Research Highlights

4000 Citations (Science Citation Index)
  • Discovered a Zeolite Based MRI Contrast Agent - Licensed Technology
  • Established Novel Methodologies for the Synthesis of Supported Catalysts
  • Discovered the First Zeolite having larger than a 12 membered Ring Pore
  • First to Prepare Zeolite Films/Membranes using Pulsed Laser Ablation
  • First to Immobilize Enzymes in Mesoporous Molecular Sieves
  • Developed a series of Novel Capacitive and Optical Chemical Sensors
  • Discovered a Novel class of Mesoporous Materials (DAM-1)
  • First to Prepare Molecular Sieve Fibers by Electrostatic Deposition

 


Research topics

 


Molecular Sieve Synthesis and Encapsulated Metal Complexes

Aspects of this work have been or supported by theDARPA, Chevron, National Science Foundation, R. A. Welch Foundation, Petroleum Research Fund , U.S. -Hungarian program & COBASE

 
 

AAAAAWe became interested in molecular sieves because of their potential as host matrices for site isolation of homogeneous catalysts and reactive intermediates. In particular, we were intrigued with the idea that one might be able to physically trap metal complexes in the oxide pores rather than immobilize the complexes by attachment to the surface of the molecular sieve. A distinct advantage would be that a trapped complex would be free to move within the confines of the channel or cage and thereby retain some of the homogeneous properties in a microreactor. The reactivity of such ship-in-a-bottle complexes would be modified by the properties of the molecular sieve, especially shape selectivity. Therefore, molecular sieve encapsulated metal complexes might be considered a bridge between homogeneous and heterogeneous systems. Some of these ideas have been realized through the encapsulation of large chelate complexes (e.g. phthalocyanines) in large pore zeolites such as NaY or X. In recent years we have spent a great deal of time studying methods of preparation and characterization of zeolite host-guest materials. Additionally, we have been working to identify system features that would enable selective catalysis. A milestone in this effort was the synthesis a perfluorophthalocyanine ruthenium complex and the encapsulation in zeolite NaX. This intrazeolite complex turns out to be an exceptional catalyst for the oxidation of alkanes and olefins. For the example the oxidation of cyclohexane to adipic acid is readily achieved at room temperature. The activity is comparable to enzymes and the stability far exceeds known catalysts or enzyme mimics. This host-guest catalyst was recently cited as possibly one of the best low-temperature peroxide based oxidation catalysts and one of the most notable developments in ruthenium chemistry This is clearly the type of result that we had hoped for and although, we continue along this avenue of investigation ( NSF ), there are several outgrowths from this work that have set us off in exciting new directions. For example, in an effort to develop synthetic strategies for the preparation of zeolite ship-in-a-bottle complexes, we found that molecular sieves could be synthesized around a metal complex ( US Patent 1992 ). This provides many advantages over more conventional in situ methods of encapsulation where the nature of the guest metal complex is poorly defined. As part of this effort we realized that metal complexes might also function as templates or structure directing agents for the synthesis of new molecular sieves. We have been particularly successful using metallocenes and derivatives as templates, which have produced over 20 new structures ( US Patent 1996, 1997, 1998 ). The most significant discovery amongst our new materials is UTD-1 which is currently the largest pore zeolite known. An exciting aspect of the UTD-1 structure is the fact that the channels are defined by 14 Si atoms. Therefore, this represents the first and only zeolite having greater than a twelve membered ring pore structure and we made it using a metal complex.

