Photocatalytic Activity of the Blends Based on TiO2 Nanoparticles and Reduced Graphene Oxide for Degradation of Acetaminophen

The aim of this work is to highlight the influence of blends based on TiO2 nanoparticles and reduced graphene oxide (RGO) on the photodegradation of acetaminophen (AC). To this end, the catalysts of TiO2/RGO blends with RGO sheet concentrations equal 5, 10, and 20 wt. % were prepared by the solid-state interaction of the two constituents. The preferential adsorption of TiO2 particles onto the RGO sheets’ surfaces via the water molecules on the TiO2 particle surface was demonstrated by FTIR spectroscopy. This adsorption process induced an increase in the disordered state of the RGO sheets in the presence of the TiO2 particles, as highlighted by Raman scattering and scanning electron microscopy (SEM). The novelty of this work lies in the demonstration that TiO2/RGO mixtures, obtained by the solid-phase interaction of the two constituents, allow an acetaminophen removal of up to 95.18% after 100 min of UV irradiation. This TiO2/RGO catalyst induced a higher photodegradation efficiency of AC than TiO2 due to the presence of RGO sheets, which acted as a capture agent for the photogenerated electrons of TiO2, hindering the electron–hole recombination. The reaction kinetics of AC aqueous solutions containing TiO2/RGO blends followed a complex first-order kinetic model. Another novelty of this work is the demonstration of the ability of PVC membranes modified with Au nanoparticles to act both as filters for the removal of TiO2/RGO blends after AC photodegradation and as potential SERS supports, which illustrate the vibrational properties of the reused catalyst. The reuse of the TiO2/RGO blends after the first cycle of AC photodegradation indicated their suitable stability during the five cycles of pharmaceutical compound photodegradation.

In this paper, the TiO 2 /RGO blends will be prepared by the solid-phase interaction of TiO 2 particles with RGO sheets. Compared to the synthesis methods reported for TiO 2 -and RGO-based catalysts [11,[14][15][16][17][18][19][20][21][22][23][24], the advantages of solid-phase interaction are the small number of reactants and the short samples preparation time that ensures the easy scaling of TiO 2 /RGO blends. In order to show the vibrational changes induced by the solid-phase interaction of the two constituents, the characterization of TiO 2 /RGO blends will be performed by Raman scattering and FTIR spectroscopy. The morphology of TiO 2 particles and TiO 2 /RGO blends will be analyzed by scanning electron microscopy (SEM). Characterization of the TiO 2 particles, RGO sheets, and TiO 2 /RGO blends by X-ray photoelectron microscopy will also be carried out. The photocatalytic properties of the TiO 2 /RGO blends in relation to AC photodegradation in the presence of UV light will be shown. In this context, the influence of the RGO sheets' weight within the TiO 2 /RGO blend, the concentration of the TiO 2 /RGO blends, as well as the presence of excipients on the AC photodegradation will be reported. The effects of TiO 2 /RGO blends on AC degradation as dependent on the AC solution's concentration as well as in real samples will also be shown.
The separation of a catalyst after the photodegradation of pollutants and its reuse are important topics today [26]. In this work, the ability of PVC membranes containing Au nanoparticles to act both as filters for the removal of TiO 2 /RGO blends after AC photodegradation and as SERS supports to illustrate the chemical adsorption of the TiO 2 /RGO blends from decontaminated waters will be shown. The recovery and reuse of the TiO 2 /RGO blends after five cycles of AC photodegradation, and the analysis of the variations in the degradation efficiency, will also be shown. The results reported in this paper will demonstrate that the solid-phase interaction method allows obtaining TiO 2 /RGO blends with a photodegradation efficiency of AC aqueous solutions equal to 95.18%, having suitable stability while reusing the catalyst in successive photodegradation cycles of this pollutant. To validate the stability of the catalyst, the ability of PVC membranes modified with Au nanoparticles to act both as filters for the removal of TiO 2 /RGO blends after each cycle of AC photodegradation and as potential SERS supports, which to illustrate the vibrational properties of the reused catalyst will be shown. Figure 1 shows the main Raman lines of TiO 2 at 147, 405, 447, 523, and 638 cm −1 , and 1178, 1205, 1329, 1373, 1583, and 1612 cm −1 , which have been assigned to the vibrational modes of E g in TiO 2 anatase (A), B 1g in TiO 2 A, E g in TiO 2 rutile (R), B 1g in TiO 2 A, Eg in TiO 2 A, a complex band of the Raman lines at 523 + 638 cm −1 , the third order of Raman lines at 401 and 447 cm −1 , the complex band of the Raman lines at 405 + 447 + 523 cm −1 , the third order of Raman line at 523 cm −1 and the presence of water molecules adsorbed onto the TiO 2 particles' surfaces [27,28]. In the range of 1000-1700 cm −1 , the Raman spectrum of RGO shows two bands with peaks at 1292 and 1597 cm −1 assigned to the breathing vibrational mode of the hexagonal rings containing carbon atoms and the E 2g phonon mode at the center of the Brillouin zone, respectively [29]. As shown in Figure 1, the solid-state interaction of TiO 2 with RGO induces a cumulative effect of the vibrational modes of the two constituents of the TiO 2 /RGO blends, simultaneously with a shift of the D band from 1292 to 1301 cm −1 . The shift of the D band indicates an increase in the disordered state in the RGO lattice.

