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.     

Probe microscope observation of platinum atoms deposited on the TiO2(110)-(1 x 1) surface

Titanium dioxide (TiO2) (110) surfaces with Pt adatoms were examined using a noncontact atomic force microscope (NC-AFM) and a Kelvin probe force microscope (KPFM). Topographic images with NC-AFM identify Pt atoms adsorbed at three different sites. These sites are on the Ti atom rows, on the O atom rows, and in O atom vacancies. Most Pt adatoms were observed on Ti atom rows. Successively recorded images show that the Pt adatoms on Ti atom rows (adatoms A) and O atom rows (adatoms C) are mobile while the adatoms in the O atom vacancies (adatoms B) are not. Adatoms A and adatoms B were identified in KPFM images. However, adatoms C were not visualized in KPFM images because they moved quickly or were swept out by the tip. The KPFM measurements reveal that the work function on adatoms A are lower than that on the surrounding (1 x 1) surface by 0.24 eV whereas adatoms B reduced the work function by 0.26 eV. The work function decrease is interpreted with an electric dipole moment directed toward the vacuum, as a result of electron transfer from the adatoms to the TiO2 substrate. In an O atom vacancy, the adatom B is in contact with two Ti atoms and therefore the electron transfer can be enhanced.     

Estimating resonance enhancement of Raman scattering by metal adatom-adsorbate complexes

Publications on surface-enhanced Raman scattering (SERS) in metal sols are perused. The discrepancy between extinction spectra for freshly formed sols and the SERS excitation spectra is found to be connected with different temperature modes used in different procedures for obtaining sols, the temperature difference leading to different concentrations of metal adatoms on sol particles. Once the presence of adatoms is accounted for, the nature of “hot” sol particles, which ensure the observation of values of the SERS enhancement coefficient G?10¹⁴?10¹⁵, can be explained and the reasons for the scarcity of such particles can be established. On compact hot sol particles, rigorous calculations of electromagnetic enhancement G _em in most cases yield G _em≤ 10¹⁴. That is why combining the coefficient σ_b of gigantic amplification of background, which is caused by electron RS in metal and which is connected with the existence of adatoms, with the coefficient σ_si of SERS enhancement in a metal-metal adatom-adsorbate adsorption complex gives σ_bσ_si ≥ 10⁸.     

Quantum chemical investigation of the interaction of the Pt<Subscript>6</Subscript> cluster with oxides of different nature

The interaction of a Pt₆ nanoparticle with different oxide supports, viz., γ-Al₂O₃, FAU and MFI zeolites, was investigated using the density functional theory. The interaction with the basic oxygen anions of the lattice and with hydroxyl groups of the support affects the electronic structure of the metal particles. The transfer of H atoms of the hydroxyl groups to the metal particle suppresses the Brönsted acidity of the support, and the activation energy of proton transfer decreases with an increase in the acidity of the support. The potential energy profiles were calculated for the transfer processes, and changes in the electronic structures and charge distribution of the supported particles were outlined. The H atom transfer results in positive charging by the metal particles, whereas the interaction with basic sites leads to the appearance of electron-enriched metal clusters.     

Bivalent ability for charge transfer in process of hydrogen interaction with surfaces of transition metals

The ability for bivalent charge transfer (CT) during hydrogen adsorption on transition metal surfaces and in the course of transition metal hydride formation is discussed. The change of the dipole moment of hydrogen adatoms, caused by transition from a strongly bound adsorption state to a weakly bound state, is demonstrated. The possibility of the CT reversion between the hydrogen adsorbate and the transition metal adsorbent, caused by a change of the surface structure, is described. The CT within the adsorbate, corresponding to the distinguished steps of the process of transition metal hydride formation, is shown.     

The Driving Force of Photoinduced Charge Separation in Metal‐Cluster‐Encapsulated Triphenylamine‐[80]fullerenes

Understanding photoinduced charge separation in fullerene‐based dye‐sensitized solar cells is crucial for the development of photovoltaic devices. We investigate here how the driving force of the charge separation process in conjugates of M@C₈₀ (M=Sc₃N, Sc₃CH, Sc₃NC, Sc₄O₂, and Sc₄O₃) with triphenylamine (TPA) depends on the nature of the metal cluster. Both singlet and triplet excited‐state electron‐transfer reactions are considered. These results based on TD‐DFT calculations demonstrate that the driving force of charge separation in TPA‐M@C₈₀ can be tuned well by varying the structure of the metal cluster encapsulated inside the fullerene cage.     