 
 

Additionally, the UTD-1 structure appears to be stable to at least 1000 o C in air suggesting this novel large pore material may have many interesting applications, especially in catalysis. The presence of Bronsted acidity generated by incorporating metal ions such as aluminum into the framework of UTD-1 is evidenced by high activity in cracking reactions and related acid catalyzed processes . Additionally, transition metals such as titanium can be incorporated into the framework. Ti-UTD-1 was found to be an effective catalyst for the room temperature oxidation of cyclohexane to adipic acid as well as the epoxidation of olefins and the oxidation of sterically
 
 
hindered phenols. We have also explored the post-synthesis metal complex modification of hexagonal Y type zeolites prepared using metal complexes. This work has resulted in the some interesting Ti silicate oxidation catalysts. We are also examining the encapsulation of Jacobson's catalyst and related chiral metal complexes in various nanoporous materials. In particular, we intercalated metal complexes in the layered precursor to MCM-22, which upon heating converts to a zeolite with vary large cages but with small 10 ring windows. The encapsulation of metal complexes during this transformation represents another unique method of preparing ship-in-a-bottle complexes. Results for the oxidation of prochiral olefins yielded ee's more than double that obtained with the homogeneous analogue. Thus MCM-22 is an interesting host zeolite that is amenable to the encapsulation of
 
 

metal complexes during synthesis. Attempts by others to immobilize catalysts after synthesis have failed because of the difficulties in both diffusing catalyst precursors through the 10 MR windows as well as self assembly of complexes within the cages. We have now extended the range of complexes that can be encapsulated in MCM-22 to include a series of ruthenium polypyridyl complexes that have promise in both optical sensing and catalysis.
AAAAAWe have also developed a method for encapsulation of chiral metal complexes and chiral dendrimers as well as polyphthalocyanine catalysts in mesoporous molecular sieves. For example, we can prepare mesoporous silica with encapsulated polyethyleneoxide coiled about chiral metal complexes. Upon extraction of the complexes the remaining polymer is imprinted such that the host molecular sieve now selectively absorbs the appropriate enantiomer from a racemic mixture. In addition, to new materials and novel reactivity, the rigid nature of our metal complexes compared with most organic templates coupled with the fact that we can systematically vary the size, shape and symmetry may provide a better understanding of the templating effect. We are also exploring the in situ synthesis of mesoporous materials with binding sites. We have prepared several tethered metal complexes ranging from metallocenes to chelate complexes. For example we have prepared DAM-1 with the pore walls decorated with propyldiphenylphosphines. After a series of steps the complex RuCl 2 (PPh 3 ) 2 PPh 2 /\/\Si- was generated. This complex will catalyze the air oxidation alcohols to ketones and aldehydes in the presence of TEMPO free radical. The observed activity was nearly twice that of the homogeneous catalyst. We anticipate these areas of molecular sieve synthesis and host/guest chemistry will continue to be quite fruitful.

 
 

AAAAAOther Host-Guest systems include zeolite encapsulated single walled carbon nanotubes (SWNT), where we are developing 2 strategies. In one case, we employ zeolites such as UTD-1 and exploit the cobalt template to catalyze the formation of SWNTs within the pores. We have prepared SWNTs <0.5nm in diameter in UTD-1, UTD-18 and UTD-12, the later maybe the smallest nanotube ever made. Such composites have shown signs of superconductivity. This is part of a larger effort in the general area of phonon engineering. The second method involves addition of the SWNT bundles to a Molecular Sieve Synthesis. The TEM (right) shows all silica DAM-1 with nanotubes protruding from the 4 nm pores. The SWNTs exhibit the highest thermal conductivity for known materials. Therefore, we will continue to study the effect of encapsulation on the thermal properties.

 

 

Enzyme Immobilization in Molecular Sieves

Supported by the National Science Foundation, PRF

 
 
AAAAAAlthough, many of our zeolite ship-in-a-bottle systems are billed as enzyme mimics or zeozymes, that type of catalytic activity and selectivity we strive for is still done better by nature. The fixation of real biologically active species onto inorganic materials would combine the high selectivity of enzymatic reactions with the chemical and mechanical properties of the support. This blend has brought to light many new applications in the fields of chemical sensing and biocatalysis. Molecular sieves as support materials offer interesting properties, such as high surface areas, hydrophobic or hydrophilic behavior and electrostatic interactions, as well as, mechanical and chemical resistance which makes them attractive for enzyme immobilization. However, inclusion of enzymes in the pores of microporous structures (i.e., zeolites) is an impossible task since the pore size of these materials is too small (<20Å). The mesoporous molecular sieves such as MCM-41 discovered by Mobil possess a regular array of uniform, unidimensional mesopores with very narrow pore size distribution, which can be systematically varied in size from approximately ~20 to 200Å. The MCM-41 materials have been used
 
