Cu4 Cluster Research Articles

Carbon vacancy-assisted stabilization of individual Cu5 clusters on graphene. Insights from ab initio molecular dynamics.

Recent advances in synthesis and characterization methods have enabled the controllable fabrication of atomically precise metal clusters (AMCs) of subnanometer size that possess unique physical and chemical properties, yet to be explored. Such AMCs have potential applications in a wide range of fields, from luminescence and sensing to photocatalysis and bioimaging, making them highly desirable for further research. Therefore, there is a need to develop innovative methods to stabilize AMCs upon surface deposition, as their special properties are lost due to sintering into larger nanoparticles. To this end, dispersion-corrected density functional theory (DFT-D3) and ab initio molecular dynamics (AIMD) simulations have been employed. Benchmarking against high-level post-Hartree-Fock approaches revealed that the DFT-D3 scheme describes very well the lowest-energy states of clusters of five and ten atoms, Cu5 and Cu10. AIMD simulations performed at 400 K illustrate how intrinsic defects of graphene sheets, carbon vacancies, are capable of confining individual Cu5 clusters, thus allowing for their stabilization. Furthermore, AIMD simulations provide evidence on the dimerization of Cu5 clusters on defect-free graphene, in agreement with the ab initio predictions of (Cu5)n aggregation in the gas phase. The findings of this study demonstrate the potential of using graphene-based substrates as an effective platform for the stabilization of monodisperse atomically precise Cu5 clusters.

Synthesis of photocatalytic cysteine-capped Cu≈10 clusters using Cu5 clusters as catalysts.

We report an easily scalable synthesis method for the preparation of cysteine-capped Cu≈10 clusters through the reduction of Cu(II) ions with NaBH4, using Cu5 clusters as catalysts. The presence of such catalytic clusters allows controlling the formation of the larger Cu≈10 clusters and prevents the production of copper oxides or Cu(I)-cysteine complexes, which are formed when Cu5 is absent or at lower concentrations, respectively. These results indicate that small catalytic clusters could be involved, as precursor species before the reduction step, in the different methods developed for the synthesis of clusters. The visible light-absorbing Cu≈10 clusters, obtained by the cluster-catalysed method, display high photocatalytic activities for the decomposition of methyl orange with visible light.

A luminescent Cu4 cluster film grown by electrospray deposition: a nitroaromatic vapour sensor.

We present the fabrication and use of a film of a carborane-thiol-protected tetranuclear copper cluster with characteristic orange luminescence using ambient electrospray deposition (ESD). Charged microdroplets of the clusters produced by an electrospray tip deposit the clusters at an air-water interface to form a film. Different microscopic and spectroscopic techniques characterized the porous surface structure of the film. Visible and rapid quenching of the emission of the film upon exposure to 2-nitrotoluene (2-NT) vapours under ambient conditions was observed. Density functional theory (DFT) calculations established the favourable binding sites of 2-NT with the cluster. Desorption of 2-NT upon heating recovered the original luminescence, demonstrating the reusability of the sensor. Stable emission upon exposure to different organic solvents and its quenching upon exposure to 2,4-dinitrotoluene and picric acid showed selectivity of the film to nitroaromatic species.

Synthesis of Square Planar Cu4 Clusters.

Template-assisted synthesis of well-defined polynuclear clusters remains a challenge for [M4 ] square planar topologies. Herein, we present a tetraamine scaffold R L(NH2 )4 , where L is a rigidified resorcin[4]arene, to direct the formation of C4 -symmetric R L(NH)4 Cu4 clusters with Cu-Cu distances around 2.7 Å, suggesting metal-metal direct interactions are operative since the sum of copper's van der Waals radii is 2.8 Å. DFT calculations display HOMO to HOMO-3 residing all within a 0.1 eV gap. These four orbitals display significant electron density contribution from the Cu centers suggesting a delocalized electronic structure. The one-electron oxidized [Cu4 ]+ species was probed by variable temperature X-band continuous wave-electron paramagnetic resonance (CW-EPR), which displays a multiline spectrum at room temperature. This work presents a novel synthetic strategy for [M4 ] clusters and a new platform to investigate activation of small molecules.

