Fullerene-modified magnetic silver phosphate (Ag3PO4/Fe3O4/C60) nanocomposites: hydrothermal synthesis, characterization and study of photocatalytic, catalytic and antibacterial activities

In this work, fullerene-modified magnetic silver phosphate (Ag3PO4/Fe3O4/C60) nanocomposites with efficient visible light photocatalytic and catalytic activity were fabricated by a simple hydrothermal approach. The composition and structure of the obtained new magnetically recyclable ternary nanocomposites were completely characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, Brunauer–Emmett–Teller (BET) specific surface area analysis, vibrating sample magnetometery (VSM), diffuse reflectance spectroscopy (DRS), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy and transmission electron microscopy (TEM). This novel magnetically recyclable heterogeneous fullerene-modified catalyst was tested for the H2O2-assisted photocatalytic degradation of MB dye under visible light. The results show that about 95% of the MB (25 mg L−1, 50 ml) was degraded by the Ag3PO4/Fe3O4/C60 nanocomposite within 5 h under visible light irradiation. The catalytic performance of the Ag3PO4/Fe3O4/C60 nanocomposite was then examined for 4-nitrophenol (4-NP) reduction using NaBH4. This new nanocomposite showed that 4-NP was reduced to 4-aminophenol (4-AP) in 98% yield with an aqueous solution of NaBH4. In both photocatalytic and catalytic reactions, the Ag3PO4/Fe3O4/C60 nanocomposite exhibited higher catalytic activity than pure Ag3PO4. Moreover, the Ag3PO4/Fe3O4/C60 nanocomposite could be magnetically separated from the reaction mixture and reused without any change in structure. The antibacterial activity of the nanocomposites was also investigated and they showed good antibacterial activity against a few human pathogenic bacteria.


Introduction
Today, water pollution is one of the main problems that human beings encounter. Every day, human activities lead to the release of contaminant substances and waste into the rivers, lakes, groundwater aquifers and oceans. This contamination affects the aquatic environmental quality for various uses and human consumption. 1 Water pollutants, including organic material such as methyl orange, methylene blue and rhodamine B dyes, are hazardous, toxic and carcinogenic for humans even at low concentrations, and they are hardly biodegradable and difficult to remove from the environment. 2 The ingestion of liquid products containing concentrated nitrophenol can cause serious gastrointestinal damage and even death. In animals, longer-term exposure to high levels of nitrophenol cause damages to the heart, kidneys, liver and lungs. It is therefore very important to nd innovative and cost-effective ways for the complete removal of organic pollutants and for monitoring water safety. 3,4 One way is the production of catalysts to eliminate water contamination, either in the dark or in visible light. Certainly, due to their high efficiency and promising economy, semiconductor-based catalytic and photocatalytic technologies have opened up new opportunities to control pollutants and deal with their effects. [5][6][7][8][9][10] To date, various metal oxides, suldes, carbon compounds and composite materials have been investigated for the development of effective photocatalysts. [11][12][13][14][15] In recent years, considerable attention has been paid to silver orthophosphate (Ag 3 PO 4 ), a new photocatalyst with an extremely high photooxidative capability for O 2 generation from water splitting. This is due to its highly positive VB position, low toxicity and superior photodegradation rate of organic dyes, which is dozens of times faster than the surface level of commercial TiO 2 under visible light irradiation. [16][17][18][19][20][21][22][23][24] Unfortunately, because of its low structural stability, it is possible for Ag 3 PO 4 to be photochemically decomposed to Ag if no sacricial reagent is involved. 25 Increasing both the stability and the catalytic activity of pure Ag 3 PO 4 by coupling Ag 3 PO 4 with other 2. Experimental

Synthesis of the Ag 3 PO 4 /Fe 3 O 4 /C 60 (m-APO/C 60 ) nanocomposites
