Nitrogen Doped Graphene Nickel Ferrite Magnetic Photocatalyst for the Visible Light Degradation of Methylene Blue

A facile approach has been devised for the preparation of magnetic NiFe2O4 photocatalyst (NiFe2O4-NG) supported on nitrogen doped graphene (NG). The NiFe2O4-NG composite was synthesized by one step hydrothermal method. The nanocomposite catalyst was characterized by Powder X-ray diffraction (PXRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), Ultraviolet–visible spectroscopy (UV-Vis) and Vibrating sample magnetometry (VSM). It is found that the combination of NiFe2O4 nanoparticles with nitrogen-doped graphene sheets converts NiFe2O4 into a good catalyst for methylene blue (MB) dye degradation by irradiation of visible light. The catalytic activity under visible light irradiation is assigned to extensive movement of photogenerated electron from NiFe2O4 to the conduction band of the reduced NG, effectively blocking direct recombination of electrons and holes. The NiFe2O4 nanoparticles alone have efficient magnetic property, so can be used for magnetic separation in the solution without additional magnetic support.


Introduction
Photocatalysis especially by TiO 2 has been widely used for the purification of waste water.The energy band gap of 3.2 eV is required for the excitation of electron by light in TiO 2 catalyst so UV light can only be used in the process of photodegradation.8][19][20][21][22] The heterogeneous systems are mostly used to perform the photodegradation reactions.The repeated use of photocatalysts after degradation is of great importance for sustainable use of the catalyst.The magnetic nanoparticles anchored on solid sup-port serve as heterogenous catalyst allowing facile separation of catalyst from reaction products. 23Superparamagnetic copper ferrite-graphene nanocomposite prepared via hydrothermal method acts as excellent catalyst for the reduction of nitroarenes.The big advantage of the catalyst is that it can be easily recovered and retains the catalytic activity even after five catalytic cycles. 24Copper-cobalt ferrites prepared by hydrothermal method from co-precipated precursor serve as efficient catalyst in the decomposition of methanol to CO and H 2 . 257][28][29] Nickel ferrite (Ni-Fe 2 O 4 ) has the inverse spinel structure.The ferrimagnetism arises due to antiparallel spin of Fe 3+ ions present at tetrahedral sites and Ni 2+ occupying octahedral sites. 30he Nickel ferrite is considered as the efficient magnetic material which has good electrical resistivity, high-Curie temperature and chemical stability.Magnetic nanoparticles of nickel ferrite have been used to manufacture titaniacoated nickel ferrite, which can act as magnetically separable photocatalyst. 31The TiO 2 doped NiFe 2 O 4 nanopar-ticles possess band gap of 2.19 eV and have displayed enhanced photocatalytic activity as compared to TiO 2 for degradation of Rhodamine B dye in aqueous solution under visible light irradiation. 32Pure nickel ferrite is photo-catalytically inactive but its composite with another semiconductor (e.g., graphene sheets) can find an effective mechanism for separation of charges leading to increased photocatalytic performance.One such example is Zn-Fe 2 O 4 -graphene photocatalyst and its great performance in the photocatalytic degradation of MB under visible light irradiation. 335][36][37] Nitrogen-doped graphene (NG) has great utility because of its higher specific capacitance matched to the pristine graphene and good durability, therefore, enabling its use as electrode materials for supercapacitors and applications in photocatalysis. 38n this paper, we report the development of one step method to design magnetically separable nitrogen doped graphene-based photocatalyst having excellent catalytic activity.The approach is designed to deposit NiFe 2 O 4 nanocrystals on nitrogen doped graphene sheets via a onestep hydrothermal method.Interestingly, in the presence of nitrogen doped graphene, the inert nanocrystals of Ni-Fe 2 O 4 have been converted into a highly efficient catalyst for the methylene blue (MB) degradation under visible light irradiation.In addition, NiFe 2 O 4 nanoparticles themselves have a magnetic property, which makes the Ni-Fe 2 O 4 -NG composite magnetically separable in liquid medium.

1. Materials
Iron(III) nitrate nonahydrate Fe(NO 3 ) 3 • 9H 2 O, Nickel(II) nitrate hexahydrate Ni(NO 3 ) 2 • 6H 2 O, graphite powder flakes, phosphoric acid and hydrogen peroxide were purchased from Alfa Aesar.All chemicals were used as received without further purification.Ethanol, urea, sodium hydroxide and sulphuric acid were purchased from Sigma Aldrich.Deionized water was used throughout.

