Patent Application
20200262705
PRIORITY DOCUMENT
The present application claims priority from Australian Provisional Patent Application No. 2016904591 titled “PROCESSES FOR CONTROLLING STRUCTURE AND/OR PROPERTIES OF CARBON AND BORON NANOMATERIALS” and filed on 10 Nov. 2017, the content of which is hereby incorporated by reference in its entirety.
The present invention relates to processes for altering the structure and/or properties of carbon nanomaterials, such as carbon nanotubes and fullerenes, and boron nanomaterials, such as boron nitride nanotubes.
Carbon and inorganic nanomaterials of various dimensionalities have attracted significant attention due to their exceptional electrical, thermal, chemical and mechanical properties. There is a need for new processes for the fabrication of new forms of carbon nanomaterials and inorganic nanomaterials where possible devoid of stabilizing agents, and avoiding the use of harsh chemicals, with control over the shape, size and morphology, as a route to tailor their properties for specific applications.
For example, carbon nanotubes (CNTs) are one-dimensional cylindrical structures consisting entirely of carbon atoms that are used for a diverse range of applications such as in electronic devices, sensors, nanocomposite materials and drug delivery. Despite exhibiting extraordinary properties, there are a number of challenges in fabricating them which can limit their potential for use in applications. CNTs are usually grown millimeters in length with high degrees of bundling and aggregation of the strands. Thus, processing them within a liquid medium typically requires the use of surface active molecules, a high degree of functionalization, the use of toxic and harsh chemicals and long and tedious processing methods, and often with limited uniformity of the resulting material2-5. Current methods to overcome the problems associated with aggregation of CNTs are directed at controlling the length of CNTs at the nanoscale dimensions, using high-energy sonication, lengthy processing times and the use of toxic chemicals. Such processing can chemically alter the surface of the CNTs with consequential change to their chemical and physical properties, thereby limiting their applications. Developing methodologies to ease the processing of CNTs while maintaining the pristine nature of the material to be incorporated in applications is an important step forward in the use of these materials.
Other carbon nanomaterials, such as carbon nanodots, C60, C70 and the like, and inorganic nanomaterials, such as boron nitride nanotubes, have wide and varied applications but can suffer from similar problems in terms of producing the materials in a desired form and with a high degree of functionalization but without the use of surface active molecules, toxic and harsh chemicals and long and tedious processing methods.
There is thus a need to provide processes for enhancing and/or controlling properties and/or structures of carbon nanomaterials such as carbon nanotubes and fullerenes and inorganic nanomaterials, such as boron nitride nanotubes.
According to a first aspect, there is provided a process for producing a carbon nanotube product comprising predominantly carbon nanotubes (CNTs) having a desired average length, the process comprising:
In some embodiments of the first aspect, the CNTs are single wall carbon nanotubes (SWCNTs). In some other embodiments of the first aspect, the CNTs are multi walled carbon nanotubes (MWCNTs).
According to a second aspect, there is provided a process for producing a single walled carbon nanotube product comprising single walled carbon nanotubes (SWCNTs) enriched in either a metallic chirality or a semiconducting chirality, the process comprising:
In some embodiments of the second aspect, the energy source is a light source. In certain of these embodiments, the light source is a laser.
According to a third aspect, there is provided a process for dethreading double walled carbon nanotubes (DWCNTs) and multi walled carbon nanotubes (MWCNTs) to produce single walled carbon nanotubes (SWCNTs) therefrom, the process comprising:
According to a fourth aspect, there is provided a process for forming toroidal carbon nanoforms from single walled carbon nanotubes (SWCNTs), the process comprising:
According to a fifth aspect, there is provided a process for fabricating carbon nanodots, the process comprising:
According to a sixth aspect, there is provided a process for slicing inorganic nanotubes or nanowires, the process comprising:
According to a seventh aspect, there is provided a process for removing defects in single walled carbon nanotubes (SWCNTs), the process comprising:
According to an eighth aspect, there is provided a process for forming supramolecular fullerene assemblies, the process comprising:
Embodiments of the present invention will be discussed with reference to the accompanying figures wherein:
As used herein, and unless expressly stated otherwise, the following abbreviations used throughout this specification have the following meanings:
We previously developed a method for laterally slicing CNTs (single, double and multi walled) in the presence of a benign solvent system, N-methyl pyrollidinone (NMP) and water12. The processing method involved controlling mechanoenergy generated within dynamic thin films in a vortex fluidic device (VFD) and a simultaneous pulsed laser operating at 1064 nm wavelength. The conditions for the effective slicing of the CNTs was optimized by varying a number of control parameters (but not extensively), including concentration of the CNT dispersion, time of exposure to both the intense shear and irradiation from the pulsed laser, dependently and independently, flow rates under the continuous flow operation, changing the wavelength of the pulsed laser (to 532 nm), varying the laser power, and changing the rotational speeds and inclination angles of the tube in the VFD. This was to obtain sufficient shear to bend the CNTs and sufficient laser power to cleave C—C bonds, which occurs during the slicing process. Shear forces created in the VFD resulted in local bending of the CNTs, as established by the observation that toroidal arrays of SWCNTs were produced in a mixture of toluene and water in the VFD in the absence of laser irradiation13. Bending is not surprising given the very high aspect ratio for SWCNTs and the departure from laminar flow in the thin film in the VFD, and with the high C—C vibrational energy imparted by the laser, bond rupture prevails. To explore this further in understanding the mechanism of slicing, molecular dynamics simulations were carried out for SWCNTs, with hairpin-shaped tubes created to mimic the bending occurring in the VFD. When relaxed near room temperature, the hairpin unfolds and no defects are created. However, when the system is raised to a high temperature (i.e. mimicking the laser irradiation) a large tear occurs in the bent region and other defects appear nearby. The tear (damage) arising from the imparted high vibrational energy (equivalent to heating to high temperatures) occurs for bonds that are already strained. These observations explain the experimental result that slicing occurs in the VFD only under laser irradiation, and that slicing does not occur in batch processing in the presence of such a laser. Without the shear forces provided by the VFD, there is no or limited localized bending or strained bonds. These initial studies produced sliced carbon nanotubes without the ability to control the length and size distribution. Further research has established a number of important control aspects of manipulating CNTs in the VFD.
The reactor used in the processes described herein is a vortex fluidic device (VFD). Details of the VFD are described in published United States patent application US 2013/0289282, the details of which are incorporated herein by reference. Briefly, the thin film tube reactor comprises a tube rotatable about its longitudinal axis by a motor. The tube is substantially cylindrical or comprises a portion that is tapered. The motor can be a variable speed motor for varying the rotational speed of the tube and can be operated in controlled set frequency and set change in speed. A generally cylindrical tube is particularly suitable but it is contemplated that the tube could also take other forms and could, for example, be a tapered tube, a stepped tube comprising a number of sections of different diameter, and the like. The tube can be made of any suitable material including glass, metal, plastic, ceramic, and the like. In certain embodiments, the tube is made from borosilicate. Optionally, the inner surface of the tube can comprise surface structures or aberrations. In embodiments, the tube is a pristine borosilicate NMR glass tube which has an internal diameter typically 17.7±0.013 mm.
