Alloy formation of supported gold nanoparticles at their transition from clusters to solids: does size matter?

Silver Decoration of Vertically Aligned MoS2-MoOx Nanosheets: A Comprehensive XPS Investigation

This study investigates the simultaneous decoration of vertically aligned molybdenum disulfide nanostructure (VA-MoS2) with Ag nanoparticles (NPs) and nitrogen functionalization. Nitrogen functionalization was achieved through physical vapor deposition (PVD) DC-magnetron sputtering using nitrogen as a reactive gas, aiming to induce p-type behavior in MoS2. The utilization of reactive sputtering resulted in the growth of three-dimensional silver structures on the surface of MoS2, promoting the formation of silver nanoparticles. A comprehensive characterization was conducted to assess surface modifications and analyze chemical and structural changes. X-ray photoelectron spectroscopy (XPS) showed the presence of silver on the MoS2 surface. Scanning electron microscopy (SEM) confirmed successful decoration with silver nanoparticles, showing that deposition time affects the size and distribution of the silver on the MoS2 surface.

Zeolite Synthesis in the Presence of Metallosiloxanes for the Quantitative Encapsulation of Metal Species for the Selective Catalytic Reduction (SCR) of NOx

Metal encapsulation in zeolitic materials through one-pot hydrothermal synthesis (HTS) is an attractive technique to prepare zeolites with a high metal dispersion. Due to its simplicity and the excellent catalytic performance observed for several catalytic systems, this method has gained a great deal of attention over the last few years. While most studies apply synthetic methods involving different organic ligands to stabilize the metal under synthesis conditions, here we report the use of metallosiloxanes as an alternative metal precursor. Metallosiloxanes can be synthesized from simple and cost-affordable chemicals and, when used in combination with zeolite building blocks under standard synthesis conditions, lead to quantitative metal loading and high dispersion. Thanks to the structural analogy of siloxane with TEOS, the synthesis gel stabilizes by forming siloxane bridges that prevent metal precipitation and clustering. When focusing on Fe-encapsulation, we demonstrate that Fe-MFI zeolites obtained by this method exhibit high catalytic activity in the NH3 -mediated selective catalytic reduction (SCR) of NOx along with a good H2 O/SO2 tolerance. This synthetic approach opens a new synthetic route for the encapsulation of transition metals within zeolite structures.

Gold nanoparticles synthesis and immobilization by atmospheric pressure DBD plasma torch method

Herein, we report the impact of plasma on gold nanoparticles synthesis. We used an atmospheric plasma torch fed with an aerosolized tetrachloroauric(iii) acid trihydrate (HAuCl₄·3H₂O) solution. The investigation showed that using pure ethanol as a solvent for the gold precursor enabled a better dispersion compared to a water-containing solution. We demonstrated here that the deposition parameters are easy to control, presenting the influence of solvent concentration and deposition time. The advantage of our method is that no capping agent was used. We assume that plasma creates a carbon-based matrix around the gold nanoparticles preventing them to agglomerate. The XPS results revealed the impact of using plasma. Metallic gold was detected in the plasma-treated sample, whereas the no-plasma sample revealed only Au(i) and Au(iii) contributions originating from the HAuCl₄ precursor. Detailed HRTEM, EDS mapping, and SAED analyses led to more insights into the structure.

Structural and optical characterization of nanoalloys mixing gold or silver with aluminium or indium: evolution under various reactive environments

