Unveiling the Morphology of Carbon-Supported Ru Nanoparticles by Multiscale Modeling.

Simulating the behavior of metal nanoparticles on supports is crucial for boosting their catalytic performance and various nanotechnology applications; however, such simulations are limited by the conflicts between accuracy and efficiency. Herein, we introduce a multiscale modeling strategy to unveil the morphology of Ru supported on pristine and N-doped graphene. Our multiscale modeling started with the electronic structures of a supported Ru single atom, revealing the strong metal-support interaction around pyridinic nitrogen sites. To determine the stable configurations of Ru2-13 clusters on three different graphene supports, global energy minimum searches were performed. The sintering of the global minimum Ru13 clusters on supports was further simulated by ab initio molecular dynamics (AIMD). The AIMD data set was then collected for deep potential molecular dynamics to study the melting of Ru nanoparticles. This study presents comprehensive descriptions of carbon-supported Ru and develops modeling approaches that bridge different scales and can be applied to various supported nanoparticle systems.

Selective Orbital Coupling: An Adsorption Mechanism in Single-Atom Catalysis.

Quantitative understanding of the chemisorption on single-atom catalysts (SACs) by their electronic properties is crucial for the catalyst design. However, the physical mechanism is still under debate. Here, the CO catalytic oxidation on single transition metal (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni) dopants is used as a theoretical model to explore the correlations between the characteristics of electronic structures and the chemisorption on SACs. For these metal dopants, their atomic d orbitals form several nondegenerate and localized electronic states that are found to be selectively coupled with the π* orbital of the adsorbed O2, which we defined as selective orbital coupling. Based on the selective orbital coupling, we find that the alignment between the selected d state and the π* state determines the bond strength, regardless of the electron occupation number of the selected d states; the electron transfer to form M-O bonding can be provided by the support. Such electron transfer can be related with the electronic metal-support interaction. We attribute the origin of the chemisorption mechanism to the coexistence of the localized orbital of the single transition metal and the continuous energy band of the Au support. Finally, we illustrate how this mechanism dominates the variation trend of the reaction barriers. Our results unravel a fundamental adsorption mechanism in SAC systems.

Mechanistic understanding of N2 activation: a comparison of unsupported and supported Ru catalysts.

N2 dissociative adsorption is commonly the rate-determining step in thermal ammonia synthesis. Herein, we performed density functional theory (DFT) calculations to understand the N2 dissociation mechanism on models of unsupported Ru(0001) terraces, Ru B5 sites, and polar MgO(111)-supported Ru8 cluster mimicking a B5 site geometry, denoted (Ru8(B5-like)/MgO(111)). The activation energy of N2 dissociative adsorption on the Ru8(B5-like)/MgO(111) model (Ea = 0.33 eV) is much lower than that on the unsupported Ru(0001) terrace (Ea = 1.74 eV) and Ru B5 (Ea = 0.62 eV) models. The lower N2 dissociation barrier on Ru B5 sites is facilitated by the enhanced σ donation and π* back-donation between N2(σ, π*) and Ru(d) orbitals resulting in the stronger activation of the molecular side-on N2* dissociation precursor. The Ru8(B5-like)/MgO(111) also exhibits enhanced σ donation because of the B5-like cluster geometry. Furthermore, the Ru cluster of the bare Ru8(B5-like)/MgO(111) model is positively charged. This induced an unusual π donation from N2(π) to Ru(d) orbitals as revealed by analyses of the density of states and partial charge densities. The combined σ and π donation resulted in an increased synergistic π* back-donation. The total interactions between N2(σ, π, π*) and Ru(d) resulted in an overall electron transfer to the adsorbed N2 from the Ru atoms in the B5-like site with no direct involvement of the MgO(111) substrate. Analyses of bond stretching vibrations and bond lengths show that the N2(σ, π, π*) and Ru(d) interactions lead to a weaker N-N bond and stronger Ru-N bonds. These correspond to a lower barrier of N2 dissociation on the Ru8(B5-like)/MgO(111) model, where the highest red-shift of N-N vibration and the longest N-N bond length were observed after side-on N2* adsorption. These results demonstrate that an electron-deficient Ru catalyst are not always inhibited from donating electrons to adsorbed N2. Rather, this study shows that the electron deficiency of Ru can promote π* back-donation and N2 activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.

Twin-Boundary Reduced Surface Diffusion on Electrically Stressed Copper Nanowires.

