Facile Synthesis of High Lateral Graphene Oxide Sheets for Visible Light-driven Photocatalytic Degradation of Industrial Dyes towards Water Treatment Applications

In this work, a facile synthesis of multilayer graphene oxide (GO) sheets having a two-dimensional structure has been realized using the modified Hummers and Offeman method. The as-synthesized GO was analyzed by UV-visible, FTIR, Powder X-ray Diffraction (PXRD), HRTEM, FESEM, and EDX for optical, chemical, structural, morphological, topographical, and elemental analysis respectively. The results reveal that GO shows an absorption band at 232 nm. The FTIR spectrum shows the oxygen-rich groups in GO, and PXRD confirms the major GO peak at 10.35° along with few minor peaks. HRTEM and FESEM confirm the two-dimensional GO sheets along with high lateral dimensions. The as-synthesized GO with a number of available functional groups and high lateral dimension was efficiently used for the photocatalytic degradation of Coralline Red BS (CR BS) and Reactive Blue 81 (RB81) dyes. This study reveals that, compared to graphene, pristine GO sheets significantly influence the degradation of CR BS and RB81 dyes. This work significantly contributes to the use of pristine GO for the removal of toxic dyes from wastewater. The evaluation of the dye degradation rate and GO reusability along with the kinetic studies is explained in detail.


Introduction
Nanomaterials offer unique physical properties and have attracted tremendous attention from researchers and scientists. The facile synthesis of nanoparticles and system production increased the understanding of nanoscience with an interface from various research fields including physics, chemistry, and material sciences 1 . The advancement in nanotechnology has led to the synthesis of novel nanomaterials that find effective use in advanced applications, such as nanoelectronics, nanomedicine, and customer products 2 . Carbon, a first-row element on the periodic table is widely available in three forms, namely diamond, graphite, and fullerenes. Graphene, an allotrope of carbon is the thinnest, strongest, and stiffest compound and has unique properties, such as being an excellent conductor of heat and electricity. Single-layer graphene that is only oneatom-thick has received a huge attracting attention because of its potential to provide extraordinary thermal, electrical, and optical properties in material science, biochemistry, and medicine 3 . Currently, the major research topics in materials sciences are the development of other allotropes of graphene 4 because of its unique properties allowing its vast application in various fields 5 like (i) in graphene, its electron transportation is labeled by Dirac's equation, allowing access to quantumelectrodynamics as part of solid-state physics. (ii) graphene-based systems are stable because of the high transport of electrons through the surface at ambient-temperature embedded with mechanical stability. (iii) Various aspects of graphite and nanotubes can be regarded as an interface of graphene.
The great interest in graphene oxide (GO) might be because of its inexpensive manufacturing rate and extensive capability to obtain from graphene 6 , and its current scalability is a desired feature. After oxidation, graphite's carbon layers are filled with 4 oxygen molecules and converted to GO. GO is a side product during oxidation of graphite and the obtained GO has high interplanar spacing between the layers as compared to graphite. After oxidation, it spreads in a base solution such as water 7 and finally the carbon layers are reduced and separated into specific layers. During these processes, massive quantities of functional groups (oxygen-containing) have been introduced onto the individual sides of the graphite sheet. The implantation of the functional groups overpowers the van-der Waals force between the sheets, increasing the interlayer spacing. The sonication easily pulls the structure and expands it further to its maximum, exfoliating the graphite into multilayer or single-layer sheets separation.
The obtained multilayer GO sheets find potential applications in elastic optoelectronics, such as organic light-emitting diodes and solar cells 8,9 . The basic requirement for these applications besides its high conductive and transparent nature, is that the electrode materials must be lightweight, flexible, low-cost, and compatible with large-scale manufacturing.
In this study, the authors described the facile synthesis of GO sheets using Hummer's method and characterized it for its structural, optical, and photochemical properties 10 .
Additionally, the photocatalytic activity of the as-synthesized GO for the degradation of Reactive blue 81 (RB81) and Coralline red (CR BS) dyes under direct sunlight is investigated in detail. The process of photocatalysis is achieved by photons with threshold energy greater than or equal to the bandgap energy semiconductor. An incident photon excites an electron from the valence band (VB) to the conduction band (CB), creating a positive hole in the VB, probably leading to the formation of hydroxyl radicals when reacting with hydroxyl-ion in water, which is finally available for oxidation. Simultaneously in the CB, an excited electron reduces the oxygen and acts as 5 an oxidizing agent. One of the main drawbacks observed is the recombination of the photo-generated electron with their hole [11][12][13] . This phenomenon dissipates almost all the photon energy resulting in the low-efficiency of photocatalysis. Therefore, the development of a reliable and efficient photocatalyst is always desired, and critical in this study.

