :N nanoparticles, four Raman lines peaked at 143 cm −1 , 395 cm −1 , 514 cm −1 , and 637 cm −1 . As seen in Figure

Photocatalytic Properties of the TiO 2 :N/CNTs Samples
Photocatalytic activity of the TiO 2 :N and TiO 2 :N/CNTs samples was investigated through RhB photodegradation. Photocatalytic reactions on TiO 2 :N surfaces were described using the Langmuir-Hinshelwood model. All the degradation experiments of RhB performed in this study followed pseudo-first-order kinetics [32]. Effects of SWNT amounts on photocatalytic degradation efficiency were investigated using the TiO 2 :N/SWNT samples with CNT concentrations between 0.25 wt.% and 5 wt.% (samples S1-S3) to optimize the best catalytic system for the degradation of RhB. Degradation efficiency was calculated from the absorption UV-VIS spectra after the exposure to UV light for 300 min, using the following equation: where A t and A init are the absorbance values of the analyzed sample corresponding to irradiation times t = 10-300 min and t = 0 min, respectively. The degradation rate constant of RhB was calculated using the following expression: where A t and A init are the absorbance values of the analyzed sample at t = 10-300 min and t = 0 min, respectively, and k represents the degradation rate constant of the reaction at t moment of time.

The Dependence of the RhB Photodegradation on CNT Concentrations in
TiO 2 :N/SWNT Samples Figure 6 illustrates the degradation efficiency of RhB for the T and S1-S3 samples as a function of UV light irradiation time, calculated using Equation (1). The obtained results indicate that the best efficiency was depicted in the case of sample S2 when the RhB was approx. 70% decomposed after the exposure to UV light, and a time of 300 min. The presence of SWNTs in the catalyst solution increased the photocatalytic activity, as observed in plots shown in Figure 6. A better analysis of Figure 6 indicates that the degradation efficiency of RhB increased with the SWNT concentration up to 2.5 wt.% CNTs (red and green curves), but at higher SWNTs concentration (5 wt.%) the degradation efficiency decreased (blue curve). As observed in Figure 6, the addition of SWNTs improved the TiO 2 :N photocatalytic activity, the values of the rate constant k certifying again this hypothesis.

Photocatalytic Properties of the TiO2:N/CNTs Samples
Photocatalytic activity of the TiO2:N and TiO2:N/CNTs samples was investigated through RhB photodegradation. Photocatalytic reactions on TiO2:N surfaces were described using the Langmuir-Hinshelwood model. All the degradation experiments of RhB performed in this study followed pseudo-first-order kinetics [32]. Effects of SWNT amounts on photocatalytic degradation efficiency were investigated using the TiO2:N/SWNT samples with CNT concentrations between 0.25 wt.% and 5 wt.% (samples S1-S3) to optimize the best catalytic system for the degradation of RhB. Degradation efficiency was calculated from the absorption UV-VIS spectra after the exposure to UV light for 300 min, using the following equation: where and are the absorbance values of the analyzed sample corresponding to irradiation times t = 10-300 min and t = 0 min, respectively.
The degradation rate constant of RhB was calculated using the following expression: where and are the absorbance values of the analyzed sample at t = 10-300 min and t = 0 min, respectively, and k represents the degradation rate constant of the reaction at t moment of time.