Surface diffusion is intimately correlated with crystal orientation and surface structure. Fast surface diffusion accelerates phase transformation and structural evolution of materials. Here, through in situ transmission electron microscopy observation, we show that a copper nanowire with dense nanoscale coherent twin-boundary (CTB) defects evolves into a zigzag configuration under electric-current driven surface diffusion. The hindrance at the CTB-intercepted concave triple junctions decreases the effective surface diffusivity by almost 1 order of magnitude. The energy barriers for atomic migration at the concave junctions and different faceted surfaces are computed using density functional theory. We proposed that such a stable zigzag surface is shaped not only by the high-diffusivity facets but also by the stalled atomic diffusion at the concave junctions. This finding provides a defect-engineering route to develop robust interconnect materials against electromigration-induced failures for nanoelectronic devices.

Cluster Nanozymes with Optimized Reactivity and Utilization of Active Sites for Effective Peroxidase (and Oxidase) Mimicking.

Single-atom catalysts have attracted attention in the past decade since they maximize the utilization of active sites and facilitate the understanding of product distribution in some catalytic reactions. Recently, this idea has been extended to single-atom nanozymes (SAzymes) for the mimicking of natural enzymes such as horseradish peroxidase (HRP) often used in bioanalytical applications. Herein, it is demonstrated that those SAzymes without constructing the reaction pocket of HRP still undergo the OH radical-mediated pathway like most of the reported nanozymes. Their positively charged single-atom centers resulting from support electronegative oxygen/nitrogen hinder the reductive conversion of H2 O2 to OH radicals and hence display low activity per site. In contrast, it is found that this step can be facilitated over their metallic counterparts on cluster nanozymes with much higher site activity and atom efficiency (cf. SAzymes with 100% atom utilization). Besides the mimicking of HRP in glucose detection, cluster nanozymes are also demonstrated as a better oxidase mimetic for glutathione detection.

Rapid Interchangeable Hydrogen, Hydride, and Proton Species at the Interface of Transition Metal Atom on Oxide Surface.

Hydrogen spillover is the phenomenon where a hydrogen atom, generated from the dissociative chemisorption of dihydrogen on the surface of a metal species, migrates from the metal to the catalytic support. This phenomenon is regarded as a promising avenue for hydrogen storage, yet the atomic mechanism for how the hydrogen atom can be transferred to the support has remained controversial for decades. As a result, the development of catalytic support for such a purpose is only limited to typical reducible oxide materials. Herein, by using a combination of in situ spectroscopic and imaging technique, we are able to visualize and observe the atomic pathway for which hydrogen travels via a frustrated Lewis pair that has been constructed on a nonreducible metal oxide. The interchangeable status between the hydrogen, proton, and hydride is carefully characterized and demonstrated. It is envisaged that this study has opened up new design criteria for hydrogen storage material.

Functionalization of Polyethyleneimine with Hollow Cyclotriveratrylene and Its Subsequent Supramolecular Interaction with Doxorubicin.

In this paper, a modified Cyclotriveratrylene was synthesized and linked to a branched Polyethylenimine, and this unique polymeric material was subsequently examined as a potential supramolecular carrier for Doxorubicin. Spectroscopic analysis in different solvents had shown that Doxorubicin was coordinated within the hollow-shaped unit of the armed Cyclotriveratrylene, and the nature of the host-guest complex revealed intrinsic Van der Waals interactions and hydrogen bonding between the host and guest. The strongest interaction was detected in water because of the hydrophobic effect shared between the aromatic groups of the Doxorubicin and Cyclotriveratrylene unit. Density functional theory calculations had also confirmed that in the most stable coordination of Doxorubicin with the cross-linked polymer, the aromatic rings of the Doxorubicin were localized toward the Cyclotriveratrylene core, while its aliphatic chains aligned closer with amino groups, thus forming a compact supramolecular assembly that may confer a shielding effect on Doxorubicin. These observations had emphasized the importance of supramolecular considerations when designing a novel drug delivery platform.

Nitrogen-doped carbon nanodots prepared from polyethylenimine for fluorometric determination of salivary uric acid.

