Affecting an Ultra‐High Work Function of Silver

An ultra‐high increase in the WF of silver, from 4.26 to 7.42 eV, that is, an increase of up to circa 3.1 eV is reported. This is the highest WF increase on record for metals and is supported by recent computational studies which predict the potential ability to affect an increase of the WF of metals by more than 4 eV. We achieved the ultra‐high increase by a new approach: Rather than using the common method of 2D adsorption of polar molecules layers on the metal surface, WF modifying components, l ‐cysteine and Zn(OH)₂, were incorporated within the metal, resulting in a 3D architecture. Detailed material characterization by a large array of analytical methods was carried out, the combination of which points to a WF enhancement mechanism which is based on directly affecting the charge transfer ability of the metal separately by cysteine and hydrolyzed zinc(II), and synergistically by the combination of the two through the known Zn‐cysteine finger redox trap effect.     

A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel‐cell electrodes, generally comes from S‐containing species such as hydrogen sulfide (H₂S). The S‐containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O₂ into gaseous SO₂. According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO₂ are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO₂ formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO₂ desorption at either room temperature or high temperatures.     

On‐surface Synthesis of a Semiconducting 2D Metal–Organic Framework Cu3(C6O6) Exhibiting Dispersive Electronic Bands

A 2D metal–organic framework (2D‐MOF) was formed on a Cu(111) substrate using benzenehexol molecules. By means of a combination of scanning tunneling microscopy and spectroscopy, X‐ray photoelectron spectroscopy and density‐functional theory, the structure of the 2D‐MOF is determined to be Cu₃(C₆O₆), which is stabilized by O–Cu–O bonding motifs. We find that upon adsorption on Cu(111), the 2D‐MOF features a semiconductor band structure with a direct band gap of 1.5 eV. The O–Cu–O bonds offer efficient charge delocalization, which gives rise to a highly dispersive conduction band with an effective mass of 0.45 m_e at the band bottom, implying a high electron mobility in this material.     

Effect of ZrS2 load single/dual-atom catalysts on the hydrogen evolution reaction: A first-principles study

The hydrogen evolution effect of ZrS₂ carrier loaded with transition metal single-atom (SA) was explored by first-principles method. ZrS₂ was constructed with transition metal single-atom and dual-atom. The structure–activity relationship of supported single-atom catalysts was described by electronic properties and hydrogen evolution kinetics. The results show that the ZrS₂ carrier-loaded atomic-level catalysts are more likely to occur in acidic environments, where the Mo SA load has a higher hydrogen precipitation capacity than the Pt SA. In the case of dual-atom adsorption, most of the hydrogen reduction processes are higher than that of single atom loading, which indicates that the outer orbital hybridization is more likely to lead to the interfacial charge recombination of the catalyst. Thereinto, Ni/Pt @ZrS₂ has the lowest Gibbs free energy (0.08 eV), and the synergistic effect of transition metals induces the deviation of the center of the d-band from the Fermi level and improves the dissociation ability of H ions. The design provides a new catalytic model for the HER and provides some ideas for understanding the two-site catalysis.     

Stability and Catalytic Performance of Single-Atom Supported on Ti2CO2 for Low-Temperature CO Oxidation: A First-Principles Study

Based on first-principles calculations, the potential of Ti₂CO₂ monolayer (MXene) as a single-atom catalyst (SAC) support for 3d transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) is studied for CO oxidation. We first screen the support effect according to the stability of a single metal atom and find that Sc and Ti supported on Ti₂CO₂ have stronger adsorption energy than the cohesive energy of their bulk counterparts and therefore, we selected Sc and Ti supported on Ti₂CO₂ for further catalytic reactions. The stability and the potential catalytic reactivity are verified by electronic structure and charge transfer analysis. Both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms are considered in this study, and lower energy barriers of 0.002 and 0.37 eV were found in the ER mechanism compared to the LH mechanism, which are 0.25 and 0.34 eV for Sc and Ti catalysts, respectively. Moreover, kinetic ER and LH mechanisms are favorable for both Sc- and Ti/Ti₂CO₂ because of the comparable energy barrier to other metals and SAC supported on 2D materials. However, Ti/Ti₂CO₂ catalyst is thermodynamically unfavorable. Based on these calculations, we propose that Sc supported on Ti₂CO₂ is the best catalyst for CO-oxidation. The current study not only broadens the scope of the single-atom Sc catalyst but also extends the consideration of MXene support for catalyst optimization.     

