U.S. patent application number 10/993569 was filed with the patent office on 2006-05-18 for mesoporous nano-crystalline titania structures for hydrogen sensing.
This patent application is currently assigned to General Electric Company. Invention is credited to Anthony Yu-Chung Ku, Sergio Paulo Martins Loureiro, James Anthony Ruud.
Application Number | 20060105141 10/993569 |
Document ID | / |
Family ID | 36386682 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060105141 |
Kind Code |
A1 |
Ku; Anthony Yu-Chung ; et
al. |
May 18, 2006 |
Mesoporous nano-crystalline titania structures for hydrogen
sensing
Abstract
A structure includes a substantially non-conductive frame having
an exterior surface. The structure defines a plurality of passages
that open to the exterior surface. Mesoporous material is disposed
in the plurality of passages and is supported therein by the frame.
In a method for making a mesoporous nanocrystalline titania hybrid
material, a templating agent, an acid, and a titania precursor is
mixed into a template liquid. A frame that defines a plurality of
passages is placed into the template liquid. A solvent is
evaporated from the template liquid, thereby forming a titania gel
encapsulating the templating agent. The gel is heated to remove
substantially the templating agent from the non-conductive frame
and the titania, thereby leaving a mesoporous titania material.
Inventors: |
Ku; Anthony Yu-Chung;
(Niskayuna, NY) ; Loureiro; Sergio Paulo Martins;
(Saratoga Springs, NY) ; Ruud; James Anthony;
(Delmar, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
General Electric Company
|
Family ID: |
36386682 |
Appl. No.: |
10/993569 |
Filed: |
November 18, 2004 |
Current U.S.
Class: |
428/131 |
Current CPC
Class: |
C04B 2111/00827
20130101; G01N 33/005 20130101; C04B 38/0045 20130101; B82Y 15/00
20130101; C04B 35/46 20130101; C04B 38/0054 20130101; C04B 38/0045
20130101; C04B 38/0003 20130101; Y10S 977/957 20130101; Y10T
428/24273 20150115; B82Y 30/00 20130101; C04B 35/46 20130101; Y10S
977/953 20130101 |
Class at
Publication: |
428/131 |
International
Class: |
B32B 3/10 20060101
B32B003/10 |
Claims
1. A structure, comprising: a. a substantially non-conductive frame
having an exterior surface and defining a plurality of passages
that open to the exterior surface; and b. mesoporous material
disposed in the plurality of passages and supported therein by the
frame.
2. The structure of claim 1, wherein the frame comprises anodic
aluminum oxide.
3. The structure of claim 2, wherein the frame comprises an anodic
aluminum oxide membrane.
4. The structure of claim 1, wherein the passages have a diameter
in a range of between 10 nm to 300 nm.
5. The structure of claim 4, wherein the passages have a diameter
in a range of between 20 nm to 210 nm.
6. The structure of claim 1, wherein the mesoporous material
comprises titania.
7. The structure of claim 1, wherein the mesoporous material has a
pore size in a range of between 2 and 20 nm.
8. The structure of claim 1, wherein the mesoporous material
comprises nanocrystals.
9. The structure of claim 1, wherein the mesoporous material
comprises a hybrid of nanocrystalline material and amorphous
material.
10. The structure of claim 1, further comprising a dopant added to
the mesoporous material.
11. The structure of claim 10, wherein the dopant is selected from
a list consisting essentially of: Ce, Co, Fe, Mn, N, Nd, Pd, Pt, S,
V, W, Eu, Cr, Tb, Er, Pr, and combinations thereof.
12. A sensor of a target substance, comprising: a. a mesoporous
titania material disposed within a frame, the mesoporous titania
material having mesopores that are capable of receiving the target
substance therein, the mesoporous titania capable of interacting
with the target substance, the mesoporous titania material having a
property that is a function of interaction with the target
substance; and b. a component that senses a change in the property
when the mesoporous nanocrystalline material is exposed to the
target substance.
13. The sensor of claim 12, wherein the target substance comprises
hydrogen.
14. The sensor of claim 12, wherein the property comprises an
electrical resistance.
15. The sensor of claim 12, wherein the frame comprises anodic
aluminum oxide.
