R-3 Repository :: Browsing by Author "Wong, Michael S."

Browsing by Author "Wong, Michael S."

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A Simple and Rapid Method of Forming Double-Sided TiO2 Nanotube Arrays
(Wiley, 2022) Conrad, Christian L.; Elias, Welman C.; Garcia-Segura, Sergi; Reynolds, Michael A.; Wong, Michael S.; Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment
Highly ordered TiO2 nanostructures, known as nanotube arrays (NTAs), exhibit potential in various energy, chemical sensing, and biomedical applications. Owing to its simplicity and high degree of control, titanium anodization serves as the prevailing NTA synthesis method. However, the practicality of this approach is marred by sluggish and inconsistent growth rates, on the order of 10 nm min−1. Growth rates strongly depend on the electrolyte conductivity, yet most reports neglect to consider this property as a measured and controllable parameter. Here, we have systematically determined a broad set of conditions (at 60 V applied potential, elevated temperatures) that allow researchers to fabricate NTAs quickly and simply. By modulating conductivity through variation of bulk electrolyte temperature and the controlled addition of several hydroxy acid species, we achieve consistent accelerated growth up to 10 times faster than traditional methods. We find that regulating the solution conductivity within a desired region (e. g., ∼800–1000 μS cm−1) enabled the fabrication of double-sided NTA layers of around 10 μm and 90 μm NTA in 10 and 180 min, respectively.
Advanced Synthesis Techniques and Characterization of Functional Semiconductor Nanomaterials
(2014-01-15) Gullapalli, Sravani; Wong, Michael S.; Verduzco, Rafael; Ajayan, Pulickel M.
Semiconductor materials are used in several modern day applications ranging from photovoltaic devices to environmental remediation. The electronic, optical, catalytic and physical properties of semiconductor nanomaterials can be precisely tuned by altering their size, shape and composition. It is thus imperative to develop simplified cost-effective techniques to synthesize functional semiconductor nanomaterials with structural and morphological control. The overall goal of this thesis is to design new synthetic schemes for well-characterized semiconductor nanomaterials and subsequently demonstrate their potential in photovoltaic and photocatalytic applications. Shape control of semiconductor nanomaterials is crucial for photovoltaic applications. Longer armed cadmium selenide (CdSe) tetrapods have demonstrated enhanced performance in hybrid solar cells. Conventional long arm tetrapod syntheses necessitate multiple injections of flammable phosphorous based chemicals. A new non-phosphorous route to long CdSe tetrapods with arm lengths > 70 nm is demonstrated by manipulating the “greener” selenium precursor temperature in the presence of a quaternary ammonium salt as the shape directing agent. Another interesting shape is the hollow morphology that provides the advantage of higher surface-to-volume ratio. However this shape for CdSe is much less investigated in photovoltaic applications. A novel molten-droplet synthesis strategy is developed to synthesize quantum confined CdSe HNPs based on the slow heating of a low melting point cadmium salt, elemental Se, alkylammonium surfactant in octadecene solvent with no external ligand. This generic technique is shown to be applicable for a variety of metal chalcogenide compositions. Further, photovoltaic device characterization of HNPs in a hybrid solar cell indicate that HNPs have improved electron transport characteristics compared to standard CdSe quantum dots. Hybrid photovoltaic device fabrication is based on low cost colloidal solution-based techniques. A new insight to understanding nanoparticle solvent interactions is provided using coarse-grained computational models and experimental characterization of oleate-capped NPs in various solvents. Solvent polarity was shown to strongly affect NP hydrodynamic diameter, colloidal stability and aggregation behavior. Photocatalytic removal of organic contaminants using semiconductor nanomaterials provides a low-cost, environmentally clean alternative for the utilization of renewable energy sources. Most photocatalytic environmental remediation techniques are oxidative and result in either partial or complete mineralization of the contaminant. A less explored reductive photocatalytic approach to organohalide removal has been demonstrated without necessitating an external co-feed of hydrogen (H2). Hydrodechlorination (HDC) of trichloroethene (TCE) as the test reaction. Bifunctional palladium-based titanium dioxide (TiO2) reduction catalysts were synthesized for the photocatalytic TCE HDC reaction with simultaneous in-situ H2 generation by photocatalytic water splitting. Extension of this reductive photocatalytic approach to other groundwater contaminants could simplify future remediation efforts.
