WO2015114668A2

WO2015114668A2 - A one pot, self-assembled process for producing individual and dispersible graphene nano-spheres

Info

Publication number: WO2015114668A2
Application number: PCT/IN2015/000059
Authority: WO
Inventors: Selvaraj KALIAPERUMAL; Suman Kumari JHAJHARIA
Original assignee: Council of Scientific and Industrial Research CSIR
Current assignee: Council of Scientific and Industrial Research CSIR
Priority date: 2014-01-31
Filing date: 2015-01-30
Publication date: 2015-08-06
Anticipated expiration: 2016-07-31
Also published as: WO2015114668A3

Abstract

The present invention discloses a one pot, self-assembled process for producing individual and dispersible graphene nano-spheres in near ambient conditions for easy confinement of cargo/active functional materials having important applications in bio imaging, optoelectronics and drug delivery.

Description

A ONE POT, SELF-ASSEMBLED PROCESS FOR PRODUCING INDIVIDUAL AND DISPERSIBLE GRAPHENE NANO-SPHERES

Field of the invention:

The present invention relates to a one pot, self-assembled process for producing individual and dispersible graphene nano-spheres in near ambient conditions for easy confinement of cargo/active functional materials having important applications in bio imaging, optoelectronics and drug delivery.

Background and prior art:

Design and assembly of nanostructures with improved properties by using novel processes is an appealing prospect for nanotechnology. Graphene and graphene oxide as nanomaterials have recently received a great deal of consideration due to their wide applications in nano-electronics, sensors, nano-composites, batteries, super-capacitors and energy storage. Graphene is also being considered for applications as diverse as plastic packaging and next-generation gigahertz transistors. Ever since its discovery in 2004 by Andre Geim and Kostya Novoselov at the University of Manchester, graphene has attracted tremendous interest due to its unique physical and chemical properties. As a single layer of graphite, graphene features a perfect two-dimensional (2D) structure with large specific surface area, high intrinsic carrier mobility, strong mechanical strength and superior flexibility. Unfortunately, the direct application of 2D pristine graphene sheets is hindered by issues such as having zero band gap, easy aggregation and poor dispersion in common solvents; hence there has been ongoing research interest in converting graphene sheets into functionalized graphene structures of different dimensions. For example, zero-dimensional (0D) graphene quantum dots, one-dimensional (ID) graphene nanoribbons and graphene fibers, 2D graphene films and three-dimensional (3D) graphene frameworks, have been widely applied in biomedicine, catalysis, imaging, photonics, quantum computing, various sensors and even more.

It is evident from diverse applications of 3D structured graphene spheres in drug delivery, super-capacitors, catalytic electrodes, sensors and active material encapsulation, that graphene spheres have emerged as nanomaterials of major interest. Therefore, several methods for preparation of 3D graphene spheres are prevalent in the art.

Common methods for the synthesis of graphene spheres usually involve co-assembly of graphene and metal oxides by electrostatic repulsion, treatment with solvent systems or reagents, and alkaline corrosion systems and using hard templates as initial reactant substrates. Few of the studies employing such methods are enlisted below.

A research study titled, "Fabrication of Graphene-Encapsulated Oxide Nanoparticles: Towards High-Performance Anode Materials for Lithium Storage" by Shubin Yang, Xinliang Feng, Sorin Ivanovici, and Klaus Mullen published in Angew. Chem. Int. Ed. 2010, 49, 8408 -8411 discloses assembly of negatively charged graphene oxide and positively charged oxide nanoparticles by electrostatic interactions, and subsequent chemical reduction, leading to metal oxides encapsulated in flexible and ultrathin graphene shells. The electrochemically active nanoparticles synthesized exhibit remarkable lithium storage performance, including highly reversible capacity with excellent cycle performance.

Yan Lin et al in Electrochimica Acta 95 (2013) 146- 154 report three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite (3D-ARGON NiAl-LDH) with super high electrochemical and capacitance performances. The process employed involves reduction of graphene oxide by hydrazine in ammonia medium to form three-dimensional reduced graphene oxide nanocup using polystyrene colloidal particle as sacrificial template followed by activation by the alkali corrosion and thermal annealing thereby providing a promising approach for design and synthesis of graphene-based materials with largely enhanced super-capacitor behaviors, which can be potentially applied in energy storage/conversion devices. However, the process for preparation of the 3D graphene nanocup involves the use of a hard template, i.e. polystyrene colloidal particle therefore restricting the size of graphene. Another research study titled, "Ternary Self- Assembly of Ordered Metal Oxide

Graphene Nanocomposites for Electrochemical Energy Storage" by Donghai Wang, et al in ACS Nano, Vol. 4, No. 3, 1587-1595, 2010 reports a ternary self-assembly approach using graphene as fundamental building blocks to construct ordered metal oxide graphene nanocomposites. The Li-ion insertion properties of the self-assembled electrodes for energy storage were investigated and indicate that Sn0₂ graphene nano-composite films can achieve near theoretical specific energy density without significant charge/discharge degradation. However, this research study is restricted to graphene-metal oxide nano-comopsite films and does not provide for the preparation of graphene spheres.