 
as hosts for a variety of guest molecules including some of our own work with metal complexes. The larger pore dimensions of these mesoporous materials offer the possibility of accommodating small enzymes and proteins within the channels, which can be several hundred nanometers long. Furthermore, the pore openings of MCM-41 materials can be modified with organosilane groups resulting in a reduction of the channel window diameter which could effectively entrap a guest molecule. This approach to enzyme
 
 
immobilization would resolve some of the disadvantages of other physical entrapment techniques, such as the leaching of adsorbed molecules, the chemical degradation of the anchoring bond of covalently attached enzymes and the barriers to diffusion of substrate and product which are encountered for large polymeric substrates in sol-gel preparations. The MCM-41 hexagonal phase or MCM-48 cubic phase could also be suitable in the evaluation of theoretical models of enzyme immobilization that assume a uniform pore size for the model supports which is in contrast with most amorphous materials that possess a significant pore size distribution. We were the first to report the immobilization of several small globular enzymes including the proteases trypsin and papain, which are important biocatalysts, as well as the heme protein cytochrome c, (shown above in MCM-41). Since then there have been well over 100 reports based on our approach. The nature of the enzymes, Molecular Sieve pore size and framework composition are all mportant aspects of on going studies.
 
 
However, more recently we have become focused on developing redox active and photoactive host molecular sieves that can be used as bifunctional catalysts as well as electrodes for biofuel cells, photoelectrochemical biofuel cells and biodensors. Interactive host materials include novel periodic mesoporous silicas (PMO) and metal organic frameworks (MOF). A model for microperoxidase MP-11 in a Cu based MOF is shown right.
 
 

We are also exploring semiconductor frameworks such as mesoporous TiO2 , which will photoreduce the encapsulated MP-11. We continue to examine a variety of intereactive mesoporous molecular sieves host materials as well as different biomolcules including enzymes, proteins and antibodies.

 

 

Redox Active Molecular Sieves and Composites

This work has been supported by NATO, NSF-INT

 
 
AAAAAWe have been interested in the redox activity of molecular sieves as electrodes, electrochromics and host-guest materials. In collaboration with Dr. Fethi Bedioui at the ENSCParis, we began evaluating cyclic voltammetry as a method of characterizing intrazeolite metal complexes. It turns out that electrochemistry provides a great deal of information not readily accessible by other analytical techniques. Additionally, we have demonstrated shape selective electrocatalysis using carbon composite electrodes containing zeolite encapsulated metal complexes. We now conduct electrochemical experiments which have become essential to the characterization of our host/guest materials in our own labs. We have also found electrochemistry can be useful in probing framework substituted molecular sieves. We will continue to examine the electrocatalytic reactivity of various MeAPO, MeAPSO and metal silicate molecular sieves as well as the metal complex composites. We have also characterized mesoporous materials modified with metal complexes and enzymes by cyclic voltammetry. We currently use these large pore host materials for the immobilization of enzymes ans we hope to address in the near future improvements in electrode design and fabrication as well as the enhancement of internal electron transfer via mediators. The issue of intrazeolite electron transfer has actually become
 
 

a subject of controversy amongst the electrochemists. Part of the issue is that aluminosilicates are insulators at room temperature such that the electrochemical response from intrazeolite species is generally quite small. Therefore we are starting to look at materials that have some reasonable degree of conductivity. This includes mesoporous indium tin oxide, fluorinated tin oxide, zinc oxide and calcium aluminates. We are also syntheisizing organic functionalized PMOs with redox active groups such as diazapyrene (shown left) and methylviologen. We have demonstrated electron transfer and charge transfer with these materials and guest molecules. Biofuel cell and sensor applications are in development.