Cluster formation between an oxadiazole derivative with metal nanoclusters (Ag/Au/Cu), graphene quantum dot sheets, SERS studies, and solvent effects

Interaction of an oxadiazole derivative, 5-(3,4-dimethoxyphenyl)-3-(3-methoxyphenyl)-1,2,4-oxadiazole (DPMO) with Ag/Au/Cu and graphene quantum dots with different solvents, is reported theoretically. The adsorption energy is maximum for the Cu6 cluster and minimum for the Ag6 cluster. The asymmetric charge redistribution between DPMO and M6s produces an improvement in dipole moment values. The decrease in energy gaps of complexes increased conductivity and metal clusters can be used as a drug sensor. The solvation energies are more negative in solvent media than in the gaseous media, indicating an enhancement in the solvent medium’s stability. Wave function studies show that there exist significant noncovalent interactions between metal clusters and oxadiazole that facilitate cluster formation. DPMO is found to form stable clusters with graphene which is evident from the enhancement of Raman activity of the system through SERS also enabling it for sensing DPMO in a mixture.

Ab initio Calculations of Structural and Electronic Properties of Pt3 and Cu3 clusters adsorbed on ZnO(000 ̅1)

This search paper is focus on the investigation of the most stability structures of Pt3 and Cu3 clusters adsorbed on ZnO (000 ̅1) surfaces in the Wurtzite stable structure. The results are obtained by using the Density Function Theory (DFT). All the calculations are performed using the Gradient Generalized Approximation to describe the exchange correlation term in SIESTA code. From this work we found new structures of Pt3 and Cu3 clusters with lowest energy. Also we have calculated the binding energy, relaxation interlayer for each cluster, bond lengths, vertical ionization potential PIv and the density of states DOS. Through the results we found that the Pt3 cluster is more stable than Cu3 cluster and confirmed by the DOS, in the other hand the ZnO-O surface is more stable than ZnO-Zn surface. The computed electron structure and DOS illustrate that the Pt3/ZnO and Cu3/ZnO clusters adsorbed on ZnO-O surfaces display metallic characteristics.

Sub-nanometer Copper Clusters as Alternative Catalysts for the Selective Oxidation of Methane to Methanol with Molecular O2.

The partial oxidation of methane to methanol with molecular O2 at mild reaction conditions is a challenging process, which is efficiently catalyzed in nature by enzymes. As an alternative to the extensively studied Cu-exchanged zeolites, small copper clusters composed by just a few atoms appear as potential specific catalysts for this transformation. Following previous work in our group that established that the reactivity of oxygen atoms adsorbed on copper clusters is closely linked to cluster size and morphology, we explore by means of DFT calculations the ability of bidimensional (2D) and three-dimensional (3D) Cu5 and Cu7 clusters to oxidize partially methane to methanol. A highly selective Eley-Rideal pathway involving homolytic C-H bond dissociation and a non-adsorbed radical-like methyl intermediate is favored when bicoordinated oxygen atoms, preferentially stabilized at the edges of 2D clusters, are available. Cluster morphology arises as a key parameter determining the nature and reactivity of adsorbed oxygen atoms, opening the possibility to design efficient catalysts for partial methane oxidation based on copper clusters.