Matias Sametband et al, in New J. Chem., 2012, 36, 36-39 demonstrate a simple, one- step method for preparing stabilized microspheres of graphene oxide (GO), by applying ultrasonic power to a biphasic system. However, further coating of the microspheres with metal oxides were not exploited by the inventors thereby not conferring energy conservation or storage properties on the resultant spheres.

Sajini Vadukumpully et al, in Carbon,_Vol 47, Issue 14, Nov 2009, 3288-94 demonstrates a simple and effective method for preparation of a few layered graphene nanoflakes directly from graphite by mild ultrasonication of highly ordered pyrolytic graphite, in presence of cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and acetic acid to afford graphene nanoflakes, however does not relate to preparation of layered 3D graphene spheres, further the preparation of graphene nano-flakes is carried out in acid conditions. Albeit several methods disclosed in the art the preparation of 3D structured graphene still remains challenging. Although many efforts have been made to fabricate graphene and carbon spheres, most pathways for sphere production rely on hard templating with metal oxides or colloidal crystals and also in non-ambient conditions. However, the size, and morphology of the replicated spheres are limited to the parent template. In spite of these studies, a well-controlled architecture of the conductive material and metal oxide is difficult to achieve.

In view of the challenges posed by existing methods for preparation of graphene spheres, the present inventors have employed a method to coerce metal/metalloid oxide and graphene into deterministic nanostructured materials by self-assembly.

Objective of the Invention: The main object of the present invention is to provide a novel process to synthesize well-controlled, ordered graphene nanospheres based on self-assembly of metal/metalloid oxide, surfactant, and graphene oxide (GO). One more object of the present invention is to provide a novel process to synthesize graphene nanospheres with particle size in the range of 200 nm to 2.5 μηι which are important for bio imaging, optoelectronics and drug delivery.

The instant inventors have devised the instant novel method to open new opportunities for designing and synthesizing large scale multifunctional carbon and carbon based nano-composite materials. While large scale production of controlled carbon nanostructures has been a great challenge, this fairly simple method offers a huge scope for the industrial scale production at near ambient conditions. SUMMARY OF THE INVENTION

The present invention discloses a one pot, self-assembled process for producing individual and dispersible graphene nano-spheres comprising: i. preparing GO sheets; and ii. converting functionalized 2D graphene sheets into 3D graphene nano-spheres to obtain the desired product. More particularly the present invention discloses synthesis of graphene nanospheres, by using a surfactant, graphene oxide as a carbon source and metal or metalloid oxide oligomers as inorganic precursors.

In a preferred aspect, the present invention discloses a one pot, self-assembled method for producing individual and dispersible graphene nano-spheres comprising: i) sonicating graphite oxide sheets followed by suspending it in an aqueous solution containing a cationic surfactant and an alkaline solution to obtain a reaction mixture; ii) ultrasonically treating the reaction mixture of step (i); iii) stirring the treated mixture of step (ii) followed by addition of an inorganic source with organic substitution slowly; and iv) washing the mixture of step (iii) with an organic solvent followed by separation and drying and by treatment with mild acid solution to afford graphene nano-spheres.

In an aspect, the present invention discloses that the cationic surfactant employed is CTAB (Cetyl trimethylammonium bromide) and the inorganic source with organic substitution is an alkoxide.

In another aspect, the present invention discloses graphene nano-spheres having uniform and easily tunable particle size in the range of 200 nm to 2.5 μιη (2500 nm).

In yet another aspect, the instant graphene nano-spheres synthesized by the instant process are a model for advanced energy storage materials, drug delivery carriers and bio- imaging.

BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 depicts step wise illustration of stripping mechanism by TEM;

Figure 2 depicts step wise illustration of stitching mechanism and formation of graphene nano-spheres by TEM;

Figure 3 depicts TEM and SEM images of graphene nanospheres; Figure 4 depicts AFM tapping mode images of (a) GO and (b) graphene nanospheres; Figure 5 depicts (a) SEM-EDX and (b) TEM-EDX of graphene nanosphers; Figure 6 depicts Raman spectra of GO and graphene nanospheres;

Figure 7 depicts XRD patterns of graphite, GO and graphene nanospheres; Figure 8 depicts the (a) C MAS NMR spectrum of GO and (b) ^zySi MAS NMR spectrum of graphene nano-spheres;

Figure 9 depicts the FT IR of GO and graphene nano-spheres;

Figure 10 depicts UV- Visible spectra of GO and graphene nanospheres; Figure 11 depicts BET N₂ adsorption-desorption isotherms of graphene nanospheres

Figure 12 depicts confinement of magnetic material (a) graphene nanospheres (b) graphene nanospheres after confining magnetic material; and

Figure 13 depicts I-V graph of reduced graphene oxide and graphene nanospheres

Figure 14 depicts Photothermal behavior of graphene nanospheres, mesoporous silica and phosphate buffer solution (PBS) by using 808 nm NIR laser source with 0.5 W/cm² power density

DETAILED DESCRIPTION OF THE INVENTION The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

In view of the above, the present invention provides a new one pot scheme to synthesize well-controlled, ordered graphene nanospheres based on self-assembly of metal/ metalloid oxide, surfactant, and GO.

