Christopher Matranga

Experimental and Computational Investigation of Au25 Clusters and CO2: A Unique Interaction and Enhanced Electrocatalytic Activity

Atomically precise, inherently charged Au(25) clusters are an exciting prospect for promoting catalytically challenging reactions, and we have studied the interaction between CO(2) and Au(25). Experimental results indicate a reversible Au(25)-CO(2) interaction that produced spectroscopic and electrochemical changes similar to those seen with cluster oxidation. Density functional theory (DFT) modeling indicates these changes stem from a CO(2)-induced redistribution of charge within the cluster. Identification of this spontaneous coupling led to the application of Au(25) as a catalyst for the electrochemical reduction of CO(2) in aqueous media. Au(25) promoted the CO(2) → CO reaction within 90 mV of the formal potential (thermodynamic limit), representing an approximate 200-300 mV improvement over larger Au nanoparticles and bulk Au. Peak CO(2) conversion occurred at -1 V (vs RHE) with approximately 100% efficiency and a rate 7-700 times higher than that for larger Au catalysts and 10-100 times higher than those for current state-of-the-art processes.

Carbon nanotube nanoreservior for controlled release of anti-inflammatory dexamethasone

On demand release of anti-inflammatory drug or neurotropic factors have great promise for maintaining a stable chronic neural interface. Here we report the development of an electrically controlled drug release system based on conducting polymer and carbon nanotubes. Drug delivery research using carbon nanotubes (CNTs) has taken advantage of the ability of CNTs to load large amounts of drug molecules on their outer surface. However, the utility of the inner cavity of CNTs, which can increase the drug loading capacity, has not yet been explored. In this paper, the use of multi-wall CNTs as nanoreserviors for drug loading and controlled release is demonstrated. The CNTs are pretreated with acid sonication to open their ends and make their outer and inner surfaces more hydrophilic. When dispersed and sonicated in a solution containing the anti-inflammatory drug dexamethasone, experiments show that the pretreated CNTs are filled with the drug solution. To prevent the unwanted release of the drug, the open ends of the drug-filled CNTs are then sealed with polypyrrole (PPy) films formed through electropolymerization. The prepared electrode coating significantly reduced the electrode impedance, which is desired for neural recording and stimulation. More importantly, the coating can effectively store drug molecules and release the bioactive drug in a controlled manner using electrical stimulation. The dexamethasone released from the PPy/CNT film was able to reduce lipopolysaccharide induced microglia activation to the same degree as the added dexamethasone.

Size-dependent photocatalytic reduction of CO2 with PbS quantum dot sensitized TiO2 heterostructured photocatalysts

The photocatalytic reduction of CO2 to value-added chemicals, such as CH4, is a promising carbon management approach which can generate revenue from chemical sales to offset the cost of implementing CO2 capture technologies. To make photocatalytic conversion approaches efficient, economically practical, and industrially scalable, catalysts capable of utilizing visible and near infrared (IR) photons need to be developed. Here we investigate the sensitization of TiO2 catalysts using PbS quantum dots (QDs) which lead to the size dependent photocatalytic reduction of CO2 at frequencies ranging from the violet to the orange-red edge of the electromagnetic spectrum (λ ∼ 420 to 610 nm). Under broad band illumination (UV- NIR), the PbS QDs enhance CO2 photoreduction rates with TiO2 by a factor of ∼5 in comparison to unsensitized photocatalysts. X-ray photoelectron spectroscopy (XPS) is used to investigate the deactivation mechanism of the QD sensitizers after prolonged photoexcitation. The synthesis, characterization, and catalytic testing of these PbS sensitized TiO2 heterostructures will aid the development of more robust, visible light active photocatalysts for carbon management applications.

