Patent Grant
11045427
Not Applicable.
The present invention relates to hollow nanoparticles. More particularly, the present invention relates to methods for fabricating double layer hollow nanoparticles.
The discovery of mesoporous silica templated by surfactants has led to tremendous interest in developing new classes of porous materials. It is now well-known now that the introduction of a templating surfactant such as cetyl trimethylammonium bromide (CTAB) into a solution containing a silica precursor such as tetraethyl orthosilicate (TEOS) leads to the formation of ordered mesoporous silica. Of significance is the rapid synthesis of such materials by the aerosol based method of making porous materials by incorporating surfactants into precursor solutions of a silica precursor. The aerosol method is advantageous because it is continuous, effective and economical. A very recent and comprehensive review (X. W. Lou, Lynden A. Archer, and Zichao Yang, Hollow Micro-/Nanostructures: Synthesis and Applications, Adv. Mater. 2008, 20, 3987-4019) describes the versatility and use of the process to produce functional inorganic materials.
This invention discloses the use of similar methods to produce unexpected non-mesoporous materials.
Hollow particles are of considerable interest because of their wide range of applications in encapsulation, catalysis, biomolecule separation, controlled drug release, and sensor technologies/biosensors. Typically, the preparation of such hollow particles requires building a desirable material layer around a core, followed by removal of the core by dissolution or high temperature calcination. These synthesis approaches generally involve multistep operations and complex components, leading to difficulties in scale up to commercially viable quantities.
The present invention discloses a scalable, rapid aerosol-based process for fabricating hollow submicrometer particles with novel morphology, one where the shell is made up of two thin layers: an outer layer of silica (or other ceramic) and an inner layer of carbon. The exposed surfaces therefore have contrasting physical characteristics, with the outer surface being hydrophilic and the inner surface being hydrophobic. Additionally, the particles contain iron nanoparticles, making them magnetically responsive.
Double-shelled hollow particles have recently been pioneered through template-based methods to form SnO2 layers for lithium storage enhancements, and these advances point to important applications of such materials.
A recent and comprehensive review of the aerosol process describes the versatility and use of the process to produce functional inorganic materials, and we refer the readers to this article for an excellent background of the process (See Boissiere, C.; Grosso, D.; Chaumonnot, A; Nicole, L.; Sanchez, C. Aerosol route to functional nanostructured inorganic and hybrid porous materials. Adv. Mater. 2011, 23, 599-623). The concept behind the present work is an important aspect of the aerosol process that has hitherto not been explored. It is well known now that the introduction of a templating surfactant such as cetyl trimethylammonium bromide (CTAB) into a solution containing a silica precursor such as tetraethyl orthosilicate (TEOS) leads to the formation of ordered mesoporous silica. In our recent work, however, we have surprisingly found that the inclusion of ferric chloride into the precursor solution completely negates the templating effect. Rather, the inclusion of the ferric salt leads to a binding of the CTAB and a phase segregation where the iron salt and CTAB become occluded within the interior of a rapidly forming shell of silica during the passage of the aerosol droplets through the heating zone of a tube furnace. Subsequent calcination of these particles leads to the burnoff of the organic surfactant species, leaving behind hollow silica particles containing magnetic iron oxides. The present invention is based on a new extension of this concept. If a rapid shell of silica is formed, can this shell act as a seal to prevent the escape of material from the interior of the particle? Specifically, if carbon precursors (sucrose) are introduced into the precursor solution, can the carbonization of sucrose be conducted in the interior of such thin-shelled silica particles? The first part of the schematic in
Various other methods have been developed to synthesize single layer hollow nanoparticles, including hard template, soft template, dual template, Ostwald ripening, as well as Kerkendall effect, but these preparation schemes generally involve multistep operations, complex components, and hence are less economical.
One-step aerosol-assisted process is an efficient approach to prepare single layer hollow nanoparticles. However, to our knowledge fabrication of double layer nanoparticles through a simple and effective aerosol assisted process have never been reported before.
Incorporated herein by reference are the following references:
X. W. Lou, Lynden A. Archer, and Zichao Yang, Hollow Micro-/Nanostructures: Synthesis and Applications, Adv. Mater. 2008, 20, 3987-4019.
Hu Wang, Jin-Gui Wang, Hui-Jing Zhou, Yu-Ping Liu, Ping-Chuan Sun and Tie-Hong Chen, Facile fabrication of noble metal nanoparticles encapsulated in hollow silica with radially oriented mesopores: multiple roles of the N-lauroylsarcosine sodium surfactant, Chem. Commun., 2011, 47, 7680-7682.
Yinqquin Wang, Bhanukiran Sunkara, Jinjing Zhan, Jibao He, Ludi Miao, Gary L. McPherson, Vijay T. John, and Leonard Spinu, Synthesis of Submicrometer Hollow Particles with Nanoscale Double-Layer Shell Structure, Langmuir 2012, 28, 13783-13787.
The present invention provides bilayer hollow nanoparticles and a method of making the same.
In a preferred embodiment, a double layered nanoparticle is fabricated in a one-step aerosol-assisted synthesis method. In one embodiment, the outer layer is silica and the inner layer is carbon.
