Kinematic HAADF-STEM image simulation of small nanoparticles

Surface oxidation protection strategy of CoS2 by V2O5 for electrocatalytic hydrogen evolution reaction

Transition metal sulfides (TMSs) are promising electrocatalysts for hydrogen evolution reaction (HER), while TMSs usually suffer from inevitable surface oxidation in air, and the impact of the surface oxidation on their HER catalytic activity remains unclear. Herein, we demonstrate an effective strategy for reducing the surface oxidation degree of easily oxidized CoS₂ by introducing glued vanadium pentoxide (V₂O₅) nanoclusters, taking advantage of the preferential adsorption and strong interaction between high-valence V and O₂. Combining oxidation protection and elaborate oxidation control experiments reveal that reduced surface oxidation degree of CoS₂ is conducive to affording promising HER catalytic performance, as the oxidized surface of CoS₂ can hinder the dissociation of water and thus is harmful to the HER process. Direct evidence is provided that surface oxidation should be carefully considered for TMS-based HER catalysts. The present work not only develops a new strategy for protecting CoS₂ from surface oxidation, but also provides deep insight into the impact of surface oxidation on the HER performance of transition metal compounds.

Revealing Size Dependent Structural Transitions in Supported Gold Nanoparticles in Hydrogen at Atmospheric Pressure

The enhancement of the catalytic activity of gold nanoparticles with their decreasing size is often attributed to the increasing proportion of low-coordinated surface sites. This correlation is based on the paradigmatic picture of working gold nanoparticles as perfect crystal forms having complete and static outer surface layers whatever their size. This picture is incomplete as catalysts can dynamically change their structure according to the reaction conditions and as such changes can be eventually size-dependent. In this work, using aberration-corrected environmental electron microscopy, size-dependent crystal structure and morphological evolution in gold nanoparticles exposed to hydrogen at atmospheric pressure, with loss of the face-centered cubic crystal structure of gold for particle size below 4 nm, are revealed for the first time. Theoretical calculations highlight the role of mobile gold atoms in the observed symmetry changes and particle reshaping in the critical size regime. An unprecedented stable surface molecular structure of hydrogenated gold decorating a highly distorted core is identified. By combining atomic scale in situ observations and modeling of nanoparticle structure under relevant reaction conditions, this work provides a fundamental understanding of the size-dependent reactivity of gold nanoparticles with a precise picture of their surface at working conditions.

Shape control of size-selected naked platinum nanocrystals

Controlled growth of far-from-equilibrium-shaped nanoparticles with size selection is essential for the exploration of their unique physical and chemical properties. Shape control by wet-chemistry preparation methods produces surfactant-covered surfaces with limited understanding due to the complexity of the processes involved. Here, we report the controlled production and transformation of octahedra to tetrahedra of size-selected platinum nanocrystals with clean surfaces in an inert gas environment. Molecular dynamics simulations of the growth reveal the key symmetry-breaking atomic mechanism for this autocatalytic shape transformation, confirming the experimental conditions required. In-situ heating experiments demonstrate the relative stability of both octahedral and tetrahedral Pt nanocrystals at least up to 700 °C and that the extended surface diffusion at higher temperature transforms the nanocrystals into equilibrium shape.

Proximity Enhanced Hydrogen Evolution Reactivity of Substitutional Doped Monolayer WS2

The development of stable and low-cost catalysts with high reactivity to replace Pt-based ones is the central focus but challenging for hydrogen evolution reaction (HER). The incorporation of single atoms into two-dimensional (2D) supports has been demonstrated as an effective strategy because of the highly active single atomic sites and extremely large surface area of two-dimensional materials. However, the doping of single atoms is normally performed on the surface suffering from low stability, especially in acidic media. Moreover, it is experimentally challenging to produce monolayered 2D materials with atomic doping. Here, we propose a strategy to incorporate single foreign Fe atoms to substitute W atoms in sandwiched two-dimensional WS₂. Because of the charge transfer between the doped Fe atom and its neighboring S atoms on the surface, the proximate S atoms become active for HER. Our theoretical prediction is later verified experimentally, showing an enhanced catalytic reactivity of Fe-doped WS₂ in HER with the Volmer-Heyrovsky mechanism involved. We refer to this strategy as proximity catalysis, which is expected to be extendable to more sandwiched two-dimensional materials as substrates and transition metals as dopants.

Discovery of face-centred cubic Os nanoparticles

The first example of the crystal structure control of Os is reported. The fcc-structured Os nanoparticles were synthesized using an Os acetylacetonate complex as a precursor although the fcc structure does not exist in the bulk state.

