Graphene-Based TiO 2 Nanocomposite for Photocatalytic Degradation of Dyes in Aqueous Solution under Solar-Like Radiation

: This study presents a novel method for the development of TiO 2 /reduced graphene oxide (rGO) nanocomposites for photocatalytic degradation of dyes in an aqueous solution. The synergistic integration of rGO and TiO 2 , through the formation of Ti–O–C bonds, offers an interesting opportunity to design photocatalyst nanocomposite materials with the maximum absorption shift to the visible region of the spectra, where photodegradation can be activated not only with UV but also with the visible part of natural solar irradiation. TiO 2 @rGO nanocomposites with different content of rGO have been self-assembled by the hydrothermal method followed by calcination treatment. The morphological and structural analysis of the synthesized photocatalysts was performed by FTIR, XRD, XPS, UV-Vis DRS, SEM/EDX, and Raman spectroscopy. The effectiveness of the synthesized nanocomposites as photocatalysts was examined through the photodegradation of methylene blue (MB) and rhodamine B (RhB) dye under artiﬁcial solar-like radiation. The inﬂuence of rGO concentration (5 and 15 wt.%) on TiO 2 performance for photodegradation of the different dyes was monitored by UV-Vis spectroscopy. The obtained results showed that the synthesized TiO 2 @rGO nanocomposites signiﬁcantly increased the decomposition of RhB and MB compared to the synthesized TiO 2 photocatalyst. Furthermore, TiO 2 @rGO nanocomposite with high contents of rGO (15 wt.%) presented an improved performance in photodegradation of MB (98.1%) and RhB (99.8%) after 120 min of exposition to solar-like radiation. These results could be mainly attributed to the decrease of the bandgap of synthesized TiO 2 @rGO nanocomposites with the increased contents of rGO. Energy gap ( E g ) values of nanocomposites are 2.71 eV and 3.03 eV, when pure TiO 2 particles have 3.15 eV. These results show the potential of graphene-based TiO 2 nanocomposite to be explored as a highly efﬁcient solar light-driven photocatalyst for water puriﬁcation. of RhB and MB in an aqueous medium showed that the chemical integration of rGO with TiO 2 promoted synergistic effects which sped up the photodegradation of selected pollutants. The photodegradation efﬁciency of MB and RhB dye by the synthesized TiO 2 @rGO (5 wt.% of rGO) and TiO 2 @rGO (15 wt.% of rGO) nanocomposite photocatalysts after 120 min of exposure to the irradiation were signiﬁcantly higher than for pure synthesized TiO 2 nanoparticles. TiO 2 @rGO (5%) presented a photodegradation of 99.6% for MB and 99.9% for RhB. The nanocomposite with a higher amount of rGO (TiO 2 @rGO (15%)) showed similar photodegradation results, 98.1% for MB and 99.8% for RhB. Meanwhile, the photoactivity evaluation of synthesized TiO 2 particles was signiﬁcantly lower. Although these nanocomposite materials showed lower photocatalytic activity than P-25, their large lay-ered structure allows a facile recovery from the aqueous medium after the photocatalytic reaction. These preliminary results show that the prepared TiO 2 @rGO nanocomposite photocatalyst may be explored as high-efﬁciency and green photocatalysts for simultane-ous sorption and degradation of dyes that can be directly applicable in real efﬂuent water treatment by solar exposition.


Introduction
Nowadays, accessible clean water and energy resources are among the highest priorities for sustainable economic growth and societal wellbeing. Water scarcity is a growing worldwide problem, so it is imperative that wastewater is treated and re-used in industrial nanocomposite materials. Moreover, the improved performance of developed composites will rely on the influence of rGO and materials committed to its surface [26][27][28].
In this study, the synthesis parameters of the one-pot cost-effective hydrothermal procedure were optimized, followed by a calcination treatment at 300 • C to prepare TiO 2 @rGO nanocomposite-based photocatalyst. As model pollutants, MB and RhB dyes were exposed to the photocatalytic decomposition initiated by the solar-like irradiation source. The aim of this research was to contribute to a broader understanding of photocatalytic degradation of MB and RhB with additional criteria to prepare TiO 2 @rGO nanocomposite, showing significant photocatalytic activity.