In another embodiment of the present invention, an outer silica layer of a bilayer nanoparticle may be etched away to fabricate hollow carbon spheres. In one embodiment a hollow sphere may encapsulate a substance. In yet another embodiment, the substance encapsulated may be a pharmaceutical compound.
In another embodiment of the present invention an inner carbon layer of hollow bilayer nanoparticles may be burnt away to fabricate silica spheres.
Another embodiment of the present invention may be to manufacture hollow bilayer nanoparticles with magnetic nanoparticles. In one embodiment, the magnetic nanoparticles may be iron. In addition to iron, it is possible to insert a variety of other metallic nanoparticles (tin, copper, palladium, chromium, zinc, rhodium, ruthenium, molybdenum—the whole series of transition metal oxides). In another embodiment, the magnetic nanoparticles may be used for drug delivery.
In another embodiment, bilayer particles may be used as amphiphilic particles to stabilize emulsions. In one embodiment, bilayer particles may be used in Pickering emulsions.
In another embodiment, bilayer particles may be used as catalytic materials. In one embodiment carbon and silica within bilayer particles may function as supports for catalytic materials.
In accordance with this invention, it is an object of this invention to incorporate iron oxide into the shell of the bilayer structure to make it magnetically responsive. The inner void allows entrapment of a high concentration of a drug agent which may be magnetically guided for targeted drug delivery.
In accordance with this invention, it is an additional object to etch away the silica layer to make hollow carbon spheres with applications to fuel cell technologies as electrode for fuel cells, for using carbon as a catalyst.
In accordance with this invention, it is an additional object to burn away carbon to make hollow silica spheres with application in drug delivery and as catalyst supports.
Some embodiments of the invention include eggshell type nanoparticles that are particles with an extremely thin outer layer that can crack or break upon a slight impact or ultrasonication. In some embodiments these eggshell particles may have a shell of 10-15 nm (though even a shell as thin as 5-7 nm and up to 20 nm thick can be useful). Some embodiments include methods of producing said eggshell particles comprising sending a precursor solution comprising a surfactant, a silica precursor, and a metal precursor such as a metal salt, through a heating zone. Some embodiments comprise a precursor solution with less silica precursor than a 1 to 8 ratio of metal salt to silica precursor.
Some embodiments of the invention include bilayer nanoparticles with protuberances referred to here as “nanohorns.” In some embodiments, there may be at least one nanohorn with the nanoparticle comprising an outer layer of silica and an inner layer of carbon. Some embodiments include methods of producing said nanoparticles with nanohorns comprising sending a precursor solution comprised of a surfactant, a silica precursor, a metal precursor such as a metal salt, and a carbon precursor through a heating zone, and then pyrolizing the particles. Some embodiments may further include calcination of the particles to remove the carbon layer, or etching of the particles to remove the silica layer.
Some embodiments of the invention include nanoparticles made with metal based precursors in place of a carbon based precursor. Some embodiments may use a titania precursor, such as titanium isopropoxide in place of a carbon precursor. In some embodiments, the silica may be etched away leaving titania nanospheres. In some embodiments, light is expected to penetrate the titania nanospheres.
Hollow nano and microparticles have a variety of applications in encapsulation, catalysis, energy storage, chemical sensing and controlled drug release. Typically they are prepared by forming a layer of the desired materials over a template which is then selectively removed by dissolution or burn-off to create a hollow core. In the present invention, a new method for manufacturing ceramic particles where a shell is created extremely rapidly, locking in chemical constituents in the interior. This is done using an aerosol based process where we have exploited salt bridging concepts to lock a surfactant (CTAB) and carbon precursors together with iron oxides in the interior of a droplet while a silica shell is allowed to form on the droplet surface. Subsequent pyrolysis results in a buildup of internal pressure forcing carbon formation as a second layer attached to the silica shell. Thus we have developed bilayer “amphiphilic” ceramic particles with a hollow interior. This new assembly method is expected to be a general approach to fabricate various hybrid double layer hollow particles with unique potential properties. In addition, the incorporation of magnetic iron oxide into the shells opens up opportunities in external stimuli responsive materials.
The present invention describes novel nanoparticles and methods of producing the same. The novel aspects of the present invention are the following: (1) the iron chloride ties up the surfactant (e.g., CTAB) so that the silica cannot grow inwards from the surface of the drop—this is why hollow particles are generated; and (2) when the carbon precursor is also enclosed in the interior, the pressure build up leads to the second shell being generated from the inside (as opposed to building shells from the outside through a layer-by-layer method). These are important differences from prior art and lead to the ability to be able to generate large quantities of hollow and double shelled particles.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the description of specific embodiments presented herein.
Detailed descriptions of one or more preferred embodiments are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in any appropriate manner.
The hollow particles of the claimed invention can be made of (a) silica; (b) silica-carbon double shelled; (c) silica-titania mixed shell; or (d) silica-titania-carbon with the outer shell being silica-titania and the inner shell being carbon. The silica in these particles can be etched out leaving (a) carbon hollow particles; (b) titania shelled hollow particles; or (c) titania-carbon particles.
