Enhancement of photocatalytic by Mn3O4 spinel ferrite decorated graphene oxide nanocomposites

The hydrothermal process was used to prepare Mn3O4/x%GO nanocomposites (NC’s) having different ratios of the Mn3O4 nanoparticles (NP’s) on the surface of graphene oxide (GO) sheet. SEM image showed that the Mn3O4 NP’s were distributed over the surface of GO sheet. HRTEM images exhibited the lattice fringe arising from the (101) plane of the Mn3O4 NP’s having the interplanar d-spacing of 0.49 nm decorating on the surface of GO. The electronic absorption spectra of Mn3O4/x%GO NC’s also show broad bands from 250 to 550 nm. These bands arise from the d–d crystal field transitions of the tetrahedral Mn3+ species and indicate a distortion in the crystal structure. Photo-catalytic activity of spinel ferrite Mn3O4 NP’s by themselves was low but photo-catalytic activity is enhanced when the NP’s are decorating the GO sheet. Moreover, the Mn3O4/10%GO NC’s showed the best photo-catalytic activity. This result comes from the formation of Mn–O–C bond that confirm by FT-IR. This bond would facilitate the transfer of the photoelectrons from the surfaces of the NP’s to the GO sheets. PL emission which is in the violet–red luminescent region shows the creation of defects in the fabricated Mn3O4 NP’s nanostructures. These defects create the defect states to which electrons in the VB can be excited to when the CB. The best degradation efficiency was achieved by the Mn3O4 NP’s when they were used to decorate the GO sheets in the Mn3O4/10%GO NC’s solution. Lattice fringe of Mn3O4 with an interplanar d-spacing of 0.49 nm for (101) plane. Photocatalytic activity of spinel ferrite Mn3O4 nanoparticles by itself is low. Number of photoelectrons created depends on number of Mn3O4 on a given area of GO The bonding of the Mn3O4 to the GO sheet would be though a Mn–O–C junction. The degradation processes were accelerated by Mn3O4/10%GO nanocomposites Lattice fringe of Mn3O4 with an interplanar d-spacing of 0.49 nm for (101) plane. Photocatalytic activity of spinel ferrite Mn3O4 nanoparticles by itself is low. Number of photoelectrons created depends on number of Mn3O4 on a given area of GO The bonding of the Mn3O4 to the GO sheet would be though a Mn–O–C junction. The degradation processes were accelerated by Mn3O4/10%GO nanocomposites