Materials and Methods
All the chemical-compounds and reagents used were of analytical-reagent grade and were practiced as procured without any additional refinement. Graphite powder (≥99%), The most promising and widely used technique for the preparation of GO is Hummers and Offeman's (1958) 14,15 . In this designed synthesis strategy, the authors can easily control the structural, morphological and optical properties by changing the oxidizing agents used to exfoliate graphite powder. For a typical experiment process, the graphite flakes and NaNO3 in 1:1 ratio were taken in 45 ml of H2SO4. The mixture-solution was reserved underneath ice immersion (0-5°C) with constant stirring for five hours. After that, a definite amount of oxidizing agent (KMnO4) was gently added to this suspension to generate the graphite solution. The addition of KMnO4 was sensibly supervised to keep the reaction-temperature below 15°C. The mixed chemical solution was stirred for another two hours and diluted by gently adding 92 ml of water. After removal from the ice-bath, the solution was stirred at 40°C until it turned deep red-brown 7 . The solution 6 was held in a reflux system at 98°C for 10-15 minutes. After this, the solution was mixed with 10 ml H2O2 to stop any further reaction. A huge amount of bubbles was released and the shade of the solution was transformed into bright yellow. The colored suspension was filtered and washed for purification with 10% of HCl and repeatedly with deionized water. Finally, the product was filtered and dried in a vacuum oven at 50°C and flakes of GO were obtained in the powder form.
RB81 and CR BS dyes procured from Parshwanath Dye Stuff Industries, Ahmedabad, India, were cast-off as a standard pollutant to evaluate the photocatalytic action of the as-synthesized GO sample. Sodium hydroxide (NaOH) and HCl, obtained from Hi-Media Laboratory Private Limited, Mumbai, India, were used to adjust the pH of the solutions for the photocatalytic test. The peak intensity at the highest absorbance wavelength (581 nm and 415 nm) was monitored during the photocatalytic experiments to evaluate the degradation of RB81 and CR BS dyes respectively in the unexposed/exposed GO samples. For a standard photocatalytic test, a 200 mL solution was prepared by dispersing 1 mML −1 of dye (RB81 or CR BS) in de-ionized water and 0.4 gL −1 of GO was added under magnetic-stirring and ultra-sonication. After that, the suspension pH was set between 4 to 11 and stirred for 30 hours in the dark to establish an adsorption/desorption equilibrium. The solution was then exposed to sunlight for 90 minutes under stirring.
During this process, in a systematic interval of 15 minutes, a sample of 5 mL from the aqueous suspension solution was taken, centrifuged (10000 rpm), and the supernatant (dye solution) was examined with a UV-vis spectrophotometer. The photocatalytic activity of GO was calculated using the following equation: where C0 is the starting dye concentration, and Ct the concentration of dye after exposure to direct sunlight for time t (0, 15, 30, 45, 60, 75, and 90 minutes).
The phase purity and the structure identification of the synthesized GO powder was determined from PXRD patterns, performed using the PAN analytical X-ray diffractometer. HF-3300 and Transmission Electron Microscope (TEM) powered by Hitachi's recorded provided the sample morphology. The UV-visible absorbance spectrum of the sample was done using Shimadzu UV 2600 spectrophotometer. The IR spectrum of the GO sample was analyzed and evaluated using the Fourier Transformed Infrared (FTIR) spectrometer powered by Bruker Alpha. The elemental configuration and chemical purity were investigated using an energy beam of 20 keV using Oxford instruments EDX.

UV-Visible absorption analysis
The UV-vis spectrum of standard GO displays a quantitative feature that could be used as an identification tool to confirm the synthesis of GO visually. Figure 1 depicts the UV-vis optical spectrum of the as-synthesized GO. The spectrum shows that GO exhibit a high absorption at a wavelength of 232 nm and a shoulder peak at 300 nm. The low and high wavelength peaks are instigated from the π − π* transition of aromatic C-C bonds and the n − π * transition of the C=O bonds, respectively 16 . These findings show that carbonyl groups are present on GO surface. Furthermore, the spectrum shows that GO absorb in the visible region and possesses a high absorption in the UV-vis range.
This could result in a good photo response of the as-synthesized GO sample in the visible range, leading to a wide possibility for different applications.    showing that carbon is not fully interconnected with the oxygen atoms 20 .

Structural and Morphological analysis
Separation in-between sets of parallel planes (interplanar spacing) are calculated from the Bragg's diffraction equation as follows: where d is the separation in-between parallel planes, θ is Bragg's angle, n is the order of reflection, and ƛ is the X-ray wavelength. Using this, d is calculated as 8.51 Å for GO, which is quite large compared to pristine graphite interplanar spacing (3.69 Å). This could be because of the presence of oxygen-containing functional groups in-between graphite layers happened during the oxidation process and injected water molecules 21 .
Based on the XRD analysis, GO is the primary material obtained in the synthesis compared to virgin graphite. Therefore, GO could be used in those applications that requires large surface-to-volume ratio or sensitive electronic properties.    shows 76% of carbon and 24% of oxygen, confirming that the GO powder has high purity and is oxidized because of the existence of the GO functional groups.
Theoretically, the expected stoichiometric mass-percentage of carbon and oxygen are 70% and 30%, respectively.    It is observed that adsorption and degradation removed 33% and 62.96% of the RB81,  Thus, a trivial reduction in photocatalytic activity enables GO as a potential candidate in the area of photocatalysis where reusability is critical. From the above discussion, it is well understood that the combined effect of adsorption and photodegradation by GO significantly influences the elimination of the two toxic dyes (RB81 and CR BS). The observed high degradation with multi dye degradation efficiency of GO might be because of its high oxygen functionalities and large surface area.

Conclusion
In this work, GO sheets were successfully synthesized by the modified Hummer's method. This work also reported the easy control over GO structural, morphological, and optical properties. The confirmation of the synthesis of GO was done by its characteristic absorption bands at 232 nm that originated from the π − π* transition of aromatic C-C bonds in the UV-vis absorption spectra. FTIR analysis showed that GO is associated with different functional groups and with different bond energies. The structural confirmation of GO is investigated by the appearance of a broad and sharp

Declaration of interest
The authors declare there is no conflict of interest. Figure 1 UV-Vis absorption spectrum of GO.

Figure 2
FT-IR spectrum of graphene oxide.

Figure 3
Powder X-Ray diffraction spectrum of the as synthesized GO.     Photocatalytic degradation plots of (a) RB81 dye and (b) CR BS dye (c) %