SI23218A - Rutile nanoparticles and process of synthesis for obtaining rutile nanoparticles - Google Patents

Abstract

The subject of the invention are rutile nanoparticles and a process of synthesis for obtaining rutile nanoparticles from metatitanium acid which is a semifinished product in the production of TiO2 pigments. The synthesis of rutile nanoparticles from metatitanium acid is based on the so called gel-sol process. Rutile nanoparticles according to the invention are very fine particles in the form of an acid/neutral suspension, where rutile nanoparticles are polycrystalline and the crystallites exhibit anisothropic morphology and are about 5 nm wide and several 10 nm long, while rutile nanoparticles themselves have a size between 60 and 100 nm in length and 15 to 50 mm in width and a specific surface between 90 and 160 m2/g.

Description

ROUTE nanoparticles and synthesis procedure for the production of rutile nanoparticles

The subject matter of the invention is rutile nanoparticles and a synthesis process for the production of rutile nanoparticles from metatitanic acid, which is a byproduct in the production of TiO ₂ pigment. The synthesis of metatitanic acid rutile nanoparticles is based on the so-called gel-salt process.

The invention contemplates a synthesis process whereby well dispersed, uniformly sized and well crystalline TiO ₂ nanoparticles with the crystalline structure of rutile can be obtained from metatitanic acid. The invention also addresses the material itself, the rutile nanoparticles, and their characteristic characteristics. The starting material of the synthesis process is metatitanic acid, which is a gel formed after hydrolysis of a so-called black solution and is a nanocrystalline agglomerate of anatase. The synthesis process describes how metatitanoic acid is appropriately converted to rutile nanoparticles to form suspensions. Synthesis of rutile nanoparticles in suspension form is very important, since no intermediate step, potentially harmful, dusty phase is formed in any synthesis step, and well crystalline nanoparticles are formed as a final product, which eliminates the need for a calcination process which energy wasteful and burdens the environment with greenhouse gas emissions.

Titanium dioxide in the form of pigment is a material with a wide range of applications, and is useful for coatings, paints, as an additive to plastics, paper, cosmetics, the pharmaceutical industry and many other applications. Today, two processes for the production of pigment titanium dioxide, i.e. sulfate and chloride process. Both the sulfate and chloride process are based on the high-temperature conversion of the corresponding components into pigment titanium dioxide. In the sulfate process, the pigment is formed in the calcination process in the hydrolysis reactions of the obtained anatase gel, while in the chloride process, the pigment is formed in the high-temperature combustion of titanium tetrachloride with oxygen. It is precisely the high-temperature calcination process present in the sulphate and chloride process that prevents the possibility of producing nano-sized particles. Also, the calcination process is heavily laden with high energy consumption and a high amount of greenhouse gas emissions is received. In addition, the resulting product is present in the form of dust that can be potentially harmful to health.

«4

-2 U.S. Pat. No. 7413762 describes a process for the production of rutile nanoparticles by the formation of titanium tetrachloride aerosol, which under the influence of water moisture hydrolyzes into an amorphous gel and / or anatase gel, which is then calcined in a furnace at a temperature of from 150 to 400 ° C for a period of from 1 to 4 hours

US 7326399 discloses a process for the production of titanium dioxide nanoparticles in the form of suspensions in which nanoparticles are formed by mixing a suitable titanium component (titanium tetrachloride, titanium sulfate) with a suitable dispersion medium, which determines the size of the nanoparticles according to the quantitative relationship with the titanium component. . Various organic components are used as suitable dispersants, e.g. hydroxyacetic acid, citric acid and various amino acids.

WO 2008/036176 describes a process for the production of low temperature rutile nanoparticles in which titanium tetrachloride or titanyl chloride reacts with hydrogen peroxide to form the corresponding titanium peroxo complex. The latter then reacts at a temperature above 55 ° C to form rutile nanoparticles.

The described examples of the production of rutile nanoparticles lead to the formation of the desired product, but are inappropriate or less suitable for industrial production, as they are either based on calcination and / or the use of various organic components which can contaminate the final product. The disadvantages described are eliminated by the present invention, which is described below.

The present invention is a simplified process for the production of rutile nanoparticles, suitable for industrial application and is based on the formation of rutile nanoparticles from raw materials present in the production of TiO ₂ pigment in the form of final suspensions, the synthesis process not based on the use of organic components, does not include the process calcination and the associated potentially harmful dry phases of the product, and consequently does not pollute the environment with greenhouse gas emissions and discharges.