Optical Properties of TiO 2 /RGO Blends
Molecules 2021, 28, x FOR PEER REVIEW 3 of 20 401 and 447 cm −1 , the complex band of the Raman lines at 405 + 447 + 523 cm −1 , the third order of Raman line at 523 cm −1 and the presence of water molecules adsorbed onto the TiO2 particles' surfaces [27,28]. In the range of 1000-1700 cm −1 , the Raman spectrum of RGO shows two bands with peaks at 1292 and 1597 cm −1 assigned to the breathing vibrational mode of the hexagonal rings containing carbon atoms and the E2g phonon mode at the center of the Brillouin zone, respectively [29]. As shown in Figure 1, the solid-state interaction of TiO2 with RGO induces a cumulative effect of the vibrational modes of the two constituents of the TiO2/RGO blends, simultaneously with a shift of the D band from 1292 to 1301 cm −1 . The shift of the D band indicates an increase in the disordered state in the RGO lattice. Additional information concerning the solid-state interaction of TiO2 with RGO is shown via FTIR spectroscopy in Figure 2. The main IR bands of TiO2 are situated in the spectral range of 500-700 cm −1 , which are accompanied by other IR bands with low absorbance peaks of 1382, 1737, and 3627-3728 cm −1 . The IR bands peaking at 692, 1737, and 3627-3728 cm −1 are assigned to the vibrational modes of stretching of the Ti-O-Ti bond water molecules, and the stretching vibrational modes of the OH bond [30]. The interaction of TiO2 with RGO induces an increase in the absorbance of IR bands localized in the spectral ranges 1000-1800 cm −1 and 3626-3730 cm −1 . The increase in absorbance of the IR bands peaking at 1740 cm −1 and 3627-3728 cm −1 indicates preferential adsorption onto the RGO sheets' surface via the water molecules on the TiO2 particles' surfaces. Figures 3 and 4 show the morphology of the TiO2/RGO blends analyzed by scanning electron microscopy (SEM) and elemental analyses of the samples by energy-dispersive X-ray (EDS). Additional information concerning the solid-state interaction of TiO 2 with RGO is shown via FTIR spectroscopy in Figure 2. The main IR bands of TiO 2 are situated in the spectral range of 500-700 cm −1 , which are accompanied by other IR bands with low absorbance peaks of 1382, 1737, and 3627-3728 cm −1 . The IR bands peaking at 692, 1737, and 3627-3728 cm −1 are assigned to the vibrational modes of stretching of the Ti-O-Ti bond, water molecules, and the stretching vibrational modes of the OH bond [30]. The interaction of TiO 2 with RGO induces an increase in the absorbance of IR bands localized in the spectral ranges 1000-1800 cm −1 and 3626-3730 cm −1 . The increase in absorbance of the IR bands peaking at 1740 cm −1 and 3627-3728 cm −1 indicates preferential adsorption onto the RGO sheets' surface via the water molecules on the TiO 2 particles' surfaces. Figures 3 and 4 show the morphology of the TiO 2 /RGO blends analyzed by scanning electron microscopy (SEM) and elemental analyses of the samples by energy-dispersive X-ray (EDS).
A careful analysis of           Figure 6a,b shows the deconvolution of the C1s and O1s XPS spectra of RGO sheets. Figure 6a highlights an intense peak at 283.9 eV, with was assigned to the bonds of the type C-C, C=C, and C-H, as well as other six of low intensity peaked at 284.9, 285.5, 286.6, 287.2, 288.1, and 289.8 eV that were assigned to the bonds of the type C-O, C=O, and -COOH [31]. Ti2p XPS spectra of TiO 2 particles are dominated by an intense peak at 464.5 eV accompanied by another peak localized at 458.8 eV (Figure 6e), which were assigned to the Ti 2p 1/2 and Ti 2p 3/2 [6]. In the case of the TiO 2 /RGO blend with a concentration of RGO sheets equal to 20 wt.%, Figure 6f-h is similar to those of C1s, O1s, and Ti2p XPS spectra of TiO 2 , the only difference being observed in the C1s XPS spectrum profile, where there is a significant decrease in the maximum located at 283.9 eV. Figure 5 shows the X-ray photoelectron spectroscopy (XPS) spectra of the RGO sheets, the TiO2 particles, and the TiO2/RGO blends with concentrations of RGO sheets equal to 5 wt. %, 10 wt. %, and 20 wt. %. Analysis of the XPS spectra indicates the following atomic concentrations: a) TiO2 particles comprise 19.4% C, 57.2% O, and 23.5% Ti; b) the RGO sheets comprise 89.9% C, 7.7% O, and 2.4% N. The atomic percentages in the TiO2/RGO blends are as follows: a) those with 5 wt. % RGO comprises 30    type C-C, C=C, and C-H, as well as other six of low intensity peaked at 284.9, 285.5, 286.6, 287.2, 288.1, and 289.8 eV that were assigned to the bonds of the type C-O, C=O, and -COOH [31]. Ti2p XPS spectra of TiO2 particles are dominated by an intense peak at 464.5 eV accompanied by another peak localized at 458.8 eV (Figure 6e), which were assigned to the Ti 2p1/2 and Ti 2p3/2 [6]. In the case of the TiO2/RGO blend with a concentration of RGO sheets equal to 20 wt.%, Figure 6f-h is similar to those of C1s, O1s, and Ti2p XPS spectra of TiO2, the only difference being observed in the C1s XPS spectrum profile, where there is a significant decrease in the maximum located at 283.9 eV. Considering this information regarding the TiO2/RGO blends, in the following, the photocatalytic properties of these materials in relation to the photodegradation of AC will be shown. Considering this information regarding the TiO 2 /RGO blends, in the following, the photocatalytic properties of these materials in relation to the photodegradation of AC will be shown.