An Investigation of Carbon-based Nanomaterials for Efficient Energy Production and Delivery
(2016-01-28) Gangoli, Varun Shenoy; Wong, Michael S.; Pasquali, Matteo; Barron, Andrew R; Adams, Wade; Hauge, Robert H
Carbon-based nanomaterials have been demonstrated to have different potential applications in the energy industry. However, there are challenges in the realization of these applications. Chirality of single wall carbon nanotubes (SWCNTs) defines their electronic properties, and obtaining an ensemble of SWCNTs of the same chirality has been a problem studied for over two decades with no clear solution yet. Other carbon-based nanomaterials, such as carbon black aggregates, are hydrophobic in nature and potential applications in the oil and gas industry require their dispersal in an aqueous solvent. Another application in the oil and gas industry is enhanced oil recovery (EOR), and here there is a need for an inexpensive, stable, and efficient surfactant compared to currently used industrial solutions. The challenge of producing SWCNTs of the same chirality is studied using two approaches- separation after synthesis of SWCNTs of mixed chiralities, and chemical control over chirality of as-synthesized SWCNTs. Agarose gel-based affinity chromatography was used as a means towards highly semiconductor- enriched SWCNTs using a family of nonionic surfactants. UV-vis-NIR spectroscopy, Raman spectroscopy and photoluminescence spectroscopy was used to quantify the separation efficiency of the metal- and semiconductor-enriched SWCNTs. This process is an improvement over other chromatography-based techniques at the time in that the nonionic surfactants used are less expensive, enable a higher purity of semiconductor SWCNTs (>95%) and decompose fully by simply heating in air thus leaving behind pristine SWCNTs. The second approach was based on using catalyst dopants to preferentially synthesize SWCNTs of a particular chirality at the expense of SWCNTs of other chiralities. Heterogeneous catalysis combined with the screw dislocation theory of SWCNT growth provided the background for this work, and both selenium and phosphorus were identified as chemical dopants for iron catalysts. Both selenium and phosphorus were demonstrated to have a direct effect on the average number density and length of SWCNTs, and selenium also was shown to have a direct control over the growth rate of SWCNTs. This, combined with some preliminary spectroscopy results, suggest chiral control over the carbon nanotubes. Collaborative work on phase transfer of hydrophobic carbon-based nanomaterials into aqueous solvents for applications including saturated oil residual (SOR) detection and quantification in underground reservoirs helped recognize the potential of hydrophobically modified polymers as surfactants for EOR. Polystyrene sulfonate was chosen as the polymer of study owing to ease of availability, low cost of the precursor material and aromatic sulfonates already being studied for EOR. Controlled desulfonation of PSS was achieved by rapid heating of an aqueous solution of PSS in a microwave reactor under acidic conditions, with the reactant temperature and pH having a strong effect on the degree of desulfonation of the product ranging from 4.9% (as-obtained PSS) to 40%. Dynamic light scattering of the desulfonated PSS (termed PDS) in brine showed good stability of the polymer aggregates at temperatures as high as 150 ºC, and tensiometry with aromatic oils such as toluene and aliphatic oils such as Isopar L showed good surface activity with interfacial tension going as low as 10-2 mN/m. Breakthrough experiments with sand packed columns at the lab scale, and core flooding at an independent facility confirmed good propagation of PDS through materials such as Berea sandstone, with minimal plugging and adsorption losses.
Anhydrosugar synthesis
(2019-07-30) Chen, Li; Wong, Michael S.; Zhang, Zongchao; Rice University; United States Patent and Trademark Office
Improved methods of making anhydrosugars by pyrolysis of a substrate sugar to remove at least one water molecule thereby producing a desired anhydrosugar and side products, the improvement being either 1) protecting one hydroxyl group of the substrate sugar before pyrolysis; or (2) pretreating the substrate sugar with a metal salt and optional acid before pyrolysis, wherein lower amounts of said side products are produced by said improved method.
Anode battery materials and methods of making the same
(2016-05-17) Biswal, Sibani Lisa; Thakur, Madhuri; Wong, Michael S.; Sinsabaugh, Steven L.; Isaacson, Mark; Rice University; United States Patent and Trademark Office
In some embodiments, the present invention provides novel methods of preparing porous silicon films and particles for lithium ion batteries. In some embodiments, such methods generally include: (1) etching a silicon material by exposure of the silicon material to a constant current density in a solution to produce a porous silicon film over a substrate; and (2) separating the porous silicon film from the substrate by gradually increasing the electric current density in sequential increments. In some embodiments, the methods of the present invention may also include a step of associating the porous silicon film with a binding material. In some embodiments, the methods of the present invention may also include a step of splitting the porous silicon film to form porous silicon particles. Additional embodiments of the present invention pertain to anode materials derived from the porous silicon films and porous silicon particles.
Assembling nanoparticle-assembled capsules on a planar substrate
(2006) Yoo, Regina Mi-Kyung; Wong, Michael S.; Laibinis, Paul E.
In this thesis, a technique to assemble nanoparticle-assembled capsules (NACs) onto a planar substrate has been demonstrated. NACs are micron-sized spherical capsules produced from organic and inorganic materials. New sensing materials of various length scales are needed for numerous sensing needs of today, and NACs on substrate are interesting to investigate. Several parameters were optimized to achieve maximum surface density of capsules (0.09 NACs/mum 2). NACs are best adsorbed onto negatively charged surface. The NaCl concentration in the poly(diallyldimethylammonium chloride) and poly(styrene sulfonic acid) solutions used to coat the glass coverslip substrate should be at least 1 M. In addition, a minimum of one bilayer of PDADMAC/PSS is required. The deposition time required to achieve the maximum density was at least 10 minutes. Finally, aging NACs before assembling them on the substrate decreased NAC coverage on the surface which could be attributed to change in NACs' surface charge.