The Dependence of the RhB Photodegradation on CNT Concentrations in
TiO2:N/SWNT Samples Figure 6 illustrates the degradation efficiency of RhB for the T and S1-S3 samples as a function of UV light irradiation time, calculated using Equation (1). The obtained results indicate that the best efficiency was depicted in the case of sample S2 when the RhB was approx. 70% decomposed after the exposure to UV light, and a time of 300 min. The presence of SWNTs in the catalyst solution increased the photocatalytic activity, as observed in plots shown in Figure 6. A better analysis of Figure 6 indicates that the degradation efficiency of RhB increased with the SWNT concentration up to 2.5 wt.% CNTs (red and green curves), but at higher SWNTs concentration (5 wt.%) the degradation efficiency decreased (blue curve). As observed in Figure 6, the addition of SWNTs improved the TiO2:N photocatalytic activity, the values of the rate constant k certifying again this hypothesis.   The reaction rate constant values for each sample ( , , and ) and the corresponding linear regression coefficients ( , , and ) are presented in Table 2.  Table 2, it can be observed that the rate constants increased with SWNT concentration up to the k value of 0.0055 min −1 in the case of 2.5 wt.% SWNTs, which was due to a higher loading of carbon nanotubes onto the TiO2:N nanoparticle surface. By increasing SWNT concentration to 5 wt.% in the TiO2:N/SWNT composite, the rate constant k decreased, the value of this constant being superior to that reported in the case of the TiO2:N nanoparticles. The linear regression coefficients had values ranging from 0.7059 to 0.9958, which indicate that RhB photodegradation took place, in agreement with the firstorder kinetics. The fastest photodegradation was obtained in the case of Sample S2. The presence of the three linear regions in Figure 7a can be explained if we consider that the RhB photocatalytic degradation was made in three steps according to the following stages shown in Scheme 1 [33,34]: Scheme 1. The stages of the RhB photocatalytic degradation.
In the first step, RhB was absorbed on the TiO2:N or TiO2/SWNT sample surface, and after the exposure to UV light the organic dye was degraded to intermediate products, The reaction rate constant values for each sample (k 1 , k 2 , and k 3 ) and the corresponding linear regression coefficients (R 2 1 , R 2 2 , and R 2 3 ) are presented in Table 2. From Table 2, it can be observed that the rate constants increased with SWNT concentration up to the k value of 0.0055 min −1 in the case of 2.5 wt.% SWNTs, which was due to a higher loading of carbon nanotubes onto the TiO 2 :N nanoparticle surface. By increasing SWNT concentration to 5 wt.% in the TiO 2 :N/SWNT composite, the rate constant k decreased, the value of this constant being superior to that reported in the case of the TiO 2 :N nanoparticles. The linear regression coefficients had values ranging from 0.7059 to 0.9958, which indicate that RhB photodegradation took place, in agreement with the first-order kinetics. The fastest photodegradation was obtained in the case of Sample S2. The presence of the three linear regions in Figure 7a can be explained if we consider that the RhB photocatalytic degradation was made in three steps according to the following stages shown in Scheme 1 [33,34]: Scheme 1. The stages of the RhB photocatalytic degradation.
In the first step, RhB was absorbed on the TiO 2 :N or TiO 2 /SWNT sample surface, and after the exposure to UV light the organic dye was degraded to intermediate products, with rate constant k 1 . The intermediate products were transformed in faded products having the rate constant k 2 . In the final step the values of k 3 rate constant indicated the saturation of the sample surfaces. The increase of RhB degradation efficiency when SWNTs were added to TiO 2 :N can be attributed to faster electron-hole recombination with increases of the SWNT concentration in the TiO 2 :N/SWNT mixture [21]. A higher SWNT concentration can block the irradiation and obstruct charge carrier generation; in addition, SWNTs can form aggregates due to van der Waals interactions between individual tubes and act as a screening agent for TiO 2 :N, blocking its exposure to UV light. [21]. This will reduce the production of reactive oxygen species, which are essential for RhB degradation. If the SWNT concentration is too high (5 wt.%), TiO 2 :N particles are insufficiently covered, and therefore photons are absorbed and scattered by SWNTs, inducing the decrease of the sample photoactivity. Figure 8 illustrates the UV-VIS absorption spectra of the T and S1-S3 samples dispersed in RhB aqueous solution, before exposure to UV light in which approximately similar values of absorbance were obtained. This fact can be explained based on the hypothesis that a similar amount of photons are absorbed regardless of SWNT concentration in the TiO 2 :N/SWNTs samples, which verifies the conclusion illustrated in the above discussion that SWNTs improve the RhB photocatalytic degradation. The absorption band situated at 554 nm corresponds to π-π* transitions of RhB [35]. were added to TiO2:N can be attributed to faster electron-hole recombination with increases of the SWNT concentration in the TiO2:N/SWNT mixture [21]. A higher SWNT concentration can block the irradiation and obstruct charge carrier generation; in addition, SWNTs can form aggregates due to van der Waals interactions between individual tubes and act as a screening agent for TiO2:N, blocking its exposure to UV light. [21]. This will reduce the production of reactive oxygen species, which are essential for RhB degradation.