In an embodiment, the present invention provides a one pot method for the synthesis of graphene nano-spheres comprising the steps of: i. preparing GO sheets (graphene oxide); and i. converting 2D graphene sheets into 3D graphene nano-spheres to obtain graphene nano-spheres. Accordingly the method for the preparation of GO sheets from natural graphite powder comprises the following steps of: i. Adding graphite and sodium nitrate to the reaction vessel to obtain a mixture; ii. Adding cone. H₂S0₄ slowly to the mixture of step (i) with stirring to obtain a mixture; iii. Stirring the mixture of step (ii) followed by addition of KMn0₄ at a temperature below 10°C to obtain a mixture; iv. Stirring the mixture of step (iii) followed by addition of KMn0₄ at a temperature below 20°C to obtain a reaction mixture; v. Stirring the mixture of step (iv) followed by addition of water which increases the temperature and gives a diluted suspension; vi. Stirring the diluted suspension of step (v) followed by treatment with H₂0 and water to obtain a warm bright yellow suspension which on filtration gave a yellow-brown filter cake; and vii. Washing the cake of step (vi) with warm aq. HC1 followed by exfoliation of product by sonication in water gave GO nano sheets which were separated by centrifugation and drying.

Accordingly, graphite oxide is prepared by the modified Hummers method as presented by Wang et al (Nanoscale Res Lett 2011, 6:8). The method is based on reacting natural graphite powder and sodium nitrate at 0 °C followed by slow addition of concentrated H₂S0₄ with stirring below 5 °C. After stirring the reaction mixture for 30 min, 0.3 g KMn0₄ is added in small portions at temperatures below 10°C. After an interval of 30 min, 7 g KMn0₄ is added to the mixture over 1 h below 20 °C. On increasing the temperature of the mixture to 35 °C and stirring for 2 h, 90 mL of water is slowly dripped into the reaction mixture (paste form), causing an increase in temperature to 70 °C and the diluted suspension is stirred at this temperature for about 15 min. Then, it is further treated with a mixture of H₂0₂ and water to afford a bright yellow suspension, which is filtered, resulting in a yellow- brown filtered cake. The cake is washed thrice with warm solution of 3% aqueous HC1. Finally, GO is exfoliated to generate GO nanosheets by sonication in water for 1 h followed by separation by centrifugation and drying in vaccum oven.

In a preferred embodiment, the present invention provides a one pot, self-assembled method for producing individual and dispersible graphene nano-spheres comprising the following steps of: a. sonicating graphite oxide sheets followed by suspending in an aqueous solution containing a cationic surfactant and an alkaline solution to obtain a reaction mixture; b. ultrasonically treating the reaction mixture of step (a); c. stirring the treated mixture of step (b) followed by addition of an inorganic source with organic substitution slowly; d. washing the mixture of step (c) with an organic solvent followed by separation and drying and to afford functionalized graphene nano-spheres; and e. treating graphene nano-spheres synthesized in step (d) with mild acid solution to afford graphene nano-spheres.

According to the preferred embodiment, the method for the conversion of 2D graphene sheets into 3D graphene nano-spheres provided by the instant method involves (a) size stabilization of small GO flakes (b) modification of zeta potential of the GO surface using a cationic surfactant and (c) functionalization of an inorganic oligomer.

The application of sonication to graphene sheets results in the size stabilization of graphene to provide graphene flakes. Stabilization is achieved through electrostatic interaction between negatively charged graphene oxide and a cationic surfactant. Ultrasonication of the graphene flakes in alkaline media containing cationic surfactant results in formation of micelles, and gets absorbed on negative surface of GO by electrostatic interaction. Decrease in zeta potential of GO surface after surfactant absorption, provides binding sites for an inorganic oligomer. This entire one pot method is conducted at 40°C. After functionalization, the graphene nano-spheres are subjected to washing with an organic solvent, mainly ethanol and are then separated and dried followed by treating it with a mild acid. Separation of the graphene nano-spheres is carried out by centrifuagation at 9200 rpm for 5 min with 3-4 times repetitive washes with warm ethanol and later subjected to drying at room temperature. The spheres are treated with mild acid (dil. HC1) to get 3D graphene spheres. In an embodiment, the present invention provides synthesis of graphene nano-spheres with inorganic exteriors wherein the inorganic layer is selected from oxides of transition metals or metalloids and other elements selected from the group consisting of Co, Ga, Ge, Hf, Fe, Ni, Nb, Mo, La, Zr, Ti, V, Cr, Mn, Cu, Zn, Sc, Si, Al, Re, Ta, W and Y. Further, in accordance with the preferred embodiment, the process for synthesis of graphene nano-spheres employed in the present invention involves the mechanism for synthesis of graphene nano-spheres comprising the steps of: a. Stripping of graphene sheets; b. Stitching of functionalized graphene sheets, and c. Progressive formation of graphene nanosphere.