16. The sensor of claim 15, wherein the frame comprises an anodic
aluminum oxide membrane.
17. The sensor of claim 12, wherein the mesoporous titania material
has a pore size in a range of between 2 nm and 20 nm.
18. The sensor of claim 12, wherein the mesoporous titania material
comprises nanocrystals.
19. The sensor of claim 12, wherein the mesoporous titania material
comprises a hybrid of nanocrystalline material and amorphous
material.
20. The sensor of claim 12, further comprising a dopant added to
the mesoporous titania material.
21. The sensor of claim 20, wherein the dopant is selected from a
list consisting essentially of: Ce, Co, Fe, Mn, N, Nd, Pd, Pt, S,
V, W, and combinations thereof.
22. A method for making a mesoporous nanocrystalline titania hybrid
material, comprising the steps of: a. mixing a templating agent
into a solvent and an acid to form a template liquid; b. adding a
titania precursor to the template liquid; c. placing into the
titania precursor and the template liquid a substantially
non-conductive frame having an exterior surface and defining a
plurality of passages that open to the exterior surface and
allowing the titania precursor and the template liquid to
infiltrate into the plurality of passages; d. evaporating the
solvent from the template liquid, thereby forming a titania gel
encapsulating the templating agent; and e. heating the gel at a
preselected temperature for a preselected period of time sufficient
to remove substantially the templating agent from the
non-conductive frame and the titania, thereby leaving a mesoporous
titania material in the plurality of passages.
23. The method of claim 22, wherein the templating agent is a
material selected from a group consisting essentially of: a
non-ionic block copolymer, a cationic surfactant, a zwitterionic
surfactant and a non-ionic surfactant and an anionic surfactant,
and combinations thereof.
24. The method of claim 22, wherein the titania precursor is a
material selected from a group consisting essentially of: titanium
ethoxide, titanium chloride, titanium isopropoxide, titanium
butoxide, titanium methoxide, titanium propoxide, and combinations
thereof.
25. The method of claim 22, wherein the heating step comprises
heating the suspension in air at 400.degree. C.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The invention relates to nanoscale structures and, more
specifically, to a nano-crystalline titania structure that may be
used in sensor applications.
[0003] 2. Description of the Prior Art
[0004] Presently, hydrogen sensors employ electrochemical, optical
or thermal detection methods. One system employs titania nanotubes
arranged in an array. Such systems generally have pore sizes of
greater than about 20 nm. Thus, the surface area of the sensing
element is limited, thereby limiting performance indicia such as
response and sensitivity. Recently, there has been renewed interest
in metal oxide semiconductor-based devices. In titania, for
example, the presence of hydrogen can dramatically change the
resistivity of the material through a variety of physical
mechanisms. The most common sensors depend on Schottky barrier
modulation in structures with Pd or Pt electrodes. Present systems
that utilize titania suffer from poor selectivity and slow response
times.
[0005] Conventional microporous materials such as zeolites have
regular pores with diameters of less than about 2 nm. Macroporous
materials have pores greater than about 50 nm, but with widely
varying pore sizes. Examples of well-known porous materials include
activated carbon used in deodorizers and silica gel used in
desiccants. The conventional porous materials with regular pore
sizes, such as zeolites, have limitations in pore diameter size,
while those with large pores have widely varying pore sizes.
Mesoporous materials are porous materials with regularly arranged,
relatively uniform mesopores (2 nm to 50 nm in diameter). They
generally exhibit a large surface area.
[0006] Existing methods are limited by a combination of high cost,
limited sensitivity, poor selectivity and slow response times.
Contamination of the sample and subsequent performance degradation
also limit existing systems use.
[0007] Therefore, there is a need for a hydrogen sensor that
exhibits good selectivity and that has a quick response.
SUMMARY OF THE INVENTION
[0008] The disadvantages of the prior art are overcome by the
present invention, which, in one aspect, is a structure that
includes a substantially non-conductive frame having an exterior
surface. The structure defines a plurality of passages that open to
the exterior surface. Mesoporous material is disposed in the
plurality of passages and is supported therein by the frame.