All these hollow particles can be made to contain iron nanoparticles. In addition to iron, it is possible to also insert a variety of other metallic nanoparticles (tin, copper, palladium, chromium, zinc, rhodium, ruthenium, molybdenum, manganese, nickel, aluminum—in fact the whole series of transition metal oxides). It is noted that these particles are in addition to iron; for example, the nanoparticles within the hollow particles are either (a) iron; or (b) iron plus a second metal. The second metal can be inserted into the hollow particles through multiple pathways, such as for example, (a) it can be added to the precursor solution prior to aerosolization and thus gets incorporated into the hollow particles; or (b) it can be allowed to diffuse using the metal salt through the pores of premade hollow particles. Tin (Sn) is especially important as it can be used effectively for Li-ion batteries. We also note that the metal inside the hollow particles is loose and not attached to the shell. This qualifies denoting the particles as “rattle” type particles.
Normally, this process results in mesoporous particles, however, in one embodiment as
As illustrated in
Typically, CTAB templates highly ordered hexagonal mesoporous silica through the aerosol process. However FeCl3 in the precursor solution disrupts the formation of mesoporous silica due to preferential partitioning of the surfactant CTAB on ferric species, therefore the dense, low porous silica layer may be formed during the aerosol process. The iron chloride ties up the surfactant CTAB so that the silica cannot grow inwards from the surface of the drop—this is why hollow particles are generated.
As depicted in the schematic of
The schematic in
The morphology of the hollow nanoparticles was evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The synthesized nanoparticles have well-defined spherical structures and the size is in the range of 100 to 1000 nm, which is consistent with characteristic droplet size distribution through aerosol process. The hollowness of synthesized nanoparticles is supported by TEM. The representative images of synthesized double layer hollow nanoparticles are shown in FIG. 6.
The nanoparticles have interesting morphology transition from ill-defined hollow structure (
The present invention demonstrates that in the presence of sucrose the much slower carbonization of the sugar would occur in the interior of a particle with a silica shell.
In one embodiment, the as-synthesized particles are then pyrolyzed at 500° C. for 3 h in a tube furnace under flowing nitrogen gas to generate carbon species through full dehydration and carbonization. Instead of nitrogen, one could use a different inert gas, such as argon or any gas which one of ordinary skill in the art would know or discover through routine experimentation. The resultant observation is remarkable, as the carbon forms as a discrete second layer adjoining the silica shell leaving a fully hollow interior (IV in
The present invention demonstrates that reason for the generation of these double-layer particles is that the silica shell seals in the carbon precursors during the aerosolization process. During pyrolysis, the off gases generated build up a high internal pressure and push the carbonaceous species to the inner surface of the silica shell. Assuming the silica shell is impermeable until pressures are built up to force out the pyrolysis gases, the internal pressures generated can be as high as 175-200 atm. The pyrolysis gases are essentially forced out through micropores in the silica layer. The pyrolysis step is depicted through the schematic (III) in
Estimation of internal pressures generated during pyrolysis-Assumptions:
a) the outer silica layer is formed extremely rapidly to seal in carbon precursors;
b) an average particle size of 190 nm with a silica shell of 35 nm;
c) precursor concentrations in a droplet are the same as that in the precursor feed solution.
With these assumptions, the molar concentration of sucrose inside a droplet is 0.137M. If we assume a droplet dimension also of 190 nm, the droplet (and eventually the particle) contains 3.94×10−18 moles of sucrose.
Appling the ideal gas law, the internal pressure can be estimated:
Thus, the formation mechanism of double-layer hollow particles involves two steps: the generation of the silica layer due to the preferred silica condensation reaction along the gas-liquid interface of an aerosol droplet and the formation of the carbon layer by the dehydration and carbonization of dissolved sucrose during the subsequent pyrolysis.
Electron Dispersive Spectrosocpy (EDS) indicates that the silica is confined to the outer layer (with a carbon background from the TEM grid), and carbon to the inner layer. Elemental analysis obtained by X-ray energy dispersive spectroscopy (EDS) of exemplary cross section sample reveals that the atomic ratio of C:O:Si:Fe of the inner carbon layer is 99.1:0.9:0:0, while that of the outer layer is 93.1:4.3:2.2:0.4 (
To understand the structural characteristics of these particles further and to prove that the outer layer is silica and the inner layer is carbon, calcination and etching treatments were conducted to selectively remove the inner and outer layers, respectively. The resulting nanoparticles were characterized with SEM and TEM.
To remove the carbon layer completely, the pyrolyzed particles were calcined at 500° C. for 3 h and an additional 2 h at 1000° C. The removal of the carbon layer from pyrolyzed particles leads to the morphology transition from double-layer particles to single silica-layer particles (
X-ray diffraction for the double layer particles (
A simple calculation assuming that the composition of the precursor solution is reflected in the relative silicon and iron atomic ratio in the bilayer particles and that the iron oxide particles are approximately 10 nm in diameter indicates that there are approximately 300 iron oxide nanoparticles in each hollow calcined particle.