Bilayer Approaches for Nanoparticle Phase Transfer
(2012) Kini, Gautam Chandrakanth; Wong, Michael S.
Nanoparticles (NPs) are often synthesized in organic solvents due to advantages of superior size and shape control obtainable in a non-polar environment. However, many applications featuring NPs require them to be in aqueous media. To transfer NPs from oil to water, surfactants with amphiphilic (hydrophobic and hydrophilic) groups have been widely used. A popular phase-transfer approach involves formation of oil-in-water emulsions upon which the oil storing the NPs is boiled off. In the process, surfactants form bilayers with hydrophobic groups on the NPs rendering them water-dispersible. This transfer route however is limited in that NPs aggregate to form clusters which results in poor colloidal stability and for the specific case of quantum dots (QDs), adversely impacts optical properties. It has ever since remained a challenge to devise approaches that transfer NPs from oil to water as single particles without compromising NP stability and properties. We have discovered that by simple addition of salt to water during the step of emulsion formation, NP transfer efficiency can be greatly enhanced in "salty-micelles" of surfactants. The strength of this approach lies in its simplicity and generic nature in that the transfer scheme is valid for different NP, surfactant and salt types. Using a model system with cadmium selenide (CdSe) QDs as NPs, Aerosol-OT (AOT) as the surfactant and NaCl as the salt in water, we found >90% of CdSe QDs transferred in salty-micelles of AOT which was significantly higher than the 45-55% QDs that transferred in deionized-water (DI-water) micelles of AOT. In the salty-micelle environment, QDs were found to exist predominantly as single NPs with narrow size distribution, as established by light scattering, analytical ultracentrifugation and electron microscopy. The effects of salt were in lowering aqueous solubility of AOT through "salting-out" action and in screening repulsions between like-charged head groups of AOT molecules. Electrophoresis, thermogravimetric analysis and photoluminescence measurements using a solvatochromic dye established higher surfactant coverage with greater lateral compaction for QDs in salty-micelles over the DI-micelle counterpart. Single NP characteristics along with a hydrophobic environment in laterally compact salty-micelles resulted in better retention of optical properties of QDs. Observations of a secondary effect by salt in inducing spontaneous emulsification of a hydrocarbon (octane)/AOT/brine system were systematically investigated by tracking time-variant octane droplet size and charge. Salinity levels that determine the spontaneous curvature and phase behavior of AOT were seen to influence the initial nucleation of octane droplets and their subsequent growth. The smallest octane drops (sub 50 nm) were nucleated at the optimum cross-over salinity and emergence of the liquid crystalline phase of AOT resulted in slowest growth rates. These factors contributed towards higher transfer efficiency of NPs in salty-micelles. Two applications from formulating aqueous NP suspensions by the new phase-transfer approach are described. In the first, QD and carbon-dot (C-Dot) "nanoreporters" were formulated for oil-field reservoir characterization using Neodol 91-7 (nonionic) and Avanel S150 CGN (hybrid nonionic and anionic) as surfactants. These NPs were stable to aggregation under reservoir-representative conditions (salinities: 1M NaCl, 1M KCl and 0.55M synthetic seawater; temperatures: 70-100 °C) and demonstrated flow and transport through crushed-calcite and quartz-sand columns with high breakthrough and recovery (> 90%). In the second application, tandem assembly of a cationic polymer, multivalent salt, and NPs was investigated in a microfluidic channel where charge ratio of the polymer/salt and shear from flow and device geometry determined their assembly into higher ordered structures such as gels and capsules.
Biomedical Nanocrystal Agents: Design, Synthesis, and Applications
(2013-09-16) Cho, Minjung; Colvin, Vicki L.; McDevitt, John T.; Wong, Michael S.
In these days, nanomaterials are applied in a variety of biomedical applications including magnetic resonance imaging (MRI), cell imaging, drug delivery, and cell separation. Most MRI contrast agents affect the longitudinal relaxation time (T1) and transverse relaxation time (T2) of water protons in the tissue and result in increased positive or negative contrast. Here, we report the optimization of r1 (1/T1) or r2 (1/T2) relaxivity dynamics with diameter controlled gadolinium oxide nanocrystals (2~22 nm) and iron based magnetic nanocrystals (4 ~33 nm). The r1 and r2 MR relaxivity values of hydrated nanocrystals were optimized and examined depending on their core diameter, surface coating, and compositions; the high r1 value of gadolinium oxide was 40-60 S-1mM-1, which is 10-15 fold higher than that of commercial Gd (III) chelates (4.3~4.6 S-1mM-1). Moreover, in vitro toxicological studies revealed that polymer coated nanocrystals suspensions had no significant effect on human dermal fibroblast (HDF) cells even at high concentration. Towards multimodal imaging or multifunctional ability, we developed the iron oxide/QDs complexes, which consist of cores of iron oxide that act as nucleation sites for fluorescent QDs. The choice of variable QDs helped to visualize and remove large iron oxide materials in a magnetic separation. Additionally, diluted materials concentrated on the magnet could be fluorescently detected even at very low concentration. The designed MRI or multifunctional nanomaterials will give great and powerful uses in biomedical applications.
