Small Gold Clusters Research Articles

Isomerization in Gold Clusters upon O2 Adsorption

A systematic investigation is performed on structural transformations in small neutral gold clusters (Au3–Au12) induced by O2 adsorption, with the use of the fully automated reaction path search techniques, i.e., anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods, implemented in the global reaction route mapping (GRRM) program. For each size of Au cluster, the most stable structure, low-energy isomers, and a network of isomerization pathways are determined. The located Aun–O2 adsorption forms can be classified into two groups: η1-AunO2, where only one oxygen atom is adsorbed on Aun, and η2-AunO2, where both oxygen atoms are adsorbed on Aun in a bridged manner. These two adsorption forms can be transformed to each other with a low barrier. The isomerization pathways of gold clusters upon O2 adsorption are compared with those obtained for the pure gold clusters without O2. It is demonstrated that O2 adsorption promotes structural transformations in gold clus...

A comparative DFT study of interactions of Au and small gold clusters Aun (n = 2–4) with CH3S and CH2 radicals

We compare DFT binding energies (BEs) of Au and small gold clusters interacting with CH3S and CH2 ligands (Aun–L complexes, n=1–4). The spin state and the binding mechanism in Aun–L varies with the participation of singly occupied non-bonding orbitals or doubly occupied lone-pair orbitals of a ligand and on the number of atoms (even or odd) of Aun. The highest BE, 354kJ/mol, exhibits the Au3–CH2 complex with the covalent bond in which participate two singly occupied orbitals of the triplet state of CH2. With CH3S the highest BE (277kJ/mol) is calculated for Au3–SCH3 with the single Au–S bond.

Effect of Size, Temperature, and Structure on the Vibrational Heat Capacity of Small Neutral Gold Clusters

The vibrational heat capacity Cvib of a re-optimized neutral gold cluster was investigated at temperatures 0.5-300 K. The vibrational frequency of an optimized cluster was revealed by small atomic displacements using a numerical finite-differentiation method. This method was implemented using density-functional tight-binding (DFTB) approach. The desired set of system Eigen frequencies (3N -6) was obtained by diagonalization of the symmetric positive semi definite Hessian matrix. Our investigation revealed that the Cvib curve is strongly influenced by temperature, size, and structure and bond-order dependency. The effect of the range of interatomic forces is studied; especially the lower frequencies make a significant contribution to the heat capacity at low temperatures. In addition to that, we have exactly predicted the vibrational frequencies (ωi) which occur between 0.55 to 370.72 cm-1, depending on the nanoparticle morphology at T=0 for small neutral gold clusters AuN=3-20. This result has been proved and confirmed by the size effect values. It was found that beside the particle size, geometric shape, defect structure and an increase in asymmetry of nanoparticles effects on heat capacity. Surprisingly, the Boson peaks are typically ascribed to an excess density of vibrational states for the small clusters. Finally, temperature dependencies of the vibrational heat capacities of the re-optimized neutral gold clusters have been studied for the first time.

Jones oxidation of glycerol catalysed by small gold clusters.

We present here a joint theoretical and experimental study on the oxidation reactivity of glycerol catalysed by chemically pure small Au clusters in the absence and presence of H2O2. From high-resolution mass spectrometry, fruitful products of glyceraldehyde, glyceric acid, tartronic acid, mesoxalic acid and glycolic acid are observed pertaining to the successive Jones oxidation process associated with C-O and C-H bond activation. We then fully demonstrate the reaction pathways on the basis of a complementary-active-sites mechanism, revealing the favourable dehydration of glycerol followed by oxidation to form glyceraldehyde and carboxylic acids in the presence of small Au clusters and H2O2. It is found that the Aun/H2O2 system undertakes a heterolytic mechanism by firstly transferring an O-atom from H2O2 to the Au cluster forming an active intermediate, on which hydride abstraction and subsequent oxygen rebound become thermodynamically possible, promoting the C-H insertion reaction and further oxidation of aldehyde to carboxylic acids.

