LiFe5O8 nanocomposites

Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112063

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Enhanced photocatalytic and antibacterial activities of RGO/LiFe5O8 nanocomposites

T

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Mahshid Chireha, Mahmoud Naseria, , Saeedeh Ghiasvandb a b

Department of Physics, Faculty of Science, Malayer University, Malayer, Iran Department of Biology, Faculty of Science, Malayer University, Malayer, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: RGO/LiFe5O8 nanocomposites Polymerization method Photocatalytic activity Antibacterial characteristics

Through a thermal treatment method, LiFe5O8 nanoparticles are prepared and by using situ polymerization method, reduced graphene oxide/lithium ferrite (RGO/LiFe5O8) nanocomposites with the ratios of 0.15, 0.20 and 0.25 are created. The structural and physical properties of prepared samples are analyzed by different techniques. By using degradation of methylene blue (MB), photocatalytic activity is investigated under visible light. The combination of LiFe5O8 nanoparticles with RGO causes the enhancement of photocatalytic activity in same conditions. Moreover, after five recycles, photocatalytic activity of RGO/LiFe5O8 does not change as compared to the photocatalytic activity of fresh catalyst, indicating the stability and reusability of RGO/LiFe5O8. Additionally, the antibacterial characteristics of studied samples were investigated against Staphylococcus aureus (S. aureus) as Gram- positive bacteria and Escherichia coli (E. coli) as a Gram- negative bacteria with inhibition zone and colony counting methods. According to obtained results, the pure LiFe5O8 do not have any antibacterial effect on S. aureus, and E. coli. RGO/LiFe5O8 nanocomposites with the maximum concentration of RGO have a greater antibacterial effect on S. aureus, and E. coli than LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites with lower RGO ratios.

1. Introduction In recent years, numerous researches have investigated the combination of nanomaterials such as ferrite nanocrystals. LiFe5O8 is one of the prospective typical iron-based materials, which is low-cost and does not damage the environment [1,2]. Recently, multifunctional nanocomposites with graphene and magnetic nanoparticles, due to their useful combinational characteristics, have been widely investigated by researchers. Graphene with its phenomenal, electrical, thermal and mechanical characteristics possesses large surface areas, which can be used as a template for following the magnetic nanoparticles [3]. Photocatalytic degradation of water pollutants is extensively studied and numerous traditional semiconductors of oxide photocatalysts, including WO and TiO are developed [4–7]. However, the band gaps of these materials are not appropriate for absorbing a large portion of solar spectrum. In comparison with wide band gap semiconductors, magnetic ferrites, due to their relatively narrow band gaps (˜2.0 eV), absorption and suitable conduction of band gap and conversion of solar energy into chemical energy under visible light irradiation are considered as good photocatalytic choices. In addition, by applying magnetic fields to solution after reaction and providing an efficient and cost effective

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method for practical operation, the magnetic ferrite materials can be recovered [8]. Graphene has the capability of accepting electrons from semiconductors and preventing the recombination of photogenerated electrons and holes. Combining well-structured photocatalysts with graphene-based materials leads to the foundation of new composites with higher photocatalytic performances [9]. Moreover, there are numerous researches about the application of these materials in nanomedicine fields including biodetection, targeted drug delivery, magnetic resonance imaging (MRI) and hyperthermia [10,11]. Grapheneoxide (GO) nanosheet is a two-dimensional material with high applicability in electronic devices [12] and its large specific surface area is efficient for immobilizing many substances such as drugs, biomolecules and nanoparticles. It was revealed that, graphene oxide sheets are biocompatible without toxicity and ideal for targeted drug delivery applications which cause an increasing interest in investigating the composite GO magnetic nanoparticles attached to GO nanosheets [13]. In current research, the synthesis of novel RGO/LiFe5O8 nanocomposites and the effect of different RGO/LiFe5O8 ratios on structural, optical, magnetic, adsorption, photocatalyst and antibacterial characteristics of prepared nanocomposites are investigated. Also, efficient factors on the enhancement of photocatalyst characteristics are closely

Corresponding author. E-mail address: [email protected] (M. Naseri).

https://doi.org/10.1016/j.jphotochem.2019.112063 Received 4 June 2019; Received in revised form 22 August 2019; Accepted 24 August 2019 Available online 26 August 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.