During calcinations treatment, oxygen molecules react with inner carbon layer of the nanoparticles and the oxidation product CO2 diffuses out of the hollow nanoparticles. The complete removal of carbon layer of hollow nanoparticles was confirmed by TEM. The removal of carbon layer from pyrolyzed nanoparticles leads to the morphology transition from double layers nanoparticles to single silica layer nanoparticles (
In addition, the EDS results of calcined particles (
X-ray diffraction (
To remove the silica layer from the double layer hollow particles, the pyrolyzed nanoparticles may be etched using 10% (v/v) HF solution (or other highly acidic solution such as HCl, sulfuric acid, or other highly acidic solution that a person having ordinary skill in the art would know or discover through routine experimentation) for 48 h. The silica layer of hollow nanoparticles reacts with HF solution, giving rise to H2SiF6, which can be washed out using deionized water. Silica can also be etched out also using a highly basic solution of for example NaOH, though one could use other highly basic solutions such as ammonium hydroxide, or other highly basic solution that a person having ordinary skill in the art would know or discover through routine experimentation. The representative TEM images (
The cross section TEM shown in panels 17d and 18d illustrate an almost intact ring, as it is difficult to section without breaking the particle. Panels 17e and 18e show the SEM of the external morphology of an etched particle and panels 17f and 18f indicate the SEM of the cross section of a particle after a single cut to demonstrate that the particles are indeed hollow.
It should be pointed out that the dense, low-porosity silica outer layer of nanoparticles is due to the fact that the relative amounts of free CTAB is not high enough compared to TEOS to template mesoporous silica. The dense silica outer layer of nanoparticles also suggests a possible mechanism for inner carbon layer formation. During pyrolysis, the escape of organic gases in the nanoparticles through the dense silica structure may force the carbonaceous species to the silica wall resulting in the formation of the inner carbon layer of hollow nanoparticles.
The double-layer hollow particles that contain an internal carbon layer and an outer silica layer have been fabricated by a simple and effective aerosol-based process. The preparation is based on the concept of rapidly forming a silica shell that retains carbon precursors within the interior of the particle. Subsequent pyrolysis jams the carbon as a second layer against the silica shell.
The generation of a silica shell by negating the templating effect of the surfactant is expected to be quite general, allowing the encapsulation of a variety of other components in the interior of the particle. In addition to the generation of a new class of hybrid materials using the aerosol technique, the fact that these systems contain iron makes them magnetically responsive.
It is also possible to add layers to the double shelled particles. Silica is hydrophilic and carbon is hydrophobic, creating an ampiphilic particle. Building additional layers can be done by adding a carbon layer to the silica shell, and then adding a silica coating on the carbon layer. The building of additional layers is done by known layer-by-layer techniques.
Another embodiment of the present invention seeks to control and exploit particle properties through modulating layer thickness as described herein. These materials are expected to have multiple applications because they are able to incorporate the benefits of both carbon and silica and additionally include magnetic materials. Their uses as catalytic materials and in stabilizing emulsions are distinct directions of continuing research.
Applications
The following are some specific applications of these materials. All these applications are elaborated upon in the Lou et al. publication, which is incorporated herein by reference, which is a review of hollow particles.
The present invention is able to make the hollow particles in large quantities because of the nature of the aerosol process.
None of the prior art processes listed in the literature is able to produce the double shelled particles. Because of the double shell, the present invention has a system that is hydrophilic on the exterior and hydrophobic on the interior. There is a distinct possibility that such systems will provide enhanced properties in catalysis and gas sensing.
The present invention is able to modulate the porosity of the shell particularly in systems with a single shell, going from an entirely nonporous to a porous system.
In addition to the applications listed in the Lou et al. publication, we propose there are environmental applications. There are potential applications to the environmental remediation of chlorinated compounds, of arsenic, and other chemicals, due to the catalytic materials in the hollow particles. Since the particles are hollow, they may have some extremely important applications in the remediation of oil spills. They can be filled with dispersants and sprayed onto oil spills. Such controlled delivery of dispersants can be efficacious in breaking up oil spills and dispersing the droplets. Additionally, the particles can be stabilized at an oil water interface to stabilize emulsion droplets. Finally the iron oxide within the particles makes them magnetically responsive and it may be possible to recover the oil through the formation of Pickering emulsions.
The hollow particles can be used to store agricultural pesticides which can be sprayed onto plants for controlled release.
The hollow particles can be used to store fertilizers which can be injected into the ground for controlled release. They can be temperature tuned by coating them with a wax that melts as the temperature increases in the growing season, releasing the fertilizer contents.
The hollow particles can be used to store enzymes for the biological breakdown of organophosphorous compounds. In application, these could be used against nerve agents.
Experimental Procedure
Synthesis of Hollow Silica-Carbon Bilayer Nanoparticles.
All chemicals are commercially available and were used as received. TEOS is used as the silica source and sucrose is used as the carbon source, together with ferric chloride and the surfactant (CTAB). Alternatively, the carbon source can be a monosaccharide or polysaccharide, such as sucrose (most preferable), glucose, cellulose, or cyclodextrins. Alternatively, the surfactant can be cetyltrimethyl ammonium bromide (CTAB), cetyltrimethyl ammonium chloride (CTAC) or other CTA-halides. Alternatively, instead of the silica source, zirconia, alumina or titania can be used.