The tube is situated on an angle of incline relative to the horizontal of above 0 degrees and less than 90 degrees. In certain embodiments, the tube is situated on an angle of incline relative to the horizontal of between 10 degrees and 90 degrees. The angle of incline can be varied. In embodiments the angle of incline is 45 degrees. For the majority of the processes described herein, the angle of incline has been optimized to be 45 degrees relative to the horizontal position, which corresponds to the maximum cross vector of centrifugal force in the tube and gravity. However, other angles of incline can be used including, but not limited to, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9 degrees, 10 degrees, 11 degrees, 12 degrees, 13 degrees, 14 degrees, 15 degrees, 16 degrees, 17 degrees, 18 degrees, 19 degrees, 20 degrees, 21 degrees, 22 degrees, 23 degrees, 24 degrees, 25 degrees, 26 degrees, 27 degrees, 28 degrees, 29 degrees, 30 degrees, 31 degrees, 32 degrees, 33 degrees, 34 degrees, 35 degrees, 36 degrees, 37 degrees, 38 degrees, 39 degrees, 40 degrees, 41 degrees, 42 degrees, 43 degrees, 44 degrees, 46 degrees, 47 degrees, 48 degrees, 49 degrees, 50 degrees, 51 degrees, 52 degrees, 53 degrees, 54 degrees, 55 degrees, 56 degrees, 57 degrees, 58 degrees, 59 degrees, 60 degrees, 61 degrees, 62 degrees, 63 degrees, 64 degrees, 65 degrees, 66 degrees, 67 degrees, 68 degrees, 69 degrees, 70 degrees, 71 degrees, 72 degrees, 73 degrees, 74 degrees, 75 degrees, 76 degrees, 77 degrees, 78 degrees, 79 degrees, 80 degrees, 81 degrees, 82 degrees, 83 degrees, 84 degrees, 85 degrees, 86 degrees, 87 degrees, 88 degrees, and 89 degrees. If necessary, the angle of incline can be adjusted so as to adjust the location of the vortex that forms in the rotating tube relative to the closed end of the tube. Optionally, the angle of incline of tube can be varied in a time-dependent way during operation for dynamic adjustment of the location and shape of the vortex.
A spinning guide or a second set of bearings assists in maintaining the angle of incline and a substantially consistent rotation around the longitudinal axis of the tube. The tube may be rotated at rotational speeds of from about 2000 rpm to about 9000 rpm.
The thin film tube reactor can be operated in a confined mode of operation for a finite amount of liquid in the tube or under a continuous flow operation whereby jet feeds are set to deliver reactant fluids into the rapidly rotating tube, depending on the flow rate. Reactant fluids are supplied to the inner surface of the tube by way of at least one feed tube. Any suitable pump can be used to pump the reactant fluid from a reactant fluid source to the feed tube(s).
A collector may be positioned substantially adjacent to the opening of the tube and can be used to collect product exiting the tube. Fluid product exiting the tube may migrate under centrifugal force to the wall of the collector where it can exit through a product outlet.
Controlling the length of CNTs within nanoscale dimensions offers a new pathway towards uptake for length specific applications. Depending on the growth process, CNTs are typically grown millimetres in length, which poses a number of challenges for processing within liquid media. These problems are often due to the low dispersibility in most organic solvents and the strong aggregation between the strands which makes them quite challenging to process, to exploit and to enhance their properties. Another key challenge is obtaining control over the lengths of the CNTs. There have been a number of attempts reported on such control, but they require the use of concentrated acids, the addition of stabilising agents, high temperature processing and lengthy processing times.
Debundled, short SWCNTs show great potential in a variety of applications, such as for drug delivery6, including the incorporation in lipid bilayers for sensing7, to increase the efficiency of solar cells10 and others. For example, short length CNTs enhance the efficiency of electronic devices8,9. Shorter CNTs provide efficient hole transportation having a few nm transportation path while maintaining high conductivity. Moreover, bundled long stranded tubes have raised concerns within the biological arena, with increasing toxicity levels in proportion with the length of the nanotubes. Shorter length CNTs within a narrow length distribution have more potential for biological applications14,15. For example, the use of CNTs with a large length range distribution, 200 to 1000 nm was observed to clog the bloodstream in vivo. Short CNTs within a narrow length distribution, approximately 50 to 300 nm is an ideal length as drug carriers in treating the Alzheimer's disease16.
With the understanding from molecular dynamic simulations of the mechanism of slicing, shear forces in the VFD cause localised bending and strained bonds with a simultaneous pulse laser providing sufficient energy to rupture the strained C—C bonds, affording sliced nanotubes within a particular length distribution12. Thus, controlling the length of the CNTs requires a method to control the extent of localised bending of the CNTs and energy input from the laser. The amount of laser power required to rupture the strained bonds is dependent on the extent of localised bending. We systematically studied the controlled bending of CNTs by altering the rotational speed of the VFD, along with varying the laser power; combining the two inputs allows one to control the length of sliced CNTs. Our results show that lower shear rates in the VFD (rotational speed 6500 rpm) and higher laser power (600 mJ) under the continuous flow mode of operation affords sliced nanotubes with much shorter lengths, with an average of 40-50 nm (
Thus, according to a first aspect there is provided a process for producing a carbon nanotube product comprising predominantly carbon nanotube (CNTs) having a desired average length. The process comprises providing a composition comprising starting CNTs. The composition comprising starting CNTs is introduced to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a predetermined rotational speed and the CNT composition in the thin film tube reactor is exposed to laser energy at a predetermined energy dose. The carbon nanotube product comprising predominantly CNTs having a desired average length is then recovered from the thin film tube reactor. The predetermined rotational speed is from about 6000 rpm to about 7500 rpm, the predetermined energy dose is from about 200 mJ to about 600 mJ and the values of the predetermined rotational speed and the predetermined energy dose are selected to produce SWCNTs having an average length of from about 50 nm to about 700 nm.
In certain embodiments of the first aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments, the CNTs having a desired average length have an average length of 40-50 nm, 75 nm, 85 nm, 150 nm, 200 nm, 300 nm, 500 nm or 680 nm. Notably, the distribution of the average length of CNTs formed according to the process of the first aspect is narrower than the distribution of the average length of CNTs formed in earlier published work12. Furthermore, in the earlier work12 the average length of the CNTs formed was ˜160-170 nm.
The composition of starting CNTs comprises a solvent or liquid phase. In certain embodiments, the solvent or liquid phase comprises water. In certain other embodiments, the solvent or liquid phase comprises a mixture of water and a solvent. In certain other embodiments, the solution of starting CNTs comprises a solvent. Suitable solvents include dipolar aprotic solvents and protic solvents. Examples of suitable solvents include, but are not limited to: N-methyl-2-pyrollidone (NMP), tetrahydrofuran, ethers, alcohols, ionic liquids, eutectic melts, and supercritical solvents.
The composition of starting CNTs may be in the form of a solution, dispersion, suspension or emulsion.
Advantageously, the composition of the composition of starting CNTs can be selected to determine the average length of the CNTs formed. For example, CNTs having an average length of 220 nm can be formed at a predetermined rotational speed of 7500 rpm, a predetermined energy dose of 260 mJ and a solution of starting CNTs comprising NMP and water in a 1:1 ratio, whilst CNTs having an average length of 150 nm can be formed at a predetermined rotational speed of 7500 rpm, a predetermined energy dose of 260 mJ and a composition of starting CNTs consisting essentially of water.
In certain embodiments, the starting CNTs are pre-treated prior to formation of the composition of starting CNTs. For example, the starting CNTs may be oxidised prior to formation of the composition of starting CNTs. The starting CNTs may be oxidised using an oxidant. The oxidant may be selected from the group consisting of: peroxides capable of producing hydroxyl radicals, such as hydrogen peroxide; singlet oxygen generated in situ or otherwise; organic peroxides; bleach materials and the like; and reactive species from an oxygen plasma generated in situ in the VFD. Oxidation may be used to increase the solubility of the starting CNTs in the solvent or liquid phase used in the composition comprising starting CNTs.
In certain embodiments, the predetermined rotational speed is 6500 rpm and the predetermined energy dose is about 600 mJ.
In certain embodiments, the composition of starting CNTs is introduced to the thin film tube reactor in a continuous flow.
In certain embodiments, the composition of starting CNTs is introduced to the thin film tube reactor as batch of fixed volume.
In certain embodiments, the CNTs are single wall carbon nanotubes (SWCNTs). In certain other embodiments, the CNTs are multi walled carbon nanotubes (MWCNTs).