Journal of Photochemistry & Photobiology A: Chemistry 385 (2019) 112063

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3. Characterization

studied. The antimicrobial effects of synthesized samples were assessed using two different methods: the colony counting and inhibition zone methods.

By using X-Ray Diffractometer (XRD, X’Pert PRO MPD, CuKα, k = 0.15406 nm), the crystalline structural phase of synthesized nanoparticles and nanocomposites is determined at room temperature and the microstructure of studied samples is investigated by FESEM (JEOL JSM-6701F). To study the chemical interactions between RGO and magnetic LiFe5O8 nanoparticles (PerkinElmer FTIR 1650 spectrometer), FTIR spectra are recorded and the transmittance, reflectance and absorbance spectra of nanoparticles and nanocomposites are determined by ultraviolet–visible spectrophotometer (UV-1650 PCSHIMADZU). By using a vibrating sample magnetometer (VSM, MDKB model), the magnetic characteristics are measured and by using a laser Raman spectrophotometer (UniRAM, wavelength of excitation laser: 785 nm), Raman spectra are measured. By using a Micrometrics Belsorp mini system, Brunauer–Emmett–Teller (BET) surface area and pore-size distribution of samples are determined by the adsorption of nitrogen at 77 K.

2. Experimental methods 2.1. Synthesis of graphene oxide Graphene oxide is synthesized by graphite powder using modified Hummers method. In present synthesis process, 2 g of graphite powder is mixed with solution with strong oxidizing agents including potassium permanganate (KMnO4, 99.5%) and concentrated sulfuric acid (H2SO4, 98%). Then, for diluting the solution at room temperature, almost 400 ml deionized water and 12 ml H2O2 (30%) are added and after 5 min, a bright yellow solution is obtained which is centrifuged and washed with 5% HCl solution and distilled water. The yellow-brown precipitate of graphene-oxide is obtained and dried at 60 °C.

4. Results and discussion

2.2. Synthesis of LiFe5O8 nanoparticles

4.1. Mechanism of formation of RGO/LiFe5O8 nanocomposites

Thermal-treatment technique has been successfully used to synthesize LiFe5O8 nanostructures. An aqueous solution of PVP was prepared by dissolving 4 g of the polymer in 100 ml of deionized water at 343 K. Then, 0.5 mmol of iron nitrate and 0.1 mmol of lithium nitrate (Fe:Li = 5:1) was added to the polymer solution and stirred for 2 h with a magnetic stirrer. At the end of the 2 h, a colorless, transparent solution was obtained. Using a glass electrode, the pH of the solution was determined to be in the range of 1–2. No precipitation was observed prior to the heat treatment. The solution was poured into a Petri dish (glass) and heated in an oven at 363 K for 24 h up to the time the water is evaporated. The remained dried precursor was crushed and ground in a mortar to obtain a powder. Calcination of the powder was performed at 1003 K for 3 h with the aim to decompose any organic compounds and crystallize the nanocrystals [1].

In Fig. 1, the synthetic mechanism for preparation of RGO/LiFe5O8 nanocomposites is shown. At first, GO sheets are dispersed in an aqueous solution containing CTAB by sonication. GO sheets surface have various oxygenated functional groups such as eOH and CeOOH. CTAB is made of a hydrophobic tail and a positively charged hydrophilic head [14], therefore, as a cationic surfactant, CTAB is an important factor in dispersion of GO in aqueous solution. There are electrostatic interactions between the positively charged hydrophilic head of CTAB and the negatively charged surface of GO sheets. Naturally, the interaction of CTAB with GO is electrostatic which breaks the van der Waals forces between GO layers [15]. By dispersing LiFe5O8 nanocomposites in GO solution, the positively charged surface of LiFe5O8 is attracted by GO surface with electronegativity and automatically, assemblies on GO sheets and finally, RGO/LiFe5O8 nanocomposites are created.

2.3. Synthesis of RGO/LiFe5O8 nanocomposites

4.2. Structural properties of the LiFe5O8 and RGO/LiFe5O8 nanocomposites

At first, GO is immersed in an aqueous solution containing CetylTrimethyl-Ammonium-Bromide (CTAB) and ultrasonicate. Then, LiFe5O8 nanoparticles (m(GO)/m(LiFe5O8) = 15, 20, 25%) are added to the mentioned mixed solution with stirring to form a homogeneous dispersion and hydrazine hydrate is added to the suspension by stirring for 60 min. The suspension is centrifuged and washed by distilled water and ethanol. Finally, in order to obtain RGO/LiFe5O8 nanocomposites, obtained product is dried at 60̊ C in vacuum for 24 h.

