Patent Application
20060293169
The invention relates to molecular structures. Particularly, the invention relates to porous molecular structures.
Multiple gas sensing requires materials with desirable selectivity and sensitivity to adsorbent gas molecules and volatile vapors. Many gas sensors in the art have problems associated with interference, when more than one gas needs to be detected. Additionally, filters or traps are needed to block gas molecules, which are not being sensed.
Microporous and mesoporous zeolites, because of their highly porous framework along with tunable pore and channel dimensions, large active surface area, variable hydrophilic and hydroscopic nature, and electrostatic behavior, are potential materials for chemical and gas sensing applications. Additionally, the open and porous structure provided by zeolites offer better accessibility to gas molecules to diffuse in and out of the material, which could reduce considerably the response time of the sensor.
In miniaturized sensors, the area available for sensing is limited. Micro and nano scale materials with high ion active surface area, in contrast to continuous thin films, may compensate for this lack of space and enable the detection of trace amounts of gases.
The challenges associated with implementing zeolites in sensing applications are generally related to synthesizing zeolites with sub-micron or nano morphologies, and to coating sensor devices with such materials without detrimental effects to the devices. In bulk form, zeolites are typically made using hydrothermal synthetic processes. Such processes typically require high pressures, high temperatures, and long hydrolysis time, rendering them cumbersome, time consuming and not device friendly. Also, the structure of zeolites formed by these processes is often difficult to control and is dictated by the reactants used, by the synthesis conditions such as temperature, time, and pH, and in particular, by the structure-directing agent used. Alternative synthesis routes including solvent evaporation techniques, surfactant template schemes, inorganic-organic cooperative assembly processes and emulsion or sol-gel chemistries have also been explored in the art. It has also been suggested in the art that efficient deposition of silica fibers through the electrospinning process may require substrates with acid-filled anapore filters. But such methods are typically incompatible with conventional semiconductor device fabrication processes and cannot typically be used without deleterious effects on device integrity. Scalability of such processes for sensor production can also prove to be very challenging.
Therefore, there remains a need for nano scale zeolite materials, which can be used for sensing applications, and methods to make them efficiently. Further, there remains a need for a method to directly deposit these materials on device structures, such as semiconductor devices and MEMS devices, to enable nano scale zeolite material-based sensors.
Embodiments of the invention meet these and other needs by providing porous nanozeolite type materials, method of making them, and nanozeolite based sensors.
Accordingly, one aspect of the invention is a porous nanozeolite material having a first dimension less than about 1 micron and a second dimension less than about 100 microns. The nanozeolite material comprises pores having an average diameter less than about 50 nm.
Another aspect of the invention is a method of making microporous nanozeolites. The method comprises the steps of providing an aqueous solution comprising at least one nanozeolite precursor material or zeolite particles, and electrospinning the aqueous solution onto a substrate to form an electrospun material. The electrospun material comprises microporous nanozeolites.
Another aspect of the invention is a method of making mesoporous nanozeolites. The method comprises the step of providing an aqueous solution comprising a nanozeolite precursor material and at least one structure directing agent, and electrospinning the aqueous solution onto a substrate to form an electrospun mesoporous nanozeolite material.
Another aspect of the invention is a sensor device. The sensor device comprises at least one material selected from the group consisting of microporous nanozeolites, mesoporous nanozeolites, and combinations thereof.
These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
Whenever particular features describe herein are said to comprise or consist of at least one element of a group and combinations thereof, it is understood that, except where otherwise so noted, the feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing a particular embodiment of the invention and are not intended to limit the invention thereto.
As defined herein, the term “nanozeolite” refers to a zeolite type material having a first dimension less than about 1 micron and a second dimension less than about 100 microns. As defined herein, the term “micropores” refers to pores having an average diameter of less than 2 nanometers (nm). As defined herein, the term “microporous” refers to materials with micropores. As defined herein, the term “mesopores” refers to pores having an average diameter of about 2 nm to about 50 nm. As defined herein, the term “mesoporous” refers to materials with micropores. As defined herein, the term “hybrid morphology” refers to mixed morphologies including but not limited to fibrous and particulate morphology. As defined herein, the term “high aspect ratio nanozeolites” refers to nanozeolites with a first dimension at least 100 times greater than a second dimension.
In one embodiment of the invention are porous nanozeolites having a first dimension less than about 1 micron and a second dimension less than about 100 microns, and wherein the nanozeolites comprise pores having an average diameter less than about 50 nm. In a further embodiment of the invention, the porous nanozeolites are microporous nanozeolites. In a still further embodiment of the invention, the porous nanozeolites are mesoporous nanozeolites.