Synthesis of Graphene Oxide (GO)
A chemical process called Hummer's method was used to manufacture the GO [29]. Firstly, flakes of graphite powder (3 g) were dispersed in a concentrated solution of H 2 SO 4 (69 mL) then NaNO 3 (1.5 g) was added to an Erlenmeyer flask. The solution was stirred on a magnetic stirrer until a homogeneous solution was obtained at a low temperature (0-5 • C). Then KMnO 4 (9.0 g) was gradually added to keep the reaction temperature below 20 • C. The mixture was warmed to 35 • C and shuffled for 30 min, during which 138 mL of water was slowly added. The mixture produced an exothermic reaction, spontaneously heating to 98 • C. During the next 15 min, the mixture was heated in order to hold the temperature at 98 • C, and then the reaction mixture was chilled in a water container for a few minutes to the room temperature. Then 420 mL of water and 3 mL of H 2 O 2 (30%) were added to the mixture. The resultant suspension was centrifugated at 3000 rpm for 10 min to remove the remaining impurities and to recover the GO. The isolated GO was first washed with HCl (10%) and then rinsed intensively with deionized water until it reached neutral pH.

Preparation of TiO 2 Colloidal Solution
The main precursor of titanium, TTIP, was added drop by drop in the i-propanol. Acetylacetone chelating agent was then added to the solution, followed by nitric acid acting as a catalyst. The chemicals were mixed in the molar ratio, 1:35:0.63:0.015. During the preparation, the mixture solution was constantly stirred, and clear yellow color of solvent was attained [30].

Preparation of TiO 2 @rGO Nanocomposites
The TiO 2 sol and GO nanosheets were mixed to produce TiO 2 @rGO nanocomposites by hydrothermal method. Different ratios of the prepared suspension of GO were added into a colloidal solution of TiO 2 sol and stirred for 1 h. After that, the black-brown mixture was homogenized for 10 min in an ultrasonic bath. The obtained product was transferred into a Teflon-lined autoclave tube. The hydrothermal reaction was performed at 180 • C for 8 h. The resultant nanocomposite material was extensible washed first with i-propanol and Appl. Sci. 2021, 11, 3966 4 of 15 then with deionized water until the neutral pH value was reached. The obtained TiO 2 @rGO nanocomposite was dried at 60 • C in the electric dryer for 1 h and then calcinated in the oven at 300 • C for 1 h.

Characterization of Photocatalysts
The crystalline phases of GO and TiO 2 @rGO were analyzed by powder X-ray diffraction (P-XRD, Rigaku) operating with CuKα radiation. Fourier transform infrared (FT-IR/ATR, Bruker Vertex 70)) spectra were performed to analyze the chemical groups in the structure of the nanocomposite materials obtained. To confirm the crystalline phase of GO and TiO 2 @rGO, Micro-Raman analyses were performed using a Bruker SENTERRA spectrometer with an Olympus microscope. X-ray photoelectron spectroscopy (XPS) performed on SPECS Phoibos 150 with AlKα radiation (1486.74 eV) was used for elemental composition determination. Binding Energy (BE) was corrected using the main peak of reduced GO as a reference, set at 284.4 eV. C 1s spectra of GO and rGO were normalized to get similar intensity to the other carbon spectra. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDX) were carried out with the aim of obtaining microstructural characteristics and elemental mapping of the samples. Measurements were made using Hitachi TM4000 plus tabletop SEM-EDX facility with 15 kV field energy in the backscattered electron (BSE) imaging mode and FEG-SEM model JSM-7600F from Jeol at 10 kV in the secondary electron (SE) mode. The bandgap energy of the prepared nanocomposite powders was acquired from UV/Vis/NIR spectroscopy measurements using a Perkin Elmer Lambda 950 spectrophotometer.