The Fe 3 O 4 nanoparticles were prepared through a hydrothermal process. 1 mmol Fe 2+ and 2 mmol Fe 3+ were dissolved in 30 ml deionized water and an appropriate amount of NaOH was added, and the pH was set to be 11 at 50 C for 10 min with continuous stirring, yielding a uniform black suspension. It was then transferred to an autoclave (50 ml) at 180 C for 20 h. Subsequently, the autoclave was cooled to room temperature naturally. The as-obtained black samples were centrifuged, washed with deionized water and ethanol three times, and dried at 70 C for 3 h. To prepare the Ag 3 PO 4 / Fe 3 O 4 /C 60 (wt 5%) nanocomposite, a mixture of 0.2 g of the Fe 3 O 4 nanoparticles dispersed in 5 ml deionization water, 1 mmol of Na 2 HPO 4 $12H 2 O, 3 mmol of AgNO 3 and 10 ml of C 60 toluene solution (1 g L À1 ) were stirred for 30 min. Aer sonication for 30 min, the homogenized suspension was transferred into a 50 ml Teon-lined stainless steel autoclave, sealed and maintained at 180 C for 20 h. The autoclave was then naturally cooled to room temperature and the resulting precipitate was separated by a magnet, washed with deionized water several times, dried at 60 C and used for further characterization. It was then transferred to a 50 ml autoclave and heated at 180 C for 20 h. Subsequently, the autoclave was cooled to room temperature naturally. The product was collected by applying an external magnetic eld, washed several times with absolute ethanol and distilled water, and nally dried at 70 C for 3 h. The samples with 10 and 20 wt% of C 60 were prepared in a similar manner. For comparison, the pure Ag 3 PO 4 nanostructure was synthesized according to the typical synthesis described above with Fe 3 O 4 and C 60 being absent. The obtained samples with 5, 10 and 20 wt% of C 60 and pure Ag 3 PO 4 are denoted as m-APO/C 60 (5), m-APO/C 60 (10), m-APO/C 60 (20) and APO, respectively.

Photocatalytic dye degradation tests
Photocatalytic degradation of the aqueous solution of methylene blue (MB) was carried out in the presence of the m-APO/C 60 photocatalyst using a 400 W high pressure mercury lamp as an irradiation source, with a cool water circulating lter to absorb the near IR and a UV light cut-off lter to avoid direct photolysis of the organic dyes (l $ 420 nm). In a typical experiment, 0.05 g of the m-APO/C 60 photocatalyst was added to 50 ml of MB (25 mg L À1 ) to perform the photocatalytic degradation. Before irradiation, the solution was stirred for 30 min to achieve an adsorption-desorption equilibrium of the dye on the photocatalyst surface. It was then subjected to visible light irradiation in the presence of H 2 O 2 . At given time intervals, 2 ml aliquots of the reaction solution were sampled, and the catalyst was immediately separated from the suspension by an external magnetic eld. The residual MB concentration was determined using a UV-Vis spectrophotometer.

Catalytic reduction tests
In order to explore the catalytic performance of the synthesized m-APO/C 60 nanocomposites, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride (NaBH 4 ) in aqueous solution was used as the model reaction. In a typical catalytic reaction, 3 ml of an aqueous solution of 4-NP (0.2 mM) and 0.7 ml of an aqueous solution of NaBH 4 (20 mM) were mixed in a standard quartz cell with a 1 cm path length, then 2.5 mg of the synthesized   nanocomposite was added to the reaction mixture. Immediately aerwards, the catalysts were transferred to a standard quartz cell while the nitrophenol concentration in the reaction mixture was monitored by UV-visible absorption spectra, recorded with a time interval of 2 min in a scanning range of 200-800 nm at ambient temperature. Aer the completion of the reaction, in order to perform the recycling experiment, the catalyst was recovered rst by an external magnet and then by centrifugation. The precipitate was washed repeatedly with deionized water and absolute ethanol in consecutive washing cycles. Aer washing and placing in a furnace in order to remove adsorbed impurities, the catalyst was used directly for the recycling test. Aer each cycle, the resulting catalyst was collected and detected by atomic absorption spectroscopy to determine the content of the synthesized nanocomposites.