Photocatalytic Properties of the TiO 2 /RGO Blends
The photodegrading efficiency of AC in the presence of catalysts comprising TiO 2 and TiO 2 /RGO blends can be calculated with the following equation: where A 0 and A t are the values of the absorbance of the band peaking at 320-338 nm, corresponding to exposure to UV light for 0 min and 100 min, respectively. Before each measurement, the UV-VIS spectra of samples under dark conditions for 30 min were recorded in order to establish the adsorption/desorption equilibrium of AC in relation to the catalysts. Figure 7 shows UV-VIS spectra of the aqueous solution of AC 0.2 mM in the presence of 0.05 mg/mL TiO 2 and 0.05 mg/mL, 0.1 mg/mL, and 0.2 mg/mL of TiO 2 /RGO blends with RGO sheet concentrations equal to 5 wt. %. measurement, the UV-VIS spectra of samples under dark conditions for 30 min were recorded in order to establish the adsorption/desorption equilibrium of AC in relation to the catalysts. Figure 7 shows UV-VIS spectra of the aqueous solution of AC 0.2 mM in the presence of 0.05 mg/mL TiO2 and 0.05 mg/mL, 0.1 mg/mL, and 0.2 mg/mL of TiO2/RGO blends with RGO sheet concentrations equal to 5 wt. %. According to Figure 7a, the UV-VIS spectrum of the TiO 2 dispersed in the AC aqueous solution shows two bands peaking at 240 nm and 338 nm, which have been assigned to electronic transitions of the types n-π* [32] and π-π* [33], respectively. Before exposure to UV light, increasing the concentration of TiO 2 /RGO blend in the AC aqueous solution from 0 mg/mL to 0.05 mg/mL, 0.1 mg/mL, and 0.2 mg/mL induced: (i) a blue shift of the band assigned to the π-π* electronic transition of the C=O bond in the amide group from 338 nm (Figure 7a Figure 7d). The exposure of the sample of TiO 2 dispersed in an AC aqueous solution to UV light leads to a gradual decrease in the absorbance of bands, as observed in Figure 7. The efficiency of AC photodegradation in the presence of TiO 2 is 43.5%. The addition of RGO sheets to TiO 2 in order to prepare TiO 2 /RGO blends with RGO concentrations of 5 wt. % leads to an increase in the efficiency of AC photodegradation. Thus, in the presence of 0.05 mg/mL, 0.1 mg/mL, and 0.2 mg/mL of TiO 2 /RGO blend with the RGO concentration of 5 wt.%, the AC photodegradation efficiency is equal to 80.41%, 87.64%, and 86.95% (Figure 7e). The greater efficiency in the case of the TiO 2 /RGO blends compared to TiO 2 can be explained by the fact that RGO has conductive properties that reduce the electronic accumulation on the surfaces of TiO 2 particles, thus leading to a reduction in the recombination of the electron-hole pair. This results in an increase in the photocatalytic properties of TiO 2 in the presence of RGO sheets. Figure 8 shows the influence of the concentration of RGO sheets in the TiO 2 /RGO blend. Before exposure to UV light, the ratio between the absorbance values of bands peaking at 240-244 nm and those at 338-320 nm is equal to~1.1 ( Figure 8a Figure 7d). The exposure of the sample of TiO2 dispersed in an AC aqueous solution to UV light leads to a gradual decrease in the absorbance of bands, as observed in Figure 7. The efficiency of AC photodegradation in the presence of TiO2 is 43.5%. The addition of RGO sheets to TiO2 in order to prepare TiO2/RGO blends with RGO concentrations of 5 wt. % leads to an increase in the efficiency of AC photodegradation. Thus, in the presence of 0.05 mg/mL, 0.1 mg/mL, and 0.2 mg/mL of TiO2/RGO blend with the RGO concentration of 5 wt.%, the AC photodegradation efficiency is equal to 80.41%, 87.64%, and 86.95% (Figure 7e). The greater efficiency in the case of the TiO2/RGO blends compared to TiO2 can be explained by the fact that RGO has conductive properties that reduce the electronic accumulation on the surfaces of TiO2 particles, thus leading to a reduction in the recombination of the electron-hole pair. This results in an increase in the photocatalytic properties of TiO2 in the presence of RGO sheets. Figure 8 shows the influence of the concentration of RGO sheets in the TiO2/RGO blend. Before exposure to UV light, the ratio between the absorbance values of bands peaking at 240-244 nm and those at 338-320 nm is equal to ~1.1 (Figure 8a After exposure to UV light, the photodegradation efficiency of the 0.2 mM AC aqueous solution with RGO concentrations equal to 10 wt. % and 20 wt. % in the TiO 2 /RGO blend (0.2 mg/mL) is equal to 76.95% and 69.69%, respectively (Figure 8c). Figure 9 shows the efficiency of TiO 2 /RGO blends with RGO concentrations of 5 wt. % in relation to an AC solution with concentrations of 0.1 mM and 0.4 mM. Before exposure to UV light, the increase in AC concentration from 0.1 mM (Figure 9a) to 0.4 mM (Figure 9b) induces a hypsochromic shift of the band assigned to the π-π* electronic transition from 322 nm (Figure 9a) to 310 nm (Figure 9b). Figure 8. Evolution of UV-VIS spectra of 0.2 mM AC aqueous solution in the presence of 0.2 mg/mL TiO2/RGO blends with RGO concentrations equal to 10 wt. % (a) and 20 wt. % (b) when the samples are exposed to UV light for 100 min. The degradation efficiency of the AC aqueous solution (0.2 M) in the presence of 0.2 mg/mL TiO2/RGO blends with RGO sheet concentrations of 10 wt. % (black curve) and 20 wt. % (red curve) after subsequent exposure to UV light (c).
