Efficient visible active ternary Bi 2 MoO 6 -rGO-TiO 2 composite material for photodecomposition of ofloxacin

The ternary Bi 2 MoO 6 -reduced graphene oxide (rGO)-TiO 2 catalyst were synthesized using a simple hydrothermal method. The improvement of the photocatalytic decomposition efficiency of Bi 2 MoO 6 -rGO-TiO 2 composite is 92.3% than the pure and binary photocatalyst. The effects of operational parameters like catalyst ratio, the different catalyst, different ratio rGO and different pH, have been analyzed. As prepared ternary photocatalyst is low Photoluminescence and high photocurrent density responsible, it exhibited that photon-induced electron and hole-recombination were suppressed and also charged separation is effective. The present study to shows the rGO is an excellent electron transfer performance and enhanced the photocatalytic reaction stability. grid (400 imaging with about KBr pellet wavelength at nm. The density responsible, that photon-induced electron-hole recombination was suppressed and charge separation is effective. The rGO is an excellent electron transfer performance and enhanced the stability of the photocatalytic reaction. The perfect cycling stability of the Bi 2 MoO 6 -rGO-TiO 2 was maintained even after five consecutive cycles on photocatalytic degradation reaction performance. The feasible degradation mechanism is proposed for the decomposition of ofloxacin under visible light illumination.


Introduction
The developing crisis of environmental contamination is a decisive imperative for the green chemistry community sustainable growth, leading to energy depletion [1,2]. Due to anthropogenic contaminants, pharmaceutical effluents are increasingly used by living beings to treat bacterial illnesses [3]. The intrinsic existence of drug chemical compounds has difficulty examining the exhibit suffering even at a low level of chemical concentrations due to inappropriate removal [4].
There were quinolone antibiotics, extensive ofloxacin use, and minimal availability in wastewater systems, the toxic risk to ecosystems, and aquatic systems [5][6][7]. Thus, the identification and corrections of these environmental pollutants are more important, and it is a matter of concern to the scientific community for developing new techniques and doing further research to eradicating the burning problems. Due to their widespread use, quinolones and their metabolites are mostly discharged and gathered in aquatic environments [8]. For example, a common 3 rd generation fluoroquinolone antibiotic ofloxacin is detected in municipal sewage up to 31.7μg/L [9]. The numerous antibiotics wastes are then charged to various aquatic environment ecosystems, drinking water, and underground water. The level of antibiotic concentrations is relatively low in aquatic environments, except for waste effluent from the polluting pharmaceutical industry and hospitals [10][11][12][13].
Although somewhat efficient, traditional water purification methods such as biodegradation and chlorination cannot efficiently eliminate ofloxacin [14], and induce more toxic byproducts [15][16]. Therefore, it is still essential to improve effective and environmentally ecofriendly strategies to remove ofloxacin from wastewater and other aquatic environments.
Photocatalyst analysis has proven to be a promising candidate for the degradation of ofloxacin due to its highly efficient method of removal of biologically resistant antibiotics. To efficiently use the visible light source, it is suitable for the ternary catalyst of visible light active photocatalysts [17][18][19][20]. TiO2 nanoparticles are widely used to decompose harmful chemicals such as phenolic, drug molecules, and dye molecules [21]. Its low functionality in a visible light source for the research society to divide strategies improves such TiO2 catalysts stimulant's performance, binding to other oxides [22][23][24]. A number of hybrid composite semiconducting materials has been analyzed to reduce photo electron-hole recombination to improve the large spectral active to wider wavelength such as BiVO4-TiO2 [25], TiO2/SiO2 [26], CdS/TiO2 [27], ZnO/TiO2 [28], TiO2/SiO2/Bi2O3 [29], CeO2/ZnO [30], WO3/TiO2 [31], and SnO2/ZnO [32]. In such a system lead to a possible gradient in the interface. This suppresses the recombination of the charge carriers and expands the spectral active to a wider wavelength. Connecting TiO2 to a short-band structure material is capable of enhancing solar consumption at the same time reducing recombination of electrons and holes created by photo-generation. In this short-band structure semiconductor material, bismuth molybdate has effective and suitable material of researchers attribute the best action to the visible source and control ability structural morphology in accordance alternative layer of [Bi2O2] 2+ and MoO4 2- [33][34][35][36].
More research was organized to develop a highly capable Bi2MoO6/TiO2 hybrid structure but still had its photodegradation efficiency underwent low visible source rarely anxious. Under weak visible light illumination, are low carriers created by photo-generation during photoreduction process than that of strong-light radiation, and therefore efficient absorption and use of visible light radiation are very important in photocatalysis. It is well known that active photogeneration, separation, and photo-generated electron holes play a significant role in photocatalytic degradation, which can be controlled by material composition and microstructure. Therefore, it is needed to purpose a good catalyst, Bi2MoO6-rGO-TiO2, to enhance the absorption of light towards photodegradation [37][38][39]. From the point of view of the catalyst constitution of Bi2MoO6/TiO2 compound, the bare Bi2MoO6 catalyst can actively absorb visible sources, so it needs to extend the active optically active of TiO2 to the visible region to enhance the production of the carriers generated by the photogeneration [40]. Moreover, these two catalytic materials can make heterojunction, thus improve the separation of the carriers generated by the photogeneration. On the other hand, specialized structures significant function in the degradation performance of photocatalyst [41].
In recent years, reduced graphene oxide-based photocatalyst has excellent and much attention for its ability to solve environmental contamination problems. Reduced Graphene oxide is a two-dimensional honeycomb structural material that is a perfect receptor and transporter for more electron transport, large surface area, and photo-generated electrons [42,43]. Many researchers have recommended that the graphene-based hybrid semiconductor improve photocatalytic performance based on the absorption of higher contamination and reduce the level of photo-generated electron-hole recombination, increase electron mobility, and enhance the absorbance light range [44]. Further, to improve the properties, noble metal nanoparticles have been introduced, which significantly enhance the photoreduction efficiency of graphene-TiO2 and improve the visible region light absorption capacity [45]. This noble metal/alloy can prevent electron-hole pairs from recombination.
The present research demonstrates that the ternary composite Bi2MoO6-rGO-TiO2 material was synthesized using hydrothermal method. Further, the introduction of Bi2MoO6-rGO-TiO2 has been to improve the photocatalytic degradation efficiency of ofloxacin under visible light illumination, and the possible photocatalytic mechanism has been deduced.

