WO2017182860A1

WO2017182860A1 - Preparation method of nanomaterials and their electrochemical properties

Info

Publication number: WO2017182860A1
Application number: PCT/IB2016/057877
Authority: WO
Inventors: Maria Cristina Fialho Oliveira; Andreia Daniela ROSA VELOSO; Pedro Manuel DE MELO BANDEIRA TAVARES; José Ramiro AFONSO FERNANDES; Romeu António VIDEIRA
Original assignee: Universidade de Tras os Montes e Alto Douro
Current assignee: Universidade de Tras os Montes e Alto Douro
Priority date: 2016-04-20
Filing date: 2016-12-21
Publication date: 2017-10-26
Anticipated expiration: 2018-10-20
Also published as: PT109335A

Abstract

The present application describes a method of preparation of nanomaterials and their electrochemical properties. The present application describes a method for producing simultaneously in an aqueous medium two distinct populations of nanomaterials, for example based on graphene, with capacitive properties, electrolytic properties and different and adjustable redox properties, without the need for separation processes and chemical modification after synthesis. The method underlying this technology enables the preparation of nanomaterials with a broad perspective of applications, including: biosensors, systems for storing and conversion of energy, photocatalysis, fluorescence imaging and novel therapeutic and diagnostic systems, commonly designated as nanomedicine.

Description

DESCRIPTION

"PREPARATION METHOD OF NANOMATERIALS AND THEIR

ELECTROCHEMICAL PROPERTIES"

Technical field

The present application describes a method of preparation of nanomaterials and their electrochemical properties.

Background

The graphene-based carbon materials consist essentially in nanotubes, quantum dots, flakes, nanoribbons and sheets, depending on size and/or shape of the graphene sheet. Herein, the smallest dimensional size materials among the graphene-based carbon materials are referred as graphene- based carbon nanomaterials. This group includes graphene quantum dots and amorphous carbon which comprises small disordered domains of sp² graphitic carbon.

The quantum dots of graphene show a good biocompatibility, high water solubility and low toxicity, which makes them a good platform for biomedical applications [1-2] . Their chemical and physical properties are dictated by the size, morphology/thickness of the graphene fragments, extension of sp² carbon hybridization and nature of functional groups bonded to carbon atoms, features which are tuned by the method of synthesis [3-4] .

Among the different synthetic methodologies that have been developed for the production of graphene-based carbon nanomaterials, the electrochemical methods have several advantages including low cost and simplicity of the operation process, enabling its scale-up without significant environmental impacts unlike other methods that require aggressive oxidizing agents, corrosive acids or elevated temperatures [5-10] . Additionally, graphene-based carbon nanomaterials synthesized by this methodology show great potential for biological and biomedical applications since their synthesis occurs in aqueous medium with biocompatible electrolytes and using nontoxic electrodes material .

To date, the electrochemical production of graphene-based carbon nanomaterials in aqueous medium is known to occur solely in one of the electrodes of the electrochemical cell, typically at the anode, and requires separation steps in the post-synthesis, such as centrifugation, filtration and/or extraction, before its use, reducing the efficiency and increasing the cost of the overall process.

In addition to these drawbacks, the graphene-based carbon nanomaterial formed at the anode of the cell are unavoidably subjected to reduction reaction at the cathode, leading to the formation of heterogeneous populations in terms of their functionalization and oxidation states.

The literature survey shows that graphene flakes or graphene sheets can be produced simultaneously and separately at the anode and cathode of an electrochemical cell [11,12], However, such processes require the use of protic ionic liquids, for example l-butyl-3-methyl imadazolium bisulfate, BMIMHS04 [11] or aprotic ionic liquids containing cations and anions able to intercalate between the layers of graphite of the cathode and anode, respectively, leading to the expansion and separation of these layers [12] . The major drawback of the use of ionic liquids is related to their toxicity, their high cost and the need for separation and purification steps after synthesis .

To date, the simultaneous production in aqueous medium, at the cathode and anode, of graphene-based carbon nanomaterials like graphene quantum dots has ever been reported.

Typically, all graphene-based carbon nanomaterials have photoluminescence properties and, in some cases, photo- induced redox properties [13, 14]. However, little is known about their intrinsic redox behavior. Although these materials may be prepared in different oxidation states [14, 15], their redox behavior, including their electrode activity, remains to be characterized, including their potential for reduction or/and oxidation. Additionally, it also remains to be invented a synthetic methodology of the graphene-based carbon nanomaterials that allows tuning their redox properties.

