Abstract: Bimetallic plasmonic nanoparticles enable tuning of the optical response and chemical stability by variation of the composition. The present numerical simulation study compares Ag–Au alloy, Ag@Au core–shell, and Au@Ag core–shell bimetallic plasmonic nanoparticles of both spherical and anisotropic (nanotriangle and nanorods) shapes. By studying both spherical and anisotropic (with LSPR in the near-infrared region) shapes, cases with and without interband transitions of Au can be decoupled. Explicit comparisons are facilitated by numerical models supported by careful validation and examination of optical constants of Au–Ag alloys reported in the literature. Although both Au–Ag core–shell and alloy nanoparticles exhibit an intermediary optical response between that of pure Ag and Au nanoparticles, there are noticeable differences in the spectral characteristics. Also, the effect of the bimetallic constitution in anisotropic nanoparticles is starkly different from that in spherical nanoparticles due to the absence of Au interband transitions in the former case. In general, the improved chemical stability of Ag nanoparticles by incorporation of Au comes with a cost of reduction in plasmonic enhancement, also applicable to anisotropic nanoparticles with a weaker effect. A photothermal heat transfer study confirms that increased absorption by the incorporation of Au in spherical Ag nanoparticles also results in an increased steady-state temperature. On the other hand, anisotropic nanoparticles are inherently better absorbers and hence better photothermal sources, and their photothermal properties are apparently not strongly affected by the incorporation of one metal in the other. This study of the optical/spectral and photothermal characteristics of bimetallic Au–Ag alloy versus core–shell nanoparticles provides detailed physical insight for development of new taylor-made plasmonic nanostructures.

Abstract: Plasma technology is gaining increasing interest for CO2 conversion. The gas temperature in (and after) the plasma reactor largely affects the performance. Therefore, we examine the effect of cooling/quenching, during and after the plasma, on the CO2 conversion and energy efficiency, for typical “warm” plasmas, by means of chemical kinetics modeling. For plasmas at low specific energy input (SEI ∼ 0.5 eV/molecule), it is best to quench at the plasma end, while for high-SEI plasmas (SEI ∼ 4 eV/molecule), quenching at maximum conversion is better. For low-SEI plasmas, quenching can even increase the conversion beyond the dissociation in the plasma, known as superideal quenching. To better understand the effects of quenching at different plasma conditions, we study the dissociation and recombination rates, as well as the vibrational distribution functions (VDFs) of CO2, CO, and O2. When a high vibrational−translational (VT) nonequilibrium exists at the moment of quenching, the dissociation and recombination reaction rates both increase. Depending on the conversion degree at the moment of quenching, this can lead to a net increase or decrease of CO2 conversion. In general, however, and certainly for equilibrium plasmas at high temperature, quenching after the plasma helps prevent recombination reactions and clearly enhances the final CO2 conversion. We also investigate the effect of different quenching cooling rates on the CO2 conversion and energy efficiency. Finally, we compare plasma-based conversion to purely thermal conversion. For warm plasmas with typical temperatures of 3000−4000 K, the conversion is roughly thermal.

Abstract: We developed a microkinetic model to reveal the effects of plasma-generated radicals, intermediates, and vibrationally excited species on the catalytic hydrogenation of CO2 to CH3OH on a Cu(111) surface. As a benchmark, we first present the mechanisms of thermal catalytic CH3OH formation. Our model predicts that the reverse water-gas shift reaction followed by CO hydrogenation, together with the formate path, mainly contribute to CH3OH formation in thermal catalysis. Adding plasma-generated radicals and intermediates results in a higher CH3OH turnover frequency (TOF) by six to seven orders of magnitude, showing the potential of plasma-catalytic CO2 hydrogenation into CH3OH, in accordance with the literature. In addition, CO2 vibrational excitation further increases the CH3OH TOF, but the effect is limited due to relatively low vibrational temperatures under typical plasma catalysis conditions. The predicted increase in CH3OH formation by plasma catalysis is mainly attributed to the increased importance of the formate path. In addition, the conversion of plasma-generated CO to HCO* and subsequent HCOO* or H2CO* formation contribute to CH3OH formation. Both pathways bypass the HCOO* formation from CO2, which is the main bottleneck in the process. Hence, our model points toward the important role of CO, but also O, OH, and H radicals, as they influence the reactions that consume CO2 and CO. In addition, our model reveals that the H pressure should not be smaller than ca. half of the O pressure in the plasma as this would cause O* poisoning, which would result in very small product TOFs. Thus, plasma conditions should be targeted with a high CO and H content as this is favorable for CH3OH formation, while the O content should be minimized.

