Nanohybrid Composites Based on TiO2 and Single-Walled Carbon Nanohorns as Promising Catalysts for Photodegradation of Amoxicillin

In this work, applications of nanohybrid composites based on titanium dioxide (TiO2) with anatase crystallin phase and single-walled carbon nanohorns (SWCNHs) as promising catalysts for the photodegradation of amoxicillin (AMOX) are reported. In this order, TiO2/SWCNH composites were prepared by the solid-state interaction of the two chemical compounds. The increase in the SWCNH concentration in the TiO2/SWCNH composite mass, from 1 wt.% to 5 wt.% and 10 wt.% induces (i) a change in the relative intensity ratio of the Raman lines located at 145 and 1595 cm−1, which are attributed to the Eg(1) vibrational mode of TiO2 and the graphitic structure of SWCNHs; and (ii) a gradual increase in the IR band absorbance at 1735 cm−1 because of the formation of new carboxylic groups on the SWCNHs’ surface. The best photocatalytic properties were obtained for the TiO2/SWCNH composite with a SWCNH concentration of 5 wt.%, when approx. 92.4% of AMOX removal was achieved after 90 min of UV irradiation. The TiO2/SWCNH composite is a more efficient catalyst in AMOX photodegradation than TiO2 as a consequence of the SWCNHs’ presence, which acts as a capture agent for the photogenerated electrons of TiO2 hindering the electron–hole recombination. The high stability of the TiO2/SWCNH composite with a SWCNH concentration of 5 wt.% is proved by the reusing of the catalyst in six photodegradation cycles of the 98.5 μM AMOX solution, when the efficiency decreases from 92.4% up to 78%.


Introduction
Amoxicillin (AMOX) is an organic compound used as an antibiotic in the treatment of various infections, such as urinary tract [1], dental [2], chest [3], sinus [4], etc. This organic compound is found in drugs commercialized under different names, including (i) Amoksiklav (AMOX-drug), Augmentin, Amoclan, when AMOX is mixed with clavulanate potassium; (ii) Voquezna Triple Pak, which contains a mixture of AMOX, clarithromycin and vonoprazan; (iii) Omeclamox-Pak, when active compounds consist of AMOX, clarithromycin and omeprazole; and (iv) Prevpac, when the ingredients include AMOX, clarithromycin and lansoprazole. One of the main pollutants reported in medical wastewater, groundwater or surface water are antibiotics. AMOX is not effectively removed by traditional organic wastewater treatment methods (for example electrolysis, activated carbon adsorption, biological methods, etc.) [5], which is why advanced oxidation processes were widely used when its complete oxidation was reported [6]. Thus, the methods of ozonation, Fenton, photo-Fenton and photocatalysis in the presence of semiconductors proved to be alternatives to the conventional methods for the degradation of AMOX into Figure 1 shows the Raman spectra of TiO 2 , SWCNHs and the TiO 2 /SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%. TiO 2 with anatase crystalline phase highlights Raman lines peaked at~147, 397, 517 and 638 cm −1 , which were assigned to the following vibrational modes: E 1g , B 1g , A 1g and E 3g , respectively [24]. SWCNHs show two Raman lines situated at 1275 and 1597 cm −1 , often labelled as D and G bands, which were attributed to SWCNH defects and graphitic structures, respectively. The analysis of the Raman spectrum of SWCNHs (magenta curve in Figure 1) indicates that the ratio between the intensities of the D and G bands (I D /I G ) is equal to 1 [25]. According to our expectations, as the SWCNH concentration in the TiO 2 /SWCNH composites mass increases, a growth in the ratio between the D and G bands intensity occurs, as shown by the red, green and blue curve in Figure 1. In the presence of TiO 2 , the value of the I D /I G ratio becomes equal to 1.1, 1.23 and 1.3 (red, green and blue curves in Figure 1). This fact indicates an increase of defects weight in the SWCNHs' structure as a consequence of the interaction between the two constituents. An increase in the full width at half maximum (FWHM) of the Raman line of TiO 2 peaking at 147 cm −1 when a SWCNH concentration in the TiO 2 /SWCNH composites mass increases from 1 wt.% to 10 wt.% is observed. Raman lines peaking at 1095, 1477 and 1691 cm −1 were assigned to the vibrational modes of the -C-O-C-and C-C bonds in aromatic ring and C=O stretching [26]. The presence of Raman lines at 1095, 1477 and 1691 cm −1 indicates an interaction of SWCNHs with TiO 2 , which leads to the appearance of C-O-C and C=O groups on the surface of SWCNHs.