Visible light plasmonic heating of Au–ZnO for the catalytic reduction of CO2

Plasmonic excitation of Au nanoparticles attached to the surface of ZnO catalysts using low power 532 nm laser illumination leads to significant heating of the catalyst and the conversion of CO2 and H2 reactants to CH4 and CO products. Temperature-calibrated Raman spectra of ZnO phonons show that intensity-dependent plasmonic excitation can controllably heat Au-ZnO from 30 to ~600 °C and simultaneously tune the CH4 : CO product ratio. The laser induced heating and resulting CH4 : CO product distribution agrees well with predictions from thermodynamic models and temperature-programmed reaction experiments indicating that the reaction is a thermally driven process resulting from the plasmonic heating of the Au-ZnO. The apparent quantum yield for CO2 conversion under continuous wave (cw) 532 nm laser illumination is 0.030%. The Au-ZnO catalysts are robust and remain active after repeated laser exposure and cycling. The light intensity required to initiate CO2 reduction is low (~2.5 × 10(5) W m(-2)) and achievable with solar concentrators. Our results illustrate the viability of plasmonic heating approaches for CO2 utilization and other practical thermal catalytic applications.

Selective Adsorption of CO2 from Light Gas Mixtures by Using a Structurally Dynamic Porous Coordination Polymer

The selective adsorption of CO{sub 2} from mixtures with N{sub 2}, CH{sub 4}, and N{sub 2}O in a dynamic porous coordination polymer (see monomer structure) was evaluated by ATR-FTIR spectroscopy, GC, and SANS. All three techniques indicate highly selective adsorption of CO{sub 2} from CO{sub 2}/CH{sub 4} and CO{sub 2}/N{sub 2} mixtures at 30 C, with no selectivity observed for the CO{sub 2}/N{sub 2}O system.

Efficient electrochemical CO2 conversion powered by renewable energy

The catalytic conversion of CO2 into industrially relevant chemicals is one strategy for mitigating greenhouse gas emissions. Along these lines, electrochemical CO2 conversion technologies are attractive because they can operate with high reaction rates at ambient conditions. However, electrochemical systems require electricity, and CO2 conversion processes must integrate with carbon-free, renewable-energy sources to be viable on larger scales. We utilize Au25 nanoclusters as renewably powered CO2 conversion electrocatalysts with CO2 → CO reaction rates between 400 and 800 L of CO2 per gram of catalytic metal per hour and product selectivities between 80 and 95%. These performance metrics correspond to conversion rates approaching 0.8-1.6 kg of CO2 per gram of catalytic metal per hour. We also present data showing CO2 conversion rates and product selectivity strongly depend on catalyst loading. Optimized systems demonstrate stable operation and reaction turnover numbers (TONs) approaching 6 × 10(6) molCO2 molcatalyst(-1) during a multiday (36 h total hours) CO2 electrolysis experiment containing multiple start/stop cycles. TONs between 1 × 10(6) and 4 × 10(6) molCO2 molcatalyst(-1) were obtained when our system was powered by consumer-grade renewable-energy sources. Daytime photovoltaic-powered CO2 conversion was demonstrated for 12 h and we mimicked low-light or nighttime operation for 24 h with a solar-rechargeable battery. This proof-of-principle study provides some of the initial performance data necessary for assessing the scalability and technical viability of electrochemical CO2 conversion technologies. Specifically, we show the following: (1) all electrochemical CO2 conversion systems will produce a net increase in CO2 emissions if they do not integrate with renewable-energy sources, (2) catalyst loading vs activity trends can be used to tune process rates and product distributions, and (3) state-of-the-art renewable-energy technologies are sufficient to power larger-scale, tonne per day CO2 conversion systems.

In Situ Observation of Water Dissociation with Lattice Incorporation at FeO Particle Edges Using Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy

The dissociation of H2O and formation of adsorbed hydroxyl groups on FeO particles grown on Au(111) were identified with in situ X-ray photoelectron spectroscopy (XPS) at water pressures ranging from 3 × 10(-8) to 0.1 Torr. The facile dissociation of H2O takes place at FeO particle edges, and it was successfully observed in situ with atomically resolved scanning tunneling microscopy (STM). The in situ STM studies show that adsorbed hydroxyl groups were formed exclusively along the edges of the FeO particles with the O atom becoming directly incorporated into the oxide crystalline lattice. The STM results are consistent with coordinatively unsaturated ferrous (CUF) sites along the FeO particle edge causing the observed reactivity with H2O. Our results also directly illustrate how structural defects and under-coordinated sites participate in chemical reactions.