Antibacterial tests
The antibacterial activity of the synthesized nanoparticles was evaluated against strains of Gram-positive bacteria (Bacillus cereus (PTCC 1015) and Staphylococcus aureus (1431)) and Gramnegative bacteria (Escherichia coli (PTCC 1330) and Klebsiella pneumoniae (PTCC 1290)) using a modied Kirby-Bauer disk diffusion method. 53 Bacteria were cultured for 18 h at 37 C in a nutrient agar medium and then adjusted with sterile saline to a concentration of 1 Â 10 6 cfu ml À1 . Bacterial suspensions in Petri dishes (8 cm) containing sterile Mueller-Hinton agar (MA) were cultured using a sterile cotton swab. The compounds were dissolved in water and sterile paper discs of 6 mm thickness were saturated with 30 ml of the samples and placed onto agar plates which had previously been immunized with the tested microorganisms. Amikacin (30 mg per disk) for Gram-negative and penicillin (10 mg per disk) for Gram-positive were used as positive This journal is © The Royal Society of Chemistry 2018 controls. Aer incubation at 37 C for 24 h, the inhibition zone diameter was measured using a meter ruler and the mean value for each organism was recorded and expressed in millimeters.

Materials characterization
FT-IR spectra were recorded on a Shimadzu FT-IR 8400S spectrophotometer in transmission mode from 4000 to 400 cm À1 using KBr pellets. The XRD patterns of the samples were obtained on an X-ray diffractometer (Rigaku D/Max C III) using Ni-ltered Cu Ka radiation (l ¼ 1.5406Å). UV-Vis diffuse reection spectroscopy (DRS) was performed on a Snico S4100 spectrophotometer over the spectral range of 200-1000 nm using BaSO 4 as the reference. The shapes and morphologies of the samples were observed by a MIRA3 TESCAN eld emission scanning electron microscope (FESEM) equipped with a link energy-dispersive X-ray (EDX) analyzer. The particle size was determined by a CM120 transmission electron microscope (TEM) at an accelerating voltage of 80 kV. TEM samples were prepared by dropping the ethanol dispersion onto a carbon coated copper grid. A PHS-1020 PHSCHINA instrument was used to measure the Brunauer-Emmett-Teller (BET) surface areas of the samples at liquid nitrogen temperature (77 K). Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Magnetic Daneshpajoh Kashan Co., Iran) with a maximum magnetic eld of 10 kOe. Raman spectroscopy was performed using a SEN-TERRA (2009) dispersive Raman microscope from BRUKER (Germany) with a laser wavenumber of 785 nm. UV-Vis spectra of the aqueous solutions during the reaction were recorded using a Cary 100 double beam spectrophotometer operated at a resolution of 2 nm using quartz cells with path length of 1 cm.  Ag 3 PO 4 , Ag 3 PO 4 /Fe 3 O 4 /C 60 (wt 5%), Ag 3 PO 4 /Fe 3 O 4 /C 60 (wt 10%) and Ag 3 PO 4 /Fe 3 O 4 /C 60 (wt 20%) are abbreviated as APO, m-APO/ C 60 (5), m-APO/C 60 (10) and m-APO/C 60 (20), respectively. In the as-prepared m-APO/C 60 samples, no other impurities could be observed. It was found that the as-prepared catalysts exhibited intense and sharp diffraction peaks ascribed to cubic crystal systems for Ag 3 PO 4 (JCPDS card no. 84-0511) and Fe 3 O 4 (JCPDS card no. 75-0449). No diffraction peaks corresponding to C 60 were observed in the m-APO/C 60 nanocomposites, which may be due to the relatively low diffraction intensity of C 60 and the high dispersion of the small amount of C 60 in the sample. The average grain sizes of the as-prepared m-APO/C 60 (5) To conrm the strong combination between the C 60 and APO/Fe 3 O 4 nanoparticles, we also investigated whether C 60 can leach from the m-APO/C 60 hybrid system in