If the SWNT concentration is too high (5 wt.%), TiO2:N particles are insufficiently covered, and therefore photons are absorbed and scattered by SWNTs, inducing the decrease of the sample photoactivity. Figure 8 illustrates the UV-VIS absorption spectra of the T and S1-S3 samples dispersed in RhB aqueous solution, before exposure to UV light in which approximately similar values of absorbance were obtained. This fact can be explained based on the hypothesis that a similar amount of photons are absorbed regardless of SWNT concentration in the TiO2:N/SWNTs samples, which verifies the conclusion illustrated in the above discussion that SWNTs improve the RhB photocatalytic degradation. The absorption band situated at 554 nm corresponds to π-π* transitions of RhB [35].  were added to TiO2:N can be attributed to faster electron-hole recombination with increases of the SWNT concentration in the TiO2:N/SWNT mixture [21]. A higher SWNT concentration can block the irradiation and obstruct charge carrier generation; in addition, SWNTs can form aggregates due to van der Waals interactions between individual tubes and act as a screening agent for TiO2:N, blocking its exposure to UV light. [21]. This will reduce the production of reactive oxygen species, which are essential for RhB degradation.
If the SWNT concentration is too high (5 wt.%), TiO2:N particles are insufficiently covered, and therefore photons are absorbed and scattered by SWNTs, inducing the decrease of the sample photoactivity. Figure 8 illustrates the UV-VIS absorption spectra of the T and S1-S3 samples dispersed in RhB aqueous solution, before exposure to UV light in which approximately similar values of absorbance were obtained. This fact can be explained based on the hypothesis that a similar amount of photons are absorbed regardless of SWNT concentration in the TiO2:N/SWNTs samples, which verifies the conclusion illustrated in the above discussion that SWNTs improve the RhB photocatalytic degradation. The absorption band situated at 554 nm corresponds to π-π* transitions of RhB [35].   The detailed evolution of RhB degradation in aqueous solutions containing the T, S2, D, and M samples after the subsequent exposure to UV light and the degradation efficiency of RhB depending on the irradiation time is illustrated in Figure 11. To further the analysis of RhB degradation efficiency in aqueous solutions containing the T, S2, D, and M samples, respectively, using Equation (1), the plots in Figure 11(a2-d2) were obtained. The detailed evolution of RhB degradation in aqueous solutions containing the T, S2, D, and M samples after the subsequent exposure to UV light and the degradation efficiency of RhB depending on the irradiation time is illustrated in Figure 11. To further the analysis of RhB degradation efficiency in aqueous solutions containing the T, S2, D, and M samples, respectively, using Equation (1), the plots in Figure 11(a 2 -d 2 ) were obtained.  Figure 10a,b demonstrates that the origin of the band localized in the spectral range 200-400 nm is in the adsorption of RhB onto the DWNs surface, when the π-π* bonds are established between the benzene rings of RhB and graphitic structure of carbon nanotubes. The detailed evolution of RhB degradation in aqueous solutions containing the T, S2, D, and M samples after the subsequent exposure to UV light and the degradation efficiency of RhB depending on the irradiation time is illustrated in Figure 11. To further the analysis of RhB degradation efficiency in aqueous solutions containing the T, S2, D, and M samples, respectively, using Equation (1), the plots in Figure 11(a2-d2) were obtained. RhB photodegradation in aqueous solutions containing the S2, D, and M samples could be analyzed based on the values obtained for reaction constants k1, k2, and k3, respectively. From Table 3, one can observe various rate constant values when the colloidal suspensions contained different types of carbon nanotubes. Taking into account the RhB photocatalytic degradation steps invoked in Scheme 1, the three-step model could also be applied in the case of the S2, D, and M samples. As described before k1 indicates the rate constant value assigned to the formation of intermediate products; therefore, from Table 3, we can observe that the highest value was found in the case of the TiO2:N/MWNT sample,  RhB photodegradation in aqueous solutions containing the S2, D, and M samples could be analyzed based on the values obtained for reaction constants k 1 , k 2 , and k 3 , respectively. From Table 3, one can