Concentration Dependences of Photocatalytic Activity in Green-Synthesis Fe3O4/TiO2 Nanocomposite Utilizing Moringa Oleifera for Methylene Blue Dye Degradation

Moringa Oleifera (MO) extract is used for green-synthesis of magnetic nanoparticles Fe3O4/TiO2 for methylene blue (MB) dye degradation. Fe3O4 was characterized using an X-ray diffractometer (XRD) showing a cubic inverse spinel structure with an average crystallite size of 5.49 nm and an average crystallite size of Fe3O4/TiO2 nanocomposites of 5.37 nm. In the UV-Vis results, the Fe3O4/TiO2 nanocomposite has band gap of 3.48 eV. MB degradation increased by increasing mass of the Fe3O4/TiO2 nanocomposite. The irradiation time of 60 minutes with a mass of 0.06 grams had the highest degradation percentage reaching 98.4%. The Fe3O4/TiO2 nanocomposites can be reused up to two times without a significant decrease in the percentage of MB due to their magnetic properties.


Introduction
Synthetic dyes such as methylene blue (MB) are used in the textile and furniture industries, with the molecular formula C16H18N3, MB is a classic cationic dye [1].To reduce organic dyes, photocatalytic techniques have been used recently.Semiconductors such as TiO2, CdO, ZnO, and SnO2 are very good at degrading organic dyes [2].TiO2 has been proven to be effective in dealing with water pollution due to its low price and high stability.Despite having excellent photocatalytic capabilities, conventional TiO2 is challenging to separate and manipulate during the water purification process due to its tiny size (nano) and high costs for further processing.Fe3O4 is one of the iron oxide nanoparticles that have unique properties such as superparamagnetic and low toxicity.Although these nanoparticles have low bandgap energy, their ability to degrade waste using photocatalytic techniques is still lacking compared to semiconductor materials.The heterojunction structure between magnetic nanoparticles and semiconductors causes an increase in the surface reactivity of the sample so that the photodegradation rate can reach ±96% [3].Magnetic nanoparticles and semiconductor materials can be combined through various synthesis techniques such as hydrothermal, sol-gel, and coprecipitation.However, these techniques require some conditions or materials that increase the cost of the process or use toxic raw materials, which makes the synthesis process unsafe or environmentally unfriendly [4].Green synthesis methods involving plant extracts are a great solution for wastewater treatment.Due to its cost- effectiveness, non-toxicity and environmental friendliness, green synthesis methods have attracted a lot of attention recently.Synthesizing nanoparticles with various plant extracts has been accomplished in some previous studies [4][5][6].Moringa oleifera (MO) leaf extract is an appropriate plant for the synthesis of nanoparticles because MO contains many of proteins, tannins, alkaloids, and flavonoids which act as reducing and stabilizing agents [6].To achieve this goal, Fe3O4/TiO2 nanoparticles were synthesized using the coprecipitation method using MO leaf extract.This research focused on analysis the crystal structure, optical properties, and photocatalytic activity of the green synthesized Fe3O4/TiO2 nanocomposite using MO extract with mass variations.

Method and experimental
Figure 1.Illustration of the synthesis process for (a) MO solution, (b) Fe3O4 NPs, and (c) Fe3O4/TiO2 using the coprecipitation method.

Moringa oleifera synthesis
MO powder was added with aquadest and stirred for 60 minutes at 60°C.The solution was cooled to room temperature then filtered using Whatman paper no-1 and the extract was stored in the refrigerator.Illustrated in Figure 1 (a).
2.3.Green-synthesis Fe3O4/TiO2 nanoparticles Figure 1 (b) shows the Fe3O4 synthesis process.Weighed 4.054 g FeCl.5H2O and 2.086 g FeSO4.7H2Odissolved in distilled water and stirred for 15 minutes at 60°C (600 rpm) then added MO solution and stirred for 30 minutes at 60°C (600 rpm).Titration was carried out using 10% ammonia solution while stirring for 90 minutes, temperature 60°C (600 rpm).The formed nanoparticles were washed 7 times using distilled water with the help of an external magnet.The precipitated nanoparticles were then dried for 2 hours at 200°C.In figure 1 (c) Fe3O4 was dissolved in 10 mL ethanol and TTIP solution plus MO solution was stirred for 15 minutes (700 rpm), both solutions were stirred for 30 minutes (700 rpm).The solution was washed using distilled water 7 times until it reached neutral pH with an external magnet.The nanoparticles were dried in a furnace at 100°C for 2 hours.