Does the Boson Peaks Exist in Small Neutral Gold Clusters?

The Boson peaks are typically ascribed to an excess density of vibrational states for the small clusters. A well resolved boson peak is observed in the low-frequency portion of the spectrum. A starting point for our discussion is the behaviour of the boson peak, derived from the vibrational density of states and the temperature dependencies of the vibrational heat capacities of the re-optimized neutral gold clusters. This Boson peak is associated with the existence of intermediate range order (IRO) in the arrangements of atoms. The low frequency (Far Infrared FIR, IR-C 200-10 cm−1) containing this dominant spectral line (Boson peak) is interpreted in terms of its relationship to the amplitude and extent of the density fluctuations in atoms and is, thereby, considered a measure of the intermediate range order in these atoms. We found a systematic relation among the boson peak energy, the boson peak intensity per atom, and the zigzag-bond density; the peak energy decreases and the peak intensity increases as zigzag-bond density decreases.

Importance of double-resonance effects in two-photon absorption properties of Au25(SR)18.

The two-photon absorption (TPA) cross-sections of small thiolate-protected gold clusters have been shown to be much larger than typical small organic molecules. In comparison with larger nanoparticles, their TPA cross-sections per gold atom are also found to be larger. Theoretical simulations have suggested that the large enhancement of these TPA cross-sections comes from a one-photon double-resonance mechanism. However, it remains difficult to simulate TPA cross-sections of thiolate-protected gold clusters due to their large system size and a high density of states. In this work, we report a time-dependent density functional theory (TDDFT) study of the TPA spectra of the Au25(SR)18- cluster based on a damped response theory formalism. Damped response theory enables a consistent treatment of on- and off-resonance molecular properties even for molecules with a high density of states, and thus is well-suited for studying the TPA properties of gold clusters. Our results indicate that the one- and two-photon double-resonance effect is much smaller than previously found, and thus is unlikely to be the main cause of the large TPA cross-sections found experimentally. The effect of symmetry breaking of the Au25(SR)18- cluster due to the ligands on the TPA cross-sections has been studied and was found to only slightly increase the cross-section. Furthermore, by comparing with larger nanoparticles we find that the TPA cross-section per gold atom scales linearly with the diameter of the particles, and that the Kerr non-linear response of the Au25(SR)18- cluster is on the same order as that of bulk gold films.

Structures, Stabilities, Reactivities, and (Hyper)Polarizabilities of Small Gold Clusters

Structures, Stabilities, Reactivities, and (Hyper)Polarizabilities of Small Gold Clusters

Small gold clusters catalyzing the conversion of glycerol to epichlorohydrin.

The conversion of glycerol to epichlorohydrin (GTE) is of great interest because the product is widely used in plastics, rubbers and adhesives, and also contributes to the disposal of the reactant glycerol, a major by-product in biodiesel production. Here we find effective catalysis by small gold clusters for the GTE reaction in water with an enhanced selectivity towards the desired product. Along with natural bond orbital (NBO) analysis rationalizing the donor-acceptor charge-transfer interactions, we illustrate the mechanism for bond activation in the reactants and intermediates over gold cluster catalysts, and present thermodynamically and kinetically favoured reaction pathways for dehydrochlorination in GTE processes.

Symmetry of Gold Neutral Clusters Au3-20 and Normal Modes of Vibrations by using the Numerical Finite Difference Method with Density-Functional Tight-Binding(DFTB) Approach

The geometries and vibrational frequency of the most stable small AuN clusters with N = 3 to 20 are presented through global structure re-optimization Study. The finite-differentiation method has been implemented within the density-functional tight-binding (DFTB) approach. The desired set of system eigenfrequencies (3N-6) is obtained by a diagonalization of the symmetric positive semi definite Hessian matrix. We have observed the vibrational modes between 0.55 and 370.72 cm-1 in wavenumbers at T=0 for the small gold clusters. The effect of the range of interatomic forces was calculated, and even the very lower frequencies were occupied in some clusters.