In a typical synthesis, about 0.8 g-1.9 g, preferably 0.95 g, of FeCl3.6H2O was first dissolved in 15 mL ethanol under vigorous stirring. Then about 0.1 g-2.2 g, preferably 1.1 g, of CTAB and about 1.0 mL-9.0 mL, preferably 4.5 mL, of TEOS were added to the solution, followed by 1.8 mL of a 0.1 M HCl solution and the dissolution of about 0.01 g-3 g, preferably 1.0 g, of sucrose. The resulting solution was then aged for 0.5 h and atomized using a commercial atomizer (Model 3076, TSI Inc.) to form aerosol droplets, which were passed through a quartz tube placed in a furnace (
It is noted that during passage through the heating zone, the coassembly of CTAB with silicate and the formation of mesoporous silica are disrupted by the preferential partitioning of CTAB on intermediate iron species such as FeO(OH). This salt bridging between the iron salt and CTAB locks the CTAB within the interior of a rapidly forming silica shell.
Synthesis of Hollow Silica Nanoparticles.
To obtain the single silica layer particles, the pyrolyzed particles were calcined at 500° C. for 3 h and additional 1000° C. for 2 h in air.
Synthesis of Hollow Carbon Nanoparticles.
To obtain the single carbon layer particles, the pyrolyzed particles were incubated with 10% HF solution for 48 h to remove silica layer.
Characterization
The morphology of the particles was characterized by field emission scanning electron microscopy (SEM, Hitachi S-4700, operated at 20 kV) and transmission electron microscopy (TEM, JEOL 2010, operated at 200 kV). The crystal phases present in the particles were identified using X-ray diffraction (XRD, Siemens, D 500, using Cu αK radiation at 1.54 Å.). The cross section samples for SEM and TEM were prepared by embedding particles within a resin (Embed 812) in 70° C. for 48 h and cut by a Leica ultracuts Microtome. Magnetic properties were characterized using a superconducting quantum interference device (SQUID, MPMS Quantum Design Inc.). The BET surface area of the particles was measured using the nitrogen sorption technique at 77K (Micromeritics, ASAP 2010).
While the double layer hollow particles constitute the key finding, there other aspects based on expanding on the conjecture that we can rapidly create a silica shell by preventing the templating effect of CTAB through salt bridging with FeCl3.
Thin Silica Shells-Template-Free Synthesis of Ultrasound Responsive Hollow Silica Microspheres with Ultrathin Nanometer-Scale Shell Structures
Novel ultrathin hollow silica microspheres have been synthesized using aerosol based process with reduced silica precursor loading (tetraethyl orthosilicate, TEOS). Hollow silica microspheres with ultrathin silica shell about 5 nm to 20 nm; or 7 nm to 20 nm, or 7 nm to 15 nm, for example 10 nm-15 nm are also conveniently cracked using ultrasonic treatment, which is one of the most promising external triggers. The ultrathin calcined hollow silica microspheres are presumably fractured by the transient cavitation, a well-known phenomenon of ultrasonication. In addition, the pore size of hollow silica microspheres can be uniquely adjusted by introducing sodium chloride into precursor solution. For example, pore sizes of 0.5 nm to 100 nm, for example 10 nm, in diameter can be obtained by with a NaCl:precursor solution ratio of 0.1:1.0 to 10:1.0. The microspheres with locked-in magnetic iron oxide open up further opportunities in magnetic stimuli responsive applications.
In some embodiments, with decreased levels of tetraethoxy silane (TEOS) in the precursor, the silica shells become progressively thinner till we get shells that are about 5 nm-20 nm thin, for example 10-15 nm thin as shown in panel d of
Hollow microspheres with controlled morphologies are of extensive interest due to their wide applications including encapsulation, biomolecule separation, catalysis, super-capacitors, gas sensing, drug delivery and energy storage. Recently, the research of hollow microspheres has been focused on design of complex structures, such as double shelled, rattle like and yolk shell structures. A variety of chemical strategies to synthesize such hollow microspheres have been applied, including the soft-template and hard-template processes. Particularly the hard template method, the most common route, requires building a desirable layer around a core and followed by the core removal by chemical etching or high temperature calcination.
Ultrasound is one of the most promising external triggers for encapsulated chemical release that can be accurately controlled by parameters, such as frequency, power density as well as duration. Although there are available methods for hollow microspheres preparation, the synthesis of ultrathin hollow microspheres that can be easily cracked by ultrasonic treatment is seldom reported. Therefore the present invention discloses a novel method to prepare ultrathin hollow microspheres and their potential applications.