To control the lengths of the CNTs, pristine (as received) CNTs were functionalised using a previously published method17. The CNTs were dispersed in two different solvent systems, (a) NMP/water and (b) water. The oxidised CNTs were then treated under intensive shear within the VFD in the presence of a pulsed laser operating at 1064 nm wavelength at 260 mJ to afford narrow length distributions of short CNTs, with average lengths of approximately 220 nm and 150 nm respectively, with a much narrower distribution in comparison to the initial published work12 (
An alternative route to control the lateral slicing of CNTs (single, double and multi-walled) is to use a pulsed laser of more than one wavelength, i.e. 532 nm wavelength or a continuous laser of other light sources. This allows systematically controlling the length of the laterally sliced CNTs. The method involves controlling the amount of power required from combined simultaneous 1064 nm and 532 nm wavelength lasers to precisely afford CNTs of specific length upon bending under intense shear. Suitable conditions include a combined laser power of 368 mJ (260 mJ from the 1064 nm wavelength and 108 mJ from the 532 nm wavelength) under optimised conditions in the VFD (i.e. a tilt angle of 45° and a rotational speed of 7500 rpm) to afford sliced CNTs with an average length of approximately 300 nm. The optimisation of the laser power from lasers of more than one wavelength offers an alternative route to control the length of the sliced CNTs.
CNTs subjected to the shear forces created in the VFD resulted in localized bending and strained bonds which then combined with heating from the laser at the point of bending resulted in rupture of the C—C bonds. Thus, the understanding of this mechanism led to the development of a method to control the lengths of CNTs down to ca 600 nm, 300 nm and 80 nm by changing the rotational speed of the VFD and the amount of laser power used to cleave the C—C bonds. These lengths are deemed important for specific applications such as in electronic devices and drug delivery applications.
A single wall carbon nanotube (SWCNT) can be thought of as a cylindrical structure formed by rolling up a graphene sheet. The electronic and optical properties of SWCNTs are dependent on the direction and magnitude of the rolling vector, being either semiconducting (s) or metallic (m) depending on the chiral angle and the diameter of the tube19. The energy bandgap of semiconducting CNTs are inversely proportional to the nanotube diameter. Many advanced applications require high purity CNTs with well-defined structures and electrical properties. For example, the semiconducting configuration is required for nanoscale field-effect transistors while the metallic configurations are used in nanoscale circuits. With the various current methods of growth consisting of a complex mixture of both the semiconducting and metallic chiralities, there is a need to separate or convert (interconvert) them, to manipulate their properties accordingly.
To avoid the need for surfactants and other chromatographic methods of separation that are low yielding and high costs, we developed a simple and novel method to enrich sliced CNTs into the metallic and semiconducting configuration. Specifically, according to a second aspect, there is provided a process for producing a single walled carbon nanotube product comprising single walled carbon nanotubes (SWCNTs) enriched in either a metallic chirality or a semiconducting chirality. The process comprises providing a composition comprising starting SWCNTs having metallic and semiconducting chiralities. The composition comprising starting SWCNTs having metallic and semiconducting chiralities is introduced to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a rotational speed, the composition comprising starting SWCNTs having metallic and semiconducting chiralities is exposed to an energy source and the tube is maintained at the rotational speed and the aqueous solution of SWCNTs is exposed to energy from the energy source for a time sufficient to produce the single walled carbon nanotube product comprising SWCNTs enriched in either a metallic chirality or a semiconducting chirality.
In certain embodiments of the second aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the second aspect, the rotational speed is 7500 rpm.
In certain embodiments of the second aspect, the energy source is a light source. The light source may be a laser, such as a Nd:YAG laser. The laser may operate at a wavelength of 1064 nm at a laser power of about 260 mJ.
In certain embodiments of the second aspect, the composition comprising starting SWCNTs comprises a mixture of water and a solvent. Suitable solvents include dipolar aprotic solvents and protic solvents. Examples of suitable solvents include, but are not limited to: N-methyl-2-pyrollidone (NMP), tetrahydrofuran, an ether, an alcohol, an ionic liquid, a eutectic melt, and a supercritical solvent.
In certain embodiments of the second aspect, the composition comprising starting SWCNTs is introduced to the thin film tube reactor in a continuous flow.
In certain embodiments of the second aspect, the composition comprising starting SWCNTs is introduced to the thin film tube reactor as batch of fixed volume.
In certain embodiments of the second aspect, the nanotube product comprises single walled carbon nanotubes (SWCNTs) enriched in metallic chirality. In certain of these embodiments, the light energy is provided by a pulsed Nd:YAG laser. In certain of these embodiments, the light energy provided by the laser is about 260 mJ.
In certain embodiments of the second aspect, the nanotube product comprises single walled carbon nanotubes (SWCNTs) enriched in semiconducting chirality. In certain of these embodiments, the light energy is provided by one or more circular polarised pulsed laser sources.
In certain embodiments of the second aspect, the method is used to generate optically pure SWCNTs of a specific (n,m).
Specifically, under both confined mode and continuous flow operations, as received SWCNTs comprising of a mixture of semiconducting and metallic chiralities are sliced in a mixture of NMP/water at a 1:1 ratio in the presence of shear in the VFD to bend the high tensile strength SWCNTs and a pulsed Nd:YAG laser to break the strained C—C bonds. The ballistic wave from the pulsed laser at 260 mJ laser power overcomes the large barrier of energy, changing the magnitude and rolling vector of the semiconducting nanotubes affording the metallic configuration.
The sliced SWCNT sample was also characterized using photoluminescence (PL) contours (
It is expected that the use of circular polarised pulsed laser sources, or other light sources, can be used to convert/interconvert SWCNTs of different chiralities, and indeed may be effective in generating optically pure SWCNTs of a specific (n,m).
Dethreading of multiwalled carbon nanotubes involves the spontaneous removal of the inner shells to gain access to single walled carbon nanotubes of progressively larger diameters. According to a third aspect, there is provided a process for dethreading double walled carbon nanotubes (DWCNTs) and/or multi walled carbon nanotubes (MWCNTs) to produce single walled carbon nanotubes (SWCNTs) therefrom. The process comprises providing a composition comprising DWCNTs and/or MWCNTs, a liquid phase and a surfactant. The composition is introduced to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a rotational speed and the composition is exposed in the thin film tube reactor to light energy. The tube is maintained at the rotational speed and the composition is exposed to the light energy for a time sufficient to produce SWCNTs.
In certain embodiments of the third aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the third aspect, the rotational speed is 7500 rpm.
In certain embodiments of the third aspect, the liquid phase comprises water.
In certain embodiments of the third aspect, the surfactant is a relatively large hydrophobic surfactant. In certain of these embodiments, the surfactant is p-phosphonated calix[n]arene, where n=4, 5, 6, and 8, but other surfactants are envisaged, including for example, and related p-sulfonated calix[n]arenes, where n=4, 5, 6 and 8, and general classes of surfactants such as dodecyl sulfate and the like, and polymer and co-polymers, including natural polymers (such as peptides and DNA) and synthetic polymers such as polyethylene glycol and the like. In specific embodiments, the surfactant is p-phosphonated calix[n]arene, where n=8.
In certain embodiments of the third aspect, the composition is introduced to the thin film tube reactor in a continuous flow.
In certain embodiments of the third aspect, the composition is introduced to the thin film tube reactor as batch of fixed volume.
In certain embodiments of the third aspect, the light energy is provided by a pulsed Nd:YAG laser. In certain of these embodiments, the light energy provided by the laser is about 260 mJ.
In certain embodiments of the third aspect, the process is used to control the length of DWCNTs within a length range of approximately 300-400 nm with and without dethreading. Dethreading of the DWCNTs and MWCNTs is possible during in situ slicing in the presence of shear in the VFD, coupled with a pulsed laser, and a surfactant, or post VFD processing (
We note (i) that reducing the length of CNTs (see above), and removal of defects, which essentially straightens them, will facilitate movement of the concentric layers of SWCNTs in the DWCNTs and MWCNTs relative to each other, (ii) this affords longer CNTs, as a further example of controlling the length.
We have also found that the VFD is effective in debundling and overcome the high flexural rigidity of the CNTs to form tightly coiled toroidal structures13. Thus, according to a fourth aspect there is provided a process for forming toroidal carbon nanoforms from single walled carbon nanotubes (SWCNTs). The process comprises providing a water/hydrocarbon solvent dispersion of SWCNTs and introducing the dispersion to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a rotational speed and in a rotational direction under conditions to form toroidal carbon nanoforms from the SWCNTs.