Stripping of graphene sheets: Treatment of graphene in the basic conditions employed modifies the surface chemistry of carbon and increases functional groups on carbon materials, however mild treatment of GO under basic condition demonstrates the reduction of GO with a loss of certain functional groups and concomitant partial restoration of graphene hexagonal structure. 6 h sonicated GO sheets, comprising a 3 to 4 layered structure (Fig. la). In basic conditions, when inorganic monomeric/oligomeric species start to get functionalized on the surface of GO, it undergoes 'peel-out' mode exfoliation due to changes in the coordination of carbon on increasing functionalization with substituted inorganic species. According to time dependent observation using TEM, as shown in fig lb to le the inorganic oligomer works as a stripper agent and initiates stripping of sheets. The stripped GO sheet surface is more exposed for progressive functionalization that bends it to convex GO sheet with inorganic coatings (fig If). These observations are systematically recorded in TEM studies of samples recovered from different reaction times. This mechanism yields bent functionalized graphene patches is substantiated through density functional theory (DFT) based quantum mechanical simulations. DFT studies support the mechanism and provide vital clues for the formation of the nanospheres.

Stitching of functionalized graphene sheets and formation of graphene nanospheres Due to exposure to heavy energy during ultrasonication various carbon atoms from the centre as well as the edges of the graphene sheets are ruptured. As the sides of the sheets are functionalized with the inorganic oligomers, the reactivity of the edge carbons is relatively higher. These exfoliated and suitable sized graphene patches, with reactive edges are systematically stitched together to form larger patches (Fig. 2a to 2d) that are further tailored to a full sphere (Fig. 2e). Once the edges are sealed to form sphere, no further modification occurs. This unique mechanism leads to synthesize uniform, stable, well controlled nanospheres whose size is tunable from 200 nm to 2500 nm. Several crucial steps in the stripping and stitching mechanism have been simulated and also supported by DFT calculations.

In an optional embodiment, the present invention provides a one pot, self-assembled method for producing, individual and dispersible graphene nano-spheres encapsulated with magnetic nano-particles comprising the following steps of: a. sonicating graphite oxide sheets followed by suspending in an aqueous solution containing a cationic surfactant and an alkaline solution to obtain a reaction mixture; b. ultrasonically treating the reaction mixture of step (a); c. stirring the treated mixture of step (b) followed by addition of an inorganic source with organic substitution slowly and magnetic nanoparticles, respectively; d. washing the mixture of step (c) with an organic solvent followed by separation and drying and to afford functionalized graphene nano-spheres; and e. treating graphene nano-spheres synthesized in step (d) with mild acid solution to afford graphene nano-spheres.

The magnetic nanoparticles employed for encapsulation are selected from the group consisting of Fe₃0_4j Sn0₂.

The concentration of the magnetic nanoparticles is in the range of 20 to 25 mg.

This route needs no template to make the core as the spheres are formed through self- assembly of functionalized graphene patches. The inorganic exterior of graphene nano- spheres provides better sites for functionalization. In addition, there is an option to have a material that poses with unique isolation of highly conducting graphene with poorly conducting inorganic exterior that separates it from other such spheres. Such materials on further modifications can be more promising with high potential to be used in nano- electronics.

The synthesis is a simple one pot , aqueous solution approach at 40 °C, a near ambient condition without using any template. The method attracts the large scale production with the use of aqueous medium, which is non-toxic and environmental friendly.

GO, Graphene nano-sphere materials synthesized in the present invention was subjected to several characterization techniques. Surface morphology of these materials was examined by transmission electron microscope (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Further techniques such as electron microscopy-EDX, powder XRD, Raman, IR, NMR, UV-visible spectroscopies have been used to understand the finer details of the structure and other aspects.

