Patent Application
20150037240
The invention relates to methods of preparing catalysts for the chirally selective synthesis of single-walled carbon nanotubes, and catalysts formed thereof. The invention also relates to methods of forming single-walled carbon nanotubes having a selected chirality.
Single-walled carbon nanotubes (SWCNT) have been widely studied since its discovery. Electronic and optical properties of single-walled carbon nanotubes correlate with their chiral structures, and many applications need chirally pure SWCNTs that current synthesis methods cannot produce. Instead, state of the art synthesis methods produce SWCNTs with different (n,m) structures, leading to mixtures with distinct electronic properties ranging from metal to semiconductors with different band gaps.
Although single chirality nanotubes may be separated from SWCNT mixtures using various separation processes, yield, scalability, and cost of such separations, as well as the property (length and functionality) of resulting SWCNTs, are dependent on the initial chirality distribution in SWCNT mixtures. This, in turn, is largely determined during SWCNT growth.
Current methods to form chiral-specific carbon nanotubes are restricted to small-diameter chiral SWCNTs, such as SWCNTS having a chiral index of (6,5) or (7,5). Furthermore, total carbon (SWCNT) yield of reported chiral specific growth thus far is very low, which translates into difficulties in achieving scalable production of specific SWCNTs for various applications. Adding to the fact that there are more than 100 different chiral SWCNTs with diameters in the range of between 0.6 nm and 1.5 nm alone, there remains a need for improved methods and catalysts that allow formation of carbon nanotubes having single chirality selectivity.
In view of the above, there is a need for improved methods of preparing catalysts for the chirally selective synthesis of single-walled carbon nanotubes, and catalysts formed thereof, as well as methods of forming single-walled carbon nanotubes having a selected chirality, that addresses at least one of the above-mentioned problems.
In a first aspect, the invention refers to a method of preparing a sulfur-containing catalyst for the chirally selective synthesis of single-walled carbon nanotubes. The method comprises:
a)
b)
In a second aspect, the invention refers to a sulfur-containing catalyst for the chirally selective synthesis of single-walled carbon nanotubes prepared by a method according to the first aspect.
In a third aspect, the invention refers to a sulfur-containing catalyst for the chirally selective synthesis of single-walled carbon nanotubes, the catalyst comprising sulfur-doped transition metal as active phase on a support, wherein the transition metal is selected from the group consisting of cobalt, iron, nickel, chromium, manganese, copper, rhodium, ruthenium, and mixtures thereof.
In a fourth aspect, the invention refers to a method of forming single-walled carbon nanotubes having a selected chirality. The method comprises:
In a fifth aspect, the invention refers to single-walled carbon nanotubes formed by a method according to the fourth aspect.
Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity.
Advantageously, methods of the invention allow synthesis of single-walled carbon nanotubes in a chirally selective manner. Carbon nanotubes having large diameters as characterized by their chiral index may be selectively formed. Sulfur present on the catalyst may serve to limit aggregation of transition metal atoms and/or to limit formation of transition metal-S compounds. In embodiments where sulfate ions are used as the sulfur source, presence of S═O bonds in sulfate ions serve to stabilize the large sulfur-doped transition metal nanoparticles, that in turn lead to the large diameter carbon nanotubes. In particular, using methods of the invention, it has been demonstrated that the carbon nanotubes formed have a mean diameter of 1.17 nm with 51.7% abundance among semiconducting tubes, and 33.5% abundance among all nanotube species.
The invention refers accordingly in a first aspect to a method of preparing a sulfur-containing catalyst for the chirally selective synthesis of single-walled carbon nanotubes.
The terms “carbon nanotube” and “nanotube” are used interchangeably throughout the entire disclosure, and refer to a cylindrical single- or multi-walled structure in which the at least one wall of the structure is predominantly made up of carbon. Carbon nanotubes may exist in different forms, such as single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT), multi-walled carbon nanotubes (MWNT), or modified multi-walled carbon nanotubes.
A single-walled carbon nanotube refers generally to a seamless cylinder formed from one graphite layer. For example, carbon nanotubes may be described as a graphite plane (so called graphene) sheet rolled into a hollow cylindrical shape so that the structure is one-dimensional with axial symmetry, and in general exhibiting a spiral conformation, called chirality.
A single-walled carbon nanotube may be defined by a cylindrical sheet with a diameter in the range from about 0.7 nm to about 20 nm, such as about 1 nm to about 20 nm, about 5 nm to about 20 nm, about 10 nm to about 20 nm, about 1 nm to about 10 nm, about 1 nm to about 5 nm, about 0.5 nm to about 1.5 nm, or about 1 nm to about 2 nm.
The single-walled carbon nanotubes formed may be of any suitable length, such as in the range from about 0.1 nm to about 10 μm, about 0.1 nm to about 5 μm, about 1 nm to about 5 μm, about 10 nm to about 5 μm, about 10 nm to about 1 μm, about 1 μm to about 5 μm, about 3 μm to about 8 μm, or about 2 μm to about 5 μm. In various embodiments, the carbon nanotubes may be at least 1 μm, at least 2 μm, between about 0.5 μm and about 1.5 gm, or between about 1 μm and about 5 μm. Atomic Force Microscopy (AFM) and/or Raman Scattering Spectroscopy may for instance be used to determine the dimensions of single-walled carbon nanotubes.
As mentioned above, carbon nanotubes may form a one-dimensional structure with axial symmetry and exhibit a spiral conformation called chirality. The chirality of the carbon hexagon rings may depend on the arrangement of the carbon hexagon rings along the surface of the nanotubes.
The arrangement of the carbon hexagon rings may be characterized by the chiral vector of the carbon nanotubes. Chiral vector is a two dimensional vector (n,m) that may be used to describe the geometry of carbon nanotubes. The values of n and m determine the chirality, or “twist” of the nanotube. For example, nanotubes with indices (m, 0) are termed “zig-zag” due to shape of the atomic configuration along the perimeter of the nanotubes. When m=n, the resulting nanotubes are termed “armchair” because of the position of the carbon atoms which are arranged in an “armchair” pattern.
The chirality in turn affects properties such as electronic and mechanical characteristics, such as conductance, density, and lattice structure of the carbon nanotubes. Depending on the arrangement of the carbon hexagon rings along the surface of the nanotube as characterized by its chiral vector, carbon nanotubes may be metallic or semiconducting.
For example, SWNTs may be metallic when n−m=3r, where r is an integer such as 0, 1, 2, 3, 4, 5, and so on, and may be semiconducting otherwise. Metallic SWNTs refer to carbon nanotubes with non-zero density of states (DOS) at its Fermi level. The term “density of states” refers to the number of states at an energy level that are available to be occupied, and the term “Fermi level” refers to an energy level with a probability of 50 percent for existence of an electron. Therefore, a SWNT may be metallic when the DOS value at its Fermi level is not zero. Semiconducting SWNTs refer to carbon nanotubes with varying band gaps, wherein the term “band gap” refers to difference in energy between the valance band and the conduction band of a material.
Chirality of the carbon nanotubes may in turn be governed by the diameter of the catalysts from which the nanotubes are grown. Diameter (d) of carbon nanotubes in nanometers may be expressed as a function of the n and m indexes, using the equation d=a[n2+m2+nm]1/2, where a=0.0783. From this equation, it may be seen that a very small change in the nanotube diameter, may result in change in chirality of the nanotube, which in turn leads to a significant effect on electronic character of the nanotube. By using a sulfur-containing catalyst prepared by methods of the first aspect, single-walled carbon nanotubes having a specific or selected chirality may be synthesized.
The method to prepare the sulfur-containing catalyst includes providing a transition metal-containing support, wherein the transition metal is selected from the group consisting of cobalt, iron, nickel, chromium, manganese, copper, rhodium, ruthenium, and mixtures thereof.
One or more of the above-mentioned transition metals may be present on the transition metal-containing support. The transition metal may be present on the support in the form of particles or nanoparticles. Of the transition metals, it has been found that iron, cobalt, and nickel, which are from Groups 8 to 10 of the Periodic Table of Elements and similar in size, are particularly suitable for forming single-walled carbon nanotubes having large diameters as characterized by a chiral index of (9,8). Accordingly, in various embodiments, the transition metal is selected from the group consisting of cobalt, nickel, iron, and mixtures thereof. The transition metal may comprise or consist essentially of cobalt. In various embodiments, the transition metal consists of cobalt.
The transition metal-containing support may be provided by impregnating a support with a solution comprising transition metal to form an impregnated support, and calcining the impregnated support at a temperature of less than 700° C. to form the transition metal-containing support.
The concentration of transition metal in the solution may be any suitable amount to render the amount of transition metal in the catalyst in the range from about 0.1 wt % to about 30 wt %. The amount of transition metal in the catalyst may also be termed as the loading level of the catalyst. In various embodiments, the loading level or the amount of transition metal in the catalyst is in the range from about 0.1 wt % to about 30 wt %, such as about 0.1 wt % to about 20 wt %, about 0.1 wt % to about 15 wt %, about 0.1 wt % to about 10 wt %, about 0.1 wt % to about 5 wt %, about 0.1 wt % to about 3 wt %, about 1 wt % to about 30 wt %, about 1 wt % to about 20 wt %, about 1 wt % to about 15 wt %, about 1 wt % to about 10 wt %, about 1 wt % to about 8 wt %, about 1 wt % to about 5 wt %, about 3 wt % to about 8 wt %, about 5 wt % to about 30 wt %, about 5 wt % to about 20 wt %, about 5 wt % to about 10 wt %, about 5 wt % to about 8 wt %, about 10 wt % to about 30 wt %, about 10 wt % to about 20 wt %, or about 30 wt %, about 20 wt %, about 10 wt %, about 5 wt %, about 4 wt %, about 3 wt %, about 2 wt %, or about 1 wt %. Generally, the chiral selectivity of single-walled carbon nanotubes is higher at a lower transition metal loading level, such as about 0.1 wt % to about 10 wt % on the catalyst, or about 0.1 wt % to about 5 wt %, or about 0.5 wt % to about 3 wt % on the catalyst. In various embodiments, the amount of transition metal in the catalyst is about 1 wt %.
The solution comprising transition metal may be an aqueous solution having dissolved therein a salt of the transition metal. For example, the solution comprising transition metal may be an aqueous solution having dissolved therein a salt of cobalt, iron, nickel, chromium, manganese, copper, rhodium and/or ruthenium. In various embodiments, the solution comprising transition metal is an aqueous solution having dissolved therein a salt of cobalt, iron and/or nickel. In further embodiments, the solution comprising transition metal is an aqueous solution having dissolved therein a salt of cobalt.
The salt may be completely or at least substantially dissolved in the aqueous solution. Generally, any salt of a transition metal that is able to dissolve in an aqueous solution may be used. In various embodiments, the salt of the transition metal is selected from the group consisting of an acetylacetonate salt, a halide salt, a nitrate salt, a phosphate salt, a carbonate salt, and mixtures thereof. In some embodiments, the salt of the transition metal is an acetylacetonate salt, a halide salt, a nitrate salt, or mixtures thereof. For example, in embodiments wherein the transition metal is cobalt, the solution comprising transition metal may be a solution comprising cobalt, provided by a solution comprising a salt selected from the group consisting of cobalt acetylacetonate, cobalt chloride, cobalt nitrate, or mixtures thereof.
A support is used as a base upon which the transition metal is dispersed upon. The transition metal may be incorporated into the support by impregnating with a solution comprising the transition metal to form a transition metal-containing support. Generally, the support is porous to provide a greater surface area upon which the sulfur-doped transition metal, which acts as active phase for carbon nanotube growth, may be dispersed. The surface area of the support may range from about 100 m2g−1 to about 1000 m2g−1, such as about 100 m2g−1 to about 800 m2 g−1, about 100 m2 g−1 to about 600 m2g−1, about 100 m2 g−1 to about 400 m2g−1, about 200 m2g−1 to about 500 m2g−1, about 200 m2g−1 to about 400 m2 about 400 m2g−1, about 300 m2g−1, or about 200 m2g−1. In various embodiments, the support is selected from the group consisting of silica, alumina, magnesia, silica-alumina, zeolite, and mixtures thereof. For example, the support may comprise or consist essentially of silica.
Porosity of the support may be characterized by the size of the pores. According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), the term “mesopore/mesoporous” refers to pore size in the range of 2 nm to 50 nm; while a pore size below 2 nm is termed a micropore range, and a pore size that is greater than 50 nm is termed a macropore range. In various embodiments, the support comprises or consists essentially of mesopores.
As mentioned above, providing the transition metal-containing support may include impregnating the support with the solution comprising transition metal to form an impregnated support. As used herein, the term “impregnate” refers to introduction of a solution into a porous material. This may take place, for example, by soaking or immersing the support into a solution such that the solution infiltrates into the pores of the support. In various embodiments, the solution is introduced into the pores of the support by capillary action.
The impregnation process is usually carried out at ambient temperature and conditions. The term “ambient temperature” as used herein refers to a temperature of between about 20° C. to about 40° C. The time required for impregnation may vary depending, for example, on the type of support used, the concentration of the impregnating solution, and the temperature at which impregnation is carried out.