Summary

The present application describes a method of preparation of nanomaterials comprising the following steps:

- selection of, at least, one source of carbon;

- selection of the electrolytic aqueous solution;

- saturating the aqueous electrolytic solution with a gas;

- electrochemical polarization of the carbon source which occurs simultaneously in both electrodes of the electrochemical cell;

- removal of the nanomaterials produced simultaneously in the anodic and cathodic compartments, which are themselves separated by an inert porous barrier. In one embodiment, the source of carbon nanomaterials used in the preparation method is graphite, pyrolytic graphite, carbon foam, carbon felt, carbon fibers, carbon black, graphene and/or carbon nanotubes .

In another embodiment, the electrolytic aqueous solution used in the nanomaterials preparation method is ultrapure water and an electrolyte agent which is individually acid, base or salt or a combination thereof.

In yet another embodiment, the electrolytic agent used in the nanomaterials preparation method is an inorganic acid or an organic acid or an amino acid or an inorganic base or a salt, or combinations thereof.

In one embodiment, the gas used in saturation of the electrolytic solution in the nanomaterials preparation method is nitrogen, argon, helium, carbon dioxide, oxygen, sulfur dioxide, individually or in mixtures.

In another embodiment, the polarization of the electrodes on the nanomaterial preparation method is performed at a constant current.

In another embodiment, the polarization of the electrodes on the nanomaterial preparation method is performed at a constant potential.

In one embodiment, the polarization of the electrodes on the nanomaterials preparation method comprises the following steps:

- to use as positive electrode a brand new electrode or an electrode that has been previously used as a negative electrode; - to use as negative electrode a brand new electrode or an electrode that has been previously used as a positive electrode;

- the electrodes are polarized for a short period of time;

- to remove and reject the aqueous electrolyte from the two compartments of the cell;

- the compartments of the cell are fulfilled with the electrolytic solution;

- Polarization of the electrodes.

In another embodiment, the preparation method further comprises the step of saturation with a gas before returning to the polarization of the electrodes.

In another embodiment, the polarization of the electrodes in the nanomaterial preparation method is performed during 500-7200 seconds.

The present application further describes nanomaterials obtained by the preparation method described above.

Brief description

The present application describes a method of producing simultaneously in an aqueous medium two distinct populations of nanomaterials, for example based on graphene, without the need of separation and chemical modifications steps after the synthesis process. These populations are distinguished by the nature and degree of functionalization, oxidation state and redox properties shown in aqueous medium. The synthesized materials reveal an electrolyte behavior in aqueous media. Another electrochemical property shown by these materials is their ability to dictate the capacitive properties of the electrode/solution interface. The underlying method comprises an electrochemical process that occurs in several stages, with the following requirements: use of a carbon material in both positive (anode) and negative (cathode) electrodes; use of an electrolytic aqueous solution; use of an electrochemical cell with at least two separate compartments; pre-polarization of the electrodes and removal of the resulting electrolytic solution; use of a gas with known composition to regulate the gas atmosphere inside of the electrochemical cell and the electrolyte solution; re-polarization of the electrodes for obtaining two populations of nanomaterials based on graphene; removal of the solutions containing the nanomaterials, from each compartment. The obtained nanomaterials may be promptly used after synthesis, but may also be submitted to subsequent operations like physical processes such as dialysis, filtration, deposition, or chemical processes, for example, by binding to biological molecules, metal, regarding specific applications.

The methodology underlying this technology has the advantage of allowing tuning qualitatively and quantitatively the functionalization of the carbon nanomaterial produced. By changing the electrolyte composition and/or the gas atmosphere of the electrochemical cell and/or the applied current/potential conditions, it allows obtaining two populations of nanomaterials with predictable redox properties, with capacitive properties in aqueous medium, and electrolytic properties. The graphene-based nanoparticles obtained by this method are always fluorescent, and may display a high fluorescence quantum yield. In general, the fluorescent properties of the nanomaterials derive from their quantum size and/or surface defects. The electrochemical exfoliation is a method which creates defects along the scission of the carbon-carbon bonds in multiple sites of the graphene network, producing many sp³ carbons and originating several oxygen functional groups . Concomitantly, it also creates particles with very small dimensions. As a result, the nanomaterials obtained by electroexfoliation are always fluorescent. The methodology underlying this technology enables a broad perspective of applications, including: biosensors, systems for storing and conversion of energy, photocatalysis, fluorescence imaging and novel therapeutic and diagnostic systems, commonly designated as nanomedicine .