Stable and low-cost carbon dots (C-dots) were prepared from polyethylenimine (PEI) by a hydrothermal method. It is found that the fluorescence of the C-dots (best measured at excitation/emission wavelengths of 365/473 nm) is quenched by selective oxidation of surface PEI by periodate but recovers in the presence of uric acid (UA). It is assumed that this is due to the selective reduction of the nitrone groups to hydroxylamine groups by UA. The findings were used to design a fluorometric method for determination of UA that has a 2.3 nM detection limit. This is lower than that of reported fluorometric and enzymatic assays. The performance of the method has been validated by determination of UA in samples of human saliva. It is found that the results agree well with those obtained by a commercial UA assay. Graphical abstract Schematic presentation of the polyethylenimine (PEI) carbon nanodots (C-dots) as a fluorescent probe for uric acid. Their fluorescence is quenched by periodate (IO4-) due to oxidative formation of nitrone groups, an subsequently restored due to reduction by uric acid (UA).

TiO2 and ZrO2 in biomass conversion: why catalyst reduction helps.

Biomass refers to plant-based materials that are not used for food or feed. As an energy source, lignocellulosic biomass (lignin, cellulose and hemicellulose) can be converted into various forms of biofuel using thermal, chemical and biochemical methods. Chemical conversion implies the use of solid catalysts, usually oxide materials. In this context, reducible oxides are considered to be more active than non-reducible oxides. But why? Using density functional theory DFT + U calculations with the inclusion of dispersion forces, we describe the properties of anatase TiO2, a reducible oxide, and tetragonal ZrO2, a non-reducible oxide, the (101) surfaces in this context. In particular, we focus on the role of surface reduction, either by direct creation of oxygen vacancies via O2 desorption, or by treatment in hydrogen. We show that the presence of reduced centres on the surface of titania or zirconia (either Ti3+ or Zr3+ ions, or oxygen vacancies) results in lower barriers and more stable intermediates in two key reactions in biomass catalytic conversion: ketonization of acetic acid (studied on ZrO2) and deoxygenation of phenol (studied on TiO2). We discuss the role of Ru nanoparticles in these processes, and in particular in favouring H2 dissociation and hydrogen spillover, which results in hydroxylated surfaces. We suggest that H2O desorption from the hydroxylated surfaces may be a relevant mechanism for the regeneration of oxygen vacancies, in particular on low-coordinated sites of oxide nanoparticles. Finally, we discuss the role of nanostructuring in favouring oxide reduction, by discussing the properties of ZrO2 nanoparticles of diameter of about 2 nm.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Size-Dependent Penetration of Gold Nanoclusters through a Defect-Free, Nonporous NaCl Membrane.

Membranes and their size-selective filtering properties are universal in nature and their behavior is exploited to design artificial membranes suited for, e.g., molecule or nanoparticle filtering and separation. Exploring and understanding penetration and transmission mechanisms of nanoparticles in thin-film systems may provide new opportunities for size selective deposition or embedding of the nanoparticles. Here, we demonstrate an unexpected finding that the sieving of metal nanoparticles through atomically thin nonporous alkali halide films on a metal support is size dependent and that this sieving effect can be tuned via the film thickness. Specifically, relying on scanning tunneling microscopy and spectroscopy techniques, combined with density functional theory calculations, we find that defect-free NaCl films on a Au(111) support act as size-dependent membranes for deposited Au nanoclusters. The observed sieving ability is found to originate from a driving force toward the metal support and from the dynamics of both the nanoparticles and the alkali halide films.

Iridicycle-Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism.

The mechanism of imine reduction by formic acid with a single-site iridicycle catalyst has been investigated by density functional theory (DFT), NMR spectroscopy, and kinetic measurements. The NMR and kinetic studies suggest that the transfer hydrogenation is turnover-limited by the hydride formation step. The calculations reveal that, amongst a number of possibilities, hydride formation from the iridicycle and formate probably proceeds by an ion-pair mechanism, whereas the hydride transfer to the imino bond occurs in an outer-sphere manner. In the gas phase, in the most favourable pathway, the activation energies in the hydride formation and transfer steps are 26-28 and 7-8 kcal mol(-1) , respectively. Introducing one explicit methanol molecule into the modelling alters the energy barrier significantly, reducing the energies to around 18 and 2 kcal mol(-1) for the two steps, respectively. The DFT investigation further shows that methanol participates in the transition state of the turnover-limiting hydride formation step by hydrogen-bonding to the formate anion and thereby stabilising the ion pair.

A DFT Study of the Reactivity of Anatase TiO2 and Tetragonal ZrO2 Stepped Surfaces Compared to the Regular (101) Terraces.