toluene solution. The color of the toluene solution turned purple aer adding C 60 and sonicating at room temperature for 15 min. However, no color change was observed when using the asobtained m-APO/C 60 composites under the same conditions. These results clearly show that the loaded C 60 clusters in the hybrid system could not be extracted by an excellent solvent (toluene), suggesting the formation of a strong interaction between the C 60 clusters and APO/Fe 3 O 4 . Raman spectroscopy is a powerful nondestructive tool to characterize the signicant structural changes in the carbon nanostructures during nanocomposite synthesis. Fig. 3 shows the Raman spectrum of the m-APO/C 60 (5) Fig. 4(a), the SEM image of Ag 3 PO 4 consists of near spherical particles with diameters in the range of 50-100 nm, which are slightly agglomerated. Fig. 4(b) shows the characteristic morphology of C 60 and Fig. 4(c) shows that pure Fe 3 O 4 consists of small, ne nanoparticles with sizes of 20-30 nm. From Fig. 4(d), it can be seen that the m-APO/C 60 (5) nanocomposite still retains the spherical morphology of Ag 3 PO 4 , and the distribution of C 60 and Fe 3 O 4 nanoparticles in the Ag 3 PO 4 matrix is homogeneous. Compared to Ag 3 PO 4 , m-APO/C 60 is more scattered. Fig. 4(e) and (f) are SEM images of the m-APO/C 60 nanocomposites with 10 and 20 wt% of C 60 . The particle size of the m-APO/C 60 nanocomposites decreased slightly aer modifying with C 60 . This may be due to the presence of C 60 between the m-Ag 3 PO 4 /Fe 3 O 4 nanoparticles. SEM images of the m-APO/C 60 nanocomposites with different amounts of C 60 indicated that the presence of C 60 had a large effect on the size of the APO nanoparticles, and as the size of particles became smaller, the amount of C 60 increased.

Results and discussion
The composition of the as-prepared m-APO/C 60 (5) composites was further investigated by EDX analysis. Fig. 5 shows the EDX spectrum and a representative SEM image of m-APO/C 60 (5) with the corresponding EDX elemental mappings. The presence of Ag, C, Fe, O and P elements can be conrmed by their peaks in the EDX elemental spectrum. The corresponding elemental mappings show that the Ag, C, Fe, O, and P elements are uniformly distributed over the nanocomposite, conrming the homogeneity of the sample. The Fe and C elements were from Fe 3 O 4 and C 60 , and the results further indicate that the Fe 3 O 4 and C 60 particles were successfully coupled with APO.
The shape and particle size of the as-synthesized APO, C 60 , Fe 3 O 4 and m-APO/C 60 (5) samples were further investigated by TEM, and the results are shown in Fig. 6. The TEM images of the Ag 3 PO 4 sample in Fig. 6(a) and (b) show nearly uniform   (20) 0.58 10. 59 50 monodispersed spheres with an average diameter of about 50-150 nm. The SEM image in Fig. 6(c) shows that the bare C 60 sample was formed from plate-like particles which were loosely aggregated. From the TEM image in Fig. 6(d), it was found that pure Fe 3 O 4 was formed of very ne and agglomerated spherelike nanoparticles with an average diameter of about 20 nm. The accumulation of these nanostructures can be attributed to the magnetic dipole interaction between them and the great surface energy due to their nanoscopic size. TEM images of the m-APO/C 60 (5) nanocomposite are shown in Fig. 6(e) and (f). By comparing with the SEM images, it is evident that the shape and morphology of the m-APO/C 60 (5) particles are similar to those of the pure components. The morphology of the prepared nanocomposite was sphere-like (APO/C 60 (5)) and plate-like (C 60 ), and the particle size