Non-classical crystallization of CeO2 by means of in situ electron microscopy

During in situ liquid-phase electron microscopy (LP-EM) observations, the application of different irradiation dose rates may considerably alter the chemistry of the studied solution and influence processes, in particular growth pathways. While many processes have been studied using LP-EM in the last decade, the extent of the influence of the electron beam is not always understood and comparisons with corresponding bulk experiments are lacking. Here, we employ the radiolytic oxidation of Ce3+ in aqueous solution as a model reaction for the in situ LP-EM study of the formation of CeO2 particles. We compare our findings to the results from our previous study where a larger volume of Ce3+ precursor solution was subjected to γ-irradiation. We systematically analyze the effects of the applied irradiation dose rates and the induced diffusion of Ce ions on the growth mechanisms and the morphology of ceria particles. Our results show that an eight orders of magnitude higher dose rate applied during homogeneous electron-radiation in LP-EM compared to the dose rate using gamma-radiation does not affect the CeO2 particle growth pathway despite the significant higher Ce3+ to Ce4+ oxidation rate. Moreover, in both cases highly ordered structures (mesocrystals) are formed. This finding is explained by the stepwise formation of ceria particles via an intermediate phase, a signature of non-classical crystallization. Furthermore, when irradiation is applied locally using LP scanning transmission electron microscopy (LP-STEM), the higher conversion rate induces Ce-ion concentration gradients affecting the CeO2 growth. The appearance of branched morphologies is associated with the change to diffusion limited growth.

Noble metal nanodendrites: growth mechanisms, synthesis strategies and applications

Inorganic nanodendrites (NDs) have become a kind of advanced nanomaterials with broad application prospects because of their unique branched architecture. The structural characteristics of nanodendrites include highly branched morphology, abundant tips/edges and high-index crystal planes, and a high atomic utilization rate, which give them great potential for usage in the fields of electrocatalysis, sensing, and therapeutics. Therefore, the rational design and controlled synthesis of inorganic (especially noble metals) nanodendrites have attracted widespread attention nowadays. The development of synthesis strategies and characterization methodology provides unprecedented opportunities for the preparation of abundant nanodendrites with interesting crystallographic structures, morphologies, and application performances. In this review, we systematically summarize the formation mechanisms of noble metal nanodendrites reported in recent years, with a special focus on surfactant-mediated mechanisms. Some typical examples obtained by innovative synthetic methods are then highlighted and recent advances in the application of noble metal nanodendrites are carefully discussed. Finally, we conclude and present the prospects for the future development of nanodendrites. This review helps to deeply understand the synthesis and application of noble metal nanodendrites and may provide some inspiration to develop novel functional nanomaterials (especially electrocatalysts) with enhanced performance.

Functional Surfactant-Induced Long-Range Compressive Strain in Curved Ultrathin Nanodendrites Boosts Electrocatalysis

Curved ultrathin PtPd nanodendrites (CNDs) with long-range compressive strain and highly branched feature are first prepared by a functional surfactant-induced strategy. Precise synthesis realized the construction of both curved and flat PtPd nanodendrites (NDs) with the same atomic ratio, which contributed to exploration of the strain effect on electrocatalytic performance alone. Abundant evidence is provided to confirm that the long-range compressive strain in curved PtPd architectures can effectively tailor the local coordination environment of active sites, lower the position of the d-band center, weaken the adsorption energy of the intermediates (e.g., H* and CO*), and ultimately increase their intrinsic activity. The density functional theory (DFT) calculations further reveal that the introduction of compressive strain weakens the Gibbs free-energy of the intermediate (ΔG_H*), which is favorable for accelerating the hydrogen evolution reaction (HER) kinetics. A similar enhanced electrocatalytic performance can also be found in the methanol oxidation reaction (MOR).

Unveiling Growth Pathways of Multiply Twinned Gold Nanoparticles by In Situ Liquid Cell Transmission Electron Microscopy

A mechanistic understanding of the growth of multiply twinned nanoparticles (MTPs), such as decahedra (Dh) and icosahedra (Ih), is crucial for precisely controlled syntheses and applications. Despite previous successes, no consensus has been reached regarding the multiple competing growth pathways for MTPs proposed thus far, in part due to the lack of information about their nucleation and growth dynamics. Here, we used decahedral and icosahedral gold nanoparticles as a model system in conjunction with in situ liquid cell transmission electron microscopy (LCTEM) to investigate the nucleation and growth dynamics of MTPs in aqueous solution; two growth pathways were successfully identified: (A) nucleation-based layer-by-layer growth from a rounded multiply twinned seed and (B) the successive twinning and growth of tetrahedra. The LCTEM results enabled us to directly and conclusively identify the growth behaviors of intermediate products. The internal strain relaxation mechanisms and growth kinetics differ for the two pathways: in pathway A, a MTP grew by the opening and closing of re-entrant grooves at the twin boundaries, which was not found in pathway B. We also analyzed different MTP growth pathways from an energetic perspective and discussed how the preferred pathway (A or B) is related to factors, such as the initial seed yield and the size- and morphology-dependent formation of MTPs. Our results contextualize the current understanding of MTP formation mechanisms and provide insightful guidance for the precisely controlled synthesis of MTPs for practical applications.