Catalytic Converters for Water Treatment
(American Chemical Society, 2019) Heck, Kimberly N.; Garcia-Segura, Sergi; Westerhoff, Paul; Wong, Michael S.; Nanotechnology Enabled Water Treatment (NEWT) Center
Fresh water demand is driven by human consumption, agricultural irrigation, and industrial usage and continues to increase along with the global population. Improved methods to inexpensively and sustainably clean water unfit for human consumption are desired, particularly at remote or rural locations. Heterogeneous catalysts offer the opportunity to directly convert toxic molecules in water to nontoxic products. Heterogeneous catalytic reaction processes may bring to mind large-scale industrial production of chemicals, but they can also be used at the small scale, like catalytic converters used in cars to break down gaseous pollutants from fuel combustion. Catalytic processes may be a competitive alternative to conventional water treatment technologies. They have much faster kinetics and are less operationally sensitive than current bioremediation-based methods. Unlike other conventional water treatment technologies (i.e., ion exchange, reverse osmosis, activated carbon filtration), they do not transfer contaminants into separate, more concentrated waste streams. In this Account, we review our efforts on the development of heterogeneous catalysts as advanced reduction technologies to treat toxic water contaminants such as chlorinated organics and nitrates. Fundamental understanding of the underlying chemistry of catalytic materials can inform the design of superior catalytic materials. We discuss the impact of the catalytic structure (i.e., the arrangement of metal atoms on the catalyst surface) on the catalyst activity and selectivity for these aqueous reactions. To explore these aspects, we used model metal-on-metal nanoparticle catalysts along with state-of-the-art in situ spectroscopic techniques and density functional theory calculations to deduce the catalyst surface structure and how it affects the reaction pathways and hence the activity and selectivity. We also discuss recent developments in photocatalysis and electrocatalysis for the treatment of nitrates, touching on fundamentals and surface reaction mechanisms. Finally, we note that despite over 20 years of growing research into heterogeneous catalytic systems for water contaminants, only a few pilot-scale studies have been conducted, with no large-scale implementation to date. We conceive of modular, on- or off-grid catalytic units that treat drinking water at the household tap, at a community well, or for larger-scale reuse of agricultural runoff. We discuss how these may be enhanced by combination with photocatalytic or electrocatalytic processes and how these reductive catalytic modules (catalytic converters for water) can be coupled with other modules for the removal of potential water contaminants.
Catalytic Oxidation Properties of Palladium-decorated Gold Nanoparticles
(2014-10-06) Zhao, Zhun; Wong, Michael S.; Gonzalez, Ramon; Zheng, Junrong
Bimetallic palladium gold (PdAu) catalysts have been shown to be superior to monometallic ones in many reactions, but the reasons for the enhancement are not thoroughly understood. In this work, palladium decorated gold nanoparticles (Pd-on-Au NPs) are used as structured model catalysts, allowing for the precise control of both size and metal distribution with Pd surface coverage (sc%). By testing reactions on a range of these catalysts, we hope to gain insight into the active site for a given reaction. In hydrodechlorination of perchloroethene (PCE), Pd surface coverage was found to be the key factor in catalyst activity, with the optimum at 80 sc%. A complete mechanistic model that coupled mass transfer processes with the surface reactions was further developed, consistent with the observed product profiles. Carbon supported Pd-on-Au NPs were tested for liquid phase glycerol oxidation for the first time. The best catalyst (80 sc%) had an initial TOF of ~6000 h-1, >10 times more active than Au/C and Pd/C. Catalytic activity, selectivity, activation energy and deactivation rate constant exhibited strong volcano-shaped dependences upon Pd sc%. Ex situ XANES results showed no to little change in surface Pd-O% for Au based catalysts, suggesting the possibility of Au suppressing Pd oxidation during reaction. Ex situ EXAFS results further confirmed the core-shell structures of 60 and 150 sc% Pd-on-Au/C catalysts via Punnett square analysis, and also ascertained no to little change in their oxidation states and coordination numbers post glycerol oxidation. EXAFS observations correlate with kinetics results, and lead to the conclusion that catalysts with a larger amount of 3-D Pd ensembles are more prone to oxidize during glycerol oxidation, making them less resistant to deactivation. Finally, Pd-on-Au/C catalysts were tested for room temperature formic acid decomposition. In situ XAS revealed that core-shell structures of 60, 150 and 300 sc% Pd-on-Au NPs maintained while oxidized Pd species was partially reduced during reaction. Catalyst with higher fraction of 3-D Pd ensembles showed much higher dehydrogenation activity than those with mostly 1-D or 2-D, correlating to the proposed mechanism that the dehydrogenation pathway is favored over metal terrace sites.