After exposure to UV light, the photodegradation efficiency of the 0.2 mM AC aqueous solution with RGO concentrations equal to 10 wt. % and 20 wt. % in the TiO2/RGO blend (0.2 mg/mL) is equal to 76.95% and 69.69%, respectively (Figure 8c). Figure 9 shows the efficiency of TiO2/RGO blends with RGO concentrations of 5 wt. % in relation to an AC solution with concentrations of 0.1 mM and 0.4 mM. Before exposure to UV light, the increase in AC concentration from 0.1 mM (Figure 9a) to 0.4 mM (Figure 9b) induces a hypsochromic shift of the band assigned to the π-π* electronic transition from 322 nm (Figure 9a) to 310 nm (Figure 9b).  Figure 10 shows the evolution of the UV-VIS spectra of the TiO 2 /RGO blend (0.2 mg/mL) with an RGO concentration equal to 5 wt. % dispersed in a 0.2 mM AC solution containing excipients resulting from the dissolution of paracetamol in distilled water. When samples are in the dark for 30 min, no changes in the UV-VIS spectrum of the TiO 2 /RGO blend dispersed in 0.2 mM paracetamol aqueous solution occurred (Figure 10a). After exposure to UV light, a gradual decrease in the absorbance can be observed (Figure 10b). The photodegradation efficiency of paracetamol in the presence of a TiO 2 /RGO blend with an RGO concentration equal to 5 wt. % is 61.3% (Figure 10c). The lower AC photodegradation efficiency in the presence of the TiO 2 /RGO blend with an RGO concentration of 5 wt. % is related to the presence of the excipients of the paracetamol (Magistra C&C), namely, povidone, corn starch, croscarmellose sodium, and stearic acid.
When samples are in the dark for 30 min, no changes in the UV-VIS spectrum of the TiO2/RGO blend dispersed in 0.2 mM paracetamol aqueous solution occurred ( Figure  10a). After exposure to UV light, a gradual decrease in the absorbance can be observed (Figure 10b). The photodegradation efficiency of paracetamol in the presence of a TiO2/RGO blend with an RGO concentration equal to 5 wt. % is 61.3% (Figure 10c). The lower AC photodegradation efficiency in the presence of the TiO2/RGO blend with an RGO concentration of 5 wt. % is related to the presence of the excipients of the paracetamol (Magistra C&C), namely, povidone, corn starch, croscarmellose sodium, and stearic acid.

Kinetic of the AC Photodegradation in the Presence of the TiO2/RGO Blends
The degradation rate constant of AC can be calculated using the equation:

Kinetic of the AC Photodegradation in the Presence of the TiO 2 /RGO Blends
The degradation rate constant of AC can be calculated using the equation: where A t and A 0 are the absorbance values of the band peaking at 320-338 nm for the sample measured at t = 2-100 min and 0 min, respectively; k is the rate constant of the photodegradation reaction of AC at time (t). Figure 11a, showing the photodegradation of AC in the presence of TiO 2 particles, adheres to a pseudo-first-order kinetic model. In the case of the TiO 2 /RGO blends, three linear regions are remarked in Figure 11b-d. The three linear regions correspond to: (i) the generation of the intermediate products caused by the photodegradation of the AC adsorbed onto the TiO 2 /RGO blend's surface, with a rate constant of k 1 ; (ii) the products resulting from ring cleavage [12] under UV light with a rate constant of k 2 ; and (iii) the surface saturation process, with a rate constant of k 3 . adheres to a pseudo-first-order kinetic model. In the case of the TiO2/RGO blends, three linear regions are remarked in Figure 11b-d. The three linear regions correspond to: i) the generation of the intermediate products caused by the photodegradation of the AC adsorbed onto the TiO2/RGO blend's surface, with a rate constant of k1; ii) the products resulting from ring cleavage [12] under UV light with a rate constant of k2; and iii) the surface saturation process, with a rate constant of k3. The reaction rate constant values for TiO2 and the three TiO2/RGO blends, as well as the corresponding linear regression coefficients for each stage, are shown in Table 1. According to Table 1, the rate constants show higher values in the cases of the TiO2/RGO blends with RGO concentrations equal to 5 wt. % and 10 wt. %, in comparison with TiO2. This suggests the higher loading of RGO sheets onto the TiO2 nanoparticles' surfaces. The photocatalytic activity of the TiO2/RGO blends can be explained via mechanisms proposed by Moctezuma et al. [34] and Tao et al. [12]. Figure 12 shows the mechanism of AC degradation in the presence of TiO2/RGO blends. The reaction rate constant values for TiO 2 and the three TiO 2 /RGO blends, as well as the corresponding linear regression coefficients for each stage, are shown in Table 1. According to Table 1, the rate constants show higher values in the cases of the TiO 2 /RGO blends with RGO concentrations equal to 5 wt. % and 10 wt. %, in comparison with TiO 2 . This suggests the higher loading of RGO sheets onto the TiO 2 nanoparticles' surfaces. The photocatalytic activity of the TiO 2 /RGO blends can be explained via mechanisms proposed by Moctezuma et al. [34] and Tao et al. [12]. Figure 12 shows the mechanism of AC degradation in the presence of TiO 2 /RGO blends. According to Figure 12, under UV light, holes are generated in the valence band (VB), and electrons appear in the conduction band (CB) of TiO 2 . Electrons from the CB of TiO 2 are transferred to the RGO sheets, which can then interact with the O 2 molecules dissolved in the AC aqueous solution, leading to the production of reactive oxygen species [35]. The holes from the VB of TiO 2 can interact with the H 2 O, resulting in hydroxyl radicals, which, as unstable species, react with the AC when degradation products are created [36]. The significant role played by the hydroxyl radicals is highlighted when using isopropyl alcohol (IPA) as the quencher in the TiO 2 /RGO blends occurs Figure 13 shows the UV-VIS spectra of 0.2 mM AC solution in the presence of 0.2 mg/mL TiO 2 /RGO blend with a concentration of RGO of 5 wt. % and IPA. The exposure of this sample to UV light induces a decrease in the absorbance of the band peaking at 346 nm as the irradiation time is increased from 0 to 100 min. After 100 min of UV light exposure, the degradation efficiency reaches 60.25%, a value smaller than that reported in the case of the TiO 2 /RGO blend with a concentration of RGO of 5 wt. %-86.95% (blue curve in Figure 3e).