Photodegradation Experiments
Methylene blue dye (MB) and Rhodamin B dye (RhB) purchased from Sigma-Aldrich (St. Louis. MO, USA) have been used for monitoring the photoactivity of the synthesized TiO 2 @rGO nanocomposites. Each pollutant was dissolved in MilliQ water in order to reach a concentration of 10 mg/L. The photocatalysts (4.5 mg) were dispersed in 9 mL of the contaminant solution within a modified beaker reactor with quartz cover. As-prepared samples were left for 60 min in a chamber without irradiation. After the irradiation was turned on, the photocatalytic efficiency was monitored over 120 min. The magnetic stirrer was continuously turned on. Concentration change was measured by the adsorption decrease using a UV/VIS spectrophotometer (PerkinElmer Lambda 950) in set time intervals while irradiated by light in a climatic chamber equipped with the cooling system, using Osram's Ultra Vitalux lamp. The lamp produces a mix of radiation intervals that can be found in the natural solar radiation spectrum. Therefore, the term solar-like radiation for this radiation mix was used. The lamp and the solution in the reactor were set up at a distance of 20 cm.
From the slope of the straight line, the first-order rate constant was calculated, represented with the following equation [31]: where k (min −1 ) is the rate constant of photodecomposition of dye, A is the absorption of dye at the time of the photocatalytic process, and A 0 is the absorption at the beginning of the experiment [32,33]. The calculation of half-life (t 1/2 ) was done using the following equation [31]: The percentage of photocatalytic degradation efficiency of the prepared photocatalysts were calculated using the following equation [31]: where η is the percentage efficiency of the photodecomposition of dye, A 0 is the absorbance of the starting content of dye, and A t is the content of dye after irradiation at the time, t (min) during the photocatalytic experiment.

Characterization of Photocatalyst
The successful preparation of materials by the combination of the hydrothermal method and calcination can be demonstrated with detailed characterization of the obtained TiO 2 @rGO nanomaterials. The FT-IR spectra of natural graphite GO and rGO are presented in Figure 1A, while the spectra of TiO 2 and the nanocomposites are shown in Figure 1B. The GO spectrum present the following peaks at 3372 cm −1 corresponded to the stretching vibration of the carboxyl group (−OH), which was ascribed on the attendance of alcohol groups and absorbed water molecules, 2857 cm −1 and 2925 cm −1 vibrations corresponded to the symmetric and asymmetric CH 2 stretching of GO [34,35]. The stretching vibration at 1715 cm −1 ascribed to the C=O from carbonyl and carboxyl group and the vibration at 1622 cm 1 (C=C) as skeletal vibration of unoxidized graphitic materials [35]. The deformation vibration of C-OH stretch from the alcohol group was displayed at 1376 cm −1 [36,37]. The stretching vibrations at 1221 cm −1 were assigned to the C-O-C of epoxy groups, and at 1039 cm −1 were ascribed to the C-O from the alkoxy group [37]. By comparing the GO and rGO spectra, displayed in Figure 1A, it could be seen that in the rGO sample, that the intensity of peaks attributed to oxygen functional groups, e.g., O-H, C=O, and C-O, were reduced [38]. The TiO 2 @rGO nanocomposite spectra exhibited peaks between 450-900 cm −1 , which belonged to the stretching vibration of Ti-O-Ti and Ti-O-C bonds, affirming the effective interaction between Ti and C [39,40]. The Raman spectrum of TiO 2 is shown in Figure 2A (inset), highlighting five sharp peaks for the anatase-phase of TiO 2 . These peaks are located at 147 cm −1 , 198 cm −1 , and 641 cm −1 as Eg, 399 cm −1 as B1g, and 517 cm −1 as A1g + B1g [41]. As the figures show, these peaks appeared for all prepared nanocomposites, but in comparison to pure TiO 2 nanoparticles, the intensity of the anatase phase was strongly reduced in the nanocomposites. Figure 2A shows, the prepared nanocomposites with a smaller amount (5 wt.%) of rGO had sharper peaks than the prepared nanocomposites with a higher amount (15 wt.