Unveiling the gas-dependent sintering behavior of Au-TiO2 catalysts via environmental transmission electron microscopy

Imaging Gas-Involved Structural Dynamics by Environmental Electron Microscopy

Understanding gas-involved physicochemical reactions is undoubtedly one of the most significant challenges in the modern chemical industry. To clarify how those reactions precede requires deep insights into the real-time visualization of reaction dynamics within a gas environment. The emergence and rapid development of in situ environmental electron microscopy (EEM) including scanning electron microscopy (ESEM) and transmission electron microscopy (ETEM) have enabled multiscale observation of dynamic gas-involved physicochemical reactions. This review examines the state-of-art EEM technologies, categorizing those gas reactions into various physical and chemical processes and detailing the corresponding dynamic behaviors. It begins by reviewing the state-of-the-art EEM techniques and is followed by detailing their application in typical physical processes. It clarifies physical vapor condensation, deposition, and geometric reshaping with gaseous involving. More importantly, all the gas-involved chemical reactions into electrochemical reactions, thermochemical reactions, chemical crystal growth, and catalytic reactions are thoroughly explored and categorized. Finally, the review highlights the technical challenges and valuable perspectives provided by in situ EEM for addressing critical gas-involved issues. Overall, this article offers a multiscale and comprehensive understanding of the physicochemical origins associated with gas-involved reactions, envisioning fundamental strategies for designing high-performance gas-involved functional materials.

Au@TiO2 Core-Shell Nanoparticles with Nanometer-Controlled Shell Thickness for Balancing Stability and Field Enhancement in Plasmon-Enhanced Photocatalysis

Plasmonic core-shell nanostructures can make photocatalysis more efficient for several reasons. The shell imparts stability to the nanoparticles, light absorption is expanded, and electron-hole pairs can be separated more effectively, thus reducing recombination losses. The synthesis of metal@TiO2 core-shell nanoparticles with nanometer control over the shell thickness and understanding its effect on the resulting photocatalytic efficiency still remains challenging. In the present study, a synthesis method is presented for preparing Au@TiO2 core-shell nanoparticles with ultrathin shells that can be accurately tuned in the range of 2-12 nm, based on the controlled slow hydrolysis of a titanium precursor. Electromagnetic simulations combined with comprehensive characterization of the opto-physical bulk properties, as well as energy electron loss spectroscopy and electron tomography reconstructions at the nanoscale, aid in understanding the crucial role of the shell in improving both the activity as well as the stability in a photocatalytic reaction. Ultrathin shells in the order of 2 nm do not suffice to prevent sintering of the nanoparticles upon annealing, with a consequent loss of plasmonic properties. After reaching an optimum for a shell of 4 nm, further increasing the shell thickness again reduces the plasmonic properties by a weakened plasmonic coupling. This trend is confirmed by photocatalytic hydrogen evolution experiments, as well as stearic acid degradation tests. With this study, we prove and emphasize the crucial importance of carefully controlling the shell thickness in plasmonic core-shell structures, so their maximum application potential may be unlocked.

Surface Self-Diffusion Induced Sintering of Nanoparticles

Despite the critical role of sintering phenomena in constraining the long-term durability of nanosized particles, a clear understanding of nanoparticle sintering has remained elusive due to the challenges in atomically tracking the neck initiation and discerning different mechanisms. Through the integration of in situ transmission electron microscopy and atomistic modeling, this study uncovers the atomic dynamics governing the neck initiation of Pt-Fe nanoparticles via a surface self-diffusion process, allowing for coalescence without significant particle movement. Real-time imaging reveals that thermally activated surface morphology changes in individual nanoparticles induce significant surface self-diffusion. The kinetic entrapment of self-diffusing atoms in the gaps between closely spaced nanoparticles leads to the nucleation and growth of atomic layers for neck formation. This surface self-diffusion-driven sintering process is activated at a relatively lower temperature compared to the classic Ostwald ripening and particle migration and coalescence processes. The fundamental insights have practical implications for manipulating the morphology, size distribution, and stability of nanostructures by leveraging surface self-diffusion processes.

In situ observation of catalyst nanoparticle sintering resistance on oxide supports via gas phase transmission electron microscopy

Oxide-supported metal catalysts are essential components in industrial processes for catalytic conversion. However, the performance of these catalysts is often compromised in high temperature reaction environments due to sintering effects. Currently, a number of studies are underway with the objective of improving the metal support interaction (MSI) effect in order to enhance sintering resistance by surface modification of the oxide support, including the formation of inhomogeneous defects on the oxide support, the addition of a rare earth element, the use of different facets, encapsulation, and other techniques. The recent developments in in situ gas phase transmission electron microscopy (TEM) have enabled direct observation of the sintering process of NPs in real time. This capability further allows to verify the efficacy of the methods used to tailor the support surface and contributes effectively to improving sintering resistance. Here, we review a few selected studies on how in situ gas phase TEM has been used to prevent the sintering of catalyst NPs on oxide supports.

