Adsorbed Au Atoms Research Articles

Morphology modulation of Cu2S nanoparticles for cleaner Au (III) recovery from aqueous media: excellent capacity, illustrious selectivity

Developing an efficient and cleaner technology to recover gold (III) from aqueous media is vital for both economic and environmental perspectives. Herein, Cu2S was first found with well Au (III) recovery ability. Rod and flower-like Cu2S nanoparticles prepared by dynamic modulation, with jaggy micro-surface and relatively hard-wearing shape, was found with well Au (III) affinity. Results proved that this adsorbent could work better over a relatively wide pH from 2 to 5, and temperature from 25 to 55 °C, and that attained high gold capacity of 5,582 mg/g which obviously exceeded other reported adsorbents, indicating well application potential. XRD, TEM, and XPS studies demonstrated that adsorbed Au (III) was reduced as Au (0) by Cu2S. DFT studies verified that Au (III) recovery is related with the formation of “Cu-S-Au” bridge-like linkage and the unceasingly produced new adsorption sites after adsorbed Au atom’s migration and agglomeration. Au (III) recovery ability and selectivity in the actual sample was proved to be well too. Also, more than 90% gold on Cu2S could be recovered with HNO3. This study is of great theoretical and practical significance for cleaner Au (III) recovery from various aqueous media.

First-principles investigations of electronic, optical, and photocatalytic properties of Au-adsorbed MoSi2N4 monolayer

The electronic, optical, and photocatalytic properties of Au-adsorbed MoSi2N4 systems are predicted by first-principles calculations. Electronic structure analysis shows that the band gap is reduced from 1.89 eV (intrinsic MoSi2N4) to 0 eV (MoSi2N4–9Au). Calculation results of density of states indicate that the valence band edge is mainly contributed by the Mo 4d state, whereas the conduction band edge is contributed by both the Mo 4d and Si 3p states. As the number of adsorbed Au atoms is increased, the absorption coefficient increases from 3.0 × 104 cm−1 to 5.6 × 104 cm−1, meaning that more photons can be absorbed. Indeed, Au-adsorbed MoSi2N4 monolayer shows greater potential for applications such as water splitting and CO2 reduction based on redox potential analysis. The Au-adsorbed MoSi2N4 structures can effectively change the properties of materials, offering great potential in different fields.

Mechanism of Gold Cyanidation in Bioleaching of Precious Metals from Waste Printed Circuit Boards

Biocyanidation is an environment-friendly technology for recovering precious metals from waste printed circuit boards (WPCBs). Although the leaching mechanism has been studied a lot, Au-release behavior still remains unknown. In this paper, density functional theory (DFT) was employed to investigate the electronic structure of the complex existing in the Au cyanidation process. Extended charge decomposition analysis (ECDA) showed that the adsorption of CN– to Au and the adsorption of O to the formed [AuCN]⁻ caused net electron transfer of 0.356 and 0.574 au, respectively. Dissociation of O from [AuCNO]⁻ had a Gibbs free energy change of −154.229 kcal/mol. Electron localization function (ELF) and localized orbital locator (LOL) confirmed that CN– covalent bonding led to a transformation of the orbit localization region of the adsorbed Au atom. It might cause electrostatic repulsion from the nondirectly contacted Au atom. This speculation was demonstrated by the Mulliken overlap population analysis, which showed that CN– bonding caused antibonding interaction. The repulsive interaction would be an important factor triggering the release of the adsorbed Au atom. This work presented a new interpretation of Au cyanidation, providing important insights into Au-release behavior. It might help construct the leaching kinetics of multimetal resources to facilitate the recovery of precious metals from waste printed circuit boards.

Using Photoionization Efficiency Spectroscopy and Density Functional Theory to Investigate Charge Transfer Interactions in AuCe3On Clusters.