Understanding the mechanism of thioguanine's binding to Ag6 and bimetallic (Ag3–Au3 and Ag3–Cu3) clusters

Using Ag6, Ag3Au3 and Ag3Cu3 clusters, density functional theory (DFT) methods were used to elucidate the interaction behavior of thione-containing thioguanine drug (TGN) on silver. The CAM-B3LYP functional with LANL2DZ basis set was used to study the different features and bonding qualities of resultant metal complexes. The drug molecule tends to anchor on the cluster with binding energies ranging from −16 to −23 kcal/mol. Due to the recovery time values and change in energy gap, the metal clusters can be turned into an electrical signal for detecting TGN. TGN-metal cluster interactions are reversible events, and a drug release mechanism has been postulated. As a result of a modest change in pH in tumor cells, the drug is able to detach from metal surfaces. Non-covalent interactions studies show that TGN's NH2 group is more important for hydrogen bonding, and the heterocyclic N and S atoms may play a role in weak interactions.

Copper(I)–Pyrazolate Complexes as Solid-State Phosphors: Deep-Blue Emission through a Remote Steric Effect

We describe a novel manifestation of rigidochromic behavior in a series of tetranuclear Cu(I)-pyrazolate (Cu4pz4) macrocycles, with implications for solid-state luminescence at deep-blue wavelengths (<460 nm). The Cu4pz4 emissions are remarkably sensitive to structural effects far from the luminescent core: when 3,5-di-tert-butylpyrazoles are used as bridging ligands, adding a C4 substituent can induce a blue shift of more than 100 nm. X-ray crystal and computational analyses reveal that C4 units influence the conformational behavior of adjacent tert-butyl groups, with a subsequent impact on the global conformation of the Cu4pz4 complex. Emissions are mediated primarily through a cluster-centered triplet (3CC) state; compression of the Cu4 cluster into a nearly close-packed geometry prevents the reorganization of its excited-state structure and preserves the 3CC energy at a high level. The remote steric effect may thus offer alternative strategies toward the design of phosphors with rigid excited-state geometries.

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Atom by atom built subnanometer copper cluster catalyst for the highly selective oxidative dehydrogenation of cyclohexene.

The effect of particle size and support on the catalytic performance of supported subnanometer copper clusters was investigated in the oxidative dehydrogenation of cyclohexene. From among the investigated seven size-selected subnanometer copper particles between a single atom and clusters containing 2-7 atoms, the highest activity was observed for the titania-supported copper tetramer with 100% selectivity toward benzene production and being about an order of magnitude more active than not only all the other investigated cluster sizes on the same support but also the same tetramer on the other supports, Al2O3, SiO2, and SnO2. In addition to the profound effect of cluster size on activity and with Cu4 outstanding from the studied series, Cu4 clusters supported on SiO2 provide an example of tuning selectivity through support effects when this particular catalyst also produces cyclohexadiene with about 30% selectivity. Titania-supported Cu5 and Cu7 clusters supported on TiO2 produce a high fraction of cyclohexadiene in contrast to their neighbors, while Cu4 and Cu6 solely produce benzene without any combustion, thus representing odd-even oscillation of selectivity with the number of atoms in the cluster.

Interpreting the Operando X-ray Absorption Near-Edge Structure of Supported Cu and CuPd Clusters in Conditions of Oxidative Dehydrogenation of Propane: Dynamic Changes in Composition and Size

Supported subnano-cluster catalysts are highly dynamic, developing true active sites only under the pressures and temperatures of reaction conditions. Operando X-ray absorption near-edge structure (XANES) spectroscopy can track changes in the oxidation state and the local environment of cluster atoms, providing insight into the development of these active sites. While bulk metal, oxide, and hydroxide standards are often used for fitting experimental XANES spectra to obtain average oxidation states, we recently showed that computed cluster standards of relevant compositions are a more suitable basis, producing more accurate fits. Here, we theoretically interpret the operando XANES of supported Cu3Pd and Cu4 clusters during temperature-programmed reaction (TPRx) of oxidative dehydrogenation of propane. We use an expanded basis set including both globally optimized computed clusters and bulk standards. Not only can we track reversible composition/oxidation state change with temperature, but also the irreversible growth of the bulk fraction upon heating, which we attribute to cluster sintering. This has important implications for the mechanism of the catalyzed reaction and the nature of the available active sites. We propose that operando XANES provides most significant insight into the nature of supported cluster catalysts in reaction conditions when interpreted using mixed computed cluster and bulk standards.