Fig. 2 indicates X-ray diffraction patterns of GO, pure LiFe5O8, and RGO/LiFe5O8 nanocomposites with different GO ratios. As it is seen, in XRD curve of GO, there is a strong diffraction peak at 2θ = 11.45° and a small peak at 2θ = 26.74°, corresponding to (001) and (121) planes, respectively. Peak (001) of GO with interlayer spacing of d001 = 7.72 Å is in good agreement with published results [16,17] indicating that, natural graphite is oxidized in GO with regular crystal structure and high oxidation degree [15]. In pure LiFe5O8, all diffraction peaks are coincident with recorded powder diffraction data of cubic spinel structure. In XRD patterns of RGO/LiFe5O8 nanocomposites, all of the diffraction peaks related to LiFe5O8 are seen and there is no important changes in XRD patterns between LiFe5O8 and RGO/LiFe5O8 nanocomposites, indicating that, after the formation of nanocomposites with RGO nanosheets, the phase purity of LiFe5O8 is saved and nanoparticles are deposited on graphene. Furthermore, the disappearance of GO diffraction peaks reveals the detachment of oxygen groups and reduction of grapheme oxide to graphene nanosheets during preparation of nanocomposites [18,19]. By the X-ray diffraction results, the average crystallite sizes were estimated to range from 29.46, 28.43, 24.28 and 24.17 nm for LiFe5O8, RGO/LiFe5O8 = 0.15, RGO/LiFe5O8 = 0.20 and RGO/LiFe5O8 = 0.25, respectively. Results show that, due to the mass ratio of GO, the average crystallite size of RGO/LiFe5O8 nanocomposites decreases. FESEM images of GO, LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites with different ratios are illustrated in Fig. 3(a–e), respectively. According to Fig. 3(a), GO has continuous and individual

2.4. Photocatalytic experiment By using photodegradation of methylene blue (MB) solution under visible light irradiation, photocatalytic characteristics of studied samples are evaluated. In all of the photocatalytic degradation experiments, 10 mg of synthesized photocatalysts are added to 100 ml MB aqueous − solution (3 × 10−6 mol l 1). Before starting light irradiation, in order to reach the adsorption-desorption equilibrium between catalyst and MB molecules, the reaction mixture is magnetically stirred for 30 min in dark. Then, the mixture is irradiated by a 400 W Hg lamp (400 < λ < 700 nm) under continuous stirring and the distance between lamp and dye solution is 10 cm. At a given time for interval irradiation, for removing all of the catalysts, 5 ml of sample solution is taken and remained solution is magnetically separated. The degradation of MB determined by UV–vis spectrometer. All experiments are repeated three times and performed at room temperature in pH = 6.7. 2

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Fig. 1. Schematic representation of the preparation of the RGO/LiFe5O8 nanocomposites.

vibration in OeH and HOHee in adsorbed water molecules. Also 1629 cm−1 band is related to the surface absorbed water and hydroxyl groups [17,20]. The intensity of RGO/LiFe5O8 nanocomposites peaks are higher than LiFe5O8 nanoparticles, indicating that, RGO/LiFe5O8 nanocomposites are highly hydrophilic and have higher photocatalytic activity. The peak at 1726 cm−1 is related to the tensional of C]O bonds in carboxylic acid and carbonyl moieties [21,22]. Between 900 and 1500 cm−1, there are some peaks related to CeO tensional with COCee peak at 1253 cm−1 and CeH peak at 1375 cm−1. Higher intense CeO peaks are located at 1053 cm−1 [23,24]. The absorption peaks located around 1375 and 1053 cm−1 are related to the functional group of RGO [25]. Additionally, graphene sheets contain functional groups such as aromatic hydrocarbons (2825–2980 cm−1), carboxyl (2350 cm−1) groups [26,27]. According to FTIR spectra of LiFe5O nanoparticles and RGO/LiFe5O8 nanocomposites, the absorption band around 520 cm−1 is a characteristic peak related to the metal-oxygen bands of ferrite groups [28]. As it is seen from Fig. 4, in FTIR spectrum of LiFe5O8 nanoparticles, peaks are weaker than the peaks of RGO/ LiFe5O8 nanocomposites.