In Situ Visualization and Mechanistic Understandings on Facet-Dependent Atomic Redispersion of Platinum on CeO2

Redispersion is an effective method for regeneration of sintered metal-supported catalysts. However, the ambiguous mechanistic understanding hinders the delicate controlling of active metals at the atomic level. Herein, the redispersion mechanism of atomically dispersed Pt on CeO2 is revealed and manipulated by in situ techniques combining well-designed model catalysts. Pt nanoparticles (NPs) sintered on CeO2 nano-octahedra under reduction and oxidation conditions, while redispersed on CeO2 nanocubes above ∼500 °C in an oxidizing atmosphere. The dynamic shrinkage and disappearance of Pt NPs on CeO2 (100) facets was directly visualized by in situ TEM. The generated atomically dispersed Pt with the square-planar [PtO4]2+ structure on CeO2 (100) facets was also confirmed by combining Cs-corrected STEM and spectroscopy techniques. The redispersion and atomic control were ascribed to the high mobility of PtO2 at high temperatures and its strong binding with square-planar O4 sites over CeO2 (100). These understandings are important for the regulation of atomically dispersed platinum catalysts.

Direct Observation of Ni Nanoparticle Growth in Carbon-Supported Nickel under Carbon Dioxide Hydrogenation Atmosphere

Understanding nanoparticle growth is crucial to increase the lifetime of supported metal catalysts. In this study, we employ in situ gas-phase transmission electron microscopy to visualize the movement and growth of ensembles of tens of nickel nanoparticles supported on carbon for CO2 hydrogenation at atmospheric pressure (H2:CO2 = 4:1) and relevant temperature (450 °C) in real time. We observe two modes of particle movement with an order of magnitude difference in velocity: fast, intermittent movement (vmax = 0.7 nm s-1) and slow, gradual movement (vaverage = 0.05 nm s-1). We visualize the two distinct particle growth mechanisms: diffusion and coalescence, and Ostwald ripening. The diffusion and coalescence mechanism dominates at small interparticle distances, whereas Ostwald ripening is driven by differences in particle size. Strikingly, we demonstrate an interplay between the two mechanisms, where first coalescence takes place, followed by fast Ostwald ripening due to the increased difference in particle size. Our direct visualization of the complex nanoparticle growth mechanisms highlights the relevance of studying nanoparticle growth in supported nanoparticle ensembles under reaction conditions and contributes to the fundamental understanding of the stability in supported metal catalysts.

Comment on "Real-time atomistic simulation of the Ostwald ripening of TiO2 supported Au nanoparticles" by B. Zhu, R. Qi, L. Yuan and Y. Gao, Nanoscale, 2020, , 19142

Using Monte Carlo simulations (MCS) in combination with an analytical model for the metal-metal interaction with the parameters based on density functional theory (DFT), Zhu, Qi, Yuan, and Gao predicted that the Ostwald ripening of Au nanoparticles on TiO₂ occurs primarily via the detachment and attachment of Au dimers. I show that this and some other predictions are not properly validated because the parameters employed in the analytical model in order to describe the Au-Au interaction are in fact inconsistent both with DFT and experimental thermodynamical data.

Molecular Dynamics Simulation of Sintering Densification of Multi-Scale Silver Layer

Based on molecular dynamics (MD), in this study, a model was established to simulate the initial coating morphology of silver paste by using a random algorithm, and the effects of different sizes of particles on sintering porosity were also analyzed. The MD result reveals that compared with the sintering process using large-scale silver particles, the sintering process using multi-scale silver particles would enhance the densification under the same sintering conditions, which authenticates the feasibility of adding small silver particles to large-scale silver particles in theory. In addition, to further verify the feasibility of the multi-scale sintering, a semi in-situ observation was prepared for a sintering experiment using micro-nano multi-scale silver paste. The feasibility of multi-scale silver sintering is proved by theoretical and experimental means, which can provide a meaningful reference for optimizing the sintering process and the preparation of silver paste for die-attach in powering electronics industry. In addition, it is hoped that better progress can be made on this basis in the future.

Tuning the shape and crystal phase of TiO2 nanoparticles for catalysis

Synthesis of TiO₂ nanoparticles with tunable shape and crystal phase has attracted considerable attention for the design of highly efficient heterogeneous catalysts. Tailoring the shape of TiO₂, in the crystal phases of anatase, rutile, brookite and TiO₂(B), allows tuning of the atomic configurations on the dominantly exposed facets for maximizing the active sites and regulating the reaction route towards a specific channel for achieving high selectivity. Moreover, the shape and crystal phase of TiO₂ nanoparticles alter their interactions with metal species, which are commonly termed as strong metal-support interactions involving interfacial strain and charge transfer. On the other hand, metal particles, clusters and single atoms interact differently with TiO₂, because of the variation of the electronic structure, while the surface of TiO₂ determines the interfacial bonding via a geometric effect. The dynamic behavior of the metal-titania interfaces, driven by the chemisorption of the reactive molecules at elevated temperatures, also plays a decisive role in elaborating the structure-reactivity relationship.