In another preferred embodiment, the present invention provides that the said graphene nano-spheres comprise of an exterior inorganic layer over at least one organic layer of carbon, most preferably graphene, wherein the layers are either continuous or discontinuous. The inorganic layer comprises of oxides of metals, metalloids or other elements selected from the group consisting of Co, Ga, Ge, Hf, Fe, Ni, Nb, Mo, La, Zr, Ti, V, Cr, Mn, Cu, Zn, Sc, Si, Al, Re, Ta, W and Y. FT IR of GO and graphene nano-spheres were compared. The most characteristic features in the FT-IR spectrum of GO are the absorption bands corresponding to the O-H stretching at 3409 cm^"1 and C=0 carbonyl stretching at 1730 cm^"1. The band at 1625 cm^"1 is assigned to the vibration of the adsorbed water molecules as well as to the contribution from the skeletal vibration of un-oxidized graphitic domains.Graphene nanospheres showed two important peaks in FT-IR spectra. The band at 1218 cnf¹ attributed to the Si-O-C asymmetric stretching appeared, while the typical carbonyl group band at 1730 cm^-1 disappeared. This proved that the carbonyl groups were converted to Si- -C bands, which have been reported in literature. The IR bands due to Si-O-Si and Si-OH framework stretching vibrations were obtained in the region of 1078, 940 and 800 cm^"1.

In yet another embodiment, the present invention provides graphene nano-spheres having electrical conductivity of the said spheres are in the range of 10^"J S/m to 10^"z S/m.

EXAMPLES

The detailed description of the inventions is explained with following examples but these should not be construed to limit the invention

Example 1:

Synthesis of Graphene nano-spheres a. Preparation of GO: Graphite (2g 500 mesh) and sodium nitrate (lg) were added to a 250 mL flask at 0 °C. Concentrated H₂S0₄ (50 mL) was added slowly with stirring below 5 °C. The mixture was stirred for 30 min and 0.3 g of KMn0₄ was added in small portions below 10 °C. The reaction mixture was stirred for an additional 30 min and 7 g of KMn0₄ was added to the mixture respectively over 1 h below 20 °C. After the temperature of the mixture was increased to 35 °C and stirred for 2 h, 90 mL of water was slowly dripped into the paste, causing an increase in temperature to 70 °C and the diluted suspension was stirred at this temperature for about 15 min. Then, it was further treated with a mixture of H₂0₂ (30%, 7 mL) and water (55 mL). The resulting suspension turned bright yellow, and the warm suspension (about 40 °C) was filtered, resulting in a yellow-brown filter cake. The cake was washed three times with a warm solution of 3% aqueous HC1 (150 mL). Finally, GO was exfoliated to generate GO nano-sheets by sonication in water for 1 h. Finally, GO was separated by centrifugation and was dried in vacuum oven at 40 °C for 24 h. Reduction of GO by oxalic acid, partially reduced GO surfaces and resulted in an enhanced electrical conductivity in the range of 10^" S/m. b. Conversion of 2D functionalised graphene sheet in to 3D graphene nano-spheres:

GO based nano spheres were prepared firstly via stabilization through electrostatic interaction between negatively charged graphene oxide and a cationic surfactant followed by functionalization of inorganic oligomer. In a typical experiment, as-synthesized 15 mg GO after 6 h sonication was firstly suspended in an aqueous solution (240 ml) containing a cationic surfactant such as CTAB (0.5 g) and NaOH (20 mg), and then ultrasonically treated for 3 h. After stirring for 2 h at 40 °C, an inorganic source with organic substitution such as tetraethylorthosilicate (TEOS) 2.5 ml was slowly added to the above mixture. After reaction for 12 h, the desired product, nano-spheres were obtained by centrifugation at 9200 rpm for 5 min with 3-4 times repetitive washing with warm ethanol, separation and drying at room temperature. Example 2:

Characterization of 3D graphene nano-spheres

GO, graphene nano-sphere materials synthesized in the present invention were subjected to several characterization techniques. Surface morphology of these materials was examined by transmission electron microscope (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). TEM was performed in the instrument Tecnai (Model F20) that was operated at 300kV. The samples were loaded on carbon coated copper TEM grids. Scanning electron microsopy (SEM) analysis was done with FEi instrument model quanta 200 3D. The samples in suspension were dried on Si wafers prior to the analysis at a temperature of 35 °C. Fig 3 depicts TEM and SEM images of graphene nano-spheres.

EDX analysis was also carried out in both microscopic techniques. Fig 5 depicts (a) SEM-EDX and (b) TEM-EDX of graphene nanospheres. AFM images were obtained from XE-100 atomic force microscope of PSIA on tapping mode. The samples for AFM measurements were prepared by ultrasonic (Equitron ultrasonic cleaner W/Htr 03.0L, watts-75) treatment of GO (in water) and Graphene nanospheres (in Methanol) dispersions of 0.25 mg ml^'1, respectively. The samples were prepared on clean silicon wafer surfaces. Fig 4 depicts AFM tapping mode images of (a) GO and (b) graphene nanospheres.

