Solvent manipulation of the pre-reduction metal-ligand complex and particle-ligand binding for controlled synthesis of Pd nanoparticles

Understanding how to control the nucleation and growth rates is crucial for designing nanoparticles with specific sizes and shapes. In this study, we show that the nucleation and growth rates are correlated with the thermodynamics of metal-ligand/solvent binding for the pre-reduction complex and the surface of the nanoparticle, respectively. To obtain these correlations, we measured the nucleation and growth rates by in situ small angle X-ray scattering during the synthesis of colloidal Pd nanoparticles in the presence of trioctylphosphine in solvents of varying coordinating ability. The results show that the nucleation rate decreased, while the growth rate increased in the following order, toluene, piperidine, 3,4-lutidine and pyridine, leading to a large increase in the final nanoparticle size (from 1.4 nm in toluene to 5.0 nm in pyridine). Using density functional theory (DFT), complemented by 31P nuclear magnetic resonance and X-ray absorption spectroscopy, we calculated the reduction Gibbs free energies of the solvent-dependent dominant pre-reduction complex and the solvent-nanoparticle binding energy. The results indicate that lower nucleation rates originate from solvent coordination which stabilizes the pre-reduction complex and increases its reduction free energy. At the same time, DFT calculations suggest that the solvent coordination affects the effective capping of the surface where stronger binding solvents slow the nanoparticle growth by lowering the number of active sites (not already bound by trioctylphosphine). The findings represent a promising advancement towards understanding the microscopic connection between the metal-ligand thermodynamic interactions and the kinetics of nucleation and growth to control the size of colloidal metal nanoparticles.

Insights into Spectator-Directed Catalysis: CO Adsorption on Amine-Capped Platinum Nanoparticles on Oxide Supports

Introducing spectator molecules to the surface of supported noble metal nanoparticles is an innovative approach to improve the selectivity of heterogeneous catalysts. Colloidal synthesis of the nanoparticles allows researchers to select the spectator and the nanoparticle size, as well as the subsequent particle loading on different supports under well-defined conditions. However, understanding the interplay of the various effects that spectators can have on the catalytic properties of metal surfaces still requires further development. In this work, dodecylamine (DDA) is used to develop insights into the influence of spectator species on the chemical properties of 1.4-3.7 nm colloidal Pt nanoparticles on different supports (powders of Al₂O₃, ZnO, and TiO₂). DDA deposition results in two chemically distinct spectator species on the Pt surface depending on temperature, as evidenced from X-ray photoelectron spectroscopy (XPS). DDA selectively blocks terrace sites on the Pt nanoparticles at room temperature, as apparent from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a surface-sensitive probe molecule. The electron donor effect of the amine group in DDA influences the electron densities of the accessible lower coordinated, reactive Pt adsorption sites as indicated from spectral shifts in DRIFTS and XPS. Furthermore, DDA suppresses CO-induced surface reconstruction of the Pt surface and metal-support interactions, although these effects depend on temperature and support composition. Therefore, spectators may be used to adjust the nature of metal nanoparticle-oxidic support interactions. The experimental findings and mechanistic explanations are supported by density functional theory calculations. These results may build a platform in understanding the fundamental properties of amine spectators in Pt-based catalysis, activating specific sites, enhancing site selectivity, acting as sensors, and directing the metal-support interaction.

Control of crystallographic phases and surface characterization of intermetallic platinum tin nanoparticles

Colloidal synthesis and characterization of intermetallic tin-rich platinum–tin nanoparticles with detailed surface characterization.

Amine Capped Gold Colloids at Oxidic Supports: Their Electronic Interactions

Colloidal deposition of noble metal nanoparticles on oxidic supports is a recent approach for the fabrication of heterogeneous catalyst materials. We present studies on the interaction of different amine ligands with gold nanoparticles before and after deposition on several oxidic supports (titania, silica, alumina, magnesia or zinc oxide), using X-ray photoelectron and Auger spectroscopy, and high-resolution transmission electron microscopy. The adsorption of amines on thin gold films as well as on nanoparticles leads to a decrease in metal photoelectron binding energies. Usually, this is explained by donor-acceptor interactions via the amine group. By additional analysis of Auger signals, which are more sensitive to changes in the oxidation state than photoelectron spectra, we demonstrate that these shifts are due to a final state effect, namely, the increased photoelectron hole screening in presence of amine adsorbates. It will be shown, that this effect is not sensitive neither to the nanoparticle size nor the sterical properties of the capping amine. After deposition on oxide supports, the photoelectron binding energies are even further decreased. The presented findings exhibit that care has to be taken to interpret binding energy shifts simply with charging, which has impact on understanding the local electronic situation on the surface of metal-loaded oxides, crucial for heterogeneous catalysis.

Colloidal nanoparticle size control: experimental and kinetic modeling investigation of the ligand-metal binding role in controlling the nucleation and growth kinetics

Despite the major advancements in colloidal metal nanoparticles synthesis, a quantitative mechanistic treatment of the ligand's role in controlling their size remains elusive. We report a methodology that combines in situ small angle X-ray scattering (SAXS) and kinetic modeling to quantitatively capture the role of ligand-metal binding (with the metal precursor and the nanoparticle surface) in controlling the synthesis kinetics. We demonstrate that accurate extraction of the kinetic rate constants requires using both, the size and number of particles obtained from in situ SAXS to decouple the contributions of particle nucleation and growth to the total metal reduction. Using Pd acetate and trioctylphosphine in different solvents, our results reveal that the binding of ligands with both the metal precursor and nanoparticle surface play a key role in controlling the rates of nucleation and growth and consequently the final size. We show that the solvent can affect the metal-ligand binding and consequently ligand coverage on the nanoparticles surface which has a strong effect on the growth rate and final size (1.4 nm in toluene and 4.3 nm in pyridine). The proposed kinetic model quantitatively predicts the effects of varying the metal concentration and ligand/metal ratio on nanoparticle size for our work and literature reports. More importantly, we demonstrate that the final size is exclusively determined by the nucleation and growth kinetics at early times and not how they change with time. Specifically, the nanoparticle size in this work and many literature reports can be predicted using a single, model independent kinetic descriptor, (growth-to-nucleation rate ratio)^1/3, despite the different metals and synthetic conditions. The proposed model and kinetic descriptor could serve as powerful tools for the design of colloidal nanoparticles with specific sizes.

Sterical ligand stabilization of nanocrystals versus electrostatic shielding by ionic compounds: a principle model study with TEM and XPS

Electrostaticversussterical ligand stabilization: competitive stabilization mechanism play a key role in the control of nanomaterial properties.