[0009] In another aspect, the invention is a sensor of a target
substance in which a mesoporous titania material is disposed within
a frame. The mesoporous titania material includes mesopores that
are capable of receiving the target substance therein. The
mesoporous titania is capable of interacting with the target
substance and has a property that is a function of interaction with
the target substance. A component senses a change in the property
when the mesoporous nanocrystalline material is exposed to the
target substance.
[0010] In yet another aspect, the invention is a method for making
a mesoporous nanocrystalline titania hybrid material. A templating
agent is mixed into a solvent and an acid to form a template
liquid. A titania precursor is added to the template liquid. A
substantially non-conductive frame having an exterior surface and
defining a plurality of passages that open to the exterior surface
is placed into the titania precursor and the template liquid. The
titania precursor and the template liquid are allowed to infiltrate
into the plurality of passages. The solvent is evaporated from the
template liquid, thereby forming a titania gel encapsulating the
templating agent. The gel is heated at a preselected temperature
for a preselected period of time sufficient to remove substantially
the templating agent from the non-conductive frame and the titania,
thereby leaving a mesoporous titania material in the plurality of
passages.
[0011] These and other aspects of the invention will become
apparent from the following description of the preferred
embodiments taken in conjunction with the following drawings. As
would be obvious to one skilled in the art, many variations and
modifications of the invention may be effected without departing
from the spirit and scope of the novel concepts of the
disclosure.
BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS
[0012] FIG. 1 is a top perspective schematic view of an exemplary
embodiment of the invention.
[0013] FIG. 2 is a cross-sectional view of the embodiment shown in
FIG. 1, taken along line 2-2.
[0014] FIG. 3A is a schematic illustration of mesopores in a cubic
arrangement.
[0015] FIG. 3B is a schematic illustration of mesopores in a
hexagonal arrangement.
[0016] FIG. 4 is a micrograph of a cross-section of a mesoporous
structure.
[0017] FIG. 5 is a micrograph of an ordered mesoporous
structure.
[0018] FIG. 6 is a cross-sectional schematic illustration of a gas
sensor employing a mesoporous structure.
[0019] FIGS. 7A-7F are schematic diagrams showing steps executed in
one method of making a mesoporous structure.
[0020] FIG. 8 is a graph of x-ray diffraction intensity of an
experimental sample of mesoporous material.
[0021] FIG. 9 is a nitrogen adsorption graph of one experimental
sample.
DETAILED DESCRIPTION OF THE INVENTION
[0022] A preferred embodiment of the invention is now described in
detail. Referring to the drawings, like numbers indicate like parts
throughout the views. As used in the description herein and
throughout the claims, the following terms take the meanings
explicitly associated herein, unless the context clearly dictates
otherwise: the meaning of "a," "an," and "the" includes plural
reference, the meaning of "in" includes "in" and "on." Unless
otherwise specified herein, the drawings are not necessarily drawn
to scale. Also, as used herein "mesoporous nanocrystalline hybrid
material" refers to a porous material with nanoscale crystals and
an amorphous matrix. Diameters of pores and passages listed herein
refer to average diameters to provide for eccentricity.
[0023] As shown in FIG. 1, one illustrative embodiment includes a
structure 100 that includes a substantially non-conductive frame
110. The frame 110 has at least one exterior surface 114 and
defines a plurality of passages 120 that open to the exterior
surface 114. The frame 110 could, for example, include an anodic
aluminum oxide membrane 112.
[0024] As shown in FIG. 2, mesoporous material 122 is disposed in
the plurality of passages 120 and is supported therein by the frame
110. The mesoporous material 122 includes a matrix, such as a
titania matrix, and a plurality of mesopores 130 that are in fluid
communication with the exterior surface 114. The titania matrix may
be crystalline, amorphous or a hybrid of the amorphous and
nanocrystalline material. Typically, the passages 120 will have a
diameter in a range of between 20 nm to 210 nm, with a range of
between 10 nm to 300 nm being possible. Depending on the size of
the passages 120 and other process-related factors, the mesopores
130 will typically have a diameter in the range of between 2 nm to
50 nm.