FT-IR spectra were recorded by using Perkin-Elmer FT-IR spectrometer in the range 500-4000 cm^"1 using KBr pellet. Fig 9 depicts the FT IR of GO and graphene nano-spheres. Raman Spectra were recorded from 300 to 3000 cm^"1 on micro Raman spectrometer

LabRAM Horiba JY HR-800using a 632.8 nm He-Ne laser beam. The dry samples were loaded on glass slide. Fig 6 depicts Raman spectra of GO and graphene nanospheres

Powder x-ray diffraction (XRD) pattern were recorded on a PANalyticalX'pert Pro dual goniometric diffractometer using reflection geometry and Cu Ka radiation (wavelength λ = 0.154 nm). Fig 7 depicts XRD patterns of graphite, GO and graphene nanospheres

UV- visible spectra were obtained from UV-visible spectrometer of Agilent/Varian CARY 50 UV-Vis spectrophotometer. Fig 10 depicts UV- Visible spectra of GO and graphene nanospheres

¹³C MAS NMR and ²⁹Si MAS NMR spectra were recorded on a Bruker ASX-300 solid state NMR spectrometer operating at 300 MHz frequency with spinning rate of 8 KHz. Fig 8 depicts the (a) ^1JC MAS NMR spectrum of GO and (b) ^ySi MAS NMR spectrum of graphene nano-spheres.

The N₂ adsorption-desorption isotherms were recorded using Quanta chrome Quadra Win instrument Version 5. 2, at liquid nitrogen temperature. Before the measurements, the samples were degassed at 100 °C for 3 h. The specific surface area was calculated using the multiple-point Brunauer-Emmett-Teller (BET) method. Pore volume was estimated from the uptake of adsorbate at a relative pressure of P/Po_. = 0.99. Barreet-Joyner-Halenda (BJH) analyses were used to calculate the surface area, pore size and pore volume, respectively. Fig 11 depicts BET N₂ adsorption-desorption isotherms of graphene nanospheres

To evaluate the photothermal behavior of graphene nanospheres, Aliquots (200 μί) were deposited in to wells of a 96 well cell culture plate. The initial temperature was 37 °C. Wells were illuminated with an 808 ± 5 nm continuous wave NIR laser source with power density,0.5 W/cm² (SDL-808-LM-1000T, Shanghai Dream Laser Technology Co., Ltd.). The NIR exposure time duration was 0-3 min. Fig: 14 depicts Photothermal behavior of graphene nanospheres, mesoporous silica and phosphate buffer solution (PBS).

Example 3:

Stripping of Graphene sheets:

When GO is made from the graphitic structure using the method mentioned, a size reduction of the sheets were observed which is believed to be reversible. A cationic surfactant was used to stabilize the size of the fine structures. GO is known as a pseudo-two-dimensional material, which contains -COH groups in the interlayer space and -COOH groups in the layer edges. In presence of appropriate alkaline condition most of the protons dissociate from the -COH and -COOH groups on GO, leaving a naked carbon layer skeleton with negative charges. In alkaline media the cationic surfactant forms micelles, get absorbed on the negative surface of GO by electrostatic interaction. Decrease in zeta potential of GO surface after surfactant absorption, provides binding site for upcoming inorganic oligomer such as hydrolyzed TEOS. Although the treatment in basic conditions is usually employed to modify the surface chemistry of carbon and to increase the population of functional groups on carbon materials, the reports on the mild treatment of GO under basic condition demonstrates the reduction of GO with a loss of certain functional groups and concomitant partial restoration of the graphene hexagonal structure. An optimal initial size of graphene flake is observed to be influencing the graphene to graphene nanosphere transformation. We use 6 h sonicated GO sheets, which contain 3 to 4 layered structure (fig. la). In basic condition, when the inorganic monomeric/oligomeric species starts to get functionalized on the surface of GO, it undergoes 'peel-out' mode exfoliation due to changes in the coordination of carbon on increasing functionalization with substituted inorganic species. According to a time dependent observation using TEM, as shown in fig lb to le inorganic oligomer works as a stripper agent and it starts the stripping of sheets. The stripped GO sheet surface is more exposed for the progressive functionalization that bends it to convex GO sheet with inorganic coatings (fig If). These observations are systematically recorded during TEM studies of samples recovered from different reaction times. This mechanism that yields bent functionalized graphene patches has also been substantiated through density functional theory (DFT) based quantum mechanical simulations. The DFT studies not only significantly support the mechanism but also provide vital clues for the formation of the nanospheres.

Example 4:

Stitching of functionalized graphene sheets and formation of graphene nanospheres

Due to exposure to heavy energy during ultrasonication that ruptures various carbon atoms from the centre as well as the edges of the graphene sheets. As the sides of the sheets are functionalized with the inorganic oligomers, the reactivity of the edge carbons is relatively higher. These exfoliated and suitable sized graphene patches, with reactive edges are systematically stitched together to form larger patches (Fig. 2a to 2d) that are further tailored to a full sphere (Fig. 2e). Once the edges are sealed to form sphere, no further modification takes place. This unique mechanism leads to synthesize uniform, stable, well controlled nanospheres whose size is tunable from 200 nm to 2500 nm. Several crucial steps in the stripping and stitching mechanism has been simulated and also supported by DFT calculations.