Photomediated Oxidation of Atomically Precise Au25(SC2H4Ph)18(-) Nanoclusters.

The anionic charge of atomically precise Au25(SC2H4Ph)18(-) nanoclusters (abbreviated as Au25(-)) is thought to facilitate the adsorption and activation of molecular species. We used optical spectroscopy, nonaqueous electrochemistry, and density functional theory to study the interaction between Au25(-) and O2. Surprisingly, the oxidation of Au25(-) by O2 was not a spontaneous process. Rather, Au25(-)-O2 charge transfer was found to be a photomediated process dependent on the relative energies of the Au25(-) LUMO and the O2 electron-accepting level. Photomediated charge transfer was not restricted to one particular electron accepting molecule or solvent system, and this phenomenon likely extends to other Au25(-)-adsorbate systems with appropriate electron donor-acceptor energy levels. These findings underscore the significant and sometimes overlooked way that photophysical processes can influence the chemistry of ligand-protected clusters. In a broader sense, the identification of photochemical pathways may help develop new cluster-adsorbate models and expand the range of catalytic reactions available to these materials.

Synthesis, characterization, and photocatalytic activity of Au–ZnO nanopyramids

Nanocrystalline Au–ZnO heterostructures were synthesized using a wet-chemical process where single-crystalline ZnO grows along the [0001] direction on top of polycrystalline Au seeds. High resolution transmission electron microscopy finds a 3.5% expansion of the ZnO (002) plane at the heterostructure interface. Rietveld analysis of X-ray diffraction patterns from ZnO and Au–ZnO powders find that the crystallographic microstrain in the metal oxide is 0.047% and 0.146%, respectively, illustrating that the crystallographic expansion at the heterostructure interface is detectable by bulk characterization techniques. Broad-band photo-degradation studies with methylene blue find that the Au–ZnO heterostructures decompose the dye 6 times faster than pure ZnO. Wavelength-dependent photodegradation studies illustrate direct gap excitation of the ZnO component of the heterostructure is required to initiate dye decomposition. The mechanistic details leading to this photocatalytic activity are discussed.

Probing active site chemistry with differently charged Au25q nanoclusters (q = −1, 0, +1)

Charged active sites are hypothesized to participate in heterogeneously-catalyzed reactions. For example, Auδ+ species at the catalyst surface or catalyst–support interface are thought to promote the thermally-driven CO oxidation reaction. However, the concept of charged active sites is rarely extended to electrochemical systems. We used atomically precise Au25q nanoclusters with different ground state charges (q = −1, 0, +1) to study the role of charged active sites in Au-catalyzed electrochemical reactions. Au25q clusters showed charge state-dependent electrocatalytic activity for CO2 reduction, CO oxidation and O2 reduction reactions in aqueous media. Experimental studies and density functional theory identified a relationship between the Au25q charge state, the stability of adsorbed reactants or products, and the catalytic reaction rate. Anionic Au25− promoted CO2 reduction by stabilizing coadsorbed CO2 and H+ reactants. Cationic Au25+ promoted CO oxidation by stabilizing coadsorbed CO and OH− reactants. Finally, stronger product adsorption at Au25+ inhibited O2 reduction rates. The participation of H+ and OH− in numerous aqueous electrocatalytic reactions likely extends the concept of charge state-mediated reactivity to a wide range of applications, including fuel cells, water splitting, batteries, and sensors. Au25q clusters have also shown photocatalytic and more traditional thermocatalytic activity, and the concept of charge state-mediated reactivity may create new opportunities for tuning reactant, intermediate and product interactions in catalytic systems extending beyond the field of electrochemistry.