It is well-known that surfactant cetyl trimethylammonium bromide (CTAB) typically templates highly ordered mesoporous silica through aerosol method when precursor solution contains silica source such as TEOS. However the present invention shows that the introducing of ferric chloride into the precursor solution disrupts the co-assembly of silicate and surfactant CTAB by preferential partitioning of CTAB and more positively charged ferric chloride under acidic condition. The iron chloride ties up the surfactant CTAB so that the silica cannot grow inwards from the surface of the drop, thereby generating hollow particles. The formation of silica rich shell is due to faster silica condensation along the gas-liquid interface of the aerosol droplets and subsequent high temperature calcination remove surfactant CTAB and converts the ferric species into iron oxides. However such synthesized hollow microspheres have relatively thick silica shell that cannot be conveniently ruptured by ultrasound irradiation. Based on this concept, can ultrathin hollow silica microspheres be formed by gradually decreasing silica precursor loading in the solution? Can the synthesized ultrathin silica microspheres be easily cracked by ultrasound treatment, so that encapsulated species can be released to the surrounding?
In the present invention, a simple and efficient aerosol based process is used to synthesize hollow silica microspheres with ultrathin shell thickness (typically approximately 10-15 nm) that can be easily cracked by external ultrasonic irradiation. In addition, the present invention discloses a uniquely tuned pore size on the hollow silica microspheres by conveniently adjusting sodium chloride concentrations in precursor solution.
Experimental
Preparation of Ultrathin Hollow Silica Microspheres
All chemicals are commercially available and were used as received. Alternatively, the surfactant can be cetyltrimethyl ammonium bromide (CTAB), cetyltrimethyl ammonium chloride (CTAC) or other CTA-halides. Alternatively, instead of the silica source, zirconia, alumina, titania or some other ceramic source can be used. In some embodiments, a carbon precursor may be included for a double-layer particle. The carbon source can be a monosaccharide or polysaccharide, such as sucrose (most preferable), glucose, cellulose, or cyclodextrins.
In a typical synthesis about 0.8 g-1.9 g, preferably 0.95 g, of FeCl3 was first dissolved in 15 mL ethanol (95%, v/v) followed by the addition of about 0.1 g-2.2 g, preferably 1.1 g, of cetyltrimethyl ammonium bromide (CTAB). Then various amounts of TEOS (about 1.0 mL-9.0 mL, for example 10, 6, 4.5 and 2 mL) were added to the above solution under vigorous stirring at room temperature. 1.8 mL of 0.1M HCl solution was also added to the solution after 3 minutes stirring. The resulting solution was aged for 30 min and the precursor was then atomized to form aerosol droplets, which were then sent through the dying zone and heating zone of quartz tube. The temperature of the heating zone was held at 400° C. and the resulting particles were collected by a filter system maintained at 80° C. The as-synthesized particles were then calcined at 500° C. for 3 h.
Ultrasonic Treatments of Ultrathin Hollow Silica Microspheres
The parameters of ultrasonication used were set as 20 kHz (frequency) and 150 W (power output). During the experiment period, the ultrasonication probe was dipped into the sample solution. Each sample solution has 5 mg synthesized microspheres dispersed in 1.5 mL deionized (DI) water and was exposed to 5 min treatment in a sonic dismembrator (Fisher Scientific, model 550). The samples were then centrifuged for 10 minutes at 12,000 rpm. The percentage of damaged microspheres was then compared to evaluate the rupture effect.
Tuning Porosity of Hollow Silica Microspheres
Similar to the synthesis of ultrathin hollow silica particles, about 0.8 g-1.9 g, preferably 0.95 g, of FeCl3 was first dissolved in 13.3 mL ethanol followed by the addition of about 0.1 g-2.2 g, preferably 1.1 g, of CTAB. Then 1.0 mL-9 mL, preferably 2 mL, TEOS were added to the above solution under stirring at room temperature. 3.5 mL of 0.1M HCl solution with about 0.01 g-1.0 g, preferably 0.4 g, NaCl was also added to the solution after 3 minutes. The as-synthesized particles from aerosol process were then calcined at 500° C. for 3 h to remove surfactant CTAB and convert iron species to iron oxides.
Dye-Encapsulation Experiment
1 mg hollow silica microspheres were dispersed into 1 mL Rhodamine solution (0.6 mg/mL) and then dried in atmospheric pressure. The dye-loaded microspheres were washed with DI water. Optical and fluorescence images of these samples were examined using inverted fluorescence microscope (model Olympus 1X71).
Hollow Silica Microspheres Characterization
The morphology and structures of the microspheres were evaluated using field emission scanning electron microscopy (SEM, Hitachi S-4700, operated at 20 kV), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) (JEOL 2010, operated at 200 kV) and X-ray diffraction (XRD, Siemens, D 500, using Cu αK radiation at 1.54 Å.). The specimens for TEM examination were obtained by dispersing microspheres in ethanol (95%, v/v) and drops of microsphere suspension were added onto a copper grid for TEM microscope.
The cross section samples for SEM and TEM were prepared by embedding silica microspheres within resin (Embed 812) in 70° C. for 48 h and cut by Leica ultracuts Microtome. The porosity of the microspheres was evaluated by the nitrogen sorption technique at 77K (Micromeritics, ASAP 2010).