Comparative study of the electrocatalytic N2 reduction property of a Cu4 cluster with μ2-/μ3-OH and a Cu2 complex with μ2-OH

Using the atomic precisely structurally characterized clusters as electrocatalysts can provide some possible structure information about the active intermediate during the electrocatalytic N2 reduction reaction (NRR) process. A Cu4 cluster, named [(bpy)4Cu4(C10H8SO3)4(OH)4]•4H2O (1), has been synthesized and fully characterized by IR, SXRD, XPS, EPR, et al. Single crystal structural analysis reveals that the core structure of the cluster has a step-scheme conformation with two μ2-OH and two μ3-OH groups. Electro-catalysis studies of this cluster show that it can achieve a highest NH3 yield of 2.18 μg h−1 mg−1 and a FE of 0.97 % at −0.6 V. The excellent NRR performance may be attributable to the additional μ3-OH groups, based on the contrastively study of the other electrocatalysts of the Cu powder and the Cu2 complex, named [(μ2-OH)(μ2-Cl)(bpy)2Cu2Cl2] (2), with one μ2-OH group in its structure. This work provides some structure information about the possible active sites during the NRR process and inspires us to develop new cluster as efficient electrocatalysts for advancing sustainable NH3 production. Using the atomic precisely structurally characterized clusters as electrocatalysts can provide some possible structure information about the active intermediate during the electrocatalytic N2 reduction reaction (NRR) process. A Cu4 cluster, named [(bpy)4Cu4(C10H8SO3)4(OH)4]•4H2O (1), has been synthesized and fully characterized by IR, SXRD, XPS, EPR, et al. Single crystal structural analysis reveals that the core structure of the cluster has a step-scheme conformation with two μ2-OH and two μ3-OH groups. Electro-catalysis studies of this cluster show that it can achieve a highest NH3 yield of 2.18 μg h−1 mg−1 and a FE of 0.97 % at −0.6 V. The excellent NRR performance may be attributable to the additional μ3-OH groups, based on the contrastively study of the other electrocatalysts of the Cu powder and the Cu2 complex, named [(μ2-OH)(μ2-Cl)(bpy)2Cu2Cl2] (2), with one μ2-OH group in its structure. This work provides some structure information about the possible active sites during the NRR process and inspires us to develop new cluster as efficient electrocatalysts for advancing sustainable NH3 production.

Mechanism of heteroatom-doped Cu5 catalysis for hydrogen evolution reaction

Developing high-performance hydrogen evolution reaction (HER) electrocatalysts is of great significance for solving the global energy crisis. Cluster has great application potential in the field of catalysis due to their unique quantum size effect and high specific surface area. Herein, the HER catalytic performance of Cu5 cluster were regulated and optimized by doping heteroatoms. The Gibbs free energy calculation shows that the catalytic activity of Cu5Ni and Cu5Pt is comparable to that of Pt-based catalysts, and the Gibbs free energy value of Cu5C can even reach 0.005 eV, indicating its much higher catalytic performance than that of other catalysts. Thus, the catalytic activity of Cu5 clusters is optimized by doping non-metal and transition metal atoms to regulate the geometric and electronic structure of Cu5. It was found that Cu5Ni, Cu5Pt and Cu5C are potential catalysts to replace Pt-based catalysts for reducing the cost and achieving large-scale hydrogen production. This work provides a new avenue to regulate the catalytic performance of clusters, which is helpful for the further development and application of clusters in the field of catalysis.