Optical and Structural Properties of the TiO 2 /SWCNH Composites
shown by the red, green and blue curve in Figure 1. In the presence of TiO2, the value of the ID/IG ratio becomes equal to 1.1, 1.23 and 1.3 (red, green and blue curves in Figures 1). This fact indicates an increase of defects weight in the SWCNHs' structure as a consequence of the interaction between the two constituents. An increase in the full width at half maximum (FWHM) of the Raman line of TiO2 peaking at 147 cm −1 when a SWCNH concentration in the TiO2/SWCNH composites mass increases from 1 wt.% to 10 wt.% is observed. Raman lines peaking at 1095, 1477 and 1691 cm −1 were assigned to the vibrational modes of the -C-O-C-and C-C bonds in aromatic ring and C=O stretching [26]. The presence of Raman lines at 1095, 1477 and 1691 cm −1 indicates an interaction of SWCNHs with TiO2, which leads to the appearance of C-O-C and C=O groups on the surface of SWCNHs. Complementary information about the SWCNH-TiO2 interaction is shown by FTIR spectroscopy in Figure 2. The FTIR spectrum of TiO2 with anatase crystalline phase (Figure 2a) is characterized by a band of high absorbance at 457 cm −1 that is accompanied by another one at 779 cm −1 and other two bands of low intensities at 1737 and 3725 cm −1 that are assigned to the stretching vibrational modes of the Ti-O and Ti-O-Ti bonds, H2O molecules and the OH bonds coming from the water adsorbed onto the TiO2 surface [16,27]. Figure 2b shows the FTIR spectra of TiO2/SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%. Regardless of SWCNH concentration, the following changes are observed in Figure 2a,b: (i) the disappearance of the IR band at 779 cm −1 ; (ii) a down-shift of the IR band from 461 cm −1 to 449 cm −1 and (iii) an up-shift of the IR bands from 3725 cm −1 and 1737 cm −1 to 3730 cm −1 and 1739 cm −1 simultaneous with their absorbance growth. In addition, Figure 2b highlights new IR bands at 816-818, 1213-1219, 1369, 1516-1518, 1593 and 3317-3323 cm −1 , which were assigned to the following vibrational modes: first order A2 [28], C-OH deformation [29], the deformation of the C-C bond of the pentagonal ring [29], E1u of the graphitic lattice [28], and the deformation of the O-H bond [29]. IR bands peaking at 625 and 3630 cm −1 were assigned to the vibrational modes of the deformation of aromatic ring of SWCNHs and free OH groups [26]. A puzzling fact is that an IR band peaked at 1730 cm −1 , which was no longer situated of the IR band at 1739 cm −1 , as shown in Figure 2b. This IR band was assigned to the C=O stretching vibrational mode of carboxylic groups present on the surface of SWCNHs [30]. The presence of the C=O and C-OH groups on the surface of SWCNHs can be explained only if we accept that the interaction of SWCNHs with TiO2 involves a charge transfer, which leads to the appearance of carboxylic groups on the surface of SWCNHs.   Figure 1. Raman spectra of TiO 2 with anatase crystalline-phase (black curve), SWCNHs (magenta curve) and the TiO 2 /SWCNH composites, with a SWCNH concentration equal to 1 wt.% (red curve), 5 wt.% (green curve) and 10 wt.% (blue curve).
Complementary information about the SWCNH-TiO 2 interaction is shown by FTIR spectroscopy in Figure 2. The FTIR spectrum of TiO 2 with anatase crystalline phase ( Figure 2a) is characterized by a band of high absorbance at 457 cm −1 that is accompanied by another one at 779 cm −1 and other two bands of low intensities at 1737 and 3725 cm −1 that are assigned to the stretching vibrational modes of the Ti-O and Ti-O-Ti bonds, H 2 O molecules and the OH bonds coming from the water adsorbed onto the TiO 2 surface [16,27]. Figure 2b shows the FTIR spectra of TiO 2 /SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%. Regardless of SWCNH concentration, the following changes are observed in Figure 2a,b: (i) the disappearance of the IR band at 779 cm −1 ; (ii) a down-shift of the IR band from 461 cm −1 to 449 cm −1 and (iii) an up-shift of the IR bands from 3725 cm −1 and 1737 cm −1 to 3730 cm −1 and 1739 cm −1 simultaneous with their absorbance growth. In addition, Figure 2b highlights new IR bands at 816-818, 1213-1219, 1369, 1516-1518, 1593 and 3317-3323 cm −1 , which were assigned to the following vibrational modes: first order A 2 [28], C-OH deformation [29], the deformation of the C-C bond of the pentagonal ring [29], E 1u of the graphitic lattice [28], and the deformation of the O-H bond [29]. IR bands peaking at 625 and 3630 cm −1 were assigned to the vibrational modes of the deformation of aromatic ring of SWCNHs and free OH groups [26]. A puzzling fact is that an IR band peaked at 1730 cm −1 , which was no longer situated of the IR band at 1739 cm −1 , as shown in Figure 2b. This IR band was assigned to the C=O stretching vibrational mode of carboxylic groups present on the surface of SWCNHs [30]. The presence of the C=O and C-OH groups on the surface of SWCNHs can be explained only if we accept that the interaction of SWCNHs with TiO 2 involves a charge transfer, which leads to the appearance of carboxylic groups on the surface of SWCNHs. Figure 3 shows the X-ray diffraction (XRD) diagrams of TiO 2 , SWCNHs and the TiO 2 /SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%.