Results and Discussion
The synthesis of ultrathin hollow silica microsphere is achieved through a simple and effective aerosol process, which is illustrated in
Based on TEM observation with high resolution (
In order to clearly demonstrate the TEOS loading effect on silica shell thickness, TEM images of hollow silica microspheres made from various TEOS loadings with comparable size (200 nm-1000 nm in diameter, more preferably ˜250 nm in diameter) are acquired (
Silica hollow microspheres prepared in different Fe:Si molar ratios (1:13 to 1:2.7) are subject to ultrasonic treatment to evaluate the rupture properties. SEM images in
To further reveal the structure of ruptured silica microspheres with ultrathin silica shell triggered by ultrasound, SEM and TEM images of calcined microspheres prepared from Fe:Si ratio of 1:13 and 1:2.7 are obtained and compared (
It appears that a Fe:Si molar ratio of 1:2.7 is ideal for creating thin silica spheres. However, the Fe:Si molar ratio can vary from 0.5:3 to 5:3 and still create acceptable thin silica spheres for sonication destruction.
The pore size tunability of hollow microspheres plays an important role in widening their applications. The porosity of hollow silica microspheres can be adjusted by varying the amount of sodium chloride loading while keeping all concentrations of other chemical species same. As shown in
The SEM images (
The XRD patterns of two groups of hollow silica microspheres: with and without washing treatments are shown in
Dye-encapsulation experiments were also conducted by dispersing hollow silica microspheres (with and without sodium chloride loading) into Rhodamine B solution (2 mg/mL). Phase contrast (
The ultrathin hollow silica particles are synthesized through a simple and effective aerosol based process using reduced TEOS loading in the precursor solution. These hollow microspheres with ultrathin shell thickness can be easily ruptured by ultrasonication treatment in a short time via cavitation mechanism, which make them a promising material for ultrasound-triggered release application. The porosity of silica hollow microspheres can be conveniently tuned by introducing sodium chloride due to simultaneous silica condensation and sodium chloride precipitation. The fact that these novel microspheres have ultrathin silica shell makes them ultrasound responsive and expected to have wide range of applications where pulsatile encapsulated release is needed.
In another embodiment of the present invention, a dense carbon particle with a net-like or cage-like thin silica shell can be created. TEOS loading in the precursor solution is reduced to make a thinner silica shell, as seen in
0 g
With the same TEOS loading (1 ml), when the sucrose in the precursor solution is increasing from 0.5 g to 1 g, the morphology of the particles is changed from hollow to dense spheres. This is because the concentration of sucrose in the aerosol droplet is too high and it obstructs the diffusion of the silica source to the gas-liquid interface. So the silica source is mixed with carbon source and other species forming dense spheres.
Nanohorns
In some embodiments, the carbon precursor concentration in the precursor solution may be increased, resulting in particles that have long protrusions, leading to a term coined as “nanohorns”
The pressure buildup during pyrolysis, rather than rupturing the shells, leads to yielding and the formation of these long protrusions some of which are longer than the particle diameter. This is an interesting structural feature as it implies that the hydrodynamics of such particles are significantly different from the hydrodynamics of spherical particles. Additionally, the protrusions may have significant consequences in the anchoring of these particles at fluid interfaces and the formation of Pickering emulsions. Such particles may not be able to easily rotate at an interface leading to the possibility of preparing a variety of Janus particles. An interesting aspect of these particles seems to be that they are not hollow internally. In other words, the excess carbon precursor loading leads to a yielding of the silica shell and the inability to firmly compress the carbon onto the silica shell.
In an exemplary synthesis, about 0.8 g-1.9 g, preferably 1.0 g, of FeCl3.6H2O is first dissolved in 15 mL of ethanol followed by the addition of about 0.1 g-2.2 g, preferably 1.1 g, of CTAB. To this solution, 1.0 mL-9 mL, preferably 4.2 g, of TEOS, 1.8 mL of 0.1 M HCl and 2.0 g sucrose are added. The resulting solution is aged for 0.5 h under stirring. The precursor is first atomized to form aerosol droplets, which are then sent through a drying zone and heating zone where preliminary solvent evaporation and silica condensation occur. The temperature of the heating zone is held at 400° C. The resulting particles are collected by a filter maintained at 80° C. The as-synthesized particles are pyrolyzed at 500° C. for 3 h to generate nanohorn structure.
Using a Titania Precursor
In some embodiments, materials other than carbon precursors may be used in the precursor solution. In one embodiment, titanium isopropoxide (TIP) rather than sucrose may be introduced to the precursor solution.
Both the double layer silica-titania particles and the etched particles have significant applications in photocatalysis. Light may be able to penetrate easily through the shell allowing efficient photocatalysis to take place. The buoyancy of these hollow particles might make them especially suitable to be used in oil spill mitigation technologies.
In an exemplary synthesis, an aerosol precursor is prepared by mixing 3.5 mmol FeCl3.6H2O and 3.0 mmol CTAB in ethanol (15 mL) first, followed by sonication for 5 min. To this solution, 3.3 mmol TIP, 20.3 mmol TEOS and 1.8 mL of 0.1 M HCl are added. The final precursor solution has a molar ratio of FeCl3.6H2O:TIP:TEOS:CTAB:HCl:EtOH=1:0.94:5.8:0.86:0.05:74. The solution is then aged for 30 min under magnetic stirring and atomized to form aerosol droplets which were sent through a drying zone and heating zone.