First-principles modelling of the new generation of subnanometric metal clusters: Recent case studies

The very recent development of highly selective techniques making possible the synthesis and experimental characterization of subnanometric (subnanometer-sized) metal clusters (even single atoms) is pushing our understanding far beyond the present knowledge in materials science, driving these clusters as a new generation of quantum materials at the lower bounds of nanotechnology. When the size of the metal cluster is reduced to a small number of atoms, the d-band of the metal splits into a subnanometric d-type molecular orbitals network in which all metal atoms are inter-connected, with the inter-connections having the length of a chemical bond (1–2 Å). These molecular characteristics are at the very core of the high stability and novel properties of the smallest metal clusters, with their integration into colloidal materials interacting with the environment having the potential to further boost their performance in applications such as luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Through the presentation of very recent case studies, this Feature Article is aimed to illustrate how first-principles modelling, including methods beyond the state-of-the-art and an interplay with cutting-edge experiments, is helping to understand the special properties of these clusters at the most fundamental level. Moreover, it will be discussed how superfluid helium droplets can act both as nano-reactors and carriers to achieve the synthesis and surface deposition of metal clusters. This concept will be illustrated with the quantum simulation of the helium droplet-assisted soft-landing of a single Au atom onto a titanium dioxide (TiO2) surface. Next, it will be shown how the application of first-principles methods have disclosed the fundamental reasons why subnanometric Cu5 clusters are resistant to irreversible oxidation, and capable of increasing and extending into the visible region the solar absorption of TiO2, of augmenting its efficiency for photo-catalysis beyond a factor of four, also considering the decomposition and photo-activation of CO2 as a prototypical (photo-) catalytic reaction. Finally, I will discuss how the modification of the same material with subnanometric Ag5 clusters has converted it into a “reporter” of a surface polaron property as well as a novel two-dimensional polaronic material.

The 2D or 3D morphology of sub-nanometer Cu5 and Cu8 clusters changes the mechanism of CO oxidation.

The mechanism of the CO oxidation reaction catalysed by planar Cu5, three dimensional (3D) Cu5, and 3D Cu8 clusters is theoretically investigated at the B3PW91/Def2TZVP level. All three clusters are able to catalyse the reaction with similar activation energies for the rate determining step, about 16-18 kcal mol-1, but with remarkable differences in the reaction mechanism depending on cluster morphology. Thus, for 3D Cu5 and Cu8 clusters, O2 dissociation is the first step of the mechanism, followed by two consecutive CO + O reaction steps, the second one being rate determining. In contrast, on planar Cu5 the reaction starts with the formation of an OOCO intermediate in what constitutes the rate determining step. The O-O bond is broken in a second step, releasing the first CO2 and leaving one bi-coordinately adsorbed O atom which reacts with CO following an Eley-Rideal mechanism with a low activation energy, in contrast to the higher barriers obtained for this step on 3D clusters.

C2 product formation in the CO2 electroreduction on boron-doped graphene anchored copper clusters.

A possible remedy for the increasing atmospheric CO2 concentration is capturing and reducing it into valuable chemicals like methane, methanol, ethylene, and ethanol. However, a suitable catalyst for this process is still under extensive research. Small sized copper clusters have gained attention in recent years due to their catalytic activity in the CO2 reduction reaction. Although C2+ products have a higher economic value, the formation of C1 products was investigated most thoroughly. Graphene is a promising support for small copper clusters in the electrochemical reduction of CO2. It exhibits good mechanical and electrical properties, but the weak interaction between copper and graphene is an issue. Our DFT computations reveal that small Cu clusters on the boron-doped graphene (BDG) support are promising catalysts for the electrochemical reduction of CO2. We found facile reaction pathways towards various C1 (carbon-monoxide, formic acid, formaldehyde, methanol or methane) and C2 (ethanol or ethylene) products on Cu4 and Cu7 clusters on BDG. The reactivity is cluster-size tunable with Cu4 being the more reactive agent, while Cu7 shows a higher selectivity towards C2 products.

Aggregation and support effects in the oxidation of fluxional atomic metal clusters. The paradigmatic Cu5 case.