%). The crystalline phase of the nanocomposite was disturbed because of the content of rGO. Figure 2B presents the Raman spectra of GO, rGO, and nanocomposites. The GO spectrum showed the presence of the two characteristic peaks at 1329 cm −1 as D-band and 1585 cm −1 as G-band. On the rGO spectrum, two peaks appeared at 1347 cm −1 (D-band) and 1593 cm −1 (G-band). The vibrations of the peaks in rGO materials were shifted significantly in relation to GO materials. The reason for the shift is the structure of the graphene material, where the G band corresponds to the sp 2 hybridization in C-C bonds, and D band matches the sp 3 defects in carbon atoms as associated with structural defects [42,43]. The spectra of the prepared nanocomposites show characteristic peaks of anatase-phase TiO 2 and the presence of the D and G band. The binding of TiO 2 nanoparticles into rGO, as well as the intensities of anatase-phases in the nanocomposites, showed a significant decrease (Figure 2A,B). As Figure 2B shows and Table 1 confirms, the content of graphene has an influence on the intensity of G and D bands in the nanocomposites. The reason for the high-intensity ratio between D and G bands was the presence of a lot of sp 2 hybridization in C-C bonds [41]. The rGO shows a higher ratio than GO, indicating that the rGO contained more defects. The nanocomposites have a higher intensity ratio compared to GO indicating higher graphene structural disorder upon binding onto TiO 2 [43]. The interactions of Ti-O-C bands could be the reason for increasing sp 3 defects in nanocomposites [42]. The intensity ratios between D and G bands were utilized to determine the crystal size parallel to basal planes (L a ), using the equation of Tuinstra, where the coefficient 38.5 was for wavelength at 633 nm [44]: The recalculated values of L a are shown in Table 1. The La values decreased from GO over rGO to the prepared nanocomposites. It could be concluded that prepared nanocomposites had decreased sp 2 domains in the structure [31].
XRD patterns of pure graphite and synthesized GO are given in Figure 3A. Both of these materials had characteristic X-ray diffraction peaks. The pure graphite showed characteristic peaks at 26.48 • and 54.58 • attributed to the (002) and (004) planes. In particular, the diffraction peak of GO located at 10.68 • corresponded to the (001) plane. On the XRD pattern of rGO, after the reduction method of GO, a new and wide diffraction peak located at 24.40 • attributed to the (002) plane appeared. The (002) plane in rGO indicates that the most oxygen-containing groups such as O-H, C=O, and C-O were removed from the nanomaterials. The new diffraction peak confirmed the successful conversion of GO into rGO (inset of Figure 3A) [44][45][46]. XRD patterns of pure TiO 2 and prepared nanocomposites are shown in Figure 3B. The pure TiO 2 nanoparticles had diffraction peaks at 25 [37]. Specified diffraction peaks correspond to the anatase-phase of TiO 2 nanoparticles. As can be seen in Figure 3B pure TiO 2 nanoparticles and both nanocomposites show only anatase-phase peaks in the X-ray diffraction patterns. This could be a consequence of the overlapping of the powerful intensity of the anatase-phase peak at 25.30 • and the low diffraction peak of the rGO at 25.40 • [46,47]. Nevertheless, we noticed that the diffraction intensities of anatase peaks decreased in both nanocomposites, in particular the diffraction peak at 25.30 • . The reason for that can be in an increasing amount of GO with defects in carbon atoms. at 24.40° attributed to the (002) plane appeared. The (002) plane in rGO indicates that the most oxygen-containing groups such as O-H, C=O, and C-O were removed from the nanomaterials. The new diffraction peak confirmed the successful conversion of GO into rGO (inset of Figure 3A) [44][45][46]. XRD patterns of pure TiO2 and prepared nanocomposites are shown in Figure 3B. The pure TiO2 nanoparticles had diffraction peaks at 25.30°, 37.86°, 47.98°, 53.94°, 55.06°, 62.62°, 68.68°, 70.18°, and 75.12° and were indexed to the (101), (103), (004), (200), (105), (211), (204), (116), (220), and (215) plane, respectively [37]. Specified diffraction peaks correspond to the anatase-phase of TiO2 nanoparticles. As can be seen in Figure 3B pure TiO2 nanoparticles and both nanocomposites show only anatasephase peaks in the X-ray diffraction patterns. This could be a consequence of the overlapping of the powerful intensity of the anatase-phase peak at 25.30° and the low diffraction peak of the rGO at 25.40° [46,47]. Nevertheless, we noticed that the diffraction intensities of anatase peaks decreased in both nanocomposites, in particular the diffraction