Characterization & Application of Immobilized Biomacromolecules using Microcantilever and QCM Sensors
(2014-04-15) Wang, Jinghui; Biswal, Sibani Lisa; Segatori, Laura; Wong, Michael S.; McDevitt, John T.; Suh, Junghae
The structure and function of immobilized biomacromolecules are likely to be altered because of the solid surface. The long-term objective of this thesis is to develop surface-based biosensors for the characterization and application of biomacromolecules at the liquid-solid interface. In this study, two analytical surface-sensitive sensors are utilized: microcantilevers and quartz crystal microbalance with dissipation (QCM-D). Each offers unique information regarding the molecules of interest. In particular, the systems that are covered in this thesis include the detection of target analytes using specific recognition elements and the characterization of supported lipid membranes. This research has led to a better understanding of the effect of solid surfaces on protein structure and function, as well as the ability to engineer biomolecular surfaces with great control. There are two detection systems that were studied: a phage-derived peptide system for the detection of pathogenic bacteria Salmonella and an antibody displacement assay for the detection of an explosive, 2,4,6-trinitrotoluene (TNT). The microcantilever responds to changes in the surface free energy on the sensor surface by monitoring changes in its deflection. The physisorption or chemisorption of molecules to the cantilever surface induces a mismatch in the surface stress, causing the cantilever to bend. The multiplexed measurement is able to quickly determine the binding affinities of various phage-derived peptides, improving the screening efficiency of the peptides derived from phage display libraries for Salmonella detection. The microcantilever-based technique provides a novel biosensor to rapidly and accurately detect pathogens and holds potential to be further developed as a screening method to identify pathogen-specific recognition elements. QCM measures mass changes on the sensor surface by monitoring the frequency change of the crystal. The combination of a competition assay with QCM using an anti-TNT antibody is able to distinguish a TNT molecule among molecules of similar structure at low concentrations, leading a sensitive and selective assay. The reliability of this method was further investigated in more real environments simulated by fertilizer solution and seawater. Furthermore, this method could be also applied in gas phase detection of TNT, as well as the detection of other chemicals, such as environmental pollutants and illegal drugs. In both of these detection assays, a mathematical model was developed to quantify the binding of the target molecules with the molecules of interest. In the second half of the thesis, the microcantilever sensor is applied to characterize supported lipid bilayers (SLBs), an interesting biomacromolecular assembly that holds great importance as a model system for membranes. Through monitoring the cantilever deflection, the formation of the SLB, its temperature induced phase transitions, and its interactions with membrane-active molecules are investigated. With increasing temperature, the lipid acyl chains transition from an ordered state to a disordered state, accompanied by a changes in the surface stress that can be readily detected using microcantilever. The phase transition temperature of SLBs is different from that of a lipid monolayer, indicating that the existence of the solid support aﬀects the monolayer structure. Two amphipathic membrane-active molecules, peptide (PEP1) and a triblock copolymer (Pluoronic), are studied for their associations with SLBs. PEP1’s association with SLBs highly depends on the ratio of peptide over lipid, while the Pluoronic interacts with SLBs as a function of temperature and the length of lipophilic block in the copolymer. Therefore, the microcantilever sensor is capable of measuring the conformational change of surface-bound molecules, as well as characterizing the kinetics of membrane-peptide interactions with great sensitivity.
Characterizing Engineered Nanomaterials: From Environmental, Health and Safety Research to the Development of Shaped Nanosphere Lithography for Metamaterials
(2012-09-05) Lewicka, Zuzanna; Colvin, Vicki L.; Tittel, Frank K.; Wong, Michael S.
In this thesis two issues in nanotechnology have been addressed. The first is the comprehensive characterization of engineered nanomaterials prior to their examination in toxicology and environmental studies. The second is the development of a method to produce nanostructure arrays over large areas and for low cost. A major challenge when assessing nanomaterial’s risks is the robust characterization of their physicochemical properties, particularly in commercial products. Such data allows the critical features for biological outcomes to be determined. This work focused on the inorganic oxides that were studied in powdered and dispersed forms as well as directly in consumer sunscreen products. The most important finding was that the commercial sunscreens that listed titania or zinc oxide as ingredients contained nanoscale materials. Cell free photochemical tests revealed that ZnO particles without any surface coating were more active at generating ROS than surface coated TiO2 nanoparticles. These studies make clear the importance of exposure studies that examine the native form of nanomaterials directly in commercial products. The second part of this thesis presents the development of a new method to fabricate gold nanoring and nanocrescent arrays over large areas; such materials have unique optical properties consonant with those described as metamaterials. A new shaped nanosphere lithography approach was used to manipulate the form of silica nanospheres packed onto a surface; the resulting array of mushroom structures provided a mask that after gold evaporation and etching left either golden rings or crescents over the surface. The structures had tunable features, with outer diameters ranging from 200 to 350 nm for rings and crescent gap angles of ten to more than a hundred degrees. The use of a double mask method ensured the uniform coverage of these structured over 1 cm2 areas. Experimental and theoretical investigations of the optical properties of the arrays revealed the optical resonances in the infrared region. Finally, in the course of developing the nanorings, etch conditions were developed to deposit large area arrays of polystyrene nanodoughnuts with diameters from 128 to 242 nm. These non-conductive structures provide an ideal template for further attachment of magnetic of optically emissive nanoparticles.