The characteristics of small cerium oxide and gold-cerium oxide clusters were investigated as models for gold attachment to various defect sites on a ceria surface. Photoionization efficiency (PIE) spectra of gas phase Ce3On (n = 0-4) and AuCe3On (n = 0-3) clusters were recorded and compared to spectral simulations based on DFT calculations. Calculated structures and PIE spectra for the Ce3O5,6 and AuCe3O4-6 clusters are also presented; however, these species were not detected during photoionization experiments. Addition of an Au atom to Ce3 was found to increase the energy of the ionization onset by ∼0.4 eV, whereas addition of one or more oxygen atoms decreases the onset by ∼0.25 eV. The optimized AuCe3On (n = 0-4) cluster geometries correlate with Au atoms adsorbed to oxygen vacancy sites while the AuCe3O5 and AuCe3O6 clusters are consistent with Au adsorption to CeO3 and CeO2 vacancies, respectively. The interactions between the cerium oxide cluster surface and the adsorbed Au atom were found to strongly depend on the nature the of the adsorption site. Au adsorbed to O vacancies are negatively charged with a Ce → Au charge transfer, whereas Au adsorbed to CeO2 and CeO3 vacancies have a reversed Au → Ce charge transfer, resulting in a positively charged Au atom. Au adsorption to the Ce3On clusters has the effect of (i) reducing the differences in the HOMO energies of the AuCe3O4, AuCe3O5, and AuCe3O6 clusters and (ii) lowering the binding energy of oxygen atoms for all AuCe3On (n = 1-6) clusters. Au adsorption appears to have a minimal effect on CeO2 vacancy formation, although CeO2 vacancies were calculated to form more readily than O vacancies on both the Ce3On and AuCe3On clusters. The low energy fragmentation calculated for the Ce3O5,6 and AuCe3O4-6 clusters, via loss of either Au, O, or CeO2, could potentially make photoionization experiments unfeasible since these clusters may simply dissociate when exposed to high energy photons above the ionization threshold.

Investigating Charge Transfer Interactions in AuCe2On Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory.

The properties of small cerium oxide and gold-cerium oxide clusters were explored as analogues for gold deposition at defect sites on a cerium oxide surface. Ce2On (n = 0-2) and AuCe2On (n = 0-2) clusters were prepared in the gas phase and investigated using photoionization efficiency spectroscopy complemented by spectral simulations based on DFT calculations; purely theoretical investigations were conducted on the Ce2O3, Ce2O4, AuCe2O3, and AuCe2O4 clusters due to these species not being detected. The optimized AuCe2On (n = 0-3) cluster geometries are consistent with Au adsorption to oxygen vacancy sites while the AuCe2O4 cluster correlates with Au adsorption to a CeO2 vacancy site. The electronic properties of the adsorbed Au atom depend strongly on the nature of the ceria adsorption site: O vacancy-adsorbed Au is negatively charged with a Ce → Au charge transfer occurring at the adsorption interface, whereas Au adsorbed to a CeO2 vacancy is positively charged with an Au → Ce charge transfer. The adsorbed Au atom is proposed to enhance the catalytic properties of the AuCe2On cluster by (i) stabilizing the negatively charged Au atom on reduced AuCe2On clusters to enhance nucleophilicity; (ii) increasing the electron accepting capability of the AuCe2O4 species; (iii) destabilizing the HOMO of the AuCe2O4 cluster; and (iv) facilitating the abstraction of additional surface oxygen atoms by reactants.

Self‐Assembly of Nanoparticle‐Spiked Pillar Arrays for Plasmonic Biosensing

AbstractPlasmonic biosensors have demonstrated superior performance in detecting various biomolecules with high sensitivity through simple assays. Scaled‐up, reproducible chip production with a high density of hotspots in a large area has been technically challenging, limiting the commercialization and clinical translation of these biosensors. A new fabrication method for 3D plasmonic nanostructures with a high density, large volume of hotspots and therefore inherently improved detection capabilities is developed. Specifically, Au nanoparticle‐spiked Au nanopillar arrays are prepared by utilizing enhanced surface diffusion of adsorbed Au atoms on a slippery Au nanopillar arrays through a simple vacuum process. This process enables the direct formation of a high density of spherical Au nanoparticles on the 1 nm‐thick dielectric coated Au nanopillar arrays without high‐temperature annealing, which results in multiple plasmonic coupling, and thereby large effective volume of hotspots in 3D spaces. The plasmonic nanostructures show signal enhancements over 8.3 × 108‐fold for surface‐enhanced Raman spectroscopy and over 2.7 × 102‐fold for plasmon‐enhanced fluorescence. The 3D plasmonic chip is used to detect avian influenza‐associated antibodies at 100 times higher sensitivity compared with unstructured Au substrates for plasmon‐enhanced fluorescence detection. Such a simple and scalable fabrication of highly sensitive 3D plasmonic nanostructures provides new opportunities to broaden plasmon‐enhanced sensing applications.