peak at 25.30°. The reason for that can be in an increasing amount of GO with defects in carbon atoms. XPS was performed in order to analyze the elemental composition and chemical environment of the elements detected at the surface of the prepared nanocomposite materials. Figure 4A shows a comparison between the high-resolution C 1s spectra of the different samples. The C 1s spectrum of GO shows the expected shape for this nanomaterial, with two intense peaks and a tail at higher BE. The peak at lower BE (284.4 eV) corresponded to the net of interconnected carbon atoms with a combination of sp 2 and sp 3 hybridization, derived from the breakdown of graphite layers [48]. The second peak and the tail were associated with carbon bonded to oxygen in a variety of functional groups, with a higher degree of oxidation as BE increased. This region of the spectrum was fitted using components centered at 286.4 eV, 288.2 eV, and 289.5 eV, corresponding to C-O, C=O, and O-C=O bonds, respectively [48]. In comparison, the rGO spectrum shows an intense peak at 284.4 eV with a smaller shoulder elongated towards higher BE values. The main peak, related to non-oxygenated carbon, was fitted using an asymmetric function according to the higher proportion of carbon sp 2 after the reduction of GO. The left side of the spectrum includes the contribution of different functional groups containing oxygen, just like in GO but with a noticeable decrease of intensity, indicating effective deoxygenation of GO nanosheets [48]. The carbon region of the TiO2 sample reveals the typical spectrum shape of adventitious carbon contamination. It was fitted using the main peak at 285.3 eV, compatible with hydrocarbons, and two smaller components at 286.7 eV and 289.4 eV, related to the C-O and O-C=O groups. C 1s spectra of the TiO2@rGO nanocomposites (5 wt.% and 15 wt.% of rGO) combine components from both rGO and TiO2 samples. This fact was easily perceptible in the spectrum of TiO2@rGO (5 wt.% of rGO), which showed a shape XPS was performed in order to analyze the elemental composition and chemical environment of the elements detected at the surface of the prepared nanocomposite materials. Figure 4A shows a comparison between the high-resolution C 1s spectra of the different samples. The C 1s spectrum of GO shows the expected shape for this nanomaterial, with two intense peaks and a tail at higher BE. The peak at lower BE (284.4 eV) corresponded to the net of interconnected carbon atoms with a combination of sp 2 and sp 3 hybridization, derived from the breakdown of graphite layers [48]. The second peak and the tail were associated with carbon bonded to oxygen in a variety of functional groups, with a higher degree of oxidation as BE increased. This region of the spectrum was fitted using components centered at 286.4 eV, 288.2 eV, and 289.5 eV, corresponding to C-O, C=O, and O-C=O bonds, respectively [48]. In comparison, the rGO spectrum shows an intense peak at 284.4 eV with a smaller shoulder elongated towards higher BE values. The main peak, related to non-oxygenated carbon, was fitted using an asymmetric function according to the higher proportion of carbon sp 2 after the reduction of GO. The left side of the spectrum includes the contribution of different functional groups containing oxygen, just like in GO but with a noticeable decrease of intensity, indicating effective deoxygenation of GO nanosheets [48]. The carbon region of the TiO 2 sample reveals the typical spectrum shape of adventitious carbon contamination. It was fitted using the main peak at 285.3 eV, compatible with hydrocarbons, and two smaller components at 286.7 eV and 289.4 eV, related to the C-O and O-C=O groups. C 1s spectra of the TiO 2 @rGO nanocomposites (5 wt.% and 15 wt.% of rGO) combine components from both rGO and TiO 2 samples. This fact was easily perceptible in the spectrum of TiO 2 @rGO (5 wt.% of rGO), which showed a shape similar to the adventitious carbon spectra but with a shoulder at lower BE, towards the position of rGO. This way, both spectra (5 wt.% and 15 wt.% of rGO) were fitted using four components, the first one related to the C net of rGO, at 284.4 eV; the second one coinciding with the energy of hydrocarbons in TiO 2 ; and two more components associated to oxygen functionalities. As expected, there is a noticeable increase in the area of the component similar to the adventitious carbon spectra but with a shoulder at lower BE, towards the position of rGO. This way, both spectra (5 wt.