In this study, the atomic and chemical structure and the optical response of A_xB_1-x bimetallic nanoparticles (BNPs) combining gold or silver (A) with aluminium or indium (B) were investigated at various stoichiometries in order to examine if stable alloyed phases could exist and promote the emergence of localized surface plasmon resonance (LSPR) in the UV range. The structure and morphology of BNPs of a few nanometres, produced by laser vaporization, were analysed by transmission electron microscopy (TEM) and optical absorption measurements were performed on matrix-embedded BNPs. Information about the oxidation state of the BNPs can be inferred from a comparison between experimental optical spectra and Mie calculations in the dipolar approximation. The BNPs' internal structures were further investigated by additional characterization techniques. Firstly, in situ X-ray photoelectron spectroscopy provided information about the chemical state of the constituent elements and their evolution with time. Secondly, synchrotron-based X-ray scattering techniques were performed on Ag-Al BNPs in a wide-angle configuration under grazing incidence, giving complementary information about structural and morphological heterogeneities in the BNPs. Finally, the restructuring of the partially oxidized Au_0.33Al_0.67 BNPs annealed in a reducing atmosphere was also attempted by environmental TEM. The complementary techniques of characterization show that silver-based Ag-In and Ag-Al BNPs form metallic silver-rich alloyed cores surrounded by an indium or aluminium oxide shell. The initial LSPR is in the UV range for both systems, but the difference in the kinetics of oxidation between indium and aluminium involves less blue-shifted LSPR for Ag-Al BNPs. In the case of gold-based BNPs, we show evidence of ordered nanoalloys just after air exposure and the appearance of gold and indium (or aluminium) demixing during oxidation. The initial LSPR of Au-In BNPs is the one the most in the UV range among the four systems, with an LSPR peak centred at 254 nm, which may be a sign of the formation of the Au_0.33In_0.67 alloy. Nevertheless, strategies to preserve BNPs from oxidation have to be developed.

Synthesis of Self-Assembled Single Atomic Layer Gold Crystals-Goldene

We report, for the first time, a technique to synthesize free-standing, one-atom thick 2D gold crystals (namely, goldene) and self-assembled 2D periodic arrays of goldene. High-resolution transmission electron microscopy (HRTEM) imaging of goldene revealed herringbone and honeycomb lattices, which are primarily gold surface features due to its reconstruction. Imaging of these surface-only features by a nonsurface characterization technique such as HRTEM is an unequivocal proof of the absence of three-dimensionality in goldene. Atomic force microscopy confirmed 1-2 Å thickness of goldene. High-resolution X-ray photoelectron spectroscopy (HR-XPS), selective area electron diffraction, and energy-dispersive X-ray spectroscopy confirmed the chemical identity of goldene. We discovered the phenomenon of electric field-induced self-assembly of goldene supracrystals with a herringbone structure and developed an electric field printing (e-print) technique for goldene arrays. Goldene showed a semiconductor response with a knee voltage of ∼3.2 V, and I/V spectroscopy revealed periodic room temperature Coulomb blockade oscillations. These observations are consistent with the theoretical calculations reported in the literature predicting enhanced Coulombic interactions between gold valence electrons and the nucleus in stable 2D gold. Goldene exhibited multiple, intense, and well-resolved optical absorption peaks and several fine bands across the UV-vis region, and we calculated its optical band gap to be 3.59 eV. Magnetic force microscopy measurements of goldene periodic arrays showed a ∼5 mV peak amplitude confirming its ferromagnetism. Optical and magnetic properties of goldene are consistent with those reported in the literature for 2D planar gold clusters with less than 12 atoms.

Au nanoparticles on Fe-modified rutile TiO2(110): Dispersion, thermal stability, and CO adsorption

Gold clusters on an iron-modified rutile TiO₂(110) surface have been characterized via scanning tunneling microscopy and x-ray photoelectron spectroscopy. This study is focused on the impact of submonolayer preadsorbed Fe on the morphologies, surface compositions, and thermal stabilities of bimetallic Au-Fe systems by comparing them to elemental Au and Fe adsorbates. We found that a submonolayer gold adsorbate followed the nucleation mode of the iron precursor, which considerably enhanced the dispersion of nano-gold while improving its thermal stability. Finally, the temperature-programmed CO desorption spectra of Au and Au-Fe nanoparticles on TiO₂(110) were compared.

Gold Nanoparticles Supported on Urchin-Like CuO: Synthesis, Characterization, and Their Catalytic Performance for CO Oxidation

Gold catalysts have been studied in-depth due to their unique activities for catalytic CO oxidation. Supports have intrinsic motivation for the high activity of gold catalysts. Thermally stable urchin-like CuO microspheres, which are potential support for gold catalysts, were prepared by facile solution-method. Then gold nanoparticles were loaded on them by deposition-precipitation method. The obtained gold catalysts were characterized by SEM, XRD, TEM, BET, ICP, and XPS. Their catalytic activity for CO oxidation was also evaluated. TEM results revealed that the gold nanoparticles with small sizes were highly distributed on the CuO surface in Au_1.0/CuO-300. XPS observations demonstrated that the gold species in Au_1.0/CuO-300 was of metallic state. Among the as-prepared catalysts, the Au_1.0/CuO-300 catalyst displayed the best performance for CO oxidation and achieved 100% CO oxidation at 80 °C. It kept 100% conversion for 20 h at a reaction temperature of 180 °C, and showed good reusability after three reaction-cycles. The possible catalytic mechanism of Au_1.0/CuO-300 catalyst for CO oxidation was also briefly proposed.