Electronic structure transformation in small bare Au clusters as seen by x-ray photoelectron spectroscopy

Free bare gold clusters in the size range from few tens to few hundred atoms (≤1 nm dimensions) have been produced in a beam, and the size-dependent development of their full valence band including the 5d and 6s parts has been mapped ‘on the fly’ by synchrotron-based photoelectron spectroscopy. The Au 4f core level has been also probed, and the cluster-specific Au 4f ionization energies have been used to estimate the cluster size. The recorded in the present work valence spectra of the small clusters are compared with the spectra of the large clusters (N ∼ 103) created by us using a magnetron-based gas aggregation source. The comparison shows a substantially narrower 5d valence band and the decrease in its splitting for gold clusters in the size range of few hundred atoms and below. Our DFT calculations involving the pseudopotential method show that the 5d band width of the ground state increases with the cluster size and by the size N = 20 becomes comparable with the experimental width of the valence photoelectron spectrum. Similar to the earlier observations on supported clusters we interpret our experimental and theoretical results as due to the undercoordination of a large fraction of atoms in the clusters with N ∼ 102 and below. The consequences of such electronic structure of small gold clusters are discussed in connection with their specific physical and chemical properties related to nanoplasmonics and nanocatalysis.

Synthesis of silica-core gold nanoshells and some modifications/variations

Gold nanoshells are particles usually composed of a spherical silica core coated with a thin gold layer. Their chemical and optical properties make them suitable and attractive for medical applications, namely cancer treatment and diagnosis, as they have been studied for biosensing, imaging, and photothermal ablation. For their synthesis, most of the reported methods are based on the first reported by Oldenburg et al. In this method, silica nanoparticles are first produced and then modified to incorporate amino groups aimed to adsorb small gold clusters, which in turn act as the nucleation sites for the reduction of additional gold until complete gold shells are formed. In this review, we examine some common conditions to synthesize gold nanoshells based on this process, along with important aspects that need to be followed to ensure the production of gold nanoshells having homogeneous size and shape that render suspensions with consistent properties in the near-infrared region of the electromagnetic spectrum. Since the customary method is laborious and time-consuming, three additional processes intended to simplify or reduce some steps are described as well. Finally, fundamental aspects on the chemistry of the synthesis and their variations involved in all the revised processes are also presented. Figures from our own findings are included to support these descriptions. Please notice that this review focuses on the synthesis; other reviews focus in optical properties and applications.

Ligand-Based Toolboxes for Tuning of the Optical Properties of Subnanometer Gold Clusters.

Recent advances in the crystal structure determination of ligand-protected metal clusters have revealed that their electronic structures and optical features are essentially governed by the nuclearity and geometries of the inorganic frameworks. In this Perspective, we point out the definite effects of the exterior ligand moieties on the properties of small gold clusters. On the basis of systematic experimental studies on the optical properties of Au8 and Au13 clusters with various anionic ligands, it was shown that not only the "through-bond" electronic effects of coordinating atoms but also the nonbonding interaction with neighboring heteroatoms and the electronic coupling with π-systems cause substantial perturbations. We also suggest that the steric rigidity of the ligand environments affects their photoluminescence efficiencies. These findings imply the feasibility of the facile modulation of the cluster properties through the appropriate choice of ligand modules, which may lead to the evolution of novel cluster-based materials with unique properties and functions.

Advanced Particle Characterization Techniques

Advanced Particle Characterization Techniques

Microwave-assisted synthesis of ultra small bare gold clusters supported over Al2O3 and TiO2 as catalysts in reduction of 4-nitrophenol to 4-aminophenol