Synthesis TiO2
Firstly, 2 mL of Tetrabutyl titanate were added slowly into concentrated HCl with strong magnetic starrier. The above-mixed solution was diluted using deionized water to adjust the volume of 75 mL. Then, the solution was stirred for 2 h, and the total solution were shifted into a 100 mL Teflon-lined autoclave and closed. The simple hydrothermal preparation was performed at 150 °C for 5 h underwent oven. After the autoclave was cooled under atmospheric condition, the yield was obtained and isolated by centrifuged, repeatedly cleaned with double distilled water, and finally heated at 110 °C underwent a vacuum oven.

Synthesis of Bi2MoO6-TiO2
The Bi2MoO6-TiO2 was synthesized by mixing 1.3 mmol of Bi(NO3)3.5H2O and 0.65 mmol of Na2MoO4.2H2O with 5 mL of ethylene glycol. Then, the 40 mL ethanol was mixed with stirring for 1 h, and 1 g of TiO2 nanorods were dispersed into the above solution. After, the autoclave was kept at 160 °C for 24 h. Then, the autoclave was cooling down to atmospheric condition. The sample was cleaned with distilled water, ethanol solution and dried at 80 °C for 12 h in a vacuum oven. Finally, catalysts were heated at 100 °C for 12 h.

Synthesis of Bi2MoO6-rGO-TiO2
The Graphene oxide (GO) is made up of graphite by Hummer method [45]. The prepared GO (200 mg) were dispersed in 120 mL of distilled water with 0.5 g of urea. The suspension solution was kept at 120 °C for 12 h with the suspension was into 150 mL Teflon lined autoclave.
The Bi2MoO6-TiO2 was synthesized by mixing 1.3 mmol of Bi(NO3)3.5H2O and 0.65 mmol of Na2MoO4.2H2O with 5 mL of ethylene glycol. Then, the 40 mL ethanol was mixed with stirring for 1 h, and 1 g of TiO2 nanorods and 200 mg of rGO was mixed into the above solution and stirred for 1 h. The above suspension was taken 100 mL stainless autoclave and kept at 180 °C for 24 h.

Photocatalytic measurement
Ofloxacin was used to investigate the photocatalytic degradation efficiency of the

Powder XRD analysis
Powder XRD is one of the non-destructive analytic techniques for analyzing the crystalline constitution of materials, which is in use since the 1950s. peaks of rGO disappeared in Bi2MoO6-rGO-TiO2 composite material due to disordered restacking of rGO [49]. The diffraction peak at 10.59 corresponds to the planes (001) relative to the facet of Graphene Oxide, and the planes of (002) correspond to the rGO. This result suggests that the XRD pattern of pure graphite and the corresponding planes of (002) indicated the graphitic nature.