This technology enables the production of graphene based carbon nanomaterials in aqueous medium, in both anode and cathode sides of an electrochemical cell under mild conditions, without the need of further processing steps. Throughout this patent application, mild conditions are considered to occur whenever it is not required oxidizing or reducing agents, such as nitric acid, sulfuric acid, hydrazine, borohydride, among others. This new synthetic strategy proves to be an important tool for synthesizing in one single step, two populations of graphene-based nanomaterials that display different redox properties, and concomitantly capacitive and electrolytic (ionic) properties in aqueous medium. Each population targets a specific application.

Compared to other electrochemical methods previously described in the literature [16-18], which are also applicable to the preparation of nanomaterials in aqueous medium, this technology presents many advantages:

- it allows obtaining in one step, and separately, two distinct populations of fluorescent nanomaterials, each one showing a high uniformity in shape, size, functionalization and redox properties;

- it allows obtaining carbon nanomaterials based on doped graphene, for example, doped with nitrogen;

- it allows manipulating the chemical composition of the electrolyte and/or the composition of the atmosphere of the electrochemical cell and/or electrochemical polarization conditions, namely the potential, current intensity and duration, in order to obtain populations of carbon nanomaterials with predictable redox properties: oxidizing and/or reducing properties;

- it provides carbon nanomaterials with high fluorescence quantum yield;

it provides carbon nanomaterials with electrolytic properties;

it provides carbon nanomaterials with capacitive properties in aqueous media.

Compared to non-electrochemical methods of production of graphene-based carbon nanomaterials, like laser ablation [19], hydrothermal method [20,21], carbohydrates dehydration in sulphuric acid [22], the present technology also presents numerous advantages:

- it is a faster method, simpler, with no negative environmental impacts as it does not require oxidizing or reducing agents, corrosive acids and other chemical substances with toxicological potential to oxidize, reduce or dope the nanomaterial;

- it is an easy method to scale up;

- it is an economically favorable methodology since its implementation does not use costly equipment and infrastructure, being able to use the equipment/infrastructure that already exists. Brief description of drawings

For an easier understanding of the technique, figures in attachment are included representing preferred embodiments. However, these are not intended to limit the scope of this application.

Figure 1 shows a schematic representation of the electrochemical cell (a) for the production of carbon nanomaterials based on graphene displaying different redox properties, as evidenced by cyclic voltammetry (b, c, d, e) .

Figure 2 shows images of High Resolution Transmission Electron Microscopy (HRTEM) of nanomaterials prepared electrochemically at the graphite cathode and anode, in a phosphate buffer solution (pH 7) : a) Graphene quantum dots - it includes lattice spacing and Fast Fourier Transform (FFT) ; b) amorphous carbon.

Figure 3 shows images obtained by Atomic Force Microscopy (AFM) of nanomaterials prepared electrochemically at the graphite anode (A) and cathode (b, c) in a phosphate buffer solution (pH 7) . The image shown in c) was obtained after diluting the solution used in b) .

Figure 4 presents X-ray Photoelectron Spectroscopy (XPS) region of C Is, 0 Is, and N Is of nanomaterials prepared electrochemically at the graphite anode and cathode in phosphate buffer solution (pH 7) under air atmosphere. The table shows the quantitative results and assignment of the binding energies (BE) .

Figure 5 shows excitation (with emission at 420 nm) and emission (with excitation at 230 nm) fluorescence spectra of nanomaterials produced electrochemically at the graphite anode (dashed line) and cathode (black line) in phosphate buffer aqueous solutions (pH 7) .

Figure 6 shows two images - a) the cyclic voltammograms containing the carbon nanomaterial produced electrochemically at the graphite anode and cathode in phosphate buffer solution, wherein the solutions were concentrated by solvent evaporation, and ; (b) the linear voltammograms of the nanomaterial obtained in the anodic compartment (solid line) and the blank solution, i.e., not containing the carbon nanomaterial (dashed line) .