Molecules 2021, 28, x FOR PEER REVIEW According to Figure 12, under UV light, holes are generated in the valence ba and electrons appear in the conduction band (CB) of TiO2. Electrons from the C are transferred to the RGO sheets, which can then interact with the O2 molecules d in the AC aqueous solution, leading to the production of reactive oxygen species holes from the VB of TiO2 can interact with the H2O, resulting in hydroxyl radical as unstable species, react with the AC when degradation products are created significant role played by the hydroxyl radicals is highlighted when using isopro hol (IPA) as the quencher in the TiO2/RGO blends occurs Figure 13 shows the spectra of 0.2 mM AC solution in the presence of 0.2 mg/mL TiO2/RGO blend w centration of RGO of 5 wt. % and IPA. The exposure of this sample to UV light i decrease in the absorbance of the band peaking at 346 nm as the irradiation ti creased from 0 to 100 min. After 100 min of UV light exposure, the degradation e reaches 60.25%, a value smaller than that reported in the case of the TiO2/RGO bl a concentration of RGO of 5 wt. %-86.95% (blue curve in Figure 3e).   According to Figure 12, under UV light, holes are generated in the valence ba and electrons appear in the conduction band (CB) of TiO2. Electrons from the CB are transferred to the RGO sheets, which can then interact with the O2 molecules d in the AC aqueous solution, leading to the production of reactive oxygen species [ holes from the VB of TiO2 can interact with the H2O, resulting in hydroxyl radicals as unstable species, react with the AC when degradation products are created [ significant role played by the hydroxyl radicals is highlighted when using isoprop hol (IPA) as the quencher in the TiO2/RGO blends occurs Figure 13 shows the spectra of 0.2 mM AC solution in the presence of 0.2 mg/mL TiO2/RGO blend wit centration of RGO of 5 wt. % and IPA. The exposure of this sample to UV light in decrease in the absorbance of the band peaking at 346 nm as the irradiation tim creased from 0 to 100 min. After 100 min of UV light exposure, the degradation effi reaches 60.25%, a value smaller than that reported in the case of the TiO2/RGO ble a concentration of RGO of 5 wt. %-86.95% (blue curve in Figure 3e).    Figure 14 shows the SERS spectra of the TiO 2 /RGO blend with a concentration of RGO of 20 wt. % after the photodegradation of a 0.2 mM AC aqueous solution.

The Vibrational Properties of the TiO 2 /RGO Blend after AC Photodegradation
The Raman spectra of the PVC membrane modified with Au nanoparticles are characterized by lines peaking at 640-694, 1435, and 2914 cm −1 , assigned to the vibrational modes of C-Cl stretching, C-H symmetrical stretching in the CH 2 group, and C-H asymmetrical stretching in the CH 2 group, respectively [37][38][39][40]. In the case of the PVC membrane modified with Au nanoparticles adsorbed onto the TiO 2 /RGO blend derived from the photodegraded AC solution, the Raman spectra show: i) the three lines that belong to PVC peak at 636-700, 1431, and 2914 cm −1 ; and ii) the other three lines peak at 145, 1302 and 1599 cm −1 , the first of which belongs to the E g vibrational mode in TiO 2 A, and the latter two of which are assigned to the breathing vibrational mode of the carbon hexagonal rings and the E 2 g phonon mode of RGO [29]. A careful analysis of the red curves in Figures 1 and 14, i.e., of the Raman lines belonging to the constituents of the TiO 2 /RGO blend with an RGO concentration of 20 wt. %, before and after AC photodegradation, shows no changes. This indicates that no changes occurred in the vibrational properties of the TiO 2 /RGO blend during the AC photodegradation and that the catalyst can be used again in the pollutant removal process. In light of the results shown in this section, we note that the Raman lines of the TiO 2 /RGO blend were observed only in the case of PVC membranes modified with Au nanoparticles. The absence of the Raman lines of the TiO 2 /RGO blend in the case of the PVC membranes that were not modified with Au nanoparticles can be explained as a consequence of the low concentration of the TiO 2 /RGO blend (0.2 mg/mL) in the AC aqueous solution. In the present work, we overcame this issue by modifying the PVC membrane with Au nanoparticles, following the generation of surface plasmons (SPs) at the interface of the Au nanoparticles and the TiO 2 /RGO blend. As is well known, SPs have a significant role in the electromagnetic mechanism of the surface-enhanced Raman scattering (SERS) process, which explains the increase in the intensity of Raman spectra of various compounds. Considering the results shown in Figure 14, we can conclude that PVC membranes modified with Au nanoparticles can be used both as filters in wastewater management and as SERS supports.
