Facile Synthesis of rGO/Mn3O4 Composite for Efficient Photodegradation of Phenol under Visible Light

To enhance reusability and to maintain higher efficiency in degradation, Mn3O4/rGO nanocomposites were synthesized by a facile thermal treatment. Initially, Mn3O4 nanoparticles were prepared and analyzed by powder XRD and HR-SEM. The composition of manganese oxide was varied to obtain different nanocomposites. The Mn3O4 ions were found to be well anchored onto the rGO surface. The obtained samples were taken for the photodegradation studies with phenol as the pollutant. Under a dynamic mode, the absorption efficiency was found to be maximum for the MnsR0.75 sample for phenol.


Introduction
Discharge of organic wastes into local water bodies has been one of the most serious issues. The highly toxic wastes in industrial effluents contain a large portion of phenolic compounds that have adverse effects on both nature and public health [1][2][3]. The common sources of phenolic pollution are paints, pesticides, coke ovens, coal conversion, polymeric resin, paper mills, petroleum refineries, and petrochemical and pharmaceutical industries [4][5][6]. These compounds have stubborn nonbiodegradable properties and toxicity and, therefore, posed to be fatally harmful to aquatic life and humans since the phenolic compounds own considerably high binding capacity in aqueous medium. Furthermore, their unpleasant odor and taste add to the detrimental effect on aqueous systems [7][8][9][10]. Thus, it is important to develop an adhesive-like material to remove the organic pollutants from water bodies to minimize pollution [11][12][13]. Many techniques have been used to remove phenol from wastewater such as photocatalytic degradation, membrane filtration, chemical oxidation, and solvent extraction [14][15][16][17][18][19][20][21]. Recently, Wang et al. prepared 2D/2D γ-MnO 2 /rGO using rGO and KMnO 4 for the catalytic ozonation of 4-chlorophenol in the presence of PMS [22]. Ramesh et al. examined the catalytic efficiency of MnO 2 /rGO nanowires for the degradation of reactive red using a sono-Fenton-assisted process in the presence of H 2 O 2 [23]. Qu [24]. Recently, the research has turned towards the removal of pollutants via photocatalysis employing either UV or visible light [25][26][27][28][29]. In recent times, rGO has gained considerable attention in this area as an effective metal-free catalyst which has been used in order to weaken up the persulphate, eventually wearing away the O-O band present at the dynamic sites and forming an excellent electron bridge (Duan et al., 2015, [30]). In a similar way, nanodiamonds, cubic mesoporous carbon, and CNT have been extensively used by researchers in degrading organic pollutants [2,31,32]. Though these materials caused degradation of the target compounds, a longer time was required to achieve a complete degradation. Hence, graphene was employed in this work to attain a higher surface area along with unique properties such as good conductivity, charge delocalization, chemical stability, and electron transfer properties thereby making it an excellent candidate for the purpose. Graphene, with these qualities, could be considered a suitable material to be used in the degradation of organic pollutants [26,33]. In addition, metal oxides based on manganese have been extensively explored owing to their superior physical and chemical properties, low toxicity, and abundance. The presence of their unique redox loops leads to a higher potential activity via the single electron transfer making its way to be a suitable catalyst [34]. The additional difficulty arises due to the superfine nature of Mn as it forms a well-dispersed solution [35,36]. In a nutshell, photocatalysis is the acceleration of photoelectrons due to the presence of a catalyst like Mn used in this case. Thus, more attention has been paid towards metal-free catalysts like carbon materials which are both cheap and environment-friendly and also for sustainable sources. In a typical heterogeneous experiment, the carbon material will be acting as an adsorbent [37], catalytic support [38], and in some cases also a catalytic support [39]. These peculiar properties make it an interesting candidate along with other properties like their stability with acids and bases, surface, texture, and mechanical and electrical properties. It is also commendable to observe that the incorporation of a carbon material induces an encouraging effect not only on the efficiency but also on the degree of degradation achieved in the reactions [40,41]. This work is aimed at creating a metal oxide-based composite with the addition of a carbon support to facilitate a better degradation of phenol.