FE-SEM analysis
The morphology of TiO2, Bi2MoO6, and Bi2MoO6-rGO-TiO2 nanocomposite materials can be identified using FE-SEM images. TiO2 depicted the rod-like structure and the length of the rod several micrometers (Fig.2a-d). The FE-SEM images of Bi2MoO6 reveals that the plate-like structure (Fig.2e-h). Bi2MoO6 nanocomposite also depicted a close interaction in composite material, the Bi2MoO6-TiO2 well dispersed on the reduced graphene oxide sheets, which may improve the photocatalytic performance. Moreover, Bi2MoO6-rGO-TiO2 ternary composite, Bi2MoO6 nanoparticles on the rGO nanosheets surface, and TiO2 rods can be observed in ternary composite material (Fig.2i-l). Reduced graphene oxide depicted that the sheet-like structure and intercalation of TiO2 and Bi2MoO6 on the sheets, demonstrate that graphite powder is exfoliated into few-layered reduced graphene oxide is obtained.

XPS chemical composition:
XPS investigation can provide more elemental constitutions and chemical states of the Bi2MoO6-rGO-TiO2 compound. The presence of all chemical components was investigated from survey XPS spectra Fig.6a. Furthermore, the Bi 4f XPS spectrum of the Bi 4f level has two intense peaks observed at 158.9 eV and 164 eV, which can be assigned as Bi 4f7/2, and Bi 4f5/2, and the peak at 160.3 and 165.7 eV are attributed to the Bi 4f7/2 and Bi 4f5/2 (Fig.6b). The observed binding energy values are perfect matched with the oxidation of Bi +3 . Furthermore, the Mo 3d level depicted strong peak positions at 236.2 eV and 232.9 eV, which can be attributed to the Mo 3d3/2 and Mo 3d5/2, respectively [50] (Fig.6c). Moreover, the binding energy of Mo 3d peak position represents the oxidation state of the Mo +6 [51]. The Ti 2p XPS spectrum depicted the two strong peak positions at 458.7, 460.2, and 464.8 eV, relative to the Ti 2p3/2, Ti 2p3/2, and Ti 2p1/2, respectively, which depicted the presence of Ti 4+ in Bi2MoO6-rGO-TiO2 composite [52] (Fig.6d).
The peak position at 528.2 eV is assigned to the Mo-O/Bi-O groups, while the peak at 529.8 and 531 eV is similar to that of OH atoms. The C1s spectra have three peaks, one is at 284.8 eV, which is relative to C=C for Sp2 hybrid carbon bonding, and another peak position at 285.8, 288.4 eV is assigned to the C=O carbon atom [53] (Fig.6f). XPS investigation offers recognition for the good attachment of carbon atoms to the surface of the Bi2MoO6-TiO2, finally in the structure of Bi2MoO6-rGO-TiO2 nanocomposites photocatalyst.

FT-Raman spectral analysis
The FT-Raman spectral analyses of GO, rGO, Bi2MoO6-rGO-TiO2, Bi2MoO6-TiO2, Bi2MoO6, rGO-Bi2MoO6, rGO-TiO2, and TiO2 catalyst are depicted in fig.8. The FT-Raman spectrum of GO illustrated well-matched with D band peak position at 1353 cm -1 , which can be sp3 defects, and the other peak position of G band at 1594 and 1586 cm -1 , which can be assigned to the sp2 vibration of carbon atoms, and E2g symmetry of Brillouin zone of phonon modes [57].
Due to the rGO self-healing nature that recovers the hexagonal of carbon atoms, it has also been exhibit that the D band peak remains unchanged when the rGO of the G band peak is reduced from 1594 to 1586 cm -1 [58]. This identified to reduced GO into rGO successfully. The 2D band position that is assigned to the graphene property of stacking nature was obtained at 2708 cm -1 for rGO.
The ID/IG ratio was used to determine the structural disorder, which is a small decrement from 0.