Charge-assembled capsules for phototherapy
(2012-12-25) Yu, Jie; Wong, Michael S.; Anvari, Bahman; Yaseen, Mohammad Abbas; Rice University; United States Patent and Trademark Office
Novel phototherapeutic methods and compositions are described herein. Nanoparticle-assembled microcapsules as a new type of delivery vehicle for photosensitive compounds may be synthesized through a two-step assembly process. Charged polymer chains and counterions may be combined with a photosensitive compound to form photosensitive aggregates, and then nanoparticles may be combined with the aggregates to form the microcapsules. The shell may be composed of nanoparticles and/or polymer, and the core interior may contain the photosensitive compound. Formation occurs rapidly (on the order of seconds) and the conditions are very mild (at room temperature, in aqueous solution, and at neutral pH). The microcapsule synthesis is highly suitable as an encapsulation method, particularly for a charged photosensitive molecule like ICG.
Chemical processing of colloidal cadmium selenide nanoparticles: New approaches to dimensional and morphological control
(2008) Asokan, Subashini; Wong, Michael S.
Cadmium selenide (CdSe) quantum dots (QDs) are colloidal semiconductor nanoparticles (NPs) that are nanometer sized fragments of the corresponding bulk crystals. They are being probed as a very interesting system for their applications in LEDs, solar cells and biomedical labeling because of their rich photophysics arising from their size dependent optical and electronic properties and flexible processing chemistry. 1-Octadecene was the only non-coordinating solvent used for the synthesis of CdSe NPs. It was imperative to understand the chemistry of synthesis of CdSe NPs using other non-coordinating solvents. Also, there is a constant search for the greener, cheaper, reproducible and scalable methods for the synthesis of CdSe NPs while not compromising on their quality. Towards the above mentioned goals, the use of heat transfer fluids was successfully demonstrated as cost-effective alternative solvents for quantum dot synthesis. Heat transfer fluids (HTF) are a class of organic liquids commonly used in chemical process industries to transport heat between unit operations. The solvents were found suitable for the hot injection synthesis of QDs while reducing the cost of the raw materials. These solvents were found to slow the growth kinetics of the CdSe NPs, leading to greater control over QD diameter. Although the chemistry of synthesis of CdSe spherical and rod shaped particles were well understood, the synthesis of tetrapod shaped NPs with uniform size and shape is especially difficult to carry out at a large scale. Post-synthesis separation can be applied though this leads to additional processing steps and reduced particle yields. Cationic surfactant ligands were discovered to lead to the successful formation of tetrapod shaped CdSe NPs with highly uniform arm lengths, arm widths and shape. Typical selectivity values for the tetrapod morphology exceeded 90%, much higher than the previously reported values of 40%. The cationic surfactant ligands were found to induce anisotropy during the growth of the CdSe NPs, with cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) leading to the specific tetrapod shape. Optimization of the synthesis procedure led to the control over the range of dimensions of the tetrapod shaped particles. The uniformity of size and shape of the CdSe tetrapodal NPs were found to be very sensitive to the ratio of the surfactants and the precursors. It was also observed that the presence of the cationic surfactants led to the exclusive formation of zinc-blende nuclei which is indispensable for the growth of nuclei into tetrapodal NPs. The growth of CdSe tetrapodal NPs from the preformed zinc-blende CdSe NPs, just by the addition of CTAB, demonstrated the crucial role of the cationic surfactants in inducing anisotropy. These findings were helpful in gaining insight in to the nucleation and growth of the anisotropic nanoparticles. These new methods should lead to improvements in current synthesis methods of tetrapodal shaped CdSe NPs, enabling the faster development of polymer/tetrapod photovoltaic devices. These results may be applicable to other compositions, leading to opportunities for large-scale application of shaped NPs.
Complete defluorination of per- and polyfluoroalkyl substances — dream or reality?
(Elsevier, 2023) Arana Juve, Jan-Max; Wang, Bo; Wong, Michael S.; Ateia, Mohammed; Wei, Zongsu; The Catalysis and Nanomaterials Laboratory
The consensus of removing per- and polyfluoroalkyl substances (PFAS) from the environment is widely recognized and enlightened by the near-zero standards released from the U.S. Environmental Protection Agency in 2023. The only way to achieve the goal of zero fluoro-pollution is to fully defluorinate or mineralize PFAS, but current technologies only partially defluorinate a limited number of PFAS, which can lead to the creation of potentially more toxic short-chain intermediates. Therefore, we discuss herein the need to broaden the scope of tested PFAS, summarize the state-of-the-art degradation technologies, and provide perspectives to achieve complete defluorination. Besides fundamental knowledge gaps in defluorination reactions, technological gaps in the aspects of water matrix effects, pilot tests, and cost analysis also limit the application and comparison of different treatment technologies. This work would shed light on further research to find solutions in the complete defluorination of PFAS.