 

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Molecular Sieve Based MRI Contrast Agents and Drug Delivery Systems

This work has been supported by Pharmacyclics, Inc. and the Texas Advanced Tech. Program

 
 

AAAAAWe have developed several diagnostic pharmaceuticals for magnetic resonance imaging (MRI) based on 2 and 3 dimensional microporous metal oxides ( US Patents 1992, 1994, 1995 ). The gastrointestinal (GI) tract is particularly difficult to image because of looping and motions of the bowels. There are currently no positive contrast agents on the market for the purpose of enhancing the contrast in the GI tract. Zeolites and clays modified with gadolinium (III) are effective positive MRI contrast agents for this application. They function by shortening the proton relaxation times of water and tissue, which results in image brightening. Pharmacyclics, Inc. licensed our technology and currently has a formulation known as Gadolite® and Citri-vu® which has completed all clinical trials and has been approved for sale in Great Britain and Canada . Final FDA approval is currently pending. We continue to conduct fundamental research in support of this product as well as in the development of new materials and biomedical applications. In relation to MRI, we are evaluating both molecular sieves and clays that contain paramagnetic species which may be in the form of exchangeable ions, metal complexes or framework atoms. This work includes developing synthetic strategies for incorporating paramagnetic ions or complexes (e.g. Gd, Mn, Fe) into these materials as well as methods for their physico-chemical characterization, including relaxivity measurements of suspensions.
AAAAA Zeolites and related nanoporous materials have potential as drug delivery systems. We have studied the immobilization of Vitamin E TPGS, which is a sticky waxy solid. The transport, storage and administration of Vitamin E would benefit from a free flowing powder form. We have immobilized Vitamin E in variety of mesoporous materials which can be recovered by extraction. To be useful for drug delivery the Vitamin E

 
 

should be immobilized in micellar form. Therefore we have used Vitamin E as a template to form the hexagonal mesoporous silica, DAM-1 (Dallas Amorphous Material No.1). We have now extended this methodology to prepare alumina and titania composites. The Al-DAM-1 completely dissolves under physiological conditions releasing the Vitamin E ( US Patent ). Additionally, we are employing Vitamin E to encapsulate other lipid soluble drugs for oral delivery. DAM-1 has also been prepared with organo silanes (NH 2 , SH, P(Ph) 2 , Ph, PhNH 2 , Cp) which will be used to imprint the pore walls for molecular recognition and sensor applications.

 

 

Molecular Sieve Based Chemical Sensors

This work has been supported by Texas Instruments, the Texas Advanced Technology program and ACS-PRF

 
 
AAAAAWe have developed a method of preparing continuous molecular sieve thin films in the nanometer range via laser ablation ( US Patents 2000, 2001 and pending ). We were the first group to report the pulsed laser deposition of low density metal oxides and aluminum phosphates in general. Although, one could envision applications for this technology in the areas of separations and catalysis, our principle effort has been in the development of molecular sieve based chemical sensors. Zeolite based SAW or QCM devices have shown
 
 

promise as chemical sensors. Unfortunately, these types of sensors only respond to a weight change upon adsorption of molecular species. Therefore, discrimination of small molecules that might be found in combustion gases (N2 , O2 , CO, CO2 , NH3 , NOx , SOx ) is quite poor. In contrast we have developed a capacitance type chemical sensor that relies on the laser ablated molecular sieve film as the dielectric phase. Since, our sensor responds to changes in the dielectric properties of the molecular sieve, a signature change can be associated with specific molecules which reflects size shape and polarity. An array of

 
 

different molecular sieves that respond differently to a specific molecule can be used to confirm the identity and concentration. In the case of AlPO4 and MeAPO molecular sieves we have shown that CO, CO2 and N2 exhibit capacitance changes that are several orders of magnitude different which allows us to detect these molecules even though they are similar in size. The sensitivity appears to be in the ppm range for most molecules of interest which is commercially acceptable. We are currently working towards a prototype device that will be inexpensive, small, stable and easy to manufacture. The materials of interest include zeolites, silicates, metal aluminum phosphates and transition metal oxides. More recently, we have extending are target materials to include mesoporous phases. We have found that Nb-TMS-1 molecular sieves are good materials for humidity sensors. In fact we have shown that such devices can be used a breath analyzers and may find application as infant respiration monitors.