The corresponding SEM and TEM images as shown in Fig. 3, respectively provides the size ranges and remarkably exhibit the features of hallow spheres through the obvious contrast between the dark edges and the pale centers, as is reported for other hollow particles with a central cavity in the literature.

Color change of GO dispersion from clear brown to dark grey after TEOS addition can be attributed to the nanospheres formation. Raman spectroscopy is always a powerful way to characterize carbon materials. Fig. 6 is the typical Raman spectra of GO and graphene nano-spheres. For GO, two intensive peaks at 1327 and 1580 cm^"1 which refer to the D band, representing a disordered sp carbon structure, and G band resulting from sp ordered crystalline graphite-like structure, respectively. After functionalization with silica, though the peak position does not shift, the relative intensities of the D and G bands undergo a considerable transformation. Increase in D-band intensity suggests an increase of defects in the larger layers to smaller ones due to the treatment.

The powder XRD pattern is a strong proof for the resulting exfoliated state from the graphitic stacked state due to the stripping process with the help of inorganic oligomer. As shown in Fig. 7 the XRD pattern of GO shows a sharp peak for 002 plane at 10.76° with an associated d-spacing of 8.21 A. Due to functionalization, as indicated in the XRD patterns, the 002 plane gradually vanishes as the GO sheets are being exfoliated from the graphitic stacks. XRD pattern of graphene nano-spheres show a hump at 26°, which reveals that the resultant graphene nano-spheres do not remain in the form of stacked crystalline plane of GO any more.

Fig. 8 shows the ¹³C MASNMR and ²⁹Si MASNMR spectra of GO and graphene nano-spheres. The C peaks of GO at 62, 73, and 138 ppm can be ascribed to 1, 2-epoxy C- O-C, alcoholic C-OH, and aromatic C=C species, respectively, which is in agreement with the structural proposal that the aromatic rings are part of the GO structure. The peak at 168 ppm is assigned to the carbonyl C=0 species existing on GO. ²⁹Si MAS NMR spectra of graphene nano-spheres show signals at -90 to -130 ppm due to cross-linked Si-O-Si bonds. A deconvulation process shows the main component to be Q⁴ (without terminal -OH) at -113 ppm and Q³ (one terminal -OH) at -102 ppm. Functionalization leads to the detachment of large number of -OH groups and greatly increases polymerization of silica particles as shown by an increased Q⁴to Q³ ratio. In Fig. 9 the FT IR of GO and graphene nano-spheres was compared. The most characteristic features in the FT-IR spectrum of GO are the absorption bands corresponding to the O-H stretching at 3409 cm^"1 and C=0 carbonyl stretching at 1730 cm^"1. The band at 1625 cm^"1 is assigned to the vibration of the adsorbed water molecules as well as to the contribution from the skeletal vibration of un-oxidized graphitic domainsGraphene nanospheres showed two important peaks in FT-IR spectra. The band at 1218 cm^""1 attributed to the Si-O-C asymmetric stretching appeared, while the typical carbonyl group band at 1730 cm^-1 disappeared. This proved that the carbonyl groups were converted to Si-O-C bands, which have been reported in literature. The IR bands due to Si-O-Si and Si-OH framework stretching vibrations were obtained in the region of 1078, 940 and 800 cm^"1. The UV-VIS spectra fig. 10, of GO exhibits a maximum absorption peak at about 230 nm, corresponding to π-π* transition of aromatic C-C bonds. The absorption peak for graphene nanospheres has red shifted to 245 nm. Generally the phenomenon of red shift has been used as a monitoring tool for reduction of GO, here it suggest that there is partial reduction of surface groups of GO due to bonding of silica to the graphene sheet. Fig. 11 shows the nitrogen adsorption- desorption isotherm of graphene nanospheres. It shows isotherm behavior near to the isotherm type IV. Graphene nanospheres show the BET surface area 438 m /g, pore volume 0.848 cm³/g, and pore diameter 39.52 A. Example 5:

Encapsulation of nano materials:

The synthesized system has capability to confine liquid and/or solid materials leading to application in electronics and biomedical techniques. In the primary stage to prove the concept of encapsulation, Fe₃0₄ nanoparticles were encapsulated in the graphene nanospheres synthesized by the instant process. The magnetic oxide nanoparticles were introduced before 3D conversion from 2D graphene sheets and subsequent characterizations suggested successful confinement of magnetic nanoparticles within the graphene spheres . The encapsulation was , understood by the magnetic behavior acquired as demonstrated in Fig. 12, where sample labeled A is the nanosphere as synthesized and the sample labeled B is the nanosphere containing the encapsulated Fe₃04 nanoparticles. Introduction of a magnet at the surface of the container containing nano-spheres synthesized by the instant process encapsulated with Fe₃0₄ nanoparticles resulted in attraction of the said spheres towards the magnet, thereby indicating the magnetic property of the magnetic oxide encapsulated graphene nano-spheres.