Molecules 2021, 28, x FOR PEER REVIEW Figure 14 shows the SERS spectra of the TiO2/RGO blend with a concentr RGO of 20 wt. % after the photodegradation of a 0.2 mM AC aqueous solution. The Raman spectra of the PVC membrane modified with Au nanoparticles a acterized by lines peaking at 640-694, 1435, and 2914 cm −1 , assigned to the vib modes of C-Cl stretching, C-H symmetrical stretching in the CH2 group, and C-H metrical stretching in the CH2 group, respectively [37][38][39][40]. In the case of the PV brane modified with Au nanoparticles adsorbed onto the TiO2/RGO blend deriv the photodegraded AC solution, the Raman spectra show: i) the three lines that b PVC peak at 636-700, 1431, and 2914 cm −1 ; and ii) the other three lines peak at 1 and 1599 cm −1 , the first of which belongs to the Eg vibrational mode in TiO2 A, latter two of which are assigned to the breathing vibrational mode of the carbon he rings and the E2g phonon mode of RGO [29]. A careful analysis of the red curves in 1 and 14, i.e., of the Raman lines belonging to the constituents of the TiO2/RGO ble an RGO concentration of 20 wt. %, before and after AC photodegradation, sh changes. This indicates that no changes occurred in the vibrational propertie TiO2/RGO blend during the AC photodegradation and that the catalyst can be use in the pollutant removal process. In light of the results shown in this section, we n  Figure 15 shows the variation in the degradation efficiency of the 0.2 mM AC aqueous solution in the presence of 0.2 mg/mL TiO 2 /RGO blend with a concentration of RGO of 5%. After the first cycle of AC photodegradation, the solution was centrifuged to separate the TiO 2 /RGO blend from the aqueous solution. Subsequently, the TiO 2 /RGO blend was redispersed in distilled water and then dried at 80 • C in a vacuum for 30 min. This protocol was also carried out after each AC photodegradation cycle.

The Stability of the TiO 2 /RGO Blends
According to Figure 15, when using the TiO 2 /RGO blend with a concentration of RGO of 5 wt.% in the five photodegradation cycles, a decrease in the degradation efficiency of AC from 95.18% to 89.2% was reported. No variation in the Raman spectra of the TiO 2 /RGO blend was observed after each photodegradation cycle of the AC solution. In the context of the photocatalytic degradation of AC in the presence of the catalysts based on TiO 2 and RGO, the best results were published in Ref. [14] when a photodegradation efficiency of 100% was reported when the concentrations of AC and catalyst were equal to 0.05 mg/mL and 2 mg/mL, respectively. In our case, the photodegradation efficiency of AC in the presence of TiO 2 /RGO blend with a concentration of RGO equal to 5 wt.% was 95.18% when the concentrations of AC and catalyst were equal to 0.2 mM (equivalent to 0.03 mg/mL) and 0.2 mg/mL, respectively. An important advantage of the TiO 2 /RGO blends, prepared by the solid-state interaction, is their higher stability. The studies previously reported by A.H.C. Khavar et al. have highlighted that the reuse of the catalyst led to a 12% decrease in efficiency after five successive photodegradation cycles of AC [14]. In the present work, the photodegradation efficiency of AC by reusing the catalyst after five successive photodegradation cycles was only 5.98% (Figure 15). The drying of the catalyst at a temperature of 100 • C in air, time of 30 min, leads to a decrease in the photodegradation efficiency from 95.18% to 91.55%, i.e., only 3.63% (Figure 16).