Optical and PL properties
The photocatalytic catalyst efficiency is associated with its capability to absorb light, and the catalyst has a visible region bandgap and enhanced photocatalytic activity. Fig.9 depicted the DRS-UV-visible spectra of the catalyst and the bandgap energy calculated using Taucs plots of the absorption spectra shown in Fig.10 respectively. The optical bandgap of the composite catalyst Bi2MoO6-rGO-TiO2 was a lower bandgap than that of pure and binary catalyst, which improved the photocatalytic activity.
The photoluminescence spectra of photo-generated electron holes in the pure and composite catalyst are shown in fig.11. The PL spectra were measured at an exciting wavelength of 320 nm. The maximum peak intensity is related to photo-induced charge carriers maximum recombination performance, decreasing photocatalytic efficiency. The lower peak intensity corresponds to the lower recombination of electrons and holes, this higher photoreduction efficiency [65]. The PL spectra of the TiO2, Bi2MoO6, Bi2MoO6-TiO2, rGO-TiO2, rGO-Bi2MoO6, and Bi2MoO6-rGO-TiO2. The PL spectra of TiO2 depicted two peaks at 450 and 466 nm, indicating the surface and irradiative electron-hole recombination of the below conduction band and valance band. Bi2MoO6 and rGO-Bi2MoO6 represented a PL peak at 466 and 467 nm, the surface cached charge carrier, whose emission energy coincides with the calculated optical bandgap energy. The PL spectra of Bi2MoO6-TiO2 and Bi2MoO6-rGO-TiO2 composite material depicted that the emission peaks at 465 nm and 466 nm, which is ascribed to the photo-generated electron and holes [63]. The emission peak intensity is lower than those of TiO2, Bi2MoO6, Bi2MoO6-TiO2, rGO-TiO2, rGO-Bi2MoO6, which depicted that the Bi2MoO6-rGO-TiO2 lower recombination rate photogenerated electron-holes.

Photocatalytic degradation of ofloxacin:
The photocatalytic efficiency of bare, TiO2, rGO-TiO2, Bi2MoO6, rGO-Bi2MoO6, Bi2MoO6-TiO2, and Bi2MoO6-rGO-TiO2 composite photocatalyst is depicted in fig.12a A series of tests were performed to estimate the optimum photocatalyst load by different photocatalyst weights from 20 to 50 mg (Fig.12b) Furthermore, the load of the photocatalyst is increasing, the adsorbed number of ofloxacin molecules increases. The catalytic removal efficiency is decreases of ofloxacin at maximum catalyst loading at 50 mg is due to the visible light resist by photocatalyst [67]. The higher amount of photocatalyst, which decreases photocatalytic removal efficiency, is due to photons resistance by catalyst particles and rGO sheets in the composite material. The maximum loading of catalyst led to the shading effect, which can reduce the visible light's absorption and strongly resist the photons, causing a decrease of photodecomposition efficiency [68].
The pH is one of the main parameters in the photocatalytic removal process. The effect of pH variation on decomposition performance of ofloxacin was performed from 5 to 9. From   Fig.12c, increasing in pH level from 5 to 9, the photocatalytic removal performance is gradually increased from 67.7%, 92.3%, and 81.1% respectively, suggesting the optimum pH value 7 for efficient photocatalytic removal of ofloxacin under visible light irradiation. The point of zero charges (PZC) for TiO2 was calculated to be 6.5 and pH level less than that of PZC, the photocatalyst surface is positive charged, and pH value is greater than PZC, it is negative charged [69,70]. The optimization of rGO in the ternary composite catalyst was analysed with different rGO percentages are 1%, 1.5%, 3% and 5% in composite material to use for the decomposition of ofloxacin, which relative to the decomposition rate were 73%, 80.7%, 92% and 88% respectively (Fig.12d). The increases of rGO content, the photocatalytic decomposition was increased up to 3% of rGO, then decreased decomposition performance with 5% of rGO, this due to the blocking of photons entering the solution by rGO sheets on the surface of the catalyst. The 5% of rGO is incited to the shading effect, which can reduce the light absorption by rGO, reduce the degradation efficiency [71]. The UV absorbance spectra depicted that the increasing time, and the decreasing UV absorbance intensity, which shows the degradation of ofloxacin (Fig.13.). is achieved maximum rate constant at 0.0177 min -1 is to be optimized level. The kinetics rate constant for different rGO content are 1%, 1.5%, 3% and 5% in composite material, which relative to the rate constant are 0.0108, 0.0135, 0.0191 and 0.0168 min -1 respectively. The optimum level is 3% of rGO in ternary composite material.