It is generally assumed that low-coordinated sites at extended defects of oxide surfaces like steps or edges are more reactive than the regular, fully coordinated sites at the flat terraces. In this work we have considered the properties of stepped surfaces of anatase TiO2 and tetragonal ZrO2 by means of periodic DFT+U calculations. For both oxides, the stability of oxygen vacancies located near the step edges is compared to that of the same defects at the regular terraces. The capability of the steps to induce nucleation of metal nanoparticles on the surface has been evaluated by simulating the adsorption of a single ruthenium adatom. We conclude that, for anatase, step edges have no particular role in favouring the reduction of the oxide by reducing the cost for oxygen abstraction; in the same way, there is no special role of the stepped anatase surface in stabilizing adsorbed Ru atoms. On the contrary, step edges on zirconia display some capability to stabilise oxygen vacancies and ruthenium adatoms.

Properties of two-dimensional insulators: a DFT study of Co adsorption on NaCl and MgO ultrathin films.

Recent experimental and theoretical results have shown that Co atoms deposited on ultrathin NaCl films grown on Au(111) result in spontaneous substitutional doping of the two-layer insulating material (Li et al., Phys. Rev. Lett., 2014, 112, 026102). This result opens the general question of the reactivity of transition metal (TM) atoms with ultrathin films consisting of few atomic layers. In this article, density functional theory with and without dispersion corrections has been used to compare the adsorption of Co atoms on various sites of unsupported and supported NaCl and MgO two-layer (2L) films. We found that Co interacts strongly with NaCl/Au(111) 2L films, and that Co incorporation in interstitial positions between the first and second NaCl layers is thermodynamically preferred compared to adsorption on the surface sites. Differently from NaCl, Co adsorbs preferentially on top of O in both unsupported and supported MgO 2L films. Co incorporation into the interstitial sites of MgO is highly unfavorable. These results show that the reactivity of TM atoms like Co is completely different on NaCl or MgO ultrathin films. The reasons for this difference, the role of dispersion, and the peculiar properties of two-dimensional insulators are discussed.

Harnessing Quantum Capacitance in 2D Material/Molecular Layer Junctions for Novel Electronic Device Functionality.

Two-dimensional (2D) materials promise advances in electronic devices beyond Moore's scaling law through extended functionality, such as non-monotonic dependence of device parameters on input parameters. However, the robustness and performance of effects like negative differential resistance (NDR) and anti-ambipolar behavior have been limited in scale and robustness by relying on atomic defects and complex heterojunctions. In this paper, we introduce a novel device concept that utilizes the quantum capacitance of junctions between 2D materials and molecular layers. We realized a variable capacitance 2D molecular junction (vc2Dmj) diode through the scalable integration of graphene and single layers of stearic acid. The vc2Dmj exhibits NDR with a substantial peak-to-valley ratio even at room temperature and an active negative resistance region. The origin of this unique behavior was identified through thermoelectric measurements and ab initio calculations to be a hybridization effect between graphene and the molecular layer. The enhancement of device parameters through morphology optimization highlights the potential of our approach toward new functionalities that advance the landscape of future electronics.

Atomic Scaled Depth Correlation to the Oxygen Reduction Reaction Performance of Single Atom Ni Alloy to the NiO2 Supported Pd Nanocrystal.

This study demonstrates the intercalation of single-atom Ni (NiSA ) substantially reduces the reaction activity of Ni oxide supported Pd nanoparticle (NiO2 /Pd) in the oxygen reduction reaction (ORR). The results indicate the transition states kinetically consolidate the adsorption energy for the chemisorbed O and OH species on the ORR activity. Notably, the NiO2 /Ni1 /Pd performs the optimum ORR behavior with the lowest barrier of 0.49 eV and moderate second-step barrier of 0.30 eV consequently confirming its utmost ORR performance. Through the stepwise cross-level demonstrations, a structure-Eads -ΔE correspondence for the proposed NiO2 /Nin /Pd systems is established. Most importantly, such a correspondence reveals that the electronic structure of heterogeneous catalysts can be significantly differed by the segregation of atomic clusters in different dimensions and locations. Besides, the doping-depth effect exploration of the NiSA in the NiO2 /Pd structure intrinsically elucidates that the Ni atom doping in the subsurface induces the most fruitful NiSA /PdML synergy combining the electronic and strain effects to optimize the ORR, whereas this desired synergy diminishes at high Pd coverages. Overall, the results not only rationalize the variation in the redox properties but most importantly provides a precision evaluation of the process window for optimizing the configuration and composition of bimetallic catalysts in practical experiments.

Dispersed surface Ru ensembles on MgO(111) for catalytic ammonia decomposition.

