Charge transfers at metal/oxide interfaces: a DFT study of formation of K delta+ and Au delta- species on MgO/Ag(100) ultra-thin films from deposition of neutral atoms

Ultra-thin oxide films grown on a metal substrate and of thickness smaller than 1 nm may exhibit unusual properties with respect to thicker films or single crystal oxide surfaces. In a previous study [G. Pacchioni, L. Giordano and M. Baistrocchi, Phys. Rev. Lett., 2005, 94, 226104] we have suggested that a Au atom adsorbed on a MgO/Mo(100) thin film becomes negatively charged by direct electron tunneling from the Mo metal and that this is related to the low MgO/Mo(100) work function. Here we show, based on periodic DFT supercell calculations, that charge transfer can occur also in the opposite direction by adsorption of electropositive K atoms on MgO/Ag(100) films. We predict the occurrence of a charge transfer also for Au on MgO/Ag(100) films despite the fact that here the work function is 1 eV larger than in MgO/Mo(100). The formation of a layer of adsorbed negative (Au delta-/MgO/Ag) or positive (K delta+/MgO/Ag) adsorbates results in an increase or decrease, respectively, of the MgO/Ag(100) work function as predicted by the classical Gurney model for ionic adsorbates on metal surfaces.     

Properties of alkali metal atoms deposited on a MgO surface: a systematic experimental and theoretical study

The adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.     

Mechanism for NO2 charging on metal supported MgO

NO2 adsorbed on MgO(100) supported by Ag or Pt is explored by density functional theory calculations. NO2 is weakly adsorbed on MgO(100), with a bond involving minor oxide to adsorbate charge transfer. However, if MgO is supported, then the adsorption energy is considerably enhanced and NO2 is adsorbed as a nitrite (N). Analysis reveals that the NO2 excess charge originates from the oxide side of the oxide/metal interface and that the electron abstraction increases the oxide/metal adhesion. The proposed mechanism is general and should apply for oxidizing surface species.     

Tin oxide-surface modified anatase titanium(IV) dioxide with enhanced UV-light photocatalytic activity

[Sn(acac)(2)]Cl(2) is chemisorbed on the surfaces of anatase TiO(2)via ion-exchange between the complex ions and H(+) released from the surface Ti-OH groups without liberation of the acetylacetonate ligand (Sn(acac)(2)/TiO(2)). The post-heating at 873 K in air forms tin oxide species on the TiO(2) surface in a highly dispersed state on a molecular scale ((SnO(2))(m)/TiO(2)). A low level of this p block metal oxide surface modification (~0.007 Sn ions nm(-2)) accelerates the UV-light-activities for the liquid- and gas-phase reactions, whereas in contrast to the surface modification with d block metal oxides such as FeO(x) and NiO, no visible-light response is induced. Electrochemical measurements and first principles density functional theory (DFT) calculations for (SnO(2))(m)/TiO(2) model clusters (m = 1, 2) indicate that the bulk (TiO(2))-to-surface interfacial electron transfer (BS-IET) enhances charge separation and the following electron transfer to O(2) to increase the photocatalytic activity.     

Adsorption of transition metal atoms on the NiO(100) surface and on NiO/Ag(100) thin films

We have studied the adsorption of Au, Pd, and Pt atoms on the NiO(100) surface and on NiO/Ag(100) thin films using plane wave DFT+U calculations. The scope of this work is to compare the adsorption properties of NiO, a reducible transition metal oxide, with those of MgO, a simple binary oxide with the same crystal structure and similar lattice parameter. At the same time, we are interested in the adsorption characteristics of NiO ultra-thin films (three atomic layers) deposited on Ag(100) single crystals. Also in this case the scope is to compare NiO/Ag(100) with the corresponding MgO/Ag(100) films which show unusual properties for the case of Au adsorption. The results show that the transition metal atoms bind in a similar way on NiO(100) and NiO/Ag(100) films, with Pt, Pd, and Au forming bonds of decreasing strength in this order. No charging effects occur for Au adsorbed on NiO/Ag(100) films, at variance with MgO/Ag(100). The reasons are analyzed in terms of work function of the metal/oxide interface. Possible ways to modify this property by growing alternate layers of MgO and NiO are discussed.     

Competitive adsorption of Ca2+ and Zn(II) ions at monodispersed SiO2/electrolyte solution interface

A study of competitive adsorption of Ca(2+) and Zn(II) ions at the monodispersed SiO(2)/electrolyte solution interface is presented. Influence of ionic strength, pH, and presence of other ions on adsorption of Ca(2+) and Zn(II) in the mentioned system are investigated. zeta potential, surface charge density, adsorption density, pH(50%), and DeltapH(10-90%) parameters for different concentrations of carrying electrolyte and adsorbed ions are also presented. A high concentration of zinc ions shifts the adsorption edge of Ca(2+) ions adsorbed from solutions with a low initial concentration at the SiO(2)/NaClO(4) solution interface to the higher pH values. This effect disappears with a concentration increase of calcium ions. The presence of Ca(2+) ions in the system slightly affects the adsorption of zinc ions on SiO(2), shifting the adsorption edge toward lower pH values and thereby increasing the adsorption slope.     

Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110)

Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (approximately 0.15 eV) energy barriers.     

Charging of Au atoms on TiO2 thin films from CO vibrational spectroscopy and DFT calculations

Au atoms have been deposited on oxidized and reduced TiO2 thin films grown on Mo(110). The gold binding sites and the occurrence of Au-TiO2 charge transfer were identified by measuring infrared spectra as a function of temperature and substrate preparation. The results have been interpreted by slab model DFT calculations. Au binds weakly to regular TiO2 sites (De < 0.5 eV) where it remains neutral, and diffuses easily even at low temperature until it gets trapped at strong binding sites such as oxygen vacancies (De = 1.7 eV). Here, a charge transfer from TiO2 to Au occurs. Au(delta-)CO complexes formed on oxygen vacancies easily lose CO (De = 0.4 eV), and the CO stretching frequency is red-shifted. On nondefective surfaces, CO adsorption induces a charge transfer from Au to TiO2 with formation of strongly bound Audelta+CO complexes (De = 2.4 eV); the corresponding CO frequency is blue-shifted with respect to free CO. We propose possible mechanisms to reconcile the observed CO desorption around 380 K with the unusually high stability of Au-CO complexes formed on regular sites predicted by the calculations. This implies: (a) diffusion of AuCO complexes above 150 K; (b) formation of gold dimers when the diffusing AuCO complex encounters a Au atom bound to an oxygen vacancy (reduced TiO2) or a second AuCO unit (oxidized TiO2); and (c) CO desorption from the resulting dimer, occurring around 350-400 K.     

Charge state of metal atoms on oxide supports: a systematic study based on simulated infrared spectroscopy and density functional theory

A long standing question in the study of supported clusters of metal atoms in the properties of metal–oxide interfaces is the extent of metal–oxide charge transfer. However, the determination of this charge transfer is far from straight forward and a combination of different methods (both experimental and theoretical) is required. In this paper, we systematically study the charging of some adsorbed transition metal atoms on two widely used metal oxides surfaces [α-Al₂O₃ (0001) and rutile TiO₂ (110)]. Two procedures are combined to this end: the computed vibrational shift of the CO molecule, that is used as a probe, and the calculation of the atoms charges from a Bader analysis of the electron density of the systems under study. At difference from previous studies that directly compared the vibrational vawenumber of adsorbed CO with that of the gas phase molecule, we have validated the procedure by comparison of the computed CO stretching wavenumbers in isolated monocarbonyls (MCO) and their singly charged ions with experimental data for these species in rare gas matrices. It is found that the computational results correctly reproduce the experimental trend for the observed shift on the CO stretching mode but that care must be taken for negatively charged complexes as in this case there is a significative difference between the total charge of the MCO complex and the charge of the M atom. For the supported adatoms, our results show that while Cu and Ag atoms show a partial charge transfer to the Al₂O₃ surface, this is not the case for Au adatoms, that are basically neutral on the most stable adsorption site. Pd and Pt adatoms also show a significative amount of charge transfer to this surface. On the TiO₂ surface our results allow an interpretation of previous contradictory data by showing that the adsorption of the probe molecule may repolarize the Au adatoms, that are basically neutral when isolated, and show the presence of highly charged Au^δ+–CO complexes. The other two coinage metal atoms are found to significatively reduce the TiO₂ surface. The combined use of the shift on the vibrational frequency of the CO molecule and the computation of the Bader charges shows to be an useful tool for the study the charge state of adsorbed transition metal atoms and allow to rationalize the information coming from complementary tools.     

Interfacial electron transfer on cytochrome-c sensitised conformally coated mesoporous TiO2 films

Hybrid protein films incorporating Cyt-c immobilized on TiO(2) films were prepared and characterised optically with UV-visible spectroscopy and electrochemically with cyclic voltammetry, and their conductivity properties were studied in detail. In addition the effects of a thin overlayer coating of a second metal oxide such as SiO(2), Al(2)O(3), ZrO(2) and MgO(2) were studied and the effects over the electrochemical properties of the hybrid working electrodes were discussed.     

DFT Study of Alkali Metal Atom Adsorption on Defect-Free MgO(001) Surface

The adsorption of isolated alkali metal atoms (Li, Na, K, Rb, and Cs) on defect-free sur-face of MgO(001) has been systemically investigated with density functional theory using a pseudopotential plane-wave approach. The adsorption energy calculated is about -0.72 eV for the lithium on top of the surface O site and about one third of this value for the other alkali metals. The relatively strong interaction of Li with the surface O can be explained by a more covalent bonding involved, evidenced by results of both the projected density of states and the charge density difference. The bonding mechanism is discussed in detail for all alkali metals.     

Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers

We report herein a methodology for conformally coating nanocrystalline TiO2 films with a thin overlayer of a second metal oxide. SiO2, Al2O3, and ZrO2 overlayers were fabricated by dipping mesoporous, nanocrystalline TiO2 films in organic solutions of their respective alkoxides, followed by sintering at 435 degrees C. These three metal oxide overlayers are shown in all cases to act as barrier layers for interfacial electron transfer processes. However, experimental measurements of film electron density and interfacial charge recombination dynamics under applied negative bias were vary significantly for the overlayers. A good correlation was observed between these observations and the point of zero charge of the different metal oxides. On this basis, it is found that the most basic overlayer coating, Al2O3 (pzc = 9.2), is optimal for retarding interfacial recombination losses under negative applied bias. These observations show good correlation with current/voltage analyses of dye sensitized solar cell fabricated from these films, with the Al2O3 resulting in an increase in V(oc) of up to 50 mV and a 35% improvement in overall device efficiency. These observations are discussed and compared with an alternative TiCl4 posttreatment of nanocrystalline TiO2 films with regard to optimizing device efficiency.     

DFT Study of Metal Atoms Adsorbed at Low-coordinated Sites of MgO （001） Surface

1 INTRODUCTION The interfaces between metals and oxide play a vital role in many industrial applications: hetero- geneous catalysis, microelectronics, thermal barriers, corrosion protection, metal processing and so on[1]. In catalysis, the choice of metal and oxide support is critical in order to obtain a desired reactivity and selectivity[2]. This is due in part to the inherent reac- tivity of the two components. Also the size and shape of the metal particle, which depend on the choice…     

Activation of ultrathin oxide films for chemical reaction by interface defects

Periodic density functional theory calculations revealed strong enhancement of chemical reactivity by defects located at the oxide-metal interface for water dissociation on ultrathin MgO films deposited on Ag(100) substrate. Accumulation of charge density at the oxide-metal interface due to irregular interface defects influences the chemical reactivity of MgO films by changing the charge distribution at the oxide surface. Our results reveal the importance of buried interface defects in controlling chemical reactions on an ultrathin oxide film supported by a metal substrate.     

Spin quenching of Mn in complexes and CO binding with Mn deposited on MgO and CaO supports: DFT calculations

An attempt has been made to analyze the spin quenching properties of Mn, as a representative of transition metals, in Mn·MgO, Mn·CaO, OC·Mn·MgO, and OC·Mn·CaO complexes formed at the regular (001) surfaces of MgO and CaO, as well as the adsorption of CO on Mn deposited on MgO and CaO by means of hybrid density functional theory calculations and embedded cluster model. Clusters of moderate sizes were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. A test has been made to examine the effect of artificial flow of charge. While the spin states of Mn in Mn·MgO and Mn·CaO complexes are preserved, the combined effects of adsorbate and substrate in OC·Mn·MgO and OC·Mn·CaO complexes are strong enough to favor the low spin states and to quench the spin. The deposited Mn atoms enhance the adsorption of CO on MgO and CaO surfaces. The significant weakening of bond strength between OC and Mn in complexes supports the concept of bond order conservation. The relation between the strength of CO adsorption and the basicity of the support is verified. The natural bond orbital analysis reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The effects of spin contamination on the geometry, Mulliken charges, and adsorption energy are examined. The binding of CO precursor is dominated by the E(i)^Mn···CO pairwise additive components in MgO and CaO complexs, and the role of the support is not restricted to supporting the metal. The adsorbed CO molecules exhibit no remarkable deviation from linearity. Finally, relations are established between the process of spin quenching and the energy gaps between frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules on the magnetic and binding properties of complexes need to be explicitly taken into account. © 2012 Wiley Periodicals, Inc.     

Infrared absorption enhancement of fluorinated poly‐acrylate thin films on Co‐alloy substrate

Fluorinated poly‐acrylate films coated on continuous Co‐based metal substrate show significant IR absorption enhancement. The degree of enhancement depends greatly on the type of chemical bonds in the polymer molecules and the corresponding vibrations of chemical dipoles. Strong polar groups like C? O, C? F and C?O show the strongest absorption enhancement that also coincides with significant blueshift of the corresponding IR bands. In this paper, the possible mechanism for IR absorption enhancement and band shift are discussed on the basis of the electric field intensification on metal surface, charge transfer and dipole reorientation of the polymer molecules adsorbed on the metal surface. Chemisorption of the polymer molecules on Co‐alloy is established through the interaction between the oxygen atoms in carboxylic group and the substrate cobalt atoms reacting through electron overlap and spin‐coupling of oxygen π‐orbital and cobalt d‐orbital. Copyright © 2006 John Wiley & Sons, Ltd.     