Anomalous variable-temperature photoluminescence of CsPbBr3 perovskite quantum dots embedded into an organic solid

All-inorganic lead halide perovskite quantum dots (PQDs) of CsPbBr₃ were synthesized at room temperature via a facile solution-based procedure. The cubic phase structure of the synthesized PQDs was judiciously identified through examining high-resolution transmission electron microscopy (TEM) images, selected area electronic diffraction (SAED) patterns and scanning TEM images of the PQDs. Variable-temperature photoluminescence (PL) spectra of the CsPbBr₃ PQDs randomly embedded into a frozen solid of methylbenzene were measured in the temperature range of 5-180 K. It is found that both the linewidth and peak position of the measured PL spectra are abnormally almost temperature independent in the temperature range of interest. Some competing mechanisms, such as a competition between the bandgap blue shift induced by thermal lattice expansion and red shift induced by thermal escaping of localized excitons, and a competition between lineshape broadening by phonon scattering and narrowing by thermal escaping of localized excitons, are proposed to interpret the phenomena. Good agreement between the theoretical fitting and the experimental data leads to a state-of-the-art understanding of the temperature-dependent luminescence of the CsPbBr₃ PQDs in a solid matrix.

Structural properties of sub-nanometer metallic clusters

At the nanoscale, the investigation of structural features becomes fundamental as we can establish relationships between cluster geometries and their physicochemical properties. The peculiarity lies in the variety of shapes often unusual and far from any geometrical and crystallographic intuition clusters can assume. In this respect, we should treat and consider nanoparticles as a new form of matter. Nanoparticle structures depend on their size, chemical composition, ordering, as well as external conditions e.g. synthesis method, pressure, temperature, support. On top of that, at finite temperatures nanoparticles can fluctuate among different structures, opening new and exciting horizons for the design of optimal nanoparticles for advanced applications. This article aims to overview geometrical features of transition metal clusters and of their various rearrangements.

Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties

Nanostructures have attracted huge interest as a rapidly growing class of materials for many applications. Several techniques have been used to characterize the size, crystal structure, elemental composition and a variety of other physical properties of nanoparticles. In several cases, there are physical properties that can be evaluated by more than one technique. Different strengths and limitations of each technique complicate the choice of the most suitable method, while often a combinatorial characterization approach is needed. In addition, given that the significance of nanoparticles in basic research and applications is constantly increasing, it is necessary that researchers from separate fields overcome the challenges in the reproducible and reliable characterization of nanomaterials, after their synthesis and further process (e.g. annealing) stages. The principal objective of this review is to summarize the present knowledge on the use, advances, advantages and weaknesses of a large number of experimental techniques that are available for the characterization of nanoparticles. Different characterization techniques are classified according to the concept/group of the technique used, the information they can provide, or the materials that they are destined for. We describe the main characteristics of the techniques and their operation principles and we give various examples of their use, presenting them in a comparative mode, when possible, in relation to the property studied in each case.

A one-pot-one-reactant synthesis of platinum compounds at the nanoscale

The preparation of inorganic nanomaterials with a desired structure and specific properties requires the ability to strictly control their size, shape and composition. A series of chemical reactions with platinum compounds carried out within the 1.5 nm wide channel of single-walled carbon nanotubes (SWNTs) have demonstrated the ability of SWNTs to act as both a very effective reaction vessel and a template for the formation of nanocrystals of platinum di-iodide and platinum di-sulphide, materials that are difficult to synthesise in the form of nanoparticles by traditional synthetic methods. The stepwise synthesis inside nanotubes has enabled the formation of Pt compounds to be monitored at each step of the reaction by aberration-corrected high resolution transmission electron microscopy (AC-HRTEM), verifying the atomic structures of the products, and by an innovative combination of fluorescence-detected X-ray absorption spectroscopy (FD-XAS) and Raman spectroscopy, monitoring the oxidation states of the platinum guest-compounds within the nanotube and the vibrational properties of the host-SWNT, respectively. This coupling of complementary spectroscopies reveals that electron transfer between the guest-compound and the host-SWNT can occur in either direction depending on the composition and structure of the guest. A new approach for nanoscale synthesis in nanotubes developed in this study utilises the versatile coordination chemistry of Pt which has enabled the insertion of the required chemical elements (e.g. metal and halogens or chalcogens) into the nanoreactor in the correct proportions for the controlled formation of PtI₂ and PtS₂ with the correct stoichiometry.

Intermetallic Nix My (M = Ga and Sn) Nanocrystals: A Non-precious Metal Catalyst for Semi-Hydrogenation of Alkynes

Intermetallic Nix My (M = Ga and Sn) nanocrystals with uniform particle size and controlled composition are successfully synthesized via a solution-based co-reduction strategy. The as-obtained nanocrystals are crystalline and structurally ordered. The active-site isolation and modified electronic structure are responsible for the excellent catalytic performance for alkyne semi-hydrogenation of the as-obtained non-precious catalysts.