Examples of anions, which may be found in nanozeolites, include but are not limited to silicate anions and aluminate anions. The porous nanozeolites have a fibrous morphology, or a particulate morphology, or hybrid morphology. In a further embodiment of the invention, the porous nanozeolites exhibit high sensitivity and high selectivity to adsorbent gas molecules and volatile vapors. The selectivity of a nanozeolite is typically determined by the diameter of the pores. Typically, molecules range in size from about 0.3 nm to about 50 nm and the nanozeolite pore diameters can be tailored to adsorb molecules within a desirable size range. The term “high selectivity” as used herein, refers to the ability of the nanozeolite to selectively adsorb only certain molecules. The term “high sensitivity” as used herein, refers to the ability of the nanozeolite to sense gas molecules present in trace amounts, typically in the parts per million range or less. Increasing the surface area of the porous nanozeolite material by making the nanozeolites with high aspect ratio enhances the sensitivity of porous nanozeolites. In a still further embodiment of the invention, the porous nanozeolites have surface areas greater than about 1000 square meters per gram.
In a further embodiment of the invention, the porous nanozeolites further comprise void spaces, which are designed by using the sol-gel chemistry of the zeolite precursor. In another embodiment, the nanozeolites are ordered porous nanozeolites. In a still further embodiment of the invention, the porous nanozeolites are short range crystalline in form, wherein the term “short range” refers to a range less than about 10 nanometers. In another embodiment, the porous nanozeolites can be designed at a molecular level by varying the silicon to aluminum ratio, and by varying the nature and level of exchanged cations. Examples of exchange ions include but are not limited to ions of alkali metals, alkaline earth metals, transition metals and rare earth metals. This enables the control of gas molecule traffic through the nanozeolite material. In a non-limiting example, porous nanozeolites of the invention exhibit selectivity in the adsorption and diffusion of different gases like CO2, O2, N2, NOx, SO2, and various hydrocarbons. The gases have different heat and kinetics of adsorption depending on the size, shape and polarity of the gas. In another embodiment, the adsorption level of the gases by the porous nanozeolites is tuned by synthesizing the nanozeolites with different pore architecture, pore diameter and channel dimensions.
In another embodiment of the invention, is a method for making microporous nanozeolites through electrospinning. In still another embodiment of the invention is a method for making mesoporous nanozeolites through electrospinning.
In another embodiment of the invention, the mesoporous nanozeolites are electrospun from sol-gels with a pH in the range of about 1 to about 3. In another embodiment of the invention, the nanozeolites are electrospun using sol-gels with a viscosity in the range of about 3000 centipoise (cP) to about 30,000 cP.
In one embodiment of the invention is a method for making microporous nanozeolites as illustrated in flow chart 128 seen in
In a further embodiment of the invention is a method for making microporous nanozeolites as illustrated in the flow chart in
In still further an embodiment of the invention is a method for making microporous nanozeolites as illustrated in the flow chart in
In another embodiment of the invention is a method for making mesoporous nanozeolites as illustrated in the flow chart in
In another embodiment of the present invention, the pore sizes and hence the selectivity of the nanozeolites is controlled by the structure directing agent. Examples of structure directing agents which can be used in embodiments of the present invention include, but are not limited to, trimethylammonium bromide (CTAB), cetyltrimethylammonium chloride, pluranic-123 C, poly(ethylene oxides), Brij® 76, and poly(ethylene oxide)x-poly(propylene oxide)y-poly(ethylene oxide)x, tetraethylammonium fluoride, quaternary ammonium ions, hexametyleneimine, tetrapropylammonium hydroxide, and combinations thereof.
Examples of zeolite precursor materials include but are not limited to tetra ethyl orthosilicate, tetra methyl orthosilicate, tetra butyl orthosilicate, SiO2 particles, sodium aluminate, and combinations thereof. Examples of zeolite materials include, but are not limited to, MCM-41, MCM-48, MCM-50, SBA-15, SBA-11, SBA-1, SBA-2, SBA-3, silicalite-1, zeolite-A, ZSM-5, ZSM-11, ZSM-23, MFI, H ferrierite, and combinations thereof. Examples of polymer materials include, but are not limited to, polyvinyl alcohol, polyethyleneimine, polycarbonate, polyethylineoxide, polyetherimide, polyamide, poly(acrylonitrile), and combinations thereof.
In one embodiment of the invention, the method to deposit nanozeolites includes the step of selecting the shape of the needle or capillary jet to provide a desired uniformity of the nanozeolite coating on the substrate. In another embodiment of the present invention, the spinning time is varied to deposit nanozeolite layers of varied thickness. In a non-limiting example, the spinning time was varied between a few seconds to 10 minutes.
In a further embodiment of the invention, the method of making nanozeolites includes the step of electrospinning using a plurality of capillary jets. In a still further embodiment, a pump system or pressure source is used to control the flow rate of the sol-gel through the capillary jet. In another embodiment, the method of making nanozeolites includes the step of using a shadow mask to selectively deposit porous nanozeolites on a substrate surface. Non-limiting examples of substrates include, quartz, semi-conducting materials such as silicon nitride, and metallic substrates.