In certain embodiments of the fourth aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the fourth aspect, the hydrocarbon solvent is selected from the group consisting of: an aromatic solvent such as toluene, o-xylene, m-xylene, p-xylene or mesitylene; an aliphatic hydrocarbon such as pentane, hexane, etc; and water immiscible liquid hydrocarbon materials such as natural oils (e.g. canola oil) and synthetic oils (e.g. biodiesel and the like).
In certain embodiments of the fourth aspect, the toroidal carbon nanoforms are in the form of figure of 8 nanoforms, the chirality of which is controlled using the rotational direction.
In certain embodiments of the fourth aspect, the rotational speed is about 7500 rpm. In these embodiments, the reaction time may be about 30 minutes.
In certain embodiments of the fourth aspect, the diameters of the rings of the figure of 8 nanoforms produced are within the range of from about 300 to about 700 nm, or from about 100 nm to about 200 nm.
We have found that the shear stress generated in the VFD provides sufficient energy to bend the CNTs to the extent where the ends come in contact and spontaneously fuse under high mechanical energy in the VFD. In addition, for long processing times, chiral “figure of 8” structures can be formed with an excess of one chirality, due to the direction of the fluid flow in the VFD under the confined mode of operation. Changing the direction of rotation during the synthesis of the “figure of 8” will change the dominance of one chirality over the other for the “figure of “8”. Passing solutions back through the VFD may further increase the enantiomeric excess of one chiral figure of 8 over another, with reversing the direction of rotation likely to reverse the chirality of the enantiomer in excess.
Cdots are carbon nanoparticles with dimensions of <10 nm in size consisting of a graphitic structure or amorphous carbon core and carbonaceous surfaces, with the basal places rich in oxygen-containing groups22. Similar to other carbon nanomaterials, Cdots exhibit exceptional properties in particular the strong quantum confinement and edge effects resulting in exceptional fluorescent properties23. A number of methods have been reported but with significant limitations affording Cdots without uniformity in shape, size and morphology24. These include using chemical ablation24, electrochemical carbonisation25, laser ablation26, arc-discharge27, ultrasound and microwave-assisted pyrolysis28, which afford Cdots in low yield and with low photoluminescence efficiency.
We developed a method using a Nd:YAG laser at a 1064 nm wavelength in the presence of different organic solvents to fabricate fluorescent carbon nanoparticles from graphite powder. The method afforded carbon nanoparticles using laser irradiation coupled with high energy sonication of a wide diameter range between 1-8 nm29. Thus, according to a fifth aspect there is provided a process for fabricating carbon nanodots. The process comprises providing or forming an aqueous composition comprising oxidised MWCNTs and introducing the aqueous composition to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a rotational speed and the aqueous composition in the thin film tube reactor is exposed to light energy. The tube is maintained at the rotational speed and the aqueous composition exposed to the light energy for a time sufficient to produce carbon nanodots.
In certain embodiments of the fifth aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the fifth aspect, the light energy is provided by a laser. In certain embodiments, the laser operates at 1064 nm, 532 nm, 266 nm, and combinations thereof. In certain embodiments, the laser is a pulsed laser. In certain embodiments, the laser operates at a power of about 260 mJ. In certain other embodiments, the laser operates at a power of about 450 mJ.
In certain embodiments of the fifth aspect, the rotational speed is about 7500 rpm.
In certain embodiments of the fifth aspect, the concentration of MWCNTs in the aqueous composition comprising oxidised MWCNTs is about 0.1 mg/mL.
In certain embodiments of the fifth aspect, the carbon nanodots produced are relatively uniform in shape and size.
In certain embodiments of the fifth aspect, the oxidised MWCNTs are formed in situ by introducing an aqueous composition comprising MWCNTs and an oxidant capable of oxidising MWCNTs to the thin film tube reactor. The oxidant may be selected from the group consisting of: peroxides capable of producing hydroxyl radicals, such as hydrogen peroxide; singlet oxygen generated in situ or otherwise; organic peroxides; bleach materials and the like; and reactive species from an oxygen plasma generated in situ in the VFD. In certain embodiments, the carbon nanodots produced have a size of about 6 nm.
In certain embodiments of the fifth aspect, the process further comprises centrifuging the reaction product mixture and separating solid product comprising carbon nanodots from the supernatant.
In certain other embodiments of the fifth aspect, the aqueous composition comprising oxidised MWCNTs is formed by dispersing oxidized MWCNTs in a mixture of water and a solvent. Suitable solvents include dipolar aprotic solvents and protic solvents. Examples of suitable solvents include, but are not limited to: N-methyl-2-pyrollidone (NMP), tetrahydrofuran, ethers, alcohols, ionic liquids, eutectic melts, and supercritical solvents.
In certain embodiments of the fifth aspect, the carbon nanodots produced have a size of less than about 4 nm, such as about 2 nm.
The newly developed process overcomes the drawbacks of conventional processing methods, to fabricate Cdots in high yield with uniformity in the shape and size, of about 6 nm. The Cdots are fabricated by debundling and disintegrating MWCNTs (or other forms of carbon) in the presence of hydrogen peroxide (30% in water), in the presence of intensive shear and a pulsed laser operating at 1064 nm (but not limited to this wavelength or the use of pulse irradiation). Aqueous H2O2 was chosen due to high amounts of hydroxyl free radicals produced in the presence of an irradiation from a pulsed laser30. The laser irradiation absorbs the photons, which then break down H2O2 into water molecules and extremely reactive radicals of oxygen. The free oxygen radicals then chemically attack CNTs, like in large organic-pigmented molecules with double bonds and long carbon chains broken into small ones via rapid oxidation14.
MWCNTs were purchased from Sigma Aldrich, prepared using the chemical vapour deposition method with an as-received purity >98%. MWCNTs (10 mg) was dispersed in 60 mL of 30% H2O2 (˜0.2 mg/mL), following ultrasonication (˜5 minutes) to afford a stable black dispersion. Under the continuous flow mode of operation, the MWCNT dispersion was introduced into the rapidly rotating tube at a flow rate of 1 mL/min using conditions of θ 45° and a rotational speed of 7500 rpm with a simultaneously nanosecond pulsed laser at 1064 nm (pulsed Q-switch Nd:YAG laser) operating at a power of ca 260 mJ (
Advantageously, the production of Cdots using the VFD is under continuous flow and thus the process is scalable.
In the presence of H2O2, the as-received MWCNTs were disintegrated into regular shaped carbon dots with an average diameter of 6 nm (
To further confirm the graphitic nature of the Cdots, Raman mapping using a 532 nm wavelength laser was conducted on a specific area with highly dense distribution of the Cdots (confirmed by SEM imaging) (
XPS spectra of the Cdots indicated a distribution of 70.5 at. % of C, 29.5 at. % of 0 compared to the as received samples with 98.46 at. % of C and 1.54 at. % of O (
The preparation of Cdots by laser-assisted VFD processing is not limited to the current reported size range. The amount of hydroxyl radical generated is dependent on the H2O2 concentration and irradiation time of the pulsed laser30,31. Thus, varying the concentration of H2O2 and the irradiation time from the pulsed laser can be used to produce Cdots with various sizes and higher yield. Controlling the size of Cdots is important in tuning the fluorescence properties of the particles. For instance, the excitation wavelength of Cdots can be red-shifted as the size of the particles increase32. In addition, Cdots fabricated using this method are ready to be employed in sequential chemical functionalisation because non-functionalised edges of Cdots are highly chemical-reactive33. This can be used for emission tuning of functionalized Cdots which can be red-shifted when adding amine34 or fluorine35 groups and blue-shifted when N-doped36.
