Nucleation and Island Growth of Alkanethiolate Ligand Domains on Gold Nanoparticles

The metal oxide cluster α-AlW₁₁O₃₉^9– (<b>1</b>), readily imaged by cryogenic transmission electron microscopy (cryo-TEM), is used as a diagnostic protecting anion to investigate the self-assembly of alkanethiolate monolayers on electrostatically stabilized gold nanoparticles in water. Monolayers of <b>1</b> on 13.8 ± 0.9 nm diameter gold nanoparticles are displaced from the gold surface by mercaptoundecacarboxylate, HS(CH₂)₁₀CO₂^– (<b>11-MU</b>). During this process, no aggregation is observed by UV–vis spectroscopy, and the intermediate ligand-shell organizations of <b>1</b> in cryo-TEM images indicate the presence of growing hydrophobic domains, or “islands”, of alkanethiolates. UV–vis spectroscopic “titrations”, based on changes in the surface plasmon resonance upon exchange of <b>1</b> by thiol, reveal that the 330 ± 30 molecules of <b>1</b> initially present on each gold nanoparticle are eventually replaced by 2800 ± 30 molecules of <b>11-MU</b>. UV–vis kinetic data for <b>11-MU</b>-monolayer formation reveal a slow phase, followed by rapid self-assembly. The Johnson, Mehl, Avrami, and Kolmogorov model gives an Avrami parameter of 2.9, indicating continuous nucleation and two-dimensional island growth. During nucleation, incoming <b>11-MU</b> ligands irreversibly displace <b>1</b> from the Au-NP surface <i>via</i> an associative mechanism, with <i>k</i>_nucleation = (6.1 ± 0.4) × 10² M^–1 s^–1, and 19 ± 8 nuclei, each comprised of <i>ca</i>. 8 alkanethiolates, appear on the gold-nanoparticle surface before rapid growth becomes kinetically dominant. Island growth is also first-order in [<b>11-MU</b>], and its larger rate constant, <i>k</i>_growth, (2.3 ± 0.2) × 10⁴ M^–1 s^–1, is consistent with destabilization of molecules of <b>1</b> at the boundaries between the hydrophobic (alkanethiolate) and the electrostatically stabilized (inorganic) domains

Nanoneedles of Mixed Transition Metal Phosphides as Bifunctional Catalysts for Electrocatalytic Water Splitting in Alkaline Media

In this work, mixed Ni/Co and Ni/Fe metal phosphides with different metal ratios were synthesized through the phosphidization of high-surface-area hydroxides grown hydrothermally on carbon cloth. The materials were characterized by means of X-ray photoemission spectroscopy, X-ray diffraction, energy dispersive X-ray analysis, and electron microscopies. The electrocatalytic performance in the electrochemical water splitting was tested in alkaline media. With the aim of determining the chemical stability of the mixed phosphides and the possible changes undergone under catalytic conditions, the materials were characterized before and after the electrochemical tests. The best performances in the hydrogen evolution reaction were achieved when synergic interactions are established among the metal centers, as suggested by the outstanding performances (50 mV to achieve 10 mA/cm2) of materials containing the highest amount of ternary compounds, i.e., NiCoP and NiFeP. The best performances in the oxygen evolution reaction were reached by the Ni-Fe materials. Under these conditions, it was demonstrated that a strong oxidation of the surface and the dissolution of the phosphide/phosphate component takes place, with the consequent formation of the corresponding metal oxides and hydroxides

Nucleation and Island Growth of Alkanethiolate Ligand Domains on Gold Nanoparticles

The metal oxide cluster α-AlW₁₁O₃₉^9– (<b>1</b>), readily imaged by cryogenic transmission electron microscopy (cryo-TEM), is used as a diagnostic protecting anion to investigate the self-assembly of alkanethiolate monolayers on electrostatically stabilized gold nanoparticles in water. Monolayers of <b>1</b> on 13.8 ± 0.9 nm diameter gold nanoparticles are displaced from the gold surface by mercaptoundecacarboxylate, HS(CH₂)₁₀CO₂^– (<b>11-MU</b>). During this process, no aggregation is observed by UV–vis spectroscopy, and the intermediate ligand-shell organizations of <b>1</b> in cryo-TEM images indicate the presence of growing hydrophobic domains, or “islands”, of alkanethiolates. UV–vis spectroscopic “titrations”, based on changes in the surface plasmon resonance upon exchange of <b>1</b> by thiol, reveal that the 330 ± 30 molecules of <b>1</b> initially present on each gold nanoparticle are eventually replaced by 2800 ± 30 molecules of <b>11-MU</b>. UV–vis kinetic data for <b>11-MU</b>-monolayer formation reveal a slow phase, followed by rapid self-assembly. The Johnson, Mehl, Avrami, and Kolmogorov model gives an Avrami parameter of 2.9, indicating continuous nucleation and two-dimensional island growth. During nucleation, incoming <b>11-MU</b> ligands irreversibly displace <b>1</b> from the Au-NP surface <i>via</i> an associative mechanism, with <i>k</i>_nucleation = (6.1 ± 0.4) × 10² M^–1 s^–1, and 19 ± 8 nuclei, each comprised of <i>ca</i>. 8 alkanethiolates, appear on the gold-nanoparticle surface before rapid growth becomes kinetically dominant. Island growth is also first-order in [<b>11-MU</b>], and its larger rate constant, <i>k</i>_growth, (2.3 ± 0.2) × 10⁴ M^–1 s^–1, is consistent with destabilization of molecules of <b>1</b> at the boundaries between the hydrophobic (alkanethiolate) and the electrostatically stabilized (inorganic) domains