Example 6:

Electrically isolated graphene nano-spheres:

The graphene nano-spheres synthesized without washing with mineral acid, the conductive graphene layers are covered with insulating inorganic oxide. The conductivity measurement performed on this sample proved these materials to be almost non-conductors. In Fig. 13 conducting behaviors of the reduced GO, the inorganic oxide coated nanosphere are presented. Due to coating of the inorganic oxide which is a poor conductor, the conducting reduced GO exhibits poor conduction in the range of 10^"9 S/m. Thus, with the ordered packing of conducting and non-conducting layers alternatively in a nanoscopic scale, the material on further exploitation shows a great potential in the field of supercapacitor and nano-electronics.

Example: 7 Photothermal behavior of graphene nanospheres

Fig. 14 shows the photothermal behavior of graphene nanospheres. To check the photothermal behavior of graphene nanospheres, 0.05 to 0.5 mg/mL concentration have been used. The temperature was increasing with extending the exposure time and we achieved the hyperthermia temperature (43 °C) within 2 min. This result demonstrated that designed graphene nanospheres are excellent material for photothermal therapy, hyperthermia at low concentration (0.05 to 0.5 mg/mL) in low exposure time and low power density of NIR laser source. Advantages of invention: a. One pot method. b. Near ambient synthesis conditions. c. Direct conversion of 2D to 3D graphene structure. d. No requirement of hard templates. e. Uniform and better size control of the nanospheres (without hard template). f. Size tunability between the size range of 200 nm to 2500 nm g. Ideal for large scale production due to (a) to (e) h. Graphene nanospheres are an ideal model for advanced energy storage materials, drug delivery carries and bio imaging. i. Encapsulation of liquid and solid materials: The synthesized system has capability to confine liquid and /or solid materials leading to application in electronics and biomedical techniques. j. Electrically isolated graphene nano-spheres: Great potential in the field of supercapacitor and nano-electronics. k. Photothermal behavior: graphene nanospheres are excellent material for photothermal therapy and hyperthermia at low concentration (0.05 to 0.5 mg/mL) in low exposure time and low power density of NIR laser source.

Claims

A one pot, self-assembled process for producing individual and dispersible graphene nano-spheres at near ambient conditions comprising: a. sonicating graphite oxide sheets followed by suspending it in an aqueous solution containing a cationic surfactant and an alkaline solution to obtain a reaction mixture; b. ultrasonically treating the reaction mixture of step (a); c. stirring the treated mixture of step (b) followed by addition of an inorganic source with organic substitution slowly; and d. washing the mixture of step (c) with an organic solvent followed by separation and drying followed by treating with mild acid solution to afford graphene nano- spheres.

The process according to claim 1 , wherein the cationic surfactant is selected from the group consisting of CTAB, Cetyl trimethyl ammonium bromide (CTAC), Cetylpyridinium chloride (CPC).

The process according to claim 1, wherein the alkaline solution is selected from the group consisting of NaOH, KOH.

The process according to claim 1, wherein the inorganic source with organic substitution is an alkoxide of transition metals, metalloids and other elements.

The process according to claim 4, wherein the alkoxide of transition metals, metalloids and other elements is selected from the group consisting of Co, Ga, Ge, Hf, Fe, Ni, Nb, Mo, La, Zr, Ti, V, Cr, Mn, Cu, Zn, Sc, Si, Al, Re, Ta, W and Y.

The process according to claim 1, wherein the process is carried out at 40°C. The process according to claim 1, wherein the said graphene nano-spheres synthesized are in the range of 200 nm to 2500nm.

The process according to claims 1 to 7, wherein the said graphene nano-spheres comprise of an exterior inorganic layer over atleast one organic layer of graphene and the layers are either continuous or discontinuous.

The process according to claim 8, wherein the said spheres are optionally encapsulated with magnetic nanoparticles.

The process according to claim 9, wherein the process for preparation of individual and dispersible graphene nano-spheres encapsulated with magnetic nano-particles selected from the group consisting of Fe₃0₄, Sn0₂ comprising the following steps of: a. sonicating graphite oxide sheets followed by suspending in an aqueous solution containing a cationic surfactant and an alkaline solution to obtain a reaction mixture; b. ultrasonically treating the reaction mixture of step (a); c. stirring the treated mixture of step (b) followed by addition of an inorganic source with organic substitution slowly and magnetic nanoparticles; d. washing the mixture of step (c) with an organic solvent followed by separation and drying and to afford functionalized graphene nano-spheres; and e. treating graphene nano-spheres synthesized in step (d) with mild acid solution to afford graphene nano-spheres.