The main XRD peaks of TiO 2 are situated at 25. . This is in accordance with JCPDF no. 00-021-1272. The XRD peaks of SWCNHs are observed at 26 • and 43 • , the first one being attributed to the crystalline plane (002), while the second one is a combination of the reflections of the crystalline plane (100), (101) and the tubular/turbostatic state of graphitic lattice [31]. One may observe that the TiO 2 /SWCNH composites are keeping the general features of the TiO 2 polycrystalline powder in regard to the positions of the diffraction peaks. The slight decrease in intensity for the composites compared to the pure TiO 2 can be understood as a plain addition effect, where SWCNHs fill in the spaces between TiO 2 crystallites, thus absorbing some of the X-rays. Due to this geometrical arrangement, the structural signature of the SWCNHs will be practically lost in comparison with pure SWCNH powder. In consequence, it is reasonable to state that the addition of SWCNHs will not modify the crystalline structure of TiO 2 ; rather, they simply wrap around the TiO 2 crystallites and fill the space between them.  Figure 3 shows the X-ray diffraction (XRD) diagrams of TiO2, SWCNHs and the TiO2/SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%. . This is in accordance with JCPDF no. 00-021-1272. The XRD peaks of SWCNHs are observed at 26° and 43°, the first one being attributed to the crystalline plane (002), while the second one is a combination of the reflections of the crystalline plane (100), (101) and the tubular/turbostatic state of graphitic lattice [31]. One may observe that the TiO2/SWCNH composites are keeping the general features of the TiO2 polycrystalline powder in regard to the positions of the diffraction peaks. The slight decrease in intensity for the composites compared to the pure TiO2 can be understood as a plain addition effect, where SWCNHs fill in the spaces between TiO2 crystallites, thus absorbing some of the X-rays. Due to this geometrical arrangement, the structural signature of the SWCNHs will be practically lost in comparison with pure SWCNH powder. In consequence, it is reasonable to state that the addition of     Figure 3 shows the X-ray diffraction (XRD) diagrams of TiO2, SWCNHs and the TiO2/SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%. . This is in accordance with JCPDF no. 00-021-1272. The XRD peaks of SWCNHs are observed at 26° and 43°, the first one being attributed to the crystalline plane (002), while the second one is a combination of the reflections of the crystalline plane (100), (101) and the tubular/turbostatic state of graphitic lattice [31]. One may observe that the TiO2/SWCNH composites are keeping the general features of the TiO2 polycrystalline powder in regard to the positions of the diffraction peaks. The slight decrease in intensity for the composites compared to the pure TiO2 can be understood as a plain addition effect, where SWCNHs fill in the spaces between TiO2 crystallites, thus absorbing some of the X-rays. Due to this geometrical arrangement, the structural signature of the SWCNHs will be practically lost in comparison with pure SWCNH powder. In consequence, it is reasonable to state that the addition of SWCNHs will not modify the crystalline structure of TiO2; rather, they simply wrap around the TiO2 crystallites and fill the space between them.   Figure 3. XRD diagrams of SWCNH (black curve), TiO 2 (red curve) and the TiO 2 /SWCNH composites with a SWCNH concentration equal to 1 wt.% (blue curve), 5 wt.% (yellow curve) and 10 wt.% (green curve).