Figure 7 shows XPS results of regions C Is, 0 Is, and N Is of the nanomaterials prepared electrochemically at the graphite anode in phosphate buffer (pH 7) under an argon atmosphere. The table shows the quantitative results and assignment of binding energies (BE) .

Figure 8 shows XPS results of regions of Cls and Ols of the nanomaterials prepared electrochemically at the graphite anode in citrate buffer (pH 6.5) under an argon atmosphere. The table shows the quantitative results and assignment of binding energies (BE) .

Figure 9 shows cyclic voltammograms of solutions containing the carbon nanomaterial produced electrochemically at the anode and cathode of graphite in a citrate buffer solution (pH 6.5) .

Figure 10 shows cyclic voltammograms of a screen printed carbon electrode in a dialyzed solution containing the carbon nanomaterial generated at a graphite anode (black line) and the cyclic voltammogram of a screen printed carbon electrode in a 0.03 M phosphate solution at pH 7

(dashed line) . v = 50 mV s^_1

Figure 11 shows the effect of the potential scan rate on the cyclic voltammograms response of a screen printed carbon electrode in a dialyzed solution containing the carbon nanomaterial produced electrochemically at an anode of graphite.

Figure 12 shows the effect, in the cyclic voltammograms of a screen printed carbon electrode, of the carbon load contained in the solution generated electrochemically at a graphite anode (after dialysis) . v = 100 mV s^_i.

Figure 13 shows the charge-discharge curve of a screen printed carbon electrode in a dialyzed solution containing the carbon nanomaterial produced electrochemically at a graphite anode (carbon load: 2.1 μg cm^-2), I = 0.25 A/g.

Description of embodiments

Next, some embodiments will be described in more detail, which however are not intended to limit the scope of this application.

Carbon Electrode

One can utilize different carbon sources as positive and negative electrode to synthesize the carbon nanomaterials based on graphene. For example, one may use graphite, pyrolytic graphite, carbon foam, carbon felt, carbon fibers, carbon black, graphene and/or carbon nanotubes. One can use the same carbon source in both electrodes or different carbon sources in the negative and positive electrodes. For example, both electrodes may be of graphite, or one of them is graphite and the other is carbon black. All combinations are likely to be used.

Electrolytic aqueous solution

The electrolytic aqueous solution should be prepared with ultrapure water, i.e, water containing only H⁺ and OH^" ions, and an electrolyte agent which is individually acid, base or salt, or combinations thereof. For example, an inorganic acid, for example sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, or an organic acid, for example acetic acid, citric acid, ascorbic acid or an amino acid, for example glycine, alanine, cysteine, or inorganic base, for example sodium hydroxide, potassium hydroxide, or a salt, or combinations thereof. The salt should contain an inorganic cation such as potassium ion, sodium ion, lithium ion, ammonium ion, and an inorganic anion, for example sulfate, phosphate, carbonate, hydrogen phosphate, dihydrogen phosphate, hydrogen carbonate, nitrate, or organic anion, for example acetate, citrate, lactate.

Electrolytes in the form of buffer agents are preferable because they allow controlling the pH during the process.

Electrochemical cell

The electrochemical cell, preferably made of glass, must have at least two separate compartments and at least two electrodes. On the separation of the compartments it is used an inert barrier which facilitates the diffusion of ions between the solutions in the anodic and cathodic compartment, such as a porous plate of sintered porcelain or glass frit. In one of these compartments it is inserted the positive electrode and in the other the negative electrode. The electrochemical cell may also have additionally a third compartment to the third electrode, the reference electrode, and no resistance between itself and the working electrode.

Saturation of the electrolyte solution and atmosphere of the cell

The synthesis may occur in the electrolytic solution under an air atmosphere or in an electrolytic solution previously saturated with a specific gas, which can be for example nitrogen, argon, helium, carbon dioxide, oxygen, sulfur dioxide, individually or in mixtures. During the electrodes polarization the gas is kept flowing over the solutions in the compartments of the positive and negative electrodes.