Hydrophobic Paper-Based SERS Sensor Using Gold Nanoparticles Arranged on Graphene Oxide Flakes

Paper-based surface-enhanced Raman scattering (SERS) sensors have garnered much attention in the past decade owing to their ubiquity, ease of fabrication, and environmentally friendly substrate. The main drawbacks of a paper substrate for a SERS sensor are its high porosity, inherent hygroscopic nature, and hydrophilic surface property, which reduce the sensitivity and reproducibility of the SERS sensor. Here, we propose a simple, quick, convenient, and economical method for hydrophilic to hydrophobic surface modification of paper, while enhancing its mechanical and moisture-resistant properties. The hydrophobic paper (h-paper) was obtained by spin-coating diluted polydimethylsiloxane (PDMS) solution onto the filter paper, resulting in h-paper with an increased contact angle of up to ≈130°. To complete the h-paper-based SERS substrate, gold nanoparticles arranged on graphene oxide (AuNPs@GO) were synthesized using UV photoreduction, followed by drop-casting of AuNPs@GO solution on the h-paper substrate. The enhancement of the SERS signal was then assessed by attaching a rhodamine 6G (R6G) molecule as a Raman probe material to the h-paper-based SERS substrate. The limit of detection was 10 nM with an R2 of 0.966. The presented SERS sensor was also tested to detect a thiram at the micromolar level. We expect that our proposed AuNPs@GO/h-paper-based SERS sensor could be applied to point-of-care diagnostics applications in daily life and in spacecraft.

Stable graphene oxide-gold nanoparticle platforms for biosensing applications

Graphene oxide-gold nanoparticle (AuNPs@GO) hybrids were fabricated in water dispersions of graphene oxide (GO) and Au precursor completely free of stabilizing agents by UV-light irradiation. Gold nanoparticle (AuNP) nucleation, growth, and stabilization mechanisms at the surface of GO are discussed on the basis of UV-Vis, Raman, IR, and X-Ray photo-spectroscopy studies. The analyses of AuNPs@GO hybrids by transmission electron microscopy (TEM), thermogravimetric (TGA) and electrochemical tests show that they exhibit outstanding chemical, thermal and electrochemical stabilities. Thus, AuNPs@GO biosensing platforms were fabricated for surface enhanced Raman spectroscopy (SERS) detection of crystal violet (CV), a SERS standard molecule, and in a different set of experiments, for flavin adenine dinucleotide (FAD), a flavoprotein coenzyme that plays an important role in many oxidoreductase and reversible redox conversions in biochemical reactions. AuNPs@GO hybrids synthesized by using UV light irradiation show exceptional stability and high intensification of the Raman signals showing that they have high potential for use as biomedical probes for the detection, monitoring, and diagnosis of medical diseases.

Impact of Nonideal Nanoparticles on X-ray Photoelectron Spectroscopic Quantitation: An Investigation Using Simulation and Modeling of Gold Nanoparticles

Quantitative X-ray photoelectron spectroscopic (XPS) analysis combined with spectral modeling of photoelectrons can be valuable while investigating the surface chemistry of nanoparticles (NPs) with different morphologies. Herein, with the use of NIST Simulation of Electron Spectra for Surface Analysis (SESSA), a comparative analysis of experimental and simulated photoelectron peak intensities in gold nanoparticles (AuNPs) of different morphologies is presented. Three sets of supported AuNPs with different morphologies were selected from a series of as synthesized Au-TiO₂ catalyst samples. Using transmission electron microscopy (TEM) analyzed morphological information on the AuNPs as input model parameters in SESSA, XPS spectra were generated from the respective input NP morphologies. A degree of greater mismatch between SESSA simulated and experimental XPS spectra was observed while using the TEM obtained average diameter of the nanoparticles. The degree of mismatch lowered when the true nonspherical shape of the nanoparticles as obtained from TEM images was taken into account for the simulation. This demonstrates the impact of surface morphology on the XPS peak intensities which needs to be incorporated to obtain precise quantified information from the supported nanoparticles. This work demonstrates the applicability of SESSA in combination with experimental XPS and TEM measurements for precise quantification of XPS spectra from complex, nonideal shaped nanoparticles. This study can be extended to include a broad range of nanoparticles with ideal or nonideal geometries, thus providing a simple method to utilize quantitative XPS analysis to a wide range of nanomaterials.

