Ab Initio Molecular Dynamical Investigation of the Finite Temperature Behavior of the Tetrahedral Au19and Au20Clusters

Pd8 Cluster: Too Small to Melt? A BOMD Study

The question of whether a solid-liquid phase transition occurs in small clusters poses a fundamental challenge. In this study, we attempt to elucidate this phenomenon through a thorough examination of the thermal behavior and structural stability of Pd8 clusters employing ab initio simulations. Initially, a systematic global search is carried out to identify the various isomers of the Pd8 cluster. This is accomplished by employing an ab initio basin-hopping algorithm and using the PBE/SDD scheme integrated in the Gaussian code. The resulting isomers are further refined through reoptimization using the deMon2k package. To ensure the structural firmness of the lowest-energy isomer, we calculated normal modes. The structural stability as a function of temperature is analyzed through the Born-Oppenheimer molecular dynamics (BOMD) approach. Multiple BOMD trajectories at distinct simulated temperatures are examined with data clustering analysis to determine cluster isomers. This analysis establishes a connection between the potential energy landscape and the simulated temperature. To address the question of cluster melting, canonical parallel-tempering BOMD runs are performed and analyzed with the multiple-histogram method. A broad maximum in the heat capacity curve indicates a melting transition between 500 and 600 K. To further examine this transition, the mean-squared displacement and the pair-distance distribution function are calculated. The results of these calculations confirm the existence of a solid-liquid phase transition, as indicated by the heat capacity curve.

Evaluation of Machine Learning Interatomic Potentials for the Properties of Gold Nanoparticles

We have investigated Machine Learning Interatomic Potentials in application to the properties of gold nanoparticles through the DeePMD package, using data generated with the ab-initio VASP program. Benchmarking was carried out on Au20 nanoclusters against ab-initio molecular dynamics simulations and show we can achieve similar accuracy with the machine learned potential at far reduced cost using LAMMPS. We have been able to reproduce structures and heat capacities of several isomeric forms. Comparison of our workflow with similar ML-IP studies is discussed and has identified areas for future improvement.

Size-dependent phase transitions boost catalytic activity of sub-nanometer gold clusters

The characterization and identification of the dynamics of cluster catalysis are crucial to unraveling the origin of catalytic activity. However, the dynamical catalytic effects during the reaction process remain unclear. Herein, we investigate the dynamic coupling effect of elementary reactions with the structural fluctuations of sub-nanometer Au clusters with different sizes using ab initio molecular dynamics and the free energy calculation method. It was found that the adsorption-induced solid-to-liquid phase transitions of the cluster catalysts give rise to abnormal entropy increase, facilitating the proceeding of reaction, and this phase transition catalysis exists in a range of clusters with different sizes. Moreover, clusters with different sizes show different transition temperatures, resulting in a non-trivial size effect. These results unveil the dynamic effect of catalysts and help understand cluster catalysis to design better catalysts rationally.

Finite temperature behavior of carbon atom-doped silicon clusters: depressed thermal stabilities, coexisting isomers, reversible dynamical pathways and fragmentation channels

BOMD simulations revealed a multifarious thermo-stimuli response (from “solid-state” to reversible dynamics to fragmentation) of experimentally identified SiC mixed clusters at finite temperature.

Adsorption-Induced Liquid-to-Solid Phase Transition of Cu Clusters in Catalytic Dissociation of CO2

Sub-nanometer metal clusters widely existing in catalysts have a large ensemble of metastable isomers that can interconvert during catalytic reactions, exhibiting complex dynamical catalytic effects. In this work, we systematically investigate the temperature dependent structural dynamics of the Cu₁₃ cluster in CO₂ dissociation using ab initio molecular dynamics and the free energy calculation method. We find an abnormal entropic effect due to adsorption-induced liquid-to-solid phase transition of the cluster during the course of the elementary dissociation step at transition temperatures. In the dissociation product, the formation of a rigid Cu₃O unit decreases the dynamical fluidity of the cluster and increases the melting temperature, causing a decrease in the entropy of the dissociation product. Our work demonstrates the nontrivial effects of surface adsorption on phase transition behaviors of dynamic clusters and offers a new perspective to dynamic catalysis.

Evidencing the relationship between isomer spectra and melting: the 20- and 55-atom silver and gold cluster cases

The present work highlights the links between melting properties and structural excitation spectra of small gold and silver clusters. The heat capacity curves are computed for Ag₂₀, Au₂₀, Ag₅₅, Au₅₅ and their ions, using a parallel-tempering molecular dynamics scheme to explore the density functional based tight binding (DFTB) potential energy surfaces and the multiple histogram method. It is found that clusters having very symmetric lowest energy structures (Au₂₀, Ag₅₅ and their ions) present sharp or relatively sharp solid-to-liquid transitions and large melting temperatures, important structural excitation energies and a discrete isomer spectrum. Opposite trends are observed for less ordered clusters (Ag₂₀, Au₅₅ and their ions). Regarding the structural evolution with temperature, very symmetric clusters exhibit minor evolution up to the starting melting temperature. The present study also highlights that, in contrast with the case of Au₂₀, a single electron excess or deficiency is not determinant regarding the melting characteristics, even quantitatively, for clusters containing 55 atoms, for gold as for silver.

Solid-to-liquid phase transitions of sub-nanometer clusters enhance chemical transformation

Understanding the nature of active sites is crucial in heterogeneous catalysis, and dynamic changes of catalyst structures during reaction turnover have brought into focus the dynamic nature of active sites. However, much less is known on how the structural dynamics couples with elementary reactions. Here we report an anomalous decrease in reaction free energies and barriers on dynamical sub-nanometer Au clusters. We calculate temperature dependence of free energies using ab initio molecular dynamics, and find significant entropic effects due to solid-to-liquid phase transitions of the Au clusters induced by adsorption of different states along the reaction coordinate. This finding demonstrates that catalyst dynamics can play an important role in catalyst activity.