对叔丁基杯[8]芳烃/钼磷杂多酸纳米杂化LB膜的制备与表征

以含有共轭大π键的对叔丁基杯[8]芳烃(Cal8)和十八(烷)铵(ODA)与Keggin结构和Dawson结构钼磷杂多酸(HPA)做成膜材料, 用LB技术制备了2种新型无机-有机杂化LB膜. 用π-A曲线、UV-Vis吸收光谱、荧光光谱和原子力显微镜(AFM)对标题LB膜的成膜性质、结构及发光性质进行了表征. 结果表明在空气/水界面Cal8/ODA/HPA杂化可形成稳定的单层与多层膜. 标题杂化LB膜的崩溃压为39.0 mN/m, 其粒子具有纳米尺寸, 在激发波长为280 nm时, 可观察到322～387 nm处由Cal8分子π-π*跃迁引起的荧光发射峰及510 nm处 杂多阴离子的配体到金属的荷移跃迁(LMCT)三重发射谱带.     

On the role of electrostatic forces in the adhesion of polymer particles to solid surfaces

Simultaneous measurements have been made of the adhesive force and double electric charge of particles after their removal from a metal surface. For the systems investigated, the adhesive force and charge on the particles increase with particle diameter according to a power law with an exponent close to 2. Such dependence can be explained on the basis of the electrostatic nature of the adhesive forces. A double electric layer exists at the interface between the particles and the metal surface. A calculation was made of the surface density of charge for the polyvinyl chloride particle-steel system.     

Changes in the interaction characteristics of polyelectrolyte complex covered silica surfaces

The adsorption of polyelectrolyte complexes, PEC, made from the cationic poly (diallyldimethylammonium) chloride (PDADMAC) and the anionic maleic acid-co-propene copolymer (MA-P) on a Si-wafer surface has been studied. The application of highly diluted colloidally dispersed PEC solutions led to the deposition of single PEC particles onto the surface of the Si-wafer. The interaction forces of the heterogeneously covered surface were monitored by direct force measurements with an atomic force microscope (AFM) in the force volume mode. On the surface of a single PEC particle drastic changes in the interaction forces were found in comparison with the unmodified Si-wafer: in all force vs. distance curves a strong increase of the adhesion was measured that can be attributed to the formation of electrostatic bonds between the negatively charged Si₃N₄-tip and the cationic excess charge of the PEC. Additionally, the behavior during approach of both surfaces has been distinct: at pH 6.1 we see a long range electrostatic attraction between the tip and the PEC particle. The attraction becomes even stronger at pH 4.1, because of an increased positive net charge. Generally, a heterogeneous surface with a wide variety of interaction features can be created by the adsorption of PEC particles.     

New complex transition metal carbides formed on an electron microscope heating stage

Particles shown to be complex carbides of copper. nickel, molybdenum.tungsten. gold, silver and titanium along with carbides of the elements in stainless steel, were formed on an electron microscope heating stage. These particles were observed to form on thin carbon films coating the electron microscope grids of the metals listed above. The d-spacings measured for these particles are essentially the same as those that are observed with a mixture of the complex carbides of general formula M₆C and M₂₃C₆. Electron microprobe analysis was performed on the particles formed in the microscope. and the analysis confirmed the presence of the respective metals within the particles. In general, the various metal carbides were observed to form at different temperatures on different metal grids.     

Modification of Surface Forces by Metal Ion Adsorption

Abstract

Sorption of ions may lead to variations in interparticle forces and, thus, changes in the stability of colloidal particles. Chemical interactions between metal ions and colloidal particles modify the molecular structure of the surface, the surface charge, and the electrical potential between colloidal particles. These modifications to the surface and to the electrical double layer due to metal ion sorption are reflected in the interaction force between a particle and another surface, which is measured in this study by atomic force microscopy (AFM). Specifically, AFM is used to investigate the sorption of copper ions from aqueous solutions by silica particles. The influence of metal ion concentration and solution ionic strength on surface forces is studied under transient conditions. Results show that as the metal ion concentration is decreased, charge reversal occurs and a longer period of time is required for the system to reach equilibrium. The ionic strength has no significant effect on sorption kinetics. Furthermore, neither metal concentration nor ionic strength exhibits any effect on sorption equilibria, indicating that for the experimental conditions used in this study, the surface sites of the silica particle are fully occupied by copper ions.     