Structure and Electronic Properties of [Ca24Al28O64]4+·4e– Surfaces: Opportunities for Termination-Controlled Electron Transfer

We investigate the structure, thermodynamic stability, and electronic properties of (001), (110), and (111) surfaces of mayenite (12CaO·7Al2O3 or C12A7) in its oxygen-deficient phase known as mayenite-derived electride Ca12Al14O32 using simulations based on the density functional theory. Seven types of surface terminations were identified, depending on the orientation of the lattice nanosize cages with respect to the surface plane. We find that the surface termination has a strong effect on the distribution of electron anions in the near-surface region, as demonstrated by the range of the ionic charges and binding energies of adsorbed Au atoms and dimers. Notably, in all cases, Au species acquire a negative charge and preferentially adsorb as isolated ions rather than aggregate into clusters. These results show that C12A7 electride can provide a simple, yet versatile substrate to achieve high adsorption capacity and steric selectivity for single atom catalysts.

First-principles study of Au-decorated carbon nanotubes

The electronic structures and spin-orbit (SO) coupling of carbon nanotubes with adsorbed Au atoms are investigated based on density functional theory. Three kinds of zigzag single-walled CNT (8,0), (10,0) and (12,0) are selected. The Au atoms prefer to adsorb on the top of C atoms. The adsorption of Au atoms can introduce impurity states in the band gap, modifying the electronic properties of systems. Furthermore, the influence of SO coupling on these impurity states is also explored. Considerable SO splitting (∼130 meV) can be obtained. We find that the SO splitting decreases with the increase of the concentration of Au atoms, which can be ascribed to the interaction between Au atoms, suppressing the SO splitting. Our work provides imperative understanding on the electronic properties and SO coupling effect in Au-decorated CNTs.

Rotational Spectromicroscopy: Imaging the Orbital Interaction between Molecular Hydrogen and an Adsorbed Molecule.

A hydrogen molecule can diffuse freely on the surface and be trapped above an adsorbed molecule within the junction of a scanning tunneling microscope. The trapped dihydrogen exhibits the properties of a free rotor. Here we show that the intermolecular interaction between dihydrogen and Mg-porphyrin (MgP) can be visualized by imaging j=0 to 2 rotational excitation of dihydrogen. The interaction leads to a weakened H-H bond and modest electron donation from the dihydrogen to the lowest unoccupied molecular orbital of MgP, a process similarly observed for the interaction between dihydrogen and an adsorbed Au atom.

Improving the Adsorption of Au Atoms and Nanoparticles on Graphite via Li Intercalation

Supported nanoclusters have an important future in chemical processes such as catalysis. However, to optimize the properties of supported nanoclusters, attention must be paid to the electronic properties of both adsorbate and substrate materials. Highly ordered pyrolytic graphite is commonly used as a substrate for Au nanoclusters; however, cluster functionality and mobility is a problem on this inert surface. Therefore, we have designed a model for Li-doped graphite and investigated the electronic properties of adsorbed Au atoms and nanoclusters on this material using density functional theory (DFT). We find that increasing the concentration of Li atoms in the substrate results in improved adsorption for both Au atoms and Au16 nanoclusters onto the surface, with adsorption energies up to 0.96 and 1.50 eV, respectively, when using the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional. In the case of the Au16 nanocluster, charge transfer of >1 e is computed, which should make this supported system functionally suitable for reactions such as CO oxidation. Furthermore, a pseudoionic bond is observed in some cases for atomic Au over a surface C atom, though the presence of such chemical interaction is dependent on the exchange-correlation functional used.