Materials and Methods
2.1. Synthesis of rGO. All the chemicals used were of analytical grade and used without any further purification. In a synthesis procedure, 1 g of graphite flakes was added to 50 ml concentrated sulfuric acid while stirring in an icewater bath and maintaining the temperature under 10°C. 0.379 M potassium permanganate was gradually added to the above solution; the reaction was quenched by the addition of 200 ml distilled water. An extra 2 h ultrasonic treatment was carried out and processed for the preparation (reduction) of rGO. After adjusting the pH to~6 with the addition of sodium hydroxide (0.1 M), the suspension was further sonicated for 1 h. 0.567 M of the reducing agent Lascorbic acid was dissolved in 100 ml distilled water and then was slowly added to the exfoliated graphite oxide suspension at room temperature, to act as a mild reducing agent. The reduction was performed by maintaining the solution at 95°C for 1 h. The resultant black precipitate was simply filtered by cellulose filter paper and was further washed with a 1 M hydrochloric acid solution to wash away the remaining impurities and followed by washes in distilled water, and the pH was brought to neutral. Finally, the filtrate was freezedried to obtain rGO powder.     2.4. Characterization. The structural, morphological, optical, compositional, and thermal data of the as-prepared samples were investigated. X-ray diffraction was carried out on a Rich Seifert X-ray diffractometer in the range 10-75°; Raman analysis was performed with the QE Pro Raman Series Spectrometer. HR-SEM and EDX analyses were carried out on a Quanta 200 FEG High-Resolution Scanning Electron Microscope equipped with an EDAX analyzer. The FT-IR spectrum was taken on Perkin Elmer Spectrum Two within a range of 500-4000 cm -1 . TG thermograms were obtained from Perkin Elmer STA 6000 in the presence of nitrogen atmosphere (20 ml/hr) in the temperature range of 100-800°C. Figure 1 shows the X-ray diffraction (XRD) pattern of the synthesized rGO sheets which exhibits a broad diffraction peak at 24°. The powder XRD pattern of the as-prepared Mn 3 O 4 is depicted in Figure 1. These peaks were conveniently indexed to a pure tetragonal phase of the Mn 3 O 4 nanostructure (JCPDS card No. 89-4837) [41]. The peaks of the composite indicate the presence of Mn 3 O 4 ; the steady increase in the intensity of the peaks indicates the increasing presence of manganese in the composites from MnsR0.25 to MnsR1. The same has been reflected in the Raman analysis depicted in Figure 2. The XRD pattern of rGO/Mn 3 O 4 nanocomposites is shown in Figure 1, and it shows no structural changes and contains perfect combinations of the above peaks corresponding to both Mn 3 O 4 and rGO. The crystallite size of Mn 3 O 4 nanospheres was calculated by the Debye-Scherrer formula:

Results and Discussion
where D is the crystalline size, k is the shape factor, λ is the wavelength of X-ray, β is the full-width half maxima of the peak, and θ the angle at which the peak has been observed. The particle size of the Mn 3 O 4 nanosphere is calculated to    Journal of Nanomaterials   Journal of Nanomaterials be between 15 and 25 nm. Raman spectroscopy being an important tool is used in exploring the structural properties of the materials. The sharp peak at 651 cm -1 depicted in Figure 2 indicates the presence of Mn 3 O 4 [42]. The Raman spectra of the reduced graphene oxide is depicted in Figure 2 with the characteristic peaks at 1359 and 1608 cm -1 corresponding to the D and G bands of reduced graphene oxide. The G band corresponds to the radial C-C stretching of the sp 2 carbon, and the D band relates to the first-order zone boundary phonon related to the defects on the edges of graphene. The intensity ratio (I D /I G ) of the D and G bands was used to qualitatively characterize the defect density of rGO to be 0.8451 and 0.8576 for the composite MnsR1 [43].
The obtained crystallite size is nearly the same when compared to those obtained from the HR-SEM images. Mn 3 O 4 exhibits a spherical morphology while rGO demon-strated a sheet-like morphology (Figures 3(a) and 3(b)), but in an agglomerated form which is attributed to the higher adsorption of moisture from the atmosphere. Mn 3 O 4 nanospheres are clearly visualized in Figure 3 The EDAX spectrum of the samples also describes the quantitative analysis of the as-prepared samples. Figures 4(a)-4(f) illustrate the EDAX spectrum of rGO, Mn 3 O 4 , and the composites. The molecular structure of the synthesized samples was studied by FT-IR spectra ( Figure 5). In the range 500-4000 cm -1 for rGO, the broad bands observed at 3338 cm -1 correspond to the strong stretching mode of the OH group [44], the absorption peak at 1635 cm -1 corresponds to the C=C stretching mode, and the peaks at 1716, 1154, and 1033 cm -1 correspond to the stretching modes of C=O, C-OH, and C-O, respectively.  Figure 6(a) depicts the TGA of the asprepared rGO which indicates a significant weight loss linearly until it reaches around 450°C, attributed to the higher thermal stability of the sample followed by a sudden weight loss up to 600°C. Further, Figure 6(b) illustrates the TGA of Mn 3 O 4 ; the TG depicts clearly three different regions of weight loss. Region 1 weight loss took place from room temperature to 300°C; this weight loss has been attributed to the removal of water from the specimen, namely, physisorbed molecular water and structural water, i.e., water of crystallization and chemically bound water. Region 2 in the range 390-610°C is attributed to the reduction of Mn from the specimen, and Region 3 from 650 to 800°C is attributed to the final decomposition of the specimen. Mn has higher thermal stability in comparison to other electrocatalysts. The composite MnsR1 has a lower total mass loss (about 15 wt%) than that of rGO ( Figure 6(c)). The mass loss from 200 to 400°C can be attributed to the decomposition of rGO. Since Mn 3 O 4 can withstand higher temperatures (about 500°C), the composite of MnsR1 only shows a gradual mass loss when the temperature is over 500°C. The difference in the total mass loss between rGO and MnsR1 is about 15 wt%. If it is assumed that Mn 3 O 4 is thermally stable and does not have a mass loss below 600°C, the amount of Mn 3 O 4 loaded in the MnsR1 is about 52.0 wt%.