distribution was narrow (from $20 nm to $40 nm) with an average particle size of 30 nm. From the images, it can be clearly seen that many spherical APO and Fe 3 O 4 particles with sizes of about 15-20 nm were well-coupled with the C 60 nanoplates. It is reasonable to assume that C 60 clusters were successfully incorporated into the m-APO/C 60 samples while retaining their special structures. The magnetic behavior of the pure Fe 3 O 4 and m-APO/C 60 nanocomposites was investigated by VSM at room temperature. The magnetization curves of the prepared nanocomposites of the m-APO/C 60 (5), m-APO/C 60 (10) and m-APO/C 60 (20) nanocomposites and pure Fe 3 O 4 are shown in Fig. 7. Increasing the applied eld from À10 000 to 10 000 Oe caused the magnetization to undergo a sharp increase. At this point, the magnetization was saturated at about 8500 Oe for the prepared nanocomposite. By comparing the magnetic remnant (M r ) and  (20). The insets show the corresponding pore size distribution curves.  Fig. 7). The results conrmed the high magnetization as well as the extremely high reusability of the m-APO/C 60 nanocomposites, implying their high potential and promising applications for water purication to avoid secondary pollution. Nitrogen adsorption-desorption experiments were used to evaluate the pore size and structure of the samples. The porosity and specic surface area were determined using the Barrett- Joyner-Halenda (BJH) method and the BET equation, respectively. 57,58 The calculated BET specic surface area of the m-APO/ C 60 (5) nanocomposite was 55.461 m 2 g À1 (see Fig. 8). The pore size and pore volume distributions of the m-APO/C 60 (5) nanocomposites were centered at 1.26 nm and 0.212 cm 3 g À1 , respectively. The isotherm in Fig. 8(a)-(c) can be classied as type IV with a H 4 hysteresis loop for the m-APO/C 60 (5), m-APO/ C 60 (10) and m-APO/C 60 (20) nanocomposites. The BET surface area values for the m-APO/C 60 (5), m-APO/C 60 (10) and m-APO/ C 60 (20) nanocomposites were higher than those for pure APO and m-APO (Table 2). 59 It can be concluded that the addition of Fe 3 O 4 and C 60 to the APO signicantly affected the microstructure of APO and greatly increased the surface area and pore volume, all of which were considered to be favorable factors for the improvement of the photocatalytic performance. In comparison to pure APO, the microporous structure and relatively high surface area of the prepared m-APO/C 60 (5) nanocomposites were expected to have higher photocatalytic activity. Fig. 9 shows the UV-Vis diffuse reectance absorption spectra (DRS) of single APO and m-APO/C 60 nanocomposites with different C 60 mass ratios. It can be seen in Fig. 9(a) that APO and the m-APO/C 60 nanocomposites show strong absorption bands above 300 nm, which are assigned to the intrinsic band gap absorption of APO. Compared to APO, the m-APO/C 60 nanocomposites show more intensive absorption over the whole visible light region, consistent with the gray color of the samples, relating to the presence of C 60 molecules. The band gap energy (E g ) of the samples can be obtained from the following formula: (ahn) 1/2 ¼ B(hn À E g ), where a, n and B are the absorption coefficient, light frequency and proportionality constant, respectively. As shown in Fig. 9(b), the value of hn extrapolated to a ¼ 0 gives the band gap energy. Based on UV-DRS spectroscopy studies, the band gaps were estimated to be 2.75 eV, 2.60 eV, 2.72 eV and 2.70 eV for pure APO, m-APO/ C 60 (5), m-APO/C 60 (10) and m-APO/C 60 (20), respectively. Among them, the m-APO/C 60 (5) nanocomposite had lower band gaps.