observe various rate constant values when the colloidal suspensions contained different types of carbon nanotubes. Taking into account the RhB photocatalytic degradation steps invoked in Scheme 1, the three-step model could also be applied in the case of the S2, D, and M samples. As described before k 1 indicates the rate constant value assigned to the formation of intermediate products; therefore, from Table 3, we can observe that the highest value was found in the case of the TiO 2 :N/MWNT sample, indicating that a faster process in the first stage takes place. This result may be due to the presence of MWNTs, which provide a higher specific interfacial area for photon absorption so that inner and outer walls can both absorb oxygen in solutions [22]. k 2 corresponds to the solution discoloration, and the corresponding values illustrated a higher discoloration capacity in the case of samples based on TiO 2 :N/DWNT and TiO 2 :N/MWNT composites. The third step of the RhB photodegradation mechanism was deduced by the k 3 rate values, indicating the saturation of final products. Additionally, k 3 rate values presented in Table 3 indicate that the saturation process took place in similar conditions for all the samples. Table 3 highlights the rate constants superior to that reported for the TiO 2 :N nanoparticles (T sample in Table 2), the k 1 value ranging from 0.00364 min −1 (T sample) to 0.0055 min −1 (S2 sample), 0.0046 min −1 (D sample), and 0.0084 min −1 (M sample), the result which indicates the highest loading of carbon nanotubes on the surface of TiO 2 :N nanoparticles occurred in the case of the M sample. As shown in the following, noticeable results were obtained in the case of the TiO 2 :N/ SWNTs-COOH samples. The exposure to UV light for 300 min of the RhB aqueous solution containing TiO 2 :N and 2.5 wt.% SWNTs-COOH implied significant modification in the UV-VIS spectra illustrated in Figure 12a: (i) a down-shift of the band of RhB from 551 nm (t = 0 min) to 497 nm (t = 300 min); (ii) the disappearance of the band shoulder situated at 515 nm; and (iii) the gradual decrease of the absorbance band situated at 551 nm from 0.69 to approx. 0.09. According to Figure 12b, the RhB photodegradation has a sinusoidal behavior and after 50 min of UV irradiation, the degradation efficiency reached rapidly 50% and continued up to 85% after 300 min of exposure to UV light. indicating that a faster process in the first stage takes place. This result may be due to the presence of MWNTs, which provide a higher specific interfacial area for photon absorption so that inner and outer walls can both absorb oxygen in solutions [22]. k2 corresponds to the solution discoloration, and the corresponding values illustrated a higher discoloration capacity in the case of samples based on TiO2:N/DWNT and TiO2:N/MWNT composites. The third step of the RhB photodegradation mechanism was deduced by the k3 rate values, indicating the saturation of final products. Additionally, k3 rate values presented in Table 3 indicate that the saturation process took place in similar conditions for all the samples. Table 3 highlights the rate constants superior to that reported for the TiO2:N nanoparticles (T sample in Table 2), the k1 value ranging from 0.00364 min −1 (T sample) to 0.0055 min −1 (S2 sample), 0.0046 min −1 (D sample), and 0.0084 min −1 (M sample), the result which indicates the highest loading of carbon nanotubes on the surface of TiO2:N nanoparticles occurred in the case of the M sample. As shown in the following, noticeable results were obtained in the case of the TiO2:N/SWNTs-COOH samples. The exposure to UV light for 300 min of the RhB aqueous solution containing TiO2:N and 2.5 wt.% SWNTs-COOH implied significant modification in the UV-VIS spectra illustrated in Figure 12a: (i) a down-shift of the band of RhB from 551 nm (t = 0 min) to 497 nm (t = 300 min); (ii) the disappearance of the band shoulder situated at 515 nm; and (iii) the gradual decrease of the absorbance band situated at 551 nm from 0.69 to approx. 0.09. According to Figure 12b, the RhB photodegradation has a sinusoidal behavior and after 50 min of UV irradiation, the degradation efficiency reached rapidly 50% and continued up to 85% after 300 min of exposure to UV light.  