Fig. 2. XRD patterns of GO sheets, LiFe5O8 nanoparticles, RGO/LiFe5O8 nanocomposites with different RGO ratio from 0.15 to 0.25.

4.3. Physical properties of the LiFe5O8 and RGO/ LiFe5O8 nanocomposites sheets which are almost stacked. As it is seen from Fig. 3(b)–(d), in RGO/LiFe5O8 nanocomposites, LiFe5O8 nanoparticles are distributed on graphene sheets surface. In Fig. 3(e), LiFe5O8 nanoparticles have relatively uniform distribution in shapes and aggregation with the average particle size of 28 nm. In addition, nanoparticles of three studied RGO/ LiFe5O8 samples have the average particle size of 18, 15 and 14 nm for RGO/LiFe5O8 = 0.15, RGO/LiFe5O8 = 0.20 and RGO/LiFe5O8 = 0.25, respectively. According to FESEM images RGO/LiFe5O8 nanocomposites, adding graphene causes the reduction of diameter and size of particles and particles aggregation. Fig. 4 indicates FTIR spectrum of LiFe5O8 nanoparticles and RGO/ LiFe5O8 = 0.25 nanocomposites. This Figure shows remarkable differences between LiFe5O8 and RGO/LiFe5O8. In RGO/LiFe5O8, the strong − and broad band at 3437 cm−1 and 609 cm 1 is related to the tensional

Investigating the optical characteristics of studied samples is demanded for their suitability in photocatalysis. Diffuse reflectance spectra (DRS) of LiFe5O8 and RGO/ LiFe5O8 samples are indicated in Fig. 5. Fig. 5(a) shows the absorbance spectra and Tauc’s plots of LiFe5O8 nanoparticles, respectively. According to this Figure, corresponding wavelength with optical energy gap for LiFe5O8 is 730 nm. The band gap energy (Eg) is calculated by (αhν)1/2 versus photon energy (hν) plots [29] (inset of Fig. 5(a)), which is approximately 1.70 eV. According to Fig. 5(b), the absorption spectrum of RGO/LiFe5O8 nanocomposites shows two peaks around 200–400 nm and 700–850 nm. The maximum absorption peak of RGO/LiFe5O8 with different ratios of RGO are observed at 216 nm. The maximum absorption peak is related to π–π* transition of aromatic CeC bonds of the reduced graphene oxide [24,30]. By increasing RGO (from 0.15 to 0.25), the light 3

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Fig. 3. FESEM microstructures of the (a) GO sheets, (b), (c), (d) RGO/LiFe5O8 nanocomposites with different RGO ratio from 0.15 to 0.25 and (e) LiFe5O8 nanoparticles.

absorption intensity in overall UV region enhances. It is seen that, the high light absorption is closely related to the acceptable photocatalytic activity of catalysts [25]. Also, by increasing RGO, the peak at 295 nm associates with RGO redshifts to lower wavelength (260 nm) indicating the enhancement of p-electron concentration with the reduction of sp3 graphene oxide to sp2 hybridization of carbon atoms [17,30]. The absorption peak at ˜380 nm can be seen which corresponds to n–π* transition due to COe bonds [31,32]. The characteristic absorption peak of lithium ferrite is also observed around 731 nm indicating a blue shift to 755 nm followed by the increase of RGO ratio. These changes can be attributed to lithium ferrite nanoparticles anchored on RGO sheets revealing a strong interaction between RGO and LiFe5O8 nanoparticles [33] evaluated by (XRD) and (SEM) analysis. It should be noted that, the absorption spectra of nanocomposites show a peak around 821 nm which can be related to the strongly reduced graphene oxide. Moreover, the increased absorption in visible light is due to the black body properties of graphene sheets [30,32]. RGO/LiFe5O8 nanocomposites with a wide adjustable absorption of light and multi-band gap energy have a great potential for being used in photocatalysis. The Raman spectra of LiFe5O8 and RGO/LiFe5O8 nanocomposites with different ratios are shown in Fig. 6. For confirming the presence

Fig. 4. FT-IR spectra of LiFe5O8 nanoparticles and RGO/LiFe5O8 = 0.25 nanocomposite.