.2. Synthesis of Magnetic NiFe O 4 -Nitrogen Doped Graphene Composite Photocatalyst
Purified natural graphite was used for the synthesis of graphene oxide (GO) by the well known method given by Hummers and Offeman. 39The graphene oxide (GO) (0.08 g) was dispersed in 20 ml of absolute ethanol and sonicated for 45 min.In a separate beaker 0.28 g of Ni(NO 3 ) 2 • 6H 2 O and 0.78g of Fe(NO 3 ) 3 • 9H 2 O mixture was added to 10 ml absolute ethanol with constant stirring for 30 min forming homogenous solution.The two solutions were mixed and pH of the mixture solution was kept 10.0 using 6 M NaOH solution and then 1 g urea was added into it.The resulting mixture was put into a 50 mL Teflon-lined stainless steel autoclave and heated to 180 °C for 18 h in an oven.After cooling the reaction mixture to room temperature and the precipitates were filtered, washed with distilled water and dried in oven at 70 °C for 12 h.The product was named as NiFe 2 O 4 -NG.Same method was applied to synthesize pure NiFe 2 O 4 with the modification that GO and urea were excluded.Sulfur was estimated as BaSO 4 by gravimetric method and Chloride was estimated as AgCl by Volhard's method. 40

3. Spectroscopic and Microscopic Measurements
The phase and size of the as-prepared samples were determined from powder X-ray diffraction (PXRD) using D8 X-ray diffractometer (Bruker) at a scanning rate of 12°m in -1 in the 2θ range from 10° to 70°, with Cu Kα radiation (λ = 0.15405 nm).Scanning electron microscopy (SEM) micrographs of the samples were recorded on FEI Nova Nano SEM 450.High Resolution Transmission Electron Microscopy (HRTEM) was recorded on Tecnai G2 20 S-TWIN Transmission Electron Microscope with a field emission gun operating at 200 kV.The samples for TEM measurements were prepared by evaporating a drop of the colloid onto a carbon coated copper grid.The infrared spectra were recorded on Shimadzu Fourier Transform Infrared Spectrometer (FT-IR) over the range of wave number 4000-400 cm -1 and the standard KBr pellet technique was employed.The magnetic moment as a function of applied field was recorded using Vibrating Sample Magnetometer (VSM), Lakeshore 7410.All the measurements were performed at room temperature.

4. Photocatalytic Activity Measurement
The catalytic activity of the as synthesized sample was performed by degradation of organic dye MB under the irradiation of visible light.For the Photo irradiation 500 W xenon lamp was used fitted with UV cut-off filters (JB450) in order to completely remove any radiation below 420 nm ensuring the exposure to only visible light.The whole procedure was performed at 25 °C.A 100 mL of MB dye solution was prepared (20 mg/L concentration) and 0.025 g of photocatalyst was mixed with dye solution.The resulting mixture was stirred for 60 min before illumination in order to establish the adsorption -desorption equilibrium between MB and catalyst surface.At same instant of time 5 mL of dye-catalyst mixture was taken out and concentration of the residual dye was determined with the help of UV-vis spectroscopy by measuring the absorption at 664 nm.The absorbance of dye at 664 nm was monitored with time after fixed intervals of time.The absor-Singh et al.: Nitrogen Doped Graphene Nickel Ferrite Magnetic ... bance of dye with time without catalyst was also recorded for reference.

1. PXRD Measurements
The structural characterization of the nanoparticles has been carried out by Powder X-ray diffraction technique using CuKα radiation.Figure 1(a-b) show the differences of phase composition between GO and NG.The doping of nitrogen in GO can be clarified easily by PXRD spectrum.The PXRD pattern of GO exhibits a characteristic (002) peak of graphite emerging at 24.2°.Compared with GO, it is found that the (002) peak of NG appears at 26.3º which indicates that nitrogen atoms have entered into the crystal lattice of graphite and caused the increased distance between the graphite layers.This confirms the formation of nitrogen-doped graphene by urea assisted hydrothermal reaction.Figures 1c, d  The average crystallite size of these nanoparticles was calculated according to the Scherrer's equation.(1)   where, L (nm) is the crystallite size, λ (nm) is the wavelength of the Cu Kα radiant, λ = 0.15405 nm, β(°) is the full-width at half-maximum (FWHM) of the diffraction peak, θ is the diffraction angle and K is the Scherrer constant equal to 0.89.All the major peaks were used to calculate the average crystallite size of the NiFe 2 O 4 and Ni-Fe 2 O 4 -NG nanoparticles.The estimated average crystallite sizes of nanoparticles are in the range of 80-120 nm.