Contact electroresistance of metals in aqueous solutions during the anion adsorption involving charge transfer

Notions about charge transfer during adsorption of anions on metals in aqueous solutions are rendered. The role played by the electron tunneling on macrocontacts during the signal formation in the method of contact electroresistance (CER) is considered. It is shown that CER depends on the metal surface coverage by adsorbed species and their effective charge. Bell-like CER vs.E curves are obtained for copper, silver, and gold in solutions containing halide ions. Potentials of maximums in the curves,E _max, correspond to the charge transfer onset and depend on the nature of the metal and anion and on the anion concentration. AtE belowE _max, halides adsorb in the form of ions, involving no substantial charge transfer. At potentials exceedingE _max by 0.1 to 0.2 V, practically complete charge transfer occurs. With changing anion nature,E _max for a given metal rises in the series I^- < Br^- ≪ Cl^-. For a given anion (say, I^-),E _max increases with the metal nature in the series Cu ≤Ag ≪ Au. The link between the charge transfer during adsorption of anions and the surface reconstruction in single-crystal electrodes is discussed.     

Electronic Oxide–Metal Strong Interaction (EOMSI)

Strong metal–support interaction of supported metal catalysts is an important concept to describe the effect of metal–support interactions on the structures and catalytic performances of supported metal particles. By using an example of CeO_x adlayers supported on Ag nanocrystals, herein a concept of electronic oxide–metal strong interaction (EOMSI) is put forward; this interaction significantly affects the electronic structures of oxide adlayers through metal-to-oxide charge transfer. The EOMSI can stabilize oxide adlayers in a low oxidation state under ambient conditions, which individually are not stable; moreover, the oxide adlayers experiencing the EOMSI are resistant to high-temperature oxidation in air to a certain extent. Such an EOMSI concept helps to generalize the strong influence of oxide–metal interactions on the structures and catalytic performance of oxide/metal inverse catalysts, which have been attracting increasing attention.     

Indirect adatom interactions via III–V semiconductor substrates

Substrate-mediated interactions between adatoms on III–V semiconductors are investigated by using the self-consistent Anderson–Newns model in the Hartree–Fock approximation. The Green function formalism of the Dyson equation approach is employed to derive Chebyshev polynomial expressions for the chemisorption energy, interaction energy, and charge transfer, in terms of the adatom separation d. An alternating s- and p-orbital model of GaSb and InAs enabled interacting hydrogen adatoms on their (100) and (111) faces to be studied. As in the metal–substrate case, the chemisorption energy decreased with increasing band widths and adbond energy and, additionally, with increasing band gap. The interaction energy was found to have a d⁻² damping factor for the (100) faces and a d⁻³ factor for the (111) faces, its magnitude being larger for smaller gaps. Self-consistency is shown to play a significant role in interaction energy calculations for small values of d. In the case of charge transfer, its variation with d is purely a self-consistent result. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 377–397, 1998.     

Integration of Plasmonic Effects and Schottky Junctions into Metal–Organic Framework Composites: Steering Charge Flow for Enhanced Visible‐Light Photocatalysis

A wide range of light absorption and rapid electron–hole separation are desired for efficient photocatalysis. Herein, on the basis of a semiconductor‐like metal–organic framework (MOF), a Pt@MOF/Au catalyst with two types of metal–MOF interfaces integrates the surface plasmon resonance excitation of Au nanorods with a Pt‐MOF Schottky junction, which not only extends the light absorption of the MOF from the UV to the visible region but also greatly accelerates charge transfer. The spatial separation of Pt and Au particles by the MOF further steers the formation of charge flow and expedites the charge migration. As a result, the Pt@MOF/Au presents an exceptionally high photocatalytic H₂ production rate by water splitting under visible light irradiation, far superior to Pt/MOF/Au, MOF/Au and other counterparts with similar Pt or Au contents, highlighting the important role of each component and the Pt location in the catalyst.     

Catalytic Activity of Thallium, Lead, and Bismuth Adatoms in the Gold Dissolution Reaction in Cyanide Solutions: A Comparative Characterization

Quartz microgravimetry is used to determine the ratio between coefficients of mass transfer (1 : 0.54 : 0.48), which characterizes relative values of rates of diffusion of hydroxy complexes of thallium, lead, and bismuth in alkaline solutions. The ratio is used when refining the condition under which on a renewable electrode in solutions containing these ions at the concentration c _i the electrode coverage by relevant adatoms _i with increasing duration of contact of the electrode with solution reaches constant values: (c _Tl t) = (0.54c _Pb t) = (0.48c _Bi t). Measured are i,t curves on a renewable gold electrode at E = const in solutions containing 0.1 M KCN, 0.1 M KOH, 0.01 M KAu(CN)₂, and 8 × 10^–6 M compounds of thallium or 1.5 × 10^–5 M, lead, or 1.6 × 10^–5 M, bismuth. Shown is that at (, E) = const the currents of dissolution of gold in these solutions increase in the series Tl < Pb < Bi, which evidences an increase in this series of the catalytic activity of adatoms of these metals. Shown is that at = const the catalytic action of adatoms of thallium and bismuth has an approximately additive character. The obtained data are analyzed with allowance made for the explanation offered earlier for the catalytic effect of adatoms on the anodic dissolution of gold based on the hypothesis about the shift of the potential of the free zero charge in the negative direction after substituting a metal atom for chemisorbed cyanide ions.     

