We use vdW-corrected density functional theory (DFT) calculations with additional electron distribution correction to study the water binding chemistry of an Au nanoparticle supported on CeO2(111) with a linear step-edge. The initial structural model of Au/CeO2 used for DFT calculations is constructed by stabilizing a Au9 nanoparticle at the linear step-edge on a CeO2(111) slab. The calculated binding energy of a water molecule clearly shows that the interfacial site between Au and CeO2 binds water more strongly than the binding sites at the surface of Au nanoparticle. Subsequent water dissociation calculation result shows that the interface-bound water can be relatively easily dissociated into–OH and –H, providing a hydroxyl group that can be utilized as an oxygen source for CO oxidation. Based on the low dissociation energy of the interface bound water molecule, we suggest that the water at the Au-CeO2 interface can facilitate further oxidation of Au-bound CO. Our results point out that Au-CeO2 interface-bound water is beneficial for low-temperature oxidation reactions such as the water-gas shift reaction or preferential CO oxidation reaction.
The charge transfer (CT) excitation energy calculations of H2N-(CH=CH)3-X and the H2N-H.....H-X structures with the various electron acceptors (-X) were performed with comparing the accuracy of various calculation methods, such as B3LYP, long-range corrected (LC)-BLYP, and EOM-CCSD. Both intra-molecular and inter-molecular systems showed a tendency for CT excitation energy to decrease as the electronic accepting property increases, and LC-BLYP showed the best accuracy in both inter- and intra-molecular CT excitation energy. In this study, it was confirmed that unexpectedly larger range separation parameter(μ) values of LC-BLYP showed better results of CT excitation energy.
Cerium oxide (ceria, CeO2) is one of the most wide-spread oxide supporting materials for the precious metal nanoparticle class of heterogeneous catalysts. Because ceria can store and release oxygen ions, it is an essential catalytic component for various oxidation reactions such as CO oxidation (2CO + O2 2CO2). Moreover, reduced ceria is known to be reactive for water activation, which is a critical step for activation of water-gas shift reaction (CO + H2O → H2 + CO2). Here, we apply van der Waals-corrected density functional theory (DFT) calculations combined with U correction to study the mechanism of water chemisorption on CeO2(111) surfaces. A stoichiometric CeO2(111) and a defected CeO2(111) surface showed different water adsorption chemistry, suggesting that defected CeO2 surfaces with oxygen vacancies are responsible for water binding and activation. An appropriate level of water-ceria chemisorption energy is deduced by vdW-corrected non-local correlation coupled with the optB86b exchange functional, whereas the conventional PBE functional describes weaker water-ceria interactions, which are insufficient to stabilize (chemisorb) water on the ceria surfaces.
Transition-metal-embedded carbon nanotubes (CNTs) have been accepted as a novel type of sensing material due to the combined advantage of the transition metal, which possesses good catalytic behavior for gas interaction, and CNTs, with large effective surface areas that present good adsorption ability towards gas molecules. In this work, we simulate the adsorption of O2 and O3 onto Rh-doped CNT in an effort to understand the adsorbing behavior of such a surface. Results indicate that the proposed material presents good adsorbing ability and capacities for these two gases, especially O3 molecules, as a result of the relatively large conductivity changes. The frontier molecular orbital theory reveals that the conductivity of Rh-CNT would undergo a decrease after the adsorption of two such oxidizing gases due to the lower electron activity and density of this media. Our calculations are meaningful as they can supply experimentalists with potential sensing material prospects with which to exploit chemical sensors.
We perform density functional theory calculations to study the CO and O2 adsorption chemistry of Pt@X core@shell bimetallic nanoparticles (X = Pd, Rh, Ru, Au, or Ag). To prevent CO-poisoning of Pt nanoparticles, we introduce a Pt@X core-shell nanoparticle model that is composed of exposed surface sites of Pt and facets of X alloying element. We find that Pt@Pd, Pt@Rh, Pt@Ru, and Pt@Ag nanoparticles spatially bind CO and O2, separately, on Pt and X, respectively. Particularly, Pt@Ag nanoparticles show the most well-balanced CO and O2 binding energy values, which are required for facile CO oxidation. On the other hand, the O2 binding energies of Pt@Pd, Pt@Ru, and Pt@Rh nanoparticles are too strong to catalyze further CO oxidation because of the strong oxygen affinity of Pd, Ru, and Rh. The Au shell of Pt@Au nanoparticles preferentially bond CO rather than O2. From a catalysis design perspective, we believe that Pt@Ag is a better-performing Ptbased CO-tolerant CO oxidation catalyst.
Through density functional theory calculations, to provide insight into the origins of the catalytic activity of Au nanoparticles (NPs) toward oxidation reactions, we have scrutinized the oxygen adsorption chemistry of 9 types of small unsupported Au NPs of around 1 nm in size (Au13, Au19, Au20, Au25, Au38, and Au55) looking at several factors (size, shape, and coordination number). We found that these NPs, except for the icosahedral Au13, do not strongly bind to O2 molecules. Energetically most feasible O2 adsorption that potentially provides high CO oxidation activity is observed in the icosahedral Au13, our smallest Au NP. In spite of the chemical inertness of bulk Au, the structural fluxionality of such very small Au NP enables strong O2 adsorption. Our results can support recent experimental findings that the exceptional catalytic activity of Au NPs comes from very small Au species consisting of around 10 atoms each.