[0025] As shown in FIG. 3A, the mesopores 302 may be exhibit a
cubic ordering. As shown in FIG. 3B, the mesopores 304 may also
exhibit a hexagonal ordering. As would be clear to those of skill
in the art, other orderings are possible and would fall within the
scope of the invention.
[0026] A micrograph 400 of a cross-section of one experimental
embodiment is shown in FIG. 4. In this micrograph, one can see
vertical walls of the frame 110 and the passages 120 filled with
mesoporous material. A micrograph 500 of a passage with highly
ordered mesopores is shown in FIG. 5.
[0027] One embodiment of a hydrogen sensor 600 employing mesoporous
material is shown in FIG. 6. Mesoporous titania 604 is disposed
within an anodic aluminum oxide frame 602. Hydrogen received in the
mesopores interacts with the mesoporous titania defining the
mesopores. The mesoporous titania has an electrical resistance that
is a function of interaction with hydrogen. A resistance sensor
620, that is electrically coupled to the mesoporous titania 604
through a pair of contacts 610 (such as platinum contacts) senses a
change in the resistance of the mesoporous titania 604 when it is
exposed to hydrogen.
[0028] As shown in FIGS. 7A through 7F, one illustrative method for
making a structure, as disclosed above, includes mixing a
templating agent 720 into a solution 710 of a solvent and an acid
to form a template liquid 722. A titania precursor is added to the
template liquid 722. The templating agent 720 may self-assemble to
form an ordered arrangement, as shown in FIG. 7B.
[0029] An anodic aluminum oxide membrane 730, or other
substantially non-conductive frame that defines a plurality of
passages, is placed in the titania precursor and the template
liquid 722. The template liquid and titania precursor are allowed
to infiltrate into the plurality of passages of the anodic aluminum
oxide membrane 730, as shown in FIG. 7C.
[0030] The solvent is allowed to evaporate from the template
liquid, thereby forming a titania gel 740 encapsulating the
templating agent 720, as shown in FIG. 7D. As shown in FIG. 7E, the
gel 740 is heated in an oven 750 for enough time and at a high
enough temperature to remove substantially all of the templating
agent from the non-conductive frame and the titania (either through
vaporization or oxidation), thereby leaving a plurality of
mesopores 760 in a matrix of titania 744. Examples of templating
agents include: a non-ionic block copolymer (e.g., polyethylene
oxide-polypropylene oxide-polyethylene oxide, including Pluronic
type P123, F127, F108, F88), a cationic surfactant, an anionic
surfactant, a zwritterionic surfactant, a non-ionic surfactant, or
a combination thereof. Examples of titania precursors include
titanium ethoxide, titanium chloride, titanium isopropoxide,
titanium butoxide, titanium methoxide, titanium propoxide, or a
combination thereof. In one experimental example, the gel was
heated in air at 400.degree. C. for 10 hours. The resulting
mesoporous titania was then allowed to cool at a rate of 60.degree.
C. per hour after completion of the heating step.
[0031] A dopant may be added to the solvent to achieve certain
desired physical properties. For example, dopants may be added to
make sensors directed to a specific element, or to fine tune the
sensitivity of a sensor to specific concentration ranges. Examples
of suitable dopants include: Ce, Co, Fe, Mn, N, Nd, Pd, Pt, S, V,
W, Eu, Cr, Tb, Er, Pr, and combinations thereof. Dopants such as
Ce, Co, Fe, Mn, N, Nd, Pd, Pt, S, V, W may be useful in fabrication
of electrical sensors, whereas dopants such as Eu, Cr, Tb, Er, Pr,
Mn and Nd may be useful in optical sensors using mesoporous
titania. Such an optical sensor could measure phosphorescence or
work according to an interferometric sensor model. Possible
mechanisms for changing an optical property in doped mesoporous
titania include the following: direct adsorption in which a
monolayer on the surface changes the index of refraction;
coordination number change from adsorption; change in oxidation
state; change in crystal field strength; and change in hydration
state.
[0032] One embodiment of the invention uses a mesoporous
nanocrystalline titania structure as the sensing element for
hydrogen. The mesoporous character of the porosity provides a large
surface area for interaction between the hydrogen and the titania.