The present invention is a process for the production of rutile nanoparticles in the form of suspensions, the process being carried out at a temperature below 100 ° C and in high yields above 90%. Anisotropic rutile nanoparticles are formed in the process, the size and morphology of which can be controlled by the degree of supersaturation, alteration of the acid concentration required for synthesis, and / or the addition of various amounts of inorganic salts that affect the final size of the nanoparticles. Depending on the synthesis process chosen, rutile nanoparticles can have sizes between 60 and 100 nanometers in length and 15 to 50 nanometers in width. In addition, their specific surface area varies from 90dol60m ² / g.

-3The process is based on the synthesis of sodium titanate from the initial methattanic acid and the subsequent reaction of the titanate at elevated temperature with an acid of appropriate mass concentration, forming only particles of the crystalline structure of rutile of very small, nano size.

The main object of the present invention is a process suitable for the industrial production of rutile nanoparticles. The main advantages of the present invention are:

- that it is carried out at a low temperature below 100 ° C

- that the nanoparticles are formed in the form of final suspensions, which excludes the dry phase of the product and therefore does not present a potential risk of dusting

- that the nanoparticles are well crystalline and therefore there is no need for calcination of the product, which means energy savings and consequently the process does not burden the environment with greenhouse gas emissions

- that the nanoparticles are well dispersed in the final suspension, which facilitates their use

- that the nanoparticles are uniformly large

- it is possible to influence the size of the nanoparticles by affecting the degree of saturation, by varying the mass concentration of the acid and / or by the addition of the corresponding inorganic salts

- yields very high and usually above 90%

Description of the invention

The invention will hereinafter be described with a description of the synthesis, figures and embodiments that adequately illustrate the synthesis process itself and the rutile nanoparticles themselves.

Pictures show:

Figure 1: The rutile nanoparticles obtained according to the invention, recorded using a scanning electron microscope,

Figure 2: X-ray powder diffraction pattern showing the most intense deflection at 27.4 °, which is characteristic of the crystalline structure of rutile,

Figure 3: High resolution image of rutile nanoparticles recorded on a transmission electron microscope obtained according to embodiment 1,

Figure 4: Plot size distribution of nanoparticles obtained according to Embodiment 1, which shows that the largest proportion of nanoparticles has a length close to 80 nm, while their width is between 15-20 nm,

Figure 5: Rutile nanoparticles obtained from embodiment 2, taken with a scanning electron microscope,

-4Figure 6: Plot size distribution of nanoparticles obtained according to Embodiment 2, showing that the largest fraction of nanoparticles has a length between 65-75 nm, while their width is between 15-20 nm,

Figure 7: Rutile nanoparticles obtained according to embodiment 3, taken with a scanning electron microscope,

Figure 8: Showing the nanoparticle size distribution obtained from the 3rd embodiment, which shows that the largest proportion of nanoparticles has a length between 80-90 nm, while the width is between 25-30 nm,

Figure 9: Rutile nanoparticles obtained from Embodiment 4, taken with a scanning electron microscope,

Figure 10: Plot size distribution of nanoparticles obtained according to Embodiment 4, which shows that the largest proportion of nanoparticles has a length between 110-130 nm and a width between 25 and 30 nm,

Figure 11: Rutile nanoparticles obtained from the 5th embodiment, recorded with a scanning electron microscope,

Figure 12: Plot size distribution of nanoparticles obtained according to Embodiment 4, which shows that the largest proportion of nanoparticles has a length between 150-160 nm and a width between 30-40 nm.

The process for the synthesis of rutile nanoparticles is based on the use of metatitanic acid, which is a by-product of the production of TiO ₂ pigment and is a nanocrystalline agglomerate of anatase, which is a titanium dioxide polymorph. Metatitanic acid is first suspended in an appropriate volume of distilled water and reacted with sodium hydroxide to a final high mass concentration of 150 to 220 g / L while stirring and heating between 80 and 115 ° C. This produces the so-called sodium titanate. Sodium titanate is a special titanium compound, characterized by its formation in a special morphological form, in the form of highly agglomerated nanotubes, which have a very high specific surface area and are at the same time poorly crystalline. The aforementioned properties largely contribute to the subsequent reaction of sodium titanate with acid, which is rapid and quantitative, resulting in the formation of rutile nanoparticles.