Photocatalytic Performance.
The photocatalytic performance of the prepared samples was carried out in the presence of visible light. The stock solution was prepared by dissolving 20 mg/l of phenol in distilled water and further taken for the analysis. 50 ml of the stock solution was placed in a dark room setup in order to block the interference of any stray light. The solution was placed on a stirrer, and 30 mg of the catalyst was added and left for 30 minutes in the dark to attain the adsorption desorption isotherm; after the collection of the sample, visible light via a 20 W bulb was introduced into the dark room for 15 minutes, and again, the sample was collected. The procedure was repeated until the completion of 120 minutes. The remaining sample was withdrawn via centrifugation after every cycle in order to carry out the repetition cycles. The degradation percentage was calculated as follows:   Journal of Nanomaterials where DE is the degradation efficiency, T is the concentration of the analyte at time 0 minute, and T 0 is the concentration of the analyte at various time intervals. The photocatalytic performance of the prepared samples was evaluated by the photocatalytic decomposition of aqueous phenol solution under visible light irradiation. From Figure 7, it is evident that rGO exhibits a lower degradation activity than the metal oxide. On further studies at the 120 th minute, a total of 33.5% was achieved in the case of pure rGO. On the other hand, pristine Mn 3 O 4 exhibits a degradation of 18.6% at the zeroth minute, and as the time increases with irradiation of visible light, a net degradation of 73.6% was achieved. The composites exhibited an average degradation of 14% at the 0 th min, and maximum degradation of 94.48% was achieved for the MnsR0.75 sample (Figures 7(a) and 7(b)).
On further increasing the concentration of Mn 3 O 4 , it is evident that the degradation percentage decreasing this could be attributed to the agglomeration occurring between the rGO sheets. The π -π stacking interaction provides a greater amount of phenol concentration near the Mn 3 O 4 nanoparticles on the surface of rGO and therefore leads to the more efficient contact between them. Due to its excellent electric conductivity and large specific surface area, it acts as an efficient electron acceptor to enhance the photoinduced charge transfer mechanism. This condition is also hindered in cases when the rGO concentration goes higher as in the MnsR1 composite. Figures 8(a) and 8(b) display the reduction ratio with respect to time. From the degradation percentage, it is evident that the sample MnsR0.75 exhibited the highest degradation, repetition of the experiment was carried out, and the sample was found to degrade around 90%, 87%, and 79% in the repetition cycle as depicted in Figure 9. On further repetition, it was found to decrease to around 52%.

Conclusion
In conclusion, Mn 3 O 4 nanoparticles were loaded onto the graphene surface by a simple thermal route. Solventethylene glycol played a vital role in the formation of the as-prepared product. The small-sized Mn 3 O 4 nanoparticles and high surface area of the reduced graphene oxide composite exhibited better catalytic activity under visible light irradiation. The photocatalyst is highly stable and has been used for four successive times without any significant loss in its activity. The nanocomposite MnsR0.75 exhibited the highest phenol degradation of 94.48%, taking into account the presence of rGO in (i) the utilization of a higher number of photoelectrons and (ii) lowering of electron hole recombination. The high activity exhibited was ascribed due to the strong interaction and progress of synergetic effect between the Mn 3 O 4 nanospheres and rGO. Thus, the MnsRnanocomposites can be considered a promising material for the removal of various harmful organic dyes from wastewater.

Data Availability
The data supporting this work is available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.