% and 15 wt.% of rGO) were fitted using four components, the first one related to the C net of rGO, at 284.4 eV; the second one coinciding with the energy of hydrocarbons in TiO2; and two more components associated to oxygen functionalities. As expected, there is a noticeable increase in the area of the component associated with rGO in the nanocomposite containing 15 wt.% of rGO with respect to one having 5 wt.%. High-resolution Ti 2p spectra of TiO2 and TiO2@rGO nanocomposites ( Figure 4B) showed two symmetric peaks at 459.2 eV and 464.9 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. Both the BE values and spin-orbit splitting of 5.7 eV were in good agreement with Ti (IV) in a TiO2 chemical environment [44]. Supporting this, Figure 4C shows the O 1s spectra of TiO2 and both TiO2@rGO nanocomposites. All of them had a main peak at 530.4 eV, assigned to oxygen bonded to titanium, and a smaller and wider component High-resolution Ti 2p spectra of TiO 2 and TiO 2 @rGO nanocomposites ( Figure 4B) showed two symmetric peaks at 459.2 eV and 464.9 eV, corresponding to Ti 2p 3/2 and Ti 2p 1/2 , respectively. Both the BE values and spin-orbit splitting of 5.7 eV were in good agreement with Ti (IV) in a TiO 2 chemical environment [44]. Supporting this, Figure 4C shows the O 1s spectra of TiO 2 and both TiO 2 @rGO nanocomposites. All of them had a main peak at 530.4 eV, assigned to oxygen bonded to titanium, and a smaller and wider component around 531.8 eV, commonly ascribed to hydroxyl groups covering the surface oxygen vacancies in the TiO 2 structure [49,50], and also compatible with oxygen in organic compounds.
The synthesized TiO 2 and TiO 2 @rGO nanocomposites and commercially available P-25 were analyzed by scanning electron microscope (SEM). Representative images of the analyzed powders are shown in Figure 5. The size of P-25 grains was around 30 nm ( Figure 5A). The synthesized TiO 2 particles were half smaller and were spherical in shape, just like P-25 ( Figure 5B). SEM images of synthesized GO and prepared rGO are displayed in Supplementary Figure S1. Figure 5C,D shows the TiO 2 @rGO nanocomposites with 5 and 15 wt.% of rGO, which aggregated into larger forms. It can be seen that TiO 2 particles had a similar size as pure powder, and they were embedded in the surface of rGO sheets (insets in Figure 5C,D) [51,52]. To confirm the homogeneous distribution of elements in the prepared nanocomposite of TiO 2 @rGO (15 wt%), elementary mapping is shown in Supplementary Figure S2.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 16 around 531.8 eV, commonly ascribed to hydroxyl groups covering the surface oxygen vacancies in the TiO2 structure [49,50], and also compatible with oxygen in organic compounds.
The synthesized TiO2 and TiO2@rGO nanocomposites and commercially available P-25 were analyzed by scanning electron microscope (SEM). Representative images of the analyzed powders are shown in Figure 5. The size of P-25 grains was around 30 nm (Figure 5A). The synthesized TiO2 particles were half smaller and were spherical in shape, just like P-25 ( Figure 5B). SEM images of synthesized GO and prepared rGO are displayed in Supplementary Figure S1. Figure 5C,D shows the TiO2@rGO nanocomposites with 5 and 15 wt.% of rGO, which aggregated into larger forms. It can be seen that TiO2 particles had a similar size as pure powder, and they were embedded in the surface of rGO sheets (insets in Figure 5C,D) [51,52]. To confirm the homogeneous distribution of elements in the prepared nanocomposite of TiO2@rGO (15 wt%), elementary mapping is shown in Supplementary Figure S2.

Photocatalytic Activity of TiO2@rGO Nanocomposites
Diffuse reflectance UV-Vis spectroscopy for modulation of the bandgap energy of the synthesized materials was calculated by Kubelka-Munk function, as shown in Figure 6. The bandgap results were obtained, giving Eg values of 2.71 eV, 3.03 eV, and 3.15 eV for synthesized TiO2@rGO nanocomposites and for pure TiO2 particles. The bandgap energy of reference P-25 material was compared and displayed in the Supplementary Figure S3. The obtained results indicate that the decrease of the bandgap of the synthesized nanocomposites occurs with the introduction of rGO sheets. It can be observed that the photoactivity could be shifted into visible light.