We investigated the adsorption of Na on graphene and graphene oxide, which are used as anode materials in sodium ion batteries, using density functional theory. The adsorption energy for Na on graphene was -0.507 eV at the hollow sites, implying that adsorption was favorable. In the case of graphene oxide, Na atoms were separately adsorbed on the epoxide and hydroxyl functional groups. The adsorption of Na on graphene oxide-epoxide (adsorption energy of -1.024 eV) was found to be stronger than the adsorption of Na on pristine graphene. However, the adsorption of Na on graphene oxide-hydroxyl resulted in the generation of NaOH as a by-product. Using density of states (DOS) calculations, we found that the DOS of the Na-adsorbed graphene was shifted down more than that of the Na-adsorbed graphene oxide-epoxide. In addition, the intensity of the DOS around the Fermi level for the Na-adsorbed graphene was higher than that for the Na-adsorbed graphene oxide-epoxide.
Zinc oxide as an optoelectronic device material was studied to utilize its wide band gap of 3.37 eV and high exciton biding energy of 60 meV. Using anti-site nitrogen to generate p-type zinc oxide has shown a deep acceptor level and low solubility. To increase the nitrogen solubility in zinc oxide, group 13 elements (aluminum, gallium, and indium) was co-added to nitrogen. The effect of aluminum on nitrogen solubility in a 3×3×2 zinc oxide super cell containing 72 atoms was investigated using density functional theory with hybrid functionals of Heyd, Scuseria, and Ernzerhof (HSE). Aluminum and nitrogen were substituted for zinc and oxygen sites in the super cell, respectively. The band gap of the undoped super cell was calculated to be 3.36 eV from the density of states, and was in good agreement with the experimentally obtained value. Formation energies of a nitrogen molecule and nitric oxide in the zinc oxide super cell in zinc-rich conditions were lower than those in oxygen-rich conditions. When the number of nitrogen molecules near the aluminum increased from one to four in the super cell, their formation energies decreased to approach the valence band maximum to some degree. However, the acceptor level of nitrogen in zinc oxide with the co-incorporation of aluminum was still deep.
Density functional theory was utilized to investigate the growth of an indium nanowire on a Si (001) buckled surface. A site between the edge of two Si dimers is most favorable when the first In atom is adsorbed on the surface at an adsorption energy level of 2.26 eV. The energy barriers for migration from other sites to the most favorable site are low. When the second In atom is adsorbed next to the first In atom to form an In dimer perpendicular to the Si dimer row, the adsorption energy is the highest among all adsorption sites. The third In atom prefers either of the sites next to the In dimer along the In dimer direction. The fourth In atom exhibited the same tendency showed by the second atom. The second and fourth In adsorption energy levels are higher than the first and third levels as the In atoms consume the third valence electron by forming In dimers. Therefore, the In nanowire grows perpendicular to the Si dimer row on the Si (001) surface, as it satisfies the bonding of the three valence electrons of the In atoms.
Calcia (CaO) stabilized cubic-HfO2 is studied by density functional theory (DFT) with generalized gradient approximation (GGA). When a Ca atom is substituted for a Hf atom, an oxygen vacancy is produced to satisfy the charge neutrality. The lattice parameter of a 2×2×2 cubic HfO2 supercell then increases by 0.02 Å. The oxygen atoms closest to the oxygen vacancy are attracted to the vacancy as the vacancy is positive compared to the oxygen ion. When the oxygen vacancy is located at the site closest to the Ca atom, the total energy of HfO2 reaches its minimum. The energy barriers for the migration of the oxygen vacancy were calculated. The energy barriers between the first and the second nearest sites, the second and the third nearest sites, and the third and fourth nearest sites are 0.2, 0.5, and 0.24 eV, respectively. The oxygen vacancies at the third and fourth nearest sites relative to the Ca atom represent the oxygen vacancies in undoped HfO2. Therefore, the energy barrier for oxygen migration in the HfO2 gate dielectric is 0.24 eV, which can explain the origin of gate dielectric leakage.
Tetragonal-Ni1-xPdxSi/Si (001) structure was studied by using density functional theory (DFT). An epitaxial interface between 2×2×4 (001) tetragonal-NiSi supercell and 1×1×2 (001) Si supercell was first constructed by adjusting the lattice parameters of B2-NiSi structure to match those of the Si structure. We chose Ni atoms as a terminating layer of the B2-NiSi; the equilibrium gap between the tetragonal-NiSi and Si was calculated to be 1.1 Å. The Ni atoms in the structure moved away from the original positions along the z-direction in a systematic way during the energy minimization. Two different Ni sites were identified at the interface and the bulk, respectively. The two Ni sites at the interface have 6 and 7 coordination numbers. The Ni sites with coordination number 6 at the interface were located farther away from the interface, and were more favorable for Pd substitution.
The total energy and strength of Mg alloy doped with Al, Ca and Zn, were calculated using thedensity functional theory. The calculations was performed by two programs; the discrete variational Xα (DV-Xα) method, which is a sort of molecular orbital full potential method; Vienna Ab-initio Simulation Package(VASP), which is a sort of pseudo potential method. The fundamental mixed orbital structure in each energylevel near the Fermi level was investigated with simple model using DV-Xα. The optimized crystal structurescalculated by VASP were compared to the measured structure. The density of state and the energy levels ofdopant elements was discussed in association with properties. When the lattice parameter obtained from thisstudy was compared, it was slightly different from the theoretical value but it was similar to Mk, and weobtained the reliability of data. A parameter Mk obtained by the DV-Xα method was proportional toelectronegativity and inversely proportional to ionic radii. We can predict the mechanical properties becauseis proportional to hardness.