Morphological and heterojunctional engineering of two-dimensional porous Mo-Ni based catalysts for highly effective catalytic reduction of aromatic nitro compounds

Hydrogenation reactions play crucial roles on chemical synthesis and pollutant elimination. The improvement of the ability to activate reactants and increase of the contact probability between the catalysts and reactants are positive to improve the catalytic performance. Herein, we have reported the design of two-dimensional porous Ni-Ni₃N-Ni Mo N heterojunction sheets(2D Mo-Ni based nanosheets) for efficient catalytic hydrogenation of the aromatic nitro-compounds. The heterojunction ...     

On-surface Synthesis of a Semiconducting 2D Metal–Organic Framework Cu3(C6O6) Exhibiting Dispersive Electronic Bands

A 2D metal–organic framework (2D-MOF) was formed on a Cu(111) substrate using benzenehexol molecules. By means of a combination of scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and density-functional theory, the structure of the 2D-MOF is determined to be Cu₃(C₆O₆), which is stabilized by O–Cu–O bonding motifs. We find that upon adsorption on Cu(111), the 2D-MOF features a semiconductor band structure with a direct band gap of 1.5 eV. The O–Cu–O bonds offer efficient charge delocalization, which gives rise to a highly dispersive conduction band with an effective mass of 0.45 m_e at the band bottom, implying a high electron mobility in this material.     

Controllable n-type doping in WSe2 monolayer via construction of anion vacancies

The successful applications of two-dimensional (2D) transition metal dichalcogenides highly rely on rational regulation of their electronic properties. The nondestructive and controllable doping strategy is of great importance to implement 2D materials in electronic devices. Herein, we propose a straightforward and effective method to realize controllable n-type doping in WSe₂ monolayer by electron beam irradiation. Electrical measurements and photoluminescence (PL) spectra verify the strong n-doping in electron beam-treated WSe₂ monolayers. The n-type doping arises from the generation of Se vacancies and the doping degree is precisely controlled by irradiation fluences. Due to the n-doping-induced narrowing of the Schottky barrier, the current of back-gated monolayer WSe₂ is enhanced by an order of magnitude and a ∼8× increase in the electron filed-effect mobility is observed. Remarkably, it is a moderate method without significant reduction in electrical performance and severe damage to lattice structures even under ultra-high doses of irradiation.     

The electronic structure of the first-row transition-metal diborides

The electronic structures of ScB₂, TiB₂, VB₂, CrB₂ and MnB₂ have been examined by theoretical investigations. The band structures and accompanying density-of-states plots are presented. The calculated Fermi Levels are, ?5.6 eV (ScB₂), ?5.7 eV (TiB₂), ?6.3 eV (VB₂), ?7.1 eV (CrB₂), and ?7.8 eV (MnB₂). The valence bands at the Fermi Edge are localised about the metal 3d orbitals. The charge distributions of the diborides are obtained from the density-of-states plots and show that the metals possess the following positive charges: Sc (+2.28), Ti (+1.99), V (+1.85), Cr (+1.52), and Mn (+1.08). The bonding within the diborides is explained with the help of solid-state calculations at a Special Point and quasi-molecular cluster calculations.     

Structural and Chemical Properties of NiOx Thin Films: Oxygen Vacancy Formation in O2 Atmosphere

NiO_x films on Si(111) were put in contact with oxygen at elevated temperatures. During heating and cooling in oxygen atmosphere Near Ambient Pressure (NAP)-XPS and -XAS and work function (WF) measurements reveal the creation and replenishing of oxygen vacancies in dependence of temperature. Oxygen vacancies manifest themselves as a distinct O1s feature at 528.9 eV on the low binding energy side of the main NiO peak as well as by a distinct deviation of the Ni2p_3/2 spectral features from the typical NiO spectra. DFT calculations reveal that the presence of oxygen vacancies leads to a charge redistribution and altered bond lengths of the atoms surrounding the vacancies causing the observed spectral changes. Furthermore, we observed that a broadening of the lowest energy peak in the O K-edge spectra can be attributed to oxygen vacancies. In the presence of oxygen vacancies, the WF is lowered by 0.1 eV.     

铈基复合氧化物催化碳烟燃烧的性能及其H2-TPR研究

采用溶胶-凝胶法或浸渍法制备了不同金属离子掺杂的铈基复合氧化物催化剂,并采用热重法考察其催化碳烟燃烧的活性,借助H₂-TPR(程序升温还原)手段探讨了催化剂氧化还原性对碳烟燃烧性能的影响. 结果表明,过渡金属的掺杂促使催化剂在低温下提供更多的表面氧和晶格氧,显著降低了碳烟的氧化温度,催化剂于200~400℃释放的活性氧数量对于碳烟燃烧性能提高至关重要; 而结构性助剂金属、碱金属或碱土金属的掺入可提高中温活性氧数量,虽然对碳烟起燃温度无明显改善,但加快了碳烟的燃烧速率.     

Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part III. Effect of metal valence band and periodic trend of CO σ-π bonding functions on transition metals

Our model of metal valence band and our new concept of σ-π coordination are further discussed and confirmed in this paper.The infrared stretching frequencies of C-O decrease in the order 2056, 1886 and 1786 cm⁻¹ in Ni(CO)₄, Co(CO)₄⁻¹ and Fe(CO)₄⁻², which parallels the increase in d electron back-donation functions (B metal bonding functions) from 1.539, 2.121 to 2.895 on Ni, Co and Fe metals, respectively. On the other hand, the M-C bond orders increase from 1.33, 1.89 to 2.16 for Ni(CO)₄, Co(CO)₄⁻¹ and Fe(CO)₄⁻², which parallel the increase in A(CO5σ-Mσ)-B(CO2π-Mπ) metal bonding functions from 24.61, 30.01 to 33.19, respectively. They are in agreement with our new concept of σ-π coordination proposed in the previous paper. This new concept has also been used to analyze the mechanism of the formation of Ni(CO)₄, Co(CO)₄⁻¹ and Fe(CO)₄⁻², and to explain why they can automotively hybridize each other despite the energy differences between 3d and 4s, 4p, which are very large.The effects of metal valence bands have been accounted for on all transition metals (d¹ to d⁸), and it is demonstrated that d orbitals increase from the Vd band upward to the Vs band, and s orbitals from the Vs band downward to the Vd band, which is equivalent to a change in orbital potential, and would modify their orbital overlap integrals with the adsorbate M.O.s and the A, B metal bonding functions significantly. The effective potentials and the percentage s, d functions of Vs, Vd and d_occ bands are the most important factors for determining the effect of the metal valence band. The effects of promoter and support are also altered by changes in the above factors. For Group VIII metals, the valence band provides various s and d orbitals at various potentials, in which a certain number of s and d orbitals can match better with CO adsorbate M.O.s, which explains why CO adsorbed species on Group VIII metals are all stable and adsorption rates are all relatively rapid.The periodic trends of metal A, B, AB and D_c bonding functions depend on the structures of the metal valence band, i.e. the potential levels and s, d percentage functions of Vs, Vd and d_occ bands. For 4d and 5d metals, the potential levels of the Vs band are high, which cannot form a strong CO 5σ-M σ bond, but the potential levels of Vd band are higher and the width of the d band is wider than those of 3d metal, so their B bonding functions are larger, and they can be used to activate saturated and unsaturated hydrocarbons. In contrast, for 3d metals, the potentials of the Vs band are lower, which favour formation of strong CO 5σ-M σ and M-C bonds, i.e. their A and D_c bonding functions are larger, which can promote coke formation. While ABD_cD_o can be used to characterize CO dissociation, B/A can be used to characterize C-C formation.The characteristics of various metal bonding functions on each transition metal are useful for designing catalyst composition. A typical example has been illustrated, using the possibility to select non-noble metals instead of noble metals in hydrocarbon reactions.     

Direct correlation between work function of indium-tin-oxide electrodes and solar cell performance influenced by ultraviolet irradiation and air exposure

We report on reversible changes of the work function (WF) values of indium-tin-oxide (ITO) under prolonged ultraviolet (UV) and air exposure. The WF of ITO is reduced from 4.7 eV to 4.2 eV by photon absorption in ITO under UV illumination or an air mass 1.5 solar simulator (100 mW cm(-2)). Air or oxygen exposure is found to increase the WF of ITO (UV-exposed) to a value of 4.6 eV. These changes of ITO's WF lead to reversible variations of the performance of organic photovoltaic devices where ITO acts primarily as the electron collecting or hole collecting electrode. These variations can be reflected in the disappearance (or appearance) of an S-shaped kink in the J-V characteristics upon continuous UV or solar simulator illumination (or air exposure). This reversible phenomenon is ascribed to the adsorption and desorption of oxygen on the surface and grain boundaries of ITO. The use of surface modifiers to either decrease or increase the WF of ITO in organic photovoltaic devices with inverted and conventional geometries is also shown to be an effective route to stabilize the device performance under UV illumination.     