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Fig. 5. UV–vis diffuse reflectance spectra of the (a) LiFe5O8 nanoparticles, (b) RGO/LiFe5O8 nanocomposites with different RGO ratio from 0.15 to 0.25. The inset of (a) is the Tauc plot of band gap energy of the LiFe5O8 nanoparticles.

domain upon the hydrothermal reduction. The intensity ratio of ID/IG is related to the quantity restoration of sp2 carbon [34]. The surface area of LiFe5O8 nanoparticles and RGO/LiFe5O8 = 0.25 nanocomposites are analyzed using nitrogen adsorption−desorption isotherms at 77 K. The BET (Barrett–Emmett–Teller) surface area and corresponding BJH (Barrett–Joyner–Halend) pore size distribution are illustrated in Fig. 7(a–d) and pore texture parameters and surface area are listed in Table 1. Fig. 7(a and b) shows that, the adsorption isotherm at relative pressure (P/P0) from 0.7 to 1.0 and from 0.3 to 1.0 are IV-type with hysteresis loop (IUPAC) for LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites, respectively. The existence of significant hysteresis loop in isotherm (Fig. 7(b)) indicates the presence of mesopore among RGO/LiFe5O8 nanocomposites. We know that, mesoporous structure is an efficient photocatalyst structure because the multi-porous structure is useful in charge carrier [36,37]. Fig. 7(c and d) shows a much broader pore size distribution at the range of 0–50 nm with dominant pore sizes of 1–10 nm. As it is seen from Table 1, LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites have specific surface areas of 22 and 13 m2/g with average pore size distribution of 30 and 40 nm, respectively. RGO/LiFe5O8 nanocomposites have lower specific surface area [38], which is due to the large number of LiFe5O8 nanoparticles occupying the active surface of RGO, which may not allow nitrogen molecules to be adsorbed in them, compared to pristine graphene [39]. Fig. 8 indicates the plot of absorbance versus the wavelength of photocatalytic degradation of MB. Also, the changes of the absorbance of MB solution in presence of RGO/LiFe5O8 = 0.25 and LiFe5O8 photocatalysts under visible light irradiation are presented in Fig. 8 and inset of Fig. 8. The concentration of remained dye are presented by measuring the absorbance of solutions at 664 nm (the main absorption peak of MB molecules) during photodegradation process. The absorbent capacity of RGO/LiFe5O8 is stronger than pure LiFe5O8. As shown in Fig. 9(a), after 100 min reaction, photodegradation rates of MB for RGO/ LiFe5O8 = 0.25 under visible light irradiation enhances up to 72%, however, in same conditions, photodegradation rates of MB for pure LiFe5O8 is only 26%. Also, according to the inset of Fig. 9(a), by frequently using RGO, the performance of photodegradation rates becomes better. Generally, RGO/LiFe5O8 nanocomposites have better photocatalytic activity than pure LiFe5O8, indicating that, RGO/LiFe5O8 is photocatalytically active and can be used as a photocatalyst under UV irradiation. In order to investigate the degradation kinetics of MB, the photodegradation reactions follow a pseudo firstorder kinetics model by using Eq. (1):

Fig. 6. Raman spectra of the LiFe5O8 nanoparticles, RGO/LiFe5O8 nanocomposites with different RGO ratio from 0.15 to 0.25.

and the role of graphene on lithium ferrite behavior, Raman analysis are performed. Raman spectroscopy is a powerful nondestructive method used for obtaining structural information of carbon-based materials [21,31]. In LiFe5O8 spectra, five peaks centered at 343, 411, 529, 616 and 689 cm−1 are related to the distinct vibrational modes of the spinel structure of LiFe5O8 nanoparticles [2]. The strongest peak is located at 689 cm−1 which is related to A1g mode [24]. Because of the interaction between LiFe5O8 and RGO, the intensity of this peak in RGO/LiFe5O8 nanocomposites is reduced compared to pure LiFe5O8 nanoparticles. In addition, the peak at 343 cm−1 in Raman spectra of pure LiFe5O8 is related to E12g mode [30]. After adding RGO, E12g and A1g modes of LiFe5O8 shift to 332 and 678 cm−1 in RGO/LiFe5O8 nanocomposites. Raman spectra of RGO/LiFe5O8 nanocomposites (15, 20 and 25 wt%) indicate the characteristics of RGO and LiFe5O8, however, two additional peaks at 1316 and 1596 cm−1 are related to D and G bands of graphene [34]. D band is related to the edge disordered band structure of k-point phonon of A1g of carbon atoms and G band is related to E2g mode of order band structure of sp2 hybridization of carbon atoms [35]. The values of ID/IG (intensity ratios of D and G band) for RGO/LiFe5O8 nanocomposites with the ratios of 0.15, 0.20 and 0.25 are 0.982, 0.989 and 0.996, respectively. By increasing RGO ratio, ID/IG ratio enhances indicating the reduction of the average size of sp2 5