Generally, impregnating the support may take place for a time period ranging from a few hours to a few days, such as about 1 hour to about 48 hours, about 1 hour to about 24 hours, about 1 hour to about 12 hours, about 1 hour to about 5 hours, about 1 hour to about 3 hours, about 3 hours to about 20 hours, about 3 hours to about 10 hours, about 3 hours to about 5 hours, about 5 hours to about 18 hours, about 12 hours to about 24 hours, about 3 hours, about 2 hours, or about 1 hour. In various embodiments, the support is allowed to age at room temperature for a few hours to allow a more uniform impregnation of the solution into the support.
A higher concentration of impregnating solution may require a longer impregnation time due to higher viscosity of the solution, thereby requiring a greater time for infiltration of the support into the pores of the support. The temperature at which impregnation is carried out may also affect the impregnation time, with a higher temperature generally having a shorter impregnation time.
Following impregnation, the impregnated support may be calcined at a temperature of less than 700° C. to form a transition metal-containing support. Calcination is normally carried out in furnaces or reactors (sometimes referred to as kilns) of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors. By the phrase “a temperature of less than 700° C.”, it is meant that the impregnated support is subjected to a furnace, kiln or reactor temperature of less than 700° C. In various embodiments, the temperature on the impregnated support is the same as or is lower than the temperature in the furnace, kiln or reactor. The calcination may be carried out under air flow. Following impregnation of the support with the solution comprising transition metal, calcination allows formation of oxidized forms of transition metal on the support.
Advantageously, it has been found by the inventor that chirally selective synthesis of single-walled carbon nanotubes may be performed by varying catalyst calcination temperatures. When catalyst is uncalcined or calcined at a lower temperature of 400° C. for example, the sulfur-containing catalyst formed demonstrated good selectivity towards larger diameter single-walled nanotubes when they are used to form the single-walled carbon nanotubes. In particular, it has been found that nanotubes having a chiral index of (9,8) form the dominating species. With an increase in calcination temperature, the chirality of SWCNTs may be shifted from large diameter tubes to small diameter tubes. Therefore, accordingly, calcination temperature may be used to affect size of the single-walled carbon nanotubes formed, and to result in formation of single-walled carbon nanotubes having a selected chirality.
As mentioned above, calcining of the impregnated support may be carried out at a temperature of less than 700° C. For example, calcining of the impregnated support may be carried out at a temperature of about 200° C. to about 700° C., about 300° C. to about 700° C., about 300° C. to about 500° C., about 400° C. to about 550° C., about 500° C., about 400° C., or about 300° C. In various embodiments, calcining the impregnated support comprises heating the impregnated support at a temperature in the range from about 300° C. to about 700° C. In some embodiments, calcining comprises heating the impregnated support at a temperature of about 400° C.
Calcination may be carried out for a time period ranging from 30 minutes to several hours, for example, 30 minutes, 1 hour, 2 hours, 3 hours or 4 hours. In various embodiments, the impregnated support is calcined for about 1 hour.
Following calcination, the transition metal-containing support is impregnated with a solution comprising sulfur to form a sulfur-doped transition metal-containing support. In various embodiments, the solution comprising sulfur comprises sulfate ions. In some embodiments, the solution comprising sulfate ions is an aqueous solution, and the sulfate ions are provided by an acid or salt selected from the group consisting of sulfuric acid, sulfurous acid, ammonium sulfate, ammonium bisulfate, and mixtures thereof. For example, the solution comprising sulfur may comprise or consist essentially of sulfuric acid.
In embodiments wherein the solution comprising sulfur comprises sulfate ions, concentration of sulfate ions in the solution may be in the range from about 0.01 mol/L to about 5 mol/L, such as about 0.01 mol/L to about 3 mol/L, about 0.01 mol/L to about 2 mol/L, about 0.01 mol/L to about 1 mol/L, about 0.01 mol/L to about 0.05 mol/L, about 0.1 mol/L to about 5 mol/L, about 0.1 mol/L to about 3 mol/L, about 0.1 mol/L to about 2 mol/L, or about 0.1 mol/L to about 1 mol/L. In various embodiments, the concentration of sulfate ions in the solution is about 0.04 mol/L.
The method of the first aspect includes calcining the sulfur-doped transition metal-containing support at a temperature of less than 700° C. to form the catalyst. Calcination conditions similar to that mentioned above for calcining impregnated support may be used. For example, calcining of the sulfur-doped transition metal-containing support may be carried out at a temperature of less than 700° C., such as about 200° C. to about 700° C., about 300° C. to about 700° C., about 300° C. to about 500° C., about 400° C. to about 550° C., about 500° C., about 400° C., or about 300° C. In various embodiments, calcining the sulfur-doped transition metal-containing support comprises heating the sulfur-doped transition metal-containing support at a temperature in the range from about 300° C. to about 700° C. In some embodiments, calcining comprises heating the sulfur-doped transition metal-containing support at a temperature of about 400° C.
In various embodiments, either of or both the impregnated support and the sulfur-doped transition metal-containing support are dried following their respective impregnation step prior to calcining. The drying may be carried out so as to remove water from the support. In doing so, shattering or destruction of the support due to rapid vaporization of water in the pores of the support at the higher calcination temperatures may be prevented. Similar drying conditions may be used for both the impregnated support and the sulfur-doped transition metal-containing support.
Generally, the drying temperature may be set at any suitable temperature that allows water to be driven off from the supports. The temperature used for drying the impregnated support and the sulfur-doped transition metal-containing support may be the same or different. In various embodiments, drying comprises heating the support at a temperature in the range from about 80° C. to about 120° C., such as about 90° C. to about 110° C., about 95° C. to about 100° C., or about 100° C. In various embodiments, drying comprises heating the support at a temperature of about 100° C.
Besides using a two-tier process as mentioned above, in which transition metal and sulfur are added separately in the form of two separate solutions to form the catalyst, the method of the first aspect also relates to a method to prepare a sulfur-containing catalyst in which a solution comprising a sulfate salt of a transition metal is used to impregnate the support. In doing so, only a single impregnation and calcination procedure is required. Examples of transition metal that may be used have already been described above.
Accordingly, when cobalt sulfate is used, for example, the method of the first aspect includes impregnating a support with a solution comprising cobalt sulfate to form a cobalt sulfate-impregnated support. The support may be impregnated with the solution comprising a sulfate salt of a transition metal under conditions similar to that detailed above for impregnating the support with a solution comprising transition metal. Following impregnation, the transition metal sulfate-impregnated support may be calcined at a temperature of less than 700° C. to form the catalyst. The transition metal sulfate-impregnated support may be calcined using conditions similar to that as mentioned above.
The invention refers in a further aspect to a sulfur-containing catalyst for the chirally selective synthesis of single-walled carbon nanotubes prepared by a method according to the first aspect. In a third aspect, the invention relates to a sulfur-containing catalyst for the chirally selective synthesis of single-walled carbon nanotubes, the catalyst comprising sulfur-doped transition metal as active phase on the support, wherein the transition metal is selected from the group consisting of cobalt, iron, nickel, chromium, manganese, copper, rhodium, ruthenium, and mixtures thereof.
As mentioned above, it has been found that iron, cobalt, and nickel have a similar size range, and are particularly suitable for forming single-walled carbon nanotubes having large diameters, such as single-walled carbon nanotubes characterized by a chiral index of (9,8). In various embodiments, the transition metal is selected from the group consisting of cobalt, nickel, iron, and mixtures thereof. The transition metal may comprise or consist essentially of cobalt. In various embodiments, the transition metal consists of cobalt.
The amount of transition metal in the catalyst may be in the range from about 0.1 wt % to about 30 wt %, such as about 0.1 wt % to about 20 wt %, about 0.1 wt % to about 15 wt %, about 0.1 wt % to about 10 wt %, about 0.1 wt % to about 8 wt %, about 0.1 wt % to about 5 wt %, about 0.1 wt % to about 3 wt %, about 1 wt % to about 30 wt %, about 1 wt % to about 20 wt %, about 1 wt % to about 15 wt %, about 1 wt % to about 10 wt %, about 1 wt % to about 8 wt %, about 1 wt % to about 5 wt %, about 3 wt % to about 8 wt %, about 5 wt % to about 30 wt %, about 5 wt % to about 20 wt %, about 5 wt % to about 10 wt %, about 5 wt % to about 8 wt %, about 10 wt % to about 30 wt %, about 10 wt % to about 20 wt %, or about 30 wt %, about 20 wt %, about 10 wt %, about 5 wt %, about 4 wt %, about 3 wt %, about 2 wt %, or about 1 wt %. Generally, the chiral selectivity of single-walled carbon nanotubes is higher at a lower transition metal loading level, such as about 0.1 wt % to about 10 wt % on the catalyst, or about 0.1 wt % to about 5 wt %, or about 0.5 wt % to about 3 wt % on the catalyst. In various embodiments, the amount of transition metal in the catalyst is about 1 wt %.
The sulfur content in the sulfur-doped transition metal may be in the range from about 0.1 wt % to about 30 wt %, such as about 0.1 wt % to about 20 wt %, about 0.1 wt % to about 15 wt %, about 0.1 wt % to about 10 wt %, about 0.1 wt % to about 5 wt %, about 0.1 wt % to about 2 wt %, about 0.1 wt % to about 1 wt %, about 0.5 wt % to about 2 wt %, about 0.5 wt % to about 15 wt %, about 1 wt % to about 30 wt %, about 1 wt % to about 20 wt %, about 1 wt % to about 15 wt %, about 1 wt % to about 10 wt %, about 1 wt % to about 5 wt %, about 1 wt % to about 3 wt %, about 1 wt % to about 2 wt %, about 5 wt % to about 30 wt %, about 5 wt % to about 20 wt %, about 5 wt % to about 15 wt %, about 5 wt % to about 10 wt %, about 10 wt % to about 30 wt %, about 10 wt % to about 20 wt %, about 10 wt % to about 15 wt %, about 15 wt % to about 30 wt %, about 15 wt % to about 20 wt %, about 20 wt % to about 30 wt %, about 20 wt %, about 15 wt %, about 10 wt %, about 5 wt %, about 3 wt %, about 2 wt %, or about 1 wt %. In various embodiments, the sulfur-doped transition metal has a sulfur content in the range from about 0.5 wt % to about 1.5 wt %. In embodiments in which the transition metal consists essentially of cobalt, the sulfur-doped cobalt comprises or consists essentially of cobalt sulfate.
In various embodiments, the sulfur-doped transition metal may be present in the form of particles or nanoparticles, and may be grafted on the support or within the pores of a porous support. Suitable supports that may be used have already been mentioned herein. In various embodiments, the support comprises or consists essentially of silica.
Size of the sulfur-doped transition metal active phase on the support may be varied to affect the size of single-walled carbon nanotubes formed, and/or to achieve chiral selective synthesis of single-walled carbon nanotubes. For example, size of the selected chirality of single-walled carbon nanotubes formed may be similar to the size of the sulfur-doped transition metal nanoparticles that are present on the support. As mentioned above, of the transition metals, it has been found that iron, cobalt, and nickel are similar in size and are particularly suitable for forming single-walled carbon nanotubes having large diameters. As an example, sulfur-doped cobalt nanoparticles, which are present as active phase on the support, are used to form single-walled carbon nanotubes having a chiral index of (9,8).
The size of the sulfur-doped transition metal active phase may be characterized by their mean maximal dimension. The term “maximal dimension” as used herein refers to the maximal length of a straight line segment passing through the center of a figure and terminating at the periphery. The term “mean maximal dimension” refers to an average maximal dimension of the nanoparticles, and may be calculated by dividing the sum of the maximal dimension of each nanoparticle by the total number of nanoparticles.
The mean maximal dimension of the sulfur-doped transition metal active phase, which may be present as nanoparticles on the support, may be in the range from about 1 nm to about 1.5 nm, such as about 1 nm to about 1.25 nm, about 1.25 nm to about 1.5 nm, about 1.2 nm to about 1.3 nm, or about 1.25 nm. In various embodiments, the mean maximal dimension of the sulfur-doped transition metal on the support is about 1.25 nm. In various embodiments, the sulfur-doped transition metal nanoparticles are essentially monodisperse.
The catalyst according to the second aspect and the third aspect may be used to form single-walled carbon nanotubes having a selected chirality. Accordingly, in a fourth aspect, the invention relates to a method of forming single-walled carbon nanotubes having a selected chirality.
The method includes reducing a catalyst according to the second aspect or the third aspect with a reducing agent. By contacting the catalyst with the reducing agent, the sulfur-doped transition metal particles that are present in the catalyst may be converted into a reduced form.
In various embodiments, reduction carried out by contacting the catalyst with a reducing agent such as hydrogen, an amine, ammonia, diborane, sulphur dioxide, hydrazine, including a flowing reducing gas such as flowing hydrogen gas. In various embodiments, the reducing agent comprises or consists essentially of hydrogen gas.
Reducing the catalyst may be carried out at any suitable temperature and conditions, which may be dependent on the type of reducing agent used. Generally, reducing the catalyst is carried out at a temperature in the range from about 300° C. to about 550° C., such as about 300° C. to about 400° C., about 300° C. to about 350° C., about 400° C. to about 550° C., about 450° C. to about 550° C., about 500° C., about 400° C., or about 300° C.
Following reduction, the method according to the fourth aspect may include purging the catalyst with an inert gas prior to contacting the gaseous source of carbon with the catalyst. In various embodiments, the inert gas is selected from the group consisting of argon, helium, neon, krypton, xenon, nitrogen, and mixtures thereof. In some embodiments, the inert gas comprises or consists essentially of argon.