Figure 12c illustrates a different profile from the results presented in the case of RhB aqueous solutions containing the T and S1-S3 samples (Figure 7), indicating that the RhB photodegradation process took place in four stages. After fitting the plot from Figure 12c, the rate constant values k 1 , k 2 , k 3 and k 4 were equal to 0.008, 0.0005, 0.0147, and 0.0013 min −1 , respectively. In the case of the SC, the value of the rate constant k 1 equal to 0.008 min −1 was higher than that reported in the case of sample S2, which was equal to 0.0055 min −1 ; this fact indicates that the presence of carboxyl functional groups onto SWNTs surface facilitated the loading of nanotubes on the surface of TiO 2 :N nanoparticles. This four-stage process might be explained based on the fact that after 170 min of UV irradiation there were unreacted species in the sample solution. Continuing with the irradiation, new chemical bonds between TiO 2 :N and SWNTs-COOH were formed, and therefore as a consequence, the k 3 rate constant value was higher than k 1 , indicating better electron transfer between TiO 2 :N and SWNTs-COOH. After 5 h UV irradiation, the sample solution was colorless. CNTs tend to aggregate in aqueous solutions; therefore, the CNT functionalization could increase the hydrophilicity and the homogeneous dispensability in the TiO 2 :N matrix. The presence of the COOH groups on the SWNTs groups enhanced the chemisorbed oxygen species on the CNTs surface and promoted the oxidation capacity of the TiO 2 :N catalyst. According to [36], the catalytic performance depends on the activity of surface-OH groups, and additionally it has a strong dependence on the concentration of surface OH radicals. The COOH groups from functionalized SWNTs broaden the specific surface area and improve the adsorption capacity of TiO 2 :N/SWNTs catalyst [37].
Based on the results of this present study regarding the photodegradation efficiency of RhB in the presence of samples containing TiO 2 :N and different types of CNTs, the diagram from Figure 13 was created. Figure 13 indicates that after 300 min of UV light irradiation, the highest degradation efficiency was obtained in the case of samples containing TiO 2 :N and 2.5 wt.% SWNTs-COOH, which ensured RhB photodegradation with more than 80% in the aqueous solution. The improved photocatalytic activity of the TiO 2 :N/2.5 wt.% SWNTs-COOH sample can be explained due to the homogeneous and dense dispersion of TiO 2 :N nanoparticles on the CNT surface. The strong contact between TiO 2 :N and CNTs induce dense heterojunctions through Ti-O-C structures [23]. The functional groups COOH in SWNTs increase the active chemisorbed oxygen amount; therefore, both the capability of oxygen delivery and the oxygen mobility on the lattice surface was also improved. The presence of SWNTs-COOH in the TiO 2 :N/SWNTs-COOH composite induced a synergy effect of TiO 2 :N, mainly attributed to the strong binding by the functional groups of carboxylic acid.  Figure 12c illustrates a different profile from the results presented in the case of RhB aqueous solutions containing the T and S1-S3 samples (Figure 7), indicating that the RhB photodegradation process took place in four stages. After fitting the plot from Figure 12c, the rate constant values k1, k2, k3 and k4 were equal to 0.008, 0.0005, 0.0147, and 0.0013 min −1 , respectively. In the case of the SC, the value of the rate constant k1 equal to 0.008 min −1 was higher than that reported in the case of sample S2, which was equal to 0.0055 min −1 ; this fact indicates that the presence of carboxyl functional groups onto SWNTs surface facilitated the loading of nanotubes on the surface of TiO2:N nanoparticles. This fourstage process might be explained based on the fact that after 170 min of UV irradiation there were unreacted species in the sample solution. Continuing with the irradiation, new chemical bonds between TiO2:N and SWNTs-COOH were formed, and therefore as a consequence, the k3 rate constant value was higher than k1, indicating better electron transfer between TiO2:N and SWNTs-COOH. After 5 h UV irradiation, the sample solution was colorless. CNTs tend to aggregate in aqueous solutions; therefore, the CNT functionalization could increase the hydrophilicity and the homogeneous dispensability in the TiO2:N matrix. The presence of the COOH groups on the SWNTs groups enhanced the chemisorbed oxygen species on the CNTs surface and promoted the oxidation capacity of the TiO2:N catalyst. According to [36], the catalytic performance depends on the activity of surface-OH groups, and additionally it has a strong dependence on the concentration of surface OH radicals. The COOH groups from functionalized SWNTs broaden the specific surface area and improve the adsorption capacity of TiO2:N/SWNTs catalyst [37].
