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activity of Au NPs with size smaller than 5 nm has been often attributed to sizedependent physical features such as the fraction of surface atoms. [5][6][7] For instance, in reactions of hydrogenation, it is widely accepted that surface atoms on small particles are much more active than those on large particles since the fraction of low coordinated sites, that is, at edges and corners, which are the hydrogen activation sites, increases when size decreases. [8][9][10][11] This concept has been largely adopted for studying the reactivity of Au NPs, especially by theoreticians and surface science researchers. [12] The common practice is to consider Au nanoparticles as perfect model crystals with well-defined symmetry and static surface facets interacting with low coverage of gas molecules. However, the effect of adsorbates, which induces surface changes, is generally neglected in these studies. [13][14][15] This is a serious drawback that may prevent reliable description of the catalyst reactivity that mainly depends on the configuration of the surface. Aside from structural effects, quantum size effects are also frequently invoked to explain the exceptional activity of Au NPs. [16,17] For instance, using Density Functional Theory, Illas et al. have reported that the minimum energy pathway of O 2 dissociation for different Au NPs is chemically similar whichThe 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.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202104571.

In this study, structural parameters of Longkou oil shale kerogen were examined and identified by the combination of pyrolysis–gas chromatography–mass spectrometry, Fourier transform infrared spectroscopy, 13C nuclear magnetic resonance spectroscopy, and X-ray photoelectron spectroscopy. Based on the experimental data, a three-dimensional kerogen model was generated using quantum chemistry and molecular dynamics methods. The optimized molecular configuration was discussed, which showed good agreement with the experimental results in terms of structural characteristics. Electron density analysis was performed to examine the bonding characteristics of kerogen, and the bond length distribution of the Longkou kerogen model was analyzed, revealing that the S atom exhibits higher affinity for the H atom compared to the aliphatic carbon from the comparison of the electron density of the C–S and S–H bonding regions. Mulliken charge analysis was carried out to evaluate the partial atomic charges of heteroatoms. The charges on the cyclic structure tended to be equally distributed because of the presence of conjugated π bond, leading to the loss of charges on the N atoms. Besides, the HOMO–LUMO properties of Longkou kerogen were calculated, and a detailed picture of the frontier orbitals of kerogen for the inter- or intramolecular chemical reactions was obtained. This study validated that polycyclic aromatic structures in kerogen play a crucial role in the reactive sites for bond cleavage during the deformation of kerogen.

Nanoparticle stabilization against detrimental aggregation is a critical parameter that needs to be well controlled. Herein, we present a facile and rapid ion-mediated dispersing technique that leads to hydrophilic aggregate-free quantum dots (QDs). Because of the shielding of the hydrogen bonds between cysteamine-capped QDs, the presence of F(-) ions disassembled the aggregates of QDs and afforded their high colloidal stability. The F(-) ions also greatly eliminated the nonspecific adsorption of the QDs on glass slides and cells. Unlike the conventional colloidal stabilized method that requires the use of any organic ligand and/or polymer for the passivation of the nanoparticle surface, the proposed approach adopts the small size and large diffusion coefficient of inorganic ions as dispersant, which offers the disaggregation a fast reaction dynamics and negligible influence on their intrinsic surface functional properties. Therefore, the ion-mediated dispersing strategy showed great potential in chemosensing and biomedical applications.

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