Photodecomposition enhancement mechanism
The photocurrent transient density of all photoelectrodes with the switch on and off visible light illumination as depicted in Fig.15a. The photocurrent density of the Bi2MoO6-rGO-TiO2 photoelectrodes higher than the others and the illumination of the Bi2MoO6-rGO-TiO2 photoelectrode has a suitable Bi2MoO6 level by managing the time of reaction. The optical photocurrent density of Bi2MoO6-rGO-TiO2 refers to the most extensive photo-generated electrons [72]. Excellent photocurrent transformation means that the synthesized photoelectrode has an efficient visible light active, macropian carrier lifespan, and a less recombination rate. The photoelectrode interface resistance illustrates photo-induced separation and charge carrier transmission performance [73,74]. A possible photodegradation mechanism was presented. This is because the conduction band and valance band levels of Bi2MoO6 and TiO2 are negative, therefore can form at the type-II heterojunction interface of Bi2MoO6-rGO-TiO2. As depicted in Scheme 1, the catalyst can absorb many photons underwent visible region by the combined absorption of Bi2MoO6-rGO-TiO2. The electrons in the VB of the photocatalyst are stimulated through the photon energy and go to the CB. In the type-II heterojunction, the electrons in Bi2MoO6 CB are transferred to TiO2, and the holes in TiO2 of VB are converted to Bi2MoO6 of VB with the help of an internal semiconducting electric field. To identification of electron migration among the Bi2MoO6 and TiO2 catalyst was studied by XPS spectra (fig.15b,c). The valance band edge positions are calculated from VB XPS spectra of the Bi2MoO6 and TiO2, relative to valance band potential 2.33 eV and 2.95 eV, respectively. The concentrated electron has enough power to reduce the surface of oxygen molecules [75], but concentrated holes have a weaker ability to oxidize OH.
The O2 plays a significant role in the decomposition of ofloxacin. Type-II heterojunction formation effectively prevents and promote the electron and hole recombination, with photocatalyst surface O2/OH to form active groups.
Reduced graphene oxide has more surface area, and it is a two dimensional integrated large bonding structure. It is highly absorptive and capable of absorbing large numbers of ofloxacin molecules on its surface are composed of π-π [76]. Due to its low resistance, electron migration, recombination of photocatalytic electron and hole are suppressed. Photo-generated electron and holes are separated, thus prolonging the life of the photo-generated electron and offering the most active free radicals to charge and contribute in photocatalytic decomposition of ofloxacin.

Detection of reactive species:
To recognize the photocatalytic removal mechanism of Bi2MoO6-rGO-TiO2 composite catalyst with ofloxacin. Triethanolamine (TEOA-1 mmol in100 mL) was used as a scavenger of the hole (h + ), benzoquinone (BQ-0.1 mmol in 100 mL) for O2 •and isopropyl alcohol (IPA-0.1 mmol in 100 mL) for • OH scavenger were used to identify the oxidative species. The ternary photocatalyst photocatalytic material stability and reusability are essential for the real application part. The successive cycle runs of ofloxacin with the Bi2MoO6-rGO-TiO2 catalyst under visible light illumination were performed to assess its cycle stability. Every cycle runs recovered the catalyst by centrifugation and then cleaned with deionized and dried at 60 °C for overnight. The obtained photocatalyst was utilized for the next catalyst runs, as depicted in the cycle test (Fig.16a). The photocatalytic decomposition efficiency was reduced slightly decreased in the fifth cycle run. In this decrement, effectiveness is attributed to the dissolution of catalyst and photo corrosion of the catalyst. Furthermore, the XRD diffraction pattern of Bi2MoO6-rGO-TiO2 ternary catalyst was investigated before the reaction and after five consecutive runs of catalyst ( fig.16b). This depicted that the XRD diffraction intensity of peak position was decreased slightly from initial decomposition. Hence, the above test results confirm that the photocatalyst has high stability.

Conclusion
The ternary Bi2MoO6-rGO-TiO2 composite catalyst was prepared by a simple hydrothermal technique. The prepared ternary composite catalyst has increased absorption in the visible region and excellent active visible catalyst than that of bare and binary catalyst for the degradation of emerging pollutant ofloxacin. As the prepared ternary composite catalyst is efficiently decomposition of ofloxacin under visible light illumination within 120 min about 92.3% and it is more effective than that of other catalyst compared with previous literature. Moreover, the ternary photocatalyst depicted low PL and high photocurrent density responsible, exhibited that photon-induced electron-hole recombination was suppressed and charge separation is effective. The rGO is an excellent electron transfer performance and enhanced the stability of the photocatalytic reaction. The perfect cycling stability of the Bi2MoO6-rGO-TiO2 was maintained even after five consecutive cycles on photocatalytic degradation reaction performance. The feasible degradation mechanism is proposed for the decomposition of ofloxacin under visible light illumination.