 
 

AAAAAWe are also PMO and MOF materials that can serve as a host matrix for redox active molecules including enzymes which we can then exploit as an optical sensor or in an electrode configuration for chemical sensing or electrocatalytic processes. For example, diazapyrene and biphenyl PMOs were found to be senstive for the detection of nitro compounds used as explosive tags. In these cases flouresence quenching is the transduction mechamism. We are also examining optical fibers coated with PLD zeolite films as optical sensors in colaboration with Dr. Paul Pantano. This can be accomplished in 3 ways. The first involves the use use of PLD to coat the end of an optical fiber with a thin zeolite film. In the case of UTD-1 we were able to prepare oriented films on the end of an optical fiber bundle. Another approach is to make polymer composites with zeolites on the ends of the fibers. The zeolite may contain encapsulated fluoresent dyes that might be sensitive to certain analytes. For example, we have composited MCM-22 encapsulated Ru(bipy) 3 2+ with nafion on the end of a fiber and monitored the fluoresence quenching by dioxygen. A third approach is to deposit different molecular sieve shapes as found with DAM-1 in the wells of an etched fiber bundle. This has the advantages that many different types of molecular sieves identified visually by shape can be patterned or randomly deposited on the end of a fiber. Such an array can be exploited in a sensor or in monitoring combinatorial syntheses.

 

 

Zeolite and Zeolite Composite Membranes

This work is supported by the PRF, DOE and Mobil Technology Co. and the Texas ATP

 
 
AAAAAWe have found that conducting polymer composite membranes exhibit enhanced permeability and selectivity upon doping. This is unusual since the two are normally inversely related. We have been examining a series of substituted thiophene based polymers which allow us to fine tune solubility and Tg as well as introduce functional groups that can bind electrolyte or can be surface modified to generate asymmetric membranes. The incorporation of zeolites into the electroactive matrix results in
 
 
facilitated transport of polar molecules such as CO2 . Ultimately, we would like to exploit the electrocatalytic properties of the intrazeolite metal complexes described above in a membrane reactor or in a more selective facilitated transport process. We are also exploring polymer composites with mesoporous molecular sieves in
 
 
an effort to improve the polymer/molecular sieve interactions. We have demonstrated that polymers can penetrate the pores which reduces that voids find in zeolite composites from poor wetting of the oxide surfaces. In collaboration with Nandika Souza, (UNT) we have measure increases in mechanical properties including modulus with increased loadings of molecular sieve. Enhanced permeability without a loss in selectivity has been achieved. Additionally, the mechanical properties of the composites are improved relative to amorphous metal oxide/polymer composites. Recently, we have found various functionalized molecular sieves
 
 