2. SEM and TEM Analysis
Figure 2a shows representative scanning electron microscopy and transmission electron microscopy images of the prepared GO.From the SEM image, morphology and structure of as-prepared graphene oxide sample was investigated.GO sheets were cast on a gold coated (100 nm) Si/SiO 2 substrate.It has been found that the graphene flakes have wrinkled surfaces.Furthermore, in the TEM image (Figure 3a) GO shows layer-by-layer stacked structure and has wrinkled paper like morphology.Such morphological changes can be attributed to the increased formation of phenolic and epoxy functional groups on the basal plane of GO.The curled and overlapped nanosheets can be clearly observed.The SEM image (Figure 2b) and TEM image (Figure 3b) reveal that nitrogen-doped grapene nanosheets exhibit a typical wrinkled structure, which results from stable bending thermodynamically. 42,43igures 2(c-d) show SEM images of the NiFe 2 O 4 and NiFe 2 O 4 -NG samples where as Figures 3(c-d

FT-IR Characterization
Figure 4(a-d) shows the FTIR spectra of GO, NG, NiFe 2 O 4 and NiFe 2 O 4 -NG.There are many O-containing groups that exist on GO sheets, such as hydroxyl, epoxy, and carboxyl groups.Majority of the O-containing groups will disappear after reduction.FTIR bands at 1050, 1220, 1405 and 1730 cm -1 were observed for GO.These bands correspond to C-O stretching, C-O-C stretching, O-H deformation vibration and C=O carbonyl stretching. 44FTIR bands at 1400 cm -1 due to C=C stretching is observed in NG and the νC=O band at 1730 cm -1 completely disappeared due to reduction.The bands located at 1180 and 1565 cm -1 in Figure 4b are assigned to the ν C-N and ν C=C respectively.The FTIR spectra suggest N doping of GO. Figure 4 (c-d

4. Photocatalytic Measurements
The adsorption of light by the photocatalysts is the key feature of photocatalysis method.Figure 5a show the  the pure MB solution.The catalyst acts as magnetic material which gives good performance in magnetic separation for the NiFe 2 O 4 -NG photocatalysts using an external magnet.

4. 1. Mechanism of Photocatalytic Activity Measurements
The photocatalytic activity for MB degradation can be best explained by the following mechanism.The notable increase in photocatalytic activity under visible light exposure can be attributed to exceptional synergistic effect between NiFe   bed water can react with holes to produce hydroxyl radical (Eq.5).At the end superoxide anion, and hydroxyl radical cause the oxidation of MB dye adsorbed on the surface of NiFe 2 O 4 -NG composite by electrostatic interaction and ππ interaction between aromatic rings of methylene blue and graphene layer (Eq.6).In the photocatalytic degradation process, the electrons of the photocatalyst i,e Ni-Fe 2 O 4 -NG nanocomposite are excited from the valence band (VB) to the conduction band (CB) by the visible light irradiation.The photogenerated holes in the VB are scavenged by OH -of water forming .OH radicals which are responsible for the MB degradation process afterwards.
The N-graphene performs two functions; (a) it acts as charge carrier to trap the delocalised electrons thereby restricting the (h-e) recombination.(b) Secondly, it increases the adsorption of MB dye on the catalyst surface thereby increasing the π-π interaction between aromatic rings of methylene blue and graphene layer. 46

5. Magnetic Characterization
Magnetization hysteresis loops of the as-prepared NiFe 2 O 4 and NiFe 2 O 4 -NG samples at room temperature were measured using vibrating sample magnetometer as shown in Figure 6(a-b).The magnetic properties of the NiFe 2 O 4 having inverse spinel structure can be described in terms of cations distribution.The magnetization originates from the Fe 3+ ions at both tetrahedral and octahedral sites and Ni 2+ is present only in octahedral sites. 47,48