Examples and methods of use are described herein as a basis for teaching one skilled in the art to employ the invention in any appropriate manner. These examples disclosed herein are not to be interpreted as limiting.
Some embodiments may utilize thin silica shells with an inner backing primarily of titania indicates for photocatalysts with buoyant properties in solution. This has tremendous applications in cleaning and remediation technologies and in the development of dye sensitized solar cells. The photocatalytic activity of each TiO2 sample will be evaluated by the degradation of Rhodamine B in deionized water. The reaction will be carried out in a RPR-100 Rayonet reactor (1.65×108 photons/s/cm3) using emission at 254 nm. In a typical experiment, 10 mg of TiO2 is added to 50 mL of a 1.0×10−5 mol·L−1 Rhodamine B solution and magnetically stirred in the dark for 30 min prior to irradiation, to achieve adsorption equilibrium of Rhodamine B with the catalyst. The samples are collected every 20 min by centrifugation to determine the degradation rate by UV-vis adsorption (553.5 nm, Shimadzu UV 1700). Comparisons with the standard photocatalyst (Degussa P25) will be made.
Embodiments can be used to solubilize/emulsify mutually immiscible phases forming emulsions that are stable over extended periods. Such surfactant free emulsions, also known as Pickering emulsions, are characterized by the degree of wettability of the particles by either the dispersed phase or the continuous phase as determined by the contact angle (θ), defined as
where γso, γsw, and γow are the interfacial tensions at the solid-oil, solid-water, and oil-water interfaces, respectively. As a general rule of thumb, hydrophobic particles (θ>90°) preferentially disperse in the oil phase and stabilize water-in-oil emulsions, while hydophilic particles wetted by water (θ<90°) solubilize oil-in-water emulsions. The contact angle of colloidal particles at the interface is analogous to the hydrophilic-lipophilic balance of surfactants, and the value of the contact angle typically determines the nature of the emulsion (oil in water or water in oil) in systems that have similar amounts of the two phases. Mechanisms involved in the stabilization of particle based emulsions and phase transitions have been extensively discussed by Binks and coworkers and in recent years, particle stabilized emulsions have been used to develop novel applications ranging from the synthesis of Janus particles and colloidosomes to drug delivery and catalysis at interfaces. The self assembly of micro and nanoparticles at interfaces is also of much interest from the perspective of creating building blocks for hierarchical structures. In understanding the nature of Pickering emulsions and self-assembly at interfaces, model systems typically used are hydrophilic colloidal silicas or hydrophilic latex particles which form oil-in-water emulsions. Water-in-oil Pickering emulsions are usually studied through the use of hydrophobically modified silicas. The rapid development of applications involving carbon based materials has led to interest in the assembly of irregular sized carbon black particles, graphene sheets, and carbon nanotubes at interfaces.
The feasibility of forming water-in-trichloroethylene emulsions using carbon microspheres has been shown. While the work was done in relevance to environmental remediation of TCE., it is straightforward to substitute an oil phase (e.g. octane) instead of TCE. Cryo-SEM of water in TCE Pickering emulsions can illustrate the assembly of particles at the interface.
The approach in the present invention will be to use the bilayer particles to stabilize oil in water emulsions and then connect up the particles through formation of the Si—O—Si bond between particles. In one embodiment, the present invention can create novel colloidosomes of bilayer particles. It is also possible to individually use the silica shells to stabilize oil in water emulsions and the carbon shells to stabilize water in oil emulsions. One embodiment may comprise combination of the two systems to lead to the formation of bicontinuous emulsions stabilized by particles.
Acronymns
This is a non provisional patent application of U.S. Provisional Patent Application Ser. No. 61/599,788, filed 16 Feb. 2012; U.S. Provisional Patent Application Ser. No. 61/610,798, filed 14 Mar. 2012; and U.S. Provisional Patent Application Ser. No. 61/621,642, filed 9 Apr. 2012. Priority of U.S. Provisional Patent Application Ser. No. 61/599,788, filed 16 Feb. 2012; U.S. Provisional Patent Application Ser. No. 61/610,798, filed 14 Mar. 2012; and U.S. Provisional Patent Application Ser. No. 61/621,642, filed 9 Apr. 2012, each of which is hereby incorporated herein by reference, is hereby claimed.
Funding was received from the US Department of Energy (grant DE-FG02-05ER46243), the National Science Foundation (grants 0933734, 1034175, and 1236089), and the Gulf of Mexico Research Initiative. The United States government has certain rights in this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20030181535 | Van Hardeveld et al. | Sep 2003 | A1 |
| 20090178589 | Yoneyama | Jul 2009 | A1 |
| 20100056366 | Lee | Mar 2010 | A1 |
| Number | Date | Country |
|---|---|---|
| 009837 | Apr 2008 | EA |
| 2366501 | Sep 2009 | RU |
| Entry |
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| Number | Date | Country | |
|---|---|---|---|
| 20180071225 A1 | Mar 2018 | US |
| Number | Date | Country | |
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| 61599788 | Feb 2012 | US | |
| 61610798 | Mar 2012 | US | |
| 61621642 | Apr 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14378921 | US | |
| Child | 15695387 | US |