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Fig. 7. Nitrogen adsorption–desorption isotherms of (a) LiFe5O8 nanoparticles and (b) RGO/LiFe5O8 = 0.25 nanocomposite. Pore size distribution of (c) LiFe5O8 nanoparticles and (d) RGO/LiFe5O8 = 0.25 nanocomposite.

ln(C0/Ct) = kt

(1)

where C0 is the concentration after the “dark” period and C is the concentration after t min of irradiation), and k = apparent rate constant [40,41]. According to Fig. 9(b), plots of ln(C/C0) versus reaction time are linear and the value of rate constant (k) equals with the corresponding slope of the fitting line. The value of k in RGO/LiFe5O8 nanocomposites is higher than LiFe5O8 nanoparticles and by increasing RGO ratio, k enhances. By adding graphene, the rate constant of LiFe5O8 (k = 0.003 min−1) increases up to 0.0126 min−1. According to presented results, in comparison with pure LiFe5O8 nanoparticles, the produced RGO/LiFe5O8 = 0.25 nanocomposites with mesoporous structure and acceptable characteristics for separating photoinduced electron–hole pairs of LiFe5O8, causes significant enhancement in photocatalytic activity in degradation of MB under visible light. Fig. 9(c) indicates the vibrating sample magnetometer (VSM) curve of LiFe5O8 and RGO/ LiFe5O8 = 0.25 and both samples show a typical hysteresis loop in their magnetic behavior, indicating that, both are soft magnetic materials. The saturation magnetization (MS) and coercivity (HC) of LiFe5O8 nanoparticles and RGO/LiFe5O8 = 0.25 nanocomposites are 28.43 emu/g, 3752 Oe, 35.70 emu/g and 4500 Oe respectively.

Fig. 8. Absorption spectra of the MB solution taken at different photocatalytic degradation times using RGO/LiFe5O8 = 0.25 nanocomposite and LiFe5O8 nanoparticles (inset).

Table 1 Textual characteristics of LiFe5O8 nanoparticles and RGO/LiFe5O8 = 0.25 nanocomposite. sample

Specific surface area SBET (m2/g)

Average pore diameter (nm)

Pore volum (cm3/g)

Total pore volume (cm3/g)

LiFe5O8 RGO/LiFe5O8 = 0.25

22 13

30 40

0.1519 0.1294

0.1717 0.1334

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Fig. 9. (a) Photocatalytic degradation of MB, inset (a): Effect of different catalysts on photocatalytic degradation of MB: LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites with different RGO ratio from 0.15 to 0.25, (b) Kinetic curves obtained for the degradation of MB, (c) Hysteresis loops of RGO/LiFe5O8 = 0.25 nanocomposite and LiFe5O8 nanoparticles. The inset is the magnetic separation property of RGO/LiFe5O8 = 0.25 nanocomposite, (d) Photodegradation rate of MB in solution for five cycles.