Photocatalytic Activity of TiO 2 @rGO Nanocomposites
Diffuse reflectance UV-Vis spectroscopy for modulation of the bandgap energy of the synthesized materials was calculated by Kubelka-Munk function, as shown in Figure 6. The bandgap results were obtained, giving E g values of 2.71 eV, 3.03 eV, and 3.15 eV for synthesized TiO 2 @rGO nanocomposites and for pure TiO 2 particles. The bandgap energy of reference P-25 material was compared and displayed in the Supplementary Figure S3. The obtained results indicate that the decrease of the bandgap of the synthesized nanocomposites occurs with the introduction of rGO sheets. It can be observed that the photoactivity could be shifted into visible light.
Firstly, the photolysis tests (without catalysts) were performed, and it can be reported that the degradation of MB and RhB dyes was negligible under the irradiation. Before the photocatalytic tests, the adsorption properties of synthesized catalysts were evaluated. The obtained results of adsorption tests for MB and RhB dyes in the dark for different catalysts are displayed in Figure 7. The adsorption test is related to adsorption-desorption equilibrium among MB and RhB dyes molecules and catalysts surface. It could be seen that prepared nanocomposites showed higher adsorption of dyes compared to pure TiO 2 particles and reference P-25 material. Indeed, the dyes adsorption for pure TiO 2 particles and reference P-25 material was negligible for both dyes. On the other hand, the RhB and MB adsorption for synthesized nanocomposites was dependent on the amount of rGO. The RhB adsorption for TiO 2 @rGO (5%) was 1.6% which was lower than for the MB at 4.8%. The highest adsorption capacity of dyes was observed for TiO 2 @rGO (15%) nanocomposite with 25.8% for RhB and 13.8% for MB. The photocatalytic performances of TiO2 and respective nanocomposites were evaluated for dyes degradation under experimental parameters as reported in Supplementary  Table S1. The photoactivity of prepared photocatalysts in comparison with reference P-25 material was monitored based on MB (cationic dye) and RhB (zwitterionic dye) decomposition under the irradiation (depicted in Figure 8A,B). The photocatalytic performances of TiO 2 and respective nanocomposites were evaluated for dyes degradation under experimental parameters as reported in Supplementary  Table S1. The photoactivity of prepared photocatalysts in comparison with reference P-25 material was monitored based on MB (cationic dye) and RhB (zwitterionic dye) decomposition under the irradiation (depicted in Figure 8A,B). The photocatalytic performances of TiO2 and respective nanocomposites were evaluated for dyes degradation under experimental parameters as reported in Supplementary  Table S1. The photoactivity of prepared photocatalysts in comparison with reference P-25 material was monitored based on MB (cationic dye) and RhB (zwitterionic dye) decomposition under the irradiation (depicted in Figure 8A,B). It can be seen in Figure 9A,B that the synthesized nanocomposites increased the decomposition of RhB and MB significantly compared to synthesized TiO2 photocatalyst. Both synthesized nanocomposites have a very efficient MB and RhB decolorization after 120 min, which was very similar to reference P-25 material. The photoactivity of TiO2@rGO (5%) was obtained with 99.6% of MB degradation and 99.9% of RhB photodecomposition. The nanocomposite with a higher amount of rGO (TiO2@rGO (15%)) showed similar photodegradation results, 98.1% for (MB) and 99.8% for (RhB). Meanwhile, the photoactivity evaluation of synthesized TiO2 particles was significantly lower than that of nanocomposites and referenced P-25 material. The degradation rate of TiO2 was 70.9% for RhB and 73.9% for MB. It can be seen in Figure 9A,B that the synthesized nanocomposites increased the decomposition of RhB and MB significantly compared to synthesized TiO 2 photocatalyst. Both synthesized nanocomposites have a very efficient MB and RhB decolorization after 120 min, which was very similar to reference P-25 material. The photoactivity of TiO 2 @rGO (5%) was obtained with 99.6% of MB degradation and 99.9% of RhB photodecomposition. The nanocomposite with a higher amount of rGO (TiO 2 @rGO (15%)) showed similar photodegradation results, 98.1% for (MB) and 99.8% for (RhB). Meanwhile, the photoactivity evaluation of synthesized TiO 2 particles was significantly lower than that of nanocomposites and referenced P-25 material. The degradation rate of TiO 2 was 70.9% for RhB and 73.9% for MB.