Purging of the catalyst with the inert gas may be carried out at any suitable temperature. For example, purging the catalyst may be carried out at a temperature in the rage from about 500° C. to about 800° C., such as about 500° C. to about 700° C., about 500° C. to about 600° C., about 600° C. to about 800° C., about 550° C. to about 750° C., about 800° C., about 700° C., about 600° C., or about 500° C.
The gaseous source of carbon may include a carbon source gas, such as carbon monoxide, methane, ethane, propane, butane, ethylene, propylene, acetylene, octane, benzene, naphthalene, toluene, xylene, mixtures of C1-C20 hydrocarbons, an organic alcohol, e.g. methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, neobutanol or tert-butanol, or any other suitable material, typically in gaseous form, that is efficacious in contact with the sulfur-containing catalyst under the process conditions suitable for growing carbon nanotubes. In various embodiments, the gaseous source of carbon is selected from the group consisting of carbon monoxide, methane, methanol, ethanol, acetylene, and mixtures thereof. In some embodiments, the gaseous source of carbon comprises or consists essentially of carbon monoxide. An inert gas such as argon may optionally be mixed with the gaseous source of carbon before contacting the catalyst.
Contacting the gaseous source of carbon with the sulfur-containing catalyst may be carried out using any suitable conditions to grow carbon nanotubes. For example, a continuous, batch, semi-batch, or other mode of processing appropriate to the specific implementation of the manufacturing operation may be employed. Contacting may, for example, be carried out in a reactor operated as a fluidized bed reactor, through which the gaseous source of carbon is flowed as the fluidizing medium. The carbon-containing gas may for example be fed into a reactor cell having catalytic particles of the sulfur-containing catalyst disposed therein.
Generally, the gaseous source of carbon is applied at a pressure or is contacted with the catalyst at a pressure in the range from about 1 bar to about 10 bar, such as about 1 bar to about 8 bar, about 1 bar to about 6 bar, about 2 bar to about 8 bar, about 3 bar to about 8 bar, about 4 bar to about 10 bar, about 5 bar to about 8 bar, about 8 bar, about 6 bar, about 4 bar, or about 2 bar. In various embodiments, the gaseous source of carbon is contacted with the catalyst at a pressure of about 6 bar.
The time required to form the carbon nanotubes may range from about 1 minute to about 4 hours, such as from about 10 minutes to about 3 hours, about 20 minutes to about 2 hours, about 30 minutes to about 1 hour, about 1 hour to about 2 hours, about 3 hours, about 2 hours, about 1 hour, or about 30 minutes. In various embodiments, the time required to form the carbon nanotubes is about 1 hour.
Using methods of the fourth aspect, majority of the single-walled carbon nanotubes thus formed have diameters within a predetermined range. Generally, the formed carbon nanotubes have a narrow diameter distribution. The narrow diameter distribution may be characterized by the chiral indices.
In various embodiments, at least 50% of the single-walled carbon nanotubes formed have the chiral indices (9,8), (9,7), (10,6), and (10,9), such as at least 55%, at least 60% or at least 70%. Of these, single-walled carbon nanotubes having a chiral index of (9,8) may be the dominating species. In various embodiments, at least 30% of the single-walled carbon nanotubes formed have the chiral index (9,8), such as at least 32%, at least 35%, at least 38%, or at least 40%. In some embodiments, at least 40% of the carbon nanotubes formed have the chiral index (9,8).
In a further aspect, the invention relates to single-walled carbon nanotubes formed by a method according to the fourth aspect. The single-walled carbon nanotubes having a selected chirality formed using a method of the invention, may be used as electrode material for forming an electrode. The electrodes formed using these chirally selective SWNTs may be used for batteries, such as metal-air batteries. Examples for metal-air batteries include a lithium, aluminium, carbon, zinc-air battery in which at least one electrode is made of carbon. They may also be used for fuel cells. In case they are used in fuel cells, catalytic noble metal materials in particulate form may be added to the electrode.
Apart from the applications mentioned above, the single-walled carbon nanotube formed using a method of the invention may also be used as an optical or an optoelectronic device, such as transistors, memory devices and optoelectronic couplers.
It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including”, “containing”, etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.
The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.
Other embodiments are within the following claims and non-limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.
CoSO4/SiO2 catalysts with about 1 wt. % cobalt were prepared by the incipient wetness impregnation method. Cobalt (II) sulphate heptahydrate (from Sigma-Aldrich) was first dissolved in deionized (DI) water, and then added to Cab-O-Sil M-5 silica powder (from Sigma-Aldrich, surface area 200 m2/g). The mixture was aged at room temperature and subsequently dried in an oven at 100° C. overnight. The resulting solids were calcined for 1 hour in an air flow. The calcination temperature was adjusted from 400° C. to 900° C.
Physical and chemical properties of CoSO4/SiO2 catalysts were characterized by H2-temperature programmed reduction (TPR), UV-vis diffuse reflectance, XAS (X-ray absorption spectroscopy), and element analysis (EA). UV-vis diffuse reflectance spectra were recorded on a Varian 5000 UV-vis near-infrared spectrophotometer. The spectra were recorded in the range of 200 nm to 800 nm with pure barium sulfate (BaSO4) as a reference. All samples were dried at 100° C. for 3 hour before performing the test. The reducibility of calcined catalysts was characterized by TPR using the thermal conductivity detector (TCD) of a gas chromatography (Techcomp, 7900). Approximately 200 mg of each sample was loaded into a quartz cell. Prior to each TPR run, the sample cell was purged by air at room temperature, then the temperature was increased to 500° C. at 5° C./min, soaked for 1 hour at the same temperature, and cooled to room temperature. This procedure produces a clean surface before running the H2-TPR. The gas flow was switched to 5% H2/Ar, and the baseline was monitored until stable. After baseline stabilization, the sample cell was heated at 5° C./min and held for 30 min at 950° C. An acetone trap was installed between the sample cell and the TCD to condense water or H2S produced during catalyst reduction. The weight percentage of S after different calcination treatments was measured by Elementarvario CHN elemental analyzer. Before the EA test, all the samples were dried at 100° C. overnight. Approximately 5 mg sample was used for each EA test, and each sample was repeated 3 times to get the mean and standard error.
The catalysts calcined at different temperatures were characterized by XAS. All X-ray absorption data were collected at beam line X23A2, National Synchrotron Light Source, Brookhaven National Laboratory. Approximately 60 mg of each sample was pressed into a self-supporting wafer (about 0.5 mm thick). Extended X-ray absorption fine structure spectroscopy (EXAFS) in the transmission mode was collected from 200 eV below the Co K edge to 900 eV above the Co K edge. Analysis of the X-ray adsorption spectra followed the procedures was described in detail in reference. The EXAFS spectra were calibrated to the edge energy of the cobalt foil reference. The background removal and edge-step normalization were performed using the IFEFFIT software. The theoretical EXAFS function for Co3O4 was used to fit the experimental data in order to obtain the corresponding Co—O first shell coordination numbers.
In a typical SWCNT growth experiment, 100 mg of CoSO4/SiO2 catalysts were first pre-reduced to a pre-reduction temperature under flowing H2 (1 bar, 50 sccm, 99.99% from Alphagaz, Soxal) using a temperature ramp of 20° C./min in a CVD reactor. Once the pre-reduction temperature of 540° C. was reached, the reactor was purged using flowing Ar (99.99% from Alphagaz, Soxal), while the temperature was further increased to 780° C. A pressured CO (99.99% from Alphagaz, Soxal) flow was introduced into the reactor at 6 bar and lasted for 1 hour. The carbonyls in CO were removed by a Nanochem Purifilter from Matheson Gas Products. All samples are used to synthesize SWCNTs under the same condition.
TPR is a useful characterization technique for investigating the metal support interaction and providing surface chemical information, such as stability, metal species, and metal distribution.
From the TPR profiles, CoSO4.7H2O exhibits a sharp peak around 585° C., which is ascribed to the reductive decomposition of bulk CoSO4. The uncalcined catalyst and those calcined at 400° C., 450° C., 500° C. and 600° C. show similar sharp peaks around 460° C. to 470° C., which is attributed to the reductive decomposition of highly dispersed CoSO4 on the SiO2 substrate, and no other reduction peaks are observed, such as CoOx and cobalt silicates. CoOx is usually reduced below 400° C., which is shown by the CoOx references (CoO and Co3O4) in
Surface cobalt silicates usually exhibit a high reduction temperature at around 600° C. to 800° C. However, when the calcination temperature increases to 700° C., CoSO4 decomposes gradually, and there are two peaks around 450° C. and 340° C. in the profile, which can be assigned to the reduction of remaining CoSO4 and CoO respectively. The TPR profiles of catalyst calcined at 800° C. and 900° C. are similar, with one peak around 310° C. located between the peaks of CoO and Co3O4, which demonstrates the formation of CoOX. In addition, there is another broad peak around 600° C. to 800° C., which is attributed to a small amount of surface cobalt silicate produced on the 800° C.-calcined catalyst, and the broad peak becomes more intense when the catalyst is calcined at 900° C. When the calcination temperature is higher than 950° C., bulk cobalt silicate would form.
UV-vis-drs spectra were used to investigate the surface chemistry of catalysts. The results of Uv-vis-drs are consistent with those of TPR. From
EXAFS spectroscopy is a technique based on the absorption of X-rays and the creation of photoelectrons scattered by neighbour atoms, which can be used to provide detailed information about the coordination number, interatomic distance, and neighbour species of the absorbing atoms.
Several changes in the EXAFS were observed. The pre-edge peaks of three catalyst samples (uncalcined, calcined at 400° C. and 600° C.) at around 7709 eV overlap, which means that Co atoms in the three samples are in a similar symmetric environment. The XAS edge jump around 7717 eV suggests that Co (II) is the dominant oxidation state for Co atoms in these catalysts. The pre-edge peak of the catalyst calcined at 800° C. located between those of CoO and Co3O4, and is more close to that of Co3O4. In addition to the pre-edge feature, the intensity of the white line also correlates with the cobalt state in the catalyst. The cobalt foil has only a very weak white line, while the uncalcined CoSO4/SiO2 catalyst has a strong white line at 7725 eV, which suggests Co in the uncalcined CoSO4/SiO2 sample is in the oxidized state. The spectrum of CoSO4/SiO2 catalyst calcined at 400° C. is almost identical to that of the uncalcined sample, indicating that a significant fraction of Co species in the catalyst are still in the same oxidized state after calcined at 400° C. The intensity of the white line of the CoSO4/SiO2 catalyst calcined at 600° C. slightly decreased, shown an intermediate state between the catalysts calcined at 400° C. and 800° C. However, after calcination at 800° C., the white line recorded for the CoSO4/SiO2 catalyst splits into two peaks. The shoulder peak around 7726 eV can be attributed to CoO and a small amount of surface cobalt silicate, and the peak at 7729 eV is similar with that of Co3O4 with respect to both position and intensity, which suggests that Co species in the catalyst were converted into CoOk after calcination at 800° C., and the majority of cobalt species are Co3O4.
EXAFS spectra in R space are shown in
aNCo—O average first-shell coordination of cobalt-oxygen.
bdR deviation from the effective half-path-length R (R is the inter-atomic distance for single scattering paths).
cσ2 (×10−2 Å2) mean-square deviation in R.
In all, according to above characterization results of the CoSO4/SiO2 catalyst at different calcination temperatures, we can conclude that CoSO4 is well dispersed on the SiO2 substrate below the calcination temperature of 400° C., and high calcination temperature results in the formation of CoOX and a small amount of cobalt silicate due to the S decomposition in the catalyst.
The filtered carbon deposits were further suspended in 2 wt % sodium dodecyl benzene sulfonate (SDBS) (Aldrich) D2O (99.9 atom % D, Sigma-Aldrich) solution by sonication in a cup-horn ultrasonicator (SONICS, VCX-130) at 20 W for 1 hour. After sonication, the suspensions were centrifuged for 1 hour at 50,000 g.
The clear SWCNT suspensions obtained after centrifugation were characterized by photoluminescence (PLE) and UV-vis-NIR absorption spectroscopy.
PLE was conducted on a Jobin-Yvon Nanolog-3 spectrofluorometer with the excitation scanned from 500 nm to 950 nm and the emission collected from 900 nm to 1600 nm.
The UV-vis-NIR absorption spectra were measured from 400 nm to 1600 nm on Varian Cary 5000 UV-vis-NIR spectrophotometer. The UV-vis-NIR spectra were conducted to confirm the results of PLE maps. All the spectra were normalized around 1420 nm.
As-grown SWCNTs were pressed into thin wafers and investigated by Raman spectroscopy. Spectra were collected with a Renishaw Ramanscope in the backscattering configuration over several random spots on samples using 514 nm and 785 nm laser. Laser energies of 2.5 mW to 5 mW were used to prevent destroying SWCNT samples during the measurement. Integration times of 20 s were adapted. There was no significant difference found in the Raman spectra compared with those from SWCNTs on filter membranes after silica support removal. Furthermore, the as-synthesized catalysts loaded with carbon deposits were further refluxed in 1.5 mol/L sodium hydroxide (NaOH) to dissolve the silica matrix and filtered on a nylon membrane (0.2 μm pore).