The resulting sodium titanate must be properly purified before reacting with the mineral acid, and filtration must remove the sulfate ions present that interfere with the formation of nanoparticles of the crystalline structure of rutile. After the filtration is carried out, the sodium titanate is resuspended in an appropriate amount of water, with an appropriate mass concentration of Ti (IV) ion, which directly • · • «

-5 determines the degree of saturation in the next reaction with the mineral acid. Typically, sodium titanate is resuspended to a T1O2 mass concentration between 90 and 180 g / L.

An appropriate amount of concentrated hydrochloric acid is then added to the suspension of sodium titanate to a pH between 4 and 5. This ensures that the sodium titanate is well dispersed in the suspension and is available as such with a large specific surface area in the next synthesis reaction of rutile nanoparticles. After adjusting the pH, adjust the mass concentration of hydrochloric acid to at least 70 g / L. Adjusting the hydrochloric acid mass concentration to at least 70 g / L is very important, since the final acid concentration directly determines the crystal structure of the T1O2 nanoparticles formed. If the final hydrochloric acid concentration was less than the indicated one, T1O2 crystalline structures of anatase or a mixture of anatase polymorphs and rutile could be produced as a product. The mass concentration of the acid can also be increased, whereby rutile nanoparticles will be obtained as a final product, the size of which will be larger than in the case of lower mineral acid concentration. Increasing the mineral acid mass concentration always leads to the formation of larger nanoparticles. This allows us to easily and effectively regulate the size of the nanoparticles by selecting the appropriate mass concentration of mineral acid.

After adjusting the hydrochloric acid mass concentration to the desired value, the suspension is slowly heated to a temperature of about 80 ° C, at which the reaction is most intensively carried out. The reaction was allowed to run for two hours, during which time the sodium titanate was completely transformed into well dispersed and uniformly large nanoparticles of rutile.

The size of rutile nanoparticles is largely controlled by the adjusted mass concentration of hydrochloric acid required to perform the synthesis. Adjusting the hydrochloric acid mass concentration to 70 g / L yields very small rutile nanoparticles of anisotropic morphology with a length of up to 80 nm and a width of about 20 nm. If the hydrochloric acid mass concentration is increased, the final size of the rutile nanoparticles is also increased accordingly. For example, At an acidic mass concentration of 100 g / L, rutile nanoparticles are obtained whose length increases to about 90 nm and width to 30 nm. Higher mineral acid concentrations further increase the final size of rutile nanoparticles.

The size of the rutile nanoparticles formed can also be largely controlled by the degree of supersaturation, that is, by adjusting the specific mass concentration of T1O2 while suspending sodium titanate in water or by adding a specific amount of inorganic NaCl salt. The addition of NaCl salt can significantly reduce the size of rutile nanoparticles, allowing the material to be obtained from a very high specific surface area.

-6In this way, the synthesis can be appropriately adapted and different shapes of rutile nanoparticles are obtained, which differ mainly in size and, consequently, in the specific surface area.

Regardless of the change in the reaction parameters of the synthesis of rutile nanoparticles, there are common characteristics for the nanoparticles themselves:

• rutile nanoparticles are polycrystalline, meaning that they are composed of single smaller rutile crystallites. The crystals exhibit anisotropic morphology and are approximately 5 nm (nanometer) wide and 10 nm long.

• rutile nanoparticles exhibit characteristic anisotropic morphology as a result of aggregation of anisotropic rutile crystallites.

At the end of the process, an acidic suspension of rutile nanoparticles is obtained, which may be the end product of the synthesis, or the acidic suspension may also be adequately purified from unwanted ions. To this end, we have developed two procedures, namely:

• Increase in pH: Acid suspensions can be adequately raised by the addition of a base to adjust the surface charge of nanoparticles near the isoelectric point, resulting in their agglomeration and deposition and filtration capacity. The filtered nanoparticles of rutile can be purified by adding an adequate amount of water, which continuously flushes out unwanted ions. This way we can quantitatively remove excess acid and any salt ions present that have been added to control the size of the nanoparticles.