Polarization of the electrodes

The polarization of the electrodes can be performed galvanostatically (constant current) or potentiostatically (constant voltage) . In the galvanostatic essays, the current applied must have a value below 100 mA cm^-2, preferably between 5 and 100 mA cm^-2 and ideally between 5 and 50 mA cm^-2. The polarization of the electrodes takes place in two steps:

1^st step or pre-polarization step - The electrodes are polarized for a short period of time, typically 400 s. The positive electrode can be a brand new electrode or an electrode which has been previously used as a negative electrode. The negative electrode can be a brand new electrode or an electrode which has been previously used as a positive electrode. The solution obtained in each of the compartments after this pre-polarization step is removed and discarded;

2^nd step or synthesis step - The electrolytic aqueous solution is added again to both compartments of the cell, and if desired, the solution is saturated with one of the gases listed above before submitting electrodes to polarization. The polarization of the electrodes is maintained for a period of time, typically 500-7200 s in order to obtain the two populations of graphene-based carbon nanomaterials .

Removal and storage of solutions

The carbon nanomaterials based on graphene solutions are removed simultaneously and at the same rate, from both the anodic and cathodic compartments, stored in vials and protected from light.

The obtained nanomaterials may be promptly used after synthesis, or may be submitted to subsequent operations like physical processes e.g., dialysis, filtration or deposition, or chemical processes, for example, binding to biological molecules, metals, envisaging a specific application.

For an easier understanding of this technology, some examples are presented, which are representative of preferred technology embodiments, but without limiting the scope of this application.

Example 1

In this example we report the results obtained with graphite electrodes in a phosphate buffer solution of pH 7, under an air atmosphere, using as polarization conditions i = 25 mA cm^-2.

Analysis by high resolution electron microscopy (HRTEM) reveals that quantum dots of graphene obtained in the cathode compartment have an average size of 17.9 ± 4.6 nm and a lattice spacing of 0.15 nm and 0.30 nm, whereas those obtained in the anode compartment have a mean size of 5.8 ± 1.4 nm and a lattice spacing of 0.223 and 0.257 nm, Figure 2a. For both nanomaterials the lattice spacing is consistent with the graphitic structure. In addition to these nanoparticles HRTEM images showed the presence of amorphous carbon in both the anode and cathode compartments, as illustrated in Figure 2b.

The results obtained by AFM, Figure 3, show that nanomaterials in the cathodic compartment have a round shape and a thickness ranging between 0.3 and 3.5 nm. In the anodic compartment nanoparticles have a non-regular shape and a thickness that varies between 0.5 nm and 1.7 nm.

The results obtained by XPS allow comparing the functionalization of nanomaterials obtained in anodic and cathodic compartments, Figure 4. In both materials it is detected at 289 eV, the peak corresponding to the loss of energy associated with the π-π* transition, characteristic of systems with delocalized electrons. The main functional group that is present on the nanomaterial obtained in the anodic compartment is the carbonyl group (C = 0) and/or the O-C-0 group wherein carbon is bonded to two oxygen atoms by single bonds, corresponding to an energy of 287.9 eV. The nanoparticles obtained in the cathodic compartment have their principal functional group with a binding energy of 286.1 eV. This energy is usually attributed to the alcohol function (C-OH) but it can be also attributed to carbon bonded to nitrogen (C-N) . Considering the atomic ratios of O/C and N/C it can be concluded that the nanomaterial obtained in the anodic compartment has three times higher oxygen content than that obtained in the cathodic compartment, but approximately half of the nitrogen content. These results indicate that it is possible to use atmospheric nitrogen to dope the carbon nanomaterial based on graphene by an electrochemical method.

Fluorescence studies show that nanomaterials obtained in anodic and cathodic compartment display fluorescence emission with a maximum at 420 nm when excited between 230 nm and 260 nm, as illustrated in Figure 5. The quantum yield of the nanomaterial obtained at the anode and cathode is 37% and 20%, respectively.

Nanomaterials were electrochemically characterized by cyclic voltammetry, showing a clear difference between the electrochemical behaviour of the material obtained at the anode and cathode, Figure 6a. Nanomaterials obtained in the anodic compartment show a quasi-reversible redox activity, like the quinones-type molecules. Typically the cyclic voltammogram shows an oxidation peak at 0.01 V and a reduction at -0.23 V. The carbon nanomaterials obtained in the anodic compartment show oxidizing power, as can be concluded from comparison between their voltammogram and the voltammogram of a solution containing the same electrolyte and pH, but without the nanomaterial, Figure 6b. Moreover the carbon nanomaterial obtained at the cathode does not discloses a faradaic behaviour, but shows a capacitive behaviour, and hence a high potential for applications in energy storage systems.