Model of the CO oxidation reaction on Au-covered Mo(112)

The adsorption of O and CO and the CO oxidation reaction on the Au-covered Mo(112) surface have been studied by means of DFT calculations of binding and activation energies. As follows from previous studies [K. Fukutani et al., Appl. Surf. Sci. 256, (2010) 4796], adsorbed Au atoms create rods lying in the furrows of the Mo(112) surface. Due to a furrowed structure of the Mo(112), a p(1 × 1)Au monolayer does not cover the surface completely, and Mo atomic rows remain available for oxygen adsorption. It is found that oxygen adsorbs dissociatively on these Mo substrate atoms. In turn, CO molecules prefer adsorption sites on the Au rows atop Au atoms, so that CO and oxygen do not hinder each other from adsorption. The Au coating significantly decreases the binding energy of O on the Mo(112) surface. This feature is essential for the lowering of the barrier for CO oxidation, which is found to be as low as 0.19 eV. In presence of adsorbed O, the binding energy of CO is relatively small (0.29 eV), but increases to 0.64 eV when CO adsorbs on bilayer Au films. Hence, the p(1 × 4) and p(1 × 3) Au bilayer structures on Mo(112) surface are predicted to be efficient catalysts for CO oxidation.

Single Gold Atom Adsorption on the Fe3O4(111) Surface

For understanding the catalytic activity of Fe3O4-supported gold catalysts, the adsorption structures and energies of a single Au atom on the six terminations of the Fe3O4(111) surface have been computed at the level of density functional theory (GGA+U). For the most stable adsorption configurations, correlation has been found between the surface stability and the Au atom adsorption energy; that is, the more stable the surface, the lower the Au atom adsorption energy. It is also found that the adsorbed Au atom is reduced and has a negative charge on the iron-terminated surfaces, whereas it is oxidized and has a positive charge on the oxygen-terminated surfaces, and the latter is in agreement with the experimental observation. No correlation between the transferred charge and the adsorption energy has been found. Regarding the experimentally observed oxidation of gold nanoparticles on the iron oxide surface, it is possible to produce an oxygen-terminated surface for gold adsorption by synthetic tuning.

Electronic states of metal (Cu, Ag, Au) atom on CeO2(1 1 1) surface: The role of local structural distortion

Surface structures and electronic states of metal (M=Cu, Ag and Au) adsorbed on CeO2(111) surface have been investigated using first-principles density-functional theory calculations with the on-site Coulomb interaction taken into account. Results indicate that the adsorption is accompanied by an electron charge transfer between neutral metal atom and neighboring Ce4+ cation. Charge redistribution with electron localized on the various Ce cations neighboring to adsorbed atom is observed and can be attributed to local structural distortion effects. For the same adsorption site, the structural configuration with the electron localized on the surface Ce cation away from the adsorbed metal atom M is energetically favorable. The strongest adsorption interaction for Cu and Ag is located at 3-fold subsurface O-top site, whereas 2-fold surface O-bridge site is the most stable for Au adsorption. The order on the adsorption energy follows the sequence of Cu>Ag>Au, indicating that Cu atom has a stronger interaction with CeO2(111) surface than Ag and Au. Previous controversies about the localization of the transferred electron and the valence state of the adsorbed Au atom have been successfully clarified via analysis of the local structural distortions.

Strong spin–orbit splitting in graphene with adsorbed Au atoms

Spin–orbit (SO) splitting in graphene with adsorbed Au atoms is investigated by using a first-principles method. Considerable (∼200 meV) Rashba-type SO splitting can be achieved in the graphene π bands. When a Au atom is adsorbed above a C–C bond, Dresselhaus-type SO splitting is found to be present due to the absence of inversion symmetry and the substantial contribution of Au 5 d xz components. The influence of strains in graphene on SO splitting is also explored. A slight strain with the strength of −5% to 5% usually does not change much the SO splitting. The variation of SO splitting versus strain strength is rationalized through structural relaxation and effective hybridization between C 2 p z and certain Au 5 d states. Our study predicts a new way to increase the SO splitting in graphene and provides useful understanding of the mechanism.