• Centrifugation of acidic suspension of rutile nanoparticles: Centrifugation of the suspension allows the separation of the liquid and solid phases and the removal of the acidic liquid phase by decantation. If spinning is not sufficient to separate the two phases, a smaller amount of aluminum sulphate solution may be added to the suspension to accelerate deposition. After casting the acidic liquid fraction, the TiO ₂ solid phase is resuspended in water and stirred and then the centrifugation cycle is repeated. This is repeated until most of the acid or water soluble salts are removed.

With both purification processes, well-purified rutile nanoparticles are ultimately obtained. These can be dried in an oven if necessary, to obtain a very fine white powder of nanoparticles, or to retain the nanoparticles as a neutral suspension.

The synthesis process for the production of high specific surface rutile nanoparticles and the anisotropic morphology of the invention is based on the so-called. gel-salt reaction where metatitanoic acid, which is a nanocrystalline anatase gel and is a by-product of production, is used as a feedstock

-Pigment TiCh is characterized in that metatitanic acid is first converted to sodium titanate in reaction with NaOH, where the plural ratio of NaOH is: T1O2 = 4: 1, to purify the suspension of sodium titanate and obtain a suspension of T1O2 mass concentration between 120 and 125 g / L and pH values slightly above 4, that the sodium titanate suspension is used as the raw material for the direct synthesis of rutile nanoparticles in the following reaction, that the synthesis of rutile nanoparticles from the sodium titanate suspension is carried out by adding hydrochloric acid with a mass concentration between 70 and 160 g / L to react at 80 ° C for two hours to obtain rutile nanoparticles in the form of an acidic suspension of an approximate mass concentration of 100 g / L so that the nanoparticles can be purified from the acidic suspension by raising the pH to achieve agglomeration of the nanoparticles or by centrifuging the acidic suspension while adding a small amount of aluminum sulfate, the cycle being centrifuged repeat the bite several times. By varying the mass concentration of hydrochloric acid involved in the reaction, it is possible to control the size of the nanoparticles. The size of rutile nanoparticles can be adjusted to about 70 nm in length and up to 160 nm in length, with the ratio of nanoparticle length to width between 5 and 6. The obtained rutile nanoparticles are in the form of an acidic suspension of an approximate mass concentration of 100 g / L and that it can isolate and clean the nanoparticles. Acid is removed from the initial suspension and the pH is adjusted to the desired value as desired. The process is constantly carried out in the wet, ie in the form of a suspension, which prevents the occurrence of potentially harmful dust. The nanoparticles obtained are polycrystalline and therefore there is no need for energy-consuming calcination of the product.

The rutile nanoparticles are in acid / neutral suspension, the rutile nanoparticles being polycrystalline and the crystallites exhibiting anisotropic morphology and about 5 nm wide and 10 nm long and the rutile nanoparticles themselves being between 60 and 100 nm in length and 15 to 50 nm in width and their specific surface area is between 90 and 160 m ² / g.

Embodiment 1: Synthesis of rutile nanoparticles with a high specific surface area and anisotropic morphology with an average nanoparticle length of 80 nm (nanometers) and a nanoparticle width of 15-20 nm.

The synthesis of rutile nanoparticles is carried out in two stages, namely:

• synthesis of a suitable precursor for the synthesis of nanoparticles, which is sodium titanate • conversion of sodium titanate into rutile nanoparticles

The starting material for the synthesis of rutile nanoparticles is so-called metatitanic acid, which is a nanocrystalline agglomerate of anatase of very high specific surface area (above 200 m ² / g). Anatase constituent crystallites in

-8metatitanoic acids are approximately 5 nm in size. Metatitanic acid is converted to sodium titanate by reaction with NaOH.

In a typical experiment, metatitanoic acid, TiO ₂ 290-300 g / L and NaOH 750 g / L, were used for the synthesis of sodium titanate. A typical reaction mixture for the synthesis of sodium titanate contains NaOH and TiO ₂ in a multiplicity of NaOH: TiO ₂ 4.2.

After the addition of NaOH to metatitanoic acid, the final mass concentration of NaOH was 220 g / L. The conversion of metatitanoic acid to sodium titanate takes 2.5 hours at 110-115 ° C.