Example 2 - Atmospheric composition effect

The experimental conditions are the same as in example 1, excepting that electrolyte solution has been saturated with argon. During the galvanostatic essay the atmosphere of the electrochemical cell was kept with this gas. The XPS analysis of the nanomaterials obtained in the anodic compartment, Figure 7, reveal that by minimizing the nitrogen content in the atmosphere and in solution, a remarkable decrease on the nitrogen content of the graphene based carbon nanomaterial is attained. It is therefore concluded that it is possible to tune the nitrogen doping of these materials, by regulating the composition of the gases dissolved in the electrolytic solution in the atmosphere of the electrochemical cell.

Example 3 - Effect of Electrolyte composition

In this example we report the results obtained with graphite electrodes in a citrate solution of pH 6.5, under an argon atmosphere using as polarization conditions i = 25 mA cm^-2. The experimental conditions are the same as those in Example 2, excepting the composition of the electrolytic solution.

The results obtained by XPS of the nanomaterials formed at the anode, Figure 8, reveal the introduction of a new functional group on the nanomaterial formed on the anode, namely the carboxylic group (COOH) , with a binding energy of 289.5 eV. The oxidation state of the nanomaterial obtained in the anodic compartment is much smaller than the one illustrated in Examples 1 and 2, as can be inferred from the O/C (0.3) .

Nanomaterials were electrochemically characterized by cyclic voltammetry, as illustrated in Figure 9, and it was observed a clear difference between the electrochemical behaviour of the material obtained in anodic and cathodic compartments. The cyclic voltammogram of carbon nanomaterials obtained in the anodic compartment displays an oxidation peak at 0.33 V, which means that these nanomaterials have the ability to be oxidized (reducing ability) . The carbon nanomaterials obtained in the cathodic compartment do not reveal a faradaic behaviour, but show a capacitive behaviour.

Example 4 - Effect of removing the supporting electrolyte after synthesis.

The experimental conditions are the same as in example 1, excepting the fact that the solution from the anodic compartment is dialyzed after the electrochemical synthesis of nanomaterials. In dialysis it was used a dialysis membrane of 3.5-5 kDa molecular weight. The XPS analysis allowed confirming that the phosphate ion was completely removed from the solution after dialysis. The dialyzed solution was characterized electrochemically without supporting electrolyte. The results of the cyclic voltammetry show that the solution containing the carbon nanomaterial displays a non-resistive behaviour, typical of an electrolytic solution, Figure 10. For comparison, Figure 10 also shows the voltammogram recorded in a solution of a conventional supporting electrolyte (0.03 M phosphate buffer, pH 7) .

Another exclusive behaviour of this carbon-based dialyzed solution is shown in Figure 11, where we can observe the effect of potential scan rate on the double-layer current. The rectangular shape of the voltammogram is characteristic of a capacitive behaviour, which is calculated from the plot of i (at 0,20 V) vs. scan rate, yielding 11 μΡ cm^-2.

The carbon-based dialyzed solution is responsible for the capacitive behaviour of the electrochemical interface, which is also evidenced by the effect of the carbon load in solution on the double layer current, as shown in Figure 12.

Capacitive properties of the electrified interface were also examined using the galvanostatic charge/discharge method, as illustrated in Figure 13, yielding a specific capacity of 10 F g^-1 at 0.25 A/g.

In the experimental results described in Examples 1 to 4 the following equipment was used:

Galvanostatic essays - The electrochemical polarization was performed under the control of an Autolab potentiostat 100 PGSTAT.

Cyclic voltammetry- The electrochemical characterization was performed inside a dark box, on a screen print carbon electrode (C110 Dropsens) under the control of an Autolab potentiostat 100. The potentials are referred to the pseudo reference electrode Ag/Ag⁺.

XPS - The XPS spectrometer used was a XSAM 800 from Kratos operated in FAT mode and with a pass energy of 20 eV. Samples were irradiated with non-monochromatic radiation Mg Ka X-radiation (hv = 1253.7 eV) using a power of 120 W. Sample solution drops were successively deposited on a Si (111) B-doped substrate (with no pretreatment) until the substrate was uniformly covered. Samples were dried under vacuum (2xl0⁴ Pa) and immediately transferred to the XPS chamber. Samples were analyzed using a take-off angle of 45° in ultrahigh vacuum (~10^-7 Pa) at room temperature. To correct the charge shift, aliphatic carbons binding energy was set to 285 eV. For quantification purposes, sensitivity factors used were 0.25 for C Is, 0 Is and 0.66 to 0.42 for C Is.