X-Ray data processing
X-ray data processing aims to determine the structure and phase of Fe3O4 nanoparticles and Fe3O4/TiO2 nanocomposites made using the green synthesis route.The data processed is characterization data in the form of diffraction peak profiles and then analyzed using MAUD (trial version) and OriginLab software.Then, the diffraction pattern data was matched with the chrystallography open database (COD).This was done to determine the crystal phase formed.From the data obtained, several parameters can be determined, such as the distance between planes, phase composition, crystal lattice, and crystal size.

Photocatalytic activity test
The testing step begins by adding catalyst with different mass variations, namely (0.02, 0.04, 0.06, 0.08, 0.1) grams into 50 mL of MB solution at neutral pH.After adding the sample, it was stirred first using a magnetic stirrer for 30 minutes in dark conditions to obtain a balance of adsorption and desorption.After stirring without light, the sample was stirred again using a magnetic stirrer under conditions exposed to light for 60 minutes.After that, the catalyst was separated from the ocean with an external magnet and then the degraded MB solution was characterized at a wavelength of 664 nm, to calculate the effectiveness of its degradation.The percentage of MB degradation can be calculated using the equation: (1) where c0 is the initial concentration of MB and ct is the final concentration at time t.

Result and discussion
3.1.Crystal structure of Fe3O4/TiO2 nanocomposites The Fe3O4/TiO2 nanocomposite XRD patterns were analyzed using the Rietveld Refinement method to obtain sharp and clear diffraction peaks without any noise or small peaks [7][8].Figure 2 shows the Xray diffraction patterns of Fe3O4 nanoparticles and Fe3O4/TiO2 nanocomposites from the Rietveld Refinement analysis that had been baselined.The diffraction peaks are located at the values (hkl): (210), (220), ( 104), (311), ( 222), (400), ( 200), (422), (511), and (440) for Fe3O4 nanoparticles.Refer to JCPDScard No. 10-11032, Fe3O4 nanoparticles have a cubic inverse spinel structure [9].In the Fe3O4/TiO2 nanocomposite XRD patterns, new diffraction peaks were observed in planes (110) and (111) indicating the presence of anatase and rutile TiO2 crystals which have a tetragonal structure [10].Lattice parameters and crystallite size of Fe3O4 and Fe3O4/TiO2 are shown in Table 1.Fe3O4 nanoparticles have a crystallite size of 5.49 nm with a lattice parameter of 7.94 A, while for Fe3O4/TiO2 nanocomposites has a crystallite size of 5.37 nm with a lattice parameter of 7.35 A. There appears to be a decrease in the average size of crystallites and lattice parameters when Fe3O4 is composited with TiO2.This is because TiO2 is formed from Ti 4+ and O 2-ions so that during the synthesis process the ions from O 2-will be dispersed into the Fe3O4 octahedral space, the loss of O 2-ions causes the substitution of Fe 2+ into the TiO2 octahedral subspace and causes the growth of anatase crystals.The growth of anatase crystals causes a decrease in crystal size and lattice parameters [11].