Raman spectroscopy is widely used to probe into the quality and structure of SWNTs based on radial breathing mode (RBM), D band and G band. Raman spectra were taken on as-synthesized SWCNT samples under 514 nm and 785 nm laser wavelength shown in
For the uncalcined CoSO4/SiO2 catalyst and catalysts calcined below 700° C., they mainly synthesize larger diameter nanotubes (dt≧1.1 nm). The RBM peaks around 193 cm−1 (
When calcination temperature is 700° C., there are a few RBM peaks in the wide Raman shift from 193 cm−1 to 310 cm−1 (
The total carbon loading was determined on as-synthesized catalysts by thermogravimetric analysis (TGA) using PerkinElmer Diamond TG/DTA equipment. For a typical measurement, about 1 mg sample (as-synthesized SWCNTs on catalysts) was loaded into an alumina pan. The sample was firstly heated to 110° C., and was held at 110° C. for 10 minutes in the air flow (200 sccm) to remove any moisture. Then the temperature was continually hiking from 110° C. to 1000° C. at a 10° C./min ramp. The weight of the sample was monitored and recorded as a function of the temperature. The same procedure was repeated after the sample was cooled to room temperature and another weight/temperature curve was obtained serving as a baseline.
TGA was used to analyze the carbon loading and different carbon species of carbon deposits. Carbon loading was directly calculated by weight loss from TGA profiles. The carbon yields of three representative carbon deposits synthesized on catalysts calcined at 400° C., 700° C. and 900° C. are 5.9%, 6.6% and 6.8% respectively, which demonstrates that carbon yields increase slightly with the calcination temperature increasing.
However, in most cases, carbon deposits contain not only SWCNTs but also other impurities like amorphous carbon, multi-walled carbon nanotubes (MWCNTs) and graphite, which can be determined based on DTG (derivative thermogravimetry) patterns obtained by taking the derivative of TGA profiles. DTG patterns of carbon deposits can be categorized into three oxidation regions: amorphous carbon below 300° C., carbon nanotubes (SWCNTs and MWCNTs) between 400° C. and 700° C., and graphite above 800° C.
However, when metal residues are present, metal residues may affect the oxidation of SWCNTs and result in the shift of oxidation temperature. In all of the three DTG profiles, there are positive peaks below 250° C. supporting the existence of metal residues which are cobalt particles resulted from the reduction of cobalt species during the synthesis of SWCNTs. The formation of graphite can also confirm the presence of metal residues. Larger metal particles are easily covered by layers of graphite. The peaks above 900° C. on DTG profiles come from the oxidation of graphite. Carbon deposits synthesized after the catalyst calcined at 900° C. have the most intense graphite peak. In addition, the small peak around 586° C. may be attributed to the existence of a small amount of MWCNTs. Therefore, with the calcination temperature increasing, the enhanced carbon yield comes from the oxidation of graphite, and furthermore, high calcination temperature can disturb the dispersion of cobalt species on the catalyst, resulting in large metal particle during the synthesis of SWCNTs, which in turn decreases the yield of SWCNTs.
Based on above SWCNT characterization results, conclusions can be obtained that the highly selective growth of (9,8) nanotubes with a narrow (n,m) distribution can be achieved on CoSO4/SiO2 catalysts calcined at a lower temperature of 400° C., and that the chirality of SWCNTs can be shifted from larger diameter (9,8) nanotubes to small diameter (7,5) and (6,5) nanotubes with the calcination temperature increasing. Because of the correlation between the SWCNT diameter and the size of the catalyst metal particle from which it grows, we attribute the (n,m) shift to the change in size of the catalyst particles resulting from the reduction of different Co species produced on the CoSO4/SiO2 catalyst by calcination. At the lower calcination temperature, the Co species are well dispersed on the catalyst, and the size of most Co nanoparticles stabilized on the substrate after reduction matches with that of (9,8) nanotubes, which therefore results in the good chiral selectivity.
TPR and XAS results have already shown CoOX and cobalt silicate are formed under high calcination temperature, and the dispersion of Co species decreases with the calcination temperature. We believe the presence of S can improve the distribution and avoid the formation of CoOX and cobalt silicate. However, the S decomposition occurs with the calcination temperature increasing, and different Co species can produce on the catalyst, which may be responsible for the SWCNT chirality shift. The calcination process of CoSO4/SiO2 catalyst at different temperatures is proposed in
When the calcination temperature is low (400° C.), S in the CoSO4/SiO2 catalyst exists in the form of two terminal S═O bonds (
(
are not stable and eventually converses into CoOx (
To further confirm the S decomposition with the calcination conditions, the S content is measured by conducting element analysis on CoSO4/SiO2 catalysts uncalcined and calcined at different temperatures.
According to the above discussion, an explanation about the effects of catalyst calcination temperature on the chirality selectivity of SWCNT synthesized on CoSO4/SiO2 catalysts is provided below. For uncalcined catalysts and catalysts calcined at 400° C., Co atom is bonded to O atom and terminated by S═O bonds, which results in the well dispersed Co nanoparticles under reduction suitable for the synthesis of (9,8) nanotubes. The uncalcined catalyst contains some water molecules, and the color of the catalyst is pink. The color of the catalyst calcined at 400° C. changes from light violet to pink when it absorbs moisture due to exposure to air at room temperature. Therefore, these water molecules in the uncalcined catalyst may have a small effect on the reduction of Co species, which results in the little difference of SWCNTs products. When CoSO4/SiO2 catalysts are calcined at higher temperature (500° C. and 600° C.), most of Co atoms are still in the distorted tetrahedron structure, however, S═O bonds start to decompose and a fraction of
forms, and when these catalysts expose to H2 during reduction, the reduced Co nanoparticles may aggregate, which result in the broader (m, m) distribution of SWCNTs. Especially, when the calcination temperature increases to 700° C., the Co nanoparticles aggregate severely to form various size of nanoclusters due to the decomposition of S═O bonds, which results in the loss of (n,m) selectivity. When the calcination temperature further increases to 800° C. and 900° C., with the complete decomposition of S═O bonds, CoOx and cobalt silicate form, which results in the synthesis of small diameter tubes, such as (6,5), (7,5), (7,6) and (8,4).
As can be seen from the above, the CoSO4/SiO2 catalyst prepared by cobalt sulphate heptahydrate was calcined in an air flow at different temperature from 400° C. to 900° C. Catalyst characterization results demonstrated that CoSO4 is well dispersed on the SiO2 substrate below the calcination temperature of 400° C., and high calcination temperature results in the formation of CoOx and cobalt silicate due to the decomposition of S═O in the catalyst. SWCNTs were synthesized on the uncalcined CoSO4/SiO2 catalyst and those catalysts calcined at different temperatures, and the chirality of SWCNTs was shifted from larger diameter nanotubes to small diameter nanotubes with the catalyst calcination temperature increasing. Co SO4/SiO2 catalysts are selective to the synthesis of (9,8) SWCNTs at a lower temperature of 400° C. The presence of S═O is proved to be critical to disperse the cobalt well on the catalysts and hence prevent efficiently the formation of cobalt oxides and cobalt silicate. Only well-dispersed Co species would aggregate into large metal clusters which are active for (9,8) SWCNT growth.
The CoSO4/SiO2 catalyst was prepared by incipient wetness impregnation method, in which metal salt dissolved in water dissolved in water is added to the catalyst support materials.
Cobalt (H) sulfate heptahydrate (CoSO4.7H2O) (Sigma-Aldrich, ≧99% purity) was first dissolved in deionized water and then added to CAB-O-SIL M-5 fumed silica with a surface area of 254 m2/g and a pore volume of 0.89 mL/g. The total Co weight loading in the catalyst is about 1.0 wt %.
The mixture was first aged at room temperature for 1 h and afterward dried in an oven at 100° C. for 2 h. The dried catalyst was further calcined under airflow of 20 sccm per gram of catalyst from room temperature to 400° C. at 1° C./min ramping rate and then kept at 400° C. for 1 h.
The catalyst was used to catalyze SWCNT growth in a continuous-flow tubular chemical vapor deposition reactor. To catalyze SWCNT growth, 200 mg of the CoSO4/SiO2 catalyst was loaded in a ceramic boat at the center of a horizontal chemical vapor deposition reactor. The catalyst was first reduced under pure H2 (1 bar, 50 sccm, 99.99% from Alphagaz, Soxal), during which the reactor temperature was increased from room temperature to an elevated temperature at 20° C./min. Once the reduction temperature reached 540° C., the reactor was purged by Ar (99.99% from Alphagaz, Soxal), while its temperature was further increased to 780° C. At 780° C., pressured CO (6 bar, 99.9% from Alphagaz, Soxal) was introduced into the reactor at 200 sccm flow rate to initiate SWCNT growth, and the growth time was 1 h. Carbonyl residues in CO gas were removed by a purifier (Nanochem, Matheson Gas Products) before entering the reactor.
In another experiment, the catalyst was reduced in H2 from room temperature to 780° C. and further reduced for 30 min at 780° C. before exposing to CO.
As-synthesized SWCNTs deposited on the CoSO4/SiO2 catalyst were first studied by Raman spectroscopy. Raman spectra were collected with a Renishaw Ramanscope in the backscattering configuration over a few random spots on samples under 514 nm, 633 nm, and 785 nm lasers with the integration time of 10 s. Laser energies of 2.5 mW to 5 mW were used to prevent sample damages during the measurement. SWCNTs were further refluxed in 1.5 mol/L NaOH aqueous solution to dissolve the SiO2 catalyst and then filtered on a nylon membrane (0.2 μm pores). No significant differences between the Raman spectra of as-synthesized SWCNTs and SWCNTs on filter membranes after catalyst removal were found.
The chiral indexes (n,m) of RBM peaks are assigned based on empirical and theoretical Kataura plots (see
A combination of empirical and theoretical Kataura plots are used because for E11 and E22 van Hove transitions of semiconducting SWCNTs, the available empirical Kataura plots are more accurate, while no empirical Kataura plots are currently available for metallic SWCNTs and higher order transitions of semiconducting SWCNTs.
213
1.11
(12, 3)
197
1.20
(9, 9), (15, 0), (14, 2), (13, 4)
202
1.18
(12, 5), (13, 3), (9, 8)
215
1.10
(9, 7)
Five RBM peaks are observed under the 633 nm laser (
There are five RBM peaks identified under the 785 nm laser (
The most intense RBM peaks belong to (12,3), (9,9), (15,0), (14,2), (13,4), (12,5), (13,3), (9,8), and (9, 7) tubes, which are highlighted as red bars ((9,8) and (9,7) are shown in blue) and hexagons in
(6,5)
0.76
27.00
983
570
209.4
3.6%
(8,7)
1.03
27.80
1263
726
209.8
3.6%
(9,7)
1.10
25.87
1321
790
861.8
14.8%
(9,8)
1.17
28.05
1414
818
3007.6
51.7%
(10,6)
1.11
21.79
1384
754
447.7
7.7%
(10,8)
1.24
26.30
1467
870
188.7
3.2%
(10,9)
1.31
28.30
1559
886
395.2
6.8%
To obtain SWCNT suspensions, carbon deposits on filter membranes were further dispersed in 2 wt % SDBS (Aldrich) D2O (99.9 atom % D, Sigma-Aldrich) solution by sonication in a cup-horn ultrasonicator (SONICS, VCX-130) at 20 W for 1 h. After sonication, SWCNT suspensions were centrifuged for 1 h at 50 000g.
SWCNT suspensions obtained after centrifugation were characterized by PL and absorption spectroscopy. PL was conducted on a Jobin-Yvon Nanolog-3 spectrofluorometer with the excitation scanned from 450 nm to 950 nm and the emission collected from 900 nm to 1600 nm.
To further evaluate the abundance of metallic tubes which cannot be observed in PL analysis, UV-vis-NIR absorbance spectroscopy was carried out. UV-vis-NIR absorption spectra were measured from 500 nm to 1600 nm on the Varian Cary 5000 spectrophotometer.
UV-vis-NIR absorbance spectrum of the sample produced after catalyst reduction at 540° C. is shown in
A method based on the electron-phonon interaction model was used to reconstruct the UV-vis-NIR absorbance spectrum (see TABLES 5 to 7).
The modified methodology from Luo et al. (Luo et al., J. Am. Chem. Soc, 2006, 128, 15511-15516) was used to reconstruct UV-vis-NIR absorbance spectra. A baseline based on the power law (that is Aλ−b) curve was subtracted from the experimental spectrum as shown in
I(n,m)expPL is the experimental PL intensity of individual (n,m) tubes extracted from
E (n,m) values were obtained from PL measurement in TABLES 4A and 4B or from theoretical Kataura plots.
Following an analysis routine used in Wang, B et al. (Wang, B et al., J. Am. Chem. Soc., 2007, 129, 9014-9019), the contribution from (n,m) tubes identified in PL analysis was first considered. Their contribution (A (n,m)) to OD was directly calculated using experimental PL intensity from TABLE 4A. However, (n,m) tubes identified in PL analysis alone cannot reconstruct the absorption spectra well. Thus, additional semiconducting tubes identified in Raman analysis from TABLE 3, as well as other tubes with similar diameters, were added. The fitting result was significantly improved, as presented in
(9,7)
7.085
(10,6)
4.714
(9,8)
33.516
(10,9)
3.933
(12,4)
4.979
(15,1)
3.757
(14,3)
3.784
The relative abundance of individual semiconducting (n,m) tubes by the equation (5) was recalculated using the reconstructed absorption ES11 peak area from each (n,m) tube, and results are also listed in TABLE 5.