AFM- The Atomic Force Microscopy (AFM) , was carried out in a Nanoscope Ilia Multimode produced by Digital Instruments (Veeco, Santa Barbara, CA) . All measurements were performed placing a drop of solution onto freshly cleaved mica. After

15 min the sample was washed with water and dried with pure N₂. The images were obtained by measuring in tapping mode, using silicon tips with a resonance frequency of ca. 300 kHz, at a scan rate of ca. 1.5 Hz at room temperature (~21°C) .

HRTEM - Images of High Resolution Transmission Electron Microscopy (HRTEM) were obtained on a JEOL 2200FS transmission microscope at 200 kV, with a digital camera of

16 Mpixel. Chemical analysis was performed by EDS from Oxford, model INCA Energy TEM 250. Formvar/carbon grids of Agar were used.

Fluorescence - Fluorescence spectra were obtained on a Varian Cary Eclipse spectrophotometer. The quantum yield measurements were made with a N₂ laser (PTI model 2000 ca. 600 ps FWHM, ~ 1.0 mJ per pulse), having used a reflection geometry to minimize re-absorption effects, as elsewhere described [23] . The optical density at the excitation wavelength (337 nm N₂ laser) for both the standard and samples was 0.1. The standard sample used was anthracene with a fluorescence quantum yield of 27%. Errors in the determination of the quantum yield do not exceed ± 2%. In order to determine the quantum efficiency the following equation was used:

F_sample and F_ref represent the fluorescence measurement on the sample and standard, respectively. A_sample and A_ref are the absorbance of sample and standard, respectively, at the same wavelength (equal to the excitation wavelength) . φ_ref is the quantum yield of the standard and n is the refractive index.

This technology is not, in any way, restricted to the forms of embodiments described herein and a person familiar with the art may provide certain changes and modifications of the technology, without departing from the scope of the invention, as defined in the claims.

All embodiments described above are obviously combinable with each other. The following claims further define preferred embodiments.

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Claims

1. Method of preparation of nanomaterials comprising the following steps:

- selection of, at least, one source of carbon;

- selection of the electrolytic aqueous solution;

- saturation of the electrolytic aqueous solution with a gas;

- removal of the nanomaterials, produced simultaneously in the anodic and cathodic compartments, which are themselves separated by an inert porous barrier.

2. Method of preparation of nanomaterials according to the previous claim, wherein the carbon source is graphite, pyrolytic graphite, carbon foam, carbon felt, carbon fibers, carbon black, graphene and/or carbon nanotubes.

3. Method of preparation of nanomaterials according to any one of the previous claims, wherein the electrolyte aqueous solution is prepared with ultrapure water and an electrolyte agent which is individually acid, base or salt or a combination thereof.

4. Method of preparation of nanomaterials according to any one of the previous claims, wherein the electrolyte agent is an inorganic acid or an organic acid or an amino acid or an inorganic base or a salt, or combinations thereof.

5. Method of preparation of nanomaterials according to any one of the previous claims, wherein the gas used in the saturation step of the electrolytic solution is nitrogen, argon, helium, carbon dioxide, oxygen, sulfur dioxide, individually or in mixtures.

6. Method of preparation of nanomaterials according to any one of the previous claims, wherein the polarization of the electrodes is performed with constant current.

7. Method of preparation of nanomaterials according to any one of claims 1 to 5, wherein the polarization of the electrodes is performed with constant potential.

8. Method of preparation of nanomaterials according to any one of the previous claims, wherein the polarization of the electrodes comprises the following steps:

- to use as positive electrode a brand new electrode or an electrode that has been previously used as a negative electrode;

- to use as negative electrode a brand new electrode or an electrode that has been previously used as a positive electrode;

- the electrodes are polarized for a short period of time;

- to add aqueous electrolytic solution to the compartments of the cell;

- polarization of the electrodes.

9. A method of preparing nanomaterials according to the previous claim, wherein the polarization of the electrodes further comprises the step of saturation with a gas before re-polarizing the electrodes.

10. Method of preparation of nanomaterials according to claim 8, wherein the polarization of the electrodes is performed during 500 and 7200 seconds.

11. Nanomaterials characterized by being obtained by the preparation method described in any one of the previous claims .