Photocatalytic Properties of the TiO 2 /SWCNH Composites
When using TiO 2 /SWCNH composites as catalysts for AMOX photodegradation, their efficiency can be calculated with the following equation: where A 0 and A t are the absorbances of the band localized at 258-276 nm when the samples were exposed for 0 min and 90 min, respectively, to UV light. To reach the equilibrium of AMOX adsorption/desorption on the catalyst's surface, before the exposure of samples to UV light, the UV-VIS spectra were recorded for 30 min in the absence of light. Figure 4a,c shows the UV-VIS spectra of the 98.5 µM AMOX solution containing 0.2 mg/mL of TiO 2 or the TiO 2 /SWCNH composite with a SWCNH concentration equal to 5 wt.%. Figure  were assigned to π-π* electronic transition in the AMOX aromatic ring [31], the amide n-π* electronic transition in the AMOX β-lactam chromophore group [31], and TiO 2 [32]. The exposure of AMOX and TiO 2 samples to UV light induces a progressive decrease in absorbance as the irradiation time gradually increases up to 90 min. The efficiency of TiO 2 (0.2 mg/mL) on the photodegradation of the 98.5 µM AMOX aqueous solution is 74.2%. This is a similar behavior that can be observed in the presence of 0.2 mg/mL TiO 2 /SWCNH composite with an SWCNH concentration of 5 wt.% in which the efficiency of this catalyst on the photodegradation of the 98.5 µM AMOX aqueous solution is of 92.4%. According to Figure 4b,d, the efficiency of AMOX photodegradation in the presence of 0.1 mg/mL and 0.3 mg/mL TiO 2 /SWCNH composites in the 98.5 µM AMOX aqueous solution is equal to 75.1% and 82.6%. ples were exposed for 0 min and 90 min, respectively, to UV light. To reach the equilibrium of AMOX adsorption/desorption on the catalyst's surface, before the exposure of samples to UV light, the UV-VIS spectra were recorded for 30 min in the absence of light. Figure 4a,c shows the UV-VIS spectra of the 98.5 µM AMOX solution containing 0.2 mg/mL of TiO2 or the TiO2/SWCNH composite with a SWCNH concentration equal to 5 wt.%. Figure 4a highlights three bands with maxima at 230 nm, 276 nm and 328 nm, which were assigned to π-π* electronic transition in the AMOX aromatic ring [31], the amide nπ* electronic transition in the AMOX β-lactam chromophore group [31], and TiO2 [32]. The exposure of AMOX and TiO2 samples to UV light induces a progressive decrease in absorbance as the irradiation time gradually increases up to 90 min. The efficiency of TiO2 (0.2 mg/mL) on the photodegradation of the 98.5 µM AMOX aqueous solution is 74.2%. This is a similar behavior that can be observed in the presence of 0.2 mg/mL TiO2/SWCNH composite with an SWCNH concentration of 5 wt.% in which the efficiency of this catalyst on the photodegradation of the 98.5 µM AMOX aqueous solution is of 92.4%. According to Figure 4b,d, the efficiency of AMOX photodegradation in the presence of 0.1 mg/mL and 0.3 mg/mL TiO2/SWCNH composites in the 98.5 µM AMOX aqueous solution is equal to 75.1% and 82.6%.  The influence of SWCNH concentration in the TiO2/SWCNH composites mass on AMOX photodegradation is analyzed by comparing the results shown in Figure 3b with those presented in Figure 5. As the SWCNH concentration in the TiO2/SWCNH composite mass increases from 1 wt.% to 5 wt.% and 10 wt.%, the photodegradation efficiency of 98.5 µM AMOX aqueous solution varies from 88.5% to 92.4% and 90.3%, respectively.  The influence of SWCNH concentration in the TiO 2 /SWCNH composites mass on AMOX photodegradation is analyzed by comparing the results shown in Figure 3b with those presented in Figure 5. As the SWCNH concentration in the TiO 2 /SWCNH composite mass increases from 1 wt.% to 5 wt.% and 10 wt.%, the photodegradation efficiency of 98.5 µM AMOX aqueous solution varies from 88.5% to 92.4% and 90.3%, respectively.