An alternative method to fabricate Cdot with size distributions of <4 nm was also developed. The method involves oxidising as received MWCNTs using the previously published method.3 The oxidised MWCNTs (O-MWCNT) were then dispersed in a mixture of NMP/water at a 1:1 ratio to obtain high yielding Cdots with a size distribution of about 1 nm. Changing the solvent system was critical in terms of controlling the size of the particles with the fabrication of Cdots in water being possible under similar conditions but with lower yields, and with the Cdots with average size of approximately 2 nm. Upon acid reflux, the as received oxidised MWCNTs are separated via ultracentrifugation based on the different lengths to obtain more control over the size distribution of the Cdots, ideally producing a much narrower size distribution (
The absence of laser radiation under the equivalent VFD conditions simply resulted in debundling of MWCNTs. To further decouple the effect of the VFD and the laser irradiation, a pulsed laser at an optimized power of 450 mJ was directed towards the CNTs dispersed in H2O2 mixed using a magnetic stirrer in a quartz cuvette rather than in a VFD tube. This resulted in minimal conversion of the MWCNTs into Cdots, with large bundles and aggregates of MWCNTs still present.
To determine the optimised conditions for fabricating the Cdots, as-processed samples were centrifuged at 1180×g to remove any aggregates or bundled nanotubes before atomic force microscopy (AFM). Operating parameters of the VFD and laser were systematically varied under continuous flow, changing one parameter at a time en route to the optimised conditions. For rotational speeds below 6500 rpm at a 45° tilt angle, apart from the presence of large bundles, short length CNTs (about 300 nm) were observed after processing (
Raman spectroscopy was used to verify the crystalline nature and degree of sp2 hybridisation of the Cdots in comparison to the as-received MWCNTs. Processing with the laser operating at 532 nm showed lower Cdot formation and poorer sample homogeneity relative to those prepared under the optimised conditions (θ 45°, 7500 rpm rotational speed) using a NIR laser operating at 1064 nm (
Post-VFD processing, centrifugation improved the sample purity by removing the large bundled CNTs but this led to a significant loss of Cdot material in the pellet. For generating practical quantities of the Cdots, no centrifugation was applied. The conversion of MWCNTs to Cdots may be further improved by lowering the starting material concentration from 0.5 to 0.1 mg/mL (
After two cycles of laser-VFD processing, the Raman spectra of Cdots show a typical graphitic spectrum with the D-band at 1352 cm−1 (1346 cm−1 for MWCNTs), and the G-band at 1594 cm−1 (1586 cm−1 for MWCNTs) (
TEM and AFM established that the as-prepared Cdots were quasi-spherical and showed an average height ca. 6 nm (from 3 to 13 nm) (
The Cdots obtained using the optimal processing conditions had good water solubility and colloidal stability, with little or no change in their optical properties over several weeks, and these are distinctly different from those of as received MWCNTs (
The scalability of the process was investigated by processing 50 mg of as received MWCNTs dispersed in 500 mL of H2O2. Approximately 40% of starting material was converted to Cdots, as deduced from residual material remaining in the syringe and the VFD tube post processing. The yield of dialysed Cdots which showed negligible cytotoxicity was ca. 10%, based on the total amount of initial MWCNT. 2D-Fluorescence maps of the Cdots showed a maximum excitation wavelength of 345 nm and an emission at 450 nm (blue in the visible region) (
AFM, TEM, Raman, FT-IR, XPS and PL of the Cdots are consistent with the proposed structure shown in
Thus, the processes described herein provide a simple and relatively benign method using a VFD to produce water soluble Cdots with scalability incorporated into the processing. At least one set of optimum operating parameters correspond to a sample concentration of 0.1 mg/mL, rotational speed of 7500 rpm, 0.45 mL/min flow rate, with a laser power of 450 mJ. The Cdots exhibit excitation wavelength dependent PL behavior with two distinctive emission peaks around 420 and 460 nm, being an integration of at least three emissive sites originated from the aromatic core, defects and functional groups. CDs are chemically reactive and could be potentially used for further chemical functionalisation. Importantly, VFD processing favours more product homogeneity in the dynamic thin film in the microfluidic platform, with product quality independent of the sample volume passing from the VFD.
It is envisaged that the intrinsic fluorescence of the Cdots may be tuned by controlling the size of Cdots which is crucial for red-shifting of the excitation wavelength. Furthermore, catalytic peroxidase enzymes, such as HRP and lignin peroxidase, may assist in accelerating the degradation of nanotubes in the presence of H2O2.
These processes for producing Cdots described herein are without precedent, with the ability to afford Cdots with uniformity in size and shape using a green chemistry approach. The method avoids the use of concentrated acids and stabilizing agents, avoiding by products and a much lower cost of processing.
Beside carbon nanotubes, there exist various inorganic nanotubes including boron nitride nanotubes (BNNTs), silicon nanotubes, gallium nitride nanotubes, titania nanotubes, tungsten(IV) sulphide nanotubes and composite boron, and carbon and nitrogen (BCN) nanotubes. Furthermore, there exist various inorganic nanowires, such as silver nanowires.
Using boron nitride nanotubes (BNNTs) as an example, BNNTs are structurally similar to CNTs, consisting of alternating B and N atoms arranged in a honeycomb crystal lattice affording a one atom thick hexagonal boron nitride layer. BNNTs are electrical insulators with a bandgap of approximately 5.5-5.8 eV which is independent of the direction and rolling vector of the BN sheets. Their wide band gap, high chemical and thermal stability and excellent mechanical properties make them ideal materials for nanodevices, high performance nanocomposite materials, biomedical applications such as drug delivery and most importantly for boron neutron capture therapy (BNCT)37 and boron nitride capture in general, for example on the walls of space craft. Similar to CNTs, the issues pertaining to the processing of BNNTs involves strong aggregation of the long strands and the need to disperse them in organic solvents, which limits their potential for applications. For biological applications in specific, the long strands, which can be several microns in length, which can be highly toxic in biological samples, when introduced into the bloodstream38.
We have developed a process to uniformly lateral slice inorganic nanotubes or nanowires and control the length devoid of surfactants, chemical functionalisation of the walls and in the presence of a benign solvent system. According to a sixth aspect, there is provided a process for slicing inorganic nanotubes or nanowires. The process comprises providing a solvent dispersion of starting inorganic nanotubes or nanowires and introducing the solvent dispersion of starting inorganic nanotubes or nanowires to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a predetermined rotational speed and the solvent dispersion of starting inorganic nanotubes or nanowires in the thin film tube reactor is exposed to light energy. Sliced inorganic nanotubes or nanowires are then recovered.
In certain embodiments of the sixth aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the sixth aspect, the light energy is provided by a laser.
In certain embodiments of the sixth aspect, the rotational speed is about 7500 rpm.
In certain embodiments, the laser operates at 1064 nm, 532 nm, 266 nm, or combinations thereof. In certain embodiments, the laser is a pulsed laser. In certain embodiments, the laser operates at a power of about 600 mJ.
In certain embodiments, the inorganic nanotubes or nanowires are selected from one or more of the group consisting of boron nitride nanotubes (BNNTs), silicon nanotubes, gallium nitride nanotubes, titania nanotubes, tungsten(IV) sulphide nanotubes and composite boron, carbon and nitrogen (BCN) nanotubes, and silver nanowires. In certain specific embodiments, the inorganic nanotubes or nanowires are BNNTs.
In certain embodiments of the sixth aspect, the solvent of the solvent dispersion is selected from one or more of the group consisting of: an alcohol, such as a C1-C6 alcohol; tetrahydrofuran; and ethers; an ionic liquid; a eutectic melt; and a supercritical solvent.
The process is scalable under the continuous flow mode of operation.
In certain embodiments of the sixth aspect, the process further comprises centrifuging the reaction product mixture and separating solid product comprising sliced inorganic nanotubes or nanowires from the supernatant.
Defects-free CNTs show significant improvement in electronic conductance and mechanical properties. According to a seventh aspect, there is provided a process for removing defects in single walled carbon nanotubes (SWCNTs). The process comprises providing a solution or dispersion of oxidised SWCNTs and introducing the solution or dispersion of oxidised SWCNTs to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube rotated about the longitudinal axis at a predetermined rotational speed and the solution or dispersion of oxidised SWCNTs in the thin film tube reactor is exposed to light energy. The reduced defect SWCNTs are then recovered.