Introduction
In recent years, many researchers are interested in graphene and graphene based materials due to its excellent physical and chemical properties such as high electron conductivity, unique transport performance [1], high mechanical strength [2], extremely high specific surface area [3] and easy functionalization make graphene a good substrate to produce graphene-based composites. Graphene is an allotrope of carbon with two-dimensional honeycomb sp 2 crystalline lattice. Also, it has been considered as the thinnest and hardest material. Due to these properties graphene has been extensively explored and utilized for the alteration of hydrogen storage [4], supercapacitor [5][6][7][8], energy storage [9][10][11], biosensors [12], electrocatalyst [13][14][15][16] and photocatalysts [17][18][19]. In addition, there are various methods used to synthesis of graphene based materials such as arc discharge method [20][21][22], laser based technique [23], microwave irradiation [24], microwave assisted materials [25][26][27], spray pyrolysis [28] and hydrothermal method [17,18]. Recently, a few novel methods have been reported. Kumar et al. [29] have been synthesized the ternary hybrids material containing manganese cobaltite (MnCo2O4) nanoparticles wrapped by microwave exfoliated-reduced graphene oxide nanosheets (ME-rGO NSs) for improved supercapacitor electrode materials. Choucair et al. [30] have shown a method to prepare graphene based on hydrothermal synthesis and sonication. The recent advances in the largescale synthesis of graphene by CVD on TiO 2 [31] and Cu [32] films open up various macroscopic applications of graphene. In addition, graphene have been led to investigate the design and the development low-cost and high yield preparation protocols for chemically-derived graphene (graphene oxide). Graphene oxide (GO) has received increasing the attention. This is because it possesses similar properties to that of graphene. The structure also consists of polar functional groups i.e. hydroxyl, carboxyl, etc. on its planar surface that will have high surface area, thermal stability, mechanical and electrical properties. Therefore, GO can be used to combine with metal oxide nanomaterials for photocatalytic activity [33][34][35][36]. A graphene layer interfacing with the distribution of nanoparticles (NPs) on the surface have been well defined. The NPs can act as a stabilizer against the aggregation of individual graphene sheets, which is generally caused by a strong Van der Waals interaction between graphene layers. Some researchers reported on the modification of graphene with metal oxide NPs such as TiO 2 , SnO 2 , and Fe 2 O 3 [37][38][39]. The incorporation of nanoparticles on the surface of graphene is highly desirable for improving the surface morphology for example: electronic structure and following intrinsic properties of graphene. Generally, various types of metal oxides have been synthesized and supported on graphene, which include ZnO, TiO 2 , CeO 2 , SnO 2 , MnO 2 , Co 3 O 4 , Fe 3 O 4 , WO 3 [40,41]. In case of TiO 2 and ZnO, they exhibited good photocatalytic activity in UV light because of their wide band gap and they are stable in aqueous conditions during photocatalysis. Furthermore, the coupling of graphene oxide with TiO 2 and ZnO increases the photocatalytic activity due to increase in the photogenerated charge carriers. The photocatalytic efficiency depends on the ratio of the photogenerated charge-carrier transfer rate to the rate of electron-hole recombination. For composite structure, M 2+ ion easily bonds with oxygen by giving an electron and super oxide radical. For NP's, when the photoelectron meets a radical in the solution immersing the NP's, a photo-chemical reaction involving the radical could occur. During photo chemical degradation of metal oxide, it found that the photo will generate holes and will react the adsorbed molecules in the water to form OH radicals that can be oxidized the organic compounds [42]. In case of waste water treatment, people use TiO 2 , ZnO and spinel ferrite (Mn 3 O 4 ) NP's. This is because of the environmental friendly. As we know, ZnO is the semiconductor. The photoelectrons can excite into the conduction band and they will interact with the holes that left behind in the valence band to form an e − /h + pair which called an exciton. When these two recombination, there will be an exciton emission close to the Near Band Edge (NBE). The nanoparticles which formed the excitons will have poor photo-catalytic performances. This is because the photoelectrons that created by the absorption of the light will not be available for the photo-catalytic activity. In case of the trimanganese tetraoxide (Mn 3 O 4 ) NP's, it has an excellent potential such as its ability to adapt to the different oxidation states which are needed for many applications in the photo-catalysis and in the solar energy conversion [43,44]. Moreover, Mn 3 O 4 has a distorted spinel structure, a unique ion exchange and a special electronic configuration. [45,46] The first excited states of the Mn 3+ ion will stay in the band gap of the host which will lead to a luminescence needed for certain photo-catalytic applications [47]. Therefore, several researchers have been recognized as an important photocatalyst that is highly effective in visible light.

Synthesis of Mn 3 O 4 nanoparticles
Mn 3 O 4 nanoparticles were prepared by the hydrothermal process. This process is cost-effective and simple. It uses hot water and high vapor pressure. In the hydrothermal process, the reaction is a chemical process and the ions are able to come into contact with each other more easily. The higher room temperature used in the autoclave allows the oxide compounds to disassociate into their ion constitutes since the solubility increases with temperature. The high pressure is needed so that the solvent does not evaporate. At the high temperature (180 °C), the ions will move faster. After the nanoparticles are formed, the solution is cooled