Next, the absorbance spectra belonging to the ES22 transition of semiconducting SWCNTs and EM11 transition of metallic SWCNTs between 500 nm and 935 nm were reconstructed.
There are two issues related to the reconstruction. First, the theoretical absorption intensity Wabscal (n,m) for ES22 transition is currently unavailable. Second, the EM11 transition of metallic SWCNTs overlaps with the ES22 transition of semiconducting SWCNTs in the same spectra range. In order to obtain an estimation of abundance for all (n,m) tubes in the SWCNT sample, the following protocol to address these two issues was proposed. Firstly, from the study of Popov et al. (Popov et al., Phys. Rev. B: Condens. Matter 2005, 72, 035436), the absorption matrix element patterns for the ES11 and ES22 transitions are similar, thus the theoretical absorption intensity Wabscal (n,m) of ES11 was directly used to approximate the theoretical absorption intensity of ES22. Secondly, a two-step reconstruction procedure was used to separate the contribution of semiconducting SWCNTs from metallic SWCNTs.
In the first step, the relative contribution to OD among each semiconducting (n,m) tubes was assumed to be similar in both ES11 and ES22 transitions, thus the A (n,m) values from ES11 transitions in TABLE 5 were used to reconstruct major ES22 peaks first. As shown in
Comparing the relative abundance of individual semiconducting tubes obtained from ES11 and ES22 reconstruction, no significant differences are observed. This supports our approach of using A (n,m) values from ES11 transitions to reconstruct major ES22 peaks first.
In the second step, the contribution of semiconducting tubes (the spectrum reconstructed by semiconducting SWCNTs only) was subtracted from the overall EM11+ES22 absorbance spectrum. Then, the remaining peaks of the absorbance spectrum (mostly between 500 nm and 800 nm) were reconstructed with possible metallic tubes. The metallic tubes are either identified in Raman analysis or tubes with similar diameters of major semiconducting tubes. All metallic tubes identified are listed in TABLE 7. The E (n,m) values of metallic tubes were obtained from the study by Maultzsch et al. (Maultzsch et al., Phys. Rev. B: Condens. Matter, 2005, 72, 205438). Their theoretical absorption intensity Wabscal (n,m) is currently not available the average value (2.155) of all semiconducting tubes identified in this study was used as an approximation for all metallic tubes. Similar to the reconstruction of ES11 spectrum, the relative contribution (A(n,m)) of individual metallic tubes to the OD was then calculated using equation (2). The reconstructed spectrum is shown in
3.349
3.100
The thin Lorentzian peaks (black) in
TGA was used to determine the yield of carbon species. As-synthesized SWCNTs together with catalyst substrates were characterized in TGA using a PerkinElmer Diamond TG/DTA Instruments. In a typical TGA, about 2 mg of the sample was loaded in an alumina pan. The sample was first heated to 200° C. and held at 200° C. for 10 min under airflow (200 sccm) to remove moisture. Afterward, its temperature was continuously increased from 200° C. to 1000° C. at a 10° C./min rate. The weight loss of the sample was monitored and recorded as a function of the temperature. The same procedure was repeated after the sample was cooled to room temperature to obtain the second weight-temperature curve for baseline correction.
TGA was used to determine the yield of carbon species.
The diameter of SWCNTs was also analyzed by TEM and AFM. TEM images of as-synthesized SWCNTs were recorded on a Philips Tecnai 12 microscope. SWCNT suspensions were dropcast on mica surfaces to form nanotube networks. AFM images of nanotubes were recorded on a MFP3D microscope (Asylum Research, Santa Barbara, Calif.) with a cantilever (Arrow NC, Nanoworld) operating in the tapping mode.
As shown in
The morphology, physical, and chemical properties of the CoSO4/SiO2 catalyst were evaluated by SEM, TEM, XRD, nitrogen physisorption, UV-vis-diffuse reflectance Spectroscopy (UV-vis-drs), H2-TPR, and element analysis.
To better understand the CoSO4/SiO2 catalyst, morphology of the catalyst was analysed using SEM and TEM. SEM images were obtained by using JEOL field-emission SEM (JSM-6701F) at 5 kV. TEM images were recorded on the Philips Tecnai 12 microscope. The solid samples were first dispersed in anhydrous ethanol by bath sonication for 30 min, and then one drop of the suspension was applied to a TEM grid covered with holey carbon film.
The physicochemical properties of the catalyst were further characterized by XRD, nitrogen physisorption, UV-vis spectroscopy, and H2-TPR. XRD measurement of CoSO4/SiO2 catalyst powders was carried out on a Bruker Axs D8 X-ray diffractometer (Cu KR, λ=0.15, 4 nm, 40 kV, 30 mA).
Nitrogen adsorption-desorption isotherms of the catalyst were measured at 77 K using a Quantachrome Autosorb-6b static volumetric instrument. Prior to the physisorption analysis, samples were degassed at 250° C. under high vacuum (<0.01 mbar). The specific surface area was calculated by the Brunauer, Emmet, and Teller (BET) method. The pore size and pore size distribution were calculated by the Barrett, Joyner, and Halenda (BJH) method using the desorption branch of the isotherms.
UV-vis diffuse reflectance spectra of the CoSO4/SiO2 catalyst and several references, such as Co3O4 (Aldrich), CoSO4 (Aldrich), and fumed silica (SiO2), were recorded on the Varian Cary 5000 spectrophotometer. The samples were first dried at 100° C. for 3 h, and then UV-vis spectra were recorded in the range of 200 nm to 800 nm with BaSO4 as a reference.
The reducibility of calcined catalysts was characterized by H2-TPR equipped with a thermal conductivity detector (TCD) of a gas chromatography (Techcomp 7900). Two-hundred milligrams of the catalyst or reference samples with equivalent Co loadings was loaded into a quartz cell. CoO, Co3O4, and CoSO4 (Sigma-Aldrich) were used as reference samples in TPR analysis. H2 (5%) in Ar was introduced to the quartz cell at 30 sccm. Pure Ar gas was used as a reference for the TCD. After the TCD baseline was stable, the temperature of the quartz cell was increased to 950° C. at 5° C./min and then held at 950° C. for 30 min. An acetone-liquid N2 trap was installed between the quartz cell and the TCD to condense water or H2S produced during the catalyst reduction.
The weight concentration of sulphur in the catalysts at different reduction conditions was determined by an Elementarvario CHN elemental analyzer. Around 5 mg of each treated catalyst was used for each test, and at least three samples from each treatment condition were measured to obtain the mean value.
The H2-TPR profile of the catalyst in
Previous experimental and theoretical studies predict a linear correlation between catalyst particle size and SWCNT diameter with their ratio ranging from 1.1 to 1.6. The (9,8) tubes at 1.17 nm produced after catalyst reduction at 540° C. suggest that catalytic particles have a narrow diameter distribution around 1.29 nm to 1.87 nm.
To verify this hypothesis, catalysts using XAS were investigated. XAS was used here because most small Co particles are under the surface of SiO2 particles, and it is difficult to quantify their diameters by TEM. The XAS spectra at the Co K-edge were recorded at the Beamline X18B at Brookhaven National Laboratory, USA. Three ex situ samples were measured, including the fresh CoSO4/SiO2 catalyst, the catalyst after SWCNT growth by reduction at 540° C., and a Co metal foil.
For catalyst samples, catalyst fine powder was pressed at about 2 tons into a round self-supporting wafer (1.5 cm in diameter) using a hydraulic pellet press to reach the optimum absorption thickness (Δμx≈1.0, Δμ is the absorption edge, x is the thickness of the catalyst wafer). Spectra were collected in a transmission mode at room temperature by scanning from 200 below the Co K-edge to 1000 eV above the Co K-edge using gas-filled ionization chamber detectors. The monochromator of this beamline was a double-crystal Si(111) with an energy resolution of approximately 0.2 eV. The XANES spectra at the sulfur K-edge were recorded at the Beamline X15B.
Four catalyst samples after different treatment conditions were measured. CoSO4.7H2O and CoS were used as references. The sample powder was brushed onto a thin strip of sulfur-free kapton tape, uncovered, facing the beam at 45°. Spectra were collected in a fluorescence mode at room temperature with the energy range of 2460 eV to 2500 eV with the step of 0.2 eV. Pure sulphur was used to calibrate the Si(111) monochromator.
The XAS experimental data at the Co K-edge were analyzed using the IFEFFIT program in three steps. (1) The XAS function (χ) was obtained by subtracting the post-edge background, and then normalized with respect to the edge jump step. (2) The normalized (E) was transferred from energy space to photoelectron wave vector k-space. The χ(k) data were multiplied by k2 to compensate for the damping of oscillations in the high k-region. Then the k2-weighted χ(k) data ink-space ranging from 2 Å−1 to 12.5 Å−1 for the Co K-edge were Fourier transformed to r-space to separate the contribution from the different coordination shells. (3) The spectra in the r-space between 1.1 Å and 3.35 Å were fitted using paths of metallic Co generated by the FEFF 9 to obtain parameters, including the first shell coordination number (NCo—Co), bond distance (R), and the Debye-Waller factor (Δσ2).
The near-edge spectra (XANES) at the Co K-edge in
The catalyst reduced at 540° C. after SWCNT growth has a NCo—Co of 7.04. The difference in bond distances with respect to the theoretical references (dR) is −0.016. The Debye-Waller factor (Δσ2) is 0.007, indicating that the fit is within acceptable limits. The first shell coordination number of nanoparticles is a nonlinear function of particle size, which has been used to quantify the nanoparticle size. Using a (111)-truncated hemispherical cubic octahedron model,
The structures of a series of Con (n=2, 3, 5, 13, 55, and 147) particles were fully relaxed to optimize without any constraint. All spin-polarized computations were performed with the Perdew-Burke-Ernzerhof (PBE) exchange correlation function using the VASP code. The interaction between an atomic core and electrons was described by the projector-augmented wave method. The plane-wave basis set energy cutoff was set to 400 eV. Periodic boundary conditions were implemented with at least 1 nm vacuum to preclude interactions between a cluster and its images. Simulation boxes were 22×22×C Å (where C is from 20 to 24 Å) for different calculated systems. The reciprocal space integration was performed with a 1×1×1 k-point mesh for all calculated systems with discrete characters.
On the basis of previous studies, Co particles with icosahedral structures are lower in energy than other structures. Co13, Co55, and Co147 adopt the icosahedral geometry. Co13 has one atom at the center and the other 12 identical atoms on the spherical shell surface with a coordination number of 6. The distance between the spherical shell and the central atom is 2.32 Å. The surface bond length is 2.44 Å. From the Co13 icosahedral structure, the Co55 was built by adding 30 atoms on the edge atoms of Co13 with a coordination of 8, and additional 12 atoms on the vertex atoms of Co13 with a coordination number of 6. Using the same methodology, Co147 was built by adding 80 atoms on the edge atoms of Co55 with a coordination of 8, and additional 12 atoms on the vertex atoms of Co55 with a coordination number of 6. Their diameters successively increase from about 0.46 nm to about 0.93 nm and 1.22 nm, respectively. The geometrical structure of these three clusters is illustrated in
The result in this work was compared with a number of previous SWCNT chiral selectivity growth studies, as listed in TABLE 9.
9Chen, Y. et al., J. Catal. 2004, 226, 351-362.
10Wei, L. et al., J. Phys. Chem. B 2008, 112, 2771-2774.
11Bachilo, S. M. et al, J. Am. Chem. Soc 2003, 125, 11186-11187.
12Miyauchi, Y. et al., Chem. Phys. Lett. 2004, 387, 198-203.
13Chiang, W. H. et al., Nature Mater. 2009, 8, 882-886.
14Yao, Y. G. et al., Nature Mater. 2007, 6, 283-286.
15Ghorannevis, Z. et al., J. Am. Chem. Soc 2010, 132, 9570-9572.
16He, M.; Chernov, A. I. et al., J. Am. Chem. Soc. 2010, 132, 13994-13996.
17Liu, B. L. et al., Chem. Commun. 2012, 48, 2409-2411.
18Loebick, C. Z. et al., J. Phys. Chem. C 2009, 113, 21611-21620.
19Zoican Loebick, C. et al., Appl. Catal., A 2009, 368, 40-49.
20Zhu, Z. et al., J. Am. Chem. Soc. 2011, 133, 1224-1227.
21Wang, H. et al., J. Am. Chem. Soc. 2010, 132, 16747-16749.
Especially, compared to the Co-TUD-1 catalyst, which has similar chiral selectivity toward (9,8) tubes, the carbon yield of the CoSO4/SiO2 catalyst is more than twice that of the Co-TUD-1 catalyst (1.5 wt %). Moreover, it would take 3 days to synthesize the Co-TUD-1 catalyst through aging, drying, and hydrothermal treatments, while the CoSO4/SiO2 catalyst can be produced by impregnation within 12 hours.
Overall, the CoSO4/SiO2 catalyst formed by a method of the invention shows several advantages: firstly, it provides a unique single chiral selectivity toward a large diameter tube; secondly, this catalyst has an adequate SWCNT yield, which is important for scalable production of SWCNTs; and thirdly, it is easy to synthesize, as compared to many mesoporous catalysts.