Abstract: We use microkinetic modeling to examine the potential of plasma-catalytic partial oxidation (POX) of CH4 as a promising new approach to produce oxygenates. We study how different plasma species affect POX of CH4 on the Pt(111) surface, and we discuss the associated kinetic and mechanistic changes. We discuss the effect of vibrationally excited CH4 and O2, as well as plasma-generated radicals and stable intermediates. Our results show that vibrational excitation enhances the turnover frequency (TOF) of catalytic CH4 dissociation and has good potential for improving the selectivities toward CH3OH, HCOOH, and C2 hydrocarbons. Nevertheless, when also considering plasma-generated radicals, we find that these species mainly govern the surface chemistry. Additionally, we find that plasma-generated radicals and stable intermediates enhance the TOFs of COx and oxygenates, increase the selectivity toward oxygenates, and make the formation of HCOOH more significant on Pt(111). We also briefly illustrate the potential impact of Eley−Rideal reactions that involve plasma-generated radicals. Finally, we reveal how various radicals affect the catalyst surface chemistry and we link this to the formation of different products. This allows us to make suggestions on how the plasma composition should be altered to improve the formation of desired products.

Abstract: The synthesis of gold nanorods requires the presence of symmetry-breaking and shape-directing additives, among which bromide ions and quaternary ammonium surfactants have been reported as essential. As a result, hexadecyltrimethylammonium bromide (CTAB) has been selected as the most efficient surfactant to direct anisotropic growth. One of the difficulties arising from this selection is the low solubility of CTAB in water at room temperature, and therefore the seeded growth of gold nanorods is usually performed at 25 degrees C or above, which has restricted so far the analysis of kinetic effects derived from lower temperatures. We report a systematic study of the synthesis of gold nanorods from pentatwinned seeds using hexadecyltrimethylammonium chloride (CTAC) as the principal surfactant and a low concentration of bromide as shape-directing agent. Under these conditions, the synthesis can be performed at temperatures as low as 8 degrees C, and the corresponding kinetic effects can be studied, resulting in temperature-controlled aspect ratio tunability.

Abstract: The adsorption of C and CHx radicals on anatase (001) was studied using DFT within the generalized gradient approximation using the Perde-Burke-Ernzerhof (PBE) functional. We have studied the influence of oxygen vacancies in and at the surface on the adsorption properties of the radicals. For the oxygen vacancies in anatase (001), the most stable vacancy is located at the surface. For this vacancy, the maximal adsorption strength of C and CH decreases compared to the adsorption on the stoichiometric surface, but it increases for CH2 and CH3. If an oxygen vacancy is present in the first subsurface layer, the maximal adsorption strength increases for C, CH, CH2, and CH3. When the vacancy is present in the next subsurface layer, we find that only the CH3 adsorption is enhanced, while the maximal adsorption energies for the other radical species decrease. Not only does the precise location of the oxygen vacancy determine the maximal adsorption interaction, it also influences the adsorption strengths of the radicals at different surface configurations. This determines the probability of finding a certain adsorption configuration at the surface, which in turn influences the possible surface reactions. We find that C preferentially adsorbs far away from the oxygen vacancy, while CH2 and CH3 adsorb preferentially at the oxygen vacancy site. A fraction of CH partially adsorbs at the oxygen vacancy, and another fraction adsorbs further away from the vacancy.

Abstract: Atomic, layer deposition,(ALD of ruthenium using two ruthenium precursors, i.e., Ru(C5H5)(2) (RuCp2) and Ru(C5H5)(C4H4N) (RuCpPy), is studied using density functional theory. By investigating the reaction mechanisms On bare ruthenium surfaces, i.e., (001), (101), and (100), and H-terminated surfaces, an atomistic insight in the Ru ALD is provided. The calculated results show that on the Ru surfaces both RuCp2 and RuCpPy an undergo dehydrogenation and ligand dissociation reactions. RuCpPy is more reactive than RuCp2. By forming a, strong, bond between N of Py and Ru of the surface, RuCpPy can easily chemisorb on the surfaces. The reactions of RuCp2,On the Surfaces are less favorable the adsorption is not strong enough This could be a,factor contributing to the higher growth-per-cycle of Ru using RuCpPy, as observed experimentally. By Studying, the adsorption on H-terminated Ru surfaces, We showed that H Can prevent the adsorption of the precursors, thus inhibiting the growth of Ru. Our calculations indicate that the H content on the surface can have an impact on the growth-per-cycle. Finally, our simulations also demonstrate large impacts of the surface structure on the reaction mechanisms. Of the three surfaces, the (100) surface, which is the less stable and has a zigzag surface structure, is also the most reactive one.