that give unprecedented separation factors for CO2 /CH3 . In this case DAM-1 with the pore walls decorated with amines was dispersed in Matramid. This composite gives a >100 for CO2 /CH4 in the presence of 300 ppm of water. More recently, we started to examine metal-organic frameworks (MOF) for facilitated transport of methane and we have observed enhanced permeability versus CO2 . We also have a program to synthesize new MOFs. This class of nanoporous material may very well change the current thinking on mixed matrix membranes. In collaboration with J. Ferraris (Polymers) and I. Musselman (Microscopist) we have assembled a state of the art membrane permeameter and a team of students that are addressing different aspects of the project.
AAAAAWe are also exploring the application of our laser ablated thin films in separations as well as membrane based catalytic reactions. For example, we have prepared thin films of UTD-1 and Ti-UTD-1 by laser ablation where the crystals are oriented with the channels perpendicular to the support. This presents an interesting opportunity to study such a catalytically interesting material in this configuration. We have demonstrated the separation of aromatics and linear paraffins with a UTD-1 membrane and the oxidation of olefins in a pervaporation experiment using a Ti-UTD-1 membrane. Oriented films of MAPO-39 , MCM-22 and MCM-41 have also been prepared for the first time where the channels are normal to the substrate which also opens up membrane based applications. In fact MAPO-39 membranes appear to effectively separate water and alcohols. We are currently attempting to prepare such films on polyelectrolyte membranes to separate methanol and water in fuel cells. There are other zeolites that also separate water and alcohols that are currently under investigation. Similarily, zeolites and layered materials that exhibit proton conductivity are being fabricated into thin membranes using the PLD method. Alternatively, we are examining the growth of molecular sieves in e-beam patterned glass, where the idea is to fill 200 nm holes with proton conducting molecular sieve. For example, mesoporous silica has been templated with polyethylimine and synthesized with meta-polyanaline, proton conductors. These molecular sieves can be grown in the shaped holes much like has been accomplished with the etched optical fibers discussed above. The zeolite films may also be patterned by laser ablation through a mask or by a novel technique known as line patterning. In this case, an image is deposited by a laser printer o copy machine then coated by PLD. After hydrothermal treatment the film is sonicated and the toner is removed leaving the pattern in the film. We have also recently developed the technology for evenly coating three dimensional objects using laser ablation. Both metal and glass beads have been coated with oriented zeolite UTD-1 which are being tested in packed columns for separations and catalysis. We can also use this methodology to coat catalyst particles and other zeolite crystals as well as optical fibers. Prof. Sarah Larson ( Iowa State ) is currently evaluating NaX
coated fibers for photooxidation of pollutants. It is also possible to prepare hollow tubular zeolite membranes using this methodology. One can envision the epitaxial growth of different zeolite layers that might then impart new types of reactivity and selectivity. We have now prepared flat membranes containing as many as 3 layers of different zeolites. The PLD method will allow one to prepare multilayer films with a cascade of different pore architectures that might be exploited in both separations and catalysis. Recently, we have used spherical

 
 

mesoporous DAM-1 particles as substrates and coated them with various zeolites by PLD. After a vapor phase treatment (ie water and template only), the DAM-1 core dissolves, leaving a shell of crystalline zeolite. These hollow zeolite spheres can be exploited in drug delivery and nanocatalysis.

 

 

Electrospun Fibers

Support from NSF and US Air Force

 
 
AAAAAWe have developed a novel method for the preparation of molecular sieve fibers using electrospinning
( US Patents pending ). In this technique a synthesis gel is subjected to 25,000 volts over a distance of 20 cm. The charged particles in the gel are accelerated to a substrate/electrode. Some of the interesting DAM-1 and SBA-15 fibers are shown right. Interesting composite meshes have also been prepared using molecular sieves
 
 
and various polymers., including the conductive, luminescent MEH-PPV. We are preparing for solar cells donor-acceptor fiber composites. Mesoporous TiO 2 fibers have been prepared for this application. PEO and PEI composites with the molecular sieves as well as hectorite and nanoscale laponite clays have been electrospun as free standing paper. This could lead to smart paper and textiles.

In particular we our focused on developing conductive fibers including TiN, ITO and
F-SnO2 . We are also developing magnetic paper based on this technology that would be white and useful in labels and packaging. Additionally we are working on plasmonic fibers based on <20nm gold shells prepared around PEI nanofibers, which are

 
 
 

transparent and highly conductive. Carbon nanotubes have also been electrospun with the polymer clay composites. We are currently growing cells (eg HELA) on scaffolds constructed from such fibers. Other applications in the areas of sensors, electrochromics, solar cells and fuel cells are in development.