Au 4f spin–orbit coupling effects in supported gold nanoparticles

We reveal that the ratio of spin–orbit components in X-ray photoelectron Au 4f spectra of titania-supported gold nanoparticles deviates from the statistical ratio 4 : 3 and demonstrates an appreciable dependence on the concentration of Au atoms on the surface of TiO₂ support and size of Au nanoparticles.

Unexpected synthesis of an Au2In2 tetrametallatricyclic complex from α-aminophosphines and formation of Au-In-P and Ag-In-P nanomaterials

Four Au-(μ-phosphinite)-In units assemble to form an unprecedented Au2In2 12-membered metallacycle which intersects at the In centres an 8-membered ring containing two In-μ-phosphinate linkages, resulting in a tricyclic structure. Thermal treatment of this complex and of its Ag(I) analog affords Au-In-P and Ag-In-P nanomaterials, respectively.

Activated carbon supported ternary gold–cesium(i)–indium(iii) catalyst for the hydrochlorination of acetylene

Promoting the effect of indium(iii) on activated carbon supported gold–cesium(i) catalyst in acetylene hydrochlorination.

Nanoscale architecture of bimetallic hybrid Fe–Au nanostructures with and without 1,4-phenylene diisocyanide pre-functionalization

The nanoscale architecture of a hybrid material has been very important in deriving a new interesting synergic property.

Controlled synthesis of ultrathin ZnO nanowires using micellar gold nanoparticles as catalyst templates

We demonstrate a simple and effective approach to control the diameter of ultrathin ZnO nanowires with high aspect ratios and high densities over large areas. Diblock copolymer-based nanoparticle arrays exhibiting a high degree of hexagonal order and offering easy control of particle size (typically 1-10 nm) and interparticle spacing (25-150 nm) are utilized as nanocatalysts for the subsequent growth of semiconductor nanowires. The as-grown ZnO nanowires exhibit a single crystal hexagonal wurtzite structure and grow along the [0002] direction. Facetted catalyst particles were observed at the tip of the nanowires after synthesis, thus suggesting a catalyst-assisted vapor-solid-solid (VSS) rather than a vapor-liquid-solid (VLS) growth mechanism, the latter being frequently used in semiconductor nanowire production. Such a growth process allows us to easily prepare ultrathin ZnO nanowires with tunable diameters well below 10 nm by taking advantage of the inherent size control of the micellar method during deposition of the catalyst nanoparticles. Raman spectroscopy reveals a phonon confinement effect as the diameter of nanowires decreases. Photoluminescence spectra of these ultrathin nanowires indicate a blue shift of the free excitons and their phonon replicas by 37 meV induced by quantum confinement.

Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au

The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure-activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.

Interaction of Au with thin ZrO2 films: influence of ZrO2 morphology on the adsorption and thermal stability of Au nanoparticles

The model catalysts of ZrO(2)-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO(2) films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO(2) films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO(2) films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO(2) film through the procedure of first spin-coating a zirconium ethoxide (Zr(OC(2)H(5))(4)) precursor onto a SiO(2)/Si(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous "sponge-like nanostructures" were obtained in this case. The second method was epitaxial growth of a ZrO(2)(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10(-6) Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO(2) film as compared to those on the ordered ZrO(2)(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO(2) surface than on the ZrO(2)(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO(2)(111) but not from porous ZrO(2). The enhanced thermal stability for Au on porous ZrO(2) can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures.

