Synthesis of Fe₂O₃/TiO₂/graphene aerogel composite as an efficient Fenton-photocatalyst for removal of methylene blue from aqueous solution

Cite this paper: Vietnam J. Chem., 2020, 58(5), 697-704 Article DOI: 10.1002/vjch.202000109 697 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Synthesis of Fe2O3/TiO2/graphene aerogel composite as an efficient Fenton-photocatalyst for removal of methylene blue from aqueous solution Tran Hoang Tu1,3, Le Tan Tai1,3, Nguyen Tan Tien1, Le Minh Huong2,3, Doan Thi Yen Oanh4, Hoang Minh Nam1,2,3, Mai Thanh Phong2,3, Nguyen Huu Hieu1,2,3* 1VNU-HCM Key Laboratory of Chemical Engineering and Petroleum Processing (CEPP Lab), Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet Street, district 10, Ho Chi Minh City 70000, Viet Nam 2Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet street, district 10, Ho Chi Minh City 70000, Viet Nam 3Vietnam National University Ho Chi Minh City, 6,Linh Trung ward, Thu Duc district, Ho Chi Minh City 70000, Viet Nam 4Publishing House for Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay district, Hanoi 10000, Viet Nam Submitted July 6, 2020; Accepted September 4, 2020 Abstract In the present study, the composites including Fe2O3/TiO2/graphene aerogel (Fe2O3/TiO2/GA) and TiO2/graphene aerogel (TiO2/GA), and graphene aerogel (GA) were synthesized by hydrothermal method. The as-prepared materials were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, Raman spectroscopy. The characterization results showed that the Fe2O3 and TiO2 particles were uniformly attached in GA structure, increasing number of active sites of materials and extending the light absorption range. The removal performance of Fe2O3/TiO2/GA is 97.38 % which is higher than of TiO2/GA and TiO2. The degradation data were well consisted with pseudo-first-order kinetic model. Accordingly, Fe2O3/TiO2/GA is potential to be used as an efficient photocatalysis for treatment of MB from water. Keywords. Fe2O3, TiO2, Graphene aerogel, nanocomposite, photocatalysis, methylene blue. 1. INTRODUCTION Recently, water pollution is an increasing global problem. The industrialization in developing countries has led to a mass discharge of organic dyes into water. These agents are important source used in various industries comprising textile, pharmaceuticals, food, paper, and cosmetic. The molecular structure of organic dyes is highly stability, persistent for a long time, and resistant to biodegradation in aqueous solution.[1] Organic matter can cause oxygen depletion, immunosuppression, reproductive failure and acute poisoning in aquatic organisms. The presence of dyes has significant impact on the human health because even the smallest amount of these agents is toxic or even carcinogenic.[2] Consequently, the developing effective handling methods are urgently necessitated. To date, various techniques have been utilized to separate these organic dyes such as advanced oxidation processes (AOPs), photocatalysis, adsorption, membranes, and biological degradation.[3] AOPs have developed based on the performance of highly reactive and nonselective hydroxyl radicals (OH•), which have the oxidizing capability to remove non-biodegradable organic compounds in water. Among a variety of AOPs, Fenton process based the catalysis effect of Fe3+/Fe2+ with H2O2 agents to generate the OH• radical, reacting with organic compounds to decompose into CO and H2O. However, the Fenton process has a few drawbacks: (i) low utilization efficiency of the generated active species, (ii) incomplete removal of dye pollutants, (iii) applicable at low pH (pH 2-4), (iv) difficult to separate and reuse catalysts.[4,5] To increase the efficiency of AOPs, the recent studies have pay attention to the development of Vietnam Journal of Chemistry Nguyen Huu Hieu et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 698 heterogeneous Fenton-like catalysts in removing organic pollutants from wastewater. Titanium oxide (TiO2) is the most common semiconductor photocatalyst used due to its potential benefits such as low cost, non-toxicity, and high stablity. The agglomeration of particles phenomenon and the recombination of photo generated electron-hole pairs decreased the degradation efficiency of TiO2. Additionally, the TiO2 has a large band gap (Eg = 3.2 eV) which absorbed in the ultraviolet light region. To tackle the mentioned above, TiO2 has been doped and coupled with various materials such as Fe2O3, ZnO, WO3, etc. to extent the light absorption and improve the degradation activity of organic pollutant.