The photodegradation rate of MB and RhB dyes followed the pseudo-first-order kinetics. The linear kinetic rate constant (k) in detail is reported in Table 2. It could be determined that pseudo-first-order rate constants of TiO 2 @rGO nanocomposites were significantly higher than for pure TiO 2 nanoparticles in the degradation of both dyes. The structure of organic pollutants played an important role in the photocatalytic process. MB as cationic dye and RhB as zwitterionic dye based on obtained photocatalytic tests showed significant photodegradation rates under the irradiation by nanocomposites which were related to the presence of rGO. In comparison to the P-25 TiO 2 , for MB photodegradation, all composite materials showed lower photocatalytic activity, and for RhB, the photocatalytic activity was almost equal to the photoactivity of P-25 TiO 2 . Pure synthesized TiO 2 showed the lowest photoactivity for both dyes. To confirm the immutability and persistence of as-prepared materials after performed photocatalytic tests, the SEM images were taken and given in Supplementary Figure S4.

Conclusions
The presented TiO 2 @rGO nanocomposites were successfully synthesized with appropriate structural features for the possible application on the photodegradation of MB and RhD dyes under natural sunlight. Graphene-based TiO 2 nanocomposites, prepared using an optimized two-step method that combined hydrothermal treatment and calcination, presented a significantly improved photocatalytic activity under solar-like irradiation. The chemical bonding of particles was closely associated with the amount of rGO, time of hydrothermal synthesis, and calcination temperature. Importantly, the obtained value of the bandgap energy (E g = 2.71 eV and 3.03 eV) of nanocomposites indicate that the wavelength value shifted to visible light when compared with pure TiO 2 (E g ≤ 3.15 eV). The application of prepared TiO 2 @rGO nanocomposites as photocatalysts for degradation of RhB and MB in an aqueous medium showed that the chemical integration of rGO with TiO 2 promoted synergistic effects which sped up the photodegradation of selected pollutants. The photodegradation efficiency of MB and RhB dye by the synthesized TiO 2 @rGO (5 wt.% of rGO) and TiO 2 @rGO (15 wt.% of rGO) nanocomposite photocatalysts after 120 min of exposure to the irradiation were significantly higher than for pure synthesized TiO 2 nanoparticles. TiO 2 @rGO (5%) presented a photodegradation of 99.6% for MB and 99.9% for RhB. The nanocomposite with a higher amount of rGO (TiO 2 @rGO (15%)) showed similar photodegradation results, 98.1% for MB and 99.8% for RhB. Meanwhile, the photoactivity evaluation of synthesized TiO 2 particles was significantly lower. Although these nanocomposite materials showed lower photocatalytic activity than P-25, their large layered structure allows a facile recovery from the aqueous medium after the photocatalytic reaction. These preliminary results show that the prepared TiO 2 @rGO nanocomposite photocatalyst may be explored as high-efficiency and green photocatalysts for simultaneous sorption and degradation of dyes that can be directly applicable in real effluent water treatment by solar exposition. Funding: This work was supported by the projects UIDB/00481/2020 and UIDP/00481/2020-FCT-Fundação para a Ciencia e a Tecnologia; and CENTRO-01-0145-FEDER-022083-Centro Portugal Regional Operational Programme (Centro 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund. Thanks, are also due for the financial support to the H 2 O Value project (PTDC/NAN-MAT/30513/2017) also supported by FCT/MEC through national funds, and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020 (CENTRO-01-0145-FEDER-030513). The present study was also supported by the Slovenian Research Agency (ARRS) under the Contracts J2-9440 and L2-1830.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

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