The recent development of new synthesis techniques has allowed the production of monodisperse metal clusters composed of a few atoms. Follow-up experimental spectroscopic characterization has indicated the stability of these atomic metal clusters (AMCs). Despite the common assumption that the occurrence of an irreversible oxidation becomes more likely as the cluster size decreases, its quenching and reversible nature has been experimentally identified in the particular case of Cu5 clusters, making them paradigmatic. This work aims to address the influence of aggregation and the effects of a chemically inert carbon-based support on the oxidation of AMCs, considering the case of Cu5 as a model system. For this purpose, we present an extended first-principles study of the oxidation of Cu5-Cu5 and circumpyrene-supported Cu5, comparing it with that of unsupported Cu5, and combine dispersion-corrected density-functionals, first principles thermochemistry, and ab initio molecular dynamics (AIMD) simulations within an adiabatic approach. Our results indicate that a molecular chemisorption/desorption model is sensible upon consideration of aggregation and support effects in such a way that the predicted (p-T)-phase diagrams do not differ significantly from those obtained for unsupported Cu5. We also provide insights into the decoupling of the Cu5-Cu5 dimer into Cu5 sub-units through activated fluxional rotational motion, upon heating, as well as the adsorption of multiple O2 molecules at high oxygen gas pressures. Furthermore, numerical evidence shows the likelihood of a support-mediated mechanism leading to the dissociation of chemisorbed peroxo O22- species, delivering states with very similar energies to those characterized by molecular chemisorption. A Boltzmann-weighted average of the free energies of formation is computed as well, coming up with a diagram of the dominant copper oxidation states as a function of temperature and oxygen gas pressure.

Evolution of atomic and electronic structures of Cun (n = 2–10) clusters supported on anatase TiO2(1 0 1) surface

Based on extensive structural search and density functional theory (DFT) calculations, the atomic and electronic structures of of Cun (n = 2–10) clusters supported on anatase TiO2(101) surface (denoted as Cun/A(101)) are systematically studied. The atomic structures exhibit obvious hereditary feature, i.e. most of the low-energy structures of larger clusters can be obtained by adding additional Cu atoms onto those of smaller clusters. Small sized clusters (n ≤ 6) prefer to quasi planar structures while large sized clusters (n > 6) prefer to complex three-dimensional structures. The thermodynamic stabilities of the supported Cun clusters are investigated by calculating the binding energies, second order energy difference, adsorption energies and nucleation energies. It is found that Cu3/A(101), Cu5/A(101), and Cu8/A(101) are particularly stable and Cu3, Cu5 and Cu9 clusters would be stabilized by the substrate mostly. All these Cun clusters can introduce in-gap states and reduce the band gaps according to the calculated density of states. Supported Cun clusters play the role of electron donor, resulting in partial oxidation state Cuδ+ and zero valence state Cu0.

Defect-rich graphene stabilized atomically dispersed Cu3 clusters with enhanced oxidase-like activity for antibacterial applications

Nanozyme has attracted great attention due to its diverse enzymatic catalytic activities. But there are still challenges in constructing novel nanozyme with robust catalytic activity. Here, we report an atomically dispersed and fully exposed Cu3 cluster stabilized on defect-rich nanodiamond-graphene hybrid support (Cu3/ND@G) with unique active adsorption sites, which benefit the adsorption and cleavage of O2, resulting in enhanced oxidase-like and antibacterial activity. The catalytic rate constant of Cu3/ND@G (Kcat = 1.474 × 10−1 s−1) is higher than those of previously reported copper single atom oxidase mimics (0.5 × 10−3 s−1) and even the commercial Pt/C mimics (1.01 × 10−2 s−1). DFT calculation revealed that the atomically dispersed Cu3 cluster as active center significantly improves the oxidase-like activity, attributing to the easy dissociation of O2 into reactive oxygen species (•OH). The atomically dispersed Cu3 cluster with an antibacterial rate of 100% in the NaAc buffer presents its potential application in the field of antibacterial materials.