Ammonia is regarded as an energy vector for hydrogen storage, transport and utilization, which links to usage of renewable energies. However, efficient catalysts for ammonia decomposition and their underlying mechanism yet remain obscure. Here we report that atomically-dispersed Ru atoms on MgO support on its polar (111) facets {denoted as MgO(111)} show the highest rate of ammonia decomposition, as far as we are aware, than all catalysts reported in literature due to the strong metal-support interaction and efficient surface coupling reaction. We have carefully investigated the loading effect of Ru from atomic form to cluster/nanoparticle on MgO(111). Progressive increase of surface Ru concentration, correlated with increase in specific activity per metal site, clearly indicates synergistic metal sites in close proximity, akin to those bimetallic N2 complexes in solution are required for the stepwise dehydrogenation of ammonia to N2/H2, as also supported by DFT modelling. Whereas, beyond surface doping, the specific activity drops substantially upon the formation of Ru cluster/nanoparticle, which challenges the classical view of allegorically higher activity of coordinated Ru atoms in cluster form (B5 sites) than isolated sites.

Gamma-Ray Irradiation Induced Ultrahigh Room-Temperature Ferromagnetism in MoS2 Sputtered Few-Layered Thin Films.

Defect engineering is of great interest to the two-dimensional (2D) materials community. If nonmagnetic transition-metal dichalcogenides can possess room-temperature ferromagnetism (RTFM) induced by defects, then they will be ideal for application as spintronic materials and also for studying the relation between electronic and magnetic properties of quantum-confined structures. Thus, in this work, we aimed to study gamma-ray irradiation effects on MoS2, which is diamagnetic in nature. We found that gamma-ray exposure up to 9 kGy on few-layered (3.5 nm) MoS2 films induces an ultrahigh saturation magnetization of around 610 emu/cm3 at RT, whereas no significant changes were observed in the structure and magnetism of bulk MoS2 (40 nm) films even after gamma-ray irradiation. The RTFM in a few-layered gamma-ray irradiated sample is most likely due to the bound magnetic polaron created by the spin interaction of Mo 4d ions with trapped electrons present at sulfur vacancies. In addition, density functional theory (DFT) calculations suggest that the defect containing one Mo and two S vacancies is the dominant defect inducing the RTFM in MoS2. These DFT results are consistent with Raman, X-ray photoelectron spectroscopy, and ESR spectroscopy results, and they confirm the breakage of Mo and S bonds and the existence of vacancies after gamma-ray irradiation. Overall, this study suggests that the occurrence of magnetism in gamma-ray irradiated MoS2 few-layered films could be attributed to the synergistic effects of magnetic moments arising from the existence of both Mo and S vacancies as well as lattice distortion of the MoS2 structure.

Manipulation of the Structure and Optoelectronic Properties through Bromine Inclusion in a Layered Lead Bromide Perovskite.

One of the great advantages of organic-inorganic metal halides is that their structures and properties are highly tuneable and this is important when optimizing materials for photovoltaics or other optoelectronic devices. One of the most common and effective ways of tuning the electronic structure is through anion substitution. Here, we report the inclusion of bromine into the layered perovskite [H3N(CH2)6NH3]PbBr4 to form [H3N(CH2)6NH3]PbBr4·Br2, which contains molecular bromine (Br2) intercalated between the layers of corner-sharing PbBr6 octahedra. Bromine intercalation in [H3N(CH2)6NH3]PbBr4·Br2 results in a decrease in the band gap of 0.85 eV and induces a structural transition from a Ruddlesden-Popper-like to Dion-Jacobson-like phase, while also changing the conformation of the amine. Electronic structure calculations show that Br2 intercalation is accompanied by the formation of a new band in the electronic structure and a significant decrease in the effective masses of around two orders of magnitude. This is backed up by our resistivity measurements that show that [H3N(CH2)6NH3]PbBr4·Br2 has a resistivity value of one order of magnitude lower than [H3N(CH2)6NH3]PbBr4, suggesting that bromine inclusion significantly increases the mobility and/or carrier concentration in the material. This work highlights the possibility of using molecular inclusion as an alternative tool to tune the electronic properties of layered organic-inorganic perovskites, while also being the first example of molecular bromine inclusion in a layered lead halide perovskite. By using a combination of crystallography and computation, we show that the key to this manipulation of the electronic structure is the formation of halogen bonds between the Br2 and Br in the [PbBr4]∞ layers, which is likely to have important effects in a range of organic-inorganic metal halides.