Based on the results of this present study regarding the photodegradation efficiency of RhB in the presence of samples containing TiO2:N and different types of CNTs, the diagram from Figure 13 was created. Figure 13 indicates that after 300 min of UV light irradiation, the highest degradation efficiency was obtained in the case of samples containing TiO2:N and 2.5 wt.% SWNTs-COOH, which ensured RhB photodegradation with more than 80% in the aqueous solution. The improved photocatalytic activity of the TiO2:N/2.5 wt.% SWNTs-COOH sample can be explained due to the homogeneous and dense dispersion of TiO2:N nanoparticles on the CNT surface. The strong contact between TiO2:N and CNTs induce dense heterojunctions through Ti-O-C structures [23]. The functional groups COOH in SWNTs increase the active chemisorbed oxygen amount; therefore, both the capability of oxygen delivery and the oxygen mobility on the lattice surface was also improved. The presence of SWNTs-COOH in the TiO2:N/SWNTs-COOH composite induced a synergy effect of TiO2:N, mainly attributed to the strong binding by the functional groups of carboxylic acid.

Mechanism of the RhB Photocatalytic Degradation Enhanced in the Presence of CNTs
RhB photodegradation in the presence of TiO 2 :N sample has more than 50% efficiency due to the doped semiconductor. Nitrogen doping of TiO 2 allows shallow states within the semiconductor band-gap; therefore, new energetic levels are introduced between the valence and conduction bands, resulting in a narrowing bandgap than the undoped TiO 2 [38]. Doping TiO 2 with non-metal elements can generate new oxygen vacancies and increase the density of the catalyst [23]. The UV radiation excites the electrons in both the mid-gap energy level and the valence band of TiO 2 :N [39]. The enhancement of the photocatalytic activity of TiO 2 :N/CNTs composites can be explained using different mechanisms: (i) the mechanism proposed by Hoffmann et al. [40] states that the incoming energy photon excites an electron from the valence band to the conduction band of TiO 2 , and then the photogenerated electrons are transferred to CNTs with the remaining holes participating in redox reactions; (ii) the mechanism proposed by Wang et al. [24] recalls that CNTs act as sensitizers and transfer the electrons to TiO 2 . The photogenerated electron is transferred to the TiO 2 conduction band and allows the formation of superoxide radicals; furthermore, the CNTs are positively charged and have the ability to remove one electron from the TiO 2 valence band, leaving a hole. The resulting positively charged TiO 2 can react with adsorbed water and form hydroxyl radicals; and (iii) the mechanism in which CNTs can act as impurity forming Ti-O-C bonds similar to carbon-doped titania [41]. The mechanism of RhB photocatalytic degradation in aqueous solutions containing TiO 2 :N/CNT samples is illustrated in Scheme 2.