 
 

 

Fuel Cells

This work is supported by DOE, the Texas ATP, Samsung and SPRING

 
 
AAAAACurrent technology for proton exchange membranes (PEM) is limited by thermal stability, conductivity, and the need to be hydrated to a high level. We are exploring several novel concepts based on inorganic/organic hybrid systems including 3-dimensional cross-linked inorganic/organic materials based on organosilanes. The highly cross-linked Si-O backbone in this material provides chemical, thermal and mechanical stability. The organic side chains are terminated in sulfonic acid groups or various organic amines, which can then be doped with trifluoromethanesulfonimide (HTFSI) to form proton conducting active site(s). We are actively looking at immobilized ionic liquids to reduce the need for water. PEMs comprising HTFSI-doped amine groups exhibited proton conductivity of 10 -3 S/cm at elevated temperature (= 130 o C), water-free conditions and 10 -2 S/cm under fully hydrated conditions. The PEM comprising sulfonic acid groups showed a proton conductivity of
 
 

>10 -2 S/cm under low humidity conditions. The polarization behavior of the PEM in a fuel cell fed with low humidified H2 /O2 (25% relative humidity or lower) was also investigated. Other strategiesinclude the preparation of free-standing mesoporous silica films synthesized using non-ionic surfactants and triflic acid, and new mesoporous tungstosilicates (MWS) synthesized using ionic surfactants and non-ionic surfactants under various conditions. Additionally, mesoporous SBA-15 containing polyaniline and mesoporous benzene silica (MBS) with aromatic rings embedded in the mesopore walls are being investigated.

 

 

Novel Hybrid White Light Emitting Diodes

 
 
AAAAADespite recent major advances in solid state lighting, further significant improvements can be expected from advancements in the fundamental science associated with white light emitting materials themselves. From our standpoint, the ideal emitting material(s) should exhibit luminescence from a single source with the following desired characteristics: (1) high quantum efficiency, (2) brightness in the solid state at room temperature, (3) chromaticity suitable for the application, (4) amenable to thin film deposition, (5) stability for long operational lifetimes, (6) suitability for conventional and flexible substrates, and (7) low manufacturing cost. The satisfaction of ALL these conditions is difficult and requires an extremely careful design that considers fundamental principles in spectroscopy, chemistry, physics, materials science, and electrical engineering. We recently discovered a 3 hour microwave synthesis of the zinc gallophosphate molecular sieve NTHU-4 which we can process as powders or as thin films by pulsed laser ablation.
AAAAA The structure of NTHU-4 shown in Figure 1 is composed of corner shared tetrahedra of GaO4 , ZnO4 and PO4 /HPO4 . The 14 membered rings (14MR) in NTHU-4 define an elliptical pore 1.13nm in diameter. Interestingly, NTHU-4 can be prepared as 2 polymorphs designated NTHU-4Y and NTHU-4W. This arises because of MO4 and HPO4 disorder sites on the rim of the channels. The NTHU-4Y material emits yellow light at 550nm regardless of the excitation frequency over the range of 280-520nm. By heating the NTHU-4Y sample at 280 o C for 4 hours or by using a different synthesis route , the more ordered NTHU-4W
 
 
was obtained which emits yellow light when excited >420nm and white light when excited below 420nm. The chromaticity coordinates for the white light generated by excitation at 390nm were 0.29,0.34 on a CIE diagram. This may be the first example of an intrinsic white light phosphor. We are currently studying hybrid composite materials comprising an organic electroluminescent material that emits at wavelengths <420nm, in turn exciting a nanoporous zinc gallophosphate molecular sieve to produce
 
 

white light. The combination of high efficiency organic light emitting materials with inorganic phosphors should provide high efficiency, long term stability, and thin-film processability. We firmly believe that this work will make major strides in all research objectives confronting the advancement of hybrid materials for practical solid state lighting applications.

 