The two-dimensional band structure of (polyphthalocyaninato)Ni(II)

The two-dimensional (2D) band structure of (polyphthalocyaninato)Ni(II), Ni(ppc), has been analyzed by a self-consistent field (SCF ) Hartree–Fock (HF ) crystal orbital (CO ) formalism based on an INDO (intermediate neglect of differential overlap) type Hamiltonian. The calculated HF band gap of Ni(ppc) amounts to 0.24 eV. The highest filled band is a ringlike a_1u combination (D_4h symmetry label) localized at the carbon sites of the organic fragment. Remarkable hybridization in the valence band leads to the considerable band width Δ?_v of 2.92 eV. This value is close to the Δ?_v numbers which are conventionally encountered in one-dimensional metallomacrocycles. The effective width of the states in Ni(ppc) is 13.8 eV. In graphite a net π interval of 13.0 eV is predicted by the present CO formalism; i.e., the energetic distribution of the π electrons is roughly comparable in both 2D solids. The Ni 3d states in Ni(ppc) are far below the Fermi level which is calculated at ?4.9 eV; they are predicted between ?12.2 and ?16.4 eV in the mean-field approximation. Quasi-particle corrections lead to a significant shift of these strongly metal-centered states. Important electronic structure properties of Ni(ppc) are compared with those of 1D metallomacrocycles with similar molecular stoichiometry. The total density of states distribution of Ni(ppc) has been fragmented into projected (ligand π and σ, Ni 3d) contributions in order to allow for a transparent interpretation of the 2D band structure.     

Synthesis of Supported Bimetal Catalysts using Galvanic Deposition Method

Supported bimetallic catalysts have been studied because of their enhanced catalytic properties due to metal‐metal interactions compared with monometallic catalysts. We focused on galvanic deposition (GD) as a bimetallization method, which achieves well‐defined metal‐metal interfaces by exchanging heterogeneous metals with different ionisation tendencies. We have developed Ni@Ag/SiO₂ catalysts for CO oxidation, Co@Ru/Al₂O₃ catalysts for automotive three‐way reactions and Pd−Co/Al₂O₃ catalysts for methane combustion by using the GD method. In all cases, the catalysts prepared by the GD method showed higher catalytic activity than the corresponding monometallic and bimetallic catalysts prepared by the conventional co‐impregnation method. The GD method provides contact between noble and base metals to improve the electronic state, surface structure and reducibility of noble metals.     

Single-Atom-Mediated Spinel Octahedral Structures for Elevated Performances of Li-Oxygen Batteries

Li-oxygen batteries have attracted much attention due to its ultra-high theoretical specific capacity, but the discharge product Li₂O₂ is easy to accumulate, leading to low battery stability. Here, we demonstrate a series of high-efficiency cathode catalysts of Co₃O₄ loaded with single-atomic metals (M=Ru, Pd, Pt, Au, Ir). The single-atomic metal could substitute the central Co atom in the octahedral coordination structure and maintain the structural stability; benefiting from the electron promoter effect, rendering more highly active Co³⁺ exposed, providing rich nucleation sites for Li₂O₂ deposition. And the loaded M atoms could separate the active Co³⁺ centers, thereby regulating the dispersion of Li₂O₂ to obtained a sheet-like morphology, which could facilitate its decomposition in the subsequent charge cycle. Our work found that the single atoms could effectively modulate the active metal oxide with which it is coordinated, thus collectively boosting the catalytic performance.     

Design of oxygen reduction bimetallic catalysts: ab-initio-derived thermodynamic guidelines

The Gibbs free energies of key elementary steps for the electrocatalytic oxygen reduction reaction (ORR) are calculated with B3LYP type of density functional theory: O2 + M + H+ + e- (0 eV) --> HOO-M (deltaG1), HOO-M + M --> HO-M + O-M (deltaG2), O2 + 2M + H+ + e- (0 eV) --> O-M + HO-M (deltaG3), and HO-M + O-M + 3H+ + 3e- (0 eV) --> 2H2O + 2M (deltaG4), where H+ is modeled as H3(+)O(H2O)3 and M stands for the adsorption site of a metal catalyst modeled by a single metal atom as well as by an M3 cluster. Taking Pt as a reference, deltaG4 is plotted against deltaG1 for 17 metals from groups V to XII. It is found that no single metal has both deltaG1 and deltaG4 more negative than Pt, although some of them have either more negative deltaG1 or more negative deltaG4. This enables us to explain thermodynamically why no other single metal catalyzes the ORR as effectively as Pt does. Moreover, a thermodynamic analysis reveals that the signs of delta deltaG (the difference between deltaG of other metals and deltaG of Pt) strongly correlate with the valence electronic structure of metals, i.e., delta deltaG1 < 0 and delta deltaG4 > 0 for metals M with vacant valence d orbitals, whereas delta deltaG1 > 0 and delta deltaG4 < 0 for metals M' with fully occupied valence d orbitals. Thus, a simple thermodynamic rule for the design of bimetallic catalysts for the ORR is proposed: couple a metal M (delta deltaG1 < 0) with a second metal M' (delta deltaG4 < 0) to form an alloy catalyst MM'3. The rationale behind this selection is based on M being more efficient for the rate-determining step, i.e., for the formation of the adsorbed species M-OOH, while M' can enhance the reductions of O and OH in the last three electron-transfer steps.     