Photocatalytic activity
The photocatalytic degradation of organic contaminants has attracted considerable attention because of its potential to solve serious environmental difficulties such as aquatic pollution and printing/textile wastewater, and the associated toxicity and perturbation to aquatic life. Various photocatalysts have been successfully developed for environmental remediation. 60 Among them, TiO 2 is one of the best, due to its non-toxicity, suitable stability and high photocatalytic activity. 61 However, TiO 2 is responsive only to UV light, which accounts for no more than 4% of the solar spectrum, greatly limiting its photocatalytic efficiency and suitable applications. Developing a novel photocatalyst with efficient visible light absorption and excellent stability remains a great challenge. The m-APO/C 60 nanocomposites prepared in this study can be an appropriate candidate. The photocatalytic activity of the m-APO/C 60 nanocomposites was evaluated by the degradation of methylene blue (MB) dye in aqueous solution under visible light irradiation and at room temperature. The UV-Vis spectral changes of the MB aqueous solution over the m-APO/C 60 (5) photocatalyst is plotted in Fig. 10 as a function of irradiation time. The degradation percentage was calculated using the equation [(C 0 À C)/C 0 ] Â 100%, where C is the concentration of the reactant aer irradiation at time t and C 0 is the concentration of the MB dye aer adsorption-desorption equilibrium. Fig. 10 shows that the intensity of the maximum absorption peak of MB at 663 nm decreases intensely as time increases and approximately disappears within 300 min. For comparison purposes, we additionally performed the experiments on the degradation of MB with m-APO/C 60 (5), m-APO/C 60 (10), m-APO/C 60 (20) and pure APO under identical experimental conditions. The degradation efficiencies (%) of these photocatalysts were found to be 95%, 89%, 80% and 33%, respectively, within 300 min under visible light irradiation. To determine the photocatalytic degradation kinetics of MB degradation, the pseudo rst order model was used: ln(C 0 /C) ¼ kt, where C 0 and C are the dye concentrations before and aer visible light irradiation, respectively, k is the pseudo rst order rate constant, and t is the reaction time. As shown in Fig. 11, the k values for the degradation of MB over the m-APO/C 60 (5), m-APO/C 60 (10), m-APO/C 60 (20) and pure APO catalysts were determined to be 1.03 Â 10 À2 , 7.70 Â 10 À3 , 5.30 Â 10 À3 and 1.20 Â 10 À3 min À1 , respectively. The results indicate that the photocatalytic activity of APO could be improved by the incorporation of Fe 3 O 4 nanoparticles and C 60 molecules. From the photocatalytic activity results, it can be further conrmed that the low optimal molar ratio of m-Ag 3 PO 4 /C 60 was good for increasing surface active sites and photocatalytic performance.
A possible mechanism for the photocatalytic degradation of MB dye is proposed as follows. The generated light with an appropriate wavelength could excite APO to produce photogenerated electrons and holes. Then, the photogenerated electrons are transferred to the surface of the C 60 particles, which act as effective electron acceptors. 62 C 60 is a conjugated structure in which the charge carriers act as massless fermions, leading to unique charge transfer properties. Therefore, the photogenerated electrons of APO could transfer easily from the CB band to the C 60 particles. As a result of the extreme inhibition of the photogenerated electrons and holes, the photocatalytic activity is enhanced. The electrons on the surface of C 60 can react with the dissolved H 2 O 2 to produce hydroxyl radicals, while the holes are scavenged by the adsorbed water or OH À to form cOH radicals. Finally, these active species could oxidize the MB molecules adsorbed on the active sites of m-APO/C 60 through electrostatic attraction or p-p stacking, resulting in dye degradation and production of CO 2 , H 2 O, etc., as acquitted materials (eqn (1)-(4)). 63 According to this, a schematic representation of the proposed mechanism is illustrated in Fig. 12.

Catalytic activity
It is necessary to develop environmentally friendly and clean techniques to remove pollutants such as nitrophenols and their derivatives from industrial wastewater. To evaluate the catalytic activity of the prepared nanocomposites in this research, the reduction of nitrophenols (4-NP) by excess NaBH 4 was used as the model pollutant in aqueous solution. The catalytic process was monitored by UV-Vis spectroscopy. This showed the concentration changes (C/C 0 ) and efficiencies of 4-NP reduction in the presence of different samples. Among these, the samples containing the prepared nanocomposite catalysts exhibited the best catalytic performance (Fig. 13).