Ferroelectric Polarization Modulated Facet-selective Charge Separation in Bi4NbO8Cl Single Crystal for Boosting Visible-light Driven Bifunctional Water Splitting

Developing bifunctional water-splitting photocatalysts is meaningful, but challenged by the harsh requirements of specific-facet single crystals with spatially separated reactive sites and anisotropic charge transfer paths contributed by well-built charge driving force. Herein, tunable ferroelectric polarization is introduced in Bi₄NbO₈Cl single crystal nanosheets to strengthen the orthogonal charge transfer channels. By manipulating the in-plane polarization from octahedral off-centering of Nb⁵⁺ and out-of-plane polarization from lone pair electron effect of anisotropic Bi³⁺, both the fast charge recombination in bulk catalyst and the process of charge trapping into surface states can be effectively modulated. Collaborating with modest polarization electric field and facet junction induced built-in electric field, cooperative charge tractive force is constructed, which reinforces the spatial separation and migration of photogenerated electrons and holes to {110} reductive site facet and {001} oxidation site facet, respectively. While excessive polarization charges impair the facet-selective charge separation characteristics and conversely promote charge recombination on the surface. As a result, polarity-optimized Bi₄NbO₈Cl shows an excellent H₂ and O₂ evolution rate of 54.21 and 36.08 μmol ⋅ h⁻¹ in the presence of sacrificial reagents under visible light irradiation. This work unveils the function of ferroelectric polarization in tuning the intrinsic facet-selective charge transfer process of photocatalysts.     

The influence of driving force on intramolecular electron transfer: A theoretical study of subphthalocyanine-AzaBODIPY-C60 supramolecular triad

The driving force of electron transfer is one of important factors for initializing inter- and intramolecular charge separation. In this work, the main goal is to understand how driving force determines electron transfer pathway in subphthalocyanine-AzaBODIPY-C₆₀ supramolecular triad. Experimental observations have suggested that there are only two intramolecular charge transfer states (subPC⁺-AzaBODIPY⁻-C₆₀ and subPC⁺-AzaBODIPY-C₆₀⁻) after photon absorption, where subPC is the donor. Through the calculations by using tuned long range corrected density functionals with polarizable continuum model, we find two more new intramolecular charge transfer states: subPC⁻-AzaBODIPY⁺-C₆₀ and subPC-AzaBODIPY⁺-C₆₀⁻, where AzaBODIPY is the donor. We compare the HOMO/LUMO energy of subPC, AzaBODIPY, and C₆₀ monomers to their corresponding orbital energy in the triad. The results indicate that the driving force (HOMO/LUMO energy offsets) is not enough for electron transfer from AzaBODIPY to subPC or C₆₀, which can explain why subPC⁻-AzaBODIPY⁺-C₆₀ and subPC-AzaBODIPY⁺-C₆₀⁻ intramolecular charge transfer states cannot be observed in the experiment. In addition, this work may provide a simple and practical method to find the intramolecular charge transport pathway of a supramolecule.     

Surface heterogeneity of polystyrene latex particles determined by dynamic force microscopy

Atomic force microscopy (AFM) was employed to characterize the surface chemistry distribution on individual polystyrene latex particles. The particles were obtained by surfactant-free emulsion polymerization and contained hydrophilic quaternary ammonium chloride, sodium sulfonate, or hydroxyethyl groups. The phase shift in dynamic force mode AFM is sensitive to charge/chemical interactions between an oscillating atomic force microscope tip and a sample surface. In this work, the phase imaging technique distinguished phase domains of 50-100 nm on the surfaces of dried latex particles in ambient air. The domains are attributed to the separation of ion-rich and ion-poor components of the polymer on the particle surface.     

Exfoliated transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2): An electrochemical impedance spectroscopic investigation

Owing to the enthralling properties which transition metal dichalcogenides present, they are facing immense scientific interest from researchers. Till date, these two-dimensional materials have been assessed for a wide array of different applications and there are various synthetic methods of attaining them in their respective bulk and exfoliated forms. Herein, we explore the effects of lithium ion intercalation exfoliation process on the charge transfer resistance of transition metal dichalcogenide materials (MoS₂, MoSe₂, WS₂ and WSe₂). We also show that electrochemical activation of the transition metal dichalcogenides results in decreased resistance towards charge transfer, as demonstrated by electrochemical impedance spectroscopy.