Abstract: The influence of triaxial in-plane strain on the electronic properties of a hexagonal boron-nitride sheet is investigated using density functional theory. Different from graphene, the triaxial strain localizes the molecular orbitals of the boron-nitride flake in its center depending on the direction of the applied strain. The proposed technique for localizing the molecular orbitals that are close to the Fermi level in the center of boron nitride flakes can be used to actualize engineered nanosensors, for instance, to selectively detect gas molecules. We show that the central part of the strained flake adsorbs polar molecules more strongly as compared with an unstrained sheet.

Abstract: A platform for producing stabilized Pt atoms and clusters through the combination of an N-doped graphene support and atomic layer deposition (ALD) for the Pt catalysts was investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was determined, using imaging and spectroscopy techniques, that a wide range of N-dopant types entered the graphene lattice through covalent bonds without largely damaging its structure. Additionally and most notably, Pt atoms and atomic clusters formed in the absence of nanoparticles. This work provides a new strategy for experimentally producing stable atomic and subnanometer cluster catalysts, which can greatly assist the proton exchange membrane fuel cell (PEMFC) development by producing the ultimate surface area to volume ratio catalyst.

Abstract: The excitonic properties of Si(core)/Ge(shell) and Ge(core)/Si(shell) nanocrystals (NC's) with diameters of similar to 1.9 nm are investigated using a combination density functional ab initio method to obtain the single particle wave functions and a configuration interaction method to compute the exciton fine structure and absorption coefficient. These core/shell structures exhibit type II confinement, which is more pronounced for the Si/Ge NC as a consequence of strain. The absorption coefficients of these NC's exhibit a single dominant peak, which has a much larger oscillator strength than the multipeaks found for pure Si and Ge NC's. The exciton lifetime in Si, Ge, and Ge/Si shows a small i:emperature dependence in the range 10-300 K, whereas in Si/Ge, the exciton lifetime decreases more than an order of magnitude in the same temperature range.

Abstract: Graphane, hydrogenated graphene, was very recently synthesized and predicted to have great potential applications. In this work, we propose a new promising approach for hydrogenation of graphene based on density functional theory (DFT) calculations through the application of a perpendicular electric field after substitutionally doping by nitrogen atoms. These DFT calculations show that the doping by nitrogen atoms into the graphene layer and applying an electrical field normal to the graphene surface induce dissociative adsorption of hydrogen. The dissociative adsorption energy barrier of an H2 molecule on a pristine graphene layer changes from 2.7 to 2.5 eV on N-doped graphene, and to 0.88 eV on N-doped graphene under an electric field of 0.005 au. When increasing the electric field above 0.01 au, the reaction barrier disappears. Therefore, N doping and applying an electric field have catalytic effects on the hydrogenation of graphene, which can be used for hydrogen storage purposes and nanoelectronic applications.

Abstract: Ab initio calculations within the density functional theory formalism are performed to investigate the stability and electronic properties of fluorinated bilayer graphene (bilayer fluorographene). A comparison is made to previously investigated graphane, bilayer graphane, and fluorographene. Bilayer fluorographene is found to be a much more stable material than bilayer graphane. Its electronic band structure is similar to that of monolayer fluorographene, but its electronic band gap is significantly larger (about 1 eV). We also calculate the effective masses around the Gamma-point for fluorographene and bilayer fluorographene and find that they are isotropic, in contrast to earlier reports. Furthermore, it is found that bilayer fluorographene is almost as strong as graphene, as its 2D Young's modulus is approximately 300 N m(-1).

Abstract: With the help of first-principles calculations, we investigate graphane/fluorographene heterostructures with special attention for graphane and fluorographene-based quantum dots. Graphane and fluorographene have large electronic band gaps, and we show that their band structures exhibit a strong type-II alignment. In this way, it is possible to obtain confined electron states in fluorographene nanostructures by embedding them in a graphane crystal. Bound hole states can be created in graphane domains embedded in a fluorographene environment. For circular graphane/fluorographene quantum dots, localized states can be observed in the band gap if the size of the radii is larger than approximately 4 to 5 Å.

Abstract: Using reactive molecular dynamics simulations, we have investigated the effect of single-impact, low-energy (thermal-100 eV) bombardment of a Si(100){2 × 1} surface by atomic and molecular oxygen. Penetration probability distributions, as well as defect formation distributions, are presented as a function of the impact energy for both species. It is found that at low impact energy, defects are created chemically due to the chemisorption process in the top layers of the surface, while at high impact energy, additional defects are created by a knock-on displacement of Si. These results are of particular importance for understanding device performances of silica-based microelectronic and photovoltaic devices.