Conclusions
In the outcome, a magnetic NiFe 2 O 4 -NG photocatalyst has been fabricated through hydrothermal route.The SEM and TEM images show that nitrogen-doped graphene sheets are flaked and furnished with NiFe 2 O 4 nanoparticles having an average diameter of 80 nm.The photocatalytic activity measurements confirm that the Ni-Fe 2 O 4 nanoparticles combined with nitrogen-doped graphene sheets lead to exciting conversion of the inactive Ni-Fe 2 O 4 into very good catalyst for the degradation of methylene blue (MB) under visible light irradiation.The notable increase in photoactivity can be ascribed to the superior conductivity of the reduced NG sheets leading to favourable and efficient separation of photogenerated carriers (hole-electron) in the NiFe 2 O 4 -NG system.Subsequently, there is very large and useful change in photocatalytic activity after coupling nickel ferrite with nitrogendoped graphene sheets.
show the PXRD diffraction patterns of the pure NiFe 2 O 4 and as prepared Ni-Fe 2 O 4 -NG.The diffraction peaks at 30.9°, 35.7°, 43.4°, 53.7°, 57.2° and 63.2° corresponding to the planes (220), (311), (400), (422), (511) and (440) are allocated to spinel-type NiFe 2 O 4 (JCPDS No. 54-0964). 41Similar diffraction patterns are observed for NiFe 2 O 4 -NG.The nitrogen doped graphene oxide can be reduced by the alcohol under hydrothermal conditions and no peak at (002) is observed in the composite.It can also be related to well exfoliation of the NG sheets in the resulting composite material.So the diffraction pattern of NG disappears in the XRD pattern of NiFe 2 O 4 -NG.

Figure 1 .
Figure2ashows representative scanning electron microscopy and transmission electron microscopy images of the prepared GO.From the SEM image, morphology and structure of as-prepared graphene oxide sample was investigated.GO sheets were cast on a gold coated (100 nm) Si/SiO 2 substrate.It has been found that the graphene flakes have wrinkled surfaces.Furthermore, in the TEM image (Figure3a) GO shows layer-by-layer stacked structure and has wrinkled paper like morphology.Such morphological changes can be attributed to the increased formation of phenolic and epoxy functional groups on the basal plane of GO.The curled and overlapped nanosheets can be clearly observed.The SEM image (Figure2b) and TEM image (Figure3b) reveal that nitrogen-doped grapene nanosheets exhibit a typical wrinkled structure, which results from stable bending thermodynamically.42,43Figures2(c-d) show SEM images of the NiFe 2 O 4 and NiFe 2 O 4 -NG samples where as Figures3(c-d) show TEM images of the NiFe 2 O 4 and NiFe 2 O 4 -NG samples.In Figure 2c and Figure 3c, NiFe 2 O 4 nanoparticles are clear- ) shows the FT-IR bands of NiFe 2 O 4 and NiFe 2 O 4 -NG.The bands observed in the range of 620-650 cm -1 corresponds to the intrinsic stretching vibrations of the M-O in the tetrahedral site.The second band around 3400-3500 cm -1 corresponds to O-H stretching vibrations. 45Furthermore, it is observed that almost all the characteristic bands of oxygen containing functional groups (C=O, O-H, C-OH and C-O-C) disappeared in the FT-IR spectrum of NiFe 2 O 4 -NG depicting the change in the surface morpholgy of NG-NiFe 2 O 4 composite.These findings show that NiFe 2 O 4 nanoparticles are bonded to the NG.The results above show the heteroatom N was entered in the graphene structure and the NiFe 2 O 4 -NG composites was prepared favourably.

Figure 3 .Figure 4 .
Figure 3. TEM images of (a) GO, (b) NG, (c) NiFe 2 O 4 and (d) NiFe 2 O 4 -NG 2 O 4 and the nitrogen-doped graphene sheets causing the effective separation of carriers generated by the light exposure in the NiFe 2 O 4 -NG composite system.A plausible mechanism for enhancement in photocatalysis process is shown as follows: When the visible-light is irradiated on the surface of Acta Chim.Slov.2017, 64, 170-178 Singh et al.: Nitrogen Doped Graphene Nickel Ferrite Magnetic ...

Figure 5a .
Figure 5a.Absorption spectra of the MB solution (C = 0.075 M and l = 1 cm) taken at different photocatalytic degradation times using Ni-Fe 2 O 4 -NG.

Figure 5b .
Figure 5b.Kinetics of photodegradation of (a) Pure MB and (b) Coercivity and saturation magnetization of NiFe 2 O 4 -NG are 47.4 G and 10.1 emu/g respectively, whereas that of Ni-Fe 2 O 4 are 33.5 G and 9.2 emu/g respectively.The values observed for NiFe 2 O 4 -NG are larger than those for Ni-Fe 2 O 4 which shows that NiFe 2 O 4 -NG is more easily separable than NiFe 2 O 4. The increase in the saturation magne-tization was possibly attributed to the increasing crystallinity and particle size of the nanoparticles.