Influence of the surface structure on the initiation of nuclear-chemical processes under laser ablation of metals in aqueous media

It is shown that the efficiency of nucleochemical transformations under conditions of laser ablation of metals in aqueous media under the influence of picosecond laser pulses with peak intensity I _E ～ 10¹⁰–10¹³ W/cm² is largely determined by features of the metal’s surface relief in the region of high spatial frequencies (nanometer range) formed under these conditions. This is found through an atomic force microscopy study of the relief features of such surfaces formed with different laser ablation modes on specially prepared model samples. Analysis of the obtained images by means of flicker-noise spectroscopy with key 3D surface parameters in the nanometer range allow us to associate the rates of nuclear processes initiated upon laser ablation with sharpness factor as a measure of the chaotic constituent of the relief profile of a forming surface at the highest spatial frequencies. It is found that it is in the neighborhood of the greatest high-frequency irregularities of the surface that electric fields with the highest voltage that lowers the energy barrier to electron injection from the metal (the Frenkel effect) are located and the elevated values of mechanical tensile stresses that also contribute to reducing the work of an electron escaping from the metal (the Zhurkov effect) are found. It is concluded that the sharpness factor must play the key role in raising kinetic energy of electrons E _e to ～5–10 eV in the subsurface regions of low-temperature plasma formed upon laser ablation in the metal subsurface region in which the above nucleochemical transformations can occur.     

Catalytic Alkyne Semihydrogenation with Polyhydride Ni/Ga Clusters

The bimetallic, decanuclear Ni₃Ga₇-cluster of the formula [Ni₃(GaTMP)₃(μ²-GaTMP)₃(μ³-GaTMP)] ( 1 , TMP=2,2,6,6-tetramethylpiperidinyl) reacts reversibly with dihydrogen under the formation of a series of (poly-)hydride clusters 2 . Low-temperature 2D NMR experiments at −80 °C show that 2 consist of a mixture of a di- ( 2_Di ), tetra- ( 2_Tetra ) and hexahydride species ( 2_Hexa ). The structures of 2_Di and 2_Tetra are assessed by a combination of 2D NMR spectroscopy and DFT calculations. The cooperation of both metals is essential for the high hydrogen uptake of the cluster. Polyhydrides 2 are catalytically active in the semihydrogenation of 4-octyne to 4-octene with good selectivity. The example is the first of its kind and conceptually relates properties of molecular, atom-precise transition metal/main group metal clusters to the respective solid-state phase in catalysis.     

Extraction Equilibria between Benzene and Water of Uni- and Bivalent Metal Picrates with Lariat 16-Crown-5: Effect of the Side Arms on the Extraction Ability and Selectivity

The overall extraction constants (K_ex) of uni- andbivalent metal picrates with 15-(2,5-dioxahexyl)-15-methyl-16-crown-5(L16C5) were determined between benzene and water at 25°C. TheK_ex values were analyzed into the constituent equilibriumconstants, i.e., the extraction constant of picric acid, the distributionconstant of the crown ether, the stability constant of the metalion–crown ether complex in water, and the ion-pair extraction constantof the complex cation with the picrate anion. The K_ex valuedecreases in the orders Ag⁺ > Na⁺ >Tl⁺ > K⁺ > Li⁺ andPb²⁺ > Ba²⁺ > Sr²⁺ for theuni- and bivalent metals, respectively, which are the same as those observedfor 16C5. The extraction selectivity was found to be governed by theselectivity of the ion-pair extraction of the L16C5–metal picratecomplex rather than by that of the complex formation in water. Theextraction ability of L16C5 is smaller for all the metals than that of 16C5,which is mostly attributed to the higher lipophilicity of L16C5. Differencesin the extraction selectivity between L16C5 and 16C5 were observed for thebivalent metals but little for the univalent metals. The side-arm effect onthe extraction selectivity was interpreted on the basis of the negativecorrelation between the effect on the complex stability constant in waterand that on the ion-pair extraction constant.