olecules 2021, 28, x FOR PEER REVIEW redispersed in distilled water and then dried at 80 °C in a vacuum for 30 min was also carried out after each AC photodegradation cycle. According to Figure 15, when using the TiO2/RGO blend with a concent of 5 wt.% in the five photodegradation cycles, a decrease in the degradatio AC from 95.18% to 89.2% was reported. No variation in the Raman s TiO2/RGO blend was observed after each photodegradation cycle of the A the context of the photocatalytic degradation of AC in the presence of the c on TiO2 and RGO, the best results were published in Ref. [14] when a pho efficiency of 100% was reported when the concentrations of AC and catalyst 0.05 mg/mL and 2 mg/mL, respectively. In our case, the photodegradatio AC in the presence of TiO2/RGO blend with a concentration of RGO equal 95.18% when the concentrations of AC and catalyst were equal to 0.2 mM 0.03 mg/mL) and 0.2 mg/mL, respectively. An important advantage of blends, prepared by the solid-state interaction, is their higher stability. The ously reported by A.H.C. Khavar et al. have highlighted that the reuse of t to a 12% decrease in efficiency after five successive photodegradation cycles the present work, the photodegradation efficiency of AC by reusing the cat successive photodegradation cycles was only 5.98% (Figure 15). The drying at a temperature of 100 °C in air, time of 30 min, leads to a decrease in the p tion efficiency from 95.18% to 91.55%, i.e., only 3.63% (Figure 16).  According to Figure 15, when using the TiO2/RGO blend with of 5 wt.% in the five photodegradation cycles, a decrease in the d AC from 95.18% to 89.2% was reported. No variation in the TiO2/RGO blend was observed after each photodegradation cycl the context of the photocatalytic degradation of AC in the presen on TiO2 and RGO, the best results were published in Ref. [14] wh efficiency of 100% was reported when the concentrations of AC an 0.05 mg/mL and 2 mg/mL, respectively. In our case, the photod AC in the presence of TiO2/RGO blend with a concentration of RG 95.18% when the concentrations of AC and catalyst were equal to 0.03 mg/mL) and 0.2 mg/mL, respectively. An important adva blends, prepared by the solid-state interaction, is their higher stab ously reported by A.H.C. Khavar et al. have highlighted that the to a 12% decrease in efficiency after five successive photodegradat the present work, the photodegradation efficiency of AC by reusi successive photodegradation cycles was only 5.98% (Figure 15). T at a temperature of 100 °C in air, time of 30 min, leads to a decrea tion efficiency from 95.18% to 91.55%, i.e., only 3.63% ( Figure 16). This fact allows us to conclude that the TiO 2 /RGO blends prepared in this work show suitable stability in the photodegradation processes of AC.
In order to highlight the performance of the catalyst in real conditions, the testing of the TiO 2 /RGO blend in the mineral spring water containing 2 mg/mL AC was carried out.
Using the TiO 2 /RGO blend with a concentration of RGO equal to 5 wt. %, the degradation efficiency of 2 mg/mL AC in mineral spring water was only 71.29% (Figure 17).
This fact allows us to conclude that the TiO2/RGO blends prep suitable stability in the photodegradation processes of AC.
In order to highlight the performance of the catalyst in real co the TiO2/RGO blend in the mineral spring water containing 2 mg/m Using the TiO2/RGO blend with a concentration of RGO equal to 5 efficiency of 2 mg/mL AC in mineral spring water was only 71.29% Figure 17. Degradation efficiency of 2 mg/mL AC in mineral spring wa mg/mL TiO2/RGO blends with an RGO concentration of 5 wt. % after sub light.
In this stage of our studies, it is significant to note that, dep wastewater, various technologies for the treatment of contamina photodegradation processes are often influenced by the che wastewater [42]. The chemical composition of the mineral spring w was as follows: 2.88 mg/L Na + , 1.18 mg/L K + , 2.63 mg/L Mg 2+ , 9.5 SO4 2− , 48.8 mg/L HCO3 − , 74 mg/L NO3 − , and 0.01 mg/L NO2, the sa to 7.04. We are tempted to explain the decrease in the photodegrad to be induced by the ions SO4 2− , NO3 − , Na + , and K + , which have b that hamper the degradation of organic pollutants [42].

Materials and Methods
The TiO2 particles, PVC, IPA, and tetrahydrofuran (THF) Sigma-Aldrich (St. Louis, MO, USA) as raw materials with a purity to our previous study, TiO2 particles contain both anatase and ru [43]. RGO sheets were prepared from graphene oxide sheets and to the protocol published by D.C. Marcano et al. [44], which wer hydrazine [29].
The TiO2/RGO blends were prepared by the solid-state interac TiO2 particles with RGO sheets by grinding the two compounds fo in the color of the TiO2 powder from white to gray was observed. T with RGO concentration of 5 wt. %, 10 wt. %, and 20 wt. % were pr In this stage of our studies, it is significant to note that, depending on the type of wastewater, various technologies for the treatment of contaminants are used [41], and photodegradation processes are often influenced by the chemical composition of wastewater [42]. The chemical composition of the mineral spring water used in this work was as follows: 2.88 mg/L Na + , 1.18 mg/L K + , 2.63 mg/L Mg 2+ , 9.53 mg/l Ca 2+ , 0.03 mg/L SO 4 2− , 48.8 mg/L HCO 3 − , 74 mg/L NO 3 − , and 0.01 mg/L NO 2 , the sample having pH equal to 7.04. We are tempted to explain the decrease in the photodegradation efficiency of AC to be induced by the ions SO 4 2− , NO 3 − , Na + , and K + , which have been reported as agents that hamper the degradation of organic pollutants [42].

Materials and Methods
The TiO 2 particles, PVC, IPA, and tetrahydrofuran (THF) were purchased from Sigma-Aldrich (St. Louis, MO, USA) as raw materials with a purity of 99.99%. According to our previous study, TiO 2 particles contain both anatase and rutile crystalline phases [43]. RGO sheets were prepared from graphene oxide sheets and synthesized according to the protocol published by D.C. Marcano et al. [44], which were then interacted with hydrazine [29].