The influence of SWCNH concentration in the TiO2/SWCNH composites mass on AMOX photodegradation is analyzed by comparing the results shown in Figure 3b with those presented in Figure 5. As the SWCNH concentration in the TiO2/SWCNH composite mass increases from 1 wt.% to 5 wt.% and 10 wt.%, the photodegradation efficiency of 98.5 µM AMOX aqueous solution varies from 88.5% to 92.4% and 90.3%, respectively. According to Figure 6, the efficiency of the TiO2/SWCNH composite (with a SWCNH concentration of 5 wt.%) on the photodegradation of the 197 µM AMOX aqueous solution is 84.1%. According to Figure 6c, the c/c0 ratio does not show variations in the dark environment, while under UV light, a significant decrease is reported.   Information regarding the reuse of the TiO 2 /SWCNH composite (with a SWCNH concentration of 10 wt.%) after six photodegradation cycles of the 98.5 µM AMOX solution is presented in Figure 7. After six cycles of the photodegradation of 98.5 µM AMOX containing the TiO2/SWCNH composite catalyst with a SWCNH concentration equal to 10 wt.%, the photodegradation efficiency is 78%. A 12.3% decrease in the efficiency of AMOX photodegradation occurs when the TiO2/SWCNH composite, with an SWCNH concentration equal to 10%, is used as catalyst, which highlights the good stability of this catalyst and its ability to be reused in the photodegradation processes of waters polluted with such pharmaceutical compounds. An explanation for this behavior must take into consideration that the TiO2/SWCNH composite can absorb the full spectrum of incident light, like TiO2 [33], thereby generating heat at the interface of the TiO2/SWCNH composite because the SWCNH powder is black. One consequence of this heat generation is that it will increase the migration of the electron-hole pair into the mass of the TiO2/SWCNH composite, thus inducing an increase in charge separation that leads to a greater photocatalytic effect than that of TiO2 [34].
The performance of the TiO2/SWCNH composite on the photodegradation of the solution containing the pharmaceutical compound Amoksiklav is presented in Figure 8. For this purpose, a solution of AMOX and CLAK (sample labeled as AMOX-drug) was prepared using the drug commercialized as Amoksiklav. After six cycles of the photodegradation of 98.5 µM AMOX containing the TiO 2 /SWCNH composite catalyst with a SWCNH concentration equal to 10 wt.%, the photodegradation efficiency is 78%. A 12.3% decrease in the efficiency of AMOX photodegradation occurs when the TiO 2 /SWCNH composite, with an SWCNH concentration equal to 10%, is used as catalyst, which highlights the good stability of this catalyst and its ability to be reused in the photodegradation processes of waters polluted with such pharmaceutical compounds. An explanation for this behavior must take into consideration that the TiO 2 /SWCNH composite can absorb the full spectrum of incident light, like TiO 2 [33], thereby generating heat at the interface of the TiO 2 /SWCNH composite because the SWCNH powder is black. One consequence of this heat generation is that it will increase the migration of the electronhole pair into the mass of the TiO 2 /SWCNH composite, thus inducing an increase in charge separation that leads to a greater photocatalytic effect than that of TiO 2 [34].
The performance of the TiO 2 /SWCNH composite on the photodegradation of the solution containing the pharmaceutical compound Amoksiklav is presented in Figure 8. For this purpose, a solution of AMOX and CLAK (sample labeled as AMOX-drug) was prepared using the drug commercialized as Amoksiklav.
The shape of the UV-VIS spectra shown in Figure 8a, as well as their behavior under UV irradiation, are similar to those of Figure 4c. According to Figure 8b, the efficiency of the TiO 2 /SWCNH composite, with a SWCNH concentration equal to 5 wt.%, on the photodegradation of Amoksiklav solution containing 98.5 µM AMOX and 25 µM CLAK is 84.3%, a value which is lower than that of the sample prepared from AMOX coming from the Sigma-Aldrich company (92.4%) (Burlington, MA, USA). An explanation for this result must consider the CLAK photodegradation in the presence of the TiO 2 /SWCNH composite with a SWCNH concentration equal to 5 wt.%. To support this statement, Figure 9 shows the variation of photodegradation efficiency of 25 µM CLAK solution, prepared from the powder from the Sigma-Aldrich company, in the presence of TiO 2 /SWCNH composite (with a SWCNH concentration of 5wt.%) under UV irradiation for 90 min.
thereby generating heat at the interface of the TiO2/SWCNH composite because the SWCNH powder is black. One consequence of this heat generation is that it will increase the migration of the electron-hole pair into the mass of the TiO2/SWCNH composite, thus inducing an increase in charge separation that leads to a greater photocatalytic effect than that of TiO2 [34].
The performance of the TiO2/SWCNH composite on the photodegradation of the solution containing the pharmaceutical compound Amoksiklav is presented in Figure 8. For this purpose, a solution of AMOX and CLAK (sample labeled as AMOX-drug) was prepared using the drug commercialized as Amoksiklav.  According to Figure 9, after a 90 min UV irradiation period of the CLAK aqueous solution containing the TiO2/SWCNH composite, the efficiency of the CLAK photodegradation is 75%.