Optical properties of Fe3O4/TiO2 nanocomposites
Figure 3 shows the absorption peak spectra and absorption edges of Fe3O4, TiO2, and Fe3O4/TiO2 nanocomposites.It was clearly observed that the Fe3O4/TiO2 nanocomposite had an absorption edge and an absorption peak which were located between the absorption wavelengths of Fe3O4 and TiO2 nanoparticles with an absorption edge located at a wavelength of 191.1 nm and a broad peak located at 384 and 396 nm.This indicates that the Fe3O4/TiO2 nanocomposite has the characteristic of absorbing light combined with its constituent materials.2 presents the energy band gap for each sample measured using the Tauc plot method as seen in Figure 4.There was an increase in the energy band gap after the Fe3O4 nanoparticles were composited with TiO2.This increase in band gap energy is closely related to the size of the crystallites resulting from XRD processing which is presented in Table 1, where it can be seen that the average crystallite size of Fe3O4/TiO2 nanocomposites is smaller when compared to Fe3O4 nanoparticles.This result is similar to that reported by Tumbelaka, 2022.The presence of TiO2 affects the size of the crystallites and also has an impact on the size of the nanoparticles [12].In addition, when the particle size decreases, the band gap energy will increase [13].

Effect of mass variation of Fe3O4/TiO2 nanocomposites on MB degradation percentage
In Figure 5 it is observed that at first MB had a distinctive color, namely blue, then after photodegradation the color changed to become clearer with a slight yellowness.Immediately it can be seen that the sample will change from blue to light blue, then change back to clear with a little bluish and so on to clear yellowish.The color in the MB solution changes due to the separation of the chromophore groups [14].
In Figure 6 and Figure 7 it can be seen that in the time range 0-20 minutes the percentage of MB degradation results increases as the mass of the Fe3O4/TiO2 Nanocomposite catalyst increases.The largest MB degradation percentage was obtained by varying the catalyst mass by 0.1 gram, where the degradation percentage could reach 96.37%.This increase may be closely related to the number of excited electrons which then produce free electrons on the surface area of the nanocomposite which interact with the MB molecules so that the photodegradation process occurs.In the time span of 20-60 minutes it was also seen that the percentage of degradation showed an increase up to a mass variation of 0.06 gram with the percentage of MB degradation reaching 98.4%.This indicates that the photocatalytic process is running effectively even though the increase is not too significant due to the agglomeration factor [15].Then, for variations in mass of more than 0.06 gram, the percentage of degradation decreased slightly, this is because the agglomeration or clumping of the photocatalyst increased with increasing mass.8 shows a bar chart of the percentage of Fe3O4/TiO2 nanocomposites in degrading MB with repeated use.It can be seen that for R1 at minute 0 (dark) there was a significant photodegradation process.The degradation percentage reached 99.18%, and it also happened in the 2nd repetition (R2) the degradation percentage reached 97.07%.This can be caused by the presence of other materials that stick during the first photocatalytic process, thus allowing for changes in the pH of the photocatalyst cycle, thereby affecting its greater adsorption properties.The magnitude of the pH will affect the adsorption of methylene blue.At acidic pH, MB tends to be protonated by H + ions and causes MB to become less colored [16].

Conclusion
Green-synthesis of Fe3O4/TiO2 nanocomposites using MO leaf extract has been successfully synthesized using the coprecipitation method.Fe3O4 has a cubic inverse spinel structure and TiO2 has a tetragonal structure.The addition of TiO2 material to Fe3O4 affects the average crystallite size.Before being composited with TiO2, Fe3O4 nanoparticles had an average crystallite size of 5.49 nm, after composited the average crystallite size was 5.37 nm.The UV-Vis results showed that there was an increase in the absorption value after Fe3O4 was composited with TiO2 accompanied by an increase in the bandgap energy value.In the first 20 minutes the photodegradation activity showed a significant value.The more mass of the catalyst, the higher the percentage of degradation.After 20 minutes the percentage of degradation tends to be constant and slightly decreases.The highest degradation is reaching 98.4%.The results of the reuse test showed that the Fe3O4/TiO2 nanocomposites is reusable and the degradation percentage value was relatively constant with a slight decrease in the second reuse.

Figure 6 .Figure 7 .
Figure 6.Percentage of MB Fe3O4/TiO2 nanocomposite degradation for each mass variation over time

Figure 8 .
Figure 8.The percentage of Fe3O4/TiO2 nanocomposites in degrading MB with the first (R1) and second (R2) repeated use.

Table 2 .
Band gap energy values of the Tauc plot of Fe3O4, TiO2, and Fe3O4/TiO2 samples