Synthesis of graphene oxide
Graphene oxide (GO) is prepared from graphite which can be used as promising starting material to generate graphene-based nanocomposites. The advantage of graphene oxide is cheaper and easier to manufacture than graphene and reduced graphene oxide. Moreover, graphene oxide can easily be mixed with different materials and polymers. It is also enhance the properties of composite materials such as tensile strength, conductivity and photocatalytic activity. Graphene oxide (GO) was synthesized using the modified Hummer's method [67] Firstly, 0.75 g of graphite flakes was mixed into 100 ml of H 2 SO 4 while being stirred for 30 min while in an ice bath. The temperature of the mixture was held below 5 °C. Then, 4.5 g of KMnO 4 and 0.31 g of NaNO 3 were added into the solution slowly for oxidizing functional group. The mixture was stirred in an ice bath for 30 min. Then, the mixture was removed from the ice bath and further stirred continuously at room temperature for 120 h. After stirred for 120 h, an aqueous 100 ml of deionized water was added into the mixture. After that the mixture was heated at 98 °C for 1 h. Then, 7.5 ml of H 2 O 2 solution was added into the mixture to stop the reaction. At this point, 50 ml of HCl, 50 ml of deionized water and 50 ml of ethanol were added to the mixture for removing the oxidance from the reaction. The black precipitates were washed with the mixture of 5% HCl and deionized water for several times until the filtrated solution reached a pH of 7. The solution was then centrifuged to remove the GO powder. The GO powders were dried using freeze dehydration at − 40 °C to obtain the GO.

Synthesis of the Mn 3 O 4 /x% GO nanocomposites
The

Characterization
Bruker D8 ADVANCE X-ray diffractometer was used to characterize the X-ray diffraction (XRD) patterns of Mn 3 O 4 /x% GO NC's. The 2θ was in the range of 5-90° using Cu-K α radiation (λ = 1.5418 A°). The crystallite size was calculated by Debye-Scherrer equation for every peak to crystal planes to get more accuracy.

Photo-catalytic activity
The visible-light photo-catalytic of the five Mn 3 O 4 /x%GO NC's was tested by looking at the degradation of methylene blue (MB) (seen as a reduction in the intensity of the blue light). Before doing this, a calibration curve was constructed. This was done by dissolving the different amounts of MB into 80 ml of deionized water. The intensities of the blue light emitted by the MB dye were than recorded by the UV-visible spectrometer. A plot of different intensities versus the amount of the MB dye which had been dissolved in the solutions is constructed. The

Structural studies
The XRD patterns of GO and Mn 3 O 4 /x% GO NC's for every samples are shown in Fig. 1. Figure 1a shows the spectra of GO peaks. A strong peak at 11.56° corresponds to the reflection of the (001) plane of graphene oxide. Another small broad peak at 41.84° corresponds to the reflections of (100) plane that also belongs to graphene oxide [68,69]. In addition, the diffraction peak (002) of GO at 30.49° was occurred when the diffraction peak (002) of graphite

Morphology of Mn 3 O 4 /x%GO NC's
The morphology of GO and Mn 3 O 4 /x%GO NC's was determined by Scanning Electron Microscope (SEM). Figure 2a shows the morphology of the GO. One can see that the surface of GO sheet is jagged, i.e., it is not smooth. In addition, the energy dispersive x-ray (EDX) measurement will be one method to determine the amount of C and O of GO. Figure 3 shows the spectrum of GO. As is seen, the EDX spectrum of GO Fig. 6. GO has intense D and G bands. The G band is more intense than the D band. The G band represents the intensity at 1596 cm −1 which is the in plane bond stretching motion of pairs of C sp 2 atoms while the D band located at 1358 cm −1 corresponds to the defects and disorder carbon in the graphite layers [73]. The intensity ratio of the D to the G band (I D /I G ) provides a sensitive