This embodiment employs thin film configurations, which can improve
the response time of the sensor. (Thinner films reduce the time
needed for gas diffusion and also decrease the electrical path
length in the titania structure.) Sensing elements fabricated
within larger pores of a template such as anodic aluminum oxide
offer the benefit of access to H.sub.2 from both sides of the
sensor, effectively reducing the thickness by half, and simplifying
integration into devices.
[0033] The use of mesoporous nanocrystalline titania also addresses
the sensitivity issue by using the a detection mechanism similar to
that observed in nanotubes. The thickness of the nanocrystalline
walls is comparable (about 2-10 nm), but the pore diameter is much
smaller (about 10 nm versus 20-100 nm). This higher effective
packing leads to a greater degree of sensitivity.
[0034] Doping of the titania with luminescent species can also lead
to improvements in the selectivity and response time. The synthesis
method used for these structures easily accommodates doping.
Surface modification of the titania mesopores with a material
catalytic for H.sub.2 such as Pd and Pt offers the potential to
increase the response time of the sensor by increasing the
adsorption kinetics.
[0035] Titania is known to exhibit a photocatalytic effect when
exposed to light with an energy higher than its bandgap.
Practically, this means it is possible to regenerate a titania
structure that has been fouled by an organic by exposing the system
to UV light. This would generate radicals at the surface of the
titania which would oxidize the organic substance. The rate of
self-cleaning would depend on the photocatalytic activity of the
titania, the incident UV intensity, and the time of UV
exposure.
[0036] In one illustrative example, a mesoporous titania sensor was
fabricated using the following steps. A precursor solution was
prepared by first completely dissolving 1.5 g of P123 block
copolymer in 24 g of ethanol. The solution was then poured into a
Petri dish, containing several elastomer spacers. The spacers were
completely submerged after adding the precursor solution. An anodic
aluminum oxide membrane (referred to herein as "AAO," 25 mm
diameter, 50 .mu.m thick, with 200 nm pores) was immersed
horizontally in the fluid on top of the spacers. The AAO used was
an ANODISC inorganic membrane available from Whatman International
Ltd. of Florham Park, N.J. (4) The solvent was allowed to evaporate
at room temperature for 20 hours. During this time, the fluid level
in the dish dropped below the level of the AAO membrane due to
evaporation of the volatile components. The AAO membrane was
removed from the spacers and heated in air at 400.degree. C. for
about 10 hours. The heating and cooling rate was 60.degree. C. per
hour. Electrical contact pads were fabricated on the top and bottom
surfaces of the membrane using a Pt powder paste and firing at
400.degree. C. for 1 hour. Pt lead wires were bonded to the Pt
contact pads using Ag paste. The sample was placed in a gas-tight
tube through which N.sub.2 gas and a mixture of H.sub.2/N.sub.2
gases could be introduced, and the electrical resistance was
measured using an ohm-meter. N.sub.2 was introduced at a rate of
200 sccm (standard cubic centimeters per minute) and a resistance
of about 14-15 mega-ohms was observed. A 4% H.sub.2/N.sub.2 gas
mixture was added at a rate of 5 sccm to the flow to make a mixture
of 975 ppm H.sub.2 in N.sub.2. After 10 minutes, the resistance
dropped to about 5 mega-ohms. The flow of the H.sub.2/N.sub.2
mixture was stopped and after 5 minutes the resistance reverted
back to a high value of about 16 mega-ohms.
[0037] FIG. 8 shows an x-ray diffraction pattern 800 of one
experimental sample of mesoporous material. The peaks are indexable
to the anatase phase and the peak broadening indicates
nanometer-sized crystallites. As shown in FIG. 9, a nitrogen
adsorption-desorption isotherm 900 of one experimental sample was
measured at 77 K. The hysteresis is typical of a type IV isotherm
and indicates mesoporosity. The BET surface area of the sample, as
fitted from the data, is about 40 m.sup.2/g.
[0038] The above described embodiments are given as illustrative
examples only. It will be readily appreciated that many deviations
may be made from the specific embodiments disclosed in this
specification without departing from the invention. Accordingly,
the scope of the invention is to be determined by the claims below
rather than being limited to the specifically described embodiments
above.
* * * * *