The photodegradation mechanism of AMOX in the presence of the TiO2 catalyst was published in 2010 [35]. This mechanism involves the generation of hydroxyl radicals ( . OH) and superoxide ion radicals ( . O2 − ), which can interact with AMOX, resulting in degradation products. A similar mechanism must also be considered for the TiO2/SWCNH composite. Thus, under UV light, holes and electrons are generated in the valence band and conduction band, respectively, of TiO2. The transfer of electrons from the conduction band of TiO2 to SWCNHs will allow for successive interactions with O2 existing in the AMOX aqueous solution, leading to the formation of reactive oxygen species. The generation of hydroxyl radicals occurs when the holes in the valence band of TiO2 interact with H2O. Subsequently, hydroxyl radicals, being unstable species, will interact with AMOX, leading to the photodegradation products reported in Ref. [35].
To highlight the role of hydroxyl radicals in AMOX photodegradation, Figure 10 shows the UV-VIS spectra of the 98.5 µM AMOX solution containing 0.2 mg/mL of the TiO2/SWCNH composite (with a SWCNH concentration of 5 wt.%) and 0.1 mL IPA when the sample is UV-irradiated for 90 min. According to Figure 9, after a 90 min UV irradiation period of the CLAK aqueous solution containing the TiO 2 /SWCNH composite, the efficiency of the CLAK photodegradation is 75%.
The photodegradation mechanism of AMOX in the presence of the TiO 2 catalyst was published in 2010 [35]. This mechanism involves the generation of hydroxyl radicals (•OH) and superoxide ion radicals (•O 2 − ), which can interact with AMOX, resulting in degradation products. A similar mechanism must also be considered for the TiO 2 /SWCNH composite. Thus, under UV light, holes and electrons are generated in the valence band and conduction band, respectively, of TiO 2 . The transfer of electrons from the conduction band of TiO 2 to SWCNHs will allow for successive interactions with O 2 existing in the AMOX aqueous solution, leading to the formation of reactive oxygen species. The generation of hydroxyl radicals occurs when the holes in the valence band of TiO 2 interact with H 2 O. Subsequently, hydroxyl radicals, being unstable species, will interact with AMOX, leading to the photodegradation products reported in Ref. [35].
To highlight the role of hydroxyl radicals in AMOX photodegradation, Figure 10 shows the UV-VIS spectra of the 98.5 µM AMOX solution containing 0.2 mg/mL of the TiO 2 /SWCNH composite (with a SWCNH concentration of 5 wt.%) and 0.1 mL IPA when the sample is UV-irradiated for 90 min. According to Figure 10, after the UV exposure of the AMOX solution containing the TiO2/SWCNH composite and IPA, the degradation efficiency becomes equal to 83.7%, a value that is smaller than 92.4%. This result highlights the role of IPA as a quencher. In order to explore the active species in the photocatalytic degradation of AMOX, we use tert-butyl alcohol (TBA) as a scavenger of •OH and benzoquinone (BQ) for O2 •− . Figure 11 shows the photocatalytic conversion ratios (PCRs), i.e., c/c0, of the degradation of the aqueous solution of 98.5 µM AMOX in the absence of scavengers and in the presence of TBA and BQ. According to Figure 11, the values of PCRs in the presence of TBA and BQ are smaller than in their absence. A careful analysis indicates that the effect of BQ is more obvious than that of TBA. AMOX degradation was largely induced by the participation of O2 •− radicals and, to a lesser extent, by the contribution of •OH radicals.
Concerning the kinetics of the photodegradation of the AMOX solution containing the TiO2/SWCNH composite, the constant of the degradation rate can be assessed with the following equation: where (i) and 0 denote the absorbance of the AMOX band localized at 258-278 nm when the time (t) of UV irradiation was between 2 and 90 min, and 0 min, respectively; and (ii) k is the rate constant of the AMOX photodegradation reaction at time (t). According to Figure 10, after the UV exposure of the AMOX solution containing the TiO 2 /SWCNH composite and IPA, the degradation efficiency becomes equal to 83.7%, a value that is smaller than 92.4%. This result highlights the role of IPA as a quencher. In order to explore the active species in the photocatalytic degradation of AMOX, we use tert-butyl alcohol (TBA) as a scavenger of •OH and benzoquinone (BQ) for O 2 •− . Figure 11 shows the photocatalytic conversion ratios (PCRs), i.e., c/c 0 , of the degradation of the aqueous solution of 98.5 µM AMOX in the absence of scavengers and in the presence of TBA and BQ. According to Figure 10, after the UV exposure of the AMOX solution containing the TiO2/SWCNH composite and IPA, the degradation efficiency becomes equal to 83.7%, a value that is smaller than 92.4%. This result highlights the role of IPA as a quencher. In order to explore the active species in the photocatalytic degradation of AMOX, we use tert-butyl alcohol (TBA) as a scavenger of •OH and benzoquinone (BQ) for O2 •− . Figure 11 shows the photocatalytic conversion ratios (PCRs), i.e., c/c0, of the degradation of the aqueous solution of 98.5 µM AMOX in the absence of scavengers and in the presence of TBA and BQ. According to Figure 11, the values of PCRs in the presence of TBA and BQ are smaller than in their absence. A careful analysis indicates that the effect of BQ is more obvious than that of TBA. AMOX degradation was largely induced by the participation of O2 •− radicals and, to a lesser extent, by the contribution of •OH radicals.