Photoluminescence spectra
The photoluminescence spectra of Mn 3 O 4 NP's and Mn 3 O 4 /x%GO NC's were measured by an excitation wavelength of 325 nm at room temperature (see Fig. 7).  Mn 3+ ions [47]. For the broad green emission, it appears at 535 nm and the broad red emission observed at 615 nm are due to the radial recombination of photo-generated hole with an electron. This may be help to produce the surface defect or surface dangling bonds on the surface of NP's [75]. From the structural analysis, we believe that the highly photoluminescent response comes from the abundant defects that builds by the Jahn-Teller distortion of Mn 3+ ion in the self-assembled NP's and it also corresponds to d-d transitions. The fact of this reason is the ground state of Mn 3+ ion is split by using the strong static Jahn-Teller effect [76]. Figure 7b shows that two separate trends are occurring when the coverage of the GO surface by the NP's. When the percentage of GO in the composite increased, the spectrum of PL emission seems to be decreased. This may be due to two reasons: (1) a decrease in the heights of the exciton peak or to (2) a decrease in the defect emission from the NP's. In Fig. 7b, the decrease in the heights of the exciton peaks in PL spectra of the 5% and 10% NC's and the complete absence in the 15% and 20% NC's means that no photoelectrons are needed for the creation of the excitons and that all the photoelectrons are available for the photo-catalytic activity. The number of photoelectrons created depends on the number of Mn 3 O 4 NP's on a given area of the GO sheet. As the amount of GO increases, the density or number of NP's decreases. The decrease in parts of the spectra due to the defect (visible light spectra) of the PL spectra with the increased coverage of the GO surface would imply that there was a lessening of the interaction between the Mn 3 O 4 crystal and the GO sheet (due to the fact that there is lesser NP per unit area of the GO surface). The results in Fig. 7c and d are different. This is because there are still Mn 3 O 4 NP crystals on the GO surface. This is indicated by the relative heights of the different Gaussian curve as seen in the Fourier decompositions of the visible light portions of the PL spectrums for the different NC's. Looking at Fig. 7c, the Gaussian peaks correspond to the green emission (504 nm, 536 nm) and yellow emission (579 nm). The PL spectra for Gaussian peaks of the Mn 3 O 4 /10%GO NC's only show the violet-red emission (605 nm) as shown in Fig. 7. This emission was ones observed only in the excitation of the host ferrite. Moreover, the intensity peak in PL spectra for the peak position at 425 nm is in blue emission and this may be the possible defect of Mn vacancy which is in the Mn 2+ tetrahedral and Mn 3+ octahedral sites. For another peak at 510 nm, it is in the green emission which comes from the oxygen vacancy. Sarma et al. [77,78] reported in their two studies that the red-shift of the Mn emission comes from the ligand field interactions and that the shifting of the Mn d-d emission may be came from the hosts which have multiple ions in the crystal lattices that would distort the Mn coordinates and have an effect on the emission energy.   Fig. 9 Tauc's plots of the absorption data for Go and the Mn 3 O 4 /x%GO NC's where ν is respectively light frequency, α is absorption coefficient, E g is band-gap energy, and A is a constant, n is ½ for direct allowed transitions and 2 indirect allowed transitions. Tauc curves which are the plots of Eq. (2) are shown in Fig. 9. We calculated by using direct band gap (n = ½). Extrapolating the linear portions of the linear part of these Tauc curves which is the energy gap can be obtained. From these results, we can see that the energy band gaps of Mn 3 O 4 /x%GO NC's changed. The values of the energy band gaps reveal in Table 4. As we seen, the changing in the energy gap is due to the change in the strain within the nano clusters when the Mn 3 O 4 NP's are linked to the GO sheets. The information on the absorbance can also be used to determine the photocatalytic reduction of methylene blue (MB). The degradation of MB can be determined by recording the initial intensities of the blue light at emitted by solutions containing a fixed amount of the MB before exposing the solution to incident beam of light when the absorbance occurs. The absorbance for Mn 3 O 4 NP's and Mn 3 O 4 /10%GO NC's are shown in Fig. 10. In Fig. 10a, no degradation by the solutions having only Mn 3 O 4 NP's was seen. This may be due to the fact that the photoelectrons created by the illumination of the NP's did not transfer to the GO since there was no GO present. Moreover, the surface of the NP's is small, the probability of a photochemical reaction between the photoelectrons on the NP's and MB molecules would very small.When the Mn 3 O 4 NP's decorate the surface of the GO sheets, photoelectrons on the NP's can migrate to the surface of the GO  Fig. 10b. Plotting the degradation of the different NC's at the different times, we get the activities of the NC's for the reduction of MB as shown in Fig. 11. From the curves, we see that the best degradation was achieved with the Mn 3 O 4 /10%GO NC's. It is interesting to note that Mn 3 O 4 prepared by hydrothermal method had no catalytic activity. This may be due to the large size of the particle, no dispersion in water and incompatible with energy band gap. Moreover, the development of the photo-catalytic activity of the Mn 3 O 4 NP's can be done by using them decorate onto the GO sheets. Then, the photo-catalytic reaction will appear on the surface of GO sheet instead of the surfaces of the NP's. Zhang et al. [79], suggested that the enhancement of photo-catalytic reaction is due to the increased absorptivity of the dye to the surface of the GO sheets. The improvement of absorptivity may come from the strong π-π conjugation of the dye molecules and the aromatic regions of graphene. The photocatalytic degradation of Mn 3 O 4 and Mn 3 O 4 /x%GO NC's was studied by using the methylene blue (MB) dye at room temperature as shown in Fig. 12 The Mn 3 O 4 NP's resting on top of GO sheet with an occupied valence band (VB) state and an unoccupied conduction band (CB) state. When Mn 3 O 4 NP's are stimulated by photon energy; the electron in VB state will excite to CB state. This can be made hole in VB and the electrons in CB state will be transferred into the GO sheet. Therefore, GO sheet will serve as good accepter and electron On the other hand, the electrons are on the GO sheet will make the reduction reaction by binding with O 2 in the water to create the • O − 2 and to react with H 2 O 2 to be • OH . This • OH is the degradation molecule of methylene blue which reacts with methylene to be CO 2 and H 2 O . Prior to the formation pair and after its annihilation, the photoelectron e − can move to the surface of the NP where it can interact with any chemical ions which are present in the solvent. Of course, the chemical reactions can only occur if the photoelectron and chemical specie meet at the same time and the same place. Since, the sizes of the NP's and chemical ions are both small and are randomly moving in the liquid, the joint encounter of the two will be small and the photo-catalytic reaction would be slow. When the NP's are attached to GO, the photo excited electron can cross the bridge (junction) connecting the NP and the GO. In the TiO 2 decorated rGO, the bridge is the Ti-O-C bond [17] while in the ZnO decorated rGO, it is the Zn-O-C bond [18]. In this present case, the junction is a Mn-O-C chemical bond. The photoelectrons can travel freely in the 2D carbon array in the GO (keeping in mind that the 2D hexagonal carbon array is a very good conductor). If the moving photoelectron encounters the same chemical ion just mentioned, the two would undergo the same chemical action as before. Because the area of the GO is much more than the area of the nanoparticle surface, the probability of the photoelectron and the chemical specie encountering would be greater on the GO surface than that on the surface of the NP. This would lead the photo-catalytic activity being enhanced. Because of the difference in the mobility's of the electrons and holes, charge separation will occur and when this happens, one encounters a suppression of the annihilation of electron/ hole pairs and therefore an increase in the photo-catalytic activity. This was the explanations given by Patra et al. [80], for the enhanced solar generation by MoS 2 /GO and MoS 2 / MoS 3-x /GO multi-functional NC's, for solar water splitting by Au-Pd/rGO/TiO 2 hetero junctions by Tudu et al. [81], and by Au-RGO/N-RGO-TiO 2 hetero junctions by Bharad et al. [82].