First principles study of interactions between Au and N atoms on SrTiO3(001) surface

First principles PWPP calculations based on the density functional theory were carried out to study the microscopic mechanism of the interaction between Au and N atoms on SrTiO3(001) surface. From analysis of the related energies and density of states, it was confirmed that there is clearly a synergy effect between the substitution of N for the surface O atoms and the adsorption of Au atoms on the SrTiO3(001) surface. In other words, the substitution of N atoms for O atoms stabilizes the Au atoms adsorbed on the surface, and the presence of the adsorbed Au atoms can also favor the process of N substitution on the surface.

Atomic structure and formation mechanism of identically sized Au clusters grown on Si(111)-(7×7) surface

Identically sized Au clusters are grown on the Si(111)-(7×7) surface by room temperature deposition of Au atoms and subsequent annealing at low-temperature. The topographical images investigated by in situ scanning tunneling microscopy show a bias-dependent feature. The current-voltage properties measured by scanning tunneling spectroscopy indicate some semiconducting characteristics of the Au adsorbed surface, which is attributable to the saturation of Si dangling bonds. These experimental results, combined with the simulated scanning tunneling microscopy images and the first-principles adsorption energy calculations, show that the Au cluster is most likely to have a Au(6)Si(3) structure. In the Au(6)Si(3) cluster, three adsorbed Au atoms replace the three Si center adatoms, forming a hollow triangle, while the replaced Si atoms and other three Au atoms connect into a hexagon locating within the triangle. The formation mechanism of this atomic configuration is intimately associated with the complicated chemical valences of Au and the specific annealing conditions.

Effect of the Support on the Electronic Structure of Au Nanoparticles Supported on Transition Metal Carbides: Choice of the Best Substrate for Au Activation

Periodic density functional theory calculations on large supercells have been carried out to investigate the atomic and electronic structure of small gold particles (Au2, Au4, Au9, Au13, and Au14) supported on the (001) surface of various transition metal carbides (TiC, ZrC, VC, and δ-MoC). All the supported Au particles exhibited strong interactions with the C sites of the metal−carbide surfaces. Nevertheless, the interactions between adsorbed Au atoms were attractive, thus ultimately facilitating nucleation of two- or three-dimensional metal particles. The presence of the underlying carbide strongly modified the electronic structure and charge density of the supported metal particles resulting in the experimentally proven improved catalytic performance of the resulting systems as compared with cases where the support is an oxide. The electronic perturbations were quite strong for two-dimensional gold particles directly in contact with the carbide substrates and gradually decreased for two-layer and three-layer thick supported particles. While all the metal carbides examined induced a qualitatively similar perturbation on the supported Au particles, the effect is significantly larger for ZrC thus suggesting that the resulting model catalyst would perform even better than the already tried Au/TiC system.

Adsorption and diffusion of Au atoms on the (001) surface of Ti, Zr, Hf, V, Nb, Ta, and Mo carbides

The adsorption of atomic Au on the (001) surface of TiC, ZrC, HfC, VC, NbC, TaC, and delta-MoC and the mechanism of diffusion of this adatom through the surface have been studied in terms of a periodic density functional theory based approach. In all the cases, the Au adsorption energies are in the range of 1.90-2.35 eV. The moderately large adsorption energies allow the Au diffusion before desorption could take place. For TiC(001), ZrC(001), and HfC(001), atomic Au is adsorbed directly on top of C atoms and diffusion takes place along the diagonal of the squares formed by M-C-M-C atoms with the transition state located above the hollow sites. For the rest of transition metal carbides the situation is less simple with the appearance of more than one stable adsorption site, as for NbC and TaC, of a small energy barrier for diffusion around the most stable adsorption site and of a more complex diffusion pathway. The small energy barrier for diffusion around the most stable site will result in a highly mobile Au species which could be observed in scanning tunnel microscope experiments. After depositing Au on metal-carbide surfaces, there is a noticeable charge transfer from the substrate to the adsorbed Au atom. The electronic perturbations on Au increase when going from TiC to ZrC or TaC. Our results indicate that metal carbides should be better supports for the chemical activation of Au than metal oxides.