Furthermore, the remained magnetization values (Mr) are 14.99 emu/g and 18.83 emu/g for LiFe5O8 nanoparticles and nanocomposites, respectively. By adding graphene in lithium ferrite, the saturation magnetization increases, the increase in saturation magnetization of RGO/ LiFe5O8 nanocomposites, compared to that of LiFe5O8 nanoparticles, originates from various defects on graphene structures, such as vacancy, topological defects or frustration, and hydrogen chemisorption or perhaps the edge [42,43]. Moreover, as discussed earlier with regard to Fig. 6, by increasing RGO ratio, ID/IG ratio enhances indicating that the defect on the RGO increasing. Therefore, the increasing of the defect creation will induce the increasing magnetism of RGO. As a result, saturation magnetization of RGO/LiFe5O8 nanocomposites increase compared to that of LiFe5O8 nanoparticles [44]. The inset of Fig. 9(c) show that the RGO/LiFe5O8 = 0.25 photocatalyst can be easily separated from the solution phase using an external magnet. The stability and recyclability of a photocatalyst is an important factor in practical applications. Therefore, the stability of RGO/ LiFe5O8 = 0.25 in photodegradation of MB is examined by repeating the photocatalytic experiment in five cycles. After each cycle, by using of an external magnet, photocatalyst is separated and recovered and washed with ethanol and deionized water, respectively and at the end of each cycle, it is dried. According to Fig. 9(d), the degradation ratio of MB in the fifth recycling was still 72% after 100 min of reaction under the same conditions. It can be clearly seen that there was no noticeable change in the photocatalytic activity in the three recycles and it was

Fig. 10. Schematic representation of the photocatalytic mechanism for MB degradation by RGO/LiFe5O8 nanocomposite.

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Table 2 The inhibition zone test results of LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites with different RGO ratio from 0.15 to 0.25 against Gram positive and Gram negative bacteria.

S. aureus E. coli

LiFe5O8

RGO/LiFe5O8 = 0.15

RGO/LiFe5O8 = 0.20

RGO/LiFe5O8 = 0.25

0 mm 0 mm

0 mm 0 mm

0 mm 16 mm

14 mm 20 mm

Fig. 11. Nutrient agar plates that show the inhibition zone test of (A) LiFe5O8, (B) RGO/LiFe5O8 = 0.15, (C) RGO/LiFe5O8 = 0.20, (D) RGO/LiFe5O8 = 0.25 nanocomposites against S. aureus and E. coli bacteria.

Fig. 12. (A) S. aureus colonies, (B) E. coli colonies on RGO/LiFe5O8 nanocomposites.

only reduced a little within five recycles. The result revealed good stability of RGO/LiFe5O8 = 0.25 and indicate that, photocatalysts of RGO/LiFe5O8 nanocomposites can be reused in wastewater treatment [41]. The photoelectron transfer mechanism in photocatalytic degradation of MB in RGO/LiFe5O8 nanocomposites is schematically indicated in Fig. 10. Under visible light irradiation, RGO/LiFe5O8 is stimulated to

create free electrons and holes. As it is seen, when RGO/LiFe5O8 nanocomposite are irradiated with light with sufficient energy (hv) equal with or greater than the band gap energy (hν≥ Eg), only LiFe5O8 nanoparticles can be activated, electrons (e−) from valence band (VB) are transferred to the conduction band (CB) and create the positively charged holes (h+) [45]. Photogenerated electrons of RGO/LiFe5O8 can easily move toward the surface of RGO sheets and provide a 8

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wall structural compositions and grow rates [51]. As colony counting and inhibition zone results shown, RGO/LiFe5O8 = 0.25 has higher toxicity compared with other synthesized nanocomposites, although RGO/LiFe5O8 = 0.20 has low toxicity effect on E. coli, other nanoparticles did not have significant toxicity.

transferring channel [46]. For increasing the transport of photogenerated electrons, the conjugated sp2 hybridized structure of RGO composite materials provides a lot of delocalized electrons [47,48]. This process causes significant improvement in separation of photogenerated electron−hole pairs and reduction of the possibility of photogenerated charge recombination. Photogenerated electrons can directly produce the superoxide anion radicals (O2−%) from molecular oxygen. Also, photogenerated holes on valence band (VB) of LiFe5O8 can be used as oxidizing agents to directly oxidize the dye or react with adsorbed water molecules for producing hydroxyl radicals (OH%). There are numerous active adsorption centers and photocatalytic reaction sites in RGO with large surface areas. As it is known, the main active radicals are photogenrated holes and OH%, and O2−% radicals in photocatalytic process. The superoxide anion radicals are formed in RGO sheets with a reaction between photoinduced electrons and adsorbed oxygen, while hydroxyl radicals are formed on LiFe5O8 surface with a reaction between photoinduced holes and water [48]. Therefore, MB molecules are degraded by continuous reactions with holes, superoxide anion radicals and hydroxyl radicals. Hence, synthesis of photo-generated electrons and holes can be prevented and consequently, the photocatalytic activity of nanoparticles and nanocomposites enhances. By increasing photo-generated electrons and holes and RGO/LiFe5O8 ratio, the photocataltic activity of nanocomposites enhances significantly.