In certain embodiments of the seventh aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the seventh aspect, the light energy is provided by a laser. In certain embodiments, the laser operates at 1064 nm, 532 nm, 266 nm, or combinations thereof In certain embodiments, the laser is a pulsed laser. In certain embodiments, the laser operates at a power of about 260 mJ.
In certain embodiments of the seventh aspect, the rotational speed is about 7500 rpm.
In certain embodiments of the seventh aspect, the solution or dispersion of oxidised SWCNTs is formed by dispersing oxidized SWCNTs in water, a solvent or a mixture of water and a solvent. Suitable solvents include dipolar aprotic solvents and protic solvents. Examples of suitable solvents include, but are not limited to: N-methyl-2-pyrollidone (NMP), tetrahydrofuran, ethers, alcohols, ionic liquids, eutectic melts, and supercritical solvents.
In certain embodiments of the seventh aspect, the process further comprises forming oxidised SWCNTs from SWCNTs by treatment with an oxidant. The oxidant may be selected from one or more of the group consisting of: nitric acid; hydrogen peroxide; singlet oxygen generated in situ or otherwise; organic peroxides; bleach materials and the like; and reactive species from an oxygen plasma generated in situ in the VFD. In certain embodiments, the oxidant is nitric acid.
We observed that post VFD-laser processing, precipitation of O-MWCNT was observed in the VFD tube (
Fullerene (C60) can assemble into a variety of architectures offering unique properties with potential specifically in photovoltaics39 and other electronic, magnetic and photonic applications.40,41 In organic photovoltaics in particular, there has been significant amounts of attention devoted towards developing novel materials of various morphologies and dimensions as donor materials. On this note, the self-assembly of fullerene, C60 molecules into three dimensional microcrystals has been one of the most favoured carbon nanomaterial for its high surface, to be used in organic solar cells due its excellent electron conductivity and efficient charge separation capabilities at the electron donor/acceptor interfaces39. The various architectures of nano and micron scale dimensions, using a green metrics approach, in being devoid of surface contaminating material, of high surface area would offer a route towards fabricating novel architectures for improved electrical conductivity and photoconductivity.
According to an eighth aspect, there is provided a process for forming supramolecular fullerene assemblies. The process comprises providing a fullerene solution comprising one or more fullerenes in a solvent and introducing the fullerene solution to a thin film tube reactor comprising a tube having a longitudinal axis, wherein the angle of the longitudinal axis relative to the horizontal is between about 0 degrees and about 90 degrees. The tube is rotated about the longitudinal axis at a predetermined rotational speed and supramolecular fullerene assemblies are recovered.
In certain embodiments of the eighth aspect, the angle of the longitudinal axis relative to the horizontal is about 45 degrees.
In certain embodiments of the eighth aspect, the rotational speed is from about 5000 rpm to about 800 ppm, such as about 5000 rpm, about 7500 rpm or about 8000 rpm.
In certain embodiments of the eighth aspect, the fullerene is selected from C60, C70, C76, C78 and C84. In certain specific embodiments, the fullerene is C69, but it is envisaged that mixtures of different fullerenes will form nano-structures of varying size, shape and morphology, and similarly for fullerene(s) in combination with other nano-materials, as detailed above, including sliced carbon nanotubes, carbon dots, and sliced boron nitride nanotubes.
In certain embodiments of the eighth aspect, the solvent is an aromatic solvent such as toluene, o-xylene, m-xylene, p-xylene and mesitylene, and/or any other solvent that solubilises C60 and other fullerenes, as well as mixtures of solvents, and solvents containing surfactants.
The ability to organise fullerene C60 molecules into flowerlike supramolecular assemblies was observed under controllable shear within dynamic thin films in the VFD. Using a solution of C60 dissolved in toluene, the size and morphology of the flowerlike microcrystals were dependent on the concentration of the C60/toluene solution and the rotational speed of the VFD. At a 45° inclination angle, the stable microcrystals rapidly form at room temperature, within minutes of processing time under the confined mode of operation. The size, dimensions and yield of the crystals was determined by the concentration of the C60/toluene solution, 0.05 mg/mL and 0.1 mg/mL at rotational speeds of 5000 rpm and 8000 rpm (
The formation of these distinct architectures devoid of surfactants is without precedent, and their accessibility is directly related to the high shear forces in the thin films in the VFD. The intense micromixing dramatically lowers the solubility of the fullerene, resulting in controlled nucleation and growth of such structures. The dynamic nature of the liquid also results in solvent evaporation under shear because of the waves and ripples in the thin film, effectively increasing the concentration of the fullerene in a given volume of liquid. However, the effect of evaporation is expected to be lower than the reduction in solubility associated with the high shear. Overall, the ability to fabricate functional nanocarbon material in this way is significant in the field, eliminating the need for annealing the nanostructures at high temperature and to remove any surfactants used to control the radial growth under diffusion controlled batch processing. Post shearing the fullerene material does not spontaneously re-dissolve, which is consistent with the well-known slow dissolution of the fullerene in a variety of solvents.42 The same outcome is then predictable for fullerene C70 and other high fullerenes. Moreover, this phenomenon of reducing the solubility has general implications in solution processing, in accessing a material with control over the nucleation and growth of complex materials.
SWCNTs were purchased from Sigma Aldrich, as chemical vapour deposition prepared material with an as-received purity >95%. Sample preparation included the addition of the SWCNTs (1 mg) into a sample vial containing a mixture of NMP and water (6 mL) at a 1:1 ratio. The solution mixture was then ultrasonicated for 5 minutes, affording a black stable suspension. Under the continuous flow of operation, jet feeds were set to deliver the CNT suspension (0.1 mg/mL) into the rapidly rotating 20 mm borosilicate NMR glass tube (ID 16.000±0.013 mm) at a rotating speed of 6500 rpm and at a tilt angle of 45 degrees. Simultaneously, a nanosecond pulsed laser processing system with an energy of approximately 600 mJ was applied to the rapidly rotating system for a period of time. Centrifugation (g 3.22) of the resulting solution for the confined mode of operation was required to remove any large agglomerates, unsliced bundled CNTs and impurities in the sample.
The method involves the use of controllable mechanoenergy within dynamic thin films in the VFD while the tube is irradiated with a pulsed Nd:YAG laser operating at a wavelength 1064 nm at a laser power of about 260 mJ. Under both confined mode and continuous flow modes of operation of the device, as received SWCNTs comprising of a mixture of semiconducting and metallic chiralities undergoes lateral slicing and in situ conversion (interconversion) to afford metallic enriched SWCNTs. For the confined mode of operation, a finite volume of total liquid is required which was set at 1 mL. This ensures that a vortex is maintained to the bottom of the tube for moderate rotational speeds to avoid different shear regimes, and without any liquid exiting at the top the tube. Stewartson/Ekman layers prevail in the dynamic thin films, which arise from the liquid accelerating up the tube with gravitational force acting against them. The effectiveness of the process was then investigated under continuous flow, using jet feeds delivering the SWCNT dispersion into the rapidly rotating tube at a flow rate of 0.45 mL/min. These experiments used similar optimised conditions to what was established for the lateral slicing of CNTs. The VFD was at an inclination angle of 45° and a rotational speed of 7500 rpm.
Preparation of aqueous suspensions of CNTs. DWCNTs were purchased from Carbon Allotropes with an as received purity >99%. p-Phosphonic acid calix[8] arene (p-H2O3 P-calix[8] arene) was synthesised following the literature method.43 Milli-Q water was used for preparing the 10 mL aqueous suspensions of CNTs. Aqueous dispersions of DWCNT (1 mg) in water (6 mL) were prepared in the presence of p-phosphonic acid calix[8] arene (1 mg/L). Each solution mixture was ultrasonicated for 5 minutes, affording a black stable dispersion. Under the confined mode of operation of the VFD, the solution mixture (1 mL) was then placed in the glass tube and rotated at 7500 rpm, at a tilt angle of 45 degrees. Simultaneously, a nanosecond pulsed laser processing system with an energy of approximately 260 mJ was applied to the rapidly rotating system for 30 minutes. Under continuous flow mode, jet feeds with a flow rate at 0.45 mL/min deliver the CNT suspension (similar concentration, as for the confined mode) into the rapidly rotating tube. Centrifugation (g=3.22) of the resulting dispersion for the confined mode of operation was required to remove any large agglomerates, bundled CNTs and impurities in the sample. The suspension of DWCNTs was then further ultracentrifugated (g ˜16900) for 30 minutes to remove the excess calixarene. The centrifuge-washing step was repeated 3 times to ensure there was no excess calixarenes present. The above method was then repeated using a mixture of NMP and water (6 mL) at a 1:1 ratio.