In this reduction process, the overall concentration of NaBH 4 was 20 mM while the overall concentration of 4-NP was 0.2 mM. Considering the higher concentration of NaBH 4 compared to   that of 4-NP, it is logical to assume that the concentration of BH 4 À remains constant during the reaction. Thus, pseudo rst order kinetics could be used to evaluate the kinetics of the catalytic reaction. The absorbance of 4-NP is proportional to its concentration in solution, and the absorbance at time t (A) and time t ¼ 0 (A 0 ) are equivalent to the concentration at time t (C) and time t ¼ 0 (C 0 ). The rate constant (k) could be obtained from the linear plot of Àln(C/C 0 ) versus the reduction time in minutes. As shown in Fig. 14, the Àln(C/C 0 ) versus time plots indicate a good linear correlation. The results show that the catalytic activity of the m-APO-C 60 (5) nanocomposite is higher than those of pure APO 4 , modied m-APO/C 60 (10) and m-APO/ C 60 (20), m-APO, APO-C 60 (5) and APO/C 60 (10). Fig. 15 shows the UV-Vis spectral changes of a 4-NP aqueous solution over different catalyst samples. It can be seen in Fig. 15(b) that the absorption peak of 4-NP undergoes a red shi from 316 to 400 nm immediately aer the addition of an aqueous solution of NaBH 4 , corresponding to a signicant change in the solution color from light yellow to yellow-green due to the formation of the 4-nitrophenolate ion. In the absence of the m-APO/C 60 (5) nanocomposite catalyst (2.5 mg), the absorption peak at about 400 nm remained unchanged for a long time, indicating that the NaBH 4 itself could not reduce the 4-nitrophenolate ion without a catalyst. In the presence of the m-APO-C 60 (5) nanocomposite catalyst and NaBH 4 , 4-NP was reduced and the intensity of the absorption peak at about 400 nm decreased progressively as time passed, and aer almost 16 min disappeared completely. Meanwhile, a new absorption peak with increasing intensity appeared at about 295 nm. This peak was attributed to the typical absorption of 4amino phenol (4-AP). Fig. 16 shows the probable reduction mechanisms of 4-NP to 4-AP in the presence of m-APO/C 60 . According to the results, it was evident that as the m-APO-C 60 content (>20%) increased, the catalytic activity deteriorated, highlighting the important role of the loading percentage and intimate contact between C 60 and Ag 3 PO 4 /Fe 3 O 4 in determining catalytic efficiency. This means that with higher amounts of C 60 , the number of active catalytic reaction sites decreases, causing a negative inuence on the catalytic processes. The excessive amount of C 60 may cover the active sites at the Ag 3 PO 4 /Fe 3 O 4 surface and also hinder the contact with 4-NP. Furthermore, from the catalytic activity results, it can be concluded that the low optimal molar ratio of m-APO/C 60 is conducive to increasing the number of active surface sites and enhancing catalytic performance. The highest catalytic performance of the m-APO/C 60 composite can be attributed to the intimate contact between Ag 3 PO 4 /Fe 3 O 4 and C 60 , which facilitates electron transfer. A probable mechanism for the reduction of 4-NP to 4-AP in the presence of m-APO-C 60 is shown in Fig. 16.
Moreover, the presence of Fe 3 O 4 in the composites caused them to be magnetically separable during the catalytic reactions. Recyclability is a very important parameter to assess how practical and reusable the catalyst is. Therefore, the recovery and reusability of the m-APO-C 60 (5) catalyst was determined for the reduction of 4-NP under the present reaction conditions. Aer the reaction was completed, the m-APO/C 60 (5) nanocomposite was separated from the reaction mixture by an external magnet. The catalyst was washed with water and ethanol several times, then dried and reused for the next reaction. Three consecutive catalyst recoveries were performed showing a good catalytic activity (Fig. 17).
For up to three catalytic cycles, no signicant loss in activity was observed, indicating that the as-prepared catalyst was stable and efficient for the reduction of nitro compounds. As observed in Fig. 18, the FT-IR spectrum, SEM image and EDX map of the recycled catalyst do not show any signicant change aer the third run, in comparison with those of the fresh catalyst (see Fig. 2, 4 and 5). This observation conrms that the m-APO/C 60 (5) nanocomposite is stable under the reaction conditions and is not affected by the reactants.