5. Conclusions In present research, LiFe5O8 nanoparticles, graphene-oxide sheets and RGO/LiFe5O8 nanocomposites are synthesized by thermal treatment method, modified Hummer’s method and polymerization method, respectively. Based on XRD and FESEM results, the formation of spinel LiFe5O8 on RGO sheets is revealed. UV–vis DRS measurements indicate RGO/LiFe5O8 nanocomposites with a wide adjustable light adsorption and multi-band gap energy have a good potential for being used in photocatalysis. Raman spectra of RGO/LiFe5O8 nanocomposites revealed that, by increasing RGO ratio, ID/IG ratio enhances. By using MB dye degradation under visible light, photocatalytic activity of studied samples are evaluated and prepared lithium ferrite nanocomposites with RGO sheets indicate the best photocatalytic activity, compared to the pure lithium ferrite nanoparticles. Also, by applying external magnetic field, RGO/LiFe5O8 nanocomposites have high magnetism, hence, they can be recovered easily. Additionally, RGO/LiFe5O8 nanocomposites show the enhancement of antibacterial activity against Gram positive and Gram negative bacteria. Investigating the characteristics of RGO/LiFe5O8 nanocomposites reveals that, these low-cost materials can be used as good applied materials in photocatalytic and antibacterial activities.

4.4. Antibacterial properties of the LiFe5O8 and RGO/LiFe5O8 nanocomposites The antibacterial activity of LiFe5O8 nanoparticles and RGO/ LiFe5O8 nanocomposites with different RGO ratios is investigated. Two bacterial strains, Gram positive, Staphylococcus aureus (S. aureus) and Gram negative (E. coli) are cultured in nutrient broth medium at 37 °C overnight. Then, 20 μl inoculums is swapped on nutrient agar medium and 4 mm disks are made by punching a sterile paper. The disks are soaked with synthesized nanoparticles and located on plates. The inoculated plates are incubated at 37 °C in an incubator for 24 h. After incubation, the inhibition zone was measured (Table 2). Fig. 11 shows nutrient agar plates that describe the effect of each nanoparticle on bacterial growth. Obtained results reveal that, the highest concentration of RGO has antibacterial effect on S. aureus and E. coli and RGO/ LiFe5O8 = 0.20 has inhibitory effect on E. coli and LiFe5O8 and RGO/ LiFe5O8 = 0.15 and 0.20 do not have any antibacterial effect on S. aureus. It is understood that, by increasing RGO concentration, the inhibition zone of nanoparticles enhances. RGO affects the bacterial membrane via direct contact and oxidative stress [26]. In addition, the oxidative stress of RGO is higher than GO. The density of functional groups of grapheme-based materials is high; therefore, there is a possibility for interacting with bacteria. Results show that, RGO/ LiFe5O8 = 0.25 nanocomposites have greater antibacterial effect on S. aureus and E. coli compared to LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites with lower RGO ratios. This issue is because of the larger surface area of graphene and better distribution of LiFe5O8 nanoparticles which culminates to better interaction between LiFe5O8 nanoparticles and bacteria. Also, the synthesized samples have greater antibacterial effect on E. coli than S. aureus. The antibacterial actions of nanoparticles for Gram positive and Gram negative bacteria are different. The main difference is membrane structure i.e. the thickness of peptidoglycan layer which is an important part of pathogenic bacteria. This thickness is 50% higher in Gram positive than Gram negative bacteria [49,50]. To examine the antibacterial effect of LiFe5O8 nanoparticles and RGO/LiFe5O8 nanocomposites with different RGO ratios on E. coli and S. aureu another antibacterial test was carried out. Fig. 12 shows the colony counting test results. In agreement with previous finding RGO/ LiFe5O8 = 0.25 has inhibitory effect on E. coli and S. aureus. It was found that the inhibition by RGO/LiFe5O8 was more significant for E. coli than S. aureus, which was attributed to the difference in their cell

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