Manganese Oxide(MnO) and Cadmium Dioxide(CdO_2) Attached to Carbon Nanotube(8, 0) as Anodes of Metal-ion Batteries: a DFT Study

Performance of carbon nanotube(CNT) and their attached metal oxides(manganese oxide(MnO) and cadmium dioxide(CdO2)) structures as anode electrodes in lithium-ion battery(LIB) and potassium-ion battery(KIB) are investigated. The Gibbs free energy of adsorption of Li and K atoms/ions on surfaces of CNT(8, 0), CNT(8, 0)-MnO and CNT(8, 0)-CdO2 are calculated. The cell voltages(Vcell) of Li and K atoms/ions adsorption on studied surfaces are examined. The Vcell of LIBs with metal-oxides attached to CNT(8, 0) as anode electrodes are higher than those KIBs. The adsorbed metal oxides(MnO and CdO2) on CNT(8, 0) increased the charges, electronic conductivity and Vcell of LIB and KIB, efficiently. The CNT(8, 0)-CdO2 as anode electrodes in LIB and KIB is proposed.     

CO和NO在Rh2-V/SiO2催化剂上共吸附的红外光谱

应用CO和NO吸附态原位红外光谱方法研究了还原态Rh2／SiO2，Rh2－V／SiO2催化剂上的活性中心铑的状态和助剂钒的作用. Rh2催化剂用Rh2（CO）4Cl2化合物制备. 在还原Rh2／SiO2催化剂上，CO吸附出现四个红外吸收峰：2085，2028cm－1（孪生态吸附RhⅠ（CO）2），2060cm－1（线式吸附RhⅡCO），1867cm－1（桥式吸附RhⅢ2CO）．在还原Rh2－V／SiO2催化剂上，CO在RhⅡ和RhⅢ中心上的吸附峰大大减弱，可以解释为Rh°向钒离子转移电子生成了带正电荷的铑中心（Rhδ＋）；同时RhⅠ（CO）2键能增加，降低了孪生CO被NO置换的程度。     

FTIR study of CO adsorption on Rh/MgO modified with Co, Ni, Fe, or CeO2 for the catalytic partial oxidation of methane

The surface state of Rh/MgO catalysts modified with Co, Ni, Fe, or CeO(2) after the reduction and partial oxidation pretreatments as well as during the catalytic partial oxidation of methane has been investigated by FTIR of adsorbed CO. The results of CO adsorption on the reduced catalysts suggest the formation of Rh-M alloy on Rh-M/MgO (M = Co, Ni, Fe) and Rh particles partially covered with reduced ceria on Rh-CeO(2)/MgO. The strength of CO adsorption on Rh/MgO is weakened by the modification with Co, Ni, Fe, or CeO(2). Partial oxidation pretreatment of Rh/MgO leads to a significant decrease in the CO adsorption due to the oxidation of Rh. In contrast, on partially oxidized Rh-M/MgO (M = Co, Ni, Fe) and Rh-CeO(2)/MgO, the preferential oxidation of the surface M atoms or reduced ceria maintains the metallic Rh and preserves the CO adsorbed on the surface Rh atoms. The CO adsorption during the reaction of catalytic partial oxidation of methane on Rh/MgO and Rh-Ni/MgO is similar to that on the reduced catalysts. On the other hand, the CO adsorption during the reaction on Rh-Co/MgO, Rh-Fe/MgO, and Rh-CeO(2)/MgO is different from that on the reduced catalysts, and this is related to the structural change of these catalysts during the reaction.     

The mechanism of emerging catalytic activity of gold nano-clusters on rutile TiO2(110) in CO oxidation reaction

This paper reveals the fact that the O adatoms (O(ad)) adsorbed on the 5-fold Ti rows of rutile TiO(2)(110) react with CO to form CO(2) at room temperature and the oxidation reaction is pronouncedly enhanced by Au nano-clusters deposited on the above O-rich TiO(2)(110) surfaces. The optimum activity is obtained for 2D clusters with a lateral size of ～1.5 nm and two-atomic layer height corresponding to ～50 Au atoms∕cluster. This strong activity emerging is attributed to an electronic charge transfer from Au clusters to O-rich TiO(2)(110) supports observed clearly by work function measurement, which results in an interface dipole. The interface dipoles lower the potential barrier for dissociative O(2) adsorption on the surface and also enhance the reaction of CO with the O(ad) atoms to form CO(2) owing to the electric field of the interface dipoles, which generate an attractive force upon polar CO molecules and thus prolong the duration time on the Au nano-clusters. This electric field is screened by the valence electrons of Au clusters except near the perimeter interfaces, thereby the activity is diminished for three-dimensional clusters with a larger size.