Dan C. Sorescu

Understanding the sensor response of metal-decorated carbon nanotubes

We have explored the room temperature response of metal nanoparticle decorated single-walled carbon nanotubes (NP-SWNTs) using a combination of electrical transport, optical spectroscopy, and electronic structure calculations. We have found that upon the electrochemical growth of Au NPs on SWNTs, there is a transfer of electron density from the SWNT to the NP species, and that adsorption of CO molecules on the NP surface is accompanied by transfer of electronic density back into the SWNT. Moreover, the electronic structure calculations indicate dramatic variations in the charge density at the NP-SWNT interface, which supports our previous observation that interfacial potential barriers dominate the electrical behavior of NP-SWNT systems.

Chemical Sensitivity of Graphene Edges Decorated with Metal Nanoparticles

Graphene is a novel two-dimensional nanomaterial that holds great potential in electronic and sensor applications. By etching the edges to form nanoribbons or introducing defects on the basal plane, it has been demonstrated that the physical and chemical properties of graphene can be drastically altered. However, the lithographic or chemical techniques required to reliably produce such nanoribbons remain challenging. Here, we report the fabrication of nanosensors based on holey reduced graphene oxide (hRGO), which can be visualized as interconnected graphene nanoribbons. In our method, enzymatic oxidation generated holes within the basal plane of graphene oxide, and after reduction with hydrazine, hRGO was formed. When decorated with Pt nanoparticles, hRGO exhibited a large and selective electronic response toward hydrogen gas. By combining experimental results and theoretical modeling, we propose that the increased edge-to-plane ratio, oxygen moieties, and Pt nanoparticle decoration were responsible for the observed gas sensing with hRGO nanostructures.

Electron-Induced Dissociation of CO2 on TiO2(110)

The electron-induced dissociation of CO(2) adsorbed at the oxygen vacancy defect on the TiO(2)(110) surface has been investigated at the single-molecular level using scanning tunneling microscopy (STM). Electron injection from the STM tip into the adsorbed CO(2) induces the dissociation of CO(2). The oxygen vacancy defect is found to be healed by the oxygen atom released during the dissociation process. Statistical analysis shows that the dissociation of CO(2) is one-electron process. The bias-dependent dissociation yield reveals that the threshold energy for electron-induced dissociation of CO(2) is 1.4 eV above the conduction-band minimum of TiO(2). The formation of a transient negative ion by the injected electron is considered to be the key process in CO(2) dissociation.

CO2 adsorption on TiO2(101) anatase: A dispersion-corrected density functional theory study

Adsorption, diffusion, and dissociation of CO(2) on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidized surface several different local minima were identified of which the most stable corresponds to a CO(2) molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surface diffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surface interstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO(2) structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO(2) molecules to the surface. CO(2) dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcal/mol. The use of such defects for catalytic activation of CO(2) on anatase (101) surface would require a mechanism for their regeneration.

Collective Reactivity of Molecular Chains Self-Assembled on a Surface

Self-assembly of molecules on surfaces is a route toward not only creating structures, but also engineering chemical reactivity afforded by the intermolecular interactions. Dimethyldisulfide (CH3SSCH3) molecules self-assemble into linear chains on single-crystal gold surfaces. Injecting low-energy electrons into individual molecules in the self-assembled structures with the tip of a scanning tunneling microscope led to a propagating chemical reaction along the molecular chain as sulfur–sulfur bonds were broken and then reformed to produce new CH3SSCH3 molecules. Theoretical and experimental evidence supports a mechanism involving electron attachment followed by dissociation of a CH3SSCH3 molecule and initiation of a chain reaction by one or both of the resulting CH3S intermediates.

CO2 adsorption on TiO2(110) rutile: Insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Adsorption of CO(2) on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy (STM). On the oxidized surface the CO(2) molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO(2) molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorption energies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations.

CO2 capture properties of lithium silicates with different ratios of Li2O/SiO2: An ab initio thermodynamic and experimental approach

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO2 capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li2O/SiO2 ratios (Li2O, Li8SiO6, Li4SiO4, Li6Si2O7, Li2SiO3, Li2Si2O5, Li2Si3O7, and α-SiO2) have been investigated by combining first-principles density functional theory with lattice phonon dynamics. All these lithium silicates examined are insulators with band-gaps larger than 4.5 eV. By decreasing the Li2O/SiO2 ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li2O/SiO2 ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO2 absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li2O/SiO2 ratio when going from Li2Si3O7 to Li8SiO6, the corresponding lithium silicates have higher CO2 capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO2 chemisorption by lithium silicates, we found that the CO2 capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li2CO3 and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO2 into the shell to continue the CO2 chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li2O and SiO2, we can obtain different lithium silicate solids which exhibit different thermodynamic behaviors. Based on our results, three mixing scenarios are discussed to provide general guidelines for designing new CO2 sorbents to fit practical needs.