Self Healing Inorganic Films

Our strategy for self healing inorganic films is shown below where a metal oxide layer is placed on top of a polymer layer that has been impregnated with polymer nanospheres or nanofibers (<100nm) that contain a reactive metal oxide precursor. When stress is applied to the metal oxide film microcracks are formed, which allow the diffusion of oxygen and moisture to the polymer layer. The nanocapsules will be composed of polymers that dissolve or degrade in the presence of water, thus releasing the reactive metal oxide precursor. Potential reactive species would be metal halides and metal akyls such as TiCl4 or Al(Me)3. This volatile liquid will quickly diffuse into the crack and hydrolyze to form TiO2, resulting in a continuous metal oxide film. An alternative to placing the nanocapsules in a polymer matrix would be to disperse them directly in the metal oxide coating. Then as stress cracks initiate the nanocapsules would dissolve and self healing of the oxide film would be realized. The prototype nanocapsules have been made from polylactic acid (PLA) and polyglycolic acid (PGA) as well as their co-polymers.  The initial metal oxide precursor studied was self heal 1TiCl4, a volatile liquid that reacts violently with water to form TiO2.  We have employed microemulsion techniques to incorporate TiCl4 in polylactic acid core-shell structures. We have prepared hollow capsules and porous nanofibers of PLA by electrostatic deposition. We have shown the evolution of TiO2 from the PLA after exposure to controlled humidity.  Next these nanocapsules were imbedded in a polymethylmethacrylate (PMMA) film, which is commonly used as a planarization layer for metal oxide coatings. This was followed by coating of the polymer with an alumina layer.  The stress induced cracks are then filled with TiO2 from the PMMA embedded fibers.

 

 

 

 

Templated nanoparticles for Solar Cells
Currently or recent supported by the Air Force and SPRING.
We have developed a novel method for the fabrication of semiconductor nanofibers by the impregnation of a mesoporous substrate with a reactive metal oxide precusror. Mesoporous fibers, films and shaped particles can be employted as templates. In the case of sphereical DAM-1 particles shown below, core shwll structures can be generated by removal of the molecular sieve template. The nanofibers are polycrystalline anatase or rutile depending upon the thermal treatment with 101 faces exposed. The nanorods2TiO2 nanorods can be doped with SnO2. We have found a significant enhancement in photooxidation activity of the doped nanorods relative to the pure anatase nanorods. These reactive surfaces can be further functionalized with metal clusters such as silver and gold or quantum dots such as PbS shown right. TiO2 nanorods are being mixed with conducting polymers for organic and Gratzel type solar cells. The PbS quantum dots are being employed to affect multiple exciton generation.
tnt        In addition to TiO2 we are examinng the synthesis and reactivity of other semiconductor nanowires. For example, silver vanadium oxide (SVO) and V2O5 nanowires have been hydrothermally synthesized. The SVO nanowires contain nanoclusters of silver that are formed in situ. The as made nanowires were over 30 µm long and 10-20 nm in diameter. The nanowires can be fabricated into free standing and flexible sheets by suction filtration. The electrical conductivity of the SVOsvo nanowires was 0.5 S/cm, compared to 0.08 S/cm for the V2O5 nanowires. The Li ion diffusion coefficient in the SVO nanowires was seven times higher than that in the V2O5 nanowires and one of the fastest ever reported. An electrochromic device (ECD) was fabricated from the SVO nanowires which displayed a color switching time of 0.2s from bleached state (green) to colored state (red-brown) and 60% transmittance contrast. These fibers have also shown promise for bolometer applications. Additionally, we are exploring supercapacitor applications with John Ferraris.
tnt 2In addition to the semiconductor nanowires, we have been working with nanotubes. For example, hollow nanotubes of various transition metal oxides such as TiO2 are easily prepared and have open pores and reactive surfaces. We have found that TiO2 nanotubes are more reactive than the nanorods. There is also interest in modifying the nanotubes with nanoparticles including, C60, quantum dots, metals and metal complexes. For example, PbS quantum dots (PbS QDs) were prepared on the inside and outside surfaces of TiO2 nanotubes by using thiolactic acid as an organic linker.  The sizes of PbS QDs are controlled by employing a dip coating process to anchor PbS QDs onto TiO2 nanotubes.  The PbS QDs with diameters of 2-10 nm were obtained by adjusting the concentration of thiolactic acid.  TiO2 nanotubes with PbS QDs located only inside the nanotubes (TEM shown right) were prepared by first coating the tubes with the double-chain cationic surfactant DDAB. We have found that the he PbQD/TiO2 nanotubes exhibit improved photocatalysis.
 
 

 

 
 

 

 

 

   
     
     
 

 

 
     

 

 

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