Concerning the kinetics of the photodegradation of the AMOX solution containing the TiO2/SWCNH composite, the constant of the degradation rate can be assessed with the following equation: where (i) and 0 denote the absorbance of the AMOX band localized at 258-278 nm when the time (t) of UV irradiation was between 2 and 90 min, and 0 min, respectively; and (ii) k is the rate constant of the AMOX photodegradation reaction at time (t). According to Figure 11, the values of PCRs in the presence of TBA and BQ are smaller than in their absence. A careful analysis indicates that the effect of BQ is more obvious than that of TBA. AMOX degradation was largely induced by the participation of O 2 •− radicals and, to a lesser extent, by the contribution of •OH radicals.
Concerning the kinetics of the photodegradation of the AMOX solution containing the TiO 2 /SWCNH composite, the constant of the degradation rate can be assessed with the following equation: where (i) A t and A 0 denote the absorbance of the AMOX band localized at 258-278 nm when the time (t) of UV irradiation was between 2 and 90 min, and 0 min, respectively; and (ii) k is the rate constant of the AMOX photodegradation reaction at time (t). According to Figure 12, three linear regions are observed for all catalysts. These correspond to (i) the intermediate products as a result of AMOX photodegradation on the catalyst surface, which are characterized by a rate constant labeled as k 1 ; (ii) the photodegradation products as a result of UV irradiation, which has a rate constant labeled as k 2 ; and (iii) the surface saturation, which occurs with a rate constant labeled as k 3 .
Molecules 2023, 28, x FOR PEER REVIEW 11 of 15 According to Figure 12, three linear regions are observed for all catalysts. These correspond to (i) the intermediate products as a result of AMOX photodegradation on the catalyst surface, which are characterized by a rate constant labeled as k1; (ii) the photodegradation products as a result of UV irradiation, which has a rate constant labeled as k2; and (iii) the surface saturation, which occurs with a rate constant labeled as k3. The rate constant of the AMOX photodegradation reaction in the case of TiO2 and the TiO2/SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%, and the linear regression coefficients for the three stages mentioned above, are shown in Table 1. A complex kinetic model of the adsorption process of pharmaceutical compound onto the catalyst's surface modeled using pseudo-first order rate is reported for the three stages. The rate constant of the AMOX photodegradation reaction in the case of TiO 2 and the TiO 2 /SWCNH composites with a SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.%, and the linear regression coefficients for the three stages mentioned above, are shown in Table 1. A complex kinetic model of the adsorption process of pharmaceutical compound onto the catalyst's surface modeled using pseudo-first order rate is reported for the three stages.

Materials and Methods
TiO 2 , SWCNHs, clavulanate potassium (CLAK, C 8 H 3 KNO 5 ), AMOX (C 16 H 19 N 3 O 4 S), p-benzoquinone (BQ) and tert-butyl alcohol (TBA) were procured from Sigma-Aldrich. According to specification sheets, (i) the TiO 2 sample corresponds to a nano-powder with a particle size < 25 nm and 99.7% trace metals basis and the crystalline phase corresponding to anatase, and (ii) the average length and diameter of the SWCNHs is 40-50 nm and 2-5 nm, respectively, and the diameter of their spherical aggregates is about 100 nm. The purity of isopropyl alcohol (IPA), from SC ATOCHIM SRK, was 99.98%, while the purity of CLAK was ≥98.0%. The purity of the anhydrous TBA and BQ was ≥99.5%. The United States Pharmacopoeia (USP) reference standard of AMOX was used for the above experiments.