Computational Simulation of Secondary Organic Aerosol (SOA) Formation from Toluene Oxidation
(2012-09-05) Liu, Ying; Griffin, Robert J.; Cohan, Daniel S.; Wong, Michael S.
Toluene is one of the most prevalent aromatic volatile organic compounds (VOCs) in the atmosphere and has large secondary organic aerosol (SOA) yields compared to many other aromatic VOCs. Recent photo-oxidation studies highlight that toluene oxidation produces more SOA than observed previously, particularly at low levels of nitrogen oxides (NOx). This study focuses on: 1.) the development of a gas-phase chemical mechanism describing toluene oxidation by hydroxyl radicals (OH); 2.) the prediction of SOA formation from toluene oxidation products; and 3.) the impact of NOx level on SOA formation. The oxidation mechanism, which includes multiple pathways after the initial OH attack, has been incorporated into the Caltech Atmospheric Chemistry Mechanism (CACM). Toluene concentrations simulated in chamber experiments by the updated CACM as a function of time are typically within 5% of observed values for most experiments. Predicted ozone and NO2 concentrations are typically within 15% of the experimental values. The gas-phase mechanism indicates the importance of bicyclic peroxy radical reactions in determining the product distribution and thus the likelihood of SOA formation. A gas-aerosol partitioning model is used in conjunction with the gas-phase mechanism to simulate SOA formation. Predicted SOA concentrations are typically within 15% of the experimental values. Under low NOx conditions, simulation shows that more than 98% of SOA mass is contributed by bicyclic products from reactions between bicyclic peroxy radicals and other peroxy radicals. Increasing NOx levels cause bicyclic peroxy radicals to react with NO or nitrate radical, leading to fragmentation products that are less likely to form SOA. SOA yield dropped from 19.26% with zero initial NOx to 13.27% with 100 ppb initial NO because of the change in the amount of toluene consumed. Composition of NOx also has an impact on SOA yield and formation, showing that NO has a greater impact on SOA yield and formation than NO2.
Contaminant Removal from Liquid Fuels and Oil-Contaminated Soils: Elucidating the Fundamental Mechanisms of Adsorptive Desulfurization and Pyrolytic Remediation
(2021-12-03) Dias da Silva, Priscilla da; Zygourakis, Kyriacos; Wong, Michael S.
Adsorptive desulfurization of liquid fuels is an emerging technology that may allow for the portable generation of electricity using jet fuel-powered fuel cell systems. Pyrolytic remediation of oil-contaminated soils is a thermal treatment that may sustainably detoxify oil-contaminated soils using less energy and preserving soil integrity. This work investigates the underlying mechanisms of these processes through advanced thermo-analytical techniques and mathematical analysis. First, adsorptive desulfurization with CuNa-Y zeolite was studied at temperatures between 30 °C and 180 °C as an approach to remotely remove sulfur present in liquid fuels for fuel cell applications. The amount of sulfur selectively removed from jet fuel and adsorbed onto the Cu sites of the zeolite increased by almost 14x as the temperature was raised from 30 °C or 80 °C to 180 °C. Elevated temperatures promoted the formation of covalent sulfur-metal bonds that displaced other aromatics that do not contain sulfur, which are present at much higher concentrations and compete for adsorption. Operating a flow-through adsorber at 180 °C could effectively reduce the sulfur content in jet fuel to ultra-low levels (1-10 ppmw) over a broad range of liquid hourly space velocities (0.13 to 3.24 h^(-1)). Detailed characterization revealed that desulfurization occurs in two stages. Sulfur is initially removed via adsorption (chemisorption) on the CuNa-Y zeolite, an assertion supported by simulations with a transient heterogeneous model. As the adsorbent becomes saturated, however, surface chemical reactions start taking place leading to the decomposition of benzothiophenes. As a result, the process continues to remove sulfur from the jet fuel feed even after it has exceeded its theoretical adsorption limit. Second, pyrolytic remediation of soils contaminated with heavy hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), was studied at temperatures between 300 °C and 420 °C. Our team developed a novel methodology that combines thermo-analytical measurements and mathematical methods to inform the reliable pyrolytic temperatures for specific soil/contaminant systems. To achieve that, we characterized the complex network of soil and contaminant transformations using thermogravimetry coupled with evolved gas analysis. Additionally, we investigated the contribution of clays during pyrolytic remediation of soils. Clays were found to be the primary component in soil that retains PAHs such as pyrene and therefore sets the remediation intensity requirements. Bentonite modified with Fe(III), Fe-bentonite, performed as a catalyst under pyrolytic conditions, decreasing the temperature at which hydrocarbon decomposition reactions were triggered. The addition of 10%wt Fe-bentonite decreased the residual total petroleum hydrocarbon (TPH) content by 65.9% for treatments at 370 °C and by 79.3% for treatments at 300 °C. Moreover, treatment at 300 °C with the addition of Fe-bentonite resulted in a similar residual TPH when compared to the treatment at 370 °C with no additives. Using such earth-abundant amendments during pyrolytic remediation of oil-contaminated soils could improve energy usage, reduce associated carbon dioxide emissions and lessen unwanted soil transformations. Overall, this work elucidates the extent and the mechanism of separation and transformation of contaminants like benzothiophenes, heavy hydrocarbons and PAHs at selected temperatures. The findings reported here contribute to the development of efficacious approaches to remove such contaminants from sensitive environments.