Chemical alignment of DNA origami to block copolymer patterned arrays of 5 nm gold nanoparticles

We have used block copolymer patterned arrays of 5 nm gold nanoparticles (AuNPs) for chemically aligned surface attachment of DNA origami. Addition of single-stranded DNA-thiol to AuNPs allowed a base paired attachment of sticky end modified DNA origami. Results indicate a stable, selective attachment between the DNA origami and ssDNA modified AuNPs. Yield data showed 74% of AuNP binding sites forming an attachment with a DNA origami rectangle, and control surfaces showed less than 0.5% nonspecific adsorption.

Study of the interface between rhodium and carbon nanotubes

X-ray photoelectron spectroscopy at 3.5 keV photon energy, in combination with high-resolution transmission electron microscopy, is used to follow the formation of the interface between rhodium and carbon nanotubes. Rh nucleates at defect sites, whether initially present or induced by oxygen-plasma treatment. More uniform Rh cluster dispersion is observed on plasma-treated CNTs. Experimental results are compared to DFT calculations of small Rh clusters on pristine and defective graphene. While Rh interacts as strongly with the carbon as Ti, it is less sensitive to the presence of oxygen, suggesting it as a good candidate for nanotube contacts.

Photoemission spectroscopy of tethered CdSe nanocrystals: shifts in ionization potential and local vacuum level as a function of nanocrystal capping ligand

We report the characterization of the frontier orbital energies and interface dipole effects for bare and ligand-capped 3.6 and 6.0 nm diameter CdSe nanocrystals (NC) tethered to smooth gold substrates, using He(I) and He(II) UV photoemission spectroscopy. Changes in the ionization potential (IP) of the NCs and local effective work function of the films were explored as a function of the dipolar nature of the NC capping ligands. The addition of thiol-capping ligands 1-hexanethiol, 1-benzenethiol, and 4-fluorothiophenol to both sizes of NCs produces negligible shifts in energy offset between the high kinetic energy edge of the CdSe NCs and the gold substrate Fermi energy. However, the local vacuum level and IP of the nanocrystal layer are altered by as much as 0.3 eV. We demonstrate the importance of determining both the local vacuum level and the high kinetic energy edge of a tethered NC sample. These studies demonstrate a method that can be used in the future to characterize the frontier orbital energy offsets for modified or unmodified nanocrystalline films, in which the NCs are incorporated into host materials, for applications ranging from photovoltaics to light-emitting diodes.

Interfacial Bonding of Gold Nanoparticles on a H-terminated Si(100) Substrate Obtained by Electro- and Electroless Deposition

Dome-shaped gold nanoparticles (with an average diameter of 10.5 nm) are grown on H-terminated Si(100) substrates by simple techniques involving electro- and electroless deposition from a 0.05 mM AuCl3 and 0.1 M NaClO4 solution. XPS depth profiling data (involving Au 4f core-level and valence band spectra) reveal for the first time the formation of gold silicide at the interface between the Au nanoparticles and Si substrate. UV-visible diffuse reflectance spectra indicate that both samples have surface plasmon resonance maxima at 558 nm, characteristic of an uniform distribution of Au nanoscale particles of sufficiently small size. Glancing-incidence XRD patterns clearly show that the deposited Au nanoparticles belong to the fcc phase, with the relative intensity of the (220) plane for Au nanoparticles obtained by electroless deposition found to be notably larger than that by electrodeposition.

Effects of charging and tunnelling in a structure based on magic and non-magic metal clusters

The effects of charging and single-electron tunnelling in a metal cluster structure are investigated theoretically. In the framework of the particle-in-a-box model for spherical and disc-shaped gold clusters, the electron spectrum and the temperature dependence of the electron chemical potential are calculated. The difference between the electron chemical potentials of massive electrodes and islands leads to the noticeable charging of the electrode. We show that the effective residual charge is equal to the non-integer value of the elementary charge e and depends on the shape of the cluster. The equations for the analysis of the current-voltage characteristic are used under the conditions of conservation of the total energy of the structure, taking into account the contact potential difference. Restrictions associated with the Coulomb instability of a cluster are introduced into the theory in a simple way. It is shown that the critical charge of the cluster in an open electron system is close to the effective residual charge. For single-electron molecular transistors based on small gold clusters the current gap and its voltage asymmetry are computed. We demonstrate that the current gap exhibits non-monotonic size dependences which are related to the quantization of the electron spectrum and the Coulomb blockade.