Conclusion
In conclusion, the Mn 3 O 4 /x%GO NC's has been successfully fabricated by the hydrothermal method. SEM image showed that the Mn 3 O 4 NP's were distributed over the surface of the GO sheet and the EDX confirmed the result of GO sheet. HRTEM investigation showed that the GO layers were interfacing with the Mn 3 O 4 NP's where a chemical interaction between the two could be created. FT-IR results investigated the bonding between Mn 3 O 4 and GO sheet and this bonding may be due to O-C vibration in the Mn-O-C bond which connects the Mn 3 O 4 NP's to the GO sheet together. PL emission which is in the violet-red luminescent region shows the creation of defects in the fabricated Mn 3 O 4 NP's nanostructures. These defects create the defect states to which electrons in the VB can be excited to when the CB. For the blue emission, it may be the possible defect of Mn vacancy which is in the Mn 2+ tetrahedral and Mn 3+ octahedral sites and the peak is in the green emission which comes from the oxygen vacancy. The best degradation efficiency was achieved by the Mn 3 O  Acknowledgements We would like to thank Faculty of Science, Kasetsart University and Department of Physics for the financial support. Finally, we also thank to King Mongkut's University of Technology Thonburi for the financial support provided by through the KMUTT 55th Anniversary Commemorative Fund.
Data and materials availability All data needed to evaluate the conclusions in the paper are present in the paper. Additional data related to this paper may be requested from the authors.

Conflicts of interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
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