The physical, mechanical, and electrical properties of the sol-gel used to electrospin the porous nanozeolites affect the characteristics of the nanozeolites. The hydrodynamic properties of the fluid depend on a combination of physical and mechanical properties, (e.g., surface tension and viscosity) and electrical properties (e.g., charge density and polarizability of the fluid).
In one embodiment of the invention, increasing the applied voltage and the spinning distance desirably favors the formation of fibrous morphology over particulate morphology. Below a certain critical voltage, micro-dripping, electro-spraying mode is favored and the liquid jet is unable to form and sustain the Taylor cone at the tip of the needle, resulting in particle spraying. For sol-gels, the spinning distance is desirably increased to reach the instability region of the jet where it splits into multiple jets to give nanozeolites with fibrous morphology. At lower spinning distances, hybrid morphologies combining fibrous and particulate nanozeolites are produced. In a still further embodiment, at sol-gel viscosities greater than about 4000 cP, predominantly nanozeolites with fibrous morphology are formed. In another embodiment of the present invention, at sol-gel viscosities lower than about 4000 cP, predominantly nanozeolites with hybrid morphology are formed.
Another embodiment of the invention is a sensor device, comprising a sensing material, wherein the sensing material comprises at least one material selected from the group consisting of microporous nanozeolites, mesoporous nanozeolites, and combinations thereof. In a further embodiment, the device comprises a gas-sensing device. In a non-limiting example, the gas-sensing device is a micro electromechanical system (MEMS) gas-sensing device. In a further example, the device is a CO2 sensing device. Non-limiting examples of the gas sensing device operate by adsorption of the gas molecules, resulting in a measurable change in mass or change in heat content or change in electrical conductivity or change in resonance wavelength or combinations thereof.
The following examples serve to illustrate the features and advantages of embodiments of the invention and are not intended to limit the invention thereto.
A schematic diagram of the electrospinning apparatus 100 is shown in
A JEOL 6335F scanning electron microscope with an Oxford EDAX detector was used to evaluate the morphology of the electrospun materials and their chemical make up. Secondary electron micrographs were used to investigate the morphology of the formed materials, while x-ray spectra were used to estimate the silica content in the formed phases.
TEOS was mixed with a structure directing agent TPAOH, water, and NaOH, in a molar ratio of about 0.25:0.09:4.8:1.0. The mixture was subsequently stirred for about 4 to about 5 hours and aged at 70° C. for about 3 to about 6 hours resulting in a sol-gel. The resulting sol-gel was mixed in equal proportions with polyvinyl alcohol (PVA) and was used to electrospin the sol-gel and polymer solution directly on to a semiconductor substrate. The spinning distance was 15 cm and the applied voltage was 15 kV. After electrospun material deposited on the substrate is calcined at 500° C. for about 4 hours, to remove the polymer template and structure-directing agent to give a microporous nanozeolite material.
TEOS was mixed with a structure directing agent TPAOH, water and NaOH in the molar ratio of about 0.25:0.09:4.8:1.0. The mixture was subsequently stirred for about 4 to 5 hrs and aged at 70° C. for 48 to 72 hours to form silicalite-1 zeolites. The synthesized silicalite-1 is powdered and calcined at 500° C. for about 4 hours to remove the structure directing agent. The powdered silicalite-1 particles are mixed with polyvinyl alcohol (PVA) to give it the required viscosity for spinning and electrospun to form microporous silicalite-1 nanozeolite. FIG.7 is a SEM micrograph showing microporous silicalite-1 with PVA after electrospinning. Silicalite-1 particles 184 of varied diameters can be seen embedded in polymeric fibers 182. The zeolite particle size can be controlled in the nanometer range to help enhance the active surface area of the sensing material and to help increase gas adsorption capability.
A sol-gel aqueous solution consisting of TEOS or tetra methyl orthosilicate (TMOS) with structure directing agent cetyltrimethylammonium bromide (CTAB) was used to synthesize mesoporous nanozeolite. The molar ratio of TEOS:Water:CTAB was fixed at about 4:8:1. TEOS was first hydrolyzed in an acidic environment (pH=1.85) and the resultant precursor solution is added drop-wise to the structure directing agent solution (CTAB) and aged for about 36 to about 48 hrs. A Brooke Field Viscometer was used to measure the sol-gel solution viscosity as a function of time and pH. The solution was electrospun and calcined at a spinning distance of about 7 cm to about 15 cm. The electrospun material was calcined at 500° C. for about 2 hour to evaporate the structure-directing agent to give mesoporous MCM-41 type nanozeolites with diameters in the micron to nanometer range with high aspect ratios.
In one embodiment of the present invention, as shown in
The previously described embodiments of the present invention have many advantages, including electrospinning micro and mesoporous nanozeolites without condensation of the electrospun material in an acidic environment and without porous filters. The sensor device embodiments of the invention show high sensitivity and selectivity.
While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 60/651,866 filed on Feb. 09, 2005, which is incorporated herein in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60651866 | Feb 2005 | US |