In this work I present a novel method to prepare ultra small bare gold clusters (Aun) loaded over different supports. Size selected gold clusters protected by glutathione ligand (Au25(SG)18) was prepared and deposited over aluminium dioxide (Al2O3 Puralox SCCa-190HPV) and anatase titanium dioxide (TiO2) by impregnation method with doping percentage 0.5 and 1 wt %. Then the doped catalysts were irradiated 1 h by microwave synthesis labstation at 80 °C and 500 W to remove the glutathione ligands (GSH) and leave the bare gold clusters (Au25) supported over alumina and titania as a strategy for enhancing their catalytic activity and selectivity. The high resolution transmission electron microscope analysis confirmed the particle size of protected (1 wt % Au25(SG)18/Al2O3) and bare (1 wt % Au25/Al2O3) gold clusters over alumina have the same size distribution (∼1 nm). The TEM images of doped gold clusters over TiO2 before and after microwave treatment are clearer than alumina. This means the gold clusters over alumina and titania did not suffer from any agglomeration by microwave treatment. From nitrogen sorption isotherms at −196 °C for the protected and bare gold clusters over supports showed two different loop types of isotherms H3 and H1, respectively. Their specific surface area SBET, pore volume and average pore diameter were calculated. The pore size distribution of supported gold clusters before and after microwave treatment was measured. These results confirmed the ligands were removed from the supported gold clusters by microwave treatment. The pores in alumina and titania which were blocked by the protected clusters have become available. Reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in presence of NaBH4 was used as a test reaction to compare the catalytic activity of supported bare and protected gold clusters over alumina. The results showed that the 1 wt % Au25/Al2O3 catalyst exhibited the best catalytic performance in the reduction of 4-NP into 4-AP and revealed 100% conversion following 90 s stirring at room temperature. The extreme catalytic activity of the ultra small gold clusters attributed to their electronic properties and geometrical configuration.

Optical Absorption of Small Palladium‐Doped Gold Clusters

The effect of Pd doping on the structure and optical absorption of small cationic gold clusters is investigated by a combined photodissociation spectroscopy and time‐dependent density functional theory study of Aun+Arp and PdAun‐1+Arp (n = 4,5; p = 0,1). While pure Au clusters are planar, the Pd‐doped clusters are 3D. UV–visible absorption is studied in the 2.0–4.7 eV photon energy range, allowing the observation of previously unreported absorption bands for Au4+ and Au4+Ar. The oscillator strength of the optical transitions is dramatically reduced upon incorporating a Pd atom in Au4+ and Au4+Ar, while this effect is less pronounced for Au5+Ar. Analysis of the electron density transfer shows a different influence of Pd with size. While Pd has a formal negative charge in Au3Pd+, in Au4Pd+ most of the charge is attracted by the highly coordinated central Au atom, leaving Pd positively charged, also affecting the induced structural changes. In addition, orbital analysis of the optical transitions is carried out in order to identify the levels involved in the optical absorption of the pure Au and Pd doped clusters. A reduction of the s density near the Fermi energy, induced by Pd doping, causes a quenching of optical absorption.

Vibrational anharmonicity of small gold and silver clusters using the VSCF method.

We study the vibrational spectra of small neutral gold (Au2-Au10) and silver (Ag2-Au5) clusters using the vibrational self-consistent field method (VSCF) in order to account for anharmonicity. We report harmonic, VSCF, and correlation-corrected VSCF calculations obtained using a vibrational configuration interaction approach (VSCF/VCI). Our implementation of the method is based on an efficient calculation of the potential energy surfaces (PES), using periodic density functional theory (DFT) with a plane-wave pseudopotential basis. In some cases, we use an efficient technique (fast-VSCF) assisted by the Voter-Chen potential in order to get an efficient reduction of the number of pair-couplings between modes. This allows us to efficiently reduce the computing time of 2D-PES without degrading the accuracy. We found that anharmonicity of the gold clusters is very small with maximum rms deviations of about 1 cm(-1), although for some particular modes anharmonicity reaches values slightly larger than 2 cm(-1). Silver clusters show slightly larger anharmonicity. In both cases, large differences between calculated and experimental vibrational frequencies (when available) stem more likely from the quality of the electronic structure method used than from vibrational anharmonicity. We show that noble gas embedding often affects the vibrational properties of these clusters more than anharmonicity, and discuss our results in the context of experimental studies.