[3] Hematite (α-Fe2O3), is an iron(III) oxide form, has attracted countless attention due to its diversity in characteristics including low cost, low toxicity, high stability, excellent optical properties, environmental friendly, narrow band gap (2.0-2.2 eV), and oxidative nature.[3] The combination of TiO2 with Fe2O3 to form the heterostructure was considered as an effective way to lower the band gap energy level and improve the absorption light efficiency of the catalyst. Simultaneously, the attaching Fe2O3 and TiO2 particles to the substrate to increase surface area and promote removal efficiency. Graphene aerogel (GA), is a three- dimensional (3D) graphene-based structure, contains many outstanding properties including low density, high porosity and surface area, low thermal conductivity, and high electric conductivity. The GA substrate enabes to create the small size nanoparticles and elevate the production of free radical and advancing the catalytic efficiency of material. Besides, GA substrate promotes the transfer of electrons to prevent the recombination rate of electrons-holes.[6,7] In addition, GA has high adsorption capacity of dye to increase the contact surface of dye with catalyst, enhancing the irradiated performance of catalyst. In this work, TiO2/GA and Fe2O3 -TiO2/GA composite were synthersized by hydrothermal method. The samples were characterized by powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electronic microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy. The samples were applied as Fenton photocatalysis to degrade methylene blue (MB) from water. Besides, the kinetic and reusability of material for methylene blue degradation were also evaluated. 2. MATERIALS AND METHODS 2.1. Materials All chemicals including graphite (99 wt.%, 20 µm), titanium(IV) isopropoxide (Ti[(OCH(CH₃)2]4, 97 wt.%), ferric nitrate (Fe(NO3)3.9H2O, 99 wt.%), sulfuric acid (H2SO4, 98 wt.%), hydrochloric acid (HCl, 36 wt.%), phosphoric acid (H3PO4, 85 wt.%), and hydrogen peroxide (H2O2, 30 wt.%) were purchased from Sigma-Aldrich, Germany. Potassium permanganate (KMnO4, 99 wt.%), MB (99 %), sodium hydroxide (NaOH, 99 wt.%), and ethanol (C2H5OH, 99 wt.%) were purchased from Vina Chemsol, Vietnam. There are no any further purification to all chemicals that were analysed and were used as received. 2.2. Synthesis of GO Graphene oxide (GO) was fabricated by improved Hummers’ method from graphite.[8] Initially, 3 g of graphite was added into 360 mL of H2SO4 and 40 mL H3PO4. After that, 18 g KMnO4 was added slowly into the mixture, and stirred for 30 minutes in an ice bath. Then, the mixture was continued to be stirred at 50 oC for 12 hours and cooled to room temperature. The resulting mixture was added with 500 mL of water, 15 mL of H2O2, centrifuged. The pH of the centrifuged mixture was adjusted to 6 by using 10 % HCl, water, and ethanol. Graphite oxide, which is the resulting solid, (GiO) was obtained, dried at 50 oC and dispersed in water (1 mg/mL). Afterward, GO suspension, which was acquired after 12 hours of ultrasonication. The suspension was centrifuged and dried at 50 oC to attain GO as the final product. 2.3. Synthesis of Fe2O3-TiO2/GA Graphene aerogel (GA) was prepared by self- assembly process of rGO sheets under hydrothermal conditions.[9] TiO2/graphene aerogel (TiO2/GA) was prepared by hydrothermal method with Ti[(OCH(CH₃)]2:GO mass ratio of 1:1. The Fe2O3- TiO2/GA nanocomposite was synthesized by hydrothermal method according to the studies. [3, 10] In a typical synthetic procedure, 0.12 g of Fe(NO3)3.9H2O was dissolved into 80 mL of GO suspension (5 mg/mL) under vigorously stirring. Then, 280 µL of Ti[(OCH(CH₃)2]4 was added dropwise into the mixture and sonicated for 2 hours. Afterward, the mixture was transferred and sealed into a Teflon-lined autoclave 100 mL, followed by a heating at 180 oC for 12 hours. The as-synthesized hydrogel was immersed in ethanol/water mixture Vietnam Journal of Chemistry Synthesis of Fe2O3/TiO2/graphene © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 699 with a ratio of 3:1 for 24 h. Then, the hydrogel was freeze-dried to obtain TiO2/GA. 2.4. Characterization The synthesized materials were characterized by XRD (Advanced X8, Bruker, Germany), FTIR (Alpha-E, Bruker, Germany), Raman spectroscopy (LabRAM HR Evolution, Horiba, Japan SEM (Hitachi S-4800, Japan), EDX (Jeol JMS 6490, JEOL, Japan), and UV-visible spectroscopy (UV- Vis, Horiba Dual FL, Japan). 