It is interesting to note that the selectivity of the CoSO4/SiO2 catalyst is toward (9,8) tubes rather than some other chiral species. The tentative nature of the following explanation is emphasized on the chiral selectivity toward (9,8) tubes in the spirit of stimulating further exploration in understanding the chiral selection mechanism in SWCNT growth. Previous theoretical studies on the structure stability of Ni2-55 and the electric dipole polarizability experimental study of Ni12-58 and Ptn (n=13, 38, and 55) showed that some nanoparticles with optimized structures are more stable than others.
Using the method of previous studies, the structure of Co particles was investigated and it was found that the optimized stable Co13, Co55, and Co147 particles adopt an icosahedral geometry. The detailed calculated results, including the average binding energy Eb, bond lengths from the central Co atom RCo-Cen, and surface bond lengths RCo—Co, are listed in TABLE 10.
The results show that the average binding energies increase with the increase of Co cluster size. The minimum Co—Co binding energy (3.67 eV for Co13) is higher than the binding energy of a Co2 dimmer (1.88 eV), and the strongest Co—Co binding energy (4.81 eV for Co147) is lower than the cohesive energy of the bulk Co (5.57 eV). The average interatomic distance also increases with the increase of the Co cluster size, varying between the bond distance of Co2 dimmer (1.97 Å) and bulk Co (2.51 Å).
As depicted in
Even though previous chirality selective growth studies may be able to tune (n,m) selectivity to some extent, none of the methods are able to achieve continuous changes of (n,m) selectivity over a wider diameter range. This suggests that matching with stable catalytic particles may be a fundamental requirement governing the growth of SWCNTs. It highlights that the efforts in achieving chiral-selective synthesis of SWCNTs should focus on growing chiral tubes with diameters similar to the most stable particles in their size range under growth conditions, other than seeking selectivity to random chiral structures. It should also be noted that adsorption and diffusion of carbon species during SWCNT growth can cause the reconstruction of catalytic particles, which may also change the (n,m) selectivity to some extent. This may explain why tubes, such as (9,7), (10,6), and (10,9) near the main (9,8), are also produced. Moreover, the chiral angle dependent growth rate could also be the reason of growing the large chiral angle (9,8) tubes, rather than other (n,m) species at the same diameter with smaller chiral angles.
From the catalyst design perspective, a key task is to find out what components in the CoSO4/SiO2 catalyst are responsible for stabilizing Co particles which leads to the growth (9,8) tubes. Cobalt oxides (CoOx) are usually reduced below 400° C., leading to large Co particles, which are easily covered by graphite layers during SWCNT synthesis. On the other hand, Co incorporated in some mesoporous SiO2 templates, such as MCM-41, or in cobalt silicates, is reduced at temperature above 700° C. They would form smaller Co particles, which are selective to smaller diameter tubes, such as (6,5) and (7,5). In our previous study of Co-TUD-1 catalyst, we proposed that Co species on the mesoporous TUD-1 can nucleate in two steps. First, Co2+ ions are partially reduced in H2 during pre-reduction, but they are still dispersed in an isolated manner on the large surface of TUD-1. Second, Co atoms aggregate quickly into clusters under CO to initiate SWCNT growth. Co ions are incorporated into the amorphous silica walls of TUD-1, and the large surface area of TUD-1 and the strong metal_support interaction are sufficient in stabilizing these clusters with a narrow diameter distribution at around 1.2 nm, responsible for the growth of (9,8) nanotubes.
However, the structure of the CoSO4/SiO2 catalyst is very different from the Co-TUD-1: first, Co ions cannot be incorporated into solid SiO2 particles by the impregnation method; secondly, the surface area of the CoSO4/SiO2 catalyst is much smaller (208 m2/g) as compared to TUD-1 (740 m2/g). Thus, the way the CoSO4/SiO2 catalyst controls the formation of Co particles is expected to be different from that of the Co-TUD-1.
Different Co precursors in catalyst synthesis, including cobalt (II) nitrate, cobalt (II) acetate, cobalt (II) acetylacetonate, and cobalt (III) acetylacetonate, were tested. None of the above Co precursors deposited on SiO2 particles showed good selectivity toward (9,8) tubes. Thus, it is postulated that the narrow reduction peak of the CoSO4/SiO2 catalyst at 470° C. may be credited to the reduction of highly dispersed CoSO4, following the chemical reaction eqs 1 and 2. The reduction of Co3O4 and CoO (chemical reaction eqs 3 and 4) was used as references to quantify the H2 consumption in CoSO4 reduction on the CoSO4/SiO2 catalyst.
Stoichiometric ratio of H2 needed for reducing the same amount of Co ions in CoSO4 over those in Co3O4 or CoO is 3.75-3 or 5-4, respectively. The integrated reduction peak area ratio between CoSO4 and Co3O4 in
CoSO4+5H2→Co+H2S+4H2O eq (1)
CoSO4+4H2→CoS+4H2O eq (2)
Co3O4+4H2→3Co+4H2O eq (3)
CoO+H2→CO+H2O eq (4)
The existence of sulfur compounds in the catalyst during SWCNT synthesis was verified using XAS and elemental analysis.
From the TPR result in
Previous studies showed that, when suitable amounts of sulfur are added in carbon precursors, not only does it promote the growth rate and the yield of carbon nanotubes it also strongly affects nanotube structures (such as shell number and diameter). One study proposed a mechanism that sulfur (from thiophene or carbon disulfide added in gas phase) would restrict the growth of Fe particles at about 1.6 nm for chiral selective growth of metallic (9,9) and (12,12) tubes. They also suggested that sulphur may form C—S bonds at the edge steps of the nanotube growth front, which lowers the activation energy of Stone-Thrower-Wales dislocation motion for SWCNT growth.
In this study, sulfur compounds are directly impregnated on the catalyst instead, and the growth temperature at 780° C. is much lower than the previous study at 1200° C. Thus, the Co particles would not be in a liquid state during SWCNT growth. It is postulated that sulfur could play two roles: First, the coexistence of sulfur atoms near Co atoms may limit the aggregation of Co atoms, which does not happen on catalysts prepared using other Co precursors without sulfur. Second, sulfur atoms may also form various Co—S compounds on Co particles, as indicated by the small CoS peak in XAS results (
In this work, it has been shown that the sulfate-promoted CoSO4/SiO2 catalyst is highly selective in growing large-diameter (9,8) SWCNTs. In contrast, the chiral selectivity reported by most previous studies is restricted to small-diameter (6,5) and (7,5) SWCNTs. After the catalyst is reduced in H2 at 540° C., it grows 51.7% (by PL, 33.5% by absorption) of (9,8) tubes. The total carbon yield over all catalyst materials used is 3.8 wt %, in which at least 90% is SWCNTs. The selectivity toward (9,8) tubes disappears if the catalyst is reduced at 780° C. The uniqueness of the CoSO4/SiO2 catalyst is that the highly dispersed CoSO4 is reduced in a narrow window near 470° C. XAS results indicate the formation of Co particles with average size of 1.23 nm, matching the diameter of (9,8) tubes. Experimental and theoretical results suggest a correlation between the most abundant (n,m) species and the stable Co particles of scattered sizes. This suggests that growing chiral tubes with diameters matching the most stable particles in their size range could be much easier than seeking selectivity to random chiral structures. Furthermore, XAS results show that the sulfur content in the catalyst changes after catalyst reduction at different conditions, which correlates with the (n,m) selectivity change observed.
Sulfur compounds incorporated in catalyst preparation may help to limit the aggregation of Co atoms and/or form various Co—S compounds, which contributes to the chiral selectivity.
The CoSO4/SiO2 catalysts with ˜1 wt % Co (based on the starting materials used) were prepared by the incipient wetness impregnation method.
Co (II) sulphate heptahydrate (Sigma-Aldrich≧99%) was dissolved in deionized water, and then added to the Cab-O-Sil M-5 silica powder (Sigma-Aldrich). Fumed silica produced by hydrolysis of SiCl4 at high temperature may be used. In the experiments, fumed silica was used because it is stable after high temperature treatment. Its porous structure provides sufficient surface areas to accommodate Co species. Fumed silica can also be easily dissolved in a NaOH solution, which facilitates SWCNT purification.
The mixture was aged at room temperature for 1 h, and dried in an open glass Petri plate at 100° C. for 2 h. The dried catalyst was ground into fine powders, calcined under a dry airflow in a fluidized bed calcinator from room temperature to a chosen calcination temperature, and kept at that temperature for 1 h before cooling to room temperature. It was found that the airflow rate, temperature increasing rate, and calcination time may affect the catalyst performance. The calcination temperature is the most critical parameter among them.
Other calcination parameters at their optimal conditions (i.e. airflow of 20 sccm per gram of catalyst from room temperature to a desired calcination temperature at 1° C./min, 5 grams of the catalyst each batch) were held, and only the calcination temperature was varied from 400° C. to 950° C.
The physiochemical properties of CoSO4/SiO2 catalysts obtained after different calcination treatments were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), nitrogen physisorption, H2—temperature programmed reduction (H2-TPR), UV-vis diffuse reflectance spectroscopy, element analysis (EA), and X-ray absorption spectroscopy (XAS).
Several reference samples were also used in catalyst characterization, including Co (II, III) oxides (99.8%, Aldrich), Co (II) oxide (99.99%, Aldrich) and Co silicate (ICN215905, MP Biomedicals).
SEM images of catalysts were obtained from a field-emission SEM (JEOL, JSM-6701F) at 5 kV.
XRD measurements of CoSO4/SiO2 catalysts were carried out on a Bruker Axs D8 X-ray diffractometer (Cu Kα, λ=0.15, 4 nm, 40 KV, 30 mA).
Nitrogen adsorption-desorption isotherms of catalysts were measured at 77 K using a Quantachrome Autosorb-6b static volumetric instrument. The samples were first degassed at 250° C. under high vacuum (<0.01 mbar). The specific surface area was calculated by the Brunauer, Emmet, and Teller (BET) method, while the pore size and pore size distribution were calculated by the Barrett, Joyner, and Halenda (BJH) method using the desorption branch of the isotherms.
H2-TPR was conducted in a TPR system equipped with a thermal conductivity detector (TCD, Techcomp 7900, Singapore). The CoSO4/SiO2 catalysts (200 mg) or Co reference samples with equivalent Co loadings were loaded into a quartz cell. H2 (5%) in Ar was introduced to the quartz cell at a flow rate of 30 sccm. Pure Ar gas was used as a reference for the TCD. After the TCD baseline was stable, the temperature of the quartz cell was increased to 950° C. at 5° C./min, and held at 950° C. for 30 min. An acetone-liquid N2 trap was installed between the quartz cell and the TCD to condense water or H2S produced during catalyst reduction.
UV-vis diffuse reflectance spectra of solid samples were collected on the Varian Cary 5000 spectrophotometer with an integrating sphere for solid-phase characterization.
The X-ray Absorption Near Edge Structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra at the Co K-edge (7709 eV) were collected at the beamline X18B, National Synchrotron Light Source at Brookhaven National Laboratory, USA. The monochromator of this beamline is a double-crystal Si (111). Catalysts were pressed into a round self-supporting wafer (1.5 cm in diameter) using a hydraulic pellet press under about 2 tons forces. The thickness of wafers was made near the optimum absorption thickness, where Δμx≈1.0 (Δμ is the absorption edge, and x is the thickness of the catalyst wafer). XAS spectra were collected in a fluorescence mode at room temperature by scanning from 200 below to 1000 eV above the Co—K edge using gas-filled ionization chamber detectors.
The XAS data at the Co K-edge were analyzed using the IFEFFIT program in three steps. First, the XAS function (χ) was obtained by subtracting the postedge background, and normalized with respect to the edge jump step. Next, the normalized χ(E) was transferred from energy space to photoelectron wave vector k-space. The χ(k) data were multiplied by k2 to compensate the damping of oscillations in the high k-region. Subsequently, the k2-weighted χ(k) data in k-space ranging from 2 Å−1 to 10 Å−1 for the Co K-edge were Fourier transformed to r-space to separate the contribution from the different coordination shells. Last, the spectra in the r-space between 0.8 Å and 2.0 Å were fitted using theoretical paths of Co3O4 and CoSO4 generated by the FEFF 9 program to obtain parameters, including the first shell coordination number (NCo—O), the bond distance (R) and the Debye-Waller factor (Δσ2).
The weight fraction of sulfur in catalysts was measured by an elemental analyzer (Elementarvario CHN). Before each test, all samples were dried at 100° C. overnight. About 5 mg sample was used in each test, and each catalyst sample was tested three times to obtain the average and standard errors.
The S K-edge XANES spectra of CoSO4/SiO2 catalysts were measured at the beamline 9-BM of the Advanced Photon Source at Argonne National Laboratory. Air absorption was eliminated by using He to purge the incident light path. The XANES spectra were collected in the total electron yield mode in the energy range of 2450 eV to 2600 eV, and up to 3 scans for each sample were collected and averaged to improve the signal-to-noise ratio. CoSO4.7H2O was used as a reference compound. The XANES data at S K-edge were processed using the EXAFSPAK software.
SWCNT growth was carried out in a horizontal chemical vapor deposition reactor. Catalysts were loaded in a ceramic boat at the center of the reactor. In typical growth conditions, 100 mg of the calcined CoSO4/SiO2 catalyst was first reduced under flowing H2 (1 bar, 50 sccm) from room temperature to 540° C. at a ramp of 20° C./min. Once the temperature reached 540° C., the reactor was purged with Ar, while its temperature was further increased to 780° C. Next, CO (99.9% from Alphagaz, Soxal, Singapore) was introduced into the reactor at 6 bar for 1 h. Carbonyl residues in CO gas were removed by a purifier (Nanochem, Matheson Gas Products, Montgomeryville, Pa., USA) before CO entered the reactor. The same growth conditions were employed for all catalysts.