At the end of the reaction, a basic suspension of sodium titanate is obtained, which is then subjected to intensive washing or washing of excess NaOH and during the formation of salts. It is of particular importance to remove the sulfate ions present in the starting material, metatitanoic acid, which disrupt the further formation of rutile nanoparticles. Rinse is carried out until excess NaOH and sulfate (SO4 ² ') ions are present. The presence of sulphate ions is verified qualitatively by means of a precipitate test with an aqueous solution of barium chloride (BaCl ₂ ). The washed sodium titanate is then resuspended in water to a mass concentration of 120-125 g / L (calculated on TiO ₂ ). It is the starting material for further synthesis of rutile nanoparticles.

Synthesis of rutile nanoparticles of sodium titanate is carried out by a simple process where the titanate suspension is heated to a suitable temperature and hydrochloric acid of a suitable concentration sufficient for quantitative conversion to rutile is formed without anatas being formed.

In the experiment, 37% hydrochloric acid was added to a 500 mL suspension of sodium titanate with a mass concentration of 120-125 g / L to a final mass concentration of 70 g / L. The resulting suspension was heated at 80 ° C for 2 hours with constant stirring of the reaction mixture at 200 rpm. An acidic suspension of rutile nanoparticles of ~ 100 g / L mass was obtained, which was then washed by adding an appropriate amount of 700 g / L NaOH mass concentration to a pH of between 4 and 5. The pH of the nanoparticles was agglomerated, since reached you isoelectric point where the charge of the nanoparticles is close to 0. The agglomerated rutile nanoparticles were then washed with water at the laboratory using a cellulose paper filter. The washed nanoparticles were then dried in an oven at 80 ° C for analysis by XRD, Sbet, and scanning (SEM) and screening (TEM) electron microscope analyzes.

The rutile nanoparticles shown in Figure 1 were captured using a scanning electron microscope. The crystal structure was determined by XRD analysis, which is shown in Figure 2 and

-9 shows only the most intense characteristic burst for rutile. The specific surface area of the nanoparticles was determined by Sbet measurement. The specific surface area of the nanoparticles obtained by the procedure described is between 115 and 130 m ² / g.

Figure 3 is a high-resolution image of a rutile nanoparticle taken with a screening electron microscope showing their specific morphology and the nature of the crystallites that make up it. As can be seen from the figure, the rutile nanoparticle is composed of elongated rutile crystals aggregated into a single polycrystalline nanoparticle.

Figure 4 shows the nanoparticle size distribution showing that the largest fraction of nanoparticles has a length close to 80 nm, while the width is between 15-20 nm.

Embodiment 2: Synthesis of rutile nanoparticles with a high specific surface area and anisotropic morphology with an average nanoparticle length of 65-75 nm (nanometers) and a nanoparticle width of 1520 nm.

The synthesis of rutile nanoparticles was carried out in the same manner as in embodiment 1, except that 15 minutes after the addition of hydrochloric acid, NaCl was added to a mass concentration of 120 g / L. NaCl is known to reduce the size of nanoparticles during the sol-gel or gel-sol reaction.

At the end of the reaction, the nanoparticles were isolated from the acidic suspension by centrifugation at 4000 rpm and repeated washing / resuspension in water and the addition of a small amount of aqueous aluminum sulfate solution. The addition of aluminum sulfate was required to achieve better deposition of nanoparticles during centrifugation. The purified rutile nanoparticles were then dried in an oven at 80 ° C for XRD, Sbet analysis and scanning and scanning electron microscope analyzes.

Figure 5 shows rutile nanoparticles imaged by a scanning electron microscope. Figure 6 shows the size distribution of the nanoparticles, which shows that the largest proportion of nanoparticles has a length between 65 and 75 nm, while the width is between 15-20 nm. The specific surface area of the nanoparticles obtained by the procedure described is between 120 and 160 m ² / g.

Embodiment 3: Synthesis of rutile nanoparticles with a high specific surface area and anisotropic morphology with an average nanoparticle length of 80-90 nm (nanometers) and a nanoparticle width of 2530 nm.

The synthesis of rutile nanoparticles was carried out in the same manner as in embodiment 1 except that the hydrochloric acid concentration was adjusted to 100 g / L.

-10The isolation of rutile nanoparticles was achieved in the same manner as in embodiment 2.

Figure 7 shows rutile nanoparticles imaged by a scanning electron microscope. Figure 8 shows the nanoparticle size distribution showing that the largest fraction of nanoparticles has a length between 80 and 90 nm while the width is between 25 - 30 nm. The specific surface area of the nanoparticles obtained by the procedure described is between 105 and 125 m / g.