Density-functional tight-binding approach for metal clusters, nanoparticles, surfaces and bulk: application to silver and gold

Density-functional based tight-binding (DFTB) is an efficient quantum mechanical method that can describe a variety of systems, going from organic and inorganic compounds to metallic and hybrid materials. The present topical review addresses the ability and performance of DFTB to investigate energetic, structural, spectroscopic and dynamical properties of gold and silver materials. After a brief overview of the theoretical basis of DFTB, its parametrization and its transferability, we report its past and recent applications to gold and silver systems, including small clusters, nanoparticles, bulk and surfaces, bare and interacting with various organic and inorganic compounds. The range of applications covered by those studies goes from plasmonics and molecular electronics, to energy conversion and surface chemistry. Finally, perspectives of DFTB in the field of gold and silver surfaces and NPs are outlined.

Au26: a case of fluxionality/co-existence

BOMD simulations are performed on experimentally identified conformations of the fluxional Au26 cluster at finite temperature. Comprehensive analysis of structural, electronic and thermal properties indicates that the highest energy tubular cage conformation (till 500 K) outperforms the ground state conformation (till 400 K) in terms of thermal stability.

Structural Rearrangement of Au-Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study

The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au-Pd nanoalloys supported on rutile TiO₂ under different conditions. We find that the Au-Pd structure is strongly dependent on the redox properties of the support, originating from strong metal-support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd-Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd-H bonds, which are stronger than Au-H bonds. Our work offers critical insights into the structure and redox properties of Au-Pd nanoalloy catalysts under working conditions.

Immobilization of Au nanoparticles on graphite tunnels through nanocapillarity

Atomistic computer simulations on the generation of nanotunnels on graphite and the subsequent immobilization of gold nanoparticles are presented in this work.

The shape of Au8: gold leaf or gold nugget?

The size at which nonplanar isomers of neutral, pristine gold nanoclusters become energetically favored over planar ones is still debated amongst theoreticians and experimentalists. Spectroscopy confirms planarity is preferred at sizes up to Au7, however, starting with Au8, the uncertainty remains for larger nanoclusters. Au8 computational studies have had different outcomes: the planar D4h "cloverleaf" isomer competes with the nonplanar Td, C2v and D2d "nugget" isomers for greatest energetic stability. We here examine the 2D vs. 3D preference in Au8 by presenting our own B2PLYP, MP2 and CCSD(T) calculations on these isomers: these methods afford a better treatment of long-range correlation, which is at the root of gold's characteristic aurophilicity. We then use findings from these high-accuracy computations to evaluate two less expensive DFT approaches, applicable to much larger nanoclusters: alongside the standard functional PBE, we consider M06-L (highly parametrized to incorporate long-range dispersive interactions). We find that increasing basis set size within the B2PLYP framework has a greater destabilizing effect on the nuggets than it has on the Au8 cloverleaf. Our CCSD(T) and B2PLYP predictions, replicated by DFT-PBE, all identify the cloverleaf as the most stable isomer; MP2 and DFT-M06-L show overestimation of aurophilicity, and favor, respectively, the nonplanar D2d and Td nuggets in its stead. We conclude that PBE, which more closely reproduces CCSD(T) findings, may be a better candidate density functional for the simulation of gold nanoclusters in this context.

The role of reducible oxide-metal cluster charge transfer in catalytic processes: new insights on the catalytic mechanism of CO oxidation on Au/TiO2 from ab initio molecular dynamics

To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au-CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of all redox steps in catalytic conversions. A detailed comparison of the current findings with previous studies is presented, and generalities about the role of surface-adsorbate charge transfer for metal cluster/reducible oxide interactions are discussed.

Probing the electronic properties and structural evolution of anionic gold clusters in the gas phase

Gold nanoparticles have been discovered to exhibit remarkable catalytic properties in contrast to the chemical inertness of bulk gold. A prerequisite to elucidate the molecular mechanisms of the catalytic effect of nanogold is a detailed understanding of the structural and electronic properties of gold clusters as a function of size. In this review, we describe joint experimental studies (mainly photoelectron spectroscopy) and theoretical calculations to probe the structural properties of anionic gold clusters. Electronic properties and structural evolutions of all known Au(n)(-) clusters as experimentally confirmed to date are summarized, covering the size ranges of n = 3-35 and 55-64. Recent experimental efforts in resolving the isomeric issues of small gold clusters using Ar-tagging, O(2)-titration and isoelectronic substitution are also discussed.

Structural transition of gold nanoclusters: from the golden cage to the golden pyramid

How nanoclusters transform from one structural type to another as a function of size is a critical issue in cluster science. Here we report a study of the structural transition from the golden cage Au(16)(-) to the pyramidal Au(20)(-). We obtained distinct experimental evidence that the cage-to-pyramid crossover occurs at Au(18)(-), for which the cage and pyramidal isomers are nearly degenerate and coexist experimentally. The two isomers are observed and identified by their different interactions with O(2) and Ar. The cage isomer is observed to be more reactive with O(2) and can be preferentially "titrated" from the cluster beam, whereas the pyramidal isomer has slightly stronger interactions with Ar and is favored in the Au(18)Ar(x)(-) van der Waals complexes. The current study allows the detailed structural evolution and growth routes from the hollow cage to the compact pyramid to be understood and provides information about the structure-function relationship of the Au(18)(-) cluster.