2.5. Photo-Fenton degradation experiments The photocatalytic activities of the synthesized materials were calculated via photocatalytic degradation in MB aqueous solution. The effect of conditions on the removal efficiency is investigated through Batch experiments. A 25W UV lamp (Natural light PT 2191-ExoTerra) with wavelength region from 280 to 320 nm as the irradiation source was placed above the reactor. In a typical, 20 mg of catalysts and the 20 mL MB solution with initial concentration of 50 mg/L were mixed together. A change to 7 occurred in the pH of the mixture. The mixture was stirred in the dark for 30 min to achieve adsorption equilibrium. Then, 2 mL of H2O2 was added in MB solution and the lamp was turned on, the photo Fenton-like reaction occurred. After the illumination time, the solution was filtered through 0.45 μm Nylon syringe filter. The residual MB concentration of solution was measured on UV-Vis spectrophotometer (Dual FL, Horiba, Japan) at 664 nm. The efficiency of materials for degradation of MB was calculated from the equation as follow: H = C0−Ct C0 100 % (1) where H (%) is the removal performance of material, C0 (mg/L) is the initial concentration of MB solution, and Ct (mg/L) is the concentration of MB after a certain irradiation time, t. The kinetics of photocatalytic degradation of MB in aqueous solution was investigated by the pseudo- first-order model. Ln Co Ct = − kt (2) where k (min-1) is the reaction rate constant; and t is irradiation time. For cyclic tests, the catalytic after degradation process was separated and washed using ethanol to remove MB residual and heated at 120 °C for 2 h. 3. RESULTS AND DISCUSSION 3.1. Characterization XRD patterns of GA, TiO2/GA, and Fe2O3-TiO2/GA are shown in figure 1. GA had a wide peak at 2θ = 24°, corresponding to the diffraction (0 0 2) peak. Based on the Bragg equation, the interlayer distance between layers in GA was determined to be 0.37 nm, which lower than of GO (0.82 nm) and higher than of graphite (0.34 nm). This phenomenon was confirmed the partially removal of oxygen- containing groups in GO structure, owing to the incomplete of reduction process of GO to rGO. Besides, the decrease in intensity of (0 0 2) peak showed to the self-assembly of rGO sheets to form GA. As for TiO2/GA, the diffraction peaks were presented at 2 = 25.39, 37.78, 47.97, 53.95, 54.90, 62.64, 68.89, 70.24, and 75.00o assigning to crystalline planes (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0), and (2 1 5) of anatase TiO2, respectively (JCPDS No. 21-1272).[11] In Fe2O3-TiO2/GA, the peaks of TiO2 were also reported. And, the characteristic diffraction peaks of α-Fe2O3 were detected at 33.21 (1 0 4), 36.12 (1 1 0), 41.75 (1 1 3), 50.08 (024), 53.68 (1 1 6), 57.73 (122), and 65.17° (3 0 0) (JCPDS No. 01-1030).[12] Thus, the Fe2O3 and TiO2 particles were docked on the GA structure. By using the Scherrer equation, the size of particles on Fe2O3-TiO2/GA was fell in the range of 0.5-3.5 nm as shown in table 1. The relatively low characteristic peak of TiO2 indicates that a reduction in size of particles in Fe2O3- TiO2/GA compared to TiO2/GA, showing the replacement of Ti4+ by Fe3+ ions.[3] Table 1: The average crystallite sizes of particles in TiO2/GA and Fe2O3-TiO2/GA Materials Crystallite sizes, nm TiO2 Fe2O3 TiO2/GA 21.20 - Fe2O3-TiO2/GA 3.43 0.38 Figure 2 shows the FTIR spectra of GA, TiO2/GA, and Fe2O3-TiO2/GA. The absorption bands at around 3550, 1600, and 1250 cm-1 corresponding to the -OH, C=C, and C-O groups were observed in samples.[2] The bands of oxygen- containing groups were almost removed, indicating that GO is reduced and self-assembled to become GA. Besides, the broad bands below 1000 cm-1 were allocated to the stretching vibration of Ti-O and Fe- O bands.[3] Moreover, the high electronegativity positions of C=O and -O- groups disappeared at 1735 and 1080 cm-1, respectively. Thus, the Ti4+ and Fe3+ ions were chemical interacted with these groups and growth to the crystal phase on GA structure.[13] Vietnam Journal of Chemistry Nguyen Huu Hieu et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 700 Figure 1: XRD patterns of GA, TiO2/GA, and Fe2O3-TiO2/GA Figure 2: FTIR spectra of GA, TiO2/GA, and Fe2O3-TiO2/GA The surface morphologies of materials are investigated by SEM imaging as indicated in Figure 3. The rGO sheets were self-assembled to form framework due to hydrogen bonds, hydrophobic and π–π interactions. For TiO2/GA and Fe2O3-TiO2/GA, the anchormen of particles were uniform on GA structure, increasing the surface area and number of active sites of the material. This result confirmed the distribution of Fe2O3-TiO2/GA as shown in figure 4. Moreover, the reduction of particles size in Fe2O3- TiO2/GA was observed because of the effect by electrostatic force between the negatively charged of residual oxygen-containing groups in rGO and positively charged of Fe3+ and Ti4+ ions.