CO2 capture properties of alkaline earth metal oxides and hydroxides: A combined density functional theory and lattice phonon dynamics study

By combining density functional theory and lattice phonon dynamics, the thermodynamic properties of CO(2) absorption/desorption reactions with alkaline earth metal oxides MO and hydroxides M(OH)(2) (where M=Be,Mg,Ca,Sr,Ba) are analyzed. The heats of reaction and the chemical potential changes of these solids upon CO(2) capture reactions have been calculated and used to evaluate the energy costs. Relative to CaO, a widely used system in practical applications, MgO and Mg(OH)(2) systems were found to be better candidates for CO(2) sorbent applications due to their lower operating temperatures (600-700 K). In the presence of H(2)O, MgCO(3) can be regenerated into Mg(OH)(2) at low temperatures or into MgO at high temperatures. This transition temperature depends not only on the CO(2) pressure but also on the H(2)O pressure. Based on our calculated results and by comparing with available experimental data, we propose a general computational search methodology which can be used as a general scheme for screening a large number of solids for use as CO(2) sorbents.

Welding of gold nanoparticles on graphitic templates for chemical sensing.

Controlled self-assembly of zero-dimensional gold nanoparticles and construction of complex gold nanostructures from these building blocks could significantly extend their applications in many fields. Carbon nanotubes are one of the most promising inorganic templates for this strategy because of their unique physical, chemical, and mechanical properties, which translate into numerous potential applications. Here we report the bottom-up synthesis of gold nanowires in aqueous solution through self-assembly of gold nanoparticles on single-walled carbon nanotubes followed by thermal-heating-induced nanowelding. We investigate the mechanism of this process by exploring different graphitic templates. The experimental work is assisted by computational studies that provide additional insight into the self-assembly and nanowelding mechanism. We also demonstrate the chemical sensitivity of the nanomaterial to parts-per-billion concentrations of hydrogen sulfide with potential applications in industrial safety and personal healthcare.

Theoretical and experimental studies of CO2 and H2 separation using the 1-ethyl-3-methylimidazolium acetate ([emim][CH3COO]) ionic liquid.

The performance of [emim][CH(3)COO] ionic liquid (IL) to separate mixtures of CO(2) and H(2) is studied using both classical and ab initio simulation methods and experiments. Simulations show that H(2) solubility and permeability in [emim][CH(3)COO] are quite low with Henrys law constants about 1 × 10(4) bar and permeabilities in the range 29-79 barrer at 313-373 K. In the case of CO(2) absorption in [emim][CH(3)COO], ab initio molecular dynamics simulations predict two types of CO(2) absorption states. In type I state, CO(2) molecules interact with the [CH(3)COO](-) anion through strong complexation leading to high CO(2) solubility. The C atom of CO(2) is located close to the O atoms of the [CH(3)COO](-) anion with an average distance of about 1.61 Å. The CO(2) bond angle (θ(OCO)) is about 138°, significantly perturbed from that of an isolated linear CO(2). In type II state, the CO(2) molecule maintains a linear configuration and is located at larger separations (>2.2 Å) from the [CH(3)COO](-) anion. The weaker interaction of CO(2) with the [CH(3)COO](-) anion in type II state is similar to the one observed when CO(2) absorbs in [bmim][PF(6)]. Simulations further demonstrate that the [emim](+) cation competes with CO(2) to interact with the [CH(3)COO](-) anion. The predicted high CO(2) permeability and low H(2) permeability in [emim][CH(3)COO] are also verified by our experiments. The experimental CO(2) permeability in [emim][CH(3)COO] is in the range of 1325-3701 barrer, and high experimental CO(2)/H(2) permeability selectivities of 21-37 at 313-373 K are observed. We propose that by replacing [emim](+) cation with 1-butyl-1-methylpyrrolidinium ([PY(14)](+)) further enhancement of CO(2) solubility in [PY(14)][CH(3)COO] IL will be obtained as well as good performance to separate CO(2) and H(2).