In these experiments, the drug Amoksiklav used was from a local pharmacy. One tablet of Amoksiklav contains 875 mg AMOX and 125 mg clavulanate potassium (CLAK). The Amoksiklav tablet core contains anhydrous colloidal SiO 2 , magnesium stearate, microcrystalline cellulose crospovidone and croscarmellose sodium, while the film which covered the core consisted of titanium dioxide (E 171), hydroxypropyl cellulose, ethyl cellulose, triethyl citrate, polysorbate 80 and talc.
The TiO 2 /SWCNH composites were prepared by the solid-state interaction of the commercial TiO 2 particles with SWCNHs by grinding the two chemical compounds for 15 min. After 15 min., the color of the TiO 2 powder changed from white to grey. Three TiO 2 /SWCNH composites with SWCNH concentration equal to 1 wt.%, 5 wt.% and 10 wt.% were prepared.
The characterization by Raman spectroscopy of the three TiO 2 /SWCNH composites, TiO 2 particles and SWCNHs were performed using a MultiRam FT-Raman spectrophotometer from Bruker, in the backscattering geometry, with a resolution of 1 cm −1 .
The FTIR spectra of the three TiO 2 /SWCNH composites and TiO 2 particles were recorded using a Vertex 80 FTIR spectrophotometer from Bruker, with a resolution of 2 cm −1 . The samples have also been investigated by X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer equipped with a Cu tube, which provides X-rays with λ Kα = 1.5406 Å. The data acquisition has been made with a step of 0.4 degrees in 2θ, with a multichannel linear detector giving an equivalent of 177 s/step.
The UV-VIS spectra of the solutions containing AMOX, TiO 2 /SWCNH composites or TiO 2 particles were recorded using Perkin Elmer Lambda 950 UV-VIS-NIR spectrophotometer at a resolution in the UV-VIS range of 0.05 nm. The photocatalytic properties of the colloidal dispersions of the catalyst for the AMOX photodegradation were prepared by adding the TiO 2 /SWCNH composite to the AMOX aqueous solution, which was ultrasonicated for 5 min. followed by magnetic stirring for 20 min at 25 • C in the dark, until reaching the adsorption-desorption equilibrium of AMOX on the surfaces of the TiO 2 /SWCNH catalysts. To assess the photodegradation efficiency of the catalysts, we will prepare the following: (a) AMOX aqueous solutions with a concentration of 98.5 µM and 197 µM, which are prepared with powder from Sigma-Aldrich; (b) colloidal dispersions of the TiO 2 /SWCNH composites, with a SWCNH concentration of 1 wt.%, 5 wt.% and 10 wt.%; and (c) colloidal dispersions with a concentration of TiO 2 /SWCNH composites in an AMOX aqueous solution equal to 0.1 mg/mL, 0.2 mg/mL and 0.3 mg/mL. For the analysis using Amoksiklav, each tablet was ground for 5 min, dispersed in 100 mL of distilled water under ultra-sonication for 10 min, and filtered to separate the excipients of active compounds, i.e., AMOX and CLAK. To assess the potential of the TiO 2 /SWCNH composites as catalyst for CLAK, a 25 µM CLAK aqueous solution was prepared. For assessing the efficiency of the TiO 2 /SWCNH composites when they are reused in successive photodegradation cycles of AMOX, the protocol published in Ref. [23] was used.
The photodegradation process of AMOX was carried out using a lamp with an emission maximum at 367 nm and power of 100 mW.

Conclusions
Novel results concerning the photocatalytic properties of TiO 2 /SWCNH composites, prepared by the solid-state interaction of the two constituents, i.e., SWCNHs and TiO 2 , on AMOX degradation are reported. The optical characterization of the TiO 2 /SWCNH composites, highlighted by Raman scattering and FTIR spectroscopy, has demonstrated the following: (i) a variation of the ratio between the intensities of the Raman lines located at 145 and 1595 cm −1 , which were attributed to the vibrational modes E g (1) of TiO 2 and the graphitic structure of SWCNHs, and a growth of the D band intensity occurring as a result of the defects' weight increase on the surface of the SWCNHs; and (ii) a gradual increase in the absorbance of the IR bands at 1735 cm −1 and 3728 cm −1 as a consequence of the C=O and -OH bonds coming from carboxylic groups being generated on the surface of the SWCNHs. The best photocatalytic properties were obtained for the TiO 2 /SWCNH composites, with a SWCNH concentration of 5 wt.%, when 92.4% AMOX removal after 90 min of UV irradiation was achieved. The reuse of the TiO 2 /SWCNH composite, with a SWCNH concentration of 10 wt.%, after six cycles of AMOX photodegradation induces a decrease in the degradation efficiency of 98.5 µM AMOX aqueous solution from 90.3% down to 78%.