SWCNTs (1.0 mg) were dispersed in toluene (3 mL) and added to MilliQ water (3 mL). Sonication for 10 minutes afforded a stable two-phase dispersion. A 1 mL portion of the mixture under sonication was collected to ensure that it was a uniform mixture of the three components, and was placed in a 20 mm (I.D=20.000±0.013 mm) or 10 mm (I.D=7.100±0.013 mm) diameter VFD tube, as standard borosilicate glass NMR tubes. The chirality of the ‘figure of 8’ was controlled by controlling the optical rotation of the borosilicate NMR tube in the VFD. A systematic evaluation of the operating parameters of the VFD was carried out to ascertain the optimal parameters for the formation of high yielding figure of 8 nanostructures to be at an inclination angle of 45° with the 20 mm VFD tube rotating at 7500 rpm, for a reaction time of 30 minutes. The diameters of the rings produced were within the range of 300 to 700 nm, as established using atomic force microscopy (AFM) and for a 10 mm diameter tube a significantly smaller diameter range, 100 to 200 nm was achieved (
MWCNTs were purchased from Sigma Aldrich, prepared using the chemical vapour deposition method with an as-received purity >98%. MWCNTs (10 mg) was dispersed in 60 mL of 30% H2O2 (˜0.2 mg/mL), following ultrasonication (˜5 minutes) to afford a stable black dispersion. Under the continuous flow mode of operation, the MWCNT dispersion was introduced into the rapidly rotating tube at a flow rate of 1 mL/min using optimized conditions of θ 45° and a rotational speed of 7500 rpm with a simultaneously nanosecond pulsed laser at 1064 nm (pulsed Q-switch Nd:YAG laser) operating at a power of ca 260 mJ. Centrifugation of the clear dispersion collected (1180×g) for 30 minutes was essential to remove bundled long MWCNTs and any impurities still present in the sample. The pellet containing the Cdots was washed multiple times with Milli-Q water. The washed Cdots were then dispersed in Milli-Q water and ultracentrifuged (11200×g) for 30 min. The Cdots with a yield of ˜62% were recovered for characterization purposes using SEM, AFM, Raman, XPS and TEM.
Boron nitride nanotubes (BNNTs) were purchased from Sigma Aldrich, with an average diameter of 5 nm±2 nm. BNNTs were dispersed in isopropanol (˜0.1 mg/mL), following ultrasonication (˜2 minutes) to afford a stable milky dispersion. Under the continuous flow mode of operation, the BNNTs dispersion was introduced into the rapidly rotating tube at a flow rate of 0.10 mL/min using an inclination angle, θ 45° and a rotational speed of 7500 rpm with a simultaneously nanosecond pulsed laser at 1064 nm (pulsed Q-switch Nd:YAG laser) operating at a power of ca 600 mJ. Centrifugation of the clear dispersion collected (1180×g) for 30 minutes was essential to remove bundled long BNNTs and any impurities still present in the sample. The sliced BNNTs were characterized SEM and AFM (
The sliced boron nitride nanotubes afforded are approximately 300-600 nm in length (
SWCNTs were purchased from Sigma Aldrich, prepared using the chemical vapour deposition method with an as-received purity >98%. As-received SWCNTs (0.3 g) were dispersed in 25 ml of the HNO3 (65 wt %) and reflux at 120° C. for 48 h. The resulting dispersion was diluted and washed using MilliQ water and filtered using a 0.45 μm membrane. The sample was dried in oven at 80° C.
For a typical experiment, the functionalized SWCNTs (0.1 mg) was dispersed in 1 ml of MilliQ water and was processed in the VFD (45 degrees inclination angle and a rotational speed of 7500 rpm) with a simultaneous pulsed laser (pulsed Q-switch Nd:YAG laser) at a 1064 nm wavelength at a laser power of 260 mJ for 30min. The post processed sample was soluble in MilliQ water and was then directly characterized using Raman spectroscopy (
In a typical experiment C60 (99685-96-8, 99+%, BuckyUSA) was added to toluene at different concentrations (0.05 mg/mL and 0.1 mg/mL) and the mixture allowed to stand overnight, whereupon it was filtered to remove any undispersed C60 and impurities. C60 dissolved in toluene (1 mL) was placed in in a glass tube, as a readily available borosilicate nuclear magnetic resonance (NMR) tube (ID 16.000±0.013 mm), which was spun for 30 minutes at an optimized speed of 5000 rpm and 8000 rpm respectively at an inclination angle of 45 degrees. For the confined mode of operation, a finite volume of total liquid is required which was set a 1 mL. The scalability of the process was then investigated under continuous flow, using one jet feeds delivering the above toluene solution of C60 at a 0.45 mL/min. The C60 nanostructures were characterized using SEM (
Fullerene C60, with the purities of 99.5% and 99+% were purchased from Sigma Aldrich and Bucky US, respectively. Fullerene C70 with 99.5% purity was purchased from Bucky US. Both fullerenes were directly used as received without any further purification. Toluene with a purity 99.9%, o-xylene, m-xylene, p-xylene, and mestlyine with purities ≥99% were also purchased from Sigma Aldrich, and used to dissolve the fullerenes. They were used to compare the influences of different solvents on the crystallisation of C60 and C70.
Solutions of C60 were prepared at different concentrations, namely 0.05, 0.1 0.2, 0.5 and 1 mg/mL. This involved added solid material to the solvent, with the mixture left for 24 hours at room temperature.
The samples were then filtered to remove undissolved particles and the supernatant were used immediately in the VFD experiments, as shown in
The two operation modes of the VFD confined mode (CM) and continuous flow mode (CF) were used in the formation of C60 nano- and micron-sized particles. For CM, 1 mL of C60 in toluene (concentration=0.05 mg/mL) was injected into the tube pre VFD processing, and this volume was used for all subsequent experiments to ensure that the fluid dynamic response is the same for a specific speed, at a fixed tilt angle of 45°. The rotational speed was varied from 5 krpm up to 8 krpm in imparting a diverse range of shear stress. Each CM experiment was carried out over 30 min, and thereafter the liquid was collected and processed. This involved centrifugation at 1.751 RCF, and collecting the precipitate by decanting, and filter it using filter paper. The solid material takes hours to redissolve (see below) such that there is sufficient time to collect the material with minimal re-dissolution post VFD processing. The optimal conditions were found at 5 krpm, and 7.5 krpm for C60 assembled into stellated and rod like structures, respectively, as shown in
Increasing the rotational speed increases the centrifugal force experienced by the liquid, based on the centrifugal force law:
F
C
=m·ω
2
·r
where FC is centrifugal force, in is mass, ω2 is angular speed and r is reduce of rotation.
Thus, the higher the centrifugal force, the greater the cross vector of this force and gravitational force, resulting higher share stress in the thin films, with gravity pulling down the liquid and rotational forces directing the liquid up the tube. The difference in shear stress clearly affects the nature of the particles formed, as the solubility under shear decreases, with onset of nucleation and growth.