Embodiment 4: Synthesis of rutile nanoparticles with a high specific surface area and anisotropic morphology with an average nanoparticle length of 110-130 nm (nanometers) and a nanoparticle width of 25-30 nm.

The synthesis of rutile nanoparticles was carried out according to the same procedure as in Embodiment 1, except that the hydrochloric acid concentration was adjusted to 130 g / L.

The isolation of rutile nanoparticles was achieved in the same manner as in embodiment 2.

Figure 9 shows the rutile nanoparticles imaged by a scanning electron microscope. Figure 10 shows the nanoparticle size distribution showing that the largest fraction of nanoparticles has a length between 110 and 130 nm while the width is between 25-30 nm. The specific surface area of the nanoparticles obtained by the procedure described is between 105 and 115 m ² / g.

Embodiment 5: Synthesis of rutile nanoparticles with a high specific surface area and anisotropic morphology with an average nanoparticle length of 150-160 nm (nanometers) and a nanoparticle width of 30-40 nm.

The synthesis of rutile nanoparticles was carried out according to the same procedure as in Embodiment 1 except that the hydrochloric acid concentration was adjusted to 160 g / L.

The isolation of rutile nanoparticles was achieved in the same manner as in embodiment 2.

Figure 11 shows the rutile nanoparticles imaged by a scanning electron microscope. Figure 12 shows the nanoparticle size distribution showing that the largest fraction of nanoparticles has a length between 150 and 160 nm, while the width is between 30 - 40 nm. The specific surface area of the nanoparticles obtained by the process described is between 90 and 100 m ² / g.

Claims (8)

Patent claims 1. Synthesis process for the production of high specific surface rutile nanoparticles and anisotropic gel-salt-based anisotropic morphology, wherein metatitanic acid, which is a nanocrystalline anatase gel and is a by-product of the production of TiO ₂ pigment, is used as a starting material, characterized in that metatitanic acid is first converted to sodium titanate by reaction with NaOH, where the plural ratio of NaOH: TiO ₂ = 4: 1, - the titanium suspension of sodium titanium is purified and a suspension of a TiO ₂ mass concentration between 120 and 125 g / L and a pH value of slightly above 4 is obtained, - that the sodium titanate suspension is used as the raw material for the direct synthesis of rutile nanoparticles in the following reaction, that the synthesis of rutile nanoparticles from the sodium titanate suspension is carried out by adding hydrochloric acid with a mass concentration between 70 and 160 g / L, - the reaction is carried out at a temperature of 80 ° C for two hours to obtain the rutile nanoparticles in the form of an acidic suspension of an approximate mass concentration of 100 g / L, - it is possible to purify the nanoparticles from the acidic suspension by raising the pH to achieve agglomeration of the nanoparticles or by centrifuging the acidic suspension while adding a small amount of aluminum sulfate, repeating the spin cycle several times Process according to claim 1, characterized in that by changing the mass concentration of the hydrochloric acid involved in the reaction, it is possible to control the size of the nanoparticles. A method according to claim 1, characterized in that the size of the rutile nanoparticles can be adjusted to about 70 nm in length and up to 160 nm in length, with the ratio of nanoparticle length to width between 5 and 6. Process according to claim 1, characterized in that the obtained rutile nanoparticles are in acidic suspension form an approximate mass concentration of 100 g / L and that the nanoparticles can be isolated and purified. Process according to claim 1, characterized in that the initial suspension is acid removed and the suspension is optionally adjusted to pH to the desired value. Method according to Claims 1 to 5, characterized in that the process itself takes place in the wet, ie in the form of a suspension, which prevents the occurrence of potentially harmful dusting. -127. Process according to Claims 1 to 6, characterized in that the nanoparticles thus obtained are polycrystalline and therefore there is no need for energy-wasteful calcination of the product. Rutile nanoparticles, characterized in that they are obtained by the process of claims 1 to 7. 9. Rutile nanoparticles, characterized in that they are in acid / neutral suspension, the rutile nanoparticles being polycrystalline and crystallites exhibiting anisotropic morphology and approximately 5 nm wide and more than 10 nm in length, and the rutile nanoparticles themselves being between 60 and 100 nm in length and 15 to 50 nm in width and their specific surface area is between 90 and 160 m ² / g.