Phosphine passivated gold clusters: how charge transfer affects electronic structure and stability

A systematic evaluation of small phosphine ligand-protected gold clusters with six to nine gold atoms using density functional theory with dispersion correction has been performed in order to understand the major factors determining stability, including its size, shape, and charge dependence. We show that the charge per atom of the cluster is much more important for the interaction between the ligand shell and gold cluster than the system size. Thus, strong charge transfer effects determine the binding strength between the ligand shell and cluster. The clusters in this series are all non-spherical and exhibit large HOMO-LUMO gaps (above 2.7 eV). Analysis of the delocalized nature of the electronic states at the centre of the clusters demonstrates the presence of nascent superatomic states. However the number of delocalized electrons in these systems is significantly influenced by the charge transfer from the phosphine ligands, contrary to the usual accounting rule for superatom complex systems. Thus, not only electron withdrawing but also charge transfer effects should be considered to influence the superatomic structure of charged ligand surrounded clusters. In consequence in the phosphine gold cluster series under consideration the systems Au7(PPh3)7+ and Au8(PPh3)82+ exhibit nearly fully filled S and P states and the HOMO-LUMO gap increases by 0.2 eV and 0.9 eV, respectively. The interpretation for the stability of the gold phosphine systems is in agreement with experimental results and demonstrates the importance of the superatomic concept.

Investigation of the Electronic Excited States of Small Gold Clusters in Rare Gas Matrices: Spin-Orbit Time-Dependent Density Functional Theory Calculation.

The effects of the weak interactions of rare gas atoms on the UV-visible absorption spectra of gold dimer and tetramer clusters are investigated. The time-dependent density functional theory based on the two-component relativistic zeroth-order regular approximation that considered spin-orbit coupling is performed to estimate the absorption spectra of Au2,4-Rgn (Rg = Ne-Xe, and n = 1-6) complexes. Using spin-orbit, including the appropriate functional, shows a close correlation between experiment and our calculations. It is also demonstrated that the weak interactions between rare gas atoms and gold clusters affect the UV-vis spectra of Au2,4 clusters by shifting the electronic transition toward the blue. Moreover, we find that the order of change in peak position, Δν̃, is proportional to the strength of interactions: Δν̃Au2,4-Xe > Δν̃Au2,4-Kr > Δν̃Au2,4-Ar > Δν̃Au2,4-Ne. In addition, comparing the UV-visible spectra of Au2,4-Rgn complexes with those of isolated Au2 and Au4 clusters shows that for Au2,4-Rg2,4,6 complexes in which Rg atoms interacted symmetrically with gold clusters no additional peaks are observed compared to isolated clusters; however, for Au2,4-Rg1,3,5 complexes, extra peaks appear because of the decrease in symmetry.

Interaction between Small Gold Clusters and Nucleobases: A Density Functional Reactivity Theory Based Study

The thermodynamic and kinetic aspects associated with the interaction of small gold clusters (Aun, where n = 3–6) with nucleobases are assessed using a density functional reactivity theory based comprehensive decomposition analysis of stabilization energy scheme. It is observed that the trend of interaction between Aun clusters and nucleobases follows the order G > A > C > T > U. Also, the Watson–Crick base pair GC interacts with Aun clusters more preferably than that of the AT pair. The observed trend is further supported by conventional binding energy and transition-state calculations at B3PW91 and MP2 levels.

An alternative methodology to assess the quality of empirical potentials for small gold clusters

We present a methodology based on local comparisons of potential energy surfaces (PES) in order to assess the quality of empirical potentials. We compare five typical empirical potentials using a criterion that shows which of these potentials resembles better a PES obtained with a high-level electronic structure method. The methodology relies on a many-body expansion in terms of normal coordinates of both the empirical and high-level theory PES. Then we investigate in a systematical way, how the features of the reference high-level theory PES are reproduced by each empirical potential in the vicinity of a given minimum energy structure. We use plane-wave density functional theory (DFT) as a reference, in particular the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional and an ultrasoft Vanderbilt pseudo potential. This study is carried out on neutral gold clusters with up to five atoms.