Mechanism of the RhB Photocatalytic Degradation Enhanced in the Presence of CNTs
RhB photodegradation in the presence of TiO2:N sample has more than 50% efficiency due to the doped semiconductor. Nitrogen doping of TiO2 allows shallow states within the semiconductor band-gap; therefore, new energetic levels are introduced between the valence and conduction bands, resulting in a narrowing bandgap than the undoped TiO2 [38]. Doping TiO2 with non-metal elements can generate new oxygen vacancies and increase the density of the catalyst [23]. The UV radiation excites the electrons in both the mid-gap energy level and the valence band of TiO2:N [39]. The enhancement of the photocatalytic activity of TiO2:N/CNTs composites can be explained using different mechanisms: (i) the mechanism proposed by Hoffmann et al. [40] states that the incoming energy photon excites an electron from the valence band to the conduction band of TiO2, and then the photogenerated electrons are transferred to CNTs with the remaining holes participating in redox reactions; (ii) the mechanism proposed by Wang et al. [24] recalls that CNTs act as sensitizers and transfer the electrons to TiO2. The photogenerated electron is transferred to the TiO2 conduction band and allows the formation of superoxide radicals; furthermore, the CNTs are positively charged and have the ability to remove one electron from the TiO2 valence band, leaving a hole. The resulting positively charged TiO2 can react with adsorbed water and form hydroxyl radicals; and (iii) the mechanism in which CNTs can act as impurity forming Ti-O-C bonds similar to carbon-doped titania [41]. The mechanism of RhB photocatalytic degradation in aqueous solutions containing TiO2:N/CNT samples is illustrated in Scheme 2. CNTs can enhance the photocatalytic efficiency of TiO 2 :N due to the electrons' adsorption and transportation. The mid-gap energy level induced in TiO 2 after the nitrogen doping is situated close to the valence band of TiO 2 . Under the exposure to UV light of the samples containing TiO 2 :N, electron (e − ) and hole (h + ) pairs are generated. Further e − are excited from the valence band and from the mid-gap energy level to the conduction band of TiO 2 :N, creating holes. According to Nila et al. [38], the highest energy level of the TiO 2 :N valence band is −6.05 eV, with a TiO 2 :N band gap of 2.3 eV. When TiO 2 :N particles are in strong contact with CNTs, the energy levels of TiO 2 :N can superpose to the HOMO-LUMO levels of CNTs, inducing the transfer of electrons from TiO 2 :N surface to the LUMO level of CNTs. From the LUMO level of CNTs, electrons can suffer successive transitions until the minimum LUMO energy level from which they are collected by the maximum energy level of HOMO, hindering the recombination of e − -h + pairs. e − from the CNTs surface react with water and oxygen leading to hydroxyl radical which will oxidize the RhB. The photodegradation of RhB can be described as follows: RhB ads + ·OH → Degradation (8) To highlight that the reactive species, i.e., hydroxyl radicals and O 2 , play a significant role in the RhB photodegradation process when the TiO 2 :N/carbon nanotube samples are used as catalysts, isopropyl alcohol (IPA) was used as quencher. Figure 14a shows the UV-VIS spectra of the RhB aqueous solution containing the SC sample and IPA as scavenger. Prior to exposure of the sample to UV light, the UV-VIS spectrum was characterized by a band with a maximum at 554 nm and another localized in the spectral range 200-400 nm. The exposure to UV light of the RhB aqueous solution containing the SC sample and IPA, in a time of 300 min, induced a gradual decrease in the absorbance of the band belonging to RhB, simultaneously with a shift from 554 nm to 502 nm. Figure 14b highlights similar behavior with that shown in Figure 12b, the degradation efficiency after 300 min of exposure to UV light being equal to 46%, a value lower than that reported in the absence of IPA. This result indicates that the hydroxyl radicals play a significant role in the photodegradation process of the RhB aqueous solution containing the SC sample. energy