As-grown SWCNTs deposited on catalysts were first examined by Raman spectroscopy. Raman spectra were collected on a Renishaw Ramanscope in the backscattering configuration over several random spots on each sample. Measurements were done under 514 nm and 785 nm lasers. The laser energy of 2.5 mW to 5 mW was used with an integration time of 10 s. The Raman signals from SWCNTs after catalyst removal were also measured, and they were similar to the signals obtained on as-grown SWCNTs.
Next, the catalysts loaded with carbon deposits were refluxed in a NaOH aqueous solution (1.5 mol/L) to dissolve silica substrates. Carbon deposits were filtered on a nylon membrane (0.2 μm pore). The filtered carbon deposits were further suspended in 2 wt % sodium dodecyl benzene sulfonate (SDBS) (Aldrich, Singapore) D2O (99.9 atom % D, Sigma-Aldrich, Singapore) solution by sonication in a cup-horn sonicator (VCX-130, SONICS, Newtown, Conn., USA) at 20 W for 1 h.
After sonication, the suspension was centrifuged at 50,000 g for 1 h. The clear SWCNT supernatant obtained after centrifugation was characterized by photoluminescence (PL) and ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectroscopy. PL signals were collected on a Jobin-Yvon Nanolog-3 spectrofluorometer with the excitation wavelength scanned from 450 nm to 950 nm and the emission wavelength collected from 900 nm to 1600 nm. The UV-vis-NIR absorption spectra were measured from 500 nm to 1600 nm on a Varian Cary 5000 spectrophotometer.
The carbon deposits were also characterized by thermogravimetric analysis (TGA). The total carbon yield was determined by analyzing weight loss of as-synthesized carbon deposits with catalysts. TGA was conducted on a PerkinElmer Diamond TG instrument. For a typical test, about 2 mg as-synthesized catalyst was placed in an alumina pan. The sample was heated to 200° C., and held for 10 min under airflow (200 sccm) to remove moisture. Subsequently, its temperature was continuously raised from 200° C. to 1000° C. at a 10° C./min rate. The weight loss was monitored and recorded as a function of the temperature. The same procedure was repeated after the sample was cooled to room temperature to get the second weight-temperature curve for baseline correction. The differential thermogravimetric (DTG) analysis was performed on the baseline corrected TG profiles.
TEM images were captured via a Philips Tecnai 12 microscope at 120 kV. The solid samples were dispersed in anhydrous ethanol by bath sonication for 30 min, and the homogenous dispersion was dropped on a TEM grid covered with holey carbon film for TEM analysis. Atomic force microscope (AFM) image of SWCNTs deposited on a silicon wafer was recorded via a MFP3D microscope (Asylum Research, Santa Barbara, Calif.) with a cantilever (Arrow NC, Nanoworld) operating in the tapping mode.
Raman spectroscopy is often used to evaluate the quality and (n,m) selectivity of SWCNTs based on their radial breathing mode (RBM), D band and G band features.
RBM frequencies are calculated as 223.5 cm−1/dt+12.5 cm−1, where dt is the diameter of SWCNTs. We used a combination of empirical and theoretical Kataura plots to identify the (n,m) structures of SWCNTs in our samples because the empirical plot is more accurate for the E11 and E22 van Hove transitions of semiconducting SWCNTs.
Corroborating with
As PL spectroscopy can only detect semiconducting SWCNTs, UV-vis-NIR absorption spectroscopy was used to complement the results from PL analysis.
Based on the relative intensity of their absorption peaks, it may be concluded that when the catalyst is calcined at low calcination temperatures, the dominant semiconducting (9,8) nanotubes have much higher abundance than metallic tubes. Raman, PL, and UV-vis-NIR absorption spectroscopy analyses consistently show that (a) the CoSO4/SiO2 catalyst is highly selective towards the large diameter single chirality (9,8) nanotubes; (b) the chiral selectivity of the catalyst is correlated with the catalyst calcination temperatures; (c) the catalyst calcined at 400° C. has the highest selectivity towards the (9,8) nanotubes; and (d) the chiral selectivity can possibly shift to small diameter nanotubes when the catalyst is calcined above 700° C.
The total carbon yield and selectivity to SWCNTs are both important in evaluating the performance of catalysts used for SWCNT synthesis. TGA was adopted to determine the carbon yield and different carbon species in the carbon deposits grown from the CoSO4/SiO2 catalyst. As depicted in
This suggests that the carbon yield from the CoSO4/SiO2 catalyst is passable for developing scalable SWCNT production processes. The carbon yield increases slightly with the increase of catalyst calcination temperature from 400° C. to 700° C., and then decreases when the calcination further increases to 900° C. The DTG profiles of the carbon deposits in
To further examine the morphology of carbon deposits, TEM images were captured on as-synthesized SWCNTs with catalysts. As seen in
Nitrogen adsorption was performed on purified SWCNTs. The SWCNTs were purified using the four-step purification method reported in Y. Chen et al., ACS Nano 1, 2007, 327-336. The purified SWCNTs have a surface area of 256 m2/g. Their adsorption isotherms as shown in
To understand how the different catalyst calcination temperature can affect the performances of the CoSO4/SiO2 catalyst in SWCNT synthesis, several characterization techniques were employed to study its physicochemical properties. The catalyst is supported on fumed silica. As rendered in
The structure of the catalyst is further characterized by XRD and nitrogen physisorption. As shown in
N2 physisorption isotherms in
H2-TPR is often used to investigate the metal support interaction and provide surface chemical information, such as stability, metal species, and metal distribution.
The uncalcined catalyst and those calcined at 400° C. and 600° C. all have a sharp peak around 460° C. to 470° C., which can be attributed to the reductive decomposition of highly dispersed CoSO4 on the SiO2 substrate. The reduction peaks from Co oxides and Co silicates are minor on their TPR profiles. When the catalyst calcination temperature rises to 800° C., there is a strong peak at 310° C., and its position lies between the peaks of CoO and Co304, advocating that the calcination at 800° C. may lead to the formation of Co oxides. When the catalyst is calcined at 950° C., a broad low intensity peak shows up from 600° C. to 950° C., which suggests the formation of various Co silicates, such as Co hydrosilicate, surface and bulk Co silicates H2-TPR results demonstrate that different Co species can be formed after catalyst calcination at different conditions. It is postulated that this may be the key reason for the observed chiral selectivity changes.
Surface chemistry of catalysts was further studied by UV-vis diffuse reflectance spectroscopy.
XAS was utilized to characterize the local chemical environment of Co atoms in the CoSO4/SiO2 catalyst.
Two spectroscopic features reveal significant differences among these catalysts. One is their preedge peaks and edge jumps shown in the insert of
In comparison, the peak at 7709 eV of the catalyst calcined at 800° C. locates between those of the CoO and Co3O4 references, and its edge jump is close to those of the Co3O4 and CoSiO3 references, implying that Co atoms in this catalyst are in a distorted tetrahedral structure. The other spectroscopic feature is the white line peak at 7725 eV, which is attributed to the unfilled d states of Co atoms at the Fermi level.
The intensity of the white line peak increases with the number of unfilled d states. Cobalt foil has a weak white line peak, while the CoSO4.7H2O has a strong white line peak. When the hydrated water is removed, the intensity of white line decreases a bit. The uncalcined CoSO4/SiO2 catalyst has a strong white line peak, which indicates that Co atoms are in an oxidized state. The white line of the catalyst calcined at 400° C. is almost identical to that of the uncalcined catalyst, suggesting that most of Co atoms in the catalyst are in an oxidized state after calcination at 400° C. The white line peak intensity of the catalyst calcined at 600° C. slightly decreases. In contrast, after the calcination at 800° C., the white line peak of the catalyst splits into two peaks, in which one at 7729 eV can be attributed to the existence of Co3O4, and the other at 7726 eV is similar to those from CoO and CoSiO3, which advocates the formation of Co oxides and Co silicates in the catalyst.
The extended X-ray absorption fine structure (EXAFS) of catalysts was Fourier transformed to r-space to separate the contribution from different coordination shells of Co atoms.
With the increase of catalyst calcination temperature, the Co—Co peak appears. The spectrum of the catalyst calcined at 800° C. is similar to those of Co3O4 and CoSiO3. The spectra in r-space were fitted using Co paths in both Co3O4 and CoSO4 generated by the FEFF 9 program to get the first shell coordination number (NCo—O) and the bond distance (RCo—O). In theoretical references, NCo—O is 4, 2, and 6, and RCo—O is 1.816 Å, 2.099 Å, and 2.133 Å in Co3O4, CoSO4, and CoO, respectively. Fitting results are listed in TABLE 19.
The Debye-Waller factors (Δo2) are 0.006-0.009, which means that the fitting is within acceptable limits. The NCo—O is in the range of 2.6-6.0, suggesting that Co atoms are in the distorted octahedral or tetrahedral environment. The NCo—O slightly increases when the catalyst calcined at 400° C. as compared to that of the uncalcined catalyst, and then drops when the calcination temperature was further increased, indicating that the catalyst is undergoing transitions. The fitting results of NCo—O obtained by using the Co paths from CoSO4 are higher than those obtained by using the Co paths from Co3O4. The deviation of the fitted bond distances (dR) is larger when the Co paths from Co3O4 are used, except for the catalyst calcined at 800° C. This indicates that the local environment of Co atoms is similar to that in CoSO4, when calcination temperature is below 600° C. The environment of Co atoms changes to become more like that in Co3O4, when the calcination temperature increases to 800° C.
Several SiO2 supported Co catalysts for SWCNT synthesis using different Co precursors have been evaluated: Co (II) nitrate, Co (II) acetate, Co (II) acetylacetonate, and Co (III) acetylacetonate. None of them shows a good selectivity to the (9,8) nanotubes. The results in this study suggest that the catalyst from CoSO4 behaves differently from the catalysts using other Co precursors. It is suspected that S plays an important role in the chiral selectivity of the CoSO4/SiO2 catalyst. The elemental analysis was first used to corroborate the existence of S in the catalyst.
Sulfur (S) content in catalysts after reduction in H2 at 540° C. during SWCNT growth was also measured. The S content in reduced catalysts is lower due to the reduction in H2. We can still observe a sharp drop when the calcination temperature changes from 600° C. to 700° C. Although the changing trend of S content does not exactly mirror the chiral selectivity change shown in
XAS was subsequently used to examine the chemical structures of S species in the catalyst. XANES spectra at the S K-edge of the CoSO4/SiO2 catalysts calcined at different temperatures are illustrated in
Based on characterization results of the CoSO4/SiO2 catalyst, it is postulated that the catalyst undergoes transitions at different calcination temperatures, as illustrated in
Previous theoretical studies predict a linear correlation between the size of metal particles and the diameter of SWCNTs with their ratio ranging from 1.1 to 1.6. It has also been proposed that the chiral selectivity comes from the different growth rates of SWCNTs, which correlates with the chiral angle of nanotubes. The selectivity towards the large chiral angle (9,8) nanotubes at 1.17 nm by the CoSO4/SiO2 catalyst suggests that the catalytic Co metal particles leading to their growth may have a narrow size distribution around 1.29-1.87 nm. Our results suggest that the unique Co and S structures formed on SiO2 surface at different catalyst calcination temperatures may influence the formation of Co particles for SWCNT growth. For the uncalcined catalyst and the catalyst calcined at 400° C., Co species are well spread on the large surface of SiO2 particles. Therefore, Co metal particles with a suitable size could be formed on SiO2 surface during SWCNT growth without severe aggregation. On one hand, the coexistence of S atoms near Co atoms may limit the aggregation of Co atoms, in contrast to catalysts prepared using other Co precursors without S. On the other hand, S atoms may also form various Co—S compounds, which could lead to the specific chiral selectivity towards the (9,8) nanotubes.
Based on current results, it cannot be concluded beyond doubt which of the above two roles played by S atoms is more important. With the increase of catalyst calcination temperature, some fractions of S atoms have been removed from the catalyst. The formation of surface Co oxides or Co silicates leads to the growth of Co metal particles in different sizes during the SWCNT synthesis. This is evident by the growth of the small diameter (6,5) nanotubes. Besides that, the abundance of the (6,5) nanotubes increases with the increasing catalyst calcination temperature and the decreasing S content in the catalyst. A previous study also reported that well dispersed Co silicates on SiO2 surface can grow small diameter tubes, such as (6,5), (7,5), (7,6) and (8,4). When the catalyst calcination temperature further raises to 800° C. and 900° C., it may lead to the formation of some bulk Co oxides and Co silicates, although we cannot detect them in XRD. The bulk Co silicates are inactive for SWCNT growth, which correlates with the drop of the observed carbon deposit yield. Furthermore, bulk Co oxides can be reduced into large Co particles, which leads to the growth of carbon fibers and graphite observed in TEM analysis. Lastly, it is postulated that the shift of (n,m) selectivity from small diameter (6,5) to large diameter (9,8) may be credited to the jump in the diameter of stable Co particles. The diameter of (6,5) and (9,8) nanotubes match with two stable Co clusters (Co55 at 0.93 nm and Co147 at 1.22 nm). Previous theoretical studies have investigated the stability of Ni and Pt clusters. The size of most stable metal clusters is at some scattered values.