For scalable, continuous flow (CF) processing, the solution of fullerene C60 was delivered into the hemispherical base of the inclined rapidly rotating tube via a jet feed, using a programmed syringe pump. This is while systematically exploring the parameter space of the VFD, namely rotational speeds, flow rate and tilt angle, along with concentration of the fullerene. The product was collected through the outlet tube at the top of the device, with the residence time for a finite volume of liquid depending on the flow rate, {dot over (v)} and rotational speed ω. With optimising conditions of concentration to 0.1 mg/mL (C60 in toluene), {dot over (v)}=0.1 mL.min−1, ω=4 krpm and θ=45°. Decreasing the flow rate results in increases the residence time and thus the time of shear stress as the liquid moves through the tube, results in self-assembled C60 as stellated particles, close to uniform in size as shown in
Studies were undertaken to further systematically explore the parameter space, in changing the speed, flow rate and tilt angles. Stellated C60 particles were the sole product formed at ω=4 krpm, {dot over (v)}=0.1 mL/mm, C=0.1 mg/mL , θ=45°, as shown in
The ability to control the nucleation and growth of both stellated and rods of self-assembled C60, without adding an anti-solvent, and without adding a surfactant is without precedent. Moreover, the stellated particles have not previously been reported. Normally the growth of particles of the fullerene requires an anti-solvent. The processes described herein were run in the absence of anti-solvent. Clearly, the shear stress in the dynamic thin film in the VFD reduces the solubility of C60. Under high shear, it is hypothesised that the solvation shell stabilising individual fullerene molecules is disturbed leading to favourable fullerene-fullerene van der Waals interactions resulting in the nucleation and growth of self-assembled fullerene C60. In addition, the ability to form different structures by changing the processing parameters of the VFD, in particular the rotational speed, 4 krpm and 7 krpm, most likely reflects different types for shear stress and fluid dynamic response in the thin film, for example transitioning from transient turbulence to turbulent flow. This shows that different rotational speeds (and different operating parameters of the VFD) can provide access to a multitude of particles with different sizes and shape.
To investigate whether other structures could be accessed using the VFD the solvent was changed. This was firstly explored for o-xylene as a related methyl substituted aromatic molecule, and resulted in the formation of uniform material comprised of spherical-like particles of self-assembled C60. The optimal operation parameters were a rotational speed of 4 krpm with the tube inclined at 45° and a flow rate of 1 mL/min, for a concentration of the solvated C60 molecules in o-xylene at 0.1 mg/mL, as shown in
The diameter of the C60 spherical-like particles can be controlled by changing the concentration of C60 in o-xylene, with the other parameters unchanged. For example, the average diameter was 3.5 μm for a concentration 0.2 mg/mL, whereas 1.8 μm and 150 nm particles were obtained by reducing the concentration to 0.1 mg/mL and 0.025 mg/mL, respectively, as shown in
Therefore, at low number ˜Re<250, the flow will be laminar. For higher Reynolds numbers the fluid will transition to turbulent flow. In conventional channel based microfluidics the Reynolds numbers are typically low, corresponding to laminar flow. For the VFD modes (confined and continuous flow) the fluid flow is regarded as at least transitioning into turbulent flow, with the greatest shear for droplets striking the base of the tube resulting in this film instability with the formation of helical flow. The Reynolds numbers is the VFD are directly proportional to high speeds.
The change in operating parameters of the VFD will affect the time taken for a finite amount of liquid to enter and exit the tube, which is the residence time, i.e. tresidence=ti−tf where, ti and tf is the time taken for a first drop of liquid to reach the bottom of the tube and exit the tube. It also clear that the residence time will increase with increasing tilt angle, and that is due to increasing the gravitational force (Fg) resulting in decreasing the centrifugal force (Fc). This results in mixtures of shapes and size of the products with small size at low value of tilt angles. As an example, when the liquid is delivered to the bottom of the tube at flow rate of 1.0 mL/min, for the tube rotating at 8 krpm, the residence time is ˜01:20 min, whereas for a flow rate of 0.1 mL/min and the same speed, the residence time is ˜12.8 min. Decreasing the speed to 4 krpm for a flow rate of 0.1 mL/min, the residence time dramatically increases to ˜44.08 min, which is shown in
The particles of C60 obtained from toluene and o-xylene, were also characterised using EDX and Raman spectroscopy. For the former, only carbon, and a small amount of silicon (25.14%) were observed. The presence of carbon is consistent with the material formed as self-assembled C60 with the silicon arising from the substrate (silicon wafer) which was used in the study.
Another technique for characterising C60 is Raman spectroscopy.
The crystal structures of both stellated and spherical like self-assembled C60 were studied using X-ray powder diffraction. Both show three characteristic 2θ peaks corresponding to fCC C60, at 12.6°, 20.6° and 24.2° of 2θ values, which correspond to the (111), (220), and (311) planes
The formation of fCC crystalline material under high shear is noteworthy. This is the same phase of C60 as formed using water as an anti-solvent in the VFD, which is the phase devoid of included solvent molecules, such that no additional processing is required of the fabricated particles of fullerene C60.
Studies using other solvents for C60 were also undertaken, using m-xylene and p-xylene and mesitylene and the results are shown in
A study was undertaken on a mixture of C60 and C70 (1:1 ratio) for different solvent systems and the results are shown in
An advantage of the processes described herein over conventional methods for forming C60 particles is that no hazardous chemicals or surfactants are required. This means that the final structure will not include a solvent, whereas in the conventional methods heat is required to remove the solvent and this can affect the structure. Similarly, no surfactant is required for the processes described herein whereas it can be difficult to remove the surfactant used in some conventional methods. Using a single solvent enables recycling of the solution back through the VFD after dissolving more pristine fullerene C60. Thus the ‘bottom up’ processing technology developed does not generate a waste stream once it is set up, with no heating or cooling required, and without the need to separate different solvents and without downstream processing to remove any included solvent.
The processes described in Example 9 can also be used for producing particles of fullerene C70. It is noteworthy that C70 has enhanced conductivity and photoconductivity, fluorescence and optical limiting performance over C60. Moreover, since C70 is more expensive than C60 and, therefore, making material from mixtures of the two fullerenes may provide access to other structures of particles. Indeed, growing novel material directly from raw fullerite (the mixture of fullerenes generated directly from graphite) may also be possible.
In the fullerene family, C70 is the second most abundant form after C60. Besides being readily available, liquid/liquid interface precipitation (LLIP) is the most conventional method for generating different shapes of crystals of self-assembled C70. LLIP has been used to generate these structures, depending on experimental conditions and methods, especially on the choice of the solvent and surfactants. Even so, one shortcoming of the LLIP method is that it involves the use of hazardous and environmentally harmful reagents in forming the interface where the crystals are formed. Moreover, the surfactants used can also pose additional problems in that they can bind to the crystals and can affect the properties of the fullerene material.
In this Example, a greener approach is provided to control the growth of self-assembled fullerene into well-defined crystals under continuous flow using a vortex fluid device (VFD). Advantageously, the method developed is without the need for an anti-solvent, and the use of more toxic chemicals or surfactants. The postulation is that the shear stress disrupts the fullerene stabilising solvation shell, resulting in aggregation of the fullerene, and this results in the growth and nucleation of particles in the thin films formed as VFD microfluidic platform.
Particles of distinct size and specific shape can be fabricated using the VFD. Changing the various processing parameters influences the outcome of shear induced nucleation and growth of C70 particles. While the tilt angle of the device was restricted to 45°, other parameters were varied, notably the flow rate, the choice of solvent, the rotational speed and the concentration of the fullerene. Clearly, the solubility of C70 is greatly reduced as a result of the shear stress in the thin film, with the nucleation and growth of C70 particles of specific size, shape and morphology, depending on the processing parameters. Uniformity in the size and shape of the particles can be achieved and indeed optimised. For instance, the particle can achieve shapes that closely resemble a uniform sphere or a cube.
Three aromatic solvents were used to investigate the impact on the different choice of solvent, in highlighting the generality of the method,
Overall our results establish a breakthrough in controlling crystallization, without the need for using surfactants or anti-solvents. The shear induced crystallization is the forth form of crystallization that have been established, the others being sublimation, evaporation of solutions, and cooling of solutions.
Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “include” and variations such as “comprising” and “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.
It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 2016904591 | Nov 2016 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2017/000237 | 11/10/2017 | WO | 00 |
