Galyna Krylova

The Role of Order, Nanocrystal Size, and Capping Ligands in the Collective Mechanical Response of Three-Dimensional Nanocrystal Solids

Chemically synthesized PbS, CdSe, and CoPt(3) nanocrystals (NCs) were self-assembled into highly periodic supercrystals. Using the combination of small-angle X-ray scattering, X-ray photoelectron spectroscopy, infrared spectroscopy, thermogravimetric analysis, and nanoindentation, we correlated the mechanical properties of the supercrystals with the NC size, capping ligands, and degree of ordering. We found that such structures have elastic moduli and hardnesses in the range of approximately 0.2-6 GPa and 10-450 MPa, respectively, which are analogous to strong polymers. The high degree of ordering characteristic to supercrystals was found to lead to more than 2-fold increase in hardnesses and elastic moduli due to tighter packing of the NCs, and smaller interparticle distance. The nature of surface ligands also significantly affects the mechanical properties of NCs solids. The experiments with series of 4.7, 7.1, and 13 nm PbS NCs revealed a direct relationship between the core size and hardness/modulus, analogous to the nanoparticle-filled polymer composites. This observation suggests that the matrices of organic ligands have properties similar to polymers. The effective moduli of the ligand matrices were calculated to be in the range of approximately 0.1-0.7 GPa.

Capping Ligands as Selectivity Switchers in Hydrogenation Reactions

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt(3) NPs can drastically increase selectivity for alkene from 0 to more than 90% with ~99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes.

Study of Nucleation and Growth Mechanism of the Metallic Nanodumbbells

We propose a general nucleation and growth model that can explain the mechanism of the formation of CoPt(3)/Au, FePt/Au, and Pt/Au nanodumbbells. Thus, we found that the nucleation event occurs as a result of reduction of Au(+) ions by partially oxidized surface Pt atoms. In cases when Au(3+) is used as a gold precursor, the surface of seeds should be terminated by ions (e.g., Co(2+), Pb(2+)) that can reduce Au(3+) to Au(+) ions, which can further participate in the nucleation of gold domain. Further growth of gold domain is a result of reduction of both Au(3+) and Au(+) by HDA at the surface of gold nuclei. We explain the different ability of CoPt(3), Pt, and FePt seeds to serve as a nucleation center for the reduction of gold and further growth of dumbbells. We report that the efficiency and reproducibility of the formation of CoPt(3)/Au, FePt/Au, and Pt/Au dumbbells can be optimized by the concentration and oxidation states of the surface ions on metallic nanocrystals used as seeds as well as by the type of the gold precursor.

High-Pressure Structural Stability and Elasticity of Supercrystals Self-Assembled from Nanocrystals

We report here combined quasi-hydrostatic high-pressure small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) studies on faceted 3D supercrystals (SCs) self-assembled from colloidal 7.0 nm spherical PbS nanocrystals (NCs). Diamond anvil cell (DAC) SAXS experiments in the pressure range from ambient to 12.5 GPa revealed nearly perfect structural stability of the SCs, with face-centered cubic organization of the NCs. Pressure-induced ordering (annealing effect) of the superstructure was observed. The ambient pressure bulk modulus of the SCs was calculated to be ∼5 GPa for compression and ∼14.5 GPa for decompression from fitting of Vinet and Birch-Murnaghan equations of state. XRD measurements revealed strong preferential crystallographic orientation of the NCs through all phase transformations to as high as 55 GPa without any indication of NC sintering. The first phase transition pressure of the NCs was found between 8.1 and 9.2 GPa and proceeds through homogeneous nucleation. Bulk modulus of PbS NCs was calculated to be ∼51 GPa based on fitting to the equations of state (K(PbS,bulk) ∼ 51-57 GPa). Closest surface-to-surface distance between the NCs in the SCs was calculated based on combined XRD and SAXS data, to reversibly tune from ∼1.56 nm to ∼0.9-0.92 nm and back to ∼1.36 nm in the ambient-12.5 GPa-ambient pressure cycle. The bulk modulus of the ligand matrix was extrapolated to be ∼2.2-2.95 GPa. These results show a general method of tuning NC interactions in packed nanoparticle solids.

In Situ Optical and Structural Studies on Photoluminesence Quenching in CdSe/CdS/Au Heterostructures

We report here detailed in situ studies of nucleation and growth of Au on CdSe/CdS nanorods using synchrotron SAXS technique and time-resolved spectroscopy. We examine structural and optical properties of CdSe/CdS/Au heterostructures formed under UV illumination. We compare the results for CdSe/CdS/Au heterostructures with the results of control experiments on CdSe/CdS nanorods exposed to gold precursor under conditions when no such heterostructures are formed (no UV illumination). Our data indicate similar photoluminescence (PL) quenching and PL decay profiles in both types of samples. Via transient absorption and PL, we show that such behavior is consistent with rapid (faster than 3 ps) hole trapping by gold-sulfur sites at the surface of semiconductor nanoparticles. This dominant process was overlooked in previous end-point studies on semiconductor/metal heterostructures.

How “Hollow” Are Hollow Nanoparticles?

Diamond anvil cell (DAC), synchrotron X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) techniques are used to probe the composition inside hollow γ-Fe(3)O(4) nanoparticles (NPs). SAXS experiments on 5.2, 13.3, and 13.8 nm hollow-shell γ-Fe(3)O(4) NPs, and 6 nm core/14.8 nm hollow-shell Au/Fe(3)O(4) NPs, reveal the significantly high (higher than solvent) electron density of the void inside the hollow shell. In high-pressure DAC experiments using Ne as pressure-transmitting medium, formation of nanocrystalline Ne inside hollow NPs is not detected by XRD, indicating that the oxide shell is impenetrable. Also, FTIR analysis on solutions of hollow-shell γ-Fe(3)O(4) NPs fragmented upon refluxing shows no evidence of organic molecules from the void inside, excluding the possibility that organic molecules get through the iron oxide shell during synthesis. High-pressure DAC experiments on Au/Fe(3)O(4) core/hollow-shell NPs show good transmittance of the external pressure to the gold core, indicating the presence of the pressure-transmitting medium in the gap between the core and the hollow shell. Overall, our data reveal the presence of most likely small fragments of iron and/or iron oxide in the void of the hollow NPs. The iron oxide shell seems to be non-porous and impenetrable by gases and liquids.

Probing the Surface of Transition-Metal Nanocrystals by Chemiluminesence

We propose a simple chemiluminescence (CL) method for investigation of the surface of Co-based nanocrystals (NCs). Using a combination of CL and spin-trap electron paramagnetic resonance techniques, we systematically studied the generation of reactive oxygen species (ROS) at the surface of differently sized CoPt(3) spherical NCs and CoPt(3)/Au nanodumbbells. We have shown that differently sized CoPt(3) NCs can promote the formation of ROS and as a result can lead to the oxidation of luminol accompanied by the emission of the light. CL allows monitoring the stability of transition-metal-based NCs against oxidation and dissolution. We found by CL that cobalt ions slowly leach from the surface of CoPt(3) NCs even under very mild conditions; however, the amount of the leached cobalt ions does not exceed the maximal concentration of cobalt at the NC surface indicating that only surface atoms can go into solution.