The TiO 2 /RGO blends were prepared by the solid-state interaction of the commercial TiO 2 particles with RGO sheets by grinding the two compounds for 15 min until a change in the color of the TiO 2 powder from white to gray was observed. Three TiO 2 /RGO blends with RGO concentration of 5 wt. %, 10 wt. %, and 20 wt. % were prepared.
In order to highlight the vibrational properties of the catalyst after AC photodegradation, a TiO 2 /RGO blend with 20% RGO was vacuum-filtered using a poly(vinyl chloride) (PVC) membrane with a diameter of 1 cm, on the surface of which 0.5 mL Au nanoparticles with a size of 10 nm were sprayed. The PVC membrane was prepared by the phase inversion method [45]. Briefly, 0.4 g PVC grains were dissolved in 10 mL THF under ultrasonication. The resulting solution was poured into a Petri vessel in order to carry out a thermal treatment at a temperature of 100 • C for 1 h to enable DMF evaporation. After this, the PVC membrane was removed from the Petri vessel and immersed in a water bath in order to remove the residual DMF. Finally, the PVC membrane was dried to a constant mass with a thickness of 40 µm. The PVC membrane modified with Au nanoparticles was used for the vacuum filtration of the TiO 2 /RGO blend after the photodegradation of the AC solution and the drying of the membrane to a constant mass. The modification of the PVC membrane with Au nanoparticles was achieved by spraying 0.5 mL of Au nanoparticles onto the PVC membrane's surface and then performing an annealing treatment at 100 • C for 20 min in order to fix the Au nanoparticles onto the PVC membrane. Using the method published by Piella et al. [46], Au nanoparticles of 10 nm were synthesized. The modification of PVC membranes with Au nanoparticles was undertaken such that these membranes could be used as supports for surface-enhanced Raman scattering (SERS) spectroscopy.
In order to assess the photocatalytic properties of the four TiO 2 /RGO blends used in the photodegradation of AC, colloidal dispersions were prepared by adding each blend to an aqueous solution of AC followed by magnetic stirring for 20 min at 25 o C in the dark, until reaching the adsorption-desorption equilibrium of AC on the surfaces of the TiO 2 /RGO blend catalysts. All colloidal dispersions were subsequently irradiated with UV light for 100 min with a halogen lamp of 100 W.
The Raman spectra of the three TiO 2 /RGO blends and TiO 2 particles were recorded with a MultiRam FT-Raman spectrophotometer from Bruker (Bruker Optik GmbH, Ettlingen, Germany), in the backscattering geometry, with a resolution of 1 cm −1 . The characterization of the TiO 2 /RGO blend with 20 wt. % RGO, after its use for AC removal, was also performed by Raman scattering.
The IR spectra of the three TiO 2 /RGO blends and TiO 2 particles were recorded with a Vertex 80 FTIR spectrophotometer from Bruker (Billerica, MA, USA), with a resolution of 2 cm −1 .
XPS spectra of the RGO sheets, TiO 2 particles, and TiO 2 /RGO blends were recorded with a SPECS spectrometer endowed with an Al Kα source and a PHOBIOS 150 analyzer.
SEM and EDS analyses of the TiO 2 particles, RGO sheets, and TiO 2 /RGO blends were undertaken with a Zeiss Gemini 500 field-emission scanning electron microscope with a Bruker EDS detector attached.
UV-VIS spectra of the solutions of AC containing TiO 2 /RGO blend catalysts, and TiO 2 particles were recorded with a Lambda 950 UV-VIS-NIR spectrophotometer from Perkin Elmer (PerkinElmer, Inc., Waltham, MA, USA), with a resolution in the UV-VIS range of 0.05 nm.

Conclusions
In this work, we have reported novel results concerning the photocatalytic properties of TiO 2 /RGO blends prepared by the solid-state interaction of their two constituents in relation to AC degradation. Using IR spectroscopy and Raman scattering, we have demonstrated that with increases in the RGO concentration in the TiO 2 /RGO blend, the following occurs: a) A shift in the IR band, assigned to the Ti-O-Ti vibrational mode, from 692 cm −1 to 698 cm −1 , simultaneously with an increase in the absorbance of the IR band peaking at 3627-3728 cm −1 , which is assigned to the stretching vibrational mode of the OH bond. The increase in absorbance of the IR bands peaking at 1737 and 3627-3728 cm −1 indicates preferential adsorption of TiO 2 particles onto the RGO sheets' surfaces via the water molecules on the TiO 2 particles' surfaces. b) A decrease in the Raman line peaking at 147 cm −1 , attributed to the E g mode in TiO 2 nanoparticles that show an anatase crystalline structure, the variation in which is accompanied by the presence of the D and G bands of the RGO sheets and the shift of the D band from 1292 cm −1 to 1301 cm −1 . The best photocatalytic properties were obtained for the TiO 2 /RGO blend with an RGO concentration of 5 wt. %, when approx. 95.18% AC removal was achieved after 100 min of UV irradiation. The reaction kinetics of AC aqueous solutions in the presence of the TiO 2 /RGO blends with various RGO concentrations show a complex kinetic model of the pseudo-first-order, separated into three stages. The reuse of the TiO 2 /RGO blends after the first cycle of AC photodegradation indicates their suitable stability during all five cycles of the photodegradation of pharmaceutical compounds. In this paper, evidence is given regarding the use of PVC membranes modified with Au nanoparticles as filters for removing the catalyst from decontaminated water and as SERS supports when seeking to identify the vibrational properties of recovered and reused catalysts.