Antibacterial activity
The antibacterial activity of the samples was analyzed against four bacteria, including Bacillus cereus, Staphylococcus aureus, Klebsiella pneumonia and Escherichia coli using the disk diffusion method. The results of the antibacterial activity tests for   Table 3. The results show that the m-APO/C 60 (5) nanocomposite has a relatively good antibacterial activity compared to pure APO, i.e. the bacteria cells were killed at a concentration of 10 mg ml À1 . The highest activity was obtained for the m-APO/C 60 (5) nanocomposite against B. cereus, while the lowest activity was observed for m-APO/C 60 (5) against E. coli. The biosynthesized m-APO/C 60 (5) nanocomposite exhibited a greater antimicrobial activity towards Gram-positive microorganisms than Gram-negative ones. The results showing the antibacterial activity of the m-APO/C 60 (5) nanocomposite with different concentrations are presented in Table 4, indicating that the m-APO/C 60 (5) nanocomposite has different antibacterial activity with different concentrations. It also represents the inhibition zone of these bacteria. The highest activity was obtained for the m-APO/C 60 (5) (1.25 mg ml À1 )  Table 4 Average size of the inhibition zones for pure APO and the m-APO/C 60 (5) nanocomposite with different concentrations nanocomposite against S. aureus, while the lowest activity was observed for the m-APO/C 60 (5) (0.625 mg ml À1 ) nanocomposite against K. pneumonia and B. cereus. The potential antimicrobial activities presented by the m-APO/C 60 (5) nanocomposite have made it a promising candidate for a novel generation of antimicrobials. The clear mechanism of the m-APO/C 60 (5) nanocomposite's interaction with bacteria is not well known. However, several main mechanisms underpin the biocidal properties of the m-APO/C 60 (5) nanocomposite against microorganisms. Firstly, the m-APO/C 60 (5) nanocomposite attaches to the negatively charged cell surface, altering the physical and chemical properties of the cell membrane and the cell wall, and disturbing important functions such as permeability, osmoregulation, electron transport and respiration. 64 Secondly, the m-APO/C 60 (5) nanocomposite can cause further damage to bacterial cells by permeating the cell and interacting with DNA, proteins and other phosphorus-and sulfur-containing cell constituents. 65 Thirdly, the m-APO/C 60 (5) nanocomposite releases silver ions, generating an amplied biocidal effect which is size-and dose-dependent. 66

Conclusions
In this study, m-APO/C 60 nanocomposites were synthesized using a facile and effective hydrothermal route. The synthesized m-APO/C 60 nanocomposites were spherical, 30 nm in size and crystalline in nature, and exhibited absorptions at $300-620 nm. The m-APO/C 60 (5) nanocomposite had a band gap of about 2.6 eV. Its photocatalytic activity was much higher than that of pure Ag 3 PO 4 or those of the other nanocomposites for degrading methylene blue. The results showed a 95% degradation of methylene blue (MB) (25 mg L À1 ) within 5 h in the presence of the Ag 3 PO 4 /Fe 3 O 4 /C 60 nanocomposite and H 2 O 2 . In addition, this new nanocomposite showed an 98% reduction of 4-nitrophenol (4-NP) (0.2 mM) with excess NaBH 4 . The formed m-APO/ C 60 nanocomposites were quite stable, showed good antibacterial activity and were utilized as catalysts for the reduction of several aromatic nitro compounds (98% reduction of 4-NP) into their corresponding amino derivatives. It is expected that this kind of m-APO/C 60 nanocomposite would provide new insights for the design and construction of high performance photocatalysts for eliminating environmental pollution damage. This nanocomposite can easily be removed using an external magnet and prevents the secondary pollution of water. The nanocomposites are therefore both economically and environmentally friendly, and they could be a good option for eliminating water contaminants.

Conflicts of interest
There are no conicts of interest to declare.