[14] This is in a good convention with XRD and FTIR data. The EDX result of Fe2O3-TiO2/GA was performed to identify the chemical compositions of composite. The present of Ti and Fe elements were recorded, indicating the anchoring of TiO2 and Fe2O3 particles in GA. (a) (b) (c) Figure 3: SEM images of (a) GA, (b) TiO2/GA, and (c) Fe2O3-TiO2/GA Figure 4: Elemental mapping of Fe2O3-TiO2/GA Vietnam Journal of Chemistry Synthesis of Fe2O3/TiO2/graphene © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 701 Table 2: The elemental compositions in materials Materials Elements (wt.%) C O Ti Fe GA 76.81 23.19 - - TiO2/GA 53.88 18.85 27.27 - Fe2O3-TiO2/GA 39.02 33.74 19.56 7.68 The Raman spectra of materials display two characteristic modes at 1335 and 1596 cm-1 with respect to the D and G-bands as shown in Figure 5a. The D-band is characterized by the crystalline defects and G-band is assigned to the sp2-hybridized carbon atoms in the carbon network. The intensity ratio of these peaks (ID/IG) reflected the degree of functional and disordered sites in graphene-based materials.[15] The ID/IG value of Fe2O3-TiO2/GA is 1.25, which is higher comparing to that of TiO2/GA (ID/IG = 1.18) and GA (ID/IG = 1.14). The number of bonds between Fe2O3, TiO2 particles and GA were higher than GA, indicating the good incorporation of the particles with the GA.[16] (a) (b) Figure 5: (a) Raman spectra of GA, TiO2/GA, and Fe2O3-TiO2/GA; (b) UV-Vis spectra of TiO2/GA and Fe2O3-TiO2/GA Besides, the absorption bands of TiO2/GA and Fe2O3-TiO2/GA were shifted to the visible region, expanding to a longer wavelength than of TiO2 as shown in figure 5b. The band-gap energies of the composites were calculated from Kubelka-Munk equation were found to be 2.63 (TiO2/GA) and 2.08 eV (Fe2O3-TiO2/GA) which are smaller than of TiO2 (3.2 eV). The band gap of composites decreased which shows the formation of bonds between the particles and carbon network of GA.[17] This result confirmed the effect of GA on the distribution of particles increase number of nuclating sites, leading to the formation of smaller particles size and expensing in the efficiency of light absorption.[3] The presence of Fe2O3 acts as a co-sensitizer to enhance the number of photo-generated electrons and holes in the Fenton reaction.[18] The Fe2O3 and TiO2 particles were successfully formed and evenly distributed on GA substrate to form Fe2O3-TiO2/GA, enhancing the absorption of light irradiation and improving the photo-activity for the removal of organic dyes.[17] 3.2. Photocatalytic performance Figure 6 shows the Fenton-photocatalytic of TiO2, TiO2/GA, and Fe2O3-TiO2/GA at different irradiation times. It is clear that the residual concentration of MB in solution obviously decreases with enhancing the time. The removal efficiencies of TiO2/GA and Fe2O3-TiO2/GA are about 76.52 and 97.38 %, respectively, which are higher than that of TiO2 (H = 55.69 %) after 60 min of UV-light irradiation. For the Fe2O3-TiO2/GA, the degradation rate rapidly accelerated within 10 min and the MB dye almost vanished after 40 min in the present of Fe2O3-TiO2/GA. Vietnam Journal of Chemistry Nguyen Huu Hieu et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 702 Figure 6: Effect of irradiation time on the MB degradation performance of TiO2, TiO2/GA, and Fe2O3-TiO2/GA Figure 7: Pseudo-first-order kinetic plots of the MB degradation The pseudo-first-order model applied to examine the degradation kinetic of materials and the linear plots were presented in figure 7. The plots are in straight line with the negative slope value. The parameters of linear pilots are showed in table 3. The degradation process is highly suitable the pseudo-first-order model for high correlation coefficients (R2 > 0.90). The values of rate constant of photocatalytic samples are 0.0558, 0.0228, and 0.0131 min-1 corresponding to the Fe2O3-TiO2/GA, TiO2/GA, and TiO2, respectively. The phenomenon could be ascribed to the synthesized composites with the distribution of particles in GA, preventing the agglomeration particles and increasing more number of active sites in the composite. The role of electron conduction in GA network promotes the migration efficiency of photo-generated electrons-hole pairs, thus the degradation activities of composites are higher than that of TiO2.[19] The removal performances of synthesized materials are compared with other materials from other studies which are presented in Table 4. The superior characteristics of Fe2O3-TiO2/GA indicated that the photo-generated electron-hole pairs separation between the band gap of Fe2O3 and TiO2, expanding the solar light absorption region of material. Under light irradiation, the electron placing in the valence band of TiO2 is stimulated and shifted to conduction band with ease, which leads to the fact that the interf