Converting nanoparticles in oil to aqueous suspensions
(2016-04-26) Wong, Michael S.; Bagaria, Hitesh Ghanshyam; Kini, Gautam Chandrakanth; Ko, Wen Yin Lynn; Rice University; United States Patent and Trademark Office
An improved process for converting an oil suspension of nanoparticles (NPs) into a water suspension of NPs, wherein water and surfactant and a non-surfactant salt is used instead of merely water and surfactant, leading to greatly improved NP aqueous suspensions.
Design of a Pd–Au Nitrite Reduction Catalyst by Identifying and Optimizing Active Ensembles
(American Chemical Society, 2019) Li, Hao; Guo, Sujin; Shin, Kihyun; Wong, Michael S.; Henkelman, Graeme
Nitrate (NO3–) is a ubiquitous contaminant in groundwater that causes serious public health issues around the world. Though various strategies are able to reduce NO3– to nitrite (NO2–), a rational catalyst design strategy for NO2– removal has not been found, in part because of the complicated reaction network of nitrate chemistry. In this study, we show, through catalytic modeling with density functional theory (DFT) calculations, that the performance of mono- and bimetallic surfaces for nitrite reduction can be rapidly screened using N, N2, and NH3 binding energies as reactivity descriptors. With a number of active surface atomic ensembles identified for nitrite reduction, we have designed a series of “metal-on-metal” bimetallics with optimized surface reactivity and a maximum number of active sites. Choosing Pd-on-Au nanoparticles (NPs) as candidate catalysts, both theory and experiment find that a thin monolayer of Pd-on-Au NPs (size: ∼4 nm) leads to high nitrite reduction performance, outperforming pure Pd NPs and the other Pd surface compositions considered. Experiments show that this thin layer of Pd-on-Au has a relatively high selectivity for N2 formation, compared to pure Pd NPs. More importantly, our study shows that a simple model, based upon DFT-calculated thermodynamic energies, can facilitate catalysts design relevant to environmental issues.
Developing gold-based nanostructures to study catalytic reactions in water
(2009) Heck, Kimberly N.; Wong, Michael S.
Gold-based catalysts are effective for reactions that occur in water. They are not well understood though, with regard to the nature of active sites and surface reaction mechanisms. Water presents interesting challenges in performing catalytic studies, as it interferes with infrared spectroscopy commonly used to detect surface intermediates under reaction conditions. Insights leading to improved catalysts can be gained if gold-based nanostructures could be designed, engineered, and tested for a given chemical reaction. Two gold-based water-phase catalytic reactions were considered for this thesis: glycerol oxidation and hydrodechlorination of chlorinated ethenes. Glycerol is a by-product of biofuel production, and is considered a possible non-petroleum feedstock for chemicals if efficient conversion processes exist. It can be oxidized using Au catalysts in alkaline solution, but the surface reaction mechanism is not known and the role of basic pH is not fully understood. Gold nanoshells (Au NSs) were used for the first time to study glycerol oxidation through surface-enhanced Raman spectroscopy (SERS). Raman bands for surface-adsorbed glyceric acid, the major reaction product, were detected. High oxygen content and high pH values led to carbon monoxide surface species, indicating carbon-carbon cleavage. When the glycerolate:O2 ratio was constant, higher pH led to advance decomposition, due to the activation of O2 by adsorbed hydroxide ions. The catalytic HDC of chlorinated ethenes can potentially remove these contaminants from groundwater. This reaction occurs at room temperature and uses palladium-coated gold nanoparticles (Pd/Au NPs), which are more efficient then pure Pd. The Au NSs were coated with Pd and used to study the surface intermediates of 1,1-dichloroethene HDC through SERS. Pathways were ascertained through careful Raman band assignments to probable chemical species and analysis of bulk reaction products, leading to the formulation of a reaction mechanism. Gold enhances Pd catalysis for HDC, presumably through the generation of palladium-based active sites, though the true active site is unknown. The effects of chloride and sulfide on the activity of Pd/Au NPs for trichloroethene HDC were studied to provide information about active sites and deactivation properties. The activity of Pd/Au NPs was unaffected by NaCl, while that of the pure Pd catalysts decreased. Pd/Au NPs were resistant to sulfide poisoning compared to pure Pd catalysts. This increased resistance is attributed to the formation of small Pd islands on the Au NPs.

Browsing by Author "Wong, Michael S."

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