Tuning Optical Properties of Magic Number Cluster (SiO2)4O2H4 by Substitutional Bonding with Gold Atoms

By bonding gold atoms to the magic number cluster (SiO(2))(4)O(2)H(4), two groups of Au-adsorbed shell-like clusters Au(n)(SiO(2))(4)O(2)H(4-n) (n = 1-4) and Au(n)(SiO(2))(4)O(2) (n = 5-8) were obtained, and their spectral properties were studied. The ground-state structures of these clusters were optimized by density functional theory, and the results show that in despite of the different numbers and types of the adsorbed Au atoms, the cluster core (SiO(2))(4)O(2) of T(d) point-group symmetry keeps almost unchanged. The absorption spectra were obtained by time-dependent density functional theory. From one group to the other, an extension of absorption wavelength from the UV-visible to the NIR region was observed, and in each group the absorption strengths vary linearly with the number of Au atoms. These features indicate their advantages for exploring novel materials with easily controlled tunable optical properties. Furthermore, due to the weak electronic charge transfer between the Au atoms, the clusters containing Au(2) dimers, especially Au(8)(SiO(2))(4)O(2), absorb strongly NIR light at 900 approximately 1200 nm. Such strong absorption suggests potential applications of these shell-like clusters in tumor cells thermal therapy, like the gold-coated silica nanoshells with larger sizes.

Structural and Electronic Properties of Au and Au2on an MgO(100) Surface: A DFT Cluster Embedding Approach

The structural and electronic properties of Au and Au2 adsorbed on five coordinated oxygen and magnesium regular terrace sites, on the neutral Fs color center (oxygen vacancy containing two electrons), and on the charged Fs+ color center (oxygen vacancy containing one electron) of the MgO(100) surface have been investigated using a density functional theory cluster embedding approach. Two types of calculations have been performed. In one, we optimized and calculated the energetics of the systems using a generalized gradient approximation, and in the second, we used a combined approach, optimizing the geometries using a local density approximation and then performing a single point energy calculation using a generalized gradient approximation. Our studies also examine the effect of relaxation of the substrate. This was accomplished through two studies: first, both the MgO surface and the Au atoms were optimized, and second, the MgO surface atoms were kept fixed while the Au atoms were allowed to relax. We report the geometry, adsorption, and dimerization energies of the lowest energy and low energy isomers at each adsorption site, charge transfers from the MgO surface to the Au atoms in the lowest energy isomers, and compare our findings with previous theoretical studies. This comparison allowed us to establish in most of the cases limits for the adsorption energies, binding energies, and geometrical parameters at each adsorption site. For the case of Au2, for each adsorption site, our studies confirm the lowest energy isomer and most of the low energy isomers, previously reported, when both the surface and the Au2 are optimized. For most cases, the studies with frozen MgO surface atoms produced similar results as the studies when the MgO surface atoms were optimized. However, for the Mg site we found that the restrictions introduced by keeping the surface frozen favored the coexistence of an additional low energy isomer, and for the Fs+ site, the restrictions favored the dissociation of the adsorbed Au2 dimer into two surface adsorbed Au atoms. In general, both fully generalized gradient and combined calculations produced similar results, though, in the case of the Fs site the combined calculations found another low lying isomer, and in the case of the Fs+ site it predicted the dissociation of the adsorbed Au2 dimer, and inverted the energetic order of the lowest energy and low energy isomers. An increase in the charge transfer from the MgO surface to the Au atoms at the different adsorption sites according with the number of localized electrons at the adsorption sites was found. Finally, we found that the site with the largest adsorption and dimerization energies, and where an Au atom is most stable and can grow, is the neutral Fs center.