In experiments, it is more likely to form stable metal clusters at certain sizes, other than continually tuning the size of metal clusters. Thus, when the size of stable Co particles changes from one (Co55) to the other (Co147), the (n,m) selectivity jumps accordingly. It should be noted that the complexity of the chemical nature of the compound catalyst, especially S may also influence the nucleation of Co particles, making it difficult to obtain a detailed mechanism at present.
The CoSO4/SiO2 catalyst prepared by impregnating 1 wt % Co from Co (II) sulphate heptahydrate on fumed silica powder is an active catalyst for SWCNT growth. The catalyst shows unique selectivity toward the large diameter single chirality (9,8) nanotubes. When the catalyst is calcined in air at 400° C., it yields 50.52% of (9,8) nanotubes among all semiconducting SWCNTs. The catalyst also possesses a passable carbon yield of 3.8 wt %, which is useful in developing a scalable SWCNT production process. The chiral selectivity of the catalyst is correlated with the catalyst calcination temperatures; the selectivity would shift to small diameter nanotubes when the catalyst is calcined above 700° C.
The catalyst calcination plays a critical role in forming active Co species on SiO2 surface for SWCNT growth. TEM, XRD and physisorption results show that the chiral selectivity change is not resulted from the morphology or physical structure changes of the catalyst. H2-TPR, UV-vis spectroscopy and XAS studies demonstrate that, at low calcination temperature (≦400° C.), Co ions adsorb on SiO2 surface through electrostatic interaction and/or form strongly bonded Co to the SiO2 surface through the oxolation reaction. Sulfur exists as chelating bidentate SO42− on the surface with Co atoms. The coexistence of S atoms near Co atoms may limit the aggregation of Co atoms or form various Co—S compounds, which may produce specific chiral selectivity towards the (9,8) nanotubes. With the increase of calcination temperature, some S atoms are removed from the catalyst, leading to the formation of surface Co oxides and Co silicates which are more selective to the small diameter SWCNTs. It is believed that novel sulfate promoted catalysts may be further developed to improve the chirality control and the yield of SWCNTs, which eventually reveal their enormous potentials in electronic and optoelectronic applications.
It is demonstrated herein that non-selective Co/SiO2 catalysts can be converted into efficient chiral selective catalysts by S doping. SWCNTs were characterized by photoluminescence (PL), UV-vis-near-infrared (UV-vis-NIR) absorption and Raman spectroscopies. Catalysts were characterized by elemental analysis, H2 temperature programmed reduction (H2-TPR), and UV-vis diffuse reflectance spectroscopy. The molecular structural changes of Co species on SiO2 caused by S doping are believed to be responsible for the chiral selectivity.
Three Co/SiO2 catalysts with 1 wt. % Co were prepared by the impregnation method using three Co precursors, including cobalt (II) acetylacetonate (Co(acac)2, Sigma-Aldrich, 97%), Co (II) chloride (CoCl2, Alfa Aesar, 97%), and Co (II) nitrate hexahydrate (Co(NO3)2.6H2O, Sigma-Aldrich, 99.999%). Co(acac)2 was dissolved in dichloromethane (Sigma-Aldrich, anhydrous, 9.8%), while Co(NO3)2.6H2O and CoCl2 was dissolved in deionized water. The Co precursor solutions were then added to fumed silicon dioxide powders (Cab-O-Sil, M-5, Sigma-Aldrich) with surface area of 254 m2/g. The mixtures were aged at room temperature for 1 h, and subsequently dried in an oven at 100° C. for 2 h. The dried catalyst was further calcined under airflow of 20 sccm per gram of catalyst from room temperature to 400° C. at 1° C./min, and then kept at 400° C. for 1 h. These three catalysts were denoted as CoACAC/SiO2, CoN/SiO2, and CoCl/SiO2.
In order to dope S into Co/SiO2 catalysts, the above calcined catalysts were impregnated by dilute sulphuric acid (H2SO4, 0.04 mol/L) at the 8 mL solution/g catalyst ratio for 1 h. Afterwards, the mixtures were dried and calcined again using the same procedure described above. The resulting S doped catalysts were denoted as CoACAC/SiO2/S, CoN/SiO2/S, and CoCl/SiO2/S, respectively.
SWCNTs were synthesized in a CVD reactor under the same condition for all catalysts. A catalyst was first reduced under pure H2 (1 bar, 50 sccm) from room temperature to 540° C. at 20° C./min, and then further heated to 780° C. under an Ar flow (1 bar, 50 sccm). At 780° C., pressured CO (6 bar, 200 sccm) replaced Ar and growth lasted for 1 h. The carbonyls in CO were removed by a Nanochem Purifilter from Matheson Gas Products.
As-synthesized SWCNTs with catalysts were first dissolved in NaOH aqueous solution (1.5 mol/L) to remove SiO2, and then filtered on a nylon membrane with 0.2 μm pores. Carbon deposits on filter membranes were further dispersed in 2 wt. % sodium dodecyl benzene sulphonate (SDBS, Aldrich) D2O solution by sonication using a cup-horn ultrasonicator (SONICS, VCX-130) at 20 W for 1 h.
SWCNT suspension obtained after centrifugation at 50,000 g for 1 h was characterised by photoluminescence (PL) and UV-vis-near-infrared (UV-vis-NIR) absorption spectroscopies.
PL was conducted on a spectrofluorometer (Jobin-Yvon, Nanolog-3) with the excitation scanned from 450 nm to 950 nm and the emission collected from 900 nm to 1600 nm.
The UV-vis-NIR absorption spectra were collected from 500 nm to 1600 nm on a spectrophotometer (Varian Cary 5000).
As-synthesized SWCNTs with catalysts and SWCNTs filtered on nylon membranes after SiO2 removal were both characterized by Raman spectroscopy. No significant differences were found on the two types of samples. Raman spectra were collected on a Ramanscope (Renishaw) in the backscattering configuration over several random spots on each sample under 514 nm and 785 nm laser excitations. The integration time of 10 s. Laser energy of 2.5 mW to 5 mW was used to prevent sample damages.
The physicochemical properties of catalysts were evaluated by elemental analysis, H2—temperature programmed reduction (H2-TPR), and UV-vis diffuse reflectance spectroscopy.
First, the weight fraction of Sin the doped catalysts was determined by an elemental analyzer (Elementarvario, CHN). Around 5 mg of catalyst sample was used for each test. Each type of catalyst was tested three times to obtain the average value. Before each test, all samples were dried at 100° C. overnight.
Next, the reducibility of Co species on undoped and S doped Co/SiO2 catalysts was characterised by TPR. CoO (Sigma-Aldrich, 99.99%), Co3O4 (Sigma-Aldrich, 99.8%), CoSiO3 (MP Biomedicals, ICN215905), CoCl2 (Alfa Aesar, 97%), and CoSO4.7H2O (Sigma-Aldrich, 99%) were used as references for TPR analysis.
The TPR experimental setup was equipped with a thermal conductivity detector (TCD) of a gas chromatography (Techcomp 7900). An acetone-liquid N2 trap was installed between a quartz cell and the TCD to condense water or H2S produced during catalyst reduction. In each test, 200 mg of catalysts or reference samples with equivalent Co loadings were loaded into a quartz cell. 5% H2 in Ar was introduced to the quartz cell at 30 sccm, and pure Ar gas was used as a reference for the TCD. After the TCD baseline was stable, the temperature of the quartz cell was increased to 950° C. at 5° C./min, and then held at 950° C. for 30 min.
Last, UV-vis diffuse reflectance spectra of catalysts and Co reference samples were recorded on the spectrophotometer (Varian Cary 5000). The samples were first dried at 100° C. for 3 h, and then UV-vis spectra were recorded in the range of 200 nm to 800 nm with BaSO4 as a reference.
PL maps in
In contrast, after doping with S, the major (n,m) products are large-80 diameter tubes (>1.1 nm), such as (9,8), (9,7), (10,6), and (10,9). The abundance of these four species calculated using their PL intensity is 52.4% to 69.1% of all semiconducting species identified, out of which, 32.7% to 40.5% is (9,8) (see TABLES 20 to 22).
PL results were corroborated by UV-vis-NIR absorption spectra.
SWCNTs were further characterized by Raman spectroscopy under two excitation lasers (785 nm and 514 nm).
The diameter of SWCNTs was calculated using the equation ΩRBM=223.5 cm−1/dt+12.5 cm−1, where ωRBM and dt are the RBM frequency and the diameter of SWCNTs. The RBM peaks in
In comparison, all S doped Co/SiO2 catalysts produce SWCNTs with high quality, as indicated by their intense RBM peaks and weak D band peaks (see
Raman results (see
UV-vis diffuse reflectance spectroscopy was used to study the surface chemistry of undoped and S doped Co/SiO2 catalysts.
One milligram of as-synthesized SWCNTs together with the catalyst (CoACAC/SiO2/S) was sonicated with 5 mL of anhydrous ethanol for 1 h, and a drop of the suspension was applied to a copper grid with holey carbon film. The grid was inserted into a Philips Tecnai 12 electron microscope, and TEM images were taken at an operation voltage of 120 kV.
TEM images in
The CoN/SiO2 catalyst was impregnated by ammonium sulfate ((NH4)2SO4, 0.2 mol/L) at the 8 mL solution/g catalyst ratio for 1 h, and subsequently dried in an oven at 100° C. for 2 h. The dried catalyst was further calcined under airflow of 20 sccm per gram of catalyst from room temperature to 400° C. at 1° C./min, and then kept at 400° C. for 1 h. The resulting S doped catalyst was denoted as CoN/SiO2/AS.
The PL map in
The strong absorption peaks at 1415 nm and 810 nm in
The TPR profile of CoN/SiO2/AS has an intense peak around 519° C., which is similar to that of CoN/SiO2/S shown in
Co species deposited on SiO2 would first be partially reduced in H2, and then nucleated into Co nanoparticles to initiate SWCNT growth. The change in (n,m) selectivity shown in
Next, H2-TPR was employed to study the reducibility of Co species.
CoN/SiO2 possesses a broad peak around 290° C. which can be attributed to Co3O4 and CoO. In contrast, all the TPR profiles of the three S doped Co/SiO2 catalysts have a sharp peak at 493° C. to 506° C. In addition, it is observed that the CoOx peaks of undoped catalysts become significantly smaller after S doping, and new peaks around 800° C. appear on CoCl/SiO2/S and CoN/SiO2/S. This observation suggests the formation Co hydrosilicate or surface Co silicate, while the 797° C. peak of CoACAC/SiO2 becomes smaller.
Lastly, UV-vis diffuse reflectance spectroscopy was used to probe the surface chemistry of the catalysts. As shown in
Doping sulfate ions to metal oxides has created various solid acid catalysts, such as SO42−/ZrO2, SO42−/TiO2, and SO42−/Fe2O3. Based on our characterization of the Co/SiO2 catalysts, the following mechanism is proposed to explain their (n,m) selectivity in SWCNT growth. As shown in
CoOx on CoN/SiO2 is reduced to form large Co particles, which are not selective to SWCNTs. Doping S through H2SO4 leads to the formation of chelating bidentate SO42−, where one S atom is linked to one Co atom through two O atoms, a common structure found in sulfate promoted metal oxide catalysts. This is supported by the sharp peaks at 493° C. to 506° C. in the TPR profiles and the broad peak around 535 nm in the UV-vis spectra.
It is proposed that the co-existence of S atoms near Co atoms may limit the nucleation of Co and/or form Co—S compounds, which change the selectivity of the catalysts to favor the formation of (9,8). The selectivity towards (9,8) may be attributed to the close match between carbon caps and the most stable Co particles in their size range, as well as the higher growth rate of high chiral angle tubes. As active Co nanoparticles are embedded under or near SiO2 surfaces, their size and composition are not able to be quantified in transmission electron microscope analysis (see
Furthermore, a reaction between H+ ions and CoOx is proposed, releasing Co ions to form well dispersed Co hydrosilicate and surface Co silicate on SiO2. This increases selectivity towards (9,8) SWCNTs. This is shown by the increased SWCNT selectivity of CoN/SiO2/S.
To further verify the proposed mechanism, CoN/SiO2 was doped using (NH4)2SO4. The same effect from SO42− was expected, but selectivity towards SWCNTs may be compromised since NH4+ is less reactive than H+. As shown in
In conclusion, a method to convert three types of Co/SiO2 catalysts has been demonstrated, which are either inactive for the SWCNT growth or only selective to small-diameter nanotubes, into chirally selective catalysts to grow SWCNTs enriched with large-diameter (9,8) tubes (up to 40.5%) by doping catalysts with S. It is also proposed in the mechanism that S atoms near Co atoms assist the formation of Co nanoparticles which are selective to (9,8) tubes. Moreover, H+ ions may react with CoOt to form well dispersed Co hydrosilicate and surface Co silicate on SiO2, which increases the selectivity to SWCNTs.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.
This application claims the benefit of priority of U.S. provisional application No. 61/609,703 filed on 12 Mar. 2012 and U.S. provisional application No. 61/753,645 filed on 17 Jan. 2013, the content of which are incorporated herein by reference in their entirety for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2013/000101 | 3/12/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61609703 | Mar 2012 | US | |
| 61753645 | Jan 2013 | US |