level of HOMO, hindering the recombination of e --h + pairs. efrom the CNTs surface react with water and oxygen leading to hydroxyl radical which will oxidize the RhB. The photodegradation of RhB can be described as follows: : + + ℎ + ℎ → ( ) + (ℎ ) : (ℎ + ℎ ) + →• + : : ( ) + →• + : ⎯ • + : To highlight that the reactive species, i.e., hydroxyl radicals and O2, play a significant role in the RhB photodegradation process when the TiO2:N/carbon nanotube samples are used as catalysts, isopropyl alcohol (IPA) was used as quencher. Figure 14a shows the UV-VIS spectra of the RhB aqueous solution containing the SC sample and IPA as scavenger. Prior to exposure of the sample to UV light, the UV-VIS spectrum was characterized by a band with a maximum at 554 nm and another localized in the spectral range 200-400 nm. The exposure to UV light of the RhB aqueous solution containing the SC sample and IPA, in a time of 300 min, induced a gradual decrease in the absorbance of the band belonging to RhB, simultaneously with a shift from 554 nm to 502 nm. Figure 14b highlights similar behavior with that shown in Figure 12b, the degradation efficiency after 300 min of exposure to UV light being equal to 46%, a value lower than that reported in the absence of IPA. This result indicates that the hydroxyl radicals play a significant role in the photodegradation process of the RhB aqueous solution containing the SC sample. Taking into account the above results, we can conclude that (i) the samples prepared in this work show high photocatalytic activity when a small concentration of carbon nanotubes (2.5 wt.%) is added to the TiO2:N matrix; and (ii) a high degradation efficiency of the RhB solution is obtained in the case of the TiO2:N/SWNTs-COOH sample.

Materials and Methods
In this paper, TiO2:N with a nitrogen concentration of 1.5% wt. was purchased from NANO-X GmbH, having purity of 99%. RhB, IPA, and carbon nanotubes (SWNTs, SWNTs-COOH, DWNTs, and MWNTs) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Taking into account the above results, we can conclude that (i) the samples prepared in this work show high photocatalytic activity when a small concentration of carbon nanotubes (2.5 wt.%) is added to the TiO 2 :N matrix; and (ii) a high degradation efficiency of the RhB solution is obtained in the case of the TiO 2 :N/SWNTs-COOH sample.

Materials and Methods
In this paper, TiO 2 :N with a nitrogen concentration of 1.5% wt. was purchased from NANO-X GmbH, having purity of 99%. RhB, IPA, and carbon nanotubes (SWNTs, SWNTs-COOH, DWNTs, and MWNTs) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The TiO 2 :N/SWNTs samples with different SWNT concentrations (0.25, 0.5, 1, 2.5, and 5 wt.%) were prepared by direct mixing of the two constituents. In the case of the TiO 2 :N/SWNTs-COOH, TiO 2 :N/DWNT, and TiO 2 :N/MWNT samples, the CNT concentration was 2.5 wt.%. The samples were labeled as described in Table 1. The colloidal dispersions were prepared by adding each sample to an aqueous solution of RhB 100 µM followed by magnetic stirring for 20 min at 25 • C. Each suspension was stirred in dark conditions to reach the adsorption-desorption equilibrium of RhB on the catalyst surface. The colloidal dispersions were subjected to subsequent UV light irradiation for 300 min with a halogen lamp of 300 W.
we have demonstrated that (i) by using the method of mixing the two constituents, which is cheaper and faster, values of a RhB photodegradation efficiency in the presence of the catalysts TiO 2 :N/SWNTs, TiO 2 :N/DWNTs, and TiO 2 :N/MWNTs of 70%, 68%, and 69%, respectively, are reported when the concentration of carbon nanotubes is only 2.5 wt.% in the mass of mixtures; and (ii) the best catalyst for the efficient photodegradation of the RhB solution is the TiO 2 :N/SWNTs-COOH sample; the photodegradation efficiency of the RhB solution in the presence of TiO 2 :N/SWNTs-COOH sample is of 85%, a value higher than that reported in the case of photodegradation of methylene blue in